Heat Calculation Considerations and Equations
"Calculations determine the amount of heater wattage required for process startup and process operation"
CONSIDERATIONS FOR PROCESS STARTUP:
- Wattage to heat material
- Wattage to heat container or tank (if applicable)
- Wattage to heat racks and hardware in container(if applicable)
- Wattage to change material state (if applicable)
- Wattage to counteract material or container heat loss (if applicable)
- Wattage to heat material transferred in and out (if applicable)
- Wattage to heat racks and hardware transfer in and out (if applicable)
- Wattage to change material state (if applicable)
- Wattage to counteract material or container heat loss (if applicable)
- The weight of the material
- The temperature change required (final temperature - initial temperature) commonly expressed as "�T"
- The specific heat of the material (see definition terminology. Specific heat charts list specific heat values for many materials.
- Heatup time required
Wattage = | WEIGHT (lbs) X SPEC. HEAT (btu/lb �F) X TEMP. CHANGE (�F) |
3.412 (BTU TO WATT/HR CONVERSION) X HEATUP TIME IN HOURS |
III. EQUATIONS FOR ADDITIONAL WATTAGE REQUIREMENTS
ADDITIONAL HEATUP REQUIREMENTS (IF APPLICABLE):
2. Wattage req'd to heat container or tank (if applicable):
Wattage = | Wt. of container (LB) x Sp. Ht. (Btu/lb. �F) x temp. changes (�F) - watts |
3.412 Btu/watt hr. x heatup time (hr) |
3. Wattage required to heat hardware in container (if applicable):
calculation similar to #1 and #2 = watts
4. Wattage required to melt a solid to a liquid (if applicable) at constant temperature:
Wattage = | Heat of fusion (Btu/lb) x weight of material melted/hr. (lb/hr) |
3.412 Btu/watt hr. |
5. Wattage required to change a liquid to a vapor state at constant temperature (if applicable):
Wattage = | Heat of vaporization (Btu/lb) x weight of mat'l vaporized/hr |
3.412 Btu/watt. hr |
Heats of fusion (solid to liquid) and vaporization (liquid to gas) are the heat energy required to change a lb. of the material from state to state at constant temperature.
6. Wattage to counteract liquid surface losses (if applicable) See Graph for loss rate of materials:
average wattage loss = | Total liquid surface area (sq. ft.) x loss rate at final temp. (watts/sq. ft.) |
2 |
7. Wattage to counteract losses from container walls, platen surfaces, etc. See Graph for losses from surfaces:
average wattage loss in heatup = | Total area of surface (sq. ft.) x Loss rate @ final temperature (watts/sq. ft.) |
2 |
IV. OPERATING WATTAGE REQUIREMENTS
Operating requirements should include some or all of the following plus some miscellaneous losses which should be estimated and included:
1. Wattage to counteract losses from open liquid surfaces: (See graph for loss rates of water and oils) Total liquid surface area (sq. ft.) x loss rate at operating temperature (watts/sq. ft.) = operating wattage losses
2. Wattage to counteract container or platen surface losses, (See graphs for loss rates) Total liquid surface area (sq. ft.) x loss rate at operating temperature (watts/sq. ft.) = operating wattage losses
3. Wattage required to heat material transferred in and out of the system (Metal dipped in heated tanks, air flows, make-up liquids, etc.)
wattage to heat material = | [Weight of material heated per hour (Lb/hr)] x [Sp. ht. (Btu/LB �F)] x [Temperature changes of material (�F.)] |
3.412 Btu/watt. hr |
4. Heatup of racks or containers, etc. transferred in and out of the system:
wattage to heat equipment = | Weight of items (Lb/hr) heated/hr. x Sp. ht. of material (Btu/LB �F) x temperature change (F) |
3.412 Btu/watt. hr |
5. Melting or vaporization of material requirements should also be included if applicable. Drying, or evaporation, requires use of the heat of vaporization.


