Drying and gas or vapor contact with solids – Apparatus – With apparatus using centrifugal force
Patent
1989-10-10
1991-01-15
Bennet, Henry A.
Drying and gas or vapor contact with solids
Apparatus
With apparatus using centrifugal force
34 46, 34133, F26B 2108
Patent
active
049843745
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention concerns the control of a rotary cut tobacco drier, of the single cylinder type described in our
STATEMENT OF PRIOR ART
UK Patent No. 1,209,929 or the double cylinder type described in our UK Patent No. 1,345,373. In particular the invention is concerned with the short term or fast response control of such driers, in which it is necessary to achieve a constant moisture content in tobacco leaving the outlet from the drier, while compensating for variation in the moisture content in tobacco fed to the drier.
Rotary cylindrical driers have on their inside a number of longitudinal paddles which first lift and then drop the material being dried. The axis of the cylinder is slightly inclined to the horizontal so that the material progresses down the cylinder each time it is dropped and will typically take 3 to 6 minutes to progress through the cylinder.
Both the cylinder and paddles are of tubular construction and heated by steam or high pressure hot water to provide the heat for drying. Alternatively in another form of single cylinder construction the cylinder is heated on the outside by flue gases from a gas or oil flame.
In either case the rate of response of the cylinder and paddles to changes in temperature is relatively slow due to the thermal capacity of the cylinder and paddles.
Air is also passed through the cylinder either in the same direction as the tobacco flow or in the opposite direction. The prime purpose of such air flow is to carry away the evaporated moisture, but in addition, if the air is heated, some small additional drying action may be achieved.
It is normal to measure the flow rate of the tobacco entering the drier by a weighing conveyor and its moisture content by means of a continuous moisture meter, and from these measurements to make a prediction of the cylinder temperature required to achieve desired output moisture content in the tobacco i.e. a feed forward control.
Further because the dwell time within the cylinder can be 3 to 6 minutes and because of the thermal capacity of the cylinder, it cannot respond readily to varying flow rate or moisture content. So input variations or inaccurate predictions can result in output errors sensed by an output moisture meter, which take 5 minutes or more to correct by alteration of cylinder temperature i.e. a feed back control.
To reduce this delay the air temperature can be raised to provide some quicker acting feed back control, particularly if the air flow is contra flow. However to be effective the air has to be at least as hot as the cylinder, and because the thermal capacity of the air is small the amount of adjustment is small. The effect is confined to the tobacco about to be discharged, so that the standard deviation of the moisture content can be doubled.
Alternatively the air flow rate can be varied to provide some quick-acting feed back control.
The main driving force for drying is the heat transfer from cylinder and paddles to the tobacco, which is dependent on the temperature difference between cylinder and tobacco. The cylinder temperature is determined by the heat supply and can be controlled to a fixed level. The tobacco temperature is comparable with a wet bulb temperature determined by the humidity of the air in the drier, in which the vapor pressure of the moisture at the surface of the tobacco exceeds the vapor pressure of the air.
A reduction in air flow through the drier results in an increase in air humidity, so the tobacco increases in temperature to increase its vapor pressure and to maintain the evaporation. The rise in tobacco temperature is a reduction in temperature difference from the cylinder and a reduction in moisture removal. It is preferable if the air flow is in-line with and in the same direction as the tobacco flow so that the highest humidity is at the delivery end, but a contra-flow arrangement may be used in some circumtances.
The air flow through a drier is normally arranged on a push pull system; that is a fan is used to blow air into the drier, via an a
REFERENCES:
patent: 2877562 (1959-03-01), Krantz
patent: 3372488 (1968-03-01), Koch et al.
patent: 3389707 (1968-06-01), Wochnowski
patent: 3409025 (1968-11-01), Wochnowski
patent: 3905123 (1975-09-01), Fowler et al.
patent: 3906961 (1975-09-01), Rowell et al.
patent: 3948277 (1976-04-01), Wochnowski et al.
patent: 4143471 (1979-03-01), Wochnowski et al.
patent: 4186755 (1980-02-01), Schmidt
patent: 4346524 (1982-08-01), Wochnowski et al.
patent: 4434563 (1984-03-01), Graalmann et al.
patent: 4513759 (1985-04-01), Wochnowski et al.
Bird Derek G.
Westlake Anthony S.
Bennet Henry A.
Creighton Wray James
GBE International plc
Gromada Denise L.
LandOfFree
Rotary drier control by adjustment of air flow or air humidity does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Rotary drier control by adjustment of air flow or air humidity, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rotary drier control by adjustment of air flow or air humidity will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-47917