The Challenge


Our customer was looking for a cartridge type heater for immersion in an airline coffeemaker. They had problems in the past with the integrity of the heli-arc weld at the heater disc end. Occasionally a pin-hole would occur in the weld allowing absorbsion of moisture resulting in premature heater failure. Some of these failures were catastrophic resulting in short circuiting out of the heater when planes were in flight.


The Solution


Working directly with the customer, Chromalox was able to develop a cartridge heater using deep draw technology. Instead of using a standard as-welded tube with a welded end disc, we used a drawn tube. Deep drawing is a sheet metal process where a sheet metal blank is radially formed into a die with a punch. Traditional cartridge heaters use "as welded" or "drawn over mandril" tube construction, using a strip that is formed into a tube and welded. Deep drawing eliminates any welded seams that are typical on traditional cartridge heaters. Deep drawing also eliminates the need for a welded end disk which if improperly manufactured can cause the ingress of moisture into the element. Specifications are strict for minimum wall thicknesses throughout the tube.

This eliminates the ingress of moisture through a faulty weld joint either on the length of the tube or the welded end. We designed a special bushing with a weld sleeve on top so the bushing is welded to the tube outside of the actual element. Should a leak occur there, the water would leak out of the bushing but not into the actual heating element eliminating the possibility of a short circuit.  The drawn tube also helps with corrosion resistance. With a welded tube, the welding can cause carbon to migrate to the HAZ (heat affected zone) encapsulating the chromium and leaving the unit with a greater possibility of corroding. We also have the heater electro polished for further corrosion resistance.




• No weld seam on the tube

• No welded end disk

• No welding that can cause carbon precipitation

• Much less likelihood of moisture ingress

• Superior corrosion resistance


Spirax-Sarco Engineering plc

Charlton House



GL53 8ER, UK

Tel | +44 (0)1242 521361

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