32.2 Crankcase & Evaporator Pressure Regulators & Unloaders

32.2 Crankcase & Evaporator Pressure Regulators & Unloaders

Trade F’s next numbered cluster is crankcase pressure regulating valves, evaporator pressure regulating valves, and unloaders. These are service and adjustment items on both Class A and Class B. They show up on supermarket racks, walk-ins, large reciprocating chillers, and some packaged recips that unload at part load. Installing the compressor itself was Chapter 23.2; repairing internals was Chapter 31.2; this section is the valves that keep the motor inside rated-load amps (RLA) and the coil at a minimum temperature. The books are still Refrigeration & Air Conditioning Technology, 9th Edition (2021), and the Troubleshooting Handbook, 2nd Edition (2003). F.S. 489.105(3)(g) still caps Class B at 25 tons cooling and 500,000 Btu heating in any one system. A 20-ton recip with unloaders can be a Class B machine. A 40-ton chiller with the same unloader story is Class A to contract. Class B still owes the theory.

Quick Answer: A crankcase pressure regulator (CPR) limits maximum suction pressure into the compressor (typically closes on rise of outlet). An evaporator pressure regulator (EPR) holds minimum pressure in the evaporator (typically opens on rise of inlet). Unloaders drop capacity — cylinders, slide valve, or inlet vanes — so the machine is not 100 percent on a 40 percent load. Set pressures from the pressure-temperature (P–T) chart and the compressor’s maximum suction rating, not from a memorized R-22 number.

Why three different valves exist

A compressor motor is sized for a design suction pressure. After a hot pull-down, after electric defrost, or on a 95°F Florida box that has been off all weekend, evaporator pressure can sit far above that design point. Mass flow through the compressor rises, amps rise, and the motor overloads even though the windings ohm out as good. That is the CPR job: hold the compressor’s inlet down so RLA is not exceeded.

A coil that is allowed to fall below the desired temperature creates a different failure: frozen product, a frozen chilled-water barrel, a sweating duct, or a comfort coil so cold it will not dehumidify the way the owner wanted. Multiple evaporators on one compressor at different temperatures cannot all run at the lowest suction the coldest box wants. That is the EPR job: hold this evaporator up while the common suction may be lower.

A machine that is too big for the present load short-cycles, over-cools, and still draws near-full power. That is the unloader job: take displacement offline so capacity matches the load. Confusing the three devices is the exam’s favorite trap. A CPR does not hold a meat-case temperature. An EPR does not keep a compressor off RLA during pull-down. An unloader does not replace either regulator.

Crankcase pressure regulating valves

The CPR is also called a close on rise of outlet (CRO) valve because the port that feeds the compressor is the valve’s outlet. As compressor-inlet pressure rises, the valve throttles closed and holds that outlet at the setpoint. Upstream (evaporator) pressure may stay high; the compressor simply does not see all of it. When the box finally pulls down and evaporator pressure falls to the setpoint, the CPR goes wide open and stays out of the way.

Setting. Read the compressor manufacturer’s maximum operating suction pressure (or the suction that corresponds to RLA at the expected condensing temperature). Convert with the P–T chart for the refrigerant in the machine. A helper who sets every CPR to an old R-22 walk-in number on an R-410A or R-454B circuit has not set anything useful — saturation pressure at a given temperature is refrigerant-specific. After the set, clamp the common or T-lead on a hot pull-down: amps should stay near RLA, not climb toward a trip. If amps are still high with the CPR throttling, the setpoint is too high, the condenser is dirty, or the motor is already damaged.

Service. A CPR that is stuck open looks like no CPR: pull-down overloads. A CPR stuck closed or set too low starves the compressor: low suction, low amps, long run times, possible oil-return failure. Bypass valves and gauge ports on the valve body are for setting, not for leaving cracked open as a permanent leak around the regulator. Isolate and replace a valve that will not stroke; do not hammer it on a charged suction line.

Worked pull-down. A Sarasota walk-in has been off. Box air is 80°F; the compressor’s maximum listed suction is the saturation pressure of 45°F for that refrigerant. Without a CPR the evaporator sits near 80°F saturation and the motor leaves RLA. With the CPR set at 45°F saturation, the compressor inlet is held there, amps stay legal, and the box temperature falls until evaporator pressure meets the setpoint and the valve opens fully. That is motor protection, not a box thermostat.

Evaporator pressure regulating valves

The EPR is also called an open on rise of inlet (ORI) or a hold-back valve. It sits on the evaporator outlet. As coil pressure tries to fall below setpoint, the valve throttles and holds the coil up. As load rises and coil pressure rises above setpoint, the valve opens and the evaporator is allowed to dump into the common suction. A suction pressure regulator (SPR) is the same family when it is applied to hold a suction pressure; the exam language you will see is evaporator pressure regulating valve.

Uses that match the outline:

• Multi-temperature systems: one compressor, a cooler and a freezer (or a meat case and a dairy case). The warmer evaporator gets an EPR so it cannot be pulled down to the colder suction.
• Freeze protection on a water coil or a comfort coil that shares a low-temperature compressor.
• Humidity / coil-temperature control: a warmer coil (higher suction) can be an intentional latent-versus-sensible choice. Do not use an EPR as a substitute for a dirty filter or a failing expansion valve.

Setting. Pick the minimum coil temperature the application allows (for example, just above freezing on a water coil, or the case manufacturer’s coil temperature). Convert on the P–T chart. Gauge on the evaporator side of the EPR, not only at the compressor. A compressor suction reading below the EPR setpoint is normal when the valve is holding — that is the pressure drop the hold-back was built to create. Calling that low compressor suction a starved TXV, and opening the stem, wrecks the coil temperature the EPR was holding.

Service. Stuck open: the coil goes as cold as common suction allows — frozen product, frozen barrel, or a comfort coil that ices. Stuck closed or set too high: the coil is warm, capacity collapses, and the compressor may short-cycle on LP if the other evaporators cannot feed it. Equalizer lines and pilots on larger EPRs must be open and in the right direction; a plugged pilot makes a large valve behave closed.

Unloaders

An unloader reduces displacement. On a reciprocating compressor the common service device is a cylinder unloader: an electric solenoid (or a pressure-operated piston) lifts the suction valve on one or more cylinders so those cylinders stop pumping. The motor still turns; those bores idle. A four-cylinder machine that unloads two cylinders is at 50 percent capacity; unloading one of four is 75 percent. The control may be a pressure switch on suction, a building-automation step, or an internal pressure operator. Oil return still needs gas velocity — long unloaded hours can dump oil into the system unless the listing includes an oil-return cycle. That is a service observation, not an excuse to defeat the unloader.

Electric unloader solenoids are electric actuating valves in the 32.1 sense, applied to capacity. Prove coil voltage and ohms on a call for unload; a solenoid that never energizes leaves the machine at 100 percent and short-cycling. A solenoid that stays energized leaves the machine at minimum capacity on a hot Florida afternoon and the space never catches up.

Screw compressors typically use a slide valve (or a VFD). Centrifugal machines use inlet guide vanes and/or a VFD. Those are the Class A large-plant install items from Chapter 21.3; Class B still has to recognize them on a theory item. Hot-gas bypass feeds discharge gas into the evaporator or suction to fake a load. It is a capacity method. It is not a cylinder unloader, and it is hard on efficiency. Do not call a hot-gas solenoid an unloader on a reciprocating head unless the item actually describes valve lift.

Worked capacity step. A legal Class B 20-ton four-cylinder recip in a Lee County medical office unloads two cylinders. Remaining capacity is about 10 tons. The control should unload when suction (or leaving-water temperature) is satisfied, not when a helper jumpered the solenoid because the oil-pressure switch was tripping. A 40-ton version of the same machine is one 40-ton system — Class A to install — even if it spends all afternoon at 50 percent.

Florida scenario

Gulfshore Refrigeration holds a certified Class B license in Sarasota County. A seafood walk-in (legal tons) pulls the condensing-unit breaker on a Monday morning after a weekend power outage. Clamp-on on a restart without a CPR shows the compressor well above RLA while the box is still warm. The CPR at the compressor inlet is stuck open (a piece of scale under the seat — foreign matter, Section 32.3). Rebuild or replace the CPR, set it to the compressor’s maximum suction from the P–T chart, and the same pull-down stays near RLA. On the same rack a dairy case is freezing product because its EPR is stuck open and the case is riding the colder common suction; holding the EPR to the case’s minimum coil temperature fixes the product, not a new thermostat. Across the parking lot a property manager wants the shop to add unloaders to a 40-ton packaged recip. Class B may describe the 50 percent step and the solenoid check. Class B may not contract that 40-ton system.

Traps: (1) Using a CPR to hold case temperature. (2) Using an EPR to stop pull-down overloads. (3) Reading compressor suction as evaporator pressure on a holding EPR. (4) One memorized psig for every refrigerant. (5) Calling hot-gas bypass a cylinder unloader. (6) Jumpering unloader solenoids. (7) Class B installing the 40-ton unloaded plant because it only runs at 20 tons after 2 p.m.