The present invention relates to a cleaning system, suitable for chemically treating and thereby removing limescale from a plate heat exchanger.

INTRODUCTION

Plate heat exchangers are well known and are used in a variety of applications, in particular in combination ("combi") boilers, which are often preferred in small domestic households such as flats where space is at a premium. The plates are typically arranged such that channels in each plate run perpendicular to channels in adjacent plates, although the Mains Water (MW) side (fresh drinking water from the mains supply) is kept entirely separate from the water making up the Central Heating (CH) system, which is typically a closed loop. However, heat is transferred from the heated Central Heating water to the cold Mains Water through the plate heat exchanger, such that the Mains Water is rapidly heated to approximately 65 degrees C.

In order to increase the surface area for heat transfer, the plate heat exchangers are typically designed with numerous plates and narrow channels. However, both the CH and MW sides of the plate heat exchanger have issues with clogging. Many solutions exist to flush out the "sludge" in Central Heating systems, but this does not address the limescale build up which is a particular problem in the MW side and, in particular, in the narrow channels of the plate heat exchanger. The channels of the plate heat exchanger on the MW side can get so badly blocked by limescale deposits that the plate heat exchanger ceases to function properly with users experiencing showers or taps that suddenly run cold then hot.

One attempt to address this is to make the plates of the plate heat exchangers out of stainless steel, such that they expand and contract as their temperature fluctuates, the idea being that this movement will cause the limescale deposits to break up before they build up to the point where they block the channels sufficiently to impair the function of the plate heat exchanger. Unfortunately, in practice, this design does not sufficiently clear enough of the limescale deposits to prevent loss of function of the plate heat exchanger. The result is that a plumbing technician is called out and the typical solution is to simply remove (plate heat exchangers are typically cartridge-like, so can be removed) and throw away the existing plate heat exchanger and replace it with a new one. This is costly and inevitably causes delays whilst parts are ordered, leaving the user without a functioning boiler in the interim. It also drives up cost for the plumbing technician who typically provide this service under insurance, if only because repeat visits are required to the same location to firstly assess and inevitably later replace the plate heat exchanger. Alternative solutions such as water softeners, although effective, are bulky and expensive, whilst there is some debate about the effectiveness of using localised magnetic fields created around pipes to address limescale build up. One can, of course, attempt to manually clean a plate heat exchanger, but this is far from trivial and is typically ineffective. Accordingly, there is a need in the art for a more cost effective solution that avoids replacement of a plate heat exchanger and ideally minimises the number of visits required by a plumbing technician and the period of time in which the boiler is ineffective.

Surprisingly, we have found that a simple pressurised two-tank cleaning system can be used to effectively clean not only the Central Heating side, but also the Mains Water side of a plate heat exchanger, preferably both sides at the same time. This is a chemical, rather than mechanical, cleaning system, so it has fewer moving parts to worry about and is less likely to cause damage to the plate heat exchanger (which is already designed to cope with elevated pressures, but not necessarily mechanical scraping). The pressurised system advantageously helps to ensure that the descaler (a limescale-removal agent, such as an acid) reaches all the channels of the plate heat exchanger.

We have also found that the gases inevitably produced from chemical treatment of limescale can be recycled back into the system to further pressurise or re-pressurise the system, thereby helping to force more of the descaler through the plate heat exchanger, helping the descaler to penetrate through to the bulk of the narrow blocked channels within the plate heat exchanger.

SUMMARY OF THE INVENTION

Thus, in a first aspect, the present invention provides a cleaning system (1), suitable for chemically removing limescale from a plate heat exchanger (9), having at least one inlet (8a, 8b) and at least one outlet (13a, 13b), by contacting and reacting the limescale in the plate heat exchanger (9) with an acidic solution (descaler, 3), the system comprising:

- a first pressurisable container (2) having:

» a descaler portion to accommodate descaler, and a gas portion (35) to accommodate gas; ^■ a primer (37) for pre-pressurising said first pressunsable container; and

^■ at least one first conduit (7a, 7b) for establishing a fluid connection between the descaler portion (34) of said first pressunsable container (2) and the inlet(s) (8a, 8b) on a plate heat exchanger (9);

- a second pressunsable container (11) having;

^■ at least one second conduit (12a, 12b) for establishing a fluid connection between the outlet(s) (13a, 13b) on the plate heat exchanger (9) and said second pressunsable container (1 1); and

^■ a reaction product portion (38) to accommodate reaction products, and a gas portion (39) to accommodate reaction product gas;

and

- a third conduit (19) comprising a valve: (21)

the third conduit (19) positioned to establish a fluid connection between the gas portion (39) of said second pressurisable container (1 1) and the gas portion (39) of said first pressurisable container (2) so as to recycle reaction product gas to the gas portion (35) of said first pressurisable container (2), and thereby re- pressurize said first pressurisable container (2);

the valve (21) restricting said fluid connection between said first and second pressurisable containers (2, 11).

The primer may be an inlet or valve connected to a pump, preferably an external pump, more preferably an external air pump. The pump is used to increase the pressure inside the first pressurisable container (prior to descaler contacting limescale in the plate heat exchanger). Accordingly, the primer is preferably an inlet or valve connected to a pump to increase the pressure inside the first pressurisable container. In some embodiments, the primer (37) comprises an inlet (6) or valve connected to a pump to increase the pressure inside the first pressurisable container (2).

Alternatively, the primer may be a plate or diaphragm within the first pressurisable container that is forced in such a direction, typically down, so as to increase the pressure on descaler in the first pressurisable container. In this embodiment, the plate or diaphragm may be forced in said direction by a pump forcing fluid (gas or liquid) into a space on the other side of the plate from any descaler which is added to the first pressurisable container. This primer is separate from the third conduit, the former helping to initiate the limescale removal reaction, the latter helping to drive the reaction forward through re-cycling of the reaction product gases (typically carbon dioxide) into the first pressurisable container to thereby re- pressurise (or maintain the pressure within it).

Preferably, the primer is a plate or diaphragm within the first pressurisable container which increases the pressure on descaler in the first pressurisable container. The plate or diaphragm is preferably urged by a pump forcing fluid into a space on the other side of the plate from any descaler.

In some embodiments, therefore, the primer (37) comprises a movable plate or diaphragm (26) positioned within the first pressurisable container (2) between the descaler portion (34) and the gas portion (39), the plate or diaphragm (26) increasing the pressure on the descaler (3) in the first pressurisable container (2). In some embodiments, the system comprises a pump to urge the plate or diaphragm to force gas into the gas portion.

The system may comprise a third pressurisable container connected to at least one or both of the first and second pressurisable containers via suitable conduit(s). This third pressurisable container may be provided to collect and more easily dispose of the waste products and excess descaler. In some embodiments, therefore, the third conduit (19) passes reaction product gas from the second pressurisable container (11) to the gas portion (39) within the first pressurisable container (2).

In some embodiments, the system comprises a heat exchanger. In other embodiments, the system does not comprise a heat exchanger.

Advantageously, in some embodiments, the system is a closed system once both the fluid connections between said first pressurisable container (2) and the inlet(s) (8) on the plate heat exchanger (9); and between said second pressurisable container (11) and the outlets(s) (13) on the plate heat exchanger (9) are established.

Each of the pressurisable containers may comprise a pressure gauge for determining the pressure of the gas within each container.

The pressurisable container(s) may be drums, tanks or other sealed or sealable vessels. They, like the rest of the system, are suitable for the limescale removal process (i.e. made of or lined with a material resistant to the descaler and the reaction products). The container(s) are pressure-resistant so that no fluid leakage occurs, even above the typical reaction pressures, or designed to withstand such pressures when sealed. The typical reaction pressures are typically at least one, preferably 2 or 3 bar. One or each container may be fitted with a suitable vent. Such vents allow gas to escape in a safe manner.

The conduit(s) may be flexible or inflexible. The conduit(s) may be pipes, tubes or hoses. It will be appreciated that the conduits connect with the inside space or area of the containers. The conduit(s) serve to convey the contents of the first pressurisable container (i.e. descaler) to the plate heat exchanger and waste reaction products including gas (and likely some descaler too) to the second pressurisable container. Thus, the conduit(s) are suitably strong, resistant and enclosed. Plastic piping or acid-resistant hoses are preferred examples. They are provided with suitable connectors, such as bungs or snap-fit connections. There may be only one first conduit, and in some embodiments, there are two, and preferably 2 or more, including 3, 4 or 5. There may be only one second conduit, and in some embodiments, there are two, and preferably 2 or more, including 3, 4 or 5.

The first pressurisable container may have one or two or three or more outlets. Similarly, the plate heat exchanger may have two or more inlets. The second pressurisable container may have one or two or three or more inlets, and/or the plate heat exchanger may have two or more outlets. A single conduit may attach to more than outlet or inlet by providing branching points and the requisite number of connections. Alternatively, one conduit may be used for each inlet/outlet pair of the pressurisable container and plate heat exchanger, or there may be a mixture of branched conduits and non-branched conduits.

An inlet or outlet may comprise a valve, such a tap.

The system is advantageously portable allowing it to be taken to the site of a boiler failure, for instance a failure that may have occurred through limescale deposition. Thus, the container(s) may be suitably sized for easy transport. They may in the range of 1-200 litres, although about 10-30 litres is ideal. Quick release connections for at least the inlet and outlet of the plate heat exchanger may also be used to improve portability and educe set-up times. However, the system may also be sized for use in a van or at a factory where plate heat exchanger are brought to the system enabling higher pressures and multiple plate heat exchangers to be connected simultaneously. Examples of typical container sizes in this instance would be in the range of 50-500 litres. The second pressurisable container may be within 10-30 per cent, preferably 15-25 per cent, more preferably around 20-25 per cent smaller in total volume than the first pressure able container. This allows more descaler to collect, or be retained, in the first container.

Multiple plate heat exchangers can be connected to the system either in series, but most preferably in parallel, though provision of suitable additional conduits, for instance branched conduits, and/or provision of additional outlets and inlets to the first and second pressurisable containers.

Descaler may be, or comprise, a limescale removal agent. The limescale removal agent may be, for instance, a weak acid. It should be sufficient to remove the limescale though chemical treatment. The system does not typically comprise descaler, when not in use as it is potentially hazardous. In some embodiments, therefore, the descaler (3) may be, or comprise, a limescale removal agent, optionally including an acidic solution, preferably a weak acid, reactable with limescale (calcium carbonate) to produce an aqueous calcium salt and a gas such as carbon dioxide

The valve in the third conduit is preferably a tap or other suitable valve that serves to reduce or entirely cut off the passage of waste fluid, preferably waste gas, between the first and second pressurisable containers, i.e. from the first pressurisable container to the second pressurisable container. It may, advantageously, be or include a non-return valve. The nonreturn valve prevents passage of waste gases from the first to the second pressurisable containers, but allows passage from the second to the first pressurisable containers. Waste gas is referred to herein and this includes reference to reaction product gas as the two are interchangeable.

The third conduit may also include a stop member, such as a valve or tap, to completely close off the third conduit (in both directions). This stop member may be part of or separate from the valve in the third conduit mentioned above.

Advantageously, both the Central Heating and Mains Water sides of the plate heat exchanger are cleaned substantially simultaneously by chemical removal of limescale.

The descaler portion 34 and the gas portion 35 may be one space or forked from one space, e.g. when the first pressurisable container (2) is empty, or may in some embodiments be divided - for example by the plate or diaphragm as described below. To accommodate descaler, gas or reaction products means, in some embodiments to collect and contain them, to prevent them from escaping the system unless this is intended. The reaction products may be reaction product foam or liquid and, potentially, some solid particles of, for example, mechanically removed limescale. The reaction product gas typically does not include liquid or foam or solid particles.

The gas portion of the containers may be described as a reaction product gas portion.

The valve (21) may be positioned along the length of the third conduit (19). The valve (21) may be restrictable in the direction of first to second containers. Preferably, it is a one-way or non-return valve. The valve should be able to prevent fluid connection, especially reaction product gas, passing between said first and second pressurisable containers.

In a related aspect, the invention provides a cleaning system, suitable for chemically removing limescale from a plate heat exchanger, the system comprising:

- a first pressurisable container having:

a primer for pre-pressurising said first pressurisable container; and

a first conduit for establishing a fluid connection between said first pressurisable container and the inlet(s) on a plate heat exchanger;

- a second pressurisable container having;

a second conduit for establishing a fluid connection between the outlet(s) on a plate heat exchanger and said second pressurisable container; and

- a third conduit, for establishing a fluid connection between said second pressurisable container and said first pressurisable container, the third conduit having a valve for restricting said fluid connection between said first and second pressurisable containers. Preferably, the third conduit recycles waste gas to, and thereby re-pressurises, said first pressurisable containers. The embodiments described herein also apply to this aspect. Methods for, or of, cleaning plate heat exchangers, or the like, are also provided. These comprise use of the present system. Preferably, the first pressurisable container is partially filled with descaler and the conduits connected to the plate heat exchanger. This may occur in any order. Once this has been completed, the first pressurisable container is primed to pre-pressurise the first pressurisable container and descaler is passed through the first conduit to the plate heat exchanger and the reaction products pass through the second conduit to the second pressurisable container, with waste gases passing through the third conduit to the first pressurisable container via the third conduit's valve. Accordingly, the present invention also provides a method of or for cleaning plate heat exchangers, or the like, comprising use of the present system. Ideally, the method includes:

• priming the first pressunsable container to pre-pressurise the first pressurisable container;

• passing descaler through the first conduit to the plate heat exchanger; and

• passing reaction products through the second conduit to the second pressurisable container.

Optionally, the method may also comprise:

• connecting the conduits to a plate heat exchanger; and/or

• passing waste gases through the third conduit to the first pressurisable container via the third conduit's valve.

In some embodiments, the method comprises:

• establishing a fluid connection between said first pressurisable container (2) and the inlet(s) (8a, 8b) on a plate heat exchanger (9); and

• establishing a fluid connection between said second pressurisable container (1 1) and the outlets(s) (13a, 13b) on the plate heat exchanger (9), to thereby provide a closed system suitable for recycling reaction product gas to, and thereby re-pressurizing, said first pressurisable container (2),

In some embodiments, the method comprises:

• priming the first pressurisable container (2) to pre-pressurise the first pressurisable container (2);

• passing descaler (3) from the first pressurisable container (2) through the first conduit (7) to the plate heat exchanger (9);

• collecting reaction products and passing said reaction products from the plate heat exchanger (9) through the second conduit (12) to the second pressurisable container (1 1); and

• passing waste gases from the second pressurisable container (1 1) through the third conduit (19) to the first pressurisable container (2), via the third conduit's valve (21), to thereby further pressurise the first pressurisable container (2). In some embodiments, the first pressurisable container is at least partially filled with descaler prior to priming. The conduits may, optionally, be connected to the plate heat exchanger at any point prior to passage of the descaler.

The method cleans not only the Central Heating side, but also the Mains Water side of a plate heat exchanger, preferably both sides at the same time. The method is preferably a chemical, rather than mechanical, cleaning method.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows an embodiment of the present invention in which the cleaning system 1 is connected (coupled) to a plate heat exchanger 9;

Figure 2 shows an embodiment of the present invention in which an alternative arrange of third conduit 19 is provided with respect to the first and second tanks (2 and 11 , respectively); and

Figure 3 shows an embodiment of the present invention in which a third (collection) tank 29 and additional conduits (hoses 30 and 31) and pressure release valves 32 and 33 are also shown.

DETAILED DESCRIPTION

As mentioned herein, advantageously, the third conduit helps to drive the reaction forward through re-cycling of the reaction product gases (typically carbon dioxide) into the first pressurisable container to thereby re-pressurise (or maintain the pressure within it). This recirculating or recycling of the waste reaction product gas, but preferably not any of the descaler or reaction product (which might be liquid or foam etc.), is under pressure (higher than atmospheric) and in a closed system. This makes the cleaning reaction faster and also drives the descaler into small spaces and far corners of the plate heat exchanger.

In some embodiments, it is advantageous to include the plate or diaphragm as this provide mechanical attenuation of any pressure spikes that may arise in the system. It also functions to separate recirculate gas from the fresh descaler. A plate or diaphragm may also be used in the second and/or third containers. A number of pressurised systems are known in the art. However, none of the art recirculates (reaction product) gas, not cleaning liquid. In particular, none of the art recirculates (reaction product) gas under pressure in a closed system. For examples, Lin (US 6305393B) does not prime the system, but later uses the primer/compressor to force the waste out, but there is no re-circulation of the waste gas.

Stevens (US 2554389A) relates to cleaning equipment for the crank case or engine block in an old V8 engine. The system is not closed, but rather the cleaning fluid is sprayed on and collected. Ho (KR 101207498B) relates to dishwasher, whilst Obata (JP 2007130613A) relates to a washing machines, but these devices are not suitable for use with plate heat exchangers.

Essentially, the cleaning system comprises first and second pressurisable containers, each pressurisable container having at least one conduit to provide a fluid connection to the inlet(s) and outlet(s) of heat exchanger, respectively.

The system works by chemically removing, in other words dissolving or reacting away the limescale, calcium carbonate. This, the system may be seen to be suitable chemically treating and thereby removing limescale.

It will be appreciated that the system does not typically comprise the plate heat exchanger, other than in use. The pressurisable container may be a tank or other vessel provide that it is suitable for the present process and therefore is sufficiently corrosion resistant to the descaler.

Suitable operating pressure ranges will include 1-3 bar for the priming in the first pressurisable container. Otherwise, suitable operating pressures may be in the range of 0.5 to 10 bar for the second pressurisable container. An upper limit might be up to 25bar, after which the tanks may explode. Safe working zones for tanks 1 and 2 will ideally be between 0.5 to 6bar of pressure. Preferably, the pressure in first tank will not exceed 2 bar. Typically, the pressure in the second tank will be slightly higher due to the gases produced. As such, it is preferred that the pressure in first tank will not exceed 3 bar or preferably 4 bar.

Although the present invention is described in relation to a plate heat exchanger for a combination boiler, it will be appreciated that it can be used on any plate heat exchanger. Suitable other examples include plate heat exchangers found in swimming pools and ground-source heat exchangers.

Although preferred, the present invention is not necessarily limited to plate heat exchangers, as there are other types of heat exchanger that could usefully be cleaned with this system. Indeed, all heat exchangers, not just plate heat exchangers, should be cleanable by the present invention. Varying nozzle sizes and tanks may be needed, but this will be apparent to the skilled person. Indeed, this invention can b used for cleaning any enclosed space, such as a pipe, tube or cavity that may comprise sludge or limescale, or container or vessel that stores, coveys or heats water.

With reference to figure 1 , it can be seen that the cleaning system 1 , firstly comprises a first pressurisable container 2, in this example, a tank similar to a hot water tank (for an immersion heater). The tank 2 comprises a descaler portion 34 and a gas portion 35. It needs to be resistant to the descaler 3 so that it does not corrode. Stainless steel or enamel would be suitable. The tank 2 comprises a visible pressure gauge 4 and ideally an indicator to show the level of descaler 3 in the tank 2 (not shown). The first tank 2 also comprises a priming inlet 6 at the top of the tank 2 to allow the tank to be primed, as described further below. There is also an internal pipe 22, within tank 2, but this is described further below.

At the bottom of the tank 2 are, in this example, two tank outlets 5a and 5b to which hoses (first conduits) 7a and 7b are attached via suitable connections, which could be as basic as an outlet spout with a snap-fit connection as found with garden hoses or a c-clip design.

3-way isolation valves are useful as they allow the heating engineer to be able to clean the whole of inner water pathways without having to remove the heat exchanger itself and even the boiler itself, if desired.

Hose 7a therefore connects at one end with the tank 2 and at the other end to an inlet 8a of the plate heat exchanger 9, said inlet 8a being for Mains Water (MW) side. Similarly, hose 7b connects at one end with the tank 2 and at the other end to an inlet 8b of the plate heat exchanger 9, said inlet 8b being for the Central Heating (CH) side. Suitable valves 10a and 10b are placed along hoses 7a and 7b, ideally at or close to the tank end of the hose to avoid excess descaler 3 being present when uncoupling the hoses 7a and 7b from the plate heat exchanger 9 after use. The second pressurisable container (tank) 11 also comprises a reaction product portion 38 and a gas portion 39. The second pressurisable container (tank) 11 has hoses (second conduits) 12a and 12b that connect the outlets 13a (MW) and 13b (CH) of the plate heat exchanger 9 to the tank 1 1 via the same mechanisms described above for tank 2. Hoses 12a and 12b also have valves 14a and 14b, again at or near the tank 11 end of the respective hoses.

Although both the MW and CH sides of the plate heat exchanger 9 need cleaning, the MW side typically has more limescale attached to it, due to the constant passage of new water through the system, bringing with it new Calcium Carbonate (limescale) in solution to deposit. In contrast, the CH side is a closed loop and the water is rarely refreshed so typically comprises of a "sludge" of Copper oxide or iron oxides (from copper or iron/steel pipes) used in the central heating system or in radiators. The result is that that, typically, more gas is produced on the MW side (see the reaction above). For this reason, separate conduits are preferred for connecting the MW and CH outlets 13a and 13b of the plate heat exchanger 9 to the tank 11. To cope with this, tank 1 1 has different internal arrangement for the MW and CH inlets from hoses 12a (MW) and 12b (CH). On the CH side, there is an internal pipe 15 inside tank 11 in fluid connection with hose 13b. Pipe 15 rises straight up and exits about a third to half way up the inside of tank 11 as shown in Figure 1 , although the exact height can vary. On the MW side, there is an internal pipe 16 inside tank 11 in fluid connection with hose 12a, but pipe 16 is an inverted U-shape and is provided with perforations 17 to allow some gas and reaction product to escape into tank 11 before the pipe bends down towards the base (bottom) 18 of tank 1 1 again where it exits again and connects with the third hose (conduit) 19 via another suitable connection.

The third hose 19 then passes back towards the base 20 of the first tank 2, via a non-return valve 21 , set (for instance at zero governor pressure - a non-return 1 bar governor valve) to allow only passage from tank 11 to tank 2 and not the return direction. This valve also has a regulator function so that it can be opened and closed as required. The third hose 19 passes back through a suitable connection in base 20 of the first tank 2 to form internal pipe 22. This rises inside tank 2 to typically 2/3 to 3/4 of the height of tank 2, where it terminates, provided that this is above the highest level of the descaler 3.

In use, the plumber will either remove a cartridge-style plate heat exchanger 9 from a boiler (not shown) or will connect the hoses 7a, 7b, 12a and 12b to suitably installed connections in the pipework surrounding the boiler. Descaler 3 will be, or will have been, added to tank 2, taking care not to overfill above the termination of internal pipe 22. Valves 10a and 10b should be closed before the descaler 3 is added. Having connected up the hoses to the inlets and outlets 8a, 8b, 13a and 13b of the plate heat exchanger 9, the system 1 is now ready for use. The primer 6 is then used to increase the air pressure inside tank 2 to approx. 1 bar. Valves 10a and 10b on hoses (first conduits) 7a and 7b and valves 14 a and 14b on hoses 12a and 12b are opened and the descaler 3 is forced through the hoses (first conduits) 7a and 7b and into and out of both MW and CH sides of the plate heat exchanger 9. The reaction products are forced along hoses 12a and 12b and into tank 1 1. Some reaction product gas and other reaction products (such as liquid, foam or even small particles of solid limescale) from the MW side of the plate heat exchanger 9 escape through the perforations 17 in internal pipe 16 (inside tank 1 1) and collect in gas portion 39. The sludge and descaler 3 from the CH side of the plate heat exchanger 9 are forced out of internal pipe 15 inside tank 1 1 and begin collecting reaction product portion 38 in tank 11. Gases pass along the third hose 19 into gas portion 35 in first tank 2 and this serves, advantageously, to pressurise or re-pressurise tank 2, thus driving more descaler 3, collected in descaler portion 34, through the system 1 and, therefore, in the hard-to-reach or substantially blocked pathways of the plate heat exchanger 9.

It will be appreciated that the effects of gravity will naturally draw the descaler (3), being a liquid, towards the base of the first container (2), namely into the descaler portion 3. Thus, the descaler portion 3 is preferably adjacent the base (20) of the first container (2) and adjacent the tank outlets (5) so that the descaler (3) can easily flow, regardless of pressure into said tank outlets. The air and ultimately the waste gas (reaction product gas) will naturally be forced into the opposite end, gas portion 35, due to its lower density.

The reaction product is a mostly a liquid, and potentially some foam and possibly small particles or pieces of limescale itself that are washed out. Similar to the situation with the descaler (3) and air or gas in the first container (2), the effects of gravity will naturally draw the reaction product (liquid and/or foam) towards the base of the second container (2), namely into reaction product portion 38. Thus, the reaction product portion 38 is preferably adjacent the base (18) of the second container (11). The waste (reaction product) gas (and any air forced through) will naturally be forced into the opposite end, gas portion 39, due to its lower density.

A non-return valve is useful for the recirculating loop but can be shut off manually to create just a pressurised tank and a receiver if required. The waste product (reaction product) gas, typically carbon dioxide, will pass in the directions shown in Arrows A, B and C in Figure 1.

An alternative arrangement may be used whereby the third conduit 19 does not pass through the base 20 of the first tank 2, but instead enters the first tank 2 above the level of the descaler. It still serves to deliver the waste gas/reaction product gas from the third conduit 19 to re-pressurise the first tank 2.

The system 1 may be reversed in the sense that the pressure can be allowed to build up in tank 1 1 through restriction of the regulator in non-return valve 21 (in third hose 19) and pressure can be carefully released in tank 2 via a release valve (not shown), such that descaler 3 (and likely some reaction products) pass back through the plate heat exchanger 9 and into tank 2.

Reverse flushing may enable the unreacted acid in the second tank to be re-used. Extraction of the unreacted acid would otherwise involve the addition of aqueous bicarbonate or sodium hydroxide to the mixture, and then the use of an organic solvent such as ethyl acetate.

The reverse flush can be useful when cleaning a large volume, such as a whole boiler, for instance.

With reference now to Figure 2, a different embodiment of the invention is described wherein the inverted U-shaped internal pipe 16 inside tank 11 is replaced with an internal pipe 23 instead and the third hose 19 then passes back from the top 24 of tank 11 towards the top 25 of the first tank 2. The internal pipe 23 terminates below a sealed plate or baffle 26 that can rise and fall within tank 11 such that gases emanating from pipe 16 force the plate up as the pressure builds within the area 27 below plate 26, thus driving the plate 26 up and increasing the pressure in the area 28 above the plate 26. This increased pressure is then used to re-pressurise tank 2 via third hose 19. Only one conduit is shown leading between the tanks and the heat exchanger, but this could be a branched conduit for instance.

Regarding Figure 2, the type of regulator shown in the middle pipe connecting tank 1 and tank 2 is a non-return valve. The four symbols connecting tank 1 and tank 2 to the heat exchanger represent isolation valves. With reference now to Figure 3, it can be seen that a third tank 29 can be provided as a collection tank for waste materials (reaction products) and gases and/or excess or unused descaler 3. Suitable hoses 30 and 31 are provided from tanks 2 and 1 1 , respectively. Pressure release valves 32 and 33 are provided to join tanks 2 and 1 1 with hoses 30 and 31. Ideally, these may be set at 3 bar. These pressure release valves, preferably in combination with the third tank 29 provide an advantageous safety feature.

In figure 3, it can also be seen that an inverted U-shaped internal pipe 16 inside tank 11 does not have to be used. The third hose 19 may also pass back towards the first tank 2 from the top 24 or bottom 18 of tank 11 and may join at the top 25 or bottom 20 of the first tank 2. Regarding Figure 3, the type of isolation valve drawn is a ballafix isolation valve.

Furthermore, in some embodiments, and as shown in Figure 3, perforations can be provided in internal pipes 15 and/or 16 inside tank 1 1. In general, perforations are advantageous as they allow gas to escape, but help to keep foam waste at the base of the tank.

Flexible hoses are advantageous as they accommodate different size plate heat exchangers 9 to be used.

A suitable example of a descaler 3 may typically include a relatively weak acidic solution, such as sulphuric acid (pH 2.75 at 1.0 mm) and this is used to exemplify the reaction process. The weak acid and the limescale can be considered to be the reagents in the limescale removal reaction. It will be appreciated that as they contact one another at the plate heat exchanger 9, a chemical reaction will take place. The weak acid will react with calcium carbonate, forming carbon dioxide, calcium sulphate and water as products:

H2SO4 (aq) + CaCOs (s)≠ C0 ₂ (g) + CaS0 ₄ (aq) + H ₂ 0 (I)

The gaseous carbon dioxide will need to be removed from the reaction site in order to avoid slowing the reaction. Removing this product, for instance by re-pressurising or recycling this gas to the first tank 2 will thereby increase the rate of the reaction, in accordance to Le Chatelier's principle (according to Le Chatelier, the position of equilibrium in the reaction will move to counteract any changes). Accordingly, not only does the re-pressurising of the first tank using the third conduit help drive more descaler into the plate heat exchanger, it also helps to draw off some of the carbon dioxide reaction product, thereby pulling the chemical reaction to the right (as written above, by removing some of the carbon dioxide reaction product as well). This synergistic effect makes the present system particularly effective at cleaning (chemically treating and thereby removing limescale). The carbon dioxide may be further vented from either tank via suitable vents.

Both the first and second pressurisable containers preferably each comprise a pressure gauge for determining the pressure of the gas, for instance in the top of the tank or at least in the space away from the liquid or waste within the tanks. This is advantageous from a safety point of view but also they can also provide an indication of when the reaction is completed and the plate heat exchanger has been cleaned. When the reaction has finished, the two pressure gauges should read same value (or within 10%).

Container sizes are discussed above, but it is advantageous for the second pressurisable container to be in the region of 20% smaller than the first pressurisable container. This ratio is thought to be optimum in terms of assisting the recycling of waste gases to re-pressurise the first pressurisable container and drive the reaction forward. The reason for this size difference is so that gases can pass back into the first tank more quickly.

It will be appreciated that the use of weak or strong acids will vary the amount of gas and liquid.

The area 27 below plate 26 and the area 28 above the plate 26 shown in Figure 2, may be used interchangeably with reaction product portion 38 and gas portion 39 described elsewhere.

Features List referred to herein and in the drawings:

• cleaning system 1

• first pressurisable container 2

• descaler 3

• pressure gauge 4

• tank outlets 5a and 5b

• a priming inlet 6

• hoses (first conduits) 7a and 7b

• inlet 8a of the plate heat exchanger 9

• inlet 8b of the plate heat exchanger 9

• plate heat exchanger 9 • valves 10a and 10b

• second pressurisable container (tank) 11

• hoses (second conduits) 12a and 12b outlets

• outlets 13a (MW) and 13b (CH) of the plate heat exchanger 9

• valves 14a and 14b

• internal pipe 15

• internal pipe 16 inside tank 1 1

• perforations 17

• base (bottom) 18

• third hose (conduit) 19

• base 20 of the first tank 2

• non-return valve 21

• internal pipe 22

• internal pipe 23

• top 24 of second tank 1 1

• top 25 of the first tank 2

• plate 26

• the area 27 below plate 26

• area 28 above the plate 26

• third tank 29

• hoses 30 and 31

• pressure release valves 32 and 33

• descaler portion 34

• gas portion 35

• primer 37

• reaction product portion 38

• gas portion (39)

• Arrows A, B and C.