Calculating COP of a Refrigerator

[skaboy607]

Hi,

I think I have confused myself over the COP of a refrigerator. I have refrigeration system and a certain set of values which enable me to plot a P-s diagram. The refrigerant is R-12. I also have another set of 4 results for varying evaporator pressures, and I have drawn all these cycles on thte P-s diagram. I have calculated the corresponding enthalpy values for all the sets of data and also h2' (based on ideal cycle) for each set. My question is...for one set of data, the h2 and h2' are almost identical which to me, made me think that this was the setup that maximised the net work output. however when calculating COP based on ideal cycle for all sets of data-this setup provide with the lowest value, the one with the worst performance?

I don't understand? Have I got my wires crossed?

Thanks

Will

 

[Valerie Park]

No, you haven't got your wires crossed. The enthalpy values for a given cycle are related to the amount of work that can be extracted from the system, so it makes sense that h2 and h2' would be nearly identical if the system is being operated at maximum efficiency. However, the COP is a measure of the efficiency of the entire system, not just the amount of energy that can be extracted from the cycle. When calculating the COP, you need to take into account other factors, such as the power input to the compressor and the losses due to friction in the system. So even though one setup may have the highest possible energy extraction from the refrigerant cycle, it may still have a lower overall COP than another setup with a lower energy extraction due to other factors.

 

[charng]

Hi Will,

Calculating the COP (Coefficient of Performance) of a refrigerator can be a bit confusing, but I'll try to explain it in a simple way. The COP of a refrigerator is the ratio of the heat removed from the refrigerated space to the work put into the system. It is a measure of the system's efficiency in removing heat.

In your case, you have plotted the P-s diagram and calculated the enthalpy values for different sets of data. It is important to note that the h2' value is the theoretical enthalpy value for an ideal cycle, while the h2 value is the actual enthalpy value for your system. So, for one set of data, if the h2 and h2' values are almost identical, it means that your system is performing close to an ideal cycle.

However, when calculating the COP based on the ideal cycle, it is expected to provide a lower value compared to the actual COP of your system. This is because the ideal cycle assumes that there are no energy losses or inefficiencies in the system, which is not the case in real life. So, the setup that maximizes the net work output may not necessarily have the best performance in terms of COP.

I hope this clarifies your confusion. It is important to keep in mind that the COP is just one measure of performance and there are other factors that need to be considered when evaluating the efficiency of a refrigerator. Keep up the good work in your research!

Best regards,