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How to Use the Evaporating Temperature Calculator
Select the refrigerant and then the mode: pressure to temperature if you have a gauge reading, or temperature to pressure if you have a target. The refrigerant has to be right, because every refrigerant has its own pressure-temperature relationship.
For a pressure reading, choose the unit that matches your gauge: bar or psi, absolute or gauge. Gauge units add atmospheric pressure. For a target, enter the evaporating temperature from -50 to 30°C.
Enter the space temperature to get the evaporator TD, which is room temperature minus evaporating temperature. Enter the suction line temperature to get superheat, which is line temperature minus the dew point.
Choose air conditioning, walk-in cooler, walk-in freezer, produce room or display case. The results compare the evaporating temperature, TD and superheat with the usual ranges for that application and explain anything outside them.
Evaporating temperature and what it tells you
Evaporating temperature is the saturation temperature of the refrigerant in the evaporator, and it sets the temperature difference available to pull heat from the space, the compression ratio, the capacity and the amount of moisture the coil removes. Technicians read suction pressure on a gauge and convert it to temperature, then compare the result with the design value for the application. A reading that is off the design value is usually the first sign of a charge, airflow or metering problem.
Converting pressure to temperature
Each refrigerant has its own pressure-temperature relationship, so the same gauge reading means a different temperature for R-410A than for R-404A. The calculator fits that relationship to real-fluid data over -50 to 30°C. Fitted saturation pressures agree with the CoolProp implementation of the reference equations to within about 0.25%, which is about 0.1 K. The property data behind such equations is the kind maintained in the NIST REFPROP database. For R-410A, 9.34 bar absolute is 5°C, and for R-404A the -6°C evaporating temperature in the cooler example is 4.94 bar absolute, 56.9 psig. Gauge readings add atmospheric pressure, 1.01325 bar or 14.7 psi, before conversion.
Dew point, bubble point and glide
Blends in the 400 series, which ASHRAE lists as zeotropes in its refrigerant designations, change temperature as they evaporate. At a given pressure the liquid starts to boil at the bubble point and finishes at the dew point, and the difference is the glide. Evaporating temperature in this calculator is the dew point, and superheat is measured from it. At -10°C the calculator uses a glide of about 6.5 K for R-407C, 6.2 K for R-448A and 1.4 K for R-454B. Measuring from the bubble point instead overstates superheat by the glide.
Evaporating temperature by application
The calculator compares your result with these ranges. Air conditioning evaporates at about 4 to 10°C and is checked against a TD of 14 to 22 K. Walk-in coolers run at -10 to -2°C with a TD of 5 to 8 K. Walk-in freezers run at -35 to -20°C with a TD of 6 to 12 K. Produce rooms run at -5 to 1°C with a TD of 3 to 5 K, and display cases at -15 to -8°C with a TD of 6 to 10 K. These are usual design ranges and not limits, so an equipment-specific rating takes priority.
TD and humidity
TD is the room temperature minus the evaporating temperature. A large TD means a cold coil, which condenses and freezes more moisture out of the air and dries the room. A small TD keeps the coil warmer and the room more humid, at the cost of a larger coil. Whether moisture is condensing at the coil depends on the coil surface temperature relative to the air's dew point, and the dew point calculator finds that figure for your room conditions. Use the evaporator sizing calculator to see what coil size a given TD requires.
Evaporating temperature and efficiency
Raising the evaporating temperature narrows the lift the compressor works against, which lowers the compression ratio and raises capacity and efficiency. The results show suction vapour density against the middle of the application range as a capacity proxy, because compressor capacity follows the density of the gas it draws in. To see how much a higher suction pressure is worth as a rating, use the EER calculator with your capacity and power figures. The compression ratio calculator gives the ratio itself.
Frequently Asked Questions
Low suction pressure, and so a low evaporating temperature, usually comes from one of three causes. Low refrigerant charge means less mass flow, and it shows up with high superheat. A frosted, dirty or blocked evaporator restricts airflow, so the coil runs colder to move the same heat. A throttled metering device passes too little refrigerant and also raises superheat. Read superheat before adding charge: high superheat with low suction pressure points to undercharge or a restricted metering device, while low superheat with low suction pressure points to an airflow problem. Check the defrost and the fans before suspecting the charge.
Evaporating temperature is the saturation temperature of the refrigerant in the coil, set by the suction pressure and the refrigerant's pressure-temperature relationship, and it is read from a gauge or chart, not measured with a probe. Suction line temperature is the actual temperature of the vapour in the line, measured with a clamp or contact probe near the compressor. The difference is the suction superheat. For R-410A at 9.34 bar absolute the evaporating temperature is 5°C, so a suction line at 12°C means 7 K of superheat. For blends with glide, use the dew point temperature for this subtraction.
Produce loses moisture to a cold, dry coil, so the room is designed for a small temperature difference between the air and the coil, 3 to 5 K in this calculator. A small TD keeps the coil only slightly colder than the air and removes less moisture on each pass. A freezer holds frozen product that does not wilt, so a TD of 6 to 12 K is accepted, and it lets the same capacity be delivered by a smaller, cheaper coil. The trade is coil size against product quality: the produce room pays for more coil surface to protect humidity, and the freezer saves on coil size and accepts more frost.
A lower evaporating temperature raises the compression ratio, which raises discharge temperature and the load on valves, bearings and the motor. Compressors are rated for a range of evaporating temperatures, and running below it for long periods accelerates wear and can break down the oil. A system with a chronically low evaporating temperature may still cool adequately, which hides the problem, so treat it as a fault to investigate and not as a stable operating point. Compare the pressures with the compressor envelope and check the compression ratio.