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Energies 2020, 13, 6446 14 of 26 The combustion air of the industrial oven can be preheated up to a temperature 10 ◦C below the exhaust temperature. Thus, in the current scenario where the exhaust is at 165 ◦C, the combustion air could be preheated up to 155 ◦C; 125 ◦C higher than the baseline temperature of 30 ◦C. Using the relation between the combustion air temperature and oven fuel consumption obtained from the pilot-scale experiments, it was estimated that a maximum fuel saving of ~6.6% i.e., 4.98 Nm3/h or 55.05 kWh could be achieved for the industrial baking oven by preheating its combustion air. The new gas usage for the oven is calculated by subtracting the gas savings obtained due to heat recovery from the baseline gas consumption of 75.55 Nm3/h. A 6.6% of fuel savings will reduce the energy consumption per kilogram of wafer production from 3.32 kWh to 3.10 kWh, thus making the oven more efficient. Contamination of the product is one of the greatest risks in a food manufacturing plant. The exhaust gases contain moisture released from the batter and trace amount of sulphur oxides if the fuel used in the oven (natural gas) contains sulphur. In this study, the lowest possible temperature of the oven exhaust gas after heat recovery was set at 110 ◦C for all the heat recovery technologies, to avoid moisture and acidic vapour condensation in the heat recovery unit. However, using this condition limits the available heat for recovery per unit time in the exhaust to 167 kW. This is because most of the vapour in the exhaust gas does not condense thus losing the latent heat of vaporisation of water. This yields an effectiveness of 33%. 3.3. Heat Recovery for Hot Water Production The industrial baking oven exhaust gases released at 165 ◦C and 3.72 m3/s are a lucrative source of free waste heat that can be utilised to reheat process hot water streams. An Aspen Plus model was developed to reheat a returning process water using exhaust’s heat. The simulation results indicate that heat can be recovered at a rate of 167 kWh from the exhaust gases by reheating 13.1 t/h of process hot water from 80 ◦C to 90 ◦C. To attain a similar output from an industrial gas boiler, 18.8 Nm3/h or 207.84 kWh of natural gas will be required to burn, assuming an 80% efficiency for the overall gas-based hot water generation system involving boiler, pressure valves, calorifier, etc. [66,67]. This gas saving does not directly occur in the baking oven, but the overall energy consumption of factory can be saved. However, the factory’s energy consumption savings were deducted from the oven’s energy consumption for simplicity of the comparison with other heat recovery options. Table 8 provides a comparison of the oven’s performance parameters before and after heat recovery. The new gas usage for the oven is obtained by subtracting the boiler gas savings of 18.8 Nm3/h, achieved due to heat recovery, from the baseline gas consumption. The effectiveness (ratio of fuel consumed by an industrial boiler to generate a similar result as the heat recovery system and the recoverable heat from a single oven exhaust) achieved by the heat recovery system reheating the process water is 124.4%. Incorporating the savings accomplished from heat recovery into the baseline parameters indicates a 25% improvement in the oven productivity. Table 8. Key performance indicators of the industrial-scale baking oven before and after heat recovery to reheat returning process water. Parameter Gas usage Load Effectiveness of heat recovery system Oven productivity Units Nm3/h kW % kWh/kg of wafer Baseline 75.55 835.25 - 3.32 After Heat Recovery 56.75 627.40 124.4 2.49 3.4. Preheating Combustion Air and Reheating Process Water The experiments revealed that preheating the combustion air flow from 30 ◦C to 155 ◦C provides a relatively small heat sink due to a much lower flowrate of combustion air compared to the oven’s exhaust gases. Hence, the waste heat in the exhaust gases are not recovered to the full potential due toPDF Image | Waste Heat Recovery Technologies for the Food Processing Industry
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