Two basic heat energy requirements should be considered in the sizing of heaters for a particular application.

All process heating applications benefit from a thorough calculation of heat requirements. The larger of the two heat energy values -- startup heat or operating heat -- will be the wattage required for the application.


When sizing heaters for a given application, two heat energy calculations should be performed: startup heat and operating heat. Taking these steps before you submit the purchase order can help ensure that the heater you receive will meet the needs of your process.

Startup heat is the heat energy required to bring a process up to operating temperature. Startup heat requirement calculations that include a material change of state should be calculated in three parts:
  • Heat requirement from ambient temperature to change of state temperature.
  • Heat requirement during change of state (latent heat).
  • Heat requirement from change of state temperature to operating temperature.
Startup heat requirements will include one or more of the following seven calculations, depending on the application.

Wattage required to heat material:

Wattage required to heat container or tank:



Wattage required to melt a solid to a liquid at constant temperature:

Heat of Fusion (Latent Heat). The amount of heat required to change one pound of a given substance from solid to liquid state without change in temperature is termed the heat of fusion. It requires 144 BTU to change one pound of ice at 32oF (0oC) to one pound of water at 32oF (the heat of fusion of ice being 144 BTU per pound).

A change of state usually is accompanied by a change of specific heat. The specific heat of ice is 0.5 while that of water is 1.0.

To calculate the wattage required to change a liquid to a vapor state at constant temperature:

Heat of Vaporization (Latent Heat). The amount of heat required to change one pound of a given substance from liquid to vapor state without a change in temperature is termed the heat of vaporization. It requires 965 BTU to change one pound of water at 212oF (100oC) to one pound of steam at 212oF.

To calculate the wattage to counteract liquid surface losses:

To calculate the wattage to counteract surface losses from container walls, platen surfaces, etc.:

Operating Heat

Operating heat is the heat energy required to maintain the desired operating temperature through normal work cycles. The larger of these two heat energy values (startup heat or operating heat) will be the wattage required for the application.

A safety factor usually is added to allow for unknown or unexpected operating conditions. The safety factor is dependent on the accuracy of the wattage calculation. A figure of 10 percent is adequate for small systems closely calculated but 20 percent additional wattage is more common. Safety factors 25 percent to 35 percent should be considered for larger systems with many unknown conditions existing.

Operating heat requirements will include one or more of the following four calculations. Also, any additional losses particular to the application should also be estimated and included.

Wattage to counteract losses from open liquid surfaces:

Wattage to counteract container or platen surface losses:

Wattage required to heat material transferred in and out of the system (for example, by metal is dipped in heated tanks, airflows, or makeup liquids):

Heatup of racks, containers, and other non-product mass transferred in and out of the system:

The ratio of the amount of heat required to increase the temperature of one pound of any substance by one degree to the amount of heat necessary to increase one pound of water is known as the specific heat of that substance. The heat necessary to increase the temperature of all other substances has been referred to water as a standard.