A low pressure refrigeration control provides two different functions in refrigeration equipment.
1. Compressor Control: The low pressure control stops the condensing unit when the suction pressure reaches a preset limit.
2. Temperature Control: Properly adjusted, the control maintains the correct product temperature.
Most commercial and industrial refrigeration equipment use automatic reset low pressure controls.
These controls open the contacts when the low pressure limit is reached. But the contacts automatically close when the pressure increases to the cut-in setting.
Therefore, they have a cut-in and a cut-out setting.
Sometimes these settings are adjusted by simply turning a screw until the desired cut-in and cut-out pressures are shown.
Other controls use cut-in and differential settings. The cut-out setting is determined by subtracting the differential from the cut-in setting.
For example: A cut-in of 30psi with a differential of 20psi gives you a cut-out setting of 10psi.
When the low pressure control is used to maintain product temperature or in a pump down system, it is always an automatic reset control.
However, automatic reset is not good when the refrigeration system has a refrigerant leak!
The condensing unit begins to short cycle. This can destroy a compressor.
What to do?
Always set the low pressure cut-out setting as far away from the cut-in setting as possible. This extra distance limits the number of on-off cycles.
I never use manual reset low pressure controls.
Their primary function is protection. When abnormal low pressure occurs, they stop the compressor.
They are normally found on small reach in coolers and freezers operating with a thermostat.
Why use low pressure controls to maintain product temperature?
Isn't a thermostat better?
Here's why! Refrigeration control using low pressure is extremely important! This little control is the workhouse of controls.
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