Refrigerator and Heat Pump question (Very Important)

1. Aug 20, 2015

KingDaniel

1. The problem statement, all variables and given/known data
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.

2. Relevant equations

3. The attempt at a solution

2. Aug 20, 2015

BvU

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

3. Aug 22, 2015

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".