How To Calculate The Flow Rate And Head Of A Water Pump, And How To Select A Water Pump?

What is the appropriate size for selecting a water pump as an important component of heating, hot water, and central air conditioning water systems?
If the selected pump head in the design is too large, the excess head will lead to an increase in flow rate, which in turn will increase the noise of the pump. In addition, it is easy to cause motor load and current to increase, leading to motor burnout and other problems. If the selection is too small, it will lead to insufficient condensation/hot water at the end, affecting the effect. Today, the editor will conduct a detailed analysis of the pump selection calculation, allowing you to fully learn the pump selection in ten minutes.
The selection and calculation of water pumps can be directly estimated using calculation formulas. A more detailed and comprehensive estimation is an accurate calculation. Selection calculation generally includes pump flow calculation, simple estimation method for pump head, practical estimation method for air conditioning pump head, etc. The specific selection algorithm is as follows:
1. Determination of pump flow rate
Flow rate is one of the important performance data for selecting water pumps, which directly affects the production and transportation capacity of the entire device. In the process design of design institutes, the normal, minimum, and maximum flow rates of pumps are usually calculated. When selecting a pump, the maximum flow rate should be taken as the basis, taking into account the normal flow rate. When there is no maximum flow rate, 1.1 times the normal flow rate is usually taken as the maximum flow rate.
Pump water flow rate: Generally, the flow rate of the pump is selected based on the numerical values provided in the product sample (for example, the flow rate can be 1.1-1.2 times the rated flow rate of the air source unit (1.1 for a single unit and 1.2 for two parallel units), or calculated according to the following formula:
Q=1.163L*ΔT
----Q is the cooling capacity of the air source host
----L is the system cooling water flow rate
----Δ T is the system temperature difference
L(m3/h)= Q(kW)/(4.5-5)℃x1.163X(1.1-1.2)
2. Calculation of Water System Pipe Diameter
In the air conditioning system, the diameter of all water pipes is generally calculated according to the following formula:
D(m)=√L(m/h) /0.785x3600xV(m/s)
L - Water flow rate of the requested pipe section (calculated in the first step)
V - Allowable water flow rate for the requested pipe section
Determination of flow velocity: In general, when the pipe diameter is between DN40 and DN50, the recommended flow velocity is around 1.0m/s; When the pipe diameter is less than DN40, the recommended flow velocity should be less than 0.8m/s; When the pipe diameter is greater than DN50, the flow velocity can be further increased. When calculating, attention should be paid to the correspondence between the pipe diameter and the recommended flow rate.
The current size specifications for pipe diameters are: DN15 DN20,DN25,DN32,DN40,DN50,DN65,DN80.......
Attention: When selecting a water pump, the inlet and outlet pipe diameters of the water pump should be one model smaller than the pipe diameter of the section where the water pump is located. For example, if the diameter of the pipe section where the water pump is located is DN25, then the inlet and outlet diameters of the selected water pump should be DN20.
3. Determination of pump head
Composition of pump head:
Hmax=△P1+△P2
----△ P1 is the resistance along the pipeline.
----△ P2 is the local resistance of the pipeline.
(1) System pressure loss along the way
The pressure loss along the pipeline refers to the continuous and consistent pressure loss in the pipeline. Generally, the most unfavorable loop in the pipeline should be identified. The farthest and most resistant branch loop in the pipeline system is the most unfavorable loop in the pipeline system.
△ P1=Least favorable pipeline length * Specific friction
Generally, the estimated loss along the length of a 100 meter pipe can be roughly taken as 5mH2O
(2) Local resistance of the system
Local pressure loss refers to the discontinuous pressure loss caused by special components in a pipeline system that change the direction of water flow or narrow the local water flow channel (such as reducing diameter, tees, valves, joints, filters, etc.). Local resistance is mainly composed of the following components
1. The water resistance of the air source unit is generally 6-8mH2O; (Specific values can be found in the product sample)
2. The water resistance of the surface cooler or evaporator of the end equipment (air handling unit, fan coil unit, etc.) is generally 5-7mH2O; (Specific values can be found in the product sample)
3. The resistance of the return water filter is generally 3-5mH2O;
4. Valve (check valve) water resistance: generally one is 3mH2O;
5. Local resistance loss of three-way connections and joints in the refrigeration system water pipeline: generally 3-4mH2O;
In summary, taking a safety factor of 10% based on the calculated total resistance, the head of the water pump can be determined.
Attention: The calculation of head should be based on the specific situation of the relevant system,