Cartridge Heater Hole Size & Bore Fit Calculator
The correct hole size for a cartridge heater balances two requirements: a bore large enough to insert and remove the heater, and tight enough that the air gap does not starve heat transfer. This calculator sizes both, using Tempco’s published watt-density and fit data for Hi-Density Cartridge Heaters. Enter your operating temperature, heater dimensions, and wattage to find the minimum hole, the recommended fit, and a ream target.
Inputs
Highlighted fields are yours to fill in.
The process operating temperature. Each Fig. 1 curve is a different temperature.
For 480 V heaters, the fit is read on the curve 200°F above the operating temperature.
Actual measured OD, not nominal. Exact values by size are in catalog Section 2.
0.002 in is a conservative default. Tempco engineering often uses 0.001 in.
Heated length is not overall length. Standard Type N deducts 3/8 in (lead) + 1/4 in (disc). A 1 in heater has only about 1/2 in heated. Enter heated length directly, or check the box to compute it.
Result
Fig. 1 assumes an oxidized mild steel block with the thermocouple 1/2 inch from the heater. For other block materials or sensor locations, the values must be extrapolated from the material’s thermal conductivity. Consult Tempco for those cases.
Minimum hole is the smallest the heater physically fits into: ream to this for the tightest practical fit. Maximum hole is the largest recommended for good heat transfer at this operating temperature and watt density.
Reference: verified fit values from Fig. 1
Recommended fit (inches) at each watt density, for an oxidized mild steel block with the thermocouple 1/2 in from the heater. Verified by Tempco engineering from catalog Fig. 1 (page 2-5). A dash means that point runs off the chart. The calculator interpolates between these points and does not extrapolate beyond them.
How proper hole size is determined
Sizing a cartridge heater bore is a heat-transfer decision, not just a clearance decision. The hole must satisfy two limits at once: a physical minimum so the heater can be installed and removed, and a thermal maximum so the air gap stays small enough to move heat out of the heater.
The physical minimum hole
The smallest hole that will still accept the heater accounts for the heater’s maximum outside diameter, its maximum camber (the slight bow along its length), and a small insertion clearance so the fit is not forced. For a 6 inch, 1/2 inch nominal Hi-Density Cartridge Heater with an actual diameter of 0.496 inch (±0.002 inch), a maximum camber of 0.005 inch, and 0.002 inch of clearance, the minimum hole is 0.505 inch. A hole below that makes installation difficult or impossible.
The thermal maximum: fit
A larger hole is easier to load, but every extra thousandth of an inch is an air gap, and air is an insulator. The hotter and higher the watt density, the smaller that gap must be. Fit is the diametral air gap between the heater and the bore, and the maximum recommended fit depends on both the operating temperature and the watt density. Higher temperature and higher watt density both call for a tighter fit to keep the internal resistance wire within its safe limits.
- A 0.496 inch OD, 6 inch heater rated 800 W, with a 5 inch heated length, runs at 600°F in a mild steel block.
- Watt density is 800 ÷ (5 × 0.496 × π) = about 103 W/in².
- At 600°F and 103 W/in², the maximum recommended fit is roughly 0.021 inch, so the maximum hole is about 0.515 inch.
- With a 0.505 inch minimum, reaming to about 0.506 inch sits near the minimum and gives excellent heat transfer.
When the required fit is not practical
At high temperature and high watt density, the recommended fit can be tighter than a heater’s camber physically allows. When that happens, there are four proven ways to bring the design back into range:
- Centerless grind the heater to a tighter diameter tolerance. Centerless grinding reduces the diameter, not the camber; how much the camber can be reduced must be discussed with Tempco.
- Lower the wattage and accept a longer heat-up time.
- Lower the wattage per heater and add more heaters to hold total output.
- Increase the heater size to spread the wattage over more area, lowering watt density.
A note on long heaters. This chart and calculator are intended for relatively short, rigid heaters. Long heaters can have camber well beyond the chart, but they also flex, so they can still seat tightly in the bore despite that camber. For long heaters, treat the chart as a guide only and consult Tempco Engineering for the right fit.
Frequently asked questions
What hole size should I use for a cartridge heater?
The hole must be large enough to insert and remove the heater, yet tight enough that the air gap does not starve heat transfer. Start from the minimum hole (maximum outside diameter plus maximum camber plus about 0.002 inch clearance), then confirm the fit is no looser than the operating temperature and watt density allow. For a 1/2 inch nominal Hi-Density Cartridge Heater with a 0.496 inch actual diameter, the minimum hole is about 0.505 inch.
What is fit in a cartridge heater installation?
Fit is the diametral air gap between the heater and the bore. Air is an insulator, so a looser fit reduces heat transfer and drives the internal element hotter. The higher the operating temperature and watt density, the tighter the fit must be to keep the heater within its safe limits.
How do I calculate cartridge heater watt density?
Watt density in watts per square inch equals the heater wattage divided by the heated surface area, where heated surface area is the heated length times the heater diameter times π. Heated length is the overall length minus the unheated cold sections at each end.
Why do cartridge heaters fail in high-temperature applications?
Above roughly 1000°F, excessive watt density is the leading cause of failure. Too much wattage in too few heaters drives the internal resistance wire toward its melting point. Lowering watt density, tightening the bore fit, and selecting an oxidation-resistant sheath such as Incoloy® 800 extend heater life.
How can I make a cartridge heater easier to remove?
At high temperature the sheath oxidizes and can bond into the bore. Designing in a threaded removal bushing at the lead end lets a technician extract the heater with a wrench, and drilling a through hole where possible allows the heater to be pushed out from behind.
