# Energy Efficient Process Heating by 8tDh90y

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1) Consider a rectangular heat treat oven with dimensions of 5 ft x 5 ft x 5 ft, an inside
air temperature of 1,000 F, an external surface temperature of 300 F, and an outer
surface emissivity of 0.9. The combustion efficiency of the oven is 50%. The
temperature of the surrounding air and surfaces is 70 F. Calculate heat lost from the
oven’s sides and the savings from insulating the sides with R = 4 hr-ft2-F/Btu insulation.

2) Consider a furnace with an opening 1 ft in diameter through a 1.5 ft thick furnace
wall. The temperature inside the furnace is 1,800 F, and combustion efficiency of the
furnace is 60%. The temperature of the surrounding surfaces is 90 F. Calculate the
radiation heat loss through the opening, and the savings if the opening is covered by a

3) Consider a furnace with inside air temperature of 700 F, combustion efficiency of
60%, and cooled by 50 gpm of water from a cooling tower. During winter, the cooling
tower supplies water to the furnace at 65 F, and the water exits the furnace at 95 F.
During summer, the cooling tower delivers 95 F water to the furnace and internal parts
are not overheated. Calculate the rate of heat removed by the cooling water in the
summer and winter, and the savings from supplying 95 F water to the furnace instead of
65 F water during winter.

4) A brazing oven with an operating temperature of 1,000 F and combustion efficiency
of 60% has a stainless steel conveyor belt weighing 10 lbs/ft traveling 30 ft per hour
when loaded with parts. The specific heat of stainless steel is 0.12 Btu/lb-F. To
prevent overheating, the conveyor must always be moving. Calculate the savings from
slowing the conveyor speed to 10 ft per hour when no parts are being brazed.

5) Steel racking with mass 100 lb and surface area 50 ft2 is used to carry products into a
batch drying oven at 500 F. The combustion efficiency of the oven is 70%. The specific
heat of steel is 0.12 Btu/lb-F, and the convection coefficient is 2 Btu/hr-ft2-F. If the
racking enters the oven at room temperature of 70 F and the drying process takes 0.25
hours, calculate the exit temperature of the racking, and the heat and fuel energy loss to
the racking with each drying cycle.

6) A well-sealed melting furnace burns 2 mmBtu/hr of natural gas. Combustion air
enters the burner at 70 F. A combustion analysis of exhaust gasses shows that the flue
temperature is 800 F and the quantity of excess air is 60%. Calculate the current
combustion efficiency, the combustion efficiency if the excess air were reduced to 5%,
and the resulting fuel savings.

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7) Consider a cure oven operating at 400 F located on the second floor of a plant. The
total area of the entrance is 60 ft2. The average exfiltration velocity is measured to be
80 ft/min over the upper half of the entrance. If ambient air temperature is 70 F and
the combustion efficiency of the oven is 80%, calculate heat loss and fuel energy savings
if the entrance was oriented horizontally on the floor of the oven.

8) Consider a cure oven with a high vertical entrance. The temperature of air is
measured to be 600 F near the ceiling of the oven 450 F near the mid-height of the
oven. Oven air at a temperature of 600 F is measured to be exfiltrating the oven at an
average velocity of 100 ft/min over an area of 10 ft2. If ambient air temperature is 70 F
and the combustion efficiency of the oven is 70%, calculate heat loss and fuel energy
savings if the entrance were lowered to mid-height.

9) Consider a glass melting furnace using 15 mmBtu/hr of natural gas. The temperature
of O2 entering the burner is 70 F, and the temperature of exhaust leaving the furnace is
2,500 F with 15% excess air. Calculate the fuel energy savings (mmBtu/hr) from
converting from air-fired to oxy-fired burners assuming 0% excess oxygen in the exhaust
and the temperature of exhaust leaving the furnace remains 2,500 F.

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