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ME354 Thermo Lab: Refrigeration

June 24, 2002

 

 

1.0 Introduction

The Refrigeration lab was conducted to gain a better understanding of the vapour-compression refrigeration cycle, and to compare the ideal cycle with the real cycle. 

In this lab dichlorofluoroethane (R141b) was used as the refrigerant, and the mass flow rate of water coming into the evaporator and the condenser was varied.  From this variation data was obtained and analysed.

2.0 Procedure

This lab was conducted according to the Thermodynamics Lab Manual [1]

3.0 Observations

Room Temperature:    24°C

Barometer:                  28.71 mm of Hg  è 96.98 Kpa

 

Electrical Power to run compressor

Volatage:                     128 V

Current                        4.8 A

 

 

Test Number

 

1

2

3

Evaporator Gauge Pressure (KPa)

-55

-51

-51

Evaporator Temperature (°C)

9.5

9.5

9.0

Evaporator Water Flow Rate (gram/s)

10

41

26

Evaporator Water Inlet Temperature (°C)

13.25

12.725

12.5

Evaporator Water Outlet Temperature (°C)

10

12

11.5

Condenser Gauge Pressure (KPa)

210

225

212.5

Condenser Temperature (°C)

28

33.1

28.25

Condenser Water Flow Rate (gram/s)

29.5

10

40

Condenser Water Inlet Temperature (°C)

12.8

13

12.1

Condenser Water Outlet Temperature (°C)

14.5

19.8

13.5

Table 1  Observations

4.0 Analysis

Figure 1 The Ideal Vapour Compression Refrigeration Cycle

In order to simplify the calculation the following assumptions were made

  • Steady operating conditions exist

  • Kinetic and potential energy changes are negligible throughout the system

  • No frictional pressure drops

  • Refrigerant  flows at constant pressure through the evaporator and condenser

  • Flow through the pipes and the expansion valve are assumed to be adiabatic

  • The refrigerant enters the compressor at state 1 as a saturated vapour

  • The refrigerant enters the condenser as a superheated vapour at state 2

  • The refrigerant leaves the condenser as a saturated liquid at state 3

  • The refrigerant is a saturated vapour when it leaves the evaporator

All calculations were done using the ideal vapour-compression refrigeration cycle models with the following considerations:

Using Newtons law of cooling the heat transfer between the condenser and the surrounding air was estimated to be

Qcondensor = 0.8(Tair – Tcondensor)

Where the difference in temperature between the condenser and the surrounding air dictates the direction of heat flow

Using Newtons law of cooling the heat transfer between the evaporator and the surrounding air was estimated to be

Qevaporator = 0.8(Tair – Tevaporator)

Where the difference in temperature between the evaporator and the surrounding air dictates the direction of heat flow

Coefficient of performance is defined as:

The benefit as a refrigerator is the heat gain in the evaporator, and the benefit as a heat pump is the heat released from the condenser.  In both cases the cost is the power drawn by the compressor defined by the following equation

Power = Voltage * Current * Power Factor – Losses

In order to calculate the heat gained by the evaporator a first law energy balance was done.  The balance took into consideration the refrigerant flow, the water flow, and the heat gained from the surroundings.  In order to calculate the heat released by the condenser a first law energy balance was done.  The balance took into consideration the refrigerant flow, the water flow, and the heat lost into the surroundings.  The key concept in performing this calculation is the fact that the heat gained by the refrigerant in the evaporator is the heat lost by the water passing flowing through the evaporator.  Furthermore the heat lost by the refrigerant in the condenser is the heat gained by the water flowing through the condenser. Appendix A presents the detailed calculation of the COP’s for different mass flow rates.  A summary of the results is displayed in section 5.0.

In order to calculate the mass flow of the refrigerant through the cycle, the state properties at state 1, state 3, and state 4 are needed only.  The ideal vapour-compression refrigeration cycle was used to define the state at critical points of the cycle, and  The equivalent values of the enthalpies at state 3 and state 4 was used to fix the states.

There are two approaches to determine the state properties at state 2.  In the first approach (M1) , the enthalpy at state 2 is found by using  the mass flow rate of the refrigerant (as describe above) in an energy balance on the condenser or the evaporator.  If an energy balance on the condenser is used the following equation is used:

Once the enthalpy is determined the temperature is determined by using a pressure-enthalpy diagram for the refrigerant. 

In the second approach  (M2) the enthalpy at state 2 is found by using  the mass flow rate of the refrigerant in an energy balance on the compressor.  The energy balance yields the following equation:

where Wcompressor = (compressor efficiency) * (Power)

Once the enthalpy is determined the temperature is determined by using a pressure-enthalpy diagram for the refrigerant.  This method is not as accurate as the first method because compressor work will vary for different mass flow rates and the efficiency given is a generalized case.

 

5.0 Results of Calculations

Refer to appendix A for detailed calculations

M1 – Method 1 of  calculating properties of state 2, as describe in section 4.0

M1 – Method 2 of  calculating properties of state 2, as describe in section 4.0

Test 1

Variable

Results

QEvaporator loss   (W)

11.6

QCondensorr loss   (W)

3.2

QH  (Kw)

0.206427

Q­L  (Kw)

0.14745

COPR

0.2450

COPHP

0.3431

WCompressor  (Kw)

0.120384

Mrefrigerant  (kg/s)

0.000753

Table  2  Results for Test 1

State

P (Kpa)

Temperature (°C)

Enthalpy (KJ/kg)

1

41.98

9.136

443.99

2

306.98

57 (M1) , More info needed (M2)

555.65 (M1), 605.20 (M2)

3

306.98

67.92

279.45

4

41.98

9.136

279.45

Table 3  State Properites for test 1

Test 2

Variable

Results

QEvaporator loss   (W)

11.6

QCondensorr loss   (W)

7.28

QH  (Kw)

0.29152

Q­L  (Kw)

0.1358505

COPR

0.857

COPHP

1.839

WCompressor  (Kw)

0.120384

Mrefrigerant  (kg/s)

0.000835

Table  4 Results for Test 2

State

P (Kpa)

Temperature (°C)

Enthalpy (KJ/kg)

1

45.98

11.31

444.117

2

321.98

??

630.58 (M1), 633.88 (M2)

3

321.98

69.56

281.46

4

45.98

11.31

281.46

Table 5 State Properties for test 2

Test 3 

Variable

Results

QEvaporator loss   (W)

 

QCondensorr loss   (W)

 

QH  (Kw)

 

Q­L  (Kw)

 

COPR

 

COPHP

 

WCompressor  (Kw)

 

Mrefrigerant  (kg/s)

 

Table 6 Results for Test 3

State

P (Kpa)

Temperature (°C)

Enthalpy (KJ/kg)

1

45.98

11.31

444.17

2

309.48

57 (M1) , More info needed (M2)

597.34 (M1), 633.88 (M2)

3

309.48

68.2

279.796

4

45.98

11.31

279.796

Table 7  State Properites for test 3

6.0 Discussion

The refrigeration cycle which was observed in the lab had a number of variables which give a deviation when calculating the COP using experimental data.  This deviation is a result of measuring equipment, the setup of the equipment and human errors in reading the different gauges.  When performing the calculations, the values used were ones that were obtained from the experiment.  However, when performing calculations in class, its assumed to have an ideal refrigeration cycle.  In which there are a few assumptions to help make the calculations easier.

Equipment

The source of errors in the lab came from the measurement equipment.  The following equipment was used for measurement

1)                  Thermometer – To measure temperature

2)                  Bourdon Gages – To measure pressure the pressure of the R141b

3)                  Ammeter – To measure the current supplied to the motor

4)                  Voltmeter – To measure the voltage to the motor

5)                  Flow Meter – To control the flow of water in the evaporator and condenser.

The thermometer which was used to measure in the experiments had a few problems with the setup.  Firstly, the thermometer is not placed in the refrigerant or the water it is placed a little above the refrigerant and water.  Therefore, the measured temperature is an average of the water/refrigerant and the surrounding air.  Secondly, the thermometer was setup such that the thermometer could be pulled out of the setup so the value could be read.  This causes a problem because the diameter of the hole must be a little bit bigger than the diameter of the thermometer.  Hence, there will be a transfer of heat to surrounding through the difference in diameter.  This can be solved by using a thermometer which is placed in the fluid and can be read without having to remove it from the setup.  The variation of the measurement of temperature has an effect on the calculated COP.  When calculating the values for the refrigerant at the states 1 and 3 we read the values on the tables based on pressure and not temperature.  This was done to minimize the error in calculations due to the variation involved with the thermometers. 

When measuring the pressure of the refrigerant the Bourdon gage was used.  Human error is a key in reading the gage properly.  The gages must be read from directly in front and not from a slight angle.  The gages should also be calibrated by manufactures.

Variation in Ideal and Experimental Refrigeration Cycle

There are a number of differences in the calculations involved in an ideal refrigeration cycle and the actual (experimental) refrigeration cycle.  In the ideal refrigeration cycle it is assumed that the pressure drops are negligible, the compression is isentropic, the evaporator and condenser are adiabatic.  However the measured values will include the pressure drops due to friction, the compressor is not isentropic and the evaporator and condenser are not adiabatic.  Since the compressor is not isentropic there will be more work required to get from state 1 to state 2.  Therefore the work input will increase which affects the COP.  Equation 4 defines COP as benefits over costs where the cost is the work input.  Hence, the COP will be greater for the ideal cycle when compared to the actual cycle for a refrigeration cycle. 

Variation of h2 Values when using different Methods

Method 1 takes uses a energy balance over the condenser to find h2.  Using equation 7 h2 was calculated to be 55.65Kj/kg [Appendix B].  The second method was use the compressor efficiency to find h2.  Using equations 8,9 and 10 the value for h2 was calculated to be 605.20 [Appendix B].  Both of these methods were using the data from the first trial.  Method 2 is a more accurate value of h2 because there is less experimental data involved in the calculation. 

Sources of Error

The equipment used to measure various data in the experiment is a main factor of error.  As discussed above the thermometer not being in the fluid plays a huge role in the temperature measured.  Human error in reading the various gages would play a role in the error.  The gages have to be read with you standing directly in front of the gage and not to an angle.  The assumption that heat transfer from the evaporator and condenser is estimated by equation 1 and 2.  This will cause errors in the calculations because this is just an estimate of the actual transfer of heat to the surroundings

6.0 Discussion

Although the collection of data, and the calculations were performed with the utmost of care, the values obtained from the actual refrigerator will not be equal to those of the ideal refrigerator.  The differences may be associated with the accuracy of the measuring equipment, and the methods used, which would impact the calculated COP values.  In the calculations, the data used were actual field measurements.  However the calculations used in the classroom are performed with the ideal refrigerator, where reasonable assumptions are made.  In an effort to make calculations easier, one of the assumptions made is that there are no irreversibility’s, which will result in a greater COP values.  Thus discussion will also include the evaluation of the apparatus for areas where possible sources of error may have arisen.

Measurement Equipment

The measurement equipment used in the laboratory consisted of the thermometer for measuring temperature; Bourdon gages to measure pressure of the refrigerant, analogue voltmeter and ammeter to measure voltage and amperage into the apparatus.

Both the condenser and the evaporator refrigerant temperature were measured with a thermometer.  Both devices had an opening from the top where the thermometer could be placed into the opening with a diameter slightly greater than that of the thermometer and a depth of less than the thermometer length.  The thermometer is never in contact with the refrigerant, but is separated by the inner wall of the device.  Thus the heat transfer must occur between the refrigerant, inner wall of the device, and the pocket of air surrounding the thermometer, to have an effect on the thermometer.  Thus reduced and increased temperatures result, with direction depending on the temperature of the environment.   The same variation occurs in the measurement of the inlet and outlet temperatures of the flow water for both the heat transfer devices.  To countermeasure this affect, the use of thermometer that is actually in the fluid to be measured will eliminate the above.

 

To ensure accurate data, the thermometer must be read correctly.  The thermometer fluid must be allowed to settle prior to taking a measurement.  The reading of the measurement is also critical since the temperature must be read from the bottom of the meniscus.  It should be noted that to read the temperature of the condenser and the evaporator, the thermometer must be removed from the opening of the device to read the scale.  Which would have allowed the temperature to try to return to room temperature.  Since the scale was readable for the water temperature, removal of thermometer was not required for them.  The possibility for variation of temperature may be reduced by incorporating the thermometer into the fluid (without disturbing the fluid flow) and allowing the temperature scale to readable without removing it, perhaps a digital readout of the temperature.

Pressure of the refrigerant was obtained from the analogue Bourdon gage.  To ensure accurate readings, the readings should be taken directly in front of the gage, and not from an angle.  The same is true of the voltage and current gage.

To ensure accurate readings gages should be calibrated in accordance with the manufacturers recommendations. 

Variations in Actual and Ideal (Textbook) Results

As mentioned earlier, differences from the actual and the ideal results of a refrigeration cycle are due to the assumptions made.  Calculations with the ideal refrigeration cycle include the following,

  • Irreversibility’s within the evaporator, condenser, and compressor are ignored

  • No frictional pressure drops

  • Refrigerant flows at constant pressure though the two heat exchangers

  • Stray heat losses to the surroundings are ignored

  • Compression process is isentropic

On the other hand, the data used for the calculations were from actual field measurements (taking into consideration irreversibility’s, pressure drops due to friction, non constant pressure across heat exchangers), but the calculations were made with the above assumptions except the compression process was not assumed to be isentropic.  Thus the entropy difference across the compressor at state points one and two reflects the entropy generated due to the irreversibility’s.  Thus the actual enthalpy and temperature at state two will always be greater than the enthalpy and temperature at state 2 for the ideal process (isentropic).  This results in a greater amount of work to be applied to the working fluid in comparison to the isentropic case.  Since COP is defined as the benefit over the cost, increasing the work will decrease the COP.  Hence the actual COP will be less than the COP obtained for the ideal system.   In the ideal system the following equation will be true,

with the assumptions made above.  However in the actual scenario, the equality does not hold due irreversibility’s.

A method to quantitatively compare the actual and ideal cycles would be with the thermal efficiency.  Similar to the COP, thermal efficiency will also be less for the actual then the ideal cycle.  A simple comparison calculation would be the percentage decrease in thermal efficiency of the actual versus the ideal cycle.

Sources of Error
Sources of error in this laboratory may include those from the measurement equipment, the actual apparatus, and assumptions made of the equipment.

As mentioned above, the measurement equipment and the methods in using them may be considered possible sources of error in obtaining measured data.  Thus the proposed countermeasures should be investigated to reduce the potential of inaccuracy in obtaining measured data.

To obtain higher COP values, the actual cycle should operate as closely as possible to the ideal cycle.  This means that irreversibility’s such as pressure drops due to friction, and heat transfer to the environment should be minimized.  Heat transfer to the surrounding, can be minimized by insulating the exposed piping and especially the condenser and evaporator.  However, the apparatus in the laboratory was possibly left insinuated to allow the student to see the contents of the condenser and evaporator.

At the condenser and the evaporator, undesirable heat transfer exists between the device and the environment, in the direction of high to low temperature.  The heat transfer for the condenser and the evaporator is approximated by  and .  The values are required to calculate the COP, however they are still approximations, which present a source of error. 

Variation

Temperature

Me

Percentage Variation In Variable

Percentage Variation In COP

Comment

Troom

Te

T5

T6

Vary Room Temperature

21

9.5

13.25

10

0.01

-12.50%

-1.63%

Small insignificant changes in COPR occur with small changes to the variable

22

9.5

13.25

10

0.01

-8.33%

-1.09%

23

9.5

13.25

10

0.01

-4.17%

-0.54%

24

9.5

13.25

10

0.01

0.00%

0.00%

25

9.5

13.25

10

0.01

4.17%

0.54%

26

9.5

13.25

10

0.01

8.33%

1.09%

27

9.5

13.25

10

0.01

12.50%

1.63%

Vary Evaporator Temperature

24

7

13.25

10

0.01

-26.32%

1.36%

Small insignificant changes in COPR occur with small changes to the variable

24

8

13.25

10

0.01

-15.79%

0.81%

24

9

13.25

10

0.01

-5.26%

0.27%

24

9.5

13.25

10

0.01

0.00%

0.00%

24

10

13.25

10

0.01

5.26%

-0.27%

24

11

13.25

10

0.01

15.79%

-0.81%

24

12

13.25

10

0.01

26.32%

-1.36%

Vary T5

24

9.5

12

10

0.01

-9.43%

-35.44%

Very large changes in COPR occur with small changes to the variable

24

9.5

12.5

10

0.01

-5.66%

-21.26%

24

9.5

13

10

0.01

-1.89%

-7.09%

24

9.5

13.25

10

0.01

0.00%

0.00%

24

9.5

13.5

10

0.01

1.89%

7.09%

24

9.5

14

10

0.01

5.66%

21.26%

24

9.5

14.5

10

0.01

9.43%

35.44%

Vary T6

24

9.5

13.25

9

0.01

-10.00%

28.35%

Very large changes in COPR occur with small changes to the variable

24

9.5

13.25

9.5

0.01

-5.00%

14.17%

24

9.5

13.25

9.75

0.01

-2.50%

7.09%

24

9.5

13.25

10

0.01

0.00%

0.00%

24

9.5

13.25

10.25

0.01

2.50%

-7.09%

24

9.5

13.25

10.5

0.01

5.00%

-14.17%

24

9.5

13.25

11

0.01

10.00%

-28.35%

Varry Mass flow rate

24

9.5

13.25

10

0.009

-10.00%

-9.21%

Large changes in COPR occur with small changes to the variable

24

9.5

13.25

10

0.0095

-5.00%

-4.61%

24

9.5

13.25

10

0.00975

-2.50%

-2.30%

24

9.5

13.25

10

0.01

0.00%

0.00%

24

9.5

13.25

10

0.01025

2.50%

2.30%

24

9.5

13.25

10

0.0105

5.00%

4.61%

24

9.5

13.25

10

0.011

10.00%

9.21%

 

 

8.0 Conclusions

From the refrigeration laboratory, the pressure, enthalpy, and temperature were calculated at the four state points of the vapour compression refrigeration cycle (VCRC).  With the state point properties and the power drawn by the compressor the COP can be determined.  However the COP can be determined from the work input to the compressor, or as the work applied to the working fluid. 

In comparison with the ideal VCRC, data obtained and calculated from the laboratory (actual results) differed from those of the ideal cycle. The differences are a result of the assumptions made when working with the ideal cycle to simplify the calculations.  The assumption that the compressor is isentropic shows the greatest impact on the variation of the ideal and the actual cycle.  

A relevant source of error involves the accuracy of the measuring equipment and of the methods used in obtaining the measured values.  The actual cycle would greatly approximate that of the ideal cycle if the assumptions could be reached, such as constant pressure heat transfer across heat exchangers, and the reduction of heat transfer to and from the environment due to insinuated devices.