Air Conditioning Laboratory Project 99.10 Design Team Members: Sean Gallagher Prathana Vannarath 260 Elkton Rd, Apt D-9 211-8 Thorne Lane Newark, DE 19711 Newark, DE 19711 (302) 369-2820 (302) 738- 8765 [email protected][email protected]Brian Davison Pamela McDowell 64 Willow Creek Lane 135 E Cleveland Ave Newark, DE 19711 Newark, DE 19711 (302) 239-1340 (302) 366-7473 [email protected][email protected]Sponsors: Dr. Tony Wexler 226 Spencer Lab
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Air Conditioning Laboratory
Project 99.10
Design Team Members:
Sean Gallagher Prathana Vannarath 260 Elkton Rd, Apt D-9 211-8 Thorne Lane Newark, DE 19711 Newark, DE 19711 (302) 369-2820 (302) 738-8765 [email protected][email protected]
Brian Davison Pamela McDowell 64 Willow Creek Lane 135 E Cleveland Ave
Newark, DE 19711 Newark, DE 19711 (302) 239-1340 (302) 366-7473
For Heat Transfer and ThermodynamicsJoint Laboratory
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Mission Statement:
Our mission is to teach the students the Thermodynamic and Heat Transfer principles of a window air conditioner with as little guidance as possible. This lab manual consists of what we hope is the only necessary information needed for the student to independently investigate these principles. We envision the lab time spent as a brainstorming session for the student to discuss the air conditioner with minimal time spent collecting data.
WARNING: Be careful when examining the air conditioner, as some of the parts get
very hot!
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Table of Contents
Introduction………………………………………………..50
Schematic of Refrigeration Cycle…………………………51
Description of Parts………………………………………..52
Sensors……………………………….……………………53
LabVIEW Controller……………………………………...54
Lab Reports………………………………………………..56-66
Compressor……………………...…..56
Throttling Valve………………....…..59
Air Flow Across the Coil……….……62
The Ideal Air-Conditioning Cycle…...65
References…………………………………………………68
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Introduction:
The basic objective of an air conditioner (or window AC as it would be) is to
remove heat from the air of a room that is being cooled. The heat is discharged to the
environment outside the room. It should be noted that the same air conditioner could be
used as a heat pump as well by simply turning it around. In this case the air conditioner
would be absorbing heat from the outside environment and rejecting it into the room.
The ideal vapor-compression refrigeration cycle is the most widely used for
refrigerators, air conditioning systems, and heat pumps. It is composed of four processes.
Starting form the compressor, refrigerant is isentropically compressed. Then, it is sent
through the condenser, where pressure remains constant. From the condenser, warm air
is rejected into the outside environment. From there, the refrigerant flows through the
throttling valve, which is an expansion device. Next, the refrigerant is sent through the
evaporator, where pressure is again constant (as in the condenser). Finally, the
refrigerant reaches the compressor where the cycle begins all over again.
It is important to recognize that the refrigerant does not simply flow through the
devices mentioned above. The refrigerant is experiencing phase changes throughout the
cycle. As the refrigerant enters the compressor, it is a saturated vapor. During the
compression process, the temperature of the refrigerant increases to well above that of the
surroundings. As it enters the condenser it is a super-heated vapor, and it leaves as a
saturated liquid. This phase change results from the refrigerant losing heat while flowing
through the condenser. It should be noted that the temperature of the refrigerant at this
phase is still well above that of the environment.
Upon entering the throttling valve, the refrigerant experiences a pressure-drop,
which in turn results in a decrease in temperature. It is at this point that the temperature
of the refrigerant finally falls below that of the surroundings. As it enters the evaporator
the refrigerant is a low quality saturated mixture. The refrigerant uses the heat from the
room to provide the necessary energy to complete the evaporation process. At this point
it is again a saturated vapor and ready to re-enter the compressor and start the cycle over.
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It is often helpful to use graphs to interpret the process, which is the air-
conditioning cycle. One such graph is the ‘T-s diagram’. The heat transfer for internally
reversible processes is represented as the area under the process curve ‘4-1’ (as shown in
figure 2).
Another commonly used graph is the ‘P-h’ diagram. As can be seen from figure
3, three of the four processes appear as straight lines. The heat transfer in the condenser
and the evaporator is proportional to the lengths of the corresponding process lines.
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Description of Parts:
Condenser/Evaporator:
In the window air conditioner, the condenser and the evaporator are actually heat
exchangers. All that can be seen of either one are the ‘U’ shaped coils attached to the
sides on the front and the back of the unit. One set of coils gets hot and the other gets
cold, so be careful when touching them.
Throttling Valve:
A throttling valve, in this case, reduces the pressure of the refrigerant. To achieve
this, the refrigerant should flow from a smaller diameter tube to a larger diameter tube. If
you still are unsure where the throttle is, ask the TA.
Compressor:
The compressor is the tall black cylinder that sits between the heat exchangers. It
gets very hot when the air conditioner has been running after several minutes.
Other:
The little black cylinder behind the compressor is a collector that has no effect on
the Thermodynamic/Heat Transfer processes of the unit, so it is ignored. Fans are needed
to move the air over the coils of the heat exchangers. There are two of them. They are
both attached to the same motor, which is located next to the collector, between the heat
exchangers. A shield covers them for your safety.
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Sensors:
Numbering of the Sensors:
The sensors are labeled with numbers. These numbers are made to correspond
with the LabVIEW program. They have no significance to any other numbering scheme
mentioned in this laboratory manual or any of the written lab instructions.
Pressure Sensors:
There are four pressure sensors placed throughout the air conditioner. Basically
one between each device (between the condenser and the compressor, etc.) They are
black with the OMEGA label on them. They measure the pressure in volts (1-5V, 1
being 0 psi and 5 being 500 psi). They are connected to the data acquisition board
through the blue wires.
Thermocouples:
There are also four thermocouples, placed the same as the pressure sensors. They
are attached to the outside wall of the tubing. They are connected to the data acquisition
board through the copper wires. Their output (in volts) is converted by LabView, and is
displayed in degrees Celsius.
Relative Humidity Sensor:
This sensor is long and cylindrical in shape, and silver in color. It mounts to the
air conditioner in three places, the outside of the evaporator, the outside of the condenser,
and in between the two. It has two functions, measuring the relative humidity of the air
and the temperature of the air. It output is also in volts, and converted by LabView to %
relative humidity and degrees Celsius, respectively.
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Mass flow sensor:
The box mounted on top of the casing is the mass flow sensor. It has a digital
display of it’s own. The number shown must be multiplied by a conversion factor of
0.4956, for refrigerant-22.
Velocity Sensor –
A hand held device use to measure the velocity and temperature of the air exiting
the condenser and the evaporator. This device is used by simply holding it in front of
desired air flow. NOTE: The fan should be perpendicular to the direction of the flow to
maximize accuracy. This device also had a digital indicator of it’s own. The output of the
temperature is in degrees Celsius, and the velocity is in meters per second.
Watt Meter:
This clamps to the power cord of the air conditioner, which is connected to the
outlet. A digital read-out shows how much power the air conditioner is using.
LabVIEW Program:
The LabVIEW program (see Appendix H) is interfaced with the data acquisition
boards. The two acquisition boards are 1) a green box, and 2) a green board with blue
boxes. The green box is connected to the pressure sensors and the mass flow sensor. The
green board with the blue boxes is connected to it; it reads the thermocouple outputs and
the relative humidity sensor outputs. Our LabVIEW program (see Appendix H) reads
and converts the signals from each of the sensors to their respective units of measure, for
example pressure is converted from volts to psi and temperature in Kelvin. LabVIEW
has the capability to read up to sixteen channels. The green board with the blue boxes is
read into the first eight channels (0-7), and the green box reads into the upper eight
channels (8-15).
The pressure sensors are labeled 1-4 and their respective channels are 8-11. The
thermocouples are also labeled 1-4, and their respective channels are 0-3. The mass flow
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is read through channel 12, and the relative humidity is read through channel 4. Be sure
the check the channel numbers BEFORE collecting any data.
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Compressor
Objectives:To understand the thermodynamic principles involved in the function of a
compressor. To use conservation of energy to find heat loss to the environment.
Background:The purpose of a compressor is to increase the pressure of a fluid. Work is done
on the fluid therefore the work term is negative when dealing with a compressor. Certain engineering assumptions are made when dealing with a compressor:
(figure 1)
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In the case of a compressor there is intentional cooling, therefore the heat transfer term cannot normally be neglected
A compressor involves a rotating shaft crossing its boundaries, therefore the work term is important.
The change in potential energy is normally quite small and thus neglected.
In a compressor the velocities involved are usually not high enoughto effect the kinetic energy at all, especially compared to the change in enthalpy.
1.
2.
3.
4.
0
0
0
0
ke
pe
W
Q
P1,T1
P2,T2
q
A compressor can be modeled as a steady flow system.
Relevant Equations:
Conservation of Energy:
q – w = h + pe + ke (eq. 1)
By making the assumptions presented previously, eq. 1 can be reduced to
q – w = h (eq. 2)
Power:
Power = (mass flow rate)*(work per unit mass w) (eq. 3)
Procedure:
1. Measure the temperature and pressure at the inlet (1) and exit (2) of the compressor.Note: The numbers (1) and (2) do not correspond to the sensor numbers only the figure 1 numbers.
2. Measure the mass flow rate of refrigerant (read from indicator on sensor).3. Determine the power input to the compressor by reading the current and voltage off the motor to the fan. OPTIONAL: Read the power from the Power Clamp on the cord, which is connected to the electrical outlet.
Analysis:
1. Do the measured values of temperature and pressure correspond to what you know is happening in the compressor? Explain in terms of thermodynamic principles.
2. Using the measured temperature and pressure values, determine the specific enthalpy h at the inlet and exit using the refrigerant tables in the back of your thermodynamics text.
3. Solve for the work done on the fluid using the power input (read from the fan motor) to the compressor and eq. 3.
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4. Solve for the heat loss to the environment using the values from parts 1 and 2 of the analysis and eq. 2.
5. Calculate the power input to the compressor using eq. 3, assuming heat loss can be neglected. Could this be a reasonable engineering assumption? Explain.
6. OPTIONAL: Compare the power calculated from reading the voltage and current off the fan motor to the power reading from the Power Clamp. Explain differences.
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Throttling Valve
Objectives:To understand the thermodynamic principles involved in the function of a
throttling valve. To determine whether approximating the fluid as saturated at the inlet of the valve is a good engineering approximation. To determine the relationship between the quality of the mixture at the exit and the temperature.
Background:The purpose of a throttling valve is to cause a significant drop in the pressure of
the fluid. A good example of this is any adjustable valve such as a sink faucet. Along with the drop in pressure comes a drop in temperature. In an air conditioner, it is this temperature drop that is the primary purpose of a throttling valve. As with any device, several engineering assumptions are made in order to simplify the analysis:
(figure 2)
A throttling valve can be modeled as a steady flow system.
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The fluid is usually moving rather quickly and throttling valves are usually small. Thus it is assumed that there is neither sufficient time nor area for significant heat transfer to occur.
A throttling valve involves no moving boundaries and therefore no work is done by or on the fluid.
The change in potential energy is normally quite small and thus neglected.
Even though the change in velocity can be quite large, the change in kinetic energy is considered insignificant.
1.
2.
3.
4. 0
0
0
0
ke
pe
w
q
fluid
Throttling Valve
Relevant Equations:
Conservation of Energy:
q – w = h + pe + ke (eq. 1)
By making the assumptions presented previously, eq. 1 can be reduced to
h = 0 or h1 = h2 (eq. 2)
It is more useful to expand eq. 2:
u1 + P1v1 = u2 + P2v2 (eq. 3)whereP is the pressureu is the internal energy, andv is the specific volume.
Qualityx = mvapor (eq. 4)
mtotal
where mvapor is the mass of the vapor, andmtotal is the total mass of the liquid and the vapor
It can be derived that:x = (hav – hf)/hfg (eq. 5a)x(exit) = (h(e) – hf(e) )/hfg(e) (eq. 5b)where hf is the specific enthalpy of the liquidhfg is the difference between the specific enthalpy of the fluid and the gas, andhav is defined as hf + xhfg.the subscript e is for the exit pressure
Procedure:
1. Measure the temperature and pressure at the inlet (1) and exit (2) of the throttling valve. NOTE: The numbers (1) and (2) do not correspond to the sensor numbers, but they do correspond to the numbers in figure 1.2. Measure the mass flow rate of refrigerant (read from indicator on the sensor).
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3. Repeat step 1 for at least 2 more settings on the air conditioner (e.g. low, med, high).
Analysis:
Questions 1 through 3 need only be evaluated at 1 air conditioner setting.7. Using only the inlet pressure reading and assuming that the refrigerant is a saturated
liquid as it flows through the throttling valve, find the specific enthalpy.8. Assuming that the enthalpy across a throttling valve does not change, use the enthalpy
from question 1 and the reading of the exit pressure, determine what state the mixture at the exit is.
9. If the refrigerant is a saturated mixture at the exit, find the quality using equation 5b.10. Using the temperature and the pressure form the inlet reading, does the previous
assumption that the inlet and of the throttling valve is a saturated refrigerant? Explain why or why not.
11. What is the temperature change for this process? A) Using the saturation temperatures from the given pressures. B) Using the data read off of the air conditioner. Is there a difference and if so, explain.
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Air Flow Across the Coil
Objective:To understand the heat transfer principles by determining the convection heat
transfer rate involved with cross flow over the coils (tubes).
Background:There is a coolant flowing inside the coils and as the air flows over the coils, heat
is transferred between the flowing coolant in the coil and the air around it. The rate at which the heat is transferred is dependent upon the heat transfer coefficient. The coil rows are either arranged in an aligned or staggered bank.
geometry of the coils
V, T (Tinlet)
ST
SL
Relevant Equations:
Maximum Velocity of the fluid around the tube
(eq. 1)
V is the measured velocity
Reynolds number for the air
(eq. 2)
D is the diameter of the tube is for the inlet of the air
Air-side Nusselt number
(eq. 3)
C2, C, and m values come from tables 7.7 and 7.8 from the Heat Transfer book
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The other properties are evaluated at the average of the inlet and outlet temperature of the fluid and the subscript s is for the surface temperature.
The average heat transfer coefficient
(eq. 4)
the k value is from the inlet of the air
Knowing only the inlet temperature of the fluid in the coil
(eq. 5)
N is the number of coils NT is the number of coils that are first hit by the air flow and cp are properties from the inlet of the air
Log mean temperature difference
(eq. 6)
Ts is the temperature of the coil surface
Heat transfer rate per unit length of the tube
(eq. 7)
Procedures:1. Measure the diameter of the coil.2. Count the number of coils that are first hit by the air and the total number of coils for
each exchanger (for every U shape seen, there are two coils running through).3. Find the coil surface temperature.4. Find velocity of the air entering the coils.5. Find the temperature of the air entering the flow over the coils. 6. With the humidity sensor, take relative humidity and temperature readings of the air
near the cool side of the air conditioner.7. Repeat step 6 at the other locations on the air conditioner.
Analysis:
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1. Using the tables in the back of the Heat Transfer book find the relevant properties of the air using table A4. Air properties using temperatures of the inlet and the coil surface.
2. Using the given equations, find the convection heat transfer rate.3. Looking at the temperature and the relative humidity data, is this a comfortable
atmosphere?
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The Ideal Air-Conditioning Cycle
Objectives:To determine the coefficient of performance (COP) of a window air-conditioner
using the assumptions of an ideal vapor-compression refrigeration cycle. To determine the rate of heat removal from the refrigerated space and heat rejected from the refrigerant to the environment. To determine the power into the compressor.
Background:The ideal vapor-compression refrigeration cycle is the most commonly
used/assumed cycle for air-conditioning. This cycle in made-up of four processes:
1-2 Isentropic compression in a compressor.2-3 Heat rejection in a condenser coil, P = constant.3-4 Expansion in the throttling valve.4-1 Heat absorption in an evaporator coil, P = constant.
(Figure 1)
For an ideal cycle the refrigerant enters the compressor as a saturated vapor and is compressed isentropically. The temperature increases during the compression. The refrigerant enters the condenser as a superheated vapor and leaves as a saturated liquid.
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Win
3
4
2
1
Evaporator
Condenser
Compressor
Throttling Valve
The pressure and temperature both drop as it passes through the throttling valve. The refrigerant then enters the evaporator as a low quality mixture and evaporates completely as it absorbs heat from the surroundings.
All four parts can be modeled as steady-flow devices. The change in kinetic and potential energy are usually small compared to the work and potential energy terms. The conservation of energy equation reduces to:
q – w = he - hi (eq. 1)
The condenser and evaporator do not involve any work. The compressor can be approximated as adiabatic. The COP of an air conditioner can be expressed as:
(eq. 2)
(eq. 3)In the ideal case h1 = hg@P1 and h3 = hf@P3.
Relevant Equations:
Rate of Heat removal
(eq. 4)
Power input
(eq. 5)
Procedure:
1. Measure the temperature and pressure each of the four states described in the background.2. Measure the mass flow rate of refrigerant (read from the indicator on the sensor).
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3. OPTIONAL: Read the voltage and current from the fan motor and the power given by the Power Clamp.
Analysis:
12. Determine the specific enthalpy h at each of the four states using the refrigerant tables in the back of your thermodynamics text.
13. Using the answers from (1):(a) Use eq. 4 to determine the rate of heat removal from the refrigerated space.(b) Use eq. 5 to determine the power input to the compressor.(c) Use eq. 4 to determine the rate of heat rejection from the refrigerant to the
environment.14. Determine the coefficient of performance of the refrigerator and the heat pump using
equations 2 and 3 respectively.15. OPTIONAL: Compare the power calculated in 2b to the power obtained by reading
voltage and current from the motor of the fan and to the power read from the Power Clamp.
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References:
Boles, Dr. Michael A., and Dr. Yunus A. Cengel, Thermodynamics, An Engineering
Approach, McGraw-Hill Inc., New York, 1994.
Dewitt, David P., and Frank P. Incropera, Introduction to Heat Transfer, Third Edition,
John Wiley & Sons, New York, 1985, pp. 351-360, 757.
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Appendix G:
The drawing package
Figure 1
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Figure 2
70
Figure 3
71
Figure 4
72
Figure 5
73
Figure 6
Brian Davison’s Responsibility
74
Figure 7
Brian Davison’s Responsibility
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Figure 8
Brian Davison’s Responsibility
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Appendix H:
The surveys
Instructor Survey
Name of Lab Preformed __________________________________________
Start time _______ End time __________
Number of students in the group _____________
What questions did the students ask?________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
How many of the above questions pertain to each of the following?
How much of the time was spent waiting, “down time”? _____________________
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Student Survey (1st Round)
Name: _________________________________
Name of Lab: ____________________________ Date: _____________
Please answer the following questions on the basis of your personal experience during the lab. Please choose a number rating between one and five, with one being the lowest and five being the highest.
Did you learn/see a number of fundamentals? 1 2 3 4 5
Were the lessons/objectives clear? 1 2 3 4 5
Did you feel “real world” connections? 1 2 3 4 5
Did the lab keep your interest? 1 2 3 4 5
Was the lab hands-on/interactive? 1 2 3 4 5
Was the procedure logical? 1 2 3 4 5
Was the lab fun? 1 2 3 4 5
What fundamentals did you see demonstrated in this experiment:________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Thank you for your time and patience, your participation is greatly appreciated.Please list any comments or suggestions you may have below; they would be extremely beneficial to us.
Name of Lab: ____________________________ Date: _____________
Please answer the following questions on the basis of your personal experience during the lab. Please choose a number rating between one and five, with one being the lowest and five being the highest.
Did you learn/see a number of fundamentals? 1 2 3 4 5
Were the lessons/objectives clear? 1 2 3 4 5
Did you feel “real world” connections? 1 2 3 4 5
Did the lab keep your interest? 1 2 3 4 5
Was the lab hands-on/interactive? 1 2 3 4 5
Was the procedure logical? 1 2 3 4 5
Was the lab fun? 1 2 3 4 5
If you participated with Dr. Wexler’s class, please comment on whether or not you found the changes beneficial.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Thank you for your time and patience, your participation is greatly appreciated.Please list any comments or suggestions you may have below; they would be extremely beneficial to us.