Oven Efficiency - Comparison of Direct and Indirect Fired Ovens
Heat transfer
The most important method of heat transfer for crackers and biscuits is radiation. Radiation penetrates the dough pieces and bakes the product, achieving the volume, structure and texture required. When baking on a heavy mesh, compound balanced weave band, conduction to the base of the product is also important. Convection, hot air impinging on the product surface, may be used in the final stages of baking to assist moisture loss and even colouring.
Direct Gas Fired Ovens
The flames from the ribbon burners radiate heat directly to the products and to the underside of the oven band. The radiation is proportional to the temperature and distance between the flame and the dough pieces. The temperature of a typical gas flame is 1950oC and the burner is approximately 150 mm from the products and so the radiation is very efficient. The first zones of a Direct Gas Fired oven will rapidly create volume and structure for crackers, particularly where a preheated oven band provides conduction to the base of the dough pieces.
FIG 1: Radiant tube arrangement in the Indirect Radiant oven
Indirect Radiant Oven
The oven has a single burner in each zone and the hot flue gas is circulated through radiant tubes above and below the oven band. Radiation is from the radiant tubes, which may be at a typical temperature of 350oC and will be at approximately 150 mm from the products. The radiant heat transfer from each tube will be around 20% of that from a ribbon gas burner in the direct gas fired oven. However due to the arrangement of the tubes along the length of the baking chamber the radiation to the products is continuous, as compared to the radiation from ribbon burners which is at several points in the oven zone, typically an average of 730 mm apart.
Extraction and movement or air in the baking chamber
For our example, we take a single oven zone 1.2 m wide x 12 m long (14.4 m2 band area) with a heat rating of 20kWh /m2 and a burner capacity of 350 kW operating at 80% capacity. Each zone will have an extraction fan of 34 m3 per minute maximum capacity.
| Average heat input required: | 288 kWh |
| Natural gas consumption per hour: | 288/10.2 = 28.2 m3 |
| Combustion air required per hour: | 285m3 |
| Total volume of flue gas per hour: | 313m3 |
Direct Gas Fired Ovens
In a Direct Fired Oven all the flue gas enters the baking chamber. If the extraction fans are operating on average at 45% capacity, approximately 918 m3 per hour of the total flue gas will be extracted with moisture from the product. In the first zones, the extraction will be at a minimum, approximately 10%, and will extract approximately 200 m3 per hour. The surplus flue gas will be drawn down the oven and will influence the subsequent zones.
FIG 2: Extraction fan on a Direct Gas Fired Oven
Indirect Radiant oven
In the Indirect Radiant oven, the flue gas is continuously circulated through the radiant tubes, headers and return ducts. The flue gas does not enter the baking chamber. The increased volume from the ingress of combustion air is exhausted through the burner flue by natural convection. In the Indirect Radiant oven, each zone is temperature controlled and operates independently of subsequent zones. The moisture in the baking chamber in each zone is extracted by a separate independently controlled fan and flue.
FIG 3: Indirect Radiant oven zone
The exhaust gas from the burner flue may be partially collected from each zone and used in a Heat Recovery System. The flue gas from each oven zone travels in a collection pipe along the length of the oven and is fed via a fan to the Heat recovery zone. The Heat Recovery System may heat one or two oven zones without additional burners.
FIG 4: Heat Recover Zone without burner
On independent tests the Heat Recovery Zone increased the oven efficiency by 15%.
| Product | Oven type | Oven size | kWh/kg of biscuits |
|---|---|---|---|
| Rotary moulded | Indirect Radiant + Heat Recovery System | 1.2m x 100m | 0.404 |
| Rotary moulded | DGF/convection | 1.2m x 60m | 0.430 |
| Rotary moulded | DGF/convection | 1.5m x 100m | 0.441 |
| Rotary moulded | Indirect Radiant | 1.2m x 100m | 0.475 |
| Rotary moulded | DGF/cyclotherm | 1.2m x 60m | 0.492 |
| Snack cracker | DGF/convection | 1.2m x 90m | 0.477 |
Summary
In the Direct Gas Fired ovens, high rate radiant heat from the burners with a preheated oven band provides rapid development of the volume, structure and texture of the products, particularly for crackers. Minimum extraction in the first zones allows flue gas to flow to the subsequent oven zones.
In the Indirect Radiant oven the radiation is lower, but is continuous. The flue gas and combustion air do not enter the baking chamber. Each oven zone has independent control of temperature and extraction of moisture. The flue gas is exhausted through the burners flues by natural convection and part may be collected and used to heat the final zone(s) of the oven. This will improve the oven efficiency by approximately 15%.
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