Manual for Refrigeration Servicing Technicians - UNEP - Division of ...

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Servicing Practices
Charging to sight glass
This method normally applies to larger systems that have a liquid
receiver. Refrigerant is charged into the system, and as it is metered
in, the technician observes the sight glass in the liquid line.
Eventually, once no more bubbles can be seen in the sight glass, the charge
size has approximately been achieved, as shown here:
Charging by sight glass
Bubbles
A Clear Glass
However, as there is always a delay between adding the refrigerant
and the effect on the sight glass, the technician should take extra
time to ensure that the correct charge has been added. It should
also be borne in mind that longer delays between adding refrigerant
and the response of the sight glass occur with larger systems.
As with all other systems, it is important to consider the ambient
temperature and the possibility of adding a little more refrigerant so
that no bubbles appear during warmer/cooler ambient conditions.
In addition, the refrigerant cylinder(s) should be weighed before and
after, and the charged amount checked against the intended charge
or compared against the size of the liquid receiver to ensure that it
will not hydraulically fill during pump-down.
Charging according to system performance
It is possible to charge a system according to the system’s
performance characteristics. This is done by monitoring the suction
pressure, discharge pressure, evaporator superheat and liquid
subcooling out of the condenser.
First, the design performance characteristics are noted: the
ambient temperature, the application temperature (to be cooled
to), the intended superheat and subcooling. From the ambient
temperature and the application temperature, a typical condenser
and evaporator temperature difference is assumed for the
equipment under consideration (say, for example, 8 K), from which
the saturated condensing and evaporating temperatures, and finally
suction and discharge pressures are estimated.
Thermometers are tightly attached to the liquid line and suction
line (using a heat transfer paste and insulated). Refrigerant is
then gradually added into the system and the pressures and
temperatures monitored. As the estimated suction and discharge
pressures are approached, and the design subcooling and
superheat values are achieved, a suitable charge is achieved.
Again, as with the sight glass, there is a delay between adding
refrigerant and the resulting performance characteristics being
achieved, so these performance characteristics should be observed
for some time to ensure that the reading are more or less constant.
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