Follow these guidelines to improve heat transfer and lengthen life.
Process Systems Product Manager
St. Louis, Mo.
Most engineers don't give much thought to the heaters in their designs — unless they fail, require too much maintenance, or cause other headaches. The fact is, heaters play a vital role in many processes, so malfunctions can easily snowball into much larger problems.
Fortunately, following a few simple guidelines reduces the likelihood of heater-related downtime, improves efficiency, cuts maintenance and, ultimately, lowers operating costs. Here are 10 ways to improve a heater's performance and service life.
1. Guard against contamination.
Contamination is the most frequent cause of heater failure. As heaters expand and contract during thermal cycling, they often draw in organic or conductive materials through the area near the terminal ends. This can lead to arcing failure between individual heater windings, or between heater windings and the electrically grounded outer case. Dirt and debris that collects at the terminal lead end of a heater can also cause electrical shorts between power pins. It's important to keep lubricants, oils, lowtemperature tapes, and processing materials from contacting the lead end of the heater. One solution is to use moisture seals such as silicone or epoxy.
2. Protect wire leads from excessive heat and movement.
Heaters with temperatures up to approximately 500°F (260°C) typically use standard fiberglassinsulated lead wire. Beyond this, leads require high-temperature wire or ceramic-bead insulation. An unheated section of the heater extending away from the system's heated region could place the leads in beneficially cooler temperatures.
For heaters in moving machinery, anchor the leads to prevent damage and specify a lead-protection option.
3. Size matters.
It might seem obvious, but heater size is important. Match a heater's wattage as closely as possible to actual load requirements to limit on/off cycling (see tip 6). A rule of thumb is to add at least a 10% safety factor above actual load requirements to the heater's power rating. For fitted parts, size holes and other features to snugly match those of the heater. A tight fit minimizes air gaps and reduces hot spots.
4. Ground the equipment.
Here's another no-brainer. It's just common sense and safe practice to electrically ground all equipment on which the heater is used. It protects machines and personnel in case of an electrical failure.
5. Regulate voltage.
Ensure the heater's rated voltage matches the voltage supply. Wattage increases (or decreases) by the square of the change in voltage to a heater. For example, a heater rated for 120 V and 1,000 W connected to a 240-V supply will generate four times the rated wattage output or 4,000 W. This will quickly lead to heater failure and may damage other equipment.
6. Prevent excessive cycling.
Excessively long switching cycles (approximately 30 to 60 sec on and off cycles, or longer) dramatically shorten heater life. These long cycles cause resistance wires to undergo repeated, large temperature excursions which, in turn, speed resistance-wire deterioration and lead to heater failure.
Eliminate this problem by using solid-state relays (SSRs) and programming the temperature controller to use a 1 or 2-sec switching time base. Better yet, use SCR controls with a zerocross variable time base. Not only will this virtually eliminate thermal cycling of the resistance wire, thus extending heater life, it also provides much better temperature control of the thermal system.
7. Ensure heated materials and heater are compatible.
This is absolutely critical to long heater life and healthy equipment. When heating solids such as metals, the operating temperature and heater-to-part fit determine sheath material and watt density. Carbon steels, aluminum, silicone rubber, and other materials are fine for lower temperatures, say a few hundred degrees. But as temperatures rise beyond this point, heater sheath materials are limited to galvanized and stainless steels and other hightemperature metal alloys. And as temperature increases, watt density must decrease accordingly. Otherwise, internal resistance wires will quickly oxidize and prematurely fail. Good heater-to-part fit ensures efficient heat transfer and prevents resistance wires from overheating.
When heating gases, the gas itself, operating temperature, and flow rate dictate sheath material and watt density. When heating hydrogen versus nitrogen, for example, hydrogen permits higher watt densities but requires Incoloy 800 sheaths, whereas 304-stainless steel generally works for nitrogen. Also, increasing flow and turbulence across the heater elements improves heat transfer and permits higher watt densities.
For liquids, the fluid and flow rates primarily drive heater materials and watt density. Water can easily handle 60 to 100 W/in.2 using a copper sheath, whereas a 50/50 water/glycol mix only handles 30 W/in.2 and must use steel sheathing.
8. Mount immersion-tank heaters horizontally near the bottom.
Installing heaters horizontally and near tank bottoms maximizes convective circulation. Vertical mounting is only advisable when space restrictions prohibit horizontal mounting. But regardless of orientation, it is essential that heaters are mounted above any sludge and debris in the tank bottom. Likewise, the entire heated length must be immersed at all times — which is one reason vertical mounting is often not recommended. Also, avoid placing heaters in restricted spaces that limit convective flow and could lead to free boiling or steam traps.
9. Prevent sludge buildup.
In liquid applications, minimize scale, coking, and sludge buildup on heater sheaths. They inhibit heat transfer if not periodically removed. This forces heater elements to operate at higher temperatures and leads to early failure. Also, make sure to keep silicone lubricant off the heated section. Silicone prevents "wetting" of the sheath by liquids, acts as an insulator, and possibly causes heater failure.
10. Ensure tight temperature control and safety-limit protection.
Matching the proper temperature controller to the heater and application is imperative for good performance and long life. Every application should, at a minimum, have a process-temperature sensor on or in the material heated and a temperature-limit sensor on the heater sheath. The process sensor should ideally be inserted or immersed into the material or snugly fit into a well in the fluid itself. For safety, use two separate control systems: one for processtemperature control and one for high-limit control. PID controls offer more-stable temperature control and faster response than on/ off switches or thermostats. The trade-off is that PID controls are often more expensive than on/off types and not necessary for applications that don't demand high accuracy.
Watlow Inc., watlow.com