Monday, August 15, 2011

Fundamentals of Cleaning: Fundamentals of Cleaning: Drying, Part 1

Critical cleaning professionals know that drying is one of the latter stages of the work called cleaning. Simply put, drying is typically the step of cleaning water from parts.
Many believe that "evaporation" and "drying" are the same. As a method of liquid removal, however, evaporation often presents serious concerns. There are other methods of removing water from parts.
In this and the following columns, we'll look at drying of parts from a mechanistic point of view.
Water can be removed from parts in three ways. This is because water can exist in three distinct phases: solid, liquid, and vapor. Drying can be thought of as being a process of phase transformation (and removal of the newly formed phase). For example, evaporative drying is removal of liquid water by converting it to a vapor.
Freeze drying plays an important role in food processing. It is also some-times used in cleaning some piping systems or small complex structures. In freeze drying, the temperature is dropped to <32 °F. Liquid water is transformed to solid water. But the drying isn't complete until the "soil water" (ice) is removed from the food (or parts). And there's the rub. If ice particles can be removed, usually via entrainment in high velocity air, the structures can be made totally free of water. But this can be difficult.
Liquid drying, an infrequently used technique, involves force--often, gravity. Because liquid droplets have more mass than the same volume of air, liquid droplets drain water from parts. Water, which has a very high surface tension, tends to form in larger droplets, films, or sheets. If these agglomerates of water are not trapped by some convex characteristic of the parts, large amount of water will drain wonderfully well. Some have harnessed surface forces to literally extrude films of water, in "sheets," from parts.
The force of gravity can be enhanced by other forces. If the parts are vibrated (and possibly rotated), sheets of water can be quickly dislodged. Usually, about half of all the water on parts can be removed through accelerated gravity-based drying.
The most common force used in drying is impact by high velocity gas, usually air. One firm makes a nozzle called a "Wind Jet" that accelerates compressed air to a velocity around 1000 ft/ sec. Pointing this nozzle at a wet part can dry it in less than one second! My data show this method of drying evaporates only ~10% of the water removed. The quality of drying achieved, called "dry to the touch," may be perfectly adequate, or a starting point for a second phase transformation.
Conversion of Liquid to Vapor (Evaporation) To most of us, evaporation is drying. Evaporation can be time-consuming. It can make the numbers on your electrical bill look like the national debt. Furthermore, soluble minerals in the water wont evaporate, but will remain as deposits (water spots) on your part surfaces.
The rate of drying parts is limited by the rate at which heat can be transferred to the water, causing it to evaporate. Slow heat transfer from gas to wet parts is normally the limiting process step. Though slow, evaporative drying is the best way to produce total dryness.
Said another way, gas-to-solid or gas-to-liquid heat transfer coefficients are quite low. They are a function of the physical properties of the air, temperature, and the air velocity. Most users expect that temperature has more effect on heat transfer coefficients than does air velocity. But the reverse is true. As shown in Figure 1, temperature has only a lightly negative effect on heat transfer coefficient. It is increased air velocity, not temperature, that can make the difference in drying rate. Air moving at 5 to 50 ft/ sec removes little water by impingement--just the opposite of air moving at 1000 ft/ sec.

Fig. 1. Drying coefficient varies little with temperature.

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