To determine power input, power output as well as coefficient of performance of heat pump.

OBJECTIVES

  1.  To determine power input, power output as well as coefficient of performance of heat pump.
  2. To draw the actual vapor-compression refrigeration cycle and compare it with an ideal cycle.

SET UP REQUIREMENTS:

  • Air and water heat pump.
  • Make : P.A Hilton pump.
  • Model: R831

 THEORY

According to the second law of thermodynamics, heat cannot spontaneously flow from a colder location to a hotter area; work is required to achieve this. Heat pumps and refrigerators are examples of machines which transfer heat from a low to high region temperature by consuming energy.

Vapor compression refrigeration cycle can be used in countless industrial, commercial and domestic situations throughout the world. The vapor compression cycle may equally be utilized to upgrade heat flow from low grade sources such as the atmosphere, a river or the soil so that it may be discharged at a more useful higher temperature for some applications or to increase temperature of specified area.

A heat pump is a device that transfers the heat from low temperature reservoir to the high temperature reservoir to the high temperature reservoir in order to maintain the temperature of a specified space higher than the surroundings by consuming energy.

The refrigerator is a device that transfers the heat from the low temperature reservoir to the high-temperature reservoir in order to maintain the temperature of a specified space lower than the surroundings by consuming energy.

The ideal vapor compression cycle is represented below in which heat is taken from a constant low temperature source and is rejected to a constant higher temperature sink.

Figure 1: vapor compression refrigeration cycle 

Above figure provides a schematic diagram of the components of a typical vapor – compression refrigeration system.

The absorption of low grade heat in either the air or water source evaporator generates HFC134a vapor which is drawn into compressor. This extraction of heat from air or water reduces the temperature of the air or water flow leaving the unit. There is increase in the pressure and temperature of the refrigerant vapor because work is done on the gas by the compressor. This hot high pressure gas flows to a concentric tube water cooled condenser.

A large volume into which excess refrigerant can flow during certain operating conditions is given by a liquid receiver as well as it ensures that the liquid is always available for changes in demand due to evaporator loading. The compressor motor has winding resistance losses, internal friction and the compression process is not isentropic. All of these conditions result in some of the electrical energy input being converted into heat. The compressor and motor are contained within thermetically sealed steel casing and run in oil which during normal operation is warmed by circulation around the casing and collects at the base of the unit. Some oil will be carried out and might even appear in the variable area flow meter as a discoloration to the flow. It is a normal thing to happen and will vanish during normal running process.

Through a panel mounted flow meter to a thermostatically controlled expansion valve, sub – cooled liquid HFC134a at high pressure is passed. The HFC134a is eco friendly gas. On passing through the valve, the pressure is reduced to that of evaporator and two phase mixture of liquid and vapor begins to evaporate within the selected evaporator.

As the compressor is specifically designed for heat pump a copper heat transfer coil is located at the base of the compressor within the oil reservoir. By passing the cold water from the mains supply through this coil before the water is transferred  to the condenser the normally waste heat from the oil can be added to that given up to the condenser.

Sub-cooled liquid HFC134a at high pressure passes through a panel mounted flow meter to a thermostatically controlled expansion valve. On passing through the valve the pressure is reduced to that of the evaporator and the two phases mixture of liquid and vapor begins to evaporate within the elected evaporator.

Control of heat pump is by variation of the condensing temperature by the source air(or water) temperature and flow rate,and by variation of the condensing temperature b the flow rate of the condenser water.The range of the source temperature can be extended directing warmed air from a fan heater at the air intake or warmed or chilled water to the source water inlet.

Relevant system temperatures are recorded by thermocouple and a panel mounted digital temperature indicator. The thermocouples used are type K (Nickel-chrome, Nickel-Aluminum).Condenser and evaporator pressures are indicated by panel mounted pressure gauges. Water and refrigerant flow rates are indicated by panel mounted variable area flow meters. The efficiency of a heat pump is given by a parameter called the coefficient of performance (COP) .

Figure 2: graph of pressure vs enthalpy

The cycle is as follows, saturated vapor at state1 and at low pressure is compressed isentropically to high pressure. Superheated vapor at state2 is passed into a condenser and heat is rejected at constant pressure to a cooling medium so that the vapor condenses and becomes saturated liquid at state 4. The high pressure saturated liquid is throttled and the resulting very wet vapor is passed into an evaporator at state 6.In the evaporator the vapor evaporates at a low temperature taking in heat from the low temperature heat reservoir and reaches state 1. The cycle now repeats.

The practical cycle differs from the idealized cycle in the following ways:

  1. Due to friction, there will be a small pressure drop between the compressor discharge and expansion valve inlet, and between the expansion valve outlet and the compressor suction.
  2. The compression process is neither adiabatic nor reversible. (There will usually be a heat loss from the compressor and, obviously, there are frictional effects.)
  3. The vapor leaving the evaporator is usually superheated. (This makes possible automatic control of the expansion valve and prevents compressor damage by ensuring no liquid enters the suction valve.)
  4. The liquid leaving the condenser is usually slightly sub-cooled, i.e., it is reduced below saturation temperature corresponding with its pressure. (This improves the COP and reduces the possibility of the formation of vapor due to the pressure drop in the pipe leading to the expansion valve.)
  5. There may be small heat inputs or losses to and from the surroundings to all parts of the circuit depending upon their temperature relative to the surrounding. The net effect of these “losses” or irreversibility on the cycle diagram is shown belo

RELEVANT EQUATIONS

The COP of a heat pump is given by the following equation:

COPHP = Desired Output/Required Input = Heating Effect/Work Input = QH/W

So,for an ideal heat pump:

COPHP = TH/ (TH-TL)

Qcomp= mcCpw(t6-t5)

Qc= mcCpw(t7-t6)

COPHP= Rate of heat delivered/compressor electrical power input

If the heat delivered to the condenser only is considered, then

COPHP=Qc/W

If the total heat delivered to the water is considered, i.e., including the waste heat from the compressor cooling coil, then

COPHP=(Qc+Qcomp)/W

Where,

COPR= Coefficient of performance of Refrigerator

COPHP= Coefficient of person of Heat Pump

Qcomp= Heat delivered to cooling water from compressor

Qc= Heat delivered to condenser cooling water

Cpw= Specific heat of water (4.18 kJ/kgoC)

OBSERVATIONS

Table no 1: For source of low grade heat: Air evaporator

S.NoPARTICULARS UNITS  
1Compressor electrical power input(W) Watts 380
2 Cooling water inlet temperature(t5) oC 16
3 Compressor cooling water outlet temperature(t6) oC 18
4 Condenser water outlet temperature (t7) oC 20
5Condenser water mass flow rate(mc) g/s2 40

Table no 2: For source of low grade heat: Water evaporator

S.NO           PARTICULARS UNITS  
  1 Compressor electrical power input(W) Watts 400
  2 Cooling water inlet temperature(t5) oC 16
  3 Compressor cooling water outlet temperature(t6) oC 19
  4 Condenser water outlet temperature (t7) oC 22
5 Condenser water mass flow rate(mc) g/s2 25

Table 3:

S.NO PARTICULARS UNITS  
1 HFC134a gauge pressure at compressor suction(p1) kN/m2 180
2 HFC134a absolute pressure at compressor suction(p1) kN/m2 285
3 HFC134a gauge pressure at compressor discharge(p2) kN/m2 800
4 HFC134a absolute pressure at compressor discharge(p2) kN/m2 905
  5 HFC134a temperature at compressor suction(t1) oC 11
  6 HFC134a temperature at compressor discharge (t2) oC 39
  7 HFC134a temperature condensed liquid (t3) oC 21
  8 HFC134a temperature at expansion valve outlet (t4) oC 11

CALCULATION

For Air Evaporator

Qcomp=mc.Cpw(t6-t5) = 40×10-3×4180×(18-16)=334.4W

Qc=mc.Cpw(t7-t6) = 40×10-3×4180×(20-18)=334.4W

When heat from both the compressor and condenser are considered,

COPHP = 1.76

If heat is delivered to condenser only                                    

COPHP = 0.88

For water evaporator

Qcomp. = m.Cp.(t6–t5) = 25×10-3×4180×(19-16) = 313.5W

Qc.      = m.Cp.(t7–t6) = 25×10-3×4180×(22-19) = 313.5 W

When heat from both the compressor and condenser are considered,                                              

COPHP = 1.5675

If heat is delivered to condenser only                                    

COPHP = 0.784

RESULTS AND ANALYSIS

From the experiment the coefficient of performance of air evaporator was found to be 1.76 and the coefficient of performance of water evaporator  was found to be 1.5675.COP or coefficient of performance is a measure of the efficiency of the heat pump. The heat pump used in the experiment had a COP greater than 1. For air source, it was 1.76 which means at this condition, 1.76 kJ of heat energy can be extracted from the system air with the input of 1 kJ of work. Similarly for water source, it was 1.5675 which means at this condition, 1.5675 kJ of heat energy can be extracted from the system water with the input of 1 kJ of work. From the experiment we found out that the value of COP of any thermodynamic cycle ranges from 0 to any higher value.  We also came to know that the practical vapor cycle differs from the idealized cycle because in idealized we don’t take account of frictional pressure drop in the system, slight internal irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

CONCLUSION

From this experiment on heat pump and refrigerant, we get familiar with the practical vapor compression cycle. We learnt to draw the p-h diagram. We know the working principle of heat engines. We studied the temperature of compressor inlet, outlet and the condenser outlet temperature and hence calculated the power input, heat output and COP. From above experimental data and calculations, it is clear that for greater amount of work or highest performance by heat pump, we have to use water evaporator rather than air. Water evaporator is more efficient than air evaporator. Similarly we got conclusion that there are several reasons to make difference between actual vapor compression cycle and ideal vapor compression cycle. The system should be ensured to be stable while taking data. Extreme high pressure should be controlled otherwise system may crash. Electrical panel should be grounded in order to control risks of possible accidents.

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