Refrigeration Cycle

REFRIGERATION CYCLE





The compressor discharges high-temperature, high-pressure vapor that contains heat absorbed in the evaporator, plus heat imparted to it by the compressor in the compression process. This vapor flows through two parallel tubes in the condenser where it releases heat to the airstream and changes to a medium-temperature, high-pressure liquid. This liquid flows through the liquid line to the orifice tube where it undergoes rapid expansion and changes from a medium-temperature, high-pressure liquid to a low-temperature, low-pressure liquid/vapor mixture. This cold, foamy refrigerant mixture enters the evaporator core at the bottom and flows through parallel tubes upward through the "U" flow core. Heat from the warm airstream passing through the core is transferred to the refrigerant, vaporizing the liquid and cooling the air. Under high-load conditions, all the liquid is vaporized in the evaporator and only heat-laden vapor flows to the accumulator and through the suction line to the compressor, where the cycle is repeated. Under lighter load conditions, the liquid and vapor are separated in the accumulator so that only vapor is drawn into the compressor.


COMPRESSOR





The vehicle uses an HD-6 compressor which is a 10 cubic inch displacement six cylinder axial-type mechanism consisting of three double-ended pistons actuated by a swash plate shaft assembly. The compressor is located on the lower right of the engine centerline (right-front corner of the engine compartment), and is belt-driven from the crankshaft pulley. The compressor performs two main functions. It provides pressure and temperature levels at which heat exchange can take place in the condenser by compressing the low-pressure refrigerant vapor from the evaporator into a high-pressure, high-temperature vapor. It also pumps refrigerant (and refrigerant oil) through the A/C system.

A magnetic clutch is used to engage the compressor shaft. When voltage is applied to the clutch coil (by closing contacts in the thermostatic switch), the clutch plate and hub assembly (which is solidly coupled to the compressor shaft), is drawn by magnetic force rearward toward the pulley which rotates freely on the front head casting. The magnetic force holds the clutch plate and pulley together as one unit. The compressor shaft then turns with the pulley. When the A/C programmer removes the voltage from the clutch coil, springs in the clutch plate and hub assembly move the clutch plate away from the pulley and the clutch plate and compressor shaft cease to rotate.


HIGH-PRESSURE RELIEF VALVE





The compressor has a pressure relief valve that opens and allows refrigerant to vent off to the atmosphere when high side pressure exceeds 3,620 ± 571 kPa (525 ± 75 psi). This is done to prevent system damage. Any condition that causes this valve to open should be corrected and the refrigerant oil and refrigerant should be replaced as necessary.

MUFFLER
The muffler is used in refrigerant Systems to reduce compressor noises and high-pressure line vibrations. A wire mesh screen inside the muffler will trap debris should the compressor fail; this will eliminate the need to flush the refrigerant system. After a compressor failure, the hose assembly will require replacement.


CONDENSER
The condenser is an aluminum tube and fin heat transfer unit located directly forward of the radiator. The function of the condenser is to transfer heat from the refrigerant flowing through its tubes to the air stream drawn through it by the engine fan. The smallest diameter refrigerant line in the system, the high-pressure liquid line, carries the high-pressure liquid refrigerant from the condenser to the evaporator. (There is no sight glass in this system.)

EXPANSION (ORIFICE) TUBE





Located in the evaporator inlet line, the expansion (orifice) tube provides resistance to the high pressure liquid, thereby controlling the flow of refrigerant to the evaporator and allowing it to enter the evaporator as a low-pressure liquid. There is also a fine mesh screen located at the expansion tube inlet, which acts as a strainer for refrigerant flowing through it.

An extended plastic screen downstream of the orifice tube serves to reduce the flow noise associated with the pressure equalization when the system is shut "OFF".

The expansion tube cannot be adjusted or repaired and must be replaced as a unit. After passing through the expansion tube, the low pressure refrigerant enters the evaporator core.

EVAPORATOR CORE
The aluminum evaporator core is located along the right-hand side of the cowl, and is the actual cooling unit of the system. The blower forces air through the evaporator core, where it is cooled and dehumidified. Condensation, which collects on the cold fins of the condenser, is drained from the housing through a small hole in the module bottom. Unlike a radiator which transfers heat to the air flowing through it, the evaporator core absorbs heat from the passing air. This is how the air is cooled. The heat that is absorbed from the air causes most of the refrigerant to boil into a vapor.

This vapor then flows out the top of the evaporator and into the accumulator, carrying with it some liquid refrigerant and refrigerant oil.

ACCUMULATOR





Connected to the evaporator outlet pipe, one of the functions of the accumulator is to separate the liquid refrigerant from the vapor, and to act as a storage device for the liquid refrigerant.

The second function of the accumulator is as a dehydrator to absorb any reasonable amount of moisture that may have entered the system. Two bags of molecular sieve desiccant are located in the bottom of the accumulator. An oil bleed hole is also located in the lower portion of the accumulator to prevent oil from being trapped, thus ensuring the return of oil to the compressor. The desiccant within the accumulator is not serviced separately, it is part of the sealed accumulator assembly which is replaced as a unit.

The low-pressure vapor then flows from the accumulator back to the compressor, where it will be compressed and the cycle will begin again.

A low-side pressure Schrader valve service fitting is located near the top of the accumulator.