REFRIGERATION SEQUENCE OF OPERATION
A good understanding of the basic operation of the refrigeration system is essential for the service technician. Without this understanding, accurate troubleshooting of refrigeration system problems will be more diffi cult and time consuming, if not (in some cases) entirely impossible. The refrigeration system uses four basic principles (laws) in its operation they are as follows:
1.“Heat always fl ows from a warmer body to a cooler body.”
2.“Heat must be added to or removed from a substance before a change in state can occur”
3.“Flow is always from a higher pressure area to a lower pressure area.”
4.“The temperature at which a liquid or gas changes state is dependent upon the pressure.”
The refrigeration cycle begins at the compressor. Starting the compressor creates a low pressure in the suction line which draws refrigerant gas (vapor) into the compressor. The compressor then “compresses” this refrigerant, raising its pressure and its (heat intensity) temperature.
The refrigerant leaves the compressor through the discharge Line as a hot High pressure gas (vapor). The refrigerant enters the condenser coil where it gives up some of its heat. The condenser fan moving air across the coil’s fi nned surface facilitates the transfer of heat from the refrigerant to the relatively cooler outdoor air.
When a suffi cient quantity of heat has been removed from the refrigerant gas (vapor), the refrigerant will “condense” (i.e. change to a liquid). Once the refrigerant has been condensed (changed) to a liquid it is cooled even further by the air that continues to fl ow across the condenser coil.
The PTAC design determines at exactly what point (in the condenser) the change of state (i.e. gas to a liquid) takes place. In all cases, however, the refrigerant must be totally condensed (changed) to a Liquid before leaving the condenser coil.
The refrigerant leaves the condenser Coil through the liquid line as a warm high pressure liquid. It next will pass through the refrigerant drier (if so equipped). It is the function of the drier to trap any moisture present in the system, contaminants, and large particulate matter.
The liquid refrigerant next enters the metering device. The metering device is a capillary tube. The purpose of the metering device is to “meter” (i.e. control or measure) the quantity of refrigerant entering the evaporator coil.
In the case of the capillary tube this is accomplished (by design) through size (and length) of device, and the pressure difference present across the device.
Since the evaporator coil is under a lower pressure (due to the suction created by the compressor) than the liquid line, the liquid refrigerant leaves the metering device entering the evaporator coil. As it enters the evaporator coil, the larger area and lower pressure allows the refrigerant to expand and lower its temperature (heat intensity). This expansion is often referred to as “boiling”. Since the unit’s blower is moving indoor air across the fi nned surface of the evaporator coil, the expanding refrigerant absorbs some of that heat. This results in a lowering of the indoor air temperature, hence the “cooling” effect.
The expansion and absorbing of heat cause the liquid refrigerant to evaporate (i.e. change to a gas). Once the refrigerant has been evaporated (changed to a gas), it is heated even further by the air that continues to fl ow across the evaporator coil.
The particular system design determines at exactly what point (in the evaporator) the change of state (i.e. liquid to a gas) takes place. In all cases, however, the refrigerant must be totally evaporated (changed) to a gas before leaving the evaporator coil.
The low pressure (suction) created by the compressor causes the refrigerant to leave the evaporator through the suction line as a cool low pressure vapor. The refrigerant then returns to the compressor, where the cycle is repeated.
Suction | Discharge | |
Line | Line | |
Evaporator | Condenser | |
Coil | ||
Coil | ||
| ||
Metering | Compressor | |
| ||
Device |
|
Refrigerant Drier Liquid
RefrigerantLine
Strainer
29
