Naked Science Forum

Non Life Sciences => Technology => Topic started by: ScorzosoLw on 05/08/2020 14:17:05

Title: How is specific enthalpy for a refrigerant calculated?
Post by: ScorzosoLw on 05/08/2020 14:17:05

A heat pump is used so that room temperature remains at 21°C, with the heat output of Q=8kW. The refrigerant R134a is isobarically condensed from state 1 (p1= 10 bar, h1= 440.58 kJ/kg) to state 2 (subcooled, t2 = 36°C). For the subcooled liquid at 10 bar, cp= const=1.464 kJ/kg.K. I need to find the mass flow of the refrigerant. Where I am confused is that the formula for specific enthalphy is h=cp*T (for cp=const), so h2 should be h2=452.59 kJ/kg. I know this is not the correct value for this, the answer is that h2=250.54 kJ/kg. Can someone explain how can I get that value?

Title: Re: How is specific enthalpy for a refrigerant calculated?
Post by: devin07 on 13/09/2021 18:37:04

You are the use of h=cpT absolutely incorrect and that method would not consist of actual fueloline effects. Enthalpy is relative so in this situation, we've got a recognized enthalpy (sat. liquid @ 1MPa)... name it kingdom a. The alternate in enthalpy from kingdom a to kingdom b (equal stress at temp = 36C) is cp(Tb-Ta).