paraphrasing

Abstract:

In this experiment, data readings of pressure and temperature of R-134A are taken from the laboratory refrigeration apparatus (Hampden Model H-RST-3B), along with the fluid flow rate and electric power input to the compressor. This data is used to find the thermodynamic state at various points within the Vapor compression refrigeration cycle. All this information provides the necessary information to draw state diagrams of the cycle and evaluate the coefficient of performance for the actual, ideal and carnot cycles.

Introduction:

The vapor-compression refrigeration system uses a liquid refrigerant as the medium, in this case R-134-A, which absorbs and removes heat from the space to be cooled and rejects that heat into the atmosphere. Figure 1 shows a standard single-stage vapor-compression system that was used in the experiment. The cycle begins with refrigerant entering the compressor in the thermodynamic state known as a saturated vapor and is compressed to a high pressure, which results in a high temperature as well (shown as #2 in figure-1). This hot, compressed vapor is then in the thermodynamic state known as a superheated vapor. It is then routed through a condenser where it is cooled and condensed into a liquid by rejecting its heat into the atmosphere (shown as #3 in figure-1). This condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the evaporation of a part of the liquid refrigerant, which causes the temperature of the liquid to lower. This results in a vapor refrigerant mixture that is colder than the temperature of the enclosed space to be refrigerated (shown as #4). The cold refrigerant mixture is then routed to the evaporator where a fan circulates the warm air in the enclosed space across the tubes carrying the cold refrigerant mixture. The warm air evaporates the liquid part of the cold refrigerant mixture as the circulating air is cooled by the tubing. This lowers the temperature of the enclosed space to the temperature set on the thermostat. To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor (shown as #1 in figure-1).

Description of Work:

· The laboratory refrigeration apparatus (Hampden Model H-RST-3B) is shown in figure-2. To start the experiment, make sure the CTV, HEV, and LRB valves are closed and the TEV, LRI, and LRO valves are open.

· Next switches S1, S2, and S3 need to be turned on. S2 and S3 need to be adjusted so that the fan speeds are at or near maximum.

· The temperature control knob needs to be set to approximately 40 oF (using knob labeled as “Temp. Control” on Figure 2). Make sure that the compressor doesn’t shut off or the temperature and pressure readings will be off.

· After the unit is turned on, it needs to run for roughly ten minutes in order to achieve steady operating conditions. Steady state is achieved when multiple instances of data are recorded and a small to no difference is seen in the recorded data.

· Finally data can be recorded. Pressure data needs to be recorded from P1-P4 located on figure-2. Next, temperature data from T1-T6 on figure-2 needs to be recorded. Also data from the flow meter and wattmeter located on figure-2 needs to recorded.

· After all the data has been collected, the unit needs to be shut down. To do this, first close valves TEV and LRO(see figure-2). The evaporator pressure will decrease, causing the automatic Low-Pressure Cut-Out switch to turn the compressor off (and the voltmeter to go to zero). Finally turn off switches S1, S2, and S3 and close valve LRI.

Discussion:

After all the data had been recorded, the enthalpies and pressure at each state in the vapor compression refrigeration cycle needed to be found with respect to the measurement locations on the laboratory refrigeration apparatus. Using figure-3 as a guide, this information was found and put into a table that is included in the results section.

Conclusions:

Looking at the COP data, the carnot and ideal COP’s are fairly close to each other and the actual COP is very low comparatively. I believe this is due the laboratory refrigeration apparatus losing and gaining heat to the atmosphere through its copper piping as the apparatus goes through its cycle. This inefficiency would greatly reduce the performance of the system and cause more work to be put into the system to achieve the same results. One suggestion would be to wrap all the copper piping in some insulating wrap to stop the heat loss/gain to the atmosphere. I believe this would increase the actual COP and bring it up around the values of the ideal and carnot COP’s.

Ideally yes we can assume that the compressor is adiabatic and is not directly transferring heat to the working fluid. However in the real world, the forces of friction within the compressor and heat transfer in the piping will cause some conduction into the compressor. This will result in the compressor heating up over time and thus some heat transfer from the compressor to the working fluid, which is not adiabatic.

Overall the lab experiment went very well. We did not run into any problems with the compressor turning off at a setting of 40° F or with inconsistent data. The only problem was a broken thermometer in one of the temperature locations. To get around this we simply changed out the broken one with a working one from a different location with roughly the same temperature, so it wouldn’t take too long to get to its operating temperature. In conclusion, the only recommendation for further improving this lab would be to get another thermometer so all data points can be measured at the same time.