Miniature Cartridge Heaters to the Rescue
- Emergency heating when an unanticipated need for additional heat is discovered late in development.
- Supplemental heating to tune heating processes for greater productivity.
- Fast response when rapid heating/cooling cycles are required.
- Equipment miniaturization.
Miniature cartridge heaters are constructed much like their larger (0.25" dia. and up) counterparts. A stainless alloy sheath encloses a compacted mass of thermally conductive electrical insulation. A high temperature resistor element is embedded in this insulation. When the power leads are connected to a voltage source, heat from the resistor is conducted through the insulation to the surface of the heater.
Miniature cartridge heaters typically have capabilities similar to their larger counterparts, generating temperatures to 1,400oF (760oC) and power densities to 500 W/in2 of sheath area. In fact, miniature cartridge heaters are designed to the same fit, temperature and watt-density parameters as full-size cartridge heaters. Manufacturers supply a single chart that applies to all sizes.
Miniature heaters, however, are designed specifically for service in tight spaces. With thin, flexible leads, miniature cartridge heaters require little space. Usually, they are mounted easily and often can tie into existing control systems.
A closer look at some of their most common uses may show you how miniature cartridge heaters can solve a problem in your process.
Emergency HeatThe size difference between standard and miniature cartridge heaters is critical when the need for supplemental heating is discovered. Because they are oriented into the part to be heated, little space is required to install the heater -- sometimes as little as 1/8" dia. by 1/2" deep. Installed in a close-fitting bore, they heat the part from the inside out. Often, they can be mounted close to the working surface of the part to be heated. They can generate wattages from just a few watts to several hundred and will operate on voltages from 6 to 120 V.
For example, a manufacturer of semiconductor processing equipment discovered that under some conditions, an outlet valve was failing to operate. An investigation showed that liquid nitrogen was freezing the valve mechanism, preventing it from closing. No space was available for a surface-mount heater, and the manufacturer was up against a delivery date. The solution was a 3/4" long, 15 W cartridge heater installed into the body of the valve. The power supply drew 24 V of local circuitry, and the machines were shipped on time.
Supplemental HeatingSupplemental heating also can be used to tune a heating process to achieve more uniform temperature or closer temperature control. While larger diameter heaters are needed to supply the majority of the heat required, their size may require them to be located a distance from the part to be heated. Distance creates thermal lag, especially in low conductivity metals, leading to control problems such as temperature overshoot and undershoot.
If the distance between heater and target area is great enough, an additional problem may arise. The large heater may have to be overdriven to push sufficient heat to the target area. This results in excessive operating temperatures at the heater, leading to shortened heater life. It also might cause undesirable heating of adjacent components and require thermal shielding.
A small supplemental heater, located closer to the target area, can boost working temperatures and alleviate these problems. An improved temperature profile is often an additional benefit.
Equipment MiniaturizationMost miniature heater designs offer several additional features to facilitate their use. Some models incorporate an internal thermocouple to provide sensing and control when space for a separate sensing element is not available. In fact, a simple 1/8" dia. thermocouple may be replaced by a heater/thermocouple combination, adding heating to sensing.
Hot-tipped heaters are effective where the area to be heated lies beyond the end of the heater. The heated tip may be located just under the skin of the surface to be heated, providing heat from a little as 1/32" away.
Miniature cartridge heaters allow shrinking of machine elements, which previously had to be arbitrarily large to accommodate a standard diameter heater. This is useful when processing small parts such as semiconductor chips. In fact, heated platens the size of a postage stamp now are possible. By taking advantage of the hot-tip feature, heated probes as small as 3/16" are available.
Rapid-Response ProcessesAdditionally, miniaturization can lead to significant benefits in systems where rapid heatup (and cooldown) are important such as thermal test equipment and soldering fixtures for electronic components.
The advantage lies in a significant reduction of thermal inertia as machine elements shrink in size. For example, a heated bar with a 3/8" square cross-section (to accommodate a 1/4" dia. heater) may be reduced to 3/16" square and accept a 1/8" heater. The reduced cross-section cuts the mass and thermal inertia of the bar by 75 percent. However, the size of the heater and its corresponding power output are reduced only by 50 percent. As a result, the ratio of input power to inertia is thereby doubled and heatup time is halved.
Alternatively, a heated platen with a relatively high profile -- 3/8" high by 1/2" wide, for example -- can be reduced to a thinner design such as 3/16" by 1/2". Two miniature heaters then can be mounted side by side, replacing a single high profile heater. Again, the ratio of input power to inertia is doubled.
One semiconductor manufacturer was able to scale the size of a chip soldering fixture to 1/2" wide by 3/4" long. Heat-up time between from ambient to the melting point of the solder reduced to 20 sec. Temperature oversshoot due to the rapid rise in temperature was handled by reducing the controller setpoint. The melting point of the solder was reached just as the impulse began to subside. Other manufacturers employ forced cooling to arrest the temperature rise and provide rapid cooldown, thus shortening the overall processing cycle.
Beyond these unique applications, miniature cartridge heaters have been the heater of choice for small heating systems. They are used to heat sample materials in analytical instruments such as chromatographs and spectrometers. Small mixing heads employ them to control material viscosity and flow rate. Could they help your process?
Sidebar: Tips for Effective ApplicationUse High Density Heaters. Many manufacturers offer low density heaters, suitable for low temperatures or low power applications. Be sure to specify a high density model when ordering.
Keep Lead Exit Temperatures Below 400oF (204oC). Lead insulating sleeves degrade rapidly above this temperature, leading to heater failure. Special constructions are available for high temperature service. Consult with the manufacturer.
Follow Manufacturer's Recommendations Regarding Maximum Power Density. When compared to the output of larger cartridge heaters, 25 W might not seem like much power but may represent a very high watt density on the surface of a miniature heater.
Strive for Balance in Rapid-Response Systems. Overwhelming the inertia of a system to gain a rapid heatup can result in extreme temperature overshoot. Experimentation may be necessary to find a compromise between rapid heatup and temperature stability.