Drying is often neglected or under-estimated when planning contamination control strategies or cleaning processes. Adding drying to an established process incurscosts and complexities that could be avoided with appropriate up-front planning.

WHEN AND WHERE TO DRY
When planning the cleaning process, recognize that:

• Particles from the surrounding air can alight onto wet components

• Particles that dry onto a surface can be particularly difficult to remove

Controlling when, where, and how drying occurs minimizes contamination issues. Premature drying can result in adherent contamination. A mist spray can prevent flash drying during tank-to-tank transport. Placing cleaning and drying equipment in the clean-room is often not a good use of costly real estate and may run counter to good contamination control practices. Locate equipment to:

• Insure that the product is sufficiently clean when it emerges from the process

• Prevent the process from re-contaminating an already clean product or to compromise the cleanroom particle specification

A commonly recommended practice is to locate all but the final drying chamber (or final rinse and dry areas) outside the most stringently controlled area. Besides the benefit of reducing contamination in the cleanest area, the more costly real estate is saved for higher-value assembly processes. However, when product is transported from a less clean to a more clean area, it is important to assure that transport mechanisms (e.g., hoists, conveyor belts, and transport baskets) do not contribute to contamination.

TIME AND ENERGY
Drying requires energy and time. Many solvents dry faster than water. Molecules in liquid are bonded by forces; these forces must be overcome by energy to convert a liquid into a vapor. Hydrogen bonds in liquid water molecules are relatively strong; the heat of vaporization at the boiling point is 540 calories/g, more than five times the energy required to heat the same quantity of water from 0 °C to 100 °C. Non-aqueous solvents, generally with weaker, non-polar bonds, have lower heats of vaporization and lower boiling temperatures.For example, isopropyl alcohol has a heat of vaporization of only 166 cal/g and a boiling point of 82 °C. Heating either the component or the drying air, and/or blowing the air, delivers the needed energy in a shorter timebut increases the required energy.

HOW TO DRY
There are many techniques for drying that include:

Air drying (not in a chamber) takes less energy and equipment but a longer time. The longer the time, the greater the chance of re-contamination.

Blow drying (not in a chamber) takes more energy than passive drying, especially if the air is heated. The air supply must be filtered. Passive or blow-off drying at ambient temperature, even in a well-controlled cleanroom, is typically not desirable. Drying is better accomplished in an environment that minimizes the likelihood of re-contamination.

Hot air drying with HEPA filtration in a drying chamber effectively provides a mini-cleanroom environment that protects the contents of the chamber and the external fabricating area. Chamber materials must be selected to avoid outgassing or re-contamination.

Vacuum drying lowers the boiling temperature of the liquid but generally does not make the vaporization occur any faster. Even in a vacuum, you still have to provide the heat of vaporization plus heat to warm the part; and heat transfer is more difficult because the heat transfer paths of air conduction and convection are reduced or eliminated. A vacuum pump takes additional energy and pumps/oils can be contamination sources.

Liquid displacement drying replaces a liquid with a high heat of vaporization like water with a low-residue miscible fluid that has a lower heat of vaporization, such as alcohol. The alcohol dries more rapidly and at lower temperatures. Isopropyl alcohol is flammable; drying equipment must be appropriately designed.

Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning,surface preparation,and contamination control.They are the editors of Handbook for Critical Cleaning,CRC Press.Contact them at BFK Solutions LLC., 310-459-3614;info@bfksolutions.com;www.bfksolutions.com.