Question

A refrigeration system with a flash chamber operates with R134a between the pressure limits of 1.0 and 0.1 MPa. The refrigerant leaves the condenser as saturated liquid and is throttled to a flash chamber operating at 0.4 MPa. The refrigerant leaving the low-pressure compressor at 0.4 MPa is also routed to the flash chamber. The vapor in the flash chamber is then compressed to the condenser pressure by the high-pressure compressor, and the liquid is throttled to the evaporator pressure. Assume the refrigerant leaves the evaporator as saturated vapor and compressors are isentropic. (The pressures are absolute pressures)

1. Draw P-h diagram for the refrigeration cycle on the attached P-h diagram.
2. The fraction of the refrigerant vapor enters the flash chamber after the first throttle valve.
3. The enthalpy of the refrigerant vapor at the flash chamber (where the two streams of vapor meet).
4. The amount of heat removed from the refrigerated space for a mass of 0.5 kg/s through condenser.
5. The power input to the compressors.
6. The coefficient of performance for the refrigeration system.

Answer 1

A Refrigeration system with a flash chamber diagram has shown below -

Specific heat capacity ratio for R134a refrigerant -

Now for High-pressure compressor-

Point 4 in the above diagram is slightly superheated.

Refrigerant R134a has a low discharge temperature. Therefore we can say that Intercooling of refrigerant vapor is not possible with a water-cooled heat exchanger, which is generally used for Ammonia refrigerant (because of its high discharge temperature).

b)

Suppose mass flow rate through 1st stage = 1 Kg/sec

From energy balance -

   (1)

(2)

By Solving equation 1 and 2,

Therefore

We know that,

Therefore,

c)

Mentioned in the diagram-

i.e. 410 KJ/Kg

d)

We have,

          (1)

    (2)

     

      (1)

   (2)

(2) - (1) - We get,

Amount of heat removed from refrigerated space =

e)

Power input to the compressor -

If

Then

Therefore,

f)

Coefficient of Performance - COP =

If you have any doubt please ask.