Cartridge heaters are most frequently used for heating metal parts by insertion into drilled holes. For easy installation, the heaters are made slightly undersize relative to their nominal diameter.
The majority of applications do not require maximum watt/in². Use a watt density only as high as needed. Take advantage of the safety margin provided by using ratings less than the maximum allowed. Select space heaters for most even heat pattern rather than the highest possible wattage per heater.
At medium Watt densities, general purpose drills are usually adequate for drilling holes. Typically, these result in holes .003" to .008" over the nominal size of the drill, resulting in fits of .010" .015". Of course, the tightest fit is desirable from a heat transfer standpoint, but somewhat looser fits aid in installing and removing cartridge heaters, especially long ones. Holes drilled completely through the part are recommended to facilitate removal of the heater. After drilling, clean or degrease the part to remove cutting lubricants.
This heating application may usually be easily accomplished by using cartridge heaters, although in some applications strip or tubular heaters can be inserted in grooved slots in the metal. When cartridge heaters are used, it is essential that the following factors are checked to insure that the heater will experience the longest possible life and provide sufficient heat for the job: 1. Sheath watt density. 2. Proper fit of cartridge heater in machined hole. 3. Provisions made to protect heater from contamination from oil, oil vapors, etc. 4. Sufficient kW is installed to accomplish work and make up for heat losses from the flat surface. The maximum permissible sheath watt densities for alloy sheath cartridge heaters at a given desired temperature on the metal are given by Figure D-1. This curve gives watt densities for various fits using CIR cartridge elements. Figure D-2 is useful for using C series elements. When cartridge heaters are installed in a drilled hole, the hole should be drilled to the nominal diameter of the heater. Cartridge heater diameters are actually .003" to .005" smaller than the nominal diameter. This allows for easy installation when cold, but upon heating the cartridge heater expands for a snug fit and excellent heat transfer. When cartridge heaters are used in plastic forming dies, extruders, etc., care must be taken to protect the heater from possible contamination entering through the terminal end. Standard end opposite terminal construction for standard cartridge heaters is a positive weld. Special moisture and abrasive resistant terminal construction is available and where moisture or contamination problems are present, hermetic seals can be supplied (see pages D-15 and D-16). CARTRIDGE HEATERS A plastic forming operation requires 2 lb. of plastic to be processed per hour; the plastic has a specific heat of 0.45 BTU/lb/°F and reaches a pliable state at 300°F. Two platens, each weighing 245 Ibs. and measuring 24 in. long x 12 in. wide x 3 in. thick, must be preheated to 300°F in 1 ⁄2 hour. The platens are not insulated. The steel platens have a specific heat of 0.12 BTU/lb°F. Heat-up is accomplished with the platens closed. In addition, losses encountered during operation (from opening and closing the platens) are negligible. Room ambient is 70°F. The total exposed surface area is considered to be 7ft2 . kW = WT x Cp x T 3412 x H where WT = weight of material to be heated Cp = specific heat (BTU/Ib/°F) T = temperature (°F) H = heat-up time in hours 3412 is conversion factor of BTU to kWh kW REQUIREMENTS FOR INITIAL HEAT-UP kW = 245 x 2x 0.12 x (300-70) 3412 x 1 ⁄2 (hr) = 13524= 7.93 1706 Losses during heat-up are 150 w/ft2 at the operating temperature of 300°F. kW = 7.0(ft2 ) x 150 (W/ft2 ) 1000(W/kW) Average loss = 1.05= .53kW (Note 1) 2 Total reqd. = 7.93 + .53 = 8.46 8.46 x 1.2(safety factor) = 10.15kW OPERATING REQUIREMENTS kW = 2(lbs) x 0.45 x (300-70)°F 3412 x 1(hr) = 207= 0.061 3412 kW = 7.0(ft2 ) x 150 (W/ft2 ) 1000(W/kW) = 1.05 Total reqd. = 0.061 + 1.05=1.111kW 1.11 x 1.2 = 1.33 kW
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