Chromalox Big Red Book

Technical

Technical Information Radiant Infrared Heating - Process Applications

Estimating Watt Density for Curing or Baking

Baking & Curing — The nomograph to the right can be used to determine the watt density required on the work product for baking and curing of paints and coating. Lacquers are cured primarily by evaporation of the solvent and can be cured by infrared in 2 - 15 minutes. Enamels are cured primarily by polymerization and require a longer time (15 - 20 minutes). Varnishes, japans and house paints cure mainly by oxidation but can usually can be accelerated by infrared heating. To find approximate watt density needed for baking: 1. Locate temperature product is to reach in five minutes (A) 2. Read across to line representing gauge of the material being heated (B) 3. Read up to ventilation air in feet per minute over surface of the product (C). If not known, estimate feet per minute based on cubic feet per minute of ventilation or circulating air divided by the the approxi- mate cross sectional area of the oven. In applications with no forced ventilation, use 2 - 5 fpm. 4. Read right to the absorption factor for the work product surface or coating (ie:  = 0.85 for enameled sheet metal) (D) 5. Read down to watt density required on the product surface (E). Determining Heater Fixture Spacing — Hav- ing determined the total required kilowatts and the desired W/in 2 on the work product, the next step is to deterine the spacing and the number of heaters. In most conveyor type oven applications, a 12" spacing from the face of the heater to the work product produces uniform distribution of the radiation. The graph to the right shows centerline to center- line spacing of Chromalox radiant heaters to obtain various intensities on the work based on a spacing of 12" from the face of the heater to the work product. Specific applications may require the distance to be increased or decreased. The graph is applicable to line or point infrared sources installed in reflectors. Refer to view factor charts for ceramic heaters and flat panel infrared sources.

0.90

0

Ventilation Air Feet Per Minute (Air at 80°F)

0.75 0.65 0.50 0.35

50

150

300

C

D

Absorption Factor (Emissivity)

E

0 1

2 3

4 5

6

7

100

Watt Density Required on Product (W/In 2 )

12 Gauge Sheet Steel

200

300

Wood

B

400

16 Gauge

A

24 Gauge

Five Minutes

500

28 Gauge

20 Gauge

600

Temperature (°F) To be Attained in

Intensity Vs. Spacing — Point & Line Infrared Sources

20

A = QRT

L

18

(3/8" Dia. Quartz Tube)

B = RAD/QRT

12"

16

(1/2" Dia. Quartz Tube)

3-11/16"

C = S-RAD D = RADD/U-RAD/QR

14

(3/8" Dia. Quartz Tube)

12

E = U-RP

E

10

8 Average W/In 2 on Work Product 6

D

C

B

A

4

2

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Heater Center to Center Distance = L (In.)

TECHNICAL

INFORMATION

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