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Screw Plug Immersion Heaters

Bucan’s screw plug immersion heaters are medium capacity heating units designed to provide heat inside tanks and pipes. They consist of a single or a set of tubular elements that are formed into hairpins and welded or brazed to a screw plug.

The sheath material of the heating elements could be steel, copper, stainless steel or Incoloy. The standard screw plug sizes utilized are 1”, 1 1/4”, 2” and 2 1/2” and are either made of steel, brass or stainless steel. Various types of electrical protective enclosures, built-in thermostats, thermocouples and high limit switches can be incorporated.

Screw Plug NPT size Plug material Standard element diameters Maximum voltage Phase Heating elements material Maximum number of elements
1" BRASS
STEEL
SS 304
SS 316
0.315" / 0.375" 300 V / 600 V 1 COPPER
STEEL
INCOLOY 800®
INCONEL 600®
SS 304
SS316
2 / 1
1 1/4" 0.315" / 0.375" 300 V / 600 V 1 / 3 3
2" 0.430"  600 V 3 3
2 1/2" 0.430" 600 V 3

3

Screw Plug Immersion Heater

Typical Heating Applications Screw Plug Immersion Heaters

  • PROCESS WATER HEATING
  • HEATING OILS, WAX, TAR, ASPHALT, PARAFFIN
  • MILD ALKALINE & ACID SOLUTIONS
  • FREEZE PROTECTION
  • EQUIPMENT WARM-UP
  • HEATING GASES
  • FOOD PROCESSING

Selection tips for screw plug Immersion heaters

Screw plug immersion heaters have a wide range of applications. It is essential to take several factors into consideration in order to select a heater that has features that match the requirements of a specific application and suitable to the environment where the heater will operate. The following are a number of criteria that should be taken into consideration:

  • Besides the heating capacity required, a heater should operate at a temperature and a watt density that is adequate to the fluid heated. Tables 2 and 3 provide maximum sheath temperatures and watt densities that are recommended for heating various fluids.
  • In order to select proper materials for screw plug heaters, general selection guides that are based on common experience in industrial usage could be used. However, this selection should always be based on a user’s experience and knowledge in a specific application. Table 3 provides recommended sheath and screw plug materials for different mediums.
  • It is important to note that at all times the temperature of the sheath material should not exceed the following temperatures:
Sheath Material Maximum Temperature
Copper 360°F (180°C)
Stainless Steel 1200°F (650°C)
Steel 750°F (400°C)
Incoloy 1500°F (815°C)
  • Taking the environment within which the heater will operate into account, safety issues should be taken into consideration.
  • Adequate temperature controlling devices, temperature and pressure high limit switches, low liquid level and flow controllers and other control/safety devices should be used. These instruments will control the heating process and protect the heater from excessive heat.
  • The classification of the electrical terminal box should match the application (NEMA 1, NEMA 4, NEMA 7, NEMA 12 etc.).
  • The amount of contamination that the heater will be exposed to should be taken into account.
  • Electrical safety codes should be considered.

For more information, please contact our technical specialists.

Application Sheath Material Screw Plug Material Typical Watt Density w/in²
Clean water Copper Brass 55-80
Process water / Weak solution Stainless Steel Stainless Steel 25-55
Oil light Steel Steel 20-25
Oil medium Steel Steel 12-15
Corrosive solutions (basic) Incoloy Stainless Steel 20-25
Corrosive solutions (acidic) Stainless Steel Stainless Steel 20-25
Vegetable oil Stainless Steel Stainless Steel 30-40
Asphalt, tar, wax Steel Steel 4-10
Air Incoloy Steel 25-30

Features of screw plug Immersion heaters

  • NEMA 4 (MOISTURE PROOF) TERMINAL BOX
  • BUILT-IN THERMOWELL
  • BUILT-IN SINGLE OR DOUBLE POLE THERMOSTATS
  • PASSIVATION FOR STAINLESS STEEL AND INCOLOY HEATERS
  • BUILT-IN PROCESS OR HIGH LIMIT THERMOCOUPLES
  • SPECIAL PLUGS OR NON-STANDARD THREAD SIZES
  • NEMA 7 (EXPLOSION PROOF) TERMINAL BOX

Thermostats and thermocouples

Thermostats that are utilized in screw plug heaters are the bulb and capillary style. These thermostats operate by the principle of expansion and contraction of a heat sensitive liquid found inside the bulb and capillary assembly, which controls the activation of an electrical switch inside the thermostat.

Thermostats in most cases open the electrical circuit when the desired temperature is attained (open on rise). The table below shows available ranges of thermostats.

A pilot duty thermostat can always be used when the ampacity of a heater is higher than the available thermostat amperages shown below.

Thermostat Contact Operation Temperature Range Volts / Amps
T SPST 10º to 120ºC 125V - 30A / 250V - 30A / 277V - 25A
T1 SPST -15,5º to 40ºC 125V - 30A / 250V - 30A / 277V - 25A
T2 SPST 70º to 280ºC 125V - 30A / 250V - 30A / 277V - 25A
T3 SPST 149º to 370ºC 125V - 30A / 250V - 30A / 277V - 25A
T4 DPST 0º to 120ºC 125V - 30A / 250V - 30A / 277V - 25A
T5 DPST 10º to 120ºC max. 277V - max. 25A
T6 DPST -15,5º to 40ºC max. 277V - max. 25A
T7 DPST -25,5º to 204,5ºC max. 277V - max. 25A
T8 DPST 10º to 120ºC max. 600V - max. 15A

It should be noted that thermostats are only temperature controllers and should not be used as fail-safe high limit switches.

Built-in thermocouples could be utilized inside screw plug heaters to give more accurate temperature indications. Thermocouples could also be attached to one of the heating elements and utilized as high-temperature limiting devices.

Terminal Boxes for Screw Plug Immersion Heaters

Terminal Box Type Fig.# Dimension "A" Dimension "B" Dimension "C" Hole Size "E"
NEMA 1 Terminal box fig. 1 3 3/4" Dia. 2 3/4" N/A 7/8" knock-out
NEMA 1 Terminal box with built-in thermostat fig. 2 3 3/4" Dia. 3 3/4" N/A 7/8" knock-out
NEMA 4 / NEMA 7 Terminal box fig. 3 3 1/2" Dia. 3 1/8" 4 1/2" 1" NPT
NEMA 4 / NEMA 7 Terminal box with built-in thermostat fig. 4 4 1/4" Dia. 5 3/16" 5 1/4" 1 1/4" NPT

For specifications, please check product catalogue

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Typical wiring digram for single phase, two elements

Typical wiring diagram for single phase, three elements

Typical wiring digram for three phase (y), three elements

Typical wiring digram for three phase (delta), three elements

Single phase, single pole, single throw line duty thermostat max. 25A max. 277V

Single phase, double pole, single throw line duty thermostat max. 15A and max. 600V

Three phase (delta), double pole, single throw line duty thermostat either max. 15A or max. 600V

Three phase (star), double pole, single throw line duty thermostate either max. 15A or max. 600V

Single phase, single pole, single throw pilot duty thermostat (SPST)

Single phase, double pole, single throw pilot duty thermostat (DPST)

Three phase, single pole, single throw pilot duty thermostat (SPST)

Three phase, double pole, single throw pilot duty thermostat (DPST)