Estimation of the Pump Head

2025-03-14

The head, flow rate, and power are important parameters for evaluating the performance of a water pump:

Flow rate
The flow rate of a water pump is also known as the water delivery volume.
It refers to the quantity of water delivered by the water pump per unit time. It is represented by the symbol Q, and its units are liters per second (L/s), cubic meters per second (m³/s), and cubic meters per hour (m³/h).
Head
The head of a water pump refers to the height to which the water pump can lift water, usually represented by the symbol H, and its unit is meters (m).
For a centrifugal pump, the head is based on the centerline of the impeller and consists of two parts. The vertical height from the centerline of the water pump impeller to the water surface of the water source, that is, the height to which the water pump can suck water up, is called the suction head, abbreviated as the suction lift; the vertical height from the centerline of the water pump impeller to the water surface of the outlet pool, that is, the height to which the water pump can press water up, is called the discharge head, abbreviated as the discharge lift. That is, the head of the water pump = suction head + discharge head.
It should be noted that the head indicated on the nameplate refers to the head that the water pump itself can generate, and it does not include the loss head caused by the frictional resistance of the water flow in the pipeline. When selecting a water pump, this should not be ignored. Otherwise, water will not be pumped up.
Power
The amount of work done by a machine per unit time is called power.
It is usually represented by the symbol N. Commonly used units include: kilogram-meter per second (kg·m/s), kilowatt (kW), and horsepower (hp). Usually, the power unit of an electric motor is expressed in kilowatts; the power unit of a diesel engine or a gasoline engine is expressed in horsepower. The power transmitted from the power machine to the shaft of the water pump is called the shaft power, which can be understood as the input power of the water pump. Generally, the power of the water pump refers to the shaft power.
Due to the frictional resistance of the bearings and packing; the friction between the impeller and water during rotation; the vortices of the water flow in the pump, the backflow in the gaps, the impact at the inlet and outlet, etc. A certain part of the power is inevitably consumed. Therefore, it is impossible for the water pump to completely convert the power input by the power machine into effective power. There must be power loss, that is to say, the sum of the effective power of the water pump and the power loss in the pump is the shaft power of the water pump.

Calculation formulas for the head and flow rate of the pump:
What does it mean when the head of the pump H = 32?
The head H = 32 means that this machine can lift water up to 32 meters at most.
The flow rate = cross-sectional area × flow velocity. The flow velocity needs to be measured by oneself using a stopwatch.

Estimation of the Pump Head1741941208175

Estimation of the head of the pump:
The head of the water pump has nothing to do with the power. It is related to the diameter of the impeller of the water pump and the number of stages of the impeller. A water pump with the same power may have a head of hundreds of meters, but the flow rate may only be a few cubic meters, or the head may only be a few meters, but the flow rate may be hundreds of cubic meters. The general rule is that under the same power, the higher the head, the smaller the flow rate; the lower the head, the larger the flow rate. There is no standard calculation formula to determine the head, which is determined according to your usage conditions and the model of the water pump from the factory.
It can be estimated according to the pressure gauge at the pump outlet. For example, if the pressure at the pump outlet is 1MPa (10kg/cm²), then the head is approximately 100 meters, but the influence of the suction pressure also needs to be considered.
For a centrifugal pump, it has three heads: the actual suction head, the actual discharge head, and the actual head. In the absence of specification, generally, the head is considered to be the height difference between the two water surfaces.

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Here, we are talking about the composition of the resistance of a closed-loop air conditioning chilled water system, because this type of system is the most commonly used example: Estimation of the head of the water pump
According to the above, the pressure loss of the air conditioning water system of a high-rise building about 100m high can be roughly estimated, that is, the head required for the circulating water pump:

Resistance of the chiller: Take 80 kPa (8m water column);
Pipeline resistance: Take the resistance of the strainer, header, manifold, and pipelines in the refrigeration machine room as 50 kPa; take the length of the pipeline on the distribution side as 300m and the specific friction resistance as 200 Pa/m, then the friction resistance is 300 * 200 = 60000 Pa = 60 kPa; if the local resistance on the distribution side is considered to be 50% of the friction resistance, then the local resistance is 60 kPa * 0.5 = 30 kPa; the total resistance of the system pipeline is 50 kPa + 60 kPa + 30 kPa = 140 kPa (14m water column);
Resistance of the air conditioning terminal device: The resistance of a packaged air conditioner is generally greater than that of a fan coil unit, so take the resistance of the former as 45 kPa (4.5m water column);
Resistance of the two-way regulating valve: Take 40 kPa (0.4m water column).
Then, the sum of the resistances of all parts of the water system is: 80 kPa + 140 kPa + 45 kPa + 40 kPa = 305 kPa (30.5m water column)
Head of the water pump: Take a safety factor of 10%, then the head H = 30.5m * 1.1 = 33.55m.
According to the above estimation results, the range of the pressure loss values of the air conditioning water systems of buildings of similar scale can be basically mastered. In particular, it is necessary to prevent the situation where, without calculation, due to excessive conservatism, the system pressure loss is overestimated and the head of the water pump is selected too large, resulting in energy waste.