# Energy Transfer homework

1. Sep 14, 2006

### onemoretomorrow

Hi guys i really need some help on this question.

Refrigerant 134a enters the compressor of a refrigeration system as saturated vapour at 0.14MPa and leaves as superheated vapour at 0.8MPa and 50 degrees celsius as a rate of 0.04kg/s. The suction area of the compressor is 10cm^2 and the discharge area 5cm^2. Determine the rates od energy transfer by mass into and out of the compressor, neglecting potential energy. Note the order of magnitude of the enthalpy and kinetic energy terms.

I cant seem to be able to get the value for the mass. can someone help? thanks

2. Sep 16, 2006

### Astronuc

Staff Emeritus
One does not need mass, since the problem seems to be asking for energy on a per unit mass basis.

The kinetic energy per unit mass is simply V2/2. Thermodynamic properties are usually given on a per unit mass basis, e.g. specific enthalpy, h (J/kg), or internal energy, u (J/kg).

Mass flow rate is given - 0.04kg/s - and assuming the continuity equation, as steady-state, mass rate (in) = mass rate (out), otherwise there would be an accumulation (if mass flow in exceeds mass flow out) of the system.