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28 MARCH 2016 COATINGSPROMAG.COM Specifying Success T he use of dehumidifcation equipment to assist in the blast and coating for interior lining applications on steel substrates has become a norm in the industry over the past 30 years. Obvious benefts to contractors have included increased productivity, decreased costs, increased scheduling accuracy, and reduced warranty claims, to name just a few. Advances in technology, equip- ment, and design have all provided the contractor with state-of-the-art methods for ensuring his or her projects are successful and cost efective. As a result of these benefts, many engineering and inspection frms have added dehumidifcation as part of their specifcation to ensure that the projects go on without a hitch and the owner receives the highest value from the coatings application for which he or she is paying. Unfortunately, it seems that over time, things have been simplifed, and confusion occurs when specifying the use of dehumidifcation. Tere are several logistical difer- ences within the dehumidifcation (DH) community, such as equipment place- ment, air change rate, and equipment type, but one idea seems to create the most confusion in the DH community and beyond: It is the dew point spread and what is appropriate to hold a blast. The Dew Point Spread We use the science of psychrometrics to measure the dynamic properties of water-air mixtures. It is used every day in the coatings world to ensure that a blast does not fash and the coating is applied at the right conditions. Psychrometrics can be used to predict and help plan for any potential work schedule conficts due to weather as well as design for specifc equipment on the site. Psychrometrics takes the guess work out of deciding when to start and stop the blasting or coatings process. Waiting for the sun to rise is not a good practice to determine one's paint schedule, for example. It is with psychrometrics that we can measure and specify a dew point and the size of the spread between that specifed dew point and the temperature of the substrate, for that matter. Te defnition for dew point is the temperature of the air at which the moisture in it contains reaches 100 percent relative humidity (aka satura- tion) and moisture begins to condense on nearby surfaces or suspended dust particles. Any surface cooler than the air dew point will result in condensa- tion being formed on that surface. For example, if the dew point on a steel tank is 40° F (4.4° C), and the steel tank surface is 40° F (4.4° C), then you should expect to see condensation beginning to form on the surface. You should then also expect that this condensation might lead to fash rusting of freshly blasted substrates or contamination of the coating materials during curing. In this scenario, you could then expect a poten- tially costly solution to a problem that may have been prevented. So, what can be done to prevent this situation? Te answer is to maintain an acceptable diference between the dew point inside the tank and the surface temperature of the steel. Controlling either one will work. However, it is much more difcult and costly to adequately raise the surface temperature of the steel than it is to lower the air dew point inside. Te question becomes then: W hat is an acceptable spread between the two? Specifying the dew point spread has often been muddy at best. Te confusion seems to come from the information that is available to the industry. Most By Russ Brown, TCS Sales Director for Polygon Don't Gamble on Specifying the Dew Point Spread Photo courtesy of Polygon