[Stoves] Calculation help

Frank Shields frank at compostlab.com
Wed Jan 18 13:44:25 CST 2012


Thanks again for all the help.

 

– I like the idea of working in the 40 to 70 deg  range.  

 

 

Regards

 

Frank

 

 

Frank Shields

42 Hangar Way

Watsonville,  CA  95076

(831) 724-5244 tel

(831) 724-3188 fax

frank at bioCharlab.com

 

 

 

 

 

 

From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Crispin
Pemberton-Pigott
Sent: Wednesday, January 18, 2012 11:25 AM
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Calculation help

 

Dear Frank

 

>Not that this has anything to do with stoves, but if water has 0 enthalpy
at 0 deg C 

 

It is no so. Water at 0° C has lots of heat in it, all the way down to 0° K.
The specific heat of ice is not constant actually because it takes different
crystal forms (at least 6). Been a while since I read about that. It is
about 2.108 so there is about 580 Joules of heat in a g of water ice. You
said it was water at 0° so you have to freeze it first (= 335 J /g). So
water at 0° contains 915 Joules per g.

 

>And is the LHV ever calculated using the temperature of the stack gas? 

 

LHV refers to the heat available from the fuel, arbitrarily calculated. The
rating for the fuel will be what it is calculated to be.  If you are trying
to measure the performance of a stove and the question of condensation
arises, you will have to compensate for it. Simple as that. 

 

>It seems it would not be because some water may have condensed on the cold
surface meaning the total water would not all be in the gas phase.

 

It increases the heat transfer efficiency in that heat was gained through
condensation). It means you have to use the LHV plus the heat gained from
condensing that mass of water. It shows up early in testing with large cold
pots and a fuel with high H2 like natural gas or alcohol. You can see water
condensing on the pot sides that looks as if it is sweating. As it is
usually re-evaporated later, there is no overall difference.

 

If you want to check the heat transfer efficiency or the system efficiency
it should be done when there is no condensation on the pot, i.e. when the
pot surface is hot enough to prevent condensation. I suggest 40-70 degrees.
As Jim Jetter pointed out recently, it is difficult to tell what the system
efficiency is at low power if the pot is already boiling. At the SeTAR
Centre we use a cold pot for all power tests and replace it if it gets hot.
The water mass and rate of temperature rise from 40-70 gives you the power
absorbed, and the fuel mass burned gives you the power generated. The two
give you the efficiency at that power level. Pretty accurately too. Remember
that it is not the fire-to-pot heat transfer efficiency, it is the
efficiency of the whole system.

 

You can thus plot the efficiency curve for any emission change as the power
is increased from low to high. Repeat this with several pot sizes and you
have characterised the stove well enough to predict the WBT performance, or
any other cooking cycle (called a ‘burn cycle’). If you instead start with
the WBT numbers, you can’t work out anything at all backwards to produce the
curves because the number are mixed together.

 

What is most interesting is to see that with some stoves, the pot is an
important part of the combustion. Changing the pot only can result in a big
change in emissions and thermal efficiency. This matters a lot if the stove
is designed for small pots and a large one either can’t be boiled or if it
interferes with the combustion.

 

Regards

Crispin

 

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