I have been following this discussion with interest & thought I would
throw some of my own thoughts into the pot:

Werner is absolutely right when he says " that 'radiant heating [is] a non
-equilibrium process', but so is heating using forced convection." This is
essentially why profiling using Datapaqs/Moles etc. is neccessary. The high
levels of heat transfer achievable by forced convection do however make the
process much nearer to being equilibrium.

This means the difference between heater zone and product temperature is less,
and tends to make the product temperature more uniform. Having some IR heating
in parallel with the convection should therefore make the overall heat
transfer
(even) higher allowing lower zone temps and further helping uniformity. The
down
side to increasing heat transfer is that you need more zones to achieve a
suitable
profile, but oven lengths/belt speeds/zone numbers have grown in parallel with
increasing convection capabilities [the extra speed also helps uniformity by
reducing the time that the product is moving from one zone to the next].

Even if IR transfer is designed down to a minimum using low emmissivity (i.e.
shiny) heater panels, as oxidation and flux accumulation occur they will start
to radiate anyway - it would be a very interesting expt. to compare the
results
from a brand new and well used machine.

>I do not see, why IR-systems utilizing radiator panels in the mid-IR range
>should be drifting; the panel temperatures can certainly as accurately
>controlled as any forced convection air/nitrogen stream.

I would go further and say radiator panel temperature can be more closely
controlled than the air - ensuring a uniform air flow rate and temperature
across an area the size of a typical heater panel is not easy & in the
absence of any feedback drifts in air speed due to e.g. flux residue build
up or fan speed variation will occur.

For info. the ratio of heat transfer modes that we have seen in our modelling
and expt. evaluation of ovens have been:

Ceramic panel heaters (Surf Systems) -  90+% IR
Early forced convection (Senju)      - ~70 % IR !
Typical more recent convection       - ~70 % Convection
"Advanced" modern machines           -  90+% Convection



David Whalley

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
##############################################################