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System balancing, System superheat, Evaporator superheat – Heatcraft Refrigeration Products PARALLEL COMPRESSOR SYSTEMS 25000102 User Manual

Page 19: Alternative superheat method

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Parallel Compressor Systems Installation & Operations Manual, October 2004

19

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 trouble-free

operation, it is necessary to balance each

and every system.

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 temperature-pressure

chart.

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.