Chilled Water Constant Flow Reverse-Return Bypass

[Btutiful]

Hey all,

I'm working on my first Cooling Tower Chilled Water system, and I was hoping to get some input on some solutions I've come up with for a reverse-return bypass. (Everything is pretty much self-taught since undergrad days and I don't have access to a mentor to get their opinion.)

The customer wants a constant flow system that'll feed a bunch of chillers they've got around the lab. Simple enough - I just put in the typical auto balancing valves at each drop and make the cooling towers closed loop with VFD fans to maintain temperature difference (also wanted by the customer). The catch comes when they say they want to add some chillers in the future. Luckily, I have information on these future chillers, and I've been able to size the pump, towers, etc. to account for these. My question comes when managing the variable amount of bypass I will need during the time they don't have these new chillers installed.

Can I simply put in an "Automatic Recirculation Valve" downstream of the cooling towers and have it's bypass go to the inlet of the pump, or do these valves not modulate but rather open and close? Or maybe put it at the end of my supply with the bypass going into the reverse-return? I've also seen "Flow Bypass Valves", but I'm not sure if these are just set at a single pressure difference and don't modulate.

Is there a different way you guys would tackle the bypass? The customer seems to want the system how I've described, or I'd just do a variable pump system.

Thanks for the help in advance.

 

[goutam_freelance]

Can you illustrate with a simplified flow diagram?

 

[LittleInch]

Why do you need a bypass?

This is only if your future flow is x 2 or x 3 your current flow?

So you're running an oversized pump, but pumps have quite a large acceptable flow range of probably 30-120% of your "rated" flow.

If all your cooling water outlets are flow controlled then you will only pump what is needed. Anything else is basically waste of energy pumping water around you don't need.

Needs a bit more data on pump capacity, current flow and how this is laid out.

 

[Btutiful]

Thanks for taking a look at this guys!

Attached is a rough draft of the P&ID that shows existing and future loads. Any criticism for this would be appreciated as well. This is a critical system for the customer, so it is setup for duty/standby as requested by the customer. I don't have the bypass roughed in yet.

To accommodate the existing load, 34.9 GPM is needed. The future load requires an additional 16.9 GPM (about 40% of the final GPM requirement of 41.8 GPM). The only flow control I have so far would be the valves at the pumps and the balancing valves at each drop. I figured that with this much flow variation, I would want some sort of bypass, but feel free to let me know if I'm wrong. It could be I didn't have a problem to begin with.

 

[LittleInch]

So some of the symbols I don't understand, but you say you have flow balancing / control at each location yes?

Your pump is good for 51.8 GPM, [was that a typo above to say 41.8?] presumably somewhere close to BEP ( A pump curve would really help here)
At 34.9 GPM, it is therefore running at ~ 65% of max duty.

I don't see a problem here that needs any fixing. Most centrifugal pumps will only start complaining when you get to about 30-35% of rated flow.

OK your pump might not be as efficient as it will be at 51.8GPM, but absorbed power will be lower as you're only pumping 35 GPM. This is almost certainly better than pumping all that water (52GPM) then just wasting the pressure by dropping the pressure back to your inlet pressure.

Either that or add a VFD and control on pressure at the end of the supply loop to maintain a min pressure for the unit furthest away. Note that VFDs use power so you might not save any money in this initial operating period.

Does that help?

But post the pump curve if you have it or advise make and model no / size.

 

[Rputvin]

Your towers don't look setup correctly to me. It might just be the schematic representation missing some detail but this appears to be setup to flow (treated) domestic water into the basin, and then pump the basin overflow and drain connections to facility drains.

The basins will hold water that will become concentrated over time. I would expect the basins to have the water treatment and monitoring instead of the supply line, and that control would trigger blowdown and run your drain pump as needed. Multiple ways to accomplish this task though.

You have a ton of strainers shown. If this system ever has issues with flow/pressure troubleshooting it will be a nightmare. I'd suggest going to a single device that can be maintained, stick a pressure alarm on it, and let it tell you when it needs to be serviced.

If you use a VFD for your main pumps - run a differential pressure between the suction and discharge headers. Your static or zero-point pressure is variable in a charged/hydronic loop. If you just use a pressure sensor at the discharge for control reference you'll have variable pump output based on that varying suction pressure.

 

[goutam_freelance]

Best plan of action is to do a techno-economic analysis considering the time when the additional flows will kick in. The options to be evaluated are as follows:

Option-1. Consider the additional cost of running the present pump(34.9 GPM) with higher power consumption(away from BEP at 51.8 GPM), provided the pump motor can take up the additional load.

Option-2. If a bypass is provided(with 51.8 GPM pump at present), consider the cost of loss of energy due to the operation of the bypass.

Option-3. Consider a VFD with additional cost and additional power consumption for the VFD with 51.8 GPM pump

Option-4. Consider the additional cost for 4x18 GPM pumps(3W+1S). 3 pumps(2W+1S) can be installed now and 1W installed in the future. Standby pumps can be 2 if required.

BTW, if the chiller loads vary significantly, you may need a minimum flow recirculation for the pumps.