Check Valve in Chilled Water Decoupler Bridge 1

[BronYrAur]

I have read opinions both ways on this forum, but I would like to bring the topic up again for discussion. I'm battling a low delta-T situation whee I don't have direct control over the chilled water once it leaves the plant. I continue to make efforts to improve the delta-T, but I have to live with the chilled water return temp that comes back to me. The plant is designed for a 20 deg F delta, but I routinely get 6 deg F deltas.

Chillers are staged on flow, not tonnage. The reason for this is that the chilled water supply temp of 39 deg in the secondary is needed at all times regardless of the return temp. It is a primary/secondary system with VFD's on the secondary pumps. Primary pumps are constant volume and come on with the chillers. If I go secondary-strong, I need to bring on another chiller to become primary-strong. This causes me to have several chillers at part load just to satisfy the flow when 1 or 2 chillers could handle the tonnage load.

So back to the check valve in the bridge. I don't have one, but I am considering it. The attached article makes a strong case for it. As I mentioned, I have read a lot of opinions in other posts on this site, but I don't know if those opinions are just a theoretical discussion, or are from practical experience.

Does anyone have a check valve in their system, and if so, how well is the plant working?

 

[Drazen]

what you described resembles beverages filling plant, where i had to dismantle whole hydronics to make it meaningful.

maintaining so low secondary temperature is simply hopeless and call for dedicated, custom-made process chillers rather than hvac chillers.

 

[EnergyProfessional]

thanks fro bringing up this question and article - i didn't know of the option with the check-valve. typically i see close-coupled Tees. I personally prefer to use these hydraulic separators because they really make for neat piping and include the dirt and air separator.

Is your piping close-coupled, or as described in the article? I mean for clsoe-coupled Tee I don't see how you easily add the checkvalve and flow meter. Do you know the system flowrates? the only way to get higher chiller dT with the given system dT would be to have the chiller flow be a third of system flow. but then the chiller has to create lower temp, which may cost you more energy. And what is the design supply temp? Do you use supply temperature reset (based on valve position)? I suspect your system has many 3-way valves.... but it seems you don't have much control over that. What are system design flowrates and allowed (or design) flowrates for each chiller?

Could you stage chillers on system supply temp? and how sophisticated can your system be?

 

[pretendfarmer]

Hi BronYrAur,

I run into this a lot but a few questions first:

1.) What is the original design Chilled water temperature for you chilled water plant? 45F? Is your system for comfort cooling primarily or is it an industrial process (as suggested above)?

2.) I'm guessing that you have a primary-secondary-tertiary system. That means the chillers have their own pumps (primary), then you have distribution pumps (secondary), then the individual loads (buildings/coils) have even more pumps (tertiary). These systems are notorious for poor Delta T performance. What is the total tonnage of your plant? Also how many distribution (secondary pumps) do you have and what is the GPM and Ft. Hd. of these pumps?

3.) If your chilled water plant is designed for a 20F Delta T, then are all the coils attached to the system designed for a 20F Delta T?

4.) Approximately how many chilled water coils are connected to your chilled water plant?

In answer to your question about the check valve, the addition of the check valve in the common piping will keep you from mixing return water with supply chilled water, but you will end up with the primary and secondary pumps being in series, possibly forcing more than design flow through the chillers. Excessive flow could lead to tube erosion and control issues. Also, the primary pumps are typically constant speed pumps and the secondary pumps are typically variable speed pumps. Trying to figure out how to control constant speed and variable speed pumps in series sounds like a lot of wasted unnecessary energy. The addition of a check valve is a band-aid, not a solution.

After reviewing the referenced article, the author seemed very reluctant to discuss Delta T performance other than to say if you are not getting design Delta T you should take remedial action by cleaning your coils (my guess is your coils are not dirty), retuning your control valves
(he doesn't talk about how to accomplish that), change out your control valves from 3 way to 2 way (now that I agree with that, although you may already have 2 way valves installed), or reduce your supply water temp (so you can immediately heat it back up to 45F with your return water temp). He does not stress the importance of the coil/control valve relationship.

My recommendation would be to address your poor Delta T performance where it occurs, at the coil/control valve. Coil performance charts tell us that the design delta T at part load (where all coils operate 99% of time) should go up. This actually occurs in less than 1% of installed chilled water systems. It can occur in all of them if properly controlled. But that control cannot be accomplished at the chiller plant. It has to occur at the coils because that is where the heat transfer occurs.

System Delta T is the quickest, easiest measure of chilled water plant operation efficiency. Most of the 10F Delta T systems I run into are operating at about 4-5F Delta T. After addressing the coil/control valve operation, this same system will typically operate at 13 to 14F Delta T for a 35% to 45% reduction in total chilled water energy savings while sacrificing nothing in terms of comfort. We have a 16F Delta T systems operating at 24F to 26F. The current record I have seen is a 16F Delta T air handler operating at a 30F Delta T while satisfying the space with 50F (not 55F because it was for an operating room) discharge air on a design day. Your part load Delta T should be 30F to 35F. That would equate to a chilled water Delta T efficiency factor of about 1.5, which is what you get when you take 30F actual Delta T divided by 20F Design Delta T. That would be excellent. You are currently 6 divided by 20, or .3, which is very poor.

Since no two systems are alike, the degree to which they can be improved depends on how over-designed they are and how poorly controlled they are. There are a number of other issues that also contribute to poor system performance. We have seen differential pressure setpoints that are way too high, air handler fan setpoints way too high, actuators that could not control against the pump differential it was seeing, etc., etc., etc.

In summary, if you are typically seeing a 6F Delta T on a 20F design Delta T system, your chilled water system control is extremely poor and can be significantly improved. Just imagine how much money you could save if you could keep additional chillers off. That would keep additional primary chiller pumps off, additional cooling tower pumps off, and additional cooling tower fans off. All of that equipment is running because of poor control valve/coil performance.

There are firms that specialize in optimizing chilled water flow in systems like yours. I work with a number of them everyday. You should consider reaching out to one of them in your area. Just be sure that they guarantee Delta T performance and that you contact some of their satisfied customers to make sure they are getting the performance they were promised. If anyone recommends more pumps and chillers, thank them for their time and call someone else. They don't have the expertise to help you.

Good Luck.

 

[EnergyProfessional]

Do what pretendfarmer said. After reading his great post, I think the author of that article really missed on some big picture items.

 

[BronYrAur]

PretendFarmer, Here are the answers to your questions:

1. The original design was 36 degF CHW supply temp with a 20 deg delta-T. We have since increased the supply temp to 39 degF with no adverse affect. We did this to help prevent chillers from dropping out on low temp. The system is a central plant for a hospital.

2. The plant chilled water feeds several flat plate heat exchanger banks. The chillers each have a primary pump (constant volume), and there are multiple secondary pumps on VFD's. All of this is in the chiller plant. The heat exchanger room are throughout the hospital campus. Each set of exchangers has pumps on the building side of the plates. In most cases, the pumps on the building side are variable speed. However, in one case, there is a complete primary-secondary system on the building side of the plates. This room in particular has been the cause of many low delta-T issues due to improper control on the building side.

3. I have no information or control of the delta-T rating of any coils on building side the system. The flat plate heat exchangers were designed from anywhere between 17 deg and 20 deg delta-T, so I am already up against challenges.

4. No idea about the coils, but there are 3 larger heat exchanger rooms and 2 smaller ones. All together, I have 12,600 tons worth of chillers.

I fully agree that correcting the delta-T problem is the best solution and makes the check valve discussion a mute point, but I have very little control of the delta-T in this building. Even though the facility is one large entity, they don't act that way. The "building" side of the plates doesn't care about the "plant" side. The building side wants its chilled water and is not concerned with the return temp. The plant side has to deliver the chilled water, but must also deal with the low delta-T issues. There are 2-way valves on the plant side of all heat exchangers that modulate to deliver a supply temp on the building side. But, as I'm sure you know, if the building side return water is too cold, there is no way to achieve the desired delta-T on the plant side. All I can do is minimize flow, but I still have poor delta-T.

This is in Chicago. Current temperature is -2 degF, and I am running a 1,400 ton chiller at a 6 degree Delta. There is a forecasted high of only 13 deg F today, but I will probably have to turn on a second chiller due to flow - not due to a real load. I have tried to wave dollar signs in front of the powers that be to show them how much they are wasting, but it doesn't seem to help.

I keep using the work "I" in this, but truth be told, I am really an outside party to this whole operation. I'm assisting the chiller operators who are beyond frustrated with the low delta-T issue. It's a shame when someone with no vested interest cares more than the ones paying the bills.

Anyway, I'm starting to think that the check valve may cause more problems than it solves. Thanks for your replies. If there are any other opinions, please share.

 

[EnergyProfessional]

Since you have HX, I think teh dT of the building sbecoems irrelevant. If they have low dT, your HX just provides less capacity and your valve throttles down. they could even have a high dT in each building if your HX are over-sized.

the HX need some minimum flow to have turbulent flow. so even at low load, the flowrate is quite high, hence low dT on your side.

You could control chillers based on supply temp to the campus. Then one chiller may have half the flowrate of the system or so. Once supply temp rises, the next chiller turns on.

Overall the entire system has an energy balance. We sometimes get distracted by temps, but each HX or Tee etc. has an energy balance. Your buildings have a specific load.... high or low dT and flow doesn't matter. How much heat they discharge in your water is the same.

Oh boy, -2°F OAT and you run chillers? Did they ever hear the word economizer and heat recovery?

 

[BronYrAur]

The problem is that the approach is too close. I have 39 deg water on the plant side, and they want 42 on the building side. If they are only coming back with say 47 on their side, I can't get a good delta-T on the plant side, no matter how much I throttle back.

That's part of my problem. Even when throttled back, my delta-T is poor.

 

[EnergyProfessional]

Could the HX on plant side get away with less flow but colder water supply temp? Then you could run fewer chillers making colder water. Obviously depends on your chiller and since that is water you get close to freezing.

do they want 42°F all year around? what is that based on? is that just some arbitrary number that one guy who retired 25 years ago chiseled ins tone? I assume in winter they don't need to de-humidfy and general load should be lower. I assume 42°F is their design supply temp.

Campus operations are notoriously inefficient. In theory they could be more efficient. I recently toured a university building. It starts that they don't have meters for heating or cooling... so no one knows (and cares) how much energy is used. Then they have chilled beam system.... since they had "rain" in the building they run it 24/7 (building only operates during business hours). 100% OA 24/7... you don't need an energy simulation to see the waste. But no one is brave enough to change something because of the "rain". The campus mechanics are not familiar with the system, so they hate it and don't want to do anything besides fixing if something breaks. that way campus systems get operated very inefficiently. Whatever energy they save by having central large chillers, they waste in operations.

I recall being a student at that very same university. It was September and over 90°F OAT. Just before that they had to shut off computer labs because the chiller plant was maxed out. but that didn't stop them from heating the classroom with the steam radiators. Instead of fixing that steam valve (or turning off stem when 90°OAT????), it is easier to just demand colder chilled water to be able to cool.

I imagine the same for your system. No one there will ever agree to higher than 42°F since no one wants to take the blame if a zone is too warm (even if that is not related to the supply temp)

 

[LowDeltaTSolver]

BronYrAur

The hospital may want to consider a Variable Primary Flow/Variable Secondary Flow (VPF-VSF) system to to mitigate the Low Delta T issues.

See ASHRAE Journal October 2014 Article "Simplified Chiller Sequencing"

http://uwf.edu/media/university-of-...ctober-2014-Simplified-Chiller-Sequencing.pdf

The article discusses an actual conversion of a Constant Primary Flow / Variable Secondary Flow (CPF-VSF) system to a VPF-VSF system at the University of West Florida. the conversion required the removal of a check valve in the decoupler piping.

Secondary Delta T range was between 6 to 10 degrees before the conversion and is now between 14 and 17 degree Delta T.

The system was operating at average wire to water chiller efficiency's greater than 0.9 kW per ton (Cooling Tower Fans, Cooling Tower Water Pumps, Primary Chilled Water Pumps & Secondary Chilled Water Pumps) before the conversion and is now operating at average chiller efficiency's less than 0.75 kW per ton. (As low as 0.57 kW per Ton)

The system is also able to provide
Supply chilled water at or below design temperature at all times
Automatically add and subtract chillers

 

[pretendfarmer]

BronYrAur,

I'm sorry that I am just getting back to you regarding your chilled water system. It sounds like you are seeing the same thing that we see a lot. The building side is operating at a low Delta T. Until that gets addressed, the plant will always run at a low Delta T. The poor performance of the building side will always drag down the plant performance.

[highlight #FCE94F]All the plant can ever do is respond to what the building side is doing. The plant responds to the low chilled water return water temperature. Poor building side Delta T performance will always lead to poor plant side performance. Until the poor Delta T performance on the building side (where the coil heat transfer in your system occurs), your plant and plant side heat exchangers will continue to perform inefficiently. No amount of tweaking the flows or playing with the controls on the plant side will address the low delta T on the building side. Methodically going through your chilled water loads and improving the coil/control valve performance on the building side will get your desired result. Since you have no control of the building side operation, you will not be able to fix the plant side delta T problem.[/highlight]

We also run into the same situation that you are seeing regarding the disconnect between the plant operators and the the people who have the authority to spend the money to fix the system. Only when we [highlight #FCE94F]guarantee[/highlight][highlight #FCE94F][/highlight] results when addressing chilled water Delta T problems do we finally get the money people's attention. You or your chiller operator need to impress upon the people who pay the bills that a lot of money is going down the drain needlessly. My guess is that you won't be needing to run additional chillers on -2F days.

To summarize, improve the building side coil/control valve performance first. This can be done one air handler at a time without a huge system shutdown issue. When the building side Delta T is performing at or above design Delta T all the time, plant Delta t performance will follow.

I compare it to tuning up your car. Without a tuneup, the car won't run well. Most (99%) chilled water systems need a tuneup. Especially if the system has been added to over time by different engineers and designers.

Good luck.

 

[BronYrAur]

Thanks all,

I know that fixing issues in the building is the only real solution. Everything else is a band-aide to attempt to makeup for a poorly performing building. I will continue to work toward that end.

 

[cry22]

One thing is not clear to me.
why use plate heat exchanger on a -2F weather?
why not eliminate the HX altogether and deliver 42F directly from the plant? it would be much more efficient.
May be an air cooled chiller using glycol is connected to the building system (such as a stand-by chiller for Operating suite using glycol), other than that, I can hardly think of anything else that would require isolation from the plant in a hospital.
It may make sense to eliminate the HX and go glycol throughout the campus if glycol is the real reason for isolation of course.

 

[BronYrAur]

That is a great question. I don't know the answer as I did not design the system. The hospital has been around for decades with multiple additions and chiller plants to go with them. A large new addition was built about 5 years ago along with a central energy plant. Chilled water was piped from the central plant to locations around the hospital where the old chiller rooms were located. They have incrementally been taking out chillers and replacing them with heat exchangers. Everything on the "secondary sides" of the heat exchangers has largely been untouched. This is of course where my actual low delta-T problem lies. The central plant doesn't really know what happens on the other sides of the plants.

So any opinions on the overall concept of having heat exchanger banks as opposed to just having hydraulic separation and tertiary loops?