table of contents
- background
- How a pump works
- Formula for calculation
- Sample calculation
- frequently asked Questions
There is sometimes confusion about the concept of the head of a pump. The delivery head has nothing to do with an actual height or a certain distance, but rather describes the pressure.
In a nutshell
- The term “delivery head” applies regardless of the purpose of the pump
- primarily affects so-called gyroscopic or Circulation pumps
- energetic term, no indication of height
- The unit is traditionally meter (m) or Meter water column (mWs)
- Formula: Delivery head (H) = (pressure loss (R) x distance (L) x resistance value (ZF)): 10000
background
The concept of delivery head causes a lot of confusion in connection with pumps. Contrary to popular belief, this is by no means something like that Suction lift meant. Rather, it is about the pressure with which the pump works or can work. Another factor contributing to the confusion is that the height in meters (m) or Meter of water column (mWs) is specified. Of course, this suggests that the value is a length specification.
Note: The use of the length information has to do with the history of mechanical engineering and is therefore historical. It would be more correct to speak of pressure and to use the term Pascal (Pa) for it.
How a pump works
It is well known that the purpose of a pump is to transport liquids from one place to another. For this to happen, kinetic energy is required, which has to be transferred to a conveying medium such as water. The kinetic energy produced by a pump must be so great that the weight of the medium and any flow resistance can be overcome.
- A heating circulation pump is a good example of this principle
- 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 does not 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 isn't overly complicated. However, certain values are required for this, which you often have to calculate yourself. In general, the formula for calculating H is:
Note: The product of these values must then be divided by the factor 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 empirical values from system builders and, in particular, from heating installers. It is basically only an estimated value that applies to a specific type of pipe system. The respective values can also be found, for example, in textbooks on heating installation.
Sample calculation
The delivery pressure or the delivery head of a heating system with the following dimensions:
- Length: 20m
- Width: 15m
- Height: 12m
We assume that the lines in the house are one Pressure loss R of 120 Pa / m cause. Of the Value ZFfor the 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 refers to the length of the house and should not be confused with the L factor for the total distance. The following calculation example then results:
- H = (120 Pa / m x 94 m x 2.2): 10,000 = 2.48 mWs
Note: In the heating example, the water not only has to flow from A to B, it also has to be returned, i.e. cover twice as much. The total distance is therefore doubled, i.e. multiplied by a factor of 2.
frequently asked Questions
As I said, this is due to historical factors. The unit meter of water column has become common and is still widely used. It has not been an official unit for printing in Germany since 1971. Rather, this is now officially Pascal (Pa). Nationwide associations have been trying for many years to enforce the Pascal unit in general use.
The concrete resistance value results from general lists or Tables that assign a certain value to a certain 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, for example in the WILO pump guide, which deals with the fundamentals of pump technology.
The value H is crucial, for example, when calculating the pump for a heating system. It can also play a role if, for example, an automatic irrigation system is installed in the garden. 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.
