GB2242935A - Flue gas extraction - Google Patents

Abstract

Flue gases entering at 10 are drawn from a gas appliance by means of a ducted Coanda effect flue system. Motive air is supplied at 9 from a remote pump. In this way the hot and corrosive gases do not come into contact with rotating fan parts. Various nozzle modifications are envisaged. <IMAGE>

Description

COANDA FLUE GAS EJECTORS This invention relates to the use of fluid driven Coanda effect ejectors for the purpose of extracting flue gases from combustion apparatus.

The use of efficient decorative fuel-effect fires for heating purposes is becoming more common in homes. Typically gas is supplied to a burner mounted within a ceramic-material bed.

Flames and combustion products cause this ceramic bed to heat up. Placed on top of the bed are non-burning coals which are also heated by the combustion. A heating effect to the room is created by this process. The combustion products of the gas, which may contain a combination of carbon monoxide, carbon dioxide, nitrous oxides and water vapour, must be removed from the room in which the heating appliance is operating. In some homes, the "fire" is placed such that the flue gases are extracted with a conventional domestic chimney.

Many modern houses do not possess a chimney and under these circumstances, the fires are equipped with a ducted fan system for fume extraction. A centrifugal fan is mounted on an external wall, often sunk into a cavity, and by means of a small electric motor, a suction force is created such that the fumes flow from the fire through the duct and away to atmosphere. These systems are on sale at present and are found to be reasonably efficient. They do, however, suffer from a disadvantage in that the hot and often corrosive exhaust gases of the fire come into contact with the fan, its bearings and even the motor itself. This effect is found to shorten bearing and motor life. For reasons of pollution control and fire efficiency it is now desired to increase the operating temperature of these fires to a level which is higher than at present.It is realised that this will shorten bearing life still further.

The need arises for a reliable efficient way of exhausting the flue gases from gas appliances without the motive unit becoming exposed to the flue gases. It is proposed in this present application to use an ejector operating from an electro-mechanical air flow generating pump. The design of ejector for this application is one operating by means of the Coanda effect.

The Coanda effect is already a well proven, documented, method of entrainment, that works with a variety of fluids.

In this application it is envisaged that the primary flow to the ejector device will be air and the secondary flow a mixture of air and flue gases.

The present use is concerned with flue gases from decorative fuel-effect fires. However, the invention of Coanda effect flue gas extraction is not limited to this application. It may be used on any combusting apparatus where it is desired to extract flue gases without the use of a conventional chimney.

The advantage that it has over present electrically driven fans is that no moving parts come into contact with the hot corrosive gases.

In this way it becomes possible to run the decorative fueleffect fires, for example, at higher temperatures, thereby increasing their efficiency. Furthermore, Coanda ejectors have the advantage of being high mixing rate devices which can enable compact units to be used.

Another advantage is that the flue gases are cooled and diluted with air before being ejected to the atmosphere.

One presently preferred embodiment is particularly described with reference to the accompanying drawings, wherein; Figure 1 shows a schematic layout of the flue system.

Figure 2 shows details of a Coanda unit.

A fire unit, 1, is mounted on an outside wall of a room. The fire unit is able to produce heat by combusting gas. The combustion products flow through a duct, 2, through a wall, 3, and into the ejector unit, 4. Motive air is supplied to the ejector, 4, by a pump, 5, which is driven by an electric motor, 6. The exit flow of air and flue gases is covered by a coarse cage, 7, which prevents the ingress of debris such as leaves. Electrical power, 8, is supplied to the motor.

Figure 2 shows a typical Coanda ejector used for such an application. Primary air, supplied by the pump, enters at 9.

Secondary flow, which is comprised of flue gases and ambient entrained air enters at 10. As the primary air emerges from the Coanda slot, 11, it entrains the secondary flow. The two flows pass through the throat and exit the nozzle at 13.

Various modifications exist within the scope of this invention. The Coanda nozzle, shown in Figure 2, is an axisymmetric unit. It might, however, have the cross section of a polygon rather than an annulus; the unit may have different primary flow inlet control devices or more than one outlet for the air supply which therefore forms more than one primary jet; the nozzle angular position relative to the body axis may be varied; the unit inlet and Coanda surface shapes may be varied in order to optimise performance. It might be that the Coanda effect is caused to occur over plane two dimensional surfaces within a rectangular duct. It may be that this embodiment will make for cost effective and robust construction.

The Coanda effect could be made to occur over an external Coanda surface constrained within the ductwork system.

Furthermore, one or more Coanda units of either internal or external design could be used in conjunction with one another to achieve the extraction function required.

Whatever combination of Coanda units is selected for a design, on/off function and user control could be provided by conventional means although a specialist control system could be developed if required.

Claims (15)

1. What we claim is: 1) A flue gas ejector system which uses the Coanda effect.
2. 2) A Coanda flue gas ejector system in which a primary fluid, which may be air, is made to exit from a primary nozzle onto a Coanda surface such that the primary flow causes the entrainment of a secondary flow which may be combustion products and ambient air.
3. 3) A Coanda flue gas ejector system according to claim 1 or 2 wherein the Coanda flow device comprises a hollow body which is provided with a) an annular nozzle arranged to direct the primary flow from a point adjacent one end of the hollow body to the other end thereof; b) a converging convex Coanda surface immediately downstream of said nozzle; and c) a diverging section immediately downstream of the Coanda surface, the arrangement being such that the primary flow along the Coanda surface and diverging section entrains the secondary flow.
4. 4) A Coanda flue gas ejector system according to claim 3 in which there exists a parallel mixing region between the converging convex Coanda surface and the diverging section.
5. 5) A Coanda flue gas ejector system according to claims 3 & BR< 4 wherein the Coanda flow device has more than one outlet for the primary flow.
6. 6) A Coanda flue gas ejector system according to claims 3,4 & 5 wherein the hollow body of the Coanda flow device is of an annular cross section.
7. 7) A Coanda flue gas ejector system according to claims 3,4 & 5 wherein the hollow body of the Coanda flow device is of a non annular cross section.
8. 8) A Coanda flue gas ejector system according to claim 2 in which in use, the primary flow is directed over an external Coanda body, said body being contained within a housing such that the primary flow and entrained secondary flow are ducted within the constraints of the housing.
9. 9) A Coanda flue gas ejector system according to claim 2 in which the primary flow is made to pass over a combination of internal and external Coanda surfaces.
10. 10) A Coanda flue gas ejector system according to any one of claims 2 to 9 comprising more than one Coanda ejector.
11. 11) A Coanda flue gas ejector system according to any preceding claim having means by which the primary and or secondary flows can be regulated.
12. 12) A Coanda flow device adapted for connection to a flue gas ejector system to produce a flue gas ejector system according to any of claims 1 to 11.
13. 13) A method of constructing a flue ejector system according to any of claims 1 to 11, comprising the step of coupling a Coanda flow device to a flue gas ejector duct.
14. 14) A direct fuel-effect fire which makes use of a Coanda effect ejector for the purpose of flue gas extraction.
15. 15) A unit as substantially described herein and as shown in the accompanying drawings.