WO2015067989A1

WO2015067989A1 - Mixing unit and method for improved production of foam for cleaning purposes

Info

Publication number: WO2015067989A1
Application number: PCT/IB2013/059912
Authority: WO
Inventors: Rune OESTERGAARD SAYK; Flemming Asp
Original assignee: Nilfisk Advance AS
Current assignee: Nilfisk AS
Priority date: 2013-11-05
Filing date: 2013-11-05
Publication date: 2015-05-14
Anticipated expiration: 2016-05-05
Also published as: EP3065851A1; EP3065851B1; DK3065851T3

Abstract

The invention pertains to a mixing unit (2) for supplying foam 4 for cleaning, the mixing unit (2) having an injector (28) positioned within it for providing a gap (34) around the injector outlet (32), said gap (34) being fluidly connected to an air inlet (14) of the housing (6) of the mixing unit (2) for allowing air to pass between the injector outlet (32 )and the housing (6) and mix with the first cleaning agent and water at the fluid outlet (18) of the housing (6). The invention also pertains to a cleaning device comprising such a mixing unit (2) and a method of producing foam for cleaning.

Description

MIXING UNIT AND METHOD FOR IMPROVED PRODUCTION OF FOAM FOR CLEANING PURPOSES

TECHNICAL FIELD

The present invention pertains to a mixing unit for producing foam for cleaning pur- poses, a cleaning device with a mixing unit and a method of producing foam for cleaning purposes, particularly for use in the food manufacturing industries, for example for cleaning heavily soiled surfaces in the food processing area, for instance in meat and fish processing industries, vegetables processing or the pastry industry. The cleaning device according to the invention may also be used for cleaning cars, tractors and other agricultural machines, which often get heavily soiled during use

BACKGROUND OF THE INVENTION

In the food processing industry, in particular in heavily soiled areas, such as slaugh- terhouses or meat and fish processing industries, tenacious soiling through grease, protein and starch residues requires the application of a series of different treatments procedures, including disinfection, in order to achieve a level of cleaning that complies with the government standards. Traditionally, the cleaning procedure would involve an initial flushing with water, wherein all larger debris is removed, and the surfaces are made wet. Then it is customary to apply a "carpet" of foam comprising a cleaning agent over these surfaces, particularly in order to dean these surfaces from grease. Finally, the areas are disinfected with yet another chemical agent, such as chlorine. This kind of industry cleaning thus requires a change between different procedures and equipment.

In FR 2 617 736 is disclosed a device which makes it possible by starting with, on the one hand, water under pressure and, on the other hand, water, to which chemical or bactericidal products have been added, to produce foam intended for cleaning surfaces or objects. The device consists of two consecutive chambers. In the first cham- ber compressed air enters by means of a tube pierced with holes. Water under pressure, to which chemical products have been added, enters by means of the tube. Then the mixture of air under pressure and of water under pressure produces foam. This foam is conveyed into a decompression chamber, which completes the mixing of the products The volume of the chambers is calculated to obtain an optimum quality of the foam. In US 5,855,217 is disclosed a device for cleaning surfaces in the food industry. The device comprises a housing body having a first inlet for feeding pressurized water; a propulsion jet positioned behind the inlet and in a direction of flow; a collection jet positioned behind the propulsion jet, which is flow-connected with a second inlet for feeding a chlorine-free alkaline foam cleaning agent, and a third inlet for feeding a hydrogen peroxide solution; and a turbulence chamber, into which an elongated jet body of the collection jet extends, the turbulence chamber being fitted with a chamber inlet for feeding compressed air into the chamber, such that a hydrogen peroxide foam is formed from a solution formed upon dosing an effective amount of the hydrogen per- oxide solution into the chlorine-free alkaline foam cleaner at a maximum of 60 seconds prior to contact of the hydrogen peroxide foam with a surface to be cleaned, the turbulence chamber further having a chamber outlet, through which the hydrogen peroxide foam leaves the chamber to contact the surface, the chamber inlet and the chamber outlet being fitted in a direction of flow ahead of an outlet orifice of the jet body.

In US 2008/271279 is disclosed a mobile foam producing unit, which includes a first hose and hose assembly, and a second hose assembly. Water is provided to the mobile foam producing unit by a supply hose. A foam gun provides for the dispensing of foam. In a first mode, a sanitizer gun dispenses a sanitizing solution together with wa- ter, and in a second mode, i.e. a rinse mode, only water is dispensed from the sanitizer gun.

In DE 197 09 098 a cleaning device is disclosed, which device can supply cleaning fluid alone or mixed with water, air under pressure and chemicals. It has a mixing and dosing unit, which has input lines for the components and an output line for the components to supply them singly or mixed. There is a setting device between the input and output lines. The setting device can mix all the components supplied and supply them indefinitely or in a dosed pattern, with at least one of the components blocked off. The setting device may be in the form of a turning roller with a number of channels in it.

Despite the above-mentioned efforts to provide a cleaning unit which can be used in several of the necessary cleaning procedures used in the food producing industry, the units so far used in the industry are not sufficiently versatile. A change between the different cleaning operations requires the use of different equipment, such as different pressure guns, and this use of different equipment is time consuming and also prone to human errors.

In the unpublished international patent application PCT/IB2012/057148, by the appli- cant of the present application, a solution to the above-mentioned problems has been proposed. However, while the above-mentioned international patent application describes a solution wherein a change between the different cleaning operations (water, foaming and disinfection) can be performed quickly - without cnange of equipment - and in an error free manner, the applicant has observed that the foam produced by this apparatus has a tendency to run off smootn surfaces, thereby impairing the cleaning process.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a mixing unit, with an improved foaming capability and thereby facilitating an improved cleaning operation

It is a further object of the invention to provide a cleaning device with an improved foaming capability and thereby facilitating an improved cleaning operation.

It is yet a further object of the invention to provide an improved method for supplying foam for cleaning purposes.

It is a yet even further object of the invention to provide a mixing unit and method. wherein the quantity of foam produced may easily be changed

According to the present invention, the above-mentioned and other objects are fulfilled by a mixing unit for supplying foam for cleaning, the mixing unit comprising the following main parts: a housing having a water inlet for receiving pressurized water, an air inlet for receiving pressurized air, a fluid outlet for said foam, and a first input line for a first cleaning agent, said mixing unit further comprising an injector positioned inside said housing, said injector having an injector inlet and an injector outlet, which injector inlet is fluidly con- nected to the first input line for a cleaning agent, and which injector outlet is fluidly connected to the fluid outlet of the housing, the injector inlet having a narrower cross section than the cross section of the injector outlet, wherein the injector is positioned within the mixing unit in such a way that a gap around the injector outlet is provided, said gap being fluidly connected to the air intet for allowing pressurized air to enter and pass through the gap and mix with the first cleaning agent at the fluid outlet of the housing. Investigations performed by the applicant reveal that, by supplying the air for producing the foam into the mixing unit around the injector outlet opening, via a gap between the injector and housing, sad air will be evenly distributed around the injector opening before it becomes mixed with the mix of water and first cleaning agent, thereby leading to a more effective foam production (se e.g. test results below). Furthermore, since the air is not led into the mixing unit after the injector, as is customary praxis in most cleaning devices today, but around the injector, a much more stable under-pressure is created by the injector, whereby the first cleaning agent is much more effectively sucked into said injector. Also, this leads to a more effective mixing of the first cleaning agent, water and air and thereby a more effective creation of foam at the outlet of the housing.

According to one embodiment of the mixing unit according to the invention, the injector has a stepwise increasing circular cross section in the direction from the injector inlet towards the injector outlet.

According to a preferred embodiment of the mixing unit according to the invention, the injector has a continuously (i.e. stepless) increasing circular cross section in the direction from the injector inlet towards the injector outlet. By providing an injector having a circular cross section, especially at or adjacent to the outlet, a more even distribution of pressurized air around the injector outlet is achieved. This causes a more effective mixing of the pressurized air with the mix of water and first cleaning agent and thereby a more effective foam creation. According to a further preferred embodiment of the mixing unit according to the invention, the inner dimensions of the housing have a cylindrical shape adjacent to the fluid outlet and wherein the injector outlet is positioned eo-axiaily relative to said housing at the fluid outlet, Hereoy is achieved that, during use. the pressurized air is distributed evenly around the injector outlet and flows in substantially the same direction as the mix of water and first cleaning agent. Preferably, these inner dimensions of the hous- ing are provided by a bore in the housing

According to a further preferred embodiment of the mixing unit according to the invention, the injector is exchangeable, for example by providing the mixing unit and injector with mutually cooperating attachment means for releasaole attachment of the injector within the mixing unit

According to a further preferred embodiment of the mixing unit according to the invention, the injector is exchangeable with another injector having a different injector inlet area and/or a different injector outlet area. This is particularly desirable in situations wherein it is desired to provide a different type of foam or a different quantity of foam. Typically, the injector inlet opening is circular with a diameter of between 1 mm and 10 mm. Preferably, the injector is chosen in dependence of a variety of parameters, such as the type of foam, which is desired to be used in a particular cleaning operation, the available water and/or air pressure, or the type of first cleaning agent, which is availa- ble. Preferably, the ratio between the area of the gap and the inlet area of the injector is varied in dependence of any of the above mentioned parameters.

According to a further embodiment of the mixing unit according to the invention, the ratio between the area of the gap and the area of the injector inlet is a constant for a given inner cross-sectional area of the housing at the fluid outlet for all injectors usable in connection with the mixing unit. Hereby is achieved that the quantity of foam to be produced can be changed without altering the pressure of the air or water. Since a change in the pressure of the water and/or air would alter the properties of the foam considerably, it would be highly undesirable to change the quantity of foam produced by an alteration of the pressure of the water and/or air. Therefore, this embodiment enables the operator using the mixing unit to alter the quantity of foam produced without changing the properties of the foam.

According to a further embodiment of the mixing unit according to the invention, the injector is exchangeable with another injector having a different injector inlet area and/or a different injector outlet area. According to a further preferred embodiment of the mixing unit according to the invention, the housing further comprises at least a second input line for a second cleaning agent, said second input line being fluidly connected to the injector inlet. Hereby is achieved a mixing unit, wherein several added properties may be added to the foam For example the first cleaning agent may mainly be responsible for the foaming, while the second may be responsible for imparting to the foam certain cleaning properties. Hence by this embodiment a more flexible mixing unit is achieved, which may be adjusted to provide a type of foam, which is optimized for the particular cleaning opera- tion at hand.

According to an alternative embodiment of the mixing unit according to the invention, the housing further comprises at least a second input line for a second cleaning agent and an additional injector having an injector inlet which is fluidly connected to the se- cond input line, and an injector outlet fluidly connected to outlet of the housing.

According to a further preferred embodiment of the mixing unit according to the invention, the mixing unit comprises switching means for switching between the provision of water alone and water mixed with the first cleaning agent and air for the provision of foam. According to a further preferred embodiment of the mixing unit according to the invention, the mixing unit comprises switching means for switching between the provision of water alone and water mixed with the first cleaning agent and air for the provision of foam, and water mixed with the second cleaning agent. Hereby is provided a mixing unit usable in a single cleaning device which can supply water, or water mixed with a first cleaning agent, or foam.

Cleaning procedures used in for example the food producing industry typically involve an initial flushing/wetting of the contaminated area, surfaces, processing machines and other equipment, which need to be cleaned. This flushing is usually followed by applⁱcation of foam to said area, surfaces, processing machines and other equipment. This foam is then flushed away after a certain period of time. Finally, said area, surfaces, processing machines and other equipment is disinfected by the application of a disinfectant. In some cases this disinfectant is removed by the flushing with clean water. This means that there is typically a successive change between the following pro- cedures: - Flushing with water alone

- Application of foam

- flushing with water alone

- Application of a disinfectant

- Flushing with water alone.

In order to facilitate a change between all these procedures, it is usually necessary to employ several cleaning devices or mixing units. For example, one device for the provision of water, e.g. a water hose or high-pressure cleaner, one device for foaming, and one device for the provision of the disinfectant. However, by providing a single mixing unit with switching means as described above, it is possible to successively change between all of the above-mentioned cleaning processes For example in the embodiment where the second cleaning agent is a disinfectant and the first cleaning agent is a foaming agent when being mixed with water and pressurized air, e.g. from compressor, an operator merely needs to switch successively between three processes. Hence there is no need to change equipment, which is both time saving and reduces the risks of errors.

According to a further embodiment of the mixing unit according to the invention, the pressurized water is provided by a water pump to the water inlet of the housing of the mixing unit.

According to a further embodiment of the mixing unit according to the invention, the mixing unit is manufactured from a metal alloy, e g stainless steel. Hereby is obtained a robust mixing unit, which can withstand pressures up to 60 bar without malfunction or any noticeable leakage.

According to a further embodiment of the mixing unit according to the invention, the provided pressurized water has a pressure of between 4 bar and 50 bar, preferably between 10 bar and 25 bar, and the provided pressurized air has a pressure of between 2 bar and 10 bar, more preferable a pressure of between 4 bar and 6 bar, wherein the provided pressurized air always has a lower pressure than the pressure of the provided pressurized water.

The above mentioned and further objects are also achieved by a cleaning device comprising a mixing unit according to any of the embodiments described above. The compressor, mixing unit, and water pump, may according to some embodiments of the cleaning device according to the invention be placed inside a cleaning device housing

According to a further embodiment of the cleaning device according to the invention, said cleaning device housing is placed on a wheeled chassis, whereby a mobile cleaning device is provided. The above mentioned and further objects are also achieved by a method of producing foam for cleaning purposes, the method comprising the steps of:

- leading pressurized water into a housing of a mixing unit and through an injector positioned within said housing, within which injector an under-pressure is created during the passage of the water through the injector towards an outlet opening of said injector, where said under-pressure causes a cleaning agent to be sucked into the injector via a first input line in fluid communication with a reservoir for said cleaning agent, in which injector the cleaning agent is being mixed with the water under its passage towards the outlet opening of the injector ,

- leading pressurized air into the housing of the mixing unit, and through a gap between said housing and the outlet opening of the injector, in a direction substantially parallel to the flow of water mixed with cleaning agent out of the injector opening, thereby causing creation of foam when said pressurized air becomes mixed with the water and cleaning agent before exiting said housing through a outlet opening.

According to a preferred embodiment of the method, the air gap completely surrounds the outlet opening of the injector. The method may according to a further embodiment comprise the step of changing the quantity of foam produced per time unit by exchanging the injector with another injector having a different injector inlet area and/or a different injector outlet area.

The method may according to a yet further embodiment comprise the step of changing the quantity of foam produced per time unit by exchanging the injector with another injector having a different injector inlet area and/or a different injector outlet area, and keeping the water pressure and air pressure unaltered.

The method may according to a further embodiment comprise the step of choosing the injector, and thereby the injector inlet and outlet area in dependence of what kind of foam is to be produced and/or what kind of first cleaning agent is to be used for creating said foam.

The method may according to a further embodiment comprise the step of choosing the injector, and thereby the injector inlet and outlet area in dependence of the available water pressure and/or available air pressure.

According to a further embodiment of the method, the ratio between the area of the gap and the area of the injector inlet is a constant for one different injector that is used to change the quantity of foam produced

Test results

A qualitative test was performed by the applicant using two different mixing units. One mixing unit A, which is a mixing unit according to the invention, where air is blown in to the mixing unit around the injector opening and in a direction parallel to the flow of fluid out of the injector opening, and a mixing unit B, wherein the air is blown into the mixing unit though a hole downstream and beside the injector outlet and in a direction substantially parallel to the flow of fluid out of the injector. The two mixing units A and B were then used to generate foam, which was blown onto a vertical, smooth wall sur- face. It was observed that the foam generated by mixing unit B was more fluent and did not stay long time on the smooth wall, because it had a tendency to run down said wall, than the foam generated by the mixing unit A, which was thicker and stayed on the smooth wall surface for a considerably longer time. This clearly indicates that the foam generated by the mixing unit according to the invention (mixing unit A) is more effective and of a higher quality, and it will therefore facilitate a better cleaning of the surfaces onto which it is applied. This experiment was repeated for various different wall surfaces, including linoleum and steel, all providing the same qualitative results.

In a second qualitative experiment, each mixing unit was used to generate foam into a 200 'iter plastic barrel, and the time it took to fill each barrel was measured. The results of these barrel tests were that the barrel which received foam from mixing unit A was filled with foam in a considerably shorter time than the barrel which received foam from mixing unit B, Since both mixing units in all experiments were supplied the same quantity of cleaning agent, water and air under equal pressure conditions, it may be concluded from these experiments that the foaming facilitated by the mixing unit ac- cording to the invention is significantly more effective than those known in the art.

BREIF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. In the following, preferred embodiments of the invention are explained in more detail with reference to the drawings, wherein

Fig. 1 shows a cross section of an embodiment of a mixing unit according to the in- vention,

Fig. 2 shows a perspective view of the same embodiment of a mixing unit as in Fig. 1 , Fig. 3 shows another cross section of a mixing unit, and

Fig. 4 shows the same cross section as in Fig. 3. but wherein the mixing unit comprises a different injector.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. Like elements will, thus, not be described in detail with respect to the description of each figure.

Fig. 1 shows a cross section of an embodiment of a mixing unit 2 for supplying foam 4 for cleaning. The illustrated mixing unit 2 comprises a housing 6 having a water inlet 8 for receiving pressurized water, indicated by the arrow 10. The water is supplied to the housing 6 via a suitable pipe 12. The housing 6 also has an air inlet 14 for receiving pressurized air. indicated by the arrow 16, a fluid outlet 18 for said foam 4, and a first input line 20 for a first cleaning agent, indicated by the arrow 22. The air is supplied to the housing 6 via a suitable pipe 24, which is preferable connected with a compressor {not shown), and the first chemical agent is supplied to the housing 6 via a suitable pipe 26 which is in fluid communication with a reservoir {not shown). The water pipe 12 is fluidly connected to a water pump {not shown) for supplying pressurized water to the housing 6 of the mixing unit 2.

The mixing unit 2 further comprises an injector 28 positioned inside the housing 6. The injector 28 has an injector inlet 30 and an injector outlet 32, which injector inlet 30 is fluidly connected to the first input line 20 for a cleaning agent, and which injector outlet 32 is fluidly connected to the fluid outlet 18 of the housing 6. As illustrated, the injector inlet 30 has a narrower cross section than the cross section of the injector outlet 32.

The injector 28 is positioned within the mixing unit 2 for providing a gap 34 around the injector outlet 32. This gap 34 is fluidly connected to the air inlet 14 of the housing 6 for allowing air to pass between the injector outlet 32 and the housing 6 and mix with the first cleaning agent at the fluid outlet 18 of the housing 6.

Fig. 2 shows a partial perspective view of the embodiment of a mixing unit 2 illustrated in Fig. 1 , By supplying the air (indicated by arrows 16) for producing the foam 4 into the mixing unit 2 around the injector outlet opening 32 via a gap 34 between the injector 28 and housing 6, said air will be evenly distributed around the injector cutlet 32 before it becomes mixed with the mix of water and first cleaning agent (said mix being indicated by arrow 36), thereby leading to a more effective foam production (se e.g. test results).

The illustrated injector 28 has a continuously (i.e. stepless) increasing circular cross section in the direction from the injector inlet 30 towards the injector outlet 32. By providing an injector 28 having a circular cross section, especially at or adjacent to the outlet 32, a more even distribution of pressurized air around the injector outlet 32 is achieved

As seen more clearly in Fig. 2, the inner dimensions of the housing 6 have a cylindrical shape adjacent to the fluid outlet 18, and the injector outlet 32 is positioned co-axially relative to said opening in the housing 6 at the fluid outlet 18. Hereby is achieved that, during use, the pressurized air (indicated by the arrows 16) is distributed evenly around the injector cutlet 32 and flows in substantially the same direction as the mix of water and first cleaning agent {said mix being indicated by the arrow 36). Preferably, these inner dimensions of the housing 6 are provided by a bore in the housing 6.

Fig. 3 and Fig. 4 shows the same cross section of a mixing unit 2 wherein two different injectors 28 are placed. The illustrated injectors 28 are exchangeable, for example by providing the mixing unit 2 and injectors 28 with mutually cooperating attachment means, such as mutually cooperating threading 41 and 43 in the injector housing 47 and housing 6 of the mixing unit 2, for releasable attachment of the injectors 28 within the housing 6 of the mixing unit 2. In fig. 3 and Fig. 4. DF is the diameter of the cylin- drical bore in the housing 6 of the mixing unit 2, wherein the injector 28 is placed. DF is fixed. dV is the diameter of the injector inlet opening 30, and DV is the outer diameter of the injector 28 at the injector outlet opening 32. While DF Is fixed, dV and DV may have varying values for different injectors 28. The mixing unit 2 illustrated in Fig. 4 is equipped with an injector having a larger dV for providing more foam 4. However, since more water is needed, DV has been made correspondingly smaller in order to provide for a larger gap 34 between the injector outlet opening 32 and the housing 6 in order to provide foam having substantially the same properties as the foam generated by the missing unit 2 illustrated in Fig 3. The ratio between the area of the gap 34 and the area of the injector inlet 30 may be a constant for a given inner cross-sectional area (defined by DF in the illustrated embodiment) of the housing 6 at the fluid outlet 18 for all injectors 28 usable in connection with the mixing unit 2. Hereby is achieved that the quantity of foam 4 to be produced can be changed without altering the pressure of the air or water. Therefore, this embodiment enables an operator using the mixing unit 4 to alter the quantity of foam 4 produced without detrimentally altering the properties of the foam 4, because the ratio between water and air is maintained. As illustrated, the air gap 34 completely surrounds the outlet opening 32 of the injector 28. However, generally the ratio between the area of the gap 34 and the inlet opening 30 of the injector 28 is not a constant, but instead chosen in dependence of what kind of foam 4 is to be produced and/or the quantities needed, and/or available pressure of the water and/or air,

Referring back to in Fig. 1 and 2. the mixing unit 2 of the housing 6 may further comprise at least one second, optional input line 38 for a second cleaning agent (indicated by the arrow 40), which second input line 38 must be fluidly connected to the injector inlet 32. This second cleaning agent is supplied to the housing of the mixing unit 2 via a pipe 42 from a reservoir (net shown) As shown in Fig. 2, a bore 46 is provided in the housing 6. This bore 46 may be used to align the Input l.nes for the first, and further optional, cleaning agents with the corresponding input apertures in the injector housing 47, when it is screwed into the housing 6 of the mixing unit 2 using the threading 41 and 43. In order to avoid leakage vari- ations two O-rings 44 and 45 manufactures from a surtable type of rubber is provided as packing means between the injector housing 47 and housing 6 of the mixing unit 2. The illustrated mixing unit 2 is manufactured from a metal alloy, e.g. stainless steel. Hereby is obtained a robust mixing unit 2, which can withstand pressures up to 60 bar without malfunction or any noticeable leakage.

LIST OF REFERENCE NUMBERS

In the following is given a list of the reference numbers that are used in the detailed description of the invention.

2 mixing unit,

4 foam,

6 housing of mixing unit,

8 water inlet of the housing.

10 arrow indicating flow of water,

12 water pipe,

14 air inlet of the housing,

16 arrows indicating air flow,

18 outlet of the housing,

20 first inlet for a first cleaning agent,

22 arrow indicating flow of cleaning agent,

24 air pipe,

26 pipe for the first cleaning agent.

28 injector,

30 inlet opening of injector,

32 outlet opening of injector,

34 gap between injector and housing.

36 arrow indicating flow of mix of water and first cleaning agent

38 inlet line for second cleaning agent,

40 arrow indicating flow of second cleaning agent,

41 threading in injector housing,

42 pipe for second cleaning agent.

43 threading in the housing of the mixing unit,

44 O-ring,

45 O-ring,

46 bore, and

47 injector housing.

Claims

1 , A mixing unit for supplying foam for cleaning, the mixing unit comprising: a housing having a water inlet for receiving pressurized water, an air inlet for receiving pressurized air, a fluid outlet for said foam, and a first input line for a first cleaning agent, said mixing unit further comprising an injector positioned inside said housing, said in- jector having an injector inlet and an injector outlet, which injector inlet is fluldly connected to the first input line for a cleaning agent, and which injector outlet is fluidly connected to the fluid outlet of the housing, the injector inlet having a narrower cross section than the cross section of the injector outlet, wherein the injector is positioned within the mixing unit in such a way that a gap around the injector outlet is provided, said gap being fluidly connected to the air inlet for allowing pressurized air to enter and pass through the gap and mix with the first cleaning agent and water at the fluid outlet of the housing.

2, The mixing unit according to claim 1 , wherein the injector has a stepwise increasing circular cross section in the direction from the injector inlet towards the injector outlet,

3- The mixing unit according to claim 1, wherein the injector has a continuously increasing circular cross section in the direction from the injector inlet towards the injec- tor outlet.

4. The mixing unit according to claim 1 , 2 or 3, wherein the inner dimensions of the housing has a cytindncal shape adjacent to the fluid outlet, and wherein the injector outlet is positioned co-axially relative to said housing at the fluid outlet.

5 The mixing unit according to claim 4, wherein these inner dimensions of the housing are provided by a bore in the housing.

6. The mixing unit according to any of the claims 1-5, wherein the injector is ex- changeable

7. The mixing unit according to claim 6, wherein the injector is exchangeable with another injector having a different injector inlet area and a different injector outlet area,

8. The mixing unit according to claim 6 or 7, wherein, for a given inner cross-sectional area of the housing at the fluid outlet, the ratio between the area of the gap and the area of the injector iniet is a constant.

9 The mixing unit according to claim 6, wherein the injector is exchangeable with another injector having a different injector inlet area or a drfferent injector outlet area.

10. The mixing unit according to claim 6 or 9, wherein the ratio between the area of the gap and the area of the injector inlet is a constant.

11. The mixing unit according to any of the claims 1-10, wherein the housing further comprises at least a second input line for a second cleaning agent, said second input line being fiuidly connected to the injector inlet

12. The mixing unit according to claim 1-10, wherein the housing further comprises at least a second input line for a second cleaning agent and an additional injector having an injector inlet that is fiuidly connected to the second input line, and an injector outlet fiuidly connected to outlet of the housing.

13. The mixing unit according to any of the claims 1-12, wherein the pressurized water is provided by a water pump to the water inlet of the housing of the mixing unit.

14. The mixing unit according to any of the claims 1-13, wherein the mixing unit is manufactured from a metal alloy, e.g. stainless steel.

15. The mixing unit according to any of the preceding claims, wherein the provided pressurized water has a pressure of between 4 bar and 50 bar. preferably between 10 bar and 25 bar.

16. The mixing unit according to any of the preceding claims, wherein the provided pressurized air has a pressure of between 2 bar and 10 bar, more preferable a pressure of between 4 bar and 6 bar.

17, The mixing unit according to claim 15 or 16, wherein the provided pressurized air has a lower pressure than the pressure of the provided pressurized water.

18 A cleaning device comprising a mixing unit according to any of the claims 1-17.

19. The cleaning device according to claim 18 when dependent on any of the claims 18-21 , wherein the compressor, mixing unit, and water pump are placed inside a cleaning device housing.

20. The cleaning device according to claim 19. wherein said cleaning device housing is placed on a wheeled chassis, whereby a mobiie cleaning device is provided.

21. A method of producing foam for cleaning purposes, the method comprising the steps of:

- leading pressurized water into a housing of a mixing unit and through an injector positioned within said housing, within which injector an under-pressure is created during the passage of the water through the injector towards an outlet opening of said injector, where said under-pressure causes a cleaning agent to be sucked into the injector via a first input line in fluid communication with a reservoir for said cleaning agent, in which injector the cleaning agent is being mixed with the water under its passage towards the outlet opening of the injector, and

- leading pressurized air into the housing of the mixing unit, and through a gap be- tween said housing and the outlet opening of the injector, in a direction substantially parallel to the flow of water mixed with cleaning agent out of the injector opening, thereby causing creation of foam when said pressurized air becomes mixed with the water and cleaning agent before exiting said housing through a outlet opening.

22 The method according to claim 21 , wherein the air gap completely surrounds the outlet opening of the injector.

23. The method according to claim 21 or 22, further comprising the step of changing the quantity of foam produced per time unit by exchanging the injector with another injector having a different injector inlet area and/or a different injector outlet area.

24 The method according to claim 21 or 22. furtner comprising the step of changing the quantity of foam produced per time unit by exchanging the injector with another injector having a different injector inlet area and/or a different injector outlet area, and keeping the water pressure and air pressure unaltered.

25. The Method according to claim 21 or 22, further comprising the step of choosing the injector, and thereby the injector inlet and outlet area in dependence of what kind of foam is to be produced and/or what kind of first cleaning agent is to be used for creating said foam.

26. The method according to claim 21 or 22, further comprising the step of choosing the injector, and thereby the injector inlet and outlet area in dependence of the available water pressure and/or available air pressure.

27. The method according to claim 23 or 24, wherein the ratio between the area of the gap and the area of the injector inlet is a constant for one different injector which is used to change the quantity of foam produced.