Heat exchange – Flexible envelope or cover type
Patent
1996-08-16
1997-09-30
Flanigan, Allen J.
Heat exchange
Flexible envelope or cover type
165115, 165166, 165DIG163, 165DIG183, 165DIG185, F28D 502, F28D 300
Patent
active
056718047
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a heat exchanger element for a film heat exchanger and a film heat exchanger constructed on the basis of this element.
The film heat exchanger presented in this description is particularly suitable for use in such heat exchanging situations where condensation of the vapour component into liquid takes place on one side of the heat exchanging surface and evaporation of the liquid component into vapour takes place on the other side of the heat surface. Heat exchangers of this type can be used e.g. in thermo-compressor evaporators and in a conventional evaporator plant which uses live steam as the source of energy.
The above film heat exchangers are particularly suitable for use in a film evaporator or distillation apparatus operating in the thermo-compressor principle. This is an apparatus where the heat exchanger unit is formed of flat bag-like elements of thin sheet material like plastic sheet placed against each other. Heat transfer takes place from the vapour condensing inside the elements through the sheet to the evaporating liquid falling on the outer surfaces of the elements. The vapour, which is introduced into the elements and which releases heat upon condensing, is generated outside the elements and its pressure and temperature has been raised by a compressor before it is introduced into the elements.
The purpose of use of film distillation apparatuses is to obtain a distillation product of desired quality like production of fresh water from sea water or purification of industrial process water before leading back to the process or to drain. Another purpose might be evaporation of excess solvent from various solutions or suspensions for obtaining a desired concentrate.
Patent publications FI 79948 and FI 86961 describe a heat exchanger suitable for a film distillation apparatus where the heat surfaces are thin plastic sheet. The plastic sheets are formed into vertical bag structures arranged adjacently. The liquid to be evaporated is led to fall from the top downwards along the outer surfaces of the bags whereupon part of it is evaporated. The pressure of the evaporated vapour and accordingly the condensation temperature is raised by a compressor after which it is introduced into the bag structures. Condensing of the vapour takes place there and the distillate or condensate obtained is discharged from the bottom of the bag. Inside the bag structure there are gorges obtained by welding the sheets together by point welds or by welding the sheets together by broken zigzag seams. The solution of the publication 86961 is provided with an end lath with a honeycomb structure at the top of the bag structure from which the liquid to be evaporated is introduced onto the outer surface of the bag structure. The end lath also comprises channels through which the vapour going into the bag structure is introduced.
The heat exchanger presented above, however, involves some points of risk. The plastic sheets may break at the welding points or at the seams since welding has changed the composition of the plastic material at these points. During operation the bags flutter due to formation of vapour and the sheets of two adjacent bags often stick together within a considerable area. When the bags come loose from each other, they readily stick at another point. This kind of continuous fluttering and sticking may lead to breaking of the plastic sheet and above all to lowering of the efficiency of the apparatus because a significant part of the heat surface is continuously out of use due to sticking. Furthermore, distribution of the liquid to be evaporated onto the outer surface of the bag is uneven because the liquid streams from the top tend to unite leaving part of the surface dry and the united liquid streams on the other hand are too fast-flowing to effect maximum evaporation. This uneven distribution of liquid leads to lowering of efficiency.
For the reasons mentioned above heat transfer is fairly low with respect to the area of the heat surfaces because approximately 40-50% of the heat surf
REFERENCES:
patent: 3256930 (1966-06-01), Norback
patent: 4216820 (1980-08-01), Andrew
patent: 4424098 (1984-01-01), Hartig
patent: 4585523 (1986-04-01), Giddings
patent: 4842049 (1989-06-01), Dodds
patent: 5340443 (1994-08-01), Heinio et al.
Flanigan Allen J.
Oy Shippax Ltd.
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