Most biscuits, cookies and crackers are baked at temperatures of 200oC – 270oC. The baking chambers contain moist air and in direct fired ovens also the products of combustion. During the baking process this hot air and gases are extracted from the baking chambers through flues to atmosphere. An extraction fan and flue is located in each zone of the oven. This is the major loss of energy in the baking process, typically accounting for 30% – 40% of the total energy usage. It is advantageous to consider recovering and re-cycling this energy.
Baker Pacific direct gas fired oven with a flue in each zone
Extraction fan and flue
For complete combustion of the gas an excess of combustion air is required for maximum efficiency. Typically for complete combustion of natural gas the air to fuel ratio is 17.2 : 1. Normally the combustion air fed to the burners is at ambient temperature and it is heated during combustion.
Highest combustion efficiency with excess air
A large percentage of the heat from the flues can be recovered and used to heat air or water. For biscuit baking the heat recovered can be used to pre-heat the combustion air fed to the burners, reducing the energy loss. Our example is based on a 6 zone oven 60m long.
Heat recovery and re-cycling unit from Econotherm Ltd.
The following data is from a gas to air heating system for a 6 zone direct gas fired oven.
Flue gas temperature: 200oC
Ambient temperature: 20oC
Mass flow rate exhaust: 5204.8 kg/h
Mass flow rate of combustion air: 4833.0 kg/h
The combustion air would be delivered to the burners at 120oC. It would be necessary to confirm that the particular burners would accept combustion air at this temperature.
The investment in the heat recovery equipment from Econotherm would be approximately £ 45,000 ex works for 4 units for a 6 zone oven.
Total recovered heat = 4 x 131kWt = 524kWht. Based on 8000 hours per year operation and £0.04/kWh for natural gas in UK, this represents a saving of circa £168k per annum in UK.
Reference: Econotherm Ltd. UK www.econotherm.co.uk