GB2081866A

GB2081866A - A thermal recuperation system and a heat exchanger for use therein

Info

Publication number: GB2081866A
Application number: GB8110278A
Authority: GB
Original assignee: Hall Thermotank International Ltd
Current assignee: Hall Thermotank International Ltd
Priority date: 1980-08-14
Filing date: 1981-04-01
Publication date: 1982-02-24
Also published as: GB2081866B

Abstract

A thermal recuperation system for a dryer is disclosed which uses a glass tubed heat exchanger (25) in the outlet duct (22) of the dryer and passes a heat transfer liquid through the glass tubes and through a conventional metal tubed heat exchanger (23) in the inlet duct (21) of the dryer. Corrosive liquid condensed on the tubes of the glass heat exchanger (25) is collected in a gutter 26. A construction for a glass tubed heat exchanger is described. <IMAGE>

Description

SPECIFICATION An improved thermal recuperation system and a heat exchanger for use therein Technical field It is known to recover some of the heat energy leaving a dryer and to return it to the inlet of the dryer. Such heat recovery has become increasingly popular with maltsters, since a malting kiln requires substantial amounts of thermal energy.

It is known to introduce sulphur vapour into a malting kiln to remove possibly harmful byproducts, and the hot gas leaving the kiln is therefore highly corrosive. Conventional metal tubed heat exchangers are unsatisfactory in such a corrosive environment.

Background art Glass tubed heat exchangers are known forthermal recuperation processes in corrosive environments and one piece of equipment which has been proposed for such applications has a glass tube cluster of general rectangular parallelepipedic shape 1600 mm long, 850 mm wide and 300 mm high. Such a heat exchanger might contain 66 litres of heat transfer liquid.In orderto provide a thermal recuperation potential for a large kiln where typically 45 m3 of saturated air per second will be flowing to atmosphere, it would be necessary to use a large number of the above-described units operating in parallel on the air side, and this would either involve dividing up the outlet air flow with duct dividers (thereby increasing the expense of the recuperation equipment and increasing the static pressure in the outlet) or else siting a pluraliyofthe heat exchangers side-by-side and one above another within the duct.

With this latter arrangement, the entire heat exchange equipment, together with the supply and return mains for the heat transfer liquid, will be subjected to the corrosive environment.

Disclosure of invention This invention, in one of its aspects, seeks to avoid these problems with a thermal recuperation system for a dryer comprising a first multitube heat exchanger in an inlet to the dryer, a second multitube heat exchanger in an outlet duct of the dryer and means to circulate a heat transfer liquid between the tubes of the two heat exchangers which is characterised in that the second heat exchanger comprises a pluraity of successive vertial banks of glass tubes, extending across the entire width of the outlet duct from at least one header box disposed outside the duct, the banks being arranged one beyond the other in the flow direction of air through the duct, adjacent banks having heat transfer liquid flowing therethrough in alternate directions, with the liquid leaving the upstream bank going to the first heat exchanger and the liquid entering the downstream bank coming from the first heat exchanger.

Each bank of tubes thus defines a pass which extends across the entire cross-section of the duct.

Each bank can consist of a single vertical row of tubes or two (or even more) rows of tubes. The number of tubes in each row will depend onthe height of the duct but in a typical case 50 tubes per row would be employed.

Desirably the spacing between the axes of adjacent tubes in each row is twice the diameter of each tube. Suitablythetubes in adjacent rows are aligned so that a common transverse plane through the duct passes through the axes of one tube in each row.

Desirably, the spacing between the axes of adjacent tubes in adjacent rows is also equal to twice the diameter of each tube.

Each bank of tubes desirably contains at least 50 litres of liquid and suitably at least 100 litres. The pressure in the tubes will normally be around 1 atmosphere gauge with a pressure drop from end to end of each tube of the order of 0.035 kg/cm2 ( psig).

The heat exchange liquid will normally be waterbased and a propylene glycol solution (e.g. 15%) is preferred.

Twenty-five per cent heat recovery can easily be obtained using the system of the invention.

Although the invention has particular application to thermal recuperation in a malting kiln it is not seen as being limited to that application. It can be used in any kiln or dryer where the gaseous effluent is corrosive and the operating temperatures are acceptable.

The key component in a system of the invention is a glass tubed heat exchanger capable of operating with many hundreds of kilograms of a pressurised liquid flowing within an array of long tubes. Such a heat exchanger represents a further feature of the present invention.

According to a further aspect of the present invention, a multitube heat exchanger comprising a plurality of rows of glass tubes extending across a gas-flow path between first and second spaced-apart tube plates, the tube ends being located in clearance holes in the respective tube plates and being bonded thereto by an annular layer of rubber-elastic material, is characterised in that the tube plates are each provided with baffle strips extending in the direction of the said rows and fixed to the tube plate between two adjacent rows to (a) strengthen the tube plate against bowing under the pressure exerted thereon by the heat transfer medium flowing in the tubes and (b) divide the rows into separate passes for the said heat transfer medium.

Desirably the baffle strips are of the same thickness as the tube plate and are welded thereto at last in regions between adjacent tubes in the rows on each side thereof.

Suitably each tube plate is provided with a perimeter wall of the same height (measured normal to the surface of the tube plate) as the baffle strips whereby a cover plate assembly secured to the perimeter wall makes sealing engagement with the edge of each baffle strip. The cover plate assembly may be divided into sections normal to the elongate direction of the baffle strips and may include a resilient sealing sheet and an overlying pressureresistant cover suitably braced on its outer surface against bowing under the influence of the pressure of the heat transfer medium.

Suitably further baffle members are provided acrdss the tube plate normal to the baffle strips to divide each row into groups of tubes, each baffle member also being fixed to the tube plate and providing a flange coplanar with the outer surface of the perimeter wall, whereby the cover plate assembly can also be secured to the baffle members. The baffle members are desirably angle section members fixed to the tube plate so that a flange thereof is spaced from the tube plate, overlies the ends of some tubes in each row, and provides a fixing surface for the cover plate assembly. Desirably where this arrangement of angle section baffle members is employed, no tube has both its ends overlain by a flange of a baffle member, so that each tube can be withdrawn through one tube plate or the other.

In a preferred heat exchanger in accordance with the invention some 1200 tubes of borosilicate glass, each some 4.8 metres long, extend between rectangulartube plates 2000 x 1000 x 9.53 mm in size.

Each tube has an OD of 20 mm and an ID of 17.6 mm and is recived in a hole of diameter 21 mm in each tube plate. Support plates of thin material (e.g.

plastics sheets) can be used at intervals between the tube plates to support the runs of glass tubing. The tubes can be bonded in clearance holes in the support plates using a rubber-elastic material (e.g. a silicone rubber).

Preferably stainless steel is used for those metal parts of the heat exchanger which are likely to come into contact with the effluent gases, but carbon steel treated and coated with an epoxy protective paint could be used.

In the case quoted above where there are some 1200 tubes, these can suitably be divided into twelve groups of 100 tubes in each group by means of six strengthening baffle strips on one tube plate of the heat exchanger and five baffle strips on the other tube plate. The provision of transverse baffle members normal to the baffle strips permits the cluster of tubes to be isolated in groups which operate in parallel, so that in the event of a single tube breaking, only one group of tubes need be isolated from the system to permit the other groups to continue to be used.

Brief description of drawings The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an isometric view of one embodiment of heat exchanger in accordance with the invention, Figure 2 is a partially broken away view of one header box in a second embodiment of heat exchanger in according with the invention, Figures 3 and 4 are, respectively, sections on the lines Ill-Ill and IV-IV of Figure 2, to an enlarged scale, Figure 5 is a side view of the cover plates on the header box of Figure 2, and Figure 6 shows, purely schematically, a malting kiln employing a thermal recuperation system according to the invention Best mode for carrying out the invention In Figure 1 the glass tubes are indicated at 1, the tube plates at 2a and 2b and the support plates at 3.

There are 1200 glass tubes in all and each end is bonded in a clearance hole in the tube plate 2a and 2b using a silicone rubber (shown at 4 in Figure 4).

Each tube plate is associated with a cover plate 5 and is stiffened by vertically extending baffle strips 7 which are welded to the tube plate and are thick enough to prevent bowing of the drilled tube plate under operating conditions. In the case illustrated each tube plate was 2000 x 1000 x 9.53 mm and each baffle strip was also 9.53 mm thick. 316 grade stainless steel is used for the tube plates and the support plates (which can be of 16 gauge material).

The heat exchanger has an inlet header 8 and an outlet header 9 mounted on the same cover plate 5, each header being connected by four valvecontrolled pipes to the baffled water-box associated with the tube plate 2a. Horizontal divisions within the waterbox split the tubes into four parallel sections for partial use in the event of a tube breakage in one of the sections.

To assist in absorbing the load between the tube plates and to facilitate the assembly of the tube bundle between the tube plates, some of the glass tubes (in practice not more than one or two per cent) can be replaced with stainless steel tubes which can be bolted through the tube plates.

The heat exchanger illustrated would contain some 2255 litres (497 gallons) of glycol solution and is designed to operate with a pressure drop of some 0.035 kg/cm2 ( psig) along each tube (i.e. with twelve passes involved there is a total pressure drop of 0.42 kg/cm2 (6 psig) between the inlet and outlet headers). The weight of solution is 2255 kg.

Figures 2 to 5 show a modified form of header box in which the cover plate is in four sections 5a...5d (only two of which are shown in Figure 2).

Figures 2 to 4 show the manner of fixing the longitudinal baffle strips 7 and transverse baffle members 10 to the tube plate 2a. These fixings strengthen the tube plate against bowing, caused by hydraulic pressure in the header box, divide the bundle of tubes 1 into passes and the passes into four, roughly equal, sections.

Each cover plate section 5a...5d has an inlet tube 8a...8dwhich opens into the spaces in the header box from which the first two vertical rows of tubes open and has an outlet tube 9a...9dwhich opens into the spaces in the header box from which the last twd vertical rows of tubes open.

The header box associated with the tube plate 2b at the other end of the heat exchanger is similar to that shown in Figure 2 but there are ony five baffle strips 7 in this header box and they are located in the positions shown by the dashed lines 7' in Figure 2.

From the above, it will be appreciated that the eleven baffle strips define twelve passes through the tube bundle, each pass constituting 100 tubes in two parallel rows, and the tube plates 2a and 2b define the sides of the duct through which the hot gas discharges from the kiln.

Each tube plate is welded to rigid stainless steel angle-section material which defines a perimeter wall 11 around the tube plate. Each baffle strip 7 is welded to the tube plate in the spaces between tubes (e.g. as shown at 12 in Figures 2 and 4). The transverse baffle members 10 are also welded to the tube plate between tube ends and the baffle strips 7 are welded between the wall 11 and a baffle member 10 in the upper and lower sections or to two baffle members 10 in the two mid-sections. The assembly of tube plate, perimeter wall, baffle strips 7, baffle members 10 and cover plate sections 5a...5d, defines a massively strong structure capable of withstanding the hydraulic forces within the header boxes without any appreciable bowing of the tube plate or cover plate sections.

The spacing of the tube plates is fixed by longitudinal angle sections 13 (see Figure 1) so that the metalwork structure of the heat exchanger protects the tubes 1 from the hydraulic pressure load in the elongate direction thereof.

From Figures 3 and 4 it can be seen that the outwardly facing edges of the baffle strips 7, and the outwardly facing flange surfaces of the perimeter wall 11 and the baffle members 10 are coplanar, so that four flat sheets of flexible resilient material 14 (see Figure 5) can serve as a seal for each pass and for the perimeter of each cover plate section 5a...5d.

Twin rows of threaded studs are provided on each baffle member 10 (not shown in Figure 2 - but see Figure 3) so that two adjacent cover plate sections rest on each baffle member. A single row of threaded studs (not shown) projects outwardly from the perimeter wall 11 to complete the securement of the cover plate sections over the respective tube plate 2a, 2b.

From Figure 3 it can be seen that some tube ends in each row are overlain by the flanges of the baffle members 10 and to ensure tht all tubes are removable from the bundle (for replacement in case of breakage), the baffle members are reversed in the other header box.

Twelve of the tubes 1 in the bundle of 1200 are of stainless steel and these can be symmetrically disposed within the bundle to initially assist in supporting the tube plates in their correct alignment prior to locating the glass tubes in place.

The angle sections 13 are provided with threaded studs or bolt holes and the vertical sections of the perimeter wall 11 are provided with threaded studs to permit the upstream and downstream duct walls 16 to be secured thereto. In this way the hostile gas is fed directly to the tube bundle and led directly away from it.

The upper angle sections 13 are overlain by a plate 15a (see Figure 1) to complete the duct wall and a surface 15b between the lower angle sections 13 defines a sloping drainage surface leading to a gutter (not shown) which extends the entire length between the tube plates on the gas outlet side of the tube bundle and drains to an outlet on one side. The gutter receives the condensate dripping off the tubes onto the drainage surface 1 sub.

The tube ends are fixed into the clearance holes in the tube plate using a known technique in which silicone rubber is poured over the tube plate while disposed horizontally, tube ends uppermost, and coats the then upper surface of the tube plate forming a fillet around each tube 1.

Figure 6 shows the kiln at 20, the air inlet at 21 and the air outlet at 22. 23 is the heat exchanger supplying heat to the cold inlet air, 24 is a source of SO2 and 25 the glass tubed heat exchanger in the kiln outlet. 26 is the gutter for dilute acid collected from the tubes of the heat exchanger 25. 27 is a pump for circulating glycol solution between the heat exchangers 23 and 25 and 28 is an optional heat pump plant which can be used to improve the efficiency of the heat recovery.

The tube plates 2a and 2b could be fabricated from carbon steel coated on both sides with a protective layer (e.g. silicone rubber).

CLAIMS (Filed on 3/8/81) 1. Athermal recuperation system for a dryer comprising a first multitube heat exchanger in an inlet to the dryer, a second multitube heat exchanger in an outlet duct of the dryer and means to circulate a heattransfer liquid between the tubes ofthetwo heat exchangers, characterised in that the second heat exchanger comprises a plurality of successive vertical banks of glass tubes, extending across the entire width of the outlet duct from at least one header box disposed outside the duct, the banks being arranged one beyond the other in the flow direction of air through the duct, adjacent banks having heat transfer liquid flowing therethrough in alternate directions, with the liquid leaving the upstream bank going to the first heat exchanger and the liquid entering the downstream bank coming from the first heat exchanger.

2. A system as claimed in claim 1, in which each bank of tubes, which defines a pass extending across the entire cross-section of the duct, consists of two vertical rows of tubes.

3. A system as claimed in claim 2, in which each bank oftubes contains at least 100 litres of heat transfer liquid.

4. A system as claimed in any preceding claim, in which the pressure in the tubes is at least 1 atmosphere gauge.

5. A system as claimed in claim 4, in which the pressure drop from end to end of each tube is of the order of 0.035 kg/cm2 (2 psig).

6. A system as claimed in claim 4 or claim 5, in which the heat transfer liquid is a water-based secondary refrigerant.

7. A system as claimed in any preceding claim in which at last 25% heat recovery from outlet duct to inlet duct is obtained.

8. A multitube heat exchanger comprising a plurality of rows of glass tubes extending across a gas-flow path between first and second spaced-apart tube plates, the tube ends being located in clearance holes in the respective tube plates and being bonded thereto by an annular layer of rubber-elastic material, characterised in that the tube plates are each provided with baffle strips extending in the direction of the said rows and fixed to the tube plate between two adjacent rows to (a) strengthen the tube plate against bowing under the pressure exerted thereon by the heat transfer medium flowing in the tubes and (b) divide the rows into separate passes for the said

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. (e.g. as shown at 12 in Figures 2 and 4). The transverse baffle members 10 are also welded to the tube plate between tube ends and the baffle strips 7 are welded between the wall 11 and a baffle member 10 in the upper and lower sections or to two baffle members 10 in the two mid-sections. The assembly of tube plate, perimeter wall, baffle strips 7, baffle members 10 and cover plate sections 5a...5d, defines a massively strong structure capable of withstanding the hydraulic forces within the header boxes without any appreciable bowing of the tube plate or cover plate sections. The spacing of the tube plates is fixed by longitudinal angle sections 13 (see Figure 1) so that the metalwork structure of the heat exchanger protects the tubes 1 from the hydraulic pressure load in the elongate direction thereof. From Figures 3 and 4 it can be seen that the outwardly facing edges of the baffle strips 7, and the outwardly facing flange surfaces of the perimeter wall 11 and the baffle members 10 are coplanar, so that four flat sheets of flexible resilient material 14 (see Figure 5) can serve as a seal for each pass and for the perimeter of each cover plate section 5a...5d. Twin rows of threaded studs are provided on each baffle member 10 (not shown in Figure 2 - but see Figure 3) so that two adjacent cover plate sections rest on each baffle member. A single row of threaded studs (not shown) projects outwardly from the perimeter wall 11 to complete the securement of the cover plate sections over the respective tube plate 2a, 2b. From Figure 3 it can be seen that some tube ends in each row are overlain by the flanges of the baffle members 10 and to ensure tht all tubes are removable from the bundle (for replacement in case of breakage), the baffle members are reversed in the other header box. Twelve of the tubes 1 in the bundle of 1200 are of stainless steel and these can be symmetrically disposed within the bundle to initially assist in supporting the tube plates in their correct alignment prior to locating the glass tubes in place. The angle sections 13 are provided with threaded studs or bolt holes and the vertical sections of the perimeter wall 11 are provided with threaded studs to permit the upstream and downstream duct walls 16 to be secured thereto. In this way the hostile gas is fed directly to the tube bundle and led directly away from it. The upper angle sections 13 are overlain by a plate 15a (see Figure 1) to complete the duct wall and a surface 15b between the lower angle sections 13 defines a sloping drainage surface leading to a gutter (not shown) which extends the entire length between the tube plates on the gas outlet side of the tube bundle and drains to an outlet on one side. The gutter receives the condensate dripping off the tubes onto the drainage surface 1 sub. The tube ends are fixed into the clearance holes in the tube plate using a known technique in which silicone rubber is poured over the tube plate while disposed horizontally, tube ends uppermost, and coats the then upper surface of the tube plate forming a fillet around each tube 1. Figure 6 shows the kiln at 20, the air inlet at 21 and the air outlet at 22. 23 is the heat exchanger supplying heat to the cold inlet air, 24 is a source of SO2 and 25 the glass tubed heat exchanger in the kiln outlet. 26 is the gutter for dilute acid collected from the tubes of the heat exchanger 25. 27 is a pump for circulating glycol solution between the heat exchangers 23 and 25 and 28 is an optional heat pump plant which can be used to improve the efficiency of the heat recovery. The tube plates 2a and 2b could be fabricated from carbon steel coated on both sides with a protective layer (e.g. silicone rubber). CLAIMS (Filed on 3/8/81)

1. Athermal recuperation system for a dryer comprising a first multitube heat exchanger in an inlet to the dryer, a second multitube heat exchanger in an outlet duct of the dryer and means to circulate a heattransfer liquid between the tubes ofthetwo heat exchangers, characterised in that the second heat exchanger comprises a plurality of successive vertical banks of glass tubes, extending across the entire width of the outlet duct from at least one header box disposed outside the duct, the banks being arranged one beyond the other in the flow direction of air through the duct, adjacent banks having heat transfer liquid flowing therethrough in alternate directions, with the liquid leaving the upstream bank going to the first heat exchanger and the liquid entering the downstream bank coming from the first heat exchanger.

heat transfer medium.

9. A heat exchanger as claimed in claim 8, in which the plurality of rows of glass tubes can be isolated in groups which operate in parallel, so that in the event of a single tube breaking, only one group of tubes need be isolated from the system to permit the other groups to continue to be used.

10. A heat exchanger as claimed in claim 8 or 9, in which the baffle strips are of the same thickness as the tube plate to which they are attached.

11. A heat exchanger as claimed in claim 8,9 or 10, in which each tube plate is provided with a perimeter wall of the same height (measured normal to the surface of the tube plate) as the baffle strips, whereby a cover assembly secured to the perimeter wall makes sealing engagement with the edge of each baffle strip.

12. A heat exchanger as claimed in claim 11, in which the cover is braced on its outer surface against bowing under the influence of the pressure of the heat transfer medium.

13. A heat exchanger as claimed in claim 9, in which further baffle members are provided across each tube plate normal to the baffle strips to divide each row into groups of tubes.

14. A heat exchanger as claimed in claim 13, in which each baffle member has a flange coplanar with a flange of a primeter wall of the tube plate and the cover is secured to said flanges.

15. A heat exchanger as claimed in claim 14, in which no tube has both its ends overlain by a flange of a baffle member, so that each tube can be withdrawn through one tube plate or the other.

16. A heat exchanger as claimed in any of claims 8 to 15, in which a gutter for dilute acid collected from the tubes is provided.

17. A heat exchanger as claimed in claim 8, substantially as illustrated in Figure 1 of the drawings.

18. A heat exchanger substantially as herein described with reference to, and as illustrated in, Figures 2 to 5 of the drawings.

19. Athermal recuperation system for a malting kiln as claimed in claim 1.

20. Athermal recuperation system as claimed in claim 19, substantially as illustrated in Figure 6 of the drawings.