Abstract:
A kiln and method of firing a kiln are provided for use in connection with the firing of small-scale pottery firing. The kiln includes a kiln wall that defines a main kiln volume that is no larger than 60 cubic feet and is less than 10 cubic feet in one embodiment. The main kiln volume is structured to receive a pottery item for firing. The kiln further includes a combustion chamber that is thermally connected to the main kiln volume. A feeder automatically feeds solid fuel to the combustion chamber to reach and maintain at least a cone 10 temperature within the main kiln volume. In use, solid fuel is automatically provided to a combustion chamber at a controlled rate and is combined with combustion air in the combustion chamber. The solid fuel is combusted in the combustion chamber to warm a thermally-connected main kiln volume.

Description:
FIELD OF INVENTION 
   The present invention relates to pottery kilns. More particularly, the present invention relates to an automated, wood-fired pottery kiln and an automated method of feeding solid fuel into a pottery kiln. 
   BACKGROUND INFORMATION 
   A pottery or ceramics kiln is an instrument used to convert clay into finished pottery. The conversion is an irreversible process, known as vitrification, that partially melts and fuses clay into glass-like pottery through the application of high temperatures. The temperature required to complete the conversion process depends on the specific clay mixtures used. For a typical stoneware clay, temperatures on the order of 2345° F. are required to complete the conversion process. 
   Various heating processes exist for vitrifying pottery and ceramics. These include electric heating, natural gas combustion, propane combustion, and wood combustion. Electric processes utilize an oxidizing air atmosphere as the heating elements typically experience short service lives in reducing atmospheres. The combustion processes allow for both oxidizing and reducing atmospheres yielding greater flexibility in glazing operations and in the use of techniques such as salt firing. Wood combustion processes are held in particularly high regard based on the unique characteristics imparted to the fired piece and the renewable nature of the fuel source. 
   Electric kilns offer the benefit of simplicity of operation and scalability to small sizes suitable for amateur and small scale production uses. Natural gas and propane fired kilns are scalable from small to large size, but require a higher level of expertise to safely operate due to the hazardous nature of these fuels and the significant volumes required to fire a kiln. For this reason, natural gas and propane fired kilns enjoy limited use among amateur and small scale production users. Wood fired kilns offer inherent safety benefits compared to natural gas or propane kilns, but their use has been limited to professional, large-scale operators because of the labor intensive nature of wood firing and the large size and cost of the kilns. Conventional wood kilns are labor-intensive because the firing process requires a large quantity of wood, frequent refueling, and long firing times. For example, a minimum-sized wood kiln might require a cord or more of wood, with small pieces fed every few minutes during peak firing, and a total attended firing time of 24 to 48 hours. The large size of wood kilns is dictated by the size of the traditional cord wood fuel source and the labor intensive nature of the firing process which lends itself to large batch sizes. As a result, conventional wood-fired kilns are not practical for use by amateur and small scale production users. 
   SUMMARY OF THE INVENTION 
   There exists a need to provide a wood-fired kiln that overcomes at least some of the above-referenced deficiencies. Accordingly, at least this and other needs have been addressed by exemplary embodiments of the kiln according to the present invention. One such embodiment is directed to a kiln including a kiln wall that defines a main kiln volume that is no larger than 60 cubic feet. The main kiln volume is structured to receive a pottery item for firing. The kiln further includes a combustion chamber that is thermally connected to the main kiln volume. A feeder automatically feeds solid fuel to the combustion chamber to reach and maintain at least a cone 10 temperature within the main kiln volume. 
   In another exemplary embodiment of the present invention, a method of firing a kiln is provided. Solid fuel is automatically provided to a combustion chamber at a controlled rate. The solid fuel is combined with combustion air in the combustion chamber. The solid fuel is combusted in the combustion chamber to warm a thermally-connected main kiln volume to at least a cone 10 temperature. 
   In yet another exemplary embodiment of the present invention, a kiln is provided for firing pottery items. The kiln includes a means for automatically providing solid fuel to a combustion chamber at a controlled rate. The kiln further includes a means for combining the solid fuel with combustion air in the combustion chamber. The kiln further includes a means for combusting the solid fuel in the combustion chamber to reach at least a 10 cone temperature in a main kiln volume. The main kiln volume is thermally connected to the combustion chamber and is no greater than 60 cubic feet. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The detailed description will refer to the following drawings, wherein like numerals refer to like elements, and wherein: 
       FIG. 1  shows a schematic diagram of one embodiment of a kiln according to the present invention; 
       FIG. 2  shows a perspective view of one embodiment of the kiln; 
       FIG. 3  shows a perspective view of the kiln shown in  FIG. 2 ; 
       FIG. 4  shows a more detailed perspective view of the connection between the hopper and the kiln wall, as shown in  FIGS. 2 and 3 ; 
       FIG. 5  shows an exploded view of one embodiment of a feeder, such as the feeder shown in  FIG. 4 ; 
       FIG. 6  shows a more detailed perspective view of the connection between the fan and the kiln wall; 
       FIG. 7  shows a perspective view of the inside of the kiln; and 
       FIG. 8  shows a perspective view of the main kiln volume of the embodiment of the kiln shown in  FIG. 7 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a schematic diagram showing one embodiment of a kiln  100  according to the present invention. Solid fuel (not shown) to supply the kiln  100  is held in a storage arrangement, such as a hopper  1 . In some applications, such as the use of a kiln  100  at home for personal or hobby use, it is desirable to minimize the size of the kiln  100 . The size of the fuel limits the minimum size of a kiln  100  so in one embodiment it is desirable to utilize finely-divided fuels to scale down the size of a kiln  100 . In a preferred embodiment of the present invention the fuel is wood pellets. The fuel (not shown) flows from a storage arrangement, such as the hopper  1  shown in  FIG. 1 , to a feeder  2 . The feeder  2  controls the rate at which fuel is fed (the “fuel feed rate”), and can be embodied in various forms including without limitation an auger, a rotary valve, a lock hopper, conveyer mechanisms, or other feeding mechanisms. The feeder  2  is powered by an actuator  3 , such as a gear motor, air motor, or other power source. 
   In the embodiment shown in  FIG. 1 , the fuel feed rate may be adjusted by adjusting the amount of fuel allowed to pass through feed mechanism  2 . In one embodiment, the feed rate through feeder  2  may be adjusted with a controller  4 . In one embodiment, the controller uses a gear-motor actuator  3  controlled by a motor-speed controller  4 . One skilled in the art will recognize that the controller  4  may be embodied in various other forms. 
   Fuel (not shown) from the feeder  2  flows through the feed tube  5 . Fuel from the feed tube  5  passes through kiln wall  6 , and drops into combustion chamber  10 . Combustion air is delivered to the kiln  100  with fan  7 , such as a forced-draft fan in one embodiment. Air is directed from the fan  7  through a duct  8  and then through an air-distribution grate  9  at or near the bottom of combustion chamber  10  of the kiln  100 . 
   Fuel combustion is completed in the combustion chamber  10  as the air reaches the fuel through the air grate  9 . The combustion chamber  10  is formed by an internal partition  11 . The internal partition  11  provides adequate combustion-chamber volume to complete combustion and to direct fully-combusted gasses to the main kiln volume  12 . In one embodiment, the main kiln volume  12  is 60 cubic feet or less. In one particular embodiment, the main kiln volume  12  is 10 cubic feet or less, and in another particular embodiment described further herein, the main kiln volume  12  is approximately 3.75 cubic feet and is still capable of achieving a cone 10 temperature. Hot combustion gases heat the main kiln volume  12  and then exit the kiln  100  through the exhaust port  13 . The exhausted gases are directed through the stack  14  into the atmosphere, at a safe location. 
   In the embodiment of  FIG. 1 , combustion air is preheated with a heat exchanger  17 . A pipe-in-pipe heat exchanger configuration is depicted in the embodiment of  FIG. 1 , formed by inner and outer pipes  14 ,  15 . Combustion gases passing through the outer pipe  15  are heated by exhaust gases passing through the inner pipe  14 . Together, the inner and outer pipes  14 ,  15  of the example of  FIG. 1  may be referred to as the stack. The hot internal pipe  14  heats cool combustion air as the combustion air passes between the outer and inner pipes  15 ,  14 . In the embodiment shown, preheated air from the heat exchanger  17  passes through a duct  16  to the fan  7 . Other embodiments may use different mechanisms to preheat combustion air before the combustion air reaches the combustion chamber  10 . 
     FIG. 2  shows a perspective view of one embodiment of the kiln  100 . In the embodiment shown, the kiln wall  6  comprises an inner enclosure of fire-resistant material, such as fire bricks  23  surrounded by insulating board  24 , encapsulated by metal walls  22  surround the brick kiln wall  6 . The fire bricks  23  provide an enclosure capable of containing high operating temperatures, such as “cone 10” temperatures of approximately 2345° F., or higher, in one embodiment. The insulating board  24  surrounding the fire bricks  23  provides insulation to help achieve and maintain the high temperatures with efficient fuel use. The metal shell  22  provides support for the fire bricks  23  and insulating board  24 . The metal shell  22  is supported by kiln frame  27 . In the embodiment shown, doors  25 ,  26  provide access to the main kiln volume  12  and to the combustion chamber  10  of the kiln  100 , respectively. A first door  25  provides access to the main kiln volume  12 , for example, for loading and unloading pottery. A second door  26  allows access to the combustion chamber  10  of the kiln  100 , for example, for starting and cleaning the combustion chamber  10 . The kiln  100  is adapted to be portable under hand power through the use of wheels, such as casters, connected to lower ends of legs  18 . In other embodiments, the kiln  100  may include skids (not shown) or similar means of allowing the kiln  100  to be moved under hand power. 
     FIG. 3  shows a perspective view of the kiln  100  shown in  FIG. 2 . In this embodiment, a power cord  20  provides power to the fan  7  and to the gear motor actuator  3 . In this embodiment, combustion air flow to the kiln  100  is controlled with a damper  21  located in the duct  16  connecting the outer pipe  15  to the fan  7 . 
     FIG. 4  shows a more detailed perspective view of the connection between the hopper  1  and the kiln wall  6 , as shown in  FIGS. 2 and 3 . In this embodiment, fuel is fed from a funnel-shaped hopper  1  using a feeder  2 . The feeder  2  is powered by an actuator, such as a gear motor. 
     FIG. 5  shows an exploded view of one embodiment of a feeder, such as the feeder  2  shown in  FIG. 4 . In this embodiment, the feeder  2  is a rotary valve. One skilled in the art will recognize that other embodiments may use different types of feeders  2 , such as augers, lock hoppers, conveyers, or other feeding mechanisms. In the embodiment of  FIG. 5 , the feeder  2  includes a cylinder  28  that has an opening  35  on an upper portion that allows fuel to fall into the cylinder  28  from the hopper ( 1  in  FIG. 4 ). Fuel flow through the feeder  2  is moderated by vanes  32 , which rotate on a shaft  31 . Fuel falls through opening  35  and onto the shaft  31  between the vanes  32 . The shaft  31  and vanes  32  rotate, thereby allowing the fuel to fall through an outlet opening  36 . The shaft  31  rotates on a bearing  30 . A disk  33  directs fuel flow to the outlet opening  36 . The actuator ( 3  in  FIG. 4 ) mounts to the feeder  2  with a mounting plate  29 . A coupling  34  connects the power source ( 3  in  FIG. 4 ) to the shaft  31 . 
     FIG. 6  shows a more detailed perspective view of an arrangement for receiving the combustion air. The embodiment shown in  FIG. 6  includes the outer pipe  15  of the pipe-in-pipe heat exchanger ( 17  in  FIG. 1 ) that preheats combustion air. Air from the outer pipe  15  is directed to the fan  7  in the duct  16 . The duct  16  includes the damper  21  that controls airflow to the fan  7 . Air from the fan  7  passes through the duct  8  and into the air distribution grate  9 . The air distribution grate  9  is located at the bottom of combustion chamber ( 10  in  FIG. 1 ). 
     FIG. 7  shows a perspective view of the inside of the kiln  100  from the combustion chamber side, without the door ( 26  in  FIG. 3 ). In this embodiment, the duct  8  passes underneath the kiln  100  to provide air to the air distribution grate  9  in the combustion chamber  10 . The combustion chamber  10  is separated from the main kiln volume  12  by a partition wall  11 . Fuel drops into the combustion chamber  10  through a fuel chute  38 , positioned through the kiln wall ( 6  in  FIG. 1 ), which is composed of fire bricks  23 , insulating board  24 , and the metal shell  22 . The fuel chute  38  directs fuel to the combustion chamber  10  using funnel-shaped chamber walls  37 . 
     FIG. 8  shows a perspective view of the main kiln volume  12  of the embodiment of the kiln  100  shown in  FIG. 7 . An exhaust opening  13  provides exhaust gases to the exhaust stack  14 . 
   By way of example, in one embodiment the kiln  100  is a small-sized kiln with an internal volume of 3.75 cubic feet. Conventional pellet-stove wood pellets are used as a solid fuel. Heat up to a full temperature of 2345° F. has been shown to be achieved using a total wood charge of 100 lbs in 9 hours. In this embodiment the volume of the hopper  1  is 0.7 cubic feet with a 30 lb capacity of wood pellets. Time between refilling the hopper  1  is approximately 2-3 hours. Fuel costs are $8.75 based on typical wood pellet costs of $3.50 per 40 lb bag. Because of the efficiency of this system, fuel costs compare favorably to a comparably sized conventional propane-fired kilns. 
   In this particular embodiment, fuel is fed using a 2-inch diameter auger as a feeder  2  and a 4-rpm gear motor as the actuator  3 . The maximum fuel rate of the auger is 27 lbs per hour with the motor running continuously at 4 rpm. The fuel rate is controlled using a commercially available repeat cycle timing relay as a controller  4  to adjust the percentage of time the motor runs during a period of time. A 40% duty cycle is near optimum for this particular embodiment with a repeating cycle of 4 seconds with the motor running in a 10 second period. The fuel firing rate may be adjusted to balance available combustion air to yield a near neutral, maximum temperature flame. In one implementation, a neutral flame is used having a balanced fuel-to-air ratio such that combustion efficiency is maximum and combustion temperature is a maximum. In this particular embodiment, approximately 11 lbs per hour of wood fuel is supplied to balance 60 lbs per hour of combustion air. 
   Combustion air is supplied by a high-temperature blower as the fan  7 , in this embodiment. The blower draws ambient air in through the annulus of the pipe-in-pipe heat exchanger  17 . In this embodiment, the inner pipe  14  is approximately 4 inches in diameter and serves as the exhaust stack  14  for the kiln  100 . The outer pipe  15  is 6 inches in diameter. The hot exhaust gasses leaving the kiln  100  provide heat to warm the incoming air. The length of the heat exchanger  17  is 4 feet in this particular embodiment and serves to heat the incoming air to a temperature of approximately 350° F. 
   Air preheat results in higher combustion temperatures which ease difficulties encountered in prior art in reaching full temperatures which are near to combustion temperatures with ambient temperature combustion air. Air preheat also recovers a portion of the exhaust gas heat which would otherwise be lost. Together, these impacts result in an efficiency improvement of approximately 46% based on fuel consumption of 100 lbs per firing compared to 187 lbs without air preheat in the same kiln  100 . 
   Operations with air preheat are improved with target temperatures more reliably achieved with the hotter combustion temperatures compared to operations without air preheat where it can be difficult to achieve target temperatures even with extended firings. 
   Although the present invention has been described with respect to particular embodiments thereof, variations are possible. The present invention may be embodied in specific forms without departing from the essential spirit or attributes thereof. It is desired that the embodiments described herein be considered in all respects illustrative and not restrictive and that reference be made to the appended claims and their equivalents for determining the scope of the invention.