© Corroventa Ltd.
2017.02
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Relative humidity and its effect on substances
All air contains more or less moisture but the naked eye cannot see it until it condenses in small
droplets on for instance a metal or glass surface. Already before it is visible however, the moisture
affects substances and production processes, causes corrosion and micro organism growth.
Air humidity is measured and referred to in terms of relative humidity (%RH) which is a measure how
much water it contains relative how much it can contain at given temperature and pressure. The
higher the temperature, the more water the air can contain but it is still the Relative Humidity that is
important and that needs to be controlled if one wants to prevent corrosion or mould growth.
At RH 100% the air is saturated – there is fog and the moisture condensates in small droplets.
Already at RH 60% steel corrodes and at 70% there is a risk for mould growth. As a rule of thumb, RH
50% is a good climate for most substances.
How to select dryer type
The adsorption principal has, compared to condensation drying, the advantage of a lower ambient
temperature dependency. Adsorption functions even well below the freezing point while the capacity
of a condensation dryer inevitably decreases rapidly with falling temperature, something that is
depicted in the diagram below to the left.
As a rule of thumb, a general aid in the selection of type, it can be said that adsorption is the primary
choice for drying of unheated spaces or when material is to be dried. The latter is motivated by the
fact that the adsorption dryer produces dryer air, generates a greater reduction of water content
measured in grams per kilogram (
Δ
x), easily deduced from the diagram below to the right and
something that is directly correlated to the drying speed. Drying of layered constructions such as floor
structures are, with advantage, dried with combined use of turbines, high pressure fans, installed for
either suction or pressure drying.
Condensation dryers are, in accordance with the diagram above, used in hot and humid conditions
provided the goal and aim is drying of rooms, ambient air.
Condensation dryer
Superior within blue area.
Functions within yellow area.
Low or no function in red area.
Adsorption dryer with air-cooled
condensor
Functions within dashed area.
Adsorption dryer
Superior within red area.
Functions in yellow and blue area.
Transition zone
Temperature,
% RH
Dry air quality, 20°C
Dry air RH%
Process air RH%
Condensation dryer
Adsorption dryer
