Sub-Cooling And Overheating Calculation - Carrier 30AJ Installation, Operation And Maintenance Manual

8.3 - SUB-COOLING AND OVERHEATING CALCULATION

SUB-COOLING
1. Definition:
Difference between saturated condensation temperature (TCD)
and liquid line temperature (TLL).
SR = TCD - TLL
2. Required measurement equipment:
• Manifold
• Bulb or electronic thermometer (with temperature sensor)
• Filter or insulating foam
• Pressure-Temperature Conversion Table for R-22.
3. Measurement steps:
1º) Place the thermometer bulb or sensor in contact with the
liquid line near the dryer filter (only for 120 size). Assure
the surface is clean. Cover the bulb or sensor with foam,
to isolate it from the ambient temperature.
2º) Install the manifold at the discharge (high manometer)
and suction (low manometer) lines.
3º) After the operation conditions stabilize, read the pressure
in the discharge line manometer.
NOTE:
Measurement shall be made with the equipment operating
within the installation project conditions to allow achieving
the desired performance.
4º) From R-22 table, get saturated condensation temperature
(TCD).
5º) Read the liquid line temperature (TLL) on the thermometer.
Subtract it from the saturated condensation temperature;
the difference is the sub-cooling.
6º) If the sub-cooling is between 8 and 11 °C, the charge is
correct. If it is below, add refrigerant; if it is above, take
away some refrigerant.
4. Calculation example:
- Discharge line pressure (manometer) ................. 260 psig
- Saturated condensation temperature (table) ............ 49°C
- Liquid line temperature (thermometer) ..................... 45°C
- Sub-cooling (subtraction) .......................................... 4°C
Add refrigerant!
OVERHEATING
1. Definition:
Difference between suction temperature (Ts) and saturated
evaporation temperature (TEV).
SA = Ts - TEV
2. Required measurement equipment:
• Manifold
• Bulb or electronic thermometer (with temperature sensor)
• Filter or insulating foam
• Pressure-Temperature conversion table for R-22.
3. Measurement steps:
1º) Place thermometer bulb or sensor in contact with the
suction line, the closest possible to the expansion valve
bulb. The surface shall be clean and the measurement
performed at the upper tube portion to prevent false
readings. Cover bulb or sensor with foam, to isolate them
from the ambient temperature.
2º) Install the manifold in the discharge (high manometer)
and suction (low manometer) lines.
3º) When the operation conditions stabilize, read the pressure
in the suction line manometer. From R-22 table, get
saturated evaporation temperature (TEV).
4º) Read the suction temperature (Ts) in the thermometer.
Perform several readings and calculate the average. This
will be the adopted temperature.
5º) Subtract the saturated evaporation temperature (TEV) from
the suction temperature: the difference is overheating.
6º) If the overheating is between 4 and 6 °C, the expansion
valve setting is correct. If it is below, much refrigerant is
being injected into the evaporator and it is necessary to
close the valve (turn set screw to the right - clockwise). If
the overheating is high, little refrigerant is being injected
into the evaporator and it is necessary to open the valve
(turn set screw to left - counterclockwise).
4. Calculation example:
- Suction line pressure (manometer) ....................... 75psig
- Suction line temperature (thermometer) .................. 15°C
- Saturated evaporation temperature (table) ................. 7 ° C
- Overheating (subtraction) .......................................... 8 ° C
High overheating: open the expansion valve!
Obs.: After to make V.E.T. adjustment don't forget to replace
the helmet.
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