Air Conditioning Lab Report 1920 PDF

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An experiment in Air conditioning...

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2019/2020 Air Conditioning Name : Suren Huan Feng A/L Anpalagam Student ID : 20095489 Module : MMME2007 Thermodynamics & Fluid Mechanics II Module : Prof. Yousif Abdalla Abakr Convenor Date : 11th December 2019 The University of Nottingham Malaysia, Department of Mechanical, Materials and Manufacturing Engineering

Contents Summary......................................................................................................................3 Background Theory.........................................................................................................4 Recorded Data............................................................................................................6 Derived & Calculated Results..........................................................................................9 Discussion...................................................................................................................11 Conclusion..................................................................................................................12 Appendix....................................................................................................................13 Calculations..............................................................................................................13 Psychometric Chart.....................................................................................................14 R134a P-h Diagram.....................................................................................................16 Reference................................................................................................................17

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Summary The experiment aims to create an understanding of air conditioning processes and the indicators of moisture and heat content of the environmental air. Besides, the experiment allows students to learn and calculate the required heat exchanges that will achieve the required air condition. Surrounding air will be channelled through the air conditioner and the temperature of four stations labelled A, B, C and D are measured and tabulated. Each station has two thermometers, a wet bulb thermometer and a dry bulb thermometer. Temperature at station A is the temperature of the inlet air (TwA=23.5°C,TdA=27.0°C), temperature at station B is the temperature of air passing through the steam injector and pre heater (TwB=27.5°C,TdB=35.5°C), temperature at station C is the temperature of air after passing through the evaporation process of the air conditioner (TwC=23.5°C,TdC=24.0°C), and temperature at station D is the temperature of the treated air (TwD=24°C,TdD=35.5°C). These values of wet and dry bulb temperature of the air enables the students to determine the relative and specific humidity, specific enthalpy, specific volume of the air via referencing the Psychometric chart. Mass flow rates of air can be calculated using the derived values (0.1192 kgs-1). These mass flow rate values will be then used to calculate the heat exchanges occurring in each station. A p-h diagram of the refrigerant (R134A) is used alongside the obtained values in order to plot the refrigerant cycle. Assumptions include a steady flow rate where differences in kinetic energy and potential energy are neglected.

Possible improvements to experimental procedure: 1) Digital thermometers should replace the analogue thermometers that were used as temperature readings can be taken within a shorter time span with a higher accuracy and parallax error can be avoided altogether. 2) Pressure gauge with a smaller range should be used as it will be more sensitive to pressure change. Usage of digital pressure gauge can further improve the sensitivity and accuracy of the pressure measurements obtained.

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Background Theory Air conditioning is a mechanism where the temperature and humidity of a controlled space is altered to satisfy the comfort of humans by removing heat. In a commercial air conditioner, initially there is work input from fan to increase the speed of air flow followed by cooling of air to dew point for condensation to occur in order to reduce air humidity. The overly-cooled air is then heated to the required temperature. In the air conditioning lab experiment, work is done to the air via fan work (station A), moisture and is added to the air (station B), air is heated (station C) and air is cooled (station D).

Figure 1: Schematic Diagram for Air Conditioning Unit

During the cooling, the R134a refrigerant goes through a cyclic process where it absorbs heat from the air in the evaporator and rejects heat back to the environment in the condenser. The refrigerant is expanded (liquid to vapour) when entering the evaporator via the expansion valve which has a very small tiny hole as outlet. This expansion reduces both the pressure and temperature of the refrigerant and thus is ideal to absorb heat from the air. The refrigerant leaves the evaporator with a raised temperature and low pressure. The cold air is blown into the room and the temperature of the room is regulated. The refrigerant will then enter the compressor which will increase the pressure as well as the temperature of the refrigerant (vapour to liquid). The refrigerant will enter the condenser where the heat will be rejected to the surrounding aided by a fan before returning back to the expansion valve. 4

Mass Flow Rate Balance & Energy Balance: Law of Conservation of Mass and Law of Conservation of Energy is used to analyse the air conditioning experiment. The power input to the air conditioner should be equivalent to the power output from the air conditioner. The sum of the energy of the condensed water and the energy of the air cooling the room must balance out the sum of the energy of air sucked in, energy absorbed and energy released by the R134a refrigerant during refrigerant cycle. As the flow is assumed to be in steady state, the rate of mass flow of dry air must be equal at both the inlet and outlet. The rate of vapour mass flow at the inlet must be equal to the sum of the rate of vapour mass flow at the outlet and the rate of flow of condensed water.

Dry air:

m a at B =¿ m a´at C ´¿

Water and vapour: Energy balance:

m´ a ωB =m´ a ωC + m´ w m´ a hB = m´ a h C + m´ w h w + H B−C

Possible factors leading to inaccurate energy and mass balance:  Heat losses of refrigerant in pipes are not accounted for  Taking temperature and pressure values and mass of condensed water over a span of time  Rate of mass flow at inlet and outlet not necessarily constant which defies the assumption of steady flow rate Experimental Results

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Recorded Data Table 1. Wet and Dry bulb temperatures at station D to visualize system stability Expected time

Actual time

T dD

T wD

0min

15:28

23.0

18.0

5 min

15:33

29.5

18.5

10 min

15:38

31.0

19.0

15 min

15:43

32.5

21.0

20 min

15:48

33.0

22.5

25 min

15:53

35.5

24.0

30 min

15:58

35.5

24.0

Temperature / ℃

Figure Graph 2: Graph of Dry Bulb and Wet Bulb

of Temperature of Dry Bulb and Wet Bulb against Time 40 35 30 25 20 15 10 5 0

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Time / s

Temperature against Time

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Table 2. Parameters and physical quantities associated with experiment Atmospheric pressure

Patm [bar]

1.0

Pre-heater heat input

´ [KW] Q p

1.0

´b Q

2.0

Water boiler heat input

[KW]

Table 3. Differential pressure & temperature data for the calculation of mass flow of air in system Orifice Differential pressure

Δ z [ mm]

5.00

Wet bulb Temperature of the air at D

T wD [˚ C]

24.50

Dry bulb Temperature of the air at D

T dD [˚ C ]

35.50

Uncertainty in

Δ z [ mm]

Uncertainty in

T wD [˚ C]

Uncertainty in

T dD [˚ C ]

0.05 0.25 0.25

Table 4. Temperature readings for the calculation of water loss and enthalpy decrease through air conditioner

A

B

C

Wet bulb Temperature of the air at A Dry bulb Temperature of the air at A Wet bulb Temperature of the air at B Dry bulb Temperature of the air at B Wet bulb Temperature of the air at C Dry bulb Temperature of the air at C

T wA [˚ C]

23.50

T dA [ ˚ C ]

27.00

T wB [ ˚ C]

27.50

T dB [˚ C ]

35.50

T wC [ ˚ C ]

23.50

T dC [˚ C]

24.00

Uncertainty in

T wA [˚ C]

Uncertainty in

T dA [ ˚ C ] Uncertainty in

T wB [ ˚ C] Uncertainty in

T dB [˚ C ] Uncertainty in

T wC [ ˚ C ] Uncertainty in

T dC [˚ C]

0.25 0.25 0.25 0.25 0.25 0.25

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Table 5. Temperature, pressure, mass flow rate, time and collected mass readings for the calculation of quantities linked with the refrigerant cycle. superheated refrigerant R134a vapour leaving evaporator

T 1 [ ˚ C]

Temperature

P1 [kN/m2]

Pressure

T 2 [ ˚ C]

Temperature R134a after compressor R134a before expansion valve, at the high pressure R134a mass flow rate Mass of condensate collected Condensate collection time

P2 [kN/m2]

Pressure

T 3 [˚ C]

Temperature

P3 [kN/m2]

Pressure

m ´ r [g/s] mc

[g]

t [s]

15.00 300.0 76.00 1200 46.00 1200 16.00

Uncertainty in T 1 [˚C] Uncertainty in P1 [kN/m2] Uncertainty in T 2 [˚C] Uncertainty in P2 [kN/m2] Uncertainty in T 3 [˚C] Uncertainty in P3 [kN/m2] Uncertainty in m ´ r [g/s]

0.25 12.5 0.25 25 0.25 25 0.05

135.0

Uncertainty in m c [g]

2.5

600.000

Uncertainty in t [s]

0.005

Table 6. Water extracted from air vs. condensate collected Mass of condensate collected after system stability Condensate collection time t (s)

m (g)

115 1800

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Derived & Calculated Results Table 3.1 Mass flow of air in system Specific volume of the air at station D. Air mass flow rate (note: orifice pressure is in m)

ν D [ m 3 /kg] ´ a=1.594 m

√

0.8943

Δz νD

0.1192

[ kg/ s]

Uncertainty in 3

ν D [ kg / m ] Uncertainty in m ´ a [ kg /s] (see equation (3))

0.0005 0.0185

Table 4.1 Calculation of water loss and enthalpy decrease through air conditioner A B C

Specific humidity at A from chart Specific humidity at B from chart Specific humidity at C from chart Mass of water lost

ω A [kg/kg]

0.0168

ω B [kg/kg]

0.0200

ωC

0.0181

[kg/kg]

m ´ w =¿ m ´ a ( ω B− ω C )

1.0728 x 10-4

[kg/s] Enthalpy of air at A from chart Enthalpy of air at B from chart Enthalpy of air at C from chart Total Cooling of air across air-conditioner

hA

[kJ/kg]

70.0373

hB

[kJ/kg]

86.9312

hC

[kJ/kg]

70.16

Q B −C =¿ m ´ a hC+ m ´ w hw ] m ´ a hB −¿

Q latent =¿ m ´ w hfg [W]

Uncertainty in m ´ w [kg/s] Uncertainty in h A [J/kg] Uncertainty in h B [J/kg] Uncertainty in hC [J/kg]

0.0001 0.0001 0.0001 5.8175 x 10-5 250 250 250

1733.4362

Uncertainty in Q B −C [W] (see equation (6))

427.63

264.4988

Uncertainty in Q Latent [W]

143.09

[W] Latent Cooling of air across air-conditioner

Uncertainty in ω A [kg/kg] Uncertainty in ω B [kg/kg] Uncertainty in ω B [kg/kg]

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Table 5.1 Calculation of quantities linked with the refrigerant cycle. superheated refrigerant R134a vapour leaving evaporator R134a after compressor R134a before expansion valve, at the high pressure Refrigerant Specific volume leaving evaporator Volumetric efficiency of compressor

Specific Enthalpy from chart h1 [kJ/kg]

312.5

Specific Enthalpy from chart h2 [kJ/kg]

355

Uncertainty in h2

[J/kg]

2.5

Specific Enthalpy from chart h3 [kJ/kg]

165

Uncertainty in h3

[J/kg]

2.5

ν1 ´r ηvol = m

[ ] m3 kg

ν1 V swept

X 100 %

0.075

Uncertainty in

Uncertainty in

h1 [kJ/kg]

ν1

[ ] m3 kg

2.5

0.005

Compressor swept volume 95.7%

−3

3

V swept =1.254 x 10 m / s

1.254x10-3

Table 6.1 Water extracted from air vs. condensate collected Rate of water extracted from air during collection time m ´ w = m /t (g/s)

0.0639

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Discussion In the first part of the experiment, the system was going through stabilization. The readings of temperature of both the dry bulb and wet bulb at station D were measured and recorded as in Table 1. The readings were taken for every 5 minutes till the temperature of both type of thermometers were about constant without much fluctuation. This is to ensure the condition inside the air conditioning unit is stable and is suitable to be used. The air conditioning unit became stable after 20 minutes (5 readings) as shown in Figure 1. Heat is removed from the air via the evaporator section of the refrigeration cycle. The refrigerant is at low pressure and low temperature in gaseous form at the evaporator and the temperature difference between the air and the refrigerant allows the heat transfer from air to the refrigerant. Higher surface area of refrigerant when in gaseous form accelerates the heat transfer even further. The temperature of air reaches the dew point and thus condensation of vapour occurs which reduces the humidity of the air. The heat gained by the refrigerant is later rejected to the environment in the condenser section.

The energy gained by the refrigeration cycle can be obtained from the Pressure – Enthalpy chart of the refrigerant R134a. The power gained in the evaporator by the refrigerant is equal to the product ´ r (11 g/s) and the difference in enthalpy between position 1 (315 of the rate of refrigerant mass flow, m kJ/kg) and 4 (160 kJ/kg). Thus, the energy gained by the refrigeration cycle in the evaporator is 1705 W. The energy lost by the air is obtained by calculating the enthalpy difference of air between station B and C where cooling occurs and multiplying it by the rate of air mass flow. The energy lost via collected condensate was also added. The energy lost is 2817.86 W as shown in Table 4. The rate of water extracted from air is 0.0167 g/s as shown in Table 6. Via calculation, the rate of water mass lost was found to be 0.3711 g/s as shown in Table 4. For both comparisons, the calculated values and the measured values have very large differences which would have been caused by multiple errors during conducting the experiment. 11

Human error includes parallax error while taking temperature readings from the thermometer and pressure readings from manometer. The time gap between reading and recording different data values adds up to the error. The range of measuring instruments are quite big and thus less sensitive to changes besides having lesser accuracy of measurement. When measuring the temperature of the refrigerant, the thermometer is placed on the copper tube which encloses the refrigerant and thus, the exact temperature of the refrigerant cannot be measured. The values obtained from the charts; Psychrometric Chart and Pressure-Enthalpy Chart includes the biggest uncertainties to the calculations. If the measurements were biased, the results obtained would be deviated even further from the actual results but will be only deviated in one direction.

Conclusion The results obtained were not reliable due to large number of errors and uncertainties. However, it can be concluded that the main objective of the experiment was achieved and a good understanding of the air conditioning system, refrigeration cycle as well as heat transfer was obtained. The refrigeration cycle plotted in the Pressure-Enthalpy Chart from experimental data in Table 5 corresponds to that of an actual cycle.

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Appendix Calculations The uncertainty of an analogue device is obtained by halving the smallest division in the scale.  To calculate air mass flow rate, ṁa=1.594

√ √

ṁa :

Δz ʋD −3

5 ×10 0.8 943 −1 ṁa=0.1192 kg s ṁa=1.594

 To calculate the uncertainty in the air mass flow rate,

√(

ϵ ṁa=

√(

ϵ ṁa =

)

2

( √ ) ) ( )( 2

2 1.594 1.594 ( ϵΔz ) + 2 √ Δz ʋD 2

1.594

2

−3 2 √ ( 5 ×10 )( 0.8943 )

ϵ ṁa=0.0185 kg s

ϵ ṁa :

2 Δz (ϵ ʋD ) 3 ʋD 2

0.05 ×10−3 +

√

)

2

1.594 5 ×10−3 ( 0.0005)2 3 2 (0. 8943)

−1

 To calculate the mass lost: ṁw=ṁ a (ω B−ω C ) ṁw=( 0.1 192)(0.0138−0.0108 ) −4 −1 ṁw =3.711 × 10 kg s  To calculate the uncertainty in the mass loss, ϵ ṁw :

√

2

2

2

ϵ ṁw= ( ωC−ωB ) (ϵ ṁa )2+ ( ṁa ) (ϵ ω C )2 +( −ṁ a ) (ϵ ω B )2 2 2 2 2 2 2 ϵ ṁw =√ (0.0138 −0.0108 ) (0.000585 ) + ( 0.1237 ) (0.0001) + (−0.1237 ) (0.0001) ϵ ṁw=1.758 x 10−5 kg s−1

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Psychometric Chart Calculation of Specific Humidity and Enthalpy of Air at Station A

Calculation of Specific Humidity and Enthalpy of Air at Station B

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Calculation of Specific Humidity and Enthalpy of Air at Station C

Calculation of Specific Humidity and Enthalpy of Air at Station D

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R134a P-h Diagram Specific Enthalpy of Refrigerant at Different Positions in the Refrigeration

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Reference   

https://www.servicechampions.net/blog/air-conditioner-works-refrigeration-cycle/ https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1228&context=icec Thermodynamics-an engineering approach by Cengal and Boles (McGraw & Hill) (ISBN 0071152474)



https://www.google.com/search? q=air+conditioner+cycle&rlz=1C1AZAA_enMY823MY823&source=lnms&tbm=isch&sa=X&ved=0ahUK Ewi7vK7qwNDeAhUXeysKHfdhCqIQ_AUIDigB&biw=1600&bih=789#imgrc=mSEMnWFaLuTbAM:



https://www.mobileair.com/tools/refrigeration-cycle-how-air-conditioner-works

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