There are essentially two ways of accomplishing dehumidification of atmospheric air. One is by chilling air below its dew point, causing moisture to condense on cool surfaces. The other is by passing air over substances having a strong affinity for moisture. These substances are called desiccants and are capable of extracting moisture directly from the atmosphere.
Equipment: There are four basic types of desiccant dehumidifiers: liquid absorbent, packed tower adsorbent, rotary-bed adsorbent, and rotary Honeycomb absorbent. While each type has certain different characteristics and uses, the basic principles involved are similar. The equipment we will describe first is a non-cyclic, dry sorption desiccant type with a single desiccant rotary structure designed for continuous operation.
Construction design shall provide counter flow of process and reactivation air streams with full face pressure seals to prevent cross leakage with static pressure differentials up to 8 inches water gauge.
The rotary structure shall consist of 100% inert silicates impregnated with an inorganic, non-granular, crystalline desiccant which transfers water to the vapor phase. The design shall assure laminar air flow through the structure for minimum pressure loss with maximum transfer surface.
The dehumidifier shall include desiccant wheel and drive system, process and reactivation air flow indicating gauges, reactivation heaters, reactivation fan/motor assembly, reactivation energy modulation system, electric/mechanical rotation fault circuitry, a factory wired NEMA 12 (NEMA 4 for outdoor systems) control enclosure including door disconnect, filter clog alarm circuits, power on and run indicating lights and all controls for safe and automatic operation.
The unit must be capable of constantly maintaining a relative humidity of 40% or less in a given area for 30 days.
The unit must be capable of maintaining the dew point at least 10 degrees below the surface temperature of the steel.
From two to three complete air changes per hour of an uncontaminated dehumidified air, with the aforementioned characteristics to the area to be dried, will assure the desired objective. The reason for using DH in tank lining operations is to create an internal environment that will drastically reduce the rate of corrosion.
The rotary Honeycomb unit uses an absorbent (usually either lithium chloride or silica gel) and the desiccant is impregnated into a honeycomb structure mounted within a cylindrical casing. The wheel rotates continuously between the dehumidifying sector and the regeneration sector.
Flexible seals separate the drying and regeneration sectors, and the airstreams flow in opposite or counter flow directions. Counter flow dehumidifies are usually more efficient than parallel flow units. As a result, pressure balances between air streams are not usually a concern. As with most dehumidifiers, after cooling may be required for space or process temperature control.
The corrosion rate is reduced by the quasi elimination of the electrolytic cell by eliminating the electrolyte which in most cases is moisture.
With no corrosion possible, the entire internal surface can be blasted before coating begins even if blasting will continue over several days or weeks. After all blasting has been done, surface areas can be cleaned thoroughly, abrasives will be removed, dust will not exist, inspection can be made and at this time coating activities will start.
Feasibility: By eliminating the necessity to change from a blasting to a coating activity on a daily basis, the contractor is able to save approximately 35% of his time.
A dust-free surface will permit better adherence of the prime coat. No surface contamination.
A continuous monolithic film will improve performance of the coating.
Blasting and coating operations can proceed regardless of weather conditions.
It is much easier to predict the amount of time necessary to complete a tank lining project.
Better working conditions improve productivity of workers.
Less coatings and solvents are used due to less clean-up of coating equipment.
The DH process will permit the use of "short window" coatings.
Here are a few examples of dehumidifiers that are commonly used during projects in the Petro Chemical industry. Delivery time is usually 4 to 6 weeks. In all cases volume discounts apply.