101

INTRODUCTION

The air conditioning is that branch of engineering science which deals with the study of conditioning of air i.e. supplying and maintaining desirable internal atmospheric conditions for human comfort, irrespective of external conditions. This subject, in its broad sense, also deals with the conditioning of air for industrial purposes, food processing, storage of food and other materials.

In another sense, the term can refer to any form of cooling, heating, ventilation, or disinfection that modifies the condition of air. An air conditioner (often referred to as AC) is an appliance, system, or machine designed to stabilize the air temperature and humidity within an area (used for cooling as well as heating depending on the air properties at a given time), typically using a refrigeration cycle but sometimes using evaporation, commonly for comfort cooling in buildings and motor vehicles.

The concept of air conditioning is known to have been applied in Ancient Rome, where aqueduct water was circulated through the walls of certain houses to cool them. Similar techniques in medieval Persia involved the use of cisterns and wind towers to cool buildings during the hot season. Modern air conditioning emerged from advances in chemistry during the 19th century, and the first large-scale electrical air conditioning was invented and used in 1902 by Willis Havilland Carrier.

This air conditioner has become one of the important items in the home appliances which require high power to operate. This is the main disadvantage with the air conditioners and has to be minimized. In order to minimize this issue we are optimizing the design of the evaporator of the air conditioner thermally and thus achieve higher efficiency.

The optimization technique used here is adopted from various design techniques followed by different air conditioning units

The structural design of a tube and fin evaporator has been modeled using the software

AIR CONDITIONING

Air-conditioning is a process that simultaneously conditions air; distributes it combined with the outdoor air to the conditioned space; and at the same time controls and maintains the required space’s temperature, humidity, air movement, air cleanliness, sound level, and pressure differential within predetermined limits for the health and comfort of the occupants, for product processing, or both.

An air conditioner (often referred to as AC or air con.) is an appliance, system, or machine designed to stabilize the air temperature and humidity within an area (used for cooling as well as heating depending on the air properties at a given time), typically using a refrigeration cycle but sometimes using evaporation, commonly for comfort cooling in buildings and motor vehicles.

REFRIGERATION

Refrigeration is the cooling effect of the process of extracting heat from a lower temperature heat source, a substance or cooling medium, and transferring it to a higher temperature heat sink, probably atmospheric air and surface water, to maintain the temperature of the heat source below that of the surroundings.

DIFFERENCES BETWEEN AIR CONDITIONING AND REFRIGERATION

The main difference between refrigeration and air conditioning is that refrigeration deals with cooling only whereas air conditioning take care of Humidity, Purity, Odour, Circulation and Temperature.

In many ways air conditioning and refrigeration systems are very similar. Both use specially designed chemicals, the physical effects of the compression and expansion of gases, and the conversion of gas to liquid to reduce the temperature of air. The varying uses of these systems, however, mean refrigeration and air conditioning systems have a handful of key differences in the design and operation.

SUPPLY:

A major difference between refrigeration and air conditioning is the point of supply for the gases. Refrigeration systems have gas installed in a series of tubes. In old refrigerators, this gas was chloro-flouro-carbon, or CFC, but this has harmful effects on people, so refrigerators not contain HFC-134a. HFC-134a is the sole gas used as a coolant in refrigeration systems. Air conditioning systems use built-in chemicals, but also air from the room or rooms being heated. Gases built into air conditioning units cool air that circulates through the unit; the unit then redistributes the cooled air through the room.

CIRCULATION:

Air conditioners have circulation systems designed to project cool air away from the units while refrigeration units have circulation systems designed to retain coolant in a confined space. Refrigeration systems circulate cool liquids and gases through a series of tubes and vents. Cool air from within a refrigerator is sucked into a compressor that recycles the gas through the tubes. Air conditioners, while also employing tubes in the coolant system, have fans for the dispersal of air. Unlike refrigeration systems, which keep gases contained to a pre-determined space, air conditioning systems disperse cool air throughout areas of unknown volume.

VAPORIZATION:

Both air conditioning and refrigeration units depend on converting liquid to gas in the cooling process, but the manner in which they achieve this is different for each system. Air conditioners use something called an evaporator to convert a liquid to a gas. An evaporator is a small, narrow hole designed to change the pressure of a liquid so that it evaporates. Refrigeration units, on the other hand, cycle HFC into a low-pressure chamber designed to boil the gas. This boiling causes HFC to vaporize. Vaporization is the process of converting a liquid to a gas and can be accomplished one of two ways: boiling or evaporation. Thus air conditioning units vaporize liquid through evaporation while refrigeration systems do so through boiling.

EQUIPMENTS USED IN AN AIR CONDITIONING SYSTEM

1) CIRCULATION FAN: The main function of this fan is to move air to and from the room.

2) AIR CONDITIONING UNIT: It is a unit, which consists of cooling and dehumidifying processes for summer air conditioning or heating and humidification processes for winter air conditioning.

3) SUPPLY DUCT: It directs the conditioned air from the circulating fan to the space to be air conditioned at proper point.

4) SUPPLY OUTLETS: These are grills, which distribute the conditioned air evenly in the room.

5) RETURN OUTLETS: These are the openings in a room surface which allow the room air to enter the return duct.

6) FILTERS: The main function of the filters is to remove dust, dirt and other harmful bacteria’s from the air.

COMPONENTS OF AIR CONDITIONING UNIT

1) COMPRESSOR: the low pressure and temperature vapor refrigerant from evaporator is drawn into the compressor through the inlet or suction valve, where it is compressed to a high pressure and temperature. The high pressure and temperature vapor refrigerant is discharged into the condenser through the delivery or discharge valve

2) CONDENSER: the condenser or cooler consists of coils of pipes in which the pressure and temperature vapor refrigerant is cooled and condensed. The refrigerant, while passing through the condenser, gives up its latent heat to the surrounding condensing medium which is normally air or water.

3) EXPANSION VALVE: it is also called as throttle valve or refrigerant control valve. The function of the expansion valve is to allow the liquid under high pressure and temperature to pass at a controlled rate after reducing its pressure and temperature. some of the liquid refrigerant evaporates as it passes through the expansion valve , but the greater portion is vaporized in the evaporator at low pressure and temperature.

4) EVAPORATOR: an evaporator consists of coils of pipes in which the liquid refrigerant at low pressure and temperature is evaporated and changed into vapor refrigerant at low pressure and temperature. In evaporating, the liquid vapor refrigerant absorbs its latent heat of vaporization from the medium (air) which is to be cooled.

FACTORS AFFECTING AIR CONDITIONING

The four factors affecting the air conditioning are:

1) Temperature of air 2) Humidity of air 3) Purity of air4) Motion of air

1) Temperature of air:

In air conditioning, the control of temperature means the maintenance of any desired temperature within an enclosed space even though the temperature of the outside air is above or below the desired room temperature. This is accomplished either by the addition or removal of heat from the enclosed space as and when demanded.

2) Humidity of air:

The control of humidity of air means the decreasing or increasing of moisture contents of air during summer or winter respectively in order to produce comfortable and healthy conditions.

3) Purity of air:

It is an important factor for the comfort of human body. It has important that proper filtration, cleaning and purification of air are essential to keep it free from dust and other impurities.

4) Motion of air:

The motion or circulation of air is another important factor which should be controlled, in order to keep constant temperature throughout the conditioned space. It is therefore, necessary that there should be equal-distribution of air throughout the space to be air conditioned.

CLASSIFICATION OF AIR CONDITIONERS

The air conditioning systems may be broadly classified as follows:

1) According to the purposea) Comfort air conditioning systemb) Industrial air conditioning system

2) According to season of the yeara) Winter air conditioning systemb) Summer air conditioning systemc) Year round air conditioning system

3) According to the arrangement of equipmenta) Unitary air conditioning systemb) Central air conditioning system

TYPES OF AIR CONDITIONERS

1) AUTOMOBILE AIR CONDITIONERS

Air conditioning systems are designed to allow the driver and or passengers to feel more comfortable during uncomfortably warm, humid, or hot trips in a vehicle.

Factors such as wind resistance, aerodynamics and engine power and weight have to be factored into finding the true variance between using the air conditioning system and not using it when figuring out difference in actual gas mileage. Other factors on the impact on the engine and an overall engine heat increase can have an impact on the cooling system of the vehicle.

2)PORTABLE AIR CONDITIONER

A portable air conditioner is one on wheels that can be easily transported inside a home or office. They are currently available with capacities of about 6,000-60,000 BTU/h (1,800-18,000 W output) and with and without electric resistance heaters. Portable air conditioners are either evaporative or refrigerative.

Portable refrigerative air conditioners come in two forms, split and hose.

A portable split system has an indoor unit on wheels connected to an outdoor unit via flexible pipes, similar to a permanently fixed installed unit.

Hose systems, which can be air-to-air or monoblock, are vented to the outside via air ducts. The monoblock type collects the water in a bucket or tray and stops when full. The air-to-air type re-evaporates the water and discharges it through the ducted hose, and can run continuously.

3)DEHUMIDIFIER

A specific type of air conditioner that is used only for dehumidifying is called a dehumidifier.

A dehumidifier is different from a regular air conditioner in that both the evaporator and condenser coils are placed in the same air path, and the entire unit is placed in the environment that is intended to be conditioned (in this case dehumidified), rather than requiring the condenser coil to be outdoors.

Having the condenser coil in the same air path as the evaporator coil produces warm, dehumidified air.

The evaporator (cold) coil is placed first in the air path, dehumidifying the air exactly as a regular air conditioner does. The air next passes over the condenser coil re-warming the now dehumidified air.

The terms "condenser coil" and "evaporator coil" do not refer to the behavior of water in the air as it passes over each coil; instead they refer to the phases of the refrigeration cycle.

Having the condenser coil in the main air path rather than in a separate, outdoor air path (as in a regular air conditioner) results in two consequences—the output air is warm rather than cold, and the unit is able to be placed anywhere in the environment to be conditioned, without a need to have the condenser outdoors.

Dehumidifiers are commonly used in cold, damp climates to prevent mold growth indoors, especially in basements. They are also sometimes used in hot, humid climates for comfort because they reduce the humidity which causes discomfort (just as a regular air conditioner, but without cooling the room). They are also used to protect sensitive equipment from the adverse effects of excessive humidity in tropical countries.

WORKING AIR CONDITIONER

1. The compressor compresses cool refrigerant gas, causing it to become hot, high-pressure refrigerant gas 

2. This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid.

3. The Freon liquid runs through an expansion valve, and in the process it evaporates to become cold, low-pressure refrigerant gas 

4. This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building.

Air conditioner

HUMIDITY CONTROL IN AIR CONDITIONER

Refrigeration air conditioning equipment usually reduces the humidity of the air processed by the system.

The relatively cold (below the dew point) evaporator coil condenses water vapor from the processed air (much like an ice-cold drink

Will condense water on the outside of a glass), sending the water to a drain and removing water vapor from the cooled space and lowering the relative humidity.

Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided.

The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space. In food retailing establishments, large open chiller cabinets act as highly effective air dehumidifying units.

AIR CONDITIONING APPLICATIONS

AN AIR CONDITIONER Air conditioning engineers broadly divide air conditioning applications into comfort and process.

Comfort applications aim to provide a building indoor environment that remains relatively constant in a range preferred by humans despite changes in external weather conditions or in internal heat loads.

In addition to buildings, air conditioning can be used for many Air conditioning makes deep plan buildings feasible, for otherwise they'd have to be built narrower or with light wells so that inner spaces receive sufficient outdoor air via natural ventilation. Air conditioning also allows buildings to be taller since wind speed increases significantly with altitude making natural ventilation impractical for very tall buildings. Comfort applications for various building types are quite different and may be categorized as

Low-Rise Residential buildings, including single family houses, duplexes, and small apartment buildings.

High-Rise Residential buildings, such as tall dormitories and apartment blocks

Commercial buildings, which are built for commerce, including offices, malls, shopping centers, restaurants, etc.

Institutional buildings, which includes hospitals, governmental, academic, and so on.

Industrial spaces where thermal comfort of workers is desired. Sports Stadiums Types of transportation — motor-cars and other land vehicles,

trains, ships, aircraft, and spacecraft.

Process applications aim to provide a suitable environment for a process being carried out, regardless of internal heat and humidity loads and external weather conditions. Although often in the comfort range, it is the needs of the process that determine conditions, not human preference. Process applications include these:

Hospital operating theatres, in which air is filtered to high levels to reduce infection risk and the humidity controlled to limit patient dehydration.

Clean rooms for the production of integrated circuits, pharmaceuticals, and the like, in which very high levels of air cleanliness and control of temperature and humidity are required for the success of the process.

Facilities for breeding laboratory animals. Since many animals normally only reproduce in spring, holding them in rooms at which conditions mirror spring all year can cause them to reproduce year-round.

Aircraft air conditioning. Although nominally aimed at providing comfort for passengers and cooling of equipment, aircraft air conditioning presents a special challenge because of the changing density associated with changes in altitude, humidity and temperature of the outside air.

Data centers Textile factories Physical testing facilities Plants and farm growing areas Nuclear facilities Chemical and biological laboratories Mines Industrial environments Food cooking and processing areas

In both comfort and process applications, the objective may be to not only control temperature, but also humidity, air quality and air movement from space to space.

HEALTH ISSUES FOR AIR CONDITIONERS

Air-conditioning system can promote the growth and spread of microorganisms, such as Legionella pneumophila, the infectious agent responsible for Legionnaires' disease, or thermophilic actinomycetes, but as long as the air conditioner is kept clean these health hazards can be avoided.

Conversely, air conditioning, including filtration, humidification, cooling, disinfection, etc., can be used to provide a clean, safe, hypoallergenic atmosphere in hospital operating rooms and other environments where an appropriate atmosphere is critical

to patient safety and well-being. Air conditioning can have a positive effect on sufferers of allergies and asthma.

ENERGY USE

In a thermodynamically closed system, any energy input into the system that is being maintained at a set temperature (which is a standard mode of operation for modern air conditioners) requires that the energy removal rate from the air conditioner increases.

This increase has the effect that for each unit of energy input into the system (say to power a light bulb in the closed system) this requires the air conditioner to remove that energy.

In order to do that the air conditioner must increase its consumption by the inverse of its efficiency times the input of energy.

REFRIGERANTS

Any substance, which absorbs heat through vaporization or expansion, is called a refrigerant. In broader sense, the term refrigerant is also applied to such cooling systems as cold water or brine solutions. As commonly interpreted, refrigerants include those working mediums that pass through the cycle of evaporation, recovery, compression and condensation. Thus, circulating cold mediums are not primary refrigerants, nor are cooling systems such as ice and solid carbon dioxide.

Desirable Properties of Refrigerants

Desirable refrigerants are those which possess chemical, physical, and thermodynamic properties that permit their efficient application and

service, in practical designs of refrigerating equipment. In addition, if the volume of charge is large, there should be no danger to health and property in case of its escape. A great variety of substances, such as butane, carbon tetrachloride, ethane, and hexane have been applied to refrigeration systems, but found to have little practical use. These materials are either highly explosive or flammable, or possess other combinations of undesirable properties.

Different Refrigerants

Air

Air is one of the earliest refrigerants and was widely used in World War I whenever a completely nontoxic material was required. Although air is free of cost and completely safe, its low coefficient of performance makes it unable to compete with the modern nontoxic refrigerants.

Ammonia

Ammonia is one of the oldest and widely used of all refrigerants. It is flammable and highly toxic. It is widely used in commercial and large industrial reciprocating compression systems where high toxicity is secondary.

Carbon Dioxide

It is a colorless and odorless gas, which is heavier than air. It is nontoxic and nonflammable but has extremely high operating pressures. In former years it was used for marine refrigeration, theatre air conditioning systems, and for hotel refrigeration systems.

Freon Refrigerants

These refrigerants use ethane and methane as bases, and are the most important group of refrigerants being used in modern technology.

These are used in a variety of applications, such as reciprocating compression refrigeration, and rotary compressors.

Others

Other refrigerants are methyl chloride, sulphur dioxide, hydrocarbon refrigerants, methyl chloride, and azeotropes.

Selection of Refrigerants

No substance has proved to be the ideal working medium, under all operating conditions. The characteristics of some refrigerants make them desirable for use with reciprocating compressors. In some applications toxicity is of negligible importance, whereas in others, such as comfort cooling, a nontoxic and nonflammable refrigerant is essential. Therefore, in selecting the correct refrigerant, it is necessary to determine those properties which are most suitable, and to choose the most closely approaching ideal for the particular application.

OZONE DEPLETION

OZONE: Ozone (O3) is a gas with molecules made of three atoms of oxygen. This is different from the oxygen we breathe (O2), which has two atoms of oxygen and makes up 21% of the Earth's atmosphere. Ozone is found throughout the Earth's atmosphere in minute quantities (about 0.6 parts per million, on average). Ozone is found in higher concentrations in pollution, and can be a health risk.

PROCESSThe ozone depletions process begins when CFCs and other ozone depleting substances are emitted into the atmosphere. The winds then mix the troposphere and evenly distribute the gases. CFCs are extremely stable and do not dissolve in rain. After several years, the ODS molecules reach the stratosphere, about 10 km above Earth's surface.

Strong UV lights break apart the ODS molecules. CFCs, HCFCs, carbon tetrachloride, methyl chloroform, and other gases release chlorine atoms. These are the atoms that actually destroy the ozone, not the intact ODS molecule. It's estimated that one chlorine atom can destroy over 100,000 ozone molecules before it is finally removed from the stratosphere.

Ozone depletion mechanism

OZONE LAYER:Ozone in the upper atmosphere is important to health. In the stratosphere, the region of the atmosphere about 12 to 45 km above the surface of the Earth, ozone exists in larger amounts. Between 20 and 40 km high, ozone makes up about 6 parts per million of the air. This higher concentration of ozone, called the ozone layer, absorbs much ultraviolet light from the Sun.

The ozone layer exists because it is routinely created from oxygen by solar ultraviolet light. Ultraviolet (UV) light is higher energy light than visible light. The ozone concentrations in the stratosphere represent equilibrium between creation of ozone and the ozone destruction that results when it absorbs UV. The stratosphere is warmer than the air above and below it as a result of these processes.

EFFECTS

If the ozone were to totally deplete then the risk of skin cancer would

be of great danger. The main skin cancers are non-melanoma skin cancers that are commonly found in the white population. There are two major forms of non-melanoma skin tumors and they are basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). However the incidence of basal cell carcinoma is generally several times greater than the incidence of squamous cell carcinoma. Squamous cell carcinomas account for as much as 4/5s of all non-melanoma skin cancer deaths (NAS, 1984). Prolonged sunlight exposure is considered to be the dominant risk factor for non-melanoma skin tumors.

The potential effects on humans and the environment have led to international resolutions designed to gradually phase out production of ozone-depleting substances. As a result, the scientific and industrial communities have collaborated to find safe and economical replacements.

An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as rice, depend on Cyan bacteria residing on their roots for the retention of nitrogen. Cyan bacteria are sensitive to UV light and they would be affected by its increase

Chlorofluorocarbons (CFCs), the manmade chemicals implicated in ozone loss.

ENERGY RATINGS FOR AIR CONDITIONER

The most important thing to look for with an air conditioner is the star rating. You need to work out what size you require for the task and then choose the most efficient model that will perform the task.

There are three kinds of Energy rating labels for air conditioners. The one for cooling-only models has a blue band of stars and a blue box for the energy consumption figure. As the name implies, these air conditioners only have a cooling mode. The label for reverse-cycle models has two bands of stars. The blue band shows the efficiency when cooling, and the red band shows the efficiency when heating. There are also two energy consumption figures - blue for cooling and red for heating. As the name implies, the term reverse cycle means that the operation of the air conditioner can be internally reversed to provide indoor heating or cooling as required - the principle used for both modes is the same.

The two main types of air conditioners for household use are window-wall systems and split systems. While both can be equally efficient, split systems tend to be more efficient for a particular size range as their components are generally less constrained by size (although this is not always true). Split systems have the advantage of being quieter indoors during operation but they are also more expensive.

A new innovation in air conditioner technology is the use of an inverter or variable speed drive in the motor system that drives the compressor. While these systems tend to look less efficient at full load (i.e. their star rating at rated capacity is not always as high as conventional air conditioners), they tend to be very efficient at part load operation, which is a more common mode in a typical household.

Sizing an air conditioner

The output capacity is a measure of the amount of heat that will be removed (cooling) or added (heating) to the room/s in your house by the air conditioner. The output range you need will depend upon your particular requirements. Air conditioner outputs are measured in kilowatts (kW). As an approximate guide for sizing room units allow:

125watts (0.125kW) per square meter of floor area to be cooled in living areas;

80 watts (0.080kW) per square meter of floor area in bedrooms.

These estimates depend on the climate and the efficiency of your house design (orientation, glazing and insulation levels).

How are air conditioner star ratings calculated?

Power Input (also called Comparative Energy Consumption or CEC)

The energy consumption or power input of an air conditioner is measured under conditions specified. Because the heating and cooling requirement is affected by climate, air conditioner use is not shown on the energy label. Instead, the cooling and/or heating Output and the Power Input is shown on the energy label at rated capacity (the units on the label are in kW which is the same as kWh/hour). To work out the likely annual energy use will require information on the climate and other factors such as occupancy (hours that cooling is required) and building shell performance (insulation, glazing, orientation etc). It is important to note that under normal usage, the air conditioner will

spend a significant amount of time at less than its rated capacity - in terms of efficiency this is really only important for variable output models which can have higher efficiency under part load conditions.

Capacity Output

The measure of energy service for an air conditioner is the rated cooling and/or heating capacity of the air conditioner, usually specified in kilowatts (kW) (some product brochures use BTUs (British Thermal Units), although this is now unusual and some retailers use compressor "horsepower", although this has no meaning in terms of the units capability). These rated values are as declared by the manufacturer and under the test conditions based on the international standard. The heating capacity of a reverse cycle air conditioner is the heat that can be put into a room. Similarly, the cooling capacity is the heat that can be removed from a room. The cooling capacity is made up of the sensible component (usually the majority of the capacity) which relates to the actual temperature reduction (cooling) of the air, plus the latent component, which is a measure of the dehumidification effect of the indoor air. Latent cooling capacity is sometimes expressed as moisture removal capacity in liters or kg of water per hour (1 kg per hour of moisture removal is equal to 683 Watts latent capacity).

How can the Capacity Output be greater than the Power Input?

Refrigerative air conditioners (the only type covered by energy labeling evaporative units are not included) use a technique called the vapor compression cycle to "move" energy in the form of heat from one space to another. This is generally a very efficient process and the amount of low grade heat that can be moved is typically 3 to 5 times (or more) the energy required to run the compressor system. This ratio is called the Energy Efficiency Ratio (EER), used for cooling, or Coefficient of Performance (COP), for heating, and is used as the basis for determining the star rating of an air conditioner . A refrigeration heat pump collects internal heat and moves it outside when in cooling mode, or collects ambient heat from outside and moves it inside when in heating mode.

Performance

To be eligible for an energy label (and to comply with MEPS), an air conditioner must meet the maximum cooling test. This ensures that the air conditioner is capable of operating under extreme conditions. The air conditioner also has to have a tested capacity of not less than 95% of the rated value and a tested energy consumption of not more than 105% of the rated value.

Star Rating

The star rating for air conditioners is determined differently to other appliances. For air conditioners, the measure of energy efficiency is the Energy Efficiency Ratio (EER) for cooling and the Coefficient of Performance (COP) for heating. The EER and COP are defined as the capacity output divided by the power input. The Star Rating Index is calculated on the measured values for energy and capacity during a rating test, rather than the nameplate or rated values.

The original star rating equations for air conditioners were developed in 1987. These were revised (re-graded) in 2000 and again in 2010 to take account of the substantial improvement in the energy efficiency of products over this period. Until 2010, all energy labels showed possible star ratings from a minimum of 1 star to a maximum of 6 stars. In 2010, the star rating system for refrigerators and air conditioners was expanded to show up to 10 stars for products that have exceptional energy efficiency. Products that achieve up to 5 stars continue to use a normal 5 star energy label.

Air conditioner energy label

The star rating for air conditioners is determined from the measured EER and COP. From 2010, for cooling, 1 star is equal to an EER of 2.75 with an extra star for an increase in EER of 0.5. For heating, 1 star is also equal to a COP of 2.75 with an extra star for an increase in COP of 0.5 Importantly, the 2010 star rating system is based on an annual efficiency calculation which includes any non-operational energy consumption such as standby and power consumption of crank case heaters (where present).

Bee star rating for air conditioners

THERMODYNAMIC CYCLES USED IN AIR CONDITIONING

A thermodynamic cycle consists of a series of thermodynamic

processes transferring heat and work, while varying pressure,

temperature, and other state variables, eventually returning

a system to its initial state. In the process of going through this cycle,

the system may perform work on its surroundings, thereby acting as

a heat engine.

State quantities depend only on the thermodynamic state, and

cumulative variation of such properties adds up to zero during a

cycle. Process quantities (or path quantities), such

as heat and work are process dependent, and cumulative heat and

work are non-zero. The first law of thermodynamics dictates that the

net heat input is equal to the net work output over any cycle. The

repeating nature of the process path allows for continuous operation,

making the cycle an important concept in thermodynamics.

Thermodynamic cycles often use quasistatic processes to model the

workings of actual devices.

The air conditioner mainly consists of vapor cycles, refrigerant

circulate in the air conditioner according these cycle itself

Vapor cycle can further be classified as:1. Vapor-compression cycle2. Vapor-absorption cycle

VAPOR-COMPRESSION CYCLE

The vapor-compression cycle is used in most household refrigerators

as well as in many large commercial and industrial refrigeration

systems. Figure below provides a schematic diagram of the processes

of a typical vapor-compression cycle

P–h diagram for refrigerant flow in air conditioning system

The thermodynamics of the cycle can be analyzed on a diagram as shown in the above figure . In this cycle, a circulating refrigerant such as Freon enters the compressor as a vapor. From point 1 to point 2, the vapor is compressed at constant entropy and exits the compressor as a vapor at a higher temperature, but still below the vapor pressure at that temperature. From point 2 to point 3 and on to point 4, the vapor travels through the condenser which cools the vapor until it starts condensing, and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature. Between points 4 And 5, the liquid refrigerant goes through the expansion valve (also called a throttle valve) where its pressure abruptly decreases, causing flash evaporation and auto-refrigeration of, typically, less than half of the liquid.

That results in a mixture of liquid and vapor at a lower temperature and pressure as shown at point 5. The cold liquid-vapor mixture then

travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

The above discussion is based on the ideal vapor-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

The thermodynamic cycles for various refrigerants are shown on the vapor dome of the particular refrigerant which depends upon various factors such as saturation temperature, boiling temperature and freezing temperature, etc.

The different vapor domes for the refrigerant R-22 are been generated by using the software cool pack are shown below

P-h diagram for R-22 refrigerant

T-s diagram for R-22 refrigerant

H-s diagram for R-22 refrigerant

EVAPORATOR

The evaporator is an important device used in the low pressure side of a refrigeration system. The liquid refrigerant from the expansion valve enters into the evaporator where it boils and changes into the evaporator where it boils and changes into vapor.

The function of an evaporator is to absorb heat from the surrounding location or medium which is to be cooled, by means of a refrigerant.

The temperature of the boiling refrigerant in the evaporator must always be less than that of the surrounding medium so that the heat flows to the refrigerant.

The evaporator becomes cold and remains cold due to the following two reasons:

The temperature of the evaporator coil is low due to the low temperature of the refrigerant inside the coil.

The low temperature of the refrigerant remains unchanged because any heat it absorbs is converted to latent heat as boiling proceeds.

Within a downstream processing system, several stages are used to further isolate and purify the desired product. The overall structure of the process includes pre-treatment, solid-liquid separation, concentration, and purification and formulation.

Evaporation falls into the concentration stage of downstream processing and is widely used to concentrate foods, chemicals, and salvage solvents.

The goal of evaporation is to vaporize most of the water from a solution containing a desired product. After initial pre-treatment and separation, a solution often contains over 85% water.

This is not suitable for industry usage because of the cost associated with processing such a large quantity of solution, such as the need for larger equipment.

ENERGETICS

Water can be removed from solutions in ways other than evaporation, including membrane processes, liquid-liquid extractions, crystallization, and precipitation.

Evaporation can be distinguished from some other drying methods in that the final product of evaporation is a concentrated liquid, not a solid. It is also relatively simple to use and understand since it has been widely used on a large scale.

In order to concentrate a product by water removal, an auxiliary phase is used which allows for easy transport of the solvent (water) rather than the solute.

Water vapor is used as the auxiliary phase when concentrating non-volatile components, such as proteins and sugars.

Heat is added to the solution and part of the solvent is converted into vapor. Heat is the main tool in evaporation, and the process occurs more readily at high temperature and low pressures.

Heat is needed to provide enough energy for the molecules of the solvent to leave the solution and move into the air surrounding the solution.

The energy needed can be expressed as an excess thermodynamic potential of the water in the solution.

Leading to one of the biggest problems in industrial evaporation, the process requires enough energy to remove the water from the solution and to supply the heat of evaporation. When removing the water, more than 99% of the energy needed goes towards supplying the heat of evaporation.

The need to overcome the surface tension of the solution also requires energy. The energy requirement of this process is very high because a

phase transition must be caused; the water must go from a liquid to a vapor.

When designing evaporators, engineers must quantify the amount of steam needed for every mass unit of water removed when a concentration is given.

An energy balance must be used based on an assumption that a negligible amount of heat is lost to the system’s surroundings. The heat that needs to be supplied by the condensing steam will approximately equal the heat needed to heat and vaporize the water. Another consideration is the size of the heat exchanger which affects the heat transfer rate.

WORKING OF AN EVAPORATOR

The solution containing the desired product is fed into the evaporator and passes a heat source. The applied heat converts the water in the solution into vapor.

The vapor is removed from the rest of the solution and is condensed while the now concentrated solution is either fed into a second evaporator or is removed.

The evaporator as a machine generally consists of four sections. The heating section contains the heating medium, which can vary. Steam is fed into this section.

The most common medium consists of parallel tubes but others have plates or coils. The concentrating and separating section removes the vapor being produced from the solution. The condenser condenses the separated vapor, then the vacuum or pump provides pressure to increase circulation.

Heat reaches the evaporator by three methods of heat transfer:

1. Thermal Conduction: is the flow of thermal energy through Substance molecule to molecule from a higher to a lower temperature region.

2. Thermal Convection: is the transfer of thermal energy by actual physical movement from one location to another of a substance such as air or water which thermal energy is stored.

3. Thermal Radiation: is the energy radiated by solids, liquid and gas in the form of electromagnetic waves, which transfer energy because of their temperature. This energy transfer heat through a space without heating the space but is absorbed by objects that it reaches.

Most of the heat in air-cooling applications is carried to the evaporator by Thermal convection currents. The Thermal convection currents set up in the refrigerant space either by action of a fan or by gravity circulation resulting from the difference in temperature between the evaporator and the space. In addition, some heat is Thermal radiated directly to the evaporator from the product and from the walls of the space. When the product is in thermal contact with the outer surface of the evaporator, heat is transferred from the product to the evaporator by direct Thermal conduction. For a liquid cooling application, where the liquid is being cooled, there must always be contact with the evaporator surface and some circulation of the cooled fluid either by gravity or by action of a pump.

Regardless of how the heat reaches the outside surface of the evaporator, it must pass through the wall of the evaporator to the refrigerant inside by conduction. Therefore, the capacity of the evaporator (the rate at which heat passes through the walls) is

determined by the same factors that governing the rate of heat flow by Thermal conduction through any heat transfer surface

Classification of evaporators:-

1. According to the type of construction(a)Bare tube coil evaporator(b)Finned tube evaporator(c) Plate evaporator (d)Shell and tube evaporator(e)Shell and coil evaporator(f) Tube in tube evaporator

2. According to the manner in which liquid refrigerants fed(a)Flooded evaporator (b)Dry expansion evaporator

3. According to the mode of heat transfer(a)Natural convection evaporator(b)Forced convection evaporator

4. According to operating conditions(a)Frosting evaporator (b) Non frosting evaporator(c) Defrosting evaporator

Bare tube coil evaporator

The bare tube evaporator is also known as prime surface evaporator. Because of simple in construction and easy to clean. In this evaporator the contact surface area is less compare to other evaporators. The amount of surface area increases by extending the tube length. The effective length of tube proportional to the capacity of expansion valve. The long tubes will also cause considerably greater pressure drop between inlet and out let of evaporator. The diameter of tube in relation to tube length may also be critical. The tube diameter is too large, the

refrigerant velocity will be too low and the volume of refrigerant will be too great in relation to surface area of tube to complete vaporization.

Bare tube & finned tube evaporator

Finned evaporators:-

It consists of bare tubes or coils over which the metal plates or fins are fastened. The metal fins are constructed of thin sheet of metal having good thermal conductivity. In this type of evaporators have high contact surface area and good heat transfer rate. This is also called as extended surface evaporators.

Shell and tube evaporators:-

This evaporator is similar to shell and tube condenser. It consists of number of horizontal tubes enclosed in a cylindrical shell. The inlet and outlet headers with perforated metal tube sheets are connected at each of the tubes. These are generally used chilled water or brine solutions. The dry expansion shell and tube evaporator used for refrigerating units of 2 to 20 TR. The flooded type shell and tube evaporator used for refrigerating units of 10 to 5000TR.

Shell and coil evaporator:-

This type of evaporators is generally dry expansion evaporators to chill water. The shell is May open or sealed. The sealed shells are usually found in shell and coil evaporators used to cool drinking water. The evaporators having flanged shells are often used to chill water in secondary refrigeration system.

Tube in tube evaporator:-

The tube in tube evaporator consists of one tube inside another tube. The liquid to be cooled flows through the inner tube while the primary refrigerant or secondary refrigerant circulates in the space between the two tubes. The tube in tube evaporator provides high heat transfer rates. However they require more space than shell and tube evaporators of the same capacity. These evaporators are used for wine cooling and in petroleum industry for chilling of oil.

Flooded type evaporator:-

It is constant liquid refrigerant level maintained and in this the float control valve used as an expansion valve to maintain constant liquid level in evaporator. The liquid refrigerant flow through the low side of the evaporator and the accumulator before entering to the evaporator. The accumulator is also known as surge drawn or surge tank. It maintains the constant liquid level to separate the liquid refrigerant to the vapor returning to the compressor. Since the evaporator is almost completely filled with liquid refrigerant, therefore the vapor refrigerant from the evaporator is not super heated but it is in a saturated condition. In order to prevent liquid refrigerant to enter into the compressor, accumulator is generally used with the flooded evaporator. The liquid refrigerant trapped in the accumulator is re circulated through the evaporator. The evaporator coil is connected to the accumulator and

the liquid flow from the accumulator to the evaporator coil is generally by gravity. The vapor formed by vaporizing the liquid in the coil being lighter, rises passes through top of accumulator from when is supplied to the suction side of the compressor. The baffle plate arrests any liquid present in the vapor. The advantage of the evaporator is whole surface is contact with the refrigerant in the evaporator. Then it gives high rate of heat transfer rate. The flooded evaporator more expensive to operate because it required more refrigerant. It applicable for industries mostly it used at chemical and food processing industries.

Dry expansion evaporator:-

The dry expansion evaporator not really dry at all. They simply used relatively little refrigerant as compared to flooded evaporator having the same coil volume. The dry expansion evaporators are usually only one forth or one third filled with liquid refrigerant. The simple bare tube dry expansion evaporator. The finned coil expansion evaporator also available. The rate at which the liquid refrigerant is fed to the evaporator generally depends upon the rate of vaporization and increases or decreases as the load on the evaporator increases or decreases. When the liquid refrigerant passes through the expansion valve, some vapour is formed. The flash gas causes bubbles can cause dry areas on the interior walls of the coil. The dry areas reduced the rate of heat transfer. Thus, the evaporator efficiency decreases as dry areas increases that when the load on the evaporator is light. If the cooling lad on the evaporator is heavy, the expansion valve allows the large volume of liquid refrigerant into the coil in order to accommodate the heavy load.

Natural convection evaporator:-

The natural convection evaporator’s arte used where air velocity low and minimum hydration of the product is desired. The domestic refrigerator, water cooler and small freezer have natural convection evaporators. The evaporator coil should be placed as high as possible in the refrigerant is because the cold air falls down as it leaves the evaporator. The velocity of air over the evaporator coil considerably affects the capacity. The velocity of air depends on the temperature of outside and inside of evaporator. The circulation of air around the coil depends its size, shape and location.

Forced convection evaporator:-

In this evaporator s, the air is forced over the refrigerant cooled coils and fins. This done by fan driven by an electric motor. The fins are provided to increases the heat transfer rate. It is more efficient to compare to natural convection evaporator because they required less cooling surface and high evaporator pressure can be used which save considerable power input to the compressor. These are mainly used for cooling units as well as for refrigerator cabinets used for store bottled beverage or foods in sealed containers.

Frosting evaporator:-

This evaporator operates at 0°c temperature. This means that the coil frosts continually when in use and it must be removed at regular intervals either manually or automatically for most efficient operation. The frost which forms on the evaporator comes from the moisture in the air. The coil or cooling efficiency decreases until the ice and frost is removed. The evaporators fall under the frosting evaporators.

Non frosting evaporator:-

It operates above the 0°c a all times. Therefore frost does not on the evaporator. The evaporator builds up a light coat of frost just before the compressor shuts off. This frost immediately melts on the off cycle. The advantages of a non frosting evaporator are its operation at a temperature close to freezing. This maintained a relative humidity from 75 to 85% in the cabinet.

Plate evaporator

Plate evaporators have a relatively large surface area. The plates are usually corrugated and are supported by frame. During evaporation, steam flows through the channels formed by the free spaces between the plates. The steam alternately climbs and falls parallel to the concentrated liquid. The steam follows a co-current, counter-current path in relation to the liquid. The concentrate and the vapor are both fed into the separation stage where the vapor is sent to a condenser.

Plate evaporators are frequently applied in the dairy and fermentation industries since they have spatial flexibility. A negative point of this type of evaporator is that it is limited in its ability to treat viscous or solid-containing products. Pillow plate from Daussiny can be used for bulk heating or cooling of solids.

Multiple effect evaporators

Unlike single-stage evaporators, these evaporators can be made of up to seven evaporator stages or effects. The energy consumption for single-effect evaporators is very high and makes up most of the cost for an evaporation system. Putting together evaporators saves heat and thus requires less energy. Adding one evaporator to the original decreases the energy consumption to 50% of the original amount. Adding another effect reduces it to 33% and so on. A heat saving % equation can be used to estimate how much one will save by adding a certain amount of effects.

The number of effects in a multiple-effect evaporator is usually restricted to seven because after that, the equipment cost starts catching up to the money saved from the energy requirement drop. There are two types of feeding that can be used when dealing with multiple-effect evaporators.

Forward feeding takes place when the product enters the system through the first effect, which is at the highest temperature. The product is then partially concentrated as some of the water is transformed into vapor and carried away. It is then fed into the second effect which is a little lower in temperature. The second effect uses the heated vapor created in the first stage as its heating source (hence the saving in energy expenditure).

The combination of lower temperatures and higher viscosities in subsequent effects provides good conditions for treating heat-sensitive products like enzymes and proteins. In using this system, an increase in the heating surface area of subsequent effects is required.

Another way to proceed is by using backward feeding. In this process, the dilute products is fed into the last effect with has the lowest temperature and is transferred from effect to effect with the temperature increasing.

The final concentrate is collected in the hottest effect which provides an advantage in that the product is highly viscous in the last stages so the heat transfer is considerably better.

APPLICATIONS

The goal of evaporation is to concentrate a target liquid, and this needs to be achieved for many different targets today.

One of the most important applications of evaporation is that on the food and drink industry. Many foods that are made to last for a considerable amount of time or food that needs a certain consistency, like coffee, need to go through an evaporation step during processing.

It is also used as a drying process and can be applied in this way to laboratories where preservation of long-term activity or stabilization is needed (for enzymes for example).

Evaporation is also used in order to recover expensive solvents such as hexane which would otherwise be wasted.

Another example of evaporation is in the recovery of sodium hydroxide in Kraft pulping.

Cutting down waste handling cost is another major application of evaporation for large companies. Legally, all producers of waste must dispose of the waste in methods that abides by environmental guidelines; these methods are costly. If up to 98% of wastes can be vaporized, industry can greatly reduce the amount of money that would otherwise be allocated towards waste handling.

PROBLEMS CAUSED BY EVAPORATORS

Technical problems can arise during evaporations, especially when the process is applied to the food industry.

Some evaporators are sensitive to differences in viscosity and consistency of the dilute solution. These evaporators could work inefficiently because of a loss of circulation.

The pump of an evaporator may need to be changed if the evaporator needs to be used to concentrate a highly viscous solution.

Fouling also occurs when hard deposits form on the surfaces of the heating mediums in the evaporators. In foods, proteins and

polysaccharides can create such deposits that reduce the efficiency of heat transfer.

Foaming can also create a problem since dealing with the excess foam can be costly in time and efficiency. Antifoam agents are to be used, but only a few can be used when food is being processed.

Corrosion can also occur when acidic solutions such as citrus juices are concentrated. The surface damage caused can shorten the long-life of evaporators. Quality and flavor of food can also suffer during evaporation.

Overall, when choosing an evaporator, the qualities of the product solution need to be taken into heavy consideration.

FACTORS AFFECTING THE HEAT TRANSFER CAPACITY OF AN

EVAPORATOR

The following are the important factors

1) MATERIAL: In order to have rapid heat transfer in an

evaporator, the material used for the construction of an

evaporator coil should be a good conductor of heat. The material

which is not affected by the refrigerant must also be selected.

Since metals are best conductors of heat, therefore they are

always used for evaporators. Iron and steel can be used with all

common refrigerants. Brass and copper are used with all

refrigerants except ammonia. Aluminum should not be used with

Freon.

2) TEMPERATURE DIFFERENCE: The temperature difference between the refrigerant within the evaporator and the product to be cooled plays an important role in the heat transfer capacity of an evaporator. Too low temperature difference may cause slime

on meat or poultry. Too high temperature difference cause excessive dehydration.

3) VELOCITY OF REFRIGERANT: The velocity of refrigerant also affects the heat transfer capacity of an evaporator. If the velocity of refrigerant flowing through the evaporator increases, the overall heat transfer coefficient also increases. But this increased velocity will cause greater pressure loss in the evaporator. Thus the only recommended velocities for different refrigerants which give high heat transfer rates and allowable pressure loss should be used.

4) THICKNESS OF THE EVAPORATOR COIL WALL: The thickness of the evaporator coil wall also affects the heat transfer capacity of the evaporator. In general, the thicker the wall, the slower is the rate of heat transfer. Since the refrigerant in the evaporator coils is under pressure, therefore the evaporator walls are thick enough to withstand the effects of that pressure. It may be noted that the thickness has only a slight effect on total heat transfer capacity because the evaporators are usually made from highly conductive materials.

5) CONTACT SURFACE AREA: An important factor affecting the evaporator capacity is the contact surface available between the walls of evaporator coil and the medium being cooled. The amount of contact surface, in turn, depends basically on the physical size and shape of the evaporator coil.

6) SUPERHEAT: Superheat is the measurement of how full the evaporator is of liquid refrigerant. High superheat means the evaporator is empty. Low superheat means the evaporator is full. Superheat should never fall below 6°F at the evaporator leaving suction line or 12°F at the compressor in inlet of a split system with 30 feet of suction line, or compressor failure will occur

Schematic diagram of a simple refrigerating system

PSYCHROMETRY

The psychrometry is that branch of engineering science, which deals with the study of moist air i.e. dry air mixed with water vapor or humidity. It also includes the study of behavior of dry air and water vapor under various sets of conditions.

PSYCHROMETRIC TERMS

1) DRY AIR: The pure dry air is a mixture of a number of gases such as nitrogen, oxygen, carbon dioxide, hydrogen, argon, neon, helium etc. But the nitrogen and oxygen have the major portion of the combination.

2) MOIST AIR: It is a mixture of dry air and water vapor. The amount of water vapor present in air depends upon the absolute pressure and temperature of the mixture.

3) SATURATED AIR: It is a mixture of dry air and water vapor, when the air has diffused the maximum amount of water vapor into it. The water vapors usually occur in the form of superheated steam as an invisible gas.

4) DEGREE OF SATURATION: It is the ratio of actual mass of water vapor in a unit mass of dry air to the mass of water vapor

in the same mass of dry air when it is saturated at the same temperature.

5) HUMIDITY: It is the mass of water vapor present in 1 kg of dry air. It is also called specific humidity or humidity ratio.

6) ABSOLUTE HUMIDITY: It is the mass of water vapor present in 1 m3 of dry air.

7) RELATIVE HUMIDITY: It is the ratio of actual mass of water vapor in a given volume of moist air to the mass of water vapor in the same volume of saturated air at the same temperature and pressure. It is briefly written as RH.

8) DRY BULB TEMPERATURE: It is the air recorded by a thermometer, when it is not affected by the moisture present in the air. The dry bulb temperature is generally denoted by td or tdb.

9) WET BULB TEMPERATURE: It is the temperature of air recorded by a thermometer, when its bulb is surrounded by a wet cloth exposed to the air. Such a thermometer is called “Wet bulb thermometer”. It is generally denoted by tw or twb.

10) WET BULB DEPRESSION: It is the difference between dry

bulb temperature and wet bulb temperature at any point. The

wet bulb depression indicates relative humidity of air.

11) DEW POINT TEMPERATURE: it is the temperature of air recorded by a thermometer, when the moisture present in it begins to condense. In other words, the dew point temperature is the saturation temperature (t sat) corresponding to the partial pressure of water vapor (pv). It is usually denoted by tdb.

12) DEW POINT DEPRESSION: It is the difference between the dry bulb temperature and dew point temperature of air.

PSYCHROMETRIC CHART

It is a graphical representation of the various thermodynamic properties of moist air. The psychrometric chart is very useful for finding out the properties of air (which are required in the field of air conditioning) and eliminate lot of calculations.

In a psychrometric chart, dry bulb temperature is taken as abscissa and specific humidity i.e. moisture contents as ordinate. Now the saturation curve is drawn by plotting the various saturation points at corresponding dry bulb temperatures. The saturation curve represents 100% relative humidity at various dry bulb temperatures. It also represents the wet bulb and dew point temperatures.

PSYCHROMETRIC CHART

The important lines in psychrometric chart are:

1) DRY BULB TEMPERATURE LINES: The dry bulb temperature lines are vertical i.e. parallel to the ordinate and uniformly spaced. Generally the temperature range of these lines on psychrometric chart is from -6 C to 45 C. The dry bulb temperature lines are drawn with difference of every 5 C and up to the saturation curve. The values of dry bulb temperatures are also shown on the saturation curve.

2) SPECIFIC HUMIDITY OR MOISTURE CONTENT LINES: The specific humidity (moisture content) lines are horizontal i.e. parallel to the abscissa and are also uniformly spaced. Generally, moisture content range of these lines on psyshrometric chart is from 0 to 30 g / kg of dry air. The moisture content lines are drawn with the difference of every 1 g (or 0.001kg) and up to the saturation curve.

3) DEW POINT TEMPERATURE LINES: The dew point temperature lines are horizontal i.e. parallel to the abscissa and non-uniformly spaced. At point on the saturation curve, the dry bulb temperature and dew point temperature are equal. The values of dew point temperatures are generally given along the saturation curve of the chart.

4) WET BULB TEMPERATURE LINES: The wet bulb temperature lines are inclined straight lines and non-uniformly spaced. At any point on the saturation curve, the dry bulb and wet bulb temperatures are equal.

5) ENTHALPY (TOTAL HEAT) LINES: The enthalpy (or total heat) lines are inclined straight lines and uniformly spaced. These lines are parallel to the wet bulb temperature lines, and are drawn up to the saturation curve. Some of these lines coincide with the wet bulb temperature lines also.

6) SPECIFIC VOLUME LINES: The specific volume lines are obliquely inclined straight lines and uniformly spaced. These lines are drawn up to the saturation curve.

7) VAPOUR PRESSURE LINES: The vapor pressure lines are horizontal and uniformly spaced. Generally, the vapor pressure lines are not drawn in the main chart. But a scale showing vapor pressure in mm of Hg is given on the extreme left side of the chart.

8) RELATIVE HUMIDITY LINES: The relative humidity lines are curved lines and follow the saturation curve. Generally, these lines are drawn with the values 10%, 20%, 30%, etc. and up to 100%. The saturation curve represents 100% relative humidity.

COOL PACK

INTRODUCTION

Coolpack is a collection of simulation models for refrigeration systems. The models each have a specific purpose e.g. cycle analysis, dimensioning of main components, energy analysis and -optimization.

Coolpack is developed by the department of mechanical engineering (mek), section of energy engineering (et)at the technical university of Denmark (dtu).

The development of coolpack has until version 1.33 been financed by the Danish energy agency.

The development of coolpack started in spring of 1998 as a part of a research project. The objective of this project was to develop simulation models to be used for energy optimization of refrigeration systems. the users of these models would be refrigeration technicians, engineers, students etc. in short all the persons with influence on the present and future energy consumption of refrigeration systems.

The first idea was to make a general and comprehensive simulation program that would give the user all the flexibility he/she could wish for in terms of handling many different system designs and investigation purposes. Some of the characteristics of very general and flexible programs are that they require many user inputs/selections

and that their numerical robustness is rather low. Experience with this type of programs has shown that this type of simulation programs is far from ideal for the main part of the users mentioned above. since most of theseUsers have limited time for carrying out the investigation, general and comprehensive programs will in many cases be very ineffective to use and they are therefore often discarded by the users.

The idea behind the development of coolpack is different from the idea described above. Instead of creating a large, general and comprehensive simulation program we have chosen to create a collection of small, easy to use, and numerically robust simulation programs. the typical simulation program in coolpack deals with only on type of refrigeration system and has a specific investigation purpose. it therefore only requires the user inputs/selections necessary to describe operating conditions etc. and not any inputs for describing the system design or for specifying the input/output structure associated with the simulation purpose.

When developing the programs for coolpack we have focused on making the underlying system models as simple, relevant and numerically robust as possible. we have preserved some flexibility in that the user can select refrigerant and also specify inputs (like pressure) in more than one way (saturation temperature or pressure).

The program in coolpack covers the following simulation purposes:

• Calculation of refrigerant properties (property plots, thermodynamic & thermo physical data, refrigerant comparisons)• Cycle analysis – e.g. comparison of one- and two-stage cycles• System dimensioning – calculation of component sizes from general dimensioning criteria• System simulation – calculation of operating conditions in a system with known components evaluation of operation – evaluation of system efficiency and suggestions for reducing the energy consumption• Component calculations – calculation of component efficiencies• Transient simulation of cooling of an object – e.g. for evaluation of cooling down periods

To make it easier to get an overview of the programs in coolpack we have chosen to divide the programs into three main groups (refrigeration utilities, EESCoolTools and dynamic).

The group refrigeration utilities consist of 3 refrigerant oriented programs, primarily used for calculating the properties of primary and secondary refrigerants, creating property plots for primary refrigerants (like p-h, t-s and h-s diagrams) and for calculating the pressure drop forFlow of secondary refrigerants in pipes. Furthermore, it is possible to create property plots for humid air (psychrometric charts).

The group EESCoolTools contains a large collection of programs for both refrigeration Systems and components. We have chosen to divide this group into four subgroups. The groups also represent the four phases of designing a refrigeration system. The programs in these four groups have almost the same type of user interface, making it easier to combine their use and also use them for comparisons. The name EESCoolTools consists of the three words EES, cool and tools:• "EES" refers to the name of the program we have implemented our simulation models in (engineering equation solver - EES). EES is developed by S.A. Klein and F.L. Alvarado, and is sold by f-chart software in Wisconsin, USA. You can get more information about EES and fchartSoftware on the internet • "cool" refers to the fact, that the simulation models are related to the area of refrigeration.• "tools" refers to that the programs are thought to be tools enabling you to make faster and more consistent (energy) design and analysis.

The group named dynamic contains the dynamic programs in coolpack. So far only a single program is available. with this program it is possible to simulate the cooling down of an object/room under various conditions and with on/off-capacity control of the compressor.

The dynamic element is modeled and solved using a DAE solver application called windali. Windali is based on the Dali-program developed in 1985, at what at that time was called the refrigeration

laboratory at the technical university of Denmark (now a part of department of energy engineering).

THERMO DYNAMICAL AND THERMO PHYSICAL PROPERTIES OF REFRIGERANTS USING COOL PACK

This model can be used to calculate the thermodynamic and thermo physical (transport) properties for refrigerants. The properties for both saturated liquid and gas at the same pressure as specified are also calculated.

Since many of the routines for transport properties are only valid for a more narrow interval than the thermodynamic properties, it is sometimes necessary to omit (select NO) the calculations of the transport properties.

NOTE: The user MUST specify a single phase state - two phase states cannot be used as input.

Calculation of the properties of R-22

Calculation of the properties of R-134a

COOLING DEMAND FOR AN AIR-CONDITIONED ROOM USING COOL PACK

a) Introduction

The model is used for calculating the cooling demand for an air-conditioned room in a steady state condition. The cooling demand consists of several individual cooling demands which will be described in the following.

b) Heat transfer through building parts

The heat load from heat transfer through building parts is calculated for all 4 walls, roof and ceiling individually. The heat transfer is calculated as follows

Q = k · A · (T - T_ROOM)

Where Q is the heat transfer in [W], A is the area of the building part [m²], k is heat transfer coefficient [W/ (m²·K)], T_ROOM is the temperature in the room in [°C] and T is the temperature on the hot side of the building part in [°C].

The heat transfer coefficient (k-value) of each building part depends on the insulation material (foam or mineral wool), how the building part is constructed and the thickness of the insulating layer of air on both sides of the building. In air-conditioned rooms there is usually a significant air movement (due to the fans) and the thickness of the air layer close to the wall is very thin. This means that the air layer (and thermal resistance of the layer) on the inside of the refrigerated room can be ignored. In most cases this is also true for on the outside of the wall. In both cases the thermal conductivity of the air layer should be taken into account if there is no air movement.

If you want to include the thermal resistance of the air layer you should use the following equation for calculating the heat transfer coefficient:

Where m_INNER is the thermal resistance [(m²·K)/W] of the air layer on the inside of the wall, m_WALL is the thermal resistance of the wall and m_OUT is the insulation property on the outside of the wall.

Typical values of m_INNER and m_OUTER are 0.04 (m²·K)/W if there is a good air movement (thin layer) and 0.08 (m²·K)/W with none or low air movement (thick layer). The insulation property of the wall can be calculated as follows

Solar radiation through windows can be specified as heat load per area [W/m² (window area)] and the window area [m²].

c) Air change (infiltration) and SHR

Due to opening of doors and leaks in the building parts warm and humid air flows into the room. This air has to be cooled and maybe also dehumidified. The flow of air infiltrating the room can be specified either directly by the flow in [m³/h] or indirectly by the Air Change Factor (ACF) specifying the flow of air as number of times the total room volume is changed per 24 hours.

d) Auxiliary loads

A part of the heat load in an air-conditioned room is caused by persons working in the room or from heat developing equipment in the room. These loads can be specified individually.

1) Persons

The heat load from persons working in the room is based on the maximal number of persons that are in the room at the same time.

There are three different types of work.

Light - typically inspection of goods

Medium - typically handling of goods using equipment

Heavy - typically manual handling of goods

The calculations are based on the assumption that there is a linear relation between the heat developed by a person and the room temperature. See H. Drees "Kühlanlagen", Table 12.18, page 376, 14. Edition, VEB Verlag Technik, Berlin 1987.

2) Lighting

The heat load from lighting equipment corresponds to the electrical power consumption of the lighting equipment .The heat load can be specified per m² or as a total heat load.

3) Fans

All of the electrical power consumption for evaporator fans is converted to heat. The heat load from this equipment is specified as the power consumption in [W].

4) other equipment

If other power consuming equipment in placed in the room the heat load of this equipment has to be estimated. Normally, the heat developed corresponds to the electrical power consumption of the equipment.

e) Total cooling demand

The total cooling demand is the sum of all the individual heat loads described. The SHR for the evaporator is calculated based on the humidity of the infiltrating air.

Cooling demand for an air conditioned room

THERMAL DESIGN OF EVAPORATOR

The phase-out of CFCs by the year 1995 and the impending phase-out of HCFCs in the future have created a need for redesigning new refrigerators and retrofitting old ones with new refrigerants. This report describes an extensive experimental and analytical effort aimed at predicting the performance of evaporators and condensers using alternative refrigerants. Heat exchanger models are also expressed in a form where heat exchanger tube diameters and lengths are explicitly specified to help analyze new configurations.

Existing refrigerator models often use a constant conductance modeling approach (e.g.ADL (Merriam et. aI., 1992), Porter and Bullard (1993)). These models are better than the single-zone constant-UA model used by the U.S. Department of Energy to set the 1993 energy standards (ADL, 1982). However, they fail to account for changes in heat transfer resistance due to changes in refrigerant flow characteristics. Characteristics that may affect the resistance to heat transfer include refrigerant mass flow rate and refrigerant properties. For instance, in our refrigerator overall heat transfer resistance may

change more than 10 percent in the two-phase region of the evaporator and more than 20 percent in the superheated region.

In addition to being more accurate than the constant conductance model, the variable Conductance model is also more flexible. When the constant conductance model is used a Conductance is determined for each zone of both the evaporator and the condenser. The Conductance’s that are determined are only useful for the refrigerant that was used in the system at the time when the conductances were determined. This is because conductances are dependent on the properties of the refrigerant in the system. The variable conductance model takes the properties of the refrigerant into account. The coefficients of the variable conductance model need to be determined once; after that the model can be used for different operating conditions, tube diameters, and refrigerants.

Finally, the model will be useful for assessing the applicability of refrigerant heat transfer Correlations to refrigerator models. The correlations that are used in our models were developed under ideal conditions in long straight tubes. The accuracy of our models will provide insight into how well the heat transfer correlations work in actual modeling applications.

Development of the variable conductance modelThe overall heat transfer equation for a heat exchanger must be written so that the Variable conductance model can be investigated. The equation is developed by identifying each component of the resistance to heat transfer between the two working fluids of the heat Exchanger. For the case of an evaporator or a condenser there are three components of heat Transfer resistance between the air and the refrigerant. The important components are the Convective resistance of the air, the conductive resistance of the heat exchanger, and the convective resistance of the refrigerant. The overall heat transfer resistance of the heat exchanger is shown below as a function of the three resistance components.

Heat transfer correlations Two-phase correlationsBoth the BoPierre correlation (Pierre, 1956) and a correlation developed by Chato and Wattelet (Smith et. aI., 1992) have been investigated for calculating the two-phase heat transfer coefficient.

Parameter estimation models have been developed using both correlations so that the two heat transfer coefficients could be compared. The BoPierre correlation was designed for use with higher Reynolds numbers. The Chatol Wattelet correlation, on the other hand, was developed for use with lower refrigerant mass flow rates. Domestic refrigeration systems have low mass flow rates, so it is likely that the Chatol Wattelet correlation will better suit our purposes.

Problem

CATIA MODELING OF TUBE AND FIN EVAPORATOR

INTRODUCTION TO CATIA

CATIA V5 is the leading solution for product success. It addresses all manufacturing organizations, from Original Equipment Manufacturers and their supply chains, to small independent producers. CATIA V5 is industry agnostic with users ranging from aerospace, automotive, and industrial machinery, to electronics, shipbuilding, plant design, and consumer goods, even architecture. CATIA V5 has the functionality to address the complete product development process, from initial specification to product-in-service, in a fully-integrated manner. It facilitates reuse of product design knowledge and shortens development cycles, helping enterprises to accelerate their response to market needs.

CATIA Version 5 Generative Shape Design allows you to quickly model both simple and complex shapes using wireframe and surface features.

It provides a large set of tools for creating and editing shape designs and, when combined with other products such as CATIA.Part Design, it meets the requirements of solid-based hybrid modeling.

The feature-based approach offers a productive and intuitive design environment to capture and re-use design methodologies and specifications.

This new application is intended for both the expert and the casual user. Its intuitive interface offers the possibility to produce precision shape designs with very few interactions. The dialog boxes are self explanatory and require practically no methodology, all defining steps being commutative.

As a scalable product, CATIA. Generative Shape Design can be used with other CATIA Version 5 products such as CATIA.Part Design and CATIA.FreeStyle Shaper and Optimizer. The widest application portfolio in the industry is also accessible through interoperability with CATIA Solutions Version 4 to enable support of the full product development process from initial concept to product in operation

CATIA (Computer Aided Three-dimensional Interactive Application) is a multi-platform CAD/CAM/CAE commercial software suite developed by the French company Dassault Systems and marketed worldwide by IBM. Written in the C++ programming language, CATIA is the cornerstone of the Dassault Systems product lifecycle management software suite.

The software was created in the late 1970s and early 1980s to develop Dassault's Mirage fighter jet, and then was adopted in the aerospace, automotive, shipbuilding, and other industries.

CATIA competes in the CAD/CAM/CAE market with Siemens NX, Pro/ENGINEER, Autodesk Inventor, and Solid Edge.

Commonly referred to as a 3D Product Lifecycle Management software suite, CATIA Supports multiple stages of product development (CAx), from conceptualization, design (CAD), manufacturing (CAM), and engineering (CAE).

CATIA can be customized via application programming interfaces (API). V4 can be Adapted in the FORTRAN and C programming languages under an API called CAA (Component Application Architecture). V5 can be adapted via the Visual Basic and C++ Programming languages, an API called CAA2 or CAA V5 that is a component object Model (COM)-like interface. Although later versions of CATIA V4 implemented NURBS, V4 principally used Piecewise polynomial surfaces. CATIA V4 uses a non-manifold solid engine.

Catia V5 features a parametric solid/surface-based package which uses NURBS as the core surface representation and has several workbenches that provide KBE support.V5 can work with other applications, including Enovia, Smarteam, and various CAE Analysis applications.

METHODOLOGY USED FOR MODELLING EVAPORATOR

The tube and fin evaporator is been modeled using the Catia software. This model has been completed in the module of part drawing (hybrid design) of the mechanical design header. This has been modeled by take y-z as the reference plane. This model has been modeled by using various tool bars like profile, operations, constraint, sketch based features, reference elements, transformation features, view, etc.

The methodology followed for completion of the design is as follows

1) Two circles have drawn on plane with fully constrained and then padded to required dimension

2) With the help transformation tool bar the required number of pipes has been generated

3) Then the perpendicular plane has been selected and by creating a reference plane a fin has been drawn and finally by following same process the number of fins have drawn throughout the length of the tube (i.e. by taking reference plane at a regular distance from each plane)

4) The bends of the tube are been completed by using the rotate option by projecting the surface of each end of the pipe

5) Finally the inlets and outlets are been generated by using rib option.

The various views of the evaporator are been shown below