pump head...help

Brad White_9

So what am I, chopped liver? :)

The reason height does not count is akin to a ferris wheel. What water you are pushing up is displaced by water coming down elsewhere in a closed system. Think of it as a self-siphon. The pump is just a circulator to that you get her moving at a rate against the pressure drop.

In an open system such as with a cooling tower, you have to add in the height from the static water line of the system up to and over the top of the tower spray nozzles (plus the nozzles themselves).

By the way, I refigured the system as I noted above but at 5" pipe size. The pipe and fittings come to about 17.5 feet and the total head FWIW comes to 54 feet.

With a real diagram of the worst circuit plus the actual pressure drops of your worst cooling coil and the chiller evaporator, I can better help you.

t.wheat

pump head.

help !
i have a contractor with a system that has a pump on the first floor. He is pumping up 64 feet to a chller on the roof, the back down to the first floor, then through two air handlers , and back to the pump. he needs 400 gpm, and the pump he has is sized for 70'hd. he isnt getting enough flow across the chiller up top. ( getting 1/2psi instead of the 6 he needs.) the system has an air seperator, and air vents that have been double and triple checked to make sure that he doesnt have an air problem. my question is i thought that he wuould have to take the 64' up, plus the loss in the piping , plus the loss thru the chiller, plus the air handlers in the basement to get his head calculation, seems like his 70 foot of head is used up almost before he gets thru the chiller....how do you figure the head in this system?

Brad White_9

Forget height in a closed system.

Firstly, make sure that the system is filled! It may be just air at the top. For the 64 feet of height plus to have 4 psig at the top you would need about 31-32 psig at the bottom. What is the lowest-elevation pressure that you have?

Rough numbers from what you posted:

You need to know the flow and pipe size of the longest circuit plus the pressure drops of any chiller and the highest pressure drop of any air handler. How many feet of pipe and fittings at size and flow rate is critical.

I assume for 400 GPM that you are running 6" pipe, is that correct?

Your chiller you say is 6 psig or about 14 feet of head which seems reasonable.

If I use a total of 200 LF of piping, 18 elbows, a handful of tees and valves, I come up with about 10 feet of head just for the pipe and fittings. Add in 14 feet of head for the chiller plus another 5 feet for the hook-up of that, 15 feet for the cooling coil, control valve and hook-up (5 feet of head each, FWIW), then add 5% for unknowns, I get about 46 feet of head.

Your 70 given the things I do not know about may be perfectly reasonable.

I would check the static pressure first.

t.wheat

im pretty sure its not air, because thats what i told him at first, and he has checked that. (water out of the new vents) also i forgot to mention that this is a 30% glycol mix, (ethel. i think) they are using 5" pipe in the main runs up and down, and the piping is the existing pipe, so im sure we have to add for that. he also has some 4" pipe and 2.5" pipe i suppose between the air handlers, the water pressure on the pump is 30psi when its just sitting there. i guess i understand that in a closed system there is no up..you just over come the loss, but are you supposed to take in consideration height ( or static pressure) at all? i cant get this through my head..

ALH_4

head

It sounds like you could use an engineer to come look at the system. It will improve the performance of the system and save money in the long run.

t.wheat

i actually have one on order...ha, i have an engineer coming out tomorrow to look. but no matter what he finds, i wont understand from him. im just trying to understand head/closed/opem systems, i used to work for the B&G rep when i was younger, but just did parts, im in a similar field now, but in electronics. all the lit i can find seems to be in relation to hot water systems or primary secondary systems,im just trying to learn about the head loss in a closed chill water system. does the rise up count??? or down for that matter.

ALH_4

open vs closed

The rise makes no difference with a closed system. The head you lose with the rise is gained on the fall.

Say you had a reservoir in the basement and you pumped water to the roof when you needed it and the pipes drained when the pump was off, you would have to overcome the elevation head initially, but once the loop was full, it becomes a closed loop because the water falling in the return pipe weighs as much as the water rising in the supply pipe. As long as there is no air in the pipes

That is the basic hydraulic design behind drainback solar heating systems.

t.wheat

wrinkle.

ok. got it. now a new wrinkle. if there is not an air problem what could it be. a restriction somewhere maybe? after talking with the contractor, i found out that the plans called for a 15hp pump and a 10hp was used. if the impeller was trimmed for a 15 hp and a 10 was used, wouldnt that overload the motor if that was the problem? i know no one has a crystal ball,im just trying to help out a good customer. bottom line this is a new job and the chiller doesnt work. not enough flow across the barrell, the test/ balance guy originally signed off on it that the pressure was there , now its not.

ALH_4

Open mouth. Insert...liver

I just meant that there is no substitute for standing there and looking at it when it is not working. 8-)

Wheat, you have one of the best on the case here in Brad.

t.wheat

pump

ive got several of Dan Holohan's books, they just all happen to be on steam and old steam systems. cuz everyone know that Memphis Tn is the steam heating capital Ha...we do have alot in our downtown, midtown areas. I really appreciate the help guys, after its all figured out , ill post and let you know what happened.

Brad White

TAB, Pumps, Impellers and HP

I have to ask did the TAB contractor do a thorough job? Is the company reputable or was it a "drive by"? Not to cast aspersions or anything but one has to ask.

The impeller trimming is just to get the head and flow to the design point. Trimming an impeller may actually increase the HP by increasing the flow (should the operating point ride out to the right side of the curve and crosses a BHP line).

If I calculate a 400 GPM pump at 70 feet of head and with 8.5 brake HP (allowing for 15% motor service factor), your pump has to be over 82% efficient. Too good. You should have a 15 if that is the case.

Try this: What is the DP across the pump? That in PSIG multiplied by 2.31 will give you feet of head. For example if the suction side says 30 PSIG and the discharge reads 60 PSIG, the delta, 30, equates to 69.3 feet of head.

If you "dead-head" the pump (close the discharge valve totally for a moment and read the pressure gauge) that will, with the help of a pump curve, tell you what the impeller diameter is. The dead-head pressure is at no-flow so it is read on the left side of the curve.

Is the pump drawing away from the expansion tank connection? Have to ask!

Brad

Brad White

Just teasing, Andrew!

All in good fun! Thanks for the Vote O'Confidence.

Cheers-

Brad