MODUL KE LIMA TEKNIK MESIN FAKULTAS TEKNOLOGI INDUSTRI UNIVERSITAS MERCU BUANA MODUL KE LIMA HEAT TRANSFER NANANG RUHYAT PROCESS HEATING PROCESS HEATING HEAT BALANCES–DETERMINIING THE HEAT NEEDS OF FURNACES AND OVENS Although rules of thumb are frequently used to size furnace and oven burners, they have to be used with care. All rules of thumb are based on certain assumptions about production rates, furnace dimensions, and insulation. If the system under consideration differs from these assumed conditions, using a rule of thumb can result in a significant error. For out-of-the ordinary conditions, or where more accurate results are required, heat balance calculations are preferred. A heat balance consists of calculating load heat requirements and adding losses to them to determine the heat input. Below is a schematic representation of the heat balance in a fuel-fired heat processing device. http://www.mercubuana.ac.id

Radiation losses – heat lost from the furnace as radiant energy escaping through openings in walls, doors, etc. Conveyor losses – heat which is stored in conveying devices such as furnace cars and conveyor belts and which is lost when the heated conveyor is removed from the furnace. Net output – this is the heat that ultimately reaches the product in the oven or furnace. On page 36 is a simplified worksheet for carrying out a heat balance. By following this format, you can determine the gross heat input required at maximum load and minimum load conditions, along with the furnace turndown and theoretical thermal efficiency. Supporting Calculations: Heat to Load Heat to load lb per hour x specific heat x temperature rise. Specific heats for many materials are listed on pages 37-39. For most common metals and alloys, use the graphs on page 40. Simply multiply lb/hr production rate by the heat content picked from the graph. Enter the heat to load under Maximum Load Conditions. Heat to load is usually zero under Minimum Load Conditions because no material is being processed through the oven or furnace. Wall Losses: Wall loss Wall Area (inside) x heat loss, Btu/sq ft/hr. Typical heat loss data are tabulated on page 44 . If the roof and floor of the furnace are insulated with different http://www.mercubuana.ac.id

% Efficiency Heat to load, maximum load conditions x 100 Gross heat input, maximum load conditions This is the maximum theoretical efficiency of the furnace, assuming it operates at 100% of rating with no production interruptions and with a properly adjusted combustion system. Heat Storage Heat Storage was left out of this analysis. Althought it is a factor in furnace efficiency, burner systems are rarely sized on the heat storage needs of the furnace. On continuous furnaces where cold startups occur infrequently, heat storage can usually be ignored without any major effect on efficiency calculations. On batch-type furnaces that cycle from hot to cold frequently, storage should be factored into efficiency calculations. Heat Storage Inside refractory surface area, ft2 x Heat Storage Capacity, Btu/ft2 Heat storage capacities for typical types of refractory construction are tabulated on Page 44. http://www.mercubuana.ac.id