Float Valves:

There are primarily two kinds of float valves- high side float valve and low side float valves.

Low-side Float Valve

It is similar to the float valves utilized for water tanks. In a water tank the float valve is set at the outlet of the water supply pipe to the tank. While in the tank the water level is low, the float ball hangs down through its own weight and the float arm keeps the valve completely open to permit water flow into the tank. Since the water level rises, the float ball (that is hollow) floats on the water and slowly rises according to the water level, throttling the water by the valve. In the end while the tank is full, the float valve fully closes the water supply. Since the water from the tank is utilized, the water level falls down; the float ball also lowers down, opening up the valve according to the level of water in the tank.

The low-side float valve also behaves in the similar way in a refrigeration system. Since the name implies the float valve is situated in the low pressure side of the system. This is fixed in a chamber (float chamber) that is linked to the evaporator. The valve assembly contains hollow ball, needle valve, a float arm, and seat. The needle valve-seat combination provides the throttling affect same to the expansion valve needle & seat. The movement of the float ball is passed on to the needle valve through the float arm. The float ball being hollow floats on the liquid refrigerant. The seat and needle valve are situated at the inlet of the float chamber. Since the liquid refrigerant vaporizes in the evaporator, its level falls down in the chamber. It causes the float ball to drop and then pull the needle away from the seat, So allowing adequate liquid refrigerant to flow into the chamber of the evaporator to built up for the amount of vaporization. While enough liquid enters, the float ball rises and at last closes the needle valve while the desired liquid level is attained. The rate of vaporization of liquid & consequent drop in the level of the liquid in the evaporator is based on the load. Therefore the movement of the float ball & amount of opening of the float valve is according to the load on the evaporator. The float valve responds to liquid level only changes and behaves to maintain a stable liquid level in the evaporator under any load without regard for the evaporator temperature and pressure.

As in the expansion valve, the capacity of the low-side float valve based on the pressure difference across the orifice in addition to the size of the orifice.

Low-side float valves are utilized for evaporators of the flooded-type system.

In greater capacity plants a small low-side float valve is utilized to pilot a liquid feed (and throttling) valve. According to the liquid level in the evaporator, the float valve pass on pressure signals to the chief liquid feed valve to decrease or increase the extent of its opening. Therefore the low-side float valve in such a system is known as a 'pilot' and the liquid-feed valve is called the pilot-operated liquid-feed valve.