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

4
Charging
System charging is adding the proper quantity of refrigerant to
refrigeration system so that it operates as intended. For a given
set of conditions (design conditions) systems have an “optimum”
RE 4 charge – this is the mass of refrigerant that the highest efficiency
and design cooling capacity (or heating capacity, in the case of a
ETRIC CHARGING
heat pump) will be achieved. At off-design conditions, for example,
at a higher or lower ambient temperatures, the optimum charge will
be different. However, it is best to add the specified charge since
this is what the system has been designed to handle.
Servicing Practices
Some systems can handle a wider variation of charge size, in particular,
those with liquid receivers. Direct expansion systems with small
condensers and capillary tube expansion devices tend to be very
sensitive to refrigerant mass and are said to be “critically charged”.
This figure illustrates the sensitivity of a critically charged system, where a
small variation in charge size can greatly reduce the equipment efficiency:
COOLING CAPACITY Qe (kW)
8
6
4
2
0
Example of how
refrigerant charge
affects performance
In all cases, the system data-plate should contain useful information
such as the design refrigerant charge size.
Qe
COP
0,1 0,2 0,3 0,4 0,5
REFRIGERANT CHARGE (kg)
4
3
2
1
0
SYSTEM COP (-)
There are several ways to charge
systems, and the most appropriate
depends upon the local conditions,
the quantity and other factors such
as the type of system.
These methods include:
Volumetric charging by graduated cylinder 4
Mass charging by balance 4
Charging to sight glass 4
Charging according to system performance 4
Electronic charging machines 4
Vapour Refrigerant Charging 4
Liquid refrigerant charging 4
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