Use and selection of water pumps for HVAC systems

Use and selection of water pumps for HVAC systems

1. Chilled water pump:

　　In the chilled water loop, a device that drives water to circulate. The end of the air-conditioned room (such as fan coil, air handling unit, etc.) needs cold water provided by the chiller, but the chilled water will not flow naturally due to the restriction of resistance, which requires a pump to drive the chilled water to circulate to achieve the purpose of heat exchange .

　　 2. Cooling water pump:

　　A device that drives water to circulate in the cooling water loop. The cooling water takes away part of the heat of the refrigerant after entering the chiller, and then flows to the cooling tower to release this part of the heat. The cooling water pump is responsible for driving the cooling water to circulate in the closed loop of the unit and the cooling tower.

　　 water pump

　　 Three. Water pump:

　　 The device used for air conditioning water replenishment is responsible for pumping treated softened water into the system.

　　 Commonly used water pumps include horizontal centrifugal pumps and vertical centrifugal pumps, both of which can be used in chilled water systems, cooling water systems and water supplement systems. Horizontal centrifugal pumps can be used for places with large machine rooms, and vertical centrifugal pumps can be considered for places with small machine rooms.

　　 Four. Parallel operation of water pumps:

　　 data

　　When the water pumps are operated in parallel, the flow rate is attenuated; when the number of parallel pumps exceeds three, the attenuation is particularly severe. Therefore, it is recommended:

　　 1. When selecting multiple pumps, consider the attenuation of the flow rate, and generally add a margin of 5% to 10%.

　　2. Large and medium-sized projects should be equipped with cold and hot water circulating pumps respectively.

　　 Generally, the number of chilled water pumps and cooling water pumps should correspond to the refrigeration host, and consider one spare. The make-up pump is generally selected according to the principle of one-use and one-preparation to ensure reliable water supply of the system.

　　 5. Calculation of pump flow:

　　 1. The formula for calculating the flow rate of the chilled water cooling water pump:

　　L(m3/h)=Q(kW)×(1.15～1.2)/(5℃×1.163)

　　 where:

　　Q——The cooling capacity of the refrigeration host, kW;

　　L——The flow rate of the freezing and cooling water pump, m3/h.

　　2, the flow rate of the make-up water pump:

The normal make-up water volume is 1% to 2% of the circulating water volume of the system. However, when selecting a make-up water pump, the flow rate of the make-up water pump should not only meet the normal replenishment volume of the above-mentioned water system, but also consider the increase in the amount of make-up water in the event of an accident. Therefore, The flow rate of the make-up water pump is usually not less than four times the normal make-up water volume. The effective volume of the make-up water tank can be considered based on the normal amount of water for 1 to 1.5 hours.

　　 VI. Determination of pump head:

　　 1. The composition of the chilled water pump lift:

　　 (1) Water resistance of the evaporator of the refrigeration unit: generally 5～7mH2O;

　　(2), terminal equipment (air handling unit, fan coil, etc.) surface cooler or evaporator water resistance: generally 5～7mH2O (for specific values, please refer to product samples);

　　(3) The resistance of the return water filter, the two-way regulating valve, etc.: generally 3～5mH2O;

　　 (4). Water resistance of water separator and water collector: generally one is 3mH2O;

　　 (5). Resistance and local resistance loss along the water pipeline of the refrigeration system: generally 7～10mH2O;

　　To sum up, the head of the chilled water pump is 26～35mH2O, generally 32～36mH2O.

　　2. The composition of the head of the cooling water pump:

　　 (1), the water resistance of the condenser of the refrigeration unit: generally 5～7mH2O;

　　 (2) The spray pressure of the cooling tower nozzle: generally 2～3mH2O;

　　(3) The height difference between the cooling tower (open cooling tower) and the nozzle: generally 2～3mH2O;

　　(4) The resistance of the return water filter, the two-way regulating valve, etc.: generally 3～5mH2O;

　　 (5). Resistance and local resistance loss along the water pipeline of the refrigeration system: generally 5～8mH2O;

　　To sum up, the head of the cooling water pump is 17～26mH2O, generally 21～25mH2O.

　　3, the head of the make-up pump:

　　The head is the distance between the fixed pressure point and the z*high point + the resistance of the suction and outlet of the pump + the rich head of 3～5mH2O.

　　 Water pipeline resistance calculation method:

　　 (1). Resistance along the way, the resistance along the way of water in the pipeline:

　　Hf=RL

　　 where:

　　Hf——the resistance along the water pipe, Pa;

　　R——the resistance along the unit length, also known as the specific friction, Pa/m;

　　L——The length of the straight section of the water pipe, m.

　　 When steel pipe or galvanized pipe is used for cold water pipe, the specific friction R is generally 100～400Pa/m, and is usually 250Pa/m. The specific friction is a quantity related to the diameter of the water pipe, the water flow velocity and the flow rate, which can be checked through the specific friction calculation chart.

　　 (2), local resistance:

　　When the water flows into elbows, tees and other accessories, the calculation formula of local resistance due to friction and eddy current energy consumption is:

　　Hd=ζ×(ρ×V2/2)

　　 where:

　　Ζ——local resistance coefficient;

　　V——Water velocity, m/s.

　　 (3). Total resistance of water pipe:

　　 The total resistance of water flow H (Pa) includes the along-path resistance Hf and the local resistance Hd, namely: H=Hf+Hd.