REFRIGERATION & AIR CONDITIONING

REFRIGERATION & AIR CONDITIONING

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REFRIGERATION

& AIR

CONDITIONING

VCR CYCLE

2020


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Vapour compression cycle is an improved type of air refrigeration cycle in which a suitable

working substance, termed as refrigerant, is used. The refrigerants generally used for this

purpose are ammonia (NH3), carbon dioxide (CO2) and sulphur-dioxide (SO2). The

refrigerant used, does not leave the system, but is circulated throughout the system alternately

condensing and evaporating. In evaporating, the refrigerant absorbs its latent heat from the

solution which is used for circulating it around the cold chamber and in condensing; it gives

out its latent heat to the circulating water of the cooler.

The vapour compression cycle which is used in vapour compression refrigeration system is now-a-

days used for all purpose refrigeration. It is used for all industrial purposes from a small domestic

refrigerator to a big air conditioning plant.

Simple Vapour Compression Refrigeration System It consists of the following essential parts:

Compressor

The low pressure and temperature vapour refrigerant from evaporator is drawn into the compressor

through the inlet or suction valve A, where it is compressed to a high pressure and temperature. This

high pressure and temperature vapour refrigerant is discharged into the condenser through the delivery

or discharge valve B.

Condenser

The condenser or cooler consists of coils of pipe in which the high pressure and temperature

vapour refrigerant is cooled and condensed.

The refrigerant, while passing through the condenser, gives up its latent heat to the surrounding

condensing medium which is normally air or water.

Receiver

The condensed liquid refrigerant from the condenser is stored in a vessel known as receiver from

where it is supplied to the evaporator through the expansion valve or refrigerant control valve.

Expansion Valve


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It is also called throttle valve or refrigerant control valve. The function of the expansion valve is to

allow the liquid refrigerant under high pressure and temperature to pass at a controlled rate after

reducing its pressure and temperature. Some of the liquid refrigerant evaporates as it passes through

the expansion valve, but the greater portion is vaporized in the evaporator at the low pressure and

temperature

Evaporator

An evaporator consists of coils of pipe in which the liquid-vapour. refrigerant at low pressure

and temperature is evaporated and changed into vapour refrigerant at low pressure and

temperature. In evaporating, the liquid vapour refrigerant absorbs its latent heat of

vaporization from the medium (air, water or brine) which is to be cooled.

Theoretical Vapour Compression Cycle with Dry Saturated Vapour after Compression

A vapour compression cycle with dry saturated vapour after compression is shown on T-s

diagrams in Figures (a) and (b) respectively. At point 1, let T1, p1 and s1 be the temperature,

pressure and entropy of the vapour refrigerant respectively. The four processes of the cycle

are as follows :

Fig.-a Fig.-b

Compression Process

The vapour refrigerant at low pressure p1 and temperatureT1 is compressed isentropically to

dry saturated vapour as shown by the vertical line 1-2 on the T-s diagram and by the curve 1-

2 on p-h diagram. The pressure and temperature rise from p1 to p2 and T1 to T2 respectively.

The work done during isentropic compression per kg of refrigerant is given by

w = h2 – h1

where h1 = Enthalpy of vapour refrigerant at temperature T1, i.e. at suction of the

compressor, and

h2 = Enthalpy of the vapour refrigerant at temperature T2. i.e. at discharge of the compressor.

Condensing Process

The high pressure and temperature vapour refrigerant from the compressor is passed through

the condenser where it is completely condensed at constant pressure p2 and temperature T2 as

shown by the horizontal line 2-3 on T-s and p-h diagrams. The vapour refrigerant is changed

into liquid refrigerant. The refrigerant, while passing through the condenser, gives its latent

heat to the surrounding condensing medium.

Expansion Process

The liquid refrigerant at pressure p3 = p2 and temperature T3 = T2, is expanded by throttling

process through the expansion valve to a low pressure p4 = p1 and Temperature T4 = T1 as

shown by the curve 3-4 on T-s diagram and by the vertical line 3-4 on p-h diagram. Some of

the liquid refrigerant evaporates as it passes through the expansion valve, but the greater

portion is vaporized in the evaporator. We know that during the throttling process, no heat is

absorbed or rejected by the liquid refrigerant.


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Vaporizing Process

The liquid-vapour mixture of the refrigerant at pressure p4 = p1 and temperature T4 = T1 is

evaporated and changed into vapour refrigerant at constant pressure and temperature, as

shown by the horizontal line 4-1 on T-s and p-h diagrams. During evaporation, the liquid-

vapour refrigerant absorbs its latent heat of vaporization from the medium (air, water or

brine) which, is to be cooled, This heat which is absorbed by the refrigerant is called

refrigerating effect and it is briefly written as RE. The process of vaporization continues up to

point 1 which is the starting point and thus the cycle is completed.

We know that the refrigerating effect or the heat absorbed or extracted by the liquid-vapour

refrigerant during evaporation per kg of refrigerant is given by

RE = h1 – h4 = h1 – hf3

where hf3 = Sensible heat at temperature T3, i.e. enthalpy of liquid refrigerant leaving the

condenser.

It may be noticed from the cycle that the liquid-vapour refrigerant has extracted heat during

evaporation and the work will be done by the compressor for isentropic compression of the

high pressure and temperature vapour refrigerant.

Coefficient of performance, C.O.P. = (Refrigerating effect)/( Work done)

12

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12

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hh

hfh

hh

hh

−

−=

−

−=

Effect of Suction Pressure

The suction pressure (or evaporator pressure) decreases due to the frictional resistance of

flow of the refrigerant. Let us consider a theoretical vapour compression cycle 1-2-3-4 when

the suction pressure decreases from ps to ps as shown on p-h diagram in Figure.

It may be noted that the decrease in suction pressure :

(a) decreases the refrigerating effect from (h1 – h4) to (h11-h4

1) and

(b) Increases the work required for compression from (h2 – h1) to

(h21-h1

1)

Since the C.O.P, of the system is the ratio of refrigerating effect to

the work done, therefore with the decrease in suction pressure, the

net effect is to decrease the C.O.P. of the refrigerating system for

the same refrigerant flow. Hence with the decrease in suction

pressure the refrigerating capacity of the system decreases and the

refrigeration cost increases.

Effect of Discharge Pressure:-

In actual practice, the discharge pressure (or condenser pressure)

increases due to frictional resistance of flow of the refrigerant. Let

us consider a theoretical vapour compression cycle l-2-3-4 when the

discharge pressure increases from pD to pD‟ as shown on p-h

diagram in Figure.resulting in increased compressor work and

reduced refrigeration effect.

Effect of Liquid Subcooling:-


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It is possible to reduce the temperature of the liquid refrigerant to within a few degrees of the

temperature of the water entering the condenser. In some condenser designs it is achieved by

installing a sub-cooler between the condenser and the expansion valve.

The effect of sub-cooling of the liquid from t3 = tk to t3/ is shown in Figure It will be seen that

sub-cooling reduces flashing of the liquid during expansion and increases the refrigerating

effect. Consequently, the piston displacement and horsepower per ton are reduced for all

refrigerants. The percent gain is less pronounced in the case of ammonia because of its larger

latent heat of vaporization as compared to liquid specific heat.

Normally, cooling water first passes through the subcooler and then through the condenser.

Thus, the coolest water comes in contact with the liquid being subcooled. But this results in a

warmer water entering the condenser and hence a higher condensing temperature and

pressure. Thus, the advantage of subcooling is offset by the increased work of compression.

This can be avoided by installing parallel cooling water inlets to the subcooler and condenser.

In that case, however, the degree of subcooling will be small and the added cost of the

subcooler and pump work may not be worthwhile. It may be more desirable to use the

cooling water effectively in the condenser itself to keep the condensing temperature as near to

the temperature of the cooling water inlet as possible

Effect of super heating:-

Superheating of the suction vapour is advisable in practice

because it ensures complete vaporization of the liquid in the

evaporator before it enters the compressor. Also, in most

refrigeration and air-conditioning systems, the degree of

superheat serves as a means of actuating and modulating the

capacity of the expansion valve.

It can be seen from Figure, that the effect of superheating of

the vapour from t1= t0 to t1/ is as follows :

(a) Increase in specific volume of suction vapour from v1 to v1/

(b) Increase in refrigerating effect from (h1 – h4) to (h1/ – h4)

(c) Increase in specific work from (h2 – h1) to (h2/ – h1/)

It is to be noted that (h2/ – h1/) is greater than (h2 – h1). This is because, although the pressure ratio is

the same for both lines, the initial temperature t1,‟ is greater than t1 and the work given by the

expression:-

𝑊 =𝛾𝑅𝑇1𝛾 − 1

[(𝑃2𝑃1)

𝛾−1𝛾− 1]

Consideration of critical factors

Ambient temperature

Compressor selection

Air flow over compressor shell

Condenser design

Air flow through condenser

Evaporator design

Air flow through evaporator


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Selection of refrigerant control device

Refrigerant used

Heat exchangers

Refrigerant piping’s.

Selection of Compressor

The compressors are normally classified as under:

a) High Back Pressure units Rating at +7.2°C Evaporator temperature [HBP]

b) Medium Back Pressure units Rating at -6.7°C Evaporator temperature [MBP]

c) Low Back Pressure units Rating at -23.3°C Evaporator temperature [LBP]

It is important to ensure that the compressor is selected on the basis of the operating back

pressure, which in turn directly related to the cooling coil operating temperatures. High back

pressure compressors should never be used on the low temperature applications, since the cooling

of the motor windings depend both on gas temperature and amount of refrigerant circulated

through the compressor. The high back pressure compressors would have inadequate refrigerant

flow over the motor winding to bring about the desired cooling. Low back pressure applications

may also require high torque motor to cope with the higher differential pressures.

Selection of Condenser

Condensers are basically heat exchangers in which the refrigerant undergoes a phase change. In

refrigeration system condenser is used in high pressure side. Its major function is to remove heat

of hot vapour refrigerant which is discharged from the compressor. In condensers the refrigerant

vapour condenses by rejecting heat to an external fluid, which acts as a heat sink [23 &48]. The

condenser should be designed to dissipate the sum of the heat absorbed by the evaporator and the

work of compression and also to provide adequate sub cooling to the liquid refrigerant in order to

improve the cycle efficiency. The design of condenser should be a compromise between economy

and safe operating pressures.

The following important factors affecting the condenser capacity:

(i) Type of material

(ii) Surface area, and

(iii) Temperature difference

According to condensing medium and application the condensers can be classified as:

(i) Air cooled condenser

(ii) Water cooled condenser, and

(iii) Evaporative condenser.

Expansion Device

All vapour compression refrigeration system uses expansion device (throttling device) like,

Automatic expansion Valve, Thermostatic expansion valve and Capillary tube. Capillary tube

is the most popular refrigerant control device used in small refrigerating system. There are

several formulas for calculating capillary tube bore and lengths, but the finer adjustments are

made by trying on the system. Capillary tube (diameter and length) influences the refrigerant

flow and characteristics. The tube diameter range from 0.5 mm to 2.25 mm and the length

ranges from 0.5m to 6 m. It is installed in the liquid line before between the condenser and

the evaporator. A fine mesh screen is provided at the inlet of the tube in order to protect it

from contaminants. A small filter drier is used on some systems to provide additional freeze

up application.

The following important advantages of capillary tube over other expansion deviser are:

1) The cost of capillary tube is less than all other forms of expansion valves.

2) When the compressor stops, the refrigerant continues to flow into the evaporator and

equalizes the pressure between the high side and low sides of the system. This considerably


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decreases the starting load on the condenser. Thus a low starting torque motor can be used to

drive the compressor.

3) Since the refrigerant charge in a capillary tube system is critical, therefore no receiver is

necessary. Problems from the blockage in the capillary tube results in lower injection of

refrigerant into the evaporator, so there will be less cooling. Typically this problem can be

solved by changing a new capillary tube.

Prb-1 Carnot refrigeration cycle absorbs heat at 270 K and rejects heat at 300 K.

(a) Calculate the coefficient of performance of this refrigeration cycle.

(b) If the cycle is absorbing 1130 kJ/min at 270 K, how many kJ of work is required per

second.

(c) If the Carnot heat pump operates between the same temperatures as the above

refrigeration cycle, what is the coefficient of performance.

(d) How many kJ/min will the heat pump deliver at 300 K if it absorbs 1130 kJ/min at 270 K.


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Prb2:- The capacity of a refrigerator is 200 TR when working between – 6°C and 25oC.

Determine the mass of ice produced per day from water at 25°C. Also find the power

required to drive the unit. Assume that the cycle operates on reversed Carnot cycle and latent

heat of ice is 335 kJ/kg.


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EXERSISE

NUMERICALS:-

1.A refrigerator works between -7oC and 27oC. The vapour is dry at the end of adiabatic

compression. There is no sub cooling and expansion is by throttle valve. Find i) condition of

refrigerant before entering the compressor, ii) refrigerating effect per kg of refrigerant, and

iii) the COP. The properties of the refrigerant are given below –

Temp in 0C Enthalpy(KJ/Kg) Entropy(KJ/Kg-K)

hf hfg Sf Sg

-7 -30 1298 -0.108 4.75

27 115 1173 427 4.33

2.In an ammonia refrigerator, the temperature range in the compressor is from 25oC to -15oC.

The vapour is dry saturated at the end of compression. There is no sub cooling and an

expansion valve used. Assume actual COP is 60% of the theoretical COP, find the actual

COP of the refrigerator. The properties of the refrigerant are given below –


hf hg Sf Sg

25 100.04 1319.92 0.35 4.49

-15 -54.56 1304.99 -2.13 5.05

3.A refrigerator works between -12oC and 22oC. The refrigerant is just dry at the end of

adiabatic compression and there is no sub cooling of the liquid refrigerant before the

expansion valve and expansion is by throttle valve. Find i) the COP of the cycle, and ii)

capacity of the refrigerator if the fluid flows at a rate of 5 kg per minute. The properties of

the refrigerant are given below –


hf hg Sf Sg

-12 151.96 293.29 0.554 1.0322

22 56.32 322.58 0.226 1.2465

THEORY:-

1. (a) With the help of a schematic labelled diagram of ideal Vapour Compression

refrigeration

system.

(b) Give the advantages of Vapour Compression system over air refrigeration system.

2. (a) List the main components required for Vapour Compression refrigeration system.

(b) Show the various processes of ideal Vapour Compression refrigeration system on T-S

and

P-H planes, also write the expression of COP of this system.

3. What are the effects of sub cooling and superheating of the refrigerant on the performance

of Vapour Compression refrigeration system ?

4. What are the effects of change in suction pressure and discharge pressure of refrigerant on


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the performance of Vapour Compression refrigeration system ?

MCQ’S QUESTIONS:-

1. In a vapour compression refrigeration cycle, heat is rejected by the refrigerant in the

(a) Compressor, (b) condenser, (c) expansion valve, (iv) evaporator. [Answer – b. ]

2. In a vapour compression refrigeration cycle, heat is absorbed by the refrigerant from

the

(a) compressor, (b) condenser,(c) expansion valve, (d)evaporator. [Answer – d. ]

3. In a vapour compression refrigeration cycle, condition of refrigerant before entering the throttle valve is

(a) saturated liquid, (b) wet vapour, (c) dry vapour, (d) superheated vapour. [Answer – a. ] 4. C.O.P. of a vapour compression refrigeration system is

(a) less than air refrigeration system,

(b) equal to air refrigeration system,

(c) more than air refrigeration system,

(d) May be more or less than air refrigeration system

[Answer – c. ]

5. Running cost of a vapour compression refrigeration system with respect to air refrigeration system is

(a) same,

(b) less,

(c) more,

(d) May be more or less.

[Answer – b. ]

6. In a vapour compression refrigeration cycle, lowest temperature occurs after .........

process.

(a) Compression,

(b) Condensation,

(c) Throttling,

(d) Evaporation.


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[Answer – c. ]

7. Under cooling or sub cooling of refrigerant in a vapour compression refrigeration

cycle

(a) decreases C.O.P,

(b) increases C.O.P,

(c) has no effect on C.O.P,

(d) May be increases or decreases.

Answer – b. ]

8. Superheating of refrigerant in a vapour compression refrigeration cycle

(a) decreases C.O.P,

(b) increases C.O.P,

(c) has no effect on C.O.P,

(d) May be increases or decreases. [Answer – a. ]

9. If the suction pressure is increased, C.O.P. of a vapour compression refrigeration

cycle will

(a) Increase,

(b) Decrease,

(c) may increase or decrease,

(d) none of the above.

[Answer – a. ]

10. If the discharge pressure is increased, C.O.P. of a vapour compression refrigeration

cycle will

(a) increase,

(b) decrease,

(c) may increase or decrease,

(d) none of the above. [Answer– b. ]

11. Now-a days In vapour compression refrigeration system uses ............... as refrigerant.

(a) Air, (b) ammonia, (c) water, (d) R134a. [Answer – d. ]

12. In vapour compression refrigeration system, capillary tube is located in between


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(a) Compressor and condenser, (b) Compressor and expansion valve, (c) Condenser and evaporator, (d) evaporator and Compressor.

[Answer – c. ]

13. In vapour compression refrigeration system, expansion of refrigerant occurs when it passes through ..................

(a) Compressor, (b) expansion valve, (c) Condenser, (d) evaporator. [Answer – b. ]

14. In an actual vapour compression refrigeration cycle, the sub cooling of refrigerant

occurs

(a) Before compression, (b) after compression, (c) before throttling, (d) after throttling. [Answer – c. ]

15. Ideal vapour compression refrigeration cycle works between ............. only.

(a) Two pressure limit, (b) Two volume limit, (c) Two enthalpy limit, (d) Two work limit. [Answer – a. ]

16. In vapour compression refrigeration cycle, enthalpy of refrigerant remain constant while passing through .......................

(a) Compressor, (b) throttle valve, (c) Condenser, (d) evaporator. [Answer – b. ]

17. If the refrigerant after condensation process is getting cooled below its saturation temperature before throttling, then it is known as ................... of refrigerant.

(a) Normal cooling, (b) sub cooling, (c) super cooling, (d) none of these.

[Answer – b. ]

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