A swimming pool heater that heats a private swimming pool and can be used to provide heating for a house, a cottage or a commercial swimming pool. Wood, that is often found around pools, is used as primary fuel. This oven has a cylindrical external appearance and is disposed horizontally. It is made of an interior wall and an exterior wall in between which water circulates. The interior wall plays the role of fire chamber and heat exchanger; the section having the shape of an annular zig-zag offers more surface for heat exchange than a simple cylinder thus increasing the overall heater efficiency. The left-over ashes insulate the bottom of the heating chamber replacing often used tiles. For general use, logs and branches are used as fuel, and an increase of 20 to 30 degrees F. of the water of a typical garden pool is attained overnight.

BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention is related to the field of water heaters, particularly those which are used as means to heat the water of outdoor swimming pools and use wood as primary fuel. 
   2. Description of the Prior Art 
   The objective of the inventor was to provide a practical means of heating pools. Originally from Quebec, this invention can make use of wood, an abundant natural resource which is often found near pools in most backyards due to the frequent use of wood for heating homes in winter. The inner wall and general design have been optimized to provide a high water heating efficiency and to be easily adaptable to most exterior pools. 
   OBJECTS OF THE INVENTION 
   It is a general objective of the inventor to provide a swimming pool water heater, the invention can be used for the heating of water in many other applications which can include water based heating of homes or cottages. Another objective is to make use of the wood which is already present around many pools, therefore using branches and slim logs, four feet long or shorter, to maintain typical pool water 20 to 30 degrees F. higher than it would be otherwise. Yet another objective is to use the ashes Left behind from the combustion of the wood, as an insulator between the fire and the bottom portion of the fire chamber. 
   A further objective is to provide an oven with a horizontal cylinder external shape, and to provide within the outside cylinder a corrugated fire chamber. Water circulates and is heated in the region between the two surfaces. The corrugated fire chamber wall provides a large heat exchange surface between the fire and the water and is typically of star shaped cross section. The particular shape of the cross section is to provide a heat exchange surface which is greater than the surface of a non-corrugated surface having the same external diameter. The greater surface of heat exchange is to increase the heat transfer across the inner wall, therefore increasing the heater's overall efficiency. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, forming a part of this specification, similar reference characters identify corresponding parts through the several, views; 
       FIG. 1  is an overall view showing the elements of the pool heater forming one embodiment of the invention; 
       FIG. 2  is a perspective view of the pool heater seen from the back; 
       FIG. 3  is a perspective view of the pool heater seen from the front; 
       FIG. 4  is also a perspective view of the pool heater seen from the front; but in which the oven's outer shell has been removed; 
       FIG. 5  is a front view; 
       FIG. 6  is a back view; 
       FIG. 7  shows a section of the heater defined by arrows  7 — 7  of  FIG. 6 ; 
       FIG. 8  illustrates the imaginary annulus which circonscribes the section of the inner shell; 
       FIGS. 9A and 9B  shows how the annulus identified  79  in  FIG. 8  may have different shapes and sizes; 
       FIG. 10  is a view which permits use of the mathematical formulae included in the description of the embodiment of FIGS.  7 , 8 , 9 A. 
   


   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   1. Detailed Description of the Drawings 
   In the following description, the numerals refer to specific items displayed in the figures. 
     FIG. 1  shows a pool heater  20  indicated by means of an arrow. An oven  22  is shown which has an opening  24  in the front for the loading of a fuel like wood. The pool heater also comprises a chimney  26  towards the rear of the oven, a water inlet  28  which brings water in from the pool  32 , an outlet  30  which sends heated water back to the pool. The pool intake can comprise different means to increase water mixing and pumping efficiency. In the case illustrated, the heater uses the standard pool input  34  and output  36 , therefore minimising need for adaptation equipment. The water flows from the pool output through the pump  40  and standard secondary filter  42  to the pool heater, by means of a tube. Part of the water then flows through a bypass tube  44  whereas the rest of the water flows through the body of the heater  22 , between its internal (not shown) and external wall where it is heated. Both flows then recombine at the outlet and return to the pool. 
     FIG. 2  shows the oven  22  with a cylindrical outer shell  46  even and preferably made of stainless steel. This shell is limited by two ends, a rear end  48  and a front end  50 , which are held together by rods  52  and sustained by means of legs  54 . A security cap  56  protects the chimney  26  from rain water and keeps sparks from coming out. An elbow  58  channels exhaust gas from the oven to the chimney, and an optionnal fan  60  with a 45° elbow  62  induces airflow in the chimney. Decoration holes  64  are also illustrated in the front and back walls. On the top of the external shell, there is an outlet  66  and a security pressure valve  68 . 
     FIG. 3  shows particularly the fact that the external shell  46  surrounds an internal shell  69 . The oven's water inlet  70  is illustrated at the bottom of the oven. In this case, the opening at the front of the oven is equipped with a door  72  held in a frame  74  by a lock  76 , and with a window  78 . 
     FIG. 4  shows the oven of  FIG. 3  without the external shell. The starlike shape of the section of the internal shell  69  is visible together with it's accordeon shaped surface  80  that is made of the shape of a staircase bended around a circumference, its section herein called star. Between the external shell and the internal shell, the heating water circulates. The surface of the internal shell forms tips  82  and valleys  81  wich form a series of pair of angles big  86  and small  88 . The bottom of the internal wall forms an ash receptacle  84 . 
     FIG. 5  shows the front end  50  and the position of the opening  24  at the front of the oven. The front end includes front legs  67 . 
     FIG. 6  shows the rear wall  65  with an exhaust hole  63 , two rear legs  64  and decorations  66 . 
     FIG. 7  shows the tips  82  of the star which form the internal shell  69 , in each tip interior an angle is formed and called small angle  88 . Between each tip there is a valley  81  that forms a bigger angle than the previous tip interior angle, this angle is called big angle  86 . This illustration shows an example of a star. It has twelve tips, the big angles are of approximately one hundred degrees whereas the small angles are of approximately seventy degrees. The distance  87  between a tip and a tangent to the external shell is of 1¼″ in this case. The figure also indicates that the internal shell can accumulate ashes which are represented by the hatched section  84  and for which the distance  85  to the center may vary. Under the ashes, it is not necessary to provide a greater heat transfer surface be cause the ashes act as an insulator, hence the bottom section of the fire chamber does not contribute to heat transfer as much as side and top regions. 
     FIG. 8  clarifies the fact that the section of the corrugated internal shell  69  can be circumscribed by an imaginary annulus  79  represented by the dashed section, and that this annulus is characterized by an internal radius  90  and an external radius  91 . The annulus defines the region where the tips and valleys of the corrugated shell are limited, the tips and valleys form a rounded accordeon shape. The big and small angles and the value of heat exchange surface area depend on the ratio of the external versus internal radius of the annulus and on the number of tips of the star. In the illustrated case, this ratio is of approximately 10:7. 
     FIG. 9A , the ratio of external to internal diameter is now of approximately 10:3 and the big and small angles appear to have approximately 40 and 10 degrees respectively. Note that the number of tips of the star shape may also vary. Although an internal radius and shape like this one would not be well suited for use with wood as fuel, oil or another fuel might prove to be excellent. 
     FIG. 9B  shows a fluid heater which uses the same principle as the one we presently study, but where the shape of the annulus has been changed to an annular rectangle. Water still flows between internal and external shell and ashes again may accumulate at the bottom of the fire chamber and provide insulation with no need for tiles. In this case, the bottom tips and valleys have been removed because they were not necessary as explained in discussion of FIG.  7 . Note also that parts of the corrugated surface section may not be circumscribed by a regular region as indicated in the figure and that this region has only been added for ease of description and understanding. 
     FIG. 10  is to be used for understanding calculations that may be done with mathematical formulae listed further down. In this representation of the annulus of  FIG. 8 , certain dimensions have been identified either by a letter of the English or the Greek alphabet. Part of the drawing has been magnified by a factor 2× and displaced from its original location along the sharp dashed lines. The reason for this is to provide increased clarity of description. 
   2. Summary and Ramifications 
   A particular objective is to provide a stainless steel water heater, stainless steel being used for its high temperature and corrosion resistance and its good thermal conductivity. 
   The external shell  46  is cylindrical and disposed horizontally, it may be equipped with external insulation. The internal shell  69  forms the combustion chamber wall and has a cross section which is star shaped. The internal shell is disposed horizontally, like the external shell; as seen in  FIGS. 3&4 . Two other surfaces, a front face and a back face enclose the volume inside the outer shell. Water flows and/or is contained in the volume delimited by these four surfaces thus forming a water tank which also acts as a conduct when water flow is activated. Hence, there are two regions inside the outer shell: the water tank, also called circulation chamber, between the outer shell and the inner shell, and the fire chamber located inside the inner shell. 
   The star shape of the corrugated shell cross section has a perimeter of approximately twice the value of one that would be formed by a non-corrugated cylindrical inner shell cross section. It is formed by tips and valleys between which are found segments of equal length. The star is circumscribed by an imaginary annulus having approximately 1½″ width between external and internal radius. This annulus  79  is delimited by the tips  82  and valleys  81  of the star cross section. Each tip is associated to a small angle  88  and each valley is associated to a big angle  86 . The difference between these angles is what creates the annulus. If both angles were equal, the corrugated cross section would define a circle of infinite radius or, in other words, a corrugated straight line. 
   Though the exterior of the oven is cylindrical, the internal shell is corrugated as indicated by the cross section described above and illustrated in  FIGS. 7&8 . The preferred annulus has a 17″ external diameter, big angles  86 , oriented towards the exterior, of approximately 100° and small angles  88 , oriented towards the interior, of approximately 70°. Though the star shape of the preferred embodiment is regular, the bottom section designed for ash accumulation may have a non corrugated cross section because due to the insulation created by the leftover ashes from the wood combustion, the importance of a greater heat exchange surface may be neglected in this region. 
   Leftover ashes are used as insulation between the fire and the fire chamber wall's bottom section. This insulation replaces the tiles which are often used in such fire burning ovens. 
   As the preferred embodiment, the fluid to be heated is a private pool water and fuel to be used is wood. 
   The means for loading the fuel comprise a door  72  mounted on a frame  74 , held closed with a lock  76 . This door is equipped by a window through which it is possible to see the degree of activity of combustion. 
   Means of water circulation to and from the pool comprise of standard pool tubing  38  having 1{fraction ( 1 / 2 )}″ diameter and the standard pool pump  40 . The entry and exit from the pool being the standard pool inlet  34  and outlet  36  as indicated by  FIG. 1 , this minimizes the need for adaptation equipment and helps reduce cost. A typical pool filter  42  is often part of the system. The heater can be placed lower than the level of the bottom of the pool, in which case the heater pumping system can benefit from the action of gravity. A bypass can be installed parallel to the pump and comprises an activator which starts if the pump stops due to a power failure. A manual valve can be added to the parallel conduct. A security valve  68  is present at the top of the oven water tank and conduct to prevent pressure from becoming too high inside the circulation chamber. In the case of flow stop caused by power failure of the pump, while combustion is taking place within the fire chamber, the water may turn to vapor and hence be evacuated to the pool although there is no pumping taking place. At pump startup, a negative pressure may occur, which may stress the oven internal and external walls. A one-way valve may be added to the outlet or to the top of the external shell to prevent such negative pressure problems by permitting outside air to enter and fill the vacuum created. 
   For gas evacuation, the oven is equipped with a chimney, the pull is insured by a 7″ duct approximately 8′ high. The chimney can be equipped with an optional fan. 
   3. Calculations 
   In the following calculations which are concentrated on dimensions of the corrugated internal surface of the preferred embodiment, the variables identified by letters refer to measurements indicated by corresponding letters on FIG.  10 . 
   The difference (D) between the big angle  86  (a) and the small angle  88  (b) is equal to 360 degress divided by the number (N) of tips of the star. To simplify calculations, we define an angle φ: the average of angles a and b. Both the numeric value of the angle φ and the length S of the segments, which permit calculation of the total perimeter of the star shaped section and thus total heat exchange surface, are function of the internal radius diameter  90  (r), the external radius diameter  91  (R), and the number of tips N. They are related by the following equations
 
φ=( a+b )/2 
 
 b=a−D  
 
 D= 360 /N  
 
 a= 360−2θ−2ψ
 
ψ= arcsin ( e *sin(θ)/ S ) 
 
 e=R−r  
 
θ=90(1−1 /N ) 
 
 S =( P   2   +e   2 −2 Pe (cos θ)) 1/2  
 
 P=R (sin(180 /N ))/sin θ
 
   Where the angles are measured in degrees, e is the width of the annulus, P is the length of the line indicated in  FIG. 10 , and θ and ψ are the angles indicated on the zoomed part of  FIG. 10  (indicated by sharp dashed lines). 
   It is to be understood that the embodiments of this invention which were described above, in reference to the annexed drawings, were given as an indication and are by no means restrictive. Modifications and adaptations thereof can be implemented without the object deviating from the framework of this invention. Other embodiments are possible and only limited by the scope of the appended claims. 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               PARTS LIST 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               20- Pool heater 
               Chauffe-piscine 
             
             
                 
               22- Oven 
               Four 
             
             
                 
               24- Loading opening 
               Ouverture de chargement 
             
             
                 
               26- Chimney 
               Cheminée 
             
             
                 
               28- Intake tubing 
               Tuyau d'entrée 
             
             
                 
               30- Exit tubing 
               Tuyau de sortie 
             
             
                 
               32- Pool 
               Piscine 
             
             
                 
               34- Pool inlet 
               Entrée de piscine 
             
             
                 
               35- Pool top 
               Haut de piscine 
             
             
                 
               36- Pool outlet 
               Sortie de piscine 
             
             
                 
               37- Pool bottom 
               Fond de piscine 
             
             
                 
               38- Tubing 
               Canalisation 
             
             
                 
               40- Pump 
               Pompe 
             
             
                 
               42- Filter 
               Filtre 
             
             
                 
               44- Extension tubing 
               Tuyau d'extension 
             
             
                 
               46- External shell 
               Coquille externe 
             
             
                 
               48- Rear end 
               Extrémité arrière 
             
             
                 
               50- Front end 
               Extrémité avant 
             
             
                 
               52- Rods 
               Tiges de maintien 
             
             
                 
               54- Rear legs 
               Pattes arrière 
             
             
                 
               56- Security cap 
               Chapeau de sécurité 
             
             
                 
               58- Elbow 
               Coude 
             
             
                 
               60- Optionnal fan 
               Ventilateur optionnel 
             
             
                 
               62- 45° Elbow 
               Coude à 45° 
             
             
                 
               64- Decorations 
               Trous de décoration 
             
             
                 
               65- Back wall 
               Mur arrière 
             
             
                 
               66- Outlet 
               Bouchon de sortie 
             
             
                 
               67- Front legs 
               Pattes avant 
             
             
                 
               68- Security valve 
               Soupape de sécurité 
             
             
                 
               69- Internal shell 
               Coquille intérieure 
             
             
                 
               70- Water inlet 
               Prise d'eau 
             
             
                 
               72- Door 
               Porte 
             
             
                 
               74- Frame 
               Cadre 
             
             
                 
               76- Lock 
               Loquet 
             
             
                 
               78- Window 
               Lunette 
             
             
                 
               79- Annulus 
               Anneau 
             
             
                 
               80- Corrugated surface 
               Paroi ondulée 
             
             
                 
               81- Valley 
               Creux 
             
             
                 
               82- Tips 
               Crêtes 
             
             
                 
               83- Segment 
               Segment 
             
             
                 
               84- Ash receptacle 
               Réceptacle à cendres 
             
             
                 
               85- Ash distance 
               Distance cendres 
             
             
                 
               86- Big angle 
               Grand dilaté 
             
             
                 
               87- Tip distance 
               Distance de pointe 
             
             
                 
               88- Small angle 
               Petit angle 
             
             
                 
               90- Internal radius 
               Rayon interne 
             
             
                 
               91- External radius 
               Rayon externe