What is a Circulator Pump?
A circulator pump
is a specific type of pump used to circulate gases, liquids, or slurries in a circuit. They are commonly found circulating water in a hydronic heating or cooling system. Because they only circulate liquid within a closed circuit, they only need to overcome the friction of a piping system (as opposed to lifting a fluid from a point of lower potential energy to a point of higher potential energy).
How are Circulator Pumps Used?
as used in hydronic systems are usually electrically-powered centrifugal pumps. As used in homes, they are often small, sealed, and rated at a fraction of a horsepower, but in commercial applications they range in size up to many horsepower and the electric motor is usually separated from the pump body by some form of mechanical coupling. The sealed units used in home applications often have the motor rotor, pump impeller, and support bearings combined and sealed within the water circuit. This avoids one of the principal challenges faced by the larger, two-part pumps: maintaining a water-tight seal at the point where the pump drive shaft enters the pump body.
Small- to medium-sized circulator pumps are usually supported entirely by the pipe flanges that join them to the rest of the hydronic plumbing. Large pumps are usually pad-mounted.
Pumps that are used solely for closed hydronic systems can be made with cast iron components as the water in the loop will either become de-oxygenated or be treated with chemicals to inhibit corrosion. But pumps that have a steady stream of oxygenated, potable water flowing through them must be made of more expensive materials such as bronze.
Circulator Pumps and Domestic Hot Water
Bronze circulator pumps
are often used to circulate domestic hot water so that a faucet will provide hot water instantly upon demand. In regions where water conservation issues are rising in importance with a rapidly expanding population, and a record economic expansion that has consumers looking for comfort, so-called Hot Water Recirculation (HWR) pumps can aid in water conservation at a relatively small expense in added energy use. In typical one-way plumbing without a circulation pump, water is simply piped from the water heater through the pipes to the tap. Once the tap is shut off, the water remaining in the pipes cools producing the familiar wait for hot water the next time the tap is opened.
By adding a circulator pump and constantly circulating a small amount of hot water through the pipes from the heater to the furthest fixture and back to the heater, the water in the pipes is always hot, and no water is wasted during the wait. The tradeoff is the energy wasted in operating the pump and the heat lost from the constantly-hot pipes. A few things can be done to help to diminish waste. Thermal insulation applied to the pipes helps mitigate this second loss and minimize the amount of water that must be pumped to keep hot water constantly available. Also, a timer can be installed to facilitate the needs of the system only when it is likely to be used. Finally, a thermostat, commonly called an aquastat, can be used to help police the water temperature.
The traditional hot water recirculation system uses a dedicated return line from the point of use located farthest from the hot water tank back to the hot water tank. In homes where this return line was not installed the cold water line is used as a return line with a temperature control device closing the connection between the hot and cold lines at a set temperature. Compared to a dedicated return line, using the cold water line as a return has the disadvantage of potentially heating the cold water pipe (and the contained water).
Reading a Pump Curve
A pump curve tells you the flow performance (measured in gallons per minute or liters per minute) of a pump relative to the head pressure. To read a pump curve, you must first examine the units of measurement used along each of the displayed axes. The pictured pump curve displays head pressure in Feet (along the left-side y-axis). Increased head pressure is indicated as you travel up the y-axis. The flow performance is indicated in gallons per minute (along the lower x-axis). This is an indication of the output flow of a pump.
The pump curve is read by first determining the head pressure of the application in which the pump will be used. Once you’ve determined your head pressure, simply follow the head value you have selected from the y-axis horizontally to where it intersects with the pump curve line. From that point, move vertically to the flow measurement on the x-axis.
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