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This invention relates to a gas powered pressureless
steam cooker, especially to a gas powered steamer for
commercial or resta~..ir~ant use. The term "pressureless steam
cooker" is intended to mean a steam cooker in which no
great pressure of steam is developed although minor
pressure of say 1/21.b. per square inch may be permissible.
Steam cookers c:~onventionally cook foods by the use of
hot steam and may be powered by a steam generator which is
either gas fired or electrical fired. No electric heating
elements ar~~ present: in the cooking compartment itself. For
gas fired units, there has always been an ongoing problem
in providing steam r:~t a suitable temperature inside the
cooking compartment without the loss of efficiency, e.g.
heat in delivering the steam to the cooking compartment.
For pressurf=less st~:am cookers steam is generally supplied
into the cooking corr~partment through an open vent thus
allowing ingress of wet steam at low temperatures.
The common design of a steam generator, or heat
exchanger, up to today utilizes an atmospheric burner
heating the outside of a steam generator. Atmospheric style
burners are general~.y larger and have low efficiency due to
their operai:ing characteristics of burning in the open
atmosphere. Usually such steam generators are located below
the oven cornpartment:lbut in some cases oven compartments
are side-by--side with steam generators.
Examples of steam ovens are shown in:
U.S. Patent No. 4,281,636 Vegh et al. Aug. 4, 1981
U.S. Patent No. 4,552,123 Bi.rkner Nov. 12, 1985
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U.S. Patent No. 3,820,524 Buckell June 28,1974
U.S. Patent No. 4,4~a0,822 Alden et al. July 17,1984
U.S. Patent No. 5,5~~9~,038 Kolvites Aug. 27,1996
U.S. Patent No. 5,732,614 Oslin Mar. 31,1998
U.S. Patent No. 7h:>2,590 Lawrence June 14,1904
U.S. Patent No. 1,226,044 Wilson May 15,1917
U.S. Patent No. 1::L3,241 Ashcroft April 4, 1871
U.S. Patent No. 2,4;1:4,587 Young et al. July 6, 1948
U.S. Patent No. 5,1'8,125 Kuen Jan. 12,1993
The present im,rentors have addressed the problem of
providing a more efsicient gas powered steam generator, by
combining t;he use ox: the latest burner technology and by
the use of ,gin innovative heat exchanger.
According to the invention there is provided a gas
powered pre;~sureless steamer cooker comprising at least one
cooking compartment having at least one steam inlet for
steam to en~er the <:ompartment for cooking, a steam
generator which is ~..ocated immediately adjacent to said
cooking com~~artment in side-by-side relation therewith, the
steam generator comprising at least one integrated heat
exchanger tube and f:'irebox within a water reservoir, the
tube opening in a gE.nerally vertical mouth, an inshot gas
burner direcJted intca said mouth, means for supplying gas to
said burner, the integrated heat exchanger tube and firebox
being in heat exchange relationship with water within the
water reser~roir to c:r~eate steam from water, the reservoir
having an outlet foz° steam, connector means between said
outlet and raid stes.m inlet of said cooking compartment
characterized in that said connector means is constrained
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to direct t:he supply of steam therethrough; and in that
said heat exchange c::ube comprises a U-shaped tube having a
lower horizontal leg, an upper horizontal leg and a
vertical we:b, said w-ertical mout:.h being located at a distal
end of the lower leg. The heat exchanger creates steam in
the steam generator anal delivers it through the connection
means from the outlE:at to the :inlet of the cooking
compartment.
Conveniently for each cooking compartment a pair of
inshot burners are provided, each one directed into a mouth
of a respective lower leg of a pair of parallel heat
exchanger U-tubes.
Utilizing the inshot burner, the flame is directed
into the parallel U--tube exchangers and resulting hot gases
flow throug)z the tuY:~es, the surfaces of which are heat
exchange surfaces with the water reservoir. Of course, one,
or more than two U-t:u:bes may be used if desired. An inshot
burner is provided for each heat exchange tube. The heat
exchangers ;ire comp7.etely immersed in water inside the
reservoir, <~ny convenient level control means being
provided to maintain the level of water. Steam is injected
into the cocking compartment i~hrough a steam inlet tube as
the connector means.
The U-tube heat: exchangers are important in the
provision of a steam oven which may have significant
advantages :in effici.e:ncy. Preferably for each U-tube heat
exchanger, i~he lowex:~ leg is of square section tube having
rounded corners and has a cross section of a size to
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enhance oxygenfair flow into the tube for utilization by
the inshot burner. ~t is, of course, possible to use tubes
with other cross sec:t.ion. Any longitudinal weld in the
lower leg may convent.ional.ly be along an upper surface.
This lower leg of tine heat exchanger tube is effectively a
firebox for the ins~ncrt burner and its dimensions and shape
are selected to enhance operation of the burner while
allowing the flow of the combustion gases into the upper
part of the heat exchanger tube.
1o Preferably, fo~:v each U-tube heat exchanger the upper
leg may be of rectangular section tube having a height
greater than the width and less than the length of the
tube. The ratio of the height o~ the upper leg of the tube
to the length of thc~ upper leg of the tube may be generally
about 1:4 and preferably about 1:2. In an extreme case the
height and the length may be equal. It may, of course, be
more or less than this.
The height of t:he upper leg of the tube should be at
least suffi~~ient to allow for the presence of channels from
side to side of the leg without interfering unduly with the
f low of hot gases ire the tube .
Preferably at least two cylindrical or other shaped
channels are provided to supply additional heat exchange
surface and to act ~~s a stay between the sides of the upper
leg of the heat excraanger tube. It is believed that the use
of a hollow stay may provide significant advantages over
conventiona:L solid stays. This is due to the fact that more
heat exchange surfac.~e is provided.
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The web of the L;~-tube may be generally vertical and it
may be advantageous to provide an angled deflection surface
for hot gases at the top corner where the web meets the
upper leg. Such def::l.ection surface may be angled at roughly
45° to each of the ~.;~p~per leg and web although any angle
will provide some benefit. A general range of suitable
angles is from 30° tvo 60°. The deflection surface may help
to reduce gas turbu:~.ence in the corner and direct the flow
of gas smoothly aro~:md the U.
The U-tube heat::. exchangers should be spaced from the
bottom of t:he steam generator tank to avoid build up of
lime scale :between the tank bottom and the U-tube.
An embodiment <:~f the invention will now be described
by way of example w~.th reference to the drawings, in which:
Figure 1 shows a general view of a two compartment
steam cooker;
Figure 2 is an isometric view showing the steam
generator for one of the compartments of Figure 1; and
Figure 3 is an isometric view of an alternate steam
generator; and
Figure 4 is an isometric view of one of the heat
exchange tubes of trre steam generators of Figures 2 and 3.
Figure 1 shows a ger:~eral view of. a two compartment steamer
10, each compartment. 12 having steam inlets 38 and being
supplied with steam from the steam generator 26 comprising
an integrated heat Exchange tube/firebox 18 (see Figure 2)
and a water reservoir 27. A one compartment unit is also
possible.
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The gas burner 14 of the exemplified embodiment is an
inshot burner type <af any convenient type. For example,
they may be gas burner nozzles .in accordance with those
described a:nd claimed in U.S. Patent No. 5,186,620 issued
February 16, 1993 tc~ Hollingshead and assigned to Beckett
Gas Inc. The inshot gas burner 14 is a relatively small
unit in comparison t:o the BTU/HR output. The gas and
primary air is mixed at the beginning of the burner inlet.
When the fl,~me exit;:=. at the end of the burner tube, it is
mixed with secondary air to form an extremely hot flame
cone.
As shown in Figures 2 and 3, for each cooking
compartment 12 a pa~.r of inshot burners 14 are directed
into mouths 16 of U--tubes 18 having their webs 20 parallel
and vertical, and hc~razontal upper legs 22 parallel with
horizontal :lower legs 24. Mouths 16 are located at the
distal ends of lower legs 24.
The lower legs 24 are each formed of stainless steel
tube of app_roximate7..y square section. The tube is formed by
bending plane into t:.he desired shape to have rounded
corners 25. A weld ~t9 is provided along the top surface.
These lower legs not:.~only form part of the heat exchanger
but also acl~ as fire tube for the inshot burners.
The upper legs 22 are deep in comparison with their
width. IndeE~d, as sY:~o~wn, the height of upper leg 22 is
approximately 1/2 its length. Two cylindrical channels 23
pass through upper leg 22 from side to side as stays
between the sides tc.~ increase the heat exchange surface.
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While two cylindrical channels 23 are shown more or less
channels are possible. These channels should not interfere
unduly with flow of gas inside the U-tube. It may be
suitable that the da.ameter of the channels be approximately
1/2 the height of upper leg 22.
The webs 20 of the U-tubes are roughly vertical and
deflecting walls 21 are provided at an upper corner to
deflect gas flow int::o upper legs 22. The angle of
to deflecting wall 21 ray be about 45°.
The or:ientatiora of U-tubes 18 is also important. U-
tubes 18 should eacY:~ be spaced from the bottom of steam
generator tank 26 to minimize build up of lime scale. The
spacing may conveniently be about from 3/4 inch to 1~ inch
although greater or even lesser spacings are possible. The
spacing is maintainE:~d by welding the lower leg 24 of the
tube 18 to r_he wall of the steam generator tank in its
mouth area and by welding the exhaust flue 17 (Figure 2),
19 (Figure 3 ) t.o thE:~ tank where it exits .
2o Figure: 2 and 5 differ from one another only in the
arrangement of the flue exhaust outlets. Figure 2 shows a
top exhaust 17 and F'i~~ure 3 shows the exhaust 19 at a side
15 of the si~eam generator tank 26.
Each pair of tubes 18 is arranged within a water/steam
tank 26 haying an ir~.lnt 31 for water. The tank 26 and an
outlet 28 for steam are also provided with at least one
probe 34 to indicate internal water level. The outlet 28 is
connected to inlet 38 through a short length of tube 44.
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The tube 44 has a small diameter to somewhat check free
flow of steam into oven compartment 12. Such constraint
tends to retain ver~r~ wet low temperature steam in the steam
generator 26 until _i.t has sufficient velocity built up to
pass through the tube 44. Preferably the tube 44 has a
diameter of 3/8 of ~:~.n inch but diameters of 1/4 of an inch
to 1/2 an inch are acceptable and diameters outside this
range are possible for some sizes of steam generators and
ovens.
To curb the flc,w of steam even further, a nozzle may
be provided to jet steam into the oven compartment. It
should be remembered that the steamer cooker is essentially
a pressurelc~ss cooker and therefore the nozzle must not be
so constrirt~ing that:. there is undue build up of pressure.
Approximate:Ly half a pound of pressure may be admissible
but more involves changes in cooking technique. Therefore
the choice of diameter of nozzle is dependent on the
pressure bu:Lld up. F'or example, a suitable nozzle diameter
z0 might be 7/:32 of an inch plus or minus 1/8 an inch.
Suitably the, nozzle may have four orifices and there are
two nozzles (and tubes 44) per oven compartment.
The proper amount of dry steam required for cooking is
generated by the laxvge heat transfer area of each U-tube 18
heat exchanger. This heat transfer is represented by all
vertical and horizontal areas in the U-tube. Flames from
burner 14 are directed into horizontal lower leg 24 through
tube openinc3 16. The end of the flame and hot gases
continue their way t.h:rough the integrated vertical web 20
and then these gases enter the upper leg 22 of the heat
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exchanger, and continue through the extension of the main
chamber in a vertica.:~l or angular direction into the flue
exhaust outlet 17 (see Figure 2) or 19 (see Figure 3). It
is believed that ut:~.lizing the inshot burners, a high
efficient, compact boiler producing dry steam may be
attained. This generator 26, being adjacent to the cooking
cavity 12 and the in;im.ediate ingress of steam into the
chamber 12, that the:: temperature for the dry steam is
maintained at generally 212°F.
The side 35 of tank 26 is flush with and juxtaposed
against a back surface 15 of cooking compartment 12 so that
the length of tubes 44 is minimized. Inshot burners 14 are
directed into the mouths of lower legs 24 of heat exchanger
~5 tubes 18 from the side allowing easy access for maintenance
and economic use of space. It it believed that by utilizing
the efficacy of inshot burner nozzles 14, the arrangement
of U-shaped heating tubes for said nozzles having
vertically arranged webs, and the immediate ingress of
steam into i:he cooking compartment 12, benefits may be
provided.
The wager level. in this generator is controlled by
probes 34. '.Chew are protected from turbulence by using a
special stainless steel box 42 in which all probes are
enclosed.
