GB1588310A

GB1588310A - Heat transfer

Info

Publication number: GB1588310A
Application number: GB5367276A
Authority: GB
Original assignee: Stone Platt Fluidfire Ltd
Current assignee: Stone Platt Fluidfire Ltd
Priority date: 1976-12-22
Filing date: 1976-12-22
Publication date: 1981-04-23
Also published as: BE864514A; DE2807110A1; AU3353978A; AU515106B2; FR2418914A1

Description

(54) IMPROVEMENTS RELATING TO HEAT TRANSFER (71) We, STONE-PLATT FLUIDFIRE LIMITED, formerly Fluidfire Development Limited, a British Company of Washington Street, Netherton, Dudley, West Midlands DY2 9RE do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method of transferring heat between a gas and a further medium which may be a solid, a liquid or a gas, and to apparatus for use in the method.

According to a first aspect of the invention, there is provided apparatus for transferring heat between a primary gas and a further medium, the apparatus comprising a quantity of solid particles, means defining a first heat exchange zone through which the particles can fall under gravity, means defining a second heat exchange zone containing a bed of the particles, particle discharging means at the bottom of the first zone for discharging particles from the first zone into the bed in the second zone, par ticle delivering means for delivering particles from the bed to the top of the first zone, the arrangement being such that particles fall substantially unimpeded through at least a part of the first zone, a primary gas inlet to the first zone, a primary gas outlet from the first zone spaced upwardly from the primary gas inlet to the first zone, whereby the primary gas flows upwardly in said part of the first zone, means for causing a fluidising gas to flow through the bed in the second zone and fluidise the particles thereof and a fluidising gas outlet from the second zone whereby the fluidising gas can leave the second zone other than by passing through the first zone and the gas outlet therefrom.

When the apparatus is in use, particles fall through the first heat exchange zone and good contact is established between the particles and the primary gas irrespective of the rate of flow of the primary gas from the primary gas inlet to the primary gas outlet. Thus, the apparatus does not rely upon the maintenance of any minimum gas velocity or gas pressure, such as is necessary in a case where contact between a gas and solid particles is established in a bed of the particles which is fluidised by the gas.

The first heat exchange zone is preferably tall, by which we mean that the height of the zone is a plurality of times greater than its mean horizontal dimension.

The first heat exchange zone is preferably substantially symmetrical about a vertical axis. The zone may be substantially cylindrical or include a frusto-conical or other tapered portion, the axis of which is vertical and the narrower end of which is an upper end.

The fluidised bed may contain heat exchange tubes for conveying the further medium through the fluidised bed. This arrangement is particularly advantageous, as compared with known heat transfer apparatus, in cases where the gas stream to or from which heat is to be transferred is such as would contaminate or corrode heat exchange tubes if brought into direct contact with such tubes.

According to a second aspect of the invention there is provided a method of transferring heat between a primary gas and a further medium wherein solid particles are caused to fall substantially unimpeded through a rising body of the primary gas, the temperature of the particles when they are introduced into the body of primary gas differing from the temperature of the primary gas, the particles then pass through a fluidised bed, heat is transferred between the particles in said bed and the further medium and a fluidising gas which is passed through the fluidised bed is maintained substantially separate from the body of primary gas through which the particles fall.

We prefer to circulate the particles along a path, a descending portion of which extends through a heat exchange zone, through which the primary gas rises, and the rate of flow of the particles along the circulatory path may be controlled to control the rate of heat transfer.

The invention will now be described, by way of example, with reference to the accompanying drawing which shows diagrammatically one apparatus in accordance with the invention.

The apparatus comprises a cylindrical duct 2 which is arranged with its axis vertical. The duct may be a chimney and the upper end of the duct may be open to the atmosphere. The bottom of the duct is closed by a floor 3 which slopes downwardly from one side of the duct to the opposite side. At a position speced somewhat above the floor 3, the duct is provided with a gas inlet 4 through which hot waste gases are fed into the duct during use.

The gases flow upwardly and leave the duct at the upper end thereof which constitutes a gas outlet 5.

The sloping floor 3 constitutes a particle discharging means and is extended at its lower part through an aperture 6 in the duct 2. The extension of the floor 3 leads into a chamber 7 situated adjacent to a lower part of the duct 2. The chamber 7 is divided by a horizontal partition 8 into upper and lower parts 9 and 10 respectively.

The partition 8 is adapted to support fluidised beds of particles on its upper surface and to admit a fluidising gas to the beds.

The partition may be formed of a perforate metal plate or of a porous refractory material.

Above but near to the partition 8 there is arranged an array of tubes 11 which are formed of a thermally conductive material and convey a gaseous or liquid medium to which heat is to be transferred from the hot waste gases. When the apparatus is in use each of the tubes 11 is substantially submerged in one or other of three fluidised beds 12, 13 and 14 of particles. The extension of the floor 3 is arranged to deliver the particles into the chamber 7 at a level above the surface of the fluidised beds and therefore above the tubes 11 and above the partition 8.

The fluidised beds 12, 13 and 14 are partly separated from each other by vertical partitions 15 and 16 which extend upwardly from the horizontal partition 8 to a level spaced somewhat below the upper surface of the fluidised beds. Although the vertical partitions do not completely prevent migration of particles from one bed to another, they do restrict such migration sufficiently to enable the beds 12, 13 and 14 to be maintained at respective different temperatures. The partition 15 extends across the chamber 7 at a position spaced from the end of the chamber communicating with the aperture 6 and the partition 16 extends across the chamber at a position spaced further from that end of the chamber, so that particles entering the chamber through the aperture 6 fall into the bed 12 and can reach the bed 14 only by passing through the bed 13.

The apparatus further includes particle delivering means for delivering particles from the chamber 7 to the top of the duct 2. Such means comprises a pneumatic conveyor 17 having an inlet 18 which extends into the bed 14 at a position therein remote from the extension of the floor 3 and an outlet 19 situated in the gas outlet 5. A pipe 20 for supplying air to the pneumatic conveyor extends into the inlet 18 through the bed 14, horizontal partition 18 and the lower part 10 of the chamber. The outlet 19 of the pneumatic conveyor is adapted to spray the particles into the heat exchange zone defined by the duct 2 so that the particles are distributed approximately evenly across the zone.

Baffles 23 are provided in the duct 2 to prevent there being established within the duct 2 substantially separate streams of particles and of gases. In the particular example illustrated, each baffle is conical, is arranged with its apex uppermost and is disposed in an upper part of the duct.

As can be seen from the drawing, these baffles interfere with the free falling of the particles in the upper part of the duct 2 but the particles can fall substantially unimpeded through a lower part of the duct.

The apparatus further includes an electrically driven fan 21 for causing a fluidising gas to flow into the lower part 10 of the chamber and thence through the horizontal partition 8 into each of the beds 12, 13 and 14. In the top of the chamber 7, at a position spaced well above the fluidised beds, there is a fluidising gas outlet 22 which is in communication, via a cyclone 24, with an inlet of the fan 21. An outlet of the fan communicates with the lower chamber part 10. Accordingly, there is defined a substantially closed path along which the fluidising gas is circulated by the fan.

During operation of the apparatus to transfer heat from hot waste gases to water, in order to raise steam, the hot waste gases enter the heat exchange zone through the gas inlet 4 and flow upwardly to emerge from the gas outlet 5. Particles are sprayed into the heat exchange zone by the outlet 19 of the pneumatic conveyor and fall downwardly through the duct 2 to the sloping floor 3. It will be noted that countercurrent flow of the particles and hot gas is established.

The particles preferably have the same constitution and substantially the same size as one another, so that the speed at which the particles descend is substantially the same for all particles. This speed depends upon the terminal velocity of the particles in the hot waste gas and the upward vel ocity of the gas. In a cylindrical heat exchange zone, the upward velocity of the gas will tend to decrease towards the upper end of the zone, as the temperature of the gas falls. In order to compensate partly or wholly for the decrease in temperature of the gas, the heat exchange zone, or at least a part thereof, may taper upwardly. Thus in a modification of the apparatus shown, the duct 2 may be of frusto-conical form with its narrow end uppermost. The duct is preferably symmetrical about its vertical axis.Typically, the downward speed of the particles in the heat exchange zone is 2 ft./sec. and the mean particle size is 630 microns.

At the top of the heat exchange zone, the particles are relatively cool, cooler than the gas in any part of the heat exchange zone. As the particles fall they extract heat from the gas so that the particles are hot when they reach the floor 3. The temperature of the waste gas is a maximum at the gas inlet 4 and falls as the gases flow upwardly through the zone.

The angle at which the floor 3 is inclined to the horizontal is such that the hot particles roll or slide downwardly into the chamber 7 where they enter the fluidised bed 12.

However, at the aperture 6 there is provided a step 30 over which the particles must pass. This step maintains on the floor 3 a layer of particles which protects the floor against abrasion by falling particles.

In the bed 12, heat is extracted from the particles and is conducted through the walls of the tubes 11 to steam flowing through the tubes. The particles migrate over the upper edge of the partition 15 into the bed 13 where heat is transferred from the particles to hot water contained in the tubes 11, thereby evaporating the water to produce steam which flows to the tubes in the bed 12. The particles eventually migrate from the bed 13 over the upper edge of the partition 16 into the bed 14 where they give up heat to water flowing through the tubes in this bed. The water which is thereby heated flows to the tubes in the bed 13.

Cool particles enter the pneumatic conveyor 17 and are returned to the top of the heat transfer zone.

The rate of heat extraction from the waste gases in the heat transfer zone is controlled by varying the rate at which particles are delivered into the zone by the pneumatic conveyor. To this end, the conveyor comprises a variable speed fan 29 for supplying air at a controlled rate through the pipe 20. To the same end, an adjustable valve may be provided in the pipe 20.

The apparatus may be used to transfer heat from the waste gases to a fluid other than water, for example to air which is to be supplied to the interior of a building or air which is to be used for combustion. In this case, the gas which is to be heated is employed as the fluidising gas and is fed into the lower part 10 of the chamber 7 by the fan 21. The heated gas is led from the gas outlet 22 to the point of use. In this case, the tubes 11 are not used and would be empty. If the apparatus is to be used exclusively for heating a gas, the tubes 11 may be omitted entirely from the apparatus.

The apparatus may be arranged so that countercurrent flow is established between the particles and a gas which is to be heated. Partitions may extend downwardly from the top of the chamber 7 to positions submerged below the surface of the fluidised beds, these partitions being aligned with the partitions 15 and 16 but being spaced somewhat therefrom to leave gaps through which particles can migrate between the beds. The lower part 10 of the chamber would also be subdivided by extensions of the partitions 15 and 16. Fluidising gas from the fan would be led through the bed 14 thence through the bed 13 and finally through the bed 12 before passing through the outlet 22.

It will be understood that the apparatus shown in the accompanying drawing may be used for heating other fluids. If gaseous, the fluid to be heated may be used as the fluidising gas. Alternatively, either a liquid or a gas may be passed through the tubes 11. In a case where the fluid which is to be heated is passed through tubes in the fluidised beds, the fluidising gas is preferably circulated along a predominantly closed path, by which we mean that, during each complete movement of the fluidising gas around the path, the majority of the fluidising gas is retained in the path. It will be appreciated that some fluidising gas may escape from the path, for example along the pneumatic conveyor 17, and that it will be necessary to introduce fresh fluidising gas continuously to make up for such loss.

In addition to the fluidised beds 12, 13, 14 in the chamber 7, there is provided in the apparatus illustrated in the accompanying drawing an additional fluidised bed 25.

The beds 14 and 25 are connected by flow and return ducts 26 and 27 respectively along which particles circulate between these beds. The bed 25 is fluidised by a gaseous mixture of a fuel and air, which mixture burns within the bed to burn off particles in the bed contaminants which can be removed by heating, for example combustible contaminants. The hot products of the combustion which occurs in the bed 25 are led through a duct 28 into the duct 2.

Although not all particles which pass through the bed 14 pass also through the bed 25, the bed 25 is effective to control the accumulation on the particles contained in the apparatus of contaminants which can be removed by heating. Gas may be admitted to the ducts 26 and 27 to fluidise the particles therein.

It will be understood, that in a case where there is no requirement for removing contaminants from the particles by heating, the bed 25 and associated ducts may be omitted.

In a modification of the apparatus illustrated in the accompanying drawing, the inlet 18 of the pneumatic conveyor is arranged to receive particles from a reservoir into which particles can flow from the bed 14 over a weir. The rate at which particles flow through the heat transfer zone can then be controlled by raising and lowering the weir to control the rate at which particles pass to the inlet of the pneumatic conveyor.

Instead of raising and lowering the weir, particles could be withdrawn from and added to the fluidised beds.

The particles may be formed of steel, silicon carbide, aluminium oxide or zircon.

Typically, the duct 2 has a height of approximately 12 metres and a diameter within the range 11 to 2 metres. Apparatus as described and having a duct of these dimensions is capable of transferring from hot waste gases at a temperature of approximately 460"C. to a further fluid energy at a rate of approximately one M.W.

WHAT WE CLATM IS: 1. Apparatus for transferring heat between a primary gas and a further medium, the apparatus comprising a quantity of solid particles, means defining a first heat exchange zone through which the particles can fall under gravity, means defining a second heat exchange zone containing a bed of the particles, particle discharging means at the bottom of the first zone for discharging particles from the first zone into the bed in the second zone, particle delivering means for delivering particles from the bed to the top of the first zone, the arrangement being such that particles fall substantially unimpeded through at least a part of the first zone, a primary gas inlet to the first zone, a primary gas outlet from the first zone spaced upwardly from the primary gas inlet to the first zone, whereby the primary gas flows upwardly in said part of the first zone, means for causing a fluidising gas to flow through the bed in the second zone and fluidise the particles thereof and a fluidizing gas outlet from the second zone whereby the fluidising gas can leave the second zone other than by passing through the first zone and the gas outlet therefrom.

2. Apparatus according to claim 1 wherein the first heat exchange zone is tall as herein defined.

3. Apparatus according to claim 1 or claim 2 wherein the first heat exchange zone is substantially symmetrical about a vertical axis.

4. Apparatus according to claim 3 wherein at least a part of the first heat exchange zone tapers upwardly.

5. Apparatus according to any preceding claim further comprising ducts which extend through the second heat exchange zone and through which ducts said further medium can be passed in heat exchange relation with the particles of the fluidised bed.

6. Apparatus according to any one of claims 1 to 4 comprising a plurality of tubes disposed within the fluidised bed in the second zone and means defining a predominantly closed path which includes the fluidised bed, the fluidising gas outlet and said means for causing a fluidising gas to flow whereby the fluidising gas is circulated along said path.

7. Apparatus according to any preceding claim comprising a plurality of fluidised beds of said particles and means for leading the fluidising gas through the beds in turn, the particles passing through the beds in countercurrent flow to the fluidising gas.

8. Apparatus according to any preceding claim wherein the particle delivering means is adjustable to vary the rate at which particles are delivered to the first heat exchange zone.

9. Apparatus according to any preceding claim wherein the particle discharging means is arranged to maintain a layer of particles at the bottom of the first heat exchange zone.

10. A method of transferring heat between a primary gas and a further medium wherein solid particles are caused to fall substantially unimpeded through a rising body of the primary gas, the temperature of the particles when they are introduced into the body of primary gas differing from the temperature of the primary gas, the particles then pass through a fluidised bed, heat is transferred between the particles in said bed and the further medium and a fluidising gas which is passed through the fluidised bed is mantained substantially separate from the body of primary gas through which the particles fall.

11. A method according to claim 10 wherein the particles are circulated along a

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the accumulation on the particles contained in the apparatus of contaminants which can be removed by heating. Gas may be admitted to the ducts 26 and 27 to fluidise the particles therein. It will be understood, that in a case where there is no requirement for removing contaminants from the particles by heating, the bed 25 and associated ducts may be omitted. In a modification of the apparatus illustrated in the accompanying drawing, the inlet 18 of the pneumatic conveyor is arranged to receive particles from a reservoir into which particles can flow from the bed 14 over a weir. The rate at which particles flow through the heat transfer zone can then be controlled by raising and lowering the weir to control the rate at which particles pass to the inlet of the pneumatic conveyor. Instead of raising and lowering the weir, particles could be withdrawn from and added to the fluidised beds. The particles may be formed of steel, silicon carbide, aluminium oxide or zircon. Typically, the duct 2 has a height of approximately 12 metres and a diameter within the range 11 to 2 metres. Apparatus as described and having a duct of these dimensions is capable of transferring from hot waste gases at a temperature of approximately 460"C. to a further fluid energy at a rate of approximately one M.W. WHAT WE CLATM IS:

1. Apparatus for transferring heat between a primary gas and a further medium, the apparatus comprising a quantity of solid particles, means defining a first heat exchange zone through which the particles can fall under gravity, means defining a second heat exchange zone containing a bed of the particles, particle discharging means at the bottom of the first zone for discharging particles from the first zone into the bed in the second zone, particle delivering means for delivering particles from the bed to the top of the first zone, the arrangement being such that particles fall substantially unimpeded through at least a part of the first zone, a primary gas inlet to the first zone, a primary gas outlet from the first zone spaced upwardly from the primary gas inlet to the first zone, whereby the primary gas flows upwardly in said part of the first zone, means for causing a fluidising gas to flow through the bed in the second zone and fluidise the particles thereof and a fluidizing gas outlet from the second zone whereby the fluidising gas can leave the second zone other than by passing through the first zone and the gas outlet therefrom.

11. A method according to claim 10 wherein the particles are circulated along a

predominantly closed path which includes a first heat exchange zone in which the particles fall substantially unimpeded through the rising body of primary gas.

12. A method according to claim 11, wherein the rate of flow of the particles along the predominantly closed path is varied to control the rate of heat transfer.

13. Apparatus substantially as herein described with reference to and as shown in the accompanying drawing.

14. A method substantially as herein described with reference to the accompanying drawing of transferring heat between a gas and a further medium.