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Tank Heaters
Screw Plug Immersion Heaters
Threaded into an NPT coupling on the tank wall. The simplest and least expensive permanent immersion heater for small and medium tanks.
What it is
Hairpin elements brazed or welded into a threaded plug. The plug screws into a matching NPT coupling on the tank wall or on a tank fitting, and the elements extend into the liquid. The terminal housing sits outside on the dry side.
It is the same idea as a flanged heater at a smaller scale and lower cost. Fewer parts, no gasket, no bolt pattern to match — just a thread size and an immersed length.
When it is the right choice
- Small to medium tanks under about 24 kW. Oil reservoirs, water tanks, small process tanks, parts washers.
- The tank already has a spare coupling. Install is a thread and sealant.
- Cost matters and the duty is modest. Cheapest permanent immersion mount.
- Service access is easy. Unthread, replace, rethread.
Where it runs into limits
Threaded joints are less forgiving than flanges at high pressure and at large diameters, and above roughly 24 kW there is not enough plug area to carry the elements at a safe watt density. Cross to a flange immersion heater at that point.
The other limit is chemistry. A screw plug in aggressive acid means a threaded penetration in a tank full of acid, which is a leak path you have to live with. For corrosive baths, an over-the-side heater or an external circulation loop keeps the penetration out of the picture entirely.
Specifications
| Specification | Range or option |
|---|---|
| Rating per unit | 0.5 kW to 24 kW |
| NPT plug sizes | 1", 1-1/4", 1-1/2", 2", 2-1/2" |
| Voltage | 120 V, 208 V, 240 V, 277 V, 480 V, 600 V |
| Phase | Single phase or three phase |
| Sheath materials | 316 stainless steel, 304 stainless steel, Incoloy 800, carbon steel, copper (clean water only) |
| Immersed length | Built to the tank — specify internal clearance |
| Element count | 1 to 3 hairpins depending on kW and plug size |
| Terminal enclosure | NEMA 1, NEMA 4X, NEMA 7 explosion proof |
| Thermostat options | Built-in adjustable thermostat with capillary bulb, or external RTD / thermocouple control |
| Options | Thermowell, high-limit cutout, stepped kW |
Common questions
What size NPT coupling do I need?
It follows the kilowatt rating. Roughly, up to about 6 kW fits a 1 inch plug, 6 to 12 kW typically needs 1-1/4 or 1-1/2 inch, and 12 to 24 kW usually needs 2 or 2-1/2 inch. The real constraint is fitting the elements in at a watt density the liquid can absorb, so send the duty and we will confirm the plug size.
Can a screw plug heater be used in a pressurized tank?
Within limits, yes, and it is common in small closed water and oil systems. State the design pressure and temperature on the inquiry. Above modest pressures and above roughly 24 kW, a flanged heater is the correct answer.
Is copper sheath ever a good idea?
Only in clean, treated water at low temperature. Copper conducts well and is inexpensive, but it corrodes in deionized water, in most chemical solutions, and anywhere chlorides are present. In doubt, specify 316 stainless or Incoloy.
Can it be installed from the bottom of the tank?
It can, but bottom mounting puts the element in the sediment layer and puts the electrical housing under the full liquid head. Side mounting near the bottom is the better practice. If the tank collects solids, an over-the-side heater positioned above the sludge line will last significantly longer.
Do you stock these or are they all built to order?
Common water and oil configurations move quickly. Anything specific to your tank — immersed length, sheath, voltage, enclosure, controls — is built to the order. Send the specifications and we will tell you the lead time with the quote.
Send the thread size and the tank duty
NPT size, immersed length available, liquid, volume, target temperature and voltage. Replacement of an existing unit only needs the nameplate and a photo.