Dumb chilled water question

[tim fox]

If you have multiple pumps in parralel on a 6 " pipe system, when a second pump is brought on line, what to happens GPM and pressure? I thought the other pumps are for system redundency, however I see places running two or more pumps all the time. If pumps are of equal size and put out equal flow at equal pressure,and the pipe size is the same with one pump or three pumps, I'm trying to figure out what is the advantage to more pumps and how to know when it's time to bring on a second one.I consider my self a good service man but a little lacking in the hot and chilled water department. Thanks Tim

[absrbrtek]

Check the pressure drop across your chiller barrel using the manufacturers flow chart for the perticular chiller. This will be a good indication of what is happening. Or get the pump curves.

[snipe70e]

Most plants the second pump is stand by. use only one at a time. some plants use the second pump as load goes up, and some plants start second pump with second chiller.

You have to know your plant and what was designed to run.

[jayguy]

i am glad that you are focusing on the whole plant and not just the chiller. to properly diagnose and service a chiller, you really need to know how the whole plant works together (or at least how it is supposed to work together).

http://www.bellgossett.com/Press/BG-unparallel.asp

good luck.

[ozone drone]

I once went on a no heat call to Jr High School in Wyoming when the outside air temp was about 15 below....When I entered the bldg it was about 38 degrees inside,naturally they had dismissed school and sent the kids home.
I went into the boiler room and all 4 Kewanee hot water boilers were running their hearts out on high fire. .... Checked inlet water temp ... 40 degree water
Checked outlet water temp... 44 degree water!!! checked the hot water pumps .. two 5 hp B&G pumps running fine ... couplers ok...

Then it dawned on me that one pump must be a back-up (this was my 1st time at this bldg).. so I shut it off... It's a wonder I didn't thermal shock the boilers...but the outlet water temp immediately jumped from 44 to 80 degrees and the loop eventualy warmed to the 180 degree setpoint..... The water was whistling through the boiler so fast it didn't have time to absorb the heat .... just illustrates how important it is to have the correct GPM. I don't know where all those btus went? up the stack? but the tubes must have been overheated since the water wasn't picking them up (btus) wonder why the tubes weren't shocked when I killed the second pump and that 40 degree water slowed down and contacted the tubes?????????

[DPSwitch]

Checked inlet water temp ... 40 degree water
Checked outlet water temp... 44 degree water!!! checked the hot water pumps .. two 5 hp B&G pumps running fine ... couplers ok..

So who piped the chilled water through the boilers? lol

[ozone drone]

So who piped the chilled water through the boilers? lol

LOL guess I did get off topic ... but it does show how gpms can affect system performance.....

[tim fox]

Thanks Jay,Snipe,Ozone,and Absrbr. The B&G info was helpful Jay.

[herschmidt]

Centrifugal pumps working in paralel increase the GPM not the pressure, pumps working in series increase pressure not GPM.

I hope this help.

Hernan

[consolidmech]

As the last post said Centrifugal pumps in parallel increase GPM and if connected in series increase head.

[du mech eng]

Centrifugal pumps working in paralel increase the GPM not the pressure, pumps working in series increase pressure not GPM.

I hope this help.

Hernan

this is a common misconception. in fact it is what i was taught entering the industry as well. take a look at the graph and the example in the link that jay guy provided. your system curve would have to be perfectly flat for your theory to hold. that means your system pressure loss would have to stay the same as you increase gpm. since this is not true and pressure losses increase as the square of flow, you get the system curve as shown. you pumps will always operate at the intersection of the pump curve and the system curve. so by bringing on a second pump in parallel you will increase both pressure and flow.

the example in the link shows 15 gpm @ 4.5 ft head with one pump running and 20 gpm @ 8 ft of head with both pumps running. so with one pump operating you are at 75% of design flow and 56% of design head.

another helpful link...
http://www.pump-zone.com/articles/52.pdf

[hvacker]

Years ago I was asked by an engineer "Give me a scientific reason why I put 2 pumps in parallel"

I felt a trap so I sugested to increase flow.
"Nope" he said, "Two smaller pumps were cheaper than one big pump"

Scientific? I guess ecconomics could be a science.

[Gib's Son]

In a parallel configuration each pump will operate at the same head and they will provide their share of flow at that pressure

In series pumping the each pump will operate at the same flow and provide their share of the pressure at that flow rate.

Some of the pitfalls to watch out for in parallel pumping with pumps of unequal pressure is that the pump with the higher pressure can cause a delta-p across the smaller pump which exceeds it’s cutoff pressure…not good.

In series pumping if the flow capacities are different than the pump with greater capacity can overflow the lesser pump which can result in cavitation by creating pressure drop in the pump instead of a pressure rise.

[Shophound]

I once went on a no heat call to Jr High School in Wyoming when the outside air temp was about 15 below....When I entered the bldg it was about 38 degrees inside,naturally they had dismissed school and sent the kids home.
I went into the boiler room and all 4 Kewanee hot water boilers were running their hearts out on high fire. .... Checked inlet water temp ... 40 degree water
Checked outlet water temp... 44 degree water!!! checked the hot water pumps .. two 5 hp B&G pumps running fine ... couplers ok...

Then it dawned on me that one pump must be a back-up (this was my 1st time at this bldg).. so I shut it off... It's a wonder I didn't thermal shock the boilers...but the outlet water temp immediately jumped from 44 to 80 degrees and the loop eventualy warmed to the 180 degree setpoint..... The water was whistling through the boiler so fast it didn't have time to absorb the heat .... just illustrates how important it is to have the correct GPM. I don't know where all those btus went? up the stack? but the tubes must have been overheated since the water wasn't picking them up (btus) wonder why the tubes weren't shocked when I killed the second pump and that 40 degree water slowed down and contacted the tubes?????????

Here's what I'm thinking, C...the tubes in the boiler never got all that warm, most of the btu's were going right up the stack. Maybe the very outer surface of the tubes were a little toasty, but if you got 40 degree water going in and 44 out, yeah, it doesn't look like much heat transfer, but since the water through the entire boiler only picked up 4 degrees, I can't see how the tubes got all that hot. The 40 degree incoming water just kept screaming in, keeping the tubes, relatively speaking, cool.

When you secured the second pump, the volume and velocity of water flow dropped, giving the water more time in the tubes to pick up heat. Since the tubes weren't all that stressed with two pumps online, there wasn't much difference between the new condition of just one pump and lighting off a cold boiler, IMO. Had the tubes been nearly dry and cooking hot, introducing that cold 40 degree water probably would've been catastrophic.

To the OP, good question! I did not intend to derail your thread, but Ozone's post does illustrate a good point regarding proper gpm through a heat exchanger, be it a chiller or boiler.