On 03/12/2014 21:02, Nick Maclaren wrote:
In article ,
newshound wrote:
On 03/12/2014 20:23, john t west wrote:
Looking at how some people are protecting their Green Houses against the
frost, i was surprised how tiny the flames were on the paraffin heaters
in the green houses. They were hardly giving off any heat at all.

I was told its not about the 'Heat', but the fact that the flame changes
the 'relative humidity' in the air'.

Could anyone explain fairly simply, how this actually works? Thanks

Well having high humidity in the greenhouse from the paraffin heater
means that as frost forms on the inside of the glass latent heat is
released, so that probably helps to maintain the internal air
temperature. Of course when it melts it promptly sucks all the heat back.

My back of envelope sums suggest that a kg of paraffin gives about 46 kJ
of energy when burned, while the potential latent heat of freezing of
the water produced would be about 26 kJ. So I would say that the heating
effect is more important.

I haven't done the calculation, but did you include the latent heat
of condensation as well? That is clearly more significant than
that of freezing from liquid.


Regards,
Nick Maclaren.

No I didn't, good point. I suppose the net heat of combustion which I
quoted from Wikipedia assumes the water is in the vapour phase. If so,
you get 46 + 122 = 168 kJ from the burn, once the water has condensed.
So the "protection" from freezing this moisture is proportionately much
less than the "heating" term.

(Relying on memory for the latent heats and J/cal conversion, 50 years
since A-level physics).