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PHONE: (978) 777-0070
Technical: Wattage Calculation Formulas
Problem 2: Paraffin Melting
| An open top uninsulated steel tank: 18" wide, 24" long, and 18" deep, weighs 140 pounds. This tank contains 168 pounds of paraffin which needs to be heated from 72°F to 150°F in 2 1/2" hours. |
From Table 1: Properties of Metals
Specific heat of steel: .12 BTU/lb-°F
From Table 2: Properties of Non-Metallic Solids
Specific heat of solid paraffin: .70 BTU/lb-°F
Melting point of paraffin: 133°F
From Table 3: Properties of Liquids and
Gases
Heat of fusion of paraffin: 63 BTU/lb
Specific heat of melted paraffin: .71 BTU/lb-°F
From Graph 1 and Graph 5: Guide for Heat
Losses
Surface loss at 150°F: 55 w/sq.ft./hr.
Surface loss at 150°F: 70 w/sq.ft./hr.
Formula A: Wattage required for heat-up.
| To Heat Tank 140 lb x .12 BTU/lb-°F x (150-72)°F = 3.412 x 2.5 |
155 watts |
| To Heat Paraffin 168 lb x .70 BTU/lb-°F x (133-72)°F = 3.412 x 2.5 |
845 watts |
| To Heat Melted Paraffin (fusion occurs at melting point) 168 lb x .71 BTU/lb-°F x (150-133)°F = 3.412 x 2.5 |
240 watts |
Formula C: Wattage for Melting or Vaporizing
| Heat Of Fusion To Melt Paraffin 168 lb x 63 BTU/lb = 3.412 x 2.5 |
1,245 watts |
Formula B: Wattage losses at operating temperature (See graphs: Guide for Heat Losses)
| Average Paraffin Surface Loss 3 sq.ft. x 70w/hr. = |
210 watts |
| Total Losses 13.5 sq.ft. x 55w/hr. = |
740 watts |
| Compensation Factor 20%(155 + 845 + 239 + 1,245 + 210 + 740) |
685 watts |
| Total wattage required = | ___________ |
In addition to calculating the watts required for initial heat-up and heat losses, operating heat requirements must be calculated. Steel pins, each weighing .175 pounds, are to be placed in a 70 pound steel rack and dip-coated in the melted paraffin. 1,750 pins can be processed per hour with 25 pounds of paraffin. |
Formula A: Wattage required for heat-up
| To Heat Pins and Rack (1750 x .175 + 70)lbs/hr x .12 BTU/lb/°F x (150 - 72)°F = 3.412 x 1 hr. |
1,030 watts |
| To Heat Additional Solid Paraffin 25lb/hr x .70 BTU/lb/°F x (133 - 72)°F = 3.412 x 1 hr. |
310 watts |
| To Heat Additional Melted Paraffin (fusion occurs at this point) 25lb/hr x .71 BTU/lb/°F x (150 - 133)°F = 3.412 x 1 hr. |
90 watts |
Formula C: Wattage for melting or vaporizing
| Heat Of Fusion, To Melt Additional Paraffin 25lb/hr x 63 BTU/lb = 3.412 x 1 hr. |
460 watts |
Formula B: Wattage losses at operating temperature (See graphs: Guide for Heat Losses)
| Paraffin Surface Loss 3sq.ft. x 70w/sq.ft./hr = . |
210 watts |
| Tank Surface Loss 13.5sq.ft. x 55w/sq.ft./hr = |
740 watts |
| Compensation Factor 20% (1,058 + 310 + 90 + 460 + 210 + 740) = |
575 watts |
| Total wattage required = | ___________ |
In the above calculations, the heat-up requirement is the greatest, therefore a heater with a wattage rating of 4,120 watts should be used in this application. The recommended watt density on the heater for this application is 16 watts per square inch (see Suggested Watt Densities). |
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