Products > Vacuum drying ovens / Vacuum ovens / Vacuum Dryer

Vacuum Drying Ovens / Vacuum Dryer are used:

   - with or without protective gas operation, combustible
     or non-flammable
   - with or without explosion proof 

 

 

 

In the chemical/pharmaceutical industry for:

 

 

 

  - Gentle and uniform drying of heat-sensitive materials under vacuum 
    pressure, as required for powders, granulates and pastes, for example.

 

 

 

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

 

   - Drying of explosive materials of class E1,
     Vacuum Drying Oven is designed acc. to VDE 0166.

     Explosive materials are dynamites, fuels and objects, which contains 
     explosive materials, as well as pyrotechnic objects.
     Pyrotechnic objects are such objects, that serve amusement
     or technical purposes and contain explosive-dangerous materials
     or material mixtures.

 

 

  - Drying of goods under clean room conditions (acc. to US Fed.
    Standard 209 / German VDI 2083) and possibly under a protective gas
    atmosphere (pure gases, cumbustible gases or inertgases).

 

 

 

 

In the electro-technical industry for:

 

 

 

 - Tempering and void-free soldering of oxidation-sensitive
    components in a flammable (e.g. hydrogen) or non-flammable
    protective-gas atmosphere (e.g. inertgas).
    Optionally, the cooling down rate is programmable. 

 

 

For the nuclear power plants for conditioning, treatment and radioactive waste management:

 

 - Drying of radioactive materials / radioactive waste management "Conditioning".
   The Vacuum Drying Ovens are designed for drying of radioactive
   contaminated waste such as filters, auxiliary means, protective clothing, sludges, materials
   of any kind which are storaged in drums. - Radioactive waste management.
   The condensed water is free of radioactive contamination. 
   The Vacuum Drying Ovens are equipped with special loading device
   for the handling of drums and with external radiation protection..  

 

 

 

 

 

 

Vacuum drying oven, heating

 

 

 

To dry the load, the liquid has to be transferred into the vapour phase, and because energy is used constantly while evaporating the liquid, energy has to be supplied continuously throughout the process.

 

 

Because, heat can only be supplied in a vacuum by means of radiation or conduction (direct contact via heating plate), heating is classified as particularly important:

 

 

 

 

 

 

 

 

 

Jacket heating:       The vacuum chamber is heated. The heat is transferred to the load for warming via the metal sheet inserts:       - Advantage:     The individual sheets are removable                           and thus allow flexible handling      - Disadvantage: The heat is not transferred from                          the heated vacuum chamber directly to the                           load, but indirectly  the brackets for insert                          sheets, i.e. high temperature gradient,                          hence longer heating and process times.

 

 

Plate heating:         The vacuum chamber sheets are designed as heating plates. The heat is transferred directly into the load via the heating plates:       - Advantage:      The heat is transferred directly from the                          heated shelf (heating plate) into the load,                          i.e. low temperature gradient, shorter heating                           and process times.       - Disadvantage: The single shelves are installed permanently                         and cannot be removed

 

 

 

 

 

 

Further Heating methods and combinations:

 

 

 

For products which, based on their design and shape (tall structures) may dry badly under vacuum conditions, the efficiency of the heating can be improved and in this way guarantee the heat supply:

 

 

 

 

 

 

 

Jacket and plate heating:         This combined heating method can improve the success rate by its uniform heat distribution. If necessary, the load is heated at atmospheric pressure (natural convection) via directly warmed heating plates. The actual drying process then takes place in a vacuum.

 

 

Jacket heating and infrared heating:  Combined heating by means of jacket heating and infrared radiator. The load is heated by means of IR radiators. Jacket heating prevents condensation inside the vacuum chamber.

 

 

Circulating air and vacuum mode:          Combined drying by means of circulating air and vacuum operation. The load is heated at atmospheric pressure with circulating air. The actual drying process takes place in a vacuum. The number of circulating air and vacuum cycles is matched to the required drying process.

 

 

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

 

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

 

 

 

Cooling of load: 

 

Vacuum Drying Ovens can also be designed with an additional cooling system (water-cooled inserts), to reduce the cooling phase and the charging temperature.  

 

 

 

 

 

Explosion Proofed Vacuum Drying Ovens / Solvent Recovery

 

 

 

Explosion proofed vacuum drying ovens according ATEX 94/9EC (ATEX 95, ATEX 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 vacuum drying process can be condensed and is recovered.

 

 

 

 

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

 

 

 

Vacuum Pumps:

 

According to the provided process requirements, we design also the associated vacuum pumps and complete pumping units for normal or increased requirements (with or without explosion proof).

We design the vacuum pumping units, depending on the process requirement, with and without solvent recovery, with diaphragm pumps, rotary vane pumps, scroll pumps, dry screw pumps, water jet pumps and turbo molecular pumps.

 

 

 

 Technical fundamentals for drying under vacuum 

 

 

Technical terms (related to vacuum): 

 

Unit of pressure:

In vacuum technology, the most commonly used unit of pressure is the millibar (mbar)

 

 

Converting units of pressure:

 

1 Pa          = 1 N/m²

1 bar         = 100.000 Pa = 10⁵ Pa

1 mbar      = 100 Pa        = 10² Pa 1 h Pa

1 Torr        = 133.32 Pa

1 mm HG  = 1 Torr          = 133.32 Pa

1 mm WS  = 9.80665 Pa

 

 

Total pressure:

The total pressure in a container is the sum of all partial pressures of gases and vapours contained inside.

For instance, air is composed of different gases such as nitrogen, oxygen, trace gases and water vapour, see below.

 

 

Composition of atmospheric air:

 

Gas type	% weight	% volume	Partial pressure, mbar
Nitrogen, N2	75.51	78.1	792
Oxygen, O2	23.01	20.93	212
Argon, Ar	1.29	0.93	9.47
Carbon dioxide, CO2	0.04	0.03	0.31
Neon, Ne	1.2 x 10-3	1.8 x 10-3	1.9 x 10-2
Helium, He	7 x 10-5	7 x 10-5	5.3 x 10-3
Methane, CH4	2 x 10-4	2 x 10-4	2 x 10-3
Krypton, Kr	3 x 10-4	1.1 x 10-4	1.1 x 10-3
Laughing gas (nitric oxide), N2O	6 x 10-5	5 x 10-5	5 x 10-4
Hydrogen, H2	5 x 10-6	5 x 10-5	5 x 10-4
Xenon, Xe	4 x 10-5	8.7 x 10-6	9 x 10-5
Ozone, O3	9 x 10-6	7 x 10-6	7 x 10-5
Total	100 %	100 %	1013 mbar
Water vapour (50% RH at 20 °C)	1.6 %	1.15 %	11.7 mbar

 

Air pressure at 50% RH / 20 °C

1024 mbar

 

 

Partial pressure:

Partial pressure of a certain gas or vapour is that pressure which the gas or vapour would have, if it was the sole substance in a container.

 

 

Vapour:

Vapour is a gas that occurs in liquid and gaseous phase side by side at a particular temperature.

 

 

Gas:

Along with solid and liquid states, gas is one of the three states of aggregation. Gas is formed from vaporisation or sublimation, i.e. if the temperature is so high or the pressure is so low that all the liquid evaporates, then it is called gas.

 

 

Saturation vapour pressure:

Saturation vapour pressure is the pressure of the vapour phase of a substance, i.e. when the liquid and vapour phases are in equilibrium.

The saturation vapour pressure for a given pure substance is a function of the vapour temperature.

Further evaporation is not possible if the partial pressure reaches the saturation vapour pressure.

 

 

Boiling point:

Boiling point is the point at which a liquid changes from liquid to gaseous form. This is also referred to as vaporisation or boiling. On the other hand, it is called evaporation when the change from liquid to gaseous form takes place below the boiling point. Evaporation takes place at the surface of the liquid.

 

The boiling point of a liquid depends on temperature and pressure. The boiling point is a pair of variates in the phase diagram, consisting of saturation temperature (boiling temperature) and the saturation vapour pressure (boiling pressure).

 

Water boils, for example, at a total pressure of 1000 mbar and a temperature of 100 °C. At a total pressure of 100 mbar, water boils at about 46 °C and at 10 mbar at 7 °C.

 

 

Boiling and melting points (freezing points) of some solvents at 1000 mbar:

 

Solvent	Melting point, °C	Boiling point, °C
Ethyl alcohol Benzol Chloroform Diethyl ether Isopropanol Methanol Nitromethane Water	-114.5 5,49 -63.5 -116.4 -89.5 -97.9 -29.2 0.0	78 >100 61 34.6 82.4 64.7 101.8 100

 

For pure elements, the melting point is the same as the freezing point. In contrast to boiling temperature, melting temperature is affected very little by pressure.

 

 

Vacuum:

Vacuum is defined as that state which if formed due to the rarefaction of gases and vapours at pressures between the atmospheric pressure (1013 mbar) and 0 mbar by pumping out the contents of a container, i.e. the pressure or the density is less than the surrounding atmosphere.

 

 

Classification of vacuum:

 

Pressure range	Pressure in mbar	Number density per cm ³
Rough vacuum	103 to 100	2.65 x 1019 to 2.65 x 1016
Fine vacuum	100 to 10-3	2.65 x 1016 to 2.65 x 1013
High vacuum	10-3 to 10-7	2.65 x 1013 to 2.65 x 109
Ultra high vacuum	10-7 to 10-12 (∞)	2.65 x 109  to 2.65 x 104

 

 

 

 

How does drying take place in a vacuum drying chamber

 

 

The drying process is explained using the example of water at a temperature of 20 °C:   As already mentioned and as can be seen in the saturation vapour pressure diagram, the saturation vapour pressure of water is 23.37 mbar at 20 °C. Or in other words: Water boils at a total pressure of less than 23.37 mbar already at 20 °C.   If the partial pressure of water is less than this value, the liquid will evaporate till the saturation vapour pressure is reached. Once this happens, further drying does not take place.   If the drying process is continued in a conventional drying oven (i.e. without vacuum), either the temperature must be increased > which also increases the value of the saturation vapour pressure< or the air, in which the partial vapour pressure has already reached the saturation pressure, should be replaced with unsaturated air.   In the vacuum drying chamber, the total pressure which is about 1000 mbar (1 bar) in the conventional drying chamber is reduced below the value of the saturation vapour pressure (23.37 mbar at 20 °C) using a vacuum pump.

Water in liquid state immediately changes into gaseous state. It starts to boil. This again increases the total pressure in the vacuum drying chamber up to a maximum of saturation vapour pressure. If this pressure is reached, further vaporisation or evaporation does not take place.

 

By continually pumping out the water vapour (H₂O gases) which is formed, it is ensured that the total pressure and thus the partial pressure of water vapour always remains below the saturation vapour pressure.

This process can be continued till all the liquid vaporises.

 

If other substances are dried using vacuum, only the values for the saturation vapour pressure changes at certain temperatures. The principle of the drying process is the same.

 

Since energy is required for changing the state from liquid to gaseous (for 1 litre of water at 20 °C, 2.454 x 10⁶ J / kg = 0.68 kWh / kg = 860 kcal / kg), energy must be supplied in the form of heat in order to maintain a particular constant temperature in the substance to be dried.

 

 

 

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 >by means of a vacuum pump<, 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.

 

 

 

 

 

 

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)

 

Water in the liquid state does convert immediately to a gaseous state. It begins to boil. Thus, the total pressure in the vacuum drying oven increases again up to the maximal possible pressure, i.e. up to the saturation vapor pressure. If this pressure is reached, no further vaporization or evaporation takes place more.

 

 

 

 

 

Vacuum Drying Ovens with and without explosion proof, examples:

 

 

 

 

 

 

		Vacuum Drying Oven Type VT          Explosion Proof Vacuum Drying Oven (ATEX 95, 94/9/EC) with solvent recovery      Designed for drying/evaporation or for heat treatment of materials which form an explosive vapour air mixture in contact with air.     Secondary Heating System:     Primary:    water steam                     alternatively electrical    Secondary: thermal-oil                    alternatively warm water                    alternatively water steam     Heating:     jacket and shelf heated
		Vacuum Drying Oven Type VT                      Explosion Proof Vacuum Drying Oven (ATEX 95, 94/9/EC)                        Designed for drying/evaporation or for heat treatment of materials which form an explosive vapour air mixture in contact with air.                             Secondary Heating System:                         Primary:    electrical                   alternatively water steam                         Secondary: thermal-oil                    alternatively warm water                    alternatively water steam                   Heating:     jacket heated                   alternatively shelf heated                   alternatively jacket and shelf heated
		Vacuum Drying Oven Type VT                           Explosion Proof Vacuum Drying Oven (ATEX 95, 94/9/EC)                           Designed for drying/evaporation or for heat treatment of materials which form an explosive vapour air mixture in contact with air.                               Secondary Heating System:                              Primary:     electrical                    alternatively water steam             Secondary: warm water                    alternatively thermal-oil                    alternatively water steam                             Heating:      jacket heated                    alternatively shelf heated                    alternatively jacket and shelf heated

Vacuum Drying Oven Type VT          Jacket Heated Vacuum Drying Oven          Designed for drying/evaporation or for heat treatment of materials which form none explosive vapour air mixture in contact with air.         Electrical heated         Heating: jacket heated               alternatively shelf heated               alternatively jacket and shelf heated

  

 

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