Film-Tech Forum ARCHIVE: Protecting xenon bulbs in power failure

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Posted by Frank Angel (Member # 248) on 02-28-2005, 07:59 AM:

Maybe I am a worrywart, but I sit and look at the 4500w lamphouses and picture a power failure in the middle of a show where we loose AC. The fans stop. The bulbs explode. Do you guys have any kind of failsafe for such an eventuality? I am thinking some kind of UPS for an auxiliary fan in the lamphouse. I know this probably doesn't come up much, but power failures do occur -- it's not like New York hasn't had a few. I mean, what happened in all the theatres during our last black out? Power failed in the afternoon when every booth in the city was running. Did bulbs explode from overheating when the fans stopped?

Or am I being paranoid? Thing is, in our park venue, it's a portable system. That's a more risky setup than permanent installations. I've taken to tapeing over the booth breakers so stagehands who need to switch circuits on and off pretty routinely in that same panel don't accidently throw the rectifier circuits.

Posted by John Walsh (Member # 168) on 02-28-2005, 09:12 AM:

I don't think there's a problem. I've worked many places where the power to the lamps were just cut at the end of the night; that is, the lamp and fan were wired this way. No one should wire a place like that, but since I've seen it done routinely for years I don't think the rare power failure will effect the bulbs.

Posted by Darryl Spicer (Member # 711) on 02-28-2005, 09:25 AM:

I don't worry about the bulb more so than the possibility that the power does not return in full phase. I have seen this happen and I have to start running turning things off before 3 phase equipment starts burning up.

Posted by John Pytlak (Member # 331) on 02-28-2005, 09:58 AM:

I'll let the lamp manufacturers answer definitively, but a few hot shutdowns due to power failure are unlikely to significantly increase the risk of lamp explosion. However, a regular practice of not letting the lamphouse cool properly should be avoided.

I agree that you want to be sure the power supply is correct and unlikely to fail again before you restart. I would hope that after any line work, the power company would endeavor to energize all three phases at the same time. [Eek!]

Posted by Robert E. Allen (Member # 1351) on 02-28-2005, 12:11 PM:

We've had two or three power failures in the seven months I've been at our ten-plex and all 10 machines shut down including the 4500w lamps and fans. It has had no visible adverse affect on the lamps.

Posted by Frank Angel (Member # 248) on 02-28-2005, 12:34 PM:

Cool.....I mean HOT. [Big Grin]

Posted by Jarryd Beard (Member # 2543) on 02-28-2005, 12:55 PM:

Why would there be any adverse effects? (From a pure temperature/fan related standpoint.) I mean, if the power to the bulb and fan are cut at the same time, it's not getting any hotter in the lamphouse. In fact, it seems the more gradual cooldown with no fan might even be better for the bulb. That's why I never understood things such as overheads or slide projectors that keep a fan running after the bulb's been shut down. Why is it necessary to cool the bulb if it no longer generates heat? Perhaps, in the case of slide projectors, it would be useful for faster storage of the machine.

Also, my theatre is on the worst possible power connection for a theatre. High winds, a distant t-storm, or even heavy rains can cause brownouts or blackouts as often as once a week during the summer months. It just takes a lot of monitoring the booth, especially the takeup platters. However, as ofter as we have electrical problems, I've never heard of a bulb exploding around here.

Posted by Dominic Espinosa (Member # 2122) on 02-28-2005, 01:11 PM:

I'd imagine the continuous cooling is better than air just standing in the lamp house since the lamps are so hot.

In power failures my biggest worry is getting everything back online fast for the next set if we're out that long.

Posted by Monte L Fullmer (Member # 2797) on 02-28-2005, 01:36 PM:

True, bulbs DO need the fan to help cool down, since bulbs cannot dissapate the heat like a vacuum bulb can. One of the only fears would be if a sudden rush of cold air hits those hot bulbs, then a good explosion could exist.

I've seen theatres immediately turn the console power off when closing after the show is over with. I jumped on these personnel hard and tell them that the bulb needs to be cooled down before turing off the console power.

I've had power drop off when running 6k bulbs with no effect when the power came back on.

-Monte

Posted by Mike Spaeth (Member # 524) on 02-28-2005, 04:50 PM:

Most theatres I've worked at have a relay installed in the console that keeps the exhaust fans running for approx. 10-15 minutes after the power to the console is cut.

Posted by Robert John Jeromson (Member # 2559) on 02-28-2005, 04:56 PM:

Our exhaust fans are all BMS controlled, they start automatically when the power to a machine is switched on in the morning and run for 30 minutes after power shut down, it is company policy to let the lamp houses cool for at least 15 minutes before tuning the power off.

Posted by Darryl Spicer (Member # 711) on 02-28-2005, 05:43 PM:

quote: John Pytlak
I would hope that after any line work, the power company would endeavor to energize all three phases at the same time.

I have had the power lose a phase on a cople occasions while working at different theatres. I think both were caused by accidents involving telephone poles. Half the lights would be on in the building we would have to run and shut down anything 3phase and shut down air conditioners before things burnt out on them.

Posted by Randy Stankey (Member # 64) on 02-28-2005, 09:15 PM:

quote: Jarryd Beard
In fact, it seems the more gradual cooldown with no fan might even be better for the bulb.

I have to agree with Jarryd on this one. I think a gradual cool down is better for a lamp in the long run.

When the lamp is LIT... Yes! Keep the fan(s) ON! I have seen, first hand, what lack off air flow can do to a xenon lamp! There is no doubt in my mind.

On the other hand, cool down after the lamp is out should be gradual as possible. In fact, I read a research report from a guy who ran an AV department at a college that said just that. Leave the fan OFF when you shut off the lamp. Basically, thermal expansion and contraction takes its toll faster if the lamp is force cooled. Letting it cool naturally lets the lamps last longer.

Caveats:

1) These results were for halogen lamps with filiaments. Allowing lamps to cool slowly allows tungsten to redeposit on the filiament instead of coating the surface of the glass. Xenon lamps, having no filiament, are not subject to this problem.

2) The projectors could NOT be moved during the cooling period. Moving a projector with a hot lamp markedly increases the chance of lamp failure because the hot filiament is being jostled around.

In my booth, I split the difference. I shut off the forced cooling fan inside the lamphouse but I leave the roof exhaust turned on. Thus, my theory is that the lamp cools slower than it would under forced air but there is some air to cool the lamp.

Posted by Mike Babb (Member # 1379) on 03-01-2005, 05:28 PM:

Randy's post made me think of this (possibly stupid) question. If the lamp power is cut and the fans are cut, does the bulb actually get hotter or does it just cool more gradually from the temp it was at when the power was cut?

Posted by Jarryd Beard (Member # 2543) on 03-01-2005, 07:26 PM:

It seems to me, the bulb couldn't get any hotter. I mean, there's no more electricity. Our fans run as soon as power is supplied to the rectifier. We use the actual breakers to power up/shutdown. As soon as the rectifier breakers are hit, the fans are on. The roof exhausts are controlled by separate circuits, of course. I don't see the necessity for the lamphouse fans to be on constantly. It just seems to ensure a constantly noisy booth, even before projectors are running.

Posted by Randy Stankey (Member # 64) on 03-01-2005, 08:54 PM:

I'm only guessing but there might be a small increase in temperature right after the fan(s) go off. The heat radiating from the interior of the lamp gets blown away by the air flow but suddenly cutting off the air flow might allow heat to build.

I can't imagine that the heat would be detrimental. Like you said, the lamp isn't PRODUCING more heat. The heat that's inside is just equalizing throughout it.

Posted by Jarryd Beard (Member # 2543) on 03-01-2005, 09:33 PM:

But also keep in mind, the fan isn't suddenly stopping. As the fan decreases in speed, it's still moving air. I know that after I've power down for the night, the fans can still be heard whirling down slowly.

Posted by Gordon McLeod (Member # 33) on 03-02-2005, 12:52 AM:

actually ther is a very large temp increase if the lamp shuts of and the fan switchs off
The total energy of the lamp has a peroid of time to disipate that is a constant if the cooling flow is interupted
I used to have a graph of it

Posted by Wolff King Morrow (Member # 2226) on 03-02-2005, 04:47 AM:

quote: Randy Stankey
In my booth, I split the difference. I shut off the forced cooling fan inside the lamphouse but I leave the roof exhaust turned on.

Question: Do you do this after each show ends? Wouldn't you have to constantly run around shutting fans off as soon as movies dropped in say an 18-screen multiplex? If it's only done at night, then I'd say the advantage gained would be negated by all the sets in between.

Posted by Randy Stankey (Member # 64) on 03-02-2005, 09:23 AM:

We only do two or three shows per day. It's not a problem for me to shut them off on in-betweens.

Yeah, if you are in a googolplex it would be a pain to shut them off all the time.

Posted by David Buckley (Member # 2600) on 03-02-2005, 04:25 PM:

I dont know a huge amount about lamp physics, but do not a bit about keeping (big) electronics cool, and thermal pathways. If you turn off the heat source (lamp arc) and the heat removal source (cooling air) simultaneously, things between the two and around them will not necesarily not get hotter.

The heat source may vanish immediately the power is removed, but things very near to it are still hot. Thus stuff inbetween and around (bulb envelope, metal of lamphouse) will still be receiving heat from the elctrodes of the lamp, but not having the heat removed by the cooling air. Thus their temperatures could increase before they fall. Although the electrodes may not have the thermal intertia for this to be noticable, I dont know that.

On the other topic - I do know a lot about electricity, and if you have a three-phase load that requires all three phases, there should be a phase failoure detection relay in the control circuit. I dont know about the USA, but in the rest of the world, all small threephase motors are connected to a Direct Online Starter (DOL) which is the box with the red and green buttons, and these detect overload or any phase going undervoltage, and switch off. Inverter driven motors are also protected by similar logic, but electronic rather than thermal-magnetic.

If you've got A/C systems getting harrassed by phase failure, you've got big problems. It should just shut down abuptly.

Posted by Rich Granata (Member # 2285) on 03-02-2005, 05:38 PM:

So what the verdict? I might be up all night pondering thermodynamics.

My theatre policy has always been to let fans run for 15 minutes.
What do the lamp manufactures say?

Posted by Dennis J. Losco (Member # 1374) on 03-03-2005, 08:48 AM:

Absolutely leave the fans running!!!! The anode is approx 2000degrees C. The quartz body is 700 degrees C. The radiant heat from the anode will continually heat the quartz body beyond recommended temperatures. Forced air is essential on the cool down stage. A power outage is awful for the lamp. Without the proper cooling additional stresses will be created within the quartz wall. A chance of an explosion will increase dramatically. It could happen immediately or the stresses will slowly increase over time and explode later down the road. [Mad]

Posted by Frank Angel (Member # 248) on 03-04-2005, 09:20 PM:

Our Osram lamphouses won't let you turn the fans off with the bulbs. You can turn all the power off to the lamphouse and the fans will keep running for about 15-20 minutes on a thermal switch, no matter what, short of your turning off the breaker. These guys make the lamphouses and the bulbs, you gotta figure they know what they are doing.

Posted by Floyd Justin Newton (Member # 1316) on 03-04-2005, 09:31 PM:

Right on, Frank. It's just damn near impossible to convince
some people of that!