Boilers

A method of disrupting vapor films formed in film boiling in boilers, in which method transient electrical discharges are effected either in the boiler liquid or in a body of liquid in acoustic communication with the boiler liquid. The electrical discharges are effected at one or more selected locations in the boiler to produce shock waves which act on surfaces of the boiler liquid space where vapor films are to be disrupted. By disrupting such vapor films there is enabled an improvement in the heat transfer rates per unit area across the interface between the fire space and the liquid space of a boiler. There are also disclosed various arrangements in boilers for generating these shock waves. Electrical discharges may be effected between a pair of electrodes mounted in the boiler or between an electrode and an adjacent wall of the boiler, and several mounting configurations for electrodes in boilers are described.

BACKGROUND OF THE INVENTION 
The present invention relates to boilers and more particularly to methods 
of disrupting vapour films formed in film boiling in boilers and to 
boilers with means for disrupting such vapour films. 
In conventional boilers, heat transfer rates across the interfaces between 
the fire space and the liquid space of the boiler can be limited by the 
phenomenon of film boiling. Film boiling is the forming of films of liquid 
vapour at the walls of the liquid space of the boiler. The presence of 
such vapour films significantly reduces the rates of heat transfer into 
the boiler liquid which can be maintained. In order to avoid film boiling, 
it has been necessary hitherto to allow for lower heat transfer rates per 
unit area across the interface between the fire space and the liquid space 
and to use, instead, relatively large areas of such interface to achieve 
desired total heat transfer rates. This results in boilers being 
relatively large, containing for instance, very great lengths and 
quantities of liquid tubes, to provide the necessary area of interface. 
SUMMARY 
According to one aspect of the present invention, a method of disrupting 
vapour films formed in film boiling in boilers comprises the step of 
effecting transient electrical discharges in the boiler liquid at at least 
one selected location in the boiler such that shock waves produced by the 
discharges act on surfaces of the boiler liquid space where vapour films 
are to be disrupted. 
The use of transient electrical discharges in a liquid to produce shock 
waves in the liquid is described in our copending Application U.S. Ser. 
No. 675,415. In the specification of that application, there is described 
a method and apparatus whereby transient electrical discharges are 
employed to increase the contact area between the phases in a multiphase 
system. 
In the present invention, the transient electrical discharges produce shock 
waves in the boiler liquid which, when they act on surfaces of the boiler 
liquid space where vapour films tend to form, are effective to disrupt 
these films and inhibit their production. Thus, with the method of the 
invention, higher rates of heat transfer can be maintained in a boiler 
without the formation of vapour films. Higher heat transfer rates can, in 
turn, enable boilers to be manufactured which are smaller for the same 
steam output, in the case of a water boiler. 
According to another aspect of the present invention, a boiler having a 
fire space and a liquid space comprises means for effecting transient 
electrical discharges in boiler liquid at at least one selected location 
in the liquid space such that, in use, shock waves produced in the boiler 
liquid by the discharges act on surfaces of the liquid space where vapour 
films formed in film boiling can be disrupted thereby. 
The present invention has its chief application in water boilers, but it 
will be understood that the invention is not limited to the boiling of 
water to make steam. Accordingly, where "liquid" is used herein in terms 
such as "boiler liquid", "liquid to be boiled", and "liquid space" it 
should be construed in the present context to cover not only water but 
also other liquids to be boiled. 
The transient electrical discharges may be effected directly in the boiler 
liquid. However, especially where the liquid to be boiled is unsuitable 
for the production of discharges therein, the discharges may be effected 
in a body or liquid, conveniently water, which is separated from the 
liquid to be boiled, but in acoustic communication therewith, by means of 
an acoustically transmissive diaphragm or membrane. In this case, it will 
be appreciated that the body of liquid in which the discharges are 
effected does not form a part of the liquid to be boiled (i.e. the boiler 
liquid). 
The electrical discharges may be formed at the or each selected location in 
the boiler between a respective pair of electrodes. However, instead, only 
a single electrode may be provided at each location, the discharge then 
taking place between the electrode and an adjacent wall of the boiler. For 
instance, the discharge may be arranged to take place between an electrode 
and the interior wall of a pipe of a liquid tube boiler. 
It will be understood that although the shock waves are generated by the 
electrical discharges in the boiler liquid itself or in the body of liquid 
in acoustic communication with the boiler liquid, these shock waves will 
be transmitted into the walls of the liquid space. The shock waves can 
therefore be transmitted along the walls of the liquid space, such as 
along a liquid tube, and will tend to generate secondary shock waves in 
the boiler liquid.

DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates an arrangement of the invention with a pair of 
electrodes incorporated in a feed drum or header for a boiler containing 
water; 
FIG. 2 illustrates an arrangement in which an independent shock wave 
generator having a pair of electrodes can be mounted adjacent a water 
tube; 
FIG. 3 illustrates an arrangement in which a single electrode is mounted 
adjacent the end of a water tube; 
FIG. 4 illustrates an arrangement similar to that of FIG. 3 but including a 
replaceable collar mounted at the end of the water tube to form a second 
electrode; 
FIG. 5 illustrates an arrangement in which a long continuously fed 
electrode is provided extending in a water tube; 
FIG. 6 illustrates an arrangement in which a single electrode is arranged 
to effect discharges to the wall of a fire tube in a fire tube boiler, and 
FIG. 7 illustrates a modified form of the shock wave generator of the 
arrangement of FIG. 2. 
DETAILED DESCRIPTION 
Referring to FIG. 1, there is illustrated a feed water drum on a header 10 
for a boiler in which there are mounted two electrodes 11 and 12 extending 
substantially coaxially with one another through openings in opposite ends 
of the drum 10. The electrodes are mounted in respective insulators 13 and 
14 so as to be electrically insulated from the walls of the drum 10. The 
insulators extend along the shaft of the electrodes 11 and 12 so as to 
expose only conical heads 15 and 16 of the electrodes. The heads 15 and 16 
are spaced apart to define a spark gap 17 which is normally immersed in 
boiler water when the boiler is in use. Water tubes 18 extend from the 
feed water drum 10. 
In operation a source 19 arranged to effect a transient high voltage 
between the electrodes to cause a discharge across the gap 17 is connected 
to the electrodes. When the boiler is in operation to produce steam, 
transient discharges are produced between the heads 15 and 16 which, in 
turn, produce shock waves in the body of boiler water in the drum 10. 
These shock waves impinge on the walls of the drum, including the wall 
portions adjacent the entrances to the water tubes 18, and are transmitted 
into the walls of the water tubes 18. The shock waves are then transmitted 
along the water tubes and tend to produce secondary shock waves in the 
water in the water tubes. The combined effect of the shock waves being 
transmitted in the water along the tubes and also in the walls of the 
tubes tends to inhibit the forming of vapour films due to film boiling at 
the water tube interior surfaces and also to disrupt such vapour films if 
formed. 
A different arrangement is shown in FIG. 2 in which a separate shock wave 
generator 20 is fastened to the wall of a header tube 21 at a location 
adjacent the entrance to a water tube 22. The shock wave generator 20 
comprises a generally cylindrical body 23 closed at one end and having a 
flange 24 at the other end which is adapted to mate with and be fastened 
to a flange 25 provided on the wall of the header tube 21. The interior 
volume of the cylindrical body 23 is in communication with the interior of 
the header tube 21 and is, when the boiler is operative, filled with 
boiler water. Two electrodes 26 and 27 are mounted diagonally opposite 
each other in the cylindrical wall of the cylindrical body 23 and have 
heads spaced apart to define a spark gap 28. When a discharge is produced 
across the gap 28 by the source 19 of high voltage, the shock waves so 
generated are transmitted in the boiler water into the water in the header 
tube 21 and also along the water tube 22. As before, the shock waves are 
partly transmitted in the boiler water itself and partly along the walls 
of the water tubes. 
Instead of the arrangement of FIG. 2, a single electrode 30 (FIG. 3) may be 
mounted in an insulator 31 so as to extend through an opening 32 in the 
wall of a header tube 33. In FIG. 3, the electrode 30 has a conical head 
34 disposed in the interior of the header tube 33 so as to be spaced from 
a swaged-over end 35 of a water tube 36. In operation, a transient high 
voltage is applied between the electrode 30 and the boiler water tube 
system, for instance at terminal 37, by means of the source 19 of 
transient high voltage. Thus, electrical discharges occur between the 
electrode head 34 and the end 35 of the water tube 36. 
In order to avoid excess corrosion of the water tube resulting from the 
discharges, a replaceable collar 40 (FIG. 4) may be provided in electrical 
contact with the end 35 of the water tube 36. The collar 40 then provides, 
in effect, a second electrode between which and the electrode 30 the 
electrical discharges take place. 
FIG. 5 illustrates a further arrangement in which a long rod electrode 50 
is provided extending through an insulator mounted in an opening in the 
wall of the header tube 33. The rod electrode 50 is arranged to extend for 
a distance substantially coaxially along the interior of the water tube 
36. Once again, a transient electrical voltage is applied by source 19 
between the electrode 50 and the water tube system so that electrical 
discharges occur between the electrode 50 and the interior surface of the 
cylindrical wall of the water tube 36. However, in this arrangement, the 
discharges may occur randomly spaced along the electrode 50 and 
electrolytic erosion of the water tube is thereby reduced. Further, the 
electrode 50 may be continuously fed in the direction of arrow 51 into the 
water tube to compensate for its consumption by electrolytic erosion 
during use. 
FIG. 6 illustrates an arrangement of the invention in a fire tube boiler. 
In the Figure, a fire tube 60 is shown extending through a water space 
region containing boiler water 61. An electrode 62 is shown extending in 
an insulator 63 through an opening 64 in a wall 65 of the boiler. The 
electrode 62 and insulator 63 are arranged so that a conical head 66 of 
the electrode is immersed in the boiler water 61 and is spaced from the 
outside wall of the fire tube 60 to form a spark gap 67. In operation, a 
transient high voltage is applied by source 19 between the electrode 62 
and the metal work of the boiler so that discharges occur between the head 
66 and the fire tube 60. Once again, the discharges produce shock waves in 
the water 61 which are effective to inhibit and disrupt vapour films which 
form at the surface of the fire tube 60. 
Although in each of the above described examples of the invention the 
electrical discharges are arranged to take place directly in boiler water, 
it may be necessary to isolate the liquid in which the discharge is to 
take place from the liquid to be boiled in the boiler. In this case the 
liquid in which the discharge is to take place may be enclosed by an 
acoustically transmissive membrane made, for example, of stainless steel 
foil or a suitable rubber, and an example of this is indicated in FIG. 2 
where an acoustically transmissive membrane 29 drawn in dashed line is 
disposed between flanges 24 and 25 to isolate the liquid in which the 
electrodes are immersed from the liquid which is to be boiled in the 
boiler. Alternatively, as shown in dashed line in FIG. 1, the electrodes 
may be sealed to a tube 38 of acoustically transmissive material. 
Whereas in FIG. 2 two electrodes are used in the shock wave generator 20, 
if desired, only one electrode can be used as shown in FIG. 7 and the 
discharge will then take place between this electrode and the cylindrical 
body 23 which will be formed of electrically conductive material, and 
which for the purpose of the present invention constitutes a wall of the 
boiler.