Screw Plug Immersion Heaters


Industrial screw plug immersion heaters provide heating solutions for freeze protection and winterizing, hot or warm water storage, parts cleaning, plating and finishing chemicals, waste oil, wash systems and numerous other thermal heating applications.

Durex Industries manufactures electric screw plug immersion heaters for industrial and process heating applications, providing economical and reliable heating for a large variety of tanks or vessels. Typical heating mediums include:

High Quality Heating Solutions with Plug Immersion Heaters

Durex process screw plug immersion heaters are constructed of hairpin bent tubular heating elements brazed or welded into threaded screw plugs. These heating elements are inserted through a threaded hole or through a threaded coupling attached to a tank or vessel for direct heating of liquids.

Electrical enclosures are used to protect the heater terminals, especially around moisture and corrosive areas. Thermostats or temperature sensors deliver precise temperature control to your thermal heating application.

Design Features:

Contact Durex Industries for Screw Plug Immersion Heaters

We have hundreds of industry standard screw plug immersion heater designs available with a 3-5 day lead-time. Durex also has over 40 years of experience with custom designed & engineered electric immersion heaters.

Our engineers can assist you in providing the best and most efficient heating solution required for you application requirements.

Buy screw plug immersion heater standard designs directly from our online store. Quick ship!

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Screw Plug Immersion Heaters - NEMA 1 Enclosures

Industry standard designed screw plug immersion heaters with general purpose NEMA 1 sheet metal terminal enclosure. 2" & 2 1/2" NPT sizes available with no thermostat or thermostat ranges of 0-100°F, 60-250°F and 100-550°F.

3-5 Day Lead Time


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Screw Plug Immersion Heaters - NEMA 4 Enclosures

Industry standard designed screw plug immersion heaters with heavy duty NEMA 4 moisture resistant cast iron terminal enclosure. 2" & 2 1/2" NPT sizes available with no thermostat or thermostat ranges of 0-100°F, 60-250°F and 100-550°F.

3-5 Day Lead Time

Screw plug immersion heater specifications and options

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Plug Orientation with Thermowell



Plug Immersion Heater Specifications


Plug NPT Size "C" Min. Insertion Hole Dia. "D" Min. Unheated (Cold) Length Element Diameter Thermowell Max T-Stat Bulb or Sensor Dia. Number of Elements Max. Voltage
1" 1.25" (32mm) 1 1/8" (29mm) 1" (25mm) 0.315" (8mm) 1/4" (6.4mm) 1 480
1 1/4" 1.25"
1 3/8" (35mm) 1" (25mm) 0.315" (8mm) 1/4" (6.4mm) 1 or 2 480
2" 1.88" (48mm) 2 1/4" (57mm) 2" (50mm) 0.475" (12mm) 3/8" (9.5mm) 2 or 3 600
2 1/2" 2" (51mm) 2 1/2" (64mm) 2" (50mm) 0.475" (12mm) 3/8" (9.5mm) 3 600

Wattage Tolerance All Units +5% -10%; Resistance Tolerance All Units +10% -5%

Immersion Length Tolerance All Units: +0 -1/2" (+0 -13mm) or +0 -1%, whichever is greater

Terminal Enclosures

Terminal enclosures (also called housings or boxes) protect the electrical terminals, bussing and connections.  Durex offers a wide variety of enclosure options to meet just about every need.  Note for hazardous location area applications: please ensure that complete /NFPA Class, Division, Gas Group, and T Code or other hazloc information is provided along with heater application details such as operating temperatures, pressures, etc.

General Purpose NEMA 1 - Sheet Metal Terminal Enclosures

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Light Duty NEMA 4 - Moisture Resistant Sheet Metal Terminal Enclosures


Heavy Duty NEMA 4 - Moisture Resistant Cast Iron Terminal Enclosures



Thermostat Options

----SPST-DPST-Thermostats.jpgThermostats can be mounted inside of the terminal enclosure to provide inexpensive, local temperature control of the screw plug immersion heater and liquid being heated. Process (automatic control) and high limit (manual reset) thermostats are available. Temperature settings are adjustable unless a fixed set point thermostat is ordered. Single pole (SPST) thermostats are typically used for single phase immersion heaters and double pole (DPST) thermostats for three phase immersion heaters. For high current draw heaters, the thermostats are often wired for "pilot duty" to a user-supplied relay. When ordering, please specify temperature range, the number of poles, the type of thermostat (process or manual reset, high limit), and if it will be used directly or wired for pilot duty.

Standard available bulb & capillary thermostats

Temperature Range Type Max. Amps to 240VAC Max. Amps at 480v
0 - 100°F Single Pole (SPST) 20 Amps N/A
85 - 230°F Single Pole (SPST) 20 Amps N/A
120 - 570°F Single Pole (SPST) 20 Amps N/A
0 - 100°F Double Pole (DPST) 30 Amps 21 Amps
60 - 250°F Double Pole (DPST) 30 Amps 21 Amps
100 - 550°F Double Pole (DPST) 30 Amps 21 Amps

Temperature Sensor Options

Process Sensors

A thermocouple or RTD sensor probe can be inserted into a thermowell in the heater to measure process fluid temperature. Type J and Type K are the most common thermocouple calibrations, while a Pt 100 ° DIN Curve sensor is the most common RTD used. Type J thermocouples and RTDs are typically used to 1000°F (540°C), whereas Type K thermocouples are used to 2000°F (1100°C) max. Other sensors are available. Standard lead length is 60" (1825mm). Specify sensor type and lead length when ordering.

High Limit Sensors

A thermocouple or RTD sensor probe is fed through the screw plug and clamped directly to a heater element to measure element temperature. This allows monitoring of the sheath temperature to prevent over temperature conditions that may shorten heater life and/or prevent fluid degradation. As an option, the high limit sensor can be inserted into a thermowell clamped or welded to the heater element sheath. This facilitates easy sensor replacement, but at the expense of a slower sensor response time. If the heater will be used vertically or in a circulation heater, contact Durex to ensure the correct sensor placement. Standard lead length is 60" (1825mm). When ordering, specify sensor type, sensor mounting style and lead length.

Construction Options

Screw Plug

Plug sizes are 1.25", 1.50", 2", 2.50", 3", 4" NPT (National Pipe Thread). Other pipe thread sizes and thread types (e.g. British Standard Pipe or BSP) are available upon request. Screw plug materials available are brass, steel, 304 and 316 stainless steels. Others are possible upon request.

Cold Length

The unheated (cold) length can be varied to suit application requirements. Longer unheated sections are often used to keep the termination area cool or to focus heat generation in a specific area of the media being heated.

Sheath Treatment and Finish

For pharmaceutical and other "clean" applications, a bright anneal finish can be supplied. Also available, depending on configuration, are passivation and electropolishing, which remove any free iron and also provides a smoother surface finish both on the heater element as well as the screw plug fitting.

Seal Options

Silicone Conformal Coating - Standard on catalog units, this seal provides general moisture / contamination protection to 220°F (105°C). However, it is porous and will allow moisture penetration over time. Not recommended for storage or use in high humidity environments.

Silicone RTV - Offers improved moisture and contamination protection to 400°F (200°C). However, it is also porous and will allow moisture ingress over time.

Epoxy Seal - Offers the best moisture and contamination protection of these seal choices. Rated for 450°F (230°C), this seal has low porosity and is recommended for higher temperature applications, high moisture environments and where long-term storage is foreseen.


How to Order

To place an order, or get a quotation, please provide the information itemized below. Contact Durex Industries for further assistance.

  • Electrical Rating: Voltage, phase, Wattage

  • Screw Plug Size and Material: 1.25", 1.50", 2", 2.50", 3", 4" NPT or other size and standard; brass, steel, 304SS or 316SS

  • Element Sheath Material: Copper, Steel, 304SS, 316SS, Incoloy® or other

  • Element Diameter and Quantity: Provide element diameter and quantity (one, two or three).

  • Element Watt Density: 8, 16, 23, 48 or 60 W/in2, or other watt density value

  • Immersion Length: Provide desired immersion length or maximum length allowable.

  • Seal Options: Silicone conformal coating (standard), RTV or epoxy

  • Electrical Enclosure: NEMA 1(standard), NEMA 4, NEMA 7, NEMA 4/7 or Other; If a hazardous location rating required, please designate location classification.

  • Sensing & Control: None (standard), thermostat type (process or high limit man. reset) and range; process sensor type or/and high limit sensor type


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Application Guidelines

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The goal is to optimize the heater configuration to ensure the longest life possible, while minimizing overall product and life cycle costs. The general temperature, sheath material and watt density guidelines below can aid in proper heater selection for some common liquids. Contact Durex for further assistance.

Liquid Max. Operating Temps °F (°C) pH Level and/or Concentration Recommended Sheath Material Typical Plug Material Max. Watt Density W/in2(W/cm2)
Clean, Potable Water
212 (100) Neutral (pH 6 to 8) Copper Brass 60-90 (9.5-14)
Process Water & Weak Solution 212 (100) pH 5 to 9; 2-3% Incoloy®, 304SS Steel or 304SS 48 (7.5)
Demineralized, Deionized Water 212 (100) Neutral 304SS or 316SS (passivated) 304SS or 316SS 60 (9.5)
Weak (Acidic) Solutions* 210 (100) Up to 5-6% Incoloy®, 316SS Steel 48 (7.5)
Mildly Corrosive (Acidic) Solutions* 180 (80) 5-15% Incoloy®, 316SS 304SS or 316SS 20-30 (3.5)
Severely Corrosive (Acidic) Solutions* 180 (80) >15% Incoloy®, titanium, Teflon® 304SS or other 15-16 (2.5)
Caustic Soda 10% 200 (100) pH 14; 10% Incoloy® Steel 20-23 (3.5)
Caustic Soda 50% 250 (120) pH 14; 50% Incoloy® Steel 16 (2.5)
Fuel Oil 1 & 2; Kerosene Lube Oil SAE 10 to 30 200-250 (95-120) - Steel Steel 20-23 (3.5)
Fuel Oil 4 & 5; Lube Oil SAE 40 to 50 200-250 (95-120) - Steel Steel 15-16 (2.5)
Fuel Oil 6 & Bunker C 160 (70) - Steel Steel 6-8 (1.2)
Hydraulic Oil 100 (40) - Steel Steel 15-16 (2.5)
Ethlyene Glycol 300 (150) - Steel Steel 23-30 (3.5-4.5)
Glycerine 500 (260) - Incoloy® Steel 8-10 (1.5)
Molton Salt Bath 800 (425) - Steel, 321SS, Monel® Steel 23-30 (3.5-4.5)
Paraffin Wax 150 (65) - Steel or 304SS Steel 15-16 (2.5)
Molasses 100 (40) - 304SS or 316SS 304SS 4-5 (0.7)

* Sheath material and watt density dependent on type of acid, concentration, temperature, aeration, movement, etc.

  • Please note that the user accepts full responsibility for validating that a heater is suitable for a given liquid or gas heating application.
  • Ensure that heater elements are completely covered in liquid AT ALL TIMES. Failure to do so may cause the heater elements to operate in open air and fail prematurely, or worse, lead to ignition of flammable vapor mixtures, causing serious personal injury and equipment damage. Liquid level sensors are highly recommended.
  • Durex recommends the use of a thermostat or process controller as well as a high limit sensor and FM approved safety controller to prevent unexpected over temperature conditions.

typical screw plug immersion heater applications

  • Boilers & Water Heating
  • Clean & Process Water
  • Freeze Protection & Winterizing
  • Heat Transfer Systems
  • Lube & Fuel Oil Heating
  • Parts Cleaning Solutions
  • Plating & Finishing Chemicals
  • Storage Tanks
  • Temperature Control Units
  • Wash Systems
  • Wax Heating
  • Waste Oil

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