Welcome to Bry-Air Asia for DEHUMIDIFIERS and PLASTIC DRYERS

In this article, we will focus on the solid granular based and fluted media based dehumidifiers as currently they are the two most used dehumidifier types.

SOLID GRANULAR BASED
Rotating Horizontal Bed

In this device, dry, granular desiccant is held in a flat, segmented rotary bed that rotates continuously between the process and reactivation airstreams. As the bed rotates through the process air, the desiccant adsorbs moisture. Then the bed rotates into the reactivation airstream, which heats the desiccant, raising its vapor pressure and releasing the moisture to the air.

The process and reactivation air heats and cools the desiccant to drive the adsorption-desorption cycle. The moisture is removed through a process of continuous physical adsorption on a continuous basis (both, counter flow and parallel flow options are available).
The adsorption of moisture and reactivation of desiccant take place continuously and simultaneously without any cross mixing of the process and reactivation air streams.

To increase capacity, the manufacturer can either increase the diameter of the rotating bed to hold more desiccant, or increase the number of beds stacked on top of one another. Both options are not practical if very large volumes of air need to be dehumidified. If the desiccant is evenly loaded through the trays, the rotating horizontal bed provides a fairly constant outlet moisture level, and a high airflow capacity can be achieved in less floor space than with dual-tower unit. The rotating horizontal bed design offers a low first cost. The design is simple, compact and easy to produce as well as install and maintain.

COMPACT DEHUMIDIFIER

COMPACT DEHUMIDIFIER OPERATING PRINCIPLE

MODULAR VERTICAL BED (MVB)

The Modular Vertical Bed (MVB) design is a 'fairly new' but 'proven' concept with the combined better features of packed tower and rotating horizontal bed designs in an arrangement that is well suited to atmospheric pressure dehumidification applications, and yet can achieve very low dewpoints. The single or double tower is replaced by a circular carrousel with eight or more vertical beds (towers) that rotate, by means of a drive system, between the process and reactivation air streams.

This design can achieve low dewpoints because leakage between process and reactivation air circuits is almost negligible. Also because the beds are separate and sealed from one another, the pressure difference between process and reactivation is not so critical; so airstreams can be arranged in the more efficient counter-flow pattern for better heat and mass transfer. Like the rotating bed, the ratcheting, semi-continuous reactivation of the desiccant provides a relatively constant outlet air moisture condition on the process side, reducing the "sawtooth" effect that can occur in packed tower units.

The "MVB" design allows for low replacement cost of desiccants as well as large savings in energy and performance improvements at low dew points, especially if the equipment incorporates a heat pipe heat exchanger in the regeneration air circuit.

Modular Vertical Bed

MVB OPERATING PRINCIPLE

FLUTED MEDIA BASED DEHUMIDIFIERS

Another dehumidifier design uses a rotating fluted wheel/rotor to present the desiccant to the process and reactivation airstreams. This is sometimes called a fluted media/honeycomb type dehumidifier. The desiccant is impregnated/ synthesised on "honeycomb" like corrugated rotor. The principle of operation is the same as the solid desiccant (granular) based system.

The process air flows through the flutes formed by the corrugations, and the desiccant in the structure adsorbs the moisture from the air. The rotating desiccant bed picks up moisture, and well before "saturation" the rotor/wheel rotates into the reactivation segment where it is heated to drive off the moisture.

The fluted design has its own advantages as it is comparatively light weight and has a smaller foot print. The fluted design is the preferred option where space is a limitation and there is a leeway to sacrifice "performance" slightly. One has to also keep in mind the higher replacement cost of the rotor compared to the desiccant in the granular systems.

FLUTED FLAT BED	FFB OPERATING PRINCIPLE

FLUTED VERTICAL BED	FVB OPERATING PRINCIPLE

Comparing Desiccant Dehumidifiers

All desiccant dehumidifiers can be made to work in virtually any application suited for desiccant dehumidifiers. The limitations of each configuration can be overcome and benefits of each design optimized by careful application engineering. No firm statement can be made about the limits of performance or amount of energy consumed or mechanical reliability of different dehumidifier types outside of a particular set of installation circumstances.

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Good application engineering, in addition to the dehumidifier type, makes a dehumidification installation reliable, efficient and low in first cost. Some of the criteria which a design engineer must cater for while designing a system are:

Installed cost

The initial cost of the dehumidifier itself is sometimes a small fraction of the cost of the installation or the running cost. Different configurations vary in their need for additional support infrastructure viz, utilities, chilled water, floor space etc.

Operating cost

The initial cost of mechanical systems is often much less as compared to their cost of operation and maintenance. The main operational cost of a dehumidification system is heat for reactivation and cooling of the desiccant and process air. Today, dehumidifiers are designed to take advantage of low-cost energy sources for these utilities. Often reactivation energy cost can offset installed cost differences in a matter of months, yielding enormous financial benefits over the typical 15 to 30 years life of this equipment. Also, designs incorporating energy recovery systems have an advantage over dehumidifying systems without energy recovery.

Design assumptions
Engineers and manufacturers make different assumptions concerning a given application. The selection of equipment configurations and size is completely dependent upon these assumptions. Often reasons for widely varying selections are the result of incomplete or erroneous data available to the system designer.