Air heating apparatus and method

An indirect fired air heater of the portable type and method of operating the same wherein the combustion gas flow is reversed in a combustion drum and passed through a heat exchanger between the drum and the outer casing, the heat exchanger being supported from the outer casing with the combustion drum floatingly supported therebetween.

BACKGROUND OF THE INVENTION 
The present invention relates to air heaters and more particularly to an 
indirect-fired air heater of the portable type suitable for field use to 
heat both personnel and equipment and a method of heating air. 
It is generally well known in the portable heater art to provide a forced 
air heater which includes an outer casing having a smaller combustion drum 
disposed therein which defines a plurality of air heating passages 
extending through the heater between the drum and the casing, the heater 
including a blower to force air through the heating passages and a burner 
to direct flame in the combustion drum toward the downstream end where 
resulting gases are evacuated. More recently attempts have been made to 
eliminate the vanes and baffles required at the downstream end of the drum 
to obtain sufficient heat transfer and to eliminate concomitant pressure 
drop brought about by such vanes and baffles. In U.S. Pat. No. 4,309,978, 
more recently issued on Jan. 12, 1982 to Charles R. Hensiek et al, a 
portable heater structure was disclosed which reversed the combustion gas 
flow at the downstream end of the combustion drum to form a flow of hot 
combustion gases moving toward discharge ports at the upstream end of the 
combustion drum, thus more effectively utilizing the entire length of the 
heater for heat exchange purposes and obtaining complete combustion 
without corrosive carbon deposits within the drum. To accomplish this and 
at the same time support the combustion drum within the outer casing, a 
particular type of ribbed drum was provided with the ribs abutting the 
casing and sometimes leading to "hot spots" at the areas of abutment, 
gathering particulate matter and interfering with heat exchange flow. 
Further, the manufacturing and assembly steps required were often 
extensive and complex and hardly economical. 
The present invention, recognizing the desirablity of reversing combustion 
gas flow at the downstream end of a combustion drum to form a flow of hot 
gases toward the upstream end of the combustion drum and at the same time 
recognizing the limitations of past structure to accomplish the same, 
provides an improved forced air heating apparatus which allows for the 
prompt and efficient heating of large volumes of air with a minimum of 
flow turbulence and pressure drop, assuring efficient utilization of 
radiant and convective heat energy and, at the same, requiring a minimum 
number of steps and parts in both manufacture and assembly. In addition, 
the present invention provides a novel method of supporting a combustion 
drum within the outer casing of a portable heater without sacrificing 
smooth, laminar heat exchange flow and without creating possible areas of 
undue wear, undesirable hot spots and unwanted crevices for particulate 
concentrations in the outer casing. 
Various other features of the present invention will become obvious to one 
skilled in the art upon reading the disclosure set forth herein. 
SUMMARY OF THE INVENTION 
More particularly, the present invention provides an improved air heating 
apparatus comprising: a longitudinally extending outer casing having 
spaced air inlet and air outlet means; a combustion drum disposed within 
the casing, the drum being closed at one end and being sized to define a 
longitudinally extending flow-through passage between the drum and outer 
casing; burner means disposed in the drum; combustion air inlet means in 
the drum communicating with the burner means; combustion gas outlet means 
in the drum; combustion gas outlet means cooperating with the outer 
casing; heat exchange means disposed to extend along the flow-through 
passage in spaced relation between the drum and the outer casing to 
connect the combustion gas outlet in the drum with the combustion gas 
outlet means cooperating with the outer casing; blower means cooperatively 
positioned with respect to the outer casing to move air from the air inlet 
of the outer casing through the flow-through passage between the drum and 
outer casing to the air outlet of the casing to warm such moving air by 
the heat exchange means and the outer surface of the drum; and, support 
means to support the heat exchange means and the combustion drum in spaced 
relation in the outer casing. In addition, the present invention provides 
a method for heating ventilating air in a heater having a casing with a 
combustion drum disposed therein in spaced relation therefrom to create a 
flow-through passage around the drum, the drum having a closed downstream 
end and an upstream burner with a combustion gas header downstream of the 
closed downstream end of the drum comprising: directing a flame of 
combustion products toward the closed downstream end of the drum while 
evacuating the gases from the peripheral region of the drum at the 
upstream end of the drum to effect a reverse flow of combustion gases 
surrounding the flame within the drum at the upstream end of the drum; 
conducting the combustion gases in separate enclosed fluid communication 
through the flow-through passage between drum and outside casing to the 
header from the upstream end of the drum; and, moving an air stream 
through the flow-through passage in separate surrounding heat exchange 
relation with the enclosed fluid combustion gases. Further, the present 
invention provides a novel combustion gas header arrangement and a novel 
support arrangement for a combustion drum and cooperating heat exchange 
means. 
It is to be understood that various changes can be made by one skilled in 
the art in one or more of the several parts of the novel apparatus 
disclosed herein and in one or more of the several steps of the disclosed 
method without departing from the scope or spirit of the invention.

DETAILED DESCRIPTION OF THE DRAWING 
As can be seen in FIG. 1 of the drawing, the inventive air heater 2 can be 
mounted on a trailer 3. Trailer 3 includes a base portion 4 which serves 
as the main fuel tank and is normally supported by a pair of wheels 6 and 
a movable handle or towbar 7. A suitable motor cowling 8 serves to house 
motor 9 which drives axial flow fan or blower 11 mounted on spider 12 at 
one end of the longitudinally extending, cylindrical outer casing 13 of 
the inventive heater 2. As known in the art and as illustrated by the flow 
arrows, venting air to be heated is drawn into cowling 8 and forced 
through outer casing 13 by axial flow fan or blower 11 driven by motor 9. 
In this regard it is to be understood that the present invention is not to 
be considered as limited to the portable arrangement heretofore described 
but that other trailer arrangements and other blower structures--such as a 
centrifugal blower--can be employed. 
As can be seen in FIGS. 2-4, the longitudinally extending, cylindrical 
outer casing 13 includes a cool air inlet 16 between the radially 
extending spokes of spider 12 at one end thereof and a warm air outlet 17 
at the opposite end thereof at the end of truncated evase 18 of outer 
casing 13. Disposed completely within cylindrical casing 13 is a 
longitudinally extending cylindrical combustion drum 19. Drum 19 is sized 
in cross-section relative the cross-section of outer casing 13 to be 
substantially smaller so as to define a flow-through annular passage 21 
therebetween. As can be seen in FIG. 2, the downstream end of combustion 
drum 19 closest to warm air outlet 17 of outer casing 13 is closed at 22 
by a suitable fire wall. 
Disposed within combustion drum 19 at the upstream end wall 23 is a 
generally frusto-conical chamber defining wall 24 coaxially aligned with 
the longitudinal axis of drum 19. A burner tube 26 is positioned to extend 
through the apex portion thereof to accommodate delivery of an axially 
directed flame or jet of combustion products into drum 19 toward wall 22. 
Although not disclosed in detail herein, burner tube 26 can include an 
appropriate nozzle and one or more openings (not shown) to meter a flow of 
venting air into the burner and drum to support the combustion process. In 
this regard, a suitable igniter (also not shown) can be employed to 
initiate combustion in the chamber and at the burner. As can be seen in 
FIG. 2, a combustion air inlet tube 27 communicating with the combustion 
chamber defined by frusto-conical wall 24 serves to introduce the 
necessary combustion air into the chamber. Adjacent the base of the 
frusto-conical chamber defining wall 24 in the upstream end wall 23 of 
drum 19 are a plurality of spaced combustion gas outlet openings 28 
extending in spaced relation therearound. 
Disposed downstream within outer cylindrical casing 13 in spaced relation 
downstream of fire wall 22 which closes the downstream end of drum 19 is 
an annular exhaust header 29. Annular header 29 is connected by an exhaust 
conduit or stack 31 to an exhaust outlet 32 in casing 19. The peripheral 
wall of stack 31 can be fixed to the wall of casing 19 at outlet 32 by 
some suitable means such as welding. Annular header 29 is provided with a 
plurality of spaced openings 33 therein along the outer peripheral wall 
thereof corresponding in number to the spaced gas outlets 28 in upstream 
drum end wall 23. A plurality of spaced heat exchange tubes 34 are 
arranged to extend in spaced relation intermediate the flow-through 
annular passage 21 defined by outer casing 13 and drum 19, each of these 
tubes 34 is so turned at its opposite extremities, that one end of each 
tube is connected to a combustion gas outlet 28 in upstream end wall 23 of 
drum 19 and the other end is connected to one of the spaced openings 33 in 
the outer peripheral wall of annular header 29. It is to be noted that 
axial fan or blower 11 is so positioned in outer casing 13 adjacent the 
upstream drum end wall 23 of drum 19 to move air through flow-through 
annular passage 21 surrounding heat exchange tubes 34 from upstream air 
inlet 16 in outer casing 13 to spaced downstream air outlet 17 of casing 
13 substantially in the same flow direction as the flow of combustion 
gases in heat exchange tubes 34. 
To support burner tubes 34 and combustion drum 19 in spaced relation to 
outer casing 13, the downstream end wall 22 of drum 19 has a plurality of 
spaced rod members 36, each fixed at one end to end wall 22 to extend in 
cantilever fashion and matingly engage in the central passage 37 defined 
by annular header 28 with the inner passage defining wall resting 
thereagainst. It is to be understood that instead of rod members 36 a 
suitable tube which can be appropriately perforated to permit air passage 
therethrough can be fixed at one end to wall 22 to extend in cantilever 
fashion therefrom and matingly engage with passage 37 at the opposite end 
thereof. 
To further support burner tubes 34 and combustion drum 19 in the desired 
position relative outer casing 19, suitable support cradles 38 (FIG. 4) 
can be appropriately mounted in spaced relation at least on the lower 
section of the inner wall of casing 19 with lower tubes 34 nesting 
thereon. Advantageously, neither cantilevered rods 36 mating with passage 
37 in annular header 29 nor tubes 34 nesting with cradles 38 are fastened 
thereto, thus allowing the tubes 34 and drum 19 to be floatingly supported 
and accommodating for the expansion and contraction which would occur 
during heating and cooling. It is to be noted that cradles 38 are 
appropriately provided with spaced apertures 39 to permit venting air to 
be heated by tubes 34 in annular passage 21 to flow therethrough. 
In a typical operation of the above described apparatus, combustion 
products are directed in axial flow from burner 26 toward downstream end 
firewall 22. The flow reverses itself to form an annular flow of 
combustion gases surrounding the combustion products directed from the 
burner 26, moving toward the upstream end of the combustion drum 19 and 
guided by truncated frusto-conical wall 24 through gas outlets 28. In this 
regard, it is to be noted that this reversal of combustion gases in drum 
19 serves to recirculate unburnt fuel until more complete combustion is 
obtained and to thus abate development of pitting and corrosive carbon 
deposits. From gas outlets 28 in the upstream end wall 23 of drum 19, the 
combustion gases are conducted by heat exchange tubes 34 in separate 
enclosed fluid communication through the annular flow-through passage 
between the combustion drum 19 and the outer casing 13 toward the 
downstream annular header 29. At the same time blower 11 driven by motor 9 
moves a venting air stream through flow-through annular passage 21 in 
separate surrounding heat exchange relation with the enclosed fluid 
combustion gases and the drum 19, the venting air flowing in the same 
direction toward the downstream end of casing 13 and warm air outlet 17 as 
the direction of flow of combustion gases toward downstream annular header 
29. It is to be noted that a portion of the venting air at the downstream 
end of drum 19 flows between fire wall 22 and header 29 through the 
central passageway 37 of the header to efficiently effect a thermal buffer 
between fire wall 22 and header 29 and along the central portion of header 
29 as well. This buffer serves to efficiently retard reheating of 
combustion gases and, at the same time, enhancing heat transfer to the 
venting air as it moves toward downstream air outlet 17. It is to be noted 
that the floating support of the heat exchanger tubes 34 and combustion 
drum 19 by cantilever rod members 36 and cradles 38 serves to accommodate 
for expansion and contraction of such parts as the temperatures increase 
and decrease therein.