1. 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.

2. 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 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

3. % 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.