1 WHAT IS HVAC HVAC HEATING VENTILATION AIR CONDITIONING The field of HVAC i.e., heating, ventilation, and air conditioning is the science and practice of controlling indoor climate to provide healthy and comfortable interior conditions. HVAC system, simultaneously control the temperature, humidity, purity, air distribution and noise in the interior spaces of the building. In the Gulf countries & Indian context, HVAC systems are generally used to ventilate and cool the building. AIR CONDITIONING FOR COOLING Air conditioning and refrigeration are provided through the removal of heat. Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. STANDARD INTERNATIONAL CODES USED IN HVAC FIELD Generally ASHRAE & CARRIER standards are used worldwide for designing of air conditioning systems. ASHRAE: - American Society of Heating, Refrigerating and Air-Conditioning Engineers. And for Air Duct Constructions there are many standard codes, but generally SMACNA standards are used. SMACNA: - Sheet Metal and Air Conditioning Contractor’s National Association.
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WHAT IS HVAC
HVAC
HEATING VENTILATION AIR CONDITIONING
The field of HVAC i.e., heating, ventilation, and air
conditioning is the science and practice of controlling
indoor climate to provide healthy and comfortable
interior conditions. HVAC system, simultaneously
control the temperature, humidity, purity, air
distribution and noise in the interior spaces of the
building.
In the Gulf countries & Indian context, HVAC systems are generally used to ventilate and
cool the building.
AIR CONDITIONING FOR COOLING
Air conditioning and refrigeration are provided through the removal of heat. Refrigeration
may be defined as lowering the temperature of an enclosed space by removing heat from
that space and transferring it elsewhere.
STANDARD INTERNATIONAL CODES USED IN HVAC FIELD
Generally ASHRAE & CARRIER standards are used worldwide for designing of air
conditioning systems.
ASHRAE: - American Society of Heating, Refrigerating and Air-Conditioning
Engineers.
And for Air Duct Constructions there are many standard codes, but generally SMACNA
standards are used.
SMACNA: - Sheet Metal and Air Conditioning Contractor’s National Association.
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FACTORS TO BE CONTROLLED IN AIR CONDITIONING PROCESS
Below are the factors has to be controlled by air conditioning process:-
1. Temperature
2. Humidity
3. Purity
4. Air distribution
5. Noise control
1.Temperature:-
Temperature measures the heat intensity or heat level of a substance. Temperature
alone does not give the amount of heat in a substance. It indicates the degree of
warmth, or how hot or cold the substance or body is. In the molecular theory of heat,
tempemture indicates the speed of motion of the molecules. Temperature is normally
measured by Thermometer.
Units of Temperature: - The two most common units of temperature are the, Fahrenheit
and the Celsius scales.
Converting 0C to 0F is:-
0C = 5/9 (0F-32)
&
Converting 0F to 0C is:-
0F = 9/5 (0C+32)
Dry-bulb temperature (DB): The dry-bulb temperature is the temperature of air
measured by a normal thermometer freely exposed to the air but shielded from radiation
and moisture. It is one of "the most important climate variables for human comfort and
building energy efficiency”.
Wet-bulb temperature (WB): The temperature registered by thermometer whose bulb
is covered by a wetted wick and exposed to a current of rapidly moving air. It is the
temperature air would have if part of its energy were used to evaporate the amount of
water it would absorb to become fully saturated.
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Indoor Comfort zone conditions for Residential & commercial applications are:-
Table-1.1
Indoor Comfort zone conditions for Industrial applications are:-
Table-1.2
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Table-1.2 (continued)
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Table-1.2 (continued)
*And Out Door Design conditions data for designing of Air Conditioning systems are
generally extracted from Carrier or ASHRAE Design Handbook.
2.Humidity:-
Humidity is the Amount of water vapor present in a given space.
Absolute Humidity:-Density of water vapor per unit volume of air is absolute humidity
(AH)--expressed in units of lbs. of water/cu-ft of dry air.
Degree of saturation:- It is water present in air/max water vapor-holding capacity of air.
Relative humidity (RH) :- Relative humidity is the ratio of (actual vapor pressure of air-
vapor mixture/pressure of water vapor when the air is completely saturated at the same
DB temperature) x 100.
Human tolerance to humidity variations is much greater than temperature
variations. In winter, the range is from 20 to 50% of RH & in summer, the range
extends up to 60% @ 75ºF.
High humidity causes condensation problems and reduces body heat loss by
evaporative cooling.
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Low humidity tends to dry throat and nasal passages also can cause static
electrical sparks.
And the standard indoor comfort levels of relative humidity for Residential, commercial &
industrial applications are given in TABLE-1.1 & TABLE-1.2.
*And Out Door Design conditions data for designing of Air Conditioning systems
are generally extracted from Carrier or ASHRAE Design Handbook.
3.Purity:-
It is the process of removing particulates (dust etc.) and biological contaminants,
(insects, pollen etc.) from the air delivered to the conditioned space for the purposes of
improving or maintaining the air quality.
Generally in air conditioning system filter are used for removing dust particulates & in
hospital HEPA (High-Efficiency Particulate Air) filters are used to filter the air which can
remove at least 99.97% of airborne particles. Please refer the picture below for a general
arrangement of an A/c Unit with Filter:-
A typical a/c unit arrangement
And for the treated Fresh air handling units Ultra violet air purifiers are used to kill the
bacteria & viruses from the air. Please refer Picture below:-
The purpose of the condenser in a vapour compression cycle is to accept the hot, high-
pressure gas from the compressor and cool it to remove first the superheat and then the
latent heat, so that the refrigerant will condense back to a liquid. In addition, the liquid is
usually slightly subcooled. In nearly all cases, the cooling medium will be air or water.
Types of condensers:- Based on the external fluid, condensers are classified into three types as mentioned below :-
i. Air cooled condensers ii. Water cooled condensers iii. Evaporative condensers
i. Air cooled condensers:-
As the name implies, in air-cooled condensers air is the external fluid, i.e., the
refrigerant rejects heat to air flowing over the condenser.
Air-cooled condensers can be further classified into two types as listed below:-
a) Natural convection type
b) Forced convection type
a) Natural convection type:-
In natural convection type condensers, heat transfer from the condenser is by buoyancy
induced natural convection and radiation. Since the flow rate of air is small and the
radiation heat transfer is also not very high, the combined heat transfer coefficient in
these condensers is small. As a result a relatively large condensing surface is required
to reject a given amount of heat. Hence these condensers are used for small capacity
refrigeration systems like household refrigerators and freezers. The natural convection
type condensers are either plate surface type or finned tube type. In plate surface type
condensers used in small refrigerators and freezers, the refrigerant carrying tubes are
attached to the outer walls of the refrigerator. The whole body of the refrigerator (except
the door) acts like a fin. Insulation is provided between the outer cover that acts like fin
and the inner plastic cover of the refrigerator. It is for this reason that outer body of the
refrigerator is always warm. Since the surface is warm, the problem of moisture
condensation on the walls of the refrigerator does not arise in these systems. These
condensers are sometimes called as flat back condensers. Figure 2.8 shows a natural
convection type condenser. Regardless of the type, refrigerators having natural
convection condenser should be located in such a way that air can flow freely over
the condenser surface.
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FIGURE 2.8 NATURAL CONVECTION TYPE CONDENSER
b) Force convection type:-
In forced convection type condensers, the circulation of air over the condenser surface is
maintained by using a fan or a blower. These condensers normally use fins on air-side
for good heat transfer. The fins can be either plate type or annular type. Forced
convection type condensers are commonly used in window air conditioners, split air
conditioners, Packaged air conditioners and air cooled chiller plants. These are either
chassis mounted or remote mounted. In chassis mounted type, the compressor,
induction motor, condenser with condenser fan, accumulator, HP/LP cut- out switch and
pressure gauges are mounted on a single chassis. It is called condensing unit of rated
capacity. Typically the air velocity varies between 2 m/s to 3.5 m/s for economic design
with airflow rates of 12 to 20 cmm per ton of refrigeration (TR).
The area of the condenser seen from outside in the airflow direction is called face area. The velocity at the face is called face velocity. This is given by the volume flow rate divided by the face area. The coils of the tube in the flow direction are called rows. A condenser may have two to eight rows of the tubes carrying the refrigerant. The moist air flows over the fins while the refrigerant flows inside the tubes. The fins are usually of aluminum and tubes are made of copper. For ammonia condensers mild steel tubes with mild steel fins are used. Figure 2.9 shows a Forced convection type condenser.
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FIGURE 2.9 FORCED CONVECTION TYPE CONDENSER
ii. Water cooled condensers:-
In water cooled condensers water is the external fluid. Many large commercial
refrigerating units use a water-cooled condenser. This condenser are classified into
three types as per there construction, as listed below:-
a) Shell & tube condensers
b) Shell & coil condensers
c) Coil in coil condensers
a) Shell & tube condensers:-
This is the most common type of condenser used in systems from 2 TR upto thousands
of TR capacity. In these condensers the refrigerant flows through the shell while water
flows through the tubes in single to four passes. The condensed refrigerant collects at
the bottom of the shell. The coldest water contacts the liquid refrigerant so that some
subcooling can also be obtained. The liquid refrigerant is drained from the bottom to the
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receiver. There might be a vent connecting the receiver to the condenser for smooth
drainage of liquid refrigerant. The shell also acts as a receiver. Further the refrigerant
also rejects heat to the surroundings from the shell. Figure 2.10 shows a shell & tube
type condenser.
FIGURE 2.10 SHELL & TUBE TYPE CONDENSER
b) Shell & coil condensers:-
The shell & coil type condensers are used in systems up to 50 TR capacities. The water
flows through multiple coils, which may have fins to increase the heat transfer
coefficient. The refrigerant flows through the shell. In smaller capacity condensers,
refrigerant flows through coils while water flows through the shell. Figure 2.11 shows a
shell & coil type condenser. When water flows through the coils, cleaning is done by
circulating suitable chemicals through the coils.
FIGURE 2.11 SHELL & COIL TYPE CONDENSER
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c) Coil in coil condensers:-
Double pipe condensers are normally used up to 10 TR capacities. Figure 2.12 shows
the Coil in coil type condenser. As shown in the figure, in these condensers the cold
water flows through the inner tube, while the refrigerant flows through the annulus in
counter flow. Headers are used at both the ends to make the length of the condenser
small and reduce pressure drop. The refrigerant in the annulus rejects a part of its heat
to the surroundings by free convection and radiation. The heat transfer coefficient is
usually low because of poor liquid refrigerant drainage if the tubes are long.
FIGURE 2.12 COIL & COIL TYPE CONDENSER
iii. Evaporative condensers:-
The evaporative condenser system carries the refrigerant into a condenser. The
evaporative condenser system is in an enclosure much like a cooling tower. The
evaporative condenser uses both air & water to cool the refrigerant. In this system as
evaporative indicates, water is sprayed or drips over the condenser. This cools it. The
water cycle is in the condenser cabinet only.
Usually, the evaporative condenser is mounted outdoors. However, it may be used
indoors if air ducts are provided to the outside.
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Some systems pumps water to a trough above the condenser. The water then drips over
coils as air is forced through them. A thermostat can be used to control water flow. A fan
blows air over the condenser, whenever the condenser is operating. The condenser is
cooled by air alone until the condenser temperature reaches 800F or more. Water
cooling is then turned on by a thermostat. Figure 2.13 shows the evaporative type
condenser.
Evaporative condensers are used in medium to large capacity systems. These are
normally cheaper compared to water cooled condensers, which require a separate
cooling tower. Evaporative condensers are used in places where water is scarce. Since
water is used in a closed loop, only a small part of the water evaporates. Make-up water
is supplied to take care of the evaporative loss. The water consumption is typically very
low, about 5 percent of an equivalent water cooled condenser with a cooling tower.
However, since condenser has to be kept outside, this type of condenser requires a
longer length of refrigerant tubing, which calls for larger refrigerant inventory and higher
pressure drops.
FIGURE 2.13 EVAPORATIVE TYPE CONDENSER
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EXPANSION DEVICES
An expansion device is another basic component of a refrigeration system. The basic
functions of an expansion device used in refrigeration systems are to:
1. Reduce pressure from condenser pressure to evaporator pressure
2. Regulate the refrigerant flow from the high-pressure liquid line into the evaporator
at a rate equal to the evaporation rate in the evaporator
Under ideal conditions, the mass flow rate of refrigerant in the system should be
proportional to the cooling load. Sometimes, the product to be cooled is such that a
constant evaporator temperature has to be maintained. In other cases, it is desirable that
liquid refrigerant should not enter the compressor. In such a case, the mass flow rate
has to be controlled in such a manner that only superheated vapour leaves the
evaporator. Again, an ideal refrigeration system should have the facility to control it in
such a way that the energy requirement is minimum and the required criterion of
temperature and cooling load are satisfied.
The expansion devices are classified into two types which are commonly used in
modern refrigeration & airconditioning systems as listed below:-
i. Capillary Tube (Cap Tube).
ii. Thermostatic Expansion Valve (TEV).
i. Capillary Tube:-
A capillary tube is a long, narrow tube of constant diameter. The word “capillary” is a
misnomer since surface tension is not important in refrigeration application of capillary
tubes. Typical tube diameters of refrigerant capillary tubes range from 0.5 mm to 3 mm
and the length ranges from 1.0 m to 6 m. Capillary tubes acts as a constant throttle on
the system. Figure 2.14 shows the capillary tube.
FIGURE 2.14 THE CAPILLARY TUBE
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Advantages and disadvantages of capillary tubes:- a.Some of the advantages of a capillary tube are:-
1. It is inexpensive. 2. It does not have any moving parts hence it does not require maintenance 3. Capillary tube provides an open connection between condenser and the
evaporator hence during off-cycle, pressure equalization occurs between condenser and evaporator.
4. Ideal for hermetic compressor based systems, which are critically charged and factory assembled.
b.Some of the disadvantages of the capillary tube are:-
1. It cannot adjust itself to changing flow conditions in response to daily and seasonal variation in ambient temperature and load. Hence, COP is usually low under off design conditions.
2. It is susceptible to clogging because of narrow bore of the tube, hence, utmost care is required at the time of assembly. A filter-drier should be used ahead of the capillary to prevent entry of moisture or any solid particles
3. During off-cycle liquid refrigerant flows to evaporator because of pressure
difference between condenser and evaporator. The evaporator may get flooded
and the liquid refrigerant may flow to compressor and damage it when it starts.
Therefore critical charge is used in capillary tube based systems. Further, it is
used only with hermetically sealed compressors where refrigerant does not leak
so that critical charge can be used. Normally an accumulator is provided after the
evaporator to prevent slugging of compressor.
ii. Thermostatic Expansion Valve (TEV):-
Thermostatic expansion valve is the
most versatile expansion valve and is
most commonly used in refrigeration
systems. A thermostatic expansion
valve maintains a constant degree of
superheat at the exit of evaporator,
therefore it is most effective for dry
evaporators in preventing the slugging
of the compressors since it does not
allow the liquid refrigerant to enter the
compressor. The schematic diagram of
the valve is given in Figure 2.15. This
consists of a sensing bulb that is
attached to the evaporator exit tube so
that it senses the temperature at the
exit of evaporator. The sensing bulb is
connected to the top of the bellows by a capillary tube. The sensing bulb and the narrow
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tube contain some fluid that is called power fluid. The power fluid may be the same as
the refrigerant in the refrigeration system, or it may be different. In case it is different
from the refrigerant, then the TEV is called TEV with cross charge.
FIGURE 2.15 THERMOSTATIC EXPANSION VALVE (TEV)
Thermostatic expansion valve showing various pressures & temperatures within valve
that operate it. F1- Sensing bulb pressure (force) tending to open valve. F2- Low side
Pressure (force) tending to close valve. F3- Spring force tending to close valve. P1-
Sensing bulb pressure tending to open valve. P2- Suction pressure (low side) tending to
close valve. T1- Sensing bulb temperature. T2- Evaporator refrigerant temperature (low
side). Valve opens when F1 is greater than combined force of F2 & F3. Valve closes when
combined F2 & F3 forces are greater than F1.
Advantages and disadvantages of TEV:- The advantages of TEV compared to other types of expansion devices are:
1. It provides excellent control of refrigeration capacity as the supply of
refrigerant to the evaporator matches the demand 2. It ensures that the evaporator operates efficiently by preventing starving
under high load conditions 3. It protects the compressor from slugging by ensuring a minimum degree of
superheat under all conditions of load, if properly selected.
The disadvantage of TEVs is, it is more expensive compared to capillary tubes.
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EVAPORATORS
An evaporator, like condenser which also acts as a heat exchanger. The purpose of the
evaporator is to receive low-pressure, low-temperature fluid from the expansion valve
and to bring it in close thermal contact with the load. The refrigerant takes up its latent
heat from the load and leaves the evaporator as a dry gas. The name evaporator refers
to the evaporation process occurring in the heat exchanger. The evaporators are
generally called as cooling coils
The evaporators are classified in several types, but only two types of evaporators are
majorly used in air-conditioning field, as listed below:-
1. The evaporators used for cooling air, by blowing air upon the cooling coil or
evaporator to keep the space conditioned. These are also called direct
expansion system.
2. The evaporators those are submerged in a liquid or liquid passed over the
cooling coil, such as water or brine solutions. These systems are also called
chilled water system.
1.Direct Expansion system (DX System):-
In direct expansion systems, air is expanded direct when it gets contact with the cooling
coil or air blown over the cooling coil by a means of fan or blower. Hence the space get
conditioned. These evaporators or cooling are used for cooling and dehumidifying the air
directly by the refrigerant flowing in the tubes. Similar to fin-and-tube type condensers,
these evaporator consists of coils placed in a number of rows with fins mounted on it to
increase the heat transfer area. In general terms air gets direct contact with the
evaporator & gets expanded.
2.Chilled water system:-
In chilled water system, a secondary refrigerant i.e. water or a brine solution is passed
over the evaporator or cooling coil just like shell and tube condenser used in water
cooled condensers, where the water gets chilled due to heat transfer & the chilled water
is circulated through chilled water pipes to the air handling unit or a fan coil unit. Through
the cooling coil of the air handling unit or a fan coil unit the space get conditioned. In
general terms cooling is achieved by a heat transfer, where water gets chilled in
evaporator & then circulated through cooling coils of air handling units or fan coil unit.