What is Vacuum drying?

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Introduction

We all have experienced a sunny day with windy weather that dries laundry quickly, while a cloudy day with humid weather in the rainy season makes it difficult to dry the laundry. Yet, drying wet objects in an industrial setting requires a system that will ensure repeated drying in order to maintain the quality of drying in the given circumstance. There shall be no need to mention the demand for ensuring drying and repeatability for complex shapes and porous machined parts. Such drying with high efficiency requires a method of vacuum drying.

Warm air drying and vacuum drying

What is warm air drying? ... Drying in daily use is called as warm air drying for which the following conditions are critical for quick drying.
(1) Object to be dried is warm.
(2) Wind is blowing.
(3) Atmosphere humidity is low.
Limiting to the drying of an object wet from water, the quality of drying is determined by the difference in water vapor partial pressure (or difference in density) between the surface and wind. The higher the surface temperature, or the lower the humidity of the drying air flow, the higher the difference in water vapor partial pressure (difference of density). The three conditions are the factors that increase the difference in water vapor partial pressure. However, the temperature of an object decreases due to the latent heat of moisture while the object dries. The drop in the water vapor partial pressure causes a smaller difference between the water vapor partial pressure and air, which makes drying more difficult. As shown in Figure 1, sunbeams normally warms laundry on sunny days with no problem. Systems for industrial purposes typically use the heat from warm air or air warmed by a heater to prevent the temperature drop in order to expedite the drying of an object. Still, this method dries moisture from the surface of the object. Drying a porous object with moisture soaked to the inside, such as a sponge, requires time to wait for the moisture inside to seep out to the surface to evaporate, which lengthens the time to complete drying.

What is vacuum drying?

What is vacuum drying? ... A drying method that places the object to be dried in an enclosed container to vent air and reduce the pressure with a vacuum pump in order to artificially increase the water vapor partial pressure difference is called vacuum drying. The vacuum drying method dries an object with a temperature of 25ºC three times faster comparing to drying with warm air of 30ºC and humidity of 50%. As shown in Figure 2, vacuum drying is completed in an enclosed container so that it has no benefit from heat supplied by the sun or warm air as in warm air drying. The method requires some type of heating in order to prevent the temperature drop of the drying object accompanying evaporation of moisture. In order to prevent the temperature drop of drying object, the following conditions must take place:
・Raising the temperature prior to placing an object in a container.
・Heating the entire container to warm the drying object with radiant heat.
・Heating the drying object with conduction heat from a heater plate in the container.
The aforementioned methods are typical. The vacuum drying forces the pressure in the narrow gaps and in the tubes to decrease, which enables the moisture in the gaps to evaporate faster. In addition, moisture trapped in the narrow gaps by a bumping phenomenon could blow out, unless the temperature inside the drying object drops excessively, which further expedites drying. Therefore, using a pump with a volume sufficient for venting evaporated moisture for vacuum drying enables any porous or even powder like object to be dried thoroughly even from the inside uniformly.

However, while it has the advantages of uniform drying even from the inside of a porous object or of more rapid drying than warm air drying, the speed of drying is determined by the temperature at which a calorie removed by evaporation and a calorie supplied is balanced. In the case of porous or powder-like objects with low thermal conductivity, while the surface temperature increases from the heating method, the internal temperature does not rise, which delays the drying in the center. Thus, drying porous material with low thermal conductivity may result in completed drying of the surface by the vacuum drying method but failure to completely dry the inside.

Figure 1 Warm air drying

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Figure 2 Vacuum drying

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Utilizing vacuum drying

Food industry ... As mentioned before, the temperature of food to be processed drops from the evaporation of moisture through drying while depressurizing an enclosed container. Storage and transportation of fresh vegetables uses this phenomenon. Fresh vegetables lose freshness by aspiration after being harvested. Suppressing this biological metabolism and cooling the products without causing any disturbance (around 3ºC) maintains freshness. Depressurizing fresh vegetables and fruit in a container while in a cardboard box cools the vegetables and fruit from the temperature drop accompanying moisture evaporation. In addition, the inside of the container is depressurized uniformly to enable quick, uniform cooling. This method is called vacuum cooling and vacuum freezing, which is used by large systems with processing volumes of several tons to several tens of tons.

Electric and electronics components ... Various coils (motor coil, transformer, and relay equipment) among the electric equipment consist of components with complex narrow gaps, such as communication cables with tiny lines twisted or covered with insulation. The condensation due to humidity and residual moisture after water washing may cause fatal defects due to failure of the insulation. Vacuum drying is the most suitable for drying coils and cables. The majority of cables are made of copper wire with high thermal conductivity, which enables raising the temperature of the portion in the deep center even for large coils to complete the drying process and satisfying the guaranteed performance.

Metal and machined components ... Drying a precision machine process component after water washing is difficult due to the complex shape and numerous tapped holes. In addition, vacuum drying is used for uniform drying of the inside of sinter components and metal powder. Still, porous objects and powder will not dry uniformly on the inside when the thermal conductivity of the material is low. In such a case, preheating the object to dry is one solution. A system fabricated to expedite raising the temperature of an object in vacuum drying is a high-speed vacuum dryer that uses both heating with warm air circulation and vacuum drying. This system uses vacuum drying after raising the temperature of an object to be dried using warm air circulation in an atmosphere environment. This method shortens the entire processing time by raising the temperature of objects with even low thermal conductivity by warm air circulation faster than in a vacuum.

Summary

Whereas we have explained in a brief outline the process of vacuum drying, the use of vacuum drying includes freeze-drying in a broad range of industrial fields. Therefore, manufacturing and supplying various types of systems to the entire industrial field by a single manufacturer may require a considerable accumulation of technical knowledge and experience. Thus, at present, the respective systems used in the respective field are supplied with the unique expertise of the respective manufactures based on user needs matching to the purposes of use.

Utilizing vacuum drying

Field of utilization Object of drying process
Food - Fresh food - Processed food Fresh vegetables, meats, fruits, instant coffee, milk products, hams, instant noodles, powder ingredients
Electronic, electronics components Print boards, relay kits, IC components, wafers, LCD panel glass boards, motor coils, transformer coils, lead line end terminals
Metal, machining parts Micro hole machined parts, micro tubes, dies for molding, sinter parts, metal filters, metal powders, bearings, precision machine parts