Experiment # 3

BASIC VAPOR COMPRESSION REFRIGERATION CYCLE (With Heat Exchanger )1.0OBJECTIVE Study the effect of variation in refrigeration load in a vapor compression refrigeration cycle system to :a) COP

b) Evaporating temperature

c) Condensing temperature

d) Refrigerant mass flow rate

e) The pressure ratio

f) The AU value of the condenser

g) Volumetric efficiency of compressor

h) Mechanical efficiency of compressor

2.0EQUIPMENTHilton Refrigeration Laboratory Unit.2.1Refrigerant:

R- 12

2.2Components:

1. Reciprocating Compressor:

No. of Cylinders = 2, Bore = 40 mm, Stroke = 30 mm, N = 460 RPM Motor Speed = 1420 RPM, Dynamometer arm length = 150 mm2. Evaporator: Tube type 3. Condenser: Tube type.

4. Expansion Valve: Thermostatic expansion valve (TXV) 5. Heat Exchanger : Liquid to vapor

Figure 1 : AXV and TXV

Automatic Expansion Valve (AXV): Capillary tube system is insensitive to load changing condition. When load increased, the suction pressure went up causing inefficient cooling and strain on the compressor. Therefore this system can be used only when the load is approximately constant. For varying load condition, a device sensible to load changes must be used. Automatic expansive valve is one of these devices. AXV keeps the pressure in the evaporator a constant and thus load on the compressor remain same. When the load on the system increases, evaporation in the evaporator also increases and hence suction pressure rises. In order to keep the suction pressure constant, the automatic expansion valve restricts the refrigerant flow therefore slowing down the evaporation and bringing the suction pressure back to its original state.

Thermostatic expansion valve (TXV): The most popular type of expansion device for moderate sized refrigeration systems is the thermostatic expansion valve. Its forces a constant superheat in the evaporator. When there is an increase of load the refrigerant will evaporate faster in the evaporator. This in turn will cause a greater superheat at the evaporator outlet. This will cause the TXV to let more refrigerant pass and drive the superheat down. The net effect is then an increase in refrigeration when there is an increase in load. A thermostatic expansion valve is shown in Fig. 1. The sensing bulb contains fluid (normally it is the same refrigerant that is used for the refrigeration system) either in a vapor or a liquid form. This bulb is usually placed after the evaporation coil and near the suction line of the compressor. When the temperature of the refrigerant rises, it causes temperature of the fluid within the sensing bulb also to rise and forces the push rod downward which in turn moves the needle or the ball from its seat allowing liquid refrigerant to flow through and admit liquid to the evaporator coil. Because of this fresh charge the temperature of the refrigerant drops which is again sensed by the bulb and causing the pressure within it to drop. This reduces pressure against the diaphragm and

the spring action from the bottom shuts the refrigerant passage. This opening and closing of the valve provides automatic regulation of refrigerant flow into the evaporator coil.3.0THEORYRefrigeration works on the principle of heat absorption due to the evaporation of refrigerant. The refrigerant is evaporated by passing from a region of high pressure to a region of low pressure, thus reducing its saturation temperature below its actual temperature. The degree of cooling can then be controlled by controlling the amount of refrigerant passing into the low pressure region. The mechanism that controls the refrigerant flow into the low pressure region is called a metering device.

The heart of the vapor compression system is the compressor. The three most common types of refrigeration compressors are the reciprocating, rotary and centrifugal. The reciprocating compressor consists of a piston moving back and forth in a cylinder with suction and discharge valves arranged to allow pumping to take place. The rotary and centrifugal compressors both have rotating members, but the rotary compressor is a positive-displacement machine, whereas the centrifugal compressor operates by centrifugal force.

Some actual refrigeration systems utilize a liquid-to-suction heat exchanger. This heat exchanger sub-cools the liquid from condenser with suction vapor coming from the evaporator. The arrangement and the corresponding ph diagram are shown in the figures on last page of the handout.

3.1Effect of Superheating1. The compression work for the superheated cycle is more than that for the saturated cycle.

2. The temperature of the discharged vapors (exit of the compressor) is considerably higher for the superheated cycle.

3. Because of the higher outlet temperature of the compressed vapors, greater quantity of heat must be rejected in the condenser. It means load on the condenser increases.

4. Superheating of the vapors at the inlet to the compressor ensures no liquid enters in the compressor.

3.2Effect of sub-cooling1. Sub-cools liquid enters in the evaporator and hence increases the refrigeration effect of the cycle.

Compared with the standard vapor compression cycle, the system using the heat exchanger may seen to have obvious advantages because of the increases refrigerating effect. But on the other hand, compressor power also increases because of the superheated vapors has to be compressed. Therefore, the COP of the cycle, which is ratio of the refrigeration effect to the compressor power of the cycle, not necessarily increases.

The heat exchanger is definitely justified, however in situations where the vapor entering the compressor must be superheated to ensure that no liquid enters the compressor.

Another practical reason for using the heat exchanger is to sub-cool the liquid form the condenser to prevent bubbles of vapor from impending the flow of refrigerant through the expansion valve.

Figure 2

Temperature

Figure 3 : Example of T-s Diagram for Steam4.0 EXPERIMENTAL RESULTSTable 1 : Experimental dataAtmospheric pressure = 1.017 bar, Ambient air dry bulb temperature = 27.5 C

UnitRun #1Run #2Run #3Run #4Run #5

T1C12369

T2C6266686969

T4C2727282930

T5C-23-18-12-7-1

mrg/s1.2234.15.5

peMPa0.030.0750.1000.1400.195

pcMPa0.880.940.981.061.14

Heater VoltageV100120140160180

Heater CurrentA2.63.23.64.14.6

Spring Force, FN6.1789.210.5

mwg/s2828282828

T7C2828282828

T8C3132333537

Evaporator pressure, pe and condenser pressure, pc are gauge pressures.

5.0 FINDINGS AND DISCUSSION

Table 2 : Experimental resultsUnitRun #1Run #2Run #3Run #4Run #5

Refrigeration LoadAV260384504656828

COP3.253.714.124.374.63

Evaporating temperature, T6C-23.53-15.93-12.38-7.30-1.25

Condensing temperature,T3C5962636269

Refrigerant mass flow rate, rg/s1.2234.15.5

Pressure ratio, rp29.3312.539.87.575.85

AU value of condenserKw0.1880.2950.4300.5720.746

Volumetric efficiency of compressor, v%28.5734.2945.7151.4357.14

Mechanical efficiency of compressor, m%30.8846.7962.9272.2080.77

Figure 4 : Graph of Refrigeration Load versus COP

Figure 5 : Graph of Refrigeration Load versus Pressure Ratio

Figure 6 : Graph of Refrigeration Load versus AU Value of Condenser

Figure 7 : Graph of Refrigeration Load versus Volumetric Efficiency of Compressor

Figure 8 : Graph of Refrigeration Load versus Mechanical Efficiency of CompressorBased on graph of Refrigeration Load versus COP, the results show that the COP value of the refrigeration unit is increasing when the refrigeration load is increase. It means that, the performance of the refrigeration system depends on the desired refrigeration load. This is because of the increasing refrigeration load will increased the work done of the compressor.Based on graph of Refrigeration Load versus Pressure Ratio, the experimental results show that the Pressure Ratio decreased when the refrigeration load is increase. Therefore, the effects of refrigeration load some how very significance to pressure ratio of the refrigeration system.

Based on graph of Refrigeration Load versus AU values of condenser, it shows that the refrigeration load is proportional to the AU values of condenser. The results show that when we increase the refrigeration load, the AU values of condenser also increase proportionally. This results also same with the volumetric and mechanical efficiency of the compressor. It means that changing the refrigeration load will give a significance effect to the AU value of condenser, volumetric and mechanical efficiency of the compressor. 5.1Sample Calculation

Run #1:

Figure 9 : Temperature-entropy diagram of the system.

Figure 10 : Pressure-enthalpy diagram of the system. Given data:

pe = 0.3 + 1.017 = 1.317 bar

pc = 8.8 + 1.017 = 9.817 bar

T5 = -23C, T7 = 28C, T8 = 31C, r = 1.2 g/s

To find T3: T = T8 - T7 = 3CGiven data: T2 = 62C

therefore, TH = T3 = 62 3 = 59CTo find T6 and enthalpies at significance states using properties table for R 12at pe = 1.317 bar , T1 = 1C (superheated)by interpolation, => Ts = T6 = -23.53C

therefore, T - Ts = 1 (-23.53) = 24.53 Kat 15K, Ts = -23.53C and Pe = 1.317 bar, by interpolation => h = 186.04 KJ/Kgat 30K, Ts = -23.53C and Pe = 1.317 bar, by interpolation => h =195.08 KJ/Kgby interpolation, => h1 = 191.78 KJ/Kgat 15K, Ts = -23.53C and Pe = 1.317 bar, by interpolation => s = 0.7461 KJ/Kg.K

at 30K, Ts = -23.53C and Pe = 1.317 bar, by interpolation => s = 0.7793 KJ/Kg.K

by interpolation, => s1 = 0.7672 KJ/Kg.K

s2 = s1 = 0.7373 KJ/Kg.K

Given data: pc = 9.817 bar , T2 = 62Cby interpolation => Ts = 40.85Ctherefore, T - Ts = 62 40.85 = 21.15Kat Ts = 40C, s2 = 0.7672 KJ/Kg.K (superheated), by interpolation => h = 231.13 KJ/Kgat Ts = Ts = 40C, s2 = 0.7672 KJ/Kg.K (superheated), by interpolation => h = 233.66 KJ/Kgby interpolation => h2 = 232.17 KJ/Kgh4 hf at T = T4 = 27C and pe = 1.317 bar

by interpolation, => h4 = 61.66 KJ/Kgh5 = h4 = 61.66 KJ/Kgat Pc = 9.817 bar => by interpolation, h3 =75.462 KJ/KgTo find the rate of heat removal from the refrigerated space, QLQL = r (h1 h5)

= (1.2/1000)(191.78 61.66) = 0.156 kwTo find the power input to the compressor, Win

in = r (h2 h1)

= (1.2/1000)(232.17 191.78)

= 0.048 kwTo find the coefficient of performance, COP

COP = (QL/in)

= 3.25To find refrigeration load

Refrigeration Load = A X V

= 2.6 X 100

= 260 AV

To find pressure ratio, rp

Pressure ratio, rp = (Pc/ Pe)

= 0.88/0.03

= 29.33

To find AU value of condenser

AU value of condenser = r (h2 - h3)

= (1.2/1000)(232.17 75.462)

= 0.188 kwTo find volumetric efficiency of the compressor, vGiven data: Compressor is reciprocating-type, single-acting twin cylinders. For each cylinders:Bore = 40 mm, Stroke = 30 mm, N = 460 rpm

Volumetric efficiency, v = (actual volume, Va / theoritical displacement of compressor, Vp)Theoritical displacement of compressor, Vp = A x S x n x N = {( x 0.04) / 4} x 0.03 x 2 x 460

= 0.035 m/minFrom Properties Table for R 12 (Superheated):At inlet compressor; T1 = 1C, Pe = 131.7 Kpa

At T = 0 C, by interpolation => = 0.1379 m/Kg

At T = 5 C, by interpolation => = 0.1408 m/Kg

Therefore, at T = 1C, by interpolation => 1 = 0.1385 m/Kg

Actual volume, Va = r 1 = {1.2 x 60)/100} Kg/min x 0.1385 m/Kg

= 0.010 m/minTherefore, volumetric efficiency, v = 28.57 %To find mechanical efficiency of the compressor, m

Given data:

Motor speed = 1420 rpm, Dynamometer arm length, r = 150 mm

No. of cylinder of compressor, n = 2, Spring force, F = 6.1 N

Mechanical efficiency, m = indicated power, Pi

shaft power, PTorque, = F x r = 6.1 N x 0.15 m

= 0.915 Nmshaft power, P = = (2N / 60) = {(2 x 1420) / 60} x 0.915

= 0.136 kwIndicated power, Pi = n r R (T2 T1) = 2 x 0.0012 x 0.287 x (335 274) n - 1

= 0.042 kwTherefore, mechanical efficiency, m = 30.88 %6.0CONCLUSIONThis experiment studied the effect of variation in refrigeration load to some significance parameters in a vapor compression refrigeration cycle system. Experiment data were collected by using the refrigeration device which is Hilton Refrigeration Laboratory Unit. As mentioned earlier, this experiment will focus on the effect of variation in refrigeration load to COP, evaporating temperature, condensing temperature, refrigerant mass flow rate, pressure ratio, AU value of condenser, volumetric efficiency and mechanical efficiency of the compressor.

The following conclusion can be drawn from the experiment. The results of this experiment has shown that refrigerant is sub cooled before its enter the throttling valve, since the refrigerant in this case enter the evaporator with a lower enthalpy and thus can absorb more heat from the refrigerated space. The throttling valve and the evaporator are usually located very close to each other, so the pressure drop in the connecting line is small. In addition, the refrigerant is superheated before it enters the compressor which increases the enthalpy.7.0RECOMMENDATIONBased on the findings and conclusions of the experiments, here are several recommendations to be considered to improve the experimental works:

1. Clearance volume and compression ratio should be optimum because both factors affect the volume of re expansion gas trapped in the clearance volume.

2. Heating effect should be maximizing. It is because when vapor refrigerant enters the compressor, heat absorbed by the vapor results in heating effect that increases the specific volume of the refrigerant and therefore the actual induced volume of suction vapor at suction pressure, Va value.

3. Leakage in compressor should be minimizing. Refrigerant leaks through the gap and the clearance across the high and low pressure side of compressor, such as the clearance between the piston ring and the cylinder in reciprocating compressor.

4. Using the Innovative Vapour Compression Refrigeration System such as Cascade and Multi-stages system. These system can reduce work done by the compressor and increase the refrigeration effect. In this system, we can also used the compressor with low compression ratio in series.REFERENCES1.Cengel, Y.A and Boles, M.A, Thermodynamics An Engineering Approach, 4th Edition, McGraw Hill, 2002.2.Eastop, T.D and McConkey, A, Applied Thermodynamics for Engineering Technologist, 5th Edition, Pearson Prentice Hall, 1993.3.Wang, S.K, Handbook of Air Conditioning and Refrigeration, 2nd Edition, McGraw Hill, 2000.4.Azlir Darisun, Pemampat Salingan, Cetakan Pertama, Dewan Bahasa Dan Pustaka, 1992.1

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T

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0.317Mpa

0.9817Mpa

P

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0.9817Mpa

0.317Mpa

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