NATE CR Service flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Superheat

The difference between the actual suction line temperature and the saturated evaporating temperature at the same pressure. Measure both at the same point and use the correct PT chart for the refrigerant.

Subcooling

The difference between the saturated condensing temperature and the actual liquid line temperature. Typical full-load values are 8-15 °F for most commercial systems with a TEV and receiver.

Target superheat

The superheat value a system should maintain at the evaporator outlet. Typically 8-12 °F for a fixed-orifice system and 4-8 °F at the compressor inlet for a TEV system.

Temperature glide

The temperature change of a zeotropic refrigerant blend as it evaporates or condenses at constant pressure. Use dew point for superheat and bubble point for subcooling calculations.

Liquid floodback

Liquid refrigerant returning to the compressor, indicated by low superheat at the compressor. Caused by an overfed TEV, a stuck-open valve, or an oversized valve.

Starved evaporator

An evaporator receiving insufficient refrigerant, indicated by high superheat. Caused by undercharge, a restricted liquid line, a stuck-closed TEV, or a clogged inlet screen.

Filter-drier

A device that removes moisture and particulate from the refrigerant. A pressure drop greater than 2-3 psi at full flow indicates restriction and the need for replacement.

Sight glass

A window in the liquid line that shows refrigerant charge and moisture content. Bubbles can indicate undercharge, a restriction, or insufficient subcooling, not always low charge.

Crankcase pressure regulator

A valve that limits suction pressure at the compressor to prevent motor overload during pull-down or high evaporator load. Set per the compressor manufacturer's rating.

Evaporator pressure regulator

A valve that maintains a minimum evaporating pressure for one or more fixtures on a common suction line, preventing coil icing and product dehydration.

Defrost termination and fan delay

A control that ends defrost on temperature and delays evaporator fans until the coil is below a set temperature, preventing warm air from blowing on product.

Pump-down cycle

A cycle that uses a liquid line solenoid valve and low-pressure switch to pump the evaporator down and store refrigerant in the receiver, preventing liquid migration to the compressor.

Oil pressure safety control

A control that trips the compressor when oil pressure fails to build within the time delay after startup, protecting against lubrication failure.

Non-condensable gases

Gases such as air or nitrogen in the system that raise head pressure and cause high subcooling with high discharge temperature. Purge or recover and recharge if found.

Dirty condenser effect

A dirty condenser raises condensing temperature and pressure, increasing compression ratio, reducing capacity, and raising amp draw. Clean coils before diagnosing other faults.

Evacuation to 500 microns

The standard for verifying a tight, dry system before charging. Hold below 1,000 microns for 10-15 minutes; a rising vacuum indicates moisture or a leak.

Charging by weight

The most accurate charging method when the nameplate charge is known. Use a scale accurate to 0.1 ounce for small systems and 1 ounce for larger systems.

ASHRAE A1 classification

A refrigerant safety group with low toxicity and no flame propagation. Examples include R-404A, R-507A, and R-134a.

ASHRAE A2L classification

A refrigerant safety group with low toxicity and lower flammability. Examples include R-454A, R-454C, and R-455A; these require ignition-source control and leak detection.

R-404A GWP

Approximately 3,922. This high global warming potential is why R-404A is being replaced by lower-GWP refrigerants such as R-448A and R-449A.

R-448A and R-449A

A1 refrigerants with GWP around 1,273-1,397, used as common R-404A retrofits. They require checking seals, filter-drier compatibility, and TXV orifice sizing.

Zeotropic blend charging

Zeotropic blends such as R-404A and R-448A must be charged as a liquid to prevent fractionation. Charge by weight and use dew point for superheat.

EPA Section 608 certification

Required for technicians who service or dispose of appliances containing refrigerant. Type I covers small appliances, Type II high-pressure systems, and Type III low-pressure systems.

Recovery cylinder limit

Recovery cylinders must be DOT-approved and never filled above 80% of capacity by volume. Use a scale to monitor the amount recovered.

Leak repair threshold

Appliances with a charge of 50 pounds or more must have leaks repaired within 30 days if the leak rate exceeds the applicable EPA threshold. Chronic leakers may require retrofit.

ARI 700 purity

The standard reclaimed refrigerant must meet before it can be resold or used in another appliance. Recycling does not restore refrigerant to this purity level.

Potential relay

A start relay in a single-phase compressor that uses back-EMF from the start winding to open the start capacitor circuit once the motor reaches about 75% speed.

Locked rotor amps

Typically 5-7 times the rated load amps. A compressor drawing LRA without starting indicates a mechanical or electrical failure.

Evaporator TD

The temperature difference between the box and the evaporator. Typically 10 °F for coolers and 10-15 °F for freezers; a higher TD lowers humidity and increases frost.

Condenser TD

The temperature difference between condensing temperature and ambient. Typically 10-15 °F for air-cooled condensers; a larger TD increases capacity but raises head pressure.

Suction line P-trap

A trap at the base of a suction riser that returns oil to the compressor. An inverted trap at the top prevents oil from draining back into the evaporator during off cycles.

Hot gas defrost

A defrost method that uses hot discharge gas to melt frost. Common in larger systems; requires a defrost differential valve and a termination control.

Electric defrost termination

A switch that ends electric defrost when the coil reaches typically 45-55 °F, preventing overheating. A failed switch causes ice buildup or heater damage.