AHS S130 Themo Grate Control - Outside Install

Hello again,

Preface: Caveat actor.

I searched the postings and could not find a thread specific to a S130 stoker installed outside that address the thermo grate settings.

Definition: An outside install is a stoker in a building that is open to ambient conditions and covered from rain. Outside air is free to move through the building and the temperature adjacent to the stoker is within a few degrees of the outside temperature."

I went to this length describing the conditions because when the blower is running the incoming air temperature can be very cold. In Central NY we normally get a cold spell with temperatures hovering around -20°F for a few days at a time. I have observed the ash with a fair amount of black coal in the ash under these conditions. To me that signifies the thermo grate setting is not optimum for high efficiency burn.

My reaction to this condition is to set the thermo grate temperature relative to the recommended setting of 130°F. The relative difference is based on the "normal" ambient temperature around a stoker installed inside. I am guessing "normal" ambient is 60°F in a basement. That puts the delta at (130 - 60 = 70°F.) The delta becomes the constant in my thermo grate setting formula, which is:

TG Set Point = Ambient +70

Here is what happens. When it's -20°F outside and the blower starts you can watch the grate (air) temperature plummet as the warm air around the stock thermocouple is replace by -20°F air from outside. This leads to early onset grate actuation. I have set my temp as low as 50°F to try and compensate. This setting did work but I was not comfortable leaving it there. Sometimes the ambient temperature changes quickley and a low TG set point like 50°F would most likely cause the coal in the feed tube to burn ... hazardous operation to say the least.

I have been thinking about ways to automatically overcome the efficiency problem without causing a hazard. I am thinking a differential controller may do the trick but they are typically expensive and not something that is easy for everybody to employ.

So I have decided to try manipulating the current sensing method to eliminate the variability of rapidly changing air temperature (convective) by measuring the ash directly (conductive). To test this hypothesis I have placed a thermocouple into the ash 8 inches in from the edge of the grate and another one adjacent to the stock sensor. Both are connected to a chart recorder. I am hopeful I will see a correlation between the conductive and convective temperatures. If I am lucky there will be a few degrees of change in the conductive mode that I can use to control the grate. Then I will simply change the mounting of the stock sensor and then tweak the set point and histerisis accordingly. Fingers crossed.

Pic (1) Sensors Pic(2) Chart Recorder I will provide the data for those interested.
Any constructive comments?

Brian

A question--have you tried the recommended setting of 140 for a sufficient amount of time at low ambient temperatures that you can reasonably conclude that it provides worse results than the lower temperatures you have tried?

I ask this because the sensor may detect radiant heat from the ash at the edge of the grate and the sensed radiant heat may be only marginally affected by ambient air temperature.

It is further possible that by setting the temperature very low you are forcing the burning portion of the coal stack higher that it should be and increasing the possibility that it will burn up into the coal supply tube.

As long as there's coal being fed to it, the coal should not burn into the feed tube. There's not enough oxygen to support it. If the auger runs low of coal, then the fire can creep up into the head.

Why not close the building up some? Then you'd have warmer air around the boiler, save a bunch of fuel, and not have to worry about ashing or freezing pipes.

I'm guessing it is hopper fed? I would say it needs to be enclosed in some sort of insulated building, whether brick block or wood, even if it is say 4 ft larger than the stoker itself. You must have so much heat lost to the outside air and wind. On another note, mine ashes almost everytime the timer runs or call for heat. If yours was running for long periods of time, I can see too much ash being taken away from the fire.

Hello ...

Tnx for the comments ... The TG Set Point is 130°F for purposes of correlation to the suggested setting.

I'll reserve my comments until I see the data across a broad range of inlet temperatures.
Here is a sample, 2 hours worth. The temps today are very mild hovering around 30°F.
The recorder is still running. I changed the bandwidth to see more amplitude in the plots. I will be adding a channel for ambient temp.
We'll let this run for a while.

Brian

MacCoon wrote:So I have decided to try manipulating the current sensing method to eliminate the variability of rapidly changing air temperature (convective) by measuring the ash directly (conductive). To test this hypothesis I have placed a thermocouple into the ash 8 inches in from the edge of the grate and another one adjacent to the stock sensor. Both are connected to a chart recorder. I am hopeful I will see a correlation between the conductive and convective temperatures. If I am lucky there will be a few degrees of change in the conductive mode that I can use to control the grate. Then I will simply change the mounting of the stock sensor and then tweak the set point and histerisis accordingly. Fingers crossed.
Brian

You could change the response time of the stock thermocouple by increasing its thermal mass. Perhaps clamping it to a sheet metal shape that shields the blowers air from quickly cooling the sense point, but still had a conductive path so it would, with elapsed time, measure the blower air temperature.

If you are going to design something special I would switch to thermisters. They are much less expensive than thermocouples and are a better dynamic range match to measuring 130 deg +/- temperatures than a type K thermocouple. Tell me something about your electronics design skills and I can make some suggestions.

Here is the data from last night. 10 hours of collection.

Pic (1) Look at the trends. On the Test Air channel you can see a very repeatable temperature profile. The boiler cycles twice per hour +/-.
On the Test Ash channel we see an interesting set of trends. My upper range was 240 and needed to be closer to 300.

I have added channels for the feed tube temperature, ambient air, and a digital signal for the grate actuator.
With that we will be able to see the big picture.

More to come ...

Bob, I did use the recommended settings and after some running time I did see patterns that I reacted to, namely the presence of unburned coal in the ash. Yes I was able to reduce the content of unburned coal by lowering the TG Set Point. Keep in mind that I was burning Barley (23 ton of 3/32-3/16), which is not recommended. This caused many control issues to overcome. I'll talk more later about this.

Yanche, I don't want to make any special electronics ... am trying to leverage existing capability and keep it simple.

Mozz, see my post in the Coal Stokers: Boilers, Furnaces and Stoves; "AHS S130 Installation by MacCoon ", where you can see my enclosure.

Freddy, The paint is cooked off the feed tube/hopper interface because the steel there was so hot from burning coal inside. I insulated the high heat loss zones of the boiler and all the water lines to and from. The garage is a pole barn. I really don’t want to change that either.

Brian

Does anyone think we take this burning coal stuff way too seriously ?

MacCoon wrote:The paint is cooked off the feed tube/hopper

Maybe the small coal causes it to creep up into the head more than it might with pea sized would? I've seen my fire many times AT the head tube, but it never goes up into it.

Here is some more data.
The TG Set Point is 130°F

I added more channels (summary):
Channel #1 Test Air (similar to the stock sensor)
Channel #2 Test Ash Temp (8 inch in from the edge of the gate)
Channel #3 Feed tube Temp (T/C fixed to the interface collar)
Channel #4 Outside Air Temp (located 1 foot above ground, 1 foot to the right of the blower, flush with the front)
Channel #5 Open
Channel #6 Grate on/off (A contact closure type input)

Each channel has a zone on the 10 cm wide paper (The abscissa is labeled 0 to 100)
Current settings:
Channel #1 Zone (0 to 20) 80°F - 180°F
Channel #2 Zone (20 to 80) +130°F +300°F (overlaps but that helps view the dynamics)
Channel #3 Zone (60 to 80) +100°F +150°F
Channel #4 Zone (80 to 90) -10°F +40°F
Channel #5 Open
Channel #6 Zone (93 to 98) On / Off

One conclusion I get from channel #3 is; I must have an air leak in my hopper significant enough to cause the feed tube temp to drop when the blower is running. In the absence of a leak you would expect the temp there to rise due to the firing of the coal. I will try to seal that up to increase the air flow thru the coal pot. I backed up this conclusion by connecting my home made manometer and saw very little delta-P across the coal pot. (Reference: I measured Pea coal (cold - e.g. no fire) at .75 inch H2O across the coal pot when the stoker was just days old.)

Normally when I setup a temperature controller I position the sensor close to the work being, touching if possible. Direct measurement is the best method. That is why I have decided to measure the ash temp. That said, I am thing about the ash temp response and how it relates to the completeness of burn. Now that I am instrumented I will begin collecting ash. When I have a consistent sample I will change the TG Set Point and collect some more.

More to come…

Brian