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The Vapour Compression Refrigeration Cycle Yousef Farfour, Lab Group E, [email protected]

Abstract The aim of this laboratory session is to observe and investigate the vapour compression refrigeration cycle by use of the P.A R714 refrigeration unit.

Introduction The earliest human experiments on vapour compression goes back to the 1st century AD where Heron of Alexandria invented the first object to produce motion via its interaction with compressed vapour (steam) it is called the Aeolipile. After roughly two millennia the refrigeration system appears, and the use of compressed vapour becomes an essential part of any refrigeration system. These systems are crucially important as they are widely used domestically by individuals in their homes and corporates for various reasons. The process of refrigeration involves the transfer of energy from low energy area to the high energy area As the 2nd law of thermodynamics conclude that heat transfer cannot be transferred from cold to hotter area without the introduction of some type of work in the process at some point. The work input could potentially be anything that’s feeding power to the system in which case its going to be electrical energy in the refrigerator. The four processes of the vapour compression cycle system are as follow Evaporation, Compression, Condensation and expansion, requiring four components. Evaporator (absorbs heat from refrigerated area), Compressor (raises pressure and temperature), Condenser (heat exchange with the surrounding refrigerant liquid) and Expansion valve(lowers temperature and pressure). The purpose of the cycle is to be able to maintain a desirable low temperature of space by absorbing the heat from it, the absorption is possible because of the liquid refrigerant that absorbs heat isothermally from the source. The report’s objective is to analyse and record the cycle process and take into account the uncertainties and errors and quantifying them to better understand the process.

Results Table 1: Coefficient of performance Test 1

Test 2

Thermodynamic COP

5.071

5.106

Overall COP

1.356

2.383

Compressor COP

1.499

2.65

Indicated COP

2.749

4.858

Table 2: Mass flow rate of refrigerant Mass flow rate of refrigerant (kg/s)

Test 1

Test 2

Calculated by heat transfer in Evaporator

0.004437

0.007475

Calculated by heat transfer Condenser

0.005531

0.0078403

4.5

7.5

Measured by rotameter

Table 3: Efficiencies of the compressor Test 1

Test 2

Mechanical Efficiency

0.905

0.899

Isentropic Efficiency

0.918

0.539

1. Derived equations for error analysis are listed below. ΔCO POV =CO POV

[| | | |]

ΔCO PTH =CO PTH

4 −( Δh2− Δh1) + |[ Δh1−Δh h1−h 4 | | (h 2−h 1) |]

Δ P e −Δ PC + Pe PC

ΔCO P comp=CO Pcomp

[| | |

Δ Pe −Δ PSHAFT + Pe PSHAFT

|]

ΔCO P IND =CO P IND

[| | |

|]

[| | |

[|

´ R 134−A ,cond =m´ R 134− A ,cond Δm

||

Δη s=η s

[|

[|

|]

Δm ´ WATER . ΔC pw .( Δ T 6− ΔT 5 ) −( Δh 2−Δh3 ) + ´ WATER . C pw .(T 6−T 5) m (h 2−h 3)

[| | |

Δm ´ R 134−A ,evap = m ´ R 134− A ,evap

Δη m=ηm

|]

ΔP e −Δ P IND Δ Pe −( Δ PSHAFT −Δ PFRIG ) + =CO P IND + P IND Pe Pe PSHAFT −P FRIG

|]

Δ P e −( Δh1−Δh 4 ) + Pe (h 1−h 4)

| | |]

Δ P SHAFT − ΔP C + P SHAFT PC

||

|]

Δh 2 s− Δh 1 −( Δh2− Δh1) + h 2 s−h 1 (h 2−h1)

2. Derivation of the uncertainty in COPTH in terms of the uncertainties in the enthalpies at states of 1 to 4, h1, h2, h3 and h4, where

CO PTH =

h1 −h4 h2−h1

CO PTH ( h 2−h 1)=h1−h 4 CO PTH ( Δh 2− Δh 1 )+ΔCO P TH (h 2−h 1 )= ( Δh1−Δh 4 ) CO P TH ( Δh 2− Δh 1 )+ ΔCO PTH (h 2−h 1 ) ( Δh 1− Δh 4 ) = h 1−h 4 CO PTH ( h 2−h1 ) CO P TH ( Δh 2− Δh 1 ) ΔCO PTH (h 2−h1 ) ( Δh1−Δh 4 ) + = CO PTH ( h 2−h 1) CO PTH (h 2−h 1 ) h1−h 4

( Δh 2− Δh 1 ) ΔCO PTH ( h 2−h1) ( Δh1−Δh 4 ) + = h1−h 4 ( h 2−h 1 ) CO PTH ( h 2−h1)

( Δh 2− Δh 1 ) ΔCO PTH ( Δh1−Δh 4 ) + = h1−h 4 ( h 2−h 1) CO PTH

[|

||

ΔCO PTH −( Δh2− Δh1) = Δh 1−Δh 4 + CO PTH h 1−h 4 (h 2−h1) ΔCO PTH =CO PTH

|]

4 −( Δh2− Δh1) + |[ Δh1−Δh h1−h 4 | | (h 2−h 1) |]

Discussion From analysing Table 1, the COPInd in theory should be the closest to the COPTh, As it includes the calculation of friction and (Pind) meaning its more detailed and not as simplified.COPov is the performance between the power that is measured. COPcomp only models the compressor. COPth, consists of four variables that are calculated and Pind in the COPind has more variables that would be calculated , as well as the denominator being difference in values. The value that should be given out t customers is COPov as this shows the customer the performance as a ratio of the power input. Shows the consumer how much energy is being used, Furthermore, it demonstrates the least error when calculated. The mechanical efficiencies calculated were 0.905 and 0.899 for test 1 and test 2 respectivaley. Knowing that a typical value for overall efficiency of a well lubricated compressor of this type would be 60% and this mechanical efficiency is 90% suggesting it was well lubricated Lose of heat due to friction could also account for some discrepancies Which leads on to another error, of human error when calculating and looking at graphs/tables, could have led to inaccurate interpretations. In addition, there were also fluctuations in the evaporator power, which is one of the major measured values. The upper and lower bound had to be looked at to make an estimate. And making estimates is not a precise way of providing data, as there could be human error, when reading it as well as precision error. Between Test 1 and 2 although there are very values input e.g. evaporator pressure – they both have similar efficiencies showing that they follow the same pattern. Making this experiment valid, and accurate.

Conclusion In conclusion, although subject to errors, the results obtained were a reliable set. As the predicted mechanical efficiency was 90%, and results calculated were very close to this. This experiment was successful. But to obtain even more reliable, and more tests should be done, to check if the compressor is as efficient as well as the mechanical efficiency.

Appendix SERIES

TEST No.

1

2

Error estimate

Condenser pressure (abs.)

pc/kN m-2

900

900

±25

Evaporator pressure (abs.)

pe/kN m-2

240

400

±12.5

Compressor suction

t1/C

-1.4

12.5

±0.1

Compressor delivery

t2/C

46.8

55.7

±0.1

Liquid leaving condenser

t3/C

31.3

32.1

±0.1

Evaporator inlet

t4/C

-2.2

9.5

±0.1

Water inlet

t 5/C

20.2

19.6

±0.1

Water outlet

t6/C

32.5

30

±0.1

Water flow rate

_w/g s-1

20

35

±2.5

R134a flow rate

_r/g s-1

4.5

7.5

±0.25

Evaporator Power

Pe/W

689

1213

±10

Pc/W

508

509

±10

Spring Balance

F/N

11

11

±0.25

Compressor speed

nc/rpm

785

782

±1

Motor speed (nm = nc x pulley ratio) nc: rpm of compressor pulley Pulley ratio = 3.08

nm/rpm

2418

2409

±1

Motor Power I (amp)⸱V(volt)...