An electric process heater (EPH) is any heater that has not already been defined by its application. There are two types of EPHs: Immersion and Circulation. The immersion heater is immersed in the process fluid and heats by natural convection. The circulation heater is inside of a pressure vessel or duct and heats by forced convection. The Gaumer Co., Houston, estimates there are approximately 10,000 new EPH designs each year in North America.
If you are one of the people designing or selecting an EPH, then you should take note of the following suggestions.
1. An EPH will continue to get hot until all of the heat is released. Therefore, you should consider how the process would cool the heater bundle in all flow scenarios, including no flow. In order to do this, you need to know the maximum tolerable film temperature of the process fluid and how to calculate the film temperature for natural flow, laminar and turbulent flow with and without radiation, and baffled flow. This will avoid coking issues in liquids and will avoid shortened heater life in high temperature gas applications. Your film temperature calculations should also consider the heater vessel temperatures to insure you don't exceed the temperature limitations of the steel vessel or the paint.
2. Over-temperature protection is the most important safety feature of an electric process heater. If multiple heaters are being installed, it is very important that the correct over-temperature thermocouple (located on each heater bundle) is connected to the correct over-temperature controller. It is also important that the over-temperature controller has a low temperature shutdown. It is not uncommon for over-temperature thermocouples to be wired backwards in the field. Having a low temperature shutdown will not allow the heater to operate when incorrectly wired.
3. An EPH should never have a remote reset. Some circumstances require an auto reset of a high condition, but the high-high condition should always require an operator to visit the control panel before resetting the heater.
4. An EPH should always have an adequate seal. The compressed magnesium oxide (MgO) power that surrounds the NiChrome wire inside of the heater element has excellent thermal conductivity and dielectric strength, but is hygroscopic in nature. That means any MgO that is exposed to air will absorb water molecules and weaken the dielectric strength of the dry MgO. A properly designed seal will prevent this problem for the life of the heater bundle.
5. Heat flux selection is not straightforward. Some instances call for a higher watt density to fit a heater bundle into a smaller heater vessel and thus force a higher process velocity that, in turn, cools the heater bundle better than a lower watt density bundle in a larger heater vessel.
Many of you who are reading this are already familiar with the consequences of these design mistakes. Those who are not familiar with EPH design should take time to fully understand the points above and also learn all of the other design details required for electric process heater selection.
SIDEBARUnderstanding these important facts about electric process heaters will go a long way in helping you design and choose the best unit for your application. Read the entire article for more details.
Facts in Brief
- An electric process heater continues to get hot until all heat is released.
- Over-temperature protection is the most important safety feature of an electric process heater.
- An electric process heater never should have a remote reset.
- An electric process heater always should have an adequate seal.
- Heat flux selection is not straightforward.