Basic Tutorial on the Selection and Design of Water Pumps for Central Air Conditioning Water Systems

2025-04-16

The so-called calculation for the selection of water pumps is actually an estimation (many calculation formulas themselves are estimations). If the estimation is done in more detail and more comprehensive contents are considered, it becomes an accurate calculation.
In particular, it should be added that when the design flow rate is near the rated flow rate of the equipment, it is often much larger than the rated flow rate of the equipment. Similarly, it is recommended to calculate the water flow velocity in the water pipe and then check the table to obtain the resistance value.
Regarding the problem of excessive pump head, if the pump head selected in the design is too large, the excess head will be exchanged for an increase in flow rate. It is the increase in flow rate that makes the noise of the water pump increase. In particular, the increase in flow rate also makes the load of the water pump motor increase, the current increase, and the heat generation increase. "Replacing the bearing countless times" is still a minor issue, and there is a great possibility that the motor will be burned out.
In addition, what does it mean that "the outlet pressure of the water pump is only 0.22MPa"? The pressure difference between the inlet and outlet of the water pump is the key to the problem. For example, if the water pump of an open system is placed on the top 100 meters high and the outlet pressure is 0.22MPa, for this system, if the water pump is placed on the ground and the water is sent to the top 100 meters high, the outlet pressure will be 0.32MPa!
Simple Estimation Method of Water Pump Head:
Selection of HVAC Water Pumps: Centrifugal clean water pumps with a specific speed ns between 130 and 150 are usually selected. The flow rate of the water pump should be 1.1 to 1.2 times the rated flow rate of the chiller (take 1.1 for a single unit and 1.2 for two units in parallel). According to the estimation, the frictional loss per 100 meters of pipe length can be roughly taken as 5mH2O. The head of the water pump (mH2O): Hmax = △P1 + △P2 + 0.05L(1 + K)
△P1 is the water pressure drop of the evaporator of the chiller.
△P2 is the water pressure drop of the air conditioning terminal device with the largest water pressure loss among those connected in parallel in this loop.
L is the pipe length of the most unfavorable loop.
K is the ratio of the total equivalent length of local resistance to the total length of the straight pipe in the most unfavorable loop. When the most unfavorable loop is long, the value of K is taken as 0.2 to 0.3, and when the most unfavorable loop is short, the value of K is taken as 0.4 to 0.6.
Practical Estimation Method of Chilled Water Pump Head:
What is discussed here is the resistance composition of the closed air conditioning chilled water system because this kind of system is the most commonly used system.

Basic Tutorial on the Selection and Design of Water Pumps for Central Air Conditioning Water Systems1744810779052

Resistance of the Chiller: Provided by the chiller manufacturer, generally 60~100kPa.
Pipeline Resistance: Including frictional resistance and local resistance. The frictional resistance per unit length, that is, the specific friction group, depends on the technical and economic comparison.
If the value is large, the pipe diameter is small, the initial investment is saved, but the energy consumption of the water pump during operation is large; if the value is small, the opposite is true. Currently, in the design, the specific friction group of the chilled water pipeline should be controlled within the range of 150~200Pa/m. When the pipe diameter is large, the value can be smaller.
Resistance of the Air Conditioning Terminal Device: The types of terminal devices include fan coil units, packaged air conditioners, etc. Their resistance is provided by the manufacturer after calculating the coil configuration according to the parameters of the air entering and leaving the air conditioning coil, the cooling capacity, the water temperature difference, etc. proposed in the design. Many rated working condition values can be found in the product catalog. This resistance is generally within the range of 20~50kPa.
Resistance of the Control Valve: The air conditioning room always requires the control of the room temperature. Setting an electric two-way control valve on the water circuit of the air conditioning terminal device is a means to achieve room temperature control. The specification of the two-way valve is selected according to the flow capacity when the valve is fully open and the allowable pressure drop. If the allowable pressure drop value is large, the control performance of the valve is good; if the value is small, the control performance is poor. The percentage of the pressure drop when the valve is fully open in the total pressure drop of the branch circuit is called the valve authority. When designing the water system, the valve authority is required to be 0.3. Therefore, the allowable pressure drop of the two-way control valve is generally not less than 40kPa.
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 by the circulating water pump:
Resistance of the Chiller: Take 80kPa (8m water column);
Pipeline Resistance: Take the resistance of the dirt remover, water collector, water divider and pipelines in the refrigeration machine room as 50kPa; take the length of the pipeline on the distribution side as 300m and the specific friction resistance as 200Pa/m, then the frictional resistance is 300×200 = 60000Pa = 60kPa; if the local resistance on the distribution side is considered to be 50% of the frictional resistance, then the local resistance is 60kPa×0.5 = 30kPa; the total resistance of the system pipeline is 50kPa + 60kPa + 30kPa = 140kPa (14m water column);
Resistance of the Air Conditioning Terminal Device: The resistance of the packaged air conditioner is generally greater than that of the fan coil unit, so take the resistance of the former as 45kPa (4.5m water column);
Resistance of the Two-way Control Valve: Take 40kPa (0.4m water column).
Therefore, the sum of the resistances of all parts of the water system is: 80kPa + 140kPa + 45kPa + 40kPa = 305kPa (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 system of buildings of similar scale can be basically mastered. In particular, it is necessary to prevent the excessive estimation of the system pressure loss and the selection of an excessively large pump head due to being too conservative without calculation, resulting in energy waste.
Design of Water Pump Head:
(1) For the cold and hot water pipeline system
Open water system: Hp = hf + hd + hm + hs
Closed water system: Hp = hf + hd + hm
In the formula, hf and hd are the total frictional resistance and local resistance loss of the water system, Pa;
hm is the resistance loss of the equipment, Pa;
hs is the hydrostatic pressure of the open water system, Pa. The value of hd/hf is between 1 and 1.5 for small residential buildings; between 0.5 and 1 for large high-rise buildings; and between 0.2 and 0.6 for long-distance transmission pipelines (centralized cooling supply). The resistance loss of the equipment is as follows:
(2) Cooling Water Pipeline System

Cooling Water Volume of the Cooling Tower
The cooling water volume of the cooling tower can be calculated according to the following formula: W = Q/c(tw1 - tw2) kg/s
In the formula, Q is the heat removed by the cooling tower, kW; for a compression refrigeration machine, take about 1.3 times the load of the refrigeration machine; for an absorption refrigeration machine, take about 2.5 times the load of the refrigeration machine;
c is the specific heat of water, kJ/(kg·℃), when at normal temperature, c = 4.1868kJ/(kg·℃); tw1 - tw2 is the temperature difference between the inlet and outlet water of the cooling tower, ℃; for a compression refrigeration machine, take 4~5℃; for an absorption refrigeration machine, take 6~9℃.
Head of the Water Pump
The required head of the cooling water pump Hp = hf + hd + hm + hs + ho
In the formula, hf and hd are the total frictional resistance and local resistance of the cooling water pipeline system, mH2O; hm is the resistance of the condenser, mH2O; hs is the lifting height of the water in the cooling tower (the height difference from the cooling water pool to the nozzle), mH2O; ho is the spray pressure of the nozzle of the cooling tower, mH2O, approximately equal to 5mH2O.
Resistance of the Chiller: Take 80kPa (8m water column);

Pipeline Resistance: Take the resistance of the dirt remover, water collector, water divider and pipelines in the refrigeration machine room as 50kPa; take the length of the pipeline on the distribution side as 300m and the specific friction resistance as 200Pa/m, then the frictional resistance is 300×200 = 60000Pa = 60kPa; if the local resistance on the distribution side is considered to be 50% of the frictional resistance, then the local resistance is 60kPa×0.5 = 30kPa; the total resistance of the system pipeline is 50kPa + 60kPa + 30kPa = 140kPa (14m water column);
Resistance of the Air Conditioning Terminal Device: The resistance of the packaged air conditioner is generally greater than that of the fan coil unit, so take the resistance of the former as 45kPa (4.5m water column);
Resistance of the Two-way Control Valve: Take 40kPa (0.4m water column).
The sum of the resistances of all parts of the water system is: 80kPa + 140kPa + 45kPa + 40kPa = 305kPa (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.