Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND ART
The aesthetic value of open fireplaces is such that
their inefficient heating abilities are endured even to
the point of reducing overall fuel efficiency for the
buildings in which they are employed. The reasons for the
latter are well known. The fireplace, itsel~, is an
inefficient heat source because most of the heat of combustion
escapes up the chimney and the strong draft thexeby created
exhausts warm air from the building thus lowering overall
building temperature outside the immediate fireplace area.
This, in turn, calls upon the central heating system to
stabilize the heat loss.
Convective heating s~stems have long been emPloyed
in conjunction with conventional fireplace structures as
a means of recovering a portion of that heat normally
lost to chimney draft and replacing, with recuperatively
heated air, at least a portion of the withdrawn room air.
Convective heating systems conventionally employ a fuel
burning stove or firebox positioned within a fireplace
enclosure in spaced relation to the back, sides, and/or
bottom walls of the enclosure. The firebox is vented to
a chimney or stack and sealed with respect to the space
between the firebox and fireplace enclosure. As fuel is
burned in the firebox limited room air is withdrawn, to
support combustion, through a firebox inlet grate and the
products of combustion are exhausted to the chimney or
stac~. When the firebox walls become heated a convective
air flow is established in the space between the firebox
and fireplace enclosure withdrawing relatively cool room
air from adjacent the floor which is heated as it passes
inwardly and upwardly within ~he fireplace enclosure prior
to its reintroduction into the room from the upper portion
of the enclosure. In addition to providing room heat
by radiation, the firebox is the heat source to establish
and heat a convective flow of room air. Firebox improve-
ments since the early "Latrobe" system ~U.S. Patent 4,744)
have included an elongate, glass fronted construction
whose generally trapezoidal shape in horizontal section
approxim~tes that of a fireplace enclosure for purposes
of improving convective flow and retaining the aesthetic
appearance of a conventional fireplace; improved combustion
air controls; and specially configured outer wall constructions
for improved heat exchange with the convective flow path
as exemplified by U.S. Patents 4/026~264; 4,026,263 and
4,015,581, respectively. The use of glass doors on the
front wall of the firebox constituted a major design
improvement which is now the accepted mode of construction
in that such doors-ameliorate draft induction of room
air while retaining the aesthetic value of an "open fire".
Aside from such basic firebox improvements the general
trend in convective heating systems has been in the
direction of improving recuperative efficiency with respect
to a given heat source. Exemplary are improved heat exchange
techniques in the form of fins and/or flow path directors
and methods for increasing convective mass flow such as
by the use of blowers and the like.
The problem has been attacked from the wrong end.
The limitations inherent in the heat source have been
accepted as more or less given parameters to be tolerated
or ignored. Stated differently, for a given quantity of
the same fuel and external factors being equal, the available
BTU's for convective heat exchange does not vary significantly
among the various systems that have been in use for years.
The key to dramatic increases in overall unit efficiency
lies with the heat source (firebox~ itself which, historically,
has been one of the most inefficient heating units ever
designed.
In the ensuing discussion explanatory of the foregoing
it must be borne in mind that the concern he~ein is for
elongate fireboxes retaining the visual aesthetics of an
open fireplace since many of their inherent limitations
derive from this general configuration.
In considering, for purposes of discussion, a
conventional elongate wood burning firebox having a generally
.
4~3
centralized flue and supplied by drafted combustion air
below a glass fronted wall; the hottest portion of the fire
is centrally of the firebox. Indeed, in many instances
the outer ends of the fuel logs either do not burn at all
and must later be stoked to the center or only become consumed
after a substantial bed of glowing embers is established.
In such conventional firebox there is a central zone which
maintains ignition temperatures while areas transverse
of the central zone remain below ignition temperature.
The reasons are twoold. The net mass flow of combustion
products is upward to a central flue creating a centrally
flowing draft (i.e. away from the outer ends of the fuel
logs) which, in turn, crea'es a centrally directed flow
of incoming combustion air to the center of the firebox
even though combustion air inlets may extend comp].etely
across the front of a closed firebox. Once the overall
central flow of combustion products and incomin~ combustion
air is visualized then the inherent creation of a central
ignition zone is readily understandable on the basis of
general thermal theory that upon attainment of flame supporte~
ignition temperature (i.e. that temperature at which the
local rate of heat generation is su~ficient to propogate
the flame throughout the combustible) the same will maintain
until fuel exhaustion or quenching occurs. Since ~uenching,
or localized quenching as applied to the present discussion,
occurs because of:
1) a rate of heat loss such as to cause local chilling
below ignition temperature; or
2) insufficient oxygen to suppor~ combusion,
it will be seen how the central flow of combustion products
and incoming combustion air contribute individually, and
collectively, to localized chilling and decreased oxygen
partial pressures transversely of the central flow zone
which quenching effect increases directly as a function of
net mass flow velocity. The result, following initial
ignition by highly combustibie materials, is transverse
34;~
--4--
quenching delimiting the central ignition zone. If, as is
the usual case, initial ignition is effected centrally of
the firebox, the remote ends of the box tend to remain well
below ignition temperature.
Expressed differently, a conventional firebox whose
central ignition zone is bounded on either side by sub-
ignition temperature zones exhibits large temperature
gradients transversely of the firebox which peak centrally
and drop off rapidly, below ignition temperature, toward
10 both ends of the firebox. The ef~ect is readily visible
from the greater amount of smoke emanating from the ends of
the logs and the greater soot and resin depositions adjacent
the ends of the firebox.
The value of vertical temperature gradients vary
greatly depending upon their position within the firebox as
would be expected from the above discussion of central draft
to flue. Considering a central portion of the firebox, the
temperature drops somewhat from the point where oxidation of
the combustible gases take place to the flue entrance but
this central, vertical gradient becomes quite small (lying
wholly within the ignition temperature range) as the firebox
interior is further heated by radiation. Similarly, vertical
gradients adjacent remote ends of the firebox are ~uite
small (lying within the sub~ignition temperature range).
Looking, however, to those diagonal temperaturP gradients
extending from outside the central area of the firebox,
upwardly toward the flue entrance (the direction of induced
draft); the value of such diagonal gradients is ~uite large
(extending from sub-ignition to ignition temperature ranges).
Similarly, those vertical temperature gradients intermediate
the central and remote portions of the firebox, i.e. lying
just outside the axis of flue exhaust, exhibit a large value
as they extend vertically from an ignition temperature range
adjacent the burning fuel source upwardly to a sub-ignition
temperature range transverse of central flue exhaust. The
minimal value of the central vertical gradient as contrasted
with the larger vertical gradient transversely thereof is,
of course, an indication of the large amount of heat being
lost up the flue.
, . .
Since the fuel source is positioned rearwardly of the
firebox to avoid overheating the glass front, it will be
seen that the aforedescribed temperature gradients define
a generally frustoconically shaped combustion zone of rela-
5 tively high (ignition) temperature as contrasted with thelower tempera~ure zones bounding either side and the front
thereof. Accordingly it is the central portions of the
top and backwalls of the firebox which provide the most
effective heat exchange for the convectiva flow path with
10 the remainder of the firebo~ walls available for heat
exchange being at a substantially lesser temperature.
Although the aforedescribed central drafting effect
of a central flue can be somewhat ameliorated and the
generally conical combustion zone somewhat elongated at the
15 truncated end thereof by ~he use of an elongated flue of
the type shown in U.S. Patent 4,026,264; the small advantage
is more than offset by the fact that down drafts from such
a flue whirl the flames transversely and forwardly over-
heating and sootin~ the doors. Additionally, thermal
20 expansion and contraction of such an elongated flue in-
evitably breaks its seal to the connected flue or chimney,
thus allowing loss of convected room air up the chimney.
The primary purpose of the invention is to substantially
reduce both the horizontal and vertical temperature gradients
25 within the firebox to the extent that the aforedescribed
combustion zone, fueled with a like charge, is expanded to
encompass a generally rectangular volume approximating
that of the firebox. This is effected by precluding the
direct escape of hot rising flue gases and momentarily
30 trapping the same to lie, in effect, as a hot air blanket
in heat exchange relation over the entire lower surface
of the upper firebox wall prior to continuing displacement
of the same to flue by subsequently rising, hotter flue
gases. The substantial elimination of direct flue escape
--6--
produces a concomitant decrease in the centralizing components
of the combustion air draft permitting comhustion air to
be introduced equally to the fuel across the length of the
firebox. The latter, taken with that radiant heat downwardly
5 directed from the overlying hot air blanket, maintains
ignition temperatures at extreme ends of the firebox the
rising flue gases from which join and supplement the hot
air blanket. The result is a generally rectangular combustion
zone maintained at ignition temperatures throughout sub-
stantially the entire firebox except immediately adjacentthe glass doors. The effect is augmented and efficiency
is further increased by preheating the combustion air
prior to its entry into the firebox via a preheat manifold
construction which not only provides a measure of air
15 shielding for the glass doors but limits the forwardmost
extent of fuel placement to prevent overheating of the glass.
The increase in both radiant and convective heating
efficiency is dramatic. The most obvious advantage is that
substantially the entire surface area of each of the back,
20 top, bottom and side walls is now maintained at a much
higher temperature than was previously possible thexeby greatly
increasing convective heat exchange efficiency w~thout
the expense of heat exchange assistants such as fins,
convoluted flow paths and the like. An ancillary advantage
25 supplementing the foregoing and desirous in and of itself
is the virtually complete combustion effected within the
firebox as a consequence of the greatly increased path
length along which the combustion products must traverse
the combustion zone prior to exiting the flue. ~his is
30 evidenced b~ the virtual elimination of both soot within
the firebox and resinous buildup in the chimney. Immediate
visual recognition, during burning, is had by virtue of
the fact that fire logs burn evenly from end to end in
a virtually smoke free environment immediately following
35 full ignition.
--7--
Although, as previously indicated, the use of glass
doors on units of the type herein proposed has become
fairly standard in the industry the problem of glass
breakage due to uneven heating is still prevalent. Major
contributing factors are continuing localized cooling
adjacent the lower edge of the glass by incoming combustion
air and momentary, intense localized heating due to
flash fires. An additional advantage in preheating the
combustion air prior to firebox entry is that it reduces
localized giass cooling. A combination of air shielding
and baffles alleviate flash fire effects an the doors.
The firebox construction herein described is adapted
for use with convective heating systems employing a free
standing, or fabricated, fireplace unit as well as a con-
15 ventional firebrick enclosure. When used with a freestanding unit over a combustible floor surface, the unusually
intense heat radiated from the firebox necessitates special
safety precautions exceeding those required for previous
units and takes the form of air cooling to supplement the
20 usual metal and insulative shielding.
Another purpose of the invention as applied to free
standing units is to utilize convected room air to effect
such cooling and then utilize the air thus heated for
separate space heating or for reintroduction into the
room heated by the conventionally convected air flow.
Secondary heat recovery is frequently effected by
directing the convective flow in heat exchange relation
with the flue pipe to extract further heat destined for loss
to atmosphere. It is a further object of the invention
30 to enhance the efficiency of this exchange by increasing
both the sensible heat availabe for exchange and the
surface area for effecting the same. This is accomplished
by creating an upper heated air trap, within the flue,
analogous to the aforedescribed entrapment of heated air
35 within the firebox.
DISCLOSURE OF INVENTION
The overall heat available from a given firebox fuel
source for recuperative exchange with convected room air
varies indirectly with net mass flow velocity to central
draft and directly with heat exchange surface area tempera-
ture which, in turn, is a direct function of the volume
ratio of combustion zone to firebox.
In the case of an elongate, centrally drafted firebox;
a baffled, deep flue ~i.e. a flue whose intake extends well
10 below the upper firebox wall) is employed to divert the hot
flow of nascent combustion products from direct escape to
draft and entrap the same as a continually renewing hot air
blanket underlying the upper firebox wall to a depth approximating
the "reach" of the flue entrance into the firebox. With
15 centralizing draft thus reduced, nascent combustion products
from the ends of the firebox rise to supplement the overlying
hot air blanket and maintain the same dynamically stable
over the length of the firebox as the hotter rising gases
continually effect displacement to the deep flue inlet.
20 Once ignition along the le~gth of the firebox is established,
merger of the heated gases instantaneously trapped above the
deep flue inlet and the rising nascent products of combustion
produce a firebox interior which, with appropriate oxygen
supply to avoid quenching, is maintained above ignition
` 25 temperature throughout substantially the entire volume
thereof. Consequently, substantially the entire surface
areas of the bounding back, top, bottom and side walls
comprising the heat exchange surface area are maintained at
those maximum temperatures characteristic of immediate
30 proximity to the combustion zone. This is in contrast with
the central areas of the back, top and bottom walls immediately
adjacent a conventional, central combustion zone as described
above.
~a~
_9_
In addition to providing a greater surface area of
high temperature exposure for convective heat exchange
an important factor in the case of a firebrick enclosure
is the greater and more even buildup of residual heat
S in the relatively massive heat sink defined by the firebrick
wall.
In order to avoid quenching at remote ends of the
firebox either from oxygen starvation or localized cooling
by incoming combustion air, the combustion air is preheated
a~d introduced along the length of the firebox from a pre-
heat manifold which is open at both ends. This open
ended construction insures against reduced combustion
air flow at remote ends of the box due to a pressure
drop along the manifold.
` While the foregoing describes a combustion zone
whose volume approaches that of the firebox with the
obviously increased heat transfer to convected air; less
obvious are the advantages considered as a function of
the combustion products flow paths thus established
within the firebox. In a conventional, elongate firebox
exhibiting a central combustion zone the mean average
flow of combustibles is upwardly and centrally and the
shortest possible flow path to flue for any discrete
volume of the mean average flow is determined by the
length of the hypotenuse of that right triangle whose
altitude extends along the flue axis from fuel source
to flue exit and whose base is determined by the hori-
zontal distance from the flue axis to the point of emanation
from the fuel source. By effectively blocking direct
escape to the flue and establishing a constantly renewing,
entrapped volume of the hottest gases above the deep
flue entrance, the mean average flow path for rising
combustibles is greatly increased and may be visualized
as an upward, transverse and re-entrant movement
respectively into, along and out of the entrapped volume
--10--
to exit the deep flue. The result is to increase the
shortest possible flow path to flue for any discrete volume
of the mean a~erage flow from a value approaching ~a2 + b2
to a value approaching a ~ b where a is that vertical
distance, along the flue axis, from fuel source to flue
exit and b is the horizontal distance from flue axis to the
point of emanation from the fuel source. Since the volume
of the combustion zone approaches that of the firebox,
the result is greatly increased residence time of com-
bustibles at ignition temperatures effecting the completenessof combustion referred to above as evidenced by decreased
smoke, soot and resinous deposition.
An elongate baffle extending downwardly and inwardly
from the upper front of the firebox to a depth exceeding
that of the deep flue "reach" cooperates on the one hand
with the deep flue construction to maintain the entrapped
volume and, on the other, with the underlying preheat
manifold to shield the doors from excessive temperatures.
The preheat mani~old lies :Elush with the firebox floor
and extends completely across the front thereof with opposed
intakes opening through the firebox endwalls. This floor
flush construction coupled with the provi~ion of manifold
exit openings positioned near the floor assure that any
ashes entering the same duxing 'Iclean out" or the like
25 will be subsequently drafted back into the firebox main-
taining a clean preheat manifold.
With the reduction of net mass flow velocity made
possible by the present invention it is not only unnecessary,
but is undesirable, to employ a separate combustion air
inlet control such as a grate or the like since, where
substantially complete combustion is taking place, control
of flue exhaust as by a conventional damper inherently
produces a stoichiometric oxygen admission if, and only
if, the supply as by way of quantity is available in
excess for any burn condition. Thus the choice of an open
ended preheat manifold exhibiting a negligible pressure
drop across the plurality of relatively large exit openings.
In further explanation of the foregoing: In a conventional
firebox exhibiting large subignition areas it is necessary
to create a strong central draft to exhaust the smoke
inherently emanating from such areas to preclude fire
extinguishment and/or their entry into the room. The draft
damper, then, must be further opened than would be necessary
if the subignition temperature æones were of lesser extent
producing less uncombusted products. This, in turn, in
drafts more room air far in excess of a simple stoichio-
metric oxygen supply which further increases net mass
flow to draft to thus maintain the large subignition tempera-
ture region as explained above. Where, on the o~her
hand as in the present invention, the combustion zone volume
approaches that of the firebo~, smoke and other uncombusted
products are practically non-existent eliminating the
necessity for their draft removal for purposes of removal
per se. Rather, draft may now be controlled as a function
of desired burn rate thus eliminating the need for an
inlet grate. As would be expectecl from the foregoing,
the optimum flue damper opening is quite small as compared
with conventional units.
The reduction of central draft and the particular
placement of the preheat manifold ser~e a further function,
in combination with the overlying baffle previously described,
of allowing more of the incoming combustion air to rise
just rearwardly of the door. This in combination with the
inherent leakage of room air over the tops of the glass
doors and the inward and downward component imparted to
the constantly renewing air blanket by the up~er baffle acts
to shield the door.
In addition to the nascent combustion products diversion
function already described, the deep flue baffle serves the
usual function of a smoke shelf. In the case oE a round
flue as herein con-templated it is important that the flue entry area between
the flue en~rance and baffle be completely open. For this reason the baffle
is suspended from the flue by small hanger assemblies which provide for vertic-
al adjustment, during installation, of the clearance between baffle and flue
entrance to take into account the usual differences in chimney draft. With a
strong drafting chimney the baffle will be placed closer to the flue entry
while a greater clearance is desired for weaker drafting chimneys. Typically,
the baffle plate will be positioned from 1 ~ - 3 inches below a deep flue
having a 3" "reach".
All of the foregoing advantages are retained in the case of free-
standing units as herein disclosed except for the increased efficiency made
possible by the greater heat storage in the firebrick heat sink. In ~he case
of free~standing units, a secondary recovery is effected and insulation of
adjacent combustible surfaces improved by a secondary convective 10w, exterior
of the,primary convective flow across the firebox walls, which ~ay be intro-
duced into the same or a different room than that heated by the primary flow.
Thus, the ~resent invention ~rovides a free-standing convective
heating unit for use with a chimney, comprising:
A) a free-standing unit having bottom, back~ sideJ and top insula-
tlve walls for defining a frontally open space, the top wall having an aperturecommunicable with a chimney for the escape of combustion products and at least
one delivery conduit for providing secondary convective air flo~ to a room;
B~ a fireplace enclosure mounted within the frontally open space of
the free-standing unit, having a bottom wall spaced above the bottom wall of the
free_standing unit so as to provide for a secondary convective air flow there-
between, side walls, a back wall, and a top pan having an aperture for passing
4~
products of combustion therethrough, the bottom wall, side walls, back wall
and top pan defining a frontally~ open enclosure, and the back wall of the
fireplace enclosure spaced away from the back wall of the free-standing unit
so as to provide for a space between these walls for the secondary convective
air flow, this space having access of air through portions of the fireplace
enclosure and also communicating with the delivery conduit in the free-standing
unit for removal of this secondar~ convective air flow to a room; and
C~ a firebox mountable ~ithin the fireplace enclosure, having a
bottom wall, top wall, back wall and side walls and openable glass doors at its
front for defining a fuel access opening to the interior of the firebox, the
firebox having its bottom wall spaced above the bottom wall of the fireplace
enclosure so as to define therebetween a space for the in~flow of primary con-
vective air, the back wall of the firebox spaced from the back wall of the
fireplace enclosure so as to define a space therebetween for the passage of
primary convective air flow, and the top wall of the firebox spaced beneath
the top pan of the fireplace enclosure so as to define a space therebetween
for the exit of the primary convective air flow into a room, the firebox having
a flue assembly passing through the top wall of the firebox to communicate with
the aperture in the top wall of the free standing unit for the escape of com-
bustion products from the firebox, the firebox further having means communicat-
ing with the primary convective air flow for bleeding combustion air from this
primar~ convective air flo~ into the interior of the firebox for combustion
purposes;
whereby the secondary convective air flow is in surrounding rela-
tionship with the primary convective air flow for the dual purpose of maintain-
lng the walls of the free-standing unit at a relatively low temperature so that
~l~a~
the unit may be enclosed ~ithin a room, and obtaining secondary heat recovery
between the fireplace enclosure walls and the free-standing unit for deliver-
.ing this secondary convective air flow into a room for heating purposes, and
wherein the primary convective air flow is delivered to the room for heating
purposes with the combustion air obtained from the primary convective air flow
and with the secondary convective air also obtained from the primary convect-
ive air flow~
`,~12b~
34~
BRIEF DESCRIPTION OF DRAWING5
-
Figure l is a perspective view of a firebox constructed
in accordance with the present invention and an associated
fireplace pan for installation of the same;
Figure 2 is an end elevation of the firebox installed in
a firebrick enclosure, with the enclosure shown in section;
Figure 3 is a perspective view of a flue trap structure;
Figures 4 and 5 are respective schematic representations
of exemplary mean average flow paths in a conventional fire-
box and the firebox of Figure l;
Figure 5 is a perspective view of a free standing
fireplace enclosure adapted to contain the firebox of
Figure l;
Figure 7 is a vertical section taken along line 7-7
of Figure 6;
Figure 8 is a vertical section taken along line 8-8
of Figure 7; and
Figure g is a horizontal section taken along line 9-9
of Figure 8 overlaid by a phantom line showing of the
overlying baffled flue construction.
Part dimensions, except for wall thickness illustrations
to permit hatching, are drawn to scale of 3/16":1'.
~io~
-14-
BEST MODE FOR ~ARRYING OUT THE INVENTION
.
A glass fronted, elongate firebox 10 having bottom,
top, back and side walls 12, 1~, 16, 18 adapted for positionment
in spaced relation to corresponding fireplace enclosure
walls for defining therewith a convective flow path is shown
in Figures 1, 2, and 7-9.
Flrebox 10 includes a baffled deep flue assembly 20
which diverts nascent combustion products from direct
escape to flue and entraps the same as a continually renewing
hot air blanket lying above the level of annular flue entry
22 which is open throughout substantially 360 between the
lower end o~ downward flue extension 24 and underlying
baffle 26. The effect of the deep flue assembly 20, following
full fuel ignition fed by preheated combustion air introduced
across the full length of the firebox via open ended preheat
manifold 28 and combustion air inlets 30, is to establish a
combustion zone whose volume approaches that of the firebox.
The Figure 5 schematic is illustrative.
The concept will best be understood by initial considera-
tion of the centralized combustion zone typical of a con-
ventional, elongate firebox 10' de~picted in Figure 4; it
being understood that the concern herein is only for elongate,
centrally drafted fireboxes of the type employed with a
convective heating system to retain the aesthetics of a
conventional open fireplace. Following full ignition in a
conventional firebox 10', the unrestricted flow of nas-
cent combustion products directly to central flue produces
a strong centralizing draft within the firebox so that the
combustion zone tends to stabilize centrally as exemplified
by the phantom line 32 of Figure 4. Subignition tempera-
ture zones stabilize at remote ends of firebox 10' either
because combustion was initiated centrally and ignition
temperatures were never attained or due to localized
quenching as previously explained. In either event, as
4~8
net mass flow to central draft increases by reason of
increased burn rate, fuel supply or combustion air inlet
area, localized quenching at remote ends of the firebox
is maintained even though the centralized combustion zone
might be somewhat expanded.
With reference to Figure 4 and considering any finite
volume of nascent combustion products arising from a
given area 34 adjacent fuel source 36 at a distance b from
the axis of flue 38; it will be seen that the shortest
possible flow path h to flue approaches ~ia2 ~ b2 where
a is the height of the firebox above the fuel source
emanation area 34. Even when this flow path h is completely
within the central combustion zone as illustrated in
Figure 4, combustion of the products is usually incomplete
prior to exiting the firebox due to the relatively short
residence time at ignition temperature. The problem
of incomplete combustion with concomitant smoke production
is, of course, more pronounced in the case of those products
arising from subignition temperature areas adjacent remote
ends of the fuel source, such as at 40 for example. Increased
smoke production, in turn, requires that draft velocity
be maintained to avoid smoke escape into the room thereby
maintaining localized quench conditions at remote ends of
firebox 10'. The obvious result is maximal heat loss
to flue and a concentration of that heat available for
recuperative exchange at the central portions of the top
and back walls of the firebox.
The firebox 10 of the present invention retains much
of that heat conventionall~ lost to flue and makes the
same available for recuperative exchange over a significantly
greater surface area in a manner wnich will be obvious
from an inspection of Figure 5 wherein most of those products
of combustion arising from a generally centralized area
42 are diverted by baffle 26 from direct flue entry to
supplement and displace a portion of that volume of
combustion products previously entrapped more or less
-16-
as a hot air blanket overlying flue entry 22. With central
drafting effect thus reduced, nascent combustion products
arising laterally of the flue exit have a significantly
lesser centralizing flow component and rise to join,
supplement and maintain the dynamic integrity of hot
air blanket 44 as continuing displacement to flue takes
place. The result is that more heat is retained within
firebox 10 and ignition temperatures are maintained sub-
stantially throughout the entire firebox just rearward
of the doors. There are two distinct effects. First,
substantially the entire cumulative areas of the bounding
firebox walls are proximate to ignition temperatures thus
greatly increasing convective exchange efficiency and,
secondly, the residence time of combustion products within
the firebox and exposed to ignition temperature is increased
as a function of their increased flow path length from a
minimal value approaching Va2 + b2 to one approaching a+b.
While a miniscule proportion of the rising combustion
products are entrained directly to flue as at 46, the
average net mass flow is upwardly into the overlying
hot air blanket, centrally ~oward flue and then downwardly
to flue entry 22 as schematically indicated by flow path
48 in ~igure 5. It will be seen that the initial rising
portion of flow path 48 approaches the height a of the firebox
above the fuel source and that the centrally and downwardly
directed portions of the path length to reach flue entry
22 approaches length b. With the idealized schematic
flow path 4a as illustrated the flow path length would
appear to be very nearly equal to a~b but actually this
path length approaches a+b from the maximal side due to
the random motion deviants from a straight llne undergone
by the combustion products in their traversal through the
hot air blanket. It will be seen, however, that the`
-17-
shortest possible flow path to flue in the firebox 10
approaches a+b when considering the integral of all
flow paths across the firebox and those minor inducted
flows, as at 46, which approach the value a+b from the
minimal side. The longer average flow path translates
to increased residence times and more complete combustion
which, of course, increases overall temperature available
for exchange from a given fuel source.
Understanding is even more pronounced when considering
flow path lengths originating from remote ends of the fuel
source in Figure 4. Assuming a centrally positioned fuel
source of length 2b', the shortest possible path length
to flue from area 40 approaches~a2 + b 2 and a significant
portion of that path length is within the subignition
temperature range. In contrast the corresponding path length
~8' of Figure 5 is not only significantly longer but takes
place at ignition temperatures.
The firebox 10 herein illustrated achieves virtually
complete ~ombustion with a normally seasoned fuel source
as evidenced by smoke free burning and lack of soot
and resin build-up.
The maintenance of the aforedescribed conditions depend:
1) Stoichiometrically, on an adequate oxygen
supply across the full length of the fuel
source;
2) Practically, on a combustion air supply
introduced to remote ends of the firebox
at such temperature as to preclude localized
quenching; and
3) Commercially, on protection of the glass
doors from the intense heat to which the
- other bounding firebox walls are exposed.
Contributory to all of the above is the particular
const~uction of preheat manifold 28 whose opposed open
ends 50 bleed combustion air from convected flow as will
be subsequently explained. The relatively large dimensions
of manifold 28, as compared with those of inlet apertures
30, and its open ended construction assures a negligible
pressure drop along the manifold length so that preheated
combustion air is available across the full length
of the firebox on a demand basis. Extending downwardly
from upper wall 14 to a "reach" exceeding that of flue
assembly 20 and inwardly to a depth approximating that
of inlet apertures 30 is a front baffle 52 whose downward
extent cooperates with downward flue extension 24 to maintain
the entrapped air blanket and whose inward extent coacts
with the particular construction of manifold 28 to shield
glass doors 54 from excessive temperatures. The rearward
extent of manifold 28 defines the fuel source placement
or hearth area, spaced from the doors, and the natural
upward component of incoming com~ustion air creates a partial
insulating curtain adjacent the lower edge of the doors
which is assisted by a natural in draft of room air which
inevitably 10ws across the tops o~ the doors and is
directed downwardly in shielding relation to the upper
portions of the doors by the rearwardly directed extension
56 of front baffle 52. The generally directed paths of
the lower and upper air curtains 58, 60, respectively,
is schematically indicated in Figure 7.
The "demand basis" availability of the preheated
combustion air, i.e. excess availability infed as a function
of combustion and therefore requiring no inlet grate control,
is important to the overall efficiency of the system. This
is so because with substantially complete combustion taking
place throughout the firebox, flue damper 62 may be kept
almost fully closed, making possible the minimal central
draft on which the increased efficiency depends, and
combustion air inducted as a function of combustion demand
rather tnan in response to flue draft. With small central
draft, the demand basis availability of combustion air
allows the same to be fed across the full length of the fuel
source which, taken with the preheat condition, avoids
1 8
--19--
localized quenching at remote ends of the firebox. The
use of preheated combustion air in the aforedescribed
shielding curtain 58 minimizes temperature extremes adjacent
the glass doors which is thought to be a significant factor
in reducing glass breakage as explained above.
Side baffles 64 are preferably employed to divert
those forwardly swixling drafts along side walls 18, which
are characteristic of flash fires and down drafts, away
from doors 54 and centrally of the firebox.
Baffle 26 is adjustably suspended from flue extension
24 by hanger straps 66 and wing nut fasteners 68 (only one
of which is shown) coacting with slots 70 in straps 66
(Figure 7) to take into account the usual differences in
chimney draft. Thus upon installation with a strong
drafting chimney, baffle 26 would be adjusted to define
a minimal entry clearance 22 while with a weak drating
chimney, the clearance would be larger.
The back wall 16 of firebox lO is conventially protected
by a replaceable plate 72.
The foregoing completes the description o flrebox lO
whose role in a convective heating system employing
a firebrick enclosure will be apparent from Figure 2.
Following sealing of the chimney entrance with a centrally
apertured fireplace pan 74 and placement of firebox 10
on support legs 76, a flue pipe section 7~ received in
central opening 80 of pan 74 is fitted over flue extension
24. The generally trapezoidal shape o the firebox as
viewed in horizontal section (Figure 9) is generally similar
to the corresponding enclosure walls from which the firebox
is spaced and, although not shown in the drawings, the spacing
of the irebox end walls 18 from the firebrick enclosure
end walls is substantially the same as that illustrated
in Figure 9 showing the end wall spacing from freestanding
enclosure wallsO
-20-
With the firebox thus vented to chimney and sealed
with respect to the space between the firebox and fireplace
enclosure, the simplest form of convective heating system
is defined. As the firebox walls becvme heated a convective
flow is established in the space between the firebox and
fireplace enclosure which withdraws relatively cool room
air from adjacent the floor which is heated as it passes
inwardly beneath bottom wall 12 and along the lower portion
of side walls 18. The heated air then rises along
the back wall 16 and the upper portion of the side walls
18 and is reintroduced into the room over approximately
the upper half of the enclosure, i.e. from across top wall
14 and the upper portion of side walls 18. Firebox 10
is usually surrounded by a decorative grate 82 which permits
free convective flow as just described. Inasmuch as the
convective flow path is across substantially the full extent
of each of the back, top, bottom and side walls it is
obvious that exchange efficiency Ls a direct function of
existent temperatures across the walls which explains the
dramatic increase in eficiency a~; compared with conventional
fireboxes where it is only the central portion of the back
and top walls which ar~ immediately adjacent ignition temperature
ranges. As would be expected, the large heat sink defined
by the firebrick enclosure, being exposed to greater tempera-
tures over a greater area is similarly, more efficientin continuing convective exchange when the firebox begins
to cool after fuel exhaustion.
A free standing, or fabricated, unit 84 incorporating
the firebox 10 is illustrated in Figures 6-9. The exterior
walls of unit 8~ are of the usual sheet metal-insulation
sandwich construction and the same is adapted for in-wall
installation to provide a primary convective flow for room
heating generally as described in connection with the
firebrick enclosure of Figure 1. Free standing unit 84
is~ however, internally configured to produce a secondary
convective flow, in surrounding relation to the primary
flow, for the dual purposes of protecting adjacent com-
bustible surfaces from excessive heat and effecting a secondary
heat recovery which may be used to supplement the room
heating effect of the primary convective flow or delivered
to an adjacent room via delivery conduits 86, 88.
In operation, the overall convected inflow of room
air through decorative grating 90 is indicated by arrows
92. Figures 8 and 9 illustrate a typical division o~ convected
flow 92 into combustion air 94, primary convected air
flow 96 and secondary convected air flow 98. The flow of
combustion air 94 into firebox 10 via preheat manifold
28 is the same as that explained in connection with the
embodiment of Figure 1. The inflow of primary convected
air 96 beneath bottom wall 12 and along the lower portions
of side wall 18 is similar to that previously described
in that it follows the same general flow pattern for return
to the room as indicated by arrows 100 (Figure 6) but
differs the~efrom in that it is supplemented by a minor
inflow of secondary convected air along the back wall 16
as indicated by ar~ow 102 (Figure 7~.
Fi.rebox 10 is supported on elongate legs 104 above
bottom wall 106 which wall 106, together with side walls
108, back wall 110 and top pan 112, define a sheetmetal
fireplace enclosure 114 directing the primary convective
flow path about firebox lQ generally as described with
respect to the firebrick enclosure. Secondary convective
flow 98 traverses the space between free standing unit
84 and fireplace enclosure 114 as best illustrated in
Figures 7-9. An initial division of convected inflow 92
is laterally through large openings 116 in side walls 108
and downwardly through openings 118 in bottom wall 106
(see Figures 9 and 8, respectively) to flow rearwardlv
and upwardly, as indicated by arrows 98, to reach upper
plenum 120 housing an upper flue trap 122 providing a
significant secondary heat exchange with air flow 98
prior to its reintroduction into the room or to a related
space via outlets 86 and 88.
Flue trap 122, illustrated in perspective in Figure 3,
momentarily entraps hot flue gases in a manner analogous
to the deep flue assembly of firebox 10 and provides an
extensive surface exchange area for the secondary convective
flow. Assembly of flue trap 122 with firebox 10 is by way
of connector section 124 (Figure 7) telescopically inter-
connecting the outlet of deep flue assembly 20 and theinlet 126 of the flue trap. Flue trap outlet 128~ inter-
connected with flue exhaust section 130 via intermediate
connector section 132 (Figure 7) is controlled by damper
134.
When employed with free standing enclosure 84, firebox
damper 62 would normally be full open with draft control
being effected by damper 134 or, in such installation,
damper 6.2 may be omitted altogether.
