GB2234421A

GB2234421A - Dampers for baking oven

Info

Publication number: GB2234421A
Application number: GB8917884A
Authority: GB
Inventors: Michael Paul Norfolk
Original assignee: Spooner Industries Ltd
Current assignee: Spooner Industries Ltd
Priority date: 1989-08-04
Filing date: 1989-08-04
Publication date: 1991-02-06
Anticipated expiration: 2009-08-04
Also published as: GB8917884D0; GB2234421B

Abstract

A baking oven employing forced convection and comprising control dampers (30, 32) for controlling the flow of gaseous medium delivered by a fan (20) from heating means (22) to upper and lower nozzle arrangements (14, 16), a bypass damper (34) for bypassing any excess heated medium past the nozzle arrangements (14, 16) is coupled to the mid point of a lever (54) whose ends are coupled to the control dampers (30, 32). On adjustment of one control damper (30 or 32) in one sense, the bypass damper (34) is adjusted in an opposite sense and to half the extent. <IMAGE>

Description

DESCRIPTION Heating or drying apparatus The present invention relates to heating or drying apparatus, such as baking ovens, and, more particularly to a damper system for such apparatus employing forced convection.

In an oven using forced convection as a method of heating a product, two factors govern the rate at which heat transfer is achieved; firstly the temperature of the air being circulated and secondly the amount of air which is impinging on the product. Common practice is that the air impinging on the top of the product comes from the same heat source as the air impinging on the bottom of the product and is therefore at the same temperature. It is important to be able to control these air flows independently to be able to create the required bake on the product, top and bottom. Furthermore, some delicate products cannot be subjected to strong jets of air, so particularly with ovens which are required to bake a range of products, a wide range of independently controllable air velocities to the top and bottom of the product is very desirable.

In a direct gas fired forced convection oven system, i.e. one in which a fan circulates products of combustion along with the circulating air, air flow can be controlled by simple independent dampers within the ductwork, which are arranged to reduce the amount of circulating air in the section. One drawback of this system is that, as a damper is closed, the air pressure within the system feeding the dampers increases and hence the flow past the other dampers increases.

In an indirect fired system where a heat exchanger is used as a heat source, or in an oven using an electric heater bank, it is essential that the flow of air through this heating means is maintained in order to prevent overheating. One method of achieving this is to have some form of damper which diverts air to the top or bottom of the product without controlling the total amount of circulating air. This is obviously unsatisfactory for delicate products, and hence ovens using this type of system are usually provided with very limited air flows.

The present invention resides in a damper system for a heating or drying apparatus employing forced convection, comprising a control damper for controlling the flow of a gaseous medium delivered from a heating means to a nozzle arrangement adapted to direct the heated medium on to a product to be heated or dried, and a bypass damper for bypassing any excess heated medium past the nozzle arrangement, in which the dampers are linked together so as to be adjustable simultaneously in an inverse sense so that the rate of delivery of the medium from the nozzle arrangement can be varied from zero or a minimum to a maximum without significantly altering the rate of delivery from the heating means.

Preferably, the arrangement is such that the back pressure upstream of the control damper remains substantially constant upon adjustment of the dampers.

The invention is useful in apparatus employing at least two nozzle arrangements, each receiving heated gaseous medium from the heating means via respective independently adjustable control dampers such that the bypass damper is linked to all control dampers to be adjusted simultaneously therewith but in an inverse sense.

In a simple arrangement employing two control dampers, the latter act on the two ends of a lever whose mid-point acts on the bypass damper. Thus on adjustment of one control damper with the other control damper stationary, the remote end of the lever acts as a fulcrum whereby the bypass damper is adjustable over substantially half its full range.

In the application of the present invention to a forced convection apparatus having several nozzle arrangements, independent control dampers are provided giving independent control of the top air flow and bottom air flow which are linked to the bypass damper in such a way as to maintain the air flow over the heat exchanger or electric heater irrespective of the top air and bottom air damper settings.

Maintaining the air flow from the fan means ensures that the pressure on the upstream side of the dampers does not significantly vary when changes are made to damper positions; hence the flow of air past a particular damper will always be the same for a given position. Additionally, as the back pressure does not increase as the dampers are closed, lower air flows are possible without the need for close fitting dampers, which can be troublesome inside an oven.

The invention is further described, by way of example, with reference to the accompanying drawing which is a diagrammatic sectional view of a continous baking oven employing forced convection.

Products 10 to be baked are transported on a continous conveyor 12 between upper and lower nozzle banks 14 and 16 in an insulating oven housing 18. A fan 20 draws air from the interior of the housing 18 through a heater 22 and delivers it to a plenum 24. The plenum 24 is connected by ducts 26 and 28 to the upper and lower nozzle banks 14 and 16 respectively.

Dampers 30 and 32 are provided in the ducts 26 and 28 respectively for individually controlling the rate of delivery of heated air to the nozzle banks. Thereby the rate of delivery of heated air on to the tops and bottoms of the products 10 to be baked can be controlled individually to suit the product. Additionally, a bypass damper 34 is disposed in a bypass 36 connecting the plenum 24 directly back to the interior of the housing 18. Such bypass air therefore does not impinge upon the products to be baked and is returned directly to the inlet side of the heater 22.

The control damper 30 is connected by its spindle 38 to a crank 40 and the control damper 32 is connected by its spindle 42 to a crank 44. The cranks 40 and 44 are coupled by respective pivots 60,62 to opposite ends of a lever 46 whose intermediate pivot 48 is connected via a link 50, a bell crank lever 52 and a link 54 to a crank 56 which is connected by a spindle 58 to the bypass damper 34.

The control dampers 30 and 32 are shown in their closed positions and the bypass damper 34 is shown in its fully open position. In these positions all of the air delivered by the fan 20 is returned via the bypass 36 to the inlet side of the heater 22 and none is directed at the products 10. If the control damper 30 is opened manually e.g. by means of an external lever attached to the spindle 38, the crank 40 is turned anticlockwise which causes the lever 46 to turn about the pivot 62 which then acts as a fulcrum for the lever 48. This pulls on the link 50, whereby the bypass damper 34 is caused to move in its closing direction. The sizes of the cranks and levers are so chosen that displacement of the damper 30 from its fully closed to its fully open position displaces the bypass damper 34 through approximately half of its full travel from its fully open to its fully closed position. Similarly adjustment of the control damper 32 by means of an external lever (not shown) attached to the spindle 42 causes the lever 46 to be turned about the pivot 60. To permit such movement the pivot 62 rides in a slot 64 in the lever 46. Again, the dimensions are so chosen that displacement of the control damper from its fully closed to its fully open position displaces the bypass damper 34 through half of its full travel. Therefore, the bypass damper 34 is only closed when both control dampers 30 and 32 are fully open. The cross sectional area of the bypass 36 is so chosen in relation to the cross sectional areas of the ducts 26 and 28 and the resistance to fluid flow through the nozzle banks 14 and 16 that the pressure in the plenum 24 remains approximately the same whatever the adjustments of the dampers 30 and 32.

By the above means it is ensured that adjustment of one control damper 30 or 32 does not influence the pressure in the plenum 24 significantly and thereby does not significantly influence either the flow of medium through the other nozzle bank nor does it influence the total rate of flow of the medium through the heater 22

Claims

1. A damper system for a heating or drying apparatus employing forced convection, comprising a control damper for controlling the flow of a gaseous medium delivered from a heating means to a nozzle arrangement adapted to direct the heated medium on to a product to be heated or dried, and a bypass damper for bypassing any excess heated medium past the nozzle arrangement, in which the dampers are linked together so as to be adjustable simultaneously in an inverse sense so that the rate of delivery of the medium from the nozzle arrangement can be varied from zero or a minimum to a maximum without significantly altering the rate of delivery from the heating means.

2. A damper system as claimed in claim 1, in which the arramgement is such that the back pressure upstream of the control damper remains substantially constant upon adjustment of the dampers.

3. A damper system as claimed in claim 1 or 2, for apparatus employing at least two nozzle arrangements, each receiving heated gaseous medium from the heating means via respective independently adjustable control dampers, in which the bypass damper is linked to all control dampers to be adjusted simultaneously therewith but in an inverse sense.

4. A damper system as claimed in claim 3 employing two control dampers, in which the latter act on the two ends of a lever whose mid-point acts on the bypass damper.

5. A damper system as claimed in any preceding claim for a forced convection apparatus having, several nozzle arrangements, in which independent control dampers are provided giving independent control of the top air flow and bottom air flow and are linked to the bypass damper in such a way as to maintain the air flow over the heating means constant irrespective of the top air and bottom air damper settings.

6. A damper system constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.