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GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale.
He holds a Bachelor of Business Administration and an associate degree in educational administration. He
has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status:
refrigeration and electrical. He has been involved with SAIRAC for 17 years and has been a Johannesburg
committee member for the past eight (chairman in 2011 and 2012) . Currently he is the treasurer
(Johannesburg council) as well as president (national council).
THE NECESSITY OF
SUBCOOLING
By Grant Laidlaw
Many people ask for assistance in the understanding of theoretical and
practical aspects of the industry. I will endeavour to enlighten.
Karel asks: Grant,can you explain subcooling please? I feel
that this is an important aspect of the system which is not
well understood. Why is subcooling necessary?
T
he term subcooling is the condition where the liquid
refrigerant is colder than the minimum temperature
(saturation temperature) required to keep it from boiling or
refers to a liquid existing below its normal boiling point. The
amount of subcooling, at a given condition, is the difference
between its saturation temperature and the actual liquid
refrigerant temperature.
Condenser subcooling ensures that there is liquid exiting from
the condenser so that only liquid refrigerant and not vapour exits
the condenser. Let us look at why subcooling is desirable.
Subcooling increases the efficiency of the system since the
amount of heat being removed per kilogram of refrigerant
circulated is greater. In other words, less refrigerant is pumped
through the system to maintain the refrigerated temperature
required. Subcooling is beneficial because it prevents the liquid
refrigerant from changing to a gas before it gets to the evaporator.
This is very important as vapour entering the evaporator can
damage the expansion device casing wire drawing on the needle
and seat, resulting in metal fatigue and possible failure.
Subcooling reduces the amount of time that the compressor
must run to maintain the temperature. The amount of capacity
boost resulting from each degree of subcooling varies with the
refrigerant being used.
Pressure drops in the liquid piping and vertical risers can
reduce the refrigerant pressure to the point where it will boil
or ‘flash’ in the liquid line. This change of phase causes the
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refrigerant to absorb heat before it reaches the evaporator.
Inadequate subcooling, therefore, prevents the expansion valve
from properly metering liquid refrigerant into the evaporator,
resulting in poor system performance.
The amount of liquid subcooling not only affects system
capacity but also the effectiveness and capacity of expansion-
type metering devices.
In the system, subcooling is sensible heat taken away from the
100% saturated liquid point in the condenser. Liquid subcooling
then occurs from the 100% saturated liquid point in the condenser
to the metering device. Once all the saturated vapour in the
condenser phase has changed to saturated liquid, subcooling
will start to occur if any sensible heat is taken away. Remember,
sensible heat is heat that causes a temperature change. Any
drop in temperature of saturated liquid in the condenser will be
considered liquid subcooling.
We find that we can identify subcooling as condenser
subcooling and total subcooling.
CONDENSER SUBCOOLING
Condenser subcooling is liquid subcooling present in the
condenser. It can be measured by subtracting the condenser-out
temperature from the condensing temperature.
When subcooled, the refrigerant is not generating or losing any
vapour pressure. Consequently, there is no pressure-temperature
relationship and a pressure-temperature chart cannot be used.
The condenser-out temperature must be measured with either a
thermistor or a thermo-couple fastened to the condenser outlet.
The condensing temperature, on the other hand, can be acquired
from the condensing pressure read from a gauge on the high
side of the refrigeration system because a pressure-temperature
relationship does exist in a saturated condition.
RACA Journal I January 2020
67