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Thermodynamics  RSS feed

 
Posts: 39
Location: NH
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I'm wondering if anyone can lead me to some of the math that goes along with the thermodynamics of heat exchange, pressure, dispersal rate, force in different diameter flue pipes, etc. I would love to be able to calculate the flow of heat in my systems and possibly what I'm losing out the exhausts, not to mention the efficiency of what I'm gaining (I hope) as radiant/convection heat in the living-space.

To begin with, one of the goals of an RMH is to surpass pyrolysis, which means reaching a burn temperature over 250 dF, yes? And this is in the burn chamber, or is it a secondary burn in the riser?

I'm also curious to see how many Newtons of force push the heat through the pipes and out the stack, which determines how hot I need the burn to be. Has anyone gone here? Has anyone been able to calculate, with any degree of accuracy, the BTUs of their homemade systems in order to determine efficiency?
 
pollinator
Posts: 2392
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Wow, that's a torrent of questions!

I would love to be able to calculate the flow of heat in my systems



First you have to start with the total heat of combustion available. This is not too difficult, you can look up heat values for the different types of fuel wood used.

what I'm losing out the exhausts



That means you are going to have to instrument the exhaust with a thermocouple so that you have some idea of the temperature of the gases exiting the stack.

((Gas temperature out) - (Gas temperature in)) x (heat capacity of air) is the amount of heat that the system is losing, and the rest of the heat you are capturing.

efficiency of what I'm gaining (I hope) as radiant/convection heat in the living-space



efficiency is 1-(heat lost/ heat of combustion).


one of the goals of an RMH is to surpass pyrolysis



I'm not sure you have stated that correctly, pyrolysis is just burning. If it's incomplete, then maybe you are losing a lot of flammable gases with more heat of combustion (like CO) up the stack. It would be better to say that the goal of a RMH is more complete pyrolysis.

Yes, one can calculate the pressure-volume work that is done by the gases expanding in the riser. But this is not a useful thing to do. You are not interested in getting work out of the RMH, just heat. If you had it connected it up to a piston arrangement that did mechanical work, well that would be another case.

I'm happy to help you work out the thermodynamics of your RMH, but let's start with the basics: do you have it instrumented so you can measure temperatures in all the areas of interest?



 
Jeff Rychwa
Posts: 39
Location: NH
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Hi, John. Thank you for responding.

I don't have the necessary equipment, yet, apparently, but now I have some direction.

I wanted to figure the force involved in order to be able to calculate, for future builds, the proportional amounts of exhaust pipe, riser heights for different cross-sectional dimensions, outdoor stack heights, etc.

This is completely new territory for me, so I'm throwing in a bunch of questions that are just brewing as I play with the concepts, and I really appreciate your taking the time to offer some guidance. This will help me start to make sense of what's coming together in my observations and future plans.
 
John Elliott
pollinator
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Temperature is only one variable you are going to have to measure. The other one is air flow. It would be nice to have some sort of aneometer so that you can measure air flow both into the RMH and how fast it is coming out the exhaust stack. Once you can measure T(emperature) and V(olume), we can calculate all the thermodynamic quantities because this is a constant pressure process.
 
Posts: 185
Location: Southeastern Connecticut, USA
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Jeff,

Have you checked out Peter Berg http://donkey32.proboards.com/thread/459/new-peter-van-berg-heater?page=1#scrollTo=4063
He has great practical research and RMH builds (and measurement tools) Search the site.

Bill

edit: spelling
l
 
Posts: 243
Location: near Houston, TX; zone 8b
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Jeff,

I am a partner in Dragon Heaters. We have the thermocouples and testing equipment necessary to ascertain the efficiency of a burn. We have built a design using our 6" burn tunnel designed by Peter van den Berg and clay chimney flue liners. The results of the testing on this build are found on our blog. http://blog.dragonheaters.com/category/flue-build-part-4/ and part 5. The draft on this system is excellent.

We didn't test the speed of the air flow. The Testo gas analyzer tests the efficiency of the combustion and that is sufficient for real world applications. I can tell you that the theoretical limit of burn efficiency in air (as opposed to pure oxygen) is 95%.

Peter van den Berg is a retired masonry heater designer from The Netherlands and has been investigating rocket heaters for several years.

Cindy
 
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