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.
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:
To calculate the wattage to counteract liquid surface losses:
Operating HeatOperating 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:
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.