Dilution air question
Dilution air question
This particular inspector was surprised to hear we intended introducing bitumen into the rotary drum as this is a drum mixer. Obviously he has only seen batch plants which only dry the aggregate and mix bitumen externally. He is worried about volatiles in the heated bitumen or asphalt mix exceeding the limit on LEL values. and requires a dilution air calc submitted. This is of the type found in NFPA 86A and copied into our Australian codes. I personally have never done this before but started working through it. I feel however that it isn't applicable to bitumen for the following reason. Bitumen has a flash point of >250degC. Our normal product temp will be 180deg to 200degC max and an overtemp cutout set at 220degC. Hence there should be no combustable vapours given off of any significance. So a dilution air requirement to keep this from reaching say 50% or even 25% of the LEL is unnecessary. Sometimes asphalt plants use RAP a recycled pavement which could contain contaminants of higher volatile compounds but this will not be the case at our plant where only virgin bitumen is used.
Does any knowledgeable person on explosive atmospheres agree or have any comments?
As an aside to this. Other asphalt plants make a product called cold mix. This is made at a low temperature of about 80degC and definitely adds diesel to the aggregate. If it was required to do a calc on minimum dilution air what values would you put in for the equation for dilution air flow in NFPA 86A.Years ago when involved with emission tests on a specific stack they had to calibrate their instrument to check for hydrocarbons from a diesel fired burner. Hexane was used as the calibrating fluid. So would you use values for hexane shown in the tables we have(ie for vapour generated by 1 litre of liquid etc) to insert in the equation. If so there is one equation quantity designated Q which is the max instantaneous evaporation rate in litres/sec at our operating temp. Where would you find that for diesel.
Hope this isn't too long to respond but the first issue on straight bitumen is the most inportant.
Thanks in advance