Static head

[AlfD1965]

In a effort to brush-up on my knowledge of building services, I have
been studying the following publication.

HEATING AND WATER SERVICES DESIGN IN BUILDINGS - SECOND EDITION
BY KEITH MOSS, SPON PRESS.

Are the following statements that extracted from the book correct with
regards to the initiation of flow in a closed system.

For closed systems such as space heating, the net developed pump
pressure must equal the hydraulic resistance in the index run. It dose
not matter if the system serves a multi-storey building, as once water
movement is initiated, circulation will take place regardless of the
static height of the system. However in such systems, the pump must
generate sufficient pressure at no-flow (ie. maximum pressure
development) to overcome the systems static height.

For closed systems the pressure developed at zero flow (ie. maximum pump
pressure) should be greater than the static height of the system to
ensure initiation of flow.

Any clarification in this matter will be much appreciated.

 

[TenPenny]

In a closed loop system, say on a 10 storey building, there will be 10 storeys of head on the suction side of the pump, as well as the 10 storeys that the pump discharge must overcome. Therefore, the only differential that the pump needs to overcome is the friction of the fluid in the system.

On initial startup of the system, however, the pump must fill the system, so there is no suction pressure on the pump, and it still must overcome 10 storeys of head on the discharge.

 

[AlfD1965]

I tend to agree with you, that's why I can't my head around the content of the text that was supposedly written by a well respected author.

 

[itsmoked]

I'm not getting it.. Doesn't it say exactly that?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BigInch]

It says the same, but its not how I would say it. The differentation between the initial fill case and the operating case isn't very clear.

It is also possible that the initial fill and subsequent refills after maintenance could be conducted via main pressure or auxiliary temporary fill pump, in which case the stationary pump would not need the initial fill head capacity, so choose your pump accordingly.

http://virtualpipeline.spaces.msn.com

 

[cbrn]

Hi,
I don't work in that field so my thoughts may be essentially wrong, but if the loop is really closed then why would you bother for startup? OK, refill may be the issue, but only in case that the pressure in the aqueduct is lower, in terms of water column head, than the highest point of your circuit. That's exactly what BigInch says, and that collimates with what I've always seen on buildings: circulators are always dimensioned only in order to match the resistance of the circuit (which is variable, since the heat exchangers may be opened or closed and not all at the same time...). Moreover, afaik the aqueduct pressure is basically high enough to reach the 18th - 20th floor, and is limited by lamination valves in case of smaller buildings or individual houses.

Regards

 

[quark]

I think the author is hinting the chance of unstable curved pumps being in use. The shut off head is lower than the BEP and if the building height is equal to the head developed by the pump at BEP (which is maximum, more or less), there will be trouble to start the flow.

Nevertheless, I agree with above posts that this is not a common phenomenon.