First , foreword
Under the stress state of evaporation experiment began in the seventeenth century, to the middle of the nineteenth century vacuum technology has been widely used in petrochemical field.
Later with the development of vacuum technology and dry technology which will use vacuum equipment or vacuum system , the application of the vacuum technology is more and more widely in the field of drying.
Vacuum is not completely without air molecules fewer state.Under the condition of vacuum heating material, make its internal moisture diffusion and inside evaporation, surface evaporation, and thus for the low temperature low pressure drying process.
Second, the vacuum pumps manufacturing
1, sealed in a vacuum drying device occupies an important place in the manufacturing, for permanent sealing connection and sealing connection, here no longer restatement, in general decompression drying process, the vacuum degree requirements is not high, can use packing seal, mainly because of the packing seal structure is simple, easy processing and assembling, low cost, also facilitate maintenance replacement, but use common packing chamber temperature, more than ninety degrees, again due to the vacuum in the vacuum chamber, packing gradually dry, lose the sealing function
2, dust removal
Phenomenon exists, we are now generally cloth bag dust catcher, every dry cleaning bag at a time.Sometimes used water bath dust, the effect is very good also.
Configuration of vacuum condenser, one is technical requirements to evaporate the solvent recycling, 2 it is can be non-condensable gas condensation down, achieve the goal of exhaust pump
Three, the choice of vacuum degree in vacuum drying process
On the practical convenience, people often think of coarse vacuum degree is divided into several regions with low vacuum in vacuum high vacuum ultra high vacuum
The general decompression drying, distillation is in low vacuum， Commonly used vacuum pump water ring pump, plunger pump, etc
In freeze drying food freeze drying in vacuum，General use rotary vane pump, roots pump and so on
Freeze drying blood and other biological products and molecular distillation under high vacuum , General use , dry vacuum pump turbomolecular pump and so on
In vacuum drying, vacuum degree is high, is advantageous to the moisture evaporation under low temperature, but the high vacuum degree and not conducive to thermal conduction, influence of materials, the heating effect of high vacuum degree is helpful to use a vacuum pressure errand such as cable came out from the internal evaporation of water through cracks movement, but high vacuum degree and cause the structure of the vacuum system is complex, increase investment in equipment.
For comprehensive advantages and disadvantages, both vacuum for heat transfer and the effect of vacuum evaporation, dry vacuum degree of work must be appropriate choice, not the higher the better.For some special materials, require moisture content is low, there are also improve the effect of vacuum to improve the drying process.
Four, vacuum drying process in vacuum system selection
(1) limiting vacuum of vacuum chamber
(2) of the vacuum chamber pressure
(3) the net speed of vacuum chamber near the bleeding point
1, according to the amount of water vapor is chosen.Vacuum unit suitable to work in the process under the condition of maximum pumping water vapor without affect working state of the unit.
2, under the condition of satisfy the vacuum degree of work, with small energy to save cost of drying unit
3, the same conditions of energy consumption, selects the pumping speed unit
4, under the vacuum drying process, the configuration of the condenser as far as possible, can greatly improve the pumping speed, reduce the cost.Under normal circumstances there is a lot of water vapor roots water ring unit more applicable.
Five, vacuum drying of distinctive features:
1: low boiling point under the vacuum state, so the evaporation area be narrowed, the entire unit is compact.
2: the drying temperature is low, can dry heat-sensitive materials.
3: due to the evaporation can realize low temperature, can use low pressure steam or vapour.
4: under the vacuum state, can be sterilized.
1, water vaporization heat required is increased with the decreasing of vaporizing temperature.
So in the true evaporation, the consumption of the same weight of moisture evaporation heat than ordinary conditions of consumption.
2, the increase of vacuum degree, lower heat transfer coefficient.
3, vacuum degree is higher, the investment of vacuum equipment such as vacuum pump is more greater.
4, and the increase of vacuum drying system more strict sealing.So, in the vacuum evaporation drying, vacuum degree of work is very important.