The variable orifice of the expansion valve can be replaced, in small systems, by a long thin tube. This is a non-modulating device and has certain limita­tions, but will give reasonably effective control over a wide range of conditions
if correctly selected and applied. Mass flow is a function of pressure difference and the degree of liquid subcooling on entry. The capillary tube is used almost exclusively in small air conditioning systems and is self regulating within certain parameters. Increasing ambient temperature results in increasing load on the conditioned space and the condensing pressure will rise, forcing more refrigerant flow.

Tube bores of 0.8-2 mm with lengths of 1-4 m are common. The capillary tube is only fitted on factory-built and tested equipment, with exact refrigerant charges. It is not applicable to field-installed systems.

The restrictor expansion device overcomes some of the limitations of the capillary tube. The orifice can be precision-drilled whereas capillary tubes can suffer from variations in internal diameter over their length which results in changes to predicted performance. Figure 8.6 shows how the device is applied in a reversible air conditioner. In Figure 8.6(a) the device is shown in normal cooling mode. A bullet which is free to move horizontally by a small amount is pressed against a seat-forcing the refrigerant through the central restriction which acts as an expansion device. When the flow reverses, Figure 8.6(b) , the bullet moves back to the other seat, but grooving allows flow around the out­side as well as through it, so that the restriction is very small.


(a) (b)

Figure 8.6 Restrictor expansion device

It is normally fitted at the outlet of the condenser rather than at the evapo­rator inlet. This means that instead of a liquid line to the evaporator, the pipe contains liquid and flash gas and must be insulated. Although heat pick up is detrimental to performance, the pressure drop, which is used to drive the fluid, would normally have occurred in the expansion valve anyway. Liquid lines to remote evaporators on split systems can be quite lengthy and in a high pressure liquid line of the type more usually used, the pressure drop can result in an increased condenser pressure and tendency to form bubbles. Also the restric — tor can be delivered as part of the condensing unit and is removable, allowing changes to be made to give optimal performance.

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