Calculating Oven Heat Load Capacity

Q. Please explain how to calculate the heat load capacity of a paint baking oven, using aluminum alloy wheels as an example. H.T.

A. Energy consumption in an oven is primarily related to radiant loss through the oven panels; energy required to raise the product, racks and conveyor to temperature; and exhaust losses. To determine the energy use, you will need to calculate these three factors and then add them to get the total.

Panel Loss

To calculate the loss of Btu/h through the oven panels, multiply the total square feet of oven panel surface area by the panel loss factor (see table) and then by the temperature difference between the oven start-up and the controller setting.

Panel Thickness	Loss Factor
3 inches	0.40
4 inches	0.35
5 inches	0.30
6 inches	0.25
8 inches	0.20

Formula: Square feet of oven surface × panel loss factor × temperature rise

Example  

Oven Size: 20 × 50 × 10 ft (W × L × H)

Operating Temperature: 300°F

Oven Start-up Temperature: 70°F

Panel Thickness: 4 inches 

Oven Surface Area 

Sides: 10 × 50 = 500 × 2 sides = 1,000 

Ends: 10 × 20 = 200 × 2 ends = 400

Top and bottom: 20 × 50 = 1,000 × 2 = 2,000

Total: 1,000 + 400 + 2,000 = 3,400 sq ft

Temperature Rise

300°F - 70°F = 230°F

Panel Loss

3,400 sq ft surface area × 0.35 panel loss factor × 230°F temperature rise = 273,700 Btu/hr

Product Loading

To calculate the Btu/hr required to support the product loading, multiply the total load (parts, rack, conveyor chain and trolleys) by the specific heat (Btu required per lb °F) by the temperature rise. The factor 0.226 is used for aluminum. Different materials will require a different multiplier, and the table below shows the specific heat values for different metals.

Metal	Specific Heat
Steel	0.12
Aluminum	0.226
Iron	0.175
Zinc	0.0931

Conveyor	Chain Weight/Ft	Trolley Weight
Enclosed Track	3.5 lbs	0.5 lbs (H-Attach)
X348	2.14 lbs	2.34 lbs
X458	3.14 lbs	5.18 lbs
X678	6.39 lbs	16.5 lbs

Formula: Total load × specific heat × temperature rise

Example: 

Chain: X348
Trolley Centers: 12 inches
Racks: 5 lbs on 12-inch centers
Parts: 5 lbs on 12-inch centers
Line Speed: 10 ft/min
Oven Temperature: 300°F
Oven Start-up Temperature: 70°F

Total Load
Chain Weight: 2.14 + 2.34 = 4.48 lbs/ft
Parts and Racks Load: 5 + 5 = 10 lbs/ft
Total: 4.48 lbs chain and trolleys + 10 lbs parts and racks = 14.48 lbs/ft
Total product load/min: 14.48 lbs/ft × 10 ft/min = 144.8 lbs/min
Product load/hr: 144.8 lbs/min × 60 min = 8,688 lbs/hr

Temperature Rise
300°F - 70°F = 230°F

Btu/Hr
8,688 lbs/hr total load × 0.12 specific heat × 230°F temperature rise = 239,789 Btu/hr

(0.12 is the multiplier used for steel.)

Exhaust Loss

The exhaust loss is determined by multiplying the cu ft/min of exhaust (cfm) by 60 min by 0.075 lbs/cu ft by 0.24 Btu/hr per pound by the temperature rise in °F.

Formula: cfm × 60 × 0.075 × 0.24 × temperature rise

Example: 
Exhaust Volume: 2,000 cfm
Set Temperature: 300°F
Oven Start-up Temperature: 70°F

Temperature Rise
300°F - 70°F = 230°F

Exhaust Loss
2,000 cfm × 60 min × 0.075 lbs/cu ft × 0.24 Btu/hr × 230°F temperature rise = 496,800 Btu/hr
 

Total Energy Use

To determine the total Btu/hr consumed, add the three factors: panel loss, product loading and exhaust loss. In the above examples, this total would be:

273,7000 + 239,789 + 496,800 = 1,010,289 Btu/hr

To determine the actual cost, you must multiply the consumption by the cost for the energy used in the oven. The cost for energy varies widely in different regions. It was an average of about $2.80 per million Btu the week ending August 15, 2012.