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HVAC pump selection

2018-06-04

  1. Chilled water pump: A device that drives water to circulate in a chilled water circuit. We know that the end of the air conditioning room (such as fan coils, air handling units, etc.) needs the cold water provided by the chiller, but the chilled water will not naturally flow due to the restriction of the resistance, which requires the pump to drive the chilled water to circulate in order to achieve the exchange. The purpose of heat.

  2. Cooling water pump: A device that drives water in a cooling water loop to make a circular flow. We know that the cooling water takes a part of the refrigerant after it enters the chiller, and then it releases the heat to the cooling tower. The cooling water pump is responsible for driving the cooling water to circulate in the closed loop of the unit and the cooling tower. Shape the same with the freezing water pump.

  3. Make-up water pump: air-conditioning replenishment device, responsible for the treatment of softened water into the system. The same shape as the water pump.

  The commonly used pumps are horizontal centrifugal pumps and vertical centrifugal pumps. They can be used in chilled water systems, cooling water systems and replenishing water systems. Horizontal centrifugal pumps can be used for large areas of the machine room, and vertical centrifugal pumps can be considered for small areas of the machine room.

  When the pumps are operated in parallel, the flow rate is attenuated; when the number of parallel units exceeds 3, the attenuation is particularly severe. Therefore, it is recommended that:

  1. When multiple pumps are used, the flow attenuation must be taken into account. Generally, a margin of 5% to 10% is added.

  2. The parallel connection of pumps should not exceed 3 sets, that is, it should not exceed 3 sets when selecting the cooling host.

  3. Large and medium-sized projects should be equipped with cold and hot water circulation pumps.

  In general, the number of chilled water pumps and cooling water pumps should be in one-to-one correspondence with the refrigeration host, and one backup should be considered. The supplemental water pump is generally selected according to the principle of one-use and one-off preparation, so as to ensure the reliable water supply of the system.

  Calculation of pump flow

  (1) Cooling water pump, cooling water pump flow calculation formula:

  L(m3/h)=Q(Kw)×(1.15~1.2)/(5°C×1.163)

  In the formula: Q - refrigeration unit cooling capacity, Kw.

  L--Flow of the refrigerated cooling water pump, m3/h.

  (2) Recharge water pump flow:

  The amount of normal recharge water is 1% to 2% of the circulating water volume of the system, but when the recharging pump is selected, the flow rate of the recharging water pump shall be in addition to the normal recharge of the above water system, and the amount of recharge water added during the accident shall also be considered. The flow rate of the recharging pump is usually not less than 4 times of the normal recharging amount. The effective volume of the supply tank can be considered as the normal amount of water for 1 to 1.5 hours.

  Determination of pump head

  (1) The composition of the lift pump lift:

  Cooling unit evaporator water resistance: generally 5 ~ 7mH2O; (specific products can refer to the sample)

  End device (air handling unit, fan coil, etc.) surface cooler or evaporator water resistance: generally 5 ~ 7mH2O; (specific values can refer to the product sample)

  Backwater filter, two-way control valve and other resistance, generally 3 ~ 5mH2O;

  Water separator, water collector water resistance: generally one is 3mH2O;

  Cooling system water pipeline resistance and local resistance loss: generally 7 ~ 10mH2O;

  In summary, the lift of the freezing water pump is 26~35mH2O, which is generally 32~36mH2O. Note: The calculation of lift must be based on the specific conditions of the refrigeration system. It is not possible to copy the experience!

  (2) The composition of cooling pump lift:

  Cooling unit condenser water resistance: generally 5 ~ 7mH2O; (specific values can refer to the product sample)

  Cooling tower nozzle spray pressure: generally 2 ~ 3mH2O;

  Cooling tower (open cooling tower) Water tray to nozzle height difference: generally 2 ~ 3mH2O;

  Backwater filter, two-way control valve and other resistance, generally 3 ~ 5mH2O;

  Cooling system water pipeline resistance and local resistance loss: generally 5~8mH2O;

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

  (3) Filling pump head: The head is the constant pressure point and the highest point distance + the suction head of the water pump and the water outlet end resistance +3 ~ 5mH2O rich head.

  Water pipeline resistance calculation method:

  Resistance along the way, resistance of water in the pipeline:

  Hf=Rl

  Where: Hf - pipe resistance along the way, Pa;

  R - resistance along the length of the unit, also known as friction resistance, Pa / m;

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

  When the cold water pipe is made of steel pipe or galvanized pipe, the specific friction resistance R is generally 100 to 400 Pa/m, and the most commonly used is 250 Pa/m. The specific friction resistance is the quantity related to the diameter of the water pipe, the flow rate of the water flow, and the flow rate, and can be found by the following calculation chart of the specific friction coefficient.

  Partial resistance: When encountering elbows, tees and other fittings when water flows, the formula for calculating the local resistance due to friction and eddy current energy consumption is: Hd=ζ×(ρ×V2/2)

  In the formula, the local resistance coefficient, V - water flow rate, m/s.

  Total Water Pipe Resistance: The total resistance H(Pa) includes the resistance along the way Hf and the partial resistance Hd, ie: H=Hf+Hd

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