Mumbai University > Mechanical Engineering > Sem8 > Refrigeration and Air Conditioning

Marks: 4M

Year: May 2013

• In case of ammonia, constant entropy lines in the superheated zone are of diverging type. So its temperature rise after single stage compression will be more than R12. Hence if water or flash intercooling is used in multistaging then work required for compression reduces significantly.
• But for R12 which is a Freon, the constant entropy lines in the superheated zone are almost parallel. So work consumed is not affected by intercooling and the temperature rise also is not large enough for proper heat rejection to take place in water intercooler.
• Also the value of $h_{fg}$ (specific enthalpy of evaporation) is high for ammonia. As a result the mass of liquid refrigerant that evaporates in the flash intercooler is negligible.
• But in case of Freon, the $h_{fg}$value is less. So mass of liquid refrigerant that would evaporate due to flash intercooling will be more. Therefore the compressor of the next stage has to compress not only the refrigerant coming from previous stage via intercooler, but also this extra refrigerant generated from the intercooler. As a result the compression work rather increases for R12 if intercooling is used. Hence, above statement stands true.