Question

A refrigerator operates on an ideal vapor compression refrigeration cycle with R-134a as the working fluid. The evaporator pressure is 0.12 MPa and the condenser pressure is 0.8 MPa. If the rate of heat removal from the refrigerated space is 32 kJ/s, the mass flow rate of refrigerant is

Answer 1

Firstly we need to relate the mass flow rate and heat transfer which being taken from refrigerant compartment

general formulae for any control volume is given by

1. LHS term : rate of change of energy in control volume

2. steady flow so we have LHS term zero and mass in equal to mass out

3. assuming low velocity change

4. no change in elevation

so we have

giving

Now refrigerant goes from expansion valve to evaporator and then to condenser

Now state at the outlet of evaporator (1) , we have saturated vapour

h1 = 385.1208 kj /kg @ P1 = 0.12 Mpa -- using standard table

s1 = 1.743 kJ / kg-k  

Now determining the inlet state for the evaporator (4), for that we need to determine the state inlet to the capillary tube(3)

At capillary inlet we have saturated liquid and we know pressure so

h3 = 243.640 kJ /kg @ P3 = 0.8 Mpa -- using table

s3 = 1.1497 Kj / kg-k

Now as we have ideal cycle then s3=s4 and we have pressure P4 = 0.12 Mpa

using table we have

h4 = 236.2 Kj /kg

= 0.215 kg /s

note : if you find difficult to use table do search online calculators for help

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