GB2099556A

GB2099556A - Electrode steam boiler

Info

Publication number: GB2099556A
Application number: GB8116513A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-05-29
Filing date: 1981-05-29
Publication date: 1982-12-08

Abstract

An electrode steam boiler comprises a boiler shell 1 and electrodes 7, 8, 9 within the shell 1 which, in use of the boiler, are at least partly immersed in water in the shell and are connectible to an electric supply source 10 for the purpose of producing a flow of electric current in the water to heat the water and generate steam which leaves the shell 1 via an outlet 18. A pipe 14 and a pump 13 are provided for supplying feed water to the shell 1 from a feed water tank 12, and a bleed system is provided for bleeding water from the shell 1 to a drain 29 via a bleed valve 30. The bleed system comprises a heat exchanger 26 having an inlet connected to the shell 1 by a pipe 24 and an outlet connected to the bleed valve 30 via a pipe 28, the heat exchanger 26 being situated in the feed water tank 12 in a position such that, in use of the boiler, it transfers heat from water in the bleed system to feed water in the tank 12, and a thermostat 34 controlling actuation of the bleed valve 30 in such a way that the bleed valve cannot be opened when the temperature of the water in the bleed system in, or downstream of, the heat exchanger 26 is above a predetermined level. <IMAGE>

Description

SPECIFICATION Electrode steam boiler This invention relates to an electrode steam boiler of the kind comprising a boiler shell, electrode means within the shell which, in use of the boiler, is at least partly immersed in water in the shell and is connectible to an electric supply source for the purpose of producing a flow of electric current in the water to heat the water and generate steam, means for supplying feed water to the shell from a feed water tank, and means for leading generated steam from the boiler.

In use of a boiler of the kind referred to, the concentration of impurities gradually increases in the water in the shell, and steps have to be taken periodically to remove water of high impurity content from the shell. This is done by removing water periodically through a bleed valve from a region of the shell where the impurity content of the water is highest, which is normally a region of the shell at the level of the electrode means. This water, which is substantially at the temperature of the generated steam at the working pressure of the boiler, has to be bled to a drain at atmospheric pressure. During the bleeding operation, the removed water will flash into steam, due to the sudden drop in pressure, and will probably cause scale to form in the bleed valve orifice.

Furthermore, the bleed valve must be designed to operate at the steam temperature. This is costly both in terms of the valve and in terms of the waste heat lost in the flash steam and the hot water which passes to the drain.

The present invention aims to provide an electrode steam boiler of the kind referred to which has a means for bleeding water from the boiler shell which does not lead to the drawbacks mentioned above.

According to the invention, a boiler of the kind referred to is provided with a bleed system for bleeding water from the boiler shell to a drain via a bleed valve, said bleed system comprising a heat exchanger having an inlet connected to the boiler shell and an outlet connected to the bleed valve, the heat exchanger being situated in the feed water tank in a position such that, in use of the boiler, it transfers heat from water in the bleed system to feed water in the feed water tank, and a thermostat controlling actuation of the bleed valve in such a way that the bleed valve cannot be opened when the temperature of the water in the bleed system in, or downstream of, the heat exchanger is above a predetermined value.

Preferably, said bleed valve is disposed externally of the feed water tank, being connected to the outlet of the heat exchanger by a pipe passing through a wall of the feed water tank. In this case, said thermostat preferably is arranged to measure the temperature of the water in the bleed system in the region where said pipe passes through the wall of the feed water tank.

Preferably, said heat exchanger is disposed close to the bottom of the feed water tank, so that it is situated in the coldest water in the feed water tank. The heat exchanger may take the form of a coiled pipe situated on, or close to, the bottom of the feed water tank. This coil, or at least that part of the coil closest to the outlet of the heat exchanger, may be disposed in a tray into which is directed cold make-up water supplied to the feed water tank.

Preferably, the bleed system is arranged to remove water from a region of the boiler shell where the boiler water has its highest impurity content, i.e. a region close to said electrode means.

In order that the bleed system does not discharge water from the boiler shell to the drain indiscriminately, it is preferred that the bleed valve should be arranged so that it can only open during periods when feed water is being supplied to the boiler shell from the feed water tank, apart from the initial period of filling the boiler shell.

In operation of an electrode steam boiler in accordance with the invention, the bleed system ensures that water at a temperature in excess of said predetermined temperature cannot pass to the drain, and the heat exchanger transfers heat from the bleed water to the water in the feed tank.

Furthermore, the bleed valve does not have to be designed to withstand steam from the boiler.

The invention will now be described, by way of example, with reference to the accompanying drawing, in which Figure 1 is a schematic sectional view and electric circuit diagram of one embodiment of an electrode steam boiler in accordance with the invention, and Figure 2 is a sectional plan taken on the line Il-Il of Figure 1.

The electrode steam boiler shown in the drawing comprises a shell, generally designated by the numeral 1, having a generally circular cylindrical wall 2, a bottom 3 and a removable top 4 secured to the upper end of the wall 2.

Mounted in the top 4 are three electrically insulating bushings 5, for example of porcelain, from which depend electrode support rods 6, which may be made of cast iron. Secured to the lower ends of the support rods 6 are electrodes 7, 8 and 9, which may also be made of cast iron. The electrodes 7-9 are connected to the three phase lines R, S, T of a 3-phase, 4-wire electrical supply 10 by terminal rods 11 connected to the support rods 6 in the bushings 5.

The numeral 12 designates a feed water tank from which feed water may be pumped to the shell 1 by means of an electrically driven pump 13 and a pipe 14. The pump 13 is electrically connected between the phase line R and the neutral line N of the supply 10 via the contact 1 5 of an electrical relay 16, the coil of which is connected to a current transformer 1 7 which measures the current supplied to one of the electrodes 7-9.

In use of the boiler, let it be assumed that the shell 1 has been filled with water to the level H, that current is being supplied to the electrodes 7-9 from the supply 10, and that the relay contact 1 5 is open. The current flowing between the electrodes heats the water in the shell 1 and generated steam passes from an outlet 1 8 in the shell 1 via a pipe 1 9 to a steam-consuming plant 20. Any condensed steam recovered in the plant 20 is returned via a pipe 21 to the feed water tank 12. As steam generation proceeds, the level of the water in the shell 1 falls and, as the upper ends of the electrodes 7-9 become increasingly exposed, the current supplied to the electrodes correspondingly decreases.When the water has dropped to a level such as that indicated by the letter L, the reduced output from the current transformer 1 7 no longer energises the relay 1 6 and its contact 1 5 closes. The pump 13 then operates to pump feed water from the tank 12 to the shell 1 to raise the water level in the latter.

When the level of the water in the shell 1 has risen to the level H once more, the relay 1 6 opens its contact 1 5 and the pump 1 3 stops.

Replenishment of the water in the feed tank 12 is effected by supplying cold make-up water to the tank via a pipe 22 and a float valve 23.

As more and more steam is generated, the concentration of impurities in the water in the shell 1 gradually rises, this being particularly the case when the plant 20 is of such a nature that little or no condensed steam is returned to the tank 12 via the pipe 21.The greatest concentration of impurities occurs in the water adjacent to the electrodes 7-9, and if this concentration is allowed to rise unchecked, there is a danger of arcing occurring at the electrodes due to the high conductivity of the impure water. It is therefore necessary, periodically, to withdraw water of high impurity content from the shell 1 and discharge it to a drain. To this end the boiler is provided with a bleed system which will now be described.

The bleed system of the boiler comprises a bleed pipe 24 leading from a region of the shell 1 close to the electrodes 7-9 to the inlet end 25 of a heat exchanger 26, in the form of a coiled pipe, situated in the feed water tank 1 2. The outlet end 27 of the heat exchanger 26 is connected by a pipe 28 to a drain 29 via an electromagnetically actuated bleed valve 30. The heat exchanger 26 is disposed in a tray 31 situated at the bottom of the tank 12 and a down-pipe 32 directs cold make-up water from the float valve 23 into the tray 31. The heat exchanger 26 is therefore situated in the coldest water in the tank 1 2.

The valve 30 is electrically connected between the phase line R and the neutrai line N of the supply 10 via the contact 15 of the relay 16 and a thermostatically controlled switch contact 33. The latter is actuated by a thermostat 34 which measures the temperature of the water at the outlet end 27 of the heat exchanger 26, the thermostat being arranged to hold the contact 33 open when the temperature of the water at the outlet end 27 of the heat exchanger exceeds a predetermined value t.

Let it be assumed that the boiler is operating as previously described, that the relay contact 1 5 is closed, that the pump 1 3 is in operation and supplying feed water to the shell 1, and that the switch contact 33 is open to maintain the bleed valve 30 closed. Under these conditions, cold make-up water then flows via the pipe 22, the float valve 23 and the pipe 32 into the tray 31 and cools the heat exchanger 26, with the result that the temperature of the water at the outlet end 27 of the heat exchanger falls below the temperature t. Switch contact 33 is then closed by the thermostat 34 to energise, and so open, the bleed valve 30.The pressure within the boiler shell 1 then forces water from the shell to the drain pipe 29 via the pipe 24 and the heat exchanger 26, this water being taken from a region of the shell 1 where the impurity content of the water is highest.

As the water bled from the shell 1 passes through the heat exchanger 26 it gives up some of its heat to the feed water in the tank 12. Eventually, the temperature of the water flowing from the outlet end 27 of the heat exchanger will rise above the value t, with the result that the thermostat 34 recloses the switch contact 33, which in turn leads to closing of the bleed valve 30.

If the boiler is under full load, there will be a considerable flow of cold make-up water coming into the feed tank 12 and consequently the bleed system will frequently discharge water from the shell 1 to the drain pipe 29. If, on the other hand, the boiler is only lightly loaded, there will be little cold make-up water entering the tank 12 and therefore little cooling effect on the heat exchanger 26, so that bleeding of water from the shell 1 only occurs infrequently. Since it is the cold make-up water that introduces impurities into the water in the shell 1, it will be appreciated that frequent bleeding from the shell 1 is desirable when there is a considerable flow of make-up water into the tank 12, whereas only infrequent bleeding is necessary when the flow of make-up water into the tank 12 is small.Any warm condensate returned from the plant 20 to the tank 12 via the pipe 21 will warm the feed water and result in a reduced frequency of bleeding water from the shell 1 to the drain pipe 29. This of course, is desirable, since the condensate returned from the plant 20 will usually be pure water.

From the above description it will be appreciated that it is desirable for the water in the feed tank 12 to assume a natural temperature gradient with the hottest water at the top of the tank and the coldest water at the bottom of the tank, and that this stratification of the water should be disturbed as little as possible by the introduction of water to the tank via the pipes 21 and 32 and the withdrawal of water via the pipe 14. It is therefore preferred to provide each of these pipes with a T-piece 35, 36 and 37, respectively, disposed so that water is introduced into, or withdrawn from, the tank 12 in a horizontal direction.

From the above description, it will be appreciated that water can only be bled from the shell 1 to the drain pipe 29 during periods when the pump 1 3 is in operation (since the contacts 1 5 and 33 are in series with one another), and that water in excess of the temperature t cannot pass to the drain pipe 29. Furthermore, the valve 30 does not have to be designed to withstand steam from the boiler.

During the initial filling of the shell 1 with water, using the pump 13, it will be appreciated that water cannot pass from the shell 1 to the drain pipe 29 until the water level reaches the level of the bleed pipe 24. When the water rises above the bleed pipe 24, since the pressure in the shell 1 is only atmospheric pressure, only a small volume of water flows to the drain pipe 29 while the pump 13 is building up the water level in the shell to the level H. However, if it is desired to prevent water flowing to the drain pipe 29 during initial filling of the shell 1, it is a simple matter to arrange a second switch contact 38 in series with the contact 33, actuation of the contact 38 being controlled by a further thermostat 39 mounted on the shell 1 and arranged to hold the contact 38 open until the temperature of the shell rises to a predetermined value.

It will, of course, be appreciated that the drawing is only a very schematic representation of a boiler in accordance with the invention. Thus, for example, in practice the electrodes 7-9 might be of spiral shape and be nested together as shown in Figures 1 and 2 of British Patent Specification No.

1,326,359 and water may be bled from the boiler shell using a plurality of bleed pipes arranged to draw water from the spiral gaps between the electrodes, for example as described in Specification No. 1,326,359.

The boiler described above with reference to the drawing may be modified in various ways. For example, the withdrawal of water from the shell 1 need not be limited to periods when the pump 13 is supplying feed,water to the shell. Instead, the periods of withdrawal of water from the shell may be controlled as described in British Patent Specification No. 1,372,176, in addition to the control exercised by the thermostat 34. Again, the feed pump 1 3 may be arranged to run continuously, in which case the pump would be arranged to be by-passed, for example by means of an electromagnetically operated valve, when the water level in the shell 1 reaches the level H.

Claims

1. An electrode steam boiler comprising a boiler shell, electrode means within the shell which, in use of the boiler, is at least partly immersed in water in the shell and is connectible to an electric supply source for the purpose of producing a flow of electric current in the water to heat the water and generate steam, means for supplying feed water to the shell from a feed water tank, means for leading generated steam from the boiler, and a bleed system for bleeding water from the boiler shell to a drain via a bleed valve, said bleed system comprising a heat exchanger having an inlet connected to the boiler shell and an outlet connected to the bleed valve, the heat exchanger being situated in the feed water tank in a position such that, in use of the boiler, it transfers heat from water in the bleed system to feed water in the feed water tank, and a thermostat controlling actuation of the bleed valve in such a way that the bleed valve cannot be opened when the temperature of the water in the bleed system in, or downstream of, the heat exchanger is above a predetermined level.

2. A boiler according to claim 1, in which said bleed valve is disposed externally of the feed water tank, being connected to the outlet of the heat exchanger by a pipe passing through a wall of the feed water tank.

3. A boiler according to claim 2, in which said thermostat is arranged to measure the temperature of the water in the bleed system in the region where said pipe passes through the wall of the feed water tank.

4. A boiler according to any of the preceding claims, in which said heat exchanger is disposed close to the bottom of the feed water tank, so that it is situated in the coldest water in the feed water tank.

5. A boiler according to claim 4, in which said heat exchanger is in the form of a coiled pipe situated on, or close to, the bottom of the feed water tank.

6. A boiler according to claim 5, in which said coiled pipe, or at least that part of the coiled pipe closest to the outlet of the heat exchanger, is disposed in a tray into which is directed cold make-up water supplied to the feed water tank.

7. A boiler according to any of the preceding claims, in which said bleed system is arranged to remove water from a region of the boiler shell close to said electrode means.

8. A boiler according to any of the preceding claims, in which said bleed valve is arranged so that it can only open during periods when feed water is being supplied to the boiler shell from the feed water tank, apart from the initial period of filling the boiler shell.

9. A boiler according to any of the preceding claims, comprising a second thermostat controlling actuation of said bleed valve in such a way that the bleed valve cannot be opened until the temperature of the water in the shell rises to a predetermined value.

10. An electrode steam boiler constructed and arranged substantially as herein described with reference to, and as illustrated in, the accompanying drawing.