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Calculating Superheat and Subcooling


A system with an expansion valve (TXV) must be charged by Subcooling. A fixed metering device system must be charged by Superheat.

SUPERHEAT

Unitary systems using fixed orifice/accurator pistons as refrigerant metering devices are charged by the suction Superheat method. Saturation temperature or boiling temperature is the temperature at which fluid changes from a vapor to a liquid or from a liquid to a vapor. Additional temperature increases during this conversion is called Superheat. Superheat is the gas temperature above the saturated temperature. Increasing the fluid's pressure will raise the saturation temperature and decreasing the pressure will lower the temperature. These systems should provide a charging chart to properly charge their systems. Sometimes these charts are available from the unit's distributor, manufacturer's web site or installation/service manuals. Most of the time they are glued inside the condenser's service panel. The charts will require an indoor wet bulb temperature reading as well as an outdoor dry bulb temperature reading. The indoor wet bulb reading indicates the total heat of the air and the total load on the indoor coil (sensible heat plus latent heat). The dry bulb temperature will only determine outdoor sensible heat. Think of it as boiling water. The water temperature of 212°F (100°C) is the sensible heat. The heat that causes a change of state (water to vapor) with no change in temperature is called latent heat. Latent heat does not affect the temperature. The sum of the sensible and latent heat of the air is called enthalpy or total heat. Cross referencing the indoor wet bulb and outdoor dry bulb temperatures, the charging chart will recommend the proper suction Superheat.


The Evaporator Superheat Method should be the first effective method of properly charging a fixed refrigerant metering system. The Condenser Superheat Method is ideal when the indoor temperatures are close (+- 5°F) to desired comfort levels would be during that particular season (Heating or Cooling).


Evaporator Superheat Method:

1. Take a dry bulb temperature of the outdoor ambient air entering the condenser coil.

2. Take a pressure reading of the suction line at the evaporator to get refrigerant saturation pressure=temperature. The refrigerant saturation pressure=temperature is when the refrigerant is turning from a liquid to a vapor. At saturation pressure=temperature, both low-pressure vapor and liquid are at the same temperature.

3. Convert saturation pressure=temperature with a pressure-to-temperature chart (Pressure-Temperature Chart).Not all evaporators have a suction service port. If a reading is obtained at the condenser, do not use the common suction pressure port on a heat pump condenser because there is a pressure drop through the reversing valve. If the suction line is longer then 25' you may have to add a few pounds of pressure in your pressure-to-temperature conversion due to pressure drop in the suction line.

4. Take a temperature reading at the suction line of the evaporator. If you use a probe-type thermometer, put a piece of pipe insulation around the probe and pipe. Record both the converted saturated temperature and the actual suction line temperature.

5. Subtract one from the other. The difference is the amount the refrigerant gas has heated above saturated temperature (Superheat).

6. A low charge will give a high Superheat. An overcharge will give a low Superheat along with a higher compression ratio. The charging chart associated with the system should indicate the amounts of Superheat designed for the system. Table 1 is a standard Superheat chart.

Table 1 EVAPORATOR SUCTION LINE TEMPERATURE - WB (F°)
54 56 58 60 62 64 66 68 70 72 74
OD - DB (F°) SUPERHEAT
115 5 8 13 16
110 5 7 11 14 18
105 4 6 8 12 15 19
100 5 8 11 14 18 20
95 4 7 11 13 16 20 23
90 4 6 9 12 15 18 22 25
85 4 6 8 12 14 17 20 25 27
80 4 6 8 12 14 16 18 23 27 28
75 5 7 10 12 14 16 18 23 26 28 30
70 8 11 12 14 16 18 22 25 28 30 33
65 11 13 15 17 18 22 25 28 30 33 36
60 13 17 18 20 24 26 28 30 33 36 39

Add refrigerant to lower temperature. Reclaim refrigerant to increase the temperature. Allow +- 2°F tolerance.


TECH NOTE: If the Superheat is correct and the suction pressure is low, the system probably has low airflow. Correct the airflow problem and check the charge again.


Condenser Superheat Method:

This method, similar to Evaporator Superheat Method, is most effective when the indoor conditions are within desired indoor comfort conditions or as a secondary method when the evaporator service port is not available.


1. Take a dry bulb temperature of the outdoor ambient air entering the condenser coil.

2. Take the suction line pressure and temperature at the condenser's suction service valve (air conditioning) or service port at the compressor (heat pump). If you use a probe-type thermometer, put a piece of pipe insulation around the probe and pipe.

3. Using Table 2, Intersect the reading of the vapor pressure and outdoor dry-bulb temperature to obtain the vapor line temperature.

Table 2 VAPOR PRESSURE AT SERVICE VALVE
52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86
OD DB (F°) VAPOR LINE TEMPERATURE (F°)
101 + 43 45 46 47 49 50 51 53 54 55
100 44 45 47 48 49 51 52 53 55 56 57
95 45 47 48 50 51 52 54 55 56 58 59 60
90 49 51 52 54 55 56 58 59 60 62
85 52 53 55 56 58 59 61 62 63
80 53 55 56 58 59 61 62 63 65
75 55 56 58 59 61 62 64 65 66
70 55 57 58 60 61 63 64 66 67
65 57 58 60 61 63 64 66 67 69

4. If the vapor line temperature is not the same, adjust the refrigerant charge. Adding refrigerant will raise the suction pressure and lower the suction line temperature. Reclaiming refrigerant will lower the suction pressure and raise the suction line temperature.


TECH NOTE: If adding refrigerant increases both the suction pressure and temperature, the unit is overcharged.


Always consider the manufacturer’s recommendations and charging charts first when choosing to use either method.


SUBCOOLING

Thermostatic expansion valves (TXVs) are metering device that controls the amount of refrigerant that enters the evaporator coil. This process causes the refrigerant to go from a liquid to a vapor. A TXV works by an internal spring, valve pin, and a diaphragm that opens and closes through a thermostatic bulb and refrigerant pressures. The bulb and the capillary tube connected to it has a charge that expands or contracts on temperature change. When the charge expands, this exerts pressure on the diaphragm that opens the valve pin allowing more refrigerant to enter the coil. The valve pin is surrounded by a spring that puts pressure on the opposite side of the diaphragm to close the valve pin when the bulb's charge contracts or refrigerant pressures drop. This spring tension is adjustable. Turning the spring's adjustment stem clockwise increases Superheat. Counterclockwise decreases Superheat.


TECH NOTE: Most TXVs are factory set and should never be manually adjusted. In most cases, Superheat can be adjusted through refrigerant charges.


Condenser Subcooling Method:

Subcooling is the amount of liquid held back in the condenser. This allows the liquid to expel heat below saturation pressure=temperature. For every 1° of Subcooling at the same condensing pressure, capacity will increase 0.5%. Increasing Subcooling with an increase of discharge pressure and compression ratio decreases capacity. Add 5° of Subcooling for every 30 ft of liquid line lift. Liquid Subcooling is normally measured at the liquid line service valve. It’s usually between 8°F and 12°F. Always use manufacturer’s recommendations first before using a standard Subcooling chart.


1. On the condenser, take a pressure reading at the liquid line service port. The refrigerant saturation pressure=temperature is when the refrigerant is turning from a vapor to a liquid. At saturation pressure=temperature, both high-pressure liquid and vapor are at the same temperature.

2. Use a temperature-to-pressure chart to convert the pressure to the saturated condensing temperature of the refrigerant (Pressure-Temperature Chart).

3. Attach a thermometer to the liquid line. If you use a probe-type thermometer, put a piece of pipe insulation around the probe and pipe. The temperature that you read with the thermometer should be lower than the saturated condensing temperature. The difference between the liquid line temperature and the saturated condensing temperature is Subcooling.


Subcooling Calculation Check:

Using Table 3, you can check the range of Subcooling by taking a wet bulb reading at the evaporator and a dry bulb reading from the air entering the condenser. This check will determine if your Subcooling calculation falls within a proper range. This chart should only be used if the manufacturer's recommendations are not available.

Table 3 EVAPORATOR SUCTION LINE TEMPERATURE - WB (F°)
57 59 61 63 65 67 69 71 73
OD - DB (F°) SUBCOOLING
115 15 14 13 12 10 8 6 4 2
110 17 16 15 13 12 10 8 6 4
105 19 18 17 16 14 12 10 8 6
100 20 19 18 17 15 13 12 10 8
95 21 20 19 18 17 15 13 12 10
90 22 21 20 19 18 16 15 14 12
85 23 22 21 20 19 18 17 16 14
80 24 23 22 21 20 19 18 17 15
75 25 24 23 22 21 20 19 18 17

Add refrigerant to increase subcooling. Reclaim refrigerant to reduce subcooling. Allow +- 2°F tolerance.


TECH NOTE: When using the Subcooling method, you can check the suction Superheat to help troubleshoot the TXV. If the expansion valve goes bad, you will have a very low suction Superheat with proper subcooling. If you get zero degrees Superheat with a TXV, then the TXV is defective and will need to be replaced. A TXV is designed to maintain a constant Superheat. Overcharging a TXV will raise Subcooling, increasing pressures, and decreasing efficiency. Undercharging a TXV will decrease Subcooling, increase Superheat, decreasing capacity, and leave oil in the evaporator.


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