Patent Publication Number: US-8535050-B2

Title: Forced flue type combustion device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a forced flue type combustion device which has a combustion chamber incorporating a burner, an air supply passage for supplying combustion air to the interior of the combustion chamber, an exhaust passage for discharging flue gas in the combustion chamber generated by combustion with a burner, and a combustion fan interposed in one of the air supply passage and the exhaust passage, and in which combustion air is forcibly supplied to the interior of the combustion chamber by the rotation of the combustion fan, while flue gas in the combustion chamber is forcibly exhausted out of the combustion chamber by the rotation of the combustion fan. 
     2. Description of the Related Art 
     When the air supply passage or the exhaust passage in this kind of combustion device clogs, the rate at which combustion air supplied to the combustion chamber is reduced relative to that corresponding to the rotational speed of the combustion fan, resulting in incomplete combustion due to deficiency of air. 
     A forced flue type combustion device is hitherto known which is designed to avoid this problem and which has an orifice provided in at least one of the air supply passage and the exhaust passage, a pressure difference sensor for detecting the difference between the gas pressure on the upstream side of the orifice and the gas pressure on the downstream side of the orifice, and a clogging determination elements configured to determine whether or not the air supply passage and/or the exhaust passage is clogged on the basis of the detected pressure difference value from pressure difference sensor (see, for example, Japanese Patent Laid-Open No. 2000-310419). The difference between the gas pressure on the upstream side of the orifice and the gas pressure on the downstream side of the orifice changes according to the rate of flow of gas through the orifice. Therefore, when the air supply passage or the exhaust passage is clogged so that the gas flow rate is reduced, the detected pressure difference value from the pressure difference sensor is reduced. Thus, it is possible to determine the existence/nonexistence of a clog on the base of the detected pressure difference value by the clogging determination elements. When it is determined that the passage is clogged, the combustion with the burner is stopped to prevent the occurrence of incomplete combustion. 
     Since the rotational speed of the combustion fan is reduced during weak combustion with the burner, the rate at which gas flows through the orifice is reduced to a small value even in a normal state in which each of the air supply passage and the exhaust passage is not clogged. During weak combustion, therefore, the amount of change in the detected pressure difference value between normal and clogged states is extremely small and the existence/nonexistence of a clog cannot be determined with accuracy. 
     SUMMARY OF THE INVENTION 
     In view of the above-described problem, an object of the present invention is to provide a forced flue type combustion device capable of accurately determining existence/nonexistence of a clog even during weak combustion. 
     To achieve the above-described object, according to the present invention, there is provided a forced flue type combustion device which has a combustion chamber incorporating a burner, an air supply passage for supplying combustion air to the interior of the combustion chamber, an exhaust passage for discharging flue gas in the combustion chamber generated by combustion with a burner, and a combustion fan interposed in one of the air supply passage and the exhaust passage, and in which combustion air is forcibly supplied to the interior of the combustion chamber by the rotation of the combustion fan, while flue gas in the combustion chamber is forcibly exhausted out of the combustion chamber by the rotation of the combustion fan, the combustion device includes an orifice provided in at least one of the air supply passage and the exhaust passage, a pressure difference sensor for detecting the difference between the gas pressure on the upstream side of the orifice and the gas pressure on the downstream side of the orifice, clogging determination means for determining whether a clog exists in at least any one of the air supply passage and the exhaust passage, a pressure difference sensor for detecting the difference between the gas pressure on the upstream side of the orifice and the gas pressure on the downstream side of the orifice, clogging determination elements configured to determine whether a clog exists in at least any one of the air supply passage and the exhaust passage on the basis of a detected pressure difference value from the pressure difference sensor, and determination control elements configured to control the execution of determination by the clogging determination elements, wherein during weak combustion in which the combustion fan rotates at a rotational speed equal to or lower than a predetermined speed, the determination control elements is arranged to intermittently execute speed increasing processing to increase the rotational speed of the combustion fan to a speed higher than the predetermined speed, and execute determination by the clogging determination elements only when the rotational speed of the combustion fan is increased by the speed increasing processing. 
     According to the present invention, the rate of flow of gas through the orifice is increased by increasing the rotational speed of the combustion fan by means of speed increasing processing during weak combustion to increase the amount of change in the detected pressure difference value between normal and clogged states. Clogging determination based on the detected pressure difference value is made when the rotational speed of the combustion fan is increased. Even during weak combustion, therefore, the existence/nonexistence of a clog can be determined with accuracy. Because the combustion fan speed increasing processing is only performed periodically, there is no bad influence on the combustion with the burner. 
     During the transient time period during which the rotational speed of the combustion fan is increased, the rate of flow of gas through the orifice is instable and the detected pressure difference value is also instable. Therefore there is a possibility of a determination error in clogging determination, if clogging determination is made during the transient time period during which the rotational speed of the combustion fan is increased. Therefore, it is desirable that the determination control elements be arranged to execute determination by the clogging determination elements after the rotational speed of the combustion fan has become stable at the speed increased by speed increasing processing. It is thereby ensured that clogging determination processing is performed while the rate of flow of gas through the orifice and the detected pressure difference value are stable. In this way, prevention of erroneous determination is achieved. 
     In the present invention, it is desirable that the determination control elements be arranged to execute determination by the clogging determination elements at all times during combustion other than weak combustion in which the combustion fan is rotating at a speed higher than the above-mentioned predetermined speed. During combustion other than weak combustion, the rate of flow of gas through the orifice is high and the amount of change in the detected pressure difference value between normal and clogged states is increased. There is no erroneous determination problem under this condition. Clogging determination is made at all times to immediately detect clogging in the air supply passage or the exhaust passage when the clogging occurs. 
     In the embodiment described below, a clogging determination control shown in  FIG. 2  corresponds to the above-described determination control elements, and steps S 2  and S 8  in  FIG. 2  correspond to the above-described clogging determination elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is sectional view of a forced flue type heater which represents an embodiment of the present invention; 
         FIG. 2  is a flowchart showing details of clogging determination control performed by a controller in the heater; and 
         FIG. 3  is a time chart showing changes in rotational speed of a combustion fan during weak combustion in the heater. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a forced flue type heater in the form of a stove which represents an embodiment of the combustion device of the present invention. This heater has a box-shaped housing  1 , in which a combustion chamber  2  is housed. The combustion chamber  2  incorporates a burner  3  and an imitation firewood  4 . An opening is formed in a front side portion of the combustion chamber  2  and covered with a glass plate  2   a . An opening facing the window portion of the combustion chamber  2  is also formed in a front side portion of the housing  1 . A window glass  1   a  is fitted in this opening. A person can see the interior of the combustion chamber  2  through the window glass  1   a  to have a visual impression such as to feel as if the imitation firewood  4  is burning during burning of the burner  3 . 
     An air supply passage  5  and an exhaust passage  6  are respectively connected to lower and upper portions of the combustion chamber  2 . The upstream end of the air supply passage  5  and the downstream end of the exhaust passage  6  communicate with the outdoor atmosphere through an air supply/exhaust top  7  having a double tube structure using inner and outer tubes. A combustion fan  8  is interposed in the exhaust passage  6 . When the combustion fan  8  is rotated, flue gas generated by combustion with the burner  3  is forcibly exhausted into the outdoor atmosphere through the exhaust passage  6 . Simultaneously, air in the outside atmosphere is forcibly supplied as combustion air to the interior of the combustion chamber  2  through the air supply passage  5  by a drawing force accompanying the forced exhaustion of the flue gas. 
     An air passage  9  is defined in the housing  1  between an inlet port  9   a  opened in an upper front portion of the housing  1  and an outlet port  9   b  opened in a lower front portion of the housing  1 . A convection fan  10  and a heat exchanger  11  interposed in the exhaust passage  6  are disposed in the air passage  9 . When the convection fan  10  is rotated, room air is drawn in through the inlet port  9   a , heated by the heat of flue gas in the heat exchanger  11 , and blown as hot air into the room through the outlet port  9   b.    
     Fuel gas is supplied to the burner  3  through a proportional valve (not shown) controlled by a controller  12  in the heater. The rate of combustion with the burner  3  is variably controlled according to the deviation of the room temperature from a set heating temperature, and the rotational speed of the combustion fan  8  is variably controlled in three stages: a high speed (H) stage, a medium speed (M) stage and a low speed (L) stage according to the rate of combustion with the burner  3 . 
     There is a possibility of the air supply passage  5  or the exhaust passage  6  being clogged, for example, by intrusion of an extraneous matter such as tree leaves, or by snow in the air supply/exhaust top  7 . In such a case, the rate at which combustion air is supplied to the combustion chamber  2  may be reduced to cause incomplete combustion with the burner  3  due to deficiency of air. 
     An orifice  13  and a pressure difference sensor  14  are therefore provided to detect the occurrence of such a clogged state in the air supply passage  5  and the exhaust passage  6 . The orifice  13  is provided in the air supply passage  5 . The pressure difference sensor  14  detects, the difference between the gas pressure on the upstream side of the orifice  13  and the gas pressure on the downstream side of the orifice  13 . A detection signal from the pressure difference sensor  14  is input to the controller  12 . The controller  12  executes control for determination of clogging based on the detected pressure difference value obtained by the pressure difference sensor  14 . 
     Clogging determination control will be described with reference to  FIG. 2 . In clogging determination control, determination is first made in step S 1  as to whether or not a value NFC indicating the rotational speed of the combustion fan  8  is L. NFC is L at the time of weak combustion when the burner  3  is burning at a combustion rate lower than a predetermined value. NFC is M or H when during combustion other than weak combustion. If NFC is M or H, the process advances to step S 2  and determination is made as to whether or not the detected pressure difference value ΔP from the pressure difference sensor  14  has become equal to or lower than a predetermined clogging discrimination value YP. 
     When the air supply passage  5  or the exhaust passage  6  is clogged, the rate at which gas flows through the orifice  13  is reduced and the detected pressure difference value ΔP is also reduced. When ΔP≦YP, it is determined that clogging has occurred. The process then advances to step S 3  to execute stoppage processing. In stoppage processing, combustion with the burner  3  is stopped and the occurrence of clogging is notified. The clogging discrimination value YP is set to a comparatively large value with respect to NFC=H, and to a comparatively small value with respect to NFC=M. 
     If it is determined in step S 2  that ΔP&gt;YP, the process returns to step S 1 . If the present combustion is not weak combustion, the process again advances to step S 2 . Thus, clogging determination processing on the basis of the detected pressure difference value ΔP in step S 2  is executed at all times during combustion other than weak combustion. 
     During weak combustion of NFC=L, the rate of flow of gas through the orifice  13  is reduced and the detected pressure difference value ΔP is also reduced. Under this condition, the amount of change in the detected pressure difference value ΔP between the normal and clogged states is so small that it is difficult to accurately determine the existence/nonexistence of a clog based on the detected pressure difference value ΔP. 
     In this embodiment, therefore, speed increasing processing for increasing the rotational speed NF of the combustion fan  8  from L to M is periodically executed during weak combustion, as shown in  FIG. 3 . That is, speed increasing processing is executed during predetermined short time periods T 2  at predetermined time intervals T 1  set comparatively long. During weak combustion, clogging determination processing based on the detected pressure difference value ΔP is executed only when the rotational speed NF of the combustion fan  8  is increased by speed increasing processing. 
     When speed increasing processing is performed, the actual rotational speed of the combustion fan  8  is changed as indicated by the broken line in  FIG. 3 . As is apparent from this, a certain amount of time period is required for stabilization of the actual rotational speed of the combustion fan  8  to M. During the transient time period during which the rotational speed of the combustion fan  8  is increased, the rate of flow of gas through the orifice  13  is instable and the detected pressure difference value ΔP is also instable. Therefore there is a possibility of a determination error in clogging determination, if clogging determination is made during the transient increasing time period. Therefore, a wait time T 3  is set according to the time period necessary for stabilization of the actual rotational speed of the combustion fan  8  to M and clogging determination is made after a lapse of this wait time T 3  from a start of speed increasing processing. 
     The above-described clogging determination control during weak combustion will be concretely described with reference to  FIG. 2 . When NFC=L is determined in step S 1  during weak combustion, the process advances to step S 4  and determination is made as to whether or not T 1  has lapsed. If T 1  has not lapsed, the process advances to step S 5  and determination is made as to whether or not NFC has been changed from L to M or H by an increase in the rate of combustion with the burner  3 . If NFC has been changed from L to M or H, the process advances to step S 2 . If NFC is still L, the process returns to step S 4 . After T 1  has lapsed when NFC=L, the process advances to step S 6  and speed increasing processing is performed to increase the rotational speed NF of the combustion fan  8  from L to M. 
     Determination is then made as to whether or not T 3  has lapsed from the start of speed increasing processing in step S 7 . After the lapse of T 3 , the process advances to step S 8  and determination is made as to whether or not the detected pressure difference value ΔP of the pressure difference sensor  14  has become equal to or lower than the predetermined clogging discrimination value YP. If ΔP≦YP, it is determined that the clogging has occurred. The process then advances to step S 3  to execute stoppage processing. If ΔP&gt;YP, it is determined that the clogging has not occurred. The process then advances to step S 9  and determination is made as to whether or not T 2  has lapsed from the start of speed increasing processing. If T 2  has not lapsed, the process advances to step S 10  and determination is made as to whether NFC has been changed from L to M or H. If NFC has been changed from L to M or H, the process advances to step S 2 . If NFC is still L, the process returns to step S 8 . If T 2  has lapsed while NFC=L, processing for returning the rotational speed NF of the combustion fan  8  from M to L is performed in step S 11  and the process thereafter returns to step S 1 . 
     In the above-described clogging determination control, the rate at which gas flows through the orifice  13  is increased by performing the combustion fan  8  speed increasing processing during weak combustion, thereby increasing the amount of change in detected pressure difference value ΔP between the normal and clogged states. Also, when the rotational speed NF of the combustion fan  8  is increased by speed increasing processing, clogging determination processing based on the detected pressure difference value ΔP is performed in step S 8 , thus enabling determination of the existence/nonexistence of a clog to be made with accuracy even during weak combustion. This clogging determination processing is performed after the rotational speed of the combustion fan  8  has become stable after being increased by speed increasing processing. Thus, clogging determination processing is performed while the rate of flow of gas through the orifice  13  and the detected pressure difference value ΔP are stable. In this way, prevention of erroneous determination is achieved. Because the combustion fan  8  speed increasing processing is only performed periodically, there is no bad influence of the processing on the combustion with the burner  3 . 
     The rate of flow of gas through the orifice is increased during combustion other than weak combustion and the amount of change in the detected pressure difference value ΔP between the normal and clogged states is increased. There is no erroneous determination problem under this condition. In the above-described clogging determination control, clogging determination processing based on the detected pressure difference value ΔP in step S 2  is executed at all times during combustion other than weak combustion to immediately detect clogging in the air supply passage  5  or the exhaust passage  6  when the clogging occurs. 
     The embodiment of the present invention has been described with reference to the drawings. The present invention, however, is not limited to the described embodiment. For example, while in the above-described embodiment the process advances to step S 8  after determining in step S 7  a lapse of the T 3  from a start of speed increasing processing, the arrangement may alternatively be such that a rotational speed sensor is provided on the combustion fan  8  and the process advances to step S 8  after determination of the completion of stabilization of the detected speed from the rotational speed sensor to M. 
     While in the above-described embodiment the combustion fan  8  is interposed in the exhaust passage  6 , it may alternatively be interposed in the air supply passage  5 . Further, while in the above-described embodiment the orifice  13  is provided in the air supply passage  5 , it may alternatively be provided in the exhaust passage  6  or in each of the air supply passage  5  and the exhaust passage  6 . 
     While the above-described embodiment is an application of the present invention to a forced flue type heater, the present invention can also be applied in a similar way to forced flue type combustion devices such as hot water supply devices other than the heater.