gardener wrote: ↑Fri. Nov. 17, 2017 9:42 am
You stroked an aspect of coal stoves I am very curious about! (efficiency)
What is a firebox liner? (is it the firebricks)
Back when I was looking at the Hitzer 983, I was still learning about the EPA certified wood stoves/inserts. I had wondered whether there was an equivalent jump in efficiency in coal stoves that the wood stoves experienced in recent decades. The Hitzer 983 does have a baffle, and it increases its efficiency by forcing the exhaust to take a longer path to the flue and heating the stove walls more in the process. Whereas on the EPA wood stoves the point of the baffle is to reflect the heat back to the firebox to inject oxygen above the fire to burn off the volatile gases that would otherwise escape up the flue. I'm saying all of this to ask this:
Is there any such parallel technology on coal stoves to increase the efficiency?
Does anthracite coal have volatile gases?
What makes a coal stove more or less efficient?
As far as idling is concerning, I know of the Hitzer 983 (hand fed), it has three grates, and the Hitzer 503 (hopper fed) has two grates. I was wondering if there are solid replacement pieces to the grates to bring the units down to just one grate. Wouldn't that idle the unit down further?
Maybe I should start a new thread for these questions?
The "liners" can be either cast firebricks, cast refractory, or ram-set refractory that is a sticky form of refractory cement that is rolled out like thick pie crust, cut into pieces to fit, and hammered into place in the firebox/firepot. Then allowed to air dry for at least 24 hours and then heat cured with a couple of small kindling fires. Coal burns best in a very hot fire. By insulating the sides of the firebox/firepot to keep the coal bed hotter it will burn more efficiently without having to run with as much damper opening as it would without a liner.
Some of the antique base burners, called "suspended firepot" designs, "wash" the outside of the firepot (which is suspended inside the stove body) in hot gases by channeling the hot exhaust down around the outside of the firepot and then up through ducts to the stove exit. That type does not need a liner and it is an extremely efficient design that, as far as I know, is not used in any modern stoves.
Wood stove efficiency is rated at the stove's maximum burn rate because that's where the most burning off of the waste gases occurs. Otherwise wood stoves send a lot of potential fuel in liquid and gas form up the chimney. Remember all the thick, white smoke the early "air tight" stoves put out when owners tried to run them slow to get longer burns ? That's wasted fuel/heat going "up in smoke".
Anthracite coal stoves are most efficient when the coal bed is kept as deep as possible, no matter if the dampers are set for idle, or for maximum heat output. Like wood stoves, many purpose-built coal stoves provide secondary air to burn off gases in a number of ways. Either though an adjustable secondary damper system, a fixed secondary air feed hole system, or as with many of the antiques, by not making them as "air tight" as today's stoves.
Anthracite does not produce deposits like wood stove creosote, or soot that some bit stoves do if they are not run hot enough. Therefore, since you don't have to run it hot to burn off those byproducts to prevent them from condensing and building up in a chimney system, you can damper a large anthracite stove down to idle just like a small stove at medium output. If you have a good drafting chimney system, it only has to stay warmer than outside the chimney enough to keep a draft going. That often means a good anthracite stove can be idled down to pipe surface temps in the very low 100F range.
Any place that air can bypass the firebed, going from under the grates to the stove collar, will make the fire sluggish or stall and die out. To reduce the firebox of an anthracite stove, you have to block off and reduce firebed width while keeping the full depth of the firebed. Depth is very important with maintaining a controllable and efficient anthracite firebed . Many newbies don't get/keep it deep enough because they think it will get too hot.
As Fred said, adding some firebricks to the sides of the firebox, to reduce the firebed width, while keeping the depth, is a common way to cut back the heat output in the shoulder months.
Paul