Recently we had a customer that was needing to “hold the blast” on a structure that he was blasting and painting. This is typical when you are doing large tank projects, as it allows you to completely blast the tank, clean up once and then paint.
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What you are trying to control is the atmospheric Relative Humidity (RH) inside the tank. This measurement is calculated as a percentage, and, in order to “hold the blast” you need the RH to be below 50%. That’s really where the size of your DH unit matters.
In this application you’re trying to protect steel because it’s bare after being blasted. If you turn the air over in that space 2-4 times an hour through a Dehumidifier, it’s going to be fine. In preparation for painting, keeping the relative humidity low enough in that space (provided it’s controlled and not opened to the atmosphere for an extended period of time) will protect you.
Let’s put those 2-4 air changes per hour into a real-life scenario. Assume you have some sort of structure that is 30 feet X 25 feet X 20 feet.
A 1,000 CFM machine will turn that air in that containment over 4 times every hour.
Now, four times per hour is pretty aggressive for most areas. What you’ll find is that no matter how big the tank is, it’s probably not going to be as many CFM as what you would expect. If the space is very air-tight, you may get away with more like 2 to 3 air-changes every hour.
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Of course, there are other considerations and limitations that you’ll need to be mindful of. As an example, there are two basic types of Dehumidifier units, one that utilizes a Desiccant wheel and the other that uses refrigerant. A Desiccant style can be used ANYTIME while the Refrigerant style cannot be used below 55F degrees.
Let’s talk about using a refrigerant type for a second. Now, it’s a little bit different when using a refrigerant based dehumidification. Slightly different than cooling. As mentioned before, it will work down to 55F. An air conditioner typically will stop removing water at 60-65 degrees. So, if you’re in a really hot environment, and it’s going to stay hot, you could use an air conditioner to pull the moisture down. An air conditioner, if it’s sized right, will typically maintain 45% relative humidity in a given space. We can’t cool air without drying it, it’s just not possible by the nature of air. Hot air holds more water than cold air. So, if we take a volume of air and we make it colder, it physically can’t hold the water. It’s just psychrometrics.
So, if you’re in a really hot environment, say 80, 85, 90 degrees and it’s going to stay that way, it’s not going to hurt you to cool it down inside there a little bit. If an air conditioner is the closest thing to the project site, just grab an air conditioner. Just make sure that you size it right. How? You’ll want to do the same type of formula as above to calculate number of air changes. Make sure you’ve got 2-4 air changes in there. And the air condition unit – we’re not using it for cooling at this point – we’re using it for drying, it’s probably never going to satisfy the temperature. In practice, you need a lot more tonnage to cool a space than you do to dry it.
Drying is going to happen the entire time, so much so that you’re going to have water pumping out of that machine non-stop. So, you can use air conditioners for drying, however, you’ll want to be careful about the dewpoint. Check the dewpoint on your DeFelsko DPM-L logger or your hygrometer before you go blasting that tank with cold air. An AC unit is going to put out 50 to 55 degree air to make sure you check the dewpoint in the space. If the dewpoint is 60 on the steel, and we hit it with 50-degree air, the water that’s there is going to condense and it’s going to rust. So, you want to make sure that when you turn on the AC, we’re not going to hit the dewpoint.
Continuous checking of the site environmental conditions is very important and having the correct tools for the job is a big advantage.
If you have some specific issues you are trying to resolve, or if you have some very unusual site conditions, feel free to reach out to BlastOne to get some qualified advice.
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