Temperature Control Systems
Service and troubleshooting of commercial refrigeration temperature controls including pressure controls, electronic controllers, programmable systems, defrost integration, and alarm systems.
- Calibrate and troubleshoot pressure-type temperature controls used in commercial refrigeration
- Program and service electronic digital temperature controllers with multiple sensor inputs
- Set cut-in, cut-out, and differential values for proper compressor cycling
- Integrate temperature controls with defrost systems, alarm systems, and energy management
Lesson 1
Pressure-Type Temperature Controls
How Pressure Controls Regulate Temperature
In commercial refrigeration, temperature is often controlled indirectly through suction pressure. Because there is a direct relationship between evaporator temperature and suction pressure (per the P-T chart), a pressure control on the suction line effectively controls box temperature.
When box temperature rises, the evaporator absorbs more heat, raising suction pressure. The pressure control closes its contacts at the cut-in pressure, starting the compressor. As the box cools, suction pressure drops. The control opens its contacts at the cut-out pressure, stopping the compressor.
Setting Pressure Control Values
To set a pressure control for a medium-temp walk-in cooler at 35 degrees F with a 10 degree F TD using R-404A:
- Target coil temperature: 35 - 10 = 25 degrees F
- Cut-out pressure: P-T chart for R-404A at 25 degrees F = approximately 40 psig
- Cut-in pressure: Allow 6-8 degrees F rise, so 31-33 degrees F = approximately 52-56 psig
- Differential: 52 - 40 = 12 psig
| Application | Cut-Out Temp | Cut-Out Pressure (R-404A) | Cut-In Pressure (R-404A) | Differential |
|---|---|---|---|---|
| Med-temp cooler (35 F) | 25 degrees F | 40 psig | 52 psig | 12 psig |
| Low-temp freezer (0 F) | -10 degrees F | 12 psig | 22 psig | 10 psig |
| Ice cream freezer (-15 F) | -25 degrees F | 4 psig | 12 psig | 8 psig |
Differential Too Narrow = Short Cycling
If the differential between cut-in and cut-out is too narrow (less than 6 psig), the compressor will short-cycle - starting and stopping every few minutes. Short cycling causes excessive wear on compressor valves, contactors, and start components. It also increases energy consumption. Minimum differential should be 8 psig for most commercial applications.
Pressure controls regulate box temperature indirectly through suction pressure - calculate cut-out from the P-T chart at the desired coil temperature (box temp minus TD), then set cut-in 8-12 psig higher to prevent compressor short cycling.