HVAC Excellence Refrigeration flashcards

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

Ton of refrigeration

A cooling rate equal to 12,000 Btu per hour, or 200 Btu per minute, or 3.517 kilowatts. It equals the heat absorbed to melt one ton of ice in 24 hours.

Sensible heat

Heat that changes a substance's temperature without changing its state. For air, the rate is Btu/h = 1.08 × CFM × temperature difference; for water, 500 × GPM × temperature difference.

Latent heat of vaporization

Heat absorbed when a liquid changes to vapor at constant temperature. For water at 212°F it is about 970 Btu per pound and rises as saturation pressure falls.

Superheat

The number of degrees a vapor is above its saturation temperature at the measured pressure. It is calculated as actual suction temperature minus saturation temperature at suction pressure.

Subcooling

The number of degrees a liquid is below its saturation temperature at the measured pressure. It is calculated as saturation temperature at liquid pressure minus actual liquid temperature.

Refrigeration cycle order

Compression, condensation, expansion, evaporation. The compressor raises pressure and temperature, the condenser rejects heat, the metering device drops pressure, and the evaporator absorbs heat.

Carnot COP

The maximum theoretical coefficient of performance: evaporating absolute temperature divided by the difference between condensing and evaporating absolute temperatures. Real systems always perform below this limit.

ASHRAE safety group A1

A refrigerant classification meaning lower toxicity (A) and no flame propagation (1). R-410A and R-134a are examples; R-32 is A2L and propane is A3.

R-410A composition

A near-azeotropic blend of 50% R-32 and 50% R-125. It has zero ozone depletion potential, must be charged as a liquid, and operates at pressures roughly 50-70% higher than R-22.

POE lubricant

Polyol ester oil required with HFC refrigerants because it is miscible with them. It is hygroscopic, so moisture must be excluded to prevent acid formation and compressor damage.

Mineral oil use

The traditional lubricant for CFC and HCFC refrigerants such as R-22. It is not miscible with HFC refrigerants, so it must not be used in HFC systems without an approved retrofit plan.

Temperature glide

The temperature range over which a zeotropic blend boils or condenses at a single pressure. Blends with glide fractionate when they leak, so systems must be recharged with virgin refrigerant.

R-744 refrigerant

Carbon dioxide, an A1 refrigerant with a global warming potential of 1. It operates at very high pressures and is used in commercial refrigeration and heat pump applications.

Recovery cylinder fill limit

Recovery cylinders must not be filled more than 80% of their capacity by liquid weight, leaving room for thermal expansion of the refrigerant as it warms.

Compressor function

The component that raises refrigerant pressure and temperature so the condenser can reject heat. Common types include reciprocating, scroll, rotary, and centrifugal; scroll compressors must not run in reverse.

Condenser subcooling check

Measuring liquid temperature below saturation temperature at the condenser outlet. A reading of about 8-15°F confirms adequate charge and heat rejection on a properly operating system.

Thermostatic expansion valve

A metering device that modulates refrigerant flow to maintain a set evaporator superheat, typically 8-12°F at the evaporator outlet, in response to bulb temperature and pressure.

Fixed orifice metering

A metering device with a fixed opening, such as a piston or capillary tube. Charge and superheat vary with load, so the system must be charged by weight or by superheat method.

Liquid-line filter-drier

A device that removes moisture and acid from the liquid refrigerant. A temperature drop across it of more than a few degrees indicates a restriction that should be corrected.

Sight glass purpose

A liquid-line window that shows bubbles when the liquid line is starved by low charge, a restriction, or flashing. A moisture indicator changes color as water content rises.

Suction accumulator

A device on the suction line that traps liquid refrigerant and meters it back to the compressor as vapor, protecting the compressor from liquid slugging during low-load or defrost operation.

Evacuation target

Pulling a system down to 500 microns or below and holding it under 500 for 10 minutes with the pump isolated. A rising micron gauge indicates moisture or a leak.

Nitrogen pressure test

Pressurizing a system with dry nitrogen, typically 150-300 psig for R-410A, before evacuation. Refrigerant or oxygen must never be used for pressure testing.

Brazing with nitrogen

Flowing dry nitrogen through piping while brazing to prevent internal oxidation. Without the purge, scale forms inside the lines and can clog the metering device and damage the compressor.

Charging by weight

Adding refrigerant measured on a scale to the manufacturer's specified amount. It is required for blends and critical systems, and is verified afterward by superheat or subcooling.

Comfort airflow rate

A design airflow of about 350-450 CFM per ton of cooling capacity. Low airflow causes low suction pressure, high superheat, and possible evaporator coil icing.

High head, high suction

A symptom pattern of high suction and high head pressure with a cool compressor, suggesting an overcharge or a condenser airflow problem. Check condenser coil and fan first.

Low suction, high superheat

A symptom pattern of low suction pressure, high superheat, and normal head pressure, suggesting a starved evaporator from low charge, a restriction, or an underfeeding expansion valve.

Overcharge symptom pattern

Low superheat with high subcooling indicates excess refrigerant. High superheat with low subcooling indicates undercharge; high superheat with high subcooling indicates a restriction or low airflow.

Liquid-line restriction test

Measuring temperature on both sides of a filter-drier or restriction. A temperature drop of more than a few degrees across the drier confirms a restriction that must be replaced.

Compressor overload trip

A compressor that trips on overload with high amp draw and hot windings. Check supply voltage, start components, and mechanical binding before condemning or replacing the compressor.

Recovery requirement

Refrigerant must be recovered rather than vented, using EPA Section 608 recovery equipment to the required vacuum level, with the recovered amount recorded on the service record.