Find out why the watt density rating of the heat source is so important when designing or troubleshooting a thermal system.
In the design of a thermal system -- and in the design of an electric heating element -- watt density is one of the most important considerations. The ability of the heater to dissipate heat from the resistance element to the heater sheath and then from the sheath to the process is determined by watt density. Too high a watt density can result in:
- Failure of the heater.
- Damage to the material being heated.
- Damage to the equipment or other components.
Watt density is the rated wattage per unit of the heated surface area, most commonly expressed in W/in2 or W/cm2. Each electric heater product has unique watt density characteristics and can range from less than 5 to more than 300 W/in2 (less than 1 to more than 46 W/cm2).
Heaters manufactured using a sheath-reduction process have higher watt density capabilities than other heaters. Swaging or roll reducing compacts the heater components to a rock-hard mass. Electrical insulation and heat transfer are at practical maximum capabilities. Tubular heaters and most insertion heaters are manufactured using this compaction process. Some band and strip heaters use a combination process that embeds the resistance element in the refractory material. High temperature processing then blends and solidifies the element and the refractory material. Known generically as mineral-insulated, band and strip heaters of this design have higher watt density and temperature capabilities than others. Both manufacturing processes will transfer heat from the resistance element to the sheath efficiently, allowing lower resistance-element temperatures and long heater life. Mica heaters and silicone rubber flexible heaters, among others, have much less watt density capability.
When determining manufacturing specifications of electric heaters, considerations include the watt density of the internal resistance element. Consistently extreme internal temperatures and wire oxidation result from high watt densities. Attention is given to the watt density from inside the heater to outside the heater. After that, those responsible for applying the heater to the process must be certain that heat is dissipated from the heater throughout the thermal system. A combination of an inappropriately high watt density and a heated medium with a low conductivity rate will cause the heat being generated to not be transferred or dissipated from the heater sheath. The heater will overheat and fail. How can this be prevented?
Avoid Excessive Watt Densities
Watt density is a process-specific variable. Certain materials have watt density limitations. Figure 1 shows recommended allowable ratings for various materials, temperature conditions and application considerations. Some materials such as water, vegetable oils and metals have high conductivity rates. The heat generated travels quickly from the element and through the medium. These materials can be heated at relatively high watt densities. Other materials such as sugar syrups, most gases, fuel oils, lubricating oils and hydraulic fluid, which have low conductivity rates, must be heated at low watt densities.
A major concern for any heat processing operation using heaters is to dissipate the heat generated by the element. This is particularly true for specific applications, including liquid immersion heating. Excessive watt densities cause several problems in liquid immersion heating:
- At higher than recommended watt densities and sheath temperatures, thermal degradation causes heat transfer fluids and hydrocarbon oils to break down. Deposits form on the element, increasing to where a barrier to the flow of heat results in overheating.
- Mineral deposits contained in water supplies build up on the heater sheath at high watt densities, insulating the heater sheath. As a result, heat transfer is prevented.
In addition, in some applications, lower watt density is required to minimize unavoidable process-related effects. For example, when heating corrosive materials, heat is a catalyst in a chemical reaction. Lowering watt densities and sheath temperatures will lessen the severe effects of heating corrosive materials.
Matching watt densities to material limitations and other application dynamics such as temperature and flow rates will ensure satisfactory heater performance. Generally, the lower the heater watt density the better.
Add Wattage to Handle Large Temperature Gradients
Temperature gradients exist in every thermal system. Some gradient is necessary for heat flow, but large differences will cause problems with control and higher-than-necessary heater temperatures. As the distance from the heat source to the ambient areas increases, temperatures decrease. To compensate for losses, additional wattage often is added to the system.
An initial step when designing a thermal system is to spread the wattage requirement over several heaters (figure 2). If 15 kW are needed, one heater might be 48 W/in2. By using two heaters at 7.5 kW each, the watt density would be reduced to 24. By using three heaters at 5 kW each, watt density is reduced to 16. Several smaller heaters also will distribute the heat better than one large heater. Agitating the liquid also will dissipate and distribute the heat better. In fact, in many applications, increasing the turbulence or flow will allow the process to be heated at higher watt densities. Sheath temperatures will be reduced. Insulation will lower the wattage requirement and is recommended wherever possible.
Increasing heater dimensions also will reduce watt densities. Consider an application using 0.375 x 12" cartridge heaters at 1,000 W each, with a watt density of 74 W/in2. If space is available, increasing the diameter to 0.5" lowers the watt density to 55 W/in2. Obviously, lengthening the heated area, if possible, also will reduce the watt density.
Extremely high sheath temperatures can be produced easily in forced convection air heating applications. Empirical charts and graphs in many electric heating element catalogs show the correlation between sheath temperature, air velocity, process temperature and watt density. Lower watt densities will result in lower element temperatures and longer life. By substituting finned tubular or finned strip heaters in the same space, wattages can be increased substantially. In some applications, it makes sense to call in a professional for assistance: Air and gas heating in pressurized ducts or circulation heaters require the density of the compressed gas and the mass velocity of the flow to be included along with watt density and other considerations.
Heater Fit Matters
In conductive heating, where the heat source is placed in a drilled or reamed hole, watt density also is important. The hole into which a cartridge-type heater is inserted typically is reamed to the nominal diameter of the heater. Insertion heaters are normally 0.002 to 0.006" undersize to ease installation and removal. Fit tolerances, or the difference between the maximum inner diameter of the hole and the minimum outer diameter of the heater, can range from 0.040 to 0.001". The exact dimension to use is derived from allowable maximum watt density graphs (figure 3). The higher the temperature and watt density, the greater the requirement for metal-to-metal contact between the outer diameter of the heater and the inner diameter of the hole. This is to be certain that the heat generated is dissipated throughout the process and heater temperatures are as low as possible.
The discussion has been purposely limited to conduction and convection heat transfer. Watt density is equally significant in radiant heat transfer applications. When designing a new thermal system, or troubleshooting problems in an existing system, watt density of the heat source is of critical importance and should never be overlooked or treated as an afterthought.
Editor's Note:After this article was published, Ogden Manufacturing Co. was acquired by Pittsburgh-based Chromalox. To learn more about Ogden brand products and advice on watt density from Chromalox, visit www.chromalox.com.