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Calculation of oven zone lengths

How to calculate the best zone lengths for a particular oven specification with examples for two different oven types.

 

In order to calculate the best zone lengths for a particular oven specification, we need to consider:

1. The control we need to maintain the best baking profile for the oven

At certain stages of the baking process, we will need short zones to provide a high heat input per m2 of oven band area and rapidly increase the baking temperature. At later stages, the requirement may be a more steady, requiring a constant, stable temperature and lower heat input and therefore longer zones will be suitable.

2. Flexibility of control

We can specify the zone lengths and heat input for a specific biscuit. However, normally we will allow some flexibility to alter baking profiles and alter conditions to bake other products. More control zones give greater flexibility.

3. The heat input

The heat input or rating from the burners in terms of kcal/m2 or kW/m2 of oven band area. This is the heat intensity which can be transferred to the dough pieces in each oven zone.

4. The construction of the oven modules 

Manufacturers often have standard module lengths and zones are normally a multiple of these standard module lengths. For particular applications, special module lengths can be made.

5. Zone lengths

Zone lengths normally vary between 8.0m and 18.0m, though shorter or longer lengths may be made for particular circumstances.

 

Example 1: Direct Gas Fired Oven for baking crackers (1.5 x 100m long)

Our baking profile requires a high heat input over the first 30% of the oven length to raise the baking temperature to 300°C and then to maintain a temperature of 280°C in the middle of the oven. The final colouring stage will require less heat input to maintain a baking temperature of 250°C.

The baker will determine the optimum baking profile for the product. Our task is now to translate the baking profile into a series of set temperatures and zone lengths. We will also consider maintaining flexibility in order to bake other products. For example, we can specify zones with the same set temperatures for one product but may require different settings for another product.

Baking profile for a soda cracker oven
Figure 1: Baking profile for a soda cracker oven

 

Heat input

Baker Pacific oven modules are 2.9m in length and contain 4 top burner positions and 4 bottom burner positions. Our DGF burners are rated at 20kW and the high rate MFB burners at 30kW. So in one module, we can input a total of 160kWh with standard burners up to a maximum of 240kWh with high rate MFB burners.

Figure 2: Oven module with 8 burner positions
Figure 2: Oven module with 8 burner positions

 

The area of the oven band in the module is 1.5 x 2.9m = 4.35m2. The maximum heat rating that can be achieved is therefore from 36.78 kWh/m2 with standard burners, up to 55.17 kW/m2 with high rate burners.

We can now divide the oven length into separate zones and specify the number and rating of the burners for each zone required to give the heat input to achieve our baking profile.

 

Figure 3: Baking profile for a soda cracker oven
Figure 3: Baking profile for a soda cracker oven

 

Our zone lengths will vary between 8.0m and 18.0m and they will be designed to achieve a rapid heat input in the first oven zones, to maintain a high temperature throughout the middle zones and then to reduce the heat input for the final zones. It should be noted that a considerable amount of heat is carried through the oven with the oven band and products, so the heat input required will be reduced in the later oven zones, 5 - 8. Our proposed oven zone configuration is shown above.

Zone lengths are:

Zone 1 2 3 4 5 6 7 8
Length (m) 8.7 8.7 11.6 11.6 11.6 14.5 14.5 17.4

Figure 4: Baker Pacific DGF oven with 2.9m modules and Eratec MFB burners
Figure 4: Baker Pacific DGF oven with 2.9m modules and Eratec MFB burners

 

Example 2: Indirect Radiant Oven for baking a short dough biscuit, Glucose biscuit (1.25m x 100m long)

The rotary moulded short dough will require a longer bake time (3.8 – 4.5min) with lower baking temperatures and heat inputs than crackers. The main aim will be to achieve a good biscuit texture, eating quality, flavour and colour. For this type of biscuit, an Indirect Radiant Oven is ideal.

Figure 5: Proposed baking profile for Glucose biscuit
Figure 5: Proposed baking profile for glucose biscuit

 

Heat input

In an Indirect Radiant Oven, there is one burner per zone. On a Baker Pacific oven, these are Weishaupt WGL-30 or Maxon OVENPAK 515SP burners with a maximum rating of 350 – 360 kW. The zones are made up from baking chamber modules of approximately 2.3m long and heater modules 2.0m long. One burner supplies heat to each zone, so the heat rating is 350 – 360 kWh / area of oven band in the zone.

Proposed heat ratings for Glucose biscuits
Figure 6: Proposed heat ratings for Glucose biscuits

 

Zone 1 2 3 4 5 6 7 8
Length (m) 8.9 11.2 11.2 11.2 13.5 13.5 13.5 15.8

Note: the use of a heat recovery system enables the last two zones to be heated without burners.

 

Figure 7: Baker Pacific Indirect Radiant Oven with Heat Recovery System
Figure 7: Baker Pacific Indirect radiant oven with Heat Recovery System

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