AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION Standard Patent Applicant(s): RINNAI CORPORATION Invention Title: HOT AIR HEATER The following statement is a full description of this invention, including the best method for performing it known to me/us: 1A HOT AIR HEATER BACKGROUND 1. Technical Field [0001] The present invention relates to a hot air heater that mixes combustion gas and air to thereby blow the mixture as hot air. 2. Related Art [00021 This kind of hot air heater has the following arrangement, i.e., the air heater is made up of a heater main body having a hot air outlet port and an air suction port, and a ventilation case disposed inside the heater main body so as to communicate the air suction port and the hot air outlet port with each other. A hot-air fan and a combustion box with a built-in burner are disposed inside the ventilation case. By the operation of the hot-air fan, the air sucked from the air suction port into the ventilation case is sent to the hot air outlet port as hot air by mixing the combustion gas discharged out of an upper end portion of the combustion box with the air sucked from the air suction port into the ventilation case. [0003] Further, in this kind of hot air heater, it is known to constitute the burner by a totally aerated combustion burner (also referred to as a fully primary aerated burner) in which a premixed gas which is leaner in fuel concentration than the theoretical air/fuel ratio is ejected out of a combustion plate which lies on an upper surface of the burner (see, e.g., Patent Document 1). In this kind of burner, it is so arranged that a combustion cover enclosing the combustion space above the combustion plate is provided, and further that part of the air from the air suction port is introduced into the combustion box. [0004] Here, if the combustion gas to be generated by the combustion of the premixed gas immediately gets mixed with the air to be sucked 2 through the air suction port, the high-temperature combustion gas will be rapidly cooled. As a result, there will be an increase in the ratio of conversion of nitrogen monoxide, out of the nitrogen oxides contained in the combustion gas, into highly toxic nitrogen dioxide. If the combustion cover is disposed as in the above-mentioned prior art example, heat exchange will take place between the air, that is introduced into the combustion box so as to flow along the outside of the combustion cover, and the combustion gas inside the combustion cover. Therefore, the temperature of the combustion gas falls and at the same time the temperature of the air rises and, as a result, the combustion gas will not be rapidly cooled at the point of confluence of the combustion gas and the air. The ratio of conversion of nitrogen monoxide to nitrogen dioxide will therefore decrease. [00051 By the way, in the above-mentioned prior art example, the height dimension of the combustion cover is made so long that the upper end of the combustion cover reaches the upper end portion of the combustion box. According to this arrangement, it has been found that the following disadvantages occur. In other words, when incomplete combustion of the premixed gas occurs inside the combustion cover at the time of oxygen deficiency, the temperature of the combustion gas that flows out of the upper end of the combustion cover (also referred to as flowing-out combustion gas) in a state of incomplete combustion becomes too low. As a result, even if the air that flows along the outside of the combustion cover gets mixed, as the secondary air, with the flowing-out combustion gas, Bunsen combustion does not take place and, consequently, the concentration of CO will increase. Prior Art Document [0006] Patent Document 1: JP-A-2004-162947 3 SUMMARY [0007] In view of the above points, this invention has a problem of providing a hot air heater which is capable of preventing the CO concentration from increasing at the time of oxygen deficiency. [0008] In order to solve the above-mentioned problems, this invention is a hot air heater comprising: a heater main body having a hot air outlet port and an air suction port; and a ventilation case disposed inside the heater main body so as to communicate the air suction port and the hot air outlet port with each other. The ventilation case has disposed therein a hot-air fan, and a combustion box with a built-in burner so that, by the operation of the hot-air fan, the air sucked from the air suction port into the ventilation case is mixed with combustion gas discharged out of an upper end portion of the combustion box so as to be sent as hot air to the hot air outlet port. The burner is constituted by a totally aerated combustion burner in which combustion takes place by ejecting, from a combustion plate on an upper surface of the burner, a premixed gas which is leaner in fuel concentration than a theoretical air fuel ratio. The burner is provided with a combustion cover which encloses a combustion space above the combustion plate and is so arranged that part of the air from the air suction port is introduced into the combustion box. The height dimension of the combustion cover is set such that, when incomplete combustion of the premixed gas occurs inside the combustion cover at the time of oxygen deficiency, the temperature of the combustion gas, flowing out of an upper end of the combustion cover, in an incomplete combustion state is above a lower-limit temperature at which Bunsen combustion is possible. The combustion cover has a flange portion which is formed on an upper end edge thereof in a manner to be bent outward. [0009] Here, in order to prevent the rapid cooling of the combustion 4 gas flowing out of the combustion cover under normal conditions in which complete combustion of the premixed gas inside the combustion cover occurs, it is desirable to make the height dimension of the combustion cover as long as possible within a range that meets the above-mentioned conditions. However, if the height dimension of the combustion cover is set in this manner, at the time of oxygen deficiency, the temperature of the flowing-out combustion gas may be above the lower-limit temperature at which Bunsen combustion is possible in the neighborhood of the upper end of the combustion cover, but will be below the lower-limit temperature once the combustion gas leaves upward from the upper end of the combustion cover. In such a case, unless the flange portion is formed at the edge of the upper end of the combustion chamber, the air that rises along the outer surface of the combustion cover will flow upward without sufficient contact with the flowing-out combustion gas near the upper end of the combustion cover, whereby Bunsen combustion near the upper end of the combustion cover will no longer take place successfully. Then, even if the air gets mixed into the flowing-out combustion gas at a position away from the upper end of the combustion cover, the temperature of the flowing-out combustion gas will be below the lower-limit temperature, whereby Bunsen combustion will no longer take place. [0010] On the other hand, according to this invention, part of the air that has flown to the outside of the combustion cover will be circulated in a manner to form swirls on the flange portion that is formed on the upper end edge of the combustion cover. Therefore, at the time of oxygen deficiency, the air circulating on the flange portion will efficiently get mixed, as secondary air, into that combustion gas in the incomplete combustion state which flows out of the upper end of the combustion cover, whereby the flowing-out combustion gas is subjected to Bunsen 5 combustion near the flange portion. Then, due to Bunsen combustion near the flange portion, the temperature of the flowing-out combustion gas rises, and Bunsen combustion takes place also in a region upwardly away from the upper end of the combustion cover. The flowing-out combustion gas is thus subjected to complete combustion, so that the CO concentration can be prevented from increasing. [0011] By the way, as a measure to prevent the problems in relation to the oxygen deficiency, it is generally practiced to provide a flame detecting element which faces an upper surface of the combustion plate so that the combustion by the burner can be stopped when the output of the flame detecting element has lowered to a predetermined threshold value due to flame lift-off at the time of oxygen deficiency. In this case, preferably the height dimension of the combustion cover is set such that, in the oxygen deficiency state before the occurrence of flame lift-off at which the output of the flame detecting element lowers to the threshold value, the temperature of the combustion gas, flowing out of the upper end of the combustion cover, in the incomplete combustion state is above a lower-limit temperature at which Bunsen combustion is possible. According to this arrangement, Bunsen combustion can be made of the combustion gas in the incomplete combustion state that flows out of the upper end of the combustion cover in a state of oxygen deficiency before the combustion is stopped due to flame lift-off. The increase in CO concentration before stopping of combustion can therefore be prevented. BRIEF DESCRIPTION OF THE DRAWINGS [0012] FIG. 1 is a perspective view of a hot air heater according to an embodiment of this invention. FIG. 2 is a sectional side view of the hot air heater according to the embodiment of this invention; 6 FIG. 3 is an enlarged sectional view taken along the line III-Ill in FIG. 2. FIG. 4 is a sectional view taken along the line IV-IV in FIG. 3. FIG. 5 is a perspective view of a burner to be disposed in the hot air heater according to the embodiment of this invention. DESCRIPTION OF EXEMPLARY EMBODIMENTS [0013] With reference to FIGS. 1 and 2, reference numeral 1 denotes a heater main body of a hot air heating apparatus. This heater main body 1 has that hot air outlet port 11 with a louver 11a which is formed in a lower portion of a front surface of the heater main body, and an air suction port 12 which is formed in a rear surface of the heater main body. Inside the heater main body 1 there is disposed a ventilation case 2 which communicates the air suction port 12 and the hot air outlet port 11 together. At a lower portion of the ventilation case 2 there is disposed a hot-air fan 3 which is driven by a motor 3a disposed on one lateral side at the lower portion of the ventilation case 2. [00141 Further, in an upper portion of the ventilation case 2 there is disposed a combustion box 5 having therein a built-in burner 4. In an upper end portion of the combustion box 5 there is formed an exhaust gas outlet port 51 which is provided to open to discharge the combustion gas from the burner 4 into the ventilation case 2. At a lower portion on a rear surface of the combustion box 5 there is formed an air introduction port 52 which opens to introduce part of the air from the air suction port 12 into the combustion box 5. It is thus so arranged that, by the operation of the hot-air fan 3, the air to be sucked from the air suction port 12 into the ventilation case 2 gets mixed with the combustion gas to be discharged from the exhaust gas outlet port 51 of the combustion box 5, thereby obtaining hot air for blowing toward the hot air outlet port 11.
7 By the way, the combustion box 5 is constituted by interposing, between side plate portions on both lateral sides of the ventilation case 2, a plate member 5a which forms a front surface portion and a bottom surface portion of the combustion box 5 as well as a plate member 5b which forms a rear surface portion of the combustion box 5. [00151 As shown in FIGS. 3 to 5, the burner 4 has a combustion plate 42 which is mounted on an upper surface of a rectangular and box-shaped burner main body 41, and is constituted by a totally aerated combustion burner in which combustion takes place by ejecting from the combustion plate 42 a premixed gas whose fuel concentration is leaner than a theoretical air-fuel ratio. By the way, in this embodiment, the combustion plate 42 is constituted by a ceramic plate in which a multiplicity of flame holes 42a are formed, but there may be used a combustion plate which is made of heat-resistant fibers that are weaved into knits. [00161 The burner main body 41 has connected thereto a gas introduction pipe 43 which is projected into the space inside the heater main body 1 by penetrating through the side plate portion on one lateral side of the ventilation case 2. The gas introduction pipe 43 has connected thereto a combustion fan 44 on an upstream end thereof and, in the middle portion thereof, has mounted a nozzle block 45 having a nozzle 45a which is elongated into the burner main body 41. The nozzle block 45 is supplied with a fuel gas through a gas supply pipe 6a which is communicated with a valve unit 6 disposed in the space inside the heater main body 1. Then, the fuel gas to be ejected from the nozzle 45a is mixed with the primary air to be supplied from the combustion fan 44, thereby generating a premixed gas whose fuel concentration is leaner than the theoretical air-fuel ratio. This premixed gas is thus arranged to be ejected from the combustion plate 42 via the burner main body 41.
8 [0017] Further, in an upper peripheral edge portion of the burner main body 41, there is vertically disposed a combustion cover 46 which encloses the combustion space above the combustion plate 42. In one side and in the other side of the side plate portion as seen in the lateral direction of the combustion cover 46, there is formed a perforation 46a, 46b, respectively. Through each of the perforations 46a, 46b there are mounted an ignition plug 47 and thermocouple 48, as a flame detecting element, both lying so as to face an upper surface of the combustion plate 42. It is thus so arranged that the combustion by the burner 4 is stopped when the flame is lifted off above the thermocouple 48 at the time of oxygen deficiency whereby the electromotive force of the thermocouple 48 has lowered to a predetermined threshold value. [00181 By the way, the air that is introduced from the air introduction port 52 into the combustion box 5 flows upward along the outer side surface of the combustion cover 46. Then, heat exchange takes place between that combustion gas inside the combustion cover 46 which is generated by the combustion of the premixed gas, and the air that flows along the outside of the combustion cover 46. Therefore, the temperature of the combustion gas lowers and, at the same time, the temperature of the air rises. As a consequence, the combustion gas will no longer be rapidly cooled at a portion where the combustion gas and the air meet together above the combustion cover 46, whereby the ratio of conversion from nitrogen monoxide to nitrogen dioxide will be reduced. [0019] Here, in order to prevent the combustion gas from getting rapidly cooled, it is necessary to make the height dimension of the combustion cover 46 long enough to a certain degree. If the height dimension is made too long, however, when incomplete combustion of the premixed gas occurs inside the combustion cover 46 at the time of oxygen deficiency, the temperature of that combustion gas in the state of oxygen 9 deficiency which flows out of the upper end of the combustion cover 46 will be too low. As a result, even if the air that has flown along the outside of the combustion cover 46 gets mixed as the secondary air with the flowing-out combustion gas, Bunsen combustion will not take place, but CO concentration will increase. By the way, if the electromotive force of the thermocouple 48 has lowered to the above-mentioned threshold value due to flame lift-off at the time of oxygen deficiency, the combustion by the burner 4 will be stopped. It is yet desirable to prevent the CO concentration before stopping the combustion from increasing. [00201 Therefore, the height dimension of the combustion cover 46 is set such that, when incomplete combustion of the premixed gas occurs inside the combustion cover 46 at the time of oxygen deficiency, the temperature of the combustion gas, flowing out of the upper end of the combustion cover 46, in the incomplete combustion state, is above the lower-limit temperature at which Bunsen combustion is possible. By the way, if the flame lift-off occurs, the premixed gas is subjected to primary combustion at a position near the upper end of the combustion cover 46, whereby the temperature of the combustion gas rises near the upper end of the combustion cover 46. As a result, the temperature of the combustion gas that flows out of the upper end of the combustion cover 46 in an oxygen deficiency state before the occurrence of the flame lift-off will be lower than the temperature of the combustion gas at the time when the flame lift-off has occurred. Therefore, in this embodiment, the height dimension of the combustion cover 46 is set such that, in the oxygen deficiency state before the occurrence of such flame lift-off at which the electromotive force of the thermocouple 48 lowers to the above mentioned threshold value, the temperature of the combustion gas, in the incomplete combustion state, flowing out of the upper end of the combustion cover 46 is above the lower-limit temperature at which 10 Bunsen combustion is possible. [00211 By the way, in order to prevent the combustion gas from getting rapidly cooled under normal operating conditions when the premixed gas performs complete combustion inside the combustion cover 46, the height dimension of the combustion cover 46 is set to as long a value as possible (e.g., 30 mm) within a range in which the above mentioned conditions are met. Further, in this embodiment, there is formed a flange portion 46c on an end edge of a plate portion in front of, and at the rear of, the combustion cover 46 in a manner to be bent outward (in a forward direction in a front plate portion, and in a rearward direction in a rear plate portion, respectively). [00221 When the height dimension of the combustion cover 46 is set as described above, in the oxygen deficiency state before the occurrence of such flame lift-off as will cause the electromotive force of the thermocouple 48 to lower down to the above-mentioned threshold value, the following will happen. That is, even if the temperature of the combustion gas, in the incomplete combustion state, flowing out of the upper end of the combustion cover 46 has exceeded the lower-limit temperature at which Bunsen combustion is possible near the upper end of the combustion cover 46, the temperature of the combustion gas will fall below this lower-limit temperature once the combustion gas is away upward from the upper end of the combustion cover 46. In this case, unless the flange portion 46c is formed at the upper end edge of the combustion cover 46, the air that flows upward along the outer side surface of the combustion cover 46 will escape upward without sufficiently coming into contact with the flowing-out combustion gas near the upper end of the combustion cover 46, whereby Bunsen combustion near the upper end of the combustion cover 46 will no longer take place successfully. In addition, even if the air gets mixed into the flowing-out 111 combustion gas at a position away from the upper end of the combustion cover 46, the temperature of the flowing-out combustion gas at this position will be below the lower-limit temperature, whereby Bunsen combustion will no longer take place. [0023] On the other hand, according to this embodiment, part of the air that goes up along the outer side surface of the combustion cover 46 will be re-circulated (flow back), as shown by arrows in FIG. 4, in a manner to swirl on the flange portion 46c that is formed on the upper end edge of the combustion cover 46. As a result, the air that circulates on the flange portion 46c will efficiently get mixed, as the secondary air, into that combustion gas in the incomplete combustion state which flows out of the upper end of the combustion cover 46. The flowing-out combustion gas will thus perform Bunsen combustion in the neighborhood of the flange portion 46c. Then due to the Bunsen combustion near the flange portion 46c, the temperature of the flowing-out combustion gas will increase and, even in the region upwardly away from the upper end of the combustion cover 46c, Bunsen combustion will also take place. The flowing-out combustion gas will thus be subjected to complete combustion. Therefore, the CO concentration can be prevented from increasing before stopping of the combustion as a result of decrease in electromotive force of the thermocouple 48 due to flame lift-off. Consequently, the height dimension of the combustion cover 46 can be set as long as possible within the range in which the above-mentioned conditions are met. The increase in the rate of conversion from nitrogen monoxide to nitrogen dioxide due to rapid cooling of the combustion gas under normal conditions can thus be restricted and, further, the increase in the CO concentration at the time of oxygen deficiency can also be prevented. [0024] Descriptions have so far been made of embodiments of this invention with reference to the drawings. This invention is however not 12 limited to the above. For example, in the above-mentioned embodiment a thermocouple 48 is used as a flame detecting element. It is however possible to use a flame rod. Further, in the above-mentioned embodiment there is employed, as a burner 4, a totally aerated combustion burner to supply the primary air from the combustion fan 44. It is, however, possible to employ a totally aerated combustion burner in which a mixing tube in communication with the burner is provided, so that the primary air is sucked by the flow of the fuel gas to be ejected from a nozzle that faces the upstream end of the mixing tube. [00251 It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. [00261 In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e., to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. EXPLANATION OF REFERENCE NUMERALS [0027] 1 heater main body 11 hot air outlet port 12 air suction port 2 ventilation case 3 hot-air fan 4 burner 42 combustion plate 46 combustion cover 46c flange portion 48 thermocouple (flame detecting element) 5 combustion box