Heat exchange – Intermediate fluent heat exchange material receiving and... – Solid fluent heat exchange material
Patent
1980-12-09
1982-08-10
Davis, Albert W.
Heat exchange
Intermediate fluent heat exchange material receiving and...
Solid fluent heat exchange material
110245, 122 4D, 422146, F28D 1300, F28C 316
Patent
active
043433529
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a heat exchanger for gases, in which a granular material is kept in motion by means of an upward gas flow through the heat exchanger, heat exchange tubes containing a flowing medium to receive or give off heat being arranged in that section of the heat exchanger in which the granular material is kept in motion.
THE PRIOR ART
Heat transfer from a gas to a contact surface in a whirling layer or fluidized bed of granular material is known within the art. A such heat transfer process may offer substantial advantages. Very high heat transfer figures may be obtained. The strongly turbulent movement of grains against contact surfaces prevents deposition of impurities on the heating surfaces. Some types of e.g. contaminated waste gases are conducive to a substantial growth on the heating surfaces of conventional heat exchangers, which in such cases substantially reduces the usefulness of conventional heat exchangers. Fluidized bed heat exchangers are disclosed e.g. in British Pat. No. 1,395,900.
However, a such fluidized bed also has disadvantages which strongly restrict its usefulness for heat exchange purposes. The resistance towards the gas flow is high, the pressure drop across the bed corresponding to the weight of the fluidized bed, and in addition comes the necessity of maintaining a substantial pressure drop across the bottom. The pressure drop across the bottom must be at least 30% of the pressure drop of the fluidized bed, preferably substantially higher, in order to ensure good gas distribution. With an individual grain density of e.g. 2.6 in the fluidized bed (quartz) the density of a conventional fluidized bed will be within the range of from 800 to 1100 kg/m.sup.3 within the conventional velocity range (of the gas) of 3-10.times.minimum fluidizing velocity. The corresponding pressure drop across the bed alone will be 800-1100 mm water column per m of bed height. This involves a very strong restriction of the height of a such bed.
Another property of a such fluidized bed is a perhaps still stronger restriction of the usefulness of the bed for heat exchange purposes. The random and strongly turbulent movement of the grains in the fluidized bed yields a continuous mixing and also an approximately complete temperature equalization throughout the bed. A such fluidized bed normally operates with through-passing gas bubbles which increase in size with increasing bed height and which give strong and random agitation throughout the bed. Several investigations have shown that the entire fluidized bed in a fluidized bed heat exchanger is approximately isothermal despite e.g. a continuous supply of heat by means of hot gas introduced through the bottom and despite a continuous cooling by means of heat transfer surfaces arranged in the bed. This isothermal state manifests itself as a substantial and easily measurable temperature drop when the gas passes through the bottom of the bed, the gas being momentarily cooled by contact with the fluidized bed. Then, as mentioned, the temperature is approximately constant throughout the bed, and the temperature of the out-going gas above the bed is approximately equal to the temperature immediately above the bottom.
If heat is to be transferred from a gas to a medium in a such bed, the efficiency of the process is restricted thereby that the temperature of the out-flowing and heated medium must always be lower than the temperature of the bed, this temperature being again approximately the same as the temperature of out-flowing cooled gas. It does not matter whether the heat transfer surface, e.g. in the form of tube sets in the bed, is arranged cocurrently, countercurrently or crosscurrently. As the temperature of the entire bed is constant, the same driving temperature difference is obtained in all cases.
The temperature profile over the height of the fluidized bed in a heat exchanger is fundamentally as shown in FIG. 1. In this figure is shown the theoretical gas temperature 1, the actual gas temperature and temperature
REFERENCES:
patent: 2640333 (1953-06-01), Bailey
patent: 2919559 (1956-09-01), Koch
patent: 3048153 (1962-08-01), Abrahamsen
patent: 3814176 (1974-06-01), Seth
patent: 3912002 (1975-10-01), Elliott
Bockman Ole K.
Rydland Kjell
A/S Norsk Viftefabrikk
Davis Albert W.
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