Drying and gas or vapor contact with solids – Apparatus – With waste gas heat and/or power conservers
Patent
1987-11-24
1988-11-01
Lacey, David L.
Drying and gas or vapor contact with solids
Apparatus
With waste gas heat and/or power conservers
622383, 62476, 162375, D21F 520, F25B 2502
Patent
active
047809671
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an apparatus for the recovery of waste heat contained in the exhaust from dryers of paper machines by means of a compression heat pump fed with the exhaust air on its low-pressure side, by which feed water process steam is produced on the high-pressure side for heating the drying cylinder, and air, which then is used for the drying, is preheated.
In the manufacture of paper in paper machines, the originally relatively watery fiber pulp is first compressed to form the fiber mat, and then dewatered by vacuum aspirators and by couch rolls, and finally in the press part by a series of wet presses. The residual moisture is then removed in the drying part of the paper machine, which is formed by a series of cylinders arranged in a drying chamber and heated with low-pressure steam at temperatures of about 140.degree. to 150.degree. C. The process steam for heating the drying cylinders has heretofore been produced in practice in low-pressure steam boilers fueled with fossil fuels, unless steam is available from other sources, such as the production of electricity. The warm exhaust air of high relative humidity produced by the drying has heretofore been emitted to the ambient atmosphere, which is wasteful of energy on the one hand, and on the other hand can pollute the environment by forming noxious vapors or fogs, especially in the case of cold outdoor temperatures.
Therefore, both for the sake of saving energy and of reducing environmental pollution, there is a need to recycle the thermal energy still contained in the warm, moist exhaust air back into the papermaking process, and thus avoid the described disadvantages. Heat pumps of themselves are suitable to a special degree for the use of exhaust heat at a relatively low temperature level, and proposals have already been disclosed for the transfer of the exhaust heat contained in the exhaust from the drying chamber to the blowing air to be fed to the drying chamber for drying purposes, by means of a compression heat pump (DE-OS 26 30 853). The use of compression heat pumps for the production of process steam from feed water for the heating of the drying cylinders and for warming the blowing air has already been described theoretically (article: "Eine Betrachtung ueber die Einsatzmoeglichkeiten von Waermepumpen bei der Papiertrocknung" by J. Kappel, F. Schweinzer and A. Weinmann in "Wochenblatt fuer Papierfabrikation" 7, 1984, pp. 133-238). However, only the use of single-substance compression heat pumps was considered, which, in order to realize a sufficient temperature elevation of the waste heat occurring at relatively low temperature, had to be formed by two compression heat pumps connected in tandem, at least when, in addition to the warming of the blowing air, steam is also to be produced for heating the drying cylinders. In the tandem heat pumps it is then necessary to operate with different working media suitable for the particular temperature range, which leads to a complex system. In the article cited, it is pointed out by the authors that the study concerning the various working media that might be used has found no thermodynamically optimum medium. In their findings, the authors therefore come to the conclusion that the use of closed compression heat pumps with electric motor drive is basically possible, but not economical at the dryer dew points presently in use.
In the manufacture of paper, furthermore, problems occur because of certain operating conditions which make it difficult to use heat pumps for the production of the process steam required for heating the drying cylinders. Due to production-related effects, a breakage of the paper in the machine often occurs, making it necessary to shut down the paper machine briefly until the paper is re-introduced into the machine. During this down time, however, no paper drying takes place, so that there is also no waste heat available from exhaust air for the evaporator of a single-substance heat pump. Upon restarting the paper machine, the heat pump would then be unable to suppl
REFERENCES:
patent: 4314668 (1982-02-01), Jansen et al.
patent: 4420942 (1983-12-01), Rojey et al.
Hastings K. M.
Lacey David L.
TCH Thermo-Consulting-Heidelberg GmbH
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