GB2109256A

GB2109256A - Improved method of drying and apparatus for drying

Info

Publication number: GB2109256A
Application number: GB08133897A
Authority: GB
Inventors: William Bradshaw
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-11-10
Filing date: 1981-11-10
Publication date: 1983-06-02

Abstract

The invention relates to a method of drying moisture containing material by subjecting the material to a pressure below atmospheric pressure e.g. by a steam ejector 18. Because of the reduced pressure, moisture is given up by the material to be dried in the form of water vapour along line 14. This vapour is passed into a condenser 24, which is vented to allow air to escape, but the vapour travelling towards the vent 48 is caused to counter flow with an incoming relatively cold fluid from a supply 44 so that the temperature of the vapour/ air mixture is reduced in order to reduce the quantity of water vapour carried away by the vented air. Consequently, there is a saving in heat energy, which manifests itself in the condensate. <IMAGE>

Description

SPECIFICATION Improved method of drying and apparatus for drying The specification of United Kingdom Patent Application No.8014313 (BRADSHAW) describes a method and apparatus for drying moisture-containing materials, especially materials in sheet form such as fabrics and paper. In that method, the material is passed through a vacuum chamber and water vapour is given off by the material: in addition condensate from the vapour evolved in the vacuum chamber is passed through a heat exchanger in the vacuum chamber to improve the drying efficiency.

In a practical application of the invention of Specification No.8014313, which is used for the drying of textile materials a steam ejector is used to provide the vacuum pump, and the output from the steam ejector, which combines the vapour supplied to operate the ejector and that given off by the fabric, is condensed in a separate condenser (or a series of condensers). There is a residual condensate at approximately 185oF, which is used, at least in part, in the heat exchanger within the drying chamber. A disadvantage of the system is that a considerable amount of heat energy is lost in the venting of air from the condenser. The proportion of water vapour entrained in the vented air increases with the temperature of the vapour/air mixture.For example, 100 pounds (1bs) of air would entrain: At 60 F- approximately 21bs water At 185 F- approximately 90 Ibs of water.

At 200"F - approximately 210 Ibs of water.

According to a first aspect of this invention in a method of drying moisture-containing material, by subjecting the material to a pressure below atmospheric pressure in a drying chamber, the water vapour given off by the material is passed into a condenser, which is vented to allow air to escape, but the vapour in travelling towards the vent is caused to counterflow with an incoming relatively cold fluid whereby the temperature of the vapour/air mixture is reduced so as to reduce the quantity of water vapour carried away by the vented air. Preferably the incoming relatively cold fluid is water, and this may be at normal mains-water temperature.

Preferably, the vapour lair flowing towards the vent is forced to pass obstructions, and/or is forced to follow a tortuous path. For example, it may have to negotiate a plurality of baffles which partly obstruct the path to the vent. In that case, it is also preferred to direct the incoming fluid on to the baffles, so as to provide a heat exchange effect in the region of countercurrent flow.

As a result of the considerable saving in heat energy, which occurs in the condenser, which maifests itself in the condensate, it is possible to provide condensate at a higher temperature than that obtained by use of the invention described in Patent Specification No. 8014313 for use in the heat exchanger of the drying chamber. It should be understood however that the present invention is not restricted to use in the method described in the said Patent Specification. It could, for example, be used in a process where the condensate obtained from the evolved vapour is not passed through a heat exchanger in the drying chamber. Obviously however the heat in the condensate (or some of it) must be used in some way- e.g. in a space heating system - if the invention is to be used to any advantage.

According to another aspect of the invention, apparatus for drying moisture-containing material comprises: a drying chamber; pump means for extracting fluid from the drying chamber to reduce the pressure therein below atmospheric pressure; a condenser adapted to receive vapour removed from aterial in the drying chamber; an air vent in or connected to the condenser; and means for supplying relatively cold fluid into the condenser, the arrangement of the vent and the cold fluid supply means being such as to produce countercurrent flow of vapour travelling towards the vent and cold fluid entering the condenser.

Preferably, the internal arrangement of the condenser is such as to cause vapour flowing towards the vent to be obstructed and/or to follow a tortuous path. It is preferred to provide a plurality of baffles within the condenser in the region where countercurrent flow will occur. It is further preferred to position the cold fluid supply means so that the cold fluid entering the condenser falls on the baffles.

In the preferred construction, fluid-flow control means is provided for regulating the supply of relatively cold fluid in response to the vapour load in the condenser, so as to control the temperature of the condensat. It is preferred to utilise a stream ejector pump to create the vacuum in the drying chamber, but this is not essential since other types of pump could be used, depending on the kind of material to be dried and the required throughput of the apparatus. Other preferred features of the invention, will appearfrom the following description of one form of apparatus, and its method of use, both in accordance wiith the invention, which is given here by way of example only, with reference to the accompanying drawing, which is a diagrammatic representation of the apparatus.

The apparatus illustrated can be used for drying a variety of moisture-containing materials. For example, it can be used for drying textile fabrics in continuous lengths, or paper after it leaves the papermaking machine. A drying chamber 10 is provided, and the mositure-containing material which is to be dried is either passed continuously through the chamber 10, or alternatively, a batch of material to be dried is placed in the drying chamber 10 for a drying operation. For present purposes, it is not necessary to discuss in detail methods of supplying material to be dried and extracting from the drying chamber, since any convenient method could be used.In the specification of United Kingdom Patent Application No: 801 431 3 for exam ple, several methods of passing sheet material in continuous form through a drying chamber such as the chamber 10 are described.

A coil 12 is provided in the drying chamber 10, and as will be hereinafter described, hot water is passed through the coil 12. Hence, the coil 12 can be regarded as the primary circuit of a heat exchanger, the purpose of which is to heat the interior of the drying chamber, and of course to heat nay moisturecontaining material within the chamber 10.

Means are provided for sealing the drying chamber 10 against the ingress of air, so that it can also be regarded as vacuum chamber. Avacuum pip 14 is connected into the chamber 10, and leads to the suction inlet 16 of a steam ejector 18. Steam ejectors are well known in themselves, and essentially they operate on the Venturi principle. Steam 20 at a relatively high pressure enters the steam ejector 18 at an inlet end, and flows out at the other end. In the Venturi, the pressure is greatly reduced, and this applies a vacuum effect through the vacuum pipe 14, to the interior of the drying chamber 10. Thus, the steam ejector operates as a pump, for evacuating the interior of the drying chamber 10.Whilst it would be possible to use other kinds of pump (e.g. liquid ring pumps) it is preferred to use the steam ejector type of pump, because it is possible to obtain quite high throughputs of vapour, and if moisture-containing material is passed continuouslythrough the drying chamber 10, it is essential to be able to remove quite high volumes of vapour from the drying chamber.

A pipe 22 leads from the outlet end of the steam ejector 18 into a cylindrical condenser 24. On top of the cylindrical portion of the condenser, there is an upstanding "chimney" 26, with a dome 28 at its upper end. A cold water inlet chamber 30 opens at its lower end into the "chimney" 26, but is closed at its top end. A large baffle plate 32 is provided in the cylindrical portion of the condenser 24, spaced below the open lower end of the "chimney" 26, and this baffle 32 is welded to the internal cylindrical wall of the condenser 24, and is provided with a closure member 34 welded to the baffle 32 and to the internal cylindrical wall of the condenser 24, and is provided with a closure member 34 welded to the baffle 32 and to the internal cylindrical wall of the condenser 24 adjacent to the inlet from the pipe 22.

This leaves an opening 36, through which vapour can pass from the cylindrical portion of the condenser into the lower part of the "chimney" 26, but it will be noted that this opening 36 is remote from the inlet at the pipe 22.

Within the "chimney" 26, there is a series of baffle plates 38 each of which extends most of the way across the width of the "chimney" and is secured at one end and along its sides to the internal wall of the "chimney". The lower baffle 32 and the series of baffles 38 provide a tortuous path for vapour passing from the region of the interior of the condenser, adjacent to the inlet from the pipe 22, to the top end of the "chimney" 38. This vapour has to travel along the length of the cylindrical portion, back under the lower baffle 38, and then in a zig-zag path between the baffles 38.

A water supply pipe 40 leads from the outlet end of the coil 12, to the interior of the dome 28, and there is a series of downwardly directed sprays 42 on the underside of the dome. Hence, hot water leaving the coil 12 is directed into the upper end of the "chimney" 26, and mixes with condensate water 44 in the lower part of the condenser 24. In addition however, there is a cold water supply pipe 44, connected at its inlet end to a water main, and supplying cold water to a spray 46 in the top end of the chamber 30. Hence, cold water supplied through the spray 46 falls on to the baffle plates 38, and helps to cool those plates. An air vent 48 is connected into the upper end of the chamber 38. It is of course necessary to allow air to escape from the interior of the condenser, through the vent.As a result of this, water vapour in the condenser tends to travel with the air out through the vent, and if the temperature of the air/water vapour mixture leaving the condenser via the vent is relatively high, then a considerable amount of water will escape through the vent, taking with it the useful energy in the form of the sensible and latent heat of the water which escapes in this manner.

It is now possible to explain the basic functioning of the apparatus so far described, and for this purpose, reference will be made to specific operating conditions. It will of course be appreciated, that the operating conditions can vary, but specific figures are useful in explaining the advantage to be obtained from the invention.

In this specific example, steam is supplied to the inlet of the steam ejector 18 at the rate of 738 kilogrammes per minute, and this steam flows straight through the steam ejector 18, and then through the pipe 22 into the condenser 24. The vacuum effect produced bythe steam ejector causes approximately 500 kilogrammes per minute of water vapour to be evolved from material being dried in the drying chamber, and this water vapour flows into the steam ejector, and mixes with the steam flowing into the condenser. In addition, approximately 82 kilogrammes per minute of air will be drawn out of the dryer 10, and mixed with the steam entering the condenser. A total of 1240 kilogrammes of water vapour enters the condenser through the pipe 22, and this is mixed with 82 kilogrammes of air.

A considerable amount of the water vapour, condenses to provide a condensate 44 at approximately 2100F in the sump of the condenser. However, the mixture of air and water vapour will attempt to flow out through the vent 48, and in so doing, will pass around the tortuous path provided by the baffles 32 and 38. At the same time, approximately 82 kilogrammes per minute of cold water at 600F enters through the cold water supply pipe 44, and through the spray 46. There is thus produced counter-current flow in the chimney" 26 and in the chamber 30, between the air and water vapour travelling upwardly, and the cold water falling downwardly. Hence, the temperature of the air/water vapour mixture will be considerably reduced, and this has the effect of reducing the quantity of hot water carried away by the air venting from the condenser.

An outlet pipe 50 leads from the sump of the condenser, to a pump 52, which passes condensate water at approximately 21 00F from the condenser sump, through a non-return valve 54, and thence through a feed pipe 56 to the inlet end of the coil 12 in the dryer 10. Again, in the specific example under consideration, it is possible to pass approxmately 409 kilogrammes per minute of hot water at 2100F through the feed pipe 56. There is a drop of approximately 20 F in the coil 12, so that the water carried away by the pipe 40 to the sprays 42, is at approximately 190 F. The energy absorbed within the dryer, from the coil 12, is therefore that given up by the temperature drop from 210 F to 1900F in the condensate water.

The condenser is also provided with a condensate water level control device 58 of known construciton, which controls a valve 60 in a condensate return pipe 62. to some apparatus in which the high temperature of the condensate water can be used (for example, in the case where the dryer is used to dry fabric), the hot water passing out through the pipe 62, can be used in a fabricwahing plant. If the level of water in the condenser 24 reaches an upper limit, the valve 60 is opened to allow excess water to pass out through the pipe 62, but if the level of condensate water in the condenser falls below a preset level, the valve 60 is closed, so that all the water being pumped out of the condenser, is used in the coil 12.

A control system 70 is also employed for regulating the supply of cold water through the inlet pipe 44. Essentially, this control system senses the temperature of the air flowing towards the vent and controls a valve 72 in the water supply pipe 44. If the temperature of the air flowing towards the vent drops, indicating a decrease in the vapour load, then the valve 72 is closed or partially closed, in order to reduce the supply of cold water into the chamber 30.

It will be appreciated that the temperature sensing can be carried out at any point in the condenser.

Claims

1. A method of drying moisture-containing material by subjecting the material to a pressure below atmospheric pressure in a drying chamber, the water vapour given off by the material being passed into a condenser, which is vented to allow air to escape, but in which the vapour intravelling towards the vent is caused to counter-flow with an incoming relatively cold fluid whereby the temperature ofthevapour/air mixture is reduced so as to reduce the quantity of water vapour carried away by the vented air.

2. A method of drying moisture-containing mate rial as claimed in Claim 1, in which the relatively cold fluid is water.

3. A method of drying moisture-containing mate rial as claimed in Claim 2, in which the incoming water is at normal mains-watertemperature.

4. A method of drying moisture-containing mate rial as claimed in any one of Claims 1 to 3, in whch the vapour/air flowing towards the vent is forced to pass obstructions.

5. A method of drying moisture-containing mate rial as claimed in any one of claims 1 to 4, in which the vapourfair flowing towards the vent is forced to pass obstructions.

6. A method of drying moisture-containing mate rial as claimed in Claim 4 or Claim 5, in which the vapourlair flowing towards the vent has to negotiate a plurality of baffles which partly obstruct the path to the vent.

7. A method of drying moisture-containing material as claimed in Claim 6, in which the incoming relatively cold fluid is directed on the the baffles, so as to provide a heat exchange effect in the region of counter-current flow.

8. Apparatus for drying moisture-containing material comprising: a drying chamber; pump means for extracting fluid from the drying chamber ot reduce the pressure therin below atmospheric pressure; a condenser adapted to receive vapour removed from material in the drying chamber; an air vent in or connected to the condenser; and means for supplying relatively cold fluid into the condenser, the arrangement of the vent and the cold fluid supply means being such as to produce counter-current flow of vapour travelling towards the vent and cold fluid entering the condenser.

9. Apparatus for drying moisture-containing material as claimed in Claim 8, in which the internal arrangement of the condenser is such as to cause vapour flowing towards the vent to be obstructed.

10. Apparatus for drying moisture-containing material as claimed in Claim 8 or Claim 9, in which the internal arrangement of the condenser is such as to cause vapour flowing towards the vent to follow a tortuous path.

11. Apparatus for drying moisture-containing material as claimed in Claim 9 or Claim 10, in which there is provided a plurality of baffles within the condenser in the region where counter-current flow will occur.

12. Apparatus for drying moisture-containing material as claimed in Claim 11, in which the cold fluid supply menas is positioned so that the cold fluid entering the condenser falls on to the baffles.

13. Apparatus for drying moisture-containing material as claimed in any one of Claims 8 to 12, in which fluid-flow control means is provided for regulating the supply of relatively cold fluid in response to the vapour load in the condenser, so as to control the temperature of the condensate.

14. Apparatus for drying moisture-containing material as claimed in any one of Claims 8 to 13, in which a steam ejector pump is utilised to create the vacuum in the drying chamber.

15. A method of drying moisture-containing materila substantially as herein described with reference to the accompanying drawing.

16. Apparatus for drying moisture-containing material constructed ans arranged substantially as herein described with reference to the accompanying drawing.