3 chilled water plate heat exchangers in parallel 2

[BronYrAur]

Looking for some comments on an idea I have about control of three chilled water plate heat exchangers in parallel. There are actually four but one is a standby. The sequence is not clear to me on how plates stage on and Stage off, but I do know that it's not satisfactory. What is happening is that an existing plate will be open with a certain percentage on the control valve on the plant side. The new plate's control valve goes to the same percentage. So the flow on the plant side essentially doubles very quickly, and this is causing issues at the plant. Some ideas have been kicked around about possibly requiring any existing valve positions to go to a minimum setting when a new plate is brought online, Etc.

Since it's not clear what is even bringing a new plate on because sometimes a plate will come on even though temperature is being satisfied, what would be wrong with running all three plates all the time? All three valves on the plant side would operate in unison to maintain a temperature on the load side. There would be no crazy flow swings because plates would never turn on or turn off. They would all be open all the time and the valves would modulate accordingly. The only danger I see is in an extreme low-flow condition that I might reach laminar flow but by then the load would be so insignificant anyway I don't think it would matter. Or perhaps a seasonal sequence could be developed where one plate runs in the winter, two in the shoulder seasons, and three in the summer.

What do you think of this idea as opposed to trying to come up with a better sequence for staging on and off plates?

 

[crshears]

I like your seasonal # of HXrs; simple and inexpensive.

I've worked with similar systems in the past, and the quick and dirty approach chosen was a manually applied seasonal sequence, in other words have the plant operators S-L-O-W-L-Y valve the incoming or retiring HXr into or out of service, allowing the control loop temperature sensors to modulate the companions as needed. Because this proved reasonably satisfactory when judiciously performed, there was no incentive to spend money developing any more complex means of accomplishing the same thing.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[mk3223]

IMO, your system has adequate equipment and can be operated either in a series or parallel. The operation of the control valves could be a factor and needs to be looked int to detail. For example, are the valves controlled by the temperature or flow control, or both? what's the valve turndown ratio?

 

[BronYrAur]

Valves only control to temperature, but the staging of plates is based on % load. Not sure how the load is calculated, but flow is no doubt part of the equation. I have seen the valves as low as 20% and as high as 100%. There doesn't seem to be a good sequence.

 

[IRstuff]

It basically comes down a few few things I'd be concerned about
> wasting cooling water -- if the cooling water is dumped, then you're paying to move water that wasn't as "used" as it could be had it been fully laden with the waste heat being exchanged.
> wasting secondary cooling -- if the water is being supplied by a chiller facility, its optimum efficiency might be designed around a much higher deltaT for the cooling water return, so lower deltaT, lower efficiency elsewhere.

Presumably, the designer of the plant had the same design decision point that you are looking at, and decided to add all this complexity that you are thinking of eliminating. You need to find out and understand why the designer choose the additional complexity. I've outlined a couple of scenarios above. Other things and considerations might have changed since the plant was commissioned, but, nevertheless, someone picked the current design point, and you need to fully understand the reasons why they choose it before you make wholesale changes.

I've been on the back end of such design changes that cost lots of time and money. At at former company we got a project to process a component, which wound up with abysmally miserable yields. After months of tiger team and experiments, it turned out a maintenance "engineer" decided to make changes to a previously qualified process to simplify his life, and a tiny 50C reduction for an 1100C furnace temperature resulted in killing the yield. It ultimately cost us 6 months and about $3M because the engineer wanted to reduce the number of times he had to replace a $2k furnace tube.

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[MikeHalloran]

I have seen the valve

That's a good sign.

As IRstuff has indicated, it's a really good idea to get out there and watch the entire process for a while, so you can figure out for yourself what's really going on.



Mike Halloran
Pembroke Pines, FL, USA

 

[lilliput1]

Make 1 or 2 chilled water flow control valves be 3 way (either mixing or bypass type) so common flow is constant but chilled water either flows to the heat exchanger or is bypassed.

 

[BronYrAur]

I avoid 3-way valves like the plague on chilled water systems. This particular plant I referred to used to have a 6 degree delta T on a design day, now it has a 17.5 degrees delta-t on a design day. this is due to a number of things but primarily because of stopping any wasted flow through three ways or valves that are manually open.

I appreciate the comments but one thing I don't want to do is introduce any 3-way valves. I'm just an outside party really on this project so I don't have authority to change the heat exchanger sequences. I just have a vested interest in making the plant run efficiently. I think what they're going to do is throttle back any open plates when they decide to open an additional plate so that the flows don't see such a dramatic increase.

 

[lilliput1]

I see but you can't say absolutely no 3 way control valve because you have to make sure the chiller do not operate below the chiller allowable minimum flow limit. The 17.5 degrees delta t is extreme. The plant must be operating at very low chilled water supply temperature and chilled water flow must be close to the minimum flow limit of the chillers. Anyway crshears idea is good if the slow valve opening is applied to the flow of the fluid being cooled in the heat exchanger being activated. It would be simpler than your idea of throttling back any open plates then throttling them back open. Operating all heat exchangers in parallel will be problematic when load is about 20% and below. One heat exchanger operating with 3 way control valve would give good control up to zero load.