HVACR317 - Electrical for Refrigeration

1. HVACR317 - Electrical for Refrigeration Pressure Controls

2. Pressure Controls • Pressure controls are designed to sense the pressure at various points in the refrigeration system and take action based on that pressure. • Pressure controls can either be operating controls or safety controls.

3. Safety Controls • Safety controls are designed to stop operation if an unsafe condition occurs. • This unsafe operation could be a high pressure condition, a blockage in a line, or voltage spikes.

4. Operating Controls • Operating controls are designed to be a part of the normal operation of a system. • Operating controls turn things on and off based on normal operating conditions. • An example of an operating control may be a thermostat or a low-ambient control.

5. Operation of Pressure Switches • Control operating pressures within design conditions. • Internal contacts change position with pressure changes.

6. Types of Pressure Switches • There are two main types of pressure switches: • Low Pressure – Used primarily for safety, and as thermostats. • High Pressure – Used as a safety control, but also for condenser fan cycling.

7. Low Pressure Control • Safety Operation • If the pressure drops too low, this indicates a lack of refrigerant charge. • Can mean no gas is returning to cool the compressor. • Can mean that there is a possible freezing condition in the evaporator. • Can indicate lack of cooling.

8. Low Pressure Control • The low pressure control will open on a pressure drop, and will close on a rise in pressure. • For temperature operation • The pressure of the suction line indicates the temperature of the evaporator. • Evaporator temperature indicates the box temperature.

9. Low Pressure Control • For Defrost Operation • If set properly the low pressure control can initiate defrost.

10. Low Pressure Control • Example: • An R12 system with a 34° F box temperature and the compressor not running has an evaporator temperature of 34° F and a pressure in the suction line of 31.7 psig.

11. Low Pressure Control • If trying to maintain a 34° F box, the temperature should fluctuate with 34° F as the average. • Temp between 36° and 32° F. • Set the low pressure control cut-in pressure to correspond with 36° F for the type of refrigerant in the system. • R12 = 33.4 psig CUT-IN.

12. Low Pressure Control • Cut-out setting will have to correspond with the evaporator temperature when the box is 32° F. • To know the exact temperature difference between the box and the evaporator in this case, the temperature of the evaporator should be measured when the box is at 34° F and the system is running. • Use a standard of 15° F between the evaporator and the box. In this example, the cut-out would have to correspond to a 17° F evaporator, or approximately 19 psig.

13. High Pressure Control • A high pressure control can be used for fan cycling and safety controls. • Fan Cycling Control • Some high pressure controls can be set to close on a rise in pressure. • The contacts can be wired in series with the condenser fan motor to turn the fan on and off depending on condenser pressures. • Used in areas where system will run in low ambient or cold conditions.

14. High Pressure Controls • Safety Control • Excessive head pressure will cause overheating of the compressor. • The high pressure control will open at a design pressure turning off the condenser. • Many controls are manual reset, meaning a reset button will open and need to be physically reset.

15. Dual Pressure Control • Will open either on low or high pressures. • Sometimes combined with a dual acting (or dual operation) control, which will open or close on rise or fall.