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There is sometimes confusion about the concept of the head of a pump. The head has nothing to do with an actual height or a specific distance, but describes the pressure.

In a nutshell

  • The term 'head' applies regardless of the intended use of the pump
  • relates primarily to so-called centrifugal or circulating pumps
  • energetic term, no elevation
  • The unit is still traditional meter (m) or meter water column (mWs)
  • Formula: head (H) = (pressure loss (R) x distance (L) x resistance value (ZF)) : 10000

background

When it comes to pumps, the concept of delivery height causes a great deal of confusion time and time again. Contrary to popular belief, this does not mean anything like the intake height. Rather, it is about the pressure with which the pump works or can work. The fact that the height is given in meters (m) or meters of water column (mWs) also contributes to the confusion. This of course suggests that the value is a length specification.

Notice: The use of length information has to do with the history of mechanical engineering and is therefore historically conditioned. It would be more correct to speak of pressure and to use the term pascal (Pa) for it.

functioning of a pump

As is well known, the purpose of a pump is to transport liquids from one place to another. In order for this to succeed, kinetic energy is required, which must be transferred to a pumping medium such as water. The kinetic energy produced by a pump must be large enough to overcome the weight of the pumped medium and any flow resistance.

heating circulating pump
  • circulating pump of a heating system is a good example of this principle
  • in order to be able to overcome resistance in the pipe system, pressure is always necessary
  • Pressure then ensures that warm water is pumped into individual radiators
  • It doesn't matter whether the radiators are on the ground floor or on the tenth floor

Formula for calculation

Calculating the value of H is not easy. In itself, the formula for this is not too complicated. However, certain values are required for this, which you often have to calculate yourself first. In general, the formula for calculating H is:

Notice: The product of these values must then be divided by a factor of 10,000 in order to convert the unit Pascal (Pa) into meters of water column (mWs). The following applies exactly: 1 mWs = 9,806.65 Pa.

The pressure loss R in the system is based on the empirical values of system manufacturers and in particular heating installers. It is basically only an estimated value that applies to a specific type of piping system. The respective values can also be found, for example, in textbooks on heating installation.

sample calculation

The delivery pressure or the delivery height of a heating system is to be calculated with the following dimensions:

  • Length: 20m
  • Width: 15m
  • Height: 12m

We assume that the wiring in the house is a Pressure drop R of 120 Pa/m cause. the Value ZF for resistance through fittings and valves should be 2.2 be. For the sake of simplicity, the total distance that has to be covered is calculated from the dimensions of the house, i.e. L+W+H. The L here refers to the length of the house and must not be confused later with the factor L for the total distance. Then the following calculation example results:

  • H = (120 Pa/m x 94 m x 2.2): 10,000 = 2.48 mwc

Notice: In the heating example, the water not only has to flow from A to B, it also has to be returned, i.e. it has to travel twice as much. The accumulated distance is therefore doubled, i.e. multiplied by a factor of 2.

frequently asked Questions

Why are old units still used?

As I said, this is historical. The meter water column unit has become established and is still commonly used. Since 1971 it has not been an official unit for printing in Germany. Rather, this is now officially Pascal (Pa). Nationwide, associations have been trying to enforce the Pascal unit in general use for many years.

How do you get the value for the factor ZF?

The concrete resistance value results from general lists or tables, which also assign a specific value to a specific equipment. If a system has fittings, fittings and thermostatic valves, the value is 2.2. If there is only one thermostatic valve, the value is 1.7, for example. These and many other values can be found in many reference works, such as the WILO Pump Guide, which deals with the basics of pump technology.

What is the specific purpose of the funding?

The value H is crucial, for example, if the pump for a heating system is to be calculated. In addition, it can also play a role if an automatic irrigation system is installed in the garden. Incidentally, for an irrigation system in the garden, the distance in the formula does not have to be doubled, since the water does not flow back.

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Category:

Garden design