Problem Solving Forum
May 24 - May 30, 2010
How do you determine the size of the dehumidification equipment required for a tank lining project?
Don Schnell of DH Tech on
July 12, 2010:
In today’s industrial coating arena, we often find ...read more In today’s industrial coating arena, we often find dehumidification (DH)recommendations thrown out using loose rules of thumb. These are based on standard equipment and the number of air changes that the unit will supply per hour in a tank or space. Air changes are calculated as interior volume x required air changes / 60 minutes = DH capacity in cubic feet per minute (cfm). Where the expertise comes in is in determining what the required air changes per hour are. This, again, is too often drawn from experience (sometimes, very little experience) or rules of thumb based on what might be considered typical conditions.
The amount and type of dehumidification required is affected by project conditions and weather conditions:
Understanding project conditions requires addressing the following.
Understanding weather conditions requires addressing the following.
Consider the most common sizing convention: two air changes per hour. This recommendation is solid if you are using desiccant units in a one-million-gallon tank in Topeka, Kansas, in May. Move the tank to Tampa, Florida, and refrigeration type dehumidification at four air changes per hour is more appropriate, but don’t try to hold the blast too long. A refrigeration unit may be capable of lowering the dew point temperature only a few degrees, and the blast may turn because you cannot maintain 50% relative humidity at the surface. Combine this refrigeration with some pre-cooled desiccant dehumidification and you might be happy with less than one air change per hour.
On a five-million-gallon water tank in Troy, New York, two air changes are probably a big waste of taxpayer money. With this large volume space, the air is stabilized and not as impacted by infiltration. Don’t try to use refrigeration on this job. No amount of cooling will hold your blast when your surface temperature is 40 F.
Let’s say you are in dry dock in Tacoma, Washington. Although the steel temperature is stabilized within the ship, you must consider the dust collector that is sucking all of your expensive air out of the space. If you are holding the blast, you need the surface temperature 17 degrees F above the dew point temperature. Maybe heat is your answer. Now put the ship in the water. What is the water temperature and what equipment will maintain the dew point temperature below the steel temperature below the water line? Depending on the time of year, this surface might be of less concern than the rest of the tank.
There is a misconception that the dehumidification volume must match the dust collector, cfm for cfm. Depending on your choice of DH system, you may be able to allow large amounts of ambient air to mix with the DH and still maintain the proper conditions. Again, what works in Toledo, Ohio, may not work in Tulsa, Oklahoma.
Have you ever been to Towner, North Dakota? The average winter temperature is about 15 F. If you heat the surface up to 40 degrees for coating, it will be 25-30 degrees above the dew point temperature. You might want to think about insulating this tank. To maintain that surface temperature at 40 F, you will need over 110 F inside without insulation.
The other extreme is when the surface temperature in very high. In Tucson, Arizona, a pre-primed tank may not require a wide dew point spread because you may not be holding the blast. Your objective is to control condensation and provide a habitable work environment. Traditional refrigeration may be a great choice. Don’t let the desert weather fool you. 65 F dew points are not uncommon in the summer months. Even if you are holding the blast, your requirements change when you are all primed out and just coating.
So, to answer this question with “it depends” is probably appropriate. Sizing DH may not be rocket science, but it is science. Very different rules apply in a boiler in Biloxi, Mississippi than in a ballast tank in Bellingham, Washington.
Lee Emerson of Therma-Stor LLC on
May 30, 2010:
Typically, the amount of dehumidification for a ta ...read more Typically, the amount of dehumidification for a tank lining project is based on the volume of the tank in cubic feet and the number of air changes required to maintain a dew point temperature at least 5 degrees below the steel temperature. An air change rate of 1.5 to 3 air changes per hour is common when using desiccant type dehumidifiers and 2-4 air changes per hour is typical when using refrigerant-based dehumidifiers. Larger tanks require fewer air changes than smaller tanks. Also, the time of year and prevailing weather conditions influence the type and size of equipment used. Refrigerant-type dehumidifiers work well in warm weather, but their performance drops significantly in temperatures below 60 degrees. Desiccant-type units work year round, but the heat added by this process may be detrimental in some warm weather applications, especially on smaller tanks. An example of sizing a one-million gallon tank for 2 air changes per hour would goes as follows: 1. A cubic foot is equal to 7.48 gallons, so the volume in cubic feet equals 133,690. 2. Dividing the cubic feet by 60 will give the number of cubic feet per minute of dehumidified air that must be supplied to the tank. In this example 2,228 cubic feet per minute must be supplied per air change. 3. Being as the example requires 2 air changes per hour, a total of 4,456 cubic feet per minute is required. Dehumidifiers for the tank lining industry are usually rated in terms of the delivered CFM. For the given example, a 4500-5500 CFM dehumidifier would be used. The number of air changes required may be specified by the owner, but if not then you may wish to consult an experienced dehumidification specialist. If you are renting equipment for your project, the vendor should be able to accurately size the equipment. If you own your own equipment, the equipment manufacturer should be able to offer advice.