"Dry point" is the location in the dryer where a coating first appears dry, even though it is not yet fully dry. It is important, especially with aqueous coatings, because the dry point occurs toward the end of the dryer, after about three-quarters of the dryer length. It is therefore a useful location from which to control the overall drying process, so the final residual solvent in the coating is at the desired level. In solvent drying, on the other hand, the dry point occurs early in the dryer—in some cases the coating already appears dry as it enters the dryer—and therefore its location is not as useful for control.

The location of the dry point is not found by observing where the coating first appears dry, which is inconvenient; it is found by noting where the temperature first rises rapidly. When the coating appears wet it behaves in some ways like a pool of solvent; the drying is fast and solvent can diffuse to the surface as rapidly as it evaporates. This is the constant-rate period of drying where, at equilibrium, the drying rate and the coating temperature remain constant. The coating remains cooler than the drying air because the heat for evaporation comes from the coating, and the lower temperature of the coating compared to the air provides the driving force for heat transfer from the air. Although during the approach to equilibrium the evaporation rate and the coating temperature are not constant but are generally increasing, this is still in the constant-rate period.

In single-sided drying, during the constant-rate period, the equilibrium coating temperature is the so-called "wet bulb" temperature of the air with respect to the particular solvent—usually water. Wet bulb temperatures for the air-water system can be found from the air temperature (the "dry bulb" temperature) and the dew point of the air, using a psychrometric chart. Such charts may be found in a number of engineering handbooks.

Once the coating appears dry the rate of drying is determined by how fast the solvent can diffuse to the surface (or move to the surface by capillary action if air intrudes into the coating) and this is a much slower process and keeps getting slower as drying proceeds. The diffusivity and the drying rate decrease by orders of magnitude as the solvent concentration goes down. This is the falling rate period of drying. And because the drying rate is low, little energy is used to evaporate the solvent and most of the heat transferred to the coating goes into increasing the coating and web temperatures. The coating temperature then rapidly rises to approach the air temperature. This rapid rise of temperature after a temperature plateau or a slow rise is easy to detect.

Infrared thermometers are often used as noncontact devices to obtain a temperature profile in the dryer. Surface thermocouples may also be used but only on the backside, and they may scratch the web. In one case a company was able to use the exhaust temperatures throughout one dryer zone to locate the rapid rise in the temperature of the coating at the dry point.

Drying software can be used to predict the location of the dry point and to set the drying conditions so the dry point occurs at the desired location. Various spreadsheets for aqueous coatings are described in the technical literature, which calculate the rate of drying during the constant rate period, which is assumed to be at equilibrium. The dry point is reached when the residual moisture drops to a certain level. The moisture in the coating at the dry point has been found to be close to 20 percent for a number of different coatings, but the actual value for a particular coating can be specified in the program. Numerous other spreadsheet programs are available. In all cases, the accuracy of the software predictions should be checked against measurements on the dryer. Although a control loop may be used to maintain the dry point at a fixed location, I am unaware of anyone doing this. Dryers tend to be quite stable and so there is less need for automatic dry-point control.

Edgar Gutoff

Consulting Chemical Engineer

617/734-7081

ebgutof@coe.neu.edu

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