Unit 6 — Refrigeration System Components
Section 4 — Accessory Devices
4.4 — Pressure Controls & Regulators
Pressure controls translate system pressure into protective actions; regulators maintain
controlled pressures in specific sections. This lesson covers pressure cutouts, EPRs,
CPRs, and head pressure regulators, and completes the section with pressure relief device
selection and the required response after a relief event.
Pressure Controls
EPR / CPR
Relief Devices
313A / 313D
4.4.1 — Pressure Controls
Pressure controls include cutouts, fan cycling controls, crankcase pressure
regulators, and other devices that translate system pressure (kPa and psig) into
electrical control actions (on/off) or mechanical modulation. They protect
compressors, condensers, and evaporators by responding to abnormal conditions before
damage occurs.
- Identify what the control protects: compressor (high/low pressure cutout), condenser (fan cycling), or suction pressure (CPR).
- Verify sensing line/port integrity: a kinked or plugged sensing line causes the control to “see” a false pressure — the device may trip or fail to trip based on a condition that does not reflect actual system state.
- Compare to the operating envelope: convert measured pressures to saturation temperatures (°C / °F) and verify they fall within the expected range for the load and ambient conditions.
4.4.2 — Regulators (EPRs, CPRs, Head Pressure Regulators)
Regulators maintain a controlled pressure in part of the system by throttling flow.
Evaporator pressure regulators (EPRs) hold minimum evaporator pressure on multi-temp
circuits; crankcase pressure regulators (CPRs) limit suction pressure on compressor
start to prevent overloading; head pressure regulators support minimum condensing
pressure during low-ambient operation.
⚠️
Regulator-Driven “Mystery” Pressures
A misadjusted or stuck regulator can create pressures that do not match what
the rest of the system predicts. For example, an EPR holding suction pressure
artificially high at the outlet of a low-temperature evaporator circuit will make
that circuit appear to be running at a warmer setpoint than intended. Always
locate measurement points upstream and downstream of the regulator,
compare pressures (kPa / psig), and verify the sensing and adjustment mechanism
before condemning the device.
4.4.3 — Pressure Relief Device Selection
Pressure relief devices protect against catastrophic overpressure and must be treated
as critical safety components. They must be correctly selected (set pressure, capacity,
compatibility), correctly installed, and kept functional — free of corrosion,
unobstructed discharge, no illegal isolation.
🚫
Never Defeat a Relief Device
Never cap, plug, or isolate a relief device in a way that defeats overpressure
protection. Where discharge piping exists, ensure it remains unobstructed,
properly supported, and directed to a safe location per applicable requirements.
💡
Selection & Compatibility
Relief devices must be selected for the correct set pressure, rated capacity
(to vent the system fast enough), and compatibility with the refrigerant in use.
A device that is correctly installed but undersized for system volume provides
inadequate protection. Always verify against equipment nameplate and applicable
code requirements.
4.4.4 — After a Relief Event
If a relief device has opened, treat it as a serious incident — the system
experienced or approached an unsafe condition. Simply replacing the device and
restarting is not an acceptable response.
- Investigate the root cause before restoring operation: heat rejection failure, valve misposition, blocked line, overcharge, or non-condensables.
- Correct the underlying condition that caused or allowed the overpressure.
- Restore overpressure protection using approved components and installation procedures.
- Document the event — repeated relief events indicate a recurring condition that requires a systematic solution.