need help with thermo lab

Condenser Water Input Flowmeter

BV-3 Valve

Compressor

Condenser

Evaporator

(Inside)

Expansion Valve

Condenser Water Input

Condenser Water Output

Figure 1 Hampden Model H-6710 Refrigeration Demonstrator

Start Up/Shut Down Procedure

The instructor will prepare the system for use by completing the following steps.

· Connect the cooling water hoses and turn on the water

· Open the valves to release the R134a refrigerant from its storage can into the system

You will complete the following steps to run the system

· Closing the BV-3 valve and turning the flow meter valve fully open (counter-clockwise) so that cooling water flow is temperature controlled

· Setting the water heater set point as desired for the lab section

· Starting the system and running it at 60 Hz for 1 hour to warm up

· Main AC

· Pump

· Heater

· Compressor

· Start the compressor by pressing the FWD button on the compressor motor control panel and adjusting to 60 Hz using the arrow keys

The unit is shut down by reversing the start up procedure after returning the compressor control frequency to 60 Hz. You must press the stop button on the compressor motor controller and wait until the motor ramps down to a stop before starting to turn off the power. For long term shut down the instructor will pump the R134a refrigerant into the storage can.

Condenser Theory

The condenser of the refrigeration cycle is a counter-flow, shell-and-tube heat exchanger. If the heat exchanger is completely insulated, we can assume that there is no heat loss in the pipes, and we can assume that the water absorbs heat lost by the refrigerant so it can later expand in the evaporator, gaining heat. The goal of the condenser in the refrigeration cycle is to cool the hot refrigerant with water. In order to calculate the heat gained by the water we must use the following equation

Eqn 1

Where Q is the heat gained from the refrigerant per unit time, is the mass flow rate (in lbm/s), c is the specific heat (in Btu/lbm°F) and ΔT is the change in temperature (in °F). The value of the specific heat can vary by temperature, but for water the change is minimal for the range of temperatures you will observe. For this lab the specific heat value will be 1 Btu/lbm °F.

To calculate the heat loss of the refrigerant, one must understand what is occurring in the condenser. The refrigerant is going through a phase change. When the refrigerant leaves the compressor, it leaves at a high pressure, high temperature gas. When the refrigerant passes though the condenser it changes from a gas to a liquid. The refrigerant will leave as a low temperature, high pressure liquid. Since the refrigerant is going through the phase change and a drop in temperature, the specific heat cannot be used. Therefore, equation 1 will be invalid for finding the heat loss of the refrigerant. In order to find the heat loss of the refrigerant you must find the change in enthalpy. The equation is as follows:

Eqn2

where is the enthalpy (measured in Btu) of the refrigerant entering the condenser and is the enthalpy of the refrigerant exiting the condenser. The change of enthalpy can be also obtained by using specific enthalpy or enthalpy per unit mass. The unit of specific enthalpy is Btu/lbm. The equation will be:

Eqn 3

Where and are the specific enthalpy of the refrigerant entering and exiting the condenser and q is the heat gained per unit mass.

Since the refrigerant is entering the condenser as a gas, the enthalpy values are obtained in the superheat tables. The refrigerant exits the condenser as a liquid, and the enthalpy are found in the saturation chart. Keep in mind that the refrigerant exiting the condenser is liquid and not vapor. Make sure that the proper value of enthalpy is selected.

The refrigerant mass flow rate must be also taken into account. The refrigerant flow meter is located on the Hampden Model H-6710 Refrigeration demonstrator. The refrigerant flowmeter has its own scale and the values must be looked up in Table A-1 excerpted from the Hampden Model H-6710 Refrigeration demonstrator user manual. When reading the flowmeter, you must read your value from the middle of the sphere located within the flowmeter.

The effectiveness can be calculated once the data has been obtained. Effectiveness is defined as: Eqn 4

The actual heat loss is relatively easy to find, but finding the maximum heat loss can be harder to find. First, what is the maximum heat loss? The maximum heat loss is the maximum amount of heat loss by the hot fluid and transferred into the cold fluid. The exit temperature of the hot fluid can only be as cool as the cold fluid entering the condenser.

For equation 4 to be valid, the enthalpy of the refrigerant at the inlet temperature of the water must be found which is the lowest enthalpy value that the refrigerant can reach. The maximum heat loss for the refrigerant will be:

Eqn 5

where Hmax is the enthalpy of the refrigerant at the water inlet temperature.

The final effectiveness equation will be:

Eqn 6

Full Cycle Theory

The refrigeration cycle can be viewed as a reversed heat engine. The difference between the two is that in the refrigeration cycle work is used to pump heat out of the system where in a heat engine heat instead is used to produce work. Depending on the flow direction of the refrigerant the system can act as a refrigerator or a heater. With a reversing valve as shown in the figure below it can provide cooling or heating. In either the cooling or heating mode this is also called a heat pump since heat is pumped into and out of the system.

According to the second law of thermodynamics, heat cannot be transferred from a medium at a lower temperature to one at a higher temperature without providing mechanical work. The goal of the heat engine is to absorb thermal energy from a high temperature region and convert it into work, while releasing heat to a lower temperature region. The heat engine must obey the second law of thermodynamics. However, the refrigeration cycle is slightly more complex. The goal of the refrigeration cycle is to remove thermal energy from a lower temperature region and transfer it to a higher temperature region. The compressor does this work to make the refrigeration cycle possible.

The refrigeration cycle has four main components: the evaporator, compressor, condenser, and expansion valve. The schematic of a simple refrigeration cycle is shown in Figure 2.

Figure 2 Schematic diagram of the refrigeration system

The purpose of the evaporator is to transfer heat from a higher temperature environment to the refrigerant. The evaporator will also cause the low-pressure refrigerant to vaporize as heat is transferred from the evaporator to the refrigerant. From the evaporator the low-pressure refrigerant gas then goes to the compressor. The compressor will compress the refrigerant from a low-pressure gas to a high pressure, high temperature gas. Work is done on the compressor by electrical means in order to achieve pressurization. After the compressor, the refrigerant then goes through a condenser. The condenser will remove any heat from the refrigerant, which will cause the gas to liquefy but remain at high pressure. Finally, the refrigerant will pass through an expander, which will cause the refrigerant to go from being high pressure to low pressure. This process causes the cooling effect and heat is transferred. The cycle will then repeat itself.

The effectiveness equation for the evaporator in terms of Figure 2 will be:

Eqn 7

Where h4 is the enthalpy of the refrigerant entering the evaporator, h1 is the enthalpy of the refrigerant leaving the evaporator, and hmax is the enthalpy of refrigerant at the temperature of the water in the evaporator.

In order to find the coefficient of performance or the coefficient of refrigeration you will need to find the work done on the cycle, the heat added to the refrigerant, and the heat rejected by the refrigerant. To accomplish this you will need to find enthalpy values at certain locations on the cycle. By assuming a steady-flow system to Figure 2, we will obtain the following equations:

1. For the evaporator,

Eqn 8

2. For the Compressor,

Eqn 9

3. For the condenser,

Eqn 10

4. For the Expansion Valve,

Eqn 11

By finding work of the compressor, heat added, and heat rejected, you can easily calculate the coefficient of performance, and the coefficient of refrigeration. The coefficient of performance for a heat pump can be defined as:

COPH Eqn 12

In addition, the coefficient of performance for refrigeration is defined as:

COPR Eqn 13

In terms of enthalpy the coefficient of performance will be:

COPH Eqn 14

In addition, the coefficient of refrigeration will be:

COPR Eqn 15

Performing the lab

1. Start the system and allow it to run for 20 minutes with the BV-3 valve closed and the flow meter fully open so the thermostat controls the water flow rate through the condenser.

2. Use this time to make sure you understand the system and determine what data you need to collect to perform the calculations.

3. Collect the data for the thermostatically controlled system.

4. Open the BV-3 valve and adjust the flow meter to a flow rate of 2 gpm (gallons per minute). Allow the system to stabilize for 5 minutes and collect the data under this condition.

5. Adjust the flow meter to a flow rate of 1 gpm, allow the system to stabilize for 5 minutes and collect the data under this condition.

6. Adjust the flow meter to a flow rate of 0.5 gpm, allow the system to stabilize for 5 minutes and collect the data under this condition.

7. Shut down the system as described earlier. Fully open the flow meter and shut off the water making sure you do not shut off the water for other experiments.

Report

This will be a formal lab report by your entire laboratory group and should include the following items in addition to the standard text describing what you did and what you learned.

· A nicely drawn block diagram of the refrigerant path showing where all of the measurements were taken.

· A plot of the COPR and COPH vs. water flow rate.

· A plot of the heat flows vs. flow rate

Pre-Lab Name: _______________________

1. What is a vapor compression cycle?

2. How is the performance of a vapor compression refrigerator measured?

3. Can a vapor compression refrigeration system operate both as a refrigerator and a heater?