One of the important requirements in heat processing charges in a tunnel kiln is to control the distributions of the pressures and temperatures of the atmosphere in the kiln throughout the length of the kiln.  The distribution of the pressure of the atmosphere in the kiln is closely related to the distribution of the flows of the atmosphere in the kiln and is predominant over the fuel economy of the kiln. 
A tunnel kiln usually has three temperature zones which consist of a preheating zone, a firing zone and a cooling zone.  The distribution of the pressure of the atmosphere in the kiln is usually such that the pressure droops from the cooling zone toward the preheating zone through the firing zone, or from the firing zone to the cooling and preheating zones, or from the cooling and firing zones toward the preheating zone.  With any of these patterns of distribution, the pressure of the atmosphere in a tunnel kiln decreases toward the preheating zone so that the atmosphere in the kiln tends to propagate toward the preheating zone.  In order to save the fuel consumption in the kiln and to process charges under optimum conditions, therefore, it is important to properly regulate the flows of the atmosphere toward the preheating zone of the kiln. 
In the firing zone of a kiln, charges are burned or fired by transfer of heat thereto directly from the flames produced by the combustion of fuel and radiation of heat from the hot walls of the kiln.  Less emphasis is thus placed on convection of heat to the charges than on radiation of heat to the charges in the kiln.  This makes it difficult to maintain the temperature of the atmosphere uniform vertically in the tunnel in the kiln so that the charges in the firing zone of the kiln tend to be fired excessively in one region and deficiently in another region of the tunnel in the kiln. 
To avoid such localized heating of charges, efforts have been paid to improve the combustion efficiency of the fuel burners to be used and to form soft flames around the charges to be fired.  The tunnel kiln of a conventional continuous combustion furnace has therefore been provided with relatively large combustion chambers for the fuel burners.  In view, however, of the fact that such combustion chambers must be formed by highly heat-resistant, extremely costly refractories and that such refractories must be shaped intricately to provide proper configurations of the combustion chambers, disproportionately large amounts of cost and labor have been required to construct the kiln with such combustion chambers.  Because, furthermore, each of the combustion chambers in the kiln is subjected to an extremely large amount of thermal load (which is usually of the order of about 200,000 to 1,000,000 k.multidot.cal/m.sup.3 .multidot.hr).  operators of the furnace are compelled to work in serious environments due to the heat dissipated from the outer walls of the kiln. 
The hot products of combustion produced in the firing zone of a tunnel kiln move upwardly from the burner tips toward the ceiling of the kiln and are permitted to flow along the ceiling toward the preheating zone without having sufficiently exchanged heat with the charges being conveyed in the firing zone.  The hot gases thus directed into the preheating zone are caused to flow downwardly away from the ceiling of the kiln by means of a drop arch or a drop air curtain formed in the preheating zone or by suitable forced recirculation fans provided in the preheating zone and are thus discharged from the tunnel in the kiln into the flue formed in the side walls of the kiln.  Problems are however encountered in a furnace having such a preheating zone in that the charges conveyed into the preheating zone, particularly, those being moved through a lower portion of the tunnel in the preheating zone can not be sufficiently preheated and are thus subjected to the attack of heat at suddenly increased temperatures in the firing zone subsequent to the preheating zone and in that the heat produced in the firing zone is wasted uselessly from the preheating zone and gives rise to an increase in the fuel consumption rate in the firing zone. 
The present invention contemplates elimination of these and other drawbacks of prior-art industrial continuous combustion furnaces.