HEAT PUMP DRYER

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CHAPTER ONE

1.0 INTRODUCTION

1.1 Preamble

All variety of foods in our day to day life needs some way of preservation for several

reasons. To reduce or stop spoilage, to make all varieties of food available throughout a year, to

maintain desired levels of nutritional properties for the longest possible time span and to add

value added products (Mjumder and Jangan, 2010). Drying of Agricultural products intended to

increase shelf life, reduce packaging, transportation expenses by reducing the weight and

volume, improved appearance and more important to maintain the original flavor and nutritional

value (Tjukup, et al., 2012). Spoilage or deterioration of food occurs during handling or due to

mechanical, physical, chemical or microbial damages. Agricultural products are perishable

because of high water content of approximately 80%. Moisture above 10% encourages

proliferation of spoilage of micro-organism in the agricultural product and causes short shelf life

(Tjukup, et al., 2012).

The food products can have wide ranges of moisture content; as low as 35 % in grains

and as high as 90% or more in some fruits (e.g. water melon has moisture content as high as

93%) which needs to be reduced to an acceptable value so as to avoid microbial growth

(Mjumder and Devastin, 2008). Traditionally food products were dried using open sun drying

method. Although this is still in common use for non-commercial purposes. It was necessary to

develop a better way of dehydration to make products of higher commercial value which resulted

in use of various forms of dryers for food applications. Use of techniques such as solar cabinet

dryers, tray dryers, and fluid bed dryers and so on has resulted in better product quality than that

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of open sun drying (Mjumder and Chen, 2008). However the process can be made cost-effective

in terms of energy consumption as well as product quality. In order to reduce energy

consumption, it is necessary to choose an efficient heating system. Heat pump provides an

efficient technology and its environmental friendly due to low energy consumption (Fatouh, et

al., 2006).

A heat pump is machine or device that moves heat from one location (heat source) to

another location (heat sink) by using mechanical work (Jahn, 2009). It is a device that can

increase the temperature of a waste-heat source readily available to a temperature where the

waste energy becomes useful. It moves waste-heat from one medium to another (Sanchaz, 2008).

Heat pumps transfer heat by circulating a substance called refrigerant through a cycle thereby

bringing about heat transfer. There are different types of heat pump depending on the criteria of

classification. Based on waste-heat source criterion, they can be classified into: geothermal heat

pump, air-source heat pump, solar heat pump and electrical driven heat pump (www.les.com).

The efficiency of a heat pump is expressed by coefficient of performance (COP), which is the

quotient between the useful heating capacity and power input. The heat pump system is highly

economical this is because industrial heat pumps uses waste-energy. Waste-energy are heat

energy derived from industrial manufacturing processes which if not used by heat pump system

will be a waste.

Refrigerants are fluid which posses the ability to transfer heat energy from one point to

another. They are often referred to as the working fluid. There are different types of refrigerants

ranging from fluorocarbons, Halocarbons, ammonia, to the environmental friendly carbon

dioxide.1, 1, 1, 2 – tetrafluoro ethane, R- 134a is a haloalkane refrigerant with thermodynamic

property similar to R- 12 (dichlorodifluoro methane ) but with less ozone depletion potential. It

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has the formula CH2 FCF3 and a boiling point of -26.3o C (- 15.34o F )

Over the years, the important of heat pump system had been emphasized. Numerious

researchers had performed various theoretical investigation on the ways to improve the

coefficient of performance of a heat pump system. The fabrication of a test rig heat pump system

will share more light on the effect of parameter such as degree of superheat , difference between

condenser and evaporator temperature , mass flow rate , and so on the coefficient of performance

of a heat pump system.

1.2 Problem Statement

Over 20% of the energy production in the world is consumed by preservation of

Agricultural products (Rhagavan et al., 2005). Therefore, there is need to invent a dryer system

which is less energy consuming and also energy saving. A refrigeration system use in heating

can provide a coefficient of performance five times higher than a conventional electric dryer

(Sarkar, 2005). Therefore, a heat pump dryer is the type of dryer that suits this requirement

Considering Heat pumps, the harmful effects of refrigerants to the environment and the

populace is a topic of international concern. A refrigerant such as hydro-chlorocarbons, Hydro-

Flourocarbons, among others causes depletion of the ozone layers and Global warming effects.

Therefore there is a need to design a heat pump using an environmental friendly refrigerant with

efficient performance characteristics.

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1.3 Aims and Objectives

1.3.1 Aim

The aim of the project is to design and fabricate a R 134a heat pump system suitable for

preservation of Agricultural products.

1.3.2 Specific Objectives

1. To design an experimental test rig for R – 134a Heat pump system.

2. To fabricate the experimental test rig designed.

3. To test the performance of the test rig.

1.4 Scope of the Project

This project is limited to the design and analysis of a Heat pump dryer test rig. The test

rig is to be investigated experimentally to observe its level of performance. The refrigerant to be

used is R- 134a. The type of heat pump system adopted is the vapour compression cycle. The

power rating of the compressor is 1.5KW. The material used for the heat exchangers (condenser

and evaporator) is copper. This project is to be carried out in a Tropical Rain forest region.

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1.5 Justification

Preservation of Agricultural products with the most effective drying mechanism is of

utmost important. This is to reduce the amount of energy consumption by the processes

involving preservation of agricultural products. Therefore there is need to investigate the

performance characteristics of the dryer, this is to measure the efficiency of the dryer. Building

into consideration how criteria such as superheat temperature, condenser temperature, evaporator

and so on, could be varied to obtain more adequate coefficient of performance.

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CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 Preservation of Agricultural Products

The astonishing fact about food preservation is that it permeated every culture at nearly

every point in time. To survive, ancient man had to harness nature. In frozen climate, he froze

seal meat on the ice. In tropical climate he dried foods in the sun. Food by its nature begins to

spoil the moment it is harvested. Food preservation enabled ancient man to make root and live in

one place and form a community. He no longer have to consume the kill or harvest immediately,

but could preserve some for later use (Nummer, 2002).

There are different methods of preserving farm produce. They includes drying, freezing,

canning, pickling, jam and jelly, hurdle technology, modified atmosphere, polling,

biopreservation and pascalization. (Eden, 1999). Of all the aforementioned methods of drying is

a method of food preservation that works by removing water from the food which inhibits the

growth of bacteria(). East and oriental cultures actively dried foods as early as12, 000 BC in the

sun. The Romans are particularly fond of any dried fruit they could make. In the middle ages

purposely built ‘still houses’ were created to dry fruits, vegetable and herbs in areas that did not

have enough strong sunlight for drying. A fire was used to create the heat needed to dry foods

and in some cases smoking them as well (Mack, 2001).

Most high value agricultural products need changing to minimize spoilage preserve

quality and reduce transportation cost (Mujumder, 2000). Drying is a complex and energy

intensive process where the water is evaporated from a product by supplying heat by convection

conduction radiation microwave and so on. About 85% of industrial driers are the convective

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type with Hot air as drying medium and 99% of them involve removal of water (Mujumder,

2008). They consume up to 25% of the material industrial energy in the develop countries

(Raghavan, et al 2005).

In the main agricultural countries drying comprises the reduction of moisture from

about 17-30% w/w to values between 8and15% w/w depending on the grain. The final moisture

content for drying must be adequate for storage. The more oil the gain has, the lower its storage

moisture content will be (though its initial moisture for drying will also be lower). Cereals are

often dried to 14% w/w, while oil seed to 12.5% (soya beans), sunflower 8%, and peanuts 9%.

(Brooker, et al, 2012). Drying is carried out as a prerequisite for safe storage, in order to inhibit

microbial growth.

2.2 Refrigeration History

Use of ice to refrigerate and thus preserve food goes back to prehistoric times. The

seasonal harvesting of snow and ice was a regular practice of the ancient centuries: Chinese,

Hebrews, Greeks, Romans, and Persians. Ice and snow were stored in caves or other insulating

materials. The Persians stored ice in pits called straw line pit. Rationing of the ice allowed the

preservation of foods over the warm periods. This practice worked well down through the

centuries, with icehouses remaining in use into the 12th century (Bolaji, 2005).

The first known method of artificial refrigeration was demonstrated by William Cullen at

the university of Glasgow in Scotland in 1756.Cullen used a pump to create a partial vacuum

over a container of diethyl ether, which then boiled, absorbing heat from the surrounding air. The

experiment even created a small amount of ice, but had no practical application at that time. In

2005, American inventor Oliver Evans designed but never built a refrigeration system based on

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vapor compression refrigeration cycle rather than chemical solution or volatile liquids such as

ethyl ether. An American living in Great Britain, Jacob Perkins obtained the first patent for a

vapor compression system 1834. Perkins built a prototype system and it usually worked,

although it did not succeed commercially (Bhatti, 1999).

In 1842, an American physician, John Gorier, designed the first system for refrigerating

water to produce ice. He also conceived the idea of using his refrigeration system to cool the air

for comforts in homes and hospitals. His system compressed air, then partially mixed the hot

compressed air with water before allowing it to expand while doing part of the work required

driving the air compressor. Alexander Twining began experimenting with vapor compression

refrigeration 1848 and obtained patents in 1850 and 1853. He is credited for having intiated

commercial refrigeration in the united states by 1856 (Arora, 2007).

Mechanical refrigeration has been around since the mid-ninth century. The first practical

machine was built by Jacob Perkins. (Thevenot, 1979). It was based on using ether as a

refrigerant in a vapor compression circuit. Carbon-Dioxide was first used as a refrigerant in 1866

and Ammonia in 1873. Other chemical used as vapor compression refrigerant include cymene

(petrol ether and naphtha), sulfur dioxide (R-764) and methyl ether. Their applications were

limited to industrial processes. Most food preservation was accomplished by using blocks of ice

collected during the winter and stored or manufactured through an industrial process. ( Mcquay,

2002). By the beginning of the twentieth century, refrigeration system was being used to provide

air conditioning in major building projects. The Milam Building in San Antonio, Texas was the

first high rise office building to be completely air conditioned. (Pauken, 1999). Sleeping Soundly

on Summer Nights.( ASHRAE,2002).

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By the mid-1970s, concern began to surface about the thinning of ozone layer and whether

CFCs may be in part responsible. This led to the ratification of the Montreal Protocol in 1987

that required the phase-out of CFCs and HFCs. New solutions were developed with HFCs taking

on major role as refrigerants.

In the 1990s global warming arose as the new threat to the well-being of the planet. While

there are many contributors to the global warming, refrigerants were included because air

conditioning and refrigeration are significant energy users and many refrigerants are themselves

greenhouse gases. (Mcquay, 2002). Hence, environment friendliness of the refrigerant is a major

factor in deciding the usefulness of a particular refrigerant.

2.3 Drying Process

Drying is a highly nonlinear coupled heat and mass transfer process. It occurs by

vaporization of a liquid by supplying heat to a wet material. Drying with hot air implies

humidification and cooling of the air in a well-insulated (adiabatic) drying chamber. Heat is

transferred by convection from drying air to the product drying surface where heat is needed for

evaporation of moisture. Figure 2.1 shows a schematic representation of energy balance of a

drying system. The material enters the drying chamber at state “a” with an initial mass

Mdry+Mwet and temperature Ta. After drying, the dried material leaves the drying chamber at

state bwith the final mass Mdry and at temperature Tb. On the other hand, in the drying chamber

enters relatively dry air (at state 2) with the flow rate mair (kgdry,air/s), It absorbs water from the

material during the isenthalpic process 2-3. If the drying air is heated at constant absolute

humidity before entering the drier, the heating thermal power (kW) will be:

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( )12, hhmQ airdry −=••

2.1

Where h1 and h2are the air mass enthalpy (kJI kg) entering and leaving the air heater,

respectively. The maximum rate of water extracted from the material will be:

( ) ( )13,23, ϖϖϖϖ −=−=•••

airdryairdrywet mmM 2.2

Where 21,ϖϖ and 3ϖ are the absolute humidity of the drying air during the process

)/( ,airdrywater kgkg . If c is the specific heat of the material, the energy balance of this

simplified drying process will be:

( )badryawetairdryairdry TTcMcTMhmhm −=

+−•••

2,3, 2.3

Heat supplied at the boundaries of the drying product diffuses into the material primarily

by conduction. The heat and mass transfer coefficients which control the heat and moisture

migration rate inside the product are strong functions of the moisture content as well as of the

product temperature. Moisture content represents the weight of water present in the product

expressed as a percentage of the weight of over-dry product. As the moisture content at the

drying surface of the product drops, liquid moisture migrates from the product bulk to the

drying surface before it is removed by the hot drying air. Local moisture diffusivity controls

liquid moisture migration rate from bulk to the drying surface of the product. The transport of

moisture can occur by one or a combination of several mechanisms as liquid (if the wet

material is at a temperature below the boiling point of water) and vapour diffusion (if the water

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vaporizes inside the material), hydrostatic pressure differences (when internal vaporization

rates exceed the rate of vapour transport through the solid to the ambient medium) or Knudsen

diffusion (in the case of freeze drying).

For materials of high diffusivity, moisture migrates rapidly to the drying surface even at low

moisture content of the product. Therefore, the drying rate of such materials is controlled by

external transport rates while the drying rate improves with improvement of the external

conditions. Evaporation of liquid moisture takes place from exposed surface by absorbing the heat

of vaporization Hot air is used both to supply the heat for evaporation and to carry away the

evaporated moisture from the solid. At low moisture content of the product, the effect of low

relative humidity becomes less significant, at low relative humidity of air, partial vapour pressure

of drying air becomes low. This leads to a higher driving potential for mass transfer resulting in

increased moisture evaporation rate.

Liquid moisture evaporates from the drying surface of the product because of the

difference of partial vapour pressures between the air and the surface of the product. Partial

vapour pressure at the drying surface is function of the drying surface temperature and its water

activity which is a function of surface moisture content and temperature. Water activity(αw)

depends on relative pressure and is defined as the ratio of the partial pressure (Pw) ofwater at

the same temperature. Thus, αw, which is also equal to the relative humidity of the ambient

humid air, is defined as: w

w p

p=α 2.4

As the moisture content and temperature of the drying surface continuously change during

drying process, partial vapour pressure at the drying surface becomes an uncontrollable

parameter. The diffusivity at the exposed drying surface significantly drops during drying

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process particularly when the moisture content of the product becomes low. The moisture

content of a wet material in equilibrium with the air of given relative humidity and temperature

represents the equilibrium moisture content. The evaporation rate of moisture depends on the mass

transfer coefficient of the drying air which depends mainly on the types of flow used such as

Stagnant, laminar or turbulent. Higher volume flow rate if drying air increases the mass transfer

coefficient, but the size of blower becomes large and power consumption rate increases.

Therefore, maintaining optimum flow of drying air is important for the economic operation of a

dryer. The isotherm obtained by exposing the solid to air of decreasing humidity is known as the

desorption isotherm. As the maximum allowable temperature of the drying air is limited

particularly for heat sensitive materials, partial vapour pressure of the drying air is usually

controlled by condensing moisture of the drying air by using heat pumps.

Fig 2.1 Diagram of Simplified schema of a drying system, (Source: Larsson, 2011)

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2.4 The Heat Pump System

A heat pump is a device that can increase the temperature of a waste-heat source readily

available to a temperature where the waste-heat becomes useful. A heat pump moves waste-heat

from one medium to another. The heat pump systems are common only in few countries, for

example: Sweden, England, United States, Canada, etc. But in quite a lot of other countries they

are not that common. For that reason the heat pump systems can be unknown for some people.

The same can happen with the heat recovery systems. Not too many people know and understand

what is a heat pump system and a heat recovery system. In the other hand refrigeration systems

are very common in all the countries. We just have to take a look in our fridges.Well then, all

these three systems have the same operation principles. The difference between them is just the

objective, the medium which is necessary to control. In a refrigeration system the objective is to

maintain the refrigerated medium at a low temperature by removing heat from it. But it is not

necessary to control the warm medium; the heat is just thrown to the environment.

In a heat pump system the objective is to maintain a heated medium at a high

temperature.(carrier, 2008). In a heat recovery system the objective is double; both mediums

need to be controlled. On the one hand a heat recovery system has to maintain the refrigerated

medium at a low temperature by removing heat from it and on the other hand it has to maintain a

heated medium at a high temperature. Although normally one of the mediums is the critical and

the other medium must adapt to changes on the other.

2.4.1 Heat Pump Heat Sources

In a heat pump the heat source is the medium donating the heat. There are a lot of mediums

which can be the heat source in heat pump systems.(cdc.gov, 2008)

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One classification of the heat sources is: Air, Water, Ground.

2.4.2 Heat Pump Sinks And Applications

The heat sink is the medium receiving the heat. Different types of liquids and gases are

both possible sinks. But the most common sinks are water and air. The possible applications of

heat pumps are very large. Some of the most common applications are:

• Space heating and water heating for washing, sanitation and cleaning.

• Steam production.

• Drying and dehumidification.

• Evaporation, distillation, concentration and desalination.

2.4.3 Heat Pump Types

I. Closed-cycle mechanical vapour compressions (MVC) heat pump

II. Open-cycle mechanical vapour compression heat pumps

III. Open-cycle thermo compression heat pumps

IV. Closed-cycle absorption heat pump

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2.4 Closed Cycle Mechanical Vapour Compression Heat Pump System

The Basic configuration

In the closed cycle MVC heat pumps, the basic heat pump has the configuration shown

below

Fig 2.2: Basic Configuration in a Closed Heat Pump System (sanchez, 2008)

In this configuration the main components are the compressor, the evaporator, the condenser and

the expansion valve. All of them are connected with pipes. Inside the pipes and components

flows the working fluid. In a close cycle MVC heat pumps it is a refrigerant. This refrigerant is

alternatively compressed and expanded and goes from the liquid to the vapour state. The

condenser and the evaporator are heat exchangers. In the evaporator heat is transferred from the

heat source to the refrigerant. In the condenser heat is transferred from the refrigerant to the heat

sink (see figure 2.3).

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Figure 2.3: Basic Heat Pump System (Source: Goodman, 2008)

The basic thermodynamic cycle with which works this heat pump configuration is the

one stage simple cycle. This thermodynamic cycle is defined by four states (points) and four

thermodynamic processes (lines) as is shown in the figure 2.5

Figure 2.4: States and Processes in a One Stage Heat pump Simple Cycle. (Source: Sanchez,

2008)

The states refer to the refrigerant condition in a specific place:

• State 1: Saturated or slightly superheated vapour.

• State 2: Superheated vapour.

• State 3: Saturated or slightly sub-cooled liquid.

• State 4: Saturated liquid-vapour mixture.

The four thermodynamic processes are: Compression, Condensation, Expansion and

Evaporation.

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Figure 2.5: States and Processes in a One Stage Heat Pump Simple Cycle (Source:Sanchez,

2008)

2.5 How Does The Heat Pump Work?

• Process ‘1’ to ‘2’: the compressor removes the gas produced in the evaporator (state ‘1’) and

compresses it, delivering it to the condenser at a higher pressure and temperature (state ‘2’). The

energy required for driving the compressor is called the power consumption ( Win [ kW] ).

• Process ‘2’ to ‘3’: in the condenser heat is transferred from the refrigerant to the heat sink, thus

the heat sink increases its temperature. The refrigerant changes its state from superheated vapour

(state ‘2’) to saturated liquid (state ‘3’). The total capacity of heat transfer in the condenser is

called the heating capacity ( Qc [ kW] ).

• Process ‘3’ to ‘4’: the expansion valve releases the pressure between the high-pressure

condensation side (state ‘3’) and the low-pressure evaporation side (state ‘4’).

Process ‘4’ to ‘1’: in the evaporator the refrigerant boils by absorbing energy from the heat

source, which reduces its temperature. The refrigerant changes its state from saturated liquid-

vapour mixture (state ‘4’) to saturated vapour (state ‘1’). The total capacity to absorb heat from

the heat source is called the cooling capacity ( Qe [kW] ).

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Figure 2.6: Heat Pump System with Input and Outputs (Source: Sanchez, 2008

2.6 Components of a Heat Pump System

In Mechanical vapour compression cycle heat pumps there are four essential components,

they are: The compressor, condenser, evaporator, and the expansion valve.

2.6.1 The Compressor:

The function of a compressor is to remove the gas produced by the evaporator and to

deliver it at a required higher pressure (see figure 3-12). The compressor can be compared to a

heart pumping the blood (the refrigerant) inside the body (close cycle MVC cycle)The

compressor can be compared to a heart pumping the blood (the refrigerant) inside the body

(close cycle MVC cycle). In the basic compression cycle, the compressor is positioned between

the evaporator and the condenser. The compressor removes the gas produced in the evaporator. It

must remove continuously the gas to maintain the same pressure. After that, the compressor

pumps the gas from the evaporator and compresses it, delivering it to the condenser at a higher

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pressure and temperature (‘1’ to ‘2’’). The energy required for the compression normally comes

from electricity.(energyblueprint.info,2008)

Fig 2.7: Close Cycle MVC Heat Pumps: Compression Process(Source : Sanchez, 2008)

The process ‘1’ to ‘2’’ symbolizes an ideal isentropic compression, in which there is not heat

exchange with the surroundings. However, in reality there are always some heat losses from

compressor and also some dissipation of energy due to mechanical friction in the equipment and

potential leakage in the compressor. The difference between isentropic and present compression

taking into account the dissipation of energy can be expressed by the isentropic compressor

efficiency: There are several types of compressors: Positive displacement compressors, Screw

compressors, Rotary compressors, Rolling piston compressors, Scroll compressors, Dynamic

compressors, and Turbo compressors.

2.6.2 The Condenser:

The function of the condenser is to transfer hot discharge gas from the compressor to a

slightly subcooled liquid flow, by transferring heat from the refrigerant to the heat sink.(

friotherm.com, 2008). The basic operation of condensers is divided into three parts:

Desuperheating, Condensation, and Sub-cooling. All three operations can be carried out

inside the condenser. Alternatively, the desuperheating or sub-cooling operation can be carried

out in a separate heat exchanger. The heat rejection can be

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Fig 2.8 : Close Cycle MVC Heat Pumps: Condensation Process

The first part of the condenser desuperheats the gas to the saturation temperature (‘1’ to

‘2’). This cooling represents 15-25% of the total heat rejection. It is one-phase heat transfer

where the temperature of the refrigerant gas decreases typically by 20-50 K, depending on the

system and refrigerant. When the refrigerant reaches its saturation temperature, the latent heat is

rejected. Normally the condensing process represents the majority (70-80%) of the total heat

rejection (‘2’ to ‘3’). Finally, the fully condensed refrigerant (state ‘3’) is sub-cooled a few

degrees (‘3’ to ‘4’) to ensure that pure liquid enters the expansion valve (state ‘4’). This is also

one-phase heat transfer operation, representing approximately 2-5 % of the total heat rejection.

The temperature of the refrigerant decreases during the desuperheating and sub-cooling

processes, but remains constant during the condensing processes

Fig 2.9: Temperature evolution along the condensation process (Sanchez, 2008)

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The energy rejected from the refrigerant heats the heat sink, whose temperature thus increases.

The temperature of the heat sink can be increased to approach or even exceed the condensing

temperature (‘6’). A temperature increases results in a smaller flow on heat sink side for the

same heat load. This reduces the required pump capacity. However, there is a minimum

temperature difference that must be considered and avoided for stable operation, it is called the

pinch temperature. This minimum occurs at the beginning of the condensation process (in a

counter-current condenser). Therefore, the temperatures of the two media in a heat exchanger

may converge but never be equal. An example of temperature difference between condensing

temperature and outlet heat sink maximum temperature (‘2’ to ‘6’) is 2 K.

2.6.3 The Expansion Valve:

The expansion valve is situated in the liquid line between the condenser and the inlet of

the evaporator. The expansion valve releases the pressure between the high-pressure

condensation side and the low-pressure evaporation side. (Icropetra F.P 2008)

2.6.3.1 Functions of Expansion Valve

1. Maintaining the pressure difference between the condenser (high pressure) and the evaporator

(low pressure): the pressure difference created by the work of the compressor is maintained by

the expansion device.

2. Controlling the amount of refrigerant entering the evaporator: if the capacity of the evaporator

increases, the expansion valve should allow a larger flow of refrigerant, and vice versa. An

expansion valve does control directly the evaporation temperature. Instead it regulates the

superheating by adjusting the mass flow of refrigerant into the evaporator, and maintains the

pressure difference between the high pressure and the low pressure sides. The evaporation

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temperature depends on the capacity of the compressor and the characteristics and efficiency of

the evaporator. There are numerous types of expansion valves, depending on the demand for

control and the type of evaporator, some of them are: Thermal expansion valves, manual valves,

Capillary valves, Automatic valves, Electronic expansion valves.

2.6.4 The Evaporator:

In the evaporator, the refrigerant boils by absorbing energy from the heat source which reduce its

temperature. The heat source may be a gas o liquid, depend on the system.(GENETRON

refrigerants, 2008) The evaporation process occurs as shown in the next Log p-h diagram.

Fig 2.10 : Close Cycle MVC Heat Pumps: Evaporation Process

When the sub-cooled liquid refrigerant at high pressure (state ‘1’) is expanded through the

expansion valve, the pressure and therefore the saturation temperature decreases (state ‘2’). The

mixture of liquid and gas from the expansion valve enters the evaporator and starts to boil,

because heat is transferred from the heat source (‘2’ to ‘3’). The evaporating refrigerant absorbs

energy from the heat source, whose temperature is reduced. After full evaporation, when 100%

of the refrigerant has become saturated vapour (state ‘3’), the temperature of the vapour will start

to increase, the vapour becomes then superheated. The refrigerant flow leaving the evaporator

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will be 100% superheated vapour (state ‘4’). The temperature change along the evaporation

process is shown in the next diagram.

Fig 2.11: Temperature Evolution along the Evaporation Process (Sanchez, 2008)

The total energy absorbed by the refrigerant consists of the latent energy of evaporation

(‘2’ to ‘3’) plus the sensible energy of superheating (‘3’ to ‘4’). The refrigerant vapour is

superheated mainly to ensure that dry gas enters the compressor (see figure 3-16). The

evaporation temperature of a pure refrigerant remains constant and corresponds to a certain

pressure level. However, in reality the evaporation temperature is never constant through the

evaporator. Inside an evaporator, the increased of velocity of the liquid-gas mixture will induce

pressure drop, which thus reduces the saturation temperature. A good evaporator is able to

provide a good, stable boiling process with a small temperature difference between the

refrigerant and the heat source. A low temperature difference means that a higher evaporation

temperature is possible, which corresponds to a higher pressure. Decreasing the pressure

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difference from the condensing pressure (high pressure) to the evaporation pressure (low

pressure) will decrease the energy use.

2.7 Refrigerants Requirements:

A refrigerant in a MVC heat pump system must satisfy some requirements. These requirements

can be divided into two groups:

• On the one hand the refrigerant should not cause any risk of injuries, fire or property damage in

case of leakage.

• On the other hand chemical, physical and thermodynamic properties of the refrigerant must be

the appropriate for the system and the working conditions at a reasonable cost.

(Heatpumpscenter.org, 2008)

2.7.1 The Ideal Properties of a Refrigerant

• Good heat transfer properties.

• High latent heat.

• Appropriate pressures for the operating temperature.

• Chemical stability.

• Low toxicity.

• Low fire risk.

• Environmentally friendly.

• Satisfactory oil solubility/miscibility.

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• Easy leak detection.

• Low cost.

The most important feature is that the refrigerant must have chemical stability within the heat

pump system. Nevertheless, when the refrigerant is emitted to the atmosphere, it should not be so

stable and decompose easily without the formation of any harmful substances (Energy.kth.se).

2.7.2 Refrigerant Impact on the Environment

In addition to being toxic or explosive and therefore dangerous to people’s health, there are other

problems associated with refrigerants. Environmental aspects are increasingly being taken into

consideration. Refrigerants can thus also be ranked according to their impact on the stratospheric

ozone layer (the Ozone Depletion Potential, ODP) or as greenhouses gases (the Global Warming

Potential, GWP).

2.7.2.1 Ozone Depletion Potential (ODP):

The ODP factor is used to reflect the refrigerants impact on the ozone layer. The ODP is the ratio

of the impact on ozone of a chemical compared with the impact of a similar mass of R11. Thus,

the ODP of R11 is 1.0 by definition. Refrigerants containing chlorine or bromine contribute to

the breakdown of the ozone layer. It breaks down and reacts with the ozone molecules repeatedly

until a more stable compound is created. Some examples of ODP are: R12 has a ODP of 1.0,

R22 has a ODP of 0.6 and R134a has a ODP of 0.0.

2.7.2.2 Global Warming Potential (GWP)

26

The GWP is used to reflect the refrigerants impact on the global warming. The GWP is the ratio

of the warming caused by a substance to the warming caused by a similar mass of carbon

dioxide. Therefore, the GWP of CO2 is 1.0 by definition. Some examples of GWP are: R12 has a

GWP of 11,000, water has a GWP of 0, R22 has a GWP of 5,310 and R134a has a GWP of

3,830. (Refrigerants.com, 2008)

27

CHAPTER THREE

3.0 METHODOLOGY

A heat pump drier for the drying of agricultural produces was constructed. The test rig will

be used for high temperature applications. The heat pump dryer is the combination of heat pump

(refrigeration circuit) and dryer unit (Air circuit). The size of heat pump was determined by

moisture removal capacity of dryer and air conditions. The system consists of six main

components: compressor, evaporator, expansion device, condenser (for conventional refrigerant

based system), fan and dryer as shown in Figure 3.1 The following methods of information

acquisition lead to the design conception required for the design and fabrication of the test rig:

1. Oral Interview:This involves visitation to Refrigeration and Air-conditioning

experts to make enquire on relevant questions on the project

2. Library: Library textbooks and journal consultation

3. Physical Observation

4. Experimental Data

5. Design: Sketches and drawing using relevant mathematical analysis were also

prepared

28

3.1 Design Consideration

1. Availability of components

2. Adequate selection of materials for components

3. Ease of operation

4. Safety Considerations

5. Cost

6. Compactness of the system

7. Choice of refrigerant

8. Ozone depletion and Global warming potential of the refrigerant

9 Mechanical properties such as durability, strength, toughness, and heat conductivity.

29

Fig 3.1 Schematic Diagram of a Heat Pump Dryer

3.2 Assumptions made for the Refrigerant Cycle Analysis

30

1. Compression process is adiabatic but non-isentropic

2. Refrigerant at evaporator outlet is considered as saturated vapor

3. Refrigerant at condenser outlet is considered as saturated liquid

4. Evaporation and heat rejection (condensation/gas cooling) processes are isobaric

5. Heat transfer with the ambient is negligible

6. Air at the evaporator outlet or the gas cooler inlet is considered as saturated

7. Outlet air condition from the dryer is the same as the inlet condition to the evaporator.

3.3 Assumptions made in the air Cycle

(a) The heat pump is operated at steady state

(b) The heat transfer with ambient for the components, dryer and connecting

tubes is negligible

(c) The ambient conditions remain constant

(d) The temperature and air velocity are uniformly distributed in transverse

direction of flow.

(e) Compression process is adiabatic but non-isentropic

(f) Mixing of air is adiabatic

3.4 Component Design

31

3.4.1 Compressor

The refrigerant mass flow rate through the compressor is given by

Mr = Vsℓℓµ �

�� (3.1)

N = Speed of the motor

VS = swept volume of the compressor cylinder

ℓ= compressor efficiency

µ= volumetric efficicency

pressure ratio rp =������� ������

������� ������ (3.2)

The work input into the compressor is given by

Wcom = Mref (h2- h1) (3.3)

The table 3.1 shows the technical specification for the Tecumseh compressor

Table 3.1 Specification for Tecumseh Compressor

32

Compressor Type: Operating range: Displacement volume: Swept volume: Max.operating pressure: Max. discharge temperature: Max. suction temperature:

Tecumseh 1800 to 7200 rpm ( 30 to 120Hz ) 3.33 cm3

rev (LP) and 1.88cm3 rev (HP) 1.439m3

h at 7200 rpm 2M Pa 850C at continuous operation 250C

Motor Maximum power input: 1400W Lubricant Type: Viscosity: Properties: Oil discharge:

Polyalkyene glycol (PAG) 100 cSt Non- soluble with CO2 Heavier than saturated CO2 liqiud above -150C Excellent thermal and chemical stability High viscosity index High flash point – low pour point Hygroscopic

Approximately 6 to 9 of total mass flow rate (100Hz). Oil discharge rate increases with rpm and lower suction discharge pressure ( Hubacher and Groll, 2002 s

3.4.2 Heat Exchanger

33

A finned tube heat exchanger are employed. The material of heat exchanger is copper. Based

max operating pressure of the compressor (2 MPa) , the diameter of the tubes is 9.5mm outer

diameter. The tube thickness is 1mm ( yielding an inner diameter of 7.5mm) which is sufficient

to sustain the expected refrigerant pressure.

3.4.3 Tubing and Fittings

All the connections for the refrigerant circuit were made by copper tube of outer diameter

6.35mm of thickness 0.8mm ( inner diameter = 4.75mm). Tubing was designed for maximum

pressure of 20bar.

3.5 Fan Model

The fan is used in a heat pump dryer system to generate a pressure head sufficient to

maintain flow of air in the system. Neglecting pressure drop in the duct, total air pressure drop in

the heat pump dryer system is given by

PF = Pe + Pg + Pd ( 3.4 )

Where PF= fan pressure

Pd = dryer pressure

The power input to the fan which can produce pressure rise of PF is

WF = ṁ.��

��.�� ( 3.5)

3.6 Design Calculation

R- 134a

Evaporator temperature = -5 to 24o C

Condensing temperature = 35 to 80o C

Condensing pressure = 26bar

Critical temperature = 101oC

34

Power input = 1400W

T4 = T1 = 10OC

h1 = 404.3

T3 = 78oC , T2 =?

S1= Sg = 1.7221 KJ KgK , h3 = h4 = constant enthalpy process, h3 = 318.63KJ KgK

To get T2, interpolation is employed

�����.�

��.�����.� =

���.���!

�.������.���!

- 0.0217 = x1 – 1.7112 X1 = 1.6895 !."

".�� =

!��.��!"

�.�"����.��!"

X2 = 1.7427KJ KgK

Similarly for y1 and y 2

Y1 = 432.77KJ KgK

Y2 = 448.58KJ KgK

#����

����� =

�.�!!���.����

�.��!���.����

T2 = 86.13oC , P2 = 30bar

Similarly , h2 = 442.46 KJ KgK

C. O. Php = �!��"

�!��� =

��!.���"��.�"

��!.������."! = 3.25

ṁ = power of compressor work net = �.�

"�.�� = 0.037 Kg s

Heat capacity =( h2 – h3) × ṁ = 123.83 × 0.037 =4.6KW

35

3.6.1 Fan Model Calculation

WF = ṁ.��

��.��

ṁa = 0.037 Kg s

Pf = Pe + Pg + Pd = 4 + 26 +1 =31 bar

Where Pd= dryer pressure = atmospheric pressure

ηf = 0.8

Pam = 1.01 bar

Wf =�.�"� ×"�

�.� �.�� = 1.41955 KW

Wf = 1.9 hp

Therefore for this design 2 horse power was selected from standard specifications

3.7 Instrumentation

Temperature and Humidity Measurement

The temperature and humidity inside the following components in the Heat pump drier

are measured: Condenser, Evaporator, Drying chamber and fan chamber. Also the ambient

temperature are measured at different time. The measuring equipment is a Lodestar LT09012.

The temperature and humidity at different tray levels are measured to determine the rate of

change in temperature and humidity in the drying chamber.

36

CHAPTER FOUR

4.0 RESULTS AND DISCUSSION

Table 4.4 shows the performance of heat pump when used for the drying of okra and

tomatoes. It was observed that the agricultural product maintains their original flavor after

drying. There is 75% reduction in weight of the tomato and 45% reduction in weight of the okra

.Agricultural produces placed at the top most part of the drying chamber takes more time to dry

due to an outlet present at the top most part used for defrostation of the evaporator. Therefore,

there is heat dissipation at the top most part. The heat pump drier exhibit higher performance on

a sunny day based on the fact that air injected into the system by the condenser fan contains

lesser humidity, therefore the system is weather sensitive. The lower the humidity in the drying

chamber the higher the rate of drying irrespective of the temperature in the drying chamber.

Figure 4.1 to 4.3 plotted from Table 4.2 shows the relationship among the time, humidity

and temperature which are the three criteria on which the drying capacity can be measured.

Therefore under zero loading, it can deduced that the lower the humidity the higher the

temperature at a specific time duration i.e the temperature is inversely proportional to the

humidity at constant time.

Figure 4.4 to Figure 4.6 plotted from table shows the relationship among the time,

humidity and temperature. During drying it can be deducted that there is higher temperature and

lower humidity in the first tray. Therefore, the rate of drying reduces from the first tray to the

topmost tray. The rate of drying is also dependent on distance. According to Figure 4.6, there is

stability in the percentage of humidity as the temperature increases which means that there is

37

little or no increase in humidity for five hours. This will produce the best coefficient of

performance of the drier. In the event of this, agricultural produce within higher water content

are advised to be placed on the first tray. Plate 4.1 and Plate 4.2 shows the picture of the heat p

ump drier at zero loading and picture of the heat pump drier during drying respectively. Table

4.5 shows the bill of material and evaluation which determines the factors considered in the

selection of the materials, analyses of various material cost and the evaluation to discover if the

costs are economical

38

Based on reading of the lodestar meter, the following deductions were made

Table 4.1: Humidity and temperature of various component of the heat pump system

Component Temperature ( o C ) Humidity ( % ) Ambient Drying chamber Condenser Evaporation Fan chamber

28 .5 75 40 55 70 15 38

35 48 63

Table 4.2 The Temperature and Humidity during zero loading(no drying)

Drying chamber Ambient Time oC % oC % 11 am 72 50 28 84 12 pm 72 48 29 80 13 pm 75 48 32 75 14 pm 78 46 35 70 15 pm 79 45 36 67

Table 4.3 The temperature and Humidity during drying

Drying Chamber Ambient Time(hrs ) Tray one Tray two Tray three Tray four oC % o C % oC % oC % oC % 11 am 75 35 66 48 60 52 52 60 28 84 12 pm 75 35 66 46 61 52 50 60 29 80 13 pm 80 36 66 48 62 55 58 60 31 75 14 pm 80 34 70 48 60 50 55 63 35 70 15 pm 82 35 71 42 65 55 52 65 36 62

Table 4.4: The performance of the heat pump dryer

Agricultural product Temperature of

drying chamber

% reduction in

weight

Duration of drying

TOMATO 78o C 70% 15 hours

Okra 78oC 45% 10hours

39

Figure 4.1 Graph of Temperature against time for zero loading

Figure 4.2 Graph of Humidity against time for zero loading

20

30

40

50

60

70

80

90

11am 12am 01pm 02pm 03pm

Te

mp

era

ture

(C

)

Time (hours)

Graph of Temperature Against Time

drying chamber

ambient

20

30

40

50

60

70

80

90

11am 12am 01pm 02pm 03pm

Hu

mo

idit

y (

%)

Time (hours)

Graph of Humidity Against Time

drying chamber

ambient

40

Figure 4.3 Graph of Temperature against Humidity for zero loading

Figure 4.4 Graph of Temperature against time under loading

20

30

40

50

60

70

80

90

72 72 75 78 79

Hu

mid

ity

(%

)

Temperature (0C)

Graph of Humidity Against Temperature

drying chamber

ambient

20

30

40

50

60

70

80

90

11AM 12AM 1PM 2PM 3PM

Te

mp

era

ture

(O

C)

Time (hours)

Graph of Time Against Temperature

tray 1

tray 2

tray 3

tray 4

ambient

41

Figure 4.5 Graph of Humidity against Time under loading

Figure 4.6 Graph of Temperature against Humidity under loading

20

30

40

50

60

70

80

90

11AM 12AM 1PM 2PM 3PM

Hu

mid

ity

(%

)

Time (hours)

Graph of Humidity Against Time

tray 1

tray 2

tray 3

tray 4

ambient

20

30

40

50

60

70

80

90

75 75 80 80 82

Hu

mid

ity

(%

)

Temperature (0C)

Graph of Humidity Against Temperature

tray 1

tray 2

tray 3

tray 4

ambient

42

Plate 4.1:Picture of the Heat P ump Drier at Zero Loading

Plate 4.2: Picture of the Heat Pump Drier during Drying

43

Table 4.4: Bill of material and Evaluation

S N Material Name Quantity Cost

1 Compressor 1 #15,000

2 Evaporator 1 #6000

3 Condenser 1 # 6000

4 Riveting nails 3packs # 1,200

5 Tubes 3yards # 1,500

6 Refrgerant 4bottles # 1,500

7 Adhesive chemical 4liters # 4000

8 Polestine foam 1 # 15,000

9

Sum total

Galvanized steel sheet 4 #22,000

# 69000

44

CHAPTER FIVE

1.0 CONCLUSIONS AND RECOMMENDATIONS

1.1 Conclusions

Based on this project, the following conclusions were drawn:

1. A locally fabricated R134a Heat pump drier was constructed.

2. Environmentally friendly refrigerant were considered in the fabrication of the Heat pump

drier

3. Invention of the most efficient equipment for the drying of Agricultural products is of

International concern and its therefore of utmost importance

4. The R134a Heat pump constructed is an energy efficient and energy saving means of

drying.

5. The fabricated heat pump drier maintains original flavor and texture of the agricultural

produces.

6. The Coefficient of performance of the heat pump drier can be up to 3.25 with a Heating

capacity of 4.6KW

45

5.2 Recommendations

The following recommendations are made

I. The heat pump drier can be produced in large scale for commercial purpose.

II. To obtain maximum performance, the system should be operated in a low humidity

environment.

III. Also, institutions and government should create environment conducive enough for

research and development of projects work.

IV. Finally, the project is subjected to further research for improvement and development.

46

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