NATE LC Refrigeration cheat sheet

Sensible heat changes temperature without a phase change; latent heat is absorbed or released during a phase change at constant temperature.

The four core components are compressor, condenser, metering device, and evaporator; the compressor raises pressure and temperature, the condenser rejects heat, the metering device drops pressure, and the evaporator absorbs heat.

Superheat = actual suction line temperature minus evaporating (saturation) temperature; it confirms vapor-only refrigerant entering the compressor.

Subcooling = condensing (saturation) temperature minus liquid line temperature; it confirms liquid-only refrigerant leaving the condenser.

A saturated condition exists whenever refrigerant is changing state; both pressure and temperature remain fixed during that change.

Pressure and saturation temperature track each other: at a given pressure a refrigerant boils and condenses at one temperature, so pressure gauges act as temperature gauges on saturated refrigerant.

Heat flows only from higher temperature to lower temperature; the system moves heat by maintaining an evaporator colder than the space and a condenser hotter than outdoor air.

Refrigerant blends that fractionate (zeotropic) must be charged as liquid to prevent composition shift; near-azeotropic blends behave closer to a single refrigerant.

Reciprocating, scroll, and rotary compressors are positive displacement; centrifugal compressors are dynamic and depend on impeller speed for lift.

Evacuation target for a tight system is typically 500 microns or lower with the pump isolated and decay held; a standing vacuum rise indicates moisture or a leak.

Nitrogen pressure testing is performed before evacuation; never pressurize with refrigerant or oxygen and never exceed component pressure ratings.

Suction line sizing prioritizes adequate refrigerant velocity to return oil to the compressor while limiting excessive pressure drop; horizontal runs slope toward the compressor.

Liquid line sizing prioritizes low pressure drop to keep liquid at the metering device; any flash gas ahead of the valve reduces capacity.

A liquid line filter-drier is installed with flow arrow toward the metering device; a suction filter-drier is used temporarily after a burnout and removed afterward.

Brazing requires flowing dry nitrogen to prevent internal oxidation (scale) that can clog the metering device and damage the compressor.

Commissioning checks include verifying superheat or subcooling charge method, recording amp draw against nameplate, checking rotation, and confirming controls sequence.

A trap (inverted P-trap or oil trap) is required on suction risers taller than a defined height to return oil against gravity.

High superheat with low suction pressure indicates a starved evaporator: restricted liquid line, clogged filter-drier, undercharge, or a failing metering device.

Low superheat or frost at the compressor indicates liquid floodback: overcharge, oversized metering device, low load, or a failed expansion valve.

High discharge pressure with normal suction points to condenser problems: dirty coil, fan failure, recirculated air, or non-condensables in the system.

Low discharge pressure with normal suction can indicate a worn compressor valve or low load on a properly charged system.

Subcooling too low with bubbles in the sight glass means the condenser is not holding enough liquid, often from undercharge or a restriction.

A pressure drop across a filter-drier greater than roughly 2-3 psi (or its rated value) signals a restriction requiring replacement.

Superheat hunting or a valve that will not open or close points to a failed power head, lost bulb charge, or a bulb improperly located or insulated.

Non-condensable gases cause abnormally high discharge pressure and high subcooling with no other obvious cause; purge and recharge after leak repair.

Always recover refrigerant before opening a system; venting is prohibited, and recovery cylinders must not be overfilled (fill by weight, typically 80 percent).

A system opened for repair must be leak-tested with nitrogen, evacuated to 500 microns or lower, and recharged by weight or by the superheat/subcooling method.

When replacing a compressor after a burnout, install a suction filter-drier, flush or replace contaminated components, and confirm acid levels before removing the temporary drier.

OEM specified oil type must be matched; mineral oil and POE are not interchangeable, and POE is highly hygroscopic so it must be kept sealed.

Leak detection sequence: visual inspection, electronic leak detector at joints, then pressure or vacuum decay testing; soap bubbles confirm the location.

Brazing near valves and controls requires wet rags or heat-sink paste to protect components from overheating.

After a repair, verify superheat or subcooling, record pressures and temperatures, and confirm amp draw and oil level at the sight glass.

Ohm's law: E = I x R; power in a resistive circuit is P = I x E, and single-phase apparent power is VA = volts x amps.

A capacitor stores charge and can retain voltage after power is removed; discharge it through a resistor before handling.

Locked rotor amps (LRA) far exceed running load amps (RLA); a compressor tripping on overload with high amps suggests a seized or shorted winding.

A start capacitor or potential relay provides starting torque; a failed start component causes the compressor to hum and trip the overload.

Controls sequence typically includes thermostat call, low-pressure or pump-down switch, defrost timer, and safety limits; verify each device in order.

Lockout/tagout must be applied before service; verify zero energy and confirm the disconnect is open.

High-pressure cutout, low-pressure cutout, and oil pressure safety switches protect the compressor; bypassing them is not a valid repair.

Personal protective equipment for refrigerant work includes gloves and eye protection; liquid refrigerant causes freeze burns and asphyxiation risk in low areas.