Problem Solving Forum

May 24 - May 30, 2010

How do you determine the size of the dehumidification equipment required for a tank lining project?

Selected Answers

From Don Schnell of DH Tech on July 12, 2010:
In today’s industrial coating arena, we often find more

  • Is the applicator attempting to preserve the cleaned surface and for how long?
  • Is the tank steel or concrete?
  • How many openings does the tank have or is it well sealed?
  • Is the tank insulated, contained, or in a building?
  • What conditions are required for coating application and cure?
  • Are there other sources of ventilation such as dust collection?

  • Understanding weather conditions requires addressing the following.

  • What are the expected dry bulb and dew point temperatures?
  • What is the expected wind speed?
  • What are the expected high and low temperatures?

  • 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.

    From Lee Emerson of Therma-Stor LLC on May 30, 2010:
    Typically, the amount of dehumidification for a ta more

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    Tagged categories: Dehumidification; Linings; Tank interiors