GB2093973A

GB2093973A - Multi-fuel heater

Info

Publication number: GB2093973A
Application number: GB8106580A
Authority: GB
Original assignee: Powrmatic Ltd
Current assignee: Powrmatic Ltd
Priority date: 1981-03-03
Filing date: 1981-03-03
Publication date: 1982-09-08

Abstract

A fluid heater comprises an oil or gas burner (2), a solid fuel combustion chamber (3) and fan means (6) disposed within an outer casing (1). The fluid, which may be air, enters the casing through inlets (4, 5), passes over the burner and exits the casing via outlet (8). A temperature sensor (10) at the burner inlet extinguishes said burner when the fluid temperature at the burner inlet exceeds a preset value, whereby heat from the solid fuel chamber is utilised preferentially to that from the burner. Since the solid fuel may be in the form of waste products and cheaper fuels, the arrangement provides a heater which runs preferentially on the cheapest fuel available. Further thermostats (9, 11) control the operation of the fan means. <IMAGE>

Description

SPECIFICATION Multi-fuel heaters The present invention relates to heaters for liquid or gaseous heat exchange fluids.

Such heaters include central heating boilers and space heaters in which the ambient air is utilized as the space heating fluid.

With the rise in energy costs, it has become more important to utilize the cheapest fuels for heating. Since the relative costs of fuels can vary widely over a comparatively short space of time, multi-fuel heaters have recently become generally available and these can be utilized to burn a variety of fuels.

One difficulty in using cheap fuels such as, for example, combustible waste products is that the heat output is dependent upon, inter alia, the stoking method and the differing heat values of the waste products utilized. It is consequently difficult to obtain a uniform fluid output temperature when using such a fuel, and in practice it can only be achieved by constant skilled attention.

The present invention, therefore, seeks to alleviate this problem by providing a method and apparatus which utilizes a cheap fuel which gives a variable output and using a more costly but more easily controlled fuel to adjust the variable output temperature to a constant predetermined temperature.

According to the present invention, therefore, there is provided in one aspect, a fluid heater comprising a burner, a heat exchange unit disposed in a chamber having an inlet and an outlet, and means for passing the current of heat exchange fluid over said heat exchange unit, characterised by the provision of means for heating the fluid prior to its entry at said inlet and temperature sensor means adapted to extinguish said burner when the temperature of the fluid at the inlet exceeds a predetermined value, whereby heat from the means for heating is utilized preferentially to that from the burner. Whereas this system is hereinafter described with reference to a space heater utilizing air as the heat exchange fluid, the system may also be readily applied to a central heating boiler or the like wherein the fluid utilized is a liquid such as water.

The fuel for the burner is preferably a higher cost fuel such as oil or gas, which may be readily controlled and which is easily fired up. The fuel for the means for heating is preferably a solid fuel such as coal, wood or a combustible waste material. By use of this device heat is derived preferentially from the cheaper fuel.

In another aspect of the invention, there is provided a method of heating a fluid to a predetermined temperature value which comprises heating a first heat exchanger utilizing a first fuel to provide a fluid at said temperature value, heating a second heat exchanger utilizing a second fuel, sensing the temperature of the heat exchange fluid from the second heat exchanger and switching off the heat from the first heat exchanger when the heat exchange fluid from the second heat exchanger reaches said predetermined value. Thus, the fuel for heating the first heat exchanger may be a higher cost fuel, while that of the second heat exchanger may be a solid fuel or combustible waste product. The temperature sensing means may be a thermostat, which may be adjustable if desired.Such thermostats are positioned, for example, in the air inlet to the burner, at the outlet from the burner and in the ambient air inlet to the the heating means. These thermostats may be used to switch on and off a blower positioned downstream of the burner unit and to adjust the amount of combustion in the burner and in the means for heating.

One aspect of the invention will now be described by way of illustration only with reference to the accompanying drawings wherein: Figure 1 is a schematic view in vertical part cross-section of a two-stage multi-fuel heater in accord with the invention; Figure 2 is a side elevation of a particular space heater in accord with the arrangement shown in Figure 1; Figures 3A and 3B are front and rear views, respectively, of the heater of Figure 2; and, Figure 4 shows a wiring diagram for the heater of Figure 2.

A space heater substantially as shown in Figures 1 and 2 is provided with an outer casing 1 of a generally rectangular cross-section. Disposed within said casing is a solid fuel combustion chamber 2 which may be loaded through door 14 as shown in Figure 2. An oil or gas burner unit is disposed adjacent the solid fuel combustion chamber; both combustion chambers being flued to the rear of the device (that is to the right hand side of Figure 2). These flues are omitted for clarity.

The solid fuel combustion chamber 3 and the burner unit combustion chamber 2 form heat exchange bodies such that air flowing in the direction of the arrows shown in Figure 1 passes first over the solid fuel chamber and thence via a fan 6 across the heat exchange unit formed by the chamber 2.

The heated air so formed is passed to the exterior via air distributors 8 which may be adjusted to expell air in any given direction.

The outer casing 1 is provided below the solid fuel loading door 14 with an air inlet 5 and above the solid fuel combustion chamber 3 with an air inlet 4. The solid fuel combustion chamber 3 is provided, below the loading door 14, with a damper 13 driven by a damper motor 1 5. An electrical connector box 7 is provided on the surface of the outer casing 1 so that the temperature thereof does not exceed a safe value.

The electrical connector box 7 is connected as shown in Figure 4 to the blower motor 12 and the damper motor 1 5 and to adjustable thermostats 9 and 11 and to preset thermostat 10.

In use the heater is controlled by the two fan and limit thermostats 9 and 1 with the simple on/off thermostat 10. The on/off thermostat 10 is located at the blower inlet and is not adjustable, providing an on/off control of the oil or gas burner and arranged such as to be activated by the temperature of the air coming into the blower from the solid fuel heat exchanger.

Assuming, therefore, that the heater has been fired up on oil or gas, and that a solid fuel fire has been started in the chamber 3, the thermostat 10 will then remain switched on until the air from the solid fuel heat exchanger reaches a value of about 55 C. before switching off the oil or gas burner.

This ensures that adequate heat can be supplied by the solid fuel alone. The adjustable theremostat 11 is positioned above the heat exchanger 2 and is a fan and limit thermostat.

This acts to automatically bring in the blower following initial start-up on oil or gas when the temperature above the oil or gas heat exchanger reaches a value of about 380C. The limit side of the theremostat is set to a value of about 930C.

and when this temperature is reached, the oil or gas burner will then switch off. If will switch on again when the temperature thereof falls to a value below 750C. The fan and limit thermostat 9, positioned above the solid fuel heat exchanger is arranged to activate theblower 6 following the lighting of the solid fuel, assuming that on oil or gas is being used. This occurs when the still air above the heat exchanger reaches 380C.

following firing up. The limit side of the thermostat is used to close the solid fuel damper when the air above the heat exchanger reaches 780C. A wall mounted thermostat may also be connected to the connector box 7, so as to operate the damper or cycle the oil or gas burner when the desired room temperature diverges from the given value. As is shown in Figure 4, a two position switch is provided to permanently isolate the oil burner connections if oil and/or gas are not available.

When the heater has been operating on solid fuel and the fire is slowly dying out, the air temperature to the blower will eventually fall below about 43 OC, at which point the oil or gas burner is reactivated. As in the previous instance, the remaining heat from the solid fuel will be added to the heat derived from the oil or gas and the limit thermostat will, or course, cycle for a short while. As the oil or gas heating continues it will be immediately apparent that every btu generated from the solid fuel heat exchanger will be extracted.

For the temperature values given, suitable fuels for the oil or gas burner unit include 35 second oils and natural gas, whereas the fuel burner unit may include literally anything burnable, for example, coal, wood, waste paper etc.

In the burner unit as shown in Figures 2 to 4 a unit rated at a nominal 150,000 btu on oil, gas or solid fuel is designed to accommodate actual output of about 200,000 btu because of the interaction of heat output from the solid fuel section while the oil and gas section is still up and running. The heater is adjusted such that the lowest output on solid fuel is approximately 80,000 btu before the oil and gas burner is automatically switched in. Air flow values in all conditions are about 18,00 cfm which, in this particular instance, gives a throw of approximately 20 feet for a terminal velocity of 250 fpm.

It will thus been seen that by use of the method and apparatus of the present invention substantial savings in heat can be achieved merely by using the solid fuel combustion chamber as an incinerator for waste material.

However, if for one reason or another gas or oil is not available, the boiler may equally be utilized as a solid fuel boiler alone, although, of course, in the absence of the oil of gas burner the heat output may be a little variable.

The invention embraces, therefore, the method and apparatus hereinbefore set forth and heating systems incorporating a heater as hereinbefore set forth.

Claims

1. A fluid heater comprising a burner, a heat exchange unit disposed in a chamber having an inlet and an outlet, and means for passing a current of heat exchange fluid over said heat exchange unit, characterized by the provision of means for heating the fluid means adapted to extinguish said burner when the temperature of the fluid at the inlet exceeds a predetermined value, whereby heat from the means for heating is utilized preferentially to that from the burner.

2. A heater as claimed in claim 1 wherein the means for heating is a solid fuel combustion chamber and wherein the heating fluid is drawn over said heat exchange unit by a fan situated between said means for heating and said heat exchange unit.

3. A heater as claimed in claim 2 wherein the heat exchange fluid is ambient air and wherein said air is drawn generally downwardly over said combustion chamber and forced upwardly over said exchange unit.

4. A heater as claimed in any one of claims 1 to 3 wherein the temperature sensor is positioned between said combustion chamber and said heat exchange unit.

5. A heater as claimed in any preceding claim wherein the combustion chamber and the heat exchange unit are separately flued.

6. A heater as claimed in any preceding claim wherein the solid fuel combustion chamber is automatically damped by a damping control means in response to values from said temperature sensor.

7. A heater as claimed in any preceding claim wherein the fuel for the burner is gas or oil.

8. A method of heating a fluid to a predetermined temperature value which comprises heating a first heat exchanger utilizing a first fuel to provide a fluid at said elevated temperature, heating a second heat exchanger utilizing a second fuel, sensing the temperature of the heat exchange fluid from the second heat exchanger and switching off the heat from the first heat exchanger when the heat exchange fluid from the second heat exchanger reaches a predetermined value.

9. A method as claimed in claim 8 wherein the first fuel is a solid fuel and the second fuel is selected form oil or gas.

10. A method as claimed in either of claims 8 or 9 which comprises additionally sensing the temperature of the fluid having the first heat exchanger.

11. A method as claimed in any one of claims 8 to 10 wherein the heating fluid is air.

12. A method as claimed in any one of claims 8 to 11 wherein each of said first and second heat exchangers is separately controlled by temperature sensors to provide means for adjusting heat output to said exchangers.

13. A heater substantially as hereinbefore set forth

14. A heater substantially as hereibefore set forth and as iilustrated in Figures 1 to 4 of the accompanying drawings.

1 5. A method substantially as hereinbefore set forth.

16. A method substantially as hereinbefore set forth and as illustrated in Figures 1 to 4 of the accompanying drawings.