Question 12.21: A refrigeration cycle uses Freon-12 as the working fluid. Th...
A refrigeration cycle uses Freon-12 as the working fluid. The temperature of the refrigerant in the evaporator is – 10°C. The condensing temperature is 40°C. The cooling load is 150 W and the volumetric efficiency of the compressor is 80%. The speed of the compressor is 720 r.p.m. Calculate the mass flow rate of the refrigerant and the displacement volume of the compressor.
Properties of Freon-12
| Temperature (°C) | Saturation pressure (MPa) |
Enthalpy (kJ/kg) | Specific volume (m³/kg) Saturated vapour |
|
| Liquid | Vapour | |||
| – 10 40 |
0.22 0.96 |
26.8 74.5 |
183.0 203.1 |
0.08 0.02 |
(GATE)
Given : Cooling load = 150 W ; η _{vol.} = 0.8 ; N = 720 r.p.m.
Mass flow rate of the refrigerant \dot{m}:
Refrigerating effect = h_{2} – h_{1}
= 183 – 74.5 = 108.5 kJ/kg
Cooling load = \dot{m} × (108.5 × 1000) = 150
or \dot{m} = \frac{150}{108.5 × 1000 } = 0.001382 kJ/s.
Displacement volume of the compressor :
Specific volume at entry to compressor,
v_{2} = 0.08 m³/kg (From table)
∴ Displacement volume of compressor = \frac{\dot{m} v_{2}}{η _{vol.}} = \frac{0.001382 × 0.08}{0.8}
= 0.0001382 m³/s.
