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At both extremes of the element there is a ‘ cold end ’ which is where the spiral is replaced with a thick piece of metal wire to which the terminal pin is welded .
Incoloy elements are made in three different surface area-to-output ratios . This is the capacity in watts per square centimetre of outside sheath area . These three types of elements with their common name and output per square centimetre are tabulated below :
Common name of element
Output ratio watts per cm 2 of heater surface area
Length per kW , mm red heat . The reason for this is that the elements would otherwise become too long . All elements can be bent by the manufacturer in any shape desired .
When purchasing an incoloy element the voltage , the wattage and the type of element – i . e . total black heat , normal black heat or red heat as well as the shape of the element – must be specified . Standard elements have a 12.5mm clinched bush , which means they are not suitable for immersion in water or for use in especially high humidity and require a 12.5mm hole for mounting . The terminals are right angle tab terminals . Any special size and shape is available from the manufacturers . Before deciding on any special size or shape it is best to discuss requirements with the manufacturer .
Total black heat 2.4 1 740 Normal black heat 3.2 1 310 Red heat 5.5 760 Stove element 7 600
8mm diameter incoloy element details .
The total black heat has the lowest surface loading and therefore the lowest surface temperature . This element will not glow at all when energised . The normal black heat element will glow a dull red when energised in still air .
The red heat element will glow red in still air . The stove element rating is given for comparative reasons . There is no restriction on the velocity over these elements . All can be installed in still air where normal convection currents can take place .
Black heat elements are often specified because their surface temperature and thus their radiant heat is so much lower . Due to their lower temperature these elements seem to last longer . Because the lower surface loading elements are longer , they are also more costly . The approximate length of the various elements per kW capacity is given . This table is for 8mm diameter elements which is the most common . The 6.6mm and 11.2mm diameter elements are available as specials .
Incoloy elements are available in the following standard ratings , 500W , 750W , 1 000W , 1 500W , 2 000W , 3 000W and 4 000W .
Elements up to 2 000W are available in total black , normal black and red heat elements ; 3 000W elements are available in normal black and red heat whilst the 4 000W elements are only available in
• Finned elements : These elements are sometimes used because their surface-to-capacity ratio is much higher than incoloy elements . These are made in mild steel or stainless steel . Steel can be prone to rusting in high temperatures and humidity . These elements are also more expensive .
• Heater banks : From the range of elements available any capacity heater bank can be made up . The total capacity required is obtained from the heat loss calculations . This load includes the heat loss from the space as well as the heat required to heat the ventilation and / or the infiltration of air . A safety factor can be applied but is often not desirable because of the extra load on the electrical system and the extra kVA requirements ( maximum demand ). In particularly cold periods it is usually sufficient to switch the heaters on a bit earlier .
Robyn , I hope that this increases your understanding of heating , in the next issue I will conclude with heating using hot water coils .
Thanks to everybody for the overwhelming response . I receive on average over 60 questions per month and cannot publish all of them . But keep them coming , as I may answer you directly . Looking forward to hearing from you . RACA
Grant Laidlaw
REFERENCES 1 . SETA training 2 . ASHRAE 3 . ACRA
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RACA Journal I February 2024 www . refrigerationandaircon . co . za