Test: Refrigeration Cycles & Systems - 1 - Mechanical Engineering MCQ

There are practical difficulties in smoothly expanding a liquid of a highly wet vapour in an expander. As vg → vf, the positive work of isentropic expansion is seldom large enough to justify the cost of an expander. The the rmodynamic and friction losses of an expander if employed, may even exceed the gain in work. Accordingly, the isentropic expansion process may be replaced by the use of an expansion device such as a throttle valve or a capillary tube.

These tubes are air-cooled condenser which are used only in small capacity machines, such as refrigerators and small water coolers which use vertical wire and tube or plate and tube construction with natural circulation, and window type and packaged air conditioners which have tubes with 5-7 fins per cm and use forced circulation of air.

Air-cooled condensers used only in small capacity machines. Air cooled condensers are seldom made in sizes over 5 TR because of high head pressure, excessive power consumption and objectionable fan noise. In air-cooled condensers, heat is removed by air using either natural or forced circulation.

COP decreases both with decreasing evaporator and increasing condenser pressures. The COP increases by installing a subcooler between the condenser and the expansion valve. The subcooling reduces flashing of the liquid during expansion and increases refrigerating effect.

DART (Dry air rated temperature): It is the temperature of the discharge air from the expander if there is no condensed moisture present and is calculated by specific procedure. The refrigerating capacity of the air cycle unit in kW has no particular relevance. The refrigerating capacity is usually expressed in terms of the mass flow rate of air at DART. In a modern jet aircraft, typical values are 1.8 kg/s of air at 10°C DART just after take-off, and 1.4 kg/s of air at 12.8°C when the aircraft is cruising.