 Rumford
understood that the only useful heat generated by a fireplace was
radiant heat.
The air that the fire heated was mixed with smoke and went up the
chimney. So he made a fireplace that had a big, tall and wide opening
and was very shallow with widely splayed covings or jambs to reflect as
much radiant heat out into the room as possible.
Rumford wanted to "reflect" the heat directly
out into the room. He saw no advantage of absorbing the heat and
re-reflecting it. Heating up the firebox only served to heat the air
that came into contact with it which just was lost up the chimney.
He advocated plastering and whitewashing the firebox to keep it clean
and reflective. He was vehemently opposed to cast iron firebacks because
they only absorbed the radiant heat and got hot which in turn heated the
air which was mixed with the smoke and was lost up the chimney.
Modern physicists, familiar with Planck's Law,
argue that what really happens is that the firebox, whether it's
whitewashed or not, does absorb heat and re-radiates it out into the
room. There isn't a significant difference between whitewashed plaster
and soot-covered brick, they say, in the infrared range. Both are pretty
good "black body radiators."
Whatever.
The objective remains to reflect or re-radiate as much heat out of
the face of the fireplace as possible. It doesn't hurt to whitewash the
firebox or to insulate behind the firebrick. It does hurt to cover the
opening with glass doors, which cut about 80% of the infrared radiation.
Tall, wide shallow fireplaces tend to smoke.
The other half of Rumford's genius is his intuitive understanding of
fluid dynamics - way ahead of his time. By rounding the breast to
"remove those local hindrances which forcibly prevent the smoke from
following its natural tendency to go up the chimney..." he essentially
created a venturi, a nozzle, like an inverted carburetor, that shot the
smoke and air up through the throat into the receiving smoke chamber.
Rumford may not have known how ingenious his
"rounded breast" really was. In our testing at McNear Brick we built a
Rumford fireplace studded with thermocouples just to see what the
temperatures were at various places. We wanted to see if the flow
through the throat was laminar, so we placed two thermocouples at the
narrowest part of the throat, one near the curved breast (5) and one a
couple on inches farther back near the fireback (6).
To our amazement, with an established 1700 degree F fire, we recorded
only 75 degrees F near the rounded throat at 5, and, only two inches
away near the back at 6, we recorded 730 degrees! The flow through the
throat certainly is laminar. The room air coming in over the fire
doesn't mix with the hot products of combustion, but goes up with them
through the throat. It's like a sheet of clean room air that acts like
an invisible glass door to keep the smoke behind it as they both go up
through the throat together.
This revelation has even more
interesting implications. Most engineers who are trying to come up with
clean-burning designs to meet EPA emission standards assume it can't be
done with an open fireplace because the excess (or dilution) air mixes
with, and cools the gasses too much to achieve secondary combustion.
What we've found is that the excess air doesn't mix turbulently with the
products of combustion in a Rumford fireplace with a streamlined throat.
The products of combustion stay isolated and hot behind
the sheet of clean room air. Because the Rumford firebox is tall, the
gasses driven off the fire in a Rumford fireplace stay hot for a long
time - long enough to attain secondary combustion. We've found that the
Rumford burns cleaner yet if we place the firewood on end, leaning
against the fireback, tipi style.
Smoke typically pours out of the ends of the logs as
they get hot and, in a log cabin style fire, some of this smoke finds
its way around and over the fire into the flue. In a tipi style fire,
however, the smoke coming out of the ends of the logs stays in the fire
and gets burned up. Tipi fires raise the height of the fire taking
advantage of the tall Rumford firebox. The "residency time" (the time
the smoke and volatile gasses are in the fire) is increased and so the
smoke is burned up, the fire gives off more heat and burns cleaner.
Most masons, used to building modern
fireplaces, have trouble believing a Rumford will draw until they see it
happen. The rules are different. In a modern fireplace the fireback is
usually sloped or rolled forward casting the products of combustion
forward. The incoming room air spills over the sharp edge of a steel
lintel and mixes turbulently with the smoke. Most masons will tell you
that you need to "cross over" or drop the lintel about eight or nine
inches below the damper to create a pocket for this smoke and in-coming
room air to "roll" or the fireplace will smoke.
Of course all this inefficiently
turbulent rolling smoke and air needs a huge throat to get through. By
keeping the fireback straight and rounding the breast to get streamlined
air flow instead of turbulence, we can build Rumford fireplaces with
openings almost a foot taller than regular fireplaces and with throats
less than half the size of a modern fireplace. No wonder Rumfords are
more efficient. They radiate more heat and waste less heated room air to
carry away the smoke. If you think that's amazing try to imagine how
much horsepower it would take to ram a small airplane through the air
with a square wing. Believe me, it helps to round off the corners of the
wings.
Rumford devoted Chapter II of his 1796 essay on
fireplaces to "Practical Directions designed for the Use of Workmen,
showing how they are to proceed in making the Alterations necessary to
improve Chimney Fire-places, and effectually to cure smoking Chimneys."
Essentially, it's laborious and anyone can read it for themselves.
Rumford apologizes to his readers "for having been so very particular in
these descriptions and explanations, but it must be remembered that this
chapter is written principally for the information of those who had few
opportunities of employing their attention in abstruse philosophical
researches..." Suffice it to say Rumford's method is to get the basic
shape right, with a plumb bob and a special device he suggests for
marking out the 135 degree angle for the covings. His masons built with
brick or stone and plastered the surfaces to make a clean reflective
surface and used plaster to round off the breast.
Because Superior Clay Corp. has been so
successful in manufacturing vitrified clay Rumford throats and smoke
chambers I think one of the footnotes in Rumford's 1796 essay is
especially interesting:
"I have of late been much engaged in these investigations, and am
now actually employed daily in making a variety of experiments with
grates and fireplaces, upon different constructions, in the room I
inhabit in the Royal Hotel in Pall Mall; and Mr. Hopkins, of Greek
Street, Soho, Ironmonger to his Majesty, and Mrs. Hempel, at her
Pottery at Chelsea, are both at work in their different lines of
business, under my direction, in the construction of fireplaces upon a
principle entirely new, and which, I flatter myself, will be found to
be not only elegant and convenient, but very economical. But as I mean
soon to publish a particular account of these fireplaces, with
drawings and ample directions for constructing them, I shall not
enlarge further on the subject in this place."
Rumford never did publish the results of
these experiments. Even though he wrote another essay on fireplaces two
years later, in 1798, and had the opportunity to discuss these
experiments in his essay on Kitchen Fireplaces, he never did.
From the difficulty Superior Clay had in manufacturing a big Rumford
throat that wouldn't warp or crack in the drying and firing process, I
suspect Rumford's potter was unsuccessful. Our "potter," the Superior
Clay Corp., may have only just accomplished what Rumford envisioned two
hundred years ago. |
