Theory and Analysis:
The air conditioning unit can be analyzed by breaking it down to resemble an ideal vapor-compression refrigeration cycle. There is a condenser, evaporator and a capillary tube that acts as the expansion valve for the system. The work input is done by the compressor which is also in the ideal cycle, and a fan which was found to be negligible in the results. 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. Although the pressure drop in the pipes was taken to be negligible there was still heat transfer from the surroundings, the refrigerant going into the compressor was superheated, and entropy increased throughout the system. There were basically two systems that were evaluated the air side which is an open system and the R22 coolant side which is a closed system. The air side has mass that can be transferred through it while the R22 side cannot.
The processes involved in the cycle were:
· 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.
Add figure of Ideal cycle
Equations used during analysis:
1. Pressures of the room air, the evaporator exit, and compressor exit
Eqn. 1
For the pressure of the gas in the room was found by linear interpolating from the values given in the air tables attached to the lab instructions. All of these pressures had to be taken from the saturated table to get the correct values.
2. The specific humidity of the room air, evap. exit and the comp. exit are needed next to further calculations.
Eqn. 2
The pressure of water vapor in the air can be found by using Eqn. 2 shown above. The relative humidity was measured at the beginning of the lab and the pressure of the gas at each point was found in Eqn. 1. Using these values for the water vapor pressure and the Eqn. below the specific humidity can be found for each point.
Eqn. 3
The relative humidity for each is found by using the solver in MathCad to rearrange the above equation for w. the pressure values in the above equations were taken by measurements while the air conditioning unit was running in steady state.
3. The enthalpy of the room can be found by using the following equation.
Eqn. 4
To solve for the enthalpy of the room the dry bulb temperature for the room and the specific humidity for the room must be known. The room dry bulb temperature was given and the specific humidity was solved for in equation 3. This equation can be repeated for each of the three air side points by substituting in the respective dry-bulb temperatures and humidity.
4. In order to find the heat load into the environment there was a series of equations that have to be used with each other.
Eqn. 5 and 6
The density and the mass flowrate of the condenser and evaporator were found using Eqn. 5 and 6. The density of both was interpolated by using the supplied tables attached to the lab instructions and the specific humidity were found using Eqn. 3. For the mass flowrate the velocity and area of the condenser and evaporator were measured during the data collection.
Eqn. 7
The total heat flow out of the system is the sum of the heat taken in by the evaporator and the heat expelled into the environment by the condenser. If the condenser outlet is not vented properly the system will be counterproductive.
5. The enthalpy of the evaporators surroundings (point 4 and 6) needs to be solved for next in order to reach the COP value.
Eqn. 8
The value at point for is saturated and a point 6 the mixture is superheated therefore the correct tables must be used for interpolation to get correct results. Once the values are interpolated the work into the system must be calculated.
6. The work of the compressor and fan need to be known in order to find the mass flowrate of R22 through the condenser.
Eqn. 9
The work of the compressor is just the current going through into the compressor times the voltage supply to the unit. Both of the values were measured in the lab so the work can be computed directly.
Eqn. 10
The above equation was found by taking an energy balance around the evaporator and solving for the mass flowrate. The mass flowrate uses the work of the fan which was split in half between the condenser and the evaporator, the enthalpy of the in and out flows and the mass flowrate through the evaporator. The enthalpy from Eqn. 4 and Eqn. 8 while the work into the system is used from Eqn. 9 to find the mass flowrate.
7. Solving for the COP of the evaporator is straight forward using values already solved for in previous equations.
Eqn. 11
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.
8. The refrigeration effect and capacitance is needed as it is the industries way of rating their system.
Eqn. 12 and 13
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.
9. Finding the entropy values for the T-s Diagram.
Eqn. 14
Using the above equation to solve for the enthalpy at point 5 due to the fact that is it a mixture with an unknown quality. Since h4=h5 Eqn. 14 can be used to find the quality.
The enthalpy of the saturated vapor and latent heat of vaporization are found from the tables that were provided for water vapor.
Eqn. 15
Since the quality is now known from Eqn. 14 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.
10. Finding the entropy at all of the remaining points to complete the T-s diagram.
Eqn. 16
The above equations use the same type of interpolation and the enthalpy equations used previously did. The only caution is to make sure that the correct tables for saturated and superheated steam are used for the correct points. Once all the values are found you can use these results and the temperatures measured to plot the Ts diagram.
Equipment, methods, procedure:
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 the experiment.
• Haier Model # HPAC9M Portable Air Conditioner used in this lab utilizes R22 as the refrigerant. The specifics for this unit are in attached sheets.
• 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 parts.
• 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.
• 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 making 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.
Photo and Sketch Needed
Bibliography:
[1] Cengel, Yunus A., and Michael A. Boles. Thermodynamics An Engineering Approach. 5th ed. New York, NY: McGraw Hill, 2006.
[2] Incropera, Frank P., and David P. Dewitt. Fundamentals of Heat and Mass Transfer. 5th ed. Hoboken, NJ: John Wiley & Sons, Inc. 2002.
[3] Moran, Michael J., Shapiro, Howard, N., Fundamentals of Engineering Thermodynamics. 5th ed. Hoboken, NJ: John Wiley & Sons, Inc. 2004.
[4] 2R12S3R126A-6A. Air Conditioning Department. Panasonic Industrial Company.
Analysis of results:
All the results that were found made sense with what was known at the start of the experiment. The rated EER for the air conditioning unit was provided as 10.75 btu/whr, the value found in the experiment was 7.865 btu/whr. This value is lower because the factory specifications were at certain conditions that were more ideal than that of the lab room during the experiment. The room temperature was a lot lower at 24.4 deg. Celsius compared to 35deg; also the evaporator and condenser temperatures were lower than the rated ones. This difference in temperature causes the water vapors to have a higher composition and make the efficiency of the system to decrease. The previous reason is also a major factor to the reason why the capacity was not the 8,000 btu/hr that the manufacturer rated the unit at instead it was -----.
Conclusion and recommendations:
All of the data collected during this experiment is enough to explain to any 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 manufactured that are purposely skewed to make their product sound 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. The cons of making the lab simpler by assuming certain constant variables is that the values are as exact as some would like them to be. For the purpose of this lab which was to show the different effects of area, velocity, temperature, humidity, and cooling fluid on an air conditioning system all simplifications were beneficial.
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 wear there would be no significant difference in the values reached.
The original hypothesis that was made for the system to have slightly lower values than that provided by Panasonic were found to be accurate with all values found to be a reasonable amount lower than the known specifications.