Category: Technical information associated with Munters Ltd
Dehumidification
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Munters
Dehumidification
Operation
Munters desiccant dehumidifiers attract moisture from the air by
creating an area of low vapour pressure at the surface of the desiccant.
The pressure exerted by the water in the air is higher, so the water
molecules move from the air to the desiccant and the air is
dehumidified.
Munters desiccant dehumidifiers make use of changing vapour pressures to
dry air continuously in a repeating cycle described by the simple
diagram Fig 1. As the desiccant picks up moisture from the surrounding
air, dry air is discharged to the process area and the reactivation air
stream discharges the wet air outside.
One subtle distinction between desiccants is their reaction to moisture.
Silica Gel desiccant wheels simply collect it like a sponge collects
water - the water is held on the surface of the material and in the
narrow passages. Another desiccant is lithium chloride which undergoes a
chemical change as it collects moisture. Whether the desiccant functions
by absorption or adsorption is not usually important to a system
designer, but the distinction exists and engineers should be aware of
the difference between the two terms.
When moisture is removed from air, the reaction liberates heat. In a
desiccant dehumidification system, the heat is transferred to the air
and to the desiccant, so the process air generally leaves the
dehumidifier warmer than when it entered the desiccant unit. The
temperature rise is directly proportional to the amount of moisture
removed from the air - the drier the air leaves the dehumidifier, the
warmer it will be. In any applications - notably product drying and
unheated storage - this temperature rise of the dry air is cooled before
being delivered to the point of use.
Dehumidification by Desiccant Wheel
The desiccant wheel is ideally suited to low temperature
dehumidification (-40ºC to +40ºC) and for exceptional low dewpoint
conditions (down to -70oC).
At the heart of the unique Munters Dry Air method is an absorbent wheel
rotating about 8 times an hour relative to a segmented housing.
In the larger segment, know as the ‘working zone’, the drying wheel
absorbs moisture from the humid storage air.
In the smaller segment, or ‘reactivation zone’ heated air passes through
the wheel absorbing the moisture and allowing it to be exhausted to the
exterior.
This operating principle provides continuous dehumidification of the
process air and the heat required to effect this drying process, since
it is strictly localised, needs an extremely small energy input.
Larger units may be equipped with a heat recovery module in which
incoming reactivation air is pre-heated using the exhaust air. This can
give energy savings of between 25 and 35 per cent.
Where exhausted or moist air to the exterior is difficult - say from
basement levels - the dehumidifier can be fitted with an air-cooled
condenser.
The wheel technology has established itself in a multitude of
applications covering the processing of materials, product drying,
storage and preservation.
Preservation
All materials can corrode, which is to say every substance can change
from one form to another through chemical reaction. Many of these
reactions, especially those which depend on oxygen, are catalyzed and
accelerated by moisture. Ferrous metals like iron and steel are well
known for their corrosion Fig 2 (Corrosion Curve) in the presence of
moisture. In the past, thousands of desiccant dehumidifiers have been
used to surround machinery and equipment with dry air, preserving
ferrous metal parts from heavy rust.
Currently, dehumidifiers are working to protect materials from subtle
and expensive forms of corrosion. Modern society depends more and more
on light equipment like computers, telecommunications gear, lightweight
composite materials and high energy batteries. While these are less
subject to gross rusting, they are very sensitive to microscopic level
corrosion. These circuits simply do not have much material to begin
with, so small amounts of corrosion create disproportionately large
problems.
Processing
Virtually every substance has some affinity for moisture. Even
plastic resins like nylon can collect six to ten per cent of their dry
weight in water vapour. In many cases, this presents no problem. In
others, moisture regain can affect critical dimensions just like thermal
expansion, or make products that would otherwise flow freely stick
together. Moisture regain on humid days clogs packaging machinery,
silo’s and conveying systems and sticky products have major economic
consequences.
Hygroscopic products are sensitive to high relative humidities rather
than absolute humidities, and relative humidity can be high at any time
of year. In fact it is often higher in winter than summer. When products
are stored at cool temperatures, problems can be especially acute.
Desiccant dehumidifiers are effective in controlling humidity at low
temperatures, and have been widely applied to prevent moisture regain.
Product Drying
Most products are dried using hot air to vaporise moisture and carry
it away. Often however, hot air is either too slow or results in damage
to the product. When there is a benefit to drying at temperatures below
46ºC, there is generally a benefit to using air which has been
dehumidified rather than just heated. The cooler the temperature, the
more the economics favour dehumidifiers. For example, in a fluidized-bed
drier. Because of the temperatures involved, the drying capacity of the
fluid bed is doubled when the air dew-point drops from 18ºC to -4ºC.
This means the size of the fluid bed can be cut in half.
Technical information associated with: Munters - Dehumidification, Desiccant
De-humidification & Condensation Control
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Munters Ltd
Blackstone Road
Huntingdon
Cambridgeshire
PE29 6EE
Tel : 08708 505202
Fax : 01480 413147 |
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