HVAC Excellence Refrigeration cheat sheet

Sensible heat changes temperature without a phase change: Q = 1.08 × CFM × ΔT for air (Btu/h), or Q = 500 × GPM × ΔT for water.

Latent heat changes state at constant temperature; for water, latent heat of vaporization is about 970 Btu/lb at 212°F and rises as pressure falls.

Total heat (enthalpy) = sensible + latent; on a pressure-enthalpy diagram, the evaporator effect is the enthalpy difference between evaporator outlet and expansion-valve inlet.

The refrigeration cycle: compression, condensation (reject heat), expansion (drop pressure), evaporation (absorb heat); the compressor only moves heat, it does not create cold.

Superheat = actual suction temperature minus saturation temperature at suction pressure; subcooling = saturation temperature at liquid pressure minus actual liquid temperature.

One ton of refrigeration = 12,000 Btu/h = 200 Btu/min = 3.517 kW; it equals melting 1 ton (2,000 lb) of ice in 24 hours.

Carnot/thermodynamic limit: COP = evaporating absolute temperature ÷ (condensing absolute temperature − evaporating absolute temperature); real COP is always lower.

Pressure-temperature relationship is fixed for a pure refrigerant at saturation; a glide refrigerant (zeotrope) has a temperature range at one pressure.

Heat flows only from warmer to cooler; the evaporator must be colder than the load, and the condenser warmer than outdoor air, for heat transfer to occur.

ASHRAE safety groups combine toxicity (A lower, B higher) and flammability (1 none, 2L low, 2 moderate, 3 high): R-410A is A1, R-32 is A2L, R-290 propane is A3.

R-22 is a HCFC (ozone-depleting, Montreal Protocol phase-out); R-410A and R-32 are HFCs with zero ODP but high GWP; R-744 (CO2) is A1 with GWP of 1.

R-410A is a near-azeotropic blend of R-32/R-125 (50/50); it must be charged as liquid to avoid fractionation, and operating pressures run roughly 50-70% higher than R-22.

R-134a is a single-component HFC used in medium-temperature and centrifugal chillers; it is not a drop-in for R-12 without an oil/retrofit review.

Mineral oil and alkylbenzene (AB) are used with CFC/HCFC refrigerants; POE (polyol ester) and PVE are required with HFCs because they are miscible with HFCs.

POE oil is hygroscopic: it absorbs moisture rapidly, so lines must be kept sealed and a liquid-line filter-drier is required; moisture forms acids that degrade the oil and windings.

A refrigerant blend with glide (e.g., R-407C) leaks fractionated; the correct repair is to recover, evacuate, and recharge with virgin refrigerant, not top off.

Never mix refrigerants or use a refrigerant with a different pressure-temperature curve; recovery cylinders are filled to 80% liquid by weight to allow thermal expansion.

The compressor raises refrigerant pressure and temperature; types include reciprocating, scroll, rotary, and centrifugal, and a scroll must not be run in reverse.

The condenser rejects heat; subcooling at the condenser outlet (typically 8-15°F) confirms adequate charge and heat rejection.

The metering device (TXV, fixed orifice, capillary, EEV) maintains the pressure difference; a TXV modulates on evaporator superheat, usually 8-12°F at the outlet.

The evaporator absorbs heat; a starved coil shows high superheat and low capacity, while a flooded coil risks liquid slugging the compressor.

A liquid-line filter-drier removes moisture and acid; a suction-line accumulator or filter protects the compressor from liquid return.

The liquid-line sight glass shows bubbles when the liquid line is starved (low charge, restriction, or flashing); moisture indicator color changes with water content.

An accumulator stores liquid on the suction side; a receiver stores liquid on the high side of systems with a receiver (e.g., some commercial and heat pump systems).

Defrost controls, crankcase heaters, and low-ambient fan cycling protect the compressor and maintain head pressure during cold weather.

Evacuation to 500 microns or below (and holding under 500 for 10 minutes with the pump isolated) confirms a dry, tight system; a rising micron gauge indicates moisture or a leak.

Nitrogen pressure test (typically 150-300 psig for R-410A systems) before evacuation; never pressure-test with refrigerant or oxygen.

Line sizing must follow manufacturer tables; undersized suction lines cause excessive pressure drop and reduced capacity, oversized lines reduce oil return.

Brazing requires flowing dry nitrogen to prevent internal oxidation (scale); use silver-phosphorus or silver brazing alloys and purge while hot.

Charge by weight for blends and critical systems; use superheat (fixed orifice/TXV) or subcooling (TXV) as the field verification method.

Electrical: match supply voltage and phase, verify compressor rotation on three-phase scrolls, and confirm correct breaker and disconnect sizing per nameplate MCA/MOP.

Airflow: 350-450 CFM per ton for comfort cooling; low airflow causes low suction pressure, high superheat, and coil icing.

Commissioning records should include static pressures, superheat/subcooling, amp draw, voltage, and refrigerant weight charged, compared to nameplate values.

High suction and high head pressure with a cool compressor and normal-to-high superheat suggests an overcharge or a condenser airflow problem; high head with normal suction points to condenser restriction.

Low suction and high superheat with a normal head pressure suggests a starved evaporator (low charge, restriction, or TXV underfeeding).

Low suction and low head pressure with low superheat and a frosted suction line suggests an overfeeding TXV or a restriction at the evaporator inlet.

A compressor that trips on overload with high amp draw and hot windings: check supply voltage, start components, and mechanical binding before replacing.

A liquid-line restriction (e.g., clogged drier) shows a temperature drop across the drier; more than a few degrees ΔT indicates a restriction.

Superheat/subcooling diagnosis: high superheat + low subcooling = undercharge; low superheat + high subcooling = overcharge; high superheat + high subcooling = restriction or low airflow.

Recovery: recover refrigerant to the required vacuum level, never vent; use EPA Section 608 recovery equipment and record amounts on service records.

Leak detection: electronic leak detectors for HFCs, nitrogen pressure tests, and soap bubbles; dye is a last resort and may void warranties.

Sensible heat formula for air: Btu/h = 1.08 × CFM × ΔT; for water: Btu/h = 500 × GPM × ΔT.

Latent heat of vaporization of water ≈ 970 Btu/lb at 212°F; it increases as saturation pressure decreases.

One ton = 12,000 Btu/h = 3.517 kW; superheat = suction temp − saturation temp; subcooling = saturation temp − liquid temp.

The compressor moves heat; it does not create cold. Heat always flows from warmer to cooler.

ASHRAE safety groups: A1 (R-410A), A2L (R-32), A3 (R-290); R-22 is an HCFC with ODP; R-744 is CO2 with GWP 1.

R-410A is a 50/50 R-32/R-125 near-azeotrope; charge as liquid; pressures run 50-70% higher than R-22.

HFCs require POE or PVE oil; CFC/HCFC systems use mineral oil or alkylbenzene. POE is hygroscopic and forms acids with moisture.

Blends with glide (e.g., R-407C) fractionate on leaks; recover, evacuate, and recharge with virgin refrigerant rather than topping off.

Compressor raises pressure/temperature; scroll compressors must not run in reverse.

Condenser rejects heat; 8-15°F subcooling confirms adequate charge and heat rejection.

TXV modulates on evaporator superheat (typically 8-12°F); a fixed orifice relies on charge and load.

Filter-drier removes moisture and acid; sight glass bubbles indicate a starved liquid line or flashing.

Evacuate to 500 microns or below and hold under 500 for 10 minutes with the pump isolated.

Nitrogen pressure test before evacuation; never pressure-test with refrigerant or oxygen.

Flow dry nitrogen while brazing to prevent internal oxidation (scale).

Charge by weight for blends; verify with superheat (fixed orifice) or subcooling (TXV); 350-450 CFM per ton for comfort cooling.

High suction + high head + cool compressor = overcharge or condenser airflow problem.

Low suction + high superheat + normal head = starved evaporator (low charge, restriction, TXV underfeeding).

A temperature drop across the liquid-line drier of more than a few degrees indicates a restriction.

High superheat + low subcooling = undercharge; low superheat + high subcooling = overcharge; high superheat + high subcooling = restriction or low airflow.