GB2116809A

GB2116809A - Heating device for liquids, particularly liquid fuels

Info

Publication number: GB2116809A
Application number: GB08300553A
Authority: GB
Inventors: Michael Ronald Bungay; Peter Francis Bradford
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1982-03-13
Filing date: 1983-01-10
Publication date: 1983-09-28
Also published as: GB2116809B; GB8300553D0

Abstract

A heating device for liquids more particularly liquid fuel comprises a pair of slab-like bodies 11 spaced by a gasket 12. Each body 11 defines a flow path for liquid, one end of each flow path terminates in a pair of aligned apertures 14 in the body and through which extends a bolt 16, the bolt defining inlet and outlet flow passages. The other ends of the flow passages are interconnected so that the two flow passages are in series. Each body has a plate 20 carrying a foil heating element, located against a face 18 thereof, an insulating sheet 19 being located between the heating element and the face. The plate 20 is located against a clamping sheet 24 which is clamped to the body. When electric current is passed through the heating elements the bodies become heated and heat the fuel in the flow passage. Means is provided to control the temperature of the bodies by controlling the current flow in the heating elements. <IMAGE>

Description

SPECIFICATION Heating device This invention relates to a heating device for liquids more particularly but not exclusively, for liquid fuels for internal combustion engines.

Liquid fuels are composed of a number of constituents which are liquid at normal temperatures but which in cold conditions, can freeze. The frozen fractions of fuel can upset the operation of the engine fuel system particularly a diesel fuel system since the latter form of system incorporates a fine filter or filters the purpose of which is to retain small particles of dirt so as to minimise damage to the working surfaces of the fuel pump. The frozen fractions of fuel are retained by the filter and gradually clog the filter thereby preventing the supply of fuel to the engine.

It is known to provide electrically operated heating devices which heat the fuel and one form of such a device is shown in the specification of United States Patent 4091265. In this case the heating device is located within a chamber through which the fuel flows before it is passed through the filter. The heating device described in the aforesaid specification comprises a heating unit carrying a pair of resistors in which most of the electrical power is dissipated, and a pair of power transistors which control the electric current flow in the resistors respectively. In addition, the unit carries control transistors and a temperature responsive device, the various components being immersed in the fuel which flows through the chamber.

A disadvantage with this type of device is the fact that the surface areas of the heated components are very low and therefore in order to achieve rapid heating the temperature of the components must be fairly high. This can cause a problem if the chamber should empty of fuel and in such a case the risk arises of the surfaces of the components becoming dangerously heated. A further disadvantage is the fact that the fuel is in contact with the electrical components which therefore have to be chosen to withstand the effect of the fuel and also any contaminant for example, water, in the fuel.

The object of the present invention is to provide a heating device for liquids in a simple and convenient form.

According to the invention a heating device for liquids comprises a metallic body defining a substantially flat face, a flow passage for the liquid to be heated defined at least in part in the body, a foil electric heating element mounted on a supporting electrically insulating plate, a clamping plate, means securing the clamping plate to the body with the insulating plate being located against the clamping plate and the foil heating element presented to said face of the body and an electrically insulating sheet disposed between the heating element and said face, said insulating sheet being of a thickness and being formed from a material such that heat generated in the element when the element is connected to a source of electric supply is conducted to the body to heat the liquid in the flow passage.

An example of a heating device will now be described with reference to the accompanying drawings in which: Figure 1 is an exploded view of the device, Figure 2 is a front elevation of part of the device shown in Fig. 1, Figure 3 is a view similar to Fig. 2 of another part of the device shown in Fig. 1, Figure 4 is a part sectional a view of part of the device seen in Fig. 1, Figure 5 is a view similar to Fig. 2 of another part of the device seen in Fig. 1, Figure 6 is an electrical circuit diagram showing one method of connecting the device to an engine electrical system, and Figure 7 is a view similar to Fig. 3 showing an alternative construction.

Referring to Figs. 1-5 of the drawings, the device which is generally indicated at 10, comprises a composite body formed from two rectangular slabs of metal such for example as an aluminium alloy. The presented faces of the slabs 11 are separated by a gasket 1 2 which is formed from a material capable of withstanding heat and the liquid to be heated.

In the presented face of the two slabs of material there is formed as shown in Fig. 3, part of a flow passage, the passage being indicated at 1 3. Each passage is of extended length, one end connecting with one through aperture 14. The other end of the flow passage terminates short of a further through aperture 1 4A but the two flow passages are interconnected by a flow aperture 1 5 formed in the gasket, the gasket having corresponding apertures to the apertures 14, the apertures in the gasket being smaller. The apertures 14 in the slabs which connect with the flow passages, have located therein a plug 16. The plug 16 is in the form of a bolt having a hexagonal head and a threaded shank which in used can be engaged with a complementary threaded hole in for example, a filter housing or a special mounting.Intermediate its ends the shank of the bolt has an enlarged portion 1 7 which in the completed assembly establishes a seal with the edge of the respective aperture in the gasket 11. Blind bores extend into the opposite ends of the bolt and these open onto the periphery of the shank of the bolt on opposite sides respectively of the enlarged portion 1 7. The blind bores are in use, connected in the fuel flow path to the filter with the result that fuel flow takes place by way of the flow passage 1 3 in one slab 11, through the aperture 1 5 in the gasket 1 2 and along the flow passage 1 3 in the other slab.In use a bolt complementary to the bolt 1 6 extends through the other apertures 1 4 and the aperture in the slab but this is of conventional form.

A modified form of slab is seen at 40 in Fig. 7. In this construction the flow passage 41 has a slightly different shape but the essential difference as compared with the example shown in Fig. 3 is that both ends of the flow passage communicate with the apertures 14 respectively. In this example the gasket 1 2 is not provided with the aperture 1 5. A similar flow arrangement to that described above can be obtained by utilising a bolt 1 6 in one pair of apertures 14 and a bolt in the other pair of apertures 1 4 which has a shank having a clearance with the edge of the aperture in the gasket. This form of flow can be regarded as being series flow since the flow passages 1 3 are connected in series.It is possible to provide parallel flow by utilising two bolts of a slightly different construction to those described. For parallel flow each bolt has a blind drilling which opens into two ports on the opposite sides of the gasket. One bolt forms the inlet for the fuel and the fuel flow is divided between the flow passages 1 3. The other bolt serves as the outlet for the fuel and two ports therein receive the fuel from the flow passages, the fuel mixing in the blind bore of the bolt.

The device is intended to be mounted on a filter unit although it can be mounted on a separate mounting. Using different forms of bolt series or parallel flow can be obtained without having to provide different basic devices.

Each slab 11 defines a flat face 1 8 which conveniently is recessed below the outer surface of the slab. In contact with the faces 1 8 respectively are a pair of electrically insulating sheets 1 9 which are formed from a material and are thin enough, to conduct heat to the associated faces 18, from heating elements which are mounted on supporting electrically insulating plates 20. Conveniently the plates are formed from an insulating laminate material of the type used in the construction of printed circuits and on the faces of the plates 20 which are presented to the sheets 19, are formed foil electric heating elements indicated at 21 in Fig. 2. The heating elements are of extended length and may be formed using conventional printed circuit techniques.The plates 20 have extended portions 22 which carry electrical terminals 23 for connection into a supply circuit as will be described.

The sheets 1 9 may be formed from a silicone based material reinforced with glass fibre. Alternatively the sheets may be formed from Nylon reinforced with glass fibre or if not reinforced coated with silicone grease.

Metal clamping sheets 24 are located against the outer surfaces of the plates 20 and the various components are held in assembled relationship by means of securing devices in the form of bolts 25 which pass through corresponding apertures in the various components.

In the example shown in Figs 1 and 2, a pair of power transistors 26 are mounted on the slabs respectively and are electrically connected into the printed circuits on the plates 20.

One of the slabs 11 is provided with a drilling to accommodate a temperature sensor 27 and the device is enclosed by means of a cover 28. The cover has an opening to allow electrical connections to be made to the terminals 23.

In use, the device is positioned in the fuel system upstream of the filter and when electrical power is supplied, the fuel becomes heated thereby minimising the risk of the filter becoming clogged in cold conditions, with frozen fractions of fuel.

It is necessary to provide some form of protection to prevent excessive heating of the slabs of material in the event that the control circuits associated with the transistors, fail. If the transistors remain in the switched on state then it is necessary to ensure rupture of the heating element once an excessive temperature has been reached. There are two ways in which this can be achieved. The first method is to relay on the expansion of the copper foil forming the heating element. When heated to an excessive temperature, the foil will expand and buckle and once this occurs the temperature of that portion of the heating element will rise substantially to the point at which it ruptures thereby breaking the electrical circuit of the heating element.A special section may be provided in the heating element which is of smaller width and which therefore has a reduced current carrying capacity. This section indicated at 29, in Fig. 2 is conviently formed on the extension 22 of the plate.

As an alternative to relying upon the buckling of the heating element, the sheets 1 9 may be formed from a thermoplastics material such as Nylon having a suitably low melting point.

In this case when the temperature of the heating element rises above a predetermined value, the plastics material will melt allowing the heating element to bear against the slabs 11. It can be arranged that there is a potential difference between the heating element and the slabs along the full length of the heating element and when such contact occurs increased heating occurs to achieve rupture of the element and thereby an interruption of the conducting path of the element.

The temperature sensor 27 is mounted in the slab adjacent to the transistors 26 which are in themselves mounted upon the slab and do become heated due to the passage of electric current. This facilitates control of the temperature of the fuel leaving the heating device and it is possible to control the fuel to within 1"C.

Turning now to Fig. 6, the two heating elements 21 are in the circuit shown to be connected in parallel and the conduction of the power transistors is controlled by a control circuit 30, one terminal of which is connected to a negative supply terminal 31 with the heating element being connected by way of a switch 32, and a fuse 33, with a positive supply terminal 34. The fuse 33 can be chosen in the event of failure of the control circuit 30, and melting of the sheet 19, to fuse in preference to fusing the heating element itself. It will be appreciated however that once the heating element has been damaged in this manner then it is probable that it cannot be used further.As will be observed from Fig. 6 two warning lamps are provided the first referenced 35, being connected between the terminal 31 and the terminal of the switch 32 which is conncted to the heating elements and the second referenced 36, to a point intermediate the control circuit 30 and the heating elements and a positive supply terminal 37 which is itself connected to the positive supply terminal of the vehicle accumulator through the normal vehicle control switch. When the control switch is closed and the temperature of the fuel as sensed by the sensor 27 which forms part of the control circuit 30, is below a predetermined value, the lamp 36 will be illuminated. This is a signal to-the driver of the vehicle to close the switch 32 whereupon the lamp 35 will also be illuminated.The two lamps will remain in an illuminated state until the fuel temperature reaches the desired value whereupon the lamp 36 will be extinguished and the operator of the vehicle can then if he so wishes, open the switch 32.

If desired the power transistors may be controlled by separate control circuits 30 and in this case two warning lamps corresponding to the lamp 36 may be provided. The two control circuits may be set to operate at different temperatures so that both heating elements are brought into operation only when the fuel is very cold.

An alternative construction employs thermostats instead of the power transistors. The thermostats are mounted on the extensions 22 of the respective plates and include a surface which is held in good thermal contact with the associated slab. The control of the fuel temperature is not so accurate using this method of control but the cost is lower and a separate housing is not needed for the control circuits. Temp- erature responsive fuses may be provided in series with the heating elements and thermostats and the thermostats may have different temperature settings.

The cover 28 is conveniently held in position by an epoxy or like potting compound which fills any voids within the cover and also holds the transistor or thermostats firmly in position.

Claims

1. A heating device for liquids comprising a metallic body, a flow passage for the liquid to be heated defined at least in part in the body, a foil electric heating element mounted on a supporting electrically insulating plate, a clamping plate, means securing the clamping plate to the body with the insulating plate being located against the clamping plate and the foil heating element presented to said face of the body and an electrically insulating sheet disposed between the heating element and said face, said insulating sheet being of a thickness and being formed from a material such that heat generated in the element when the element is connected to a source of electric supply, is conducted to the body to heat the liquid in the flow passage.

2. A heating device according to Claim 1 in which said metallic body comprises a generally rectangular slab, the device having two such slabs, a gasket disposed between said slabs, each slab having formed in its surface presented to the gasket, part of a flow passage the remaining part of the flow passage being defined by the adjacent surface of the gasket, each slab having associated therewith a foil electric heating element mounted on a supporting insulating plate, a clamping plate and an electrically insulating sheet disposed between the heating element and the slab, the components being held in assembled relationship by bolts passing through apertures in the components.

3. A heating device according to Claim 2 in which each slab is provided with a through aperture communicating with one end of the respective flow passage, said apertures being aligned to allow a bolt to be passed therethrough, the gasket having an aperture therein which is aligned with the apertures in the slabs but is smaller in size.

4. A heating device according to Claim 3 in which said gasket is provided with a flow aperture, said flow aperture acting to place the other ends of the flow passage in communication with each other.

5. A heating device according to Claim 3 in which each slab is provided with a further through aperture communicating with the other end of the respective flow path, said further apertures being aligned to allow a bolt to be passed therethrough, the gasket having a further aperture therein which is aligned with said further apertures in the slabs but smaller in size.

6. A heating device according to Claim 5 including temperature responsive means for controlling the flow of electric current in the heating elements.

7. A heating device according to Claim 6 in which said means includes a power transis tor and a control circuit for the power transistor, said control circuit including a temperature sensor responsive to the temperature of one of said slabs.

8. A heating device according to Claim 6 in which said foil heating element has a portion of reduced section whereby if an electric fault develops, said portion of reduced section will fuse.

9. A heating device according to Claim 6 in which said temperature responsive means comprises thermostats connected in circuit with said heating elements respectively, said thermostats including surfaces which have good thermal contact with the respective slabs.

1 0. A heating device according to Claim 7 in which said thermostats have different temperature settings.

11. A heating device according to Claim 9 including temperature responsive fuses in circuit with the heating elements respectively.

12. A heating device according to Claim 4 or Claim 5 in which said bolt has an enlarged portion formed on its shank intermediate the ends thereof said enlarged portion co-operating with the edge of the aperture to form a seal, the bolt being provided with blind bores extending inwardly from its opposite ends and passages formed in the shank communicating with said bores and opening onto the periphery of the shank on opposite sides of said enlarged portion.

1 3. A heating device according to Claim 1 2 when dependent upon Claim 5 including a further bolt located in said further apertures.

the shank of said bolt being smaller in diameter than the further aperture in the gasket.

14. A heating device according to Claim 5 in which said bolt and a further bolt in said further apertures have shanks smaller in diameter than the apertures in said gasket, said bolts each having a drilling which communicates with ports opening onto the periphery of the shank on opposite sides of said gasket.

1 5. A heating device for liquids comprising the combination and arrangement of parts substantially as hereinbefore described with reference to the accompanying drawings.