One Student Question

[7ramy]

designing an evaporator qith thermal load 30 kWatt
to cool air from 45 to 21 °c
with operating temperature of 10 °c inside tubes
temperature,pressure of refrigerant at outlet inlet is unknown

i need to calculate mass flow rate as following:

flow rate = thermal load /
enthalpy out - enthalpy in


what enthalpy values should i use

those of refrigerant ?
or of (water) saturated liquid,vapor ?

difference in enthalpy for refrigerant is much lower than those of water and it confuses me
help plz

[US Iceman]

For the refrigerant enthalpy difference you need to know the condensing temperature of the refrigerant being used. Let's say the condensing temperature is 35°C. You provided the evaporator temperature of +10°C.


Also, to keep this simple we will assume there is no superheat being done in the evaporator.

So, you need to find the liquid enthalpy at 35°C. This is referred to as hf, or the liquid enthalpy.

Next you need to find the saturated vapor enthalpy at +10°C. This is referred to hg, or vapor enthalpy.

thermal load / (hg-hf) = mass flow

If the vapor is superheated in the evaporator then the value for hg is based on the vapor enthalpy at the superheated condition. Use the evaporating pressure @ +10°C (in the superheat tables) and look up the vapor enthalpy at that pressure and actual vapor temperature.

The actual vapor temperature is the saturated temperature plus the degrees of superheated created in the evaporator.