WO2010015258A1

WO2010015258A1 - Heat recovery system

Info

Publication number: WO2010015258A1
Application number: PCT/DK2009/050194
Authority: WO
Inventors: Allan Kjaer
Original assignee: Unic Air 2008 Aps
Priority date: 2008-08-04
Filing date: 2009-08-04
Publication date: 2010-02-11
Also published as: DK176758B1

Abstract

A heat recovery system (1) for use in heat recovery from exhaust air (2), preferably fatty exhaust air (2) from kitchens, where the system (1) includes: - a connection (3) for connecting with a suction unit from the kitchen and a connection (4) for connecting with an exhaust unit to the open air, and first duct (5) provided there between; - a connection (6) for connecting with a fresh air supply unit for introducing fresh air (7) from the open and a connection (8) for connecting with an injection unit for the kitchen, and a second duct (9) provided there between; - a tube heat exchanger (10), wherein the two ducts (5, 9) are in heat exchanging contact for establishing heat exchange between the exhaust air (2) and the fresh air (7), - a washing system (11) with at least one nozzle (12) in the first duct (5) for injecting cleaning liquid in the first duct (5) for cleaning deposits off the heat exchanger (10); - a connection (13) for a cleaning liquid supply unit (14) for the nozzle (12); - a valve (15) for regulating the cleaning liquid flow; and - a device (32) for collecting cleaning liquid at a position after the heat exchanger (10).

Description

HEAT RECOVERY SYSTEM

Field of the Invention

The present invention concerns a heat recovery system for use in heat recovery from exhaust air, preferably fatty exhaust air from kitchens, where the system includes: - a connection for connecting with a suction unit from the kitchen and a connection for connecting with an exhaust unit to the open air, and a first duct provided there between;

- a connection for connecting with a fresh air supply unit for introducing fresh air from the open and a connection for connecting with an injection unit for the kitchen, and a second duct provided there between;

- a heat exchanger, wherein the two ducts are in heat exchanging contact for establishing heat exchange between the exhaust air and the fresh air.

Background of the Invention

It has been known for a long time to make heat recovery systems for use in residential buildings and office buildings. It is an acknowledged fact that it is necessary by ventilation to renew the air in rooms where people stay with fresh air in order to provide acceptable comfort. This ventilation with air from the open causes a loss of energy as it is necessary to heat the fresh air which is often colder than the desired room temperature, before it is blown into the room. It is acknowledged that it is possible to reduce this energy loss by letting the air removed from the room pass through one side of a heat exchanger and simultaneously let the air to go into the room pass the other side of the same heat exchanger. The two airflows exchange heat so that part of the energy removed from the room is recovered and used in the injected air.

The exhaust from deep fat or roasting plates in fast-food restaurants or stoves in kitchens in other restaurants, or in other places where there is a warm fatty exhaust air contains a large energy potential that may provide great savings in the energy consumption for heating if heat recovery is introduced.

In the present application, the term kitchen is used but the system may be used for exhaust air from other premises in other connections, as e.g. industrial food production or in connection with processing oil. The exhaust air from the kitchen contains fatty water vapour where the fat rapidly clogs conventional heat exchangers such that the system has to be cleaned.

CA 2551268 is an example of a system for heat recovery from a restaurant kitchen. The system has a ventilator sucking the exhaust air out of the kitchen. The ventilator is adapted with a design of impeller blades and a housing which is suited for mechanical removal of the fat from the exhaust airflow and to collect it at the bottom of the housing. The exhaust air is passed through a heat exchanger with a liquid-filled coil with fins, where the heat from the exhaust air is transmitted to a liquid before the exhaust air is conducted out of the system. The liquid is passed to another heat exchanger where it transmits the heat to a fresh air flow or water for use in the restaurant.

The above system may not remove all fat from the exhaust air, and the efficiency of the heat exchanger will be reduced over time. Cleaning the heat exchanger has to be done manually and is very time-consuming. Besides, heat transmission between the exhaust air and the injection air is effected by a liquid-borne intermediate step. Losses stemming from the efficiency of the heat exchangers thus count twice. Furthermore, the mechanical way of removing fat in the air implies a large pressure loss. This means that relatively more energy is consumed for sucking the air out through the system than if the fat was not to be removed.

Object of the Invention

It is the object of the invention to indicate a system for heat recovery which reliably and with modest maintenance and a large degree of automation is capable of utilising the heat content in a fatty exhaust air for heating an inwards going fresh air flow which is used for ventilating a room. The system is to be tolerant to fat content in the exhaust air and not dependent on the air being cleaned from fat.

Description of the Invention This is achieved according to the present invention by a heat recovery system of the kind mentioned in the introduction which is peculiar in that the said heat exchanger is a tube heat exchanger and that the system further includes: - a washing system with at least one nozzle in the first duct for injecting cleaning liquid in the first duct for cleaning deposits off the heat exchanger;

- a connection for a cleaning liquid supply unit for the nozzle;

- a valve for regulating the cleaning liquid flow; and - a device for collecting cleaning liquid at a position after the heat exchanger.

Hereby is achieved that it is possible to recover heat from a fatty exhaust air with great reliability and low maintenance without having to remove the fat from the air. The heat is used for heating a fresh air supply before conducting it into a room. In that way it may be avoided to use resources for other heating in order to cover a ventilation loss.

The heat exchanger is a tube heat exchanger where the fatty exhaust air flows at the outer side of the tubes and the fresh air flows inside the tubes. Since a tube heat exchanger is relatively open compared with e.g. a plate heat exchanger, the fat particles in the exhaust air will only slowly clog the heat exchanger to such a degree that its efficiency becomes too low. Besides, the tube heat exchanger is particularly tolerant to fat particles in the exhaust air when the fresh air is sufficiently cold to condense the water vapour. The condensate will form a film on the tubes, inhibiting fat in sticking on the tubes and thereby forming deposits.

The condensate is collected in the device for collecting cleaning liquid which may be a drip tray under the heat exchanger. The remaining part of the fatty water vapour continues in the exhaust air and the fat is not actively removed. Thereby possible pressure losses from devices for removing fat from the exhaust air are obviated.

In good time before the heat exchanger is clogged, the washing system is used to remove the deposited fat particles in the heat exchanger.

The washing system is activated by opening a valve controlling the cleaning liquid flow. Hereby, the cleaning liquid starts flowing to and out of the at least one nozzle of the washing system. The position of the at least one nozzle and possible additional nozzles is arranged such that all tubes in the heat exchanger are irrigated with cleaning liquid. The nozzle is preferably a round cone nozzle, but other nozzle types, such as mist and jet nozzles may also be used.

The speed and pressure of the cleaning liquid is not the most important parameter with regard to the cleaning effect of the washing system as it only acts on the uppermost layers of tubes in the heat exchanger. However, the distribution of cleaning liquid across the top of the heat exchanger is of great significance in relation to the cleaning of the lowermost tubes. Therefore, it is important that the nozzle or nozzles provide good distribution of cleaning liquid across the entire heat exchanger.

The cleaning liquid supply may be a connection to a cold or a warm fresh water supply from private or public water supply. The cleaning liquid may advantageously be softened by e.g. reversed osmosis.

The cleaning liquid is collected in the device for collecting cleaning liquid at the bottom of the heat exchanger. The cleaning liquid may either be conducted to a sewer, directly to a purifying plant or be collected in a container for later cleaning.

The washing system may be automated. Starting the washing cycle is initiated automatically, either at a certain time or based on measured parameters in the system, such as the efficiency of the heat exchanger or pressure loss.

The system forms part of a ventilation system for a kitchen. The system may be disposed at a distance from the kitchen. This may e.g. be a technical room or at the roof of a building. The air is possibly conducted in pipes from the kitchen to the system and from the system to the open in pipes. This piping is not part of the system.

Practical experiments with a prototype system of the described type have been performed in a period which is typical for the Danish weather in the course of the four seasons. This experiment has shown that by heat recovery of the exhaust air from the kitchen in an average restaurant, there may be expected a reduction of the total expenses for heating the fresh air of the kitchen to ¹A, which is only constituted by power consumption for operating the system. Directly derived therefrom, installing such a system will result in a positive environmental effect, as the required resources for heating the kitchen, e.g. in the form of burning fossil fuels, will be reduced to 0. The experiment has shown that a payback time of about 1.5 years can be expected by installation of a system according to the invention.

The prototype has appeared to be very reliable. The system has been washed once a day, but it is expected that the washing interval may be increased to a longer time interval without problems. The daily cleaning may be performed in the closing time such that it does not influence the operation of the system when the restaurant is open. Operation of the system has required very few manhours per hour of operation, as control of temperature and cleaning is automated.

According to a further embodiment, the heat recovery system according to the invention is peculiar in that the washing system includes a device for admixing cleaning agent in the cleaning liquid.

Hereby is achieved a more efficient cleaning of the heat exchanger as the cleaning agent will cause rapid dissolution of the deposited fat particles. The cleaning agent is chosen such that it is suited for dissolving fat.

According to a further embodiment, the heat recovery system according to the invention is peculiar in that the washing system includes a device for admixing surfactant in the cleaning liquid.

Surfactant contributes to soften the cleaning liquid and reduce the surface tension. The reduced surface tension causes the droplets to run off the tubes more easily and prevents the droplets from sticking and drying on the tubes. Formation of calcareous deposits is hereby avoided which otherwise would reduce the efficiency of the heat exchanger. Such calcareous deposits can be difficult to remove.

According to a further embodiment, the heat recovery system according to the invention is peculiar in that the tubes in the heat exchanger are optionally made from glass, stainless steel, plastic or aluminium. It is advantageous that the tubes in the heat exchanger have a very smooth surface which by itself prevents fat from being deposited in the heat exchanger. The selected tubes are therefore to be made of a material that enables formation of such a surface. In addition, the material is selected by parameters like accessibility, corrosion resistance, requirements to hygiene, thermal conductivity, handling and heat resistance. Glass is the preferred material since it has a very smooth surface, high accessibility, it is corrosion resistant and its surface has a very low porosity, limiting bacterial growth and facilitating cleaning. However, handling and processing glass may be associated with certain difficulties as it may be very fragile with a low tolerance to impacts. Stainless steel is a suitable material as it has high accessibility, a smooth surface, where the smoothness can be enhanced by polishing, good temperature resistance, corrosion resistance, may resist impacts and is easy to handle and process. Aluminium and plastic are not quite as suitable materials as the previous ones, but may find application if there are special requirement to the weight of the system. Furthermore, the materials may be combined. For example, one may use tubes of stainless steel that are coated with a smooth Teflon surface.

According to a further embodiment, the heat recovery system according to the invention is peculiar in that that the system has a bypass line with a first connection for the first duct at a position before the washing system, and a second connection for the first duct after the washing system, and that the bypass line includes a bypass damper for regulating the air flow in the bypass line, and that the first duct includes a first regulating damper for regulating the exhaust air flow in the heat exchanger.

Hereby is achieved that it becomes possible to continue operation of the system while the heat exchanger is washed or during a service check. The bypass damper is opened and the first regulating damper is closed. Hereby, the exhaust air flows around the heat exchanger and through the bypass line to the other connection to the first duct.

It is very important to avoid disruption of operation with regard to the work environment in the room, as the temperature in the room very quickly will rise to an intolerable level, and imply great smell nuisances for the persons staying in the room. Furthermore, there is an increased fire hazard.

Moreover, one may use the bypass line for regulating the heat output in the heat exchanger such that less heat is supplied to the fresh air so that the fresh air will not be too warm.

According to a further embodiment, the heat recovery system according to the invention is peculiar in that the system in the second duct includes a cooling unit and a heating unit for conditioning the fresh air.

If the fresh air is warmer than the desired temperature in the room to which the system supplies fresh air, it is necessary to be able to cool the fresh air. The heating unit is used for heating the fresh air if it is colder than the desired temperature in the room supplied with the fresh air by the system. This feature will ensure comfort in the room. The heating unit will most often only be used in connection with disruption of operation as the heat capacity of the exhaust air is great enough to ensure the required heat supply all the year round. Thus there will be an instance of extreme cold before the heating is to be used together with the heat exchanger, but it is possible, however.

The above thus provides the option of washing the heat exchanger or performing maintenance on the system without persons staying in the supplied room feeling any difference in the room temperature.

According to a further embodiment, the heat recovery system according to the invention is peculiar in that the system in the second duct includes a noise attenuation unit for noise reduction and a ventilator producing a fresh air flow.

Hereby may be achieved that the system itself may drive the fresh air flow. A ventilator with fixed or adjustable speed may be chosen if it is necessary with an additional control parameter. In connection with such a ventilator, it is necessary to install a noise attenuation unit for attenuating the noise from the ventilator. In an alternative embodiment of the invention, the noise attenuation unit is adapted with several ducts providing changes of direction and speed of the fresh air flow, which has appeared to contribute to noise reduction. At the same time, the noise attenuation unit is provided with a number of baffles in the duct providing additional noise attenuation. An advantage of this construction is the possibility of a compact reconfiguration of the noise attenuation unit in the system. The length may be reduced to about half of that of an inline noise attenuation unit in an exhaust air duct.

According to a further embodiment, the heat recovery system according to the invention is peculiar in that the system in the second duct includes at least one filter at a position before the heat exchanger for cleaning the fresh air.

Hereby is achieved that one avoids contaminants in the fresh air that otherwise would clog the interior of the tubes in the heat exchanger, or possibly other units farther inside the second duct. Moreover, the filter may be provided with an charcoal filter such that annoying smells in the fresh air supply are avoided. This may often be desirable in connection with restaurants in connection with motorway service stations where there will be a great concentration of annoying petrol fumes.

According to a further embodiment, the heat recovery system according to the invention is peculiar in that the system has a bypass duct with a first connection for the second duct at a position before the heat exchanger, and a second connection for the second duct after the heat exchanger, and that the bypass line includes a bypass damper for regulating the fresh air flow in the bypass line, and that the second duct includes a first shut-off valve at a position before the heat exchange for interrupting the fresh air flow, and a second regulating damper for regulating the fresh air flow through the heat exchanger.

Hereby is achieved that one may regulate the temperature of the fresh air when delivered to the room which is supplied with fresh air from the system. The bypass line enables conduction of more or less of the fresh air around the heat exchanger, depending on how much heat is needed to be supplied to the fresh air. The second regulating damper interacts with the bypass damper so that it is closed when the bypass damper is open and open when the bypass damper is closed, and correspondingly for the intermediate positions. These two dampers are modulating and will continuously adapt their position such that the mixture of the two airflows provides the desired temperature.

Typically, the first shut-off valve will be located immediately within the connection for the fresh air supply. The shut-off valve will be closed when the system is not operating. Hereby is avoided that air is drawn through the second duct when the system is not operating if there is a subpressure in the room. This prevents draft from being formed in the room supplied by the system.

Moreover, the heat exchanger is protected against frost bursts that may occur at times where the fresh air is under or close to the freezing point, and where a subpressure is present simultaneously in the room, hi this case, the cold fresh air is sucked through the system and the heat exchanger due to the subpressure in the room. The cold fresh air will make possible water drops freeze in the heat exchanger. This gives a risk of frost bursts in the heat exchanger.

During start of the system, opening the shut-off valve for the fresh air supply may be delayed until the heat exchanger has reached a temperature where the risk of frost bursts is eliminated.

According to a further embodiment, the heat recovery system according to the invention is peculiar in that the system in the first duct includes a second shut-off valve for regulating the exhaust air flow.

This second shut-off valve has the same purpose as the first shut-off valve. The second shut-off valve may close the exhaust air flow such that draft is avoided in the room supplied by the system if there is a subpressure in the room. Moreover, it is avoided that air is sucked into the system when it is not operating, with consequent frost burst as described previously. According to a further embodiment, the heat recovery system according to the invention is peculiar in that the system in the first duct includes a ventilator for generating an exhaust air flow.

Hereby is achieved that the system itself may produce the required exhaust air flow such that it does not become dependent of other external units for producing this flow.

As mentioned previously, the system is not just limited to include exhaust air from kitchens. The tolerance of the system to fatty particles in the exhaust air due to the formed film of condensate on the heat exchanger tubes may be utilised in other connections, of which e.g. may be mentioned exhaust air from rooms in slaughterhouses, laundries, industrial production of food and processing of oil.

Description of the Drawing

The invention will be explained in more detail below with reference to the accompanying drawing, where:

Fig. 1 shows a flow chart of the system; and

Fig. 2 shows a schematic view of a system with positions of the various units.

In the explanation of the Figures, identical or corresponding elements will be provided with the same designations in different Figures. Therefore, no explanation of all details will be given in connection with each single Figure/embodiment.

Detailed Description of the Invention

The heat recovery system 1 has a first duct 5 for the fatty exhaust air 2 from a kitchen. The suction will typically be mounted in connection with deep fat or a stove. The first duct 5 is provided between a connection 3 to a suction unit in the kitchen and a connection 4 for an exhaust unit to the open air. The connections 3, 4 may be connected with ventilation ducts outside the system 1 such that the system 1 can be located at a distance from the kitchen and the outdoors. The first duct 5 is delimited by internal partitionings in the system 1. When looking in the direction of flow of the exhaust air 2, the exhaust air flow first meets a first connection 21 for a bypass line 20. On Fig. 2, the bypass line 20 is hidden behind the first duct 5 and the heat exchanger 10. The flow in the first duct 5 is regulated by a first regulating damper 30 and regulated in the bypass line 20 with a bypass damper 23. In the shown embodiment on Fig. 2, these two dampers 23, 30 are integrated in one unit.

The cross-section of the bypass line 20 is dimensioned such that the pressure loss in the bypass line 20 corresponds to the pressure loss in the first duct 5. Hereby it becomes possible to regulate the part of the exhaust air 2 through the first duct 5 and through the bypass pipe 20. During service or washing, the first regulating damper 30 will be completely closed and the bypass damper 23 will be open.

The exhaust air flow is turned 90° and passed down through the heat exchanger 10 where it passes the outer side of the tubes in the heat exchanger 10. In the shown embodiment, the heat exchanger 10 is a glass tube heat exchanger. Part of the fatty water vapour condenses on the tubes, forming a water film which inhibits depositing of fat on the tubes. However, over time fat deposits will appear in the heat exchanger 10. The rest of the fatty water runs down into a drip tray 32 under the heat exchanger 10. The part of the fat not deposited on the tubes or led out with the fatty water in the drip tray 32 continues with the exhaust air 2.

The exhaust air flow is again turned 90° and passes through a second shut-off valve 37. This shut-off valve 37 is entirely closed during service or washing. After the second shut-off valve 37, the exhaust air flow is joined again in the first duct 5 at the point where the bypass line 20 is provided with a second connection 22 to the first duct 5.

In the embodiment on Fig. 1, the system 1 is provided with a ventilator 31 in the first duct 5 for driving the exhaust air 2. The embodiment in Fig. 2 does not have this ventilator 31. The suction air 2 now leaves the system through the connection 4 to an exhaust unit.

The system 1 has a second duct 9 for fresh air 7. The second duct 9 is provided between a connection 6 to a fresh air supply unit and a connection 8 to an injection unit for the kitchen. The connections 6, 4 may be connected with ventilation ducts outside the system 1 such that the system 1 can be disposed at a distance from the kitchen and the outdoors.

The second duct 9 is delimited by internal partitionings in the system 1. As seen in the direction of flow for the fresh air 7, it will first meet the first shut-off valve 42 which is open when the system 1 is operating, and closed when not operating. It is used for shutting off the fresh air 7 when the system 1 is not operating in order to prevent fresh air 7 from penetrating into the system 1 if there is subpressure in the room.

Then it runs through a filter 28 where contaminants and possible smells are removed. The fresh air 7 meets a connection 39 for a bypass line 38 which leads around the heat exchanger through a bypass damper 41 to a second connection 40 to the second duci. On Fig. 2, the bypass line 38 is not shown. In the system 1 on Fig. 2, the bypass duct 38 is delimited by a compartment in front of the heat exchanger 10. If the bypass damper 41 is open, a part of the fresh air 7 will flow through the bypass line 38. The remaining part of the fresh air 7 flows into the glass tubes in the heat exchanger 10. Here it is heated by heat exchanging with the exhaust air 2.

The cold fresh air 7 is provided some of the fatty water vapour in the exhaust air 2 to condense externally on the glass tubes. The formed water film makes it difficult for the fat to be deposited. The deposited fat reduces the efficiency of the heat exchanger 10 which is therefore to be dimensioned to make allowance for this fact.

The fresh air 7 then continues through a second regulating damper 29 which cooperates with the bypass damper 41 so as to regulate the temperature of the fresh air 7. In the embodiment on Fig. 2, these two dampers 29, 41 are integrated in one unit. When the bypass damper 41 is opened, the second regulating damper is closed, and a larger part of the fresh air 7 will be conducted around the heat exchanger 10 with the result that less heat is supplied to the fresh air 7. Conversely, more heat can be supplied to the fresh air 7 by opening the second regulating damper 29 more and by closing the bypass damper 41 more. Next, a heat exchanger for a cooling unit 24 is passed which may be used for cooling the fresh air 7, a ventilator 26 producing a flow of fresh air, a two-part noise attenuation unit 27 in the form of a duct with 90° bends at each side of a heat exchanger for a heating unit 25 for heating fresh air 7, and a noise attenuation part with baffles 27 in the duct. The fresh air 7 leaves the system 1 through a connection 8 for connecting to an injection unit in the kitchen.

The washing system 11 is used for cleaning the heat exchanger 10 by removing deposits of fat. The washing system 11 consists of a number of nozzles 12. In the shown embodiment, the system 11 has one nozzle 12, but it is preferred to have at least two nozzles in order to achieve satisfactory coverage of the heat exchanger. The nozzle 12 has a connection 13 for a cleaning liquid supply unit 14. This unit 14 may just be a connection to the normal hot or cold water supply of the building. However, it is preferred using a cleaning liquid of softened water.

A solenoid valve 15 regulates the cleaning liquid flow. In the shown embodiment, the washing system 11 has a device 16 for cleaning agent 17 and a device 18 for surfactant 19 which can be admixed the cleaning liquid. The device 16 for cleaning agent comprises a container 33 with a supply of cleaning agent 17, a dosing pump 35 for dosing cleaning agent 17 and a connection to the connection 13. The device 18 for surfactant 19 comprises a container 34 with a supply of surfactant 19, a dosing pump 36 for dosing surfactant 19 and a connection to the connection 13.

The washing system 11 operates in that the solenoid valve 15 is opened whereby the cleaning liquid begins flowing towards and out of the nozzle 12. Cleaning agent 17 and surfactant 19 are added according to need.

The washing sequence for a system 1 in the shown embodiment is as follows:

1. The first regulating damper 30 and the second shut-off valve 37 are closed.

2. The solenoid valve 15 is opened and the dosing pump 35 for cleaning agent is switched on so that cleaning liquid with cleaning agent 17 is sprayed over the heat exchanger. 3. The dosing pump 35 for cleaning agent is switched off and the solenoid valve 15 is closed.

4. There is a break while the cleaning liquid softens the deposits.

5. The solenoid valve 15 is opened, and the heat exchanger 10 is flushed free from unclean cleaning liquid.

6. The dosing pump 35 for cleaning agent is switched on and the heat exchanger 10 is washed clean.

7. The dosing pump 35 for cleaning agent is switched off and the solenoid valve 15 is closed. 8. There is a break while the cleaning liquid softens the deposits.

9. The solenoid valve 15 is opened, and flushing is performed until the heat exchanger 10 is free from cleaning liquid.

10. The dosing pump 36 for surfactant 19 is switched on and wetting is performed.

11. The dosing pump 36 for surfactant 19 is switched off and the solenoid valve 15 is closed.

The system 1 is now cleaned and ready for operation.

Before operation, the first regulating damper 30 and the second shut-off valve 37 are to be opened.

Claims

1. A heat recovery system (1) for use in heat recovery from exhaust air (2), preferably fatty exhaust air (2) from kitchens, where the system (1) includes: - a connection (3) for connecting with a suction unit from the kitchen and a connection (4) for connecting with an exhaust unit to the open air, and a first duct (5) provided there between;

- a connection (6) for connecting with a fresh air supply unit for introducing fresh air (7) from the open and a connection (8) for connecting with an injection unit for the kitchen, and a second duct (9) provided there between; - a heat exchanger (10), wherein the two ducts (5, 9) are in heat exchanging contact for establishing heat exchange between the exhaust air (2) and the fresh air (7), characterised in that the heat exchanger (10) is a tube heat exchanger, and that the system further includes:

- a washing system (11) with at least one nozzle (12) in the first duct (5) for injecting cleaning liquid in the first duct (5) for cleaning deposits off the heat exchanger (10);

- a connection (13) for a cleaning liquid supply unit (14) for the nozzle (12);

- a valve (15) for regulating the cleaning liquid flow; and

- a device (32) for collecting cleaning liquid at a position after the heat exchanger (10).

2. Heat recovery system according to claim 1, characterised in that the washing system (11) includes a device (16) for admixing cleaning agent (17) in the cleaning liquid.

3. Heat recovery system according to claim 1-2, characterised in that the washing system (11) includes a device (18) for admixing surfactant (19) in the cleaning liquid.

4. Heat recovery system according to any preceding claim, characterised in that the tubes in the heat exchanger (10) are optionally formed of glass, stainless steel, plastic or aluminium.

5. Heat recovery system according to any preceding claim, characterised in that the system has a bypass line (20) with a first connection (21) for the first duct (5) at a position before the washing system (11), and a second connection (22) for the first duct (5) after the washing system (11), and that the bypass line (20) includes a bypass damper (23) for regulating the exhaust air flow in the bypass line (20), and that the first duct (5) includes a first regulating damper (30) for regulating the exhaust air flow in the heat exchanger (10).

6. Heat recovery system according to any preceding claim, characterised in that the system (1) in the second duct (9) includes a cooling unit (24) and a heating unit (25) for conditioning the fresh air (7).

7. Heat recovery system according to any preceding claim, characterised in that the system (1) in the second duct (9) includes a noise attenuation unit (27) for noise reduction and a ventilator (26) for producing a fresh air flow.

8. Heat recovery system according to any preceding claim, characterised in that the system (1) in the second duct (9) at a position before the heat exchanger (10) includes at least one filter (28) for cleaning the fresh air (7).

9. Heat recovery system according to any preceding claim, characterised in that the system has a bypass duct (38) with a first connection (39) for the second duct (9) at a position before the heat exchanger (10), and a second connection (40) for the second duct (9) after the heat exchanger, and that the bypass line (38) includes a bypass damper (41) for regulating the fresh air flow in the bypass line (38), and that the second duct (9) includes a first shut-off valve (42) at a position before the heat exchange (10) for interrupting the fresh air flow, and a second regulating damper (29) for regulating the fresh air flow through the heat exchanger (10).

10. Heat recovery system according to any preceding claim, characterised in that the system (1) in the first duct (5) includes a second shut-off valve (37) for regulating the exhaust air flow.

11. Heat recovery system according to any preceding claim, characterised in that the system (1) in the first duct (5) includes a ventilator (31) for producing an exhaust air flow.