Counter-flow VS. Parallel coil arrangements

[tab-tech]

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?

[cxagent]

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.

[lew2]

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.

[alex_in_fl]

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

Alex

[jon harmon]

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.

[Carnak]

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?

[tab-tech]

..... 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.

[tab-tech]

...... the supply air sensor is duct mounted about 25 feet downstream.
Hot water preheat coils are before the chilled coils.

[jon harmon]

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.

[Islander]

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.

[flemsteele]

This may be a stupid question? Any chane the coils can be re-piped ?

[tab-tech]

Originally posted by flemsteele
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.

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.

[sirtab]

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.
http://www.colmaccoil.com/
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]

[flemsteele]

I believe your gonna lose about 25%-30% efficiency between the two . My memory is a little foggy though?

[tab-tech]

..........concerning the high humidity levels in the building (typically 70 - 75%) they have finally decided to change all chilled coils to counter-flow arrangements.
The biggest wrench in the works has been the controls contractor stating that IF the paralleled coils was the problem with high humidity levels, then one would expect to find ONLY those areas served by the coils in question at high moisture levels.
I enlightened him that 6 out of 9 coils are definately piped parallel flow and that 2/3 of the building being impounded with high humidity levels would very easily migrate that effect throughout the entire building envelope.
The remaining 3 coils that are not confirmed could very well be backwards too.
He kept going on about the air being 100% saturated at 55 degrees and that changing pipe arrangement to counter-flow will not do anything but waste time and money.
We'll see..................... ;)

[sirtab]

Not a waste of money they will get their money back in energy savings. Cross your fingers on the humidity, hope you didn't stick your neck out too far. You could end up a hero though. Why would the controls contractor give a $hit?

[NormChris]

The problem with parallel flow is the two flows reach approximately the same temperature about 75% of the way though the coil. That leaves the last 25% of the coil doing nothing much. The previous statement that you loose about 25 to 30% of your heat transfer area is correct.

Repiping to counterflow is the correct solution. This applies to water cooled condensers as well as water to air cooling coils. It does not apply to DX coils!

Norm

[dmedley]

trane is correct on thier info of the coils. however the whole reason for counter flow is so that the air will see colder water on the leaving side of the coil to help remove the moisture from the air. on parralell flow the air sees warmer water on the leaving side of the coil and less humidity is removed. the only way you can remove close to as much humidity as counter flow is if you increase gpm through the coil. if all the coils are piped this way and sizing on the units(tonage) is tight you could definitely see a dramatic humidity problem in the building. recommend reducing the amount of air coming thru the outside air dampers as possible until problem is solved.

[tab-tech]

....... in the efficiency of the coil performance can be realized in the removal of latent heat, that the current 70 - 75% humidity levels will reduce to 40 - 50%. That would be sweet.
Could anyone please explain what the controls guy was possibly referring to in his statement about the 55 degree air leaving the coil being nearly 100% saturated and how that might correspond with humidity levels and his claim that changing the coil piping arrangement won't effect the concern?

[Islander]

so what happened when they lowered the economizer temp setpoint from 67 degrees? just curious.

[tab-tech]

Originally posted by Islander
so what happened when they lowered the economizer temp setpoint from 67 degrees? just curious.


...... from the high 80 to 90 percent, to the present 67 percent. They are installing enthalpy control which will help too. This morning was 60 degess with a heavy fog bank that didn't burn off til around 10 am, then temps jumped to high 80's.

[tab-tech]

....that the sensed outside air temperature was 10 degrees below the actual measured at any given point in time.
That probably doesn't help matters when free cooling is being allowed at 10 degrees higher then design condition intent.

[sysint]

Originally posted by tab-tech
....... in the efficiency of the coil performance can be realized in the removal of latent heat, that the current 70 - 75% humidity levels will reduce to 40 - 50%. That would be sweet.
Could anyone please explain what the controls guy was possibly referring to in his statement about the 55 degree air leaving the coil being nearly 100% saturated and how that might correspond with humidity levels and his claim that changing the coil piping arrangement won't effect the concern? -because he doesn't understand what happens when that air dumps into a room. Hand him a psyc. chart.

[cxagent]

Originally posted by tab-tech
....... in the efficiency of the coil performance can be realized in the removal of latent heat, that the current 70 - 75% humidity levels will reduce to 40 - 50%. That would be sweet.
Could anyone please explain what the controls guy was possibly referring to in his statement about the 55 degree air leaving the coil being nearly 100% saturated and how that might correspond with humidity levels and his claim that changing the coil piping arrangement won't effect the concern?

I'll take a stab at what he was saying. If you look at a psychrometric chart the humidity ratio (lb water/lb dry air) is roughly the same at 74F & 50% RH as it is at 55F & 100% RH. (The easier way to say this is the dew point is 55F. That way you don't have to keep track of the decimal point and number of zeros to talk about humidity ratio.)

Now I'm assuming what his point was - If the air was cooled to 55F, the dew point could be no higher than 55F. So no matter how the coil was piped, the air leaving the coil had been dried to 55F dew point. That 55F dewpoint is a common design condition.

Now what seems to be happening to the building and Islander was driving at - The 55F dewpoint will be the lowest dewpoint your system/building can ever reach in a humid environment (humid enviroment cannot dry to the outdoors). And it will only reach that if there is little to no moisture generated (people) or moisture leaking into the space. In humid climates, or where there is a lot of internal moisture generation, or leaky building, the air leaving the coil must be DRIER to offset the moisture in the space from other means.

I hope that helps. Also, the 25 - 30% difference in efficiency will not be a 25 - 30 % change in RH. You have to look at the psych chart for a 25 - 30% change in heat transfer. It takes a lot of heat transfer to remove a small amount of water from the air.

[Islander]

single or double sensor enthalpy control along with calibrated outside air temp sensor ought to help nicely
given don't have the the leaky bldg, negative space pressure, too much min o.a., not cold enough coil for long enough, correctly functioning economizer dampers..
can't think of anything else that could be a drastic cause offhand.

Keep posting the cure i'm learning from this one.