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Would a 70w halogen bulb produce less heat than 60w normal bulb?

  1. Dec 21, 2011 #1
    On Light bulb appliances there is often a 60w limit. I would guess that a halogen bulb of 70w would produce less heat so should be fine to put in similar fittings.
     
  2. jcsd
  3. Dec 21, 2011 #2

    russ_watters

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    Staff: Mentor

    60W and 70W are both energy outputs, so no, 70W is more than 60W.
     
  4. Dec 21, 2011 #3
    It will produce more light and less heat (IR) but it will produce 10w more as the total sum of visible and IR power which may heat or may not result is a hotter fixture. It depends on how much of the energy is captured by the fixture instead of radiating out.
     
  5. Dec 21, 2011 #4
    You should access and read the following two sites, both of which are informative about light sources:
    http://en.wikipedia.org/wiki/Incandescent_light_bulb
    http://en.wikipedia.org/wiki/Luminous_efficacy#Overall_luminous_efficacy

    There you will find out that for a thermal radiator such as the incadescent bulb the theoretical luminous [efficacy] is 95 lumens per watt, or about 14%, but this is at a temperature of 6300 C. For a typical tunsten bulb, where the filament remains solid, (below 3683 kelvins), most of its emission is in the infrared and the overall luminous efficiency is about 2%.

    From the wikapedia article, the halogen lamp operating at 230 v , 100 W, would have an overall luminous efficacy (lm/W) of 2.4 % versus that of a regular incadescent of 2%. This means that this halogen bulb gives out about 20% more visable light, with the same power input.

    Your two bulbs operate at 60W and 70W.
    Using the above figures as a rough guide ( you would have to consult manufacturer fact sheets to obtain actual figures for voltage, wattage )
    60W x 98% = 58,8 watts is given off as heat or not visable em radiation.
    70W x 97.6 = 68.3 watts as heat or non visable em radiation.

    The 70 W halogen gives off more heat than the 60 W regular incadescent.
     
    Last edited: Dec 21, 2011
  6. Dec 22, 2011 #5
    But you don't want the luminous efficiency, because that accounts for the sensitivity of the human eye, wich is irrelevant here. You only want to know what percentage of the energy
    is radiated away, and not stopped by the glass, or the fixture. This will include lots of
    red light, for wich the eye is not very sensitive, and some infrared as well.
     
  7. Dec 22, 2011 #6
    Exactly, Willem.

    A similar miscalculation by manufacturers of the newer LED lighting has lead to building fires and a change of building electrical codes (in Europe at least).

    A further comment.

    The higher the element temperature the more visible light you get per watt input.
    Tungsten is used as the element because it has the highest known melting point of all conductors.
    The higher the temperature of the tungsten element the more it evaporates off tungsten atoms.
    These coat the inside of the bulb and darken it, reducing the light transmission. This also reduces the life of the element.
    So the actual element temperature is an engineering compromise between useful life and output.

    Introducing halogens into the bulb's atmosphere partially suppress these evaporated atoms, allowing the compromise to be shifted towards a higher temperature and more light per watt.
     
  8. Dec 22, 2011 #7

    russ_watters

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    Staff: Mentor

    ....er, note though, guys, that all of the light energy is heat energy too. It's all just different frequencies of the same thing and ultimately, the 70W lamp adds 70W of heat to the room.
     
  9. Dec 22, 2011 #8
    70 watts is 70 watts, yes.

    But to the room?

    There is a practical difference between 70watts spread throughout the room and 70 (or a bit less) confined to the solid fitting and concentrated near some flammable building component.
     
  10. Dec 22, 2011 #9

    I like Serena

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    I believe the OP's question is whether a 70 W halogen bulb can be put in a 60 W appliance fitting.
    As yet there is no answer.
    However, from the posts I've seen here, I would tentatively conclude that one shouldn't.
     
  11. Dec 22, 2011 #10
    I think this is one of those situations where it is a case of

    "If you don't know the answer then don't do it"

    Halogen lamps are run at higher temperatures than standard.

    This means that the lampholder will experience higher temperatures.

    This is why halogen lamps are (should be) fitted only into ceramic holders and the immediate connecting leads should have a high temperature cover.

    So it is not only a question of wattage.
    You should not (in general) use a 25 watt halogen lamp in a fitting designed for a standard 60 watt lamp.
    Many fittings designed for standard lamps are made of or contain plastic, unsuitable for use at the temperatures encountered in a halogen lamp.
     
  12. Dec 22, 2011 #11
    Post #7 is the correct theory: no matter how you slice it, a 70 watt bulb is consuming more electrical power than a 60 watt bulb since those are measures of power consumption...not direct measures of light production or efficiency. Whether the heat [power] consumed is given off via conduction, convection or radiation, it IS given off.

    So I think Studiot and Serena have the correct final conclusion.
     
  13. Dec 22, 2011 #12
    Hello Naty, let us be more dramatic about the answer since I say it is also a question of temperature as well as wattage.

    Suppose we had two lamp fittings

    1) A conventional fitting with plastic insulating parts and a 60watt bulb

    2) A 'special' fitting with insulating parts made of ice (yes ice) and a 5 watt bulb

    Which would fail first?
     
  14. Dec 22, 2011 #13

    cmb

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    In practice, I suspect it will make no difference, especially if the fitting is 'all metal' rather than some cheap plastic affair. I have desk lights with a '60W' sticker on them, but that is really implying not to put a 100W in there (that being the next size up, for the incandescent bulb range to which it is referring). With a proliferation of new bulb technolgies and varying power outputs, chances are that it will make not the blindest bit of difference, even more so if the bulb in question is a 'spot/reflector' bulb.

    The other thing not mentioned here so far is that the physical size of the equivalent halogen is now typically smaller. The fitment rating for an incandescent bulb would, I am quite sure, also take into account the physical size of the bulb within the envelope of the fitting. With halogen types being physically smaller, the convection is much improved within the light fitting, again suggesting that a '60W' fitment rating may be looked upon with a degree of 'flexibility' with the new range of power ratings.

    Bear in mind also that such power ratings are unlikely to produce an outright 'hazard', simply that higher powers result in higher surface temperatures, and at some point someone has made a judgement call as to how hot they are permitting surfaces to get before it raises a 'burn-injury' risk. One might expect some margin has been built into that decision.

    Disclaimer; we cannot see nor know what you have, nor the engineering design margins on it, so what you choose to fit to what fitting is your call, and yours alone.
     
  15. Dec 22, 2011 #14
    Try reading the information supplied in this thread again and see if you still feel this way.
     
  16. Dec 22, 2011 #15

    cmb

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    I'll not counter you directly on that, but I have never heard of such a caution ever being made. Do you have a link to some consumer/safety website, or such, that puts forward this caution?

    There is a caution on the type of switchgear you need to run such bulbs. You generally need twice the equivalent rating for a switch, because of the initial high current loads of the halogen.

    Consumers will, no doubt, make up their own minds on this, so without clear information you'll get comments like mine, and like Studiot's in conflict.

    I'm not so sure about the 25W versus 60W thing though. This sounds unlikely. For sure, the filament itself runs hotter, but we are talking about an amount of power feeding into an envelope and then a flux of that energy coming back out of it. If the two envelopes are the same size and the same energy flux (notwithstanding the discussions on the spectrum content, as above) I don't see why either would be much different to the other in regards heating of the immediate surroundings.
     
  17. Dec 22, 2011 #16

    cmb

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    I did. Which bit do you think impacts that conclusion?
     
    Last edited: Dec 22, 2011
  18. Dec 22, 2011 #17
    cmb, you seem to focus on heat input alone.

    1) The smaller the bulb the smaller the surface area to effect heat transfer to the surroundings tending to reduce its rate.

    2) Filaments in halogen lamps run at considerably higher temperatures than standard. Actual figures are already given here by others. Since the filament is hotter the surrounding lamp internal atmosphere will be at a higher temperature, as will its envelope material.

    3) Newton's law of cooling tells us that the higher temperature promotes more rapid heat transfer.

    4) Regardless of the rate of heat transfer contact with a body above its softening or melting temperature will eventually destroy insulation. (By the way what lampholders do you know that contain only metal parts?). Hence my ice (an insulator) example.

    So there are some competing effects in respect of the rate of cooling, but the issue of heat damage to insulating material is absolute and still occurs with conventional bulbs in non-ceramic insulators.
     
  19. Dec 22, 2011 #18

    cmb

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    Right, but there is an equivalently 'less of' a halogen bulb. Newton's rate of cooling doesn't describe that you'd have twice the heat flow for twice as much stuff.

    If you were to draw a bubble around the bulb with a total surface area of 100cm^2 and you stick 100W into it and it attains thermal equilibrium, then I do not see how it matters at all whether that 100W has been used to heat something to 80C or to 10,000K, because you will still have 1 W/cm^2 heat flux from it.

    The consequence of the 'spectral temperature' difference is accounted for above - we're talking 97.6% of the total heat flux under consideration for a halogen, compared with 98% for an incandescent. I'm generally discounting that as being essentially a statistical irrelevance, compared with all the other unknown variables in the OP's light fitting.

    So, I don't comprehend what you are saying. Why does a thermal flux of 1 W/cm^2 from one bulb differ to 1 W/cm^2 from another?
     
  20. Dec 22, 2011 #19
    Because the heat is being produced by sources of different temperatures.

    The problem is not with the heat dissipated to the general surroundings (the room) but the temperature of the heat source in intimate physical contact with very local surroundings (the holder) and its effect on the temperature sensitive parts of that holder. Yes the heat passes through the holder and onto the general surroundings, but what damage does it do on the way?
     
  21. Dec 22, 2011 #20

    cmb

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    So, if you're saying that you need to run a 25W halogen in place of a 60W incandescent, and that it is because of heat conduction up the bulb stalk, then how much power are you saying is going up the stalk in each case?

    Do you have a reliable source for this assertion? It just sounds very wrong to me.
     
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