Advantages of the Process
The temperature range of the baking oven's flue gas ranges from 300 to 350 degrees Celsius. The waste heat available ranges from 70-80kW and the proofing oven requires energy between 20-45kW. With an air velocity for the system of 1.5 metres per second and an heat exchanger effectiveness of around 65 percent, the waste heat recovery system was able to supply all the heat needed by the proofing oven, thus eliminating the need for any natural gas heating.
The predicted annual waste heat recovery from one eight hour shift working six days a week is estimated to be around 500GJ.
The heat exchanger was manufactured locally from steel pipes charged with distilled water. The RMIT research team supervised the manufacturing process to ensure that the equipment would withstand the high temperature and corrosive environment of the baking oven's exhaust gases.
The total cost of manufacturing and constructing the 50kW prototype heat exchanger, excluding data monitoring operations, is estimated to be about $10,000.
Based on annual saving of 500GJ per year for one shift, cost of natural gas at $4 per GJ, and boiler efficiency of 70 percent, the pay back period is less than 3.5 years. For a three shift operation, the pay back period is reduced to 1.5 years.
Additional benefits resulted from the more efficient operation of the boilers in terms of lower steam requirement, less water to steam off, less gas used, less water treatment, and less maintenance on the boilers.
Following the success of the project at its Clayton plant, it was decided to install a second heat pipe heat exchanger system in a new baking plant just completed. This second heat recovery system employs a new arrangement - called a "split heat pipe heat exchanger" - in which the evaporator and condenser coils are located some distance apart, and connected with sections of heat pipes. This arrangement reduces the cost and complexity of ducting that would otherwise be required