AU691924B2

AU691924B2 - Drying process

Info

Publication number: AU691924B2
Application number: AU25583/95A
Authority: AU
Inventors: Patrick Constantine
Original assignee: PJC AIRCONDITIONING MANUFACTUR
Current assignee: PJC AIRCONDITIONING MANUFACTURERS Pty Ltd
Priority date: 1994-05-31
Filing date: 1995-05-31
Publication date: 1998-05-28
Anticipated expiration: 2015-05-31
Also published as: AU2558395A

Description

WO 95/33169 PCT/AU95/00324 1.

TITLE OF INVENTION: Drying Process FIELD OF INVENTION: This invention relates to a drying process which has particular applicability to, but is not restricted to, foodstuffs.

BACKGROUND OF THE INVENTION: In this specification, the term "drying" will be used rather than "dehydration", which is sometimes used in connection with the drying of foods. Dehydration can imply removal of water accompanied by a chemical change.

It is known to dry many materials including pharmaceutical, biological materials and foodstuffs. Drying processes include evaporating liquids at rates varying from a few grams to many tonnes per hour from a single dryer. Drying temperatures for pharmaceutical, biological materials and foodstuffs may vary from 100 to 700C depending on the process used. Dryers range in size from small cabinets to large towers.

The materials dried may be in the form of thin solutions, suspensions, slurries, pastes, granular materials, bulk objects, fibres or sheets. Drying may be accomplished by convective heat transfer, by conduction from heated surfaces, by radiation, and by dielectric heating.

It is an object of the present invention to provide a drying plocess which has particular applicability to, but is not restricted to, foodstuffs and which is of improved efficiency as compared with known processes but does not harm the material to be dried.

BRIEF SUMMARY OF THE INVENTION: This invention provides a drying process which has particular applicability to, but is not restricted to, foodstuffs which comprises the steps of: warming ambient air;, passing the warmed ambient air to a primary heat exchanger coupled with a dehumidifier where it is cooled, water removed and then reheated: passing the warm dry air produced by step to a heat reclaim coil wherein the temperature of the air is increased and the relative humidity decreased; SUBSTITUTE SHEET (RULE 26)

I-

r o-l WO 95133169 PCT/AU9500324 2 passing the hot dry air produced by step over or through a product for a sufficient time to achieve the desired drying effect; and venting the warm moist air produced by step to the atmosphere.

This invention also provides an apparatus for carrying the process according to the invention into effect.

BRIEF DESCRIPTION OF THE DRAWINGS: The accompanying drawings illustrate one embodiment of the invention in which: Figure 1 is a cross-sectional view of an apparatus used to carry the invention into effect; Figure 2 is a diagrammatic view of a secondary heat exchanger and associated apparatus; and Figure 3 is a frontal diagrammatic view of a rotating wheel heat exchanger forming part of the apparatus according to the invention.

DETAILED DESCRIPTION OF THE INVENTION: The ambient air used in step may be warmed by heat-exchange in a secondary heat-exchanger with the warm moist air passed to step The result is warm ambient air with a high relative humidity.

With reference to step the primary heat exchanger is conveniently a rotary heat exchanger which is located adjacent a dehumidifier whereby warmed ambient air enters through one part of the primary heat exchanger rotary mechanism, in which it is cooled, passes through the dehumidifier, in which it loses moisture, and then to another part of the primary heat exchanger rotary mechanism, in which it gains heat, Such dehumidifiers were developed to provide continuous dry air.

In step the product may be dried in bulk or in some broken-up or comminuted form.

The product may be sampled from time to time to check whether sufficient drying has been achieved.

SUBSTITUTE SHEET (RULE 26) WO 95/33169 PCT/AU95/00324 3 A proportion of the warm moist air from step may be passed to the primary heat exchanger to mix with incoming ambient air.

The process according to the invention may be used to evaporate liquids at rates varying from 2 to 5 grams to 4 tonnes per hour, for example. Greater amounts of liquid can be removed depending on the load and the size of the apparatus.

The drying temperature may vary from 100 to 700C depending on the application.

A typical power input in a process and apparatus according to the invention is from 10 to 120 kWh.

Turning to the rotary heat exchanger, incoming air is passed through the bottom of the rotary heat exchanger where its temperature is lowered sufficiently to enable it to be passed to an evaporator coil. This evaporator coil further lowers the temperature of the air to allow water to be removed. The resulting cold air is then returned to the top of the rotary heat exchanger where it is warmed to produce warm dry air.

Heat may be reclaimed from the evaporator coil and passed to a reclaim coil. The warm dry air from the top of the rotary heat exchanger may be passed to the reclaim coil where it is heated to produce hot dry air which may then be used to dry the product.

The product may be treated with electromagnetic waves which are adapted to bring moisture to the surface of the product. Suitable electromagnetic waves are those of microwave or radiofrequency. Infra-red waves may be used but are less preferred.

The invention is particularly effective in controlling bacterial growth. It is thought that one reason for the effectiveness of this control is the rapid change of temperature in the airflow which interferes with the metabolism of bacteria. Thus ambient air at, for example 20 0 C may be hated to, for example, 40 0 C in an external heat exchanger. It may then be cooled to, for example, 15 0 C in the bottom of a rotary heat exchanger and to, for example, 10°C in the evaporator coil. This cold air may then be heated to, for example, 0 C in the top of the rotary heat exchanger. The heated air may then be further heated to, for example, 70 0 C in a reclaim coil before being passed to the product to be dried.

Finally, the warm moisture-laden air is passed to, for example, an external heatexchanger. This heat-exchanger cools the air from, for example, 50 0 C to, for example, before it is vented to the atmosphere.

SUBSTITUTE SHEET (RULE 26)- WO 95/33169 PCT/AU95/00324 4 Turning to the drawings, an external structure 10 encloses elements of the apparatus according to the invention and which will be described in sequence with reference to the airflow. Thus numeral 11 indicates an air-intake duct which leads downwardly to a rotary heat exchanger 12 located at one end of the structure 10. Rotary heat exchanger 12 is divided into a bottom portion 12A and a top portion 12B. Warmed ambient air passes down through inlet duct 11 and is mixed with a proportion of warm moisture-laden air coming off the product. Heat is extracted from the mixture in bottom portion 12A of the rotary heat exchanger and passed to upper portion 12B.

Cooled air passes to evaporating coil 13 where water is remo cd. Evaporating coil 13 is located near the bottom of an ancillary chamber 14. The further cooled air is reflected from the rear of ancillary chamber 14, passes above evaporating coil 13 and then through upper portion 12B of rotary heat exchanger 12. The heated air is then passed upwards, directed by baffle 15, to a horizontal, upper duct 15A located at the top of structure A first fan 16 is located in horizontal upper duct 15A. Heated air is forced by first fan 16 through reclaim coil 17 and second fan 18. The now hot air is passed down through a vertical duct 18A located at an end of structure 10 distal from heat exchanger 12.

The hot air is passed from vertical duct 18A into an internal housing 19 in which is located an upper conveyor 20, conveying from right to left as viewed in the drawing.

Part of the air-stream is forced into housing 19 by means of a bottom-mounted, third fan 18B. A lower conveyor 21, conveying from left to right for recovery of product conveyed on the upper conveyor 20, is located below upper conveyor 20. Hot air is passed over and/or through product on upper conveyor Hot moisture-laden air is then passed to a downwardly sloping duct 22 and an upper internal duct 23. Sloping duct 22 conducts part of the warm moisture-laden air to lower portion 12A of the rotary heat exchanger 12. Internal duct 23 leads the remaining part of the warm moisture-laden air, through first filter 23A, to an exit duct 24. It should be noted that heating equipment such as microwave apparatus may be located on the structure of internal duct 23.

Numeral 25 refers to a heat control bypass linked to reclaim coil 17 and located outside external structure 10. The purpose of this integer is to dump excess heat from reclaim coil 17 to the atmosphere.

SUBSTITUTE SHEET (RULE 26) WO 95/33169 PCT/AU95/00324 Turning to Figure 2, an external heat exchanger 30 is supplied with warm moisture-laden air from exit duct 24 passing through second filter 31. The cooled air is returned to atmosphere through first exhaust fan 32. Incoming ambient air passes to heat exchanger through third filter 33. The heated air is then passed to second exhaust fan 34 and then through heating coil 35, which receives some heat from reclaim coil 17. The warm ambient air is then passed to inlet duct 11.

Turning to Figure 3, numerals 12, 12A and 12B refer to the same integers discussed above. Numeral 40 refers to a barrier ltbtween lower portion 12A and upper portion 12B of the rotary heat exchanger 12. Nr.merals 41 and 42 indicate the arrows showing the direction of flow of warm ambient air to lower portion 12A. Numeral 43 indicates the direction of flow of hot air from upper portion 12B. Finally, numeral 44 indicates the direction of rotation of the w'.,el constituting the essential operative part of the rotary heat exchanger 12.

SUBSTITUTE SHEET (RULE 26)

Claims

6- The claims defining the invention are as follows: 1. A drying process which includes the steps of: taking in ambient air via an air intake duct; passing said ambient air to a primary heat exchanger couple with a dehumidifier, where it is cooled, dehumidified and then reheated; passing the heated air produced by step to a heat reclaim coil where the air is heated further and the relative humidity decreased; passing the hot dry air produced by step over or through product to achieve the desired drying effect; and venting the warm moist air produced by step to the atmosphere. 2. A process as claimed in claim 1, wherein the ambient air used in step is warmed by heat-exchange in a secondary heat-exchanger with the warm moist air passed to step 3. A process as claimed in claim 1, wherein the primary heat exchanger of step 15 is conveniently a rotary heat exchanger which is located adjacent a dehumidifier whereby warmed ambient air enters through one part of the primary heat exchanger rotary mechanism, in which it is cooled, passes through the dehumidifier, in which it loses moisture, and then to another part of the primary heat exchanger rotary mechanism, in which it gains heat. 4. A process as claimed in claim 1, wherein in step the product may be dried in bulk or in some broken-up or comminuted form. 4. A p KLM:NP #26465RSI 24 Math 1998 WO 95/33169 PCT/AU95100324 7 A process as claimed in claim 1, wherein a proportion of the warm moist air from step may be passed to the primary heat exchanger to mix with incoming ambient air. 6. A process as claimed in claim 1, wherein the drying temperature may vary from 100 to 70 0 C depending on the application.
7. An apparatus for carrying the process according to claim 1 into effect.
8. An apparatus as claimed in claim 7, wherein in a rotary heat exchanger, incoming air is passed through the bottom of the rotary heat exchanger where its temperature is lowered sufficiently to enable it to be passed to an evaporator coil, and wherein the evaporator coil further lowers the temperature of the air to allow water to be removed, and wherein the resulting cold air is then returned to the top of the rotary heat exchanger where it is warmed to produce warm dry air.
9. An apparatus as claimed in claim 8, wherein heat is reclaimed from the evaporator coil and passed to a reclaim coil. An apparatus as claimed in claim 9, wherein warm dry air from the top of the rotary heat exchanger is passed to the reclaim coil where it is heated to produce hot dry air which may then be used to dry the product.
11. An apparatus as claimed in claim 7, which con, 1 rises means for the product to be treated with electromagnetic waves which are adapted to bring moisture to the surface of the product. SUBSTITUTE SHEET (RULE 26) I-