Numerous combinations of bore and length are available to obtain the desired restriction. Once the capillary tube has been selected and installed, however, the tube cannot adjust to variations in discharge pressure, suction pressure, or load. The compressor and expansion device must arrive at suction and discharge conditions which allow the compressor to pump from the evaporator the same flow rate of refrigerant that the expansion device feeds to the evaporator. A condition of unbalanced flow between these two components must necessarily be temporary.

For a closer look at balance points the mass rate of flow fed by the capillary tube can be plotted on the same graph as the mass rate of flow pumped by the compressor. Figure 13-1 is such a plot with the flow through the capillary tube shown in dashed lines and the pumping capacity of a reciprocating compressor shown in solid lines. At high condensing pressures the capillary tube feeds more refrigerant to the evaporator than it does at low condensing pressures because of the increase in pressure difference across the tube. The compressor-capacity curves are the same as those explained in Chap. 11 in the study of compressors. At a 30°C condensing temperature, for example, the compressor and capillary tube must search for a suction pressure which allows them both to pass equal mass rates of flow. This suction pressure is found at point 1, which is the balance point at a 30°C condensing temperature. Points 2 and 3 are the balance points at 40°C and 50°C condensing temperatures, respectively.

The compressor and capillary tube do not have complete liberty to fix the suction pressure because the heat-transfer relationships of the evaporator must also be satisfied. If the evaporator heat transfer is not satisfied at the compressor-capillary-tube balance point, an unbalanced condition results which can starve the evaporator or overfeed the evaporator.