Two major categories of liquid-level controllers used in industrial refrigeration systems are (1) high-side float valves, and (2) low-side float valves. These classes of valves, as shown schematically in Fig. 11.10, differ in the placement of the valve. The flow regulation in a high-side float (Fig. 11.10a) is downstream of the controlled liquid level and the flow regulation in the low-side float (Fig. 11.10b) is upstream of the controlled liquid level.
The low-side level controllers are much more common in industrial refrigeration systems than are high-side controllers, but there are roles for the high-side float valve, which are primarily to allow liquid to pass but prevent the flow of vapor. Three applications for the high-side float valve are (1) to drain condensed liquid from a coil during hot-gas defrost as is shown in Fig. 6.56, (2) to drain condensed liquid from a hot-gas main, and (3) to be placed at the outlet of a condenser in a critically charged system. The function of the valve in the latter situation is to allow condensed refrigerant to leave the condenser, but prohibit the passage of vapor. These critically-charged systems sometimes have no high-pressure receiver, but there must be a vessel somewhere on the low side to accommodate changes in liquid volume in the evaporators.
The low-side liquid level controller is the type commonly encountered in industrial refrigeration systems, because it is the class used for control of the liquid level in low-pressure receivers and flash-tank/desuperheaters.