How was Energy wasted in Glass Tempering ?


Glass tempering energy consumption is used as a crooked way to increase sales. There are many ways to do it.

According to GLASTON FC-series Chinook Pro brochure (2018) energy consumption for 4 mm clear glass is 2,70 kWh/m². The article tells the same, (2,70 kWh/m²), but that convection energy consumption is not included. GLASSROBOTS RoboTemp had about 30% higher capacity than Chinook Pro but energy consumption was 3,3 kWh/m²! Loading efficiency was the same, 65% in both cases. Maybe GLASTON has not included electrical and mechanical efficiency or heat losses either? They are remarkable, heat loss 2% + tempering blower motor and inverter min. 5%. Graph 1 describes my thinking.

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GLASTON explained this well in articles. GLASTON used in the brochure 2018 65% loading efficiency but in the articles various ones. The articles are from years 2015 and 2017. First CONVAIR was made in 1996. GLASTON bought GLASSROBOTS RoboTemp know how in 2014 and employed GLASSROBOTS engineers.  65% loading efficiency is generally used and this is used later, too.

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However, certain things must be known.


  1. Energy consumption for heating is very clear, but the loading efficiency must be accurate, for example 65%.
  2. GLASTON gives in article “thumb rule” for tempering energy. It is given for loading efficiency of 90%. This is adjusted to 65%. This information is important since the “limit” of tempering and cooling is difficult to say for glasses over 5 mm thick.
  3. Everything else is “unknown”. Fortunately, they are nearly the same for every manufacturer. Therefore, the comparison is is quite accurate but the correct capacity must be known.


Note: A part of the cooling energy is necessary. For thin glasses heating time is always longer than tempering and cooling time. Thus, the result is always correct. For thicker glasses this is correct, too as the cooling time can always be shortened to the heating time. The exception is thickest glasses.

Tempering furnace manufacturers know many tricks to confuse customers for energy consumption and capacity. For example, giving capacities depending on emissivity values of 0,08 or 0,04 and single and double silver glasses. Another one is that the tempering machine can temper “even E 0,01 glass”. Even radiation heating machine can do this. The quality of glass, energy consumption and capacity are another thing.

This table shows the calculation results as the the above principle and graphs 1 and 2.

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CONVAIR raised my interest in convection in 1996. Since then, I felt that high heating speed is the key for low energy consumption as per graph 3.

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GLASTON article has a following sentence.  “The example above does not take into account energy required by convection because different technologies use different amounts of energy, which is why the figures in reality would be slightly higher”. In this article energy consumption is the same as in brochure, 2,70 kW/m². FC-series has high convection power, (13 kW/m²), The correct explanation would have been that in high temperature it uses 1/3 of energy and convection blowers are not used average 50% power.  The difference to CONVAIR is in the range 0,1 – 0,3 kWh/m² glass thicknesses 3 – 10 mm.

For comparison, CONVAIR convection power was 5 kW/m² and RoboTemp, 10 kW/m², They  had approx. 30% higher capacity than GLASTON FC-series Chinook Pro! According to my understanding and graph 1 this should be on the contrary. For capacity GLASTON patented their development.

When GLASTON successfully launched FC-series in 2010 the main sales arguments were:

“Up to 30% lower energy consumption” and that “40% capacity increase”. GLASTON did not specify to what glass types or heating process.

Also, convection air circulation illustration was such that it would have made a disaster of the good process of FC-series.

These were successful marketing jargon, not based on technical realities.



Eventually poor technology is replaced by better technology. CONVAIR and RoboTemp disappeared from the market as the change of heaters was almost impossible to make. Price paid a big role in this. The competitors started to sell ”convection heating” furnaces. Even such which do not heat convection air at all! Land Glass Technology Co., Ltd success was based since they started early. I had license negotiations with them in 2002 and 2003. As a result they started to violate FERACITAS patents US 7,290,405 and Chinese and Finnish respective patents. The improvement they made was short heaters inside of convection box and a matrix type heating of convection air. Later came FC-series after 2010 and few years after HEGLA-Taifin.

CONVAIR reached extremely high capacities in 1996. For 5 mm clear glass 26 loads/hour and E 0,02 glass 22 loads/hour. The reason was unknown 25 years. Risto Nikander invented and understood that CONVAIR heated convection air twice in 2021. Already in 2020, he understood to heat convection air once + radiation heating. Now these patent applications are combined and  cover also heating convection twice like CONVAIR. This I call “pure convection” since it has not effective colorful radiation at all.  However, radiation increases clear glass capacity remarkably. Based on patent application interim decision it is almost certain that the patents will be granted.

Post time: Mar-07-2022