Steam generator controlled by pressure switch

The steam generator according to the invention consists of a main vessel 1 equipped with heating devices 12 controlled by control devices 13. The water level in the steam generator is kept at an average level 5 by control devices 13 which control a water inlet solenoid valve 10. Measurement of the average water level 5 is achieved by a pressure switch 20 that is exposed to the pressure inside a measuring vessel 18 that communicates with the main vessel 1 via a lower measuring opening 22. Balancing pipework 34 links the pressure switch 20 to the pressure of the steam produced in the steam generator. Pressure switch 20 generates measuring signals that are sent via conductors 17 to control devices 13 in order to actuate solenoid valve 10.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to steam generators intended primarily to 
supply natural steam or pressurized steam that is injected into a cooking 
chamber for foodstuffs. 
2. Description of the Prior Art 
Steam generators generally consist of a main vessel bounded by a watertight 
wall that contains water. The water inlet pipework includes a water inlet 
control solenoid valve in order to inject water into the vessel via at 
least one water inlet opening. Steam outlet pipework communicates with the 
upper part of the main vessel via at least one steam outlet opening. 
Heating devices that can be connected to an external energy source are 
provided in order to heat the water contained in the main vessel. Control 
devices control the heating devices and water inlet control solenoid valve 
in accordance with input signals generated by water level detectors in the 
main vessel. 
In known steam generators, the water level is kept essentially constant in 
the main vessel. To achieve this, means of detecting the water level are 
fitted in order to measure the water level and generate control signals 
for the water inlet solenoid valve. 
Various types of methods of detecting the water level are in common use: 
according to document EP-A-O 323 939, the water level is detected by a 
resistive sensor situated in the main vessel which comes into contact with 
the upper water level; 
according to a second embodiment, the heating devices consist of vertical 
hollow heating tubes that contain the water to be heated and the upper 
part is equipped with a temperature measuring sensor mounted on the 
external peripheral surface of the tube; a drop in the water level below 
the area occupied by the temperature measuring sensor causes a rise in the 
temperature of the corresponding tube wall, this temperature rise is 
detected by the temperature sensor and the temperature rise is interpreted 
as a drop in the water level below the permitted level; 
the article entitled LEVEL MONITORING, by Thomas C. Eliott which appeared 
in POWER No. 9 in September 1990, page 41 ff. states how to measure the 
water level in a steam generator by means of a pressure gauge but without 
specifying the means used; 
document DE-C-662 932 long ago described how to measure the water level in 
a tank by using a device with a diaphragm that is in contact with the 
water. 
These water level measuring devices have proved satisfactory up to now, at 
least during the initial periods of use. However, these known methods have 
drawbacks which become apparent either at the start of operation or after 
a period of prolonged operation. 
For instance, detecting the water level by a resistive sensor does not 
allow correct detection if there is a requirement to produce steam using 
demineralised water. The resistivity of demineralised water is too high to 
allow correct operation of a resistive sensor. 
In addition, all known methods, regardless of whether they involve a 
resistive sensor or the detection of temperature or pressure, are 
particularly sensitive to the presence of deposits of boiler scale or 
calcareous fur which inevitably form after the steam generator has been 
used for a more or less prolonged period. Deposited boiler scale or fur on 
a resistive sensor significantly alters the electrical signals produced by 
the sensor. Similarly, deposited boiler scale or fur on the internal wall 
of a heating tube in the area occupied by the temperature sensor 
significantly alters the operation of said sensor because the film of 
boiler scale or fur acts as a thermal insulator. Deposited boiler scale on 
the diaphragm of a pressure gauge significantly alters the ability of the 
diaphragm to change shape and affects detection accuracy. 
SUMMARY OF THE INVENTION 
The problem to be solved by the present invention is to eliminate the 
drawbacks associated with the presence of deposited boiler scale or fur 
that occurs after a steam generator has operated for an extended period of 
time. 
The invention also makes it possible to correctly detect the water level 
even in the presence of demineralised water. 
To achieve this, the invention proposes the detection of the water level in 
the steam generator by using special means to detect the pressure produced 
by the head of water in the steam generator, with such means generally 
being arranged so as to prevent any deposition of boiler scale. One 
particular difficulty is the fact that the pressure detected depends on 
the pressure of the steam generated. The invention therefore proposes to 
provide a means of compensating the effects that the pressure of the 
generated steam might have on the measurement of the water level. 
According to another objective of the invention, the means of detecting the 
water level are provided by very inexpensive components that make it 
possible to achieve a significant reduction in the cost of manufacturing a 
steam generator. 
In order to achieve these objectives as well as others, the steam generator 
according to the invention includes special means of detecting the water 
level. These means comprise: 
A measuring vessel, comprising a lower communicating opening, that is 
associated with a pressure switch that is exposed to the pressure inside 
the measuring vessel; the pressure switch generates input signals as a 
function of said pressure and sends them to the means of controlling the 
injection of water into the steam generator; 
Measuring pipework having a first end connected to a measuring opening 
located in the lower part of the main vessel below the water level and a 
second end that is connected to said lower communicating opening in the 
measuring vessel; said lower communicating opening is also located below 
the level in the main vessel, there is always a volume of air in the 
measuring vessel between the water and the pressure switch and this air is 
trapped in the measuring vessel because the normal water level is higher 
than the lower communicating opening in the measuring vessel. This avoids 
any contact between the water and the pressure switch. In particular, it 
prevents the formation of deposited boiler scale or fur on the active 
components of the pressure switch such as the pressure measuring 
diaphragm. 
In this way, detection is obtained by a pressure measurement, and this 
measurement is practically insensitive to whether or not the water is 
demineralised and the fact that deposits of boiler scale or fur may form 
in certain parts of the steam generator. The pressure that is measured is 
the pressure produced by the head of water in the steam generator above a 
reference level which is close to the pressure switch. 
In addition, the trapped air ensures thermal insulation between the 
diaphragm of the pressure switch and the water which is at boiling point. 
The pressure switch ideally consists of a diaphragm that is capable of 
elastic deformation and which is entirely located above the water level in 
the measuring vessel having its first face exposed to the air pressure 
inside the measuring vessel and its second face exposed to the pressure 
inside a balancing chamber. The diaphragm is connected to electrical 
conductors that form switches which close and open depending on the 
deformation of the diaphragm under the effects of the pressure difference 
between the measuring vessel and the balancing chamber. 
In the case of a steam generator which must produce pressurized steam, the 
balancing chamber is ideally connected to the first opening of balancing 
pipework, the second opening of which is connected to an upper opening in 
the main vessel, said upper opening being located above the water level. 
The pressure in the balancing chamber is therefore the same as the 
pressure of the steam produced in the generator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the figures, a steam generator according to the present 
invention includes a main vessel 1 that is bounded by a watertight wall to 
contain water. 
In the embodiments shown, main vessel 1 consists of a lower compartment 2 
and an upper compartment 3. Lower compartment 2 and upper 
via a compartment 3 are connected to each other baffle 4 or transverse part 
so that the lower compartment 2 and the upper compartment 3 are laterally 
offset in relation to each other. The steam generator is intended to 
contain water up to an average level 5 situated preferably below baffle 4 
in the upper part of lower compartment 2. Upper compartment 3 is intended 
to contain steam and exclude water in the liquid phase. 
In the embodiment shown, lower compartment 2 and upper compartment 3 form a 
main chamber which is itself connected to a secondary chamber 6 that is 
connected in parallel to the main chamber. Secondary chamber 6 is 
connected in parallel between lower communicating opening 7 situated below 
the average water level 5 and upper communicating opening 8 situated above 
the average water level 5. Secondary chamber 6 is an area where the water 
level is stable in contrast to the water level in the main chamber which 
is exposed to the effects of turbulent convection in the water during the 
production of steam. 
Main vessel 1, preferably in upper compartment 3, is fitted with a steam 
outlet opening 9 that is connected to steam outlet pipework (not 
represented). Steam outlet opening 9 is situated in the upper part of the 
main vessel. 
Water inlet pipework that includes water inlet control solenoid valve 10 is 
used to inject water into vessel 1 via at least one water inlet opening 
11. 
Heating devices 12, shown diagrammatically in the Figures in the form of 
two immersion heating elements, are provided in order to heat the water 
contained in the main vessel. The heating devices can be connected to an 
external energy source. Heating devices 12 are preferably positioned to 
heat the water in lower compartment 2 in main vessel 1. 
Control devices 13 are used to control the various functional units of the 
steam generator. In particular, control devices 13 are used to switch on 
or switch off the transmission of electric energy from electricity supply 
line 14 to heating devices 12 to which they are connected by conductors 
15. Heating devices 13 are also used to switch on or switch off the supply 
of electric power to solenoid valve 10 to which they are connected by a 
pair of conductors 16. Heating devices 13 respond depending on the input 
signals that are present on input conductors 17. 
The steam generator according to the invention includes measuring vessel 18 
which has a lower communicating opening 19 and a pressure switch 20. 
Pressure switch 20 is exposed to the pressure inside measuring vessel 18 
and generates input signals on input conductors 17 that transmit the 
signals to control devices 13. 
Measuring pipework 21 has its first end connected to a measuring opening 22 
situated in the lower part of main vessel 1 below average water level 5 
and its second end is connected to said lower communicating opening 19 in 
measuring vessel 18. Said lower communicating opening 19 is also located 
below average water level 5 in main vessel 1. Measuring opening 22 can 
ideally be situated in the lower part of secondary chamber 6 of main 
vessel 1. 
As shown in the figures, the water level in measuring pipework 21 and 
measuring vessel 18 is located essentially at the level of lower 
communicating opening 19 in the measuring vessel and pressure switch 20 is 
situated in the upper part of the measuring vessel, i.e. above lower 
communicating opening 19. Air can therefore be introduced into the 
measuring chamber and remains trapped in the chamber and cannot escape via 
communicating opening 19 which is sealed by water. In this way, there is 
always a volume of air in measuring vessel 18 between the water and 
pressure switch 20. 
The structure of the means of measuring the water level in the embodiment 
in FIG. 1 is shown on an enlarged scale in FIG. 4. In this embodiment, 
pressure switch 20 consists of a diaphragm 23 that is capable of elastic 
deformation having its first face 24 exposed to the air contained in the 
measuring vessel and therefore to the pressure inside measuring vessel 18 
and its second face 25 exposed to the pressure inside balancing chamber 
26. Diaphragm 23 is connected to electrical conductors, such as conductors 
27 and 28, that form switches which close and open depending on the 
deformation of diaphragm 23 under the effects of the pressure difference 
between measuring vessel 18 and balancing chamber 26. 
In the embodiment in FIGS. 1 and 4 in which the steam generator is intended 
to produce natural steam that escapes via outlet opening 9, balancing 
chamber 26 may either be a watertight chamber containing a constant 
quantity of air capable of compressing or expanding depending on the 
movement of diaphragm 23 or, ideally, a chamber that is vented to 
atmospheric pressure by a duct (not represented). The pressure inside 
measuring vessel 18 equals the steam pressure above average water level 5 
in main vessel 1 plus the pressure of the head of water H situated between 
average water level 5 and the water level in lower communicating opening 
19 in the measuring vessel. It is clear that a change in average water 
level 5 causes a change in pressure due to head of water H, and that this 
produces a movement of diaphragm 23 and electrical conductors 27 and 28, 
thus producing electrical signals that are sent to control devices 13 via 
input conductors 17. 
In the embodiment shown, the steam generator according to the invention 
also has means of draining. This means of draining consists of drain 
pipework 29 connected to lower drain opening 30 in main vessel 1 with a 
water seal 31 that leads to outlet opening 32. Pump 33 is fitted in drain 
pipework 29 and discharges into water seal 31. Water seal 31 is located at 
a higher level than average water level 5 in the steam generator. 
The embodiment in FIGS. 2 and 5 makes it possible to produce pressurized 
steam that escapes via steam outlet opening 9. The steam generator 
according to this embodiment has the same functional units as those in the 
embodiment in FIGS. 1 and 4. These functional units are identified by the 
same numerical references and include, in particular: main vessel 1 with 
its lower compartment 2 and upper compartment 3, secondary chamber 6 
connected in parallel between lower communicating opening 7 and upper 
communicating opening 8, steam outlet opening 9, solenoid valve 10 to 
control the inlet of water via water inlet opening 11, control devices 13, 
measuring vessel 18 with pressure switch 20 and drain pipework 29. 
Compared with the previous embodiment in FIGS. 1 and 4, this embodiment in 
FIGS. 2 and 5 also contains balancing pipework 34 equipped with a first 
opening 35 and a second opening 36. First opening 35 is connected to a 
second measuring vessel 180 of pressure switch 20. Second opening 36 is 
connected to an upper opening in main vessel 1 in a position so that 
second opening 36 is situated above average water level 5. Solenoid valve 
37 may be fitted in balancing pipework 34 and is controlled by control 
devices 13 to which it is linked by control conductors 38. 
In the embodiment in FIG. 5, pressure switch 20 consists of two diaphragms 
capable of elastic deformation, namely first diaphragm 23 which is similar 
to that in the embodiment in FIG. 4 and second diaphragm 123 which is 
parallel to first diaphragm 23. Electrical conductors 27 and 28 form 
switches and constitute mobile spacers that move with either of diaphragms 
23 and 123 and open and close electrical contacts depending on the 
deformation of the diaphragms under the effect of the pressure difference 
between measuring vessel 18 and second measuring vessel 180. Electrical 
conductors 27 and 28 are therefore isolated from the atmosphere inside 
measuring vessel 18 and in second measuring vessel 180. Balancing chamber 
26 may ideally be at atmospheric pressure. 
In the embodiment in FIGS. 2 and 5, the steam generator must also have a 
drain solenoid valve 39 fitted in drain pipework 29 in order to allow or 
stop the flow of water in drain pipework 29. Solenoid valve 39 prevents 
the production of steam pressure from causing the removal of water via 
water seal 31 if the steam pressure exceeds the weight of the head of 
water between average water level 5 and water seal 31. Solenoid valve 39 
is controlled by control devices 13 to which it is linked by control 
conductors 40. 
FIG. 3 shows an alternative embodiment of a steam generator according to 
the present invention to produce pressurized steam. In this embodiment, 
the steam generator has the same functional units as those described in 
connection with FIG. 1 with main vessel 1, heating devices 12, control 
devices 13, measuring vessel 18, pressure switch 20 which generates input 
signals that are sent to the control devices via input conductors 17, pump 
33 fitted in drain pipework 29 equipped with a water seal 31. In this 
embodiment in FIG. 3, drain pipework 29 also includes drain solenoid valve 
39 which is controlled by conductors 40 that link it to control devices 
13. Also, balancing pipework 34 is connected via second opening 36 to the 
upper part of main vessel 1 in the same way as in the embodiment in FIG. 
2. However, in the embodiment in FIG. 3, first opening 35 of drain 
pipework 34 is not connected to second measuring vessel 180 of pressure 
switch 20, but to one of the inputs of 3-way solenoid valve 41 that is 
controlled by control devices 13 via conductors 42. 3-way solenoid valve 
41 is fitted between pressure switch 20 and measuring vessel 18 as shown 
in the Figure and makes it possible to connect pressure switch 20 
alternately to the pressure inside measuring vessel 18 or the pressure of 
the steam produced by the steam generator and carried by balancing 
pipework 34. 
In all the embodiments, pressure switch 20 ideally generates input signals 
comprising at least four different signals which correspond respectively 
to four different pressure levels in measuring vessel 18. In the Figures, 
these four pressure levels are denoted by the letters A, B, C and D. 
Control devices 13 are suitable to: 
order the opening of water inlet solenoid valve 10 in the presence of an 
input signal corresponding to a pressure lower than that produced by the 
height of the head of water H if the water level equals lower level A; the 
solenoid valve can be closed again as soon as level A is reached unless an 
additional command signal to open the valve is generated by other devices 
stated below: 
enable the switching on of heating devices 12 in the presence of an input 
signal corresponding to a pressure higher than that produced by head of 
water H if the water level equals intermediate level B and to prevent 
operation of heating devices 12 if the input signal indicates a lower 
pressure; 
order the switching on of heating devices 12 and the closing of water inlet 
solenoid valve 10 in the presence of an input signal corresponding to a 
pressure higher than that produced by head of water H if the water level 
is at upper intermediate level C and switch off the heating devices 12 and 
open water inlet solenoid valve 10 if the input signal indicates a lower 
pressure; 
order the closing of water inlet solenoid valve 10 in the presence of an 
input signal corresponding to a pressure higher than that produced by the 
head of water if the water level is at upper level D. 
During normal operation, average water level 5 in the steam generator is 
close to level C. Control devices 13 ensure that electric power is 
supplied to heating devices 12. The water level tends to drop due to 
vaporisation and, when it drops below level C, control devices 13 open 
solenoid valve 10 and inject water into the steam generator. The level 
then rises above average level C and control devices 13 close solenoid 
valve 10. If solenoid valve 10 is not switched off, the water level 
reaches level D and this causes the sending of an overflow safety signal 
and the closing of solenoid valve 10. 
If, when average water level 5 drops, the opening of solenoid valve 10 is 
not ordered by the usual means of regulating the water level around level 
C, the surface of the water reaches level B which is detected by pressure 
switch 20. The signal produced by the pressure switch then, via control 
devices 13, switches off the power supply to heating devices 12. In the 
event of a subsequent drop in the water level as far as level A, control 
devices 13 then open solenoid valve 10 in order to inject water. 
In addition, in the embodiment in FIGS. 2 and 3, solenoid valve 39 is 
closed during all the stages when there is a requirement to produce 
pressurized steam and solenoid valve 29 is only open during drain cycles. 
In the embodiment in FIG. 2, solenoid valve 37 may be closed during 
operating cycles in order to produce natural steam and must be opened 
during operating cycles to produce pressurized steam. 
FIG. 6 shows an alternative embodiment to FIG. 2. This alternative uses the 
same functional components identified by the same numeric references. In 
addition, a separator 134 is fitted in balancing pipework 34. Separator 
134 has the task of transmitting the pressure throughout the balancing 
pipework and preventing steam from main vessel 1 from reaching pressure 
switch 20. The diaphragm of pressure switch 20 is therefore exposed to the 
balancing pressure without coming into contact with hot, aggressive steam. 
In the embodiment in FIG. 3, 3-way solenoid valve 41 can have two operating 
modes: to produce natural steam, solenoid valve 41 can continuously 
connect pressure switch 20 and measuring vessel 18; to produce pressurized 
steam, solenoid valve 41 alternately connects pressure switch 20 to 
measuring vessel 18 or balancing pipework 34. 
The present invention is not confined to the embodiments explicitly 
described and includes the various alternatives within the scope of the 
invention as defined in the appended claims.