WO1992013456A1

WO1992013456A1 - A convection oven

Info

Publication number: WO1992013456A1
Application number: PCT/SE1992/000066
Authority: WO
Inventors: Leif Johansson; Nils Pers
Original assignee: Revent International Ab
Priority date: 1991-02-04
Filing date: 1992-02-04
Publication date: 1992-08-20
Also published as: SE9100336D0; AU1234592A; SE9100336L

Abstract

In a convection oven, particularly a so-called rack baking oven, comprising a circulating fan (F) for moving heated air in a substantially closed path including the oven chamber (12), and means (S) for introducing steam into the oven chamber (12), an outlet passage (21) which is constantly open toward an extraction device (24) is provided at the top of a door (14) which extends over substantially the full height of the oven chamber. When the door (14) is closed the extraction device (24) essentially communicates with the oven chamber only through the outlet passage (21), preferably by way of an air duct (22) which is provided on the door and extends from the lower portion of the door to the top thereof. Preferably, an air intake (15) provided with a damper (16) is connected to the suction side of the circulating fan.

Description

A convection oven

This invention relates to a convection oven of the kind defined in the pre-characterising part of the independent claim, particularly a so-called rack oven, especially a rack oven for baking bread products.

When baking bread products in convection ovens, that is, commercial baking ovens in which heated air is caused to flow over and around the bread products to effect the baking, it is common practice to introduce steam into the oven chamber during a short period of time at the commencement of the baking process. As the bread products are relatively cool during this phase of the baking, the steam is condensed on the bread products. Initially, the bread products therefore develop an elastic surface layer and then, in the course of the subsequent main baking phase, they develop a crust having the desired colour and crispiness.

During this initial phase of the baking process the door of the oven chamber is closed, but the fan which maintains a forced air circulation in a closed path including the oven chamber during the main baking phase is still inoperative. Usually, the steam is produced by pouring water over heated metal bodies which are disposed in the closed circula¬ tion path but outside the oven chamber. Steam so produced enters the oven chamber through openings in the side-walls thereof.

During the baking, there is no substantial exchange of the air forming the oven atmosphere, because the air flows in an essentially closed circulation path. Consequently, the moisture that has been added to the oven atmosphere as a result of the addition of steam during the initial phase is still in the oven atmosphere when the baking process is completed. Additional moisture is added to the oven atmos¬ phere from the bread products being baked.

During the steaming phase a certain overpressure is developed in the oven chamber. For that reason, a certain amount of steam inevitably is forced out of the oven chamber at the door even though the door may be sealed against the door frame. To obviate this problem it is common to connect a separate extraction duct to the oven chamber but such an extraction duct is no satisfactory solution to the problem.

To prevent the escaping steam from spreading in the room which accommodates the oven it is common to provide an ex- traction hood outside the oven chamber above the door to extract air from the region in front of the doorway so that as much as possible of the escaping steam is carried away.

However, because the doorway generally is relatively high, normally of about the height of a man, the extraction hood is unable to carry away all of the escaping steam in an efficient manner.

When the oven chamber is opened upon completion of the baking, the space in front of the doorway is filled with the very moist oven atmosphere. The extraction hood then is even less capable of efficiently removing the steam from this region.

An object of the invention is to obviate the problem explained above. In accordance with the invention, this problem is solved by embodying in the oven the features set forth in the independent claim. Advantageous embodiments of the oven according to the invention may include the features set forth in one or more of the independent claims.

The invention will now be described in greater detail with reference to the accompanying diagrammatic drawing, in which:

Fig. 1 shows an oven according to the invention in verti¬ cal sectional view, the sectional plane passing through the oven chamber at the door thereof;

Fig. 2 is a horizontal sectional view on line II-II of Fig. 1.

Apart from the portions described in detail below, the commercial convection oven diagrammatically shown in the drawing is essentially identical with that which is disclosed in US-A-3 954 053 and US-A-4 202 259. For a more detailed description of the portions not described here, reference is made to the just-mentioned patent specifications, which are incorporated herein by reference. Referring to the drawing, the oven comprises a heat- insulating enclosure 11 in which an oven chamber 12 is pro¬ vided; the oven chamber is of about the height of a man and extends over the major portion of the height of the oven. Th oven chamber 12 is designed to receive a so-called rack trolley (not shown) which is a rack provided with wheels and a stack of baking plates or other carriers for the goods to be treated in the oven, particularly bread products.

The rack trolley is wheeled into and out of the oven chamber 12 through a doorway 13 provided on the front wall o the oven and having associated therewith a side-hung door 14 As is described in greater detail in the above-mentioned patent specifications, the oven chamber 12 forms part of a closed circulation path in which air heated by means of a heater (not shown) disposed in the enclosure 11 is circulate at high velocity by means of a circulating fan F. The heated air enters the oven chamber 12 through one of the side-walls thereof and leaves the oven chamber through an opposed side- wall. Within the enclosure 11, outside one of the side-walls o the oven chamber, a steaming device S is provided. This steaming device includes a heat-accumulating metal mass and system for pouring water over the heated metal mass. When th water is poured, the circulating fan F is inoperative, and the steam produced by the accumulated heat is forced into th oven chamber 12 through the openings in the adjacent side- wall through which the heated air flows during the main baking phase.

Disregarding the doorway 13 and a short tube 15 which extends through the top wall of the enclosure 11 and has its lower end connected with the suction side of the circulating fan F, the enclosure 11 is essentially air-tight at least as far as the portions defining the closed air circulation path for the oven atmosphere are concerned. At the upper end of the tube 15 a damper 16 is provided which can be opened and closed by means of a handle at the front side of the oven, or by means of an automatic control device, to permit or preven flow of air through the tube. Along the horizontal upper side of the doorway 13 a sealing strip 17 is secured to the enclosure 11. When the door 14 is closed, the inside of the top portion of the door sealingly engages the sealing strip 17. Similar sealing strips (not shown) extend along the vertical sides of the doorway 13 or the door 14 to prevent air from escaping from the oven chamber 12 when the door is closed. Finally, along its horizontal bottom side the door 14 is provided with a flexible sealing lip 18 which cooperates with a bottom plate 19 in the oven chamber 12 to prevent air from escaping at th bottom when the door is closed.

At the top of the doorway 13, the door frame has a reces 21 which forms an opening above the horizontal top side of the door 14 when the door is closed. This opening communi- cates with the interior of the oven chamber 12 by way of a pair of air channels 22 provided on the inside of the door 14 and extending vertically between the top side of the door and the lower door portion.

The two air channels 22 are formed by a pair of sheet metal channel members secured to the inner side of the door along the vertical sides thereof. Through these channels air can flow freely from the region adjacent the floor of the oven chamber 12 into and out of the opening or outlet passage formed by the recess 21. At the top of the door 14 a filling member 23 is posi¬ tioned between the channel members 22. Through the interme¬ diary of this filling member 23 and the air channel members 22 the door 14 sealingly engages the sealing strip 17 in the closed position so that air can escape from the oven chamber 13 through the doorway 13 substantially only by way of the air channels 22 and the outlet passage formed by the recess 21.

On the front side of the oven enclosure 11 an extraction box or hood 24 is mounted above the doorway 13. This extrac- tion device is provided with a two-speed fan 25 the outlet of which is connected to an outlet duct which is not shown in the drawing. At the lower portion of the extraction device 24 a pair of suction slits 26 and 27 are provided which extend horizontally and parallel to the main plane of the doorway 13. The suction slit 26 is positioned directly above and in front of the air outlet passage formed by the recess 21 whil the other suction slit is spaced forwardly from the doorway 13. Similar slits (not shown) are provided at the sides of the extraction device 24 which extend transversely of the main plane of the doorway 13 so that there is an inlet forme by the suction slits which extends all the way around the lower portion of the extraction device. When a baking cycle is to be carried out after the rack trolley with the bread products thereon has been wheeled int the oven chamber 12, the door 14 has been closed and the damper 16 has been set to a closed position, water is poured over the metal mass in the steaming device S which has been heated during the preceding baking cycle. The steam formed a a result of the pouring of water expands into the oven chamber 12 and a portion of it is condensed on the bread products. During the steaming, the circulating fan F of the oven is inoperative while the fan 25 of the extraction devic 24 runs at low speed.

The air which tends to escape from the oven chamber 12 because of the overpressure therein can escape relatively freely, namely from the bottom region of the oven chamber, the escape path being formed by the air channels 22 and the outlet passage defined by the recess 21. The escaping air an any entrained steam is thereby efficiently extracted by the extraction device 24 but because the extraction fan 25 runs at low speed and the circumferential inlet slits of the extraction device 24 form a side inlet for the air, no exces sive amount of steam is extracted from the oven chamber 12.

The steam formed during the initial steaming phase of th baking cycle has a strong tendency to rise in the oven chamber 12. The air extracted from the oven chamber 12 at th bottom thereof therefore is primarily the dry air which remains in the oven chamber after the preceding baking cycle and which when extracted provides additional space for the steam so that the steam can fill the oven chamber from the top and down. In this way it is ensured that most of the steam remains in the oven chamber so that a sufficient amoun of water can precipitate on the bread even in the low por¬ tions of the rack trolley.

When the steaming phase is completed, the circulating fan S is switched on to circulate the moist oven atmosphere in the closed path during the main phase of the baking. During the main baking phase there is no substantial exchange of the oven atmosphere, that is, no substantial amounts of air escape from the closed path. In accordance with the law of least resistance the small amount of air which does escape from the closed path during the main baking phase will escape through the outlet passage formed at the top of the door 14 by the recess 21 where the escaping air is immediately received by the extraction device 24 through the inlet slit 26; the fan 25 of the extraction device 24 is still running at relatively low speed.

During the final baking phase it is desirable to remove the steam from the oven before the oven chamber door is swung open to permit withdrawal of the rack trolley with the baked bread products. In accordance with the invention this is accomplished by opening the damper 16 a few minutes before the completion of the baking so that relatively dry air is drawn into the circulation path to replace a portion of the moist atmosphere. To the extent that such dry ambient air is drawn into the oven atmosphere, moist air will be extracted from the circulation path through the extraction device 24 by way of the air channels 22 and the outlet passage defined by the recess 21. The extraction fan 25 can then operate at a higher speed. When the oven chamber door 14 is to be swung open to permit withdrawal of the rack trolley, this can thus be done without a large amount of steam escaping from the oven chamber.

In accordance with the invention the escape of oven atmosphere that does occur, takes place in such a manner that the loss of steam in the initial phase is minimized and occurs at a place where the escaping air and steam can be disposed of efficiently. As is indicated diagrammatically in the drawing the damper 16 is in the form of a plate which is positioned over the open upper end of the air intake tube 15. This construc¬ tion of the damper not only is very simple but also virtuall eliminates the danger that the person who installs the oven confuses the intake tube 15 with the extraction tube which i common in ovens of the type with which the invention is concerned but which is superfluous in the oven according to the invention. Accordingly, there is little danger that the intake tube 15 will be mistakenly connected to an extraction duct.

Claims

1. A convection oven comprising an oven chamber (12) having a doorway (13) with a door (14) , means (F) for moving heated air in an essentially closed path including the oven chamber (12) , means (S) for introducing steam into the oven chamber, and an extraction device (24) disposed at the top of the doorway (13) and adapted to extract air from the region in front of the doorway (13) , characterised by an outlet passage (21) provided at the top of the doorway (13) through which outlet passage the oven chamber (12) is in substan¬ tially open communication with the extraction device (24) .

2. A convection oven according to claim 1, characterised in that the inner side of the door (14) is provided with an air channel (22) extending from the lower portion of the door to near the top thereof, and in that the outlet passage (21) communicates with the oven chamber (12) essentially only through the air channel (22) .

3. A convection oven according to claim 1 or 2, characterised in that an air intake (15) provided with a dam- per (16) is connected with the suction side of an air circu¬ lating fan (F) for moving the heated air in the closed path.