Open Loop system is one in which the water flows into a reservoir open to the atmosphere. Eg: Transfer Pump network, Condenser water pump network. Transfer pump network: In buildings water is transferred from the underground tank to roof tank, where the water is opened to the atmosphere.

 

Closed Loop system is one in which water flow is circulated in a closed loop without exposure to the atmosphere at any point.

Eg: Chilled water pump network. In a chilled water pipe network, the water is re-circulating around the network i.e. from pump to chiller to load (FCU/AHU/FAHU) and then back to pump. Water in this network is never open to atmosphere.

Total Pump Head = Static Head + Friction Head + Pressure head + Velocity Head

Static head is the pressure resulting from the weight of water in pipe acting under gravity. The weight of water in the pipe exerts pressure on the pipe sides and bottom. This is called static head pressure and is caused by Earth's gravitational pull.

Static Head in Closed Loop System is zero, because the fluid on one side of the system pushes the fluid up the other side of the system, so the pump does not need to overcome any elevation.

Friction Head is also called pressure drop, while water flows through chilled water pipe network (pipe, fittings, valves, coils, etc.) there will friction losses, this causes loss in pressure. The pump has to overcome this friction.  Friction head is usually expressed in units called "feet of head." A foot of friction head is equal to lifting the fluid one foot of static height.

          

Velocity Head: Accelerating water from a standstill or low velocity at the starting point to a higher velocity at an ending point requires energy. In closed systems the starting point is the same as the ending point. Therefore the beginning velocity equals the final velocity, so velocity head is not a consideration.

Pressure Head: While liquid is pumped from a vessel at one pressure to a vessel at another pressure, pressure head exists, these applications include condensate pumps and boiler feed pumps. Pressure head is a consideration only in some open systems and not applicable in chilled water network.

Total chilled Water Pump Head is the total resistance that a pump must overcome. It consists of the static head, Friction head, Pressure head & Velocity head. As chilled water is a closed loop network, we need to consider only friction head and all other components will have negligible impact in chilled water network as detailed above.

Points to consider while doing Pump head calculation of Chilled water network.

  1. Head calculation shall flow along the chilled water flow. Starting from the Pump to chiller, to riser, to the unit in the index circuit and return back to pump suction.
  2. Index point may be either the last unit in the network or the unit with maximum coil pressure drop nearby the last unit. We need to consider pipe length plus unit with maximum coil pressure drop at the end of the circuit.

 It is not necessary that the farthest unit in the network will have the maximum pressure drop in all cases.