8.4 Pressure and Safety Controls: High/Low Pressure, Oil Safety, Freeze Protection, and Head Pressure Control

Cut-in, cut-out, and differential - the two settings behind every pressure control

Every pressure control, mechanical or electronic, is defined by two numbers. Cut-out is the pressure at which the contacts open. Cut-in is the pressure at which they close again. The differential is the span between them:

$Differential = Cut_{in} - Cut_{out}$

The direction matters and it is the single most common source of wrong answers. A low-pressure control opens on falling pressure at the cut-out setting and closes on rising pressure at cut-out plus differential. A high-pressure control opens on rising pressure at cut-out and closes on falling pressure at cut-out minus differential.

Worked example. A walk-in cooler's low-pressure control is set for a 20 psig cut-out with a 30 psi differential. The compressor runs until suction falls to 20 psig, then stops. It restarts only after suction climbs back to $20 + 30 = 50$ psig. Convert both numbers to saturation temperature on the pressure-temperature chart for the actual refrigerant in the system, and you have the coil temperature the control is really enforcing.

Most mechanical controls give you a range (or cut-out) scale and a separate differential scale, and the third number is whatever the arithmetic makes it. Set the differential too narrow and the compressor short-cycles; set it too wide on a refrigeration application and the box temperature swings.

High-pressure cutouts

The high-pressure cutout (HPC) senses discharge or high-side pressure and opens the control circuit before the system reaches the setting of its relief device or the maximum allowable working pressure (MAWP) of the high side. ANSI/ASHRAE Standard 15 frames the hierarchy: the pressure-limiting device is set below the pressure-relief device, and a positive-displacement compressor with a stop valve in its discharge must have relief protection sized so pressure cannot exceed 10 percent above the MAWP of components between the compressor and that valve.

Settings are manufacturer- and refrigerant-dependent, and the open-book exam expects you to look them up rather than recite them. As a sense of scale, residential and light-commercial R-410A high-pressure switches commonly open somewhere between 590 and 650 psig and reset near 420-450 psig, while R-22 and R-404A equipment - which has much lower design pressures - cuts out several hundred psi lower. Take the number from the data plate, the wiring diagram, or the manufacturer's service literature every time.

Automatic reset controls close again once pressure falls through the differential. Manual reset (lockout) controls require a technician to push a button, and the button will not latch until pressure has actually dropped. Manual reset is specified where a trip signals a condition someone must investigate - large equipment, systems where repeated cycling would drive a relief device to vent refrigerant, and any application where the equipment listing or the authority having jurisdiction calls for it. If you find a manual-reset HPC tripped, the diagnosis list is short: dirty or blocked condenser, failed condenser fan or motor, restricted airflow, overcharge, non-condensables, a service valve left partly closed, or on water-cooled equipment, loss of condenser water flow or a scaled tube bundle. Texas summer design conditions push rooftop equipment close to these settings all afternoon, so a unit that trips only at 4 p.m. in August is usually telling you about condenser airflow, not about the switch.

Low-pressure cutouts, and the loss-of-charge switch that is not one

A low-pressure cutout (LPC) does double duty:

• As a safety it protects against a lost charge, a plugged metering device, an evaporator iced solid, or a compressor pulling itself into a vacuum.
• As an operating control it terminates the pump-down cycle. The thermostat opens the normally closed liquid-line solenoid circuit, the compressor keeps running until suction reaches cut-out, and the charge is parked on the high side. On many refrigeration systems the LPC also serves as the box temperature control, with the coil's saturation pressure standing in for the space temperature.

Setting rules follow from that. The cut-out must be below the lowest normal running suction pressure at design box temperature, or the compressor short-cycles at the end of every pull-down. The cut-in must be low enough that the compressor does not restart on normal off-cycle pressure creep, but high enough that it actually restarts when the solenoid opens.

A loss-of-charge switch looks similar and is not the same device. It is installed in the liquid line rather than the suction line, it trips at a much lower pressure - roughly 5-20 psig - and it is frequently manual reset. Its only job is to prove there is still refrigerant in the system. Heat pumps in particular cannot use a conventional suction LPC as a year-round safety, because heating-mode suction pressure is normally very low in cold weather and a standard control would nuisance-trip on the coldest nights of the year.

Oil pressure safety switches

On semi-hermetic compressors with a positive-displacement oil pump, lubrication is proved by a differential measurement, not by a gauge reading. The oil pump discharges into the crankcase environment, so the pump has to overcome crankcase pressure before any oil moves:

$P_{net} = P_{oil} - P_{crankcase}$

If the oil pump gauge reads 55 psig and the crankcase (suction) reads 22 psig, net oil pressure is 33 psid - and 33 psid is the only number that matters. A mechanical oil pressure safety control uses two opposing bellows to make that subtraction mechanically. When net oil pressure falls below the control's minimum - around 9 psid on common controls - a set of contacts energizes a small heater near a bimetal element. If the low-pressure condition persists for the control's time delay, the bimetal opens the compressor control circuit and the control locks out on manual reset. Delay versions of 45, 90, and 120 seconds are standard; the delay exists so the compressor has time to build oil pressure on start-up.

A so-called nuisance trip almost never means a bad switch. Work the real list: low oil level, oil diluted or foaming from refrigerant migration into the crankcase, no crankcase heater or one that is open, a plugged oil pickup screen, worn bearings or a worn oil pump, short-cycling that never lets the oil return, an oil separator or oil-level regulator problem on a rack, or poor oil return from undersized or badly pitched suction risers. Electronic oil protection modules perform the same subtraction internally and lock out the same way. Jumping out an oil safety control to keep a compressor running is how a compressor gets destroyed between the service call and the parts delivery.

Freeze protection

Freeze protection covers three different situations, and the exam distinguishes them:

• Air-side low-limit (freezestat). A long capillary - commonly 20 feet, serpentined across the coil face - senses the coldest 8 to 12 inch segment anywhere along its length, so stratified air cannot hide a freezing spot. Typical setpoints are near 36-38 degrees F. Most are manual reset and will not reset until the sensed temperature is roughly 5 degrees F above setpoint, which forces someone to find the outside-air damper, valve, or pump problem before the unit restarts. The contacts are wired into the safety circuit to stop the supply fan, close the outdoor air damper, and open the heating valve.
• Refrigerant-side coil frost protection. On direct-expansion equipment, evaporator icing is prevented either by a low-pressure control set at the saturation pressure corresponding to about 32 degrees F coil temperature, or by a thermostat clamped to the suction line or inserted in the coil. Iced coils usually trace back to low airflow, a dirty filter, a low charge, or a failing metering device - fix the cause, not the setpoint.
• Hydronic freeze protection. Chilled- and hot-water coils exposed to outdoor air need freeze stats, glycol, or both. Texas jobs are not exempt: the February 2021 statewide freeze put outdoor-air-exposed coils and piping in Houston and Dallas below freezing for days.

Head pressure control and crankcase heaters

A thermostatic expansion valve (TXV) is a pressure-driven device. It needs an adequate pressure difference across its port to push the rated mass flow through. When the ambient drops and condensing pressure falls too far, that difference disappears and the valve underfeeds the evaporator: suction pressure sags, superheat climbs, capacity collapses, the coil may frost, and the low-pressure control may trip. Low head pressure also lets liquid flash in the liquid line, which starves the valve further. Low head pressure is a problem, not a bonus.

The common corrections, roughly in order of cost:

1. Condenser fan cycling - a pressure control (or an outdoor thermostat) cycles one or more condenser fans off. Cheapest and coarsest; head pressure swings between cut-in and cut-out.
2. Fan speed control - a variable-speed drive, a head-pressure-actuated fan speed control, or an electronically commutated motor (ECM) modulates fan rpm for a much steadier condensing pressure.
3. Condenser air dampers - modulating dampers or shutters restrict airflow across the coil.
4. Flooded-condenser control (headmaster valves) - a three-way valve backs liquid up into the condenser, reducing the effective condensing surface until the set minimum condensing pressure is restored, while feeding hot gas to the receiver to keep liquid moving to the TXV. These valves are nonadjustable and ordered by setting (180 psig and 215 psig versions are common), and the system must carry an oversized receiver and the extra charge needed to flood the condenser at the design minimum ambient.
5. Water-regulating valves - on water-cooled condensers, a pressure-actuated valve throttles condenser water to hold head pressure.

In Texas the design low ambient is mild compared with the northern states, but north and west Texas nights and post-frontal mornings routinely drop rooftop refrigeration condensers below their minimum condensing temperature, and outdoor commercial refrigeration operates year-round. Head pressure control is specified for the coldest expected condition, not the average one.

Crankcase heaters address the related off-cycle problem. Refrigerant migrates to the coldest part of a system, and on a cold night that is the compressor crankcase, where vapor is absorbed into the oil. On the next start, crankcase pressure drops, the oil foams, lubrication is lost, and the oil safety control trips - or the compressor slugs liquid. A crankcase heater keeps the oil warmer than the rest of the system. Manufacturers require the heater to be energized for a period before start-up on a compressor that has been off or is new to the site - commonly 12 to 24 hours - and pump-down control is the other standard defense against migration.

Troubleshooting a safety string with a meter

Safety controls are wired in series with the compressor contactor coil, so one open device stops everything and none of them announces itself. Work the string in order:

1. Confirm control voltage at the source - typically 24 V at the transformer secondary, or line voltage on a refrigeration control circuit.
2. Leave the circuit energized and measure across each control's two terminals. A closed switch drops essentially 0 V. The open device is the one showing full control voltage across it, because the entire source voltage appears across the break.
3. Once you find the open control, put gauges on the system before you touch it. The control may be doing its job correctly and reporting a real fault.
4. Momentarily jumping a control is a legitimate confirmation test. Leaving the jumper in is not, and on an oil safety or manual-reset high-pressure control it will destroy the compressor.
5. With power off, an ohmmeter reads continuity through a closed control. Remember that a manual-reset device may refuse to latch until the sensed pressure or temperature has moved back through its reset band.