Refrigerator and Heat Pump question (Very Important)

[KingDaniel]

Homework Statement

Hi,
How come, for a refrigerator and a heat pump, we don't consider the changes in energy for the refrigerant for when the pressure drops (at expansion valve) and when it rises (at the compression valve)? We only consider the changes in energy when it evaporates and condenses, and the electrical work.

Homework Equations

The Attempt at a Solution

 

[BvU]

Who says we don't ? Are you familiar with the enthalpy concept ?

 

[rude man]

Depends on how you look at it.
By the 1st law,
Q1 = Q2 + W for each complete cycle. So you can for example find efficiency e = Q2/W = Q2/(Q1-Q2) if you knew Q1 ("energy by condensation") and Q2 ("energy by evaporation").

However, Q1 and Q2 are not readily obtainable from a p-V diagram since the process is not reversible. But as BvU suggests, knowing the enthalpies associated with each part of the cycle allows you to compute e and other parameters like "quality".