Re: Cooling fans and projector lamps

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

Kenneth Luker writes

>>The quartz cools too quickly if the fan is left on after the lamps 
>>have been on a while. This causes small cracks to form. These are 
>>many times visible to the eye. After a number of these fast heat-
>>cool cycles, the envelope springs a leak and oxygen enters the 
>>envelope. IT won't take long after that for the lamp to burn out. 
>
>That makes sense, but what about the sudden heating when the lamp is 
>turned on?  Doesn't the rapid heating phase cause as much or more 
>thermal stress as a rapid cooling phase?  It certainly is of shorter 
>duration than the cooling, with or without fan.  It would seem that 
>the life extension due to turning the fan off when the light is 
>turned off would be very minimal compared to the life extension that 
>might be possible if there could be a slow heating phase at turn-on 
>time. Of course in the limit, if one never heated the lamps at all 
>(kept them turned off all the time) the life extension could approach 
>being infinite. :) :) 

Quartz bulbs will develop cracks during turn on cycle as well but the
fan does reduce the temperature jump from sudden room temperature to
full operating temperature of the bulb. But don't think the impact is
that great thou. During shut down the fan is recommended to stay on
only if the lamp housing and components can't take the heat build up
if the fan was not on. It's not for the lamps life extension (other
than when it's on) and probably actually reduces its life span. Glass
or Quartz it would seem to me that if the lamp housing was able to
take the heat it would make more sense to then switch off the lamp and
fan at the same time. Now I don't remember my physics but when the lamp
be it quartz or even glass, (and fan) is turned off don't the lamps not
increase in temperature in the housing since it is not generating any
more heat. The only thing that would be building up heat is the
surrounding area.
The slow turn on would be a good idea and Hershel Saylor just posted 
an interesting tip on using a thermistor. I think alot of us have
noticed that bulbs in general burn out when switched on. Be it 
16mm film projectors or household bulbs. Hershel care to elaborate
on what type of thermistor is required and how it's put on. This of
course all applies to filament lamps and not Xe lamps. Question here
now I know they use Xe lamps in the Imax theatres but do they use them
also in the movie houses or in regular slide projectors. A Xe lamp
has it's own switch on circuitry and doesn't use a filament but uses
the Xe gas to generate the light similiar to a tube black light. 
Side note, I use a monochromator at work which uses either a 600W QTH 
(quartz tungsten halogen) lamp or 450kW Xe arc lamp.
In both cases the fan stays on after switching the power supplies off.
I will have to check with the manufacter why this is so since a few
good points were raised. As was mentioned its preferable to extend
the life of the projector housing than the lamps and I would agree but
in my case, Xe lamps go for a couple of hundred bucks so it isn't 
trivial. I'll check when I get back to work next week. BTW I used to
think the Xe lamp I use in the monochromator looked impressive, it's
about 8 inches long, until I saw what they use for the Imax movies 
which are roughly 3 feet long!

Gabriel 


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