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Posted by Victor Roberts on March 27, 2007, 10:46 am
On Tue, 27 Mar 2007 04:14:33 +0000 (UTC), don@manx.misty.com
(Don Klipstein) wrote:
>>On 26 Mar 2007 17:24:14 GMT, dhinds@sonic.net wrote:
><edit for space>
>
>>>I've heard the claim that CF bulbs heat up a fixture more
>>>than an incandescent bulb of higher wattage. How that is possible is
>>>a little mysterious to me since it seems to me that the CF bulb is
>>>producing a much smaller percentage of a much smaller wattage as waste
>>>heat. But maybe the CF heat is somehow better captured.
>>
>>As Don has said, most of the "waste heat" in an incandescent
>>lamps is IR radiation at wavelengths just longer than
>>visible light. It can escape the fixture just as the light
>>does.
>>
>>All of the waste heat from a CFL is very long wavelength IR
>>that tends to get trapped in the fixture. But, as has been
>>stated by others, the lower power of the CFL tends to offset
>>this issue so any increased fixture temperature is small for
>>equal amounts of light. If you used equal input power for
>>the CFL and incandescent sources, then the fixture with the
>>CFL would get hotter.
>
> I would like to add that for equal light output, a CFL typically has so
>much less power input that a CFL heats the fixture less than an
>incandescent does despite CFL being more efficient at fixture heating than
>incandescent.
Don, I agree. I was thinking last night that my post on
this subject was incorrect on this point. In spite of the
higher fraction of non-radiated power, a CFL will create
lower in-fixture temperatures than an incandescent lamp of
the same light output.
The issue that got those of us working on CFLs interested in
this subject was not an equal light comparison, but an equal
power comparison. At one point in time, we naively thought
we could predict the in-fixture ambient temperature of a CFL
by installing an incandescent lamp of the same POWER in the
fixture and measuring the ambient temperature. We then
realized the now obvious fact that due to the high fraction
of "short wavelength" IR radiation from an incandescent
lamp, a CFL would produce higher in-fixture ambient
temperatures than an incandescent lamp of the SAME POWER.
> I have posted one data point saying that CFLs are about 50% more
>efficient at heating a fixture than incandescents are. (This will vary,
>because different incandescents have different percentage loss by heat
>conduction/convection by the fill gas or lack thereof. The loss is
>highest in the lowest current models that use a fill gas.) CFLs are
>generally 3-4 times as efficacious at producing visible light as
>incandescents are. So a CFL should be able to produce somewhere around
>or over twice as much light as an incandescent that causes equal fixture
>heating.
>
> On the other hand, half as much fixture heating may still be too much
>for CFLs since CFLs have a more restrictive upper limit on operating
>temperature than incandescents have. CFL life may be shortened. Also,
>excessively high temperature can cause the light output of a CFL to
>decrease, color to shift, and color rendering index to decrease due to
>above-optimum concentration of mercury vapor.
>
>>>So... throwing caution to the wind, I accidentally left a CFL bulb
>>>(Feit 23w spiral, 100w equivalent, available all over in California
>>>for $0.25 due to PG&E credits) on overnight in an enclosed, recessed
>>>ceiling fixture. This morning I tentatively touched the glass cover
>>>plate... a little warm, not hot. I opened it up, felt around inside,
>>>the reflector and casing are only warm. I grab the bulb itself by the
>>>ballast and by the spiral itself... warmer, but not too hot to hold.
>>
>>But you are not a semiconductor component.
>>
>>>I know from experience that these fixtures (unvented metal boxes) get
>>>VERY hot with a standard 75w incandescent bulb. Can I conclude from
>>>these observations that the CF bulbs are fine? What sort of reduction
>>>in life should I expect from what seems like a very modest increase in
>>>operating temperature?
>>
>>Well, one lamp I know about was designed for 15,000 hours
>>life in free air at 25C so it would reach 10,000 hours in a
>>recessed can.
>
> Based on how much of a heat hellhole recessed cans often are, I suspect
>the electronic ballast in that lamp would have to have a life expectancy
>of a goodly 50,000-100,000 hours, probably closer to or maybe even above
>100,000 hours, in free air in order to have that low an impact on life
>expectancy of a CFL in a recessed can. In a recessed can, air warmed by
>the lamp rises and the ballast will get hotter than anything else. And
>hot air around the ballast may be at least somewhat trapped there, and the
>ballast often gets pretty toasty warm to put it mildly. That is true even
>if the tip end of the tubing is not all that hot.
I am guilty once again. I should have pointed out that
screw-base CFL lifetime is function of both the life of the
"wire lamp" and the ballast, with most failures in free air
being caused by failure of the "wire lamp." The "wire
lamp" failure rate is not a significant function of ambient
temperature.
The term "wire lamp" is GE-jargon for the fully functional
lamp portion of the CFL excluding the ballast, ballast
housing and base. Perhaps others use the same jargon. I
would love to have a better name for this part of a CFL, but
the term "lamp" is already taken since it refers to the
whole, fully-functional CFL and the term "bulb" is already
used to refer to the formed glassware before processing into
a functional lamp.
--
Vic Roberts
http://www.RobertsResearchInc.com
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