![]() radial impellers have low specific speeds, mixed flow ("diagonal") impellers have a higher specific speed range and axial impellers have the highest specific speeds. The fluid flow through the impeller changes with increasing specific speed, i.e. The values to be inserted in the above equation are the optimum head Hopt for one stage in the case of multistage pumps, and the optimum flow rate Qopt for one impeller half in the case of double-entry impellers. The following relationship exists between the numerical values of the dimensional and dimensionless coefficients: N in s rpm -1 Rotational speed Q opt in m 3/s Flow rate at η max H opt in m Head at η max g = 9.81 m/s 2 Acceleration due to gravity Q opt in m 3/s Flow rate at ηmax H opt in m Head at ηmax n in rpm Pump speed n s in rpm Specific speedĪ dimensionless characteristic coefficient in accordance with DIN 24260 can be established using the following equation: The specific speed can be used to classify the optimum impeller design and the corresponding pump characteristic curves.ĭefined as the theoretical rotational speed at which a geometrically similar impeller would run if it were of such a size as to produce 1 m of head (H opt) at a flow rate (Q opt m 3/s at the best efficiency point, the specific speed is expressed in the same units as the rotational speed: I use what is called the “modeling law” or “model law” or “factoring law.” This “law” is used when a designer wishes to “model” a pump to one of a different size.The specific speed (ns) is a characteristic coefficient derived from the similarity conditions which allows a comparison of impellers of various pump sizes even when their operating data differ ( flow rate and head at best efficiency point, rotational speed). (Don't start one of these guys with the discharge valve closed.) Derivationįor those who wish to see a derivation of NS, the following is offered. When NS exceeds about 5,000, the slope of the power curve reverses, with the maximum value being at shut-off, and the head curve is very steep, with the shut-off value being as much as three times the BEP value. In the midrange of NS values, the head curve continually rises to shut-off, and the power curve changes little from shut-off to BEP. It increases significantly from shut-off to best efficiency point (BEP). Such a droop does not make the pump unstable. Note that pumps with a low specific speed have a flat head curve, sometimes with a slight droop at shut-off (zero capacity). The small performance curves across the top of the graph illustrate the typical shapes of the performance curves correspondent to the values of specific speed. The small drawings below the graph show the profiles of the impellers that correspond to the specific speed numbers. units, are tabulated along the bottom of the graph. Note that the values of specific speed, in U.S. The chart was developed years ago by the Worthington Group and is used extensively by the industry. An impeller with a high specific speed has a fat profile (the shrouds are far apart) and has an eye diameter that is closer in size to the impeller OD.įigure 1 helps illustrate the concept. It indicates the shapes of the performance curves and also determines, to a large degree, the profile shape of the impeller.Īn impeller with a low specific speed has a thin profile (the shrouds are close together) and a large outside diameter (OD) relative to the eye diameter. In Europe, specific speed is sometimes called the “shape number,” and I prefer that name. I prefer to think of it as an index number. ![]() One of the definitions of specific speed is that it is the speed that a modeled pump would run to produce a one foot head when pumping one gallon per minute, but I find that definition awkward. I later learned that the concept is essential to the designer, and-because it indicates the shape of the head, power and efficiency curves and the maximum achievable efficiency-it is also of value to end users. It appeared to me that the concept might be useful to pump designers, but I could see no value to the end user. When I was first introduced to specific speed, I must admit to being unimpressed.
0 Comments
Leave a Reply. |