Parallel Compressor Systems
Electronic Control System
The electronic controller has become the standard on parallel compressor systems. The increased capabilities of the controllers magnify the efficiency of the parallel compressor system making it a very attractive accessory item.
The electronic control system preferred by Heatcraft is the Computer Process Control (RMCC) controller. The RMCC offers a complete control and monitoring package through one or more input boards (16AI). The controller continuously monitors the parameters for refrigerant pressures, defrost operation, temperature control, and system alarms.
As the RMCC monitors the system in operation, it compares the reported values against programmed set points it is to maintain, thus cycling compressors, unloaders, condenser fans, defrost periods and sounding alarms as required.
Interface with the actual devices being controlled is through one or more pressure transducers, watt transducer, temperature sensors, refrigerant sensors, humidity sensors, refrigerant level sensor, phase loss and output boards (8R0). The 8RO boards can be mounted remotely for lower installation cost, when controlling devices such as air cooled condensers. These boards are connected to the RMCC via a three wire network and are purchased as needed for the application. Remote communications capabilities is standard with the RMCC through a modem that is supplied.
System Balancing
Important: In order to obtain the maximum capacity from a system, and to ensure
System Superheat
This is extremely important with any refrigeration system. The critical value that must be checked is suction superheat
Superheat is not preset at the factory.
Suction superheat should be checked at the compressor as follows:
1.Measure the suction pressure at the suction service valve of the compressor and determine the saturation temperature corresponding to this pressure from a “Temperature- Pressure” chart.
2.Measure the suction temperature of the suction line about one foot back from the compressor using an accurate thermometer.
3.Subtract the Saturated temperature from the actual suction line temperature. The difference is Superheat.
If suction superheat is too low, it can result in liquid refrigerant being returned to the compressor. This will cause dilution of the oil and eventual failure of the bearings and piston rings. In extreme cases, the compressor will fail as a result of the diluted oil.
High suction superheat will result in excessive discharge temperatures that can cause a breakdown of the oil. This action results in piston ring wear, piston and cylinder wall damage.
Also, as the superheat increases, the suction pressure decreases resulting in reduced capacity. For maximum system capacity, suction superheat should be kept as low as is practical. Heatcraft recommends that the superheat at the compressor be no lower than 20°F.
If adjustments to the suction superheat need to be made, the expansion valve at the evaporator should be adjusted. See instructions in next section.
Evaporator Superheat
Check Your Superheat after the box temperature has reached or is close to reaching the desired temperature, the evaporator superheat should be checked and adjustments made if necessary. Generally, systems with a design TD of 10°F should have a superheat value of 6° to 10° F for maximum efficiency. To properly determine the superheat of the evaporator, the following procedure is the method Heatcraft recommends.
1.Measure the temperature of the suction line at the point the bulb is clamped.
2.Obtain the suction pressure that exists in the suction line at the bulb location by either of the following methods:
a)A gauge in the external equalized line will indicate the pressure directly and accurately
b)A gauge directly in the suction line near the evaporator or directly in the suction header of the evaporator will yield the same reading as above
3.Convert the pressure obtained in 2a or 2b above to saturated evaporator temperature by using a
4.Subtract the Saturated temperature from the actual suction line temperature. The difference is Superheat.
Alternative Superheat Method
The most accurate method of measuring superheat is found by following the previous procedure, Temperature/Pressure method. However, that method may not always be practical. An alternative method which will yield fairly accurate results is the temperature/temperature method.
1.Measure the temperature of the suction line at the point the bulb is clamped (outlet).
2.Measure the temperature of one of the distributor tubes close to the evaporator coil (inlet).
3.Subtract the outlet temperature from the inlet temperature. The difference is approximate Superheat.
This method will yield fairly accurate results as long as the pressure drop through the evaporator coil is low.
Parallel Compressor Systems Installation & Operations Manual, October 2004 | 19 |