Air Conditioning Lab
ME 4122 – Thermo Fluids Lab
Justin Gregerson
Caleb Glantz
Tyler Watson
3/7/06
Presentation Outline
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Lab Objective
Summary
Theory and Analysis
Equipment
Methods of Measurement
Procedure
Results and Discussion
Conclusion and Recommendations
Questions?
Objective
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The objective of this lab was to analyze the
performance of an actual air conditioning
unit based on experimental measurements.
Summary
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The air conditioning unit was analyzed and the performance was evaluated.
Based on the measured dry bulb temperatures and pressures, enthalpies and
entropies were calculated for each state point. These values were used to
calculate the Refrigeration Effect of 5283 BTU/hr, Coefficient of Performance of
2.304 and a Energy Efficiency Ratio (EER) of 7.865 BTU/W * hr.
For the air side analysis the dry bulb temperatures and relative humidity of the
room, condenser outlet and evaporator outlet were used to calculate specific
humidity, enthalpies and wet bulb temperatures. In conjunction with these
calculated values the mass flow-rate of the air passing by the evaporator and
condenser was used to determine the heat load rejected to the environment.
The total heat load amounted to 6755 BTU/hr.
In order to achieve performance values closer to the manufactures rated
performance the same conditions they used should be used for this lab. The
ambient air temperature they used was 35°C and for these calculations the
room temperature was 24.4°C. Since our temperature was lower there was less
heat transfer to the R22 effectively decreasing our heat capacity and
performance. The following report contains further analysis and complete
results of the experiment performed.
Theory and Analysis
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The air conditioning unit can be analyzed by breaking
it down to resemble an ideal vapor-compression
refrigeration cycle.
The processes involved in the cycle were:
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Isentropic Compression from points 1-2 where it was
assumed to be adiabatic.
Constant pressure heat transfer out of the system in the
piping from points 2-3.
Adiabatic throttling process through the condenser from
points 4-5 where the h values remained constant.
From points 5-6 there is a constant pressure heat transfer
into the system through the evaporator.
Departures from the Ideal Cycle
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There were some differences that the A.C unit
had with respect to the ideal cycle so it became
more reasonable to model it after the actual
vapor-compression refrigeration cycle.
There were two systems that were evaluated,
the air side which is an open system where
mass can be transferred and the R22 coolant
side which is a closed system that has no
mass transfer
T-s Diagram
• The evaporator outlet and the compressor inlet (points 1 and 6)
and the compressor outlet and the condenser inlet (points 2 and 3)
are too close to discern on a reasonably scaled chart.
Energy Balance of Evaporator
Energy Balance Across Evaporator
R22
Air Entering
½ Fan Work
Evaporator
Water Condensate
Air exiting
Quality of Evaporator Inlet and COP
h4
hfg x  hf
•The above equation was used to solve for the enthalpy at point 5 due to
the fact that is it a mixture with an unknown quality. Since the enthalpy at
evaporator inlet and the condenser outlet are equal. The enthalpy of the
saturated vapor and latent heat of vaporization are found from the tables
that were provided for water vapor.
COPe 


mr22e h 6  h 4
Wcom
The coefficient of performance is directly solved for using the answers
found in Eqns. 8-10. This value is one of the rating factors for the air
conditioning unit.
s 5  s fg x5  s f
Since the quality is now known from the enthalpy equation the
entropy at point 5 can be calculated by using the above equation and
the known values from the tables attached to the lab instructions.
Capacitance and EER
kg
kJ
kJ
Qe  .01   250.748
 97.245 
s 
kg
kg 
EER  3.413 COPe
The capacitance equation used the values found in Eqn.
10 and 8. The capacitance should be lower than the rated
valued due to the lab conditions. The EER is COP times
a constant ratio that gives the industry rating of the unit.
The EER found in the lab was reasonably smaller than
that provided by Panasonic.
Equipment
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Haier Model # HPAC9M Portable Air Conditioner
Omega RH83 Digital Sling Psychrometer/Thermo Hygrometer
FLUKE 336 Clamp Meter (Ammeter)
Data Acquisition System described in the Instrumentation Lab Background
EXTECH Instruments Model No. 451112 Vane Anemometer
Control Company Model No. 14-648-51 Aneroid Barometer
Mitutoyo Model No. CD-6”CS Digital Caliper
Extech Multimaster 560 digital multimeter
AC Unit Important Points
6
1
5
2
3
4
AC unit views
AC unit views
Methods of Measurement
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Six type K thermocouples are connected to the data acquisition system and mounted at key points in the
refrigerant piping; the evaporator outlet, the compressor inlet, the compressor outlet, the condenser inlet,
the condenser outlet, and the evaporator inlet. This placement was predetermined for this experiment
and correlate to the points 1-6 in Figures 2 and 3.
Omega RH83 Digital Sling Psychrometer/Thermo Hygrometer was used to find the room dry-bulb
temperature and the relative humidity of the air surrounding the unit by holding the unit anywhere that is
reasonably far from the exit points of the unit. This device is also used to find the dry bulb temperature
and relative humidity of the evaporator and the condenser outlet points by holding the psychrometer in
front of the air flows leaving both outlets.
• FLUKE 336 Clamp Meter (Ammeter) was used to find the current flow into the compressor by clamping
around the red wire that is easiest to access behind the control panel of the air conditioning unit.
• Extech Instruments Model No. 451112 Vane Anemometer used to measure the outlet velocity from the
condenser and the evaporator. This device must be held with the yellow dot on the side with the air flow
hitting it.
Control Company Model No. 14-648-51 Aneroid Barometer used for taking the pressure of the
atmosphere in the room. Device is factory calibrated so the values can be read directly off of the device
and used in calculations with an uncertainty of .05 kpa.
Mitutoyo Model No. CD-6”CS Digital Caliper measured the outlet areas of the condenser and evaporator
air flows. Make sure that multiple measurements are taken of the inner diameters of the outlet ducts
since they aren’t exactly circular. The uncertainty of this device can be neglected do to the multiple
measurements and averaging done in finding the values.
Extech Multimaster 560 digital multimeter found the wall outlet voltage that the air conditioning unit was
taking its power from. This is done by sticking the positive and negative probes into the wall socket and
reading the value from the meter.
More Methods of Measurement
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Two Bourdon tube pressure gauges are attached to the
refrigerant piping at the evaporator outlet and the compressor
outlet. The bourdon tubes were used to measure the evaporator
and compressor outlet pressures of the R22 refrigerant. The
pressure gauges can be read with two different scales, the psi
scale was used with an uncertainty of .5psi.
All measurements were taken at least twice and the average was
found to minimize the error in measurement going into the
calculations. Constant pressure through the condenser and the
evaporator allows the assumption of inlet and outlet pressures in
the evaporator and condenser being equal valid. The air
conditioner unit must be allowed to run until the displays are
shown to be as close to steady state as possible.
Procedure
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Visually inspected all components of AC unit
Plugged in and turned on AC unit
Used digital caliper to measure condenser and
evaporator outlets
Monitored R22 temperature until steady-state was
reached
Recorded following values for the air side while AC unit
is at steady-state: Room Pressure, Dry Bulb Temp,
Relative Humidity; Condenser Outlet Velocity, Dry Bulb
Temp, Relative Humidity; Evaporator Outlet Velocity, Dry
Bulb Temp, Relative Humidity;
Results and Discussion
R22 Analysis
compressor
R22 States
In (1)
condenser
Out (2)
In (3)
evaporator
Out (4)
In (5)
Out (6)
Tdb (°C)
1.466
69.948
69.689
42.032
4.154
0.39
P (MPa)
0.359
1.9
1.9
1.9
0.359
0.359
s (kJ/kg-K)
0.944
0.917
0.916
0.351
0.365
0.939
252.09
281.31
281.05
97.245
97.245
250.748
h (kJ/kg)
Air Side Analysis
State
Position
Room / Entering air
Tdb (˚C)
Twb (˚C)
φ (%)
ω (10^-3)
h (kJ/kg)
24.4
11.5
14.3
2.78
32.04
Evaporator exit
4.4
1.25
56.6
0.645
11.93
Condenser exit
40.4
16.5
4.8
0.113
48.12
Results and Discussion
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Analysis of the data obtained in the lab is made possible with the state
postulate. If two independent properties are known than everything may
be determined off of the measured values [1]. Examining the state of
the R-22 refrigerant the working fluid flowing through the system
requires the temperature and pressure at certain positions. In
conjunction with the R22 properties tables the enthalpy and entropy of
the fluid may be determined by interpolation [1].
At state point five, the evaporator inlet, the temperature and pressure
are relatively low thus the enthalpy and entropy values are
correspondingly low (Table 1). At this point the fluid is a saturated
mixture with a quality of 23.5 % water vapor. As the fluid flows through
the evaporator the difference between the low evaporator temperature
and the higher room air temp allow for heat transfer to the refrigerant.
This effectively increases the energy contained in the fluid shown by the
increase in enthalpy at the evaporator exit represented by state point 6.
The specific enthalpy of the refrigerant entering the evaporator is 97.25
kJ/kg, while the exiting is a much greater 250.75 kJ/kg.
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As desired the R22 is now on the superheated region giving the fluid a
higher quality so moisture doesn’t harm the compressor blades. State
points one and two respectively represent the inlet and outlet conditions
of the R22 over the compressor (Table 1). Over the compressor the
pressure increases raising the temperature which also increases the
specific enthalpy from 252.09 kJ/kg entering to 281.31 exiting the
compressor. The idea is to raise the temperature going into the
condenser as high as possible so more heat can be rejected to a heat
sink. Increasing the high temp of rejection overall increases the
efficiency of heat transfer or the coefficient of performance.
The condenser is the next component the R22 will travel through,
corresponding to state points three and four. The goal is to reject heat
from the R22 so more thermal energy may be absorbed when the fluid
makes its way back down to the evaporator. This is represented by the
decrease in temperature and enthalpy from state point three to four.
Here the specific enthalpy decreases from 281.05 kJ/kg for the
refrigerant entering the condenser to 97.25 for the refrigerant exiting the
condenser.
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Following the condenser are the capillary tubes, represented as
state points four to five. Capillary tubes act as a throttling device
which utilizes the Joule-Thompson Effect where a large drop in
temperature is associated with a large drop in pressure from
expansion [1]. Our capillary tubes are idealized assuming that it
is an adiabatic throttling process. This allows the tubes to be
viewed as an isenthalpic device where the enthalpy remains
constant. This is an important assumption because it allows the
enthalpy value obtained at the condenser exit to be used at the
evaporator entrance.
Along with enthalpy the entropy of the system at different state
points may also be interpolated using the temperature and
pressure measurements obtained experimentally. The values of
specific entropy noted as “s” may be viewed in the above Table
1. These values are used along with the dry-bulb temperatures to
graphically represent the cycle (Fig 3).
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Now the refrigeration effect or capacity may be determined by
examining the change in specific enthalpy over the evaporator since
this is the location of thermal energy addition. The calculated rate of
heat transfer from the air to the refrigerant was 5.238*10^3 BTU/hr. This
is lower than the 8,000 BTU/hr rated performance of the air conditioner
because the room air temperature used in the experiment was 24.4 °C,
which is lower than the room air temp of 35 °C used by Panasonic
Industrial Company [4]. Since the difference of air temp and R22 temp
entering the evaporator found experimentally was lower than the
temperature difference the manufacturer used there will be less heat
transferred to the refrigerant. The general rule is that the greater the
temperature difference the greater the heat transfer.
The “efficiency” of the refrigerator was determined by Coefficient Of
Performance (COP) and the Energy Efficiency Ratio (EER). The COP
calculated was 2.304 and the EER was 7.865 BTU/W*hr. This is also
lower than the manufacturer’s rated performance of EER which is 10.75
BTU/W*hr for the same reason that the refrigerating capacity was lower,
the same testing conditions were not used to evaluate the system.
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Another part of the analysis was to determine the states of the air entering the AC or the
room’s air and air exiting through either the evaporator or condenser. This encompasses
using the relative humidity and dry bulb temperatures measured during the experiment to
determine the wet-bulb temp, specific humidity, and the specific enthalpy of the air and
water vapor mixture (Table 2). The incoming air temp is greater than the air temp exiting
the evaporator since thermal energy is extracted from the air by the refrigerant shown by
the decrease of specific enthalpy. This is the desired effect of an air conditioner, take air
from the room, cool it off and return it into the room. Over the evaporator and condenser
moisture is removed from the incoming air, represented by the decreases in specific
humidity.
Water condenses out of the room air on the cold evaporator coils thus decreasing the ratio
of mass of water vapor to mass of dry air. This is a cooling and dehumidifying process. The
increased air temp leaving the condenser is to the thermal rejection of heat since the
condenser coil temperature is greater than the surrounding room air temp. The enthalpy of
the air also increases relative to the increase in air temp. The relative humidity values
measured increase and decrease according to the air temp and the actual amount of
moisture present in the air. When the air temp and moisture content exiting decreases such
as over the evaporator the relative humidity will increase since the air is less able to
contain moisture. While the opposite occurs over the condenser and the relative humidity
decreases. The wet-bulb temperatures were estimated by examining a psychrometric
chart. These values are supplemental and not actually used for calculation purposes.
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The heat rejected to the surroundings may be obtained by using
the values of specific enthalpy obtained along with the mass
flow-rate of air going across the evaporator and the condenser.
The rate of heat load from the room air entering to the room air
exiting the evaporator is -5,291 BTU/hr. The negative sign
convention accounts for energy being taken out of the air. This
value is greater than the heat capacity taken in by the refrigerant
because the energy transfer for the air accounts for the energy
taken out by the refrigerant in addition to the latent heat of
vaporization released by the condensing water over the
evaporator. The heat load of the air entering and exiting the
condenser is 6,755 BTU/hr. So the overall heat load rejected is
the sum of the two amounting to 1,465 BTU/hr. In this experiment
the heat is being added back into the room since the condenser
isn’t placed outside.
Conclusions and Recommendations
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All of the data collected during this experiment is enough to explain to any
engineer or layman what the air conditioning unit will do when it is put into use
in the lab settings. Anyone can use the same type of system to analyze any
air conditioning unit to see what the actual output of the unit into the
surrounding environment will be instead of going by what the optimum values
are that are provided by the manufacturer that are purposely skewed to make
their product seem better.
The pros of doing the lab this way was that it can be performed by anyone as
long as supporting instructions and directions are given in a clear manner. It is
simplified to a point where the valuable information emerges. Simplifying a
system allows the important areas to be evaluated with less stress on
obtaining the so called “right” answer. The bad part of making these
simplifying assumptions are that certain error in the calculations are
introduced making the values mere estimates. For the purpose of this lab
which was to analyze the performance of an air conditioning system based on
measurements, these certain assumptions were all simplifications that proved
to be beneficial.
Conclusion and Recommendations
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In order to get this lab to be more effective it should be in a controlled environment. The air conditioning
unit should be placed in a room with a temperature like the rated temp and as close to ideal air conditions
as possible. Also limiting the uncertainty in the measurement devices would decrease the difference in
the known values and the found values for the unit. The measurements were taken in the positions that
would account for the right amount of heat load and enthalpy for each state that the flow goes through.
Simplifying assumptions were made during the experiment to make the data easier to collect and
analyze. The effects of the pressure loss could be analyzed but should be small enough to where there
would be no significant difference in the values reached.
After a complete analysis was performed the original hypothesis pertaining to the heat capacity and EER
provided by Panasonic being greater than the values we obtained experimentally was accepted. It was
assumed that the heat capacity for our lab settings was going to be lower then the manufacturer’s since
the temperature in our lab was lower than the ambient air temperature they used minimizing the heat
transfer from the air to the refrigerant. Through measurements we obtained a heat capacity of 5,238
BTU/hr compared to the claimed 8,000 BTU/hr put out by Panasonic, which shows that a significant
amount of thermal energy is still being absorbed by the refrigerant over the evaporator.
Since the COP and the EER are directly related to the heat capacity and the work going into the
compressor to transfer heat from a low temperature region to a high temperature, the values obtained
experimentally will also be lower than the ones claimed. Experimentally the COP was found to be 2.304
and the EER was 7.865 compared to Panasonics COP of 3.145 and EER of 10.75 BTU/W/hr. The lab
effectively demonstrated how an air conditioner functioned and how certain efficiencies are determined.
QUESTIONS/COMMENTS?