Chilled water pumps that are commonly used in HVAC industry are the centrifugal pumps. It basically consists of a pump impeller which is connected to the shaft of a motor. The centrifugal force created when the motor turns causes the liquid to be drawn towards it and discharged to the opening of the volute.
The pump create a differential pressure between the water inlet and outlet of the pump. This pressure differential enables the water to flow through the pipes.
The motor used can be single speed, dual speed or variable speed. The more complicated variable speed motor which can vary its speed and connected to the building management system is increasingly being used due to its better efficiency and control. By varying the speed of the pump motor, the impeller speed can be optimized to the load of the system.
The moving parts of the pumps are usually made of bronze or other non-ferrous material to prevent corrosion. The body is made from cast-iron for stability and durability.
These pumps are used in:
In-Line Pumps and the Closed Coupled Pump
There are two main design of the chilled water pumps. They are the In-Line Pumps and the Closed Coupled Pump.
The in-line type is smaller which is used for a lower head applications. It is also known as booster pumps. The suction and discharge are connected in a straight line making it light in weight and hence can be supported directly by the piping. It is also less costly and simpler to install.
The closed coupled pump has its impeller mounted on and supported by the motor shaft. It has mounting flange with base that supports the motor and pump. There is also an end suction connection. It is available from small to medium capacities and is not too costly.
The design of the blade has different effect on the Pump Head vs Discharge. If the design is forward-curved where the outlet blade angle > 90°, the positive slope is obtained. This positive condition is not stable and cause the pump to surge. The pump surge is a condition where it oscillates to find the proper operating point. Due to this, this design is seldom used.
If the design is backward-curved where the outlet blade angle < 90°, the negative slope is obtained. This is the most stable pump characteristics and hence is the most preferred design in centrifugal pumps.
In the design and installation consideration of chilled water pumps, it is always a good practice to provide for venting air from the system to prevent air from blocking the proper operation of the fluid flow. Efficiency of the pump will be reduced with the introduction of air in the piping system.
When selecting pump, choose one with 60-80% of the maximum flow as it is not advisable to operate at its maximum capacity. This is to cater for the sudden increase in flow when extra capacity is needed.
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