Heater Guides

Submerged arc welding (SAW) is widely used for boiler tubes, pressure vessels, pipes and heavy fabrication. The quality of the weld depends heavily on the condition of the granular flux. Moist flux is one of the main causes of hydrogen-related weld defects. That is why many automated welding lines heat the flux in the hopper itself.

Why flux must be kept dry

Welding flux absorbs moisture from the air. When damp flux enters the arc, the moisture breaks down and hydrogen enters the weld metal. This can lead to:

• porosity and blow holes
• hydrogen-induced cracking
• rejected welds and costly rework

Flux manufacturers specify baking or re-drying temperatures for their products, and holding temperatures for storage. Keeping flux hot right up to the point of use prevents it from picking up moisture again.

Why heat the hopper and not just the oven?

Flux is often re-dried in an oven, but once it sits in an unheated hopper on the machine it starts cooling and absorbing moisture again. A heated hopper keeps the flux at the required temperature during the whole welding shift, which is especially important on long automated runs such as boiler pipe welding.

Design points for a flux hopper heater

Heating a granular material is different from heating a liquid or a metal block. Flux does not flow or conduct heat well, so the heater design must account for:

• Uniform heating – heaters placed around and sometimes inside the hopper so that no cold zones remain
• Target temperature – often in the range of a few hundred °C, for example around 300 °C, depending on the flux maker’s recommendation
• Watt density – kept moderate so that flux in contact with the heater is not overheated
• Temperature control – a thermocouple placed in the flux and a controller to hold the set point
• Space and mounting – the heater must fit the existing hopper geometry on the welding machine
• Robust construction – to handle vibration, abrasion and the workshop environment

Common heater types for this job include strip heaters or band heaters on the hopper wall, and tubular or cartridge heaters placed in the flux.

Custom flux hopper heating from Thermosys

Thermosys Technologies designs and manufactures custom heaters for welding flux hoppers on automated welding machines, including projects heating flux to around 300 °C. Send us a sketch or photo of your hopper with its dimensions and the required temperature, and we will propose a heater layout with control.

Strip heaters · Tubular heaters · Contact us

J type and K type are the two most common thermocouples in Indian industry. Both are inexpensive and reliable, but they are not interchangeable. Using the wrong type – or the wrong extension cable – gives wrong temperature readings.

How a thermocouple works

A thermocouple is two different metal wires joined at one end. When that junction is heated, a small voltage is produced. The temperature controller reads this voltage and converts it into temperature. Each thermocouple type uses a different pair of metals, so each produces a different voltage for the same temperature.

J type thermocouple

• Materials: iron (+) and constantan (−)
• Typical range: up to about 750 °C (lower in continuous use)
• Strengths: low cost, good sensitivity, very common on plastic processing machines
• Limitations: the iron wire can rust in moist or oxidising conditions; not for high temperatures

Typical uses: injection moulding and extrusion barrels, hot runners, moulds, dies and packaging machines.

K type thermocouple

• Materials: chromel (+) and alumel (−)
• Typical range: up to about 1200 °C (depending on sheath and wire size)
• Strengths: wide temperature range, good oxidation resistance, general-purpose
• Limitations: slightly less sensitive than J type at low temperatures

Typical uses: furnaces, heat treatment, ovens, kilns, exhaust gas and general process measurement.

Quick comparison

	J type	K type
Wires	Iron / Constantan	Chromel / Alumel
Approx. max temperature	~750 °C	~1200 °C
Best for	Plastic machinery, moulds	Furnaces, ovens, high temperature
Cost	Low	Low
Moisture resistance	Lower (iron can rust)	Better

The most common mistake

The controller, the thermocouple and the extension/compensating cable must all be the same type. A K type sensor connected to a controller set for J type – or joined with J type cable – will show a wrong temperature, and your heaters may run much hotter or colder than you think.

When to choose an RTD (PT100) instead

An RTD such as a PT100 gives better accuracy and stability at lower temperatures (roughly up to 400–500 °C). It is a good choice when precise control matters – for example in food, pharma and liquid heating – but it is less rugged and more expensive than a thermocouple.

Thermocouples and RTDs from Thermosys

Thermosys Technologies supplies J and K type thermocouples and PT100 RTD sensors in Pune, in the probe length, sheath and termination your machine needs. Cartridge heaters can also be supplied with a built-in thermocouple.

See thermocouples and RTDs or contact us.

If you replace cartridge heaters in the same mould or die again and again, the problem is rarely “bad luck”. In most cases the failure has a clear cause that can be fixed. Here are the seven reasons we see most often at our workshop in Pune.

1. Loose fit in the hole

This is the number one cause. A cartridge heater pushes its heat into the metal by conduction. An air gap – even a small one – acts as insulation, so the heater runs much hotter inside to deliver the same power. The resistance wire overheats and fails.

Fix: measure the real hole size, keep the fit close, and ream holes for high-density heaters. If the hole is worn, order a slightly larger heater diameter or reduce the watt density.

2. Watt density too high

A heater that packs too many watts into a small surface runs hot even in a good hole. Combined with a loose fit or high process temperature, life drops sharply.

Fix: use a longer heater, more heaters, or a lower wattage to reduce the watt density.

3. Moisture in the insulation

The magnesium oxide (MgO) insulation inside a heater can absorb moisture from the air, especially when heaters sit in storage or in humid conditions. This can cause low insulation resistance and leakage current.

Fix: store heaters in a dry place, and heat up slowly at low voltage if a heater has been stored for a long time. Specify moisture-resistant end seals for humid or wash-down areas.

4. Oil, plastic or other contamination at the lead end

Oil, hydraulic fluid, molten plastic or cleaning chemicals entering the lead end can carbonise and cause a short circuit.

Fix: choose sealed lead ends and protect the leads with sleeving or conduit.

5. Lead wire damage and flexing

In moving tools, the lead wires bend every cycle and eventually break where they exit the heater.

Fix: use flexible leads, stainless steel braid or conduit, right-angle exits, or swaged-in leads that resist flexing. Clamp the leads so that the movement happens away from the heater.

6. Poor temperature control

A faulty thermocouple, a thermocouple placed too far from the heater, or an on/off controller with a wide band can let the heater run far above the required temperature.

Fix: check the thermocouple and its position, or use a heater with a built-in thermocouple for faster, more accurate control.

7. Heated section outside the metal

If part of the heated length sticks out of the block, that section has nowhere to send its heat and burns out.

Fix: specify an unheated (cold) zone at the lead end that matches the part outside the block.

Quick checklist when ordering replacements

• Measured hole diameter and depth
• Wattage, voltage and process temperature
• Length that is actually inside the metal
• How the old heater failed (photo helps)
• Lead type and protection needed

We can help you solve repeated failures

Send us a photo of the failed heater and your application details. Thermosys Technologies manufactures custom cartridge heaters in Pune and can suggest a design that lasts longer. See our cartridge heaters, read how to choose the right cartridge heater, or contact us.

Choosing too small a heater means the tank takes forever to reach temperature. Choosing too large a heater wastes money and can shorten heater life. Fortunately, the calculation is simple.

The basic formula

The energy needed to raise the temperature of a liquid is:

Energy (kWh) = Mass (kg) × Specific heat (kJ/kg·°C) × Temperature rise (°C) ÷ 3600

To find the heater power, divide by the heat-up time you want:

Power (kW) = Energy (kWh) ÷ Heat-up time (hours)

Then add an allowance for heat losses (explained below).

Useful values

Liquid	Density (approx.)	Specific heat (approx.)
Water	1.0 kg/litre	4.19 kJ/kg·°C
Mineral / thermal oil	0.85–0.9 kg/litre	about 1.9–2.1 kJ/kg·°C

(Values vary with the exact liquid and temperature – check your liquid’s data sheet for accurate numbers.)

Example 1: Heating water

A tank holds 500 litres of water. You want to heat it from 25 °C to 75 °C in 2 hours.

• Mass = 500 litres × 1.0 = 500 kg
• Temperature rise = 75 − 25 = 50 °C
• Energy = 500 × 4.19 × 50 ÷ 3600 = 29.1 kWh
• Power = 29.1 ÷ 2 = 14.5 kW

Example 2: Heating oil

A tank holds 300 litres of oil (density 0.87, specific heat 2.0). Heat from 30 °C to 150 °C in 3 hours.

• Mass = 300 × 0.87 = 261 kg
• Temperature rise = 120 °C
• Energy = 261 × 2.0 × 120 ÷ 3600 = 17.4 kWh
• Power = 17.4 ÷ 3 = 5.8 kW

Remember: oil needs a low watt density heater to avoid carbon build-up, so the elements will be longer or more in number than for water at the same kW.

Don’t forget heat losses

The formula above only covers heating the liquid. In practice, heat is also lost:

• from the tank walls and open surface,
• to the tank material itself during heat-up,
• through any liquid that is drawn off and replaced.

An open, uninsulated tank can lose a lot of heat, especially at higher temperatures. A common practical approach is to add a safety margin of around 20–30 % for an insulated, covered tank, and more for open or uninsulated tanks. For critical applications, the losses should be calculated properly.

For Example 1, adding 25 % gives about 18 kW – so an 18 kW three-phase immersion heater would be a sensible choice.

Maintaining temperature

Once the liquid is hot, the heater only needs to replace the losses (plus any heat taken by product or fresh liquid). A temperature controller switches the heater on and off to hold the set point.

Let us size it for you

Send us the tank size, liquid, start and final temperature, heat-up time and whether the tank is insulated or open. Thermosys Technologies will recommend the right immersion heater – screw-plug, flanged or over-the-side – and manufacture it in Pune.

See immersion heaters or contact us.

Many buyers ask us for an “immersion heater” when they need a tubular heater, or the other way round. The two are closely related, so the confusion is understandable. Here is a simple explanation.

What is a tubular heater?

A tubular heater is the basic building block of many industrial heaters. It has a nickel-chrome resistance coil placed in the centre of a metal tube. The space around the coil is filled with high-grade magnesium oxide (MgO) powder, and the tube is then compacted (swaged). The MgO insulates the coil electrically but conducts heat very well to the tube.

Because the tube can be bent into almost any shape, tubular heaters are used for:

• Air and gas heating – in ducts, ovens, dryers and hot air generators (often with fins added)
• Surface heating – clamped onto plates, platens and dies, or placed in machined grooves
• Radiant heating – spaced away from the work
• Liquid heating – when fitted with a flange or screw plug, the tubular heater becomes an immersion heater
• Casting into metal – for hot plates and heated blocks

Typical sheath materials are stainless steel, Incoloy and copper, in tube diameters roughly from 6 mm to 16 mm.

What is an immersion heater?

An immersion heater is designed to sit directly in the liquid it heats – water, oil, chemicals or solutions. In most industrial immersion heaters, the heating elements are tubular heaters, mounted on:

• a screw plug (for smaller tanks),
• a flange (for larger kW ratings and pressure vessels), or
• an over-the-side bracket (for open tanks where you can’t drill the wall).

So, an immersion heater is usually a set of tubular heaters plus a mounting and a terminal enclosure.

Key differences at a glance

	Tubular heater	Immersion heater
What it is	A single heating element	A complete assembly for liquids
Heats	Air, surfaces, liquids, metal	Liquids only
Mounting	Clamps, brackets, fittings	Screw plug, flange, over-the-side
Shapes	Straight, U, W, coil, custom	Usually hairpin elements on a head
Terminal box	Optional	Normally included

How to choose for liquid heating

When heating liquids, the most important choice is watt density – how much power each square centimetre of the element surface puts out.

• Water can take a relatively high watt density.
• Oils and viscous liquids need a much lower watt density. Too high and the oil carbonises on the element, insulates it, and the element burns out.
• Corrosive chemicals need the right sheath – SS 316, Incoloy, titanium or other materials depending on the chemical.

Also decide the required kW (see our guide on calculating heating power for a tank), the voltage and phase, the mounting type and whether you need a thermostat or thermowell.

How to choose for air heating

For air, consider air velocity and outlet temperature. Finned tubular heaters give more surface area and run cooler, which improves life in forced-air applications such as duct heaters and hot air generators.

Get it made to your requirement

Thermosys Technologies manufactures tubular heaters, finned heaters and immersion heaters in Narhe, Pune, with in-house swaging, MgO filling and bending. Tell us what you are heating and we will suggest the right design.

Tubular heaters · Immersion heaters · Contact us

Band heaters wrap around a cylinder – usually the barrel or nozzle of an injection moulding machine, extruder or blow moulding machine – and heat it from the outside. Most plastic processing machines in India use either mica band heaters or ceramic band heaters. Both work, but they suit different situations.

Here is a simple comparison to help you choose.

Mica band heaters

A mica band heater has a resistance wire or ribbon wound on mica insulation sheets, enclosed in a metal sheath (usually stainless steel).

Advantages

• Lower cost
• Thin and compact – fits where space is tight
• Fast heat-up
• Good heat transfer when clamped tightly to the barrel

Limitations

• Typically used up to around 280–300 °C sheath temperature
• Very sensitive to fit: a loose band overheats and fails quickly
• Not ideal where the heater is frequently removed and refitted

Best for: nozzles, smaller barrels, machines running standard plastics like PP, PE and ABS.

Ceramic band heaters

A ceramic band heater uses a coiled resistance wire threaded through interlocking ceramic tiles, with an insulating layer and an outer sheath.

Advantages

• Higher operating temperatures (commonly up to around 600–700 °C, depending on design)
• Heats by both conduction and radiation, so it tolerates a less-than-perfect fit better
• Insulation layer reduces heat loss to the surroundings – often saves energy
• Flexible construction – easy to install on large barrels
• Long life

Limitations

• Higher initial cost
• Bulkier than mica bands

Best for: large barrels, extruders, engineering plastics like nylon, PC and PET, and anywhere energy saving matters.

Nozzle band heaters

Nozzle heaters are small-diameter band heaters made for injection nozzles. They can be mica, ceramic or mineral-insulated. Since nozzles need tight temperature control, a good fit and a correctly placed thermocouple are especially important.

Quick comparison

	Mica band heater	Ceramic band heater
Typical max temperature	~300 °C	~600–700 °C
Initial cost	Lower	Higher
Energy efficiency	Good	Better (insulated)
Fit sensitivity	High	Moderate
Size	Thin	Bulkier
Life	Good with proper fit	Long

(Exact limits depend on construction and watt density – ask us for your application.)

Tips to make any band heater last longer

1. Clamp it tight. Re-tighten after the first heat-up, because the barrel expands.
2. Keep the barrel surface clean – no plastic, oil or scale under the band.
3. Choose the right watt density for the process temperature.
4. Protect the terminals from plastic drool and moisture. Use terminal boxes where needed.
5. Use a proper temperature controller and a working thermocouple. An overheated band is usually a control problem, not a heater problem.

Information needed for a quotation

• Inner diameter and width
• Wattage and voltage
• Type: mica, ceramic or nozzle
• Terminal type (screw terminal, leads, box) and position
• Clamping type and any cut-outs or holes for thermocouples
• Quantity

Band heaters made in Pune

Thermosys Technologies manufactures mica, ceramic and nozzle band heaters in custom sizes at our facility in Narhe, Pune, for plastic processing machines, extruders and other industrial equipment.

See our band heaters or send us your requirement.

Cartridge heaters are among the most widely used heating elements in industry. You will find them inside injection moulds, hot runner systems, packaging sealing jaws, die-casting dies, hot stamping tools, laboratory equipment and many other machines. They are small, but choosing the wrong one is one of the most common reasons for early heater failure.

As a cartridge heater manufacturer in Pune, we see the same questions from maintenance and design engineers again and again. This guide explains the key points to check before you place an order.

1. Start with the hole: diameter and fit

A cartridge heater transfers its heat to the metal block mainly by conduction. Any air gap between the heater sheath and the hole acts as insulation. The heater then runs hotter internally to deliver the same heat, and its life drops sharply.

• Measure the actual hole diameter, not the drawing value, especially for older moulds and dies.
• For high-watt-density heaters, a close fit is essential. A reamed hole gives a much better result than a drilled one.
• If the hole is worn or oversized, tell your supplier. A heater can be made to a slightly larger diameter, or a lower watt density can be chosen.

2. Watt density – the most important number

Watt density is the heater wattage divided by its heated surface area (W/cm² or W/in²). It decides how hot the heater runs internally.

• Higher watt density = faster heat-up, but higher internal temperature and less tolerance for a loose fit.
• Lower watt density = longer life, but you may need a longer heater or more heaters to get the same total power.

As a rule, the hotter the process temperature and the looser the fit, the lower the watt density should be. If you are replacing a heater that keeps failing, reducing the watt density is often the simplest fix.

3. Low-density vs high-density cartridge heaters

• Low-density cartridge heaters use a simpler construction and suit moderate temperatures and lower watt densities – for example platens, sealing bars and general heating.
• High-density (swaged) cartridge heaters are compacted after filling with magnesium oxide (MgO). The compaction gives excellent heat transfer from the coil to the sheath, so they can run at much higher watt densities and temperatures. They are the usual choice for moulds, dies and hot runners.

4. Sheath material

• SS 304 / SS 321 – the standard choice for most applications.
• Incoloy 800 / 840 – for higher temperatures and better resistance to oxidation.
• Special sheaths are possible for corrosive environments.

5. Heated length and unheated zones

Not every part of the heater needs to produce heat. Unheated zones at the lead end prevent heat being wasted where the heater exits the block, and protect the lead wires. If part of the heater sits outside the metal, make sure that portion is unheated.

6. Lead wire and termination options

The lead end is where many heaters fail, especially in moulds with movement, oil or moisture. Common options:

• Fibreglass or PTFE insulated leads
• SS braided or flexible conduit protection for abrasion
• Right-angle leads where space is limited
• Swaged-in leads for vibration and flexing
• Moisture-resistant sealing for humid or oily environments

7. Built-in thermocouple

For precise temperature control, a cartridge heater can be supplied with a built-in J-type or K-type thermocouple at the tip or at a chosen location. This saves space and gives a faster control response.

8. Information to share when ordering

To get an accurate quotation quickly, send us:

1. Diameter and total length (and the measured hole size)
2. Wattage and voltage
3. Heated length / unheated zones
4. Operating temperature
5. Lead length and type of protection
6. Thermocouple requirement, if any
7. Quantity

A photo or drawing of the old heater also helps a lot.

Need cartridge heaters in Pune or anywhere in India?

Thermosys Technologies manufactures low-density and high-density cartridge heaters to your exact size, wattage and lead requirement at our facility in Narhe, Pune. Our in-house test bench includes high-voltage, resistance, leakage and moisture testing.

View our cartridge heaters or contact us for a quotation.