GB2135034A

GB2135034A - Drying apparatus

Info

Publication number: GB2135034A
Application number: GB08304058A
Authority: GB
Inventors: Russel Benstead; Alan Vernon Heaton
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1983-02-14
Filing date: 1983-02-14
Publication date: 1984-08-22
Also published as: GB2135034B; GB8304058D0

Abstract

Material fed through chamber 10 is dried by contact with heating elements 13, in the form of fins or a screw arranged to transport the material through the chamber 10. Heating element 14 superheats vapour in the chamber 10. Vapour is withdrawn from the chamber by compressor 16, then fed back to the central pipe of the heating elements 14, while pressurized liquid is injected from line 18, so that the compressor output is de-superheated, and its latent heat of evaporation given off. Excess pressurized water from the pipe 14 may be flashed off via a throttle 20, trap 21, and line 22 to make up the vapour fed from the chamber 10 to the compressor 16. Heat exchanger 24 increases the temperature of the superheated vapour returned to the chamber 10. <IMAGE>

Description

SPECIFICATION Drying apparatus The present invention is concerned with apparatus for drying a material by evaporation of a vapour therefrom. Although the invention has particular application to the drying of material containing water, it is generally applicable to the drying of materials carrying other liquid which can be removed as a vapour. Thus, in considering the invention broadly, reference will be made to a liquid and to a vapour, it being understood that the terms "liquid" and "vapour" refer to the same substance in its different phases.

The specification of U.K. Patent 1558913 describes a method and apparatus for drying in which the latent heat of evaporation is at least partially recovered from the vapour given off by the material being dried, and utalised to maintain the temperature of the apparatus.

This is achieved by withdrawing excess vapour from the drying chamber, compressing the vapour to increase its temperature and supplying the compressed vapour to a heat exchanger arranged to heat recirculating superheated vapour used for drying the material.

In this apparatus, the vapour is compressed such that the compressed vapour tends to condense in the heat exchanger to give up latent heat of evaporation. Pressurized liquid leaving the heat exchanger is flashed-off, to reduce its temperature, and the vapour produced thereby is recycled into the uncompressed circulating vapour.

In the arrangement of this prior art specification the pressurized vapour supplied to the heat exchanger is normally superheated.

United States patent specification 2192168 also discloses apparatus for drying paper which utilises the latent heat of the vapour given off by the paper. The vapour derived from the drying paper is compressed and supplied to drying drums over which the paper is rolled, the vapour then condensing within the drums to give off latent heat of evaporation.

According to the present invention, apparatus for drying a material by evaporation of vapour therefrom comprises a drying chamber, heat exchanger means associated with the drying chamber, means for feeding the material into the chamber into heat exchanging relationship with the heater to evaporate vapour from the material, a compresser to compress excess vapour from the chamber to raise its temperature, means to inject liquid to de-superheat the compressed vapour, the compressed saturated vapour being fed to the heat exchanger means to condense therein giving up latent heat of evaporation, a proportion of the pressurized liquid from the heat exchanger being supplied for injection by said means to inject, and means to reduce the pressure of the balance of the pressurized liquid.By injecting liquid to de-superheat the compressed vapour, heat transfer from the compressed saturated vapour to the heat exchanging means is very much improved. Substantially all the heat given off by the compressed vapour to the heat exchanging means is latent heat of evapoaration and this heat is evolved directly on contact with the walls of the heat exchanging means. It will be appreciated that the drying chamber may be at any desired pressure.

Preferably the means to reduce the pressure comprises throttling means at which vapour is flashed-off and at least a proportion of the vapour flashed-off is fed back to make up the uncompressed vapour from the chamber. Conveniently, the apparatus may include a preheater using residual hot liquid from the means to reduce pressure to preheat the material to be dried. Any excess flashed-off vapour may also be employed for preheating.

The means to inject may include a pump to inject liquid into the compressed vapour.

However, instead the means to inject may be arranged to inject liquid into the uncompressed or partially compressed vapour. By injecting the pressurized liquid into the suction line of the compressor, or alternatively into the compressor body, the compressor and the vapour during compression is cooled. Furthermore, it may be preferable to avoid the use of the pump required to inject liquid into the compressed vapour, since such a pump may suffer from cavitation problems with liquid so close to saturation conditions.

In a preferred embodiment, the heat exchanger means comprises the heater element of a contact dryer. There may be additionally provided means to recirculate vapour through the chamber and means to heat the recirculated vapour. These additional means can assist in the removal of vapour evaporating from the material being dried and can additionally enhance the drying process. Said means to heat recirculated vapour may comprise a heat exchanger supplied with hot compressed saturated vapour either derived from said above mentioned compressor or from a second compressor.

It will be appreciated that it is important for proper operation of the above described apparatus to substantially exclude air from the recirculation system. Preferably, therefore, the contact dryer and the drying chamber are arranged such that air ingress into the chamber is limited to less than 1% of the rate of evaporation from material being dried.

An example of the present invention will now be described with reference to the accompanying drawing which is a schematic diagram illustrating the recirculating liquid and vapour flows through a contact dryer embodying the present invention.

Referring to the drawing, a contact dryer of the kind known as a trough dryer is incorporated in a pressurized recirculating vapour system. The trough dryer is formed of a trough like chamber 10 into which material to be dried is fed at one end through an inlet port 11 and dry material is emitted at the other end through an outlet port 1 2. The material in the chamber 10 is dried by contact with heating elements 1 3. The heating elements 1 3 conveniently take the form of fins or flights or preferably a screw arranged to transport the material being dried along the length of the chamber 10. The heating elements 1 3 are heated by hot vapour passing through a central closed pipe 1 4 extending from one end of chamber 10 to the other.

Vapour given off by drying the material in the chamber 10 fills the chamber 10 which is arranged to be at only slightly above atmospheric pressure. The temperature of the heating element 1 4 is such that the vapour in the chamber 10 is superheated.

Superheated vapour from the chamber 10 is withdrawn from the chamber on a line 1 5 by a compressor 1 6 which is arranged to compress the superheated vapour from the chamber 10 raising its temperature. The compressed vapour from the compressor 1 6 is fed back on a line 1 7 into the central pipe 14 of the heating elements in the chamber 10.

Pressurized hot water (assuming the material to be dried is carrying water) is injected from a line 1 8 into the low pressure superheated vapour immediately upstream of the compressor 16. The arrangement is such that sufficient water is injected from the line 1 8 so that the compressed heated vapour at the output of the compressor 1 6 is de-superheated so that saturated vapour is supplied to the heater of the contact dryer. As a result, latent heat of evaporation is given off by the saturated vapour condensing in the pipe 14 of the contact dryer. This provides a very effecient mechanism for transferring heat from the vapour to the hot elements of the contact dryer. Pressurized hot water leaves the pipe 1 4 and is fed, via an air separator 19, as the injection water on the line 18.Excess pressurized water from the pipe 1 4 is passed through a pressure relief valve or throttle 20 where vapour flashes-off in a trap 21.

With the example of the invention as described thus far it can be seen that the heating elements 1 3 of the contact dryer in the chamber 10 are heated by saturated compressed vapour derived from the vapour evaporated from the material being dried. The heat given off by the heating element 1 3 is latent heat of evaporation as the saturated vapour condenses. In this way, the latent heat of evaporation from the water evaporated out of the material being dried is substantially retained in the system.

The illustrated example also shows low pressure vapour derived from the trap 21 and supplied on a line 22 to make up the vapour from the chamber 10 fed to the compressor 16.

Additionally, in the example illustrated, a circulating fan 23 is provided to recirculate superheated vapour through the interior of the chamber 20. In the arrangement shown the circulation is counter-flow. Conveniently, the recirculated superheated vapour is passed through a heat exchanger 24 which may be arranged to further increase the temperature of the superheated vapour being returned to the chamber 10 thereby to enhance the drying processes in the chamber. The heat exchanger 24 is fed with saturated pressurized vapour on a line 25 from a second compressor 26 arranged in parallel with the compressor 1 6 also comprssing low pressure vapour from the chamber 10, made up by additional vapour on the line 22.Again, high pressure water is injected from a line 27 on the low pressure side of the compressor 26 to ensure that the output vapour on the line 25 is saturated.

In this way, heat from the saturated pressurized vapour on the line 25 is given up in the heat exchanger 24 by condensation, again ensuring efficient transfer in the exchanger 24. Pressurized water condensed in the exchanger 24 is fed on a line 28 to an air separator 29 and thence returned on the line 27 for injection at the compressor 26. The excess water from the separator 29 is passed through a pressure relief valve 30 where vapour is flashed-off in a trap 31. Flashed-off vapour from the trap 31 may also be added to the make-up vapour on line 22.

Hot water from the traps 21 and 31 may flow to waste on lines 32 and 33. However, this hot water may be employed to preheat the material to be dried supplied to the input port 11 of the contact dryer chamber 10.

Furthermore, excess vapour in the system can escape through a pressure relief valve 34 and this vapour also may be employed to preheat the material to be dried.

It will be appreciated that the pumped system illustrated in the drawing is not self starting. The temperature of the apparatus must first be raised to achieve the requisite vapour pressure before further heat can be added to the apparatus as necessary by operation of the compressors 1 6 and 26. To start up the apparatus, a boiler 35 is provided connected to the lines 15, 1 7 and 25 of the recirculating vapour system and arranged to produce water vapour to heat up the apparatus until further water vapour is being evolved by drying of material in the chamber 10.

Then, the compressor 1 6 and 26 can be started and the valves 36 connecting the boiler 35 to the apparatus can be shut off.

Claims

1. Apparatus for drying a material by eva poration of vapour therefrom comprising a drying chamber, heat exchanger means associated with the drying chamber, means for feeding the material into the chamber into heat exchanging relationship with the heat exchanger means to evaporate vapour from the material, a compressor to compress excess vapour from the chamber to raise its temperature, means to inject liquid to de-superheat the compressed vapour, the compressed saturated vapour being fed to the heat exchanger means to condense therein giving up latent heat of evaporation, a proportion of the pressurised liquid from the heat exchanger being supplied for injection by said means to inject, and means to reduce the pressure of the balance of the pressurized liquid.

2. Drying apparatus as claimed in claim 1 wherein the means to reduce the pressure comprises throttling means at which vapour is flashed-off and at least a proportion of the vapour flashed-off is fed back to make up the uncompressed vapour from the chamber.

3. Drying apparatus as claimed in claim 2 and including a preheater using hot liquid from the means to reduce pressure to preheat the material to be dried.

4. Drying apparatus as claimed in any preceding claim wherein the means to inject includes a pump to inject liquid into the compressed vapour.

5. Drying apparatus as claimed in any of claims 1 to 3 wherein the means to inject is arranged to inject liquid into the uncompressed or partially compressed vapour.

6. Drying apparatus as claimed in any preceding claim wherein the heat-exchanger means comprises the heater element of a contact dryer.

7. Drying apparatus as claimed in any preceding claim and including means to recirculate vapour through the chamber and means to heat the recirculated vapour.

8. Drying apparatus as claimed in claim 7 wherein said means to heat the recirculated vapour comprises a heat exchanger supplied with hot compressed saturated vapour.

9. Drying apparatus as claimed in claim 8 wherein the hot compressed saturated vapour is derived from said compressor.

10. Drying apparatus as claimed in claim 8 and including a second compressor to compress vapour to supply to the heat exchanger.

11. Drying apparatus as claimed in any of claims 1 to 5 and including means to recirculate vapour through the chamber and wherein the heat exchanger means is arranged to heat the recirculated vapour and thereby to heat and evaporate further vapour from the material.