GB2185803A - Steam boiler - Google Patents

Abstract

A boiler 10 comprises an outer jacket 12 capable of being exposed to a source of heat e.g. the flue gasses of a furnace, and, within the jacket and spaced away from the walls thereof a chamber 14 having porous walls 18. The jacket and the chamber are preferably formed as concentric tubes having one end blanked off with an end cap 20. Liquid, for example water, is fed from within the chamber 14 and percolates through the porous walls 18 where it evaporates at the surface thereof exposed to the heated outer jacket 12. Steam is collected from the gap 16 between the two. In normal operation, at no time does liquid water come in contact with the heated surface of the outer jacket 12. Thus, heat transfer is not reduced by an oxide or vapour layer building up on the inner surface of the jacket 12 and the boiler may be placed in the flaming region of a furnace without quenching occurring. <IMAGE>

Description

SPECIFICATION Steam boiler The present invention relates to a steam boiler.

Traditional steam boiler designs fall into two main classes, namely water tube and fire tube boilers according to whether the water to be boiled is led in tubes through the heating zone or hot gasses are led in tubes through the water. In both cases heat transfer is from the hot gasses, generated by burning materials, through the metal wall to liquid water. There are several disadvantages to these designs.

Firstly, the heat transfer through the tube to the water is inefficient owing to build up of oxide on the metal surface and to the existence of a static vapour layer trapped between the tube wall and the liquid, both of which inhibit efficient transfer of heat. Where water tubes are employed, these must be kept out of the flaming area of the furnace since the presence of water within the tube will quench the furnace. Thus the tubes are located further up the flue in a cooler and therefore less efficient part. Traditional boilers employ a reservoir of water which must be heated to boiling point before any steam is generated, thus there is considerable lead time before steam can be generated on starting up of the boiler. Also, the water level must be carefully monitored since, if it is allowed to go out of control, explosions can result.

The invention seeks to provide a design of steam boiler in which the above disadvantages are reduced or eliminated.

According to the present invention there is provided a boiler which comprises an outer jacket capable of being exposed to a source of heat and, within the jacket and spaced from the walls thereof, a chamber having porous walls.

The boiler of the invention is advantageously employed with water to produce steam.

Water is fed into the chamber and percolates through the porous walls. The outer surfaces of the porous walls are exposed to heat from the inner surfaces of the jacket which is heated, e.g. in flue gasses, evaporating the water as it 'perspires' through or apears as perspiration on the porous wall surface. In this way, the heated surface (the outer jacket) does not come into contact at any stage with liquid water. The rate at which water percolates through is matched to the rate at which heat is transfered across the wall of the outer jacket and through the vapour gap to cause evaporation of liquid.

Preferably the outer jacket is in the form of an elongated tube and the inner chamber is also in the form of a tube of smaller diameter than the outer jacket. The outer jacket is preferably thin walled and formed from a metal such as stainless steel, aluminium or copper, or a so-called refractory metal, depending on the temperature conditions in the furnace with which it is to be used and operating steam pressures. The inner chamber is preferably formed from a sintered metallic material to have the necessary porosity coupled with heat resistant characteristics. Sintered metallic materials available for, inter alia, filtration purposes have been found suitable, for example a cupro-nickel sintered alloy.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional view of a steam boiler in accordance with the invention; and Figure 2 is a diagrammatic representation of the boiler in use.

Referring to the drawings, it can be seen that a steam boiler 10 comprises an outer jacket 12 in the form of an elongate tube within which is contained a chamber 14. The outer jacket is made from a thin walled metal such as stainless steel and is spaced apart from the inner chamber 14 so as to leave a vapour gap 16.

The inner chamber 14 is also generally tubular in nature and has porous walls 18 of sintered material, for example cupro-nickel alloy.

As illustrated, two grades of sintered material are employed to ensure even distribution of feed water. A gap may be provided between outer surface of inner sintered material and the inner surface of outer porous wall 18 in the interest of achieving a uniformly distributed liquie front facing the outer porous wall.

The inner chamber and outer jacket are both terminated with an end cap 20 preferably having a high temperature ceramic coating since this is the portion of the boiler which in use will be closest to the heart of the furnace. In an alternative construction the inner diameter 14 is led though the end cap 20. An inlet 22 for feed water is provided at the upper end of the chamber 14. The inlet 22 passes through a collar/heat shield 24 preferably made of a material such as cast iron. The function of the spacer collar is to contribute to the bulk mass of the assembly and it also acts as a heat shield for the inner chamber and in general contributes to the even heat distribution throughout the tube. The spacer collar 24 connects to the inner chamber 14 via a support collar 26 again preferably made of cast iron.

In use, the steam boiler of the invention may be inserted in a furnace close to or within the flaming area. The outer jacket 12 quickly heats up and the boiler is ready for use. Feed water in led into the chamber 14 either under the action of gravity or under pressure and it advances and percolates through the porous walls 18. As it reaches the outer surface, termed the 'perspiration surface' heat radiated from the inner surface of the outer jacket 12 causes the water to evaporate and turn into steam filling the vapour gap 16. The steam so generated is removed through the gap 28 between the collar 24 and the outer jacket 12. The size of this gap is chosen, in combination with the other design parameters, to give steam of the desired pressure. The steam is then led away for use as known conventionally.

The steam production can be regulated by regulating the feed of water. However, since the device can run 'dry' that is without any water present, there is little or no danger of explosion with the boiler of the invention. In order to ensure correct operation, care should be taken that the water feed rate does not exceed the evaporation rate at the surface of the chamber 14 so that liquid water does not at any point enter the vapour gap 16 or come into contact with the outer jacekt 12. A thermostatic device (not shown) can be employed to ensure that the vapour gap is not flooded by the feed water. Sampling of the output steam and the use of known thermostatic or other control devices may regulate the water flow to maintain the proper evaporation thereof under normal running conditions.

Referring now to Fig. 2, a number of steam boilers 10 in accordance with the invention are positioned within a furnace close to the heart of the burning zone 30. Water is fed into the boilers 10 by means of a feed pump 32 and a thermostatic feed regulator 34 (which as previously explained prevents flooding of the vapour gap 14). Steam will be generated almost immediately (and will be generated at once if the boilers 10 are already preheated). The steam is led by pipes 36 to the steam main 38 for use as conventionally known.

Unlike the water and fire tube steam boiler designs discussed previously, where the liquid (water) is in contact with the heating surface, the boiler of the invention has a design in which feed water evaporates some distance away from the heating surface causing the outer jacket which is exposed to flame im pingement to be comparatively dry at ail times. This ensures relatively high ignition temperatures and consequently 'cleaner' flue gasses. The boiler may generally be located in the flaming region of the furnace, again unlike previous boiler designs which place the ele ments in upper flue regions that are at a lower temperature. The quenching effect of the boiler of the invention is negligible, parti cularly after having been exposed for a short length of time in the furnace prior to steam ing.Maximum evaporation efficiency is obtained when the vapour/liquid interface lies at a small distance below within the outer layers of the porous surface of the inner chamber 14. The proportion of heating surface exposed to the evaporation surface is maximised for high efficiency particularly at upper temperature limits. The vapour gap 14 can be adjusted so a to avoid heat damage to the nonferrous porous walls 18. The sintered nickel alloy referred to above can withstand a maximum temperature of 500"C. without damage.

The steam boiler of the invention does not require a pressure vessel with accompanying steam and water space and consequently no gauge glass for continuous monitoring of the water level is required. The boiler can be flushed periodically by closing the water feed off and allowing the steam pressure to escape through the fear tube in a reverse flush operation which is sufficient to dislodge scale and other matter in the walls 18 and the feed tube. It is preferred, of course, to use a purer feed water which will reduce the necessity for such blow down.

Since, in the boiler of the invention there is no actual contact between liquid and metal heating surface, heat transfer losses causes by stagnant oxide and vapour films on the liquid side are significantly reduced. Stagnant films of gas on the outside of the jacket 12 may be reduced by introducing a small measure of turbulence in the furnace. Heat transfer within the steam boiler of the invention 10 is therefore initially by radiation and, during operation, the vapour within the gap 14 also transmits heat by conduction. Convective heat transfer is not thought to be a significant contributor.

The boiler of the invention is fully compatible with furnaces burning all types of fuels from low grade biomass to high grade fuels such as propane, liquid hydrogen, etc. High furnace efficiencies are maintained owing to the dryness of the metal jacket which permits high ignition temperatures required for burning fossil fuels efficiently. Steaming capacity may be increased by increasing the number of boiler tubes of the invention or regulated by cutting in or out one or more of the tubes already in situ. Similarly failure of one tube does not inerfere with generation from the remaining tubes.

The steam boiler of the invention is of sim ple design and is virtually industructible. The only part likely to need replacement is the porous chamber wall 18. This may readily be replaced with a spare. The boiler does not therefore demand critical human attention dur ing its operation and can be made self starting and stopping with the deployment of suitable control systems.

It will be apparent that the boiler of the invention has numerous applications where steam is required in industry, agriculture or in the heating of domestic and public buildings.

In particular may be mentioned the use of the boiler in industry for supplying process steam, or its application as a modular unit in pollutant rich flue streams for steam scrubbing atmo spheric contaminants. In agriculture, the boiler of the invention may be used with straw burn ing furnaces. The low heat release of com pacted straw renders the boiler of the invention ideal for the purpose as does the low maintenance required. Owing to the durability and simplicity of maintenance of the boiler it may be used for these purposes in third world countries in particular to power irrigation units and generate electricity burning low grade biomass fuels. Compact portable boiler units may be used in hospitals, laboratories, or even for domestic heating where steam has significant economic advantages over hot water systems, and the boiler appliance is virtually explosion proof.Furthermore, one or more steam boiler tubes of the invention may be employed in an existing boiler located in the furnace flaming zone where it will not interfere with the main boiler installed further up in gaseous flue zone and will give enhanced steaming for peak load demands.

Being of compact construction the boiler of the invention may be used as a critical heat extraction unit and may be made to function equally well using various liquids of diverse physical properties, such as Dowtherm, glycol, common hydrocarbons etc.

Further the boiler of the invention may be especially suitale for operation in the coolant system of an internal combustion engine wherein the heat absorbed would be released by means of evaperative cooling in a condensing cycle which is more efficient than cooling using a radiator. The system therefore permits controlled higher engine operatng temperatures which has a favourable reducing effect on exhaust emissions.

Claims (10)

1. A boiler which comprises an outer jacket capable of being exposed to a source of heat and, within the jacket and spaced from the walls thereof, a chamber having porous walls.

2. A boiler as claimed in claim 1 in which the outer jacket is in the form of an elongated tube and the inner chamber is also in the form of a tube of smaller diameter than the outer jacket.

3. A boiler as claimed in either of claims 1 and 2 in which the outer jacket is preferably thin walled and formed from a metal.

4. A boiler as claimed in any of claims 1 to 3 in which the inner chamber is formed from a sintered metallic material having the necessary porosity coupled with heat resistant characteristics.

5. A boiler as claimed in claim 4 in which the sintered material is a cupro-nickel sintered alloy.

6. A boiler as claimed in either of claims 4 or 5 in which two grades of sintered material are employed to ensure an even distribution of the liquid.

7. A boiler as claimed in claim 6 in which there is a liquid gap between the two grades of material.

8. A method of operating a boiler as claimed in any of claims 1 to 7 which comprises exposing the outer jacket to a source of heat and forcing liquid through the porous walls of the chamber to evaporate in the gap between the chamber and the outer jacket.

9. A method as claimed in claim 8 in which the rate at which water percolates through the walls of the chamber is matched to the rate at which heat is transferred across the wall of the outer jacket and through the vapour gap to cause evaporation of the liquid.

10. A method as claimed in either of claims 8 or 9 in which the boiler is positioned within the flue gasses of a furnace close to or within the flaming area.