RADIANT HEATING WITH INFRARED - Watlow

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18 Comments/Questions What is the required heater watt density? If selecting a <strong>Watlow</strong> RAYMAX 1120 for this application, Figure 18 shows that this heater requires about 9 W/in 2 to maintain 780°F face temperature in open air. In this application, slightly less would be required since some of the radiant energy that is reflected off of the 0.80 emissivity surface of the metal is reflected back into the heater and re-absorbed. This would be a much more significant factor if the product surface had a lower emissivity, say 0.5. What about losses off the surface of the metal as it heats up? Generally, the air temperature between the heater and the product is higher than the product temperature, so some convection heating takes place. In this application, assume the plate is resting on a good heat insulator and there is very little air movement. If this is not the case, then these losses must be estimated and added to the required wattage determined above. Example #2: Drying a Moving Web of Cloth A fabric dying process requires that water be evaporated from a five foot web of cotton cloth moving at 10 feet per minute. The dry cloth weighs 0.0356 pounds per square foot. The wet cloth weighs 0.0762 pounds per square foot. The wet cloth will enter the drying oven at 60°F. Heaters can be located within four inches above and below the cloth. 1. Collect the data, make assumptions. Assume a cloth exit temperature of 250°F -∆T cloth = 250°F - 60°F = 190°F -∆T water = 212°F - 60°F = 152°F -∆T steam = 250°F - 212°F = 38°F -Specific heat cloth * = 0.31 BTU/lb°F -Specific heat water * = 1.0 BTU/lb°F -Specific heat steam * = 0.482 BTU/lb°F -Heat of vaporization of water * = 965 BTU/lb -Weight of dry cloth = 0.0356 lb/ft 2 -Weight of water = wet cloth - dry cloth = 0.0762 lb/ft 2 - 0.0356 lb/ft 2 = 0.0406 lb/ft 2 -Emissivity of textiles (dry) = 0.77 -Emissivity of water = 0.92 -Emissivity of wet cloth (best guess) = 0.85 *These values can be found in the Application Guide.
2. Determine the wattage required to completely dry the wet cloth. There are four parts to this problem. A. Watts required to raise the temperature of the cloth to 250°F. The area per hour being heated is: 10 ft/min x 5 ft wide X 60 min/hr = 3000 ft 2 /hr The weight of cloth = 3000 ft 2 /hr x 0.0356 lb/ft 2 = 106.8 lb/hr Watts to heat cloth = Wt x Specific Heat x ∆ T Time x 3.412 BTU/watt hr Watts to heat cloth = 106.8 lb. x 0.31 BTU/lb°F x 190°F 1 hr x 3.412 BTU/watt hr Watts to heat cloth = 1,844 watts B. Watts required to raise the temperature of the water to 212°F. The weight of the water = 3000 ft 2 /hr x 0.0406 lb/ft 2 = 121.8 lb/hr Watts to heat water = 121.8 lb x 1 BTU/lb°F x 152°F 1 hr x 3.412 BTU/watt hr Watts to heat water = 5426 watts C. Watts required to vaporize the water. Watts to vaporize water = Weight of Water x Heat of Vaporization Time x 3.412 BTU/watt hr Watts to vaporize water = 121.8 lb x 965 BTU/lb 1 hr x 3.412 BTU/watt hr Watts to vaporize water = 34,448 watts D. Watts required to raise the temperature of the steam from 212°F to 250°F. Watts to heat steam = 121.8 lb x 0.482 BTU/lb °F x 38°F 1 hr x 3.412 BTU/watt hr Watts to heat steam = 654 watts Heat cloth 1,844 watts Heat water 5,426 watts Vaporize water 34,448 watts Heat steam 654 watts Total watts 42,372 watts 3. Determine the heater temperature and array size required to transfer the needed wattage. Note: The energy required to vaporize the water far exceeds all the other requirements combined. 19
Page 1 and 2: ed W A T L O W RADIANT HEATING WITH
Page 3 and 4: The Advantages of Radiant Heat Elec
Page 5 and 6: 10 6 X 10 1.4 X 10 1.2 X 10 0.4 0.7
Page 7 and 8: Radiated Power vs. Wavelength (Fig.
Page 9 and 10: W/in 2 Radiated Net In a typical ra
Page 11 and 12: Looking at Figure 8, the new view f
Page 13 and 14: Heater Emissivity Just as the surfa
Page 15 and 16: This information is useful for sele
Page 17 and 18: Step 3: Compute the Absorbed Wattag
Page 19: E = 1 = 1 = 0.70 1 + 1 - 1 1 + 1 -
Page 23 and 24: D. The required oven length can now
Page 25 and 26: C. Use the Stefan Boltzman equation
Page 27 and 28: Controlling Radiant Heaters The fol
Page 29 and 30: Usually this sensor temperature doe
Page 31 and 32: Tips On Oven Design Efficiency The
Page 33 and 34: Temperature Uniformity: Edge Losses
Page 35 and 36: Wiring Do not use copper wire or ri
Page 37 and 38: (Fig. 18) Time vs. Temperature Pane
Page 39: 14 15 13 12 11 10 9 8 6 4 2 7 3 5 1

RADIANT HEATING WITH INFRARED - Watlow

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