GB1593711A - Apparatus for indicating or controlling the combustion efficiency of oil furnaces or similar enclosef fire places - Google Patents

Description

(54) AN APPARATUS FOR INDICATING OR CONTROLLING THE COMBUSTION EFFICIENCY OF OIL FURNACES OR SIMILAR ENCLOSED FIRE PLACES (71) We, HENNING JENSON, a citizen of Denmark, of No. 191 T) sevej, Nr.

Vallenderd, 4340 TBllse, Denmark; and Svend Aage Markland, a citizen of Denmark, of No. 4 Havnsvej, 4591 F)llenslev, Denmark; 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:: The invention relates to an apparatus for indicating or controlling the combustion efficiency of oil furnaces or similar enclosed fire places, by which the content of free oxygen in the combustion gas is measured by means of a measuring probe containing a high-purity oxide of zirconium or hafnium, which is conductive to oxygen ions in the solid state, and having electrodes for deriving an electromotive force produced by conduction of oxygen ions through the probe as a measuring voltage for an electrical measuring circuit in order to indicate the oxygen content or a quantity derived therefrom.

BACKGROUND OF THE INVENTION With the growing need for energy economizing measures, an increasing interest has developed in recent years for a continuous control of the heating economy of oil furnaces and similar enclosed fire places for domestic use.

Since a correct adjustment of oil furnace installations will result in considerable savings in fuel consumption, efforts are also made by the authorities to encourage periodic voluntary inspections in order to prepare the way for a compulsory control. However, in practice, only a limited number of oil furnace installations are subjected to periodic inspections, and ordinary users have no possibility to benefit from such inspections, nor to obtain their own continuous indication or control of the combustion efficiency.

Oxygen measuring probes of the kind mentioned above have been applied in recent years to the control of industrial combustion processes and supervision of oven atmospheres in heat treatments. The principle of this measurement is that in case of different oxygen pressures on the two sides of a wall of an oxide of the kind referred to, oxygen ions will be conducted through the wall, whereby an electromotive force is produced from which a measuring voltage for an electrical circuit may be derived by means of electrodes arranged on the two sides of the wall.The conductivity to oxygen ions results from the fact, for example, that zirconium oxide by the addition, for example, of calcium oxide, magnesium oxide, yttrium oxide, ytterbium oxide or other rare earth metal oxides or mixtures thereof may be stabilized in a manner known per se into a cubic form with vacant oxygen ion sites in the crystal lattice.

The physical conditions involved in this conduction and prior art applications of the principle for measuring purposes have been described in more detail, inter alia, in an article "Festkorperionenleiter fir Messfühler und Energie speicher" by Franz Gross in the periodical BBC-Nachrichten, 1975, vol. 5/6, pages 386 to 391.

However, the conductivity to oxygen ions is only present to a sufficient extent at higher temperatures, and in the prior art applications, it has been prescribed that the measuring probe should be kept at a temperature above 600"C.

Therefore, the arrangement of a measuring probe directly in the gas mixture, the oxygen content of which is to be measured, has only been possible in combustion or heat treatment processes which are taking place at a very high and constant temperature. Furthermore, the electromotive force produced by the oxygen ion conduction is given by the well known Nernst' equation and will depend, therefore, not only of the logarithmic ratio between the oxygen pressures occurring on the two sides of the oxide wall, but also of the absolute temperature of the probe, so that in order to secure comparable measuring results, care must be taken that the oxygen content is always measured at substantially the same temperature.In continuous control, this may be obtained in a manner known per se by keeping the probe at a constant temperature by means of an internal heating element, such as explained, for example, in an article "Speeding up the on-line measurement of oxygen in pro cess gases" by E. Neuberger in the periodical Control and Instrumentation, June 1975, pages 24-25. In case of gas mixtures having a lower temperature, it has been suggested to arrange the measuring probe in a separate, thermostatcontrolled oven, which is kept at the necessary high temperature and is supplied with samples of the gas mixture.

In published German patent specification No. 2.461.565, a somewhat similar principle has been suggested for an oxygen measuring probe in a smaller combustion equipment for domestic use, whereby the probe is kept at the necessary working temperature, which in this case has been indicated to be about 500"C, by means of an electric heating element. Due to this additional electrical heating element and the temperature control circuit necessary in connection therewith, this construction is so expensive that in most cases it will not be attractive to ordinary users.

SUMMARY OF THE INVENTION By the invention it is possible, at least in some embodiments, to avoid this drawback of prior art devices and to provide an apparatus which allows a continuous effective indication or control of the combustion efficiency and, in addition, is of a sufficiently simple and cheap construction with respect to production as well as arrangement in existing furnace installations to be attractive to ordinary users.

According to the invention there is provided an apparatus for indicating or controlling the combustion efficiency of oil furnaces or similar enclosed fire places by measuring the content of free oxygen in the combustion gas, comprising a measuring probe formed as a tube which is closed at one end and constructed to be arranged in and heated by the combustion gas, means for providing an oxygen reference pressure in the interior of said tube, temperature sensing means such as a thermo-couple being arranged in the interior of said tube to sense the inside temperature thereof, at least part of the wall of said tube being made of a high-purity oxide of zirconium or hafnium, which is conductive for oxygen ions in the solid state, measuring electrodes being attached to the inner and outer sides of said wall part for sensing an electric measuring voltage arising therebetween in response to oxygen ions migrating through said wall part, electrode conductors connected to said temperature sensing means and said electrodes, and an electric measuring circuit connected with said electrode conductors, the circuit providing an output in accordance with said oxygen content, or a quantity derived therefrom in response to said measuring signal only at a value of said inside temperature corresponding to a predetermined temperature in each temperature cycle of combustion.

The invention is based on the recognition of the fact that, in practice, it has appeared possible to manufacture stabilized oxides of the kind referred to, particularly zirconium oxides, with such a high purity that with a probe designed according to the invention, a conductivity to oxygen ions which is usable for practical purposes may be obtained even at a probe temperature of 200-5000C or lower. Thereby, it is made possible in practice to utilize the measuring method, for example, in ordinary oil furnace installations by arranging the measuring probe directly in the combustion chamber.Since no additional installations, such as a separate oven or an additional heating element, are required in order to make the probe operating in a satisfactory manner, installation of the control apparatus according to the invention even in connection with existing oil furnaces will be very simple, the only requirement being the arrangement of the tubular sensitive part of the probe in a suitable place in the combustion chamber, which may be obtained, for example, by means of a cutting sleeve in the conventional inspection cover.

Thereby, it will also be easy to remove the probe for cleaning or replacement.

However, the measuring probe need not necessarily be arranged in the combustion chamber proper, but may be arranged in another place in a direct contact with the hot combustion gas, such as in the smoke box or the smoke pipe. Thereby, the probe may be utilized by means of its thermo-couple for continuous control of the temperature of the smoke gas, which together with the free oxygen in the combustion gas is a measure for the efficiency of the combustion.

It will appear from the foregoing that the oxide of zirconium or hafnium may constitute, if desired, only part of the tube which is closed in one end, and for cost reasons it will be advantageous that the part constituted by the oxide material is as small as possible. In an embodiment of the apparatus according to the invention, the oxide material constitutes the end wall of a tube made of another material which is relatively resistive to influences from the hot combustion gas. Such an end wall may be provided, for example, by an oxide disc which is tightly arranged at the end of the tube by means, for example, of a tight sealing in a cut-out in the tube wall. Resistive materials, from which the tube wall may be constructed, are well known, but such a material will preferably be a metal or a metallic conductive material, preferably steel such as stainless steel.

In such an embodiment, the tube material may suitably constitute one electrode conductor for the wall part made of oxide. Furthermore, a tube made of a metal or a metallic conductive material provides the best possible heat conduction between the interior of the tube and the combustion gas outside the tube. This is advantageous, since it is to be desired that a temperature difference between the two sides of the tube, which is as small as possible, should be obtained as quickly as possible, whereby the reference medium in the tube will attain the same temperature as the combustion gas as quickly as possible.

Stainless steel is particularly advantageous as a tube material, since it will only be slowly attacked by the corrosive combustion gas at temperatures of up to about 500"C occurring in the combustion gas.

In a particularly advantageous embodiment, the interior of the probe tube is open to the atmosphere outside the place of installation of the tube, so that the oxygen reference pressure is provided by this atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, embodiments of the invention will be described in detail with reference to the accompanying schematical drawings, in which: Figure 1 is a longitudinal sectional view of a first embodiment of the oxygen measuring probe in an apparatus according to the invention, Figure 2 shows a separate temperature probe for measuring smoke gas temperature, Figure 3 is a block diagram showing the principal construction of an electrical measuring circuit, Figure 4 is a graphic illustration of the temperature variation during a combustion cycle, Figure 5 is a graphic illustration of the combustion efficiency expressed in terms of percentage of chimney losses as a function of the temperature and carbon dioxide content of the smoke gas, Figure 6 shows a simple embodiment of an indicator panel, and Figure 7 shows a second, particularly cheap embodiment of the oxygen measuring probe.

DETAILED DESCRIPTION The oxygen measuring probe in Figure 1 comprises a tubular stick 1 which may be of stainless steel and on which a bushing 3 is secured by means of a cutting sleeve with a union 2, said bushing having an external threading for nuts 4 and 5 for securing the probe relative to a part shown at 6 of a wall of a combustion chamber, in which wall part a bore 7 has been formed for the bushing 3.

At the open end of the stick 1 positioned in the combustion chamber, there is tightly inserted an oxygen measuring probe in the form of a tube 8 which is closed in one end and preferably made of zirconium oxide which by the addition, for example, of an yttrium-containing material has been stabilized into the cubic lattice structure for obtaining a high conductivity to oxygen ions.

In order to secure a sufficient conductivity to oxygen ions at the temperatures to which the probe is exposed in usual oil furnace installations, the material used for the tubular oxygen measuring probe 8 must have a high purity. A suitable material may be yttriumcontaining zirconium oxide having a purity exceeding 99.5%. In order to obtain a stable crystal lattice structure, it is particularly preferred that individual impurity contributions do not exceed 0.1%, and, furthermore, metallic impurities which oxidize at the temperatures to which the probe is exposed and may, thereby, distort the measuring result should preferably not be present. In practice, it has appeared possible to manufacture measuring probes of zirconium oxide having such a high purity.

The tubular measuring probe 8 is provided both on the external and the internal side with electrodes, which are not shown in detail, but may have the form of coatings of an electrode material which does not oxidize at the temperatures to which the probe is exposed, the coating of said material having, furthermore, been made in such a way that on a considerable part of both the external and the internal surface of the measuring tube 8, the zirconium oxide is exposed to the combustion atmosphere in the combustion chamber and a medium, respectively, serving to provide a constant oxygen reference pressure in the interior of the measuring tube 8. The electrode material may, for example, be platinum.

In most cases, the reference medium may be atmospheric air which has a well defined oxygen content, although care must be taken that the interior of the measuring tube 8 may communicate with the ambient air outside the combustion chamber through a suitable air passage in the probe stick 1. In certain cases, however, it may be preferred to use as a reference medium in the interior of the measuring tube 8 a powdered metal-metal oxide mixture, such as indicated by shading 9 in Figure 1, which mixture may, for example, be a mixture of copper and cuprous oxide (Cu2 O), which will result in a somewhat lower oxygen reference pressure than atmospheric air and, thus, in a somewhat greater sensitivity to small changes in the oxygen content in the examined atmosphere.

Furthermore, in the interior of the measuring tube 8, there is arranged, for the purpose of measuring the temperature of the measuring probe, a thermo-couple 10, one conductor of which serves simultaneously as a conductor for the electrode coating on the innerside of the zirconium oxide tube 8 in order that this conductor may serve as a common reference conductor for the measuring signals obtained by the oxygen measurement and from the thermocouple. By means of a cable 14, the three electrode conductors shown at 11, 12 and 13 are taken out of the measuring probe to an electrical measuring circuit, the construction of which will be explained in the following.

The probe operates in the manner that in case of a difference between the oxygen pressure in the combustion atmosphere outside the measuring tube 8 and the oxygen reference pressure in the interior of the measuring tube 8, there arises as a result of oxygen ion conduction through the zirconium oxide an electromotive force, the magnitude of which depends on the logarithmic ratio between the two oxygen pressure values and the temperature of the probe. At a suitable high probe temperature, this electromotive force will result in a measuring signal on conductors 11 and 12 in Figure 1.

In order to complete the oxygen measurement performed by means of the probe in Figure I,the apparatus according to the invention may further comprise a temperature probe 15, shown in Figure 2, to be arranged in a smoke pipe or a smoke box for an oil furnace, for example in the opening which is usually present for manual control of the combustion efficiency, in order to obtain a measurement of the temperature of the smoke gas. The temperature probe 15 may have conductors 16 and 17 which lead through a cable 18 to the electrical measuring circuit.

In the schematical block diagram shown in Figure 3, the electromotive force obtained by the oxygen measurement is supplied as a measuring signal on conductors 11 and 12 to an amplifier 18', whereas the measuring signal from the thermo-couple 10 is supplied through conductors 12 and 13 to an amplifier 19. A detector 20 is connected to the output of amplifier 19 and provides at a predetermined value of the temperature output signal from amplifier 19 a gating signal for a gate 21, which is thereby opened for passage of the output signal from amplifier 18 proportional to the oxygen content in the examined atmosphere to an indicator unit 22.Dependent on the magnitude of this input signal, the indicator unit supplies an actuating signal on one of a number of control lines 23 for illuminating one of a number of indicator lamps 24 each corresponding to a predetermined value of the oxygen content in the examined atmosphere, in this case the combustion gas in the combustion chamber, in which the probe shown in Figure 1 is arranged. In this manner, there is obtained, even for the ordinary user who does not have special qualifications, an unambiguous indication of the fact whether the oxygen content or a quantity derived therefrom, such as the carbon dioxide content in the smoke gas from the oil furnace in question, is inside or outside an allowable variation range.

In a similar manner, actuation of one of a number of indicator lamps 27 each corresponding to a predetermined value of the smoke gas temperature may be obtained by supplying the measuring signal from the temperature probe 15 in Figure 2 through an amplifier 25 to an indicator unit 26.

By utilizing the measuring signal provided by the thermo-couple 10 for the temperature of the probe in Figure 1 as a gating signal for passage of the oxygen measuring signal to the indicator unit 22, particular measures for keeping the probe in Figure 1 on a constant working temperature are avoided. In Figure 4, the temperature in the combustion chamber is indicated as a function of time during a combustion cycle. The point A on the curve represents a suitable value of the probe temperature for actuation of the detector 20 in Figure 3 to supply the gating signal to gate 21. Thereby, in each combustion cycle, the oxygen content in the combustion gas occurring precisely at this temperature value will be indicated by one of the lamps 24.In connection therewith, the measuring circuit may be constructed in a manner not shown in detail, so that the oxygen indication is maintained until the actuation time in the next combustion cycle.

In Figure 5, a graphic illustration is shown of the combustion efficiency of an oil furnace expressed in terms of percentages of chimney losses as a function of the carbon dioxide content in the smoke gas and the difference between the temperature of the smoke gas and the air temperature, respectively. In practice, a satisfactory combustion efficiency and heating economy is considered to have been achieved, if the carbon dioxide content and the relative smoke gas temperature are within the rectangular variation range B shown in the figure.

In a particularly simple embodiment of the control apparatus according to the invention, the indicator lamps 24 and 27, shown in Figure 3, may be arranged in an indicator panel 28 and may be designed such that lamps corresponding to measuring values outside the allowable variation range show an indication which is clearly distinguishable from the indication by lamps corresponding to measuring values inside the allowable variation range, such as by red light in the first mentioned lamps and green light in the last mentioned lamps.

Thereby, the possibility is opened for an easy control which does not put demands on special qualifications of the user. Furthermore, since the values of carbon dioxide content and smoke gas temperature indicated by the lamps will usually change gradually, there may be obtained in this manner a preliminary warning in due time before inspection and adjustment of the furnace installation in question ought to take place.

The schematically indicated construction of the electrical measuring circuit represents a particularly cheap and simple embodiment, by which the total costs of procuring and installation of the control apparatus will relatively quickly be balanced by corresponding savings in the fuel consumption. However, this embodiment is not limiting to the invention, since the measuring circuit may also be constructed for other types of visual indication, such as a digital display, just as nothing could prevent utilization of the measuring signals provided by the probes directly in a feed back control of the fuel supply to the furnace. In addition, the probe construction itself may be modified relative to the embodiment shown within the scope of the invention.

In Figure 7, another embodiment of an oxygen measuring probe is shown, which comprise a tube 3 1 preferably made of a resistive metal or metallic conductive material, such as stainless steel. This tube may be secured in a wall 36 which is in contact with the hot combustion gas in the same manner as described in the foregoing. A disc 32 of zirconium oxide of the kind mentioned is arranged in a cut-out in the end of the tube positioned in the combustion gas. This disc 32 is arranged tightly by means of a suitable sealing 33, which may be of asbestos. On the external side of the zirconium oxide disc and in connection with the tube 31, an electrode coating 34 is provided, which may be of platinum, and on the internal side of the disc there is arranged a similar electrode coating 35 which is connected to one electrode conductor 37 for the measuring probe.The thermocouple 40 is arranged near the oxide disc 32.

Conductors 42 and 43 for the thermo-couple, the electrode conductor 37 and a conductor 44 secured to the tube 31 may be assembled into a cable in the same manner as described in the foregoing.

Even this modification of the measuring probe may be formed otherwise than shown.

Thus, the wall part constituted by the oxide need not necessarily be an end wall in the tube, but may form an annular part thereof, even if such a construction is not as simple. Instead of being made of stainless steel or another resistive metal, the tube may also be constructed, for example, from metallized or metal containing ceramic materials or one of the many wellknown dispersion reinforced metals.

WHAT WE CLAIM IS: 1. An apparatus for indicating or controlling the combustion efficiency of oil furnaces or similar enclosed fire places by measuring the content of free oxygen in the combustion gas, comprising a measuring probe formed as a tube which is closed at one end and constructed to be arranged in and heated by the combustion gas, means for providing an oxygen reference pressure in the interior of said tube, temperature sensing means being arranged in the interior of said tube to sense the inside temperature thereof, at least part of the wall of said tube being made of a high-purity oxide of zirconium or hafnium, which is conductive for oxygen ions in the solid state, measuring electrodes being attached to the inner and outer sides of said wall part for sensing an electric measuring voltage arising therebetween in response to oxygen ions migrating through said wall part, electrode conductors connected to said temperature sensing means and said electrodes, and an electric measuring circuit connected with said electrode conductors, the circuit providing an output in accordance with said oxygen content, or a quantity derived therefrom in response to said measuring signal only at a value of said inside temperature corresponding to a predetermined temperature in each temperature cycle of combustion.

2. An apparatus as claimed in claim 1 wherein the temperature sensing mean is a thermo-couple.

3. An apparatus as claimed in claim 1 or 2 wherein said tube is open at its opposite end to allow communication between the interior of the tube and the atmosphere outside the place of installation of the probe for providing said oxygen reference pressure.

4. An apparatus as claimed in claim 1 or 2 wherein a powdered metal oxide mixture is placed in the interior of said tube for providing said oxygen reference pressure.

5. An apparatus as claimed in claim 4, wherein said mixture consists of copper and cuprous oxide (Cu2 0).

6. An apparatus as claimed in any one of claims 1 to 5, wherein said wall part constitutes the closed end wall only of said tube, and the other wall parts of the tube are made of a material which is relatively resistive to influences from said combustion gas.

7. An apparatus as claimed in claim 6, wherein said material is a metal or a metallic conductive material.

8. An apparatus as claimed in claim 7 wherein said metal is steel.

9. An apparatus as claimed in claim 7 or 8 wherein the electrode conductor connected with one of said measuring electrodes is constituted by said other wall parts of the tube.

10. An apparatus as claimed in any one of claim 1 to 9 wherein said high purity oxide consists of zirconium oxide or hafnium oxide or a mixture thereof, which has been stabilized into the cubic crystal lattice structure with a relatively high conductivity for oxygen ions and the purity of which exceeds 99.5 per cent with individual impurity concentrations below 0.1 per cent and free from metallic impurities which oxidize at the highest temperature in said temperature cycle.

11. An apparatus as claimed in any one of claims 1 to 10, wherein said electric measuring circuit comprises an indicator unit having a number of separate visual indicator means, each responsive to its own predetermined value of said oxygen content or said quantity derived therefrom.

12. An apparatus for controlling the combustion efficiency of oil furnaces or similar enclosed fire places substantially as described hereinbefore with reference to the accompanying drawings.

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

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. relative to the embodiment shown within the scope of the invention. In Figure 7, another embodiment of an oxygen measuring probe is shown, which comprise a tube 3 1 preferably made of a resistive metal or metallic conductive material, such as stainless steel. This tube may be secured in a wall 36 which is in contact with the hot combustion gas in the same manner as described in the foregoing. A disc 32 of zirconium oxide of the kind mentioned is arranged in a cut-out in the end of the tube positioned in the combustion gas. This disc 32 is arranged tightly by means of a suitable sealing 33, which may be of asbestos. On the external side of the zirconium oxide disc and in connection with the tube 31, an electrode coating 34 is provided, which may be of platinum, and on the internal side of the disc there is arranged a similar electrode coating 35 which is connected to one electrode conductor 37 for the measuring probe.The thermocouple 40 is arranged near the oxide disc 32. Conductors 42 and 43 for the thermo-couple, the electrode conductor 37 and a conductor 44 secured to the tube 31 may be assembled into a cable in the same manner as described in the foregoing. Even this modification of the measuring probe may be formed otherwise than shown. Thus, the wall part constituted by the oxide need not necessarily be an end wall in the tube, but may form an annular part thereof, even if such a construction is not as simple. Instead of being made of stainless steel or another resistive metal, the tube may also be constructed, for example, from metallized or metal containing ceramic materials or one of the many wellknown dispersion reinforced metals. WHAT WE CLAIM IS:

1. An apparatus for indicating or controlling the combustion efficiency of oil furnaces or similar enclosed fire places by measuring the content of free oxygen in the combustion gas, comprising a measuring probe formed as a tube which is closed at one end and constructed to be arranged in and heated by the combustion gas, means for providing an oxygen reference pressure in the interior of said tube, temperature sensing means being arranged in the interior of said tube to sense the inside temperature thereof, at least part of the wall of said tube being made of a high-purity oxide of zirconium or hafnium, which is conductive for oxygen ions in the solid state, measuring electrodes being attached to the inner and outer sides of said wall part for sensing an electric measuring voltage arising therebetween in response to oxygen ions migrating through said wall part, electrode conductors connected to said temperature sensing means and said electrodes, and an electric measuring circuit connected with said electrode conductors, the circuit providing an output in accordance with said oxygen content, or a quantity derived therefrom in response to said measuring signal only at a value of said inside temperature corresponding to a predetermined temperature in each temperature cycle of combustion.

2. An apparatus as claimed in claim 1 wherein the temperature sensing mean is a thermo-couple.

3. An apparatus as claimed in claim 1 or 2 wherein said tube is open at its opposite end to allow communication between the interior of the tube and the atmosphere outside the place of installation of the probe for providing said oxygen reference pressure.

4. An apparatus as claimed in claim 1 or 2 wherein a powdered metal oxide mixture is placed in the interior of said tube for providing said oxygen reference pressure.

5. An apparatus as claimed in claim 4, wherein said mixture consists of copper and cuprous oxide (Cu2 0).

6. An apparatus as claimed in any one of claims 1 to 5, wherein said wall part constitutes the closed end wall only of said tube, and the other wall parts of the tube are made of a material which is relatively resistive to influences from said combustion gas.

7. An apparatus as claimed in claim 6, wherein said material is a metal or a metallic conductive material.

8. An apparatus as claimed in claim 7 wherein said metal is steel.

9. An apparatus as claimed in claim 7 or 8 wherein the electrode conductor connected with one of said measuring electrodes is constituted by said other wall parts of the tube.

10. An apparatus as claimed in any one of claim 1 to 9 wherein said high purity oxide consists of zirconium oxide or hafnium oxide or a mixture thereof, which has been stabilized into the cubic crystal lattice structure with a relatively high conductivity for oxygen ions and the purity of which exceeds 99.5 per cent with individual impurity concentrations below 0.1 per cent and free from metallic impurities which oxidize at the highest temperature in said temperature cycle.

11. An apparatus as claimed in any one of claims 1 to 10, wherein said electric measuring circuit comprises an indicator unit having a number of separate visual indicator means, each responsive to its own predetermined value of said oxygen content or said quantity derived therefrom.

12. An apparatus for controlling the combustion efficiency of oil furnaces or similar enclosed fire places substantially as described hereinbefore with reference to the accompanying drawings.