Setting refrigeration pressure switches

By Grant Laidlaw
A pressure switch serves as a reliable operating and/or safety control device and all HVAC&R technicians should be able to set one.

James asks: Hi, Grant. We really have difficulties when working on refrigeration equipment that does not operate with a thermostat. The underbar systems are a typical example. Temperature is controlled by a pressure switch. The older technicians just give out settings but cannot describe how they come to these settings. This is somewhat of a thumbsuck method. If you could describe how to determine the settings and please, if possible, describe a method for the practical setting of a pressure switch. Maybe this is outdated?

Hi, James. Modern systems tend towards electronics for control, but yes, the underbar systems and many smaller walk-in cold rooms still operate on a pressure switch for temperature control.

A good understanding of a refrigerant’s pressure temperature relationship is essential in understanding how this type of system is controlled.
These systems do not have defrost heaters or timers.

As we know, the formation of frost on an evaporator surface is very undesirable. In the case of a pump-down system, heater elements will be installed below the evaporator. This is, of course, controlled by a defrost timer. In the situation where we have an installation temperature of above 0°C, the air flowing through the evaporator will remove frost build-up during the compressor off cycle.

This type of installation uses a pressure switch for temperature control and operates on what is called the automatic defrosting cycle.
The automatic defrosting cycle operates as follows:

  • During the “ON” cycle the coil is usually at a temperature below the freezing point of water (0°C). This will cause a frost accumulation on the evaporator.
  • During the “OFF” cycle the evaporator warms up enough so that the frost melts from the coil before the compressor starts again.

To enable the frost to melt during the “OFF” cycle it is obvious that the installation temperature must be above the melting point of ice. As a general rule, the minimum installation temperature of a system operating on the automatic defrosting cycle is 1°C.

One of the aspects that we do need to understand is that the suction pressure does not remain constant. The average refrigerant pressure is the pressure of the refrigerant in the evaporator taken halfway through the running cycle as measured on a time basis. This should not be confused with the halfway point between the cut-on and cut-off pressure.

Referring to the temperature/pressure chart, the average refrigerant temperature is obtained.

A general rule, which may be followed for this type of system, is that the average suction pressure is usually about 14kPa above the cutout point, excluding the pressure drop in the coil and suction line.

The average suction pressure is therefore 14kPa above the minimum suction pressure or the cut out pressure. This point is called the minimum suction pressure. The average suction pressure is the refrigerant pressure at the evaporator but the pressure switch that controls the system is located at the condensing unit. Due to the effect of gas friction losses in the coils there is a pressure drop in the evaporator and suction line. This pressure drop in an average installation is about 21kPa.

To calculate the suction pressure in the evaporator from a reading taken at the condensing unit, we must consider the pressure drop in the coil and suction line.

The cut-in point of the pressure switch is determined by the installation temperature when automatic defrosting is required. Water freezes and ice melts at approximately 0°C, but in practice a slightly higher temperature is necessary to ensure fairly rapid defrosting of the evaporator. If the required installation temperature is 3°C, the evaporator can safely be allowed to reach about 4°C during the defrost cycle.

Therefore to determine the “range” (cut in setting) for this type of system one simply adds 1°C to the required installation temperature. Converting this temperature to pressure will give you the correct setting for the pressure switch. This does not dramatically change the product stored temperatures, which remain fairly constant.

Grant01

Calculation of low-pressure switch settings in practice

Assuming that we have a cold room fitted with a forced air evaporator connected to a condensing unit, the desired cold room temperature (BT) is 3°C, and automatic defrosting is required. The plant has been designed to give a temperature difference (TD) of 9K. This gives an ET of -6°C, converted to pressure equals 132.9kPa.

Grant02

James, this gives you the answer to the first part of your question.

It is also important to understand that the Temperature Difference (TD) of an evaporator has an impact on the humidity within the installation, which in turn affects the products stored.

It is imperative that the proper TD be employed. The TDs required for forced air circulation are closer than those for gravity air circulation, but the same general rules apply to both. The technician should be familiar with these TD figures, as he will check many installations to determine whether or not a job is properly adjusted for the best food preserving conditions. The method used to check the TD of any installation is to first determine the fixture or box temperature (BT). This is very easily done by using a pocket thermometer to check the temperature of some product that has been in the fixture for at least 10 hours. In this way a peak temperature reading (either high or low) will be avoided.

The next step is to determine the average refrigerant temperature (ET). For example, consider a kitchen refrigerator using R134a on which the switch is set to cut off at 90kPa and to cut in at 234kPa to hold a cabinet temperature of 3°C.

As discussed before, a general method which may be followed in determining the average refrigerant temperature is that the average suction pressure is usually about 35kPa above the point at which the switch cuts out. The temperature corresponding to this pressure may then be assumed to be the average refrigerant temperature. Therefore, the average refrigerant pressure for the above application is:
90kPa + 35kPa = 125kPa, and the average refrigerant temperature (ET) will be -7°C.
Now to determine the TD on this application we merely subtract the -7°C from the 3°C fixture temperature and we find the TD to be:
TD = BT - ET
= 3°C - (-7°C)
= 10K

Grant03

Suppose in checking the TD on a particular installation it is found that it is too great. The complaint in this case would very well be one or more of the following:

  1. Long running time, resulting in high operating costs.
  2. Products drying out.
  3. Meat discolouration.
  4. Generally unsatisfactory refrigeration.

When too small a TD is encountered, the usual result is excessive humidity in the fixture or the system efficiency may be low.

In practice, after setting a low-pressure switch the operation and fixture temperature must be observed for several cycles. Particular attention must be paid to the following points:

  • The settings must not cause the condensing unit to short cycle.
  • In an installation operating on an automatic defrosting cycle, the evaporator must be completely defrosted before the switch will cut in.

Practical setting of a pressure switch

One can set a pressure switch using nitrogen or set on the system. When setting on the system manually pump down the system to see at what pressure the compressor cuts in and out at.

It is important to note that on the low side the range must be set first.
You must use a manifold gauge set.

  1. Set “R2” (Low pressure cut-in value)
  2. Then set “D” (Low pressure cut-out value)
  3. Then set “R1” (High pressure safely cut-out)

Example:
Assume the low pressure (LP) cut-in pressure needed is 185kPa and the cut out pressure is 60kPa.

Grant04 

James, I hope that this helps you with pressure switch setting. A pressure switch serves as a reliable operating and or safety control device and all HVAC&R technicians should be able to set one. As to your comment about pressure switches being outdated, no, I think they are going to be around for a while!

Grant05Here is a new condensing unit operating on R290 (small flammable label on compressor) and that uses pressure switches. Not outdated yet!

References:
ACRA

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