The present invention relates to a method for controlling an air handling unit, i.e. a unit for controlling air circulation in a controlled space. The method of the invention allows the air handling unit to be controlled in a stable manner.

BACKGROUND OF THE INVENTION

Air handling units are sometimes used for controlling indoor climate in a room or a building. For instance, the air handling unit may be used for obtaining a desired temperature, a desired humidity, a desired level of fresh air supply, etc., to the room or building .

Accordingly, air handling units may be capable of supplying fresh air from the outside into the room or building, while conveying exhaust air from the room or building to the outside. Air handling units may further be capable of recirculating air inside the room or building, e.g. in order to maintain a comfortable temperature level in the case that the outdoor temperature is significantly higher or lower than the comfortable temperature level . US 8,393,549 discloses a system for controlling temperature and humidity of a controlled space. The system includes a supply air path for supplying an outside air stream to the controlled space and an exhaust air path for conveying an exhaust air stream from the controlled space. The system further includes a total energy recovery device, e.g. in the form of an enthalpy wheel, in contact with the outside air stream and the exhaust air stream, a dehumidification wheel in contact with two spaced portions of the outside air stream, and a cooler in contact with the outside air stream between the two spaced portions.

As described above, air handling units may be used for controlling an air temperature inside a controlled space, such as a room . Several devices of the air handling unit may be operated in order to obtain this, e.g . the total energy recovery device, the dehumidification wheel and the cooler disclosed in US 8,393,549. However, adjusting one of the devices will affect operation of the other devices, and thereby the control of the various devices of the air handling unit may counteract each other, potentially leading to an unstable control of the air handling unit and/or to a situation in which a target temperature is not reached within a reasonable time. DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a method for controlling an air handling unit in a stable manner.

According to a first aspect the invention provides a method for controlling an air handling unit, the air handling unit comprising an ingoing duct for supplying ingoing air to a controlled space, an outgoing duct for conveying outgoing air from the controlled space, and at least one heating coil for heating air flowing in the ingoing duct and/or at least one cooling coil for cooling air flowing in the ingoing duct, the air handling unit further comprising a mixing damper arrangement for controlling mixing of a part of the outgoing air into the ingoing duct, and/or an enthalpy wheel providing heat exchange between the ingoing air flowing in the ingoing duct and the outgoing air flowing in the outgoing duct, the method comprising the steps of: measuring an intermediate temperature value in the ingoing duct at a position upstream relative to the heating coil(s) and/or cooling coil(s), and downstream relative to the mixing damper arrangement and/or the enthalpy wheel, and controlling the mixing damper arrangement and/or the enthalpy wheel in accordance with an intermediate temperature setpoint value, and in order to obtain an intermediate temperature value which is equal to the intermediate temperature setpoint value, wherein the intermediate temperature setpoint value depends on an ingoing air temperature setpoint value at an inlet passage from the ingoing duct to the controlled space.

According to the first aspect of the invention, a method for controlling an air handling unit is provided. As described above, in the present context the term 'air handling unit' should be interpreted to mean a unit or a system used for controlling indoor climate of a controlled space, e.g. in the form of a room or a building . This may, e.g ., include controlling temperature, humidity, air circulation, etc., in the controlled space.

The air handling unit comprises an ingoing duct and an outgoing duct. Ingoing air is supplied to the controlled space via the ingoing duct, and outgoing air is conveyed from the controlled space via the outgoing duct. The ingoing air may be in the form of intake air being drawn from the outside. As an alternative, the ingoing air may be in the form of recirculated air, which is supplied from the outgoing duct to the ingoing duct, possibly via one or more filters. As another alternative, the ingoing air may be an appropriate mixture of intake air and recirculated air. Furthermore, the nature of the ingoing air may change along the ingoing duct. For instance, only intake air may be present upstream relative to a mixing damper arrangement, and a mixture of intake air and recirculated air may be present downstream relative to the mixing damper arrangement.

Similarly, the outgoing air being conveyed from the controlled space via the outgoing duct may be delivered to the outside and/or supplied to the ingoing duct for recirculation.

The air flowing in the ingoing duct and/or in the outgoing duct may advantageously be moved along the duct(s) by means of one or more fans.

The air handling unit further comprises at least one heating coil and/or at least one cooling coil arranged in the ingoing duct. In the case that the air handling unit comprises at least one heating coil, the heating coil(s) may be used for heating the air flowing in the ingoing duct, thereby increasing the temperature of air being supplied to the controlled space. Similarly, in the case that the air handling unit comprises at least one cooling coil, the cooling coil(s) may be used for decreasing the temperature of air being supplied to the controlled space. In the case that the air handling unit comprises at least one heating coil as well as at least one cooling coil, the temperature of the air being supplied to the controlled space may be increased as well as decreased, depending on whether the heating coil(s) or the cooling coil(s) is/are activated, and in accordance with given requirements.

The air handling unit further comprises a mixing damper arrangement and/or an enthalpy wheel. The mixing damper arrangement is for controlling mixing of a part of the outgoing air into the ingoing duct. Thus, by means of the mixing damper arrangement it can be controlled how large a portion of the air being conveyed from the controlled space is to be supplied to the ingoing duct, and thereby recirculated, and how large a portion is to be delivered to the outside. Thereby, the mixing damper arrangement is also used for controlling how large a portion of the air flowing in the ingoing duct, and thereby being supplied to the controlled space, is recirculated air, and how large a portion is intake air, drawn from the outside.

The enthalpy wheel provides heat exchange between the ingoing air flowing in the ingoing duct and the outgoing air flowing in the outgoing duct. In the case that the outdoor temperature is lower than a comfortable indoor temperature level, then intake air drawn from outside and into the ingoing duct will probably have to be heated before being supplied to the controlled space. Furthermore, the temperature of the intake air will most likely be lower than the temperature of air being conveyed from the controlled space, i.e. air flowing in the outgoing duct. Therefore, providing heat exchange between the ingoing air and the outgoing air, via the enthalpy wheel, will, in this case, result in an increase of the temperature of the ingoing air and a decrease in the temperature of the outgoing air. Thereby some of the heat being removed from the controlled space is reclaimed and used for heating the incoming air.

Similarly, in the case that the outdoor temperature is higher than a comfortable indoor temperature level, then the intake air will probably need to be cooled before being supplied to the controlled space, and it will most likely have a temperature which is higher than the temperature of air being conveyed from the controlled space. Therefore, providing heat exchange between the ingoing air and the outgoing air, via the enthalpy wheel, will, in this case, result in a decrease of the temperature of the ingoing air and an increase in the temperature of the outgoing air. It is within the scope of the present invention that the air handling unit may comprise only a mixing damper arrangement, only an enthalpy wheel, or a mixing damper arrangement as well as an enthalpy wheel.

According to the method of the first aspect of the invention, an intermediate temperature value is initially measured. The intermediate temperature value is measured in the ingoing duct at a position upstream relative to the heating coil(s) and/or cooling coil(s), and downstream relative to the mixing damper arrangement and/or the enthalpy wheel. In the case that the air handling unit comprises only a mixing damper arrangement or only an enthalpy wheel, then the intermediate temperature value is measured in the ingoing duct, downstream with the respect to the mixing damper arrangement or enthalpy wheel. In the case that the air handling unit comprises a mixing damper arrangement as well as an enthalpy wheel, then the intermediate temperature value may be measured in the ingoing duct at a position which is downstream with respect to the mixing damper arrangement as well as with respect to the enthalpy wheel, or it may be measured at a position which is between the mixing damper arrangement and the enthalpy wheel, i.e. downstream with respect to one of the mixing damper arrangement and the enthalpy wheel, and upstream with respect to the other.

The mixing damper arrangement and/or the enthalpy wheel is/are then controlled in accordance with an intermediate temperature setpoint value, and in order to obtain an intermediate temperature value which is equal to the intermediate temperature setpoint value. In the case that the air handling unit comprises only a mixing damper arrangement or only an enthalpy wheel, then the mixing damper arrangement or enthalpy wheel is controlled. In the case that the air handling unit comprises a mixing damper arrangement as well as an enthalpy wheel, then either one or both of the mixing damper arrangement and the enthalpy wheel may be controlled . It should, however, be noted that in the case that the intermediate temperature is measured at a position between the mixing damper arrangement and the enthalpy wheel, then only the part which is arranged upstream with respect to the measurement position is controlled .

The intermediate temperature setpoint value depends on an ingoing air temperature setpoint value at an inlet passage from the ingoing duct to the controlled space. Thus, the ingoing air temperature setpoint value is a setpoint value for the temperature of the air which enters the controlled space from the ingoing duct. This temperature is not necessarily the same as a representative temperature inside the controlled space, or as a desired temperature level inside the controlled space. For instance, in the case that the outdoor temperature is lower than a comfortable temperature level inside the controlled space, then ingoing air temperature should be slightly higher than the comfortable temperature level, in order to heat the air inside the controlled space in order to reach or maintain the comfortable temperature level. Similarly, the ingoing air temperature should be slightly lower than the comfortable temperature level, in the case that the outdoor temperature is higher than the comfortable temperature level, in order to cool the air inside the controlled space.

Accordingly, the ingoing air temperature setpoint value is selected in such a manner that, when controlling the air handling unit in order to obtain an ingoing air temperature which is equal to the ingoing air temperature setpoint value, then a desired temperature level is obtained inside the controlled space.

As mentioned above, the intermediate temperature setpoint value depends on the ingoing air temperature setpoint value. Thereby the intermediate temperature setpoint value is preferably selected in such a manner that, if the intermediate temperature setpoint value is reached at the point where the intermediate temperature value is measured, then the ingoing air temperature setpoint is reached at the inlet passage, preferably without using the heating coil(s) and/or cooling coil(s) . Thus, the mixing damper arrangement and/or the enthalpy wheel is/are controlled on the basis of the intermediate temperature, and the heating coil(s) and/or cooling coil(s) is/are controlled on the basis of the ingoing air temperature. Thereby it is ensured that the control of the mixing damper arrangement and/or the enthalpy wheel, on the one hand, is not counteracted by the control of the heating coil(s) and/or cooling coil(s), on the other hand . Accordingly, a stable control of the air handling unit is obtained, and the air handling unit, as a whole, is controlled in order to obtain a desired ingoing air temperature.

Furthermore, if an intermediate temperature can be obtained, by controlling the mixing damper arrangement and/or the enthalpy wheel, which is close to the ingoing air temperature setpoint value, then the ingoing air temperature setpoint value can be reached with no, or only limited, use of the heating coil(s) and/or cooling coil(s) . Thereby the air handling unit can be operated in a more energy efficient manner.

As described above, the air handling unit may comprise a mixing damper arrangement as well as an enthalpy wheel, and in this case the method may comprise the steps of: - measuring a first intermediate temperature value in the ingoing duct at a position upstream relative to the mixing damper arrangement, and downstream relative to the enthalpy wheel, controlling the enthalpy wheel in accordance with a first intermediate temperature setpoint value, and in order to obtain a first intermediate temperature value which is equal to the first intermediate temperature setpoint value, measuring a second intermediate temperature value in the ingoing duct at a position upstream relative to the heating coil(s) and/or cooling coil(s), and downstream relative to the mixing damper arrangement, and controlling the mixing damper arrangement in accordance with a second intermediate temperature setpoint value, and in order to obtain a second intermediate temperature value which is equal to the second intermediate temperature setpoint value.

According to this embodiment, the enthalpy wheel and the mixing damper arrangement are controlled independently of each other, and essentially in the manner described above. The enthalpy wheel is arranged upstream relative to the mixing damper arrangement, along a flow direction of air flowing in the ingoing duct. Accordingly, air flowing in the ingoing duct, from an air intake for supplying outside air to the air handling unit, towards the controlled space, will first encounter the enthalpy wheel, then the mixing damper arrangement, and finally the heating coil(s) and/or the cooling coils(s), before being supplied to the controlled space. The first intermediate temperature value is measured in the ingoing duct at a position upstream relative to the mixing damper arrangement and downstream relative to the enthalpy wheel, i.e. at a position between the enthalpy wheel and the mixing damper arrangement. Accordingly, the first intermediate temperature value represents the temperature of air leaving the enthalpy wheel and entering the mixing damper arrangement. The enthalpy wheel is then controlled in accordance with a first intermediate temperature setpoint value, and in order to obtain a first intermediate temperature value which is equal to the first intermediate temperature setpoint value. Accordingly, the enthalpy wheel is controlled in order to supply air to the mixing damper arrangement, having a temperature which is as close to the desired ingoing air temperature as possible.

The second intermediate temperature value is measured in the ingoing duct at a position upstream relative to the heating coil(s) and/or cooling coil(s), and downstream relative to the mixing damper arrangement, i.e. at a position between the mixing damper arrangement and the heating coil(s) and/or cooling coil(s) . Accordingly, the second intermediate temperature value represents the temperature of air leaving the mixing damper arrangement and entering the heating coil(s) and/or cooling coil(s) . The mixing damper arrangement is then controlled in accordance with a second intermediate temperature setpoint value, and in order to obtain a second intermediate temperature value which is equal to the second intermediate temperature setpoint value. Accordingly, the mixing damper arrangement is controlled in order to supply air to the heating coil(s) and/or cooling coil(s), having a temperature which is as close to the desired ingoing air temperature as possible.

Thus, according to this embodiment, it is attempted to obtain a desired ingoing air temperature by appropriately controlling the enthalpy wheel, thereby obtaining a first intermediate temperature value which is equal to the first intermediate temperature setpoint value. Ideally, this would result in the second intermediate temperature value being equal to the second intermediate temperature setpoint value, and the ingoing air temperature being equal to the ingoing air temperature setpoint value, without having to control the mixing damper arrangement or the heating coil(s) and/or cooling coil(s) .

However, if controlling the enthalpy wheel as described above is not sufficient, then the mixing damper arrangement is appropriately controlled in order to obtain a second intermediate temperature value which is equal to the second intermediate temperature setpoint value.

If this is also not sufficient, then the heating coil(s) and/or cooling coil(s) is/are appropriately controlled in order to obtain an ingoing air temperature which is equal to the ingoing air temperature setpoint value. Thus, the enthalpy wheel, the mixing damper arrangement and the heating coil(s) and/or cooling coil(s) are controlled independently of each other, in the sense that they are controlled on the basis of three different temperature values. Thereby controlling one of the devices does not counteract the control of the other two devices, and a stable control of the air handling unit is obtained.

The intermediate temperature described above could be either of the first intermediate temperature and the second intermediate temperature described here. The method may further comprise the steps of: measuring an ingoing air temperature at the inlet passage from the ingoing duct to the controlled space, and controlling the heating coil(s) and/or cooling coil(s) in accordance with an ingoing air temperature setpoint value, and in order to obtain an ingoing air temperature at the inlet passage from the ingoing duct to the controlled space which is equal to the ingoing air temperature setpoint value.

As described above, the heating coil(s) and/or cooling coil(s) can be controlled in order to obtain a desired ingoing air temperature, if controlling the enthalpy wheel and/or the mixing damper arrangement as described above is not sufficient. The step of controlling the enthalpy wheel may comprise controlling a rotating speed of the enthalpy wheel. In this case, the enthalpy wheel is of a kind which rotates, and is in thermal contact with air flowing in the ingoing duct as well as air flowing in the outgoing duct.

Thereby heat is transferred from air flowing in the ingoing duct to air flowing in the outgoing duct, or vice versa, at a rate which is determined by the rotating speed of the enthalpy wheel. Accordingly, the heat transfer between the ducts can be controlled by controlling the rotating speed of the enthalpy wheel.

The step of controlling the mixing damper arrangement may comprise controlling an opening degree of one or more dampers of the mixing damper arrangement, in order to adjust a fraction of outgoing air being supplied from the outgoing duct to the ingoing duct. According to this embodiment, the mixing damper arrangement may comprise a first damper determining an air flow from the outgoing duct to the ingoing duct, a second damper determining an air flow in the outgoing duct towards the outside, and a third damper controlling an air flow in the ingoing duct from the outside. The three dampers are controlled in a synchronized manner, in order to appropriately divide the air flow in the outgoing duct into a portion being supplied to the ingoing duct and a portion being conveyed to the outside, and in order to appropriately mix outside air with recirculated air from the outgoing duct in the ingoing duct.

For instance, in the case that it is desired to supply only recirculated air to the controlled space, then the first damper is fully opened, and the second and third dampers are fully closed . Similarly, in the case that it is desired to supply only fresh air to the controlled space, then the first damper is fully closed, and the second and third dampers are fully opened . However, in the case that it is desired to supply a mixture of fresh air and recirculated air to the controlled space, then all of the dampers are partly opened at opening degrees which provide the desired mixture of fresh and recirculated air. The method may further comprise the step of calibrating the intermediate temperature setpoint value with respect to the ingoing air temperature setpoint value at the inlet passage from the ingoing duct to the controlled space. According to this embodiment, for a given air handling unit, and taking the specific operating conditions into account, an appropriate intermediate temperature setpoint value is determined, which allows the ingoing air temperature setpoint value to be reached with no or only limited use of the heating coil(s) and/or cooling coil(s) .

The step of calibrating the intermediate temperature setpoint value may comprise the steps of: deactivating the heating coil(s) and/or cooling coil(s), - operating the air handling unit until a steady state condition is reached, measuring the ingoing air temperature at the inlet passage from the ingoing duct to the controlled space and the intermediate temperature, and calibrating the intermediate temperature setpoint value based on the measured ingoing air temperature and the measured intermediate temperature. Deactivating the heating coil(s) and/or cooling coil(s) has the effect that the temperature of air flowing in the ingoing duct will not be actively affected when passing through the coil(s) . This allows a direct correlation between the intermediate temperature and the ingoing air temperature to be derived . When the heating coil(s) and/or cooling coil(s) has/have been deactivated, the air handling unit is operated until a steady state condition is reached. In the present context the term 'steady state condition' should be interpreted to mean a stable condition in which some or all of the quantities which describe the system are independent of time. The steady state condition could include that thermal equilibrium has been obtained, but this is not necessarily the case.

Operating the air handling unit in this manner may include selecting settings for the enthalpy wheel and/or the mixing damper arrangement, such as a rotating speed of the enthalpy wheel and opening degrees of one or more dampers of the mixing damper arrangement, and leaving the enthalpy wheel and/or the mixing damper with these settings until the steady state condition has been reached .

When the steady state condition has been reached, the ingoing air temperature at the inlet passage from the ingoing duct to the controlled space and the intermediate temperature are measured, and the intermediate temperature setpoint value is calibrated based on the two measured temperature values. The two measured temperature values provide information regarding the correlation between the intermediate temperature and the ingoing air temperature during a steady state condition, i.e. when nothing is disturbing the system. Thereby the two measured temperature values can be used for deriving at which level the intermediate temperature should be in order to ensure that a desired ingoing air temperature is reached, without having to operate the heating coil(s) and/or cooling coil(s) . Accordingly, an appropriate intermediate temperature setpoint value can be derived, which depends on an ingoing air temperature setpoint value, and thereby a calibration is obtained.

According to a second aspect the invention provides a method for calibrating an intermediate temperature setpoint value of an air handling unit, the air handling unit comprising an ingoing duct for supplying ingoing air to a controlled space, an outgoing duct for conveying outgoing air from the controlled space, and at least one heating coil for heating air flowing in the ingoing duct and/or at least one cooling coil for cooling air flowing in the ingoing duct, the air handling unit further comprising a mixing damper arrangement for controlling mixing of a part of the outgoing air into the ingoing duct, and/or an enthalpy wheel providing heat exchange between the ingoing air flowing in the ingoing duct and the outgoing air flowing in the outgoing duct, the method comprising the steps of: deactivating the heating coil(s) and/or cooling coil(s), operating the air handling unit until a steady state condition is reached, measuring an ingoing air temperature value at an inlet passage from the ingoing duct to the controlled space, measuring an intermediate temperature value in the ingoing duct at a position upstream relative to the heating coil(s) and/or cooling coil(s), and downstream relative to the mixing damper arrangement and/or the enthalpy wheel, and calibrating an intermediate temperature setpoint value based on the measured ingoing air temperature value, the measured intermediate temperature value, and an ingoing air temperature setpoint value at the inlet passage from the ingoing duct to the controlled space. It should be noted that a person skilled in the art would readily recognise that any feature described in combination with the first aspect of the invention could also be combined with the second aspect of the invention, and vice versa . The remarks set forth above are therefore equally applicable here.

For instance, the method according the second aspect of the invention is performed using an air handling unit as described above with reference to the first aspect of the invention .

The method according to the second aspect of the invention provides calibration of the intermediate temperature setpoint value with respect to the ingoing air temperature setpoint value, essentially as described above with reference to the first aspect of the invention.

The step of operating the air handling unit may comprise the steps of: - preventing rotations of the enthalpy wheel,

- setting at least one damper of the mixing damper arrangement in a position in which no outgoing air is supplied from the outgoing duct to the ingoing duct, and

- waiting until a steady state condition is reached.

According to this embodiment, the settings for the enthalpy wheel and the mixing damper arrangement are selected in such a manner that no heat exchange takes place between the air flowing in the outgoing duct and the air flowing in the ingoing duct, and in such a manner that none of the air flowing in the outgoing duct is recirculated, i.e. only outside air is supplied to the controlled space. This will allow a steady state condition to be reached, which reflects the current outdoor temperature level.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1 is a diagrammatic view of an air handling unit being controlled in accordance with a method according to an embodiment of the invention, and

Fig. 2 is a block diagram illustrating a method according to an embodiment of the invention .

DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagrammatic view of an air handling unit 1 being controlled in accordance with an embodiment of the invention . The air handling unit 1 is used for controlling indoor climate inside a controlled space 2, e.g. in the form of a room or a building . For instance, the air handling unit 1 may control temperature, humidity etc. inside the controlled space 2.

The air handling unit 1 comprises an ingoing duct 3 for supplying ingoing air to the controlled space 2, and an outgoing duct 4 for conveying outgoing air from the controlled space 2. The air flowing in the ingoing duct 3 and/or in the outgoing duct 4 may advantageously be moved along the duct(s) 3, 4 by means of one or more fans 19.

An enthalpy wheel 5 is arranged in thermal contact with air flowing in the outgoing duct 4 as well as with air flowing in the ingoing duct 3. Accordingly, heat exchange can take place between the air flowing in the outgoing duct 4 and the air flowing in the ingoing duct 3, by rotating the enthalpy wheel 5. The rotating speed of the enthalpy wheel determines the rate of the heat exchange.

For instance, in the case that the outdoor temperature is lower than a comfortable temperature level being desired inside the controlled space 2 (or at least lower than an ingoing air setpoint value, which could be higher or lower than the comfortable temperature), then the temperature of air drawn from outside and into the ingoing duct 3 will most likely need to be increased before the air is supplied to the controlled space 2. This may be at least partly achieved by transferring heat from the air flowing in the outgoing duct 4 to the air flowing in the ingoing duct 3, by means of the enthalpy wheel 5, since the outgoing air must be expected to have a higher temperature than the outdoor air in the ingoing duct 3.

Similarly, in the case that the outdoor temperature is higher than a comfortable temperature level being desired inside the controlled space 2 (or at least higher than an ingoing air temperature setpoint value), then the temperature of air drawn from outside and into the ingoing duct 3 will most likely need to be decreased before the air is supplied to the controlled space 2. Similarly to the situation described above, this may be at least partly obtained by transferring heat from the, warmer, air flowing in the ingoing duct 3 to the, cooler, air flowing in the outgoing duct 4, via the enthalpy wheel 5. The air handling unit 1 further comprises a mixing damper arrangement 6. The mixing damper arrangement 6 comprises a first damper 7 controlling an air flow from the outgoing duct 4 to the ingoing duct 3, a second damper 8 controlling an air flow in the outgoing duct 4 towards the outside, and a third damper 9 controlling an air flow in the ingoing duct 3 from the outside towards the controlled space 2. Each of the dampers 7, 8, 9 have an adjustable opening degree, and by controlling the opening degrees of the dampers 7, 8, 9 in a synchronous manner, it is possible to adjust how large a portion of the air being conveyed from the controlled space 2, via the outgoing duct 4, is to be supplied to the ingoing duct 3, and thereby recirculated, and how large a portion is to be conveyed to the outside.

Accordingly, it is also possible to adjust a mixture of the ingoing air flowing in the ingoing duct 3 of recirculated air and fresh air, drawn from the outside.

For instance, in the case that it is desired to fully recirculate the air in the system, then the first damper 7 should be fully opened, and the second damper 8 and the third damper 9 should be fully closed. On the other hand, in the case that it is desired to supply only fresh air to the controlled space 2, i .e. no recirculation is desired, then the first damper 7 should be fully closed, and the second damper 8 and the third damper 9 should be fully opened. Finally, any desired mixture of recirculated air and fresh air in the ingoing duct 3 can be obtained by partly opening all three dampers 7, 8, 9, and by selecting the opening degrees of the dampers 7, 8, 9 in an appropriate manner.

The air handling unit 1 further comprises two heating coils 10 and a cooling coil 11 arranged in the ingoing duct 3. The air flowing in the ingoing duct 3 can be heated by activating one or both of the heating coils 10, and cooled by activating the cooling coil 11. The heating coils 10 and the cooling coil 11 may further be used for controlling the humidity of the air being supplied to the controlled space 2. The enthalpy wheel 5, the mixing damper arrangement 6, the heating coils 10 and the cooling coil 11 can all be used during control of the air handling unit 1 in order to obtain a desired temperature of air being supplied to the controlled space 2.

The air handling unit 1 may be operated in the following manner. A first intermediate temperature value is measured in the ingoing duct 3 at a position 12 upstream relative to the mixing damper arrangement 6 and downstream relative to the enthalpy wheel 5. Accordingly, the first intermediate temperature value represents the temperature of the air leaving the enthalpy wheel 5 and entering the mixing damper arrangement 6, in the ingoing duct 3.

The enthalpy wheel 5 is then controlled in accordance with a first intermediate temperature setpoint value, and in order to obtain a first intermediate temperature value which is equal to the first intermediate temperature setpoint value. Accordingly, the enthalpy wheel 5 is controlled on the basis of the temperature of air flowing in the ingoing duct at position 12, i.e. the enthalpy wheel 5 is controlled in order to supply air of a desired temperature to the mixing damper arrangement 6. The enthalpy wheel 5 may advantageously be controlled by controlling the rotating speed of the enthalpy wheel 5, thereby controlling the heat transfer between the air flowing in the outgoing duct 4 and the air flowing in the ingoing duct 3.

Furthermore, a second intermediate temperature value is measured in the ingoing duct 3 at a position 13 upstream relative to the heating coils 10 and the cooling coil 11 and downstream relative to the mixing damper arrangement 6. Accordingly, the second intermediate temperature value represents the temperature of air leaving the mixing damper arrangement 6 and entering the heating and cooling coils 10, 11, in the ingoing duct 3.

The mixing damper arrangement 6 is then controlled in accordance with a second

intermediate temperature setpoint value, and in order to obtain a second intermediate temperature value which is equal to the second intermediate temperature setpoint value. Accordingly, the mixing damper arrangement 6 is controlled on the basis of the temperature of air flowing in the ingoing duct 3 at position 13, i.e. the mixing damper arrangement 6 is controlled in order to supply air of a desired temperature to the heating coils 10 and the cooling coil 11. The mixing damper arrangement 6 may advantageously be controlled by adjusting the opening degrees of the first damper 7, the second damper 8 and the third damper 9, thereby controlling the portion of air being recirculated from the outgoing duct 4 to the ingoing duct 3, and thereby the mixture of recirculated air and fresh air in the ingoing duct 3, after the mixing damper arrangement 6.

Furthermore, an ingoing air temperature is measured at an inlet passage 14 from the ingoing duct 3 to the controlled space 2, i.e. downstream relative to the heating coils 10 and the cooling coil 11. Accordingly, the ingoing air temperature represents the temperature of air leaving the heating coils 10 and the cooling coil 11, and being supplied to the controlled space 2, from the ingoing duct 3. This is the temperature which it is desired to control by means of the air handling unit 1. The heating coils 10 and the cooling coil 11 are then controlled in accordance with an ingoing air temperature setpoint value, and in order to obtain an ingoing air temperature which is equal to the ingoing air temperature setpoint value. The heating coils 10 and the cooling coil 11 may advantageously be controlled by activating and deactivating the coils 10, 11, depending on whether an increase or a decrease in the ingoing air temperature is required. Thus, a desired ingoing air temperature is obtained by appropriately controlling the enthalpy wheel 5, the mixing damper arrangement 6, the heating coils 10 and the cooling coil 11. However, each of the enthalpy wheel 5, the mixing damper arrangement 6 and the heating and cooling coils 10, 11 are controlled on the basis of a separate temperature value, i.e. the first intermediate temperature value, the second intermediate temperature value and the ingoing air temperature, respectively. This provides a more stable control of the air handling unit 1 than would be the case if the enthalpy wheel 5, the mixing damper arrangement 6 and the coils 10, 11 were all controlled on the basis of the ingoing air temperature, because in the latter case there is a risk that control of one of the devices 5, 6, 10, 11 interferes with the control of one or more of the other devices 5, 6, 10, 11. The first intermediate temperature setpoint value and the second intermediate temperature setpoint value are selected on the basis of the ingoing air temperature setpoint value.

Thereby it is ensured that the temperature of air supplied to the mixing damper arrangement 6 as well as the temperature of air supplied to the heating coils 10 and the cooling coil 11 is controlled in such a manner that the desired ingoing air temperature can be easily obtained, e.g. with no or only limited activation of the heating coils 10 and/or the cooling coil 11. A calibration process for the first intermediate temperature setpoint value and/or the second intermediate temperature setpoint value, with respect to the ingoing air temperature setpoint value, may be performed, e.g . in the manner described above.

Fig. 2 is a block diagram illustrating a method according to an embodiment of the invention . The method may, e.g ., be used for controlling the air handling unit 1 of Fig. 1.

A temperature, T _r∞m , of air inside a controlled space, in the form of a room, is measured and supplied to a first controller 15 along with a temperature setpoint value, S _r∞m , for the air temperature inside the room. Based thereon the first controller 15 calculates a temperature setpoint value, S for ingoing air being supplied to the room. In the case that the measured temperature, T _room , is lower than the setpoint temperature, S _r∞m , then the temperature of the air inside the room needs to be increased, and therefore an ingoing air temperature setpoint value, S _supp i _y , which is higher than the temperature setpoint value, S _r∞m , is selected, in order to increase the air temperature, T _r∞m . Similarly, in the case that the measured temperature, T _r∞m , is higher than the setpoint temperature, S _r∞m , then the temperature of the air inside the room needs to be decreased, and therefore an ingoing air temperature setpoint value, S _supp i _y , is selected which is lower than the temperature setpoint value, S _r∞m , in order to decrease the air temperature, T _r∞m . Furthermore, in the case that there is a net heat flow in or out of the room vi other channels than the air handling system, e.g . due to people being present in the room, sun shining through windows, heat generating machinery, etc., then such net heat flow may advantageously be taken into account, when the ingoing air temperature setpoint value, S _supp i _y , is selected. For instance, in the case that there is a net heat flow into the room, then the ingoing air temperature setpoint value, S _supp i _y , should always be below the setpoint temperature value, S _r∞m , and in the case that there is a net heat flow out of the room, then the ingoing air temperature setpoint value, S _supp!y , should always be above the setpoint temperature value, S _r∞m .

The ingoing air temperature setpoint value, S _supp!y , is supplied to an adder 16. A bias signal is also supplied to the adder 16. The bias signal provides information regarding a correlation between the ingoing air temperature setpoint value, S _supp!y , and an intermediate temperature of air in an ingoing duct, at a position downstream with respect to a mixing damper arrangement, and upstream with respect to one or more heating coils and/or cooling coils. Thus, the bias signal provides a calibration of the intermediate temperature with respect to the ingoing air temperature.

Based thereon the adder 16 calculates an intermediate air temperature setpoint value and supplies this to a second controller 17. The intermediate temperature, T _mixer , is measured, and is also supplied to the second controller 17. Based on the intermediate temperature setpoint value, provided by the adder 16, and the measured intermediate temperature value, Tmixer, the second controller 17 generates a control signals for a mixing damper arrangement. Thereby the mixing damper arrangement is controlled on the basis of the intermediate temperature value, T _m i _X e _r .

The first controller 15 further supplies the ingoing air temperature setpoint value, S _supp!y , to a third controller 18. The actual ingoing air temperature, T _supp!y , is measured, and is also supplied to the third controller 18. Based on the ingoing air temperature setpoint value, Ssuppiy, and the measured ingoing air temperature value, T _supp i _y , the third controller 18 generates a control signal for one or more heating coils and/or cooling coils. Thereby the heating coil(s) and/or cooling coil(s) is/are controlled on the basis of the ingoing air temperature, T

Accordingly, the mixing damper arrangement and the heating coil(s) and/or cooling coil(s) are controlled using two separate control loops, and on the basis of two different control parameters. Thereby a stable control of the air handling unit is obtained. Yet, controlling the mixing damper arrangement as well as controlling the heating coil(s) and/or cooling coil(s) forms part of controlling the air handling unit in order to obtain a desired ingoing air temperature, T and eventually a desired air temperature, T _room , inside the room.