Low Flow in Large Pipes - head calcualtion 1

[BronYrAur]

I am a little uncertain about this application. I am looking into putting a heat pump on the 25th floor of a 54 story building. During the normal day the building supplies condenser water. During off-hours, however, I am responsible to pump my own condenser water.

There is a cooling tower on the roof, so the discharge nozzles are probably at the equivalent of 56 stories. I will need to pump up from the 25th to the 56th floor, but I will have a full column of return water from the sump (let's assume at the 54th floor) down to my heat pump on the 25th floor.

So I need to figure all of my branch piping, the PD through the heat pump, cooling tower nozzles, and the additional elevation between tower sump and nozzles. No problem there. But how do I figure the PD through my building risers? They are probably 12" risers, and I am only pumping 5 GPM. The friction loss is nothing, but won't there be resistance?

I know I'm not asking the question too well, so let me just throw it out for discussion. Will I actually push my 5 GPM through the tower?

For reference, here is what I am figuring:

Heat pump PD: 11' from manufacturer
Nozzels: 10' estimated
tower elevation diff 12' measured
200 TEL branch pipe 10' calculated
600' of 12" Risers ????

I selected a 5 GPM at 60' pump to get me started and it had a 1/2" HP motor. Someone questioned that such a small motor can handle the job and that got me second-guessing.

Thanks for your help.

 

[BigInch]

There is gravity and friction loss. There is no other resistance.

Friction loss for 5 gpm isn't much, in the risers, or in the fittings.

If you have some 28 stories of water column on the suction side of the pump, you can expect a free ride up to the 53rd floor, in a static situation. There will be some friction loss in the downcomer when things are flowing, but with 5 gpm, again not much.

Provided you can fill the riser from the 25th to 56th floor and the top of the highest pipe from some other water source having enough pressure to do that, you won't have to do that with your pump, so all your pump really has to do is pump against gravity for the few stories above the 54th floor to the 56th + the high point pipe elevation, so the required differential head won't be much at all, although you will need 31 stories + some additional feet of pressure rating on the pump discharge; some 150 psig more or less.


**********************
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[BronYrAur]

BigInch,

I'm still a little confused. The pipes are full. The building just shuts down the pumps after hours. So I shouldn't need to fill anything.

Your last statement about pump discharge is also puzzling. Don't I have almost as much static pressure on my suction side? I don't see your concern about the discharge static. Please elaborate.

 

[BigInch]

I said, "if" you have to fill. If you don't, no problem.

Static = "stopped" pressure, no friction losses when stopped. You will have +/- 150 psig on both suction and discharge when the system is stopped, and it won't change all that much when its started and running, maybe +15 to +20 psi on the discharge and minus the same, or maybe less, on the suction side.

**********************
"Being GREEN isn't easy" ..Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com/

 

[katmar]

You certainly don't need 12" pipe if you are only pumping 5 gpm. If there is other water going through the risers then your pressure drop calculation becomes much more complicated and in that case there is not enough information for us to comment. For 5 gpm, inch and a half or maybe 2" is all you need.

Don't forget that all your pipe, the pump seals, the condenser etc must be rated for the maximum pressure which will be around 150 psi.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

BTW, change "+/-150" to "approximately 150"

**********************
"Being GREEN isn't easy" ..Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com/

 

[CTDon]

Heat pump systems typically employ closed circuit cooling towers. If so, you don't have to worry about static lift- It's like a continuous pipe up to the roof and back. The bad news is the cooling tower probably won't be working. Even if it was, the flow would be laminar and thermal capacity would be limited. The good news is that 12" pipe holds a lot of water. So, if you tapped the pump suction into the supply line and discharged into the return line it would be like having access to a big lake- by the standards of your 5gpm cooling requirement. You can do the math but I expect you could run your heat pump for quite awhile and not have much impact on the loop temperature. When the building engineer starts the tower it will dissipate your puny heat load in a matter of seconds.

 

[racookpe1978]

Could his 12" line be part of the fire hydrant supply?