What is a circulator pump?
A pump designed to circulate a fluid through a closed system. A closed system is one which runs in a loop, with the pump discharge line eventually returning back to the pump suction, often without ever being exposed to atmospheric pressure. They are generally considered centrifugal pumps, although there are also a few types that use positive displacement technology.
How do they work?
Circulator pumps are most commonly single stage. The pump works like any centrifugal pump, with the rotating impeller building up the velocity of the fluid as the fluid passes along the impeller vanes. Then the high velocity fluid is converted into high pressure fluid in the diffusion portion of the pump casing. The most common casing design for circulator is vertical inline. Smaller versions, such as those used in home and small commercial building hydronic heating systems, have a centrifugal wet rotor, and closed coupled type of construction that eliminates the mechanical seal. Higher flow versions use vertical inline pumps but with mechanical seals.
When a circulator pump is used in a completely closed system, i.e., in a system where the fluid never sees atmospheric pressure but operates in a completely closed loop, the system usually must include an expansion tank. This is a closed tank that is attached to the suction line of the pump. The tank has a diaphragm in the middle, with the volume above the diaphragm being filled with air under a small amount of pressure. As the liquid circulates in the closed loop, it is often heated or cooled (depending on the application), which causes the fluid to expand or contract. The air space above the diaphragm in the expansion tank adjusts its volume to accommodate the expansion and contraction of the water. If there were no expansion tank, the pipes could be over pressurized and burst as the water expanded in the closed system.
Where are circulator pumps used?
They are commonly used in hot water heating and cooling systems, as well as in applications that require chemicals to be regularly mixed into the fluid, such as pool and spa pumps. Pumps used to circulate water in a pond or tank also qualify as circulating pumps.
In a hot water heating system, water heated by the boiler is circulated through pipes to all the rooms in the building, often using radiators to heat the rooms. The water then returns to the source and is sent back through the boiler by the pump. For most types of systems, the pump is turned on and off automatically by the building's thermostat.
For other types, the process is much simpler. A swimming pool pump, for instance, the water is usually circulated for an hour or so every 24 hours, so the size and power of the pump depends upon the volume of the pool. A pool pump is also an example of a type of circulating pump that mixes a chemical—chlorine, in this case—into the fluid as it circulates. The pump also moves the pool water through a heater to heat it up as it circulates.
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