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Temperature difference across thermal expansion valve

Temperature difference across thermal expansion valve

Temperature difference across thermal expansion valve

(OP)
Why is there a temperature difference across a thermal expansion valve, such as one used in a refrigeration cycle? If there is no work or heat transfer in the device then the internal energy of the fluid remains the same and so should the temperature, regardless of pressure difference across the valve.

RE: Temperature difference across thermal expansion valve

The trasformation through the valve is such that starting and final enthalpy can be assumed to be the same.

h1 = c*T1 + p1*v
h2 = c*T2 + p2*v

being:

h1 = enthalpy of state 1
h2 = enthalpy of state 2
c = specific heat
T1 = temperature of state 1
p1 = pressure of state 1
v = specific volume
T2 = temperature of state 2
p2 = pressure of state 2


h1 = h2 implies

c*T1 + p1*v = c*T2 + p2*v

rearranging

T2-T1 = (p2-p1)*v/c

RE: Temperature difference across thermal expansion valve

(OP)
I think this clears up my confusion. I had in mind an ideal gases for which I do not think there would be a temperature change, since h1=h2 and h is a function of temperature only. But, I guess ideal gasses aren't used in refrigeration cycles, so this makes sense to me now. Thanks ione.

RE: Temperature difference across thermal expansion valve

Look at the last equation, the only way to get no temperature change is to have o zero volume. Nitrogen is darn close to an ideal gas and it is used as a refrigerant. Helium is even closer and it is used to liquefy itself, as a refrigerant

If you are talking about why say freon gets cold during an expansion, since the energy is zero, 1 molecule of freon transfers it's energy to another molecule. The first loses energy so it gets cold, the second one gains energy and changes energy state from liquid to gas (it vaporizes). Energy is conserved, cold is not flowing to hot, laws of thermo are maintained.

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