Enemark and Roelsgaard demonstrated that the amount of refrigerant charge in a hermetically sealed compressor system would affect the starting load applied to the compressor motor. This load is a critical factor in the selection of compressor motors, particularly when the refrigeration plant has to work in tropical areas where ambient temperatures of 104F (4OC) to 113F (45C) are found and supply voltage can be as low as 80% of the rated voltage.

In the majority of cases the use of a receiver to ensure adequate supply of refrigerant is perfectly satisfactory. But there are instances, namely in aero-space applications, where it is of fundamental importance that the weight of the plant should be maintained at a :minimum. In some cases, it might be necessary to charge a plant under conditions vastly different than those appertaining to the normal working conditions of the plant. There are two methods available to determin!9 the charge required by these plants:

1. To simulate the working conditions of the plant in the factory
2. To calculate the accurate charge required.

The later method has an advantage in that accurate calculations carried out during the design stage alleviate the need for modifications of the plant when placed in operation. Secondly, while it is possible to test factory packaged equipment at various evaporator and condenser loads, this is often impossible for field erected equipment: in this case, the use of a computer simulation of the plant during the design stage is desirable.

In the past, designers of refrigeration systems have based their designs on a heat and mass flow balance in the various system components. However, it will be seen from the formulae we derive that both the amount of refrigerant charge and the internal volume of the condenser and evaporator can influence the overall performance of a vapor compression refrigeration plant.

Most modern shell-and-tube condensers are designed so that they can act as liquid receivers. In these dual purpose condensers the lower tube bundles are submerged in the liquid refrigerant. Chaddock proved that this liquid refrigerant serves as an insulating film, reducing the heat transferred through these lower tube bundles.

The formulae derived can be used to calculate the amount of liquid and vapor in both the condenser and evaporator. Knowing the amount of liquid refrigerant present, it is a simple procedure to calculate the total heat transfer surface submerged in the liquid so an optimum heat transfer surface configuration can be designed.