Cluster boiler

A cartridge boiler designed for use in multiples in a furnace, each cartridge boiler having a vertically oriented cylindrical drum top and bottom with hemispherical end caps, with a plurality of free-expansion boiler tubes having internal feed water supply tubes depending in a cluster through the bottom end cap. The supply tubes are gravity fed by an open feed water pan within the drum, the feed water supply being controlled by a boiler level monitoring device. The boilers are used in multiples to generate the steam required, the relative small size of individual cartridge boiler being designed to minimize fabrication costs by minimizing drum diameters, and maximize system performance by allowing select cartridge replacement without system shutdown.

BACKGROUND OF THE INVENTION 
This invention relates to a cartridge boiler designed for use in multiples 
in a steam generation plant for power production, cogeneration or steam 
use. Small and medium size steam generating plants have long been used in 
factories and institutions for combusting waste products as well as fossil 
fuels for generation of steam. The generated steam was used primarily for 
heating and in many instances for processing of manufactured goods or 
agricultural products. With the design of small efficient steam turbines 
the combination of a steam generating system with a power production unit 
has economic merit. However, conventional designs of boilers used for 
steam heating are customerily low pressure systems ineffective for driving 
a steam turbine. The requirement for an inexpensive, medium pressure 
boiler that is safely operable with a variety of often low Btu fuels of 
inconsistent composition has led to the design of free-expansion tube 
arrangements and systems using multiple cartridge-like boiler units. While 
certain prior cartridge boiler designs are advantageous, they pose 
fabrication complications and do not represent the most effective designs 
for absolute minimum cost. The boiler system of the present invention is 
designed to generate steam suitable for electrical power generation from 
fuels that are of inconsistent composition and vary in thermal potential 
from 4000 Btu to 20,000 Btu. The boilers are designed primarily for use in 
multiples enabling flexibility in the design of a plant having a capacity 
suited for the desired use. 
Naturally, however, the boiler as a stand alone unit will find use in 
situations where maximizing system efficiencies is not a primary goal. 
SUMMARY OF THE INVENTION 
The cartridge boiler of this invention is designed for use in multiples for 
example, in a double row in an elongated boiler furnace. The use of 
multiple boilers instead of a single boiler in a furnace has advantages in 
overall plant efficiency by permitting individual boilers to be removed 
for repair or cleaning with only a partial reduction in the plant output. 
Elimination of shutdown for boiler attention vastly increases the 
reliability of the plant and its suitability for power generation and 
industrial or agricultural processing where an unplanned shutdown could be 
disasterous, and a planned shutdown inconveniently expensive. Because of 
the multiplicity of boilers in a cluster boiler steam generating plant, 
each boiler must be inexpensive to fabricate. Key to economical cost 
construction is the relatively small diameter vertical drum design, the 
compact fire-tube cluster design, the flat plate tube connection scheme, 
and the circulating scheme, all of which allow moderate to high steam 
temperatures to be obtained with standard drum structures, but with a 
novel orientation and arrangement. Sizing is flexible with departures from 
a two inch, twenty foot fire-tube comparative standard, being design 
trades of performance and material costs. It is common knowledge that as 
drum sizes increase, costs rise exponentially to accommodate strength and 
material problems inherent in pressure vessels. The cartridge boiler is 
designed to minimize costs by minimizing drum sizes and fabrication 
complexities. 
The cluster boiler is of a cartridge-type, having a compact cluster of 
free-expansion tubes connected to a single over-head drum. This design 
allows fuels of incosistent Btu value to be used in the furnace since each 
tube can expand freely according to its thermal exposure. Thermal 
differentials therefore place no expansion stresses common with double 
drum systems. The dependent, comb-like arrangement of the tubes allows the 
boiler to be easily removed from the furnace for servicing or repair. The 
vertical configuration of the drum minimizes the space necessary for each 
boiler permitting access to the drum for disconnection and ease in 
extracting the boiler from the furnace. The configuration permits the drum 
to be oriented on the outside top of the furnace with the tubes 
penetrating into the furnace. In this matter the boiler can be vertically 
hoisted, withdrawing the tubes from the furnace. 
The cartridge concept allows furnace designers an opportunity to position 
tube clusters at various locations within the furnace according to the 
combustion and flow characteristics of the furnace. In this manner 
controlled pyrolysis designs with low emission characteristics can be 
effectively matched with steam production requirements in an extended 
chamber furnace. 
Internally, the boiler utilizes an open, suspended feed-water distribution 
pan. A plurality of feed-water tubes are connected to the pan and extend 
down into the fire tubes. The feed-water tube includes perforation in its 
lower section to supply water to the fire tubes for transformation into 
steam. The fire tubes are inexpensive to fabricate and easily replaceable 
making them ideal for potentially corrosive furnace feedstock such as 
refuse derived fuel or blow-fired coal powders. These and other, features 
are described in greater detail in the detailed description of the 
preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a cartridge boiler, identified by the reference 
numeral 10 is shown. The boiler is constructed with a water drum 12 with 
depending free-expansion fire tubes 13. The drum 12 has a vertically 
oriented cylinder section 14 with top and bottom integral flanges 16. The 
drum is coupled to a top hemispherical pressure cap 18 with a perimeter 
flange 20 by a series of flange bolts 22. The top cap 18 has a water inlet 
24 and a steam outlet 26 having tube extensions, 28 and 30 respectively, 
terminating within the interior of the boiler. The water tube extension 28 
terminates within the perimeter of a open feedwater distribution pan 32 
suspended within the boiler by straps 34 connected to the pan 36 and the 
inner wall 36 of the top cap 18. A plurality of feed water tubes 38 are 
threadably connected to the pan allowing the feed water tubes to be 
withdrawn with the cap from the fire tubes 13 on separation of the cap 
from the cylinder section 14 of the drum 12. The feedwater distribution 
pan 32 is located below the level of the drum water 40 such that the 
cooler feed water from the feed water extensions 28 is delivered to the 
feedwater tubes by the baffle effect of the pan and the natural downward 
flow by convection. In this manner a special circulating pump for the 
boiler water within the drum is eliminated. Water level is controlled by a 
sensor 41 which regulates the feed water supply by monitoring a 
conventional valve system. A low level warning sensor 42 senses a 
dangerously low water level and signals an alarm and/or activates a backup 
water supply to correct the situation. 
The elongated feed water tubes have perforations 43 in the lower portions 
of the tubes to permit the feed water to flow directly to the fire tubes 
to insure a supply of water to the fire tubes. Without the perforations 
there is the potential flow can become restricted on a flash boiling of 
the supply from the bottom of the water tube during excessively hot 
conditions resulting in a back pressure surge. 
The steam outlet extension 30 terminates in the upper portion of the top 
cap 18 and is encased by a steam pervious moisture separator 44. The 
moisture separation 44 removes the moisture entrained in the outward 
flowing steam the condensate dripping back to the boiler water. 
The cluster of fire tubes 13 depending from the boiler drum are sized 
according to the design of the furnace and may extend twenty feet or more 
in length. The straight fire tubes have a distal end 46 that is pinched 
and welded, and a connecting end 48 that is threaded. In this manner, 
fabrication of the fire tubes is simple and can be accomplished in the 
field without special equipment. The connecting end 48 of the tubes is 
threaded into a flat, fire tube mounting plate 50, which comprises a 
diaphram-like seal for the boiler. The flat mounting plate 50 is 
convenient for mounting the tubes and easily fabricated, but its 
configuration is inadequate to withstand the pressures encountered in the 
boiler without deforming and possibly failing. Since the hemispherical 
shape is ideal for withstanding the high pressures of a boiler, a 
hemispherically configured pressure cap 52 with integrally cast cross ribs 
54 is designed to couple to the cylinder section 14 of the drum 12 and 
reinforce the flat mounting plate 50. The pressure cap 52 has a flange 56, 
which couples to the flange 16 of the cylinder section 14 by a series of 
bolts 22, with the fire tube mounting plate sandwiched therebetween. 
Appropriate gaskets or seals (not shown) are also used. 
The fire tubes extend down between the ribs 54 and through holes 56 in the 
hemispherical surface of the cap. Since it is the mounting plate 50 that 
acts as the sealing diaphram, the holes 56 through the cap allow for 
expansion of the tubes and do not perform any pressure sealing function. 
The cap, however, provides a shield to protect the fire tube connections 
from direct exposure to the furnace fire when the boilers are mounted over 
a furnace chamber 58 as shown in FIG. 5. 
Referring to FIGS. 4 and 5, the boilers 10 are illustrated schematically in 
conjunction with a controlled pyrolysis furnace 60 for a power plant. The 
furnace 60 has a rotary descending stoker grate 62 which feeds a solid 
fuel 64 to the entrance of the combustion chamber 58 where it is gasified 
by partial pyrolysis under oxygen lean conditions. Regulation of 
combustion is accomplished by flow control of air through air feed 
conduits 66 connected to a blower (not shown) which distribute the air 
supply over the grate 62 and along the length of the combustion chamber 
58. Complete pyrolysis is delayed to maintain controlled furnace 
temperatures generally below the nitrogen fixation temperature of 
2500.degree. F. The combustion gases swirl through the tubes by action of 
baffles 67 oriented along the chamber 58. The baffles have sections 68 on 
pivots 69 for adjustments. The fully pyrolized gases are exhausted through 
an exhaust conduit 70. The exhaust conduit 70 may be connected to heat 
scavengers to heat the air supply and fuel before scrubbing and exhaust to 
atmosphere as is known in the art. 
As shown in FIG. 5, the boilers are mounted on the top of the furnace with 
the tubes extending into the chamber 58. A hoist hook 72 helps an 
individual boiler to be removed and the opening covered without shutdown 
of the plant. The plant continues operating at corresponding reduction in 
capacity. The eight boilers shown are representative of a multiple boiler 
plant for generation of steam for use in a steam turbine power station. 
The plant can be elongated for additional boilers for increased capacity. 
Details of the plant are beyond the scope of this invention the plant 
being shown schematically to illustrate one configuration of multiple 
cluster boilers for a controlled pyrolysis furnace. 
While in the foregoing embodiments of the present invention have been set 
forth in considerable detail for the purposes of making a complete 
disclosure of the invention, it may be apparent to those of skill in the 
art that numerous changes may be made in such detail without departing 
from the spirit and principles of the invention.