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HVAC Practical Report...

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HVAC LABORATORY EXPERIMENT by

MOHAMMED VARACHIA 201470930 A practical report submitted to the Faculty of Engineering and the Built Environment in partial fulfilment of the requirements for the degree of BACCALAUREUS INGENERIAE in MECHANICAL ENGINEERING at the UNIVERSITY OF JOHANNESBURG

TML4B21 October 2018

Declaration I, Mohammed Varachia, hereby declare that this mini-dissertation is wholly my own work and has not been submitted anywhere else for academic credit either by myself or another person. I understand what plagiarism implies and declare that this mini-dissertation is my own ideas, words, phrases, arguments, graphics, figures, results and organisation except where reference is explicitly made to another’s work. I understand further that any unethical academic behaviour, which includes plagiarism, is seen in a serious light by the University of Johannesburg and is punishable by disciplinary action.

Signature ........................................

Date .......................................

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Abstract This report describes the experimental analysis of a single zone air condition system which must be analysed for cooling load capacity. A relevant literature study is conducted and set out in Chapter 2. The experiment was conducted for two cases namely: Case 1 in which inlet and outlet valves were open allowing for no air mixing, Case 2 in which the inlet valve was closed and the outlet valve was open releasing conditioned air to the ambient. For Case 1: the psychrometric analysis showed that a total capacity of 127.48 W is achieved while the calculated analysis showed a capacity of 130.23 W is achieved. These results are relatively close with only a 2% deviation. For Case 2: The psychrometric analysis showed that a total capacity of 402.55 W is now achieved while the calculated analysis showed that a capacity of 401.32 W is achieved. These results have only an 0.3% deviation from each other. It was determined that the recirculating air from the closed loop system provided a better cooling capacity from the coil, this is due to the air being cooler and thus a greater cooling effect, for the constant input power and mass flow of refrigerant supplied to the coil. This result was also achieved due to the fact that for Case 1 the outside air was significantly dry and thus required more load from coil, this reduced the cooling capacity whereas the recirculated air had a better humidity allowing for a better cooling effect.

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Table of Contents Abstract ..................................................................................................................................... II List of Figures ...........................................................................................................................IV List of Tables .............................................................................................................................. V 1

2

3

4

5

Introduction .......................................................................................................................1 1.1

Background .................................................................................................................1

1.2

Aim .............................................................................................................................. 1

1.3

Objectives .................................................................................................................... 1

Literature review ................................................................................................................ 3 2.1

Refrigeration Cycle ..................................................................................................... 3

2.2

The Psychrometric Chart............................................................................................. 4

2.3

Comfort zone ............................................................................................................... 6

Experimental setup ............................................................................................................ 7 3.1

Apparatus used ............................................................................................................ 7

3.2

Experimental design .................................................................................................... 9

3.3

Experimental procedure .............................................................................................. 9

Results and Discussion .....................................................................................................11 4.1

Case 1: Inlet and Outlet Open ................................................................................... 11

4.2

Case 2: Inlet Closed and Outlet Open ....................................................................... 16

4.3

Discussion of results.................................................................................................. 20

Conclusion ........................................................................................................................ 23 5.1

Recommendations ..................................................................................................... 23

References ................................................................................................................................ 24 Appendix A: Psychrometric charts ........................................................................................... 26

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List of Figures Figure 2.1.1: Principle of operation for vapour-compression refrigeration cycle [5] ................ 3 Figure 2.1.2: Vapour-compression cycle [6] ............................................................................. 4 Figure 2.2.1: Properties and line identification on a psychrometric chart [9] ...........................5 Figure 3.1.1: HVAC test system ................................................................................................ 7 Figure 3.1.2: Experimental setup 1 ............................................................................................ 8 Figure 3.1.3: Experimental setup 2 ............................................................................................ 8 Figure 3.3.1: R134a Refrigerant used ...................................................................................... 10 Figure 3.3.2: Close-up of the cooling coil ............................................................................... 10

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List of Tables Table 3.2.1: Experimental matrix .............................................................................................. 9 Table 4.1.1: Obtained results ................................................................................................... 11 Table 4.2.1: Obtained results ................................................................................................... 16 Table 4.3.1: Measured parameters for Case 1.......................................................................... 21 Table 4.3.2: Measured parameters for Case 2.......................................................................... 21

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1 Introduction 1.1 Background Heating, ventilation and air conditioning (HVAC) systems are responsible for maintaining the internal environment of buildings by controlling the cooling, heating, humidifying and ventilation. Homes, businesses and schools often require an air conditioning system to achieve the desired environment for both comfort and good health [1]. Within a working environment, the primary goal of the HVAC system is to provide optimal conditions to ensure human comfort, health and safety. If a working environment possesses an uncomfortable setting for its occupants, the discomfort felt tends to hinder the performance of these occupants. Therefore, it is necessary for the HVAC system to control the indoor environment in a work space and provide thermal comfort to the occupants to ensure good health and productivity [2]. The basic functions of an HVAC system include maintaining acceptable air quality, temperature and humidity within the space. The air quality is kept clean by utilising filters which prevent particles from outside air entering the conditioned space. The temperature within the space to be controlled is altered by the HVAC system through its heating and cooling capability. The relative humidity of the environment is also a major factor in the thermal comfort experienced by occupants of a building, the humidity increases how warm an environment feels and must therefore also be controlled. Humidity is controlled by an HVAC system by controlling the movement as well as the distribution of air within the space [3], [4].

1.2 Aim The aim of this practical is to analyse a simple air conditioning system by determining the enthalpy values at the “on-coil” and “off-coil” conditions from temperature and relative humidity at these points. The enthalpy values need to be determined from the psychrometric chart and also via the relevant formulae to obtain the total cooling capacity of the coil. These cooling capacities are then compared to determine if the calculations are correct.

1.3 Objectives The following objectives are expected to be achieved at the end of this report:  

A literature study on the relevant theoretical concepts for this practical is conducted. Perform the experiment in accordance to the experimental method and matrix.

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Plot the obtained temperatures and relative humidity values on the psychrometric chart to determine the enthalpies for each state (“on-coil” and “off-coil”).  Obtain the mass flow of air from the volumetric flow of air over the coil.  Determine the enthalpy for state points from first principles using the “on-coil” and “off-coil” conditions.  Confirm that the total capacity using the enthalpy method is equivalent to the sensible and latent contribution. 

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2 Literature review 2.1 Refrigeration Cycle Air conditioners using the refrigeration cycle need to be economical and therefore use the refrigerant repeatedly in the system by means of a closed circuit operation. The refrigeration cycle is used to lower the inlet air temperatures and decrease the relative humidity of the incoming air. Figure 2.1.1 below shows the working of a refrigeration cycle [5].

Figure 2.1.1: Principle of operation for vapour-compression refrigeration cycle [5]

Refrigeration by the cycle is accomplished as a result of heat rejection and absorption properties of the refrigerant as given below: 

Liquids absorb heat when a phase change from liquid to gas occurs



Gases reject heat when a phase change from gas to liquid occurs

As mentioned above, refrigerant in the cycle is reused and thus air conditioning systems using the refrigeration cycle employ this closed cycle of compression, condensation, expansion and evaporation. This refrigerant is used as a heat transfer medium to move heat from space to cool said space and reject this heat to another area [5].

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Figure 2.1.2: Vapour-compression cycle [6]

A brief summary of the processes in the refrigeration cycle shown in Figure 2.1.2 is given below [6]: 

State 1 - The refrigerant enters the compressor as a low-pressure gas. It is then compressed and moves out of the compressor as a high pressure gas.



State 2 – The high-pressure gas then enters the condenser. The condenser changes the phase from a high pressure gas to a high pressure liquid, ideally as an isobaric process, and heat is given off by the refrigerant.



State 3 - The cooled high pressure liquid then enters the expansion device which acts as a restriction in the system, this causes the refrigerant to experience a pressure drop and exit the device at a low pressure. The pressure reduction results in a flash evaporation of part of the refrigerant, lowering temperature of the refrigerant.



State 4 - The low-pressure refrigerant now enters the evaporator. Here it absorbs the heat from the conditioned space (cold region). The refrigerant experiences another phase change from liquid to a gas.



The low-pressure refrigerant gas moves into the compressor where the cycle repeats itself.

2.2 The Psychrometric Chart Psychrometric charts are graphical representations of the psychrometric properties of air and are given for a certain altitude. These charts are useful tools to analyse psychrometric processes 4|Page

and are used to determine the relevant properties of moist air at constant pressure. The properties of air that may be obtained from a psychrometric chart are the dry-bulb and wetbulb temperature, enthalpy, specific volume, relative humidity, humidity ratio and the sensible heat ratio. Figure 2.2.1 below shows how the properties may be obtained from a psychrometric chart and what the relevant lines represent [7], [8].

Figure 2.2.1: Properties and line identification on a psychrometric chart [9]

The properties that can be obtained from a psychrometric chart are discussed below [10]: 

Dry-bulb temperature – This a measure of the temperature when the sensing bulb of



Wet-bulb temperature – This a measure of the temperature when the sensing bulb of

a thermometer has no moisture present.

the thermometer is covered with a wet cloth or cotton wick and is exposed to air velocities greater than 3m/s. 

Enthalpy – This is the total heat energy of the moist air. It is the sum of the heat in dry air and the heat in the moisture of the air.



Specific volume – The volume of air per unit mass of air.



Relative humidity – Expresses the amount of moisture in air with respect to the maximum amount of moisture that the air can contain at a particular temperature.

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

Humidity ratio – Indicates the mass content of moisture present and is given by the ratio of the mass of water vapour to the mass of dry air.

2.3 Comfort zone HVAC systems have a primary aim of providing thermal comfort to occupants in the conditioned space. There is a specific range of temperatures and humidity for which humans experience thermal comfort and any drastic deviations from this specified zone results in discomfort for the occupants. The elements for human comfort are fresh air, moderate temperatures, moderate humidity and moderate lighting [11]. The minimum level of relative humidity for human comfort is 20% and values below this result in dry nasal passages, dry mouth and skin. The maximum relative humidity comfort level varies with season. In summer, it is best for the relative humidity to be kept below 60% as the need to expel heat from the body is more important. In winter, the need to expel heat is less important and thus a relative humidity of 80% is tolerable. In general, high humidity in a zone results in the human body losing moisture at a slower rate, leading to occupants retaining heat and thus feeling warmer than normal whereas low humidity results in the human body losing moisture at a faster rate which in turn makes the occupants feel colder [11], [12]. According to ISO 7730 or ASHRAE 55, the thermal comfort zone for humans is when the

operative environment is maintained at temperatures between 20 ℃ and 23.5℃ in winter and 22.5℃ to 26℃ in summer with a relative humidity between 30% to 60% [13].

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3 Experimental setup 3.1 Apparatus used The experiment was conducted in a single zone HVAC system which includes:       

Cooling coil “Push through” circular duct fan Orifice plate Testo 425 Hot Wire Anemometer Refrigerant R134-a Ducts Manoair 500 Humidity sensor

Figure 3.1.1 below shows a schematic representation of the experimental system.

Figure 3.1.1: HVAC test system

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Orifice Pressure Gauges

Orifice Plate 1 Circular Duct Fan

Outside Air Intake Valve

Cooling Coil

Figure 3.1.2: Experimental setup 1

Discharge Valve

Refrigerant Lines Orifice Plate 2

ManoAir 500 Humidity Sensor Cooling Coil

Testo 425 Figure 3.1.3: Experimental setup 2

Anemometer

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3.2 Experimental design Constant values:    

Ambient temperature Ambient pressure Air mass flow rate Refrigerant mass flow rate

Dependent variables:  

Pressure drop Outlet relative humidity and temperature

Independent variables:  

Apparatus due point temperature Humidity supplied

Experimental matrix: Table 3.2.1: Experimental matrix

Position of CASE

Temperature (°C)

Relative humidity

measurement

(%)

1) Inlet open Outlet open

Inlet

T1

Φ1

Outlet

T2

Φ2

2) Inlet closed Outlet open

Inlet

T3

Φ3

Outlet

T4

Φ4

3.3 Experimental procedure 



Case 1 o The inlet valve and outlet valve were opened. o The fan was switched on and allowed to run for a small period of time to ensure the system reached stability o The pressure gauge readings for the differential pressure across each orifice plate was recorded. o Relative humidity and temperature values were measured at the inlet and outlet positions before the cooling coil and after the cooling coil Case 2 o The outlet valve was left open and inlet valve closed. o The system was left running for a short period of time to allow the system to stabilise once again. 9|Page

o The pressure gauge readings for the differential pressure across each orifice plate was recorded. o Relative humidity and temperature values were measured at the inlet and outlet positions again.

Figure 3.3.1: R134a Refrigerant used

Figure 3.3.2: Close-up of the cooling coil

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4 Results and Discussion

The results obtained for the two cases are as follows are given next. It is noted that all ℎ𝑣 values are obtained from the ASHRAE Steam Tables at the respective temperatures. The density of air is taken as 0.975 𝑘𝑔/𝑚3 . The psychrometric chart used is for an altitude of 1700m above sea level.

4.1 Case 1: Inlet and Outlet Open Table 4.1.1 below shows the results obtained for Case i: Inlet open and Outlet open. The pressure drop was observed to be 200 Pa at the outlet and 10 Pa at the inlet. Table 4.1.1: Obtained results

CASE

Position of measurement 1. Inlet

1) Inlet open Outlet open

Temperature (°C)

2. Outlet

23.5

Relative humidity (%) 28.9

17.3

52

Using the inlet and outlet values as the “on coil” and “off coil” conditions respectively, the enthalpy values from the psychrometric chart in Appendix A are found to be:   

kJ

On-coil: ℎ1 = 40.1 kg

kJ

Off-coil: ℎ2 = 37.6 kg

ℎ𝑥 = 44.2

𝑘𝑗 𝑘𝑔

𝑄 = 0.00258 𝑥 √

2𝑆𝑃𝑜𝑟𝑜𝑓𝑖𝑐𝑒 𝜌

𝑄 = 0.00258 𝑥 √ 𝑄 ...