Heat Requirement Calculations
When sizing the heaters for a particular application there are two energy requirements; the startup heat and the operating heat.
StartUp Heat is the heat energy required to bring a process up to operating temperature. Startup heat requirement calculations, including material change of state, should be done in three parts.

Operating Heat is the heat energy required to maintain the desired operating temperature through normal work cycles. A safety factor is usually added to allow for unknown or unexpected operating conditions. 10% is adequate for small systems with relatively few unknowns, while 20% additional wattage is more common, and figures of 25% to 35% may be considered for larger systems with many unknowns.

Startup Heat Requirements
Heat requirements to heat up system
1. Wattage required to heat material:
2. Wattage required to heat container or tank:
3. Wattage required to heat hardware in container:

Specific Heat is the amount of heat required to change a unit mass of a substance by one degree in temperature. Specific Heat of Metals, Nonmetals and Liquids and Gases can be found here.
Heat of Fusion – the amount of heat required to change a unit mass of a substance from solid to liquid without temperature change. Heat of fusion figures can be found here. Heat of Vaporization  the amount of heat required to change a unit mass of a substance from liquid to vapor without temperature change. Heat of vaporization figures can be found here. 
Heat Requirements for phase changes
4. Wattage required to melt a solid to a liquid at constant temperature
Weight of Material to be Melted (lbs/hr) X Heat of Fusion (Btu/lb)  
Watts =  
3.412 Btu/watt hr. 
5. Wattage required to change a liquid to a vapor at constant temperature
Weight of Material to be Vaporized (lbs/hr) X Heat of Vaporization (Btu/lb)  
Watts =  
3.412 Btu/watt hr. 
Heat Requirements to counteract surface losses.
Use Loss Rate graphs in the equations below.
6. Wattage to counteract liquid surface losses
Use Loss Rate graphs in the equations below.
6. Wattage to counteract liquid surface losses
Total Liquid Surface Area (sq. ft.) X Loss Rate at Final Temperature (Watts/sq. ft.)  
Watts =  
2 
7. Wattage to counteract surface losses from container walls, platen surfaces, etc.
Total Surface Area (sq. ft.) X Loss Rate at Final Temperature (Watts/sq. ft.)  
Watts =  
2 
Operating Heat
Heat Requirements to counteract losses
1. Wattage to counteract losses from open liquid surfaces:
1. Wattage to counteract losses from open liquid surfaces:
Watts =  Total Liquid Surface Area (sq. ft.) X Loss Rate at Operating Temperature (Watts/sq. ft.) 
2. Wattage to counteract container or platen surface losses, either insulated or uninsulated:
Watts =  Total Surface Area (sq. ft.) X Loss Rate at Operating Temperature (Watts/sq. ft.) 
Heat Requirements to heat items transferred in and out of system
3. Wattage required to heat material transferred in and out of the system:
3. Wattage required to heat material transferred in and out of the system:
Weight of Material to be Heated (lbs) X Specific Heat (Btu/lb °F) X Temperature Rise (°F)  
Watts =  
3.412 Btu/watt hr. X Heat  up time (hr.) 
4. Heatup of racks of containers, etc. transferred in and out of the system:
Weight of Items to be Heated (lbs) X Specific Heat (Btu/lb °F) X Temperature Rise (°F)  
Watts =  
3.412 Btu/watt hr. X Heat  up time (hr.) 
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