Refrigerant 134a R134a enters a compressor Figure 1 steadily

Refrigerant 134a (R134a) enters a compressor, Figure 1, steadily as a saturated vapour at 10 degree C with a mass flow rate of 5 kg/s. It leaves the compressor at a pressure of 1400 kPa and temperature of 60 degree C. A heat loss of 2.5 kJ/kg occurs during the process. The inlet pipe to the compressor has a diameter of 250 mm and the exit pipe is 50 mm diameter Power input vs heat loss for exit pipe sizes of 25, 50 and 65 mm. power input against exit temperature for exit pipe sizes of 25, 50 and 65 mm Discuss the results (consider each plot). What would you recommend, in terms of minimising the compressor work in regards to R134a exit temperature, compressor cooling and exit pipe size?

Solution

The first abservation I get from the graphs plotted is that there is a significant change of work done in the compressor when transitioning from 25mm to 50 mm but the transition from 50mm to 65mm is hardly noticeable. Hence, for practical purposes there is no difference between a 50mm and a 65mm pipe. However, the 65mm pipe is more expensive to make and consumes a lot of material. The benefits derived would easily be overtook by the costs of manufacture. Hence, 65 mm pipe can be safely ignored.

Between the 50mm pipe and 25mm pipe, a look at the graph tells me that the work done by the compressor in case of 25mm pipe as output pipe is significantly lower than the work done in the case of 50mm pipe. I can only assume that to maintain the same output conditions, the compressor should work harder and increase the flow rate to fill up the extra space available in a 50mm diameter pipe.

The answer therefore becomes obvious that for the minimum work for the compressor, the 25mm pipe should be selected for exit.

It can also be seen that as the exit temperature increases, the work done by the compressore decreases. Hence, I would suggest increasing the outlet temperature. But one thing to be kept in mind is that as the exit temperature increases, there is the need for a bigger and more effective heat exchanger to cool down the liquid.

From the graph power input vs heat loss, we can see that as the heat dissipated inside the compressor increases, the power input required decreases. Hence, it would be suggestible to increase the heat dissipation rate to reduce the power required to run the compressor.

I am speculating here that the increase in output temperature is causing a bigger difference between the compressor temperature and the atmospheric temperature causing a greater heat dissipation rate. If that is the case and the extra temperature after compression can be compensated, then it is even better because there is no need for a bigger colling system as well as the work input to the compressor is quite low.