Refrigerator Air Conditioner

8/2/2019 Refrigerator Air Conditioner

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CONTENTS

1. ACKNOWLEDGEMENT

2. SYNOPSIS

3. INTRODUCTION

4. BLOCK DIAGRAM

5. WORKING PROCEDURE

6. APPLICATIONS AND ADVANTAGES

7. LIST OF MATERIALS

8. COST ESTIMATION

9. CONCLUSION

10. BIBLIOGRAPHY

11. PHOTOGRAPHY


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SYNOPSIS

A refrigerant heating type air conditioner operable in a cooling mode and a heating mode

includes a first external heat exchanger which works as a condenser in the cooling mode and a

second external heat-exchanger which works as an evaporator. Bypass means is disposed

between a discharge side of a compressor and the second external heat-exchanger. An expansion

valve is connected to an inner heat-exchanger, which works as a condenser in the heating mode,

and to both the first and second external heat-exchangers. A control valve is disposed between a

suction port side of the second external heat-exchanger for preventing the refrigerant from

flowing to the second external heat-exchanger in the cooling mode. Pressure of the refrigerant

which is trapped between the control valve and the expansion valve is increased by applying

heat, by heating means, to the second external heat-exchanger. The heated refrigerant is released

to the bypass means when its pressure exceeds to the pressure of discharged refrigerant from the

compressor.


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INTRODUCTION

What is Refrigeration and Air ConditioningRefrigeration and air conditioning is used to cool products or a building environment. The

refrigeration or air conditioning system (R) transfers heat from a cooler low-energy reservoir

to a warmer high-energy reservoir (see figure 1).

High Temperature Reservoir

Low Temperature Reservoir

R

Work Input

Heat Absorbed

Heat Rejected

Figure 1. Schematic representation of refrigeration system

Electrical Energy Equipment: Refrigeration and Air Conditioning Energy Efficiency Guide for

Industry in Asia www.energyefficiencyasia.org UNEP 2


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There are several heat transfer loops in a refrigeration system as shown in Figure 2. Thermal

energy moves from left to right as it is extracted from the space and expelled into theoutdoors through five loops of heat transfer:

Indoor air loop. In the left loop, indoor air is driven by the supply air fan through a

cooling coil, where it transfers its heat to chilled water. The cool air then cools thebuilding space.

Chilled water loop. Driven by the chilled water pump, water returns from the cooling coil

to the chillers evaporator to be re-cooled.

Refrigerant loop. Using a phase-change refrigerant, the chillers compressor pumps heatfrom the chilled water to the condenser water.

Condenser water loop. Water absorbs heat from the chillers condenser, and the

condenser water pump sends it to the cooling tower.

Cooling tower loop. The cooling towers fan drives air across an open flow of the hot

condenser water, transferring the heat to the outdoors.

1.2 Air-Conditioning Systems

Depending on applications, there are several options / combinations of air conditioning,

which are available for use:

Air conditioning (for space or machines)

Split air conditioners

Fan coil units in a larger system

Air handling units in a larger system

1.3 Refrigeration Systems (for processes)

The following refrigeration systems exists for industrial processes (e.g. chilling plants) and

domestic purposes (modular units, i.e. refrigerators):

Small capacity modular units of the direct expansion type similar to domestic

refrigerators.

Centralized chilled water plants with chilled water as a secondary coolant for a

temperature range over typically 5 oC. They can also be used for ice bank formation.


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Brine plants, which use brines as a lower temperature, secondary coolant for typically

sub- zero temperature applications, which come as modular unit capacities as well as large

centralized plant capacities.

The plant capacities up to 50 TR (tons of refrigeration) are usually considered as small

capacity, 50250 TR as medium capacity and over 250 TR as large capacity units.

A large company may have a bank of units, often with common chilled water pumps,

condenser water pumps, cooling towers, as an off site utility. The same company may also

have two or three levels of refrigeration and air conditioning such as a combination of:

Comfort air conditioning (2025 oC)

Chilled water system (80100 C)

Brine system (sub-zero applications)

TYPES OF REFRIGERATION AND AIR CONDITIONING

This section describes the two principle types of refrigeration plants found in industry:

Vapour Compression Refrigeration (VCR) and Vapour Absorption Refrigeration (VAR).VCR uses mechanical energy as the driving force for refrigeration, while VAR uses thermal

energy as the driving force for refrigeration.

2.1 Vapour Compression Refrigeration System

2.1.1 Description

Compression refrigeration cycles take advantage of the fact that highly compressed fluids at a

certain temperature tend to get colder when they are allowed to expand. If the pressurechange is high enough, then the compressed gas will be hotter than our source of cooling

(outside air, for instance) and the expand ed gas will be cooler than our desired cold

temperature. In this case, fluid is used to cool a low temperature environment and reject the

heat to a high temperature environment.

Vapour compression refrigeration cycles have two advantages. First, a large amount of

thermal energy is required to change a liquid to a vapor, and therefore a lot of heat can be

removed from the air-conditioned space. Second, the isothermal nature of the vaporizationallows extraction of heat without raising the temperature of the working fluid to the

temperature of whatever is being cooled. This means that the heat transfer rate remains high,

because the closer the working fluid temperature approaches that of the surroundings, thelower the rate of heat transfer.

The refrigeration cycle is shown in Figure 3 and 4 and can be broken down into the followingstages:


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12. Low-pressure liquid refrigerant in the evaporator absorbs heat from its

surroundings, usually air, water or some other process liquid. During this process itchanges its state from a liquid to a gas, and at the evaporator exit is slightly superheated.

23. The superheated vapour enters the compressor where its pressure is raised. The

temperature will also increase, because a proportion of the energy put into the compressionprocess is transferred to the refrigerant.

34. The high pressure superheated gas passes from the compressor into the condenser.

The initial part of the cooling process (3-3a) de-superheats the gas before it is then turned back

into liquid (3a-3b). The cooling for this process is usually achieved by using air or water. A

further reduction in temperature happens in the pipe work and liquid receiver (3b - 4), so that the

refrigerant liquid is sub-cooled as it enters the expansion device.

4 - 1 The high-pressure sub-cooled liquid passes through the expansion device, which

both reduces its pressure and controls the flow into the evaporator.

Schematic representation of the vapour compression refrigeration cycle


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The condenser has to be capable of rejecting the combined heat inputs of the evaporator and

the compressor. In other words: (1 - 2) + (2 - 3) has to be the same as (3 - 4). There is no heat

loss or gain through the expansion device.

Types of refrigerant used in vapour compression systems

A variety of refrigerants are used in vapor compression systems. The required cooling

temperature largely determines the choice of fluid. Commonly used refrigerants are in thefamily of chlorinated fluorocarbons (CFCs, also called Freons): R-11, R-12, R-21, R-22 and

R-502. The properties of these refrigerants are summarized in Table 1 and the performance of

these refrigerants is given in Table 2 below.

Vapour Absorption Refrigeration System

2.2.1 Description

The vapour absorption refrigeration system consists of:

Absorber: Absorption of refrigerant vapour by a suitable absorbent or adsorbent, forming

a strong or rich solution of the refrigerant in the absorbent/ adsorbent

Pump: Pumping of the rich solution and raising its pressure to the pressure of the

Condenser

Generator: Distillation of the vapour from the rich solution leaving the poor solution forRecycling

The absorption chiller is a machine, which produces chilled water by using heat such as

steam, hot water, gas, oil etc. Chilled water is produced based on the principle that liquid (i.e.

refrigerant, which evaporates at a low temperature) absorbs heat from its surroundings when

it evaporates. Pure water is used as refrigerant and lithium bromide solution is used asabsorbent.

Heat for the vapour absorption refrigeration system can be provided by waste heat extracted

from the process, diesel generator sets etc. In that case absorption systems require electricity

for running pumps only. Depending on the temperature required and the power cost, it mayeven be economical to generate heat / steam to operate the absorption system.

Condenser


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A simple schematic of a vapour absorption refrigeration system

The absorption chiller is a machine, which produces chilled water by using heat such assteam, hot water, gas, oil etc. Chilled water is produced based on the principle that liquid (i.e.

refrigerant, which evaporates at a low temperature) absorbs heat from its surroundings when

it evaporates. Pure water is used as refrigerant and lithium bromide solution is used asabsorbent.

Heat for the vapour absorption refrigeration system can be provided by waste heat extracted

from the process, diesel generator sets etc. In that case absorption systems require electricityfor running pumps only. Depending on the temperature required and the power cost, it may

even be economical to generate heat / steam to operate the absorption system.

A description of the absorption refrigeration concept is given below (references for thepictures are unknown)

Evaporator

The refrigerant (water) evaporates at around 4oC under a high vacuum condition of 754 mm Hg

in the evaporator. Chilled water goes through heat exchanger tubes in the evaporator and

transfers heat to the evaporated refrigerant.

The evaporated refrigerant (vapor) turns into liquid again, while the latent heat from thisvaporization process cools the chilled water (in the diagram from 12 oC to 7 oC). The chilled

water is then used for cooling purpose.


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Absorber

In order to keep evaporating, the refrigerant vapor must be discharged from the evaporator and

refrigerant (water) must be supplied. The refrigerant vapor is absorbed into lithium bromidesolution, which is convenient to absorb the refrigerant vapor in the absorber. The heat generated

in the absorption process is continuously removed from the system by cooling water. The

absorption also maintains the vacuum inside the evaporator

High Pressure Generator

As lithium bromide solution is diluted, the ability to absorb the refrigerant vapor reduces. In

order to keep the absorption process going, the diluted lithium bromide solution must beconcentrated again..

An absorption chiller is provided with a solution concentrating system, called a generator.

Heating media such as steam, hot water, gas or oil perform the function of concentratingsolutions.


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The concentrated solution is returned to the absorber to absorb refrigerant vapor again

Condenser

To complete the refrigeration cycle, and thereby ensuring the refrigeration takes place

continuously, the following two functions are required.

1. To concentrate and liquefy the evaporated refrigerant vapor, which is generated in the

highpressure generator.

2. To supply the condensed water to the evaporator as refrigerant (water) For these two functionsa condenser is installed.


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Absorption refrigeration systems that use Li-Br-water as a refrigerant have a Coefficient of

Performance (COP) in the range of 0.65 - 0.70 and can provide chilled water at 6.7 oC with acooling water temperature of 30 oC. Systems capable of providing chilled water at 3 oC are

also available. Ammonia based systems operate at above atmospheric pressures and are

capable of low temperature operation (below 0oC). Absorption machines are available with

capacities in the range of 10-1500 tons. Although the initial cost of an absorption system ishigher than that of a compression system, operational costs are much lower if waste heat is

used.

Evaporative cooling in vapor absorption refrigeration systems.

There are occasions where air conditioning, which stipulates control of humidity of up to

50% for human comfort or for processes, can be replaced by a much cheaper and less energy

intensive evaporative cooling.

Sprinkling Water


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The concept is very simple and is the same as that used in a cooling tower. Air is brought in

close contact with water to cool it to a temperature close to the wet bulb temperature. Thecool air can be used for comfort or process cooling. The disadvantage is that the air is rich in

moisture. Nevertheless, it is an extremely efficient means of cooling at very low cost. Large

commercial systems employ cellulose filled pads over which water is sprayed. Thetemperature can be controlled by controlling the airflow and the water circula tion rate. The

possibility of evaporative cooling is especially attractive for comfort cooling in dry regions.

This principle is practiced in textile industries for certain processes.

OF REFRIGERATION AND AIR CONDITIONING

This section describes how the performance of refrigeration / air conditioning plants and be

assessed.

3.1 Assessment of Refrigeration

3.1.1 TR

We start with the definition of TR.

TR: the cooling effect produced is quantified as tons of refrigeration, also referred to as

chiller tonnage.

TR = Q xCp x(TiTo) / 3024

Where Q is mass flow rate of coolant in kg/hr

Cp is coolant specific heat in kCal /kg deg CTi is inlet, temperature of coolant to evaporator (chiller) in 0C

To is outlet temperature of coolant from evaporator (chiller) in 0C.

1 TR of refrigeration = 3024 kCal/hr heat rejected

3.1.2 Specific Power Consumption

The specific power consumption kW/TR is a useful indicator of the performance of a

refrigeration system. By measuring the refrigeration duty performed in TR and the kW

inputs, kW/TR is used as an energy performance indicator.

In a centralized chilled water system, apart from the compressor unit, power is also

consumed by the chilled water (secondary) coolant pump, the condenser water pump (for

heat rejection to cooling tower) and the fan in the cooling tower. Effectively, the overall

energy consumption would be the sum of:


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Compressor kW

Chilled water pump kW

Condenser water pump kW

Cooling tower fan kW, for induced / forced draft towersThe kW/TR, or the specific power consumption for a certain TR output is the sum of:

Compressor kW/TRChilled water pump kW/TR

Condenser water pump kW/TR

Cooling tower fan kW/TR

Coefficient of Performance

The theoretical Coefficient of Performance (Carnot), (COPCarnot, a standard measure of

refrigeration efficiency of an ideal refrigeration system) depends on two key systemtemperatures: evaporator temperature Te and condenser temperature Tc. COP is given as:

COPCarnot = Te/ (Tc - Te)

This expression also indicates that higher COPCarnot is achieved with higher evaporator

temperatures and lower condenser temperatures. But COPCarnot is only a ratio of

temperatures, and does not take into account the type of compressor. Hence the COPnormally used in industry is calculated as follows:

Cooling effect (kW)

COP =Power input to compressor (kW)

where the cooling effect is the difference in enthalpy across the evaporator and expressed

as kW.

Figure


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Assessment of Air ConditioningFor air conditioning units, the airflow at the Fan Coil Units (FCU) or the Air Handling Units(AHU) can be measured with an anemometer. Dry bulb and wet bulb temperatures are

measured at the inlet and outlet of the AHU or the FCU and the refrigeration load in TR isassessed as:

Where, Q is the air flow in m3/h

is density of air kg/m3hin is enthalpy of inlet air kCal/kg

hout is enthalpy of outlet air kCal/kg

Use of psychometric charts can help to calculate h in and hout from dry bulb and wet bulb

temperature values which are measured during trials by a whirling psychrometer. Power

measurements at compressor, pumps, AHU fans, cooling tower fans can be taken with aportable load analyzer.

Estimation of the air conditioning load is also possible by calculating various heat loads,sensible and latent, based on inlet and outlet air parameters, air ingress factors, air flow,

number of people and type of materials stored.

An indicative TR load profile for air conditioning is presented as follows:

Small office cabins = 0.1 TR/m2

Medium size office i.e., = 0.06 TR/m2

1030 people occupancywith central A/C

Large multistoried office = 0.04 TR/m2

complexes with central A/C

3.3 Considerations when Assessing Performance

3.3.1 Accuracy of flow and temperature measurements

In a field performance assessment, accurate instruments are required to measure the inlet and

outlet temperatures of chilled water and condenser water, preferably with a count of at least

0.1 oC. Flow measurements of chilled water can be made with an ultrasonic flow meter

directly or can be determined based on pump duty parameters. Adequacy checks of chilledwater are often needed and most units are designed for a typical 0.68 m3/hr per TR (3

gpm/TR) chilled water flow. Condenser water flow can also be measured with a non-contactflow meter directly or determined by using pump duty parameters. Adequacy checks of

condenser water are also needed often, and most units are designed for a typical 0.91 m 3/hr

per TR (4 gpm / TR) condenser water flow.


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3.3.2 Integrated Part Load Value (IPLV)

Although the kW/ TR can serve as an initial reference, it should not be taken as an absolute

since this value is based on a 100% equipment capacity level and on design conditions that

are considered most critical. These conditions may only occur during % of the total time the

equipment is in operation throughout the year. For this reason, it is essential to have data thatreflects how the equipment operates with partial loads or under conditions that demand less

than 100% capacity. To overcome this, an average kW/TR with partial loads has to bedetermined, which is called the Integrated Part Load Value (IPLV).

The IPLV is the most appropriate reference, although not considered the best, because it only

captures four points within the operational cycle: 100%, 75%, 50% and 25%. Furthermore, it

assigns the same weight to each value, and most equipment operate between 50% and 75%

oftheir capacity. This is why it is so important to prepare a specific analysis for each case that

addresses the four points mentioned, as well as developing a profile of the heat exchanger's

operations during the year.

ENERGY EFFICIENCY OPPORTUNITIES

This section includes areas for energy conservation in refrigeration plants.

4.1 Optimization of Process Heat Exchangers

There is a tendency to apply high safety margins to operations, which influence the

compressor suction pressure / evaporator set point. For instance, a process-coolingrequirement of 15 oC would need chilled water at a lower temperature, but the range can vary

from 6 oC to about 10 oC. At chilled water of 10 oC, the refrigerant side temperature has to be

lower (about5oC to +5oC). The refrigerant temperature determines the correspondingsuction pressure of the refrigerant, which in turn determines the inlet duty conditions for the

refrigerant compressor. Applying the optimum / minimum driving force (temperature

difference) can thus help to reach the highest possible suction pressure at the compressor,

thereby minimizing energy consumption. This requires proper sizing of heat transfer areas ofprocess heat exchangers and evaporators as well as rationalizing the temperature requirement

to highest possible value. A 1oC raise in evaporator temperature can save almost 3 % of the

power consumed. The TR capacity of the same machine will also increase with the

evaporator temperature, as given in the table below.


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In order to rationalize the heat transfer areas, the heat transfer coefficient on the refrige rant

side can range from 14002800 watts /m2K. The refrigerant side heat transfer areas are ofthe order of 0.5 m2/TR and above in evaporators.

Condensers in a refrigeration plant are critical equipment that influence the TR capacity and

power consumption demands. For any refrigerant, the condensation temperature andcorresponding condenser pressure are dependent on the heat transfer area, the effectiveness of

heat exchange and the type of cooling chosen. A lower condensation temperature means that

the compressor has to work between a lower pressure differential as the discharge pressure isfixed by design and performance of the condenser.

The choice of condensers in practice is between air-cooled, air-cooled with water spray, and

heat exchanger cooled. Larger shell and tube heat exchangers that are used as condensers and

that are equipped with good cooling tower operations allow operation at low discharge pressure

values and improve the TR capacity of the refrigeration plant.

If the refrigerant R22 is used in a water-cooled shell and tube condenser then the dischargepressure is 15 kg/cm2. If the same refrigerant is used in an air-cooled condenser then thedischarge pressure is 20 kg/cm2. This shows how much additional compression duty is

required, which results in almost 30 % additional energy consumption by the plant.

One of the best options at the design stage would be to select large sized (0.65 m 2/TR andabove) shell and tube condensers with water-cooling, rather than less expensive alternatives

like air cooled condensers or water spray atmospheric condenser units.

Maintenance of Heat Exchanger Surfaces

Once compressors have been purchased, effective maintenance is the key to optimizingpower consumption. Heat transfe r can also be improved by ensuring proper separation of the

lubricating oil and the refrigerant, timely defrosting of coils, and increasing the velocity of

the secondary coolant (air, water, etc.). However, increased velocity results in larger pressuredrops in the distribution system and higher power consumption in pumps / fans. Therefore,

careful analysis is required to determine the optimum velocity.

Fouled condenser tubes force the compressor to work harder to attain the desired capacity.

For example, a 0.8 mm scale build-up in condenser tubes can increase energy consumption

by as much as 35 %. Similarly, fouled evaporators (due to residual lubricating oil or

infiltration of air) result in increased power consumption. Equally important is properselection, sizing, and maintenance of cooling towers. A reduction of 0.55oC in temperature

of the water returning from the cooling tower reduces compressor power consumption by 3%.

* 15 ton reciprocating compressor based system. The power consumption is lower than that for

systems typically available in India. However, the percentage change in power consumption is

indicative of the effect of poor maintenance


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Multi -Staging For Efficiency

Efficient compressor operation requires that the compression ratio be kept low, to reduce

discharge pressure and temperature. For low temperature applications involving highcompression ratios, and for wide temperature range requirements, it is preferable (due toequipment design limitations) and often economical to employ multi-stage reciprocating

machines or centrifugal / screw compressors.

There are two types of multi-staging systems, which are applicable to all types of

compressors: compound and cascade. With reciprocating or rotary compressors, two-stage

compressors are preferable for load temperatures from20oC to58oC, and with centrifugal

machines for temperatures around43oC.

In a multi-stage operation, a first-stage compressor that sized to meet the cooling load, feeds

into the suction of a second-stage compressor after inter-cooling of the gas. A part of thehigh-pressure liquid from the condenser is flashed and used for liquid sub-cooling. The

second compressor, therefore, has to meet the load of the evaporator and the flash gas. A

single refrigerant is used in the system, and the two compressors share the compression task

equally. Therefore, a combination of two compressors with low compression ratios canprovide a high compression ratio.

For temperatures in the range of46oC to101oC, cascaded systems are preferable. In thissystem, two separate systems using different refrigerants are connected so that one rejects

heat to the other. The main advantage of this system is that a low temperature refrigerant,

which has a high suction temperature and low specific volume, can be selected for the lowstage

to meet very low temperature requirements.

4.4 Matching Capacity to System Load

During part- load operation, the evaporator temperature rises and the condenser temperature

falls, effectively increasing the COP. But at the same time, deviation from the design

operation point and the fact that mechanical losses form a greater proportion of the totalpower negate the effect of improved COP, resulting in lower part- load efficiency.

Therefore, consideration of part-load operation is important, because most refrigeration

applications have varying loads. The load may vary due to variations in temperature and

process cooling needs. Matching refrigeration capacity to the load is a difficult exercise,

requiring knowledge of compressor performance, and variations in ambient conditions, anddetailed knowledge of the cooling load.


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4.5 Capacity Control and Energy Efficiency

The capacity of compressors is controlled in a number of ways. Capacity control of

reciprocating compressors through cylinder unloading results in incremental (step-by-step)

modulation. In contrast, continuous modulation occurs in centrifugal compressors through

vane control and in screw compressors through sliding valves. Therefore, temperature controlrequires careful system design. Usually, when using reciprocating compressors in applications

with widely varying loads, it is desirable to control the compressor by monitoring the returnwater (or other secondary coolant) temperature rather than the temperature of the water leaving

the chiller.

This prevents excessive on-off cycling or unnecessary loading / unloading of the compressor.

However, if load fluctuations are not high, the temperature of the water leaving the chiller should

be monitored. This has the advantage of preventing operation at very low water temperatures,

especially when flow reduces at low loads. The outgoing water temperature should be monitoredfor centrifugal and screw chillers.

Capacity regulation through speed control is the most efficient option. However, when

employing speed control for reciprocating compressors, it should be ensured that thelubrication system is not affected. In the case of centrifugal compressors, it is usually

desirable to restrict speed control to about 50 % of the capacity to prevent surging. Below

50%, vane control or hot gas bypass can be used for capacity modulation.

The efficiency of screw compressors operating at part load is generally higher than either

centrifugal compressors or reciprocating compressors, which may make them attractive in

situations where part-load operation is common. Screw compressor performance can beoptimized by changing the volume ratio. In some cases, this may result in higher full-load

efficiencies as compared to reciprocating and centrifugal compressors. Also, the ability ofscrew compressors to tolerate oil and liquid refrigerant slugs makes them preferred in somesituations.

4.6 Multi -level Refrigeration for Plant Needs

The selection of refrigeration systems also depends on the range of temperatures required inthe plant. For diverse applications requiring a wide range of temperatures, it is generally

more economical to provide several packaged units (several units distributed throughout the

plant) instead of one large central plant. Another advantage would be the flexibility and

reliability. The selection of packaged units could also be made depending on the distance at

which cooling loads need to be met. Packaged units at load centers reduce distribution lossesin the system. Despite the advantages of packaged units, central plants generally have lower

power consumption since at reduced loads power consumption can reduce significantly due

to the large condenser and evaporator surfaces.


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Many industries use a bank of compressors at a central location to meet the load. Usually

the chillers feed into a common header from which branch lines are taken to differentlocations in the plant. In such situations, operation at part- load requires extreme care. For

efficient operation, the cooling load, and the load on each chiller must be monitored closely.

It is more efficient to operate a single chiller at full load than to operate two chillers at partload.The distribution system should be designed such that individual chillers can feed all branch lines.

Isolation valves must be provided to ensure that chilled water (or other coolant) does not flow

through chillers not in operation. Valves should also be provided on branch lines to isolatesections where cooling is not required. This reduces pressure drops in the system and reduces

power consumption in the pumping system.

Individual compressors should be loaded to their full capacity before operating the second

compressor. In some cases it is economical to provide a separate smaller capacity chiller,

which can be operated on an on-off control to meet peak demands, with larger chillers

meeting the base load.

Flow control is also commonly used to meet varying demands. In such cases the savings inpumping at reduced flow should be weighed against the reduced heat transfer in coils due to

reduced velocity. In some cases, operation at normal flow rates, with subsequent longer

periods of no- load (or shut-off) operation of the compressor, may result in larger savings.

4.7 Chilled Water Storage

Depending on the nature of the load, it is economical to provide a chilled water storagefacility with very good cold insulation. Also, the storage facility can be fully filled to meet

the process requirements so that chillers need not be operated continuously. This system is

usually economical if small variations in temperature are acceptable. This system has the

added advantage of allowing the chillers to be operated at periods of low electricity demandto reduce peak demand charges. Low tariffs offered by some electric utilities for operation at

nighttime can also be taken advantage of by using a storage facility. An added benefit is that

lower ambient temperature at night lowers condenser temperature and thereby increases theCOP.

If temperature variations cannot be tolerated, it may not be economical to provide a storagefacility since the secondary coolant would have to be stored at a temperature much lower than

required to provide for heat gain. The additional cost of cooling to a lower temperature may

offset the benefits. The solutions are case specific. For example, in some cases it may be

possible to employ large heat exchangers, at a lower cost burden than low temperature chiller

operation, to take advantage of the storage facility even when temperature variations are notacceptable. Ice bank systems, which store ice rather than water, are often economical.


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4.8 System Design Features

In overall plant design, adoption of good practices improves the energy efficiency

significantly. Some areas for consideration are:

Design of cooling towers with FRP impellers and film fills, PVC drift eliminators, etc.

Use of softened water for condensers in place of raw water.

Use of economic insulation thickness on cold lines, heat exchangers, considering cost of

heat gains and adopting practices like infrared thermography for monitoring - applicableespecially in large chemical / fertilizer / process industry.

Adoption of roof coatings / cooling systems, false ceilings / as applicable, to minimize

refrigeration load.

Adoption of energy efficient heat recovery devices like air to air heat exchangers to precoolthe fresh air by indirect heat exchange; control of relative humidity through indirect

heat exchange rather than use of duct heaters after chilling.

Adopting of variable air volume systems; adopting of sun film application for heat reflection;

optimizing lighting loads in the air conditioned areas; optimizing number of air changes in

the air conditioned areas are few other examples.

OPTION CHECKLIST

This section includes most important energy efficiency options.

Cold Insulation: Insulate all cold lines / vessels using economic insulation thickness to

minimize heat gains; and choose appropriate (correct) insulation.

Building Envelope: Optimize air conditioning volumes by measures such as use of false

ceiling and segregation of critical areas for air conditioning by air curtains.

Building Heat Loads Minimization: minimize the air conditioning loads by measures such

as roof cooling, roof painting, efficient lighting, pre-cooling of fresh air by air- to-air heat

exchangers, variable volume air system, optimal thermo-static setting of temperature ofair conditioned spaces, sun film applications, etc.

Process Heat Loads Minimization: Minimize process heat loads in terms of TR capacityas well as refrigeration level, i.e., temperature required, by way of:


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Flow optimization

Heat transfer area increase to accept higher temperature coolant

Avoiding wastages like heat gains, loss of chilled water, idle flows.

Frequent cleaning / de-scaling of all heat exchangers

At the Refrigeration A/C Plant Area:

Ensure regular maintenance of all A/C plant components as per manufacturerguidelines.

Ensure adequate quantity of chilled water and cooling water flows and avoidbypass flows by closing valves of idle equipment.

Minimize part load operations by matching loads and plant capacity on lineand adopt variable speed drives for varying process load.

Make efforts to continuously optimize condenser and evaporator parametersfor minimizing specific energy consumption and maximizing capacity.

Adopt a VAR system where economics permit as a non-CFC solution.

Ensure that the AC does not get overloaded and check the fuse or circuit breaker if theAC does not operate.

Replace or clean the filter and clean the evaporator and condenser coils regularly, for theair conditioner to cool efficiently.

Clean the thermostat regularly and replace it if necessary.

If a compressor does not work properly, call a service person immediately

Any noise that your AC makes needs to be checked by your mechanic.

A good air filter will extend the life of your air conditioner because the important parts,

like the blower assembly, the cooling coil, and other inner parts will stay cleaner, operatemore efficiently and last longer.

Avoid frequent opening of doors/windows. A door kept open can result in doubling the

power consumption of your AC.

Ensure direct sunlight and heat do not enter the air-conditioned space, particularly in the

afternoons.

Most people believe that a thermostat set to a lower temperature than desired will forceyour air-conditioner to cool faster, not really, all it does, is make your air-conditioner

operate for longer. Moreover, you will have an unnecessarily chilly room and wastedpower. Every degree lower on the temperature setting results in an extra 3-4% of power

consumed. Hence, once you found yourself a comfortable temperature and set the

thermostat at that level, avoid changing the thermostat settings.


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Once an air-conditioning system has been designed and installed avoid any major change

in the heat-load on the AC. This will add to wasted power

A clogged drain line is usually caused by algae (the green moss- like stuff!) build-up

inside the drain line. The air handler provides a cool, damp environment for development

of molds and mildew and if left untreated these growths can spread into your ductwork.Get rid of these molds by using a disinfectant (consult your dealer). Make sure that the

face of the cooling or evaporator coil is clean so that air can pass through freely.

If you have an air return duct in a hot area such as an attic or garage, make sure that this

duct is not broken, split, or disconnected and sucking in hot air.

Window unit should tilt down slightly on the outside. The part that removes humidity

(where water accumulates) is the front coil, which is inside your home. Normally, there is

a trough and/or a drain tube that lets the water run to the rear of the unit. If the drain gets

clogged, water will back up and leak inside. Ask your mechanic to clean the chassis and

make sure all screws are tight.

Heat load can be reduced by keeping a false ceiling in offices. Curtains/ blinds /sun filmon windows reduces heat input into the room. Insulating the ceiling, which is exposed to

the sun with 50- mm thermocole drastically, reduces heat input into the room.

Check for duct leaks and crushed ductwork. All air leaks should be sealed with a good

quality duct sealant (not duct tape).

Inspect the chiller as recommended by the chiller manufacturer. Typically, this should bedone at least quarterly.

Routinely inspect for refrigerant leaks.

Check the compressor operating pressures.

Check all oil levels and pressures.

Examine all motor voltages and amps.

Check all electrical starters, contactors, and relays.

Check all hot gas and unloader operations.

Use superheat and subcooling temperature readings to obtain a chiller's maximum

efficiency.

Take discharge line temperature readings.

Some Rules of Thumb are:


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Refrigeration capacity reduces by 6 percent for every 3.5 C increase in condensing

temperature.

Reducing condensing temperature by 5.5 C results in a 2025 percent decrease in

compressor power consumption.

A reduction of 0.55 C in cooling water temperature at condenser inlet reduces

compressor power consumption by 3 percent.

1 mm scale build-up on condenser tubes can increase energy consumption by 40 percent.

5.5 C increase in evaporator temperature reduces compressor power consumption by 20

25 percent.

The condensed refrigerant in the condenser is in condition A which lies on the line for theboiling point of the liquid. The liquid has thus a temperature tc, a pressure pc also called

saturated temperature and pressure.

The condensed liquid in the condenser is further cooled down in the condenser to a lower

temperature A1 and now has a temperature tl and an enthalpy h0. The liquid is now sub-cooled

which means that it is cooled to a lower temperature than the saturated temperature.

The condensed liquid in the receiver is in condition A1 which is sub-cooled liquid. This liquid

temperature can change if the receiver and liquid is either heated or cooled by the ambient

temperature. If the liquid is cooled the sub-cooling will increase and visa versa.

When the liquid passes through the expansion valve its condition will change from A1 to B. Thisconditional change is brought about by the boiling liquid because of the drop in pressure to p0.

At the same time a lower boiling point is produced, t0, because of the drop in pressure. In theexpansion valve the enthalpy is constant h0, as heat is neither applied nor removed.

At the evaporator inlet, point B, there is a mixture of liquid and vapour while in the evaporator atC there is saturated vapour. At the evaporator outlet.

4. Refrigeration process, pressure/enthalpy diagram

point C1 there is super-heated vapour which means that the suction gas is heated to a higher

temperature than the saturated temperature. Pressure and temperature are the same at point Band at outlet point C1 where the gas is super-heated the evaporator has absorbed heat from thesurroundings and the enthalpy has changed to h1.


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When the refrigerant passes through the compressor its condition changes from C1 to D.

Pressure rises to condensing pressure pc. The temperature rises to thot-gas which is higher thanthe condensing temperature tc because the vapour has been strongly superheated. More energy

(from the electrical motor) in the form of heat has also been introduced and the enthalpy

therefore changes to h2.

At the condenser inlet, point D, the condition is thus one of superheated vapour at pressure pc.

Heat is given off from the condenser to the surroundings so that the enthalpy again changes to

main point A1. First in the condenser there occurs a conditional change from stronglysuperheated vapour to saturated vapour (point E), then a condensation of the saturated vapour.

From point E to point A the temperature (condensing temperature) remains the same, in that

condensation and evaporation occurs at constant temperature.

From point A to point A1 in the condenser the condensed liquid is further cooled down, but the

pressure remains the same and the liquid is now sub-cooled.

tc = condensing temperaturepc = condensing pressure

tl = liquid temperaturet0 = evaporating temperature

p0 = evaporating pressure

Expansionvalve

Receiver

Pressure

Heat

Refrigeration plant main components

The job of the compressor is to suck vapour from the evaporator and force it into the condenser.

The most common type is the piston compressor, but other types have won acceptance, e.g.

centrifugal scroll and screw compressors.

The piston compressor covers a very large capacity range, right from small single cylinder

modeIs for household refrigerators up to 8 to 12 cylinder modeIs with a large swept volume for

industrial applications.

In the smallest applications the hermetic compressor is used, where compressor and

motor are built together as a complete hermetic unit.


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For medium sized plants one of the most common compressors is the larger sizes of

hermetic compressors in either piston or scroll versions. The applications are both airconditioning, general commercial refrigeration and chillers.

For larger plants the most common is the semihermetic compressor. The advantage here is that

shaft glands can be avoided; these are very difficult to replace when they begin to leak.However, the design cannot be used on ammonia plants since this refrigerant attacks motor

windings.

Still larger HFC compressors, and all ammonia compressors, are designed as opencompressors, i.e. with the motor outside the crankcase. Power transmission can be direct to the

crankshaft or through a V-belt drive.

For quite special applications there is the oil-free compressor. But lubrication of bearings and

cylinder walls with oil is normally always necessary. On large refrigeration compressors oil is

circulated by an oil pump.

Vapor compression refrigeration, as the name suggests, employs a compression process to

raise the pressure of a refrigerant vapor flowing from an evaporator at pressurep1 top2, as

shown in Figure 8.2. The refrigerant then flows through a heat exchanger called a condenserat

the high pressure,p2 =p3, through a throttling device, and back to the low pressure, p1, in theevaporator. The pressuresp2 =p3 andp4 =p1 correspond to refrigerant saturation temperatures,T3 and T1 = T4, respectively. These temperatures allow natural heat exchange with adjacent hot

and cold regions from high temperature to low. That is, T1 is less than TL; so that heat, QL, willflow from the cold region into the evaporatorto vaporize the working fluid. Similarly, the

temperature T3 allows heat, QH, to be transferred from the working fluid in the condenserto the

hot region at TH.

This is indicated by the arrows of Figure 8.2.

Thus the resulting device is one in which heat is transferred from a low temperature, TL, to ahigh temperature, TH, using a compressor that receives work from the surroundings, therein

satisfying the Clausius statement.

The throttling device, as shown in Figure 8.2, restrains the flow of refrigerant from the

condenser to the evaporator. Its primary purpose is to provide the flow resistance necessary to

maintain the pressure difference between the two heat exchangers. It also serves to control the

rate of flow from condenser to evaporator. The throttling device may be a thermostatic

expansion valve (TEV) controlled by evaporator exit temperature or a long, fine-bore pipe calleda capillary tube.


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For an adiabatic throttling device, the First Law of Thermodynamics requires that h3 = h4 forthe irreversible process, because Q and Ware zero and kinetic energy change is negligible. Thus

saturated liquid at T3 flashes to a mixture of liquid and vapor at the evaporator inlet at the

enthalpy h4 = h3 and pressurep4 =p1. Also the evaporator entrance has the qualityx4 and

temperature T4 = T1. Heat from the cold source at TL > T4 boils the mixture in the evaporator toa saturated or slightly superheated vapor that passes to the suction side of the compressor.

The compressor in small and medium-sized refrigeration units is usually a reciprocating orother positive-displacement type, but centrifugal compressors often are used in large systems

designed for commercial and industrial service.

It may be noted from the T-s diagram in Figure 8.2 that the vapor compression cycle is a

reversed Rankine cycle, except that the pressure drop occurs through a throttling device rather

than a turbine. In principle, a turbine or expansion device of some kind could be used to

simultaneously lower the refrigerant pressure and produce work to reduce the net work required

to operate the compressor. This is very unlikely because of the difficulty of deriving work from amixture of liquid and vapor and because of the low cost and simplicity of refrigeration throttling

devices.

An exploded view of a through-the-wall type room air conditioner commonly used in motels

and businesses is shown in Figure 8.3. A fan coil unit on the space side is the evaporator. Athermally insulating barrier separates a hermetically sealed, electric-motor-driven positive

displacement compressor unit and a finned-tube heat exchanger condenser from the room on the

outdoor side.


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Figure 8.4 shows a packaged air-conditioning unit designed for in-space use or for a nearby

space with short duct runs. Units sometimes are designed to operate with either one or twocompressors, coils, and fans to better accommodate varying cooling demands. A unit with

watercooled condensers such as that shown requires an external heat sink, usually provided by a

nearby ground-level or rooftop evaporative cooling tower.

Rather than being combined in a single enclosure, refrigeration units frequently are installed

as split systems. Figure 8.5 shows an uncovered rooftop condensing unitthat contains acompressor and air cooled condenser. Such units are, of course, covered to resist the outdoorenvironment over many years. Cooled refrigerant is piped in a closed circuit to remote air

distribution units that contain cooling coils (evaporators) and throttling devices. Figure 8.6

shows a skid-mounted air-cooled condensing unit also designed to function with remote

evaporators in applications such as walk-in coolers.


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Refrigerants

Refrigerants are specially selected substances that have certain important characteristics

including good refrigeration performance, low flammability and toxicity, compatibility with

compressor lubricating oils and metals, and good heat transfer characteristics. They are usually

identified by a number that relates to their molecular composition.


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The ASHRAE Handbook (ref. 1) identifies a large number of refrigerants by number, as

shown in Table 8.1. Inorganic refrigerants are designated by 700, plus their molecular weight.For hydrocarbon and halocarbon refrigerants, the number schemeXYZworks as follows: (1)Z,

on the right is the number of fluorine atoms; (2) Yis the number of hydrogen atoms plus one; and

(3) the leftmost digit,X, is one less than the number of carbon atoms in the compound.

Two important examples are refrigerants R-12 and R-22. R-12, dichlorodifluoromethane, has

two fluorine, one carbon, and two chlorine atoms in a methane-type structure. Thus the halogens


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chlorine and fluorine, replace hydrogen atoms in the CH4 molecular structure as shown in Figure

8.7. R-22, monochlorodifluoromethane, has a similar structure to R-12, except for a singlehydrogen atom replacing a chlorine atom. Charts of the thermodynamic properties of these

refrigerants are given in Appendix F.

The commonly used chlorofluorocarbon (CFC) refrigerants are a cause of great concern,

because their accumulation in the upper atmosphere creates a .hole. in the ozone layer that

normally shields the earth from solar ultraviolet radiation (refs. 8 and 9). In 1987, more than 35

countries, including the United States, signed the Montreal Protocol on Substances that Depletethe Ozone Layer. The Montreal Protocol called for a freeze in 1989 and reductions in the 1990s

on the production levels of R-11, R-12, R-113, R-114, and R-115. The halocarbon refrigerants,

some of which are also widely used as aerosol propellants, foams, and solvents, are nowcategorized as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), or

hydrofluorocarbons (HFCs). The HFCs, lacking chlorine, are no threat to the ozone layer but are

not in common usage as refrigerants. CFCs, which contain more chlorine than do HCFCs, are the

most serious offenders, are very stable, and do not break down rapidly in the lower atmosphere.

The Clean Air Act of 1990 (ref. 15) mandated termination of production in the United States of

all CFCs such as R-12 by the year 2000. Government data indicate that, because of the structuraldifference between them, R-12 has twenty times the ozone-depletion potential in the upper

atmosphere of R-22. Nevertheless, R-22 and other HCFCs are also scheduled by the law for

phaseout of production by the year 2030.


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Thus, the search for alternate refrigerants to replace those used in existing applications

(worth hundreds of billions of dollars) has assumed enormous importance. It is a difficult,expensive, and continuing task to which industry is vigorously applying its efforts. Charts of

thermodynamic properties for two of the newer refrigerants, R-123 and R134a, are given in

Appendix F.

Vapor-Compression Cycle Analysis

A vapor-compression cycle was shown in Figure 8.2, The work required by the refrigeration

compressor, assuming adiabatic compression, is given by the First Law of Thermodynamics:w = h1 . h2 [Btu/lbm | kJ/kg] (8.1)

where the usual thermodynamic sign convention has been employed. The enthalpies h1 and h2

usually are related to the temperatures and pressures of the cycle through the use of charts ofrefrigerant thermodynamic properties such as those given in Appendix F.

In the ideal vapor compression cycle, the compressor suction state 1 is assumed to be a

saturated vapor. The state is determined when the evaporator temperature or pressure is given.For the ideal cycle, for which compression is isentropic, and for cycles for which the

compression is determined using a compressor efficiency, state 2 may be defined from state 1

and the condensing temperature or pressure by using the chart of refrigerant thermodynamicproperties.

Assuming no heat exchanger pressure losses, the evaporator and condenser heat transfers are

easily determined per unit mass of refrigerant by application of the First Law ofThermodynamics:

qL = h1 . h4 [Btu/lbm | kJ/kg] (8.2)qH = h3 . h2 [Btu/lbm | kJ/kg] (8.3)

The evaporator heat transferred, qL, is commonly referred to as the refrigeration effect, RE.The product of the refrigerant mass flow rate and RE, the rate of cooling produced by the unit, is

called the refrigeration capacity [Btu/hr | kW].

Applying the First Law to the refrigerant in the system as a whole, we find that the work andheat-transfer terms are related by

qL + qH = w [Btu/lbm | kJ/kg] (8.4)

where qH and w are negative for both refrigerators and heat pumps. HenceqL + |w| = |qH| [Btu/lbm | kJ/kg] (8.5)

Equation (8.5) is written here with absolute values to show that the sum of the compressor work

and the heat from the low-temperature source is the energy transferred by the condenser to thehigh-temperature region.


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This may be seen graphically by addition of the enthalpy increments representing Equations (8.1)

to (8.3) in the pressure-enthalpy diagram shown in Figure 8.8. The p.h diagram is applied oftenin refrigeration work because of its ease of use in dealing with enthalpy differences and constant-

pressure processes.


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For an ideal vapor compression refrigeration system operating with refrigerant 22 at an

evaporator temperature of 0F and condensing at 100F, find the following: the compressorsuction and discharge pressures, enthalpies, and specific volumes; the condenser discharge

pressure and enthalpy; the refrigeration COP; and the refrigerant mass flow rate and power

requirement for a 10-ton refrigeration unit.

Solution

Following the notation of Figures 8.2 and 8.8, from the chart (Appendix F) for refrigerant 22

at T1 = 0F, the other properties at state 1 arep1 = 38 psia, h1 = 104 Btu/lbm, v1 = 1.4 ft3/lbm,and s1 = 0.229 Btu/lbm-R.

The saturated-liquid condenser discharge properties at T3 = 100F arep3 = 210 psia and h3 =

39 Btu/lbm.

The compressor discharge-state properties at s2 = s1 andp2 =p3 = 210 psia are h2 = 123

Btu/lbm, T2 = 155F, and v2 = 0.31 ft3/lbm.

The evaporator inlet enthalpy is the same as that at condenser discharge, h4 = h3 = 39

Btu/lbm.

The refrigeration effect and the compressor work are then

RE = h1 . h4 = 104 . 39 = 65 Btu/lbm

w = h2 . h1 = 123 - 104 = 19 Btu/lbm

Thus

COPr = RE /w = 65/19 = 3.42.The rate of cooling, or cooling capacity, for a 10-ton unit is 10200 = 2000 Btu/min. The

refrigerant mass flow rate is the capacity divided by the refrigeration effect = 2000/65 = 30.8lbm/min.

The power required by the compressor is the product of the mass flow rate and thecompressor work = 30.819 = 585.2 Btu/min, or 585.260/ 3.413 = 10,290 W, or 10.29 kW.

The ideal EER may then be calculated from the capacity and power as 200060/10,290 =

11.7 Btu/Watt-hr, or from the COP as 3.4133.42 = 11.7 Btu/Watt-hr.


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Compressors

While most small- and medium-capacity refrigeration systems use hermetically sealed,

electricmotor- driven compressor units or open (externally powered) reciprocating compressors,

centrifugal compressors are frequently found in large units for cooling buildings and for

industrial applications.

The reciprocating compressor has much in common geometrically with a simple two-strokereciprocating engine with intake and exhaust valves. As in that case, the compressor clearance

volume Vc is the volume at top center, and the piston sweeps out the displacement volume, as

indicated in Figure 8.9. The processes 1-2-3-4-1 on the idealized pressure-volume diagramrepresent the following:

_ 1.2 Both valves are closed. Compression of the maximum cylinder volume V1 = Vc + Vd

of refrigerant vapor through the pressure ratiop2/p1 to a volume V2._ 2.3 Exhaust valve is open. Discharge of refrigerant through the exhaust valve at

condenser pressurep3 until only the clearance volume V3 = Vc remains when the piston isat top center.

_ 3.4 Both valves are closed. Expansion of the clearance gas with both valves closed fromV3 to V4. Note that the inlet valve cannot open until the cylinder pressure drops to p4 =p1

without discharging refrigerant back into the evaporator.

_ 4.1 Intake valve is open. Refrigerant is drawn from the evaporator into the cylinder at

constant pressurep1 through an intake valve by the motion of piston. Refrigerant in the

amount V1 . V4 is processed per cycle.

Assuming polytropic compression and expansion processes with the same exponent k:V4 = V3(p3/p4)1/ k = Vc(p2/p1)1/


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Then the volume of refrigerant vapor processed per cycle (or per shaft revolution)V1 . V4 = Vd + Vc . Vc (p2/p1)1/ k= Vd . Vc [(p2/p1)1/ k. 1] is less than the displacement volume anddepends on the compressor pressure ratio.

Neglecting the difference between the refrigerant density leaving the evaporator and that in thecompressor cylinder just before compression, we may write the compressor volumetric

efficiency as the ratio ofV1 . V4 to the displacement:

_v = (V1 . V4)/Vd = 1 . (Vc/Vd) [(p2/p1)1/ k. 1]Examination of the compressor processes for different pressure ratios, as in Figure 8.9 (p2 /p1,for example), shows that the refrigerant volume processed per cycle, and thus the volumetric

298 efficiency, decreases with increasing pressure ratio. It is also evident that the clearance

volume must be kept small to attain high volumetric efficiency. It is clear that, for a given

positive displacement compressor, the volumetric efficiency limits the usable pressure ratio and

thus the difference between the condensing and evaporating temperatures.

Suction and Subcooling Considerations

Lets examine two items of concern with respect to some vapor compression systems. In systems

with reciprocating compressors, there is a danger that, due to changing cooling loads, that theliquid refrigerant in the evaporator may not be completely vaporized, causing slugs of liquid to

299 enter the compressor.

Because liquids are essentially incompressible, positive-displacement compressors with fixed

clearance volumes can be damaged when such "slugging" occurs.

The use of a thermostatic expansion valve (TEV) that responds to change in the degree ofsuperheat in the suction line provides one solution to this problem. A bulb filled with refrigerant

attached to the suction line, when heated by superheated vapor, transmits an increasing pressure

signal to a diaphragm in the TEV, which adjusts the valve flow area and in turn changes the mass


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flow rate of refrigerant. This control is usually set to maintain a minimum of about five degrees

of superheat to avoid liquid slugs entering the compressor inlet.

A second concern is the possibility of entry of vapor into the throttling valve if the refrigerant

at the condenser exit is not completely condensed. Because vapor occupies much more spacethan liquid, the throttling valve will not function properly if vapor can enter from the condenser.

One approach to dealing with this is to locate a liquid receiver downstream of the condenser to

assure the availability of liquid to the expansion device.

Both of the above concerns may be dealt with simultaneously by the addition of a suction-line

heat exchanger that superheats the evaporator discharge about five degrees, ensuring that only

vapor enters the compressor. The heat exchanger that provides suction superheat from state 6 to

state 1 in Figure 8.12 may be set up to receive heat from the subcooling of the condenserdischarge from state 3 to state 4. This ensures the absence both of vapor entering the throttling

valve and of liquid slugs entering the compressor. Note that the subcooling also tends to increase

the refrigeration effect over that of the ideal cycle by decreasing the enthalpy entering the

Combining Heating and Cooling in a Single System

It is possible to combine both heating and cooling functions in a single system by providing heatexchangers that can operate as both evaporator and condenser and a control system that can

reroute the flow of refrigerant when switching functions is required. Figure 8-13 presents a

302

schematic diagram for such a system, commonly called a heat pump (context usually determines

whether the term .heat pump. refers to a device that heats only or that combines heating and

cooling functions). The key component in a commercial heat pump is a reversing valve. With thevalve shown in the figure, rotation through an angle of 90 reroutes the flow of refrigerant from

the indoor coil to the outdoor coil, and vice versa. As a result of this change, the indoor coil that

served as a condenser in the winter becomes an evaporator in the summer. The outdoor coil

changes accordingly. Separate throttling devices may be used to accommodate differing loadconditions in winter and summer. One-way check valves ensure that refrigerant flow is through

the appropriate throttling device during each season.

8.3 Absorption RefrigerationExample 8.3 shows that vapor compression refrigeration requires a significant supply of workfrom an electric motor or other source of mechanical power. Absorption refrigeration is an

alternate approach to cooling that is largely thermally driven and requires little external work.

This form of refrigeration is growing in importance as energy conservation considerations

demand.


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closer scrutiny of the disposition of heat rejection from thermal processes. Absorption

refrigeration provides a constructive means of utilizing waste heat or heat from inexpensivesources at a temperature of a few hundred degrees, as well as directly from fossil fuels. The

eventual abolition of the use of CFCs may also boost absorption refrigeration technology.

This system relies on the fact that certain refrigerant vapors may be dissolved in liquids called

absorbents. For instance, water vapor is a refrigerantthat tends to dissolve in liquid lithium

bromide, an absorbent. Just as when they condense, vapors release heat when they go intosolution. This heat must be removed from the system in order to maintain a constant temperature.

Thus, cooling causes vapor to be absorbed in absorbents, just as cooling causes vapor to

condense. On the other hand, heating tends to drive vapor out of solution just as it turns liquid tovapor. This solution phenomenon and the fact that pumping liquid requires a relatively small

amount of work compared with that required to compress a gas are the secrets of absorption

refrigeration.

Consider the schematic diagram in Figure 8.14, which shows a basic absorption refrigerationunit. The condenser / throttling valve / evaporator subsystem is essentially the same as in the

vapor compression system diagram of Figure 8.2. The major difference is the replacement of thecompressor with a different form of pressurization system. This system consists primarily of an

absorberat the pressure of the evaporator, a vapor generatorat the pressure of the condenser,

and a solution pump. A second throttling valve maintains the pressure difference between theabsorber and the generator.

The system operates as follows: Refrigerant vapor from the evaporator flows into the

absorber, where it mixes with the absorbent. The mixture is cooled by heat transfer QA to air orwater at the temperature of the environment, causing the vapor to go into solution. The

refrigerant-absorbent solution flows to the solution pump where it is pressurized to the pressure

level of the generator and condenser. Heat from an energy source, QG, then drives the vapor from

the cold liquid solution.


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BLOCK DIAGRAM


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WORKING PROCEDURE

Refrigeration cycle

In the refrigeration cycle, aheat pumptransfers heat from a lower-temperatureheat source into a

higher-temperatureheat sink. Heat would naturally flow in the opposite direction. This is the

most common type of air conditioning. Arefrigeratorworks in much the same way, as it pumpsthe heat out of the interior and into the room in which it stands.

This cycle takes advantage of the wayphase changeswork, wherelatent heatis released at a

constant temperature during aliquid/gasphase change, and where varying thepressureof a pure

substance also varies itscondensation/boiling point.

The most common refrigeration cycle uses anelectric motorto drive acompressor. In an

automobile, the compressor is driven by abeltover apulley, the belt being driven by the engine'scrankshaft(similar to the driving of the pulleys for thealternator,power steering, etc.). Whetherin a car or building, both use electric fan motors for air circulation. Since evaporationoccurs

when heat is absorbed, and condensation occurs when heat is released, air conditioners use a

compressor to causepressurechanges between two compartments, and actively condense and

pump arefrigerantaround. A refrigerant is pumped into theevaporatorcoil, located in thecompartment to be cooled, where the low pressure causes the refrigerant to evaporate into a

vapor, taking heat with it. At the opposite side of the cycle is thecondenser, which is located

outside of the cooled compartment, where the refrigerant vapor is compressed and forcedthrough another heat exchange coil, condensing the refrigerant into a liquid, thus rejecting the

heat previously absorbed from the cooled space.

By placing the condenser (where the heat is rejected) inside a compartment, and the evaporator

(which absorbs heat) in the ambient environment (such as outside), or merely running a normal

air conditioner's refrigerant in the opposite direction, the overall effect is the opposite, and thecompartment is heated. This is usually called aheat pump, and is capable of heating a home to

comfortable temperatures (25 C; 70 F), even when the outside air is below the freezing point of

water (0 C; 32 F).

Cylinder unloaders are a method of load control used mainly in commercial air conditioning

systems. On a semi-hermetic(or open) compressor, the heads can be fitted with unloaders which

remove a portion of the load from the compressor so that it can run better when full cooling is

not needed. Unloaders can be electrical or mechanical.

] Humidity

Air conditioning equipment usually reduces thehumidityof the air processed by the system. Therelatively cold (below thedew point) evaporator coil condenseswater vaporfrom the processed

air, much as a cold drink will condense water on the outside of a glass. The water is drained,

removing water vapor from the cooled space and thereby lowering itsrelative humidity.
http://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Heat_sinkhttp://en.wikipedia.org/wiki/Heat_sinkhttp://en.wikipedia.org/wiki/Heat_sinkhttp://en.wikipedia.org/wiki/Refrigeratorhttp://en.wikipedia.org/wiki/Refrigeratorhttp://en.wikipedia.org/wiki/Refrigeratorhttp://en.wikipedia.org/wiki/Phase_changehttp://en.wikipedia.org/wiki/Phase_changehttp://en.wikipedia.org/wiki/Phase_changehttp://en.wikipedia.org/wiki/Latent_heathttp://en.wikipedia.org/wiki/Latent_heathttp://en.wikipedia.org/wiki/Latent_heathttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Condensationhttp://en.wikipedia.org/wiki/Condensationhttp://en.wikipedia.org/wiki/Boiling_pointhttp://en.wikipedia.org/wiki/Boiling_pointhttp://en.wikipedia.org/wiki/Boiling_pointhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Belt_%28mechanical%29http://en.wikipedia.org/wiki/Belt_%28mechanical%29http://en.wikipedia.org/wiki/Belt_%28mechanical%29http://en.wikipedia.org/wiki/Pulleyhttp://en.wikipedia.org/wiki/Pulleyhttp://en.wikipedia.org/wiki/Pulleyhttp://en.wikipedia.org/wiki/Crankshafthttp://en.wikipedia.org/wiki/Crankshafthttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Power_steeringhttp://en.wikipedia.org/wiki/Power_steeringhttp://en.wikipedia.org/wiki/Power_steeringhttp://en.wikipedia.org/wiki/Evaporationhttp://en.wikipedia.org/wiki/Evaporationhttp://en.wikipedia.org/wiki/Evaporationhttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Refrigeranthttp://en.wikipedia.org/wiki/Refrigeranthttp://en.wikipedia.org/wiki/Refrigeranthttp://en.wikipedia.org/wiki/Evaporatorhttp://en.wikipedia.org/wiki/Evaporatorhttp://en.wikipedia.org/wiki/Evaporatorhttp://en.wikipedia.org/wiki/Heat_exchanger#HVAC_air_coilshttp://en.wikipedia.org/wiki/Heat_exchanger#HVAC_air_coilshttp://en.wikipedia.org/wiki/Heat_exchanger#HVAC_air_coilshttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Hermetic_sealhttp://en.wikipedia.org/wiki/Hermetic_sealhttp://en.wikipedia.org/wiki/Hermetic_sealhttp://en.wikipedia.org/wiki/Humidityhttp://en.wikipedia.org/wiki/Humidityhttp://en.wikipedia.org/wiki/Humidityhttp://en.wikipedia.org/wiki/Dew_pointhttp://en.wikipedia.org/wiki/Dew_pointhttp://en.wikipedia.org/wiki/Dew_pointhttp://en.wikipedia.org/wiki/Water_vaporhttp://en.wikipedia.org/wiki/Water_vaporhttp://en.wikipedia.org/wiki/Water_vaporhttp://en.wikipedia.org/wiki/Relative_humidityhttp://en.wikipedia.org/wiki/Relative_humidityhttp://en.wikipedia.org/wiki/Relative_humidityhttp://en.wikipedia.org/wiki/Relative_humidityhttp://en.wikipedia.org/wiki/Water_vaporhttp://en.wikipedia.org/wiki/Dew_pointhttp://en.wikipedia.org/wiki/Humidityhttp://en.wikipedia.org/wiki/Hermetic_sealhttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Heat_exchanger#HVAC_air_coilshttp://en.wikipedia.org/wiki/Evaporatorhttp://en.wikipedia.org/wiki/Refrigeranthttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Evaporationhttp://en.wikipedia.org/wiki/Power_steeringhttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Crankshafthttp://en.wikipedia.org/wiki/Pulleyhttp://en.wikipedia.org/wiki/Belt_%28mechanical%29http://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Boiling_pointhttp://en.wikipedia.org/wiki/Condensationhttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Latent_heathttp://en.wikipedia.org/wiki/Phase_changehttp://en.wikipedia.org/wiki/Refrigeratorhttp://en.wikipedia.org/wiki/Heat_sinkhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Heat_pump


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Some air conditioning units dry the air without cooling it. These work like a normal air

conditioner, except that aheat exchangeris placed between the intake and exhaust. Incombination withconvectionfans, they achieve a similar level of coolness as anair coolerin

humidtropical climates, but only consume about one-third the energy.
http://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Fan_%28mechanical%29http://en.wikipedia.org/wiki/Fan_%28mechanical%29http://en.wikipedia.org/wiki/Fan_%28mechanical%29http://en.wikipedia.org/wiki/Air_coolerhttp://en.wikipedia.org/wiki/Air_coolerhttp://en.wikipedia.org/wiki/Air_coolerhttp://en.wikipedia.org/wiki/Tropical_climatehttp://en.wikipedia.org/wiki/Tropical_climatehttp://en.wikipedia.org/wiki/Tropical_climatehttp://en.wikipedia.org/wiki/Tropical_climatehttp://en.wikipedia.org/wiki/Air_coolerhttp://en.wikipedia.org/wiki/Fan_%28mechanical%29http://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Heat_exchanger


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ADVANTAGES


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BIBLIOGRAPHY

REFERENCESAmerican Society Heating Refrigeration and Air Conditioning.ASHRAE Hand Book. 2001

Arora, C.P.Refrigeration and Air Conditioning. Second edition. Tata McGraw-Hill

Publishing Company Ltd. 2000.Bureau of Energy Efficiency, Ministry of Power, India.HVAC and Refrigeration Systems. In:

Energy Efficiency in Electrical Utilities, chapter 4. 2004Compare India. www.compareindia.com

Munters. Pre-Cooling of Gas TurbinesEvaporative Cooling. 2001.

www.munters.com/home.nsf/FS1?ReadForm&content=/products.nsf/ByKey/OHAA-

55GSWHNational Productivity Council, Ministry of Industries, India. Technology Menu on Energy

Efficiency.

Plant Services Magazine. www.plantservices.com

US Department of Energy, Energy Efficiency and Renewable Energy.

www.eere.energy.gov


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Refrigerator Air Conditioner