Humidity/Ventilation Control for "Car Wash"

[Harvin_K]

Hello,

I'm interning at a large automotive manufacturing plant, and I'm working on solutions for controlling humidity in one of our equipment booths. Each of our two booths houses a "Waterleak Tester" which is a set of spray nozzles that sprays water onto the cars for about 80 seconds. The equipment itself is stainless steel, and hasn't deteriorated. The issue is, the building (booth) that houses the equipment was not originally designed to do so. Mild steel columns and trusses were used, with no protection. These have corroded to the point of needing reinforcement. We have repaired and re coated to structural members of the building, but I'm looking into solutions that would significantly slow down the corrosion going forward.

General Info:
- 2 booths beside each other, with an open man-door in between the booths.
- Roll up doors to maintain somewhat of an airlock between outside the building and inside.
- 4800 CFM roof mounted exhaust fan that is shared between the two booths.
- each booth is about 31ft long, 16ft wide, 24ft high.
- Vehicles using the equipment continuously for 16hr a day.
- Temperature set to 22 deg C (heated make up air)
- 4000 CFM heated make up air per booth (only when temperature is below setpoint)

I've been looking into either specifying a dehumidifier or a beefier exhaust fan to help reduce the moisture, which in theory should slow the rate of corrosion down.

Can anyone help me decide which soltuion(s) is best? I'm pretty inexperienced with HVAC, and would appreciate any help.

Thanks.

 

[DBronson]

Harvin_K:

It sounds like the moisture laden air is escaping the booth and causing the problems with the plant steel. I would seem to me that the booth walls need to be made vapor tight. You might perhaps look at installing a spray applied vapor barrier Link on the insides of the booth and take steps to ensure any penetrations through the walls and ceilings are also sealed. Where the walls meet the floor should be carefully sealed. Also, I'm assuming that the the exhaust system is adjusted to keep the booths pressure slightly negative relative to the plant's pressure. Again, if anything, you want the plant air to tend to leak into the booth - not the booth air to leak out. If there's significant moist air leaking out of the booths when the doors are open a simple collector hood above each door and tied to the exhaust system would help capture any leaked air. Are vfd's provided on the exhaust fans?

Regards,

DB
NB: I would think dehumidifying that amount of air would be pretty expensive - especially on the operating cost side.

 

[Harvin_K]

DBronson:

Thanks for the response, I appreciate the help.

My apologies, I don't think I explained the layout/setup of the booth well enough, but we arent having issues with moisture leaking out of the building (booth) where the water leak testers are installed. Please see the attached picture for a better idea of the equipment setup. I took the picture from outside the building looking in. Those set of arches you see in the picture are referred to as the "Tester" and the building which houses those arches is referred to as the "Booth".

We are having troubles with exposed structural columns and overhead trusses in the booth which are exposed to overspray and a humid environment 24/7. My thought is that by installing a dehumidifier, we can allow the booth to at least dry up for the 8 hours a day that its not in use. I don't expect it to keep the humidity way down during prodcution hours, as that would require a massive dehu.

I could contract this out for a third party firm to look at it, but being an intern, I thought it may be a good learning experience if I could learn more about HVAC and figure out a solution in house.

I am also looking into reducing soluble salt levels in the booth, as their hygroscopic properties pose a big problem for the coating on the structural members of the building.

If you have any suggestions or insight on how to reduce corrosion, given our situation, I would appreciate it.


You are correct, there is a slight negative pressure in the booth compared. The exhaust fans do have VFDs installed.

Thanks,

Harvin

 

[EnergyProfessional]

Are the corroding embers being sprayed on, or are they corroding from humidity only? I suspect some droplets will hit them no matter what. so dehumidifying won't do much. and you mention salt, once salt is deposited, a dehumidifier won't help.
and drying during off-times will depend on the ratio. Are they only spraying once a week, then yes, it may help. If they spray daily as i suspect, having some night drying time won't help much. it may delay the inevitable by some short time.

Based on your picture it looks like sprayers are int hat plastic tunnel. You may look into exhausting air out of that tunnel, and supply fresh air outside that tunnel. That way less aerosol will travel to the structure. Continue running exhaust after the sprayers shut off until all water on the floor is evaporated. i guess deploring some really huge dehumdifiers will help. you also could look into some good epoxy or or other treatment of the structural members.

 

[Harvin_K]

The corroding members are not being spray on directly, however they do get a decent amount of over spray when the equipment is running. I mentioned the salt because it is another area we are looking into, but I understand that dehumidifying wont address that, we are looking into interval based maintenance to spray them with the soluble salt solvent for starters.

These booths spray pretty much continuously for 16 hours. Every car that is built has to go through these testers, so it is essentially always on. There is no production from 1 am to 6:30 am, so that would be the interval where the equipment stays off.

The corroding members were recently vapor blasted to near white and coated with a cold galvanizing compound, and an epoxy topcoat. We conducted soluble salt tests before vapor blasting and coating. My goal is to ensure that the coating lasts as long as possible. These structural members were sandblasted and coated back in 2013, and have since failed significantly. Coating these members only dealt with the symptom. My hope is that the dehumidifier or exhaust would help deal with the root cause.

I have played around with the idea of routing an exhaust to the top of that plastic panel, and cutting a hole in the panel to allow the duct to exhaust air directly out of the water leak tester (spray chamber). You can see a concept image of what I am talking about in the attached picture. My issue with this concept is that if we exhaust air while the equipment is spraying water onto the vehicle, it could potentially alter the point where the nozzle is spraying because of the air being pulled into the exhaust. Each nozzle is specifically calibrated to hit a certain spot on the vehicle for quality purposes.


Also, in the picture I sent in my last post, there are roll up doors at the entrance and exit of the booth, which stay closed when the equipment is running.