GB1587733A

GB1587733A - Heat exchangers

Info

Publication number: GB1587733A
Application number: GB4492576A
Authority: GB
Original assignee: UK Secretary of State for Industry
Current assignee: UK Secretary of State for Industry
Priority date: 1976-10-28
Filing date: 1976-10-28
Publication date: 1981-04-08
Also published as: FR2369528A1

Description

(54) IMPROVEMENTS IN OR RELATING TO HEAT EXCHANGERS (71) I, THE SECRETARY OF STATE FOR INDUSTRY, London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to heat exchangers, and is especially but not exclusively concerned with the provision of a heat exchanger suitable for heating the interior passenger and/or driver compartment of a vehicle using heat from the exhaust of an engine by which the vehicle is propelled.

In the case of vehicles having water-cooled engines, it is a relatively simple matter to divert a proportion of the engine coolant to flow through a heat exchanger in which some of the heat of the coolant is given up to a supply of air flowing into the vehicle interior.

The possibility of escape of working fluid (i.e. the engine coolant) into the vehicle interior does not in general pose a major hazard to the occupants. However, in the case of an air cooled engine, a supply of heat from a non-hazardous working fluid is not so readily available. A convenient approach would be to make use of waste heat present in the exhaust gas from such an engine, by causing fresh air to be drawn across a hot region of the duct through which hot exhaust gases leave the engine and hence into the interior of the vehicle. With this type of heater there is, however, a danger that if leaks develop in the exhaust system in the region of heat exchange, toxic gases such as Carbon Monoxide present in the exhaust stream could escape into the heater airstream. Toxic gases could thus be drawn into the interior of the vehicle and thereby cause a grave hazard to the occupants.

The present invention seeks to provide a means of effectively transferring heat from hot exhaust gases of a vehicle's engine to a stream of fresh air used to heat the interior of the vehicle, whilst at the same time reducing the risk that a leak in the duct carrying the hot exhaust gases will result in the entry of toxic exhaust gases into the vehicle interior.

According to the present invention, a heat exchanger for transferring heat form a stream of hot engine exhaust gas to a stream of air for heating the interior of a motor vehicle comprises an exhaust duct through which hot engine exhaust gas can be constrained to flow, an air duct which communicates with the interior of the motor vehicle and through which air for heating the interior of the motor vehicle can be constrained to flow, and chamber separating said exhaust direct form said air duct, said chamber containing a fluid having a thermal conductivity greater than that of air.

Preferably the fluid is a non-toxic gas chemically stable at elevated temperatures normally encountered in the exhaust of an internal combustion engine and having a thermal conductivity considerably greater than that of air, such as helium.

In order for toxic exhaust gas to enter the air stream via the heat exchanger, it is necessary that the chamber becomes perforated in two places so that the chamber is in fluid communication with both the exhaust duct and the air duct. In practice, two such perforations are extremely unlikely to occur simultaneously (except possibly in the event of an impact, when the system would be checked). Should a single perforation occur, however, the consequent escape of working fluid can provide an indication to the occupants of the vehicle that all is not well. The most direct indication will, of course, be the loss of heating efficiency consequent upon a perforation of the chamber.

It can be envisaged, however, that a still more positive indication might be provided by arranging that the fluid in the chamber is maintained at a pressure which is above or below the range of atmospheric pressure normally encountered for example above 16 pounds per square inch or below 14 pounds per square inch. In this case, a pressure sensing device arranged to sense the pressure of fluid in the chamber may be provided.

Such a sensing device is preferably so arranged as to operate a visual, audible, or other form of alarm perceptible by an occupant of the vehicle's interior.

One embodiment of the invention will now be described by way of example only, with reference to the drawing filed with the Provisional Specification, which is a schematic view of a heat exchanger suitable for use with a car having an air-cooled engine.

The heat exchanger comprises three concentric pipes 1, 2, 3, of which the innermost pipe, 1 is of stainless steel and forms a section of the exhaust pipe for carrying hot exhaust gases from an internal combustion engine (not shown) by means of which the car is propelled. Between the innermost pipe 1 and the intermediate pipe 2, (which is also of stainless steel), there is formed a sealed annular chamber 4 containing -helium gas at approximately atmospheric pressure. A small loading tube 5 is provided through which the chamber 4 can be evacuated and subsequently charged with helium, whereafter the loading tube can be sealed. The pipes 1 and 2 are so spaced that the thickness of the layer of helium contained therein is approximately 1/32 inch, although another thickness might be more suited for different specific circumstances.

The outermost tube 3 coacts with the intermediate tube 2 to define an annular passage 6 through which air may be caused to flow. The outermost tube 3 is provided near one end with a side-branch pipe 7 having a flange 8 for connection to ducting by which air flowing in through the passage 6 can be directed to the interior of the car to be heated.

As shown schematically towards the left hand side of the Figure, the innermost pipe 1 and the intermediate pipe 2 can be formed with corresponding helical corrugations 9 which serve to increase the surface area available for heat exchange for any particular length of heat exchanger, whilst maintaining the thickness of the chamber 4 approximately constant in the direction of heat flow therethrough. Also, by use of this corrugated construction, differential expansion between the pipes can be accommodated with less stressing of the pipe walls and welded joints. A helical baffle 10 is provided on the passage 6, so that air flowing therethrough is forced to flow along a helical path, thereby increasing the heat transfer effect.

The baffle 10 is conveniently wound at the same pitch as the corrugations 9. These measures serve to provide a more compact and efficient heat exchanger.

In use of the heat exchanger, cold fresh air is drawn in through the passage 6, in which it is forced to follow a helical path over the exterior surface of the corrugations 9. The direction of fresh air flow is generally from right to left as shown in the Figure. Hot exhaust gas flows through the pipe 1 in counter-current flow relationship to the incoming fresh air. Heat flows from the hot exhaust gas through the pipe walls and through the counductive buffer of helium gas to increase the temperature of the incoming air stream, which then flows via branch pipe 7 to the interior of the car. A control valve arrangement (not shown) may be provided for limiting the flow of air through passage 6 so as to facilitate control of the temperature of the vehicle's interior.

Should a perforation of the exhaust pipe 1 occur, exhaust gases may penetrate the chamber 4, but will be prevented from entering the fresh air stream within passage 6, by reason of the intermediate pipe 2. Two perforations of the walls of chamber 4 are necessary before exhaust gas can enter the sir stream. However should a single perforation of the chamber walls occur, this will result in the escape of helium and the ingress of air into the chamber 4. This condition will make itself apparent by a loss of heating performance, giving a warning that investigation is necessary, and so allowing the prevention of any exposure of the vehicle occupant to the hazard of toxic exhaust gas.

Modifications within the scope of the invention will be readily apparent to the skilled artisan. Thus, for example, the pipes 1, 2 need not be of corrugated form as shown schematically on the left hand side of the Figure, but could be of plain cylindrical form as shown on the right of the Figure. The helical baffle 10 could be omitted, or alternatively a plurality parallel helical baffles could be used. The heat exchanger can be of any desired length, as indicated by the broken-off schematic illustration in the Figure. Other possible modifications within the scope of the invention will be readily apparent.

WHAT I CLAIM IS: 1. A heat exchanger for transferring heat from a stream of hot gas to a stream of air for heating the interior of a motor vehicle comprising an exhaust duct through which hot engine exhaust gas can be constrained to flow, an air duct which communicates with the interior of the motor vehicle and through which air for heating the interior of the motor vehicle can be constrained to flow, and a chamber separating said exhaust duct from said air duct, said chamber containing a fluid having a thermal conductivity greater than that of air.

2. A heat exchanger according to claim 1 wherein said fluid in the chamber is a nontoxic gas chemically stable at elevated temperatures encountered in the exhaust of an internal combustion engine and having a thermal conductivity greater than that of air.

3. A heat exchanger according to claim 2 wherein the said non toxic gas is helium.

4. A heat exchanger according to any one preceding claim wherein the chamber contains gas at a pressure above or below the range of atmospheric pressures normally encountered.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. with reference to the drawing filed with the Provisional Specification, which is a schematic view of a heat exchanger suitable for use with a car having an air-cooled engine. The heat exchanger comprises three concentric pipes 1, 2, 3, of which the innermost pipe, 1 is of stainless steel and forms a section of the exhaust pipe for carrying hot exhaust gases from an internal combustion engine (not shown) by means of which the car is propelled. Between the innermost pipe 1 and the intermediate pipe 2, (which is also of stainless steel), there is formed a sealed annular chamber 4 containing -helium gas at approximately atmospheric pressure. A small loading tube 5 is provided through which the chamber 4 can be evacuated and subsequently charged with helium, whereafter the loading tube can be sealed. The pipes 1 and 2 are so spaced that the thickness of the layer of helium contained therein is approximately 1/32 inch, although another thickness might be more suited for different specific circumstances. The outermost tube 3 coacts with the intermediate tube 2 to define an annular passage 6 through which air may be caused to flow. The outermost tube 3 is provided near one end with a side-branch pipe 7 having a flange 8 for connection to ducting by which air flowing in through the passage 6 can be directed to the interior of the car to be heated. As shown schematically towards the left hand side of the Figure, the innermost pipe 1 and the intermediate pipe 2 can be formed with corresponding helical corrugations 9 which serve to increase the surface area available for heat exchange for any particular length of heat exchanger, whilst maintaining the thickness of the chamber 4 approximately constant in the direction of heat flow therethrough. Also, by use of this corrugated construction, differential expansion between the pipes can be accommodated with less stressing of the pipe walls and welded joints. A helical baffle 10 is provided on the passage 6, so that air flowing therethrough is forced to flow along a helical path, thereby increasing the heat transfer effect. The baffle 10 is conveniently wound at the same pitch as the corrugations 9. These measures serve to provide a more compact and efficient heat exchanger. In use of the heat exchanger, cold fresh air is drawn in through the passage 6, in which it is forced to follow a helical path over the exterior surface of the corrugations 9. The direction of fresh air flow is generally from right to left as shown in the Figure. Hot exhaust gas flows through the pipe 1 in counter-current flow relationship to the incoming fresh air. Heat flows from the hot exhaust gas through the pipe walls and through the counductive buffer of helium gas to increase the temperature of the incoming air stream, which then flows via branch pipe 7 to the interior of the car. A control valve arrangement (not shown) may be provided for limiting the flow of air through passage 6 so as to facilitate control of the temperature of the vehicle's interior. Should a perforation of the exhaust pipe 1 occur, exhaust gases may penetrate the chamber 4, but will be prevented from entering the fresh air stream within passage 6, by reason of the intermediate pipe 2. Two perforations of the walls of chamber 4 are necessary before exhaust gas can enter the sir stream. However should a single perforation of the chamber walls occur, this will result in the escape of helium and the ingress of air into the chamber 4. This condition will make itself apparent by a loss of heating performance, giving a warning that investigation is necessary, and so allowing the prevention of any exposure of the vehicle occupant to the hazard of toxic exhaust gas. Modifications within the scope of the invention will be readily apparent to the skilled artisan. Thus, for example, the pipes 1, 2 need not be of corrugated form as shown schematically on the left hand side of the Figure, but could be of plain cylindrical form as shown on the right of the Figure. The helical baffle 10 could be omitted, or alternatively a plurality parallel helical baffles could be used. The heat exchanger can be of any desired length, as indicated by the broken-off schematic illustration in the Figure. Other possible modifications within the scope of the invention will be readily apparent. WHAT I CLAIM IS:

1. A heat exchanger for transferring heat from a stream of hot gas to a stream of air for heating the interior of a motor vehicle comprising an exhaust duct through which hot engine exhaust gas can be constrained to flow, an air duct which communicates with the interior of the motor vehicle and through which air for heating the interior of the motor vehicle can be constrained to flow, and a chamber separating said exhaust duct from said air duct, said chamber containing a fluid having a thermal conductivity greater than that of air.

5. A heat exchanger according to any

one of claims 1 to 3 in which the chamber contains gas at a pressure less than 14 pounds per square inch or more then 16 pounds per square inch.

6. A heat exchanger according to claim 4 or claim 5 wherein there is provided a pressure sensing device arranged to sense the pressure of fluid in the chamber.

7. A heat exchanger according to any one preceding claim wherein the air duct and the chamber are of annular cross-section and encircle the exhaust duct.

8. A heat exchanger according to claim 7 wherein there is provided a helical baffle within the air duct which serves to define a helical flowpath for the flow of air there through.

9. A heat exchanger according to any one preceding claim wherein the chamber has a corrugated wall adjacent the exhaust duct.

10. A heat exchanger according to any one preceding claim wherein the chamber has a corrugated wall adjacent the air duct.

11. A heat exchanger substantially as hereinbefore described with reference to the drawing filed with the Provisional Specification.