ReplyPump should be selected based on the flow necessary for the circuit.
Regular Guest
Hi guys, should secondary pumps flow be equal or greater than the primary flow, i mean, when choosing pumps for chilled water system, should i install secondary pumps that are greater in flow capacity than the primary pumps or both should be equal?
Many thanks,
Professional Member*
Pump should be selected based on the flow necessary for the circuit.
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Professional Member*
This thread should be in one of the "Chillers" section. Ask a moderator to move it. Try clicking on the "!" in the lower left corner of your post. I'm thinking the secondary pump(s) should be sized based on the load (and lift?), and the primary pump(s) should be sized based on their respective chiller(s). But I'm not certain. Maybe I'm wrong about moving this thread. Is it a residential application? I was not aware primary/secondary loops were used there.
Regular Guest
as i understand, the primary pumps are sized based on chillers water flow, but im not sure about secondary pumps, should secondary pumps be equal to primary pumps flow or to the flow demand of the cooling units they serve for (AHU, fan coil unit ...)Originally Posted by comfortdoc
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Moved to Tech to Tech.
Professional Member
they are almost never equal, the primary pump is sized to move fluid throughout the heat exchanger, the secondary pump is sized to move fluid throughout the loads.
just figure the pump head needed by each circuit.
Professional Member*
Primary pump= flow of the device doing the work.
Secondary pump= sized for the load.
Think of it this way, there can be numerous primary and/or secondary pumps..... This should clarify the thought process...
I have a building with two primary pumps (one for each chiller) and two secondary pumps (building split into two phases) but only one water system.
GT
If a day goes by and you have learned nothing, I hope you got a lot of sleep.
Regular Guest
This may help
Regular Guest
thanks for the pdf, very informative.
Regular Guest
You are welcome. It was posted by someone else.
Professional Member
You would always want primary flow to be larger than secondary flow. You can violate that for short periods but doing so for any degree of time will cause lots of problems with humidity and such.
Professional Member
Based on your question, you need an engineer. If you're planning on choosing pumps based on information given over an open internet forum and not on your own proven ability to do so and do so correctly, you're openin' a can of worms that may be very hard to close.
Professional Member*
This is not 100% accurate...
All depends on the temperatures and approaches of the chilled (or hot water) medium and overall controls scheme.
The secondary loop flows can exceed the primary loop flows indefinitely. In fact, they do most of the time.
If a day goes by and you have learned nothing, I hope you got a lot of sleep.
Professional Member
times used to be, primary flow was greater than or equal to secondary. these days, it really depends on system design, delta t etc. engineers, and regular joe's like me play with delta t, pipe sizing to do different things. this is even more true on boiler systems running multiple temps on secondary loops. you need to look at the requirements of your primary equipment(chiller), and that of the secondary loop(s).
Professional Member
If the secondary flow is greater than the primary flow the return from the loads will mix with the supply from the chillers. This will raise the temperature of the supply water to the loads. This doesnt seem like a good thing.
I always thought the primary should be more than the secondary. Some engineers like a check valve in the decoupler that will not allow the secondary flow to be greater than the primary.
Professional Member*
While in a common sense world I would agree with you, however, with some of today's systems and controls schemes things have really been set outside the box.
The one I did recently had two chillers, building divided into two separate loops and all tied together using four pumps. Basically with three way valves at the control level, I can literally run both building sections on a mild day with less than half of one chiller.
The return water temperature sensor controlling the chillers is literally in the decoupling run. This creates a blended return temp of both buildings and it's reset by building total load (using a return water sensor for each building section). The energy used for cooling has dropped in half and the second chiller seldom runs for more than a few hours on the warmest of days.
The system used to be one chiller, one building section with a single pump with full flow going to each chiller/building section.... Ran that way for 12 years. I replaced one of the 40 ton air cooled chillers and talked them into piping it this way and never looked back.
Don't get me wrong, I see your point, but it is not that "cookie cutter" anymore.
I could tell you about the 20 ton FHP system we did, but you wouldn't believe that it actually works and I am too tired to try and explain it right now....
If a day goes by and you have learned nothing, I hope you got a lot of sleep.
Professional Member
Sounds like you definately improved things. I would enjoy seeing a schematic of the system.
I'm just have a hard time seeing how it can be a good thing to have return water from the ahu's mixing with supply from the chillers, and then going back to the ahu's. Oh well, Im not a cw piping expert by any means, just trying to think through it.
Professional Member
Bigtime, just all depends on conditions (temps/humdity/etc). You could get away with stuff in some parts of the country that would never fly in others.
These days I wouldn't even design with a primary and secondary loop if I had a remotely constant load.
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