CAN YOU MAKE IT IN MY SIZE? HOW TEMPCO BUILDS CUSTOM CARTRIDGE AND BAND HEATERS TO FIT YOUR EQUIPMENT

A Catalog Table Is a Starting Point, Not the Limit

A catalog table lists the sizes a manufacturer builds most often. It does not mark the edge of what can be built. If the heater you need is a little shorter than anything on the page, or your barrel has a thermocouple port right where a standard band puts its heat, the table is the wrong place to stop looking.

“Can you make it in my size?” comes up often on calls to Tempco. A caller put the common version of this request plainly:

“If possible, I’d like to get them to be shorter so that they could fit inside the actual housing as opposed to having to flare the housing out to fit the standards.”

For cartridge heaters, the catalog answers it in one line: for sizes and ratings not listed, Tempco will design and manufacture a Hi-Density® cartridge heater to your requirements. The band heater pages say the same thing about mica insulated designs. The catalog shows standard specifications and the most popular designs, and many applications need something non-standard.

That doesn’t mean any dimension is free. Every construction has physical limits, and changing one dimension changes how the heater runs. Below: what you can change, where the limits sit, and what to send so the first quote is the right one.

Custom Cartridge Heaters: What “Your Size” Can Mean

Standard Hi-Density® cartridge heaters run from 1/4 to 1 in. (6.4 to 25.4 mm) nominal diameter. Maximum length depends on diameter: 36 in. (914 mm) on the 1/4 and 5/16 in. sizes, rising to 72 in. (1829 mm) on 5/8 in. and larger. Here is what moves inside and around those ranges.

• Smaller diameters. Miniature heaters in 1/8, 5/32 and 3/16 in. nominal diameters (0.122, 0.153 and 0.184 in. (3.10, 3.89 and 4.67 mm) actual) are made to order, from 1 in. (25.4 mm) long for the 1/8 in. diameter and 1-1/4 in. (31.8 mm) for the 5/32 and 3/16 in. diameters, up to 12 in. (305 mm). They are rated to a maximum temperature of 1200°F (649°C). Larger Hi-Density® sizes carry a 1400°F (760°C) maximum, with a maximum recommended sheath temperature of 1200°F (649°C) for a standard heater; for operation up to 1400°F (760°C), an Incoloy® 800 sheath is available.
• Metric diameters. If your equipment was designed in millimeters, Hi-Density® heaters are also built in metric nominal diameters from 6.5 to 20 mm.
• Tighter diameter tolerance. Standard tolerance is ±0.002 in. (±0.051 mm) through 5/8 in. and ±0.003 in. (±0.076 mm) on 3/4 and 1 in. Where precise temperature control and heat transfer matter, heaters can be centerless ground to ±0.0005 in. (±0.013 mm).
• Cold sections where you need them. Tempco can put unheated sections anywhere along the length. This matters when part of the heater projects out of the platen or mold. A heated section with nothing around it to absorb the heat gets hotter than the rest of the sheath and fails early.
• Electrical configuration. On 1/2 in. and larger diameters, dual voltage, three-phase, dual circuits and up to three heat zones are available. A 3/8 in. heater can take dual voltage or multiple heat zones. Some of these options may require a larger diameter transition area at the lead end.
• Sheath material. The standard sheath is 321 stainless steel, with high-temperature strength up to 1200°F (649°C). Incoloy® 800 is available for temperatures up to 1400°F (760°C) or for corrosive immersion heating. Ask about other materials. Miniature heaters use Type 304 stainless steel, with Inconel® 600 available on the 1/8 in. size.
• Terminations. There are more than 40 standard termination styles. Give this choice real attention: Tempco’s catalog names the wrong termination as the primary reason for heater failures.

The special-applications pages of the catalog show how far a cartridge heater can move from standard. One example has a built-in thermal fuse and ground wire for X-ray processing equipment. Another, for medical diagnostic instruments, has an integrated heat sink and thermal fuse. A third was designed to run continuously at 1202°F (650°C) with a built-in isolated thermocouple. Cartridge heaters can also be formed into complex shapes.

If your design has to clear a review, the paperwork can be specified too. On custom Hi-Density® heaters, Tempco can perform dye penetrant testing of weld joints with certified test reports, hydrostatic pressure testing of heaters with pipe fittings, and dielectric withstand (hipot) testing up to 5,000 volts with leakage current measured in microamps. Heaters can be serialized, with test reports sent with each shipment if required.

Custom Band Heaters: What “Your Size” Can Mean

A band heater is sized to a cylinder: inside diameter, width, and how it goes on (slipped over the end, opened up and closed around the barrel, or clamped on as two halves). Each Tempco construction covers a different envelope.

• Mi-Plus® mineral insulated bands run 1 to 14 in. (25.4 to 355.6 mm) inside diameter in one piece and up to 25 in. (635.0 mm) in two-piece construction. Above 25 in. (635.0 mm), Tempco recommends three or four segments. With standard brackets, widths run 1 to 8 in. (25.4 to 203.2 mm). Below 4 in. (101.6 mm) diameter, the maximum width is twice the diameter. If you need a non-standard width or a tighter tolerance, ask. Mi-Plus® bands are rated to 1400°F (760°C).
• Duraband® mica bands start at 7/8 in. (22.2 mm) inside diameter and 3/4 in. (19.1 mm) width. Recommended segment layouts reach 96 in. (2438.4 mm) inside diameter in eight segments. The standard sheath is rated to 900°F (482°C).
• Ceramic bands are fully flexible. A single wider ceramic band can stand in for several narrow or two-piece bands.

Mica insulated construction is where a band heater can stop looking like a band. The catalog’s custom-design page shows boat-shaped and long conical two-piece heaters, a fully flexible one-piece heater that varies in width, a two-piece rectangular heater with a corner radius and cutouts, an arch-shaped heater for a special die, ring and disc heaters, and a dual circuit unit with two heaters in one envelope.

Smaller adjustments count too. If a probe or thermocouple has to pass through the gap between the band’s ends, specify a larger gap when you order.

The Catch: Every Change Moves the Watt Density

Watt density is the wattage a heater dissipates per square inch of heated surface. Change the size and keep the wattage, and the watt density changes with it.

Cartridge Heater Watt Density Formula

For a cartridge heater:

Watt density (W/in²) = wattage ÷ (heated length × diameter × 3.1416)

Make the heater shorter or slimmer at the same wattage, and every square inch of sheath has to carry more watts. The resistance wire inside runs hotter, and heater life depends on the actual temperature of that wire, not on the process temperature you set. Shortening or slimming the heater is often possible. It just has to be done with the watt density in view.

Two other factors set the ceiling. The higher the operating temperature, the lower the maximum recommended watt density. And the tighter the heater fits its hole, the higher the watt density it can handle. That is why Hi-Density® cartridge heaters carry a maximum of 100 to 300 W/in² (15.5 to 46.5 W/cm²) rather than a single number: it depends on heater size and operating temperature.

If the size you need pushes the watt density close to the maximum, the catalog lists three corrections:

• Increase the number, diameter or length of heaters.
• Lower the total wattage, which may increase the heat-up time.
• Get a tighter fit in the hole.

Band Heater Watt Density: Holes and Cutouts

On band heaters, the same arithmetic applies to holes and cutouts. Every hole removes heated area, so the watts that would have gone there land on the area that’s left. For a Mi-Plus® band:

Watt density (W/in²) = wattage ÷ [(3.14 × band ID × band width) − cold area]

The cold area is 1 in. (25.4 mm) × width for a one-piece band, 1-1/2 in. (38.1 mm) × width for a one-piece expandable band, and 2 in. (50.8 mm) × width for a two-piece band. For each hole or cutout, add (hole size + 1/2 in. (12.7 mm)) × width.

Here is how much one small hole moves the number. This is an illustration of the formula, not a sizing recommendation. Take a one-piece Mi-Plus® band, 4 in. (101.6 mm) ID and 2 in. (50.8 mm) wide, rated at 1,000 W:

One half-inch (12.7 mm) hole raised the watt density by about 9 percent. The next step is to check the result against the maximum allowable watt density chart for that diameter and operating temperature. Then derate: reduce the chart value by 20 percent for bands wider than 3 in. (76.2 mm), and by 25 percent if the band runs under an insulating shroud. If the calculated figure is still too high, use more bands or lower the wattage.

The temperature and watt density figures in this article are catalog capability ratings, not warranty terms. Tempco warrants that at the time of shipment, the products manufactured by Tempco will be free from defects in material and workmanship and will be in conformity with the applicable printed or written specifications. The warranty period is one (1) year from shipment from Tempco’s plant.

What to Send for a Custom Heater Quote

The fastest custom quote starts with complete information. Lists like these keep the first exchange from turning into five.

Replacing an old heater with no readable part number? Send a photo of the heater and any nameplate or tag, along with measured dimensions and the watts and volts if you can find them.

Custom heaters are built to order, so ask for the lead time on your specific design when you request the quote.

Inconel® and Incoloy® are trademarks of the Special Metals Corporation group of companies.