Tom Lyons
Thick Film Products Manager
Watlow
St. Louis, Mo.
It’s no secret that off-the-shelf products are no longer adequate for many designs. More and more, customers demand engineered solutions tailored precisely to their needs. Industrial heaters are a case in point, where users increasingly look for special configurations that offer high efficiency and precise control. Conventional heaters often cannot meet these needs.
Fortunately there is a new solution to consider — thick-film heaters. These low-profile devices put heat exactly where it’s needed, offering greater control and the ability to precisely customize an application. They provide uniform heating at high temperatures and, with low thermal mass, respond as fast as 150°C/sec.
Heater basics
Thick-film resistance heaters consist of a sandwich of several different materials. These layers include a substrate, a glassy dielectric material, a resistor, and a final dielectric layer.
Substrate thickness is usually a function of the external loads and stresses the heater will experience in use. A thick-film dielectric ink is deposited on the substrate in an even layer, dried at 120°C, and fired to 850°C. During firing, the glass “frit” suspended in the ink softens and reforms into a continuous layer forming an electrical barrier. The current-carrying layer, on the other hand, contains electrically conductive suspended metal oxides. When fired, the glass and metal oxide form a conductive matrix. The circuit resistance and surface area determine heat output.
The glass-based film is nonporous and does not absorb water. However, because of the materials and processing required, this type of resistive heater tends to be more expensive than silicone-rubber or tubular heaters.
Several different dielectric/resistor combinations are available, usually based on operating temperature, cycle rates, and other such considerations. However, it is important that the base and substrate have similar thermal-expansion characteristics to prevent delamination. Some materials currently used include 300 and 400 Series stainless steels, Inconel and Incoloy, titanium, alumina, quartz, aluminum nitride, and beryllium oxide.
Thick-film heaters currently operate at a maximum temperature of 500°C with voltages to 480 V, three phase. Watt densities can reach 25 W/in.2 in radiant or open air applications, 65 W/in.2 in conductive or clamp-on applications, and 175 W/in.2 when immersed. Circuit resistances are held to a tolerance of ±5%.
Control considerations
Devices ranging from standard thermostats to sophisticated ramping and PID controllers can safely control these products, but proportional controls set for soft start-up optimize performance and maximize life. Feedback is available through a variety of sensing options which include temperature sensors epoxy bonded directly to the heated surface.
High-temperature alarms in the PID controller as well as independent limit controllers can prevent overtemperature conditions and premature heater failure.
Many thick-film heater applications require tight temperature control.In these cases, advantages come from a power-switching method called burst firing.
Burst firing provides more even heating than traditional controller output switching, and reduces temperature cycling that puts thermal stress on the heater elements. Burst firing, also called zero cross firing or cycle proportioning, provides a proportional output by switching the heater on and off 50 to 60 times/sec based on the ac power supplied to the system.
In air or liquid applications, cascade control uses temperature measurements at both the heater and the process to modify the traditional PID control algorithm for a more controlled ramp to the final set point.
Application options
Designers should consider thick-film heaters when more-conventional heaters cannot adequately solve a problem, but they are not for every application. Here are some important considerations.
• Thick-film heaters are best suited for applications that require high thermal uniformity across a heated surface. By optimally distributing the resistor layer, more heat can be put in a specific area.
• Where space is at a premium, thick-film heaters are an excellent alternative due to their low profile. This is especially true in applications where the voltage/wattage combination requires small wires.
• The film and heater substrate can be quite thin, so heat transfers throughout the component quickly after applying power. Low heater mass means fast response.
• Consider thick-film heaters when annual unit volumes range from small quantities to 10,000, but note that engineering and tooling charges can make small-quantity orders expensive on a per-unit basis.
• Avoid applications subject to excessive impact forces or other physical abuse that may crack the film.
Thick-film heaters are generally produced in two-dimensional shapes and cylinders, but most any other configuration can be made. Standard lengths or widths up to 20 in. can be printed in a single pass and heaters can be joined into larger assemblies. Independent testing shows that some specific thick-film materials experience minimal outgassing in vacuum conditions, making them well suited for semiconductor applications.
Due to the unique applications of thick-film heater technology to date, almost all products are custom made. Watlow is developing standard square, stainless-steel heaters for several markets. Standard lead time for prototypes is currently six or seven weeks. Once approved, three to five-week delivery is the norm.
A look at heaters and materials Thick film on stainless steel is a good, inexpensive thermal conductor. The combination provides a low-profile heater that can be custom fit to most any CAD-generated, 2D shape. Parts are fabricated through precision laser cutting or conventional machining techniques. Thick film on ceramics provides relatively low-thermal expansion, high-temperature tolerance, high dielectric strength, and dimensional stability, making them a preferred substrate for more-demanding applications. Thick film on quartz substrates provide a low-profile heater that can be fabricated or welded to a variety of shapes and sizes. Excellent corrosion resistance makes quartz a desirable substrate for aggressive wet chemistries or gases. Substrate materials are usually chosen based on factors such as processing concerns, operating temperature, and cycle rates. Stainless steel is the lowest cost option while ceramics have good thermal stability at a higher cost. 300 and 400 Series stainless steels provide good thermal stability at thicknesses above 20 gage on cylinder-shaped products. Stainless steel’s moderate price makes it a common substrate for thick-film heaters. Incoloy and Inconel are high-temperature alloys that resist stress cracking, sulfur attack, internal oxidation, scaling, and corrosion in a wide variety of industrial applications. Alumina (Al2O3) is the most widely used substrate material due to ready availability in a variety of purity levels, relatively low cost, and stable physical properties. The material is fairly easy to fabricate into a range of shapes while remaining strong at high temperatures. Aluminum nitride (AlN) has high thermal conductivity, making it an excellent choice where an application demands fast response or precise temperature uniformity. Aluminum nitride, however, is expensive to fabricate due to high-temperature atmosphere firing requirements and costly materials. Beryllium oxide (BeO) has the highest thermal conductivity available today and superior dielectric strength needed in some cases. This material is available only in small sizes, and safety can be a concern when dealing with toxic beryllium-oxide powder. Quartz is ideal for applications that involve aggressive chemicals, require thermal shock resistance, and low thermal conductivity. Quartz also offers excellent corrosion resistance. |