Heating device, fluid circulating heating apparatus and method for enhancing and controlling heat production of a heating device

The present invention relates to a heating device comprising a housing and a rotatably mounted rotor having a rotation axis, the housing (12) having a fluid inlet and a fluid outlet, a clearance space is provided between the rotor and the housing for allowing fluid communication, the surface of the rotor is provided with recesses for generating cavitation.

The invention further relates to a fluid circulation type heating apparatus having a primary circuit and a secondary circuit, and having a heat exchanger between the primary circuit and the secondary circuit.

The invention further relates to a method for enhancing and controlling the heat production of a heating device of the above type.

Numerous devices and apparatuses are known in which rotating components are used for increasing the pressure and/or temperature of fluids. A particular type of such fluid heating devices makes use of the phenomenon of cavitation. When the flow rate of a fluid is locally accelerated the static pressure may drop to such an extent that cavitation bubbles, i.e. steam voids comprising vapour gases of the fluid are formed. If subsequently the fluid enters a region where its flow rate decreases the cavitation bubbles collapse instantaneously, which is accompanied by energy release in the form of heat. This phenomenon is called cavitation. Cavitation typically occurs at the edges of rotating components where the flow rate is rapidly accelerated and the generated

cavitation bubbles subsequently collapse. On the other hand cavitation is strongly destructive as the instantaneous collapse of the cavitation bubbles produce strong shock waves which may cause substantial damage in the surrounding material. Another phenomenon is also termed cavitation wherein the flow rate of a fluid decreases, as a consequence of which small gas bubbles within the fluid adhere and form larger bubbles, which in turn abruptly collapse, or the volume of such small gas bubbles increases and then rapidly decreases again. In both cases heat is being produced. In the context of the present invention cavitation is understood to cover all kinds of turbulent motion wherein heat is generated.

US 5,385,298 patent discloses a heating device for heating fluids wherein a rotor is mounted on a rotatable shaft within a housing. A clearance space is provided between the rotor and the housing for allowing fluid communication and the surface of the rotor is provided with recesses for generating cavitation bubbles. The housing has a fluid inlet and a fluid outlet allowing a fluid to flow through the clearance space between the housing and the rotor. The rotor is rotated via an external motor drive, whereby cavitation occurs at the edges of the recesses formed on the surface of the rotor causing the temperature of the fluid to increase substantially. The heating device may be used in a fluid circulation type heater having a primary and a secondary circuit, wherein the heating device is used for heating the fluid circulated in the primary circuit.

The heating device according to US 5,385,298 has the disadvantage that cavitation occurs in a non-controlled manner, hence the destructive side effect of cavitation can only be reduced by limiting the efficiency of the heating device. At maximum power output both the inner wall of the housing and the surface of the rotor is subject to strong abrasion.

It is an object of the present invention to provide a heating device and heating apparatus, which overcomes the disadvantages associated with the prior art heating devices.

In particular, it is an object of the present invention to provide a heating device which makes use of the phenomenon of cavitation, however wherein cavitation occurs in a controlled manner.

It is a further object of the invention to enhance heat production in heating devices employing cavitation.

This object is achieved by a heating device comprising a housing and a rotatably mounted rotor therein having a rotation axis, the housing having a fluid inlet and a fluid outlet, a clearance space being provided between the rotor and the housing for allowing fluid communication, the surface of the rotor being provided with recesses for generating cavitation, wherein the heating device further comprises a gas inlet duct for supplying gas to a fluid which is flown across the clearance space between the housing and the rotor

Various possibilities are at hand for supplying gas to the fluid. In a preferred embodiment the gas inlet duct terminates in a space between the housing and the rotor, thus gas is supplied to the fluid by feeding gas into said space.

In another preferred embodiment the gas inlet duct is arranged along the rotation axis of the rotor and comprises a plurality of branch pieces each of them terminating in at least one recess, thus gas is supplied to the fluid by feeding gas into the recesses.

In another preferred embodiment the gas inlet duct is connected to a fluid inlet duct, which is connected to the fluid inlet, thus gas is supplied to the fluid even before it enters the housing.

Heat production occurs when the bubbles collapse, however the amount of bubbles formed depends on the amount of gas present in the fluid flown across the heating device. The gas may be present in solute form or in the form of small bubbles. By supplying extra gas to the fluid, heat production can be enhanced and controlled.

In a second aspect the invention provides fluid circulation type heating apparatus having a primary circuit and a secondary circuit, and having a heat exchanger between the primary circuit and the secondary circuit, wherein the primary circuit comprises a heating device according to any of claims 1 to 10.

In a third aspect the invention provides a method for enhancing and controlling the heat production of a heating device, which comprises a housing and a rotatably mounted rotor therein having a rotation axis, the housing having a fluid inlet and a fluid outlet, a clearance space being provided between the rotor and the housing for allowing fluid communication, the surface of the rotor being provided with recesses for generating cavitation. The method is characterised by supplying gas in a controlled way to a fluid flown within the clearance space between the rotor and the housing.

Further advantageous embodiments of the invention are defined in the attached dependent claims.

Further details of the invention will be apparent from the accompanying figures and exemplary embodiments.

Fig. 1 is a schematic partial cross-sectional view of an embodiment of a heating device according to the invention. Fig. 1a is a schematic partial cross-sectional view perpendicular to the rotation axis of a rotor according to the invention and illustrating an alternative gas inlet.

Fig. 1b is a schematic partial cross-sectional view parallel to the rotation axis of the rotor shown in Fig. 1a. Fig. 1c is a schematic perspective view of another possible construction of the rotor shown in Fig. 1a.

Fig. 2 is a schematic top plan view of the lateral face of the rotor being spread out for the purpose of illustrating the pattern of the blind holes.

Fig. 3 is a schematic top plan view of the lateral face of the rotor being spread out for the purpose of illustrating another possible pattern of the blind holes.

Fig. 4 is a cross sectional view illustrating a blind hole.

Fig. 5 is a schematic partial cross-sectional view of another preferred embodiment of the heating device according to the invention. Fig. 6 is a schematic block diagram of an embodiment of the heating apparatus according to the invention.

Fig. 1 shows a schematic partial cross-sectional view of a preferred embodiment of a heating device 10 according to the invention. The heating device 10 comprises a cylindrical housing 12 and cylindrical rotor 14 rotatably mounted therein and having a rotation axis t. Securing of the rotor 14 may be realised by any known fastening which allows the rotor 14 to rotate around the rotation axis t, for example the schematically illustrated driving shaft 15 fitted with bearings can be applied. A narrow clearance spacing 16 is provided between the housing 12 and the rotor 14 having a depth (thickness) of approx. 0.5 to 2 mm. The exact depth of the clearance spacing 16 can be determined taking into account the fluid quantity to be heated. The inner wall surface of the housing 12 facing the clearance space 16 is preferably smooth in order to decrease the damaging effect of the cavitation, and the depth of the clearance space 16 is preferably even. For example for a clearance space depth falling within the range of 0.5 to 2 mm the deviation from the nominal value of the depth is preferably less than 0.1 mm. The housing 12 may be made of heat resisting polymer or any kind of metal having a smooth inner surface as described above. Advantageously the chosen material has good temperature holding capacity and advantageously the housing 12 is provided with heat insulation from the outside. The rotor 14 is preferably made from moulded polymer, for example polymers sold under the brand name "danamid", "danamit", "teramid" and "coramid", or any other material having similar properties. Such materials have satisfying elastic properties for withstanding caviation.

The housing 12 has a fluid inlet 18 and a fluid outlet 20, which are preferably situated in the vicinity of one end 22 of the rotor 14 and in the vicinity of the other end 24 of the rotor 22 respectively.

Recesses are provided on the outer surface (lateral face) of the rotor 14 in the form of blind holes 26. In a particularly preferred embodiment the blind holes 26 situated along spiral paths on the outer surface have an increasing and decreasing size (cross-section in the present example). In the illustrated embodiment the depth of the blind holes 26 is constant, thus the increasing and decreasing cross-section implies increasing and decreasing blind hole size. The

increasing and decreasing size along such spiral paths is achieved in the present embodiment as follows. The blind holes 26 are arranged in rows around the circumference of the planes perpendicular to the rotation axis t, and the neighbouring blind holes 26 of such rows form columns running parallel to the rotation axis t. The size of the blind holes 26 of the same row is constant, while the blind holes 26 in a column are of increasing and decreasing size. In Fig.3 the increasing and decreasing blind holes 26 are illustrated with reference numbers 26a, 26b, 26c, 26d, and 26e. It will be apparent to the skilled person that depending on the size of the rotor 14 the blind hole 26e may be followed by further blind holes 26 of increasing and decreasing size forming further rows.

In a preferred embodiment of the heating device 10 according to the invention a gas inlet duct 28 reaches into the interior of the rotor 14 through the housing 12. The gas inlet duct 28 may be held within the drive shaft 15 of the rotor 14, such that it is rotated together with the rotor 14. Branch pieces 30 are connected to the gas inlet duct 28 within the rotor 14, for conducting gas arriving via the gas inlet duct 28 to the bottom of the blind holes 26. In Fig. 1 a separate branch piece 30 leads to all the illustrated blind holes 26, however this is not a requirement for achieving the desired effect, moreover it may increase the manufacturing costs of the heating device 10. It is sufficient to provide branch pieces 30 to a portion of all the blind holes 26, for example to 40-60 % of the blind holes 26. In the embodiment illustrated in Fig. 1a the rotor 14 has a central gas inlet duct 28 and the neighbouring branch pieces 30 connected to the central gas inlet duct 28 are at an angle of 120 degrees with respect to each other. Three such branch pieces 30 which are at an angle of 120 degrees to each other (i.e. with an even angle distribution) may be arranged in a single plane perpendicular to the gas inlet duct 28 (and the rotation axis t) and the planes containing the triple branch pieces 30 may be spaced at a distance of one or more blind holes 26 from each other (Fig. 1b). Alternatively the branch pieces 30 may be arranged in different planes as illustrated in Fig. 1c. Numerous other variations are conceivable, for example branch pieces 30 may be provided only for blind holes 26 of a certain size. It is clear that the branch pieces 30 need not run radially (i.e. perpendicular to the rotation axis t) also, the

branch pieces 30 running from the gas inlet duct 28 may comprise further branches and may feed a plurality of blind holes 26 with the gas supplied through the gas inlet duct 28. The gas is preferably air, thus gas supply can be achieved in a cost efficient manner, moreover, no environmental pollution occurs when the fluid exiting via the fluid outlet is degasified. When using air it is sufficient to connect a gas pump 31 to the external end of the gas inlet duct 28 for ensuring the required gas pressure (air pressure).

The benefit of supplying gas via the gas inlet duct 28 and the branch pieces 30 connected thereto lies in the fact that the gas is supplied directly to the scene of cavitation, i.e. to the blind holes 26 as will be discussed in more detail later on. However, the cavitational bubble formation and thereby the cavitational heat production may also be effectively influenced (enhanced and controlled) by supplying gas (e.g. air) to the fluid entering the housing 12 at any point. For example the gas may be supplied through a gas inlet duct 28 terminating in a space 32 between the housing 12 and the rotor 14 (Fig. 1). It is also possible to supply gas to the fluid before it even reaches the fluid inlet 18, i.e. gas is fed into a fluid duct terminating in the fluid inlet (18). In the later case the gas inlet duct 28 can be regarded as coinciding with the fluid duct connected to the fluid inlet 18, and the gas may enter the fluid duct via a gas pump 31. The gas inlet ducts 28 illustrated by way of example may be used independently or in any desired combination. The rate of the gas supply and thereby the intensity of cavitation may be controlled by controlling the gas pumps 31. The inner diameter of the gas inlet duct 28 and/or the branch pieces 30 is preferably small, preferably about 20% of the cross section of an average blind hole 26, but preferably not less than 2 mm, for example in case of the above given dimensions of the heating device 10 the inner diameter may be of approx. 3 mm, allowing the gas to disperse well in the fluid. Dispersed gas has the benefit that very small gas bubbles do not lead to strong vapour formation, which would otherwise decrease the efficiency of the heating device 10. Fig. 2 illustrates a possible matrix-like pattern of the blind holes 26 on the outer surface (lateral face) 39 of the rotor 14 according to the invention. The

rows 40 of the matrix are made up of blind holes 26 of constant size, while the blind holes 26 of each column 42 are of increasing and decreasing size.

In the exemplary embodiment depicted in Fig. 2, the rotor 14 has the following dimensions. The diameter D of the rotor 14 is 180 mm, hence the longer side of the depicted lateral face 39 - which corresponds to the circumference K of the rotor 14 - has a length of K=D ^* ττ, the rounded-off value of which is 560 mm. The length L of the rotor 14 along the rotation axis t is 200 mm. The diameters d1 - d5 of the blind holes 26 in the subsequent rows 40 are 7 mm, 9 mm, 11 mm, 13 mm and 15 mm respectively while the blind holes 26 of the following rows 40 are of decreasing diameter d4, d3, d2 and d1 respectively. In the present matrix-like pattern of the blind holes 26 there is one blind hole 26 per each approx. 2 mm x 2 mm surface region of the lateral face 39. If the diameter D of the rotor 14 is greater, the lateral face has a correspondingly greater area, in which case preferably one blind hole 26 may be present in a correspondingly greater surface region. The dimensions of the heating device 10 can be determined with regard to the required heat output. By proportionally increasing the dimensional parameters of the housing 12 and of the rotor 14 the heat output of the heating device 10 may be increased.

The above described matrix-like pattern forms a plurality of spiral paths 44 on the surface of the rotor 14, where along each spiral path 44 the blind holes 26 are of increasing d1 - d5 diameter followed by blind holes 26 of decreasing d4 - d1 diameter. By increasing the length L of the rotor 14 or by decreasing the circumference K of the rotor 14, the spiral path 44 may include a whole loop around the lateral surface 39 of the rotor 14, or even several loops. The revolution number of the motor 14 may preferably vary between

2800 and 3500/min in order to maintain the cavitational effect below the threshold of destruction.

Due to the rotation of the rotor 14 the fluid flowing within the clearance space 16 between the rotor 14 and the housing 12 is flowing along a spiral trajectory as well (fluid spiral trajectory 46), which may in certain cases coincide with the spiral path 44. It should be noted that the pitch of the fluid spiral trajectory 46 depends on the flow rate of the fluid as well as on the

circumferential speed of the rotor 14, thus the arising fluid spiral trajectory 46 does not necessarily correspond to the spiral path 44 comprising blind holes 26 of strictly increasing and decreasing size. In order to achieve the goals of the invention the strictly increasing and decreasing blind hole 26 size along the actual fluid spiral trajectory 46 is not an absolute requirement, all the more so since the fluid does not strictly follow the theoretical fluid spiral trajectory 46 calculated from the fluid flow rate within the clearance space 16 and from the circumferential speed of the rotor 14. It will be apparent to the skilled person however, that if the blind holes 26 along the spiral path 44 (and along every other path parallel therewith) are of periodically increasing and decreasing size, then the fluid flows along blind holes 26 of gradually increasing and decreasing size as well, the degree of gradual size increase/decrease (i.e. the rate at which the size of the blind holes 26 increase or decrease along the fluid spiral trajectory) depends on the flow rate of the fluid and on the circumferential speed of the rotor 14.

The benefit of providing blind holes 26 of increasing and decreasing size along a spiral path 44 is as follows. Due to the rotation of the rotor 14 the fluid entering via the fluid inlet 18 and exiting via the fluid outlet 20 flows along a quasi-spiral path within the clearance space 16 between the rotor 14 and the housing 12 with respect to the rotor 14, hence it encounters blind holes 26 of periodically increasing/decreasing cross-section (size). The blind holes 26 of greater cross-section induce stronger cavitational effect, while the blind holes 26 of smaller cross-section induce weaker cavitational effect. By providing blind holes 26 of increasing size the strength of the cavitational effect is gradually increased, while providing blind holes 26 of decreasing size leads to the damping of the cavitational effect. In this way the cavitational effect can be kept below the threshold of destruction while ensuring continuous energy production at the same time. In order to achieve the above benefit it is not necessary that the first row 40 of the blind holes 26 of periodically increasing and decreasing size (cross-section) start with the blind holes 26 of the smallest size at the end 22 of the rotor 14 nearest to the fluid inlet 18. Gradual excitation and damping of the cavitational effect can be achieved just as well if the fluid entering via the

fluid inlet 18 meets a blind hole 26 of any size first, which blind hole 26 may equally belong to a series of increasing size blind holes 26 as well as to a series of decreasing size blind holes 26. From this it follows that it is not required of the rotor 14 according to the invention that the rows 40 comprising the blind holes 26 of equal size be perpendicular to the rotation axis t, and the columns comprising the blind holes 26 of periodically increasing and decreasing size be parallel to the rotation axis t. Fig. 3 illustrates the lateral face 39 of the rotor 14 of an alternative embodiment, wherein the rows 40 comprising blind holes 26 of the same size are parallel to the length L of the rotor 14, and the columns 42 comprising the series of increasing and decreasing size blind holes 26 are parallel to the outspread circumference K of the rotor 14. As can be observed, the blind holes 26 form a spiral path 44 in this arrangement as well. In this case the fluid entering the housing 12 via the fluid inlet 18 may first encounter a blind hole 26 of smaller or greater size depending on the region of the circumference K of the rotating rotor 14 which is currently in the vicinity of the fluid inlet 18. For this reason blind holes 26 of (periodically) increasing and decreasing size is understood to comprise series of blind holes of increasing size and series of blind holes 26 of decreasing size regardless of the order of the increasing series and the decreasing series as well as of the number of blind holes 26 comprised in each series. Such series of blind holes 26 of increasing and decreasing size are more generally referred to as having varying size as opposed to the prior art uniform recesses.

Furthermore, it is clear that the so-called rows 40 and columns 42 of the matrix may just as well be diagonal to the rotation axis t. The optimal pitch of the spiral path 44 comprising the blind holes 26 of periodically increasing and decreasing cross-section should be determined and designed with regard to the operating parameters (such as the rotor revolution number, the rotor diameter, flow rate at which the fluid is fed in and taken out, quantity of the fluid within the housing 12). For example the spiral path 44 comprising the blind holes 26 of increasing/decreasing size may be designed to correspond to the fluid spiral trajectory 46 in case of the planned operation parameters. A further possibility of controlling the strength of cavitation is to

decrease or increase the revolution of the rotor 14, whereby the fluid spiral trajectory 46 resulting from the modified revolution will differ from the nominal fluid spiral trajectory 46 corresponding to the spiral path 44 of increasing/decreasing size blind holes 26 as depicted in Fig. 2. The cross-sectional view of the blind holes 26 can be seen in Fig. 4, wherein only one single blind hole 26 is shown for the sake of simplicity. The blind hole 26 of any desired diameter d can be formed with an appropriate boring bit. The depth h of the blind holes 26 can (in combination with the above given parameters) be about 15 mm for all the blind holes 26, or it may vary proportionally with the diameter d of each blind hole 26. The branch pieces 30 of the gas inlet duct 28 arranged parallel to the rotation axis t preferably terminate in the bottom 34 of the blind holes 26, thereby increasing the impact velocity of the produced gas bubbles, thus increasing the efficiency of the cavitation. Although one of the main benefits of the invention resides in keeping the cavitational effect below the threshold of destruction, however wearing of the rotor 14 can be further decreased by providing a bevel edge 38 at the outer opening of the blind holes 26. The bevel edge 38 can have for example a bevel angle of 45 degrees created by any conventional means. The importance and requirement of the bevel edge 38 depends strongly on the used material. For example if the rotor 14 is made of aluminium then the application of the bevel edges 38 is recommended. However, when using the above noted polyamides, for example when using danamid as the material of the rotor 14, danamid has high elasticity and stability allowing omission of the bevel edges 38, which results in more simple and less expensive manufacturing.

Fig. 5 is a schematic partial cross-sectional view of another preferred embodiment of the heating device 10' according to the invention. The heating device 10' differs mainly from the heating device 10 illustrated in Fig. 1 in that the blind holes 26 are arranged at an angle α, and in that the rotor 14 comprises two rotor sections 50, 50', having therebetween means for accelerating the flow rate of the fluid. The later is typically a means producing suction effect, for example in the present embodiment a three-blade turbine 52 is used the blades

of which are positioned at 120 degrees with a torsion of 40 degrees, and the turbine 52 is also mounted rotatably around the rotation axis t. The turbine 52 allows for accelerating the flow rate of the fluid, thereby allowing for modifying the resulting fluid spiral trajectory 46 and for increasing the cavitation. The angle α is such that the blind holes 26 reach into the wall of the rotor 14 in a direction opposed to the pitch of the spiral path 44 as can be seen in Fig. 5. In a preferred embodiment the axis of the blind holes 26 may be arranged such that the axes are inclined along the spiral path 44 such that the axes form skew lines (not intersecting lines) with the rotation axis t of the rotor 14. By orienting the blind holes 26 at the selected angle α the speed at which the fluid is cast out of the blind holes 26 (due to the centrifugal effect) is increased as well as the pressure within the clearance space 16 between the housing 12 and the rotor 14 making the bubbles collapse which are formed. The operation of the heating device 10 is as follows. Fluid (e.g. water) is fed into the housing 12 through the fluid inlet 18, from here the fluid flows to the other end 24 of the housing 12 through the clearance space 16 between the housing 12 and the rotor 14, and exits the housing 18 at the fluid outlet 20. Water is cast out from the inside of the blind holes 26 as a result of the rotation of the rotor 14 (centrifugal force) and is forced into the narrow (e.g. 0.5-2 mm) clearance space 16 between the housing 12 and the rotor 14. At the same time the rapid change in the flow parameters at the blind holes 26 induce bubbles, which upon entering the higher pressure clearance space 16 break down or collapse. This is the cavitational phenomenon which leads to heat production. Thereby the temperature of the fluid flowing through the clearance space 16 is substantially increased hence heated hot fluid exits via the fluid outlet 20.

Bubble formation - and thereby efficiency - is increased and controlled in accordance with the invention by feeding gas to the fluid. This can be carried out inside the housing 12, within the space 32 between the housing 12 and the rotor 14; or gas can be fed to the blind holes 26 via the gas inlet duct 28 arranged along the rotation axis t of the rotor 14 and comprising branch pieces 30; or gas can be fed to the fluid before it even enters the housing 12 through

the fluid inlet 18. Due to the controlled gas supply lower revolution of the rotor 14 suffices for the bubble formation and the resulting cavitation is less damaging for the material of the rotor 14 as compared to the case where heating is achieved via cavitation induced by spontaneous bubble formation. The supplied gas bubbles represent extra bubbles with regard to the amount of bubbles that would be formed by the blind holes 26 spontaneously. Since heat production is a result of bubbles collapsing, by supplying gas artificially to the fluid the quantity of bubbles can be controlled, which amounts to controlling the heat production. Gas supply is preferably controlled by controlling the gas pump 31. For example the gas pump 31 is operated for 3-4 sec supplying gas to the fluid, then the gas pump is shut down for the next 3-4 sec. When using the gas pump 31 in combination with the gas inlet duct 28 having branch pieces 30 leading to the blind holes 26 relatively small pressure (approx. 0.3-0.5 bar) is sufficient because due to the centrifugal forces associated with the rotation of the rotor 14 the fluid is forced out of the blind holes 26.

The optional increasing/decreasing size of the blind holes 26 play a role in controlling the cavitational effect as explained above. The increasing size (cross-section) of the blind holes 26 "excite" the cavitation, and as the fluid flows to blind holes 26 of gradually decreasing size (cross-section) the intensity of the cavitation becomes more moderate. It is thereby ensured that the cavitational effect does not reach the threshold of destruction in order to protect the rotor 14 and the housing 12 against damaging. Partially switching off cavitation and restarting (exciting) it again allows the material of the rotor 14 and the housing 16 to relax partially as opposed to being constantly under the stress of the damaging effect of the cavitation. It has been found that the above described polyamides are particularly suitable for withstanding the stress caused by the cavitation. The periodical excitation and damping (restraining) of the cavitation can also be achieved by series of blind holes 26 of periodically increasing and decreasing size wherein the number of blind holes 26 in the increasing series and the number of blind holes 26 in the decreasing series are different. For example in case of the rotor 14 consisting of two rotor sections 50 and 50', the number of blind holes in the excitation series (i.e. the blind holes 26

having increasing cross-section) on the second rotor section 50' may be fewer since there the cavitation is to be induced in an already heated fluid. Similarly, it is also possible in the case of a single piece rotor 14 to design excitation and damping series of different blind hole 26 number, for example rows 40 comprising blind holes 26 of gradually increasing cross-section may be provided upto the 2/3rds of the length L of the lateral face 39 of the rotor 14 and rows 40 comprising blind holes 26 of decreasing size may be provided on the last 1/3rd of the length L.

The periodical excitation and damping of the cavitational effect increases the life of the rotating components (in particular the rotor 14) to such an extent that the heating device 10 is thereby rendered suitable for daily use. On the other hand substantial temperature increase can be achieved in the fluid without waste gas (steam) formation. Moreover due to the strong collision between the walls of the bubbles the so called Casimir effect arises, which further increases the efficiency of the heating device 10 by a small extent.

Fig. 6 is a schematic block diagram of an embodiment of the fluid circulation type heating apparatus 60 according to the invention. The heating apparatus 60 comprises a primary circuit 62 and a secondary circuit 64, and a heat exchanger 66 is provided there between for allowing heat exchange between the two circuits 62, 64. A heating device 10 according to the invention is connected to the primary circuit 62 for heating the fluid circulated in the primary circuit. Any type of known circulating pump 68 can be used for circulating the fluid, however, providing the blind holes 26 at an inclined angle α may be sufficient for ensuring the required fluid flow, since as the fluid is forced out of the blind holes 26 (due to the rotation) the angle α has the effect of accelerating the fluid, practically "propelling" the fluid and contributing to the circulation. The above described turbine 52 further enhances this effect.

The circulated fluid may be for example water, distillate water or any other liquid which does not deteriorate the components of the heating device 10 and of the heating apparatus 60.

In case of the depicted embodiment the heat exchanger 66 is connected to the fluid inlet 18 of the heating device 10 via a first duct 70, and it

is connected to the fluid outlet 20 of the heating device 10 via a second duct 72. An optional circulating pump 68 can be connected to one or both of the two ducts 70, 72. In the present example supply of the gas for enhancing the bubble formation is carried out via the gas pump 31 , which is connected to the first duct 70. The gas pump 31 can be an air pump, thus the duct 70 also serves as the gas inlet duct 28. As explained above the gas may be fed in other ways as well, for example via the gas inlet duct 28 arranged along the rotation axis t and having branch pieces 30 (not illustrated). Gas supply (in the present case air supply) is preferably done in a controlled manner, for example with the help of control electronics 74 controlling the gas pump 31. One or more degassing units

75 may be arranged along the first and/or second fluid ducts 70, 72, which degassing unit 75 may be a known check valve.

The rotor 14 within the heating device 10 is driven by an exterior motor

76 in a known way, for example the rotation of a first V-belt sheave 80 driven by the rotor 14 is carried over to a second V-belt sheave 84 mounted on the drive shaft 15 of the rotor 14 via a V-belt 82. For the purpose of driving the rotor 14 having the above specified parameters the motor 76 can be a monophase electromotor of 1000-3000 W power, operated at 2000-3500 revolution/min the later being preferably controlled by a revolution control unit 86, for example a known thyristor control.

Since only a relatively small amount of fluid resides within the clearance space 16 between the housing 12 and the rotor 14 as well as in the space 32, therefore, in case of a heating device 10 with the above given dimensions, it is sufficient to circulate about 20-25 litres of fluid (water) in the primary circuit 62. Such an amount of the primary circuit fluid allows for heating as much as 300- 700 litres of fluid (e.g. water) per hour in the secondary circuit 64 or for holding it at temperature.

One or more devices to be heated are connected to the secondary circuit 64, for example the radiator 90 illustrated in Fig. 6. A circulating pump 92 is connected to the duct 88 of the secondary circuit 64 for circulating the fluid therein.

The operation of the heating apparatus 60 according to the invention is as follows. The fluid which is circulated in the primary circuit 62 is fed into the heating device 10 driven by the motor 76, where the fluid is heated as explained above. The bubble formation and thereby the efficiency of the heating device 10 may be influenced by controlling the gas pump 31 supplying air to the primary circuit via the control electronics 74. The gas (air) fed into the primary circuit 62 is preferably vented out at the degassing unit 75, thereby excess gas build-up in the circulated fluid is prevented, which would otherwise lead to steam formation within the heating device 10. Ideal is to provide heating fluid heated to a temperature of 70-90 degrees. The heated fluid exists the heating device 10 via the fluid outlet 20 and flows to the heat exchanger 66 where it transmits heat to the fluid circulated in the secondary circuit 64, which in turn heats the radiator 90 depicted in the present example.

Apart from controlling the gas supply, heat production can be limited by decelerating the rotation of the rotor 14 via the revolution control unit 86 controlling the revolution of the motor 76, or even by temporarily stopping the rotor 14 if the fluid circulated in the primary circuit 62 has reached its desired temperature.

The above-described embodiments are intended only as illustrating examples and are not to be considered as limiting the invention. Various modifications will be apparent to a person skilled in the art without departing from the scope of protection determined by the attached claims.