6.4 Pressure Controls, Capacity Controls, and System Protection

6.4 Pressure Controls, Capacity Controls, and System Protection

The Commercial Air Conditioning and Commercial Refrigeration sheets of the HVAC Excellence task list name a long series of controls by function: head pressure controls, low pressure controls, pump down solenoid, oil pressure safety control, hot gas bypass valve, evaporator pressure regulator, crankcase pressure regulator, low ambient temperature control, lockout relay, Head Master, and compressor capacity control methods. Each one exists because a compressor is expensive and fragile, and each one has a distinct location, a distinct sensing point, and a distinct failure signature.

---

1. The Safety Switches

Low-pressure switch (LPS)

Senses suction pressure and opens on a fall.

• Purpose: protects against loss of charge and against evaporator freeze-up on systems where suction pressure tracks box temperature.
• Cut-out and cut-in are the two settings. On a fixed-differential control you set cut-out; on an adjustable-differential control you set both.
• Typical R-410A comfort cooling: cut-out near 50 psig, cut-in near 100 psig.
• Dual duty: on many refrigeration systems the low-pressure switch also serves as the temperature control, cycling the compressor as box temperature and therefore suction pressure fall.

High-pressure switch (HPS)

Senses discharge pressure and opens on a rise.

• Purpose: protects against condenser fan failure, a blocked or filthy condenser, non-condensables, overcharge, or a closed discharge service valve.
• Typical R-410A cut-out: around 610 psig, cut-in around 420 psig. R-22 equipment used roughly 400/300 psig.
• Many are manual reset by design, because an automatically resetting high-pressure trip hides a real fault and lets the compressor cycle itself to death.

Lockout relay

An impulse relay that latches the control circuit off after a safety trip, so the equipment cannot short-cycle through a fault. Resetting requires interrupting power at the thermostat or disconnect. Its purpose is diagnostic as much as protective: a locked-out unit tells the technician a safety operated.

Internal and external overloads

• Internal (klixon) overloads sit in the winding and sense both temperature and current. They cannot be reset from outside and may take hours to cool.
• External line-break overloads mount on the compressor shell.
• A compressor that reads open C-to-R and C-to-S but recovers after cooling is almost always on an internal overload, not failed.

---

2. Oil Pressure Safety Control

Semi-hermetic and larger compressors use a positive-displacement oil pump. The control does not read oil pump discharge pressure directly — it reads net oil pressure:

$\text{Net oil pressure} = \text{Oil pump discharge pressure} - \text{Crankcase (suction) pressure}$

Because the pump discharges into a crankcase already at suction pressure, only the difference does useful work at the bearings. A compressor showing 60 psig oil pump discharge with 45 psig suction has a net oil pressure of only 15 psi — often below the manufacturer's minimum.

The control includes a time delay, typically 90 to 120 seconds, implemented with a small heater and bimetal. On start-up net oil pressure is momentarily zero, so the delay allows the pump to build pressure. If net pressure has not reached the cut-in point when the delay expires, the heater warms the bimetal and the contacts trip. Oil safety controls are manual reset.

Common causes of a trip: low oil level, oil logged in the evaporator (refrigerant migration or wrong oil after a retrofit), refrigerant flooding back and diluting the oil so the pump cavitates, a worn oil pump, or a plugged oil strainer.

---

3. Pressure-Regulating Valves

Evaporator pressure regulator (EPR)

• Location: in the suction line leaving the evaporator.
• Senses: its own inlet pressure — that is, evaporator pressure.
• Action: throttles closed to prevent evaporator pressure from falling below its setting.
• Purpose: maintains a minimum coil temperature. On a multi-temperature rack — a produce case at 32°F, a meat case at 28°F, a dairy case at 34°F all feeding one suction main — each higher-temperature evaporator gets an EPR so it does not pull down to the lowest suction pressure on the system. It is also used on a single evaporator to prevent frost or to hold humidity.
• Adjustment: turning the stem in (clockwise) raises the pressure the valve maintains, which raises coil temperature.
• Pilot-operated EPRs are used on large evaporators where the pressure drop across a direct-acting valve would be excessive.

Crankcase pressure regulator (CPR), also called a suction pressure regulator or hold-back valve

• Location: in the suction line at the compressor inlet.
• Senses: its own outlet pressure — compressor suction pressure.
• Action: throttles closed to prevent compressor suction pressure from rising above its setting.
• Purpose: protects the compressor motor from overload. A low-temperature compressor is sized for a 5 psig suction; after a defrost or a hot pull-down, suction pressure can reach 40 psig, and the resulting mass flow would draw current far above nameplate. The CPR holds it back until the box pulls down.
• Memory aid: EPR protects the Evaporator (minimum pressure, inlet-sensing). CPR protects the Compressor (maximum pressure, outlet-sensing).

Head pressure / low ambient control

An air-cooled condenser in cold weather condenses too well: head pressure falls, the pressure difference across the metering device collapses, the TXV cannot feed the coil, and the evaporator starves. Three solutions, in order of sophistication:

1. Fan cycling — a pressure switch or thermostat cycles condenser fans off as head pressure falls. Cheap, but coarse, and cycling stresses the motor.
2. Fan speed control — a pressure-actuated or electronic control modulates fan speed continuously. Smoother and now common with ECM condenser fans.
3. Flooding-type head pressure control (a "Head Master" or three-way flood valve) — a three-way valve that backs liquid refrigerant up into the condenser, reducing effective condensing surface so head pressure rises, while simultaneously bypassing hot discharge gas into the liquid line to keep the receiver pressurized. This is the only method that works reliably in severe cold, and it requires a receiver with extra charge to hold the flooded volume — typically the system holds a significantly larger charge than a non-flooded design.

Hot gas bypass valve

• Location: a line from the discharge (hot gas) side to the evaporator inlet (preferred, downstream of the metering device with a distributor connection) or to the suction line.
• Senses: suction pressure.
• Action: opens as suction pressure falls below setpoint, admitting hot gas to create a false load.
• Purpose: capacity control — it lets a fixed-capacity compressor run continuously at part load without short-cycling or freezing the coil. It is an efficiency compromise, not an efficiency device: the compressor works while producing no useful cooling.
• Bypassing into the evaporator inlet is preferred because the liquid feeding the coil desuperheats the bypassed gas and keeps the compressor's return vapor cool. Bypassing directly into the suction line risks overheating the compressor unless a liquid injection valve is added.

---

4. Pump-Down and Migration Control

Off-cycle migration is the mechanism that destroys compressors quietly. During the off cycle, refrigerant vapor migrates to the coldest point in the system. Refrigeration oil has a strong affinity for refrigerant, so vapor condenses into the oil in the crankcase. On the next start, crankcase pressure drops instantly, the dissolved refrigerant boils violently, the oil foams, and the pump loses prime — bearings run dry and liquid slugs reach the valves.

Two defenses:

1. Crankcase heater — keeps the crankcase 20–30°F warmer than the coldest system component so refrigerant will not condense there. It must be energized during the off cycle, which is why a crankcase heater is wired ahead of the contactor and why a compressor that has been de-energized should be heated for several hours before starting.
2. Pump-down cycle — the preferred method on commercial refrigeration.

Pump-down sequence:

1. The temperature control (thermostat) is satisfied and de-energizes the liquid-line solenoid valve, which closes.
2. The compressor keeps running, pumping the remaining low-side refrigerant into the condenser and receiver.
3. Suction pressure falls until the low-pressure switch opens and stops the compressor.
4. Nearly all refrigerant is now stored on the high side, so there is little left in the low side to migrate.
5. On a call for cooling the solenoid opens, pressure rises, the low-pressure switch closes, and the compressor restarts.

A recycling pump-down repeats this whenever pressure creeps up during the off cycle. A non-recycling pump-down pumps down once at shutdown only.

Diagnostic note: on a pump-down system, a compressor that will not start with a satisfied box may simply be pumped down normally. A compressor that short-cycles on the low-pressure switch every few minutes with the solenoid open usually has a leaking solenoid seat or a low charge.

---

5. Compressor Capacity Control

Modulating capacity is more efficient than cycling because it avoids repeated start losses and holds the evaporator at a stable temperature, which improves dehumidification and reduces coil frosting.