Dehumidification of Condair

Klaus Achenbach, Market Development Manager
Klaus Achenbach (M.Sc. in Engineering) 

Market Development Manager

Operating characteristics of adsorption drying and condensation dehumidifiers

Functional principle Condensation dehumidifier

Functional principle Adsorption drying

Planning criteria for TGA (Technical Building Services) and specialist planners

Air dehumidification and drying for industrial and commercial companies

Air dehumidification and drying for industrial and commercial companies

Drying through sorption
Adsorption of water vapor contained in humid air by hygroscopic surfaces (silica gel rotor). This brochure refers only to the dehumidification and drying solutions of stand-alone systems. Therefore, dehumidification by means of surface coolers supplied with cold water, as used in ventilation systems, are not discussed further.

Air dehumidification through condensation
This process involves cooling the humid air below the dew point by conducting the air stream across the cold surface of a heat exchanger (evaporator of a cooling circuit).

Condensation air dehumidifier
Condensation air dehumidifiers as ready-to-use aggregates are often used in industrial and commercial applications, and for dehumidifying swimming pools. Everywhere that air humidity levels must be kept between 45 and 60% RH, condensation dehumidifiers present an energy-efficient and cost-effective solution. Swimming pool air dehumidifiers are special types of condensation air dehumidifier. They are protected against air containing chlorine through special measures (e.g. coating of the heat exchangers) and can be equipped with additional heat exchangers for indoor heating, pool water condensers etc.

Limiting factors regarding the achievable final moisture content are mainly the characteristics of the coolant used (pressure, temperature) and the constructive layout of the evaporator-heat exchanger (bypass factor). In general, the following principle applies: condensation dehumidifiers can be usefully deployed in temperature ranges between approx. +5 and +36°C at an achievable relative humidity of approx. 45% RH.

Condensation air dehumidifier operation
The basis of any condensation air dehumidifier is a closed cooling circuit, which works in accordance with the heating pump principle. A fan sucks in the humid ambient air. It firstly passes through a filter which is installed to protect the heat exchangers, before it is conducted over the evaporator. On this cold surface, it is cooled to below its dew point, with a large proportion of the water vapor it contains being converted to condensation.

The water that has become condensation is contained in a condensation tray installed under the evaporator and then diverted directly into the outflow or collected in a corresponding container. Next, the air stream which has now been dehumidified passes through the condenser, where it is heated through the condensation heat of the cooling circuit. The waste heat from the fan and the compressor is partly absorbed by the air stream conducted via the dehumidifier. As a result, the dehumidified air introduced into the area is always warmer than it was when it entered the dehumidifier.

Desiccant dryers
Desiccant dryers are used wherever condensation air dehumidifiers physically reach their limits and compliance with minimum absolute humidity or water vapor levels is required. In this technology, the water vapor level is no longer stated in % RH, rather always in the absolute humidity x in g/kg of dry air or in the associated dew point temperature in °C.

The principle of sorption refers to the capacity of certain substances to bind water vapor to its surface. The inner surface of these substances is on a scale of between 600 and 1,000 g/m2. An extremely low water vapor partial pressure exists in the immediate surroundings of these chemical substances. Due to the laws of thermodynamics, water vapor diffuses from areas of higher partial pressure (in this case from ambient air) to areas of lower partial pressure (sorbent).

Silica gel, aluminum oxide or sometimes molecular sieves are used as sorbents. The further examination focus exclusively on adsorption using silica gel, as this is by far the most widely-used sorbent on the HVAC market.

Desiccant dryer operation
Humid ambient air (process air) is sucked out by a fan and conducted through an adsorption rotor. The adsorption rotor consists of a corrugated and finely laminated storage mass with an enormous inner surface, which is coated with the highly hygroscopic silica gel. The entire cross section of the rotor is divided into a drying sector of 270° and a regeneration sector of 90°. The sectors are insulated from each other.

A continual, slow turning of the adsorption rotor is carried out by the motor, the turning speed is in the range of 5–30 turns per hour. The air stream to be dried is continuously conducted through the drying sector of the rotor.

In the process, the water vapor it contains is almost fully adsorbed. The 90° regeneration sector of the rotor is conducted into the countercurrent of regeneration air, which was previously heated to approx. 120°C via a regeneration pack. As a result, adsorbent water vapor bound in the rotor is forced back out and discharged to the outside with the humid air stream. The regeneration air stream amounts to approx. 1/3 of the process air stream.

This adsorption/desorption process can be repeated as often as required without the degree of effectiveness of the sorbent being significantly influenced. The adsorption capacity of silica gel is so high that dew points of -70°C can easily be attained.

Why use an air dehumidifier?

Particularly in the industrial and commercial sectors, swimming bath and warehousing industries, users are often confronted with a pressing need to control the humidity of the air.

Ensuring product quality
Being able to precisely configure the air humidity throughout production processes is often an essential factor in ensuring product quality remains consistently high. Using air dehumidifiers and dryers helps to ensure that these processes remain safe and stable.

Maintaining operations and preventing
Dehumidifiers can protect pipework, installations, operating materials and technical appliances from moisture damage. This helps to ensure that equipment is always ready for use and reduces the need for expensive renovations. The risk of production downtime is therefore far lower.

Protecting valuables in storage and archives
In the storage and archive area, air dehumidifiers protect valuable items from the expensive consequences of all humidity damages, in extreme cases from their destruction.

Conservation of out of service machinery
Machines and equipment that are taken out of service periodically can be protected from corrosion damage with air dehumidifiers. This keeps them in peak condition to ensure that they can be put back into service more quickly when the time comes.

Protecting building structures
Air dehumidifiers can be used to prevent/minimize water vapor diffusion through building structures, and so protect them from deterioration over the long term. This in turn reduces the risk of expensive building renovations becoming necessary.

Operational safety and hygiene
Condensation on walkways can lead to an increased risk of accidents and encourage bacterial growth. Air dehumidifiers help to maintain a safe and hygienic environment.
Klaus Achenbach (M.Sc. in Engineering), Market Development Manager