Water Balance Fun

[WAYNE3298]

I'm doing TAB on a job that has been interesting from start to finish. I thought you guys might like to hear about some of the fun.
The building has a history of not heating or cooling properly. For this reason the redesign is over kill of the highest order. Every space has a fan powered box and every space has a design airflow and heating design that considerably exceeds the need.
The present problem is that most of the FPB water heating coils were downsized at the last minute but they forgot to down size the circuit setters and control valves. A few coils were up sized. The circuit setters are Nexus Orturi and the design orifice DP'S vary from 0.81 inches of water to 129 inches of water to provide design GPM. The control valves were provided the same size as the circuit setters.
I'll give my take on this after getting some of your comments but thought you guys would like to add your two cents.

[BBeerme]

Do you really mean 129 " water?

"The control valves were provided the same size as the circuit setters." I learned fairly early on to not size by line size; you size by the specs of the flow. Strainers was the area I first learned that in, sizing strainers to the line size often resulted in lack of flow. Especially with a finer mesh screen

[WAYNE3298]

Yeah 129 inches of water. That isn't the drop across the valve but the drop through the circuit setter orifice.
Control valves are normally one pipe size smaller than the line size but in this case where the flows are small they are normally the same size as the circuit setter.

[WAYNE3298]

One of the problems is there is a 1" valve that has a design water flow of 0.6 GPM. Another one is a 3/4" valve with 0.6 GPM design.

[crazzycajun]

[bradluke0]

Sounds like an interesting job. Are these series or parallel boxes? Will the hwc's produce a useable leaving air temperature ? Are the hot water control valves 2 or 3 way? Sizing the control valves based on the CV is the way it should be done . I will certainly have more thoughts on this one , probably a bit unorthodox though .

[WAYNE3298]

The boxes are parallel. The leaving air temperature is good. The control valves were originally sized by Cv. The control valves are two-way equal percentage.
The coil sizes were changed but everybody forgot about the circuit setters and control valves.

[bradluke0]

Is this a multiple story building ? How many boxes and coils are you dealing with? The reason I ask is you maybe better off not balancing each coil and letting the two way valves do their thing . Slowing down the pid loop helps when valves are oversized. Being the fan is your 1stage of heat you could have the valve open 2 degrees lower than the fan comes on .

[WAYNE3298]

It is a two story building with 22 FPB'S. I have advised them that since the control valves are two-way equal percentage that balancing the water is not only a waste of time but the system will work better if not balanced. On the oversized valves the equal percentage aspect is out the window. The valves will hunt and their life will be shortened somewhat but there is only two choices at this point. Replace the valves now or replace them later after they fail with the right valve. The oversized valves are better off not balanced also because the circuit setters will be so near to closed that they will plug. The control valves won't because they will open enough to clear any debris that might plug them.

[bradluke0]

Slowing the loop speed down will help a bunch with the hunting . Most all loop speeds are way too fast anyway as the control guys want to see stuff move when they change a setpoint . The problem is that the system doesn't really want to work that way . You might want to drive the valves open and set the flow to each floor . The other thing I would do is while they are driven wide open in the heat mode ( and the parallel fans on ) is get a snap shot of the leaving air temperatures of each box. If the control system doesn't have that feature I would just shoot one grille off each box with a Raytek . Very good move by suggesting they not balance the water . As these are two way valves I assume there is a vfd or a bypass for the pump . Getting the right setpoint is much better than trying to balance this turd . You are definitely steering them in the right direction , we will see if they listen .

[WAYNE3298]

I can't proportion the water to each floor because there are only circuit setters at each coil and none for the loop. As oversized as some of the control valves are slowing the loop speed will not help much. The control guy is really good and after I alerted him to the problem has already slowed it down.
This situation is rare with the folks I'm working with. The engineer is good and values what I do and gives me a free hand. The installing contractor is real smart and particular and also trusts me. The owner's people don't know much about this kind of thing but trusts all of us. The problem is a result of last minute changes and the water got overlooked.
I ran the calculations for air discharge temperature and all the boxes looked good but one and it was borderline at design airflow. I cut the primary air minimum a little and got the design up to 95 F with 180 F water. Even with outside air reset the box should heat with no problem.
I also calculated the threshold of cavitation and it is about 24 PSI. Given I only need about 9 PSI max cavitation should not be a problem unless vena contracta recovery is a problem due to the valves throttling so near to closed. What do you think about having the control guy not control flow on the grossly oversized valves? I've thought about having him throw them half open and holding them there as long as heat is needed.
Looks like we are thinking a lot alike brad on this. Welcome aboard.

[bradluke0]

On the one that you had to reduce the primary minimum airflow down , why not just let it go to zero ? There are enough boxes that have minimums to provide ventilation so I wouldn't hesitate to do this to achieve a good LAT . Actually , you might take the worst 3 to 5 of them and go to zero minimum. Not really sure about driving the valves half open on a call for heat . I didn't think of that so let me ponder a bit . I think I would question the control guy a bit about how slow the valves will drive . To me I would want at least 6 minutes for them to go from closed to wide open . Every control guy I know thinks 120 seconds is plenty . Thinking further , you could balance the flow ( kinda ) on each floor by taking the return water temp on each one and tweaking the isolation valve for one floor a bit . If you are thinking about cavitation I assume there is no VFD or bypass ? Is there diversity between the coils and pump ? Yes , we do think alike and thanks for making me feel welcome . Cheers , bud .

[WAYNE3298]

You hit on how I would prefer to handle this when you suggested setting the total flow to each floor. If I had the branch circuit setters I would set each floor to 2/3 design and have the outside reset minimum bumped up about 10 F on the minimum side. That would allow me to have the pressure to deliver the water where ever needed but the max flow would be limited. That would in essence give me a rarely used diversity that would only be used during warm-up on not so cold days. I love hot water because it is so forgiving but at the same time the forgiveness opens so many possibilities in situations like this it gets more complicated than chilled water. Almost any way you do it will work so you have to look at the long run.
It really gets interesting.
I forgot to answer about my concern with cavitation. There is a VFD but I am wondering if the vena contracta will extend beyond the valve into the pipe when the valve is throttling close to the closed position. The recovery factor on these valves is 0.7 but that is with the valve full open. What happens when controlling almost closed? In this application I really don't know for sure but suspect some cavitation will occur but will probably not be to the level of damaging anything. I am concerned about cavitation as a result of velocity not pressure.

[icy78]

Curious...where is this cavitation possibly happening? In the coil after the 2-way? Flashing from the sudden pressure drop?

How does vena contracta function in a small pipe?

What is 'recovery factor' on a valve?

Sent from my SM-G950U using Tapatalk

[WAYNE3298]

The cavitation I'm concerned that might happen is at the outlet of the control valve. The valves that are controlling almost closed will have a lot higher velocity at the outlet than normal. The sudden pressure drop may cause flashing because the velocity will be high. The high velocity may push the vena contracta past the valve outlet before it recovers. If that happens will I get noise from the collapsing of the gas bubbles? I don't know the answer to that.
Vena contracta functions about the same way in any pipe size because the size of the contraction is proportional to the pipe size.
The recovery factor is a factor used when calculating the maximum pressure drop of a control valve before damaging cavitation results. Different size valves have different factors. The factor was empirically determined and is dimensionless.
The formula for determining the damaging cavitation threshold pressure drop is
(Water Entering Pressure in PSIA x the recovery factor) quantity divided by the vapor pressure. The results is PSIG.

That is with the valve wide open.

[bradluke0]

Since you have a VFD I would set the total to each floor by the return water temp . Then you can play with it a bit to find the DP setpoint for the VFD . Being you have infinite control of the pump I wouldn't worry about cavitation ( yet …lol ) You mention the design fan cfm is too high . I always use the following if the design numbers are wacky......Design max primary cfm - Design min primary cfm = fan cfm . Good luck , wish this job was close I would come over and between the two of us we could really create a mess .

[WAYNE3298]

True brad we could discuss it to death if nothing else. To do what you said about return temperature it would require balance of the individual circuit setters. There are so many miss-sized I don't want to do that. I don't disagree with the theory but in this application it isn't feasible.
I finally decided to not balance any of the system except DP set point. The control valves will take care of heating the building with no problem even though some of them will hunt. The control guy reset the PID to 180 seconds. That should help some. He also said the valves would go full open upon a call for heat then throttle back. That will keep debris from collecting on the valve seat of the oversized valves. I don't know how to explain all this in the balance summary without getting technical and losing everybody. Nobody on this job has a good technical understanding of hydronics so how do I communicate? If you have words that will help I'm interested.
On the FPB fan airflow the minimum airflow is too high but the outlet temperature is good because the coils are also oversized. The only way to get the airflows lower is to reset the manufacturer's SCR minimum adjustment. I can set the minimums down to 60% of the motor input voltage without damaging the motor but don't want to do that unless there are complaints.

[BBeerme]

You need to tell it like it is. As gently as possible, while stating the facts. Start out something like . . . Due to improper engineering and oversized equipment and valves, a conventional balancing and commissioning is/was not possible.

I've walked up to a few jobs in my day that were messed up. You just look for the easiest way out. But it always starts the same way. You gotta state right up front that things are messed up. In these/those cases, I usually do everything in steps. Meaning, step one is the cheapest and fastest. Steps two, three, and on get more expensive if required.

You could probably incorporate something along those lines. Just to show that you did your due diligence under the circumstance you were given.

True brad we could discuss it to death if nothing else. To do what you said about return temperature it would require balance of the individual circuit setters. There are so many miss-sized I don't want to do that. I don't disagree with the theory but in this application it isn't feasible.
I finally decided to not balance any of the system except DP set point. The control valves will take care of heating the building with no problem even though some of them will hunt. The control guy reset the PID to 180 seconds. That should help some. He also said the valves would go full open upon a call for heat then throttle back. That will keep debris from collecting on the valve seat of the oversized valves. I don't know how to explain all this in the balance summary without getting technical and losing everybody. Nobody on this job has a good technical understanding of hydronics so how do I communicate? If you have words that will help I'm interested.
On the FPB fan airflow the minimum airflow is too high but the outlet temperature is good because the coils are also oversized. The only way to get the airflows lower is to reset the manufacturer's SCR minimum adjustment. I can set the minimums down to 60% of the motor input voltage without damaging the motor but don't want to do that unless there are complaints.

[WAYNE3298]

What has made it difficult BBeerme is the system heats really good and that has everyone convinced everything is OK. I basically have to accurately predict the future in regards to how the valves will stand up to being abused when I'm the only one that knows it is happening. In their minds I can't even prove the system isn't right.
This is a redesign of a building HVAC system that never made the building comfortable. It does now and everyone is happy. My job is to bad mouth part of the design. It's like telling someone they have a disease when they have no symptoms.
I'm trying to do this without looking like a smart ass trying to baffle them with BS.
The hardest part is convincing them there is good reason not to attempt balancing the heating water system. There are balancers a dime a dozen that don't know how to design these systems or even how they work that will jump at the chance to do the balance. They won't back anything with facts or theory just set the circuit setters without concern for the outcome.

Do you think pointing out the obvious over sized control valves and circuit setters is enough without getting any more technical than explaining how equal percentage works? That seems pretty simple to understand to me.

[BBeerme]

Keeping it simple is always important. Much of what you have said in this thread could even be used. Saying what you did about the programmer opening the valves 100% to keep them clear due to them being oversized and running near closed also seems rather simple. Things like that would show that you did the best you could under the circumstances.

Probably wouldn't work in your case here, but often when I am talking to customers about a piece of air conditioning equipment, I make the relation to an automobile. Because everybody knows what a car is so complicated things seem to make more sense to them.