Cooling Coil Fouling

[DouginMB]

I am investigating a cooling problem at a local hospital. They have an air handler and associated coil that is about 24 years old. The original specs are as follows:

Airflow = 48,000 cfm
EAT = 80 degrees F
LAT = 50 degrees F
Fluid (Water) = 42 degrees F
Fluid Flow = 380 gpm

The actual measured airflow is about the same.
The EAT is approximtely 80 degrees.
The fluid temperature is 43 degrees.
The flow rate through the coil is 420 gpm.

The problem is the leaving air temperature is 58 degrees. They recently cleaned the coil (chemically and with a pressure sprayer). The water has been chemically treated all long. I suspect fouling in the tubes, but it seems that such a high discharge temperature would be an indication of severe fouling. Like I said, the water is treated, so i can't believe there would be that much build-up. The question i have is: how much of a deposit build-up in the coil tubes would there have to be to cause such a severe drop in performance? Any thoughts would be appreciated.

 

[25362]

I have some questions that may help one to think about the problem:

a. Since the HT resistance is mainly on the air side, is the air side clean ?
b. Is the air side finned or are the tubes bare ?
c. Is the pressure drop for the increased water flow rate more than expected ?
d. Is there a chance that part of the air is by-passing the cooler area ?
e. Are the measuring instruments (temp., flow, press.) reliable ?

When answering these questions, you may find additional queries yourself and discover other probable reasons for the drop in performance.

Good luck.

 

[speco]

DouginMB,

I agree with Mr. (or Ms?) 25362, but let me elaborate and try to answer your question. It appears that your overall heat transfer coefficient has roughly dropped in half. In order for this to happen, the fouling on the inside of the tubes would have to be very significant. Since you say you are now pumping 420 GPM instead of 380 GPM, the pressure drop should be about 22% higher than in the design case.

There are two possible scenarios which might explain this apparent loss of performance:

1. There has been a loss of contact between the fin and the tube. This could easily cause this kind of loss of performance. Check to see if the fins are loose, if possible. Sometimes it's hard to tell with a platefin type tube bundle.

2. Is the relative humidity much higher than that for which the coil was designed? If so, then the effective temperature difference between the air and the cooling water could be reduced somewhat.

I would recheck the air side to make sure that it is not fouled. Using af flashlight, you should be able to see some light shining through the tube bundle. Note how the tubes are arranged, either in-line or staggered. If they are staggered, then shine the light diagonally through the bundle to check it.

One other possibility: Did it EVER work right? Sometimes designs can be a little marginal, especially in the HVAC field.

Regards,

Speco

http://www.stoneprocess.com

 

[DouginMB]

I completed my investigation of the problem and found that the coil was only partially to blame for the elevated discharge temperature. Fortunately, there were two other almost identical air handlers that I used for comparison. The discharge air temperature I listed in the original post was taken at the air handler discharge (past the supply fan), not off the cooling coil. The coil in question had a discharge temperature of 53.8 degF. Although this is higher than the 50 degF specification, this was similar to the coils in the other air handlers. I couldn't inspect the intake side of the cooling coil because there is a pre-heat coil butted up against it. The discharge side looked clean and in good condition. I wasn't able to inspect the coil any closer because i couldn't shut down the air handler.

It sounds like the rise in temperature from 53.8 to 58 is due to fan motor heat. The 53.8 degF number might be due to leaking control valves at the pre-heat coils, but i imagine some is due to fouling. I just can't find a way to prove it (without cutting open the coil).

Thanks for your input.

 

[quark]

Dougin!

Are you sure about fan motor heat? In that case it comes out to be 217728 Btu/Hr (18 Tons approx.) What is a preheat coil? I suspect something up at the preheat coil. One way is to check the temperature drop and flow of the preheating fluid. Balance the standard equations mCp(Tw1-Tw2)and 1.08cfm(Ta2-Ta1) to check increase in air temperature after preheat(?) coil.

Regards,

 

[DouginMB]

quark,

Each of the three air handlers has a 125 hp supply fan motor. The air handlers are the large built-up walk-in type. The fan/motor assembly is mounted on an inertia pad inside the air handler and is therefore in the airstream. If you reference the ASHRAE fan motor heat gain data, you can see that 125 hp motors emit quite alot of heat. The problem I have is that each of the three air handlers are the same, except the temperature rise across the motors in the other two air handlers is negligible, while the temperature rise in the problem air handler is almost 4 degrees.

The pre-heat coil is there for freeze protection of the cooling coil. If the air hanlders are operating in economizer mode (cooling with outside air) in the winter months, the pre-heat coils keep the cooling coils from freezing-up.

 

[ChasBean1]

With hospitals, I tend to think of humidifiers... is one installed?