Continuous oven for making baked ware

In a continuous oven for making baked ware comprises a baking belt, which extends through an elongate oven space, and upper and lower radiant heaters, which are flown through by heating gas and are included in a heating gas circulating circuit, which comprises a heat exchanger, which is arranged to be heated by the nesting gases being recycled and serves to neat the air that is to be supplied to the oven space The heat exchanger is included in the heating gas circulating circuit between the radiant heaters and the exhaust gas outlet and in an air circulating circuit provided with a water vapor outlet and a fresh air inlet is disposed between the fresh air inlet and the oven space.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a continuous oven for making baked ware, 
comprising an elongate oven space provided with upper and lower radiant 
heaters, which are connected to be flown through in operation by heating 
gases, a revolving endless baking belt having a baking belt course for 
transporting the material to be baked through the oven space, and a heat 
exchanger for heating the air for the oven space, wherein the radiant 
heaters are connected to at least one heating gas circulating circuit, 
which is provided with an exhaust gas outlet, a fresh air inlet, and at 
least one heating gas generator. 
2. Description of the Prior Art 
In a traveling-hearth baking oven disclosed in Published German Application 
1,532,904 the oven space is heated by heating gases, which are generated 
in a combustion chamber and are passed through the heating gas ducts, 
which are disposed above and below the oven space and consist of radiant 
heaters, and the gases are subsequently discharged as exhaust gases. The 
air for the oven space is sucked off and is replaced by preheated fresh 
air, which is initially preheated by the returning baking belt or in a 
heat exchanger, which is flown through by the air which has been sucked 
from the oven space, and the fresh air is subsequently passed through a 
recuperator, which is heated by the exhaust gases from the oven. 
Thereafter the fresh air is intermittently injected into the oven space. 
The fresh air may be heated up further in the combustion chamber in which 
the heating gases for the oven are generated. There is no circuit for 
circulating the heating gases. 
EP 0 334 001 A1 discloses a continuous oven for making baked ware, which 
comprises two separate and separately controllable heating gas circuits 
for the upper heat and the lower heat, respectively. In that oven the 
burners for generating the respective heating gases are spaced apart along 
the oven space in the associated heating gas duct, which is disposed 
directly above and below the oven space, respectively. That continuous 
oven is provided with two water vapor exhaust ducts, which extend through 
the upper heating gas duct, and with water vapor-circulating means, which 
are disposed in the last one-third of the oven and serve to circulate 
water vapors transversely to the direction of travel of the baking belt. A 
supply of preheated air for the oven space has not been contemplated. 
Swiss Patent Specification 356,100 discloses a travelling-hearth 
breadbaking oven that comprises an indirectly heated oven space and 
variable means for circulating air through the oven space along the 
latter. The oven space is heated by upper and lower heating gas ducts, 
which are flown through by heating gases. The air for the oven space is 
circulated along length sections of the oven space, which length sections 
differ in length by means of two fans and a duct system, which opens into 
the oven space through shut-off valves, which are distributed over the 
length of the oven space. A single circuit for circulating air for the 
oven space or two separate ones of such circuits, which are connected in 
series may be provided. The duct system comprises two ducts for supplying 
fresh air, which may be preheated. A supply of air which has been 
preheated by a heat exchanger to the oven space has not been contemplated. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide for making bakeware a 
continuous oven having a higher oven efficiency. 
In a continuous oven which is of the kind described first hereinbefore this 
is accomplished in that the heat exchanger is connected to be flown 
through in operation by the air which is to be heated and is to be 
supplied to the oven space and is also connected to be flown through in 
operation by the heating gases which are being recycled to the heating gas 
generator of a heating gas circulating circuit, that a circuit for 
circulating air for the oven space is provided with a water vapor outlet 
and a fresh air inlet and communicates with the oven space and contains 
the heat exchanger between the fresh air inlet and the oven space, and 
that a heating gas circulating circuit includes the heat exchanger between 
the radiant heaters and the exhaust gas outlet. 
Owing to the design in accordance with the invention the heating gases 
which are being recycled to the heating gas generator are cooled as far as 
possible before they reach the exhaust gas outlet so that the heating 
gases withdrawn through the exhaust gas outlet have the lowest temperature 
in the heating gas circulating circuit and the heat loss which is due to 
the withdrawal of heating gases will be minimized. At the same time the 
air which is intended for the oven space and is to be preheated is heated 
to the desired inlet temperature by the heating gases which are being 
recycled in the heating gas circulating circuit before said air enters the 
oven space.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An illustrative embodiment of the invention will be described more in 
detail hereinafter with reference to the drawing. 
The continuous oven in accordance with the invention comprises an elongate 
oven space 1 and heating gas ducts 2, 3, which extend above and below the 
oven space 1, respectively, and constitute radiant heaters for emitting 
radiant heat into the oven space 1. The upper course of a revolving 
endless baking belt 4 extends through the oven space 1 and transports 
material to be baked through the oven space 1. That belt 4 is deflected in 
reversing stations 4', 4" disposed outside the enclosure surrounding the 
oven space and returns below said enclosure. 
The heating gases are generated by a heating gas generator 5, which 
consists, e.g., of a gas or oil burner, and through a forward plenum 
chamber 6 that communicates with the heating gas ducts 2 and 3 are 
supplied to the inlet end of the heating gas ducts 2, 3. At their outlet 
end the heating gas ducts 2, 3 open into a rear plenum chamber 7, from 
which the heating gases are sucked by a far and are then recycled in a 
recycle duct 9 to the burner 5. The burner 5 is supplied with the heating 
gases which have been recycled in the recycle duct 9 and with fresh air 
coming from a fresh air inlet 10. An exhaust gas outlet duct 12 provided 
with an controllable exhaust gas damper 11 branches from the recycle duct 
9 before the burner 5. The recycle duct 9 extends through a heat exchanger 
14, which precedes the junction 13 at which the exhaust gas outlet duct 12 
is connected. In the heat exchanger 14 the heating gases being recycled 
deliver heat to the air which flows through the chamber 15 of the heat 
exchanger 14 before being supplied to the oven space. The upper and lower 
heating gas ducts 2 and 3 constitute radiant heaters and together with the 
forward plenum chamber 6, the rear plenum chamber 7 and the recycle duct 9 
constitute a circuit for circulating the heating gases for the upper heat 
and the lower heat in the continuous oven. The super-atmospheric pressure 
which exists in the heating gas circulating circuit owing to the supply of 
fresh air to the burner 5 is relieved by the exhaust gas outlet duct 12 
and by the natural draft in the chimney (not shown), which is connected to 
the outlet duct 12. As a result, the rate at which heating gas or exhaust 
gas escapes through the exhaust gas outlet duct 12 is exactly equal to the 
rate at which fresh air supplied through the fresh air inlet 10 is 
required for the combustion. The rate at which heating gas is circulated 
in the heating gas circulating circuit of the continuous oven will depend 
on the size of the continuous oven and on the nature of the product which 
is to be baked in a given case. In dependence on the size of the oven the 
heating gases are circulated at a rate in the range between 3000 m.sup.3 
/h and 10,000 m.sup.3 /h. The heating gases are at a temperature in the 
range from 400.degree. to 600.degree. C. immediately after the burner 5 
and at a temperature in the range from 300.degree. to 400.degree. C. in 
the rear plenum chamber 7. 
Air is sucked from the oven space 1 by a fan 17 through a rear manifold 16, 
which directly precedes the rear plenum chamber 7 for heating gases. The 
fan 17 is disposed in a rear air duct 18, with leads from the rear 
manifold 16 to the heat exchanger 14. The air which has been sucked from 
the oven space 1 is forced by the fan 17 through the chamber 15 of the 
heat exchanger 14 and through a forward air duct 19, which extends from 
the forward end of the heat exchanger 14 and leads to a forward injecting 
chamber 20, which opens downwardly into the oven space 1. A fresh air 
supply duct 22 opens into the rear air duct 18 and is provided with a 
controllable fresh air damper 21 and has an outlet 23, which precedes the 
fan 17. A water vapor discharge duct 24 branches from the rear air duct 18 
between the rear manifold 16 and the fresh air supply duct 22. The water 
vapor discharge duct 24 contains an exhaust damper 25 and a fan 26. 
In dependence on the nature of the product to be baked and on the size of 
the oven, the fan 17 disposed in the rear air duct 18 blows air at a rate 
of up to 3000 m.sup.3 /h through the heat exchanger 14 into the oven space 
i and sucks off said air through the rear manifold 16. The air for the 
oven space is heated in the heat exchanger 14 to 250.degree. to 
300.degree. C. In dependence on the nature of the product to be baked in a 
given case, 1 to 100% of the air which has been blown into and sucked from 
the oven space 1 are withdrawn through the water vapor discharge duct 24 
and fresh air at a corresponding rate is supplied through the supply duct 
22 to the rear air duct 18. The remaining air is recycled to the heat 
exchanger so that in dependence on the nature of the product up to 99% of 
the air for the oven space are recycled from the rear manifold 16 through 
the rear air duct 18, through the heat exchanger 14 and through the 
forward air duct 19 to the forward injecting chamber 20.