represent the cycle on P-h diagram.

Mumbai University > Mechanical Engineering > Sem 8 > Refrigeration and air conditioning

Marks: 06M

Year: May 2016

Fig.(a) and (b) shows a two stage vapour compression refrigeration system with flash intercooler and water intercooler interposed between two stages. the superheated vapour from the water cooled heat exchanger bubbles through the refrigerant liquid in the flash tank. it is assumed that in this process the superheated refrigerant vapour gets completely de-superheated and emerges out as a saturated vapour at state 4. however, in practice completer de-superheating may not be possible. as mentioned the use of combination of water cooling with flash tank for intercooling reduces the vapour generated in the flash tank. the performance of this system can be obtained easily by applying mass and energy balance equations to the individual components. it is assumed that the flash tank is perfectly insulated and potential and kinetic energy changes of refrigerant across each component are negligible.

Fig (a): Two stage VCRS with flash and water intercooler Fig(b): Two stage vapour compression refrigeration system with water and flash intercooling- P-h diagram

From mass and energy balance of the flash tank:

where $Q_I$ is the heat transferred by the refrigerant to the cooling water in the intercooler. It can be seen from the above expression that the refrigerant flow through the high stage compression $m_{II}$ can be reduced by reducing the enthalpy of refrigerant vapour entering into the flash tank, $h_3$ from the water-cooled intercooler.

The amount of additional vapour generated due to de-superheating of the refrigerant vapour from the water-cooled intercooler is given by:

thus the vapour generated $m_{gen}$ will be zero, if the refrigerant vapour is completely de-superheated in the water-cooled intercooler itself. however, this may not be possible in practice.

For the above system, the COP is given by:

The above system offers several advantages,

a) Quality of refrigerant entering the evaporator reduces thus giving rise to higher refrigerating effect, lower pressure drop and better heat transfer in the evaporator.

b) Throttling losses are reduced as vapour generated during throttling from $P_c$ to $P_i$ is separated in the flash tank and recompressed by Compressor-II.

c) Volumetric efficiency of compressors will be high due to reduced pressure ratios.

d) Compressor discharge temperature is reduced considerably.