HW Pumping 1

[ChasBean1]

Greetings All, I'm working on a project where a heating water (HW) pump is used at the basement level of a 12-story building. The HW pump draws suction from a steam-to-HW heat exchanger. This point (the pump suction) is the maximum loop temperature point, as it is right at the outlet to the heat exchanger.

This is a closed loop. I already know that these HW pumps can't be used for filling the loop as their total head is 90 feet and the building height is over 200 feet.

My question: Should the HW pumps be placed on the cold side of the heat exchanger? Would the added net positive suction head (NPSH) increase efficiency? Would the pump and heat exchanger be better in the penthouse - it's easier to move the LP steam heating medium up 12 floors than the HW! But then again, this is a closed loop... thoughts are appreciated. -Chas

 

[TBP]

The circ pump should be near the outlet from the heat exchanger. The air separator should be located between the outlet of the HX, and the pump suction. This will give it the best opportunity to pull dissolved air out of the system. The closer water is to it's boiling point, the less dissolved air it will hold. The highest temp (HX outlet) combined with the lowest pressure (pump suction - if the expansion tank is located just ahead of the pump) is this point. If you could locate the whole assembly in the penthouse, it would be the best location for the air separator. However, the steam must be run up there, and the condensate returned. It's quite possible that this will be more trouble than it's worth. You'll need to allow for drip traps and expansion.

Hot water boilers in newer buildings are located in penthouses for a couple of reasons. One is the boiler doesn't have to withstand the pressure that it would if the whole X-number of stories was over it, and for dissolved air removal.

It's also important that the expansion tank for the sytem be tied into the system between the HX and the pump, and that the make-up water be connected to the short section of pipe between the loop and the expansion tank. This is the only place the circ pump can't fool the make-up water feeder from adding water when it's not needed.

An alternative in this case would be to leave the HX in the basement, and locate the circ pump, air separator, expansion tank and make-up in the penthouse.

An important thing to remember is that it's not just heat transfer and hydraulics, it's also effective removal of dissolved air from the loop water.

 

[ChasBean1]

TBP - excellent points and thank you for your thoughts. -CB

 

[PacificSteve]

By the way, who keeps putting that dissolved air into the water, and why is that a problem?

Steve

 

[TBP]

At the initial filling of the sytem, atmospheric air in the system is vented manually. When the system is fired for the first time, dissolved air is released from the water as it heats up, and will collect in annoying places. If the air separation equipment isn't arranged correctly, you keep getting air bound rads and coils. The system also sounds like it's full of B-Bs. People working in offices, or trying to sleep in apartments or houses with hot water heat don't like this noise.

Also, if water is added to the system, like after it's been drained for a repair, or addition, more dissolved air arrives with the incoming water.

 

[pardal]

You wrote

"This is a closed loop. I already know that these HW pumps can't be used for filling the loop as their total head is 90 feet and the building height is over 200 feet."
I tell you
The head pressure in a closed loop have to do with the head resistance by piping and fittins.
As it is closed the water is at the same level both sides of the pump, so the power use to up the water , the water return it as hidraulic power at the intake.

One thign to corcenr is about case pressure allowed by the pump.

Just as a example a normal hot water circulating pump
has a head presurre of 63 Ft water column [wc] (19 meters) and a Max service pressure of : 330 Ft wc or about 100 M wc.




You can reach me at k281969@hotmail.com
but keep your post here for mutual benefits.
Pardal