Products > Drying ovens with or without air circulation

Drying Ovens, Application

   - with or without circulating air
   - with or without protective gas operation, combustible
     or non-flammable/inertgas
   - with or without explosion proof

 

Drying Ovens with or without Circulating Air are used for:

  

 

 - Drying and heat-treating of goods in a fresh-air or protective gas
   atmosphere (e.g. combustible gas like hydrogen or inertgas).

 

 

 - Drying or heat-treating of goods under clean room conditions
   (acc. US Fed. Standard 209 / German VDI 2083) and possibly under
   a protective gas atmosphere (pure gases, combustible or
   non-flammable gas / inertgas).

 

 

 - Drying of materials containing solvents which do build an explosive
   mixture in contact with air acc. to German ATEX Directive 94/9/EC (ATEX 95,
   ATEX 100a) or EN1539.

 

 

 - Drying of explosive materials, Drying Oven is designed acc.
   to German VDE 0166, drying chamber: Class E1

   Explosive materials are explosives, fuels, flammable materials and
   objects that include explosives as well as pyrotechnic compositions.
 
   Pyrotechnic articles are those things that serve for pleasure or for 
   technical purposes and where explosive hazardous substances or
   mixtures are included.

 

 

 

 - Lacquer Drying Ovens, Drying Ovens for Coating Materials

 

   For drying of coated materials where flammable substances are
   released, which do not build an explosive vapour air mixture, due
   to controlled technical ventilation

 

   The amount of flammable solvent per load is limited – The amount of
   flammable solvent is calculated acc. to the European Regulation EN 1539

  

   With the technical ventilation, it is guaranteed that the forming of
   dangerous explosive atmosphere is prevented under all operating
   conditions. Therefore, the solvent steam concentration must not
   exceed a limiting value of 25 percent of the lower explosion limit
   of the used solvent, or 40 g/m³ (at 20 °C), if the lower explosion limit
   of the used solvent is unknown.

 

 

 - Explosion Proofed Circulating Air Drying Ovens

 

   Designed for heat treatment applications containing large quantities of 
   flammable solvents or heat treatment of materials emitting vapours which
   build an explosive vapour air mixture in contact with air

 

   Drying chamber: Zone 1, Category 2, suitable for combustible solvents of the 
   temperature class T1 to T4 and explosion group IIA and IIB

 

   The quantity of combustible solvent per load is not subject to any limitation

 

 

 - Explosion Proofed Fresh Air Drying Ovens

 

   Designed for heat treatment applications containing large quantities of flammable
   solvents or heat treatment of materials emitting vapours which build an explosive 
   vapour air mixture in contact with air

 

   Drying chamber: Zone 1, Category 2, suitable for combustible solvents of the 
   temperature class T1 to T4 and explosion group IIA and IIB

 

   The quantity of combustible solvent per load is not subject to any limitation

 

 

 - Tempering and Heat Treatment Ovens for Optical Glasses with highest standards
    regarding pure protective gas atmosphere and temperature behaviour.

 

    Designed for contamination-free, silicone-free and none-ferros material  tempering
   and heat treatment of optical components, such as laser optics, optical mirrors and lenses
   under pure air as well as high-purity inert gas atmosphere with higest standards of purity
   and temperature behavior; precise compliance with the temperature and the heating and
   cooling rates.
   With decontamination program for the interior to insure the required purity class.

 

 

 

 Heating and Drying Ovens, Kind of Heating

 

 

Normally, the drying ovens are fitted with electrical heating. Depending on the application, other types of heating can be chosen:

 

 

- Electrical heating, standard
- Infrared heating, IR-Radiator
- Warm water heating
- Thermal oil heating
- Steam heating
- Steam/warm water heating

 

The Heating and Drying Ovens can also be designed with an additional cooling system, to reduce the cooling phase and the charging temperature.

 

 

 

Explosion Proofed Drying Ovens / Solvent Recovery

 

Explosion proofed drying ovens according ATEX 94/9/EC (ATEX 95 / 100a), correspond to equipment Category 2 and can be installed in Zone 1. According the request, the supply aggregate can be installed in a neutral or in a potentially explosive atmosphere.

 

The solvent that results with the drying process can be condensed and is recovered.

 

The exhaust air cleaning can be executed, depending on the solvent, over condensation/ cryo-condensation. The prepared exhaust air stream meets the requests of the German TA Luft "Technical Instructions on Air Quality Control".

 

 

Heat supply in drying ovens:

 

As described above, the drying ovens can be used for heat treatment and/or for drying goods.

 

The heat can be supplied through heat convection, radiant heat or heat conduction (direct contact via hot plate).

 

 

Heat conduction Heat transfer through direct contact     Heat radiation Heat transfer through radiation     Heat convection Heat transfer through direct contact and continuous transportation of heat transferring medium

 

Heat convection:

 

Natural heat convection:

In drying cabinets with natural heat convection (natural ventilation), the heated air moves on account of natural buoyancy due to density differences.

 

Forced heat convection through air agitation:

In drying cabinets with forced heat convection (air agitation), the heated air moves undirected after being agitated by a fan placed in the working chamber.

 

Forced heat convection through forced ventilation:

In drying cabinets with forced heat convection (forced ventilation), the heated air moves (horizontally or vertically) as directed by a fan placed outside the working chamber.

 

 

natural convection		forced heat convection though air agitation		forced heat convection through forced ventilation

 

 

Radiant heat:

 

The product is heated primarily via infra-red radiation. Secondary heating is via heat convection.

 

 

Heat conduction:

 

The inlays of the drying cabinet are designed as hot plates. Heat is transported to the product to be heated directly, primarily via the hot plates. Secondary heating is via heat convection.

 

 

Composition of atmosphere in the drying oven: 

 

Circulating air operation:

The air in the system is circulated continuously as directed (horizontal or vertical) without being renewed.

Area of application: For heat treatment of goods.

 

 

Fresh air operation:

The air located in the system is replaced continuously. The air enriched with humidity is exchanged continuously.

Area of application: For heat treatment and/or drying of goods.

 

 

Circulating air operation with addition of fresh air:

The air located in the system is circulated continuously as directed (horizontal or vertical). Fresh air is added continuously, so that the humid air is renewed and replaced partially.

Area of application: For heat treatment and/or drying of goods.

 

The volume of the fresh air can be adjusted with the help of a fresh air valve. The air replacement rate shows how often the air in the drying cabinet is replaced within a certain period of time.

 

 

 

 

 

 

 

 

How drying takes place in a Drying Oven / Circulating Air Drying Oven:

 

The basis of drying is the vapour pressure of the liquid.

 

In the convection-based drying process, moisture in the drying oven is converted into vapour. Hence, it is mainly the vapour pressure of the liquid to be vaporised that is crucial.

 

If one increases the temperature to such an extent that the vapour pressure becomes equal to the atmospheric pressure, the liquid starts to boil. Vapour bubbles are formed inside the liquid against the external pressure, which leave the liquid. Continuous boiling is possible only when the vapour is discharged through the air vent.

 

 

Drying can take place only by virtue of the pressure difference between the ambient air pressure (atmosphere) and the vapour pressure of the liquid to be vaporised.

 

During drying the ambient air pressure is always below the vapour pressure, PU < PU

 

 

 

 

 

TU   PU   TG   PG

Capillary action   Diffusion   Evaporation, vapourisation   Providing heat       Ambient temperature   Ambient pressure   Temperature of product   Pressure in the product   PU  <   PG

 

 

 

 

From the practical viewpoint: in an open pot, heated water boils when its vapour pressure rises above the ambient air pressure.

 

 

As the temperature increases, the vapour pressure also increases, because more molecules in the liquid are brought to the boil, and hence more vapour is produced.

 

To dry a substance, the liquid must therefore be converted into the vapour state, and as heat energy is consumed continuously during vaporisation of a liquid, this energy must also be supplied continuously.

 

 

Liquid pure substances have a constant but pressure-dependent boiling point:

 

At a constant ambient pressure of 1013mbar, diethyl ether boils at 34.6°C, ethanol at 78.4°C and water at 100°C.

 

Continuous vaporisation (boiling) however, is possible only when the vapour is discharged, i.e., the atmosphere in the drying oven must be replaced continuously.

Because drying is not possible (evaporation and vaporisation) if the liquid content in the atmosphere is equal to the vapour pressure, or even exceeds it.

 

 

 

relatively dry (unsaturated) air     relatively moist (saturated) air

 

 

 

The higher the temperature and the greater the temperature differences between the ambient atmosphere and the vapour pressure of a substance to be vaporised, the faster the drying.

 

 

 

 

 

The drying can be started or accelerated by:   - Increasing the temperature - Increasing the pressure difference (vacuum chamber)

 

 

 

 

 

 

 Drying Ovens / Circulating Air Drying Ovens with and without explosion proof, examples:

 

 

 

	Drying Oven Type UW Circulating Air Drying Oven designed for drying and heat-treating of materials in a fresh air or protective gas atmosphere Heating System: electrical
	Drying Oven Type UW Circulating Air Drying Oven designed for drying and heat-treating of materials   Heating System: electrical
	Drying Oven Type UW-EN1539 Circulating Air Drying Oven designed for lacquer drying processes according safety regulations EN 1539 (VBG 24) in clean room design Designed for drying of coated materials where flammable substances are released, which do not build an explosive vapour air mixture, due to controlled technical ventilation.        Heating System: electrical

 

 

 

 

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