Pump Curve Analysis
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CIRCULATOR CHARACTERISTICS
All circulators used in hydronic heating systems have an operating characteristic in which the flow they produce depends on the resistance they are working against. The resistance is generated by the friction of the fluid moving through the pip- ing systems. The greater this resistance is, the slower a given circulator can move fluid through the piping system.The term that in part describes the characteristic of a particular circulator is called “head”. The most accurate way to describe head is mechanical energy. All operating circulators add head (or mechanical energy) to the fluid passing through them. This added mechanical energy does not increase the speed of the fluid (other than for a few seconds when the circulator first starts). It also does not increase the temperature of the fluid (as does thermal energy added to the fluid). What the added head energy does do is increase the pressure of the fluid. The fact that the pressure on the outlet side of an operating circulator is higher than the pressure on the inlet side (differential pressure) is the “evidence” that head energy has been added to the fluid.
INTERPRETING PUMP CURVES
Reading and interpreting a pump curve is essential to any HVAC professional, whether you're a residential installer working on a hydronic home heating system. If you were an HVAC engineer designing a massive complex commercial operation, you need to understand pump curves to select the right size pump to meet the requirements of your order. This article covers the necessary information on how to read a pump curve based on the pump head and flow a pump performance curve. A pump curve represents the performance of a pump graphically. Every pump has its unique curve, and those curves vary widely for different pumps. The variations depend on many factors such as the horsepower of the pump and the shape, and the size of the impeller pump curves can be used to provide information on many other elements of pump performance. Some of these are of interest only for designing commercial systems. In the most widely used form, a pump curve shows two vital pump performance factors flow and head. Flow measured in gallons per minute is the rate at which water must be moved to carry heat throughout a hydraulic system. Too much flow results in a system noise too little flow mean insufficient. The temperature will reach at least some areas of the building. The pressure is usually measured in psi (pounds per square inch), but in hydraulics, the pressure is calculated in terms of head pressure or head loss as measured in head feet. Technically, the head is the total mechanical energy content of a fluid at a given point. In a piping system, a circulator has to produce a sufficient pressure differential to overcome the pressure or head loss created in the piping by friction and by the various valves and fittings in the system. Converting psi to head loss is very simple. A column of water 2.31 feet or 28 inches high will have a gauge pressure at the bottom of one psi. Therefore one psi of pressure drop in a system equals 2.31 feet of head.
The above figure enables us to see how the pump will perform at any given point within its performance range. For a flow rate of 10 gallons per minute, the pump-007 will produce between 7 and 8 feet of the head, whereas, at 4 feet of the head, this pump-007 will generate a flow rate of just about 17 gallons per minute. Reading the pump curve makes it easy to choose a pump for a particular hydraulic system. Technically, if the head and flow operating point of the system is located on or below a given pump performance curve, then that pump will be sufficient to do that job. So if the system requires a flow rate of 12 gallons per minute and ahead of 4 feet, the pump-007 will quickly meet those requirements. It will also meet the system requirements if the system needs a flow rate of 6 GPM and a head of 2 feet or 5 feet or any other intersection of flow rate and head in the area below the curve. For a system that requires 12 gallons per minute flow at 4 feet of the head, several models of circulators will meet those requirements like the 0010 or the 0011 or the 0012 and the 0013. The curve for 009 is a steep curve, and 0012 has a very different shaped curve. 0012 can produce a lot of flow up to 52 GPM according to its curve but can produce a maximum of only 14 feet of head pressure. Such a curve is called a flat pump curve. It can be seen by the curves that they can produce high flow rates but relatively low pressure. A smaller flat curve pump such as the 005 or 007 is ideal as a system circulator for zone valves in standard residential jobs. The larger flat curve pumps like the 0010 and 0012 are designed for larger residential jobs or light commercial applications. On the other hand, pumps 008, 0011, 0013 and 0014 are all in the steep curve family. Compared to pumps with a flat curve, these circulators can generate a lot of head pressure but comparatively low flow. These circulators are best for higher head applications such as high head loss fan coils air handlers and heat exchangers. The third family includes 003 and 006. Both these circulators are designed for domestic hot water applications.
References[edit]
noraweb.org
tacocomfort.com
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