The Estimation Method For The Pump's Head Height

One, what is head?
Head, as defined in the textbook, is the work done by the pump on a unit weight of liquid, which is also the increase in energy of a unit weight of liquid after passing through the pump.
However, in practical applications, we do not use energy units to represent the head; instead, we use the liquid column height H to represent it, with the unit being meters (m). Besides meters, other commonly used units for head include kilograms (kg) and megapascals (mpa). The conversion relationship is as follows:
1 MPa = 10 kg = 100 m.
II. The Importance of Head Calculation
The pump's head is an important working parameter of the pump and is a key factor in pump selection. It indicates whether the pump can convey water to the designated location as required.
If the lift height is set too low, water cannot be delivered; if it is set too high, on the one hand, the power consumption will increase and the electricity bill will rise; on the other hand, it may cause the motor to exceed its current limit, and even damage the motor.
For professionals in the field, calculating the pump's head is a necessary skill. Below, we will focus on introducing the parameters of head, flow rate, and power, which are crucial for evaluating the performance of a pump:
The flow rate of the water pump is also known as the discharge volume.
It refers to the amount of water that the pump can convey in a unit of time. It is represented by the symbol Q, and its units are liters/second, cubic meters/second, or cubic meters/hour.
2. The head of a pump refers to the height to which the pump can lift water. It is usually denoted by the symbol H and is measured in meters.
The head of a centrifugal pump is based on the centerline of the impeller and is composed of two parts. The vertical height from the centerline of the pump impeller to the water surface of the water source, that is, the height to which the pump can draw water, is called the suction head, or simply the suction head; the vertical height from the centerline of the pump impeller to the water surface of the discharge tank, that is, the height to which the pump can pressurize water, is called the pressure head, or simply the pressure head. That is, the pump head = suction head + pressure head. It should be noted that the head indicated on the nameplate refers to the head that the pump itself can generate, and it does not include the loss head caused by the friction resistance of the water flow in the pipeline. When selecting a pump, be sure not to ignore this. Otherwise, water will not be able to be pumped.
3. Power refers to the amount of work done by a machine within a unit of time.
It is usually represented by the symbol N. Common units include: kilogram·meter/second, kilowatt, horsepower. The power unit of an electric motor is usually expressed in kilowatts, while the power unit of a diesel engine or gasoline engine is expressed in horsepower. The power transmitted from the power machine to the pump shaft is called shaft power, which can be understood as the input power of the pump. Generally speaking, the pump power refers to the shaft power. Due to the frictional resistance of the bearings and packing; the friction between the impeller and water when it rotates; the vortices in the pump, the backflow in the gaps, the inlet and outlet, and the impact at the inlet, etc. A certain amount of power is inevitably consumed. Therefore, the pump cannot convert all the power input by the power machine into effective power. There must be power loss, that is to say, the effective power of the pump plus the loss power within the pump is the shaft power of the pump.
III. Calculation Method for Pump Head
What does H = 32 for the pump's head mean?
The head H = 32 means that this machine can lift water to a maximum height of 32 meters.
Flow rate = Cross-sectional area * Flow velocity. The flow velocity needs to be measured by yourself: stopwatch.
Estimation of pump head:
The head (lift) of a pump has no relation to its power. It is related to the diameter of the pump's impeller and the number of stages of the impeller. Even pumps with the same power can have head values ranging from hundreds of meters to just a few cubic meters, or from just a few meters to hundreds of cubic meters. The general rule is that under the same power, a higher head results in a lower flow rate, while a lower head leads to a higher flow rate. There is no standard formula to determine the head. It is determined based on your usage conditions and the model of the pump manufactured. You can estimate it using the pressure gauge at the pump outlet. For example, if the pump outlet pressure is 1 MPa (10 kg/cm²), the head is approximately 100 meters. However, the suction pressure should also be taken into account. For centrifugal pumps, there are three heads: the actual suction head, the actual pressure head, and the actual head. Without specific indication, it is generally considered that the head refers to the height difference between the two water surfaces.
What is discussed here is the resistance composition of the closed air conditioning chilled water system, as this type of system is widely used.
Estimate the head of the water pump
Based on the above information, a rough estimate can be made of the pressure loss of the air conditioning water system in a high-rise building approximately 100 meters tall, that is, the head required by the circulating pump:
1. Resistance of the water-cooling unit: 80 kPa (8 meters of water column)
2. Pipeline resistance: The resistance of the purifier, collector, distributor and pipeline in the refrigeration machine room is taken as 50 kPa; taking the length of the pipeline on the distribution side as 300m and the specific frictional resistance as 200 Pa/m, the frictional resistance is 300 * 200 = 60,000 Pa = 60 kPa; if considering that the local resistance on the distribution side is 50% of the frictional 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 (14 meters of water column).
3. Resistance of air conditioning terminal devices: The resistance of a combined air conditioner is generally greater than that of a fan coil unit. Therefore, the resistance of the former is taken as 45 kPa (4.5 water columns).
4. Resistance of the two-way regulating valve: 40 kPa (0.4 water column)
5. Therefore, the total resistance of all parts of the water system is: 80 kPa + 140 kPa + 45 kPa + 40 kPa = 305 kPa (30.5 meters of water column)
6. Pump head: Taking a 10% safety factor, the head H = 30.5m * 1.1 = 33.55m.
Based on the above estimation results, it is possible to roughly determine the pressure loss range of the air conditioning water system for buildings of similar scale. In particular, it is necessary to prevent overestimating the system pressure loss due to insufficient calculation and overly conservative assumptions, which could result in an excessively high selection of pump head and energy waste.