A hollow float is sometimes used to control the level of refrigerant (see Fig. 11-43). The float is fastened to a lever arm. The arm is pivoted at a given point and connected to a needle that seats at the valve opening. If there is no liquid in the evaporator, the ball-lever arm rests on a stop and the needle is not seated, thus leaving he valve open. Once liquid refrigerant under pressure from the compressor enters the float chamber, the float rises with the liquid level until, at a predetermined level, the needle closes the needle-valve opening.
In some large size plants where Freon-12 is used as a refrigerant, multiple ports are provided for handling the larger quantities of liquid.
Installation. The following precautions must be observed before installation of a float valve:
? Most float controls are designed for a maximum differential pressure of 200 lb.
? If the pressure will exceed 190 psi, there are stems and orifices of special size available for low-temperature use.
? In any application, keep the bottom equalizing line above the bottom of the evaporator to avoid oil logging.
? Make sure there are no traps in the equalizing line.
? The stems of a globe valve must be in a horizontal plane.
? Refrigerant flow must be kept to less than 100 ft/min where a bottom float equalizing connection is made to the header or accumulator return. That means the header and accumulator pipe must be properly sized.
? Accumulators of a small diameter with a velocity of over 50 fpm are not suitable for accurate float application. However, the float may control within wider limits with higher velocities. The top equalizing connection must be connected to a point of practically zero gas velocity.
? In automatic plants, always provide a solenoid valve in the liquid line ahead of the float control. This solenoid valve is to close either when the temperatures are satisfactory or when the compressor stops.
Figure 11-44 illustrates the connections for a high-pressure float control. There have been new developments in the control of liquid level since the early days of refrigeration.