For Heat Transfer and Thermodynamics

Air Flow Across the Coil……….……62

The Ideal Air-Conditioning Cycle…...65

References…………………………………………………68

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.

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.

Condenser/Evaporator:

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.

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.

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 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.

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)

A compressor can be modeled as a steady flow system.

Relevant Equations:

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