Trabalho de Ar condicionado

Monash University

CHE2162: Air Conditioning Lab Report

Group 16

Thomas Parker, Laura De Rango, Arthur Parreira Silva Medeiros and Yasiru Karunaratne

9/12/2014

Laura, Tom, Arthur and Yasiru

Contents1. Executive Summary:.................................................................................................................... 3

2. Introduction:.................................................................................................................................. 3

2.1. Background Information........................................................................................................... 3

2.2. Theory.............................................................................................................................................. 3

2.3. Scope and Motivation for study...............................................................................................3

3. Aims:................................................................................................................................................. 4

4. Experimental Work:.................................................................................................................... 4

4.1. Experimental Apparatus............................................................................................................4

4.1.1 HDL Software................................................................................................................................. 5

4.1.2 Thermocouples............................................................................................................................. 5

4.1.3 Fan..................................................................................................................................................... 5

4.1.4 Humidification System............................................................................................................... 5

4.1.5 Pre-heaters and Re-heaters...................................................................................................... 6

4.1.6 Cooling System.............................................................................................................................. 6

4.1.7 Air Differential Pressure............................................................................................................7

4.1.8 Exhaust and Volume Controller...............................................................................................8

4.2 Experimental Procedure............................................................................................................8

4.2.1 Safety [Monash University, AC Lab Manual 2014].............................................................8

4.2.2 Procedure........................................................................................................................................ 9

5. Results:............................................................................................................................................. 9

6. Mass and Energy Balance Calculations:..............................................................................12

6.1. In-duct Coefficient Calculation:.............................................................................................12

6.2. Mass Balance Calculations:.....................................................................................................13

6.2.1. Human Comfort Condition......................................................................................................13

6.2.2. Maturing Cheese Condition.................................................................................................... 15

6.3. Energy Balance Calculations:.................................................................................................16

6.3.1. Human Condition:...................................................................................................................... 16

6.3.2. Maturing Cheese:........................................................................................................................ 16

7. Discussion:................................................................................................................................... 17

7.1. Achievement of Aim:................................................................................................................. 17

7.2. Mass Balance Calculations:.....................................................................................................17

7.3 Energy Balance Calculations:...........................................................................................................18

8. Conclusion:................................................................................................................................... 19

1 | P a g e


9. References.................................................................................................................................... 19

10. Appendix....................................................................................................................................... 21

2 | P a g e


1. Executive Summary:

2. Introduction:2.1. Background Information

Air conditioning is used to control and regulate many aspects of a local atmosphere in order to hold the ideal conditions constant for a particular purpose. These conditions range from the temperature of the room to the humidity, as well as the air circulation speed. The techniques used for controlling these conditions are able to be altered to maintain conditions for a wide range of purposes.

2.2. TheoryThe mechanism of an air conditioner is similar to that of a refrigerator, with the most significant difference being the air conditioner relies on the walls of a house or system to keep the heat locked in [1].The air conditioning process relies on the condensation and evaporation of a special chemical (refrigerant) to cool natural air and moderate its temperature. These changes of phase processes occurring in the refrigerant, cause a gain/loss of energy from or to the air as it passes through the ducts and closed cycle. This process forms the underlying principles of air conditioning [1].

In this experiment numerous control switches are turned on/off in order to achieve a specified temperature and humidity at a specific location (Thermocouple 8/7: wet and dry bulb respectively). The first component of the cycle that could be altered was the fan speed, which dictates the speed at which the air is sucked into the system. To increase/decrease the humidity level there are 3 boilers available to be turned on or off. To increase the temperature of the humidified air, are two pre-heaters present which may be turned on if necessary. Next are two re-heaters available after the evaporator, to heat the cooled air. The last component that can be turned on/off is the compressor, which condenses the water in humidified air and cools the air.

To assist in reaching the objectives of this practical the Psychrometric chart is utilized. This requires a ‘wet’ and ‘dry’ bulb temperature, and from this the relative humidity and mass of water per mass of dry air can be determined.

2.3. Scope and Motivation for study

There are many factors that affect the ideal conditions for a certain process in a local atmosphere. However, the scope of this research is limited to studying the effect on the following variables.

The wet bulb temperature The dry bulb temperature Enthalpy of the system Relative humidity of the system.

3 | P a g e


These four variables were chosen as each of them impact where the system lies on the Psychrometric chart and whether or not it lies in the specified ideal range.

Altering just one of any of the variables can completely change the characteristics of the environment.

Looking at the Psychrometric Chart, if dry bulb temperature is held constant, we see that increasing the relative humidity of the system increases the wet bulb temperature. We can also see by looking at this chart that decreasing the relative humidity also decreases the wet bulb temperature. If the air is saturated (100% relative humidity), the dry and wet bulb temperatures are equal (textbook). It is the relationship between these factors and how they can be controlled to produce an environment that is required for a particular purpose that will be studied in this experiment.

3. Aims:

The aim of this laboratory was to achieve two specific steady state operation conditions.

The first of these aims were to achieve a steady temperature between 20 and 25°C and relative humidity of 40-50% to be suited to the purpose of human comfort. For this system, the recycle rate of air was set to 75%.

The second of these aims was again, to achieve a steady temperature but this time between 10 and 15°C and relative humidity of 70-80% to be suitable for the purpose of maturing cheese. For this system, the recycle rate of air was set to 25%.

4. Experimental Work:4.1. Experimental Apparatus

Figure 1. Illustrates a schematic of the experimental apparatus

4 | P a g e


Figure 1: Schematic diagram of the air-conditioning system (Department of Chemical Engineering, Monash University,2014).

4.1.1 HDL Software

Software provided by P.A. Hilton Ltd is responsible for the whole acquisition of data for the experiment. For instance, it records the temperature assessed by each thermocouple, the mass flow rate of the refrigerant component, as well as the differential pressure in the fresh air inlet. Furthermore, this HDL software make possible to visualize which heaters and boilers are switched on and off.

4.1.2 Thermocouples

A series of twelve duplex thermocouples (number “2” in the diagram) are installed throughout the air conditioning system in order to measure air temperature. These instruments are placed in pairs so that the dry and wet bulb temperature can be measured in a certain point of the system. Based on these values, other intensive properties of the air can be determined by the Psychrometric chart. The temperature of the cooling system is also measured by 3 other duplex thermocouples. During the experiment, temperature of each sensor is recorded in the HDL software for latter analysis.

4.1.3 Fan

The fan (depicted as number “9” in the diagram) is installed right after the first two thermocouples and it sucks the fresh and recycle air to the air conditioning system. The fan speed can be increased or decreased in the experiment by a device located on the left side of panel control Figure 2.

Figure 2: Control panel (Courtesy: Monash University).

4.1.4 Humidification System

After the fan, air entering the system can be humidified by injection of steam (number “3” in the diagram). For this purpose, fresh water is collected in a tank (“15”), located below the control panel. This water can be heated by three boilers and vapour is generated by this process. During the experiment, each boiler is switched on and off in the control panel. The electrical resistance, as well as the power of each boiler is shown in Table 1.

5 | P a g e


Table 1: Power and electrical resistance of boilers.

Power ResistanceBoiler, Lower 2 kW 25.2 ΩBoiler, Upper 2kW 24.6 ΩBoiler 1 kW 57.8 Ω

4.1.5 Pre-heaters and Re-heaters

Four heaters – each half placed before and after the cooling system – can be operated in the control panel to warm the system. These instruments convert electrical energy into thermal energy, based on the fact that current travels through resistive systems, creating what is called the Joule effect. The electrical resistance, as well as the power of each heater is shown in Table 2.

Table 2: Power and electrical resistance of each heater.

Power Resistance1st Pre-heater 1 kW 47.3 Ω2nd Pre-heater 1 kW 46.8 Ω1st Re-heater 1 kW 47.4 Ω2nd Re-heater 1 kW 47.9 Ω

4.1.6 Cooling System

The cooling system is broadly used to decrease air temperature to a desired thermal condition. For this purpose, a refrigerant component – named R134a or 1,1,2-tetrafluoroethane – removes heat from air. This component is subjected to what is called a refrigeration cycle (Hundy, Trott, & Welch, 2008): R134a is at liquid state and goes through an evaporator (number “5” in the diagram) to remove heat from air. As a result, the refrigerant vaporizes and is sent to a compressor (“24”). The superheated vapour that leaves the compressor travels through a condenser (“25”) and it is converted back to liquid state. This liquid passes through a valve (“12”) and it is suddenly expanded, causing a drop in its temperature. This process can be represented by Figure 3.

6 | P a g e


Figure 3: Refrigeration cycle with pressure and enthalpy values for R134a [3].

The refrigerant flow rate can be visualized by a rotameter (“28”), installed on the right side of the control panel. R134a mass flow rate is also recorded in HDL software.

Furthermore, three pressure gauges (similar to illustrated by Figure 4) to monitor the pressure of the refrigerant through the system. These instruments are installed after the evaporator (number “11” in the diagram) at the condenser inlet (“27”) and outlet (“29”).

4.1.7 Air Differential Pressure

Fresh air enters through an orifice plate - number “1” in the diagram. One inclined manometer (“14”) and its corresponding transducer are placed at this section in order to measure the differential pressure of air flowing into the system. The differential pressure of return air (“7”) is also measured by an inclined manometer similar to Figure 5

.

Figure 5: Inclined manometer (Courtesy: Monash University).

7 | P a g e

Figure 4: Pressure gauge [4]


4.1.8 Exhaust and Volume Controller

Air leaves the system through an exhaust (number “41” in the diagram) placed at the bottom of the equipment (Figure 6). The flow rate of exhausted air is mainly determined by a volume controller (number “42” in the diagram), which is installed on the left side of the exhaust. This device determines the percentage of air (in volume) that is recycled to the system.

Figure 6: Volume controller and exhaust (Courtesy: Monash University).

4.2 Experimental Procedure4.2.1 Safety [2]

Icing at Evaporator

If the air flow rate is too low, the refrigerant is capable to decrease temperature in the evaporator so that ice can be formed on the tubes. If these conditions persist, more ice formed might block the passage of air through the system. Therefore, it is advisable to keep air flow rate high enough or turn on the pre-heaters.

High Pressure Cut-out

The compressor (cooling system) is automatically turned off if the condenser pressure is higher than 1800 kN/m2 gauge. This system is re-started after the pressure drops to 1200-1400 kN/m2 gauge.

High Temperature Cut-out (In Duct)

If air temperature close to pre-heaters or re-heaters exceeds 55ºC, power to all heaters (including boilers) is turned off by the action of thermostats. The fan continues working to help cool the system. After air temperature decreases, power to the heaters is re-supplied.

High Temperature Cut-Out (Steam Generator)

If water is not supplied to the tank of humidification system and the boilers are eventually turned on, thermostats will switch of the power supply to these heaters. This cut-out occurs to avoid a dangerous situation by overheating the electrical resistances.

It is highly advisable to check if water is being regularly supplied to the tank so this emergency is not repeated, as it may cause the failure of the heaters.

8 | P a g e


4.2.2 Procedure

Before beginning the experiment, the wet bulb reservoir must be filled to the level mark. The manometer fluid must also be ensured to be topped up and checked that the level of this fluid is zero before the system is turned on. The water supply to the water must be turned on to the boiler and then the main switch to the system must be turned on. All the eight switched on the control panel must be turned off at the beginning of the experiment. The water must be visible in the sight glass of the steam generator water heaters.

After these above steps have been carried out the HDL software must be loaded and calibrated. The steps of calibration can be found in the AC lab manual.

In order to obtain an environment suitable to our objectives the fan speed can be increased or decreased, and the eight switches on the control panel can be turned on and off in different combinations in order to try and obtain the steady state temperatures desired.

5. Results:

Table 3: Data obtained for Initial Steady State, Human Comfort and Maturing Cheese Conditions

Atmospheric Pressure: 1026.3 hPa [5]

Initial Human Comfort

Maturing Cheese

A Air at Fan Inlet Dry T1 C 18.39 19.79 17.04Wet T2 C 12.28 14.77 11.85

B After Preheat & Steam Injection

Dry T3 C 18.90 20.75 33.65Wet T4 C 13.07 16.64 20.92

C After Cooling & Dehumidification

Dry T5 C 18.62 13.10 14.00Wet T6 C 12.29 11.90 11.41

D After Reheating Dry T7 C 18.63 20.06 14.61Wet T8 C 12.19 14.85 11.97

E Return Air Dry T9 C 18.63 19.90 15.60Wet T10 C 12.51 14.99 12.36

F Fresh Air Intake Dry T11 C 18.45 19.68 17.39Wet T12 C 17.26 15.92 12.05

Evaporator Outlet T13 C 18.13 5.52 4.89Condensor Inlet T14 C 18.30 66.71 67.90Condensor Outlet T15 C 18.21 34.32 34.45Evaporator Outlet Pressure P1 kN/m2 448.54 150.62 93.25Condensor Inlet Pressure P2 kN/m2 450.69 830.79 673.75Condensor Outlet Pressure P3 kN/m2 435.21 826.34 820.37Duct Differential Pressure Ze mmH20 3.77 11.72 3.10Fresh Air Intake Differential Pressure Zf mmH20 0.15 0.19 2.01Fan Supply Voltage Vf V AC 141.28 249.60 140.90

9 | P a g e


Time Interval x s 30 30 30R134a Mass Flow Rate m(ref) g/s 1.03 13.49 13.08

Table 4: Final achieved Relative Humidity values for each steady state condition

Human Comfort Maturing Cheese

D After Reheating

Dry T7 °C 20.06 14.61Wet T8 °C 14.85 11.97Relative Humidity % 57.4 73.8

Table 5: Mass Fraction of Water at each point in AC unit. [6]

Human Comfort Maturing Cheese

A xA kg H20/kg 0.00834118 0.00646579

B xB kg H20/kg 0.01004249 0.01015575

C xC kg H20/kg 0.00811163 0.00727226

D xD kg H20/kg 0.00831675 0.00757182

The values for the water mass fraction are found using the dry and wet bulb temperatures at each point. However this is given in the psychrometric chart as kg H20 per kg of dry air, so this value is converted to an overall mass fraction and entered into the table above as kg H20/kg.

Example for point A, Human Comfort:

Using T1 = 19.79 and T2 = 14.77, the mass fraction of water is found to be 0.00841134 kg H20/kg DA, therefore:

x A=0.00841134 kg

H20kg

DA

0.00841134 kg H20+1kg DA=0.00834118kg

H20kg

Table 6 : Total Mass Flow Rates in kg/s

10 | P a g e

Human Comfort Condition (kg/s)

Maturing Cheese Condition (kg/s)

A 0.02912 0.02814B 0.02917 0.02824C *0.02911 *0.02816D - -E *0.02894 *0.01499F 0.007419 0.02439

Water in 5.004 x 10-5 10.16 x 10-5

Water Out 5.677 x 10-5 7.879 x 10-5

Total Water Difference

0.673 x 10-5 2.281 x 10-5

removed from system added to system


**Note: ideally these values should be equal as no mass is added or removed from the system between points D and E.

The values for enthalpy from the psychrometric chart are given in kJ/kg of dry air. Hence, to find the total amount of energy of the stream at any point it if first necessary to convert the mass flow rate of the dry air, minus the water content, at these points.

H A=H A( kJkg DA )∗m( kgs )∗(1−x )

Example: Point A, Human comfort:.

H A=41.24 ( kJkgDA )∗0.02912( kgs )∗(1−x)¿1.1910 kJ /s

All values for the total energy brought in by the mass of the air flow are tabulated below:

Table 7: Enthalpy of the streams at points A – D (kJ/s)

Human Comfort kJ/s Maturing Cheese kJ/s

A 1.191008507 0.940049702

B 1.346112654 1.680351107

C 0.976737788 0.911235285

D 1.196706735 1.680351107

Water in 0.13390704 0.15191652

Table 8: Total Energy Balance for Human Comfort Condition

ENERGY IN ENERGY OUT

Enthalpy IN Additional components Enthalpy OUT

A B 1.191008507 Fan: 0.220 kW 1.346112654

B C 1.346112654 Evaporator 0.976737788

C D 0.976737788 1kW Reheater 1.196706735

Table 9: Total Energy Balance for Maturing Cheese Condition

ENERGY IN ENERGY OUT

11 | P a g e


Enthalpy IN Additional components Enthalpy OUT

A B 0.940049702 Fan: 140 kW Reheater: 1kW 1.680351107

B C 1.680351107 evaporator 0.911235285C D 0.911235285 - 1.680351107

Table 10: Final Report of Water addition and Heating/Cooling Duty required for each steady state condition

Human Condition Maturing CheeseTotal cooling Load (kW) 0.1494 0.0402

Total Water Difference (kg/s)

0.673 x 10-5 2.281 x 10-5

removed from system added to system

6. Mass and Energy Balance Calculations:6.1. In-duct Coefficient Calculation:

It is given that the mass flow of the fresh feed stream of air, at point F, can be calculated at any time using the equation:

mF=0.0157 √ ZV̂

Where Z is the differential pressure (obtained from AC unit output data) ,and V is the specific volume (obtained from psychometric chart using the dry and wet bulb temperatures at points F).

For the initial steady state condition, from table 1.1 this is evaluated as:

mF=0.0157 √ 3.7725210.84096=0.03325 kg/ s

Assuming that the initial condition was completely at a steady state and no recycling of the air occurred, it is clear from Figure __ that, due to the conservation of mass, the mass flows at point E and F are equal, ie

mF=mE0.03325kgs

=c f √ ZV̂

Where Cf is the unknown induct coefficient at point E. This can be calculated as seen below:

0.03325=c f √ 0.153590.83444c f=0.07751

Now the mass flow rates can be calculated at points E and F at any time using the equations

12 | P a g e

A B C D

F EFresh Air intake

Exhaust

Recycle stream

Water in Water out


mF=0.0157 √ ZV̂

and mE=0.007751√ ZV̂

Where Z is the differential pressure (obtained from AC unit output data) ,and V is the specific volume (obtained from psychometric chart using the dry and wet bulb temperatures at points E and F respectively).

6.2. Mass Balance Calculations:

Figure 7: Schematic diagram of AC Unit with A to F labelled for future reference.

6.2.1. Human Comfort Condition

To find the mass of water added and removed from the system by the boiler and condenser respectively, it is first necessary to find the mass flow rates at points F and E of Figure 7.

mF=0.0157 √ ZV̂

mE=0.007751√ ZV̂

¿0.0157√ 0.190.84174

¿0.007751√ 11.720.84084

¿0.007419 kg /s ¿0.02894 kg/s

The AC unit was set so that 75% of the air flowing at point E was recycled:

mA=mF+0.75mE¿0.007419+0.75∗(0.02894 )¿0.02912kg/ s

A B

13 | P a g e


Water component balance x AmA+mw ,∈¿=xBmB¿

Overall mass balance mA+mw ,∈¿=mB ¿

Where xA = 0.008341, xB= 0.010042 and mA = 0.02912 kg/s0.008341∗0.02912+mw ,∈¿=0.010042mB ¿

0.02912+mw ,∈¿=mB¿

Solving simultaneously yields

mw ,∈¿=0.0000500356 kg / s¿mB=0.02917 kg /s

B C

Water component balance xBmB=mw ,out+xCmC

Overall mass balance mB=mw ,out+mC

Where xB = 0.010042, xC= 0.0081116 and mB =0.02917 kg/s

0.010042∗0.02917=mw , out+0.0081116 ¿mC0.02917=mw ,out+mC


mw , out=0.0000567703 kg /s

mC=0.0291132 kg/s

C D E

No mass is added or removed to the system between points C D E so ideally the mass flow between these points should be equal. However, it was originally found that the mass flow at point E was 0.02894 kg/s so it is clear that poor data collection has led to inconsistencies. For the purposes of the energy balance calculations, the mass flow rate at point D was considered the same as the previous point C.

6.2.2. Maturing Cheese Condition

The same steps (as explained in detail above) are now used to calculate the mass flow of water added and removed from the system by the boiler and condenser respectively for the ‘Maturing Cheese’ condition.

mF=0.0157 √ ZV̂

mE=0.007751√ ZV̂

¿0.0157√ 2.010.83096

¿0.007751√ 3.100.82718

¿0.02439 kg /s ¿0.01499 kg /s

14 | P a g e


In this case, the AC unit was set so that 25% of the air flowing at point E was recycled:

mA=mF+0.25mE

¿0.02439+0.25∗0.01499

¿0. 0281375kg /s

A B

Water component balance x AmA+mw ,∈¿=xBmB¿

Overall mass balance mA+mw ,∈¿=mB ¿

Where xA = 0.006465791, xB= 0.010042 and mA = 0.0281375 kg/s0.006465791∗0.0281375+mw ,∈¿=0.010042mB ¿

0.0281375+mw ,∈¿=mB ¿


mw ,∈¿=0.000101646 kg / s ¿mB=0.0282391 kg/ s

B C

Water component balance xBmB=mw ,out+xCmC

Overall mass balance mB=mw ,out+mC

Where xB = 0.010155752, xC= 0.007272257and mB =0.0282391 kg/s

0.010042∗0.0282391=mw ,out+0.007272257 ¿mC0.0282391=mw, out+mC


mw , out=0.000078788 kg /smC=0.0281603 kg /s

C D E

No mass is added or removed to the system between points C D E so ideally the mass flow between these points should be equal. However, it was originally found that the mass flow at point E was 0.01499 kg/s so it is clear that poor data collection has led to inconsistencies. For the purposes of the energy balance calculations, the mass flow rate at point D was considered the same as the previous point C.

6.3. Energy Balance Calculations: 6.3.1. Human Condition:

A B

15 | P a g e


mB HB−mAH A−mwHw=Q̇−W s1.346−1.191−0.1339=0+0.2200.0211≠0.220

B C

mCH C−mB HB+mw Hw= ˙Q evap0.9767−1.3461+(5.677×10−5 ) Hw= ˙Qevap

˙Qevap=−0.3694 kW

C D

mDH D−mC HC= ˙Q reheater1.1967−0.9767= ˙Q reheater ˙Qreheater=0.2200 kW

Total Cooling:

Qreheater+Qevap=0.2200−0.3694 kW¿−0.1494 kW

6.3.2. Maturing Cheese:

A B

mB∆ H B−mA ∆H A−mw Hw=Q̇−W s1.6804−0.9400−0.1519=Q̇−0.140˙Qheater=0.729kWB C

mC∆HC−mB ∆H B= ˙Q evap0.9112−1.6804= ˙Qevap ˙Qevap=−0.7692 kWC D

mD ∆HD−mC∆HC=Q−W S1.6804−0.9112=00.7692 kW ≠0Total Cooling:

Qheater+Qevap=0.729−0.7692 kW¿−0.0402 kW

7. Discussion: 7.1. Achievement of Aim:

The main objective of the experiment was to ‘condition’ the air to a specific steady state temperature and humidity range, for two cases; human comfort and maturing cheese.

For human comfort the temperature and relative humidity range required was 20-25 ° C and 40-50% respectively. We achieved a steady state temperature of approximately 20.06°C and a relative humidity of 57.4%, which is acceptable for temperature however higher than what was required with respect to humidity. To lower the humidity the boiler needed to be turned off, however turning it off would completely eliminate the presence of generated humidity in the air. Another method of reducing humidity was decreasing the wet-bulb temperature. To decrease the wet bulb temperature the pre-heaters and re-heaters were turned off. This significantly decreased the temperature of the system outside of the desired range. The compressor was turned on to reduce the temperature of the system, with the pre-heater turned on however this proved to be ineffective due to the compressor working to condense the water in the air rather than cooling the air itself. The use of pre-heaters and

16 | P a g e


re-heaters proved to create large oscillations in temperature, which deviated to a great extent from the range that was required.

For maturing cheese condition the required temperature and relative humidity range was 10-15°C and 70-80% respectively. The steady state temperature attained was 14.61°C and the relative humidity was 73.8%. Both conditions were within the chosen range. One boiler was not enough to produce the desired relative humidity, however when two boilers were operating too much water was being produced, significantly increasing the humidity. The compressor was not used because of its unpredictability, in particular when there was more than one boiler operating. When this was the case, the compressor was working to condense the excess water, in preference to dropping the temperature as was required.

7.2. Mass Balance Calculations:No mass is added or removed to the system between points C D E so ideally the mass flow between these points should be equal. However, in both the human comfort and Maturing Cheese Condition, the mass flows at C and E were found to differ.

The mass at point E was calculated from the induct coefficient formula and hence the only variable here was the assumption that the data from the initial condition was entirely at a steady state and the mass flows at point F and E for the initial condition were the same. However, this may not have been the case and hence all future calculations, based on the induct coefficient value, would be incorrect.

Alternatively, if it was assumed that the induct coefficient at E was calculated correctly, then problems will have arisen from the data collection of the dry and wet bulb temperatures at points A, B and C. This is probable, as throughout the experiment there were issues with the wet-bulbs drying out and giving incorrect temperature values.

In fact, the inability to verify that mass was conserved throughout the system is likely to have been combination of both factors. For the purposes of the energy balances, it was assumed that the mass flow at point D was equal to the calculated value at C, rather than E.

7.3 Energy Balance Calculations:Due to the inability to verify many results, the approach for each of the energy balances between A to D is outlined below. Although the heaters were known to produce 1kW of heat, these values were assumed to be ‘unknown’ and solved for in the energy balances in section 6 on the assumption that the enthalpy values of the air stream were correct at each of the points.

A-B: The fan voltage was converted from volts to RMS Watts using the chart from the CHE2162 AC Laboratory Manual. It was assumed that the steam added to the system due to the boiler was saturated at 100˚C and atmospheric pressure, and steam tables (using the same reference state as the psychrometric chart, of liquid water at 0˚C) gave a specific enthalpy value of 2676 kJ/kg. This was incorporated into the energy balance, and the conservation of energy could not be verified, perhaps

17 | P a g e


because heat is being lost to the environment, but most likely because the issues with the mass flow calculation has been carried through to give incorrect values.

A similar approach was used for the Cheese maturation condition, however with the additional energy component from the 1kW reheater. In this case, assuming conservation of energy gave a reheater wattage of 0.729 kW, an overall efficiency of 72.9% was determined.

B-C: Between points B and C, the energy taken out of the system by the condensation of water was neglected. This is because the mass of the condensing water was minimal, and also because the enthalpy of liquid water is minor. Note that, in comparison, the enthalpy of the saturation steam was incorporated into part A-B as steam has a much higher enthalpy value than that of liquid water. For the human comfort and maturation of cheese condition, the evaporator removed 0.3694 kW and 0.729kW of energy respectively.

C-D: For the human comfort maturation condition, a 1kW re-heater added energy to the system. Assuming the enthalpy values of the air were correct at points C and D, it was found that the difference in these values was only 0.22 kW. It seems that either a large portion of the energy was lost to the environment, or the heater did not provide the total 1kW of energy that it was supposed to, operating at an efficiency of 22%.

For the cheese maturation condition, there were no additional energy components between points C and D so ideally the total enthalpy of the air at these points should be equal. However, this was not found to be the case, and this is most likely due to the residual heat of the heater which was turned on previously when attempting to achieve a steady state human comfort condition.

8. Conclusion:

Only one of the two major aims of this laboratory task, to achieve appropriate steady states for a Human Comfort and Maturing Cheese condition, were achieved in this laboratory.

As seen in Table 4, for the Human Comfort condition, the final steady state had a dry bulb temperature 20.06C which was in the desired range of 20-25C, however the conditions were too humid, with 57.4% relative humidity. In comparison, the steady state aim for the maturing cheese condition was achieved, with both the temperature (14.61C) and relative humidity (73.8%) being in the desired ranges of 10-15C and 70-80% respectively.

Finally, Table 10 reports the final water difference and heating and cooling duty required for each steady state. A total of 0.673x10-5 kg/s of water was removed from the system to achieve human comfort condition from the initial steady state. In comparison 2.285x10 -5 kg/s of water

18 | P a g e


was added to the system to achieve the more humid maturing cheese condition, giving a total difference of an extra 2.958 kg/s between the human and cheese maturation states.

In both cases, the evaporator removed more heat than the heaters provided, with the human comfort and maturing cheese condition having overall cooling loads of 0.1494 and 0.0402 kW respectively.

9. References

[1] How Stuff Works, ‘How Air Conditioners Work’ [Online]. Viewed 19 September 2014. Available: http://home.howstuffworks.com/ac.htm

[2] CHE2162: Air Conditioning Laboratory Manual, Monash University 2014

[3] Hundy G. F., Trott, A. R., Welch, T. Refrigeration and air-conditioning 4th Edition. 2008. Butterworth Heinemann

[4] Micro Brand Pressure Gauges. [2013 online]. Viewed 19 September 2014. Available: http://trade.indiamart.com/details.mp?offer=4698153212

[5] Bureau of Meteorology, ‘Moorabbin Victoria September’ [Online]. Viewed 13 September 2014. Available: http://www.bom.gov.au/climate/dwo/IDCJDW3052.latest.shtml [6] The Sugar Engineers, ‘Psychrometric Calculations.’ [Online]. Viewed 20 September 2014. Available: http://www.sugartech.co.za/psychro/index.php

19 | P a g e

http://www.sugartech.co.za/psychro/index.php

http://www.bom.gov.au/climate/dwo/IDCJDW3052.latest.shtml

http://home.howstuffworks.com/ac.htm


10.Appendix

Table 11: Compiled Stream Data used for Energy balances

Human Comfort

Maturing Cheese

A Air at Fan Inlet

total mass flow mA kg/s 0.02912 0.02814water mass fraction xA kg H20/kg 0.00834118 0.006465791specific enthalpy ĤA kJ/kG DA 41.2440414 33.6235758

B After Preheat & Steam Injection

total mass flow mB kg/s 0.02917 0.02824

water mass fraction xB kg H20/kg 0.01004249 0.010155752specific enthalpy ĤB kJ/kG DA 46.6152935 60.1130108

C After Cooling & Dehumidification

total mass flow mC kg/s 0.02911 0.02816

water mass fraction xC kg H20/kg 0.00811163 0.007272257specific enthalpy ĤC kJ/kG DA 33.8277402 32.5962559

D After Reheating

total mass flow mD kg/s *0.02911 *0.02911water mass fraction xD kg H20/kg 0.00831675 0.00757182specific enthalpy ĤD kJ/kG DA 41.4545831 33.9856734

E Return Air total mass flow mE kg/s 0.02894 0.01499

F Fresh Feed total mass flow mF kg/s 0.007419 0.02439

Water In due to boiler total mass flow mw,in kg/s 5.004 x 10-5 10.16 x 10-5

Water Out due to condenser total mass flow mw,out kg/s 5.677 x 10-5 7.879 x 10-5

*Note: For the purposes of the energy balances, it was assumed that the mass flow at point D was equal to the calculated value at C, rather than E. See the Mass Balance discussion for more details.

20 | P a g e

Date post:	10-Feb-2016
Category:	Documents
Upload:	arthur-parreira-silva-medeiros
View:	228 times
Download:	0 times

Trabalho de Ar condicionado

Documents