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10
Subcooling and Overheating Calculation
SUBCOOLING
1. Definition:
Difference between the saturated condensation
temperature (T
) and the temperature of the liquid
CD
line (T
).
LL
SR = T
2. Required equipment for measurement:
•
Manifold;
•
Bulb or electronic thermometer (with
temperature sensor);
•
Filter or insulating foam;
•
Conversion Table: Pressure-Temperature for R-22.
3. Measurement steps:
1
) Place the thermometer bulb or sensor in contact
st
with the liquid line near the dryer filter. Ensure
the surface is clean. Cover the bulb or sensor
with the foam, in order to isolate it from the
ambient temperature.
2
) Install the manifold in the discharge lines (high
nd
pressure gauge) and suction (low pressure gauge).
3
) After the operation conditions stabilize, read the
rd
pressure in the pressure gauge of the discharge
line.
The measurements shall be made with the
equipment operating within the conditions of the
installation project, in order to allow achieving
the desired performance.
4
) From the R-22 table, get the saturated
th
condensation temperature (T
5
) In the thermometer, read the temperature of the
th
liquid line (T
). Subtract it from the saturated
LL
liquid line temperature: the difference is the
subcooling.
6
) If the subcooling is between 8° and 11°C the
th
charge is correct. If it is below, add refrigerant. If
it is above, remove refrigerant.
4. Example of calculation:
— Discharge line pressure (pressure gauge) ....... 260 psig
— Saturated condensation temperature (table) .... 49°C
— Liquid line temperature (thermometer) ............. 45°C
— Subcooling (subtraction) ........................................ 4°C
— Add refrigerant.
54
M A N U A L D E I N S TA L A C I Ó N , O P E R A C I Ó N Y M A N T E N I M I E N T O
- T
CD
LL
).
CD
OVERHEATING
1. Definition:
Difference between the suction temperature (T
the temperature of the saturated evaporation (T
SA = T
2. Required equipment for measurement:
•
Manifold;
•
Bulb or electronic thermometer (with
temperature sensor);
•
Filter or insulating foam;
•
Conversion Table: Pressure-Temperature for R-
22.
3. Measurement steps:
1
) Place the thermometer bulb or sensor in contact
st
with the suction line, as near as possible of the
compressor (10 to 20 cm). The surface must be
clean and the measurement must be performed
in the upper portion of the tube, in order to
prevent false readings. Cover the bulb or sensor
with the foam, in order to isolate it from the
ambient temperature.
2
) Install the manifold in the discharge lines (high
nd
pressure gauge) and suction (low pressure gauge).
3
) After the operation conditions stabilize, read the
rd
pressure in the pressure gauge of the suction line.
From the R-22 table, get the saturated evaporation
temperature (T
).
EV
4th) Read the suction temperature (T
thermometer, 10 or 20 cm before the compressor.
Perform several readings and calculate the average,
which will be the adopted temperature.
5
) Subtract the saturated evaporation temperature
th
(T
) from the suction temperature, the difference
EV
is the overheating.
6th) If the overheating is between 4°C to 6°C, the
adjustment of the expansion valve is correct. If it
is below, much refrigerant is being injected in the
evaporator, and it is necessary to close the valve
(turn the adjustment screw to the right -
clockwise). If the overheating is high, little
refrigerant is being injected in the evaporator, and
it is necessary to open the valve (turn the
adjustment screw to the left - counterclockwise).
4. Example of calculation:
— Suction line pressure (pressure gauge) ..... 75psig
— Suction line temperature (thermometer) .. 15°C
— Saturated evaporation temperature (table) . 7°C
— Overheating (subtraction) ................................ 8°C
— High overheating: open the expansion valve.
After setting the EXV, do not forget to put back
the helmet.
Only set the overheating after the adjustment of
the subcooling.
) and
S
).
EV
- T
S
EV
) in the
S
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