Commercial HVAC Cooling Coil Performance Issue

I am not an HVAC engineer, so I'm just guessing here from my home central air experiences. Can you measure your water temperature before and after the exchanger? Maybe you are getting some hot air leaking into the cooled air downstream of your heat exchanger.

First of all, I was in Austin last weekend and it was HOT! That might have something to do with it.

Second, south Texas has an abundance of calcium and magnesium ions (commonly called hard water). Although you didn't find anything in your inspection there could still be a layer of this material adhering to the interior walls of your coil strangling flow and hampering proper thermal transfer. Does your plant utilize a water softener system? I'm no A/C expert either but maybe this will give you another avenue to explore.

Do you have any historical records? Is it getting worse, or was it always like this? How long has the plant been working? As has been suggested, can you check the inlet & outlet water and air temperatures?

It's also been pointed out that it's dam' hot down your way - is your plant up to the job?

How about coating the entire unit with an Energy Star 'Cool Roof' coating that is also used as a paint? Raise the solar reflectivity and emmissivity of the exterior surface of the unit and the air inside will not absorb that heat and it will stay cooler.

I have been coating exposed AC units and ducts since 1988 and have dropped the air temp coiming into the buildings by 5 to 25 degrees. The least I have ever done is 5 degrees, the most was 25 degrees.

We have several statements and pictures on our site. Of course, this goes against the laws of physics but, that would not be the first time we did that.

http://www.ct-texas.com

Hal Skinner

Is the coil piped correctly for countercurrent flow of air and water?

Do you have info on water temp in and out of the coil, and gpm; and air temp on and off the coil, and cfm? Also outdoor air temp and RH (or wet-bulb temp)?

Need a bit more data.

What is the supply temp you are getting after the primary coil?

What is the entering and leaving CHW temps?

When you say that outside design conditions appear to be in range- is that dry bulb temp only or do you have both dry bulb and wet bulb data- if so, what is it? Higher than programmed wet bulb could be causing significantly higher latent load- which will affect dry bulb supply temp (higher than design).

If the unit was designed for 100% outside air, does it have pre-heat coils? If so, you could re-route leaving CHW through those coils to pre-cool air. You will not begin to condense, so no worry about drain pans, but you will take some load off primary cooling coil, allowing higher performance.

One of the major reasons for poor performance of the coil is its heat transfer properties. Few check lists are:- Check the condition of fins;Fouling inside the tubes;Poor heat transfer coefficient;Change in by pass factor; Performance of blower etc. To carry out detail analysis need still some more data.

As I mentioned earlier, I am not a HVAC engineer. I do not understand this chart. How can the leaving air be cooler than the leaving water. Unless, the leaving water is making contact with the entering air while the entering water is making contact with the leaving air.

Air and water are counter flow- warmest air is contacting warmest water (which is still cooler than the air) so air is being continuously cooled as water is being continuously heated.

The two conditions shown are "normal" peak load- high dry bulb temperature with humidity level consistent with that temperature and "high humidity" load where air is "cooler" but humidity level is much higher.

High humidity air has much higher total energy per unit. In HVAC, we have two forms of energy- sensible (the temperature difference between two items times specific heat times mass flow) and latent heat (the difference between the energy contained in the water vapor in the air at the start and at the end)- much like when steam turns into water. The cumulative value of this energy is called enthalpy.

Humid air has a much higher latent load than drier air, and latent energy is a much bigger portion of the overall enthalpy. For example- air from 98F with 74F wet bulb to air at 52F with a 51F wet bulb has a sensible load of 49.7 BTU per cubic foot but has an enthalpy load of 75.1 BTU per CuFt, so the moisture load (latent) is 24.4.

The same numbers using 89F with 78F wet bulb air yields a sensible difference of 40.0 and an enthalpy difference of 93.1 BTU per CuFt, so the moisture load (latent) is 53.1.

Good morning, MrGeneRall.

1. It appears your coil is designed for a maximum of 98º Entering Air db. If in an exceptionally hot few days of say 108º db of outside air has existed, one might expect the Leaving Air to be ~62º. Might this adversely affect your Inside Air?

2. I don't suppose you have the luxury of adjusting a damper to temporarily recirculate a certain portion of Inside Air during these unusual hot few days?

If the db was in excessive of 98, I would not expect to get the 52 degree db discharge.

Unfortunately, I do not have the luxury of recirculating the air as I am trying to maintain positive pressure on manufacturing floor space and I must use almost the entire volumetric capacity of the air handler to offset my losses from building exhaust.

Thanks for the suggestion though.

Based on the sensible capacity indicated for case 1, you are moving about 22,500 SCFM of air.

Based on the "current" air temps given in your Post #23 below, your inlet enthalpy (h) is about 34.56 BTU / Lb and your leaving air enthalpy is about 23.32 BTU/Lb.

Using the formula Q = SCFM x 4.5 x Delta-h, you are currently doing about 1,138,050 BTUH of cooling.

Assuming that CHW flow is say, 15% above the max indicated of 199 GPM (or 230 GPM), and using the indicated CHW DeltaT from Post #9 of (54-45) or 9F, it is doing GPM x 500 x DeltaT or 230x500x9 = 1,035,000 BTUH (damn close to the airside values).

Since the air side values AND the water side values are reasonably close, and between the rated capacities of 1118 to 1391 MBH, it looks like your system is essentially doing ALL that it can.

Going back to the ratings you posted, 98FDB/74 FWB has an enthalpy of about 37.577 BTU / Lb and 89FDB/78FWB has an enthalpy of about 41.501 BTU / Lb. Defined leaving air of 52FDB/51FWB has an enthalpy of about 20.878 BTU /Lb.

Based on these values, your COIL load should have been listed as

Case1- 22500 x 4.5 x (37.577 - 20.878) = 1,690,774 BTUH (only 51% off) that would require 242 GPM of CHW at 45F to 59F.

Case2- 22500 x 4.5 x (41.501 - 20.878) = 2,088,079 BTUH (only 50% off) that would require 298 GPM of CHW at 45F to 59F.

Net effect- your lawyer needs to send a very nasty letter to your consulting engineer, who is going to have to pay whatever it takes to meet your loads. AND- needless to say, find another engineer for any other projects that you have coming up.

Not what you want to hear, but at least now you know that you have been chasing an impossible dream- If you got by last year, it is because, like us Midwest Yankees, you had a "good" year in 2009.

By the way- doing the repipe I suggested for your "per-heat" coil will at least "help" you get to where you want to be.

You might also add an inline pump (rated near the higher values- check your current pump curve to see how far out in GPM you can go) between your current CHWR line and the connection to the "pre-cool" coil to boost CHW flow as well (be sure to add the pressure drop for the higher flow (DeltaP changes as the square of the ratio between new and old flow)- the existing circ pump at the "pre" coil might have to be changed as well.

Good morning energygod.

For a novice, would you pls explain how you arrived at the 22,500 SCFM figure? Thx.

In post 8, the design operations for Case 1 were- 98F Dry Bulb cooled to 52F Dry Bulb- a 46F differential.

Additionally, rated sensible capacity was 745,000 BTUH.

1.08 is a constant for CFM that includes air mass flow rate (based on standard air density), 60 minutes per hour, and air specific heat value that yields BTUH when multiplied by SCFM and Air DeltaT.

745,000 / ( 46 x 1.08 ) =OH SHIT, I SCREWED UP AND HIT THE WRONG BUTTON ON THE CALCULATOR. (That's the problem with electronic calculators- no print out to verify proper input data)

ACTUAL SCFM is about 15,000 not the 22,500 that I calculated earlier. Anyway "flyinghigh", that is the way you can "back into" a fans performance.

DAMN- That totally blows my previous posting.

OK- TOTAL REWRITE OF MY PRIOR POST.

FIRST- At 15,000 SCFM, the design thermal performance would be inline with data provided- MY calculation was 50% off.

At 15,000 SCFM, the load (based on the inlet and outlet air data provided) would be 758,700 BTUH and the required CHW flow (based on the posted actual DeltaT) would be 169 GPM. THIS IS MUCH LOWER THAN THE "DESIGN" PEAK WATER FLOW.

Comparing enthalpy difference of "current" inlet air and Design 52 FDB/51 FWB leaving air (13.68 BTU per Lb) with Design (Case 1) inlet to leaving air difference (16.70 BTU per Lb) yields an 81.9% ratio. Comparing GPM calculated (properly this time) with current coil performance (169 GPM) and the peak design GPM (since the poster indicated that the CHW supply valve was at Full Open) of 199 GPM, that is 84.9%.

Since the current CHW flow is at maximum available, and since the current coil performance is reasonably equal to the same %-age deviation of performance as the CHW flow appears to be, it looks like the culprit is LOW WATER FLOW. IF the water flow was as high as it SHOULD be, the current CHW DeltaT would be much lower.

MrGeneRall- You have not stated that this AHU was supported by a dedicated CHW pump. Assuming that it is fed from a central distribution system, several of the suggestions from my (somewhat blown) earlier post are still valid.

You need to boost flow through the coil. Adding a local in-line booster pump on the downstream side of the coil is likely the easiest way to accomplish that. Be sure that you calculate the TOTAL added head created by the increased flow rate. I would select the new pump for maybe 10% over the current peak design 199 GPM (say 220 GPM) and let your CHW valve control actual flow through the coil.

After the new pump is in place, and you can see how the coil is working, you may STILL need to use your "pre-heat" coil as a booster cooling system. Pipe it as I indicated with a possible second pump change-out.

Also- the issue of the possible slightly active reheat coil, based on the air temperature numbers and valve position I thought I saw on your flow schematic print-out, is still an issue.

SORRY for the busted calculation indicating an even BIGGER problem.

Upgrading and expanding on my earlier statements, and trying to help-

Case 1's design inlet air has an enthalpy of 37.43.

Case 2's design inlet air has an enthalpy of 41.47.

The design leaving air enthalpy is 20.86.

Based on 15,000 SCFM supply, Case 1 would have a load of 1,118,000 BTUH and Case 2's load would be 1,391,000 BTUH.

The "current" reported entering air has an enthalpy of 34.78 and the reported leaving air has an enthalpy of 23.84, which produced the 758,700 BTUH load referred to earlier.

Based on the reported DeltaT of the CHW (8 - 9F), the calculated load of 758,700 BTUH will require 169 (9F) to 190 (8) GPM of CHW.

758,700 BTUH is the "current" performance, which is 66% of Case 1 peak and 53% of Case 2. IF the recorded entering air was cooled to the DESIGN temperature of 52/51, the load would be 939,600 BTUH. That potential load is 84% of Case 1's 1,118,000 BTUH and 68% of Case 2's 1,391,000 BTUH, AND the entering CHW temperature is the design 45F, so- since 169 is higher than the 160 GPM indicated for Case 1 and 190 is very close to Case 2's 199, there is NO reason that the coil should not be able to meet the current load.

Assuming that the design maximum coil DeltaP for the CHW would occur at the maximum 199 GPM, 169 GPM should produce a pressure drop of 10.8 Feet (4.6 PSIG) and 190 GPM should produce a pressure drop of 13.7 (5.9 PSIG). Therefore, the noted DeltaP of 6-6.5 PSIG is higher than should be seen.

Using the average flow of 180 GPM (DeltaP of 5.0 PSIG), and an average observed DeltaP of 6.25 PSIG, the observed pressure drop is 125% higher than it should be. In order for the flow to produce the indicated pressure drop, its velocity must be 11.8% higher than "normal" so the opening of the coil tubing must be 11.8% smaller. Using the formula for area of a circle (D²/4 X Pi), that means that effective D²/4 must be 89.4% of normal. Assuming the standard 5/8" OD tubing typically used for coils, that means an inside diameter of 0.447" compared to a typical 0.500", yielding a potential film of 0.026".

Using the 66% performance, based on Case 1, and a chart describing the effect of fouling factors, the indicated fouling factor for this coil is 0.0032, vs. the standard "clean" fouling factor of 0.0005.

Treated CHW does not NORMALLY get any film, but something may have gone wrong.

Once a coil or other HXU gets fouled, typically the only solution is some form of physical brush cleaning. Several small bore brushes on long flexible shafts are available. BUT- you still have several months of hot weather to go before you can clean this coil.

Which goes back to an earlier suggestion to "use" your Pre-heat coil as a pre-cool coil.

Another ADDED action is to turn down your chillers set-point and start making 42F CHW (to give the log-mean-delta-T a significant boost) OR you could rent a small chiller and use a primary-secondary connection to just sub-cool the water to this coil to 40-42F for a few months. 190 GPM cooled from 45 to 41 would be 30 tons.

Another option, since renting a chiller is suggested, is to rent a "dedicated" chiller for this installation and circulate directly into and out of just this coil. You could use a glycol mix to make 30F brine (a 125-ton standard air cooled chiller would do it) to get cold enough water to work "past" the film.

Totally beyond THIS problem- if your CHW system is supporting OTHER systems, are they still operating without problem, or has this been you biggest issue?

CHW Supply Temp = 45 Deg F

CHW Return Temp = 53 to 54 Deg F

Discharge Air Temp = 57 to 58 Deg F

So it looks like your water is drawing the proper amount of energy out of your airflow. The last pieces of the puzzle maybe the entry air wet and dry temperatures.

You are correct in that the problem is in the coil.

Based on the selection data (both "normal" and high humidity) you should be able to make 52F air. The fact that your return water is cooler than design means that it should be even easier to make low temp air, but it also says that you coil is not currently capable of producing enough cooling effect to heat the return water sufficiently. The lower CHW DeltaT essentially matches your lower supply air DeltaT, which again points to the fact that the coil is not performing as "designed".

Since your original post did NOT state that this system USED TO meet its loads, but now has a problem leads me to only one possibility- the coil was not built with the correct arrangement of piping circuits necessary to present the correct mix of cold water surfaces to the airstream.

My first recommendation is for you to pull the submittal drawings for the AHU and verify that the coil was submitted to meet the loads. If you do not have those forms, your consulting engineer certainly does. Then, if the submittal says that the coil should perform as specified, contact the coil manufacturer- which MAY be the same as the AHU manufacturer- and present your operating conditions compared to the design criteria. If the submittal does NOT say the coil should meet the specs, have your attorney contact the consulting engineer about your problem and demand that THEY fix their breach of fiduciary duty by allowing non-conforming materials to be installed at you site.

The AHU manufacturer and the coil manufacturer "sold" you a product that was supposed to meet the specifications and does not. That is a breach of contract that they must fix, or literally "pay" for that violation. You will want to contact your attorney and have them draft your letter to the manufacturers (both the AHU and the coil) as a way to show them that you mean business. By the way, breach of contract is not a warranty issue covered by a one year time period.

While you are waiting for resolution- and your attorney's letter will definitely get things moving quickly- you can go back to my earlier suggestion and repipe the preheat coil to add some more cooling surface and get you closer to your desired output.

I know this is going to sound bad, but I think the system performed as designed last year.

I know - hey it has been a long year, I'm severely understaffed, roll excuse no. 34572, etc.

The coil has been in service about 2 years and some change. I need to rule out the other possibilities before I do as you have suggested. Good suggestions by the way.

So.....it sounds like I need to focus on some film on the interior of the coil?

I will be doing some bacteria slides later today and should have the results by Monday. This should let me know if a bioogical film is the culprit.

Check with US Weather Bureau RE- temperatures for last year compared to this year.

Not sure where you have been, but WE have seen significantly higher temperatures AND dew point (wet bulb issues) here in Ohio and around the Midwest. Last year, we had 3 "heat emergency" days- this year we have had 29 (with more to come next week)- both high temps AND high humidity.

You MIGHT have done OK last year because you were not being "pushed".

Go ahead and check the water quality, but I don't think you will have a problem- film inside coil should affect pressure drop significantly, and you have not indicated any problem there.

You didn't mention measuring the pressure drop on the water side to assure that you have the specified GPM of chilled water. If you haven't done so, I would recommend having a certified balancing company check the parameters to be certain that both air and water flow are correct. It's not rocket science but field measurements, particularly airside, by personnel who are not TAB certified can be misleading. I assume you have 30% or so concentration of glycol on the waterside for freeze protection. That diminishes coil performance significantly.

chw absorbs heat as it passes through the coil. if the leaving water temp exceeds 52'f increase you flow rate. if your latent load exceeds the design you lose capacity to latent(humidity) load. I believe you posted design spec's but put the entering leaving water and air reads on line and add your indoor/ outdoor db/wb and we have your answer imediately. sounds like your cooling with oa. if this is right you would usually be running the hell out of your reheats with 52 leaving. you hit my specialty with this one, give me all your data and i'll give you your answers gratis.

I am running 100% water with a corrosion inhibitor.

I have no way to increase flow. CHW valve is at max and there are no restrictions that I have been able to find.

Pd across the coil is running about 6 to 6.5 PSI - right inline with the 15 TDH listed on the design spec.

Getting other info right now - back soon.

Your reheat coil 3-way valve is indicating partially open- very hard to read %-age. Looks like 54.83 off CHW coil and 58.34 after reheat coil.

If you connect CHWR to preheat HWS upstream of pump, and then connect HWS between coil and pump tee back to CHWR, and run pump- you should begin the "pre-cooling" process. Air temp will not drop enough to start condensation, so no worry about drain pan at "pre" coil.

During "cold" weather, use automatic shut-off valve to isolate CHW from "pre" coil below some temperature- 60F, whatever. "Pre" hot water valve will stay closed because air temp after "pre" coil will still be well above actuation temperature.

Here is what I am running right now:

OA DP = 65.05

OA T = 82.44

OA H = 55.94%

EAT Chilled Water Coil = 82.81 Deg F

LAT Chilled Water Coil = 56.83 Deg F

Chilled Water Valve = 100% open

Definitely go back to Mfr. and engineer and get resolution.

when your bug test comes back you can eliminate fouling. it does not appear to be the issue because your lat and lwt indicate a good rate of transfer.if there is no fouling the answer is to increase the flow rate of the water or decrease the flow rate of the air. i am assuming your strainers are clear and all valves are open and unobstructed. some pumps give you more when you over speed (vfd) most won't but the manufacturer can tell you that. your fps is low, 3fps won't competely push out all the air bubbles (min 4.25) and your reynolds number is low. 5.4 fps =2500 optimum on a reynolds scale. you can go to 8fps but after that your rate of heat transfer falls off rapidly. Reynolds is a measure of turbulence which is required for good heat transfer. your specification staes that the design is for 59'f leaving wat. your lat cannot be lower than this because it would start giving back once it went above 52. check your sensor calibration, I see a minor discrepancy. You are running a small pump so even if you had to replace with lager it's still a relatively minor cost. @ 360 gpm I would assume that it's 15 hp with a cut from a 10" impeller with 5" supply and return mains. For a quick easy one I would add 40% to gpm and head put on a vfd and reconfigure my controller to push the vfd according to lat. It's a few hours and 3-3.5 k if you are doing in house. check the cv on your valves if you make the change, should be around 110.

After three days, it looks like I have an Iron Related Bacteria issue.

Last night we administered a gallon of biocide to our Chilled Water System (which will be followed by additional corrosion inhibitor in two days).

Waiting to see if performance improves.

Will keep you posted.