Coalgun- Puff backs & Explosions

rychw wrote:I have operated the boiler for 5 seasons on the factory settings and have had explosions (too many to count) during that time. That led me to make the changes to prevent the explosions. Yes, the explosions are a result of too much new coal being allowed to enter the burn chamber too fast. That is why slowing the rate of ash dumping will slow the rate of new coal allowed to enter the chamber. It is important to have the fire as high up the burn chamber, tube, as possible. I needed to lower the thermo ash dump temperature to accive that. The factory settings set by AHS are not correct, especially if you have a domestic hot water coil and thermo grate combo. Again, this is trial by error with no help from AHS.

I also had puffbacks for several seasons with factory settings. It was only when I came to understand the concept of fire height and searched for a SV that gave an appropriate fire height that my problem was resolved.

I know that one of the theories about the problem is that it is caused by the ashing running when the blower is not running. I don't have the data to conclusively accept or reject that theory. But I do know that my ashing does continue after the blower shuts down and for the past 1.5 years I haven't had a puffback.

I noted that both you and lsayre have your boilers set up so that the ashing cannot occur when the blower is not running.
I was only trying to suggest that you might try--if you haven't already--running the unit with ashing allowed when the boiler is not running but ensuring that the SV is set to maintain the appropriate fire height.

In my opinion it's a defective design. One of the fundamental principles of a control system is that the control sensor be strongly related to the variable you are trying to control. Such is not the case here. All seem to agree the height in the Anthratube of the burning glowing coal is what is to be controlled. Having it just below the inspection/flapper door is the ideal location. Too high and it will burn into the hopper, too low and CO gases are produced and when the right conditions, oxygen and flame are both present, boom.

So how is a single point temperature measurement related to glowing coal height? Not very well. Look where the sensor is, i.e., the tip of the thermocouple. What temperature does it see? Ash temperature, bottom of the boiler water temperature and burning glowing coal temperature. But how far away is the glowing coal bed? It's up the Anthratube, many inches away from the thermocouple tip. There is considerable thermal resistance, the stack of ash below the burning bed. Is it a fixed thermal resistance? No. It varies, varies according to thickness and coal type (white vs. red). So we have a feedback sensor and it's electronics trying to make a decision based on temperature. But that temperature is falsely influence by temperatures not related to the parameter your trying to control. I would expect the change in boiler water temperature as radiated down to the thermocouple has too much influence and swamps out the temperature path from the glowing coal in the Anthratube. Perhaps it could be made to work but first I would want to see a multi-node thermal computer simulation analysis. This is just a fancy way of saying today's engineering design methods and tools need to be applied, not those from a half century ago.

What would I design? I'd put a linear thermocouple array vertically in the Anthratube. Then the hottest spot would be the location of the burning coal bed. Operate the ashing motor to keep it high, but not too high. Change the combustion blower to a variable speed and slowly purge the gases, before opening the inspection port to idle position. Once I gained engineering data on this control, I'd redesign it to use only two thermocouples, one each at the top and bottom of the Anthratube. Using an algorithm based on both the ratio and absolute values of the temperatures I would determine the burning coal bed position.

My two cents worth of design, AHS go run with it. Express some comment. Critique my design. Offer yours. What are you waiting for?

As I have posted I don't have serious buff back issues using a percentage timer method of control. But, I also heat my domestic hot water using a indirect tank. This likely buffers any rapid boiler water temperature drop, that would occur using a domestic coil. Which would falsely lower the temperature seen by the thermocouple.

I've started measuring the duration my boilers firing (fan) cycles, and it seems that they range (so far) from 6 minutes to 14 minutes, with most of them clustered within the lower end of this range (6-7 minutes). My initial assumption is that 6 minutes must be the minimum amount of time that it takes the boiler to recover from 170 degrees to 180 degrees if only a single T-Stat (zone) is calling for heat. Therefore I'm thinking that if I put my Tork EJWT "Percentage Timer" to work for ashing, my initial settings would be 6 minutes on the "Time Cycle" dial, and 20% on the "Percentage On" dial. This would cause the boiler to ash for 20% of every 6 minute period of time that the fan is running. The EJWT resets itself each time current is dropped, so each time the fan shuts off the timer will reset and when the fan starts again it will begin counting down its repeating cycles of 6 minutes anew. For every "full" 6 minutes of firing, the boiler would ash for 1.2 minutes. A longer 14 minute firing cycle would see 2 ashing cycles of 1.2 minutes per each.

Just an observation from someone looking through the glass - What has AHS actually done to try and correct this problem?

Rob R. wrote:Just an observation from someone looking through the glass - What has AHS actually done to try and correct this problem?

So far they keep telling me to lower my ashing SV and/or decrease my hysteresis. I think I've gone as far as they are willing to take me with these settings. They have also suggested that I try a different coal (or try blending my coal), since Harmony has too high a percentage of volatiles, and so little ash (perhaps this is because it is so dense due to fusing into clinkers, or perhaps it is a combination of clinkering and truly low ash content). Both low ash "volume" and high volatiles are apparently considered major negatives for the AHS. A certain volume of ash is always needed to keep the fire high in the tube, else detonations, and the higher volatiles contribute to increased detonation intensity.

With the 130M Axeman-Anderson the fire column/tube is 11" dia. and ashing cycle is done w/ an Anthrastat temp. switch combined w/ an adjustable linkage to select the number of *clicks* to control the ashing speed/fire position...this would be much like what *rychw* is doing w/ his slowing down of the ashing motor's speed on a 130S AHS.

lsayre wrote:

Rob R. wrote:Just an observation from someone looking through the glass - What has AHS actually done to try and correct this problem?

They have also suggested that I try a different coal (or try blending my coal), since Harmony has too high a percentage of volatiles, and so little ash (perhaps this is because it is so dense due to fusing into clinkers, or perhaps it is a combination of clinkering and truly low ash content). Both low ash "volume" and high volatiles are apparently considered major negatives for the AHS. A certain volume of ash is always needed to keep the fire high in the tube, else detonations, and the higher volatiles contribute to increased detonation intensity.

But I'm burning the same UAE Harmony mine coal with a % time control without serious problems. Adjust the thermocouple controller all you want, it's not going provide a solution. The thermocouple sensor is the wrong place and the logic of the control system is incomplete.

Yanche wrote:But I'm burning the same UAE Harmony mine coal with a % time control without serious problems. Adjust the thermocouple controller all you want, it's not going provide a solution. The thermocouple sensor is the wrong place and the logic of the control system is incomplete.

Are you ashing for 2 minutes out of every 10? Does your cycle timer remember its place if a firing cycle runs for only 6-7 minutes, or does it reset after each fan cut-off?

Example: Boiler fires for 6 minutes. Does your boiler subsequently ash 2 minutes into the next firing cycle (ashing 2 minutes out of every 10 minutes of cumulative firing)? Or does it start counting time all over again instead and ash only if the firing cycle extends beyond 8 minutes (with ashing beginning at the 8 minute mark)? Or does it do something different from these two options?

My last two firing cycles were of 9 minutes and 8 minutes in duration.

As previously posted in this thread:

"My boiler ashing is controlled by a ten minute cycle timer. It is powered only when the combustion blower is running. I have it set to 1.5 minutes. So the ashing grate moves 1.5/10 % of the time the combustion blower is running. My fire height is near the top of the "Anthratube". I'm burning UAE pea coal and I have no puff backs."

Because the timer motor is not powered when the blower turns off, it remembers it's previous state. If it was ashing and didn't finish, it will start again when the blower is next powered.

Yanche wrote:As previously posted in this thread:

"My boiler ashing is controlled by a ten minute cycle timer. It is powered only when the combustion blower is running. I have it set to 1.5 minutes. So the ashing grate moves 1.5/10 % of the time the combustion blower is running. My fire height is near the top of the "Anthratube". I'm burning UAE pea coal and I have no puff backs."

Because the timer motor is not powered when the blower turns off, it remembers it's previous state. If it was ashing and didn't finish, it will start again when the blower is next powered.

Thanks much Yanche!

Keep your fingers crossed the Puff -Back season will start soon as the warmer weather arrives and the boilers sit idle during the day while running hard at night. The perfect storm for puff back seems to be more during the mild days...

I do not want to jinx myself since I have been running without any puff backs and did a mid winter cleaning two weeks ago . I have to admit I have been a little nervous in the boiler room during the ash cycle and I hear the coal dropping in the hopper. My wife calls my boiler "The Boom Box".. LOL

Yanche wrote:In my opinion it's a defective design. One of the fundamental principles of a control system is that the control sensor be strongly related to the variable you are trying to control. Such is not the case here.

I'm not so sure the design is defective.

First the AA uses the same design--a temperature probe to sense ash temperature-- for control of ashing and neither you nor anyone else so far as I know say the AA design is defective. I simply don't see how the use of more modern temperature sensors and control could be considered a defective design. I am aware that it has been suggested that selection of an alternate thermocouple may be desirable--but is there any reason to believe the thermocouple selection is causing puff backs?

Second, I see no evidence that there has been a complete examination of the boilers that experience the problem to rule out sample variation in the construction of the boilers or a defective component or electrical connection problems. Without these steps I think it is entirely premature to conclude that the design is defective. I do think it is fair to be critical of AHS for the failing to step in and help the owners having problems with a careful step by step approach to identifying the problem and eliminating it.

I agree that direct measurement of temperatures in the column of coal/ash as you suggest could be an improvement--but it seems to me it would be more expensive to implement--both in manufacturing of the basic boiler and for the additional components required along with a more complex controller and that it may not be necessary based on the long history of success of a simple temperature probe in the AA boiler.

It seems to me Freddy pin pointed the problem and fixed it with a more graduated and simple shut down of the air supply. Why go off on tangents? The excessive accumulation of unburned gas at certain times and conditions is the problem. The question I would have is, does Freddy's fix burn the gas or just dilute and exhaust it? The problem Yanche alludes to might be another problem entirely.

Bob wrote:I'm not so sure the design is defective.

First the AA uses the same design--a temperature probe to sense ash temperature-- for control of ashing and neither you nor anyone else so far as I know say the AA design is defective. I simply don't see how the use of more modern temperature sensors and control could be considered a defective design.

If you look at the measurement characteristics of the A-A temperature sensor and the AHS thermocouple, there is a world of difference. The A-A sensor is basically a bi-metal snap switch-like sensor. It integrates temperature changes and has a slow to respond characteristic. A thermocouple, on the other hand, is a tiny sensor, and does not integrate temperatures at all. It's response to change is instant. The electronics amplifies the milli-volt thermocouple voltages to something useful, also includes a hysteresis function, which allows a similar integration-like capability. But it's really different and in my opinion not what's needed. As has been pointed out many times none of us has the engineering data for a definitive answer. I still stand by my opinion, that it's not the right design.

I welcome an design explanation of either the A-A or AHS system by anyone. Temperature design set points, for rising and falling temperatures, hysteresis values, CO concentrations that cause or do not cause puff backs. Even if you don't have numbers, just explain how it's supposed to work. Is the idle draft what is expected to clear out the CO boom-boom mix? Etc.

My opinion is as follows:

The anthrastat is simply a device to make the boiler more effiecient. You only ash once the coal is burnt and the ash has cooled to set point. In the AA130's case, that is 140 degrees. You drop below 140, you ash until you go above 140. I do not know the span of the device but for most mechanical thermostats it could be anywhere from 1 to 8 degrees F.

What has to do with puff backs is ashing rate and amount. Too much ashing in relation to the firing rate, and the fire level drops creating more air and fresh coal volume in the combustion chamber, then ka-boom (technical term). Too little ashing rises the fire level to a point of chocking the fire. The right fire level is the key.

The reason for the explosions is not CO but fresh coal. Anthracite has 5 to 10 % volitiles. Put fresh coal in a hot zone and the first thing that happens is the volitiles bake off. This happens when the blower stops on the boiler. The only air drawn in is from natural draft. The excess heat rises and bakes the fresh coal releasing the volitiles. If your fire is too low, there is more volume of un-baked coal in the combustion chamber. This coal bakes, give off volitiles at a rate that overwhelms the natural draft's ability to draw them off. You hit the Lower Explosive Level (LEL) of the volitiles and then Ka-boom. There is also more air between the coal when the fire is low. That makes the Ka-boom even bigger.

No matter how you control it, the key is to match the firing rate and the ashing rate such that the fire level remains constant. The AA anthrastat acts as a limit if you are ashing too fast and hot coals start down the plate.

If ashing doesn't have a temperature input, you have no way to know if you are ashing right. The ashing rate has to be linked to the burn rate, more you burn, more you ash. If you were to ash at the end of a burn cycle with a timer, then the problem is under light load you cycle more and then ash more but burn less coal. Visa versa when under heavy load. Yanche has linked his timer to the blower such that he has it set to match his burn rate.

If you were to ash simply with a thermocouple or thermostat, then you again ash too much as it doesn't know when the blower shuts off and reduces the fire. It must be set to kill ashing when the blower shut off.

Again, just my view on things from operating my AA-130.