Adding a Water Side Economizer in a Chiller Plant

[yyj10]

Hi,

Would add a water side economizer in a chiller plant cause low delta_T symptom for the chillers? The water side HX is in series with the chiller evaporators, therefore, i am a little worried about low delta_T symptom. Anyone has any experience before?

Thanks a lot!

 

[willard3]

I don't think a waterside economizer would be in series with the evaporator.

 

[yyj10]

Thanks Willard3, This is a integrated water side free cooling. So the heat exchanger is in seriers with the evaporaters and it is controlled by a bypass valve to determine if the chilled water is going to pass the heat exchanger or not.

 

[DRWeig]

Hi yyj10,

Perhaps provide us with a sketch (one-line) and the capacities and flow rates in your chilled water loop, along with the design delta-T of the chillers and heat exchanger. I'd be willing to study it a bit more.

Like willard3, I've never seen the heat exchanger in series, only in parallel (as if it were an additional chiller).

Good on ya,

Goober Dave

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[yyj10]

Hi Dave,

It is in the ASHRAE Journal September Volume, article: Optimizing Design & Control of Chilled Water Plants Part 2: Condenser Water System Design

I attachedd the snapshot.


The water side economizer is piped in series with chiller evaporators.

 

[Drazen]

i dare thinking that only parallel connection with condensers would make sense.

It would be interesting to read that article, while from general sense point of view, low delta t is not issue while in free cooling mode as everything you can get from free cooling is at premium, and at the point where free cooling is not sufficient, load is high enough and no low delta t will occur.

Of course, if chiller mode would modulate to add only extra load needed to free cooling, that could create lot of troubles, but I am not sure whether manufacturers offer that - normally, when free cooling is not sufficient, cycle switches to full refrigeration.

That would be interesting idea to install very small chiller to cover "free cooling" peaks, but I would not come into that before reading many, many articles

 

[DRWeig]

Thanks yyj10, your snapshot didn't attach but I have that journal issue. I'll look it up!

Good on ya,

Goober Dave

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[yyj10]

Hi Drazen,

Thanks a lot for your input, I agreed with your idea that the chiller will take over when the free cooling is not sufficient. One thing I like to point out too is that the article also mentioned that the free cooling heat exchanger can be used to precool the chilled water for the chiller, which means that we are running free cooling and chiller simultaneously. That makes me worry about the low delta_T symptom, but i guess a VFD chiller may not have this issue.

hi DRWeig, sorry that I could not attach the snapshort for some reason. Hope you read the article already. Thanks

 

[DRWeig]

yyj10,

I did read the article, it's a good one. The definition of integrated-vs-nonintegrated is different what I have learned in the past, but the author defines his wording very well.

Good on ya,

Goober Dave

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[DRWeig]

Sorry, enter key got me for some reason. Here's the rest of my thought.

The definition of integrated I have been around for years had to do with controls, not piping. The author says that nonintegrated means that the HX is in parallel with the chillers, and it cannot be operated at the same time that the chillers are running. Note that it can be operated in parallel with the other chillers, as long as the outdoor air is cold enough to make CHW supply that matches the temperature of the primary loop. At that point, it becomes like just like an additional chiller in the plant. That's my only disagreement.

I think that either series or parallel lineup can be used and managed with the work of a skilled designer and skilled controls person. However, I agree that the series arrangement allows economizing at higher outdoor temperatures, so you get more hours' use per year and thus save more energy (which is probably why parallel piping is not allowed in the newer 90.1 code). Your chiller won't mind a low delta-T, that's what it has done for decades in constant-speed primary loop pumping when the load is small.

I'll defer to the MEs on this one, though -- my experience is more thorough in controls and simulation with chiller plants. I really don't know what the effect of low delta-T at low loads might be with the innards of a chiller in a variable speed pumping loop.

Good on ya,

Goober Dave

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