In this article we will discuss about simple vapour compression refrigeration system.
Introduction to Vapour Compression System:
For a closed cycle of refrigeration employing the condensable refrigerant vapour, the following processes are required:
(i) Compression of the vapour, thereby increasing pressure.
(ii) Condensing these vapours and rejecting heating to the cooling medium (usually water or atmospheric air).
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(iii) Expanding the condensed liquid refrigerant thereby lowering the pressure and corresponding saturation temperature.
(iv) Evaporating the liquid refrigerant thereby absorbing heat from the body or space to be cooled or refrigerated. It is due to this requirements of compression that the system is called Vapour Compression System and the cycle of operation is called Vapour Compression Cycle of Refrigeration.
This cycle, incorporating the compressor and condenser is shown in Fig. 36.19. Here the liquid at state D, the discharge of the condenser, is still at the same pressure of compressor discharge. If it were allowed to boil as it is, it will do so at the saturation temperature and will require heat to be supplied to it. Heat could possibly flow only from some temperature higher than this saturation temperature. But the requirement is to achieve a low temperature.
In order to produce the temperature less than the temperature of the body, say -10°C, the liquid at state D should be throttled down to the pressure which is the saturation pressure corresponding to -10°C. For this purpose an expansion device is provided at point between condenser and evaporator. The state of the liquid at A, is low pressure liquid having the saturation temperature of-10°C.
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Hence, the system once charged with Freon or Ammonia will continuously work, where the refrigerant will liquefy and evaporate alternatively, producing refrigeration effect at evaporator and heat rejection at condenser. Neither the cylinder of Freon nor any fresh supply of the same is required.
A simple vapour compression system of refrigeration consists of the equipment as follows:
(a) Compressor
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(b) Condenser
(c) Expansion valve
(d) Evaporator
The schematic diagram of the arrangement is shown in Fig. 36.20. The low temperature, low pressure vapour at state B is compressed by the compressor. This vapour is condensed into high pressure liquid at state D in the condenser and then passes through the expansion valve. Here the liquid is throttled down to a lower pressure and passes on to the evaporator, where it absorbs heat from the surroundings or the circulating fluid (being refrigerated) and vaporises to low pressure vapour of state B. The cycle then repeats.
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The exchange of energy is as follows:
(a) Compressor requires work W. The work energy is supplied by the system from the surroundings.
(b) During condensation heat Qv equivalent of latent heat of condensation etc. is lost. This heat energy flows out from the system to the surroundings.
(c) During evaporation heat equivalent of latent heat of vaporisation is absorbed by the refrigerant. This heat is again an inflow of energy into the system.
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(d) There is no exchange of energy during the throttling process through the expansion valve, as this process occurs at constant enthalpy.
Hence, by making an energy balance we get,
Figure 36.21 shows on T-S diagram for the equipment arrangement of Fig. 36.20. The cycle works between the temperatures T1 and T2 representing the condenser and evaporator temperature respectively.
The various processes of the cycle A-B-C-D are as given below:
(i) Process B-C:
Isentropic compression of the vapour from state is B to C. If the vapour state, B or superheated, (B”), the compression is called Dry Compression. If initial state is wet, (B’), the compression is called wet compression as represented by B’-C’.
(ii) Process C-D:
Heat is rejected by the refrigerant, in condenser, to the cooling medium (water and air), at constant pressure. It is carried out in two stages. The first is through C-C’ or C’-C” where the heat is rejected by desuperheating of the vapour and the second is through C’-D where latent heat is rejected at constant temperature (and constant pressure).
(iii) Process D-A:
An irreversible adiabatic expansion of liquid takes place through expansion valve. The pressure and temperature of the liquid are reduced. The process is accompanied by partial evaporation of some liquid. Due to its irreversible nature, the process is shown by a dotted line.
(iv) Process A-B:
Heat absorption by the refrigerant takes place in evaporator at constant pressure. The final state depends on the quantity of heat absorbed and the same may be wet (B’), dry and saturated (B), or superheated (B”) as shown in Fig. 36.21.
Analysis of Vapour Compression Cycle:
The purpose of a mechanical compression refrigeration machine is to produce the maximum amount of refrigerating effect for a given amount of work consumed. Its performance may, therefore, be estimated by calculating the efficiency of the machine which will be equal to the ratio of these two energy quantities. This efficiency in refrigeration is called a Coefficient of Performance (C.O.P.) to distinguish from the power generator efficiency.
COP is defined as the ratio of the desired effect (in refrigerating machine refrigerating effect is the desired effect) to the energy supplied to produce that desired effect.
Work of compression/kg is given by W = h2 – h1, and is supplied. (Fig. 36.22)
Similarly, the refrigerating effect is the heat absorbed by the refrigerant in the evaporator at constant temperature and pressure and is equal to the change in enthalpy during the process.
Pressure-Enthalpy (Total Heat) Chart:
The pressure-total heat, or pressure-enthalpy chart is probably the most convenient chart for refrigeration calculations; this is the chart recommended by the refrigeration sub-committee of IME. Such a chart giving common features is shown in Fig. 36.23. Horizontal lines represent constant pressure lines, vertical lines are constant enthalpy lines, while lines of constant temperature and of constant entropy are also plotted and are shown in Fig. 36.23.
For different refrigerants, different P-h charts are available:
(a) Constant pressure process is shown by the horizontal line.
(b) Constant enthalpy lines represent throttling or isenthalpic process.
(c) Constant entropy line represents isentropic or adiabatic (reversible) process.
(d) Constant temperature lines represent isothermal processes.
Simple Vapour Compression Cycle on P-h Chart:
A simple vapour compression cycle is shown by 1—2—3—4—1 on P-h chart of Fig. 36.24.
1- 2 Isentropic compression in compressor.
2- 3 Constant pressure cooling (Heat rejection).
3- 4 Isenthalpic expansion through expansion valve.
4- 1 Constant pressure heat absorption.
Actual Vapour Compression Refrigeration Cycle:
The theoretical vapour compression cycle. The cycle as applied in practice, however, differs considerably from the theoretical cycle.
An actual vapour compression cycle is shown in Fig. 36.25 on T-S diagram.
A few of the important points of deviations of this cycle as compared to the ideal theoretical cycle are explained below:
(i) Process 1-2-3:
This process represents the passage of refrigerant through the evaporator coil at Pevaporator and T., represents the entry of vapour to compressor in superheated condition. Superheating upto this point may be either due to larger removal of heat from the evaporator; (this is an advantage) or due to heat picked up by the vapour in suction piping which is a disadvantage.
(ii) Process 3-4-5-6-7-8:
This process represents the passage of vapour through the compressor, and shows several departures from the theoretical process.
(a) Both the suctions and discharge valves of the compressor are actuated by pressure difference. Hence the actual suction pressure ps in the cylinder is lower than the evaporator pressure (pevap). Similarly the actual discharge pressure (pd) will be higher than the condenser pressure (pcond). There is thus throttling effect in both the suction and discharge valves. Pressure 3-4 and 7-8 represent this throttling effect through Δps and Δpd respectively.
(b) As soon as the cold vapour enters the cylinder it gets heated by coming in contact with the hot cylinder walls. Hence the temperature of the vapour increases. This heating is represented by process 4—5. Similarly there is a cooling effect at the discharge which is represented by process 6-7.
(c) States 5 and 6, therefore, represent the initial and final condition of vapour during actual compression. The compression may neither be isentropic nor polytropic. It may be assumed that the heat absorbed by the gas during the first part of compression is equal to the heat rejected during the latter part. The actual compression process is as shown by process 5-6 in Fig. 36.25.
(iii) Process 8-9-10-11:
This process represents the passage of refrigerant through the condenser at pressure (pcond)- Process 8-9 is actually the removal of superheat, 9-10 is the removal of latent heat and 10-11 represents the sub-cooling of refrigerant liquid.
(iv) Process 11:
This process represents the throttling of sub-cooled liquid from state 11 so as to result in state 1 at the entry to the evaporator, after expansion.
The following example illustrates the method of considering most of the departures of actual cycle from the theoretical cycle discussed above.
Illustrative Example:
An ammonia compression machine is required to cater for a load of 100 tons. The cooling water temperature requires the condenser to work at 35°C and the brine temperature requires evaporator to work at -30°C.
The other data is given below:
(i) Temperature at entry to expansion valve 30°C.
(ii) The vapour is superheated 5.5°C in evaporator and further superheats by 14°C in suction line.
(iii) Pressure drop in suction and discharge valves is 0.02 bar and 0.03 bar respectively.
(iv) The vapour heats up by 10°C by picking up heat from cylinder walls.
(v) Compression index n = 1.2.
(vi) Discharge vapour cools down by 55°C in discharge line before entry to condenser.
(vii) Compressor is 2-cylinder single acting running at 560 rpm and volumetric efficiency is 0.57.
Ignore the losses e.g., pressure drop in condenser, evaporator and piping and heat transfer in liquid line etc.
Draw the cycle on a P-h diagram and determine:
(a) B.P. of the compressor if Mech. efficiency is 75%
(b) Bore and stroke of the cylinder if UD = 1.25
(c) Condenser water required for a temperature rise of 6°C.
Solution:
The various state points of the cycle shown in Fig. 36.26 may be obtained with the help of P-h chart as given below:
Effect of Under Cooling or Sub-Cooling on Vapour Compression System:
Undercooling or sub-cooling of the liquid takes place when the liquid is cooled below the saturation temperature corresponding to condenser pressure, before admitted to the throttle valve or expansion valve. Undercooling of the liquid is generally along the constant pressure line.
This is represented by line 3-3′ both on T-S and P-h diagrams of Fig. 36.27. Many times this undercooling is shown wrongly along the liquid line of T-S diagram (3-3″). Undercooling is brought about by circulating greater quantity of cooling water through the condenser.
Many times undercooling of liquid refrigerant coming out of the condenser is brought about by the vapours coming from the evaporator thus vapours are superheated to some extent. This arrangement is shown in Fig. 36.28.
The coefficient of performance COP may be increased by what is known as precooling or undercooling. It will be noticed from the diagrams of Fig. 36.27 that the net refrigerating effect has been increased by the area 4’—4—6—5’— 4′. The work done in compressed is not increased in that proportion and hence the coefficient of performance is increased by undercooling the liquid.
Effect of an Expansion Cylinder on Vapour Compression System:
If, instead of expanding the high-pressure liquid at 3 (coming from the condenser) through an expansion valve, it is expanded in an expansion cylinder or expander, thus driving the piston, work may be obtained which could be utilized in helping to drive the compression cylinder or compressor. By this method less external power would be required to drive or run the refrigerator.
The work saved by the expansion cylinder is represented by the area 3—3’—4′. The high pressure liquid at 3 is expanded isentropically to 4′ and the wet vapour is then forced through the brine tank or refrigerator. The net refrigerating effect is now increased by the area 4’—4—5—6—4′ and the work required to drive the compression is less and hence the coefficient of performance is increased to a large extent.
The expansion cylinder is not used in practice, as the work saved is insufficient to overcome the friction of the necessary mechanism.
The Effect of Superheating on Vapour Compression System:
If the vapour is compressed after it has become dry and saturated then the vapour becomes superheated. This is shown on T-S diagram of Fig. 36.30. The effect of this superheating of vapour on the performance of the cycle COP can be studied from this diagram.
The vapour is drawn in the compressor cylinder at the condition of 1′ and is compressed to 2′, the temperature being Tsup. Remaining cycle processes remain unchanged as shown.
Referring to Fig. 36.30, Additional work done due to superheating is say W1 and given by-
Coefficient of Performance of Vapour Compression System:
Coefficient of performance when T1, T2, specific heat of liquid and latent heat at higher temperature given:
Let Fig. 36.31 represents the T-S diagram of the cycle and in this case let the vapour have a dryness fraction x at the end of compression.
From the diagram of Fig. 36.31, we have-
Approximate Solution for Coefficient of Performance:
An approximate solution for the coefficient of performance may be obtained by assuming the liquid line (3-5) to be a straight line. (Fig. 36.30). This means that the area 3-5-6-3 is assumed to be a triangle.
Then triangle 3-5-6-3
By using this value of (4-6), we can calculate work done W and refrigerating effect N by using the value of (4-6).
Note here that for CO2 as a refrigerant liquid line 3-5 cannot be regarded as a straight line.