Pump Head in a Closed Loop 2

[moeg]

Hi,

In a Chilled water system, how do you calculate your head? ( what are the different types of head)
- The building is 150 meters high. Does that go in for the head caluclation?

Another question, if the pump is switched off, system full of water, what would be the reading on the gauge before and after the pump.


Regards,

Mo

 

[bimr]

If the pump was switched off, the pressure readings on either side of the pump would be the static head of 150 meters.

Refer to the discussion on water systems in the link. High rise buildings are generally zoned off to minimize large pressure differentials.

http://www.ctaspe.com/uploads/TS_400-1-1.pdf

 

[LittleInch]

In a closed loop system the static pressure at the lowest point in the system must be at least equal to the static head of the fluid (water) at the highest point, in your case 150m. However to avoid cavitation and air bubbles forming, the system is usually pressured to a bit more so that the highest point the pressure is always > 0 barg, often 1-2 barg. This increase the static head at the lowest point.

The differential head of the pump though, regardless of where it is located, is the head required to pump the water around the system at the flow rate required through the elements required (AHU, chiller etc). Often the pump on large building is located at the top to reduce the pressure rating of the pump and its seals. The height of building is irrelevant to this calcualtion as the height issue cancels each other out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Correction to my post. If the pump was switched off, the pressure reading on the discharge side of the pump would be the static pressure head created by the difference between the elevations of the uppermost pipe (assume that it is on the roof) and the pump plus any residual pressure that was put into the closed loop by the pump. The pressure on the inlet side would be more or less the same since it is a closed loop.

 

[moeg]

While calculating the pump head in a closed loop chilled water system, you consider
1) the height of the building, in our case the 150 m, as static pressure.
2) dynamic head, the system losses (chillers,thus...etc)

Assume the longest route with all its fitting losses is 10 bar, and building height is 150 m, so 15 bar head. Total pump head is 25 Bar?

Please correct this information for me if it is wrong.

 

[bimr]

It is 10 Bar.

The term pump head however, has nothing to do with height. It is a term that is used that is the pumps ability to overcome the friction that is created when fluid flows through a pipe. Once a system is filled with fluid – the pump merely has to overcome this friction. An easy way to think about it is that as water goes up one side of the loop, it also comes down the other side of the loop. The two cancel each other out.

A good illustration is like a Ferris wheel. One car going up balances the other car coming down. The cars are in balance and the motor only has to overcome the friction in the bearings and we are off and spinning. The pump in a closed loop system in simple terms is like the motor on a Ferris wheel. All the pump has to do is overcome the friction that is created when the water rubs
against the pipes – and that friction has nothing to do with height. In fact we could take a ten story building, size a pump for it, knock the building over on its side and we would still need the same exact pump. Pump head has to do with the friction caused by flow, that is all. We have to have a pump with enough head to move the fluid through the longest loop.

 

[nabeel3]

In a closed loop system,like in a chilled water circuit the height of the building is irrelevant because the work done(Pressure) by the pump in lifting the water column is gained when the water column falls by the same height. So the +ve and - ve pressure cancels each other. Hence consider only the dynamic pressure head only. i.e The pressure that the pump has to develop to circulate the fluid overcoming the friction loss and fitting pressure loss. Also consider the pressure loss in the most remote AHU or FCU. That will give the Head required by the Pump.