The challenge will be to develop a method to keep the oxygen sensor from fouling-up from the combustion by-products. Also, probably need a heated ox sensor, as the stove exhaust temps may not be hot enough for the ox sensor to function, especially if the stove is made more efficient (which will lower the exhaust temp). I suspect because of the above reasons it is not practical to utilize and ox sensor for a pellet stove.
"the correct Stoichiometric ratio"
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The challenge will be to develop a method to keep the oxygen sensor from fouling-up from the combustion by-products. The exhaust from a gasoline car is way cleaner than what comes out of a pellet stove! Also, probably need a heated ox sensor, as the stove exhaust temps may not be hot enough for the ox sensor to function, especially if the stove is made more efficient (which will lower the exhaust temp). I suspect because of the above reasons it is not practical to utilize and ox sensor for a pellet stove.
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maple1
Minister of Fire
NYBurner
Burning Hunk
John - I read the article regarding efficiency but was left wondering what the conclusion of the testing was? I see that the usage was ~1 ton per stove over a month cycle, but it does not discuss how the stoves were configured, feed rates, damper settings, etc.. so what was the final conclusion?
I understand the idea of 'excess' 02 measured at output but were any steps taken to adjust feed to compensate? Also flue temp was not explained in relevance to efficiency, is this explained as more efficient heat exchange or result of improper combustion?
Also in terms of this thread discussion relative to intentionally running rich for wood pellet stoich combustion, is the intent to slow air travel and allow a longer heat transfer time or more a function of burn time relative to the fuel to create a complete burn?
Sorry for all of the questions, I am just trying to understand this correctly.
Ravelli uses a hall sensors on the motor housing of the combustion blower to maintain variable known RPMs in the system logic. The older controllers and I'm not sure on the latest models, there was a recipe the installer had to set according to the vent length. This increased or decreased overall percentages of full power to the combustion blower across all heat settings. However, when fire is achieved, the stove creates positive pressure in the vent via heat (draw) so the hall sensors allow the controller to change output to the combustion blower.
The newer Ravelli / Ecoteck stoves, with the new RDS feature, don't need the "install recipe" setting (based on the installers opinion of the flue vent restriction/configuration). That was replaced with the RDS initial setup routine, done one-time when the stove is installed, which actually measures the restriction of the flue vent system that was installed, and sets the baseline RPM of the combustion blower (for each of the "power" heat settings P1 thru P5). The RDS system can do this because it has a mass-airflow sensor in the intake air path, which can measure the volume of intake air. The controller adjusts the RPM of the combustion blower to get the measured intake air volume to match the factory preset values, one for each P power level. This RDS system is effectively an auto-adjusting combustion air damper, and can maintain the same amount of combustion airflow even as the flue and burn-pot becomes more restrictive due to ash build-up. When the ash build-up becomes too large to be compensated for (by ever increasing the combustion blower RPMs), the stove then trips an alarm that tells the user to clean out the stove.
The "old" Rika Austroflam Integra stoves were the first stove to use an intake mass-airflow sensor, but they did not have an adaptation routine like the Ravelli RDS, to adjust for different exhaust flue pipe restrictions/configurations that are unique for each installation.
The "old" Rika Austroflam Integra stoves were the first stove to use an intake mass-airflow sensor, but they did not have an adaptation routine like the Ravelli RDS, to adjust for different exhaust flue pipe restrictions/configurations that are unique for each installation.
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NYBurner
Burning Hunk
Hey Guys - just hoping to bump this so someone in the know can answer my questions on this (post #54 right above). With discussions bouncing back and forth regarding o2 sensors, mass air, etc....I feel there is 3 different focus of discussions taking place that all have different requirements, clean emissions, ideal burn, and stove heat efficiency. Either that or I am totally missing something very apparent to everyone else so would really appreciate the clarification/education!
Clean emissions and ideal burn are kinda the same thing.
Stoitiometry as it relates to the air and fuel in the combustion of a pellet stove has to take in to consideration a lot of variables that are not known or may be a variable. The common ratio in wood combustion is 35 parts air to 1 part fuel as they meet and combust in the combustion zone. Pellet stoves, because they have controls and induced draft for a very long time enjoyed a default efficiency of 78% as it was thought that through these controls, optimum efficiency was going to be achieved. I believe this myth as been busted. All manufacturers seems to go about this achievement in all sorts of ways and if I had to put money on it, from the testing I've been privy to in the lab as well as my vast knowledge of equipment out there, I'd have to say that very few, if any, are able to get to and maintain that sweet spot and here's why. As I said above, there are variables.
For example, a pellet stove rated for 3" diameter venting that has 3 90 degree offsets, rises 7 feet in height and travels 2 feet horizontal will have much more restriction than a direct vent installation of 3' straight out horizontal through a wall. These two installation types are common and both quite short runs but will perform different, especially once they are dirty, whether they are warm or cold and whether there may be additional influences such as leeward or windward issues relating to the side of the house they may be installed on. Additionally the same unit may be vented in to a masonry liner, insulated or not and extending 40 feet above a roof pitch. All the same stove.
Some installation requirements want the installer to start and run the stove and set dampers, such as the case with all Sherwood products. Harman may have a default impeller that is used below 4000 feet in altitude or another one used above.
All of this plays into how much air is allowed to enter the machine. Most stoves have an excess of air. This allows the unit to run and operate the same when it's clean as when it's slightly neglected but this approach will lower the overall combustion efficiency as well as emissions.
The correct ratio of air required cannot be attained in an all-in-one blanket approach. As stated above, Sherwood allows a damper to be set for the installation by measuring the draft and making the required corrections but j can tell you most installers just don't do it. It adds an additional 1 or 2'hours to the install, most don't own the equipment etc etc
Back in the day or Austroflamme, now Rika and also the engine for todays Hearthstone pellet stoves, a mass airflow sensor was used in the intake path that allowed the controls to measure and ramp up or down the combustion fan to try and create a perfect air to fuel ratio. If the vent was in a tall chimney liner, there would be an assist once the liner became warm and this would pull air through the machine. The control board can see the RPMs rising and slow the motor. If the system had a direct vent and it was very dirty, the airflow would be restricted and to some extent the draft blower could speed up and deliver more air....however, there are some drawbacks; the controls are ONLY measuring the air flow and bouncing that info off a preprogrammed known variable written in to the logic. Also, it can only measure the intake air from the intake. Of the stove has an air wash, gasket leak etc, combustion efficiency is affected. This system is far from perfect. While it seems to know what going on in the stove for air, it doesn't really know what going on on the fuel side.
From a measurement standpoint it seems to me that if one is trying to create the "perfect burn" one MUST measure the stove's combustion efficiency such as with a oxygen sensor as well as mass air flow and perhaps even a fit of temp sensor and white your at it, toss a sensor on the vent too.
It's a lot to consider and try and work out. I can tell you this, no manufacturer seems to want to let the end user know too much about what actually going on and perhaps that's an issue the service sector of this industry needs to try and figure out.
I spent a couple days with this dude Norbert, a builder of masonry heaters up in Canada. We was saying that they passed some regulations in his parts where technicians as part of a chimney service actually have to test the system for efficiency and make adjustments as necessary. The test adds $100 or so to the cost of a service call and requires an upfront investment of $6k or so for the equipment, but in his view it's worth it. For one it gives a bit of credibility back to the trade as we are actually measuring something and making improvements AND it kind of thins out those who are charging for a service that they may or may not be doing correctly. That's a whole new thread.
NYBurner
Burning Hunk
Scott- I agree with what you are saying (except that emissions focus and perfect burn focus are identical......often similar but not the same) however, that is my question. What is the focus of determining the parameters of a 'perfect burn'?? The test paper posted above had some very interesting information but the relevance and intended data support was never given. (ie...If my emissions have high relative O2 but my flue temps are safe.....why is that inefficient/bad and how does it correlate to heat output into my house?)
I, like everyone on this forum love to tinker with the stove and see what it can do..learn how to make it better. At the same time its main objective in my opinion is to heat and reading what is in this thread, I am being advised that I need to choke down my stove for a 'better' burn. So I am asking....what constitutes a 'better burn' relative to direction given by the Green Heat Team and the info posted? Is this more emissions oriented, or better for me heat transfer oriented?? When I am thinking efficiency I am thinking of thermal efficiency into my home.
Running lean in a race motor is associated with running hot. However a race motor runs under compression and a pellet stove running under very low vacuum are two completely different things and I am trying to understand.
I, like everyone on this forum love to tinker with the stove and see what it can do..learn how to make it better. At the same time its main objective in my opinion is to heat and reading what is in this thread, I am being advised that I need to choke down my stove for a 'better' burn. So I am asking....what constitutes a 'better burn' relative to direction given by the Green Heat Team and the info posted? Is this more emissions oriented, or better for me heat transfer oriented?? When I am thinking efficiency I am thinking of thermal efficiency into my home.
Running lean in a race motor is associated with running hot. However a race motor runs under compression and a pellet stove running under very low vacuum are two completely different things and I am trying to understand.
As far as I understand it, my pellet boiler takes the inputs of water temperature, flu temperature, and time and varies pellet feed rate and induced fan speed according to some magic algorithm. Windhager touts its system as requiring less maintenance and provide just as good results as, for example, Fröling's lambda sensor, which I'm sure Fröling would contest. I would imagine one could stick a probe from a device similar, or maybe the same, as that used by an oil burner service tech, to determine efficiency. I think one indicator of efficiency could be flue temperature-the lower the better, above the condensation point. I have read that there are condensing pellet boilers in Europe.
Yes agree, heat transfer efficiency and emissions efficiency are two completely different things. Harman stoves, specifically speaking, the 52i insert (that's what we tested ourselves) has a fantastic heat transfer on low to medium burns, it could be seen by monitoring the flue temps. But as we increased up to high burn rates, the flue temps went up considerably as compared to the lower and medium settings. This really has all to do with the heat exchangers thermal transfer capability. One the one hand, we want the stove produce as much heat as required but on the other hand one is limited by the design. Suppose that's one of the reasons why we tested stoves.
A perfect burn is subjective and subject to interpretation I suppose. I can tell you this, stoves are primarily "decorative" appliances. If manufacturers were super interested in achieving the highest efficiencies, both transfer and emissions, they would get rid of the glass. Glass is a problem as its not insulated and to keep it clean air that could be used solely for combustion is diverted to that glass suave to keep it clean via friction, which I'll add also cools the firebox. This is another topic altogether but part of getting these efficiencies up, you need to insulate the firebox and direct the whole amount of heat to the exchanger. Focus there. From my point of view, a stove really needs to monitor efficiency and have some kind of a variable heat exchanger that can transfer heat at as close to 100% as possible across all heat levels. Water jackets do this fairly well in boilers. I've done a few prototype drawings using mineral oil.
Perfect burn... Nothing's perfect. One thing for sure, there's plenty of room for new technology in a system that hasn't seen much innovation since it was conceived 30 years ago.
Awe I thinks it's more complicated than "sticking a probe from a similar device"
! There needs to be some logic to read the input and then all that data needs to be bounced off all of the other data that's being considered. I've written some ladder logic and man, you have like three lines of of code for one event and then you add another data set and then you have 12 lines of code and the. You add another data set and you have 36 lines of code....to a system that may have 1200 lines of code or more adding another data set is like rewriting 3600 lines of code!
I was referring to the Bacharach combustion analyzer, or whatever, that the oil techs use to measure the efficiency of an oil burner. You referred to that earlier to analyze the efficiency of a biomass burner.
Right, gotcha, like a Testo or such. But I agree, someone should use something to see what's relevant to the issues atbhand
NYBurner
Burning Hunk
Ok this kind of walks back to what I was thinking and I wanted to make sure of in my head before I commented in this direction. Moving away from the glass, liquid transfer, etc...the reality is we all bought pellet stoves for the ambiance as well the heat. Attempting to turn these into blast furnace style stoves would defeat the purpose really, and people looking for something of that style can get one (furnace/boiler). So basically at the end of the day this all really comes back to a thread I had posted a month or two ago regarding turning up the convection blower manually.
Via the test data, high flue temps indicate that the maximum heat transfer is occuring in most all of the stoves via their thermal capacity, except the Enviro (which is more apparent by the o2 data suggesting non ideal burn although Im not sure I agree with this). In reality, creating guidelines or research into a 'more efficient' burn are some what pointless. Either a catalyst afterburn can be added similar to a blazeking wood stove to increase both thermal output and emissions cleanliness, greater mass needs to be added to the stoves, and/or higher output convection blowers to increase the temp delta between burn and exchanger. Correct?
I really like the idea of improving efficiency, and tinkering in general.....but this smells like a big marketing push via the industry to 'add on' a bunch of sensors/complexity/service and COST to a system that really just needs a bigger fan/plate of metal added. I love innovation, but at this point the basic crude design is not even perfected.
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