Introduction to the Process of Dehumidification-
Here are the answers to why that there is a need for dehumidification:
It also deals with questions like : Where does the moisture come from ? 
Does it help if I throw a pack of silica gel in my drawers ?
Does Heating up a place reduces the moisture ?

Why dehumidification?

Iron and steel will not rust if the air in contact with the surface has a relative humidity below 50% relative humidity (RH). Dehumidification is often a better and less expensive way to protect investments than painting is. Objects that are protected by dehumidification are for example bridges, power stations , boats (during standstill) and off-shore constructions.

Moisture will not condensate on a surface if the air in contact with it has a dew-point lower than the surface temperature. Condensation can lead to many problems such as corrosion or short-circuit in electrical systems.

Ice formation
Ice will not form on a surface if the air in contact with it has a dew-point lower than the surface temperature. Ice formation is a problem in for example cold storages where it will cause a lower efficiency in the cooling machinery. Dehumidification will keep the ice away.

Handling of hygroscope materials
The quality of dry drugs, dry food, hard candy and
 other hygroscope materials can only be maintained through production to the consumer 
if it is kept in contact with air at a low relative
humidity. If your products form lumps, degrade
quickly, or get sticky you should consider 
dehumidification. Most materials demand 
dew-points so low that they can only be 
achieved by sorption dehumidifiers.

Drying buildings
When drying moisture from a building, whether 
it is a new building or a building with water
damages, the most effective way is to use 
sorption dehumidifying. Heating will only move 
the moisture to another part of the building,
heating in combination with outdoor 
ventilation will create high energy costs. 
With sorption dehumidifying the moisture 
is moved out of the building in a very 
energy efficient way.

Mould and fungus formation is prevented if the
surrounding air is kept below 70%RH. This is
important in many situations, for example in
storage of wooden products. In houses 
designed with a crawl space it is often 
necessary to use a dehumidifier to keep 
mould and fungus from forming.

Product drying and cooling
When drying and cooling products a low relative
humidity is essential for a fast process. If the
products to be dried are sensitive to high
temperature, the low relative humidity can 
only be obtained with dehumidification. In 
cooling processes dehumidification allows 
for a lower temperature without condensation.
The result is faster cooling and money saved.

Bacteria need humidity to survive and multiply.
 Often the humidity required for bacteria to
 multiply is found on hygroscope materials. 
If the surrounding air is held below 50% relative
 humidity most bacteria will not find a suitable
 environment to multiply.

Bad smell will drastically be reduced if the relative
humidity is kept below 50%RH. In for example
sewage stations the odor can often be controlled
by installing a dehumidifier.



Click the following for the answers to the questions:

How does a dehumidifier (DH) reduce moisture ?

What is Relative Humidity ( or RH ) ?

What is Absolute Humidity ?

How to calculate Humidity load in general cases ?


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Dehumidification is an on-going process to reduce and keep the air moisture low.at an enclosed space.

How does moisture come into the space ?  Normally, there is a certain air getting into the space
 via cracks or gaps ( Infiltration) , or even through the wall (Permeation). Human being gives 
moisture through' breathing and evaporation from the skin too. If the place stores things that give 
up water like plants, wet towers, exposed water containers, wet sink etc. In the  basement of 
buildings, usually there are more moisture problems than other floors. This is caused by moisture
in the earth below and surrounding the walls of the basement. Though concrete wall is supposed 
to be able to keep water from flooding in, moisture permeation has a very high power draw to 
penetrate even this thick walls. As long as there is a small opening, more humid air will flow into 
the areas with lower humidity. This is a physical process   calls Diffusion. The Rate of Diffusion  
( or the speed of molecular movement ) is determined by the vapour pressure difference between
the two spaces. A high moisture concentration constitute higher vapour pressure.   

G-bridge.JPG (11712 bytes) If the space is left alone, the infiltration, permeation and other moisture contributing process will take place until the space reaches an equilibrium "wetness" ( or Humidity ) level that these process no longer take place. This level of wetness may usually be too damp for the equipment which may cause damage or human discomfort. ( see Dehumidifier Applications )

To get rid of this moisture, some form of Dehumidification process is needed to remove the moisture. As the moisture infiltration is process is a continuous one, by putting in a pack of silica gel usually does not work for a long time. Silica gel is a well known moisture absorbing material. It sucks up moisture like a sponge !. However, it will stop working once it is SATURATED.  It does not suck up moisture forever ! That's why some camera box silica gel has blue (when dry)  /  red (wet) colour indication feature. Silica Gel, however, is very useful and cost effective for tight sealed packed product which has water proof skin like plastic or aluminium foil.

Heating as a form of reducing relative humidity
is found  in simple devices like a piano heater, or instrument box /cabinet with built-in heaters. Heating raises the space temperature and thus at the same moisture content of a space lower the relative humidity. It has, however, never removed a single droplet of moisture from the air ! It is relatively workable in cool and dry countries. Its effectiveness in hot and humid countries not as good. The good news is, the moisture damaging effect on product is proportional to the Relative Humidity of the Space rather than the actual amount of moisture content
( Absolute Humidity ).
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Some of the modern bridges with welded box construction on cold countries are protected against corrosion by the help
of dehumidifiers, saving a lot of maintenance. Another area will be in the marine application. Ships are very sensitive to
moisture. Partly due to the salty atmosphere and partly caused by the vast temperature fluctuations. The upper deck
which is exposed to the air may be tens of degrees different in temperature compared to the lower part which is touching
the sea water. This variation usually causes large humidity differences and resulted in condensation of water on the metal
surface. Dehumidifiers are usually used to keep the air dry.