A recent concern has been flagged by me in regards to a chilled water system I balanced that has several large coils piped in a "parallel flow" arrangement as opposed to the typically found "counter-flow" arrangement.
Complaints now from the building management is that high humidity levels in the building are causing doors to swell and bind,along with comfort complaints of "cold and clammy" which I feel is directly related to the "parallel piping at the coils" not removing the proper amount of latent heat and carrying the high moisture levels to the occupied spaces.
The recommend "cure" by the controls contractor is to simply reduce the discharge air temperature from 55 to 50 and that will solve that problem.
My concern is that lowering the discharge temp by 5 degrees will "help" the high moisture levels, but will not cure it. I also think that "over-cooling" complaints will result in applying this bandaid fix to a proplem related to coil piping arrangements.
Your comments are requested.
Also, what is the expected difference in efficient and effective air conditions in regards to "counter-flow" VS. "parallel flow" coil piping?
FIrst I will try to address the easy question. A parallel flow arangement cannot acheive the heat transfer a counterflow arrangement can. The best it could ever do is reach the average temperature of the two inlet streams. Since a temperature difference is required to drive heat transfer, the parallel flow will have to start with a larger temperature difference to get to the same leaving conditions. If the chiller water setpoint can be lowered enough the result will be higher energy consumption to get to the lower chilled water setpoint. I can expand on this, but you seem to understand it already. I can't think of any reason someone would intentionally use a parallel flow system except to limit heat transfer. Was this an 'opps'?
"Overcooling" the air to reduce humidity shouldn't cause too many problems, in some cases. On average, the colder air will cause the supply flow to be reduced (VAV system) or other means to reduce cooling capacity so the space setpoint is maintained. One possible problem is if the VAV's go to minimum position and the diffusers start dumping instead of mixing the supply air. Then you will get very cold spots near the supply diffusers and a little warmer than comfortable other places. The space temperature sensor should remain close to setpoint while many people are complaining about both too hot and too cold. At minimum flow on the VAVs they may start bring on reheat. That will drive the energy cost up. But most people really don't care about energy cost as much as comfort, IAQ, and moisture induced problems.
Good luck and post what you find.
Another thread is on this site regarding this problem.
The control contractor may find it is not an easy task to simply lower the supply air temp. The chilled water temp may need to be much lower than normal to make things work. Then they will perhaps find that the chiller was not selected for the lower water temp, and will have problems operating. Sometimes when things go wrong they just keep going that way.
The real answer may be to repipe the coils.
Just my opinion.
CXagent was right about counterflow vs parallel. Counterflow is much more efficient. Give me the line temps and I'll make you a rough guess-atimate. Counterflow should provide better heat transfer.
Have you considered a heatpipe? This will help remove more moisture without sending ice cold air into the room
Also make sure that inlet and outlet are marked correctly. I was at Carrier for a couple and had to make a few warranty repairs on some chilled water coils due to mislabeling from the factory.
Is this a draw through air handler?
If it is draw through where is the discharge air sensor located, upstream or down stream of blower?
What is the entering and leaving db/wb across the coil?
The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.
Several other concerns...............
..... are being addressed first.
The economizer setpoint has been 67 degrees which is a bit high for Northern New England. We've had quite a few weeks of "dog days" with outside temps of 65 - 70 degrees and 99% humidity.
Should both the entering water temp AND the discharge air setpoint be lowered, or will just the CHW do?
The units monitor inside/outside DB temps only, not enthalpy (not good for building moisture levels). 60 is recommended.
The RTU's are Tranes with OSA flow stations that are not functional through controls and need to be wired and programmed.
It makes sense, lowering the chilled water setpoint from the present 45, which it's maintaining, down 5 degrees would have a good impact on the latent heat removed, and the vav mounted reheats should handle any over-cooling concerns. Efficiency probably takes a nose dive throughout any mechanical cooling condition & especially heavy loading, and this being a State Building that didn't call for a commissioning process- only adds to the possibility of problems beyond my scope of testing and responsibility.
The roof tops are Trane TSCA050 AF plug; blow through and FC returns. The indoor units are Trane(MCCA035 plug fans) having external ducted COOK SQN-B's. The indoor return fan tee's left and right at fan discharge- one going towards the unit mixing box- the other ducted to outside for relief/exhaust across a modulating MOD at the wall.
I have to wait now until controls have all their stuff done before I schedule a revisit. More later and thanks for the advice so far.
Oh yeah, btw................
...... the supply air sensor is duct mounted about 25 feet downstream.
Hot water preheat coils are before the chilled coils.
Why would you not want to use enthalpy sensors as opposed to dry bulb? Other than min. postion I dont want my OA dampers open above 55 degrees with low humidity. On Honeywell W7200's I use setting D. I'm in Indiana and we can have some fairly humid weather.
imo if the coil is efficient enough to produce 55 degree air (15-20 dt)then it is also removing moisture/lheat from the air.
65-70 degrees outside air temp with a 67 degree economizer setpoint sounds like the problem to me...chill water valve is staying closed too long.. and when it does open all that moisture that was pumped in is condensed.
even if the chill water setpoint is lowered 5 degrees...the economizer will still be open at 67 degree setpoint @ 65-70 degree outside air temp when the chill water valve shuts off as space temp is satisfied.
along with lowering the economizer setpoint...might want to reverify economizer damper acheives minimum position.
a simple test would be to close the economizers and recheck space rh after an hour or two under same weather conditons...if the building does not have a negative pressure you should get nice readings.
..if i understood the prob correctly.
This may be a stupid question? Any chane the coils can be re-piped ?
IMO (regarding mechanical systems) the only stupid questions are the ones that don't get asked.
Originally posted by flemsteele
This may be a stupid question? Any chane the coils can be re-piped ?
The RTU's could be repiped at their present connected side of the coils, as they are fed by risers from the floor below. They could be repiped simply by swapping the present configuration.
The AHU (indoor units) are all "right hand" units, 2 per mech room with the chilled system piping points being at the access alley locations in between. The coils that are piped wrong should have also been connected at the right side of the unit for counter flow. The present piping looks "symetrically pleasing" to the unknowing eye that installed it that way, but very "concerning" from an efficient and effective operational view point (IMO).
Someone is now saying that the manufacturer (Trane) claims that a parallel arrangement only drastically effects "small coils" and that with larger coils the efficient differs only by 7%. I find that hard to understand "why" that should make any difference, and would want to see documents backing up that claim if I was the one to be held responsible for a final decision. Glad I'm just the balancer who covered his butt by mentioning at the job site(early on) to the mech contractor and, in the report as an "inconsistant with normal arrangement considerations" concern.
?? Does anyone know where I could review any on-line resources regarding counter flow vs. parallel efficiency data comparisons ??
I understand the "how it works" enough to enable me to do my job, and would like to dig a little deeper into "why it works" to back that up if asked.
Thanks for all the responces so far.
The way I picture this my head is in theory if you have 2 pipes strapped together with 2 different temps and the flow is parrallel the best you can achieve is an average temp.
But if it is counterflow, and if the pipes were long enough they could approach each others temp. So I believe the bigger the coil the greater the loss in heat transfer if piped parrallel, the same applies with heat exchangers.
I had a similar situation with a flatplate heat exchanger piped wrong and called them they said on that particular model it would reduce efficiancy by up to 50%.
I agree with you tab-tek I think Trane is trying to blow air up somebodys a$$.
Here is a good coil site.
Some good pdf files.
Don't know if it will have what your looking for though?
[Edited by sirtab on 07-31-2004 at 04:45 PM]