Heat Pump

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Energy Engineering Performance of a Heat Pump

Introduction

Aim The aim of this laboratory is for you to understand the operation and some of the characteristics of a heat pump.

Equipment

Figure 1: The configuration of the heat pump showing the position of the thermometers (T1 – T9), pressure gages

(p1 and p2), and the flow meters. The position of the state points (1 – 4) corresponding to the temperature

measurements T1 – T4 are also shown on a pressure enthalpy chart.

The Hilton Air and Water Heat Pump is a small-scale demonstration unit designed especially for educational

purposes. Heat may be exchanged from either atmospheric air or to/from water supplied from the cold mains

water supply. The work input is in the form of electrical energy supplied to a hermetically sealed compressor.

The unit comprises the following components:

Compressor: Hermetically sealed and fitted with an oil-cooling coil. The swept volume is 15cm3. The

operating speed is approximately 2800rev/min. It is powered by an electric motor.

Condenser: This consists of concentrically coiled tubes, with the refrigerant flowing through the inner tube

and the cooling water flowing through the annular space. The system is thermally insulated.

Expansion valve: This is thermostatically controlled to provide a small amount of superheat at the compressor

inlet (to prevent liquid entering the compressor).

Evaporators: This consists of concentrically coiled tubes, the refrigerant passing through the inner tube and

water (which acts as the heat source), passing through the annular space.

Refrigerant: R134a (tetrafluoroethane CH2F-CF3).

�� ��

�� �

�� ��

p-h diagram

Specific Enthalpy

Pressure

1

2 3

4

Circuit Diagram

T1, p1

T2, p2 T3

T4

T5 T6

T7 T8

T9 Compressor

Throttle

Condenser

Evaporator

�� �

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Energy Engineering Performance of a Heat Pump

These are shown in figure 1, which also indicates the position of state points 1, 2, 3 and 4 in the refrigerant and

also on a p-h diagram.

Instrumentation

• An electricity meter for measuring the power input to the compressor,

• Glass thermometers at ten temperature measuring locations,

• Two pressure gauges; one for measuring the compressor inlet pressure, and the other the compressor outlet

pressure,

• Two water flow rotameters; one for measuring the water flow to condenser, and the other to the compressor

and the evaporator,

• A rotameter for measuring the refrigerant flow rate at the inlet to the expansion valve (or throttle).

The position of these instruments is indicated in figure 1.

Background and Analysis The machine is called a refrigerator when the desired effect is to remove heat from the low temperature

reservoir, and is called a heat pump when the desired effect is to supply heat at the high temperature; both

processes occur simultaneously, the only difference is the desired effect. In either case, the useful heat transfer

is usually greater than the work input, so the “efficiency” of the machine is called a coefficient of performance.

For a heat pump, the coefficient of performance is the ratio of the heat transfer rate to the high temperature

reservoir �� to the rate of work input �� ,

COP�� � ��

�� ,

and for a refrigerator, the coefficient of performance is the ratio of the heat transfer rate to the low temperature

reservoir �� to the rate of work input �� ,

COP� � ��

�� .

If there are no other heat transfers, then

�� � �� ��� ,

and therefore

COP�� � �� ���

�� � COP� � 1.

Procedure • Turn on the two water supply taps.

• Adjust the water flow by means of the needle valves on the rotameters to read maximum values.

• Switch on the power supply and wait for equilibrium conditions to be reached.

• Record all pressure, temperature and flow rate measurements (see table at the back of the lab sheet).

• Measure the time for one revolution of the electricity meter.

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Energy Engineering Performance of a Heat Pump

• Reduce the condenser water flow rate to about 7.5g/s. This will cause the condenser pressure to rise to a

maximum of about 1400kN/m2 (gauge pressure).

• Adjust the evaporator water flow to keep T4 the same as in the first test. Remember that an increase in

evaporator water flow will raise the evaporator temperature.

• Wait for stable conditions, and record all measurements.

• When finished, turn off the power supply but leave the water taps on.

• Record the atmospheric pressure on the laboratory barometer.

Analysis of Results The coefficients of performance can be calculated using the heat transfer rates based on the water flow or the

refrigerant flow. Since heat is transferred from the water to the refrigerant (or vice versa), the calculated heat

transfer rates should be equal in magnitude, but opposite in sign.

Performance based on the water flow

Work input to the compressor

In the electricity meter, 1kWh causes 1662/3 revolutions of the disc. Therefore one revolution indicates the

consumption 6 � 10��kWh. If this is consumed in time t (s), the electrical power �� � can be calculated as

�� � � 6 � 10 �� �

3600

� .

This is the power input to the motor driving the compressor, not the power input to the compressor itself. If we

assume an efficiency of 80%, then the actual power from the compressor is

�� � 0.8 ��� � .

Heat transfer rate in the compressor

Applying the steady flow energy equation (SFEE) to the compressor water flow gives

��� ! � �� ��"#$ % #&' � �� ��(!)"*$ % *&',

where �� �� is the water flow rate through the compressor (note we need to change all flow rates to kg/s, i.e. the

reading in g/s divided by 1000), and (!) � 4.19kJ/kgK is the specific heat capacity of water.

Heat transfer rate from the cold temperature reservoir (evaporator)

Applying the steady flow energy equation (SFEE) to the evaporator water flow gives

�� � �� ��"#2 % #3' � �� ��(!)"*2 % *3',

where �� �� is the water flow rate in the evaporator.

Heat transfer rate from the high temperature reservoir (condenser)

Applying the steady flow energy equation (SFEE) to the condenser water flow gives

�� � �� ��"#4 % #$' � �� ��(!)"*4 % *$'.

Coefficients of performance

The coefficient of performance of the heat pump based on the water flow is given by

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Energy Engineering Performance of a Heat Pump

COP�� � ��

�� .

The coefficient of performance of the refrigerator based on the water flow is given by

COP5 � ��

�� .

In calculating the COPs, ignore the sign of the heat transfers.

Performance based on the refrigerant flow

Plotting the cycle diagram

The thermodynamic cycle can be plotted on the pressure-enthalpy (6-#) diagram for R134a provided as shown in

figure 2. Note that you should join up state points 1-4 to show the thermodynamic cycle.

Figure 2: Pressure-Enthalpy chart. State points are shown as crosses (black), and temperatures are shown as

temperatures circles (red); the short(red) lines attached to these circles are lines of constant temperature. The two

long horizontal lines (blue) are lines of constant pressure, the vertical lines (green) are lines of constant enthalpy.

To find your enthalpy values from the chart you need to

1. Convert the readings of gauge pressure 67 and 68 into absolute values by adding atmospheric pressure.

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Energy Engineering Performance of a Heat Pump

2. Plot state points 1 and 2 in the superheat region using values of 67, *7 and 68, *8. Join points 1 and 2 to

represent the compression process.

3. Locate *� on the constant pressure line (horizontal) corresponding to 68 (it’s expected to be on or near the

saturated liquid line); this is state point 3. The horizontal line between points 2 and 3 represents cooling

and condensation at constant pressure.

4. Draw a vertical line downwards from point 3 until it reaches an isotherm corresponding to *9. Inside the

saturation dome an isotherm (line of constant temperature) corresponds to a horizontal line of constant

pressure. This vertical line represents the (constant enthalpy) throttling process between state points 3

and 4 (no heat transfer and no work done).

5. Notice that the pressure at point 4 is higher than at point 1, mainly because of the flow resistance of the

non-return valve between the evaporator and the compressor. Join points 4 and 1 with a straight line

representing the evaporation process.

6. Read the values of specific enthalpies #7, #8, #�, and #9 from the 6-# diagram, noting that #9 � #�.

Rate of work done by the compressor

Appplying the SFEE to the refrigerant flow through the compressor gives

�78 ��� 78 � �� �"#8 % #7',

where �� � is the refrigerant flow rate (again in kg/s), and �78 � % ��� ! (evaluated from the water flow).

Therefore, the rate of work done compressing the refrigerant vapour is given by

�� 78 � �� �"#8 % #7' � ��� !.

Heat transfer rate through the condenser

The heat transfer rate from the refrigerant is given by

�8� � �� �"#� % #8'.

Heat transfer rate through the expansion valve

There are no heat or work transfers in the throttling process, so #� � #9.

Heat transfer rate through the evaporator

The heat transfer to the refrigerant is given by

�97 � �� �"#7 % #9'.

Coefficients of performance

The coefficient of performance of the heat pump based on the refrigerant flow is given by

COP�� � �8�

�� 78 .

The coefficient of performance of the refrigerator based on the refrigerant flow is given by

COP5 � �97

�� 78 .

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Energy Engineering Performance of a Heat Pump

In calculating the COPs, ignore the sign of the heat transfers.

Discussion How do your results relate to theory?

Discuss and suggest reasons for the differences.

What are the sources of error in the experiment? Could these be reduced or eliminated?

Quantify the effect of the errors on your results.

Conclusions Summarise the main points from your discussion.

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Energy Engineering Performance of a Heat Pump

Use this table to record your results

Measured quantities Units Test 1 Test 2

*7 °C

*8 °C

*� °C

*9 °C

*3 °C

*2 °C

*& °C

*$ °C

*4 °C

67 bar

68 bar

Mass flow rate of refrigerant (�� �)

g/s

Mass flow rate of water to evaporator (�� ��)

g/s

Mass flow rate of water to condenser/compressor (�� ��)

g/s

Time for one revolution of the electricity meter (�)

s

Atmospheric pressure = bar

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Energy Engineering Performance of a Heat Pump

Derived quantities

Enthalpy Units Test 1 Test 2

#7 kJ/kg

#8 kJ/kg

#� kJ/kg

#9 kJ/kg

Test 1 Test 2

Based on the

water flow Based on the

refrigerant flow Based on the

water flow Based on the

refrigerant flow

Compressor power

�� , �� 78 (kW)

Heat transfer is the compressor

��� !, �78

(kW)

Heat transfer in the condenser

��, �8� (kW)

Heat transfer in the evaporator

��, �97 (kW)

COP�� (no units)

COP� (no units)