WO2012011865A2

WO2012011865A2 - Air handling unit with bypass to the rotary heat exchanger

Info

Publication number: WO2012011865A2
Application number: PCT/SE2011/050961
Authority: WO
Inventors: Andrew Lawrenson
Original assignee: Swegon Ab
Priority date: 2010-07-23
Filing date: 2011-07-21
Publication date: 2012-01-26
Also published as: WO2012011865A3

Abstract

A traditional packaged Air handling unit (AHU) with a perpendicular rotary heat exchanger (rotor) design uses excessive fan energy to overcome the pressure drop of the rotary heat exchanger when the full recovery effect of the heat exchanger is not required. The present invention relates to an Air Handling Unit (AHU) comprising at least one rotary heat exchanger (6) tilted at an angle, one or more air inlets (2), air outlets (3), air filters (5), fan enclosures (4), and one or more centrifugal plenum fans (8) producing an air flow through the AHU. The AHU further includes one or more bypass channels (7), with bypass dampers (10) being placed on one or both sides of the rotary heat exchanger (6), said bypass dampers (10) when arranged in an open, partially closed or completely closed position directs the airflow produced by the centrifugal plenum fans (8) to pass through either the bypass channel (7), or the rotary heat exchanger (6), or both. This combination provides an improved fan energy consumption over existing designs with rotary heat exchangers without extending the cross sectional dimensions of the AHU.

Description

AIR HANDLING UNIT WITH BYPASS TO THE ROTARY HEAT EXCHANGER

TECHNICAL FIELD

The present invention relates to ventilation and air conditioning of commercial, industrial and private buildings, in particular it relates to adapted air handling units comprising a heat exchanger with a parallel bypass function that reduces the pressure losses associated with the heat exchanger when full energy recovery is not required.

BACKGROUND OF THE INVENTION

Air handling units (AHU) are used to provide ventilation in buildings. So that the energy in the air being replaced is not lost, AHUs have a heat recovery exchanger to transfer the energy from the extracted air and pass it back into the supply air.

A rotary air-to-air heat exchanger (rotor), otherwise known as heat or enthalpy wheel is a regenerative heat exchanger where a mass made of corrugated channels rotates through airstreams. Adjacent supply and exhaust air streams each flow through each half of the rotor in a counter flow direction. A rotary heat exchanger offers high efficiency heat recovery and is a common choice of heat exchanger in an AHU due to a favourable ratio of heat recovery efficiency to pressure drop ratio. The actual choice of rotor size is a compromise of performance where a larger rotor provides less pressure drop with greater efficiency but also greater capital costs since the required AHU framing and installation space is greater for larger rotors. Life Cycle Costs (LCC) analysis, where the installed investment costs and the energy costs through the units life, is used to determine the optimal solution.

A typical AHU with a rotary heat exchanger that is optimised for LCC has a typical square shape with the largest practical rotor diameter as possible is placed in the square cross section such that the velocities are kept low and the efficiency high. There are many parts of the year when the outside weather is mild and the full recovery of the rotary heat exchanger is not required. During these periods it would be advantageous to bypass part of the flow around the rotor such that the rotor's pressure drop is reduced. A prior art solution is to incorporate the bypass in the AHU at the side of the rotor, which can be done in either of two ways. The first is to reduce the diameter of the rotor freeing space for the bypass channel. This option however, is not optimal in terms of energy consumption since the velocities through the rotor increase, and hence the pressure losses and energy consumption through the rotor also increase. The second prior art method is to extend the width of the unit to accommodate a bypass by the side of the rotor, and is hereinafter referred to in the text as the "extended width bypass" solution. Since one needs to maintain good flow angles when the air flow bypasses the rotor, this design also necessitates that the AHU to be lengthened so that the flow into the open bypass channel and also from the open bypass channel to the fan inlet does not create a flow angle which deviates too greatly from the mean flow direction through the unit as this would create unnecessary flow losses and increase energy consumption and noise. The present invention places a rotor at an angle with a bypass at the side and thereby enables a practical bypass functionality to be incorporated into the unit without the need to extend the width of the unit. The angled rotor of the design in the present invention necessitates a similar AHU length increase as the extended width bypass solution. A further embodiment of the invention is to use a flow guiding structure at the fans inlet. This improves substantially the turbulence and swirl associated with a flow from a partially or fully open bypass flow which is offset from the centreline of the fans inlet, such that the noise from the unit is substantially the same as a prior art unit with a perpendicular rotor of the same type and diameter. US patent 4727931 refers to a rotary heat exchanger that is inclined in a similar way as in the present invention. What is different, however, and which makes the solution of the present invention a much better design in terms of energy performance and noise, is the combined use of the centrifugal plenum fan and further the use of the bypass channel optimisation and a flow guiding (honeycomb) structure at the fans inlet.

SUMMARY OF THE INVENTION

The AHU of the invention will typically provide a balanced decentralised ventilation solution to a building or part thereof as the aim of the present invention is to provide an AHU specifically designed to have an optimised ventilation solution both in terms of "Life Cycle Cost", (LCC): with the outer dimensions of the unit being optimised so as to reduce capital costs; with a heat exchanger bypass means so that fan energy is saved when the full energy recovery is not required; and with the minimum noise levels so that the unit can be used in close proximity to occupied spaces where the customer requires it.

This objective is achieved by combining a centrifugal plenum fan of either backwards curved or airfoil type with a rotary heat exchanger that is tilted at an angle between 18 to 45°, more preferably at an angle of 25 to 40° to the flow direction through the unit in either of the horizontal or vertical planes. Angling the rotor at an angle between 18 and 45^° enables a bypass channel having an area of between 10 to 40% of the original cross sectional rotor area being arranged on at least one side of the rotary heat exchanger without the need to extend the dimensions of the AHU's cross section, neither in the width nor in the height direction. The length of the unit is increased to accommodate the angled rotor, however, this length is not greater than what would be required with the prior art "extended width bypass solution" where the width of the AHU is increased to incorporate a bypass.

Generally the casing of an AHU has the shape of a rectangular box, wherein the air flows through the unit in the longitudinal direction. The AHU of the invention may furthermore typically include one or more air inlets through which air enters the AHU and one or more air outlets wherefrom air exits the AHU. The casing, may further include fan enclosures, one or more air filters covering the air inlets on the inside of the casing. The casing also includes one or more centrifugal plenum fans and optionally one or more flow

straightening structures, all of which will be described in more detail below.

One or more bypass valves or dampeners are located in the one or more bypass channels. The bypass valves or dampeners can be open, partially closed or completely closed depending upon the required amount of bypass flow. With the bypass damper in the fully open or regulated (partly open) position, the airflow, divides with one portion bypassing the rotor through the bypass channel and one portion passing through the rotary heat exchanger. When the rotor is turning, heat is exchanged between air passing through the rotary heat exchanger. Air passing from the bypass channel and the exit of the rotor, is mixed by turbulence as it is drawn into the inlet of a fan and thereafter passes through a centrifugal plenum fan to an air outlet of the AHU. The temperature of the mixed air is determined by the ratio of outside air passing through the bypass channel and the air passing through the rotary heat exchanger. When the bypass damper is in the fully closed position the air flow passes through the rotary heat exchanger only.

It has been found that for the air flow side where the bypass valve is located the furthest away from the fan wall, the distance between the edge of the rotor and the fan wall can be reduced to zero such that the corner of the rotor can make contact with the fan wall. In such a manner the overall increase in length caused by the angling of the rotor is decreased to reduce to cover unit costs. A bypass damper may be made from a bypass blade mounted on a bypass blade shaft. The bypass damper is perpendicular to the air flow direction through the AHU when in the closed position and opens in a clockwise manner so that the damper blade directs the flow towards the inlet of the fan, as in the partially open case position. Fully open the damper blade lies essentially in the same plane as the direction of the air flow, or forms an angle of less than 10° to the direction of the air flow.

To maintain the correct pressure balance over the rotor during regulation and the diversion of the air flow through the bypass channels, both bypass dampers on each air flow sides, will need to open to the same degree. This means that the bypass dampers will need to be connected. In order for the damper blades to rotate correctly, they need to turn in opposite directions to each other. This may for example be achieved by connecting the two blades of the bypass damper with damper blades shafts via two equally sized cog wheels. This means that the dampers for each of the air sides would be connected such that turn simultaneously. The person skilled in the art realizes that there may be other solutions for connecting the damper blades to each other.

The bypass means may be located on only one side of the rotor. However, preferably both bypass dampers, on each air flow side, should be connected together and operated by one motor, thereby the bypass dampers act as guides to the bypass airflow towards the inlet of the fan. Alternatively bypass dampers are placed at both sides of the rotor. The preferred method of opening these would be the same as for the single side bypass design in that they rotate to direct flow towards the inlet of the fan.

As with all the bypass options the inclusion of a bypass leads to increased noise and some instability in the fan operation when the bypass is open. This is due to the offset of the bypass flow from the centre line of the inlet of the fan, which causes turbulence and swirl in the inlet of the fan. The increase in noise levels can be quite substantial.

Advantageously a flow straightening structure may be placed at the inlet to the fan. This has the effect of straightening the swirl in the air flow through the inlet plenum and in reducing the turbulent structures in the flow. The combined effect of this is that the fan noise is significantly reduced. The flow straightening structure may have a honeycomb cell structure. The cell structure can have hydraulic diameters (D_Cen) in the range from 2 to 20mm with a cell length L=2 to 30*D_cen with a maximum length of 0.2 x fan diameter or 100mm, whichever is the shortest. Preferably the honeycomb structure has a cell size of 6.4mm and channel length of 30mm resulting in improvements that are substantial. In summary an AHU design as described above which employs a tilted rotary heat exchanger (rotor) with centrifugal plenum fans and bypass means added in parallel to the rotor, significantly reduces fan energy consumption and results in a unit that has the same cross sectional profile as prior art units with a standard perpendicular rotor placement. The AHU as described above provides a much improved yearly fan power consumption whilst maintaining the cooling and heating energy recovery from the rotor usage and noise levels over the prior art.

The present invention will be described more in detail in following with reference to the attached drawing showing some preferred embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows general views of the invention

Figure 2 shows general views of the prior art AHU without bypass

Figure 3 shows alterative bypass options

Figure 4 shows the extended side bypass prior art solution

Figure 5 shows results of a computation fluid dynamics (CFD) analysis for the invention Figure 6 shows the dimensions used in the performance simulation comparison

Figure 7 shows the flow guiding structure and the sound level improvements of the invention with the use of the flow guiding structure

DETAILED DESCRIPTION OF THE INVENTION

As used herein the term "Life Cycle Cost", (LCC) refers to an estimation of the energy and investment costs and is commonly used to compare different air handling unit solutions. As used herein the term "fan diameter" is largest diameter that the trailing edge of the fan blades at the outlet of the fan. This diameter is commonly used to characterize the size of a centrifugal fan.

As used herein the term "temperature transfer efficiency" is the heat recovery efficiency exchanger and refers only to the temperature recovery (otherwise known as sensible recovery) nt = (t2 - 11 ) / (t3 - 11 ), where qt = temperature transfer efficiency, t1 = temperature outside air before the heat exchanger (^°C), t2 = temperature outside air after the heat exchanger (°C) and t3 = temperature inside air before the heat exchanger (°C) As used herein the term "hydraulic diameter", DH, is a commonly used term when handling flow in noncircular flow channels and gives the equivalent diameter of round tube in terms of resistance to flow. Definition: DH=4A/P, where A is the cross sectional area and P is the wetted perimeter of the cross-section.

As used herein the term "Rotor" is intended to mean Rotary Heat Exchanger (other names: Heat wheel, Enthalpy wheel)

As used herein the term "air handling unit (AHU)" is intended to mean a unit providing balanced supply and extract ventilation which has all the necessary components (i.e. extract and supply fans, heat recovery, filters and control) integrated into one packaged unit. The advantages of the Air Handling Unit (AHU) with angled rotor and bypass invention will now be described in detail with reference to the accompanying drawings. However, the described embodiments mentioned below are only given as examples and should not be limiting to the present invention. Other solutions, uses, objectives, and functions within the scope of the invention as claimed in the below described patent claims should be apparent for the person skilled in the art.

Fig. 1 a shows a general plan view of an AHU of the invention. Generally the casing 1 of an AHU has the shape of a rectangular box with the dimensions length (L) x width (W) x height (H), wherein the air flows through the unit in the longitudinal (L) direction. In the present invention when incorporating the bypass design, the height (H) and width of the casing (W) of the casing remain unchanged compared to a prior art AHU which is shown in a general plan view in Fig 2a. The length (L) however needs to be increased for the bypass invention.

On each short end, the casing 1 includes one or more air inlets 2 through which air enters the AHU and one or more air outlets 3 where from air exits the AHU. The casing 1 , further includes, fan enclosures 4, one or more air filters 5 covering the air inlets 2 on the inside of the casing 1 . The air filters are generally composed of a fibrous material which removes solid particulates such as dust, pollen, mold, and bacteria from the air before it enters the AHU. The casing further comprises at least one rotary heat exchanger 6 (hereafter also referred to as a rotor), one or more bypass channels 7 at one or both sides of the rotor(s). The casing also included one or more centrifugal plenum fans 8 and one or more flow straightening structures 9, all of which will be described in more detail below. A bypass damper or valve 10 is located in the bypass channel 7 which can be open, partially closed or closed depending upon the required amount of bypass flow.

Figure 1 b shows a sectional plane (viewed from A-A in Fig. 1 a) through the centre of the rotor towards one half of the unit. Looking at the air flow from this view, and with the bypass valve in the closed position the air would enter the AHU at the inlet 2, pass through the air filter 5 then through a filter plenum 1 1 and to the rotor 6. Heat is exchanged between air and the rotor as the air passes through channels of the rotor. Upon exiting the rotor the air passes through an inlet plenum 12, through the flow straightening structure 9 to the inlet of the fan 13, whereupon the centrifugal plenum fan 8 increases the delivery pressure of the air and then blows the air out through a fan enclosure 4 and to the AHU's outlet 3.

With the bypass valve 10 in the fully open or regulated (part open) position, the airflow after the air filter 5 divides with one portion bypassing the rotor through the bypass channel 7, show by the arrow marked (ii) and one portion passing through the rotary heat exchanger 6, shown by the arrow marked (i). With the rotor turning heat is exchanged between the air passing through the heat exchanger. The air passing from the bypass channel 7 and the exit of the rotor 6, is mixed by turbulence as it is drawn into the inlet of the fan 13 and passes through the centrifugal plenum fan 8 to the air outlet 3 of the AHU. The temperature of the mixed air is determined by the ratio of outside air passing through the bypass channel 7 and the air passing through the rotary heat exchanger 6. It can be concluded from figure 1 a that this air flow is reversed in the other half of the unit .

Referring to Figure 1 d which shows a sectional plane (viewed from B-B in Fig. 1 a) through the centre of the rotor towards one half of the unit. The air flow enters air inlet 2^' situated at the same short end of the housing 1 ^' as the air outlet 3. The air flows past the air filter 5^' then through the filter plenum 1 1 ^', and divides to pass through the rotor 6 and bypass channel 7' determined by the position of the bypass valve 10', to the inlet plenum 12^', through the flow straightening structure 9^' to the inlet of the fan 13^', past the centrifugal plenum fan 8^', through the fan enclosure 4^' and to the AHU's air outlet 3^', where the air exits the AHU.

There are three possible prior art bypass solutions which are improved upon by this invention which are show in the schematic drawings of Fig 3 taken at section C-C of figure 2b. The first is to use the space between the outer diameter of the rotor and the casing 1 of the unit. In a unit which is optimised for LCC in terms of unit cost and rotor

performance, the rotor 6 would be as large as possible, sitting inside a square profile of the AHU, shown in cross section in fig 3a there would be space for the bypass channel 7 only in the corners. Considering the easiest design solution of inserting round ducts fig 3b then there would be an bypass channel 7 area of approximately 10% of the rotor 7 area. This solution is therefore not optimal as a bypass area of only 10% would not significantly lower the pressure drop across the rotor when the bypass is open.

The second and a prior art solution is to reduce the diameter of the rotor 6 so that a bypass channel 7 can be placed at the side of the rotor 6, fig 3c. This solution has a negative impact of the energy consumption since the increased velocity through the rotor leads to an increase in the pressure losses, and hence fan energy consumption, when the bypass is closed. The third and prior art solution is to leave the rotor 6 diameter the same and to place a bypass channel 7 at the side of the unit as in fig 3d. This bypass solution is referred to as the "extended side bypass". Fig 4 shows schematic views of this solution.

This solution requires that the width of the unit (W) in the immediate region of the rotor 6 is increased together with the length before and after the rotor section to allow for a reasonable flow path from the bypass channel 7. This is the only prior art solution that offers significant energy savings, over the prior art AHU without bypass, since the area of bypass channel 7 can be large enough (over 20% of the rotor 6 area) so as that the pressure drop over the rotor 6 can be reduced significantly (by a factor of 0.5) with the bypass valve 0 fully open. The invention provides similar energy savings to this prior art solution, however, the invention has the benefit of allowing for the bypass design to be incorporated in the existing cross sectional profile of the unit so that the width (W) is unchanged. For the invention the length (L) of the prior art unit needs to be lengthened to accommodate the tilted rotor. However, the additional length required is similar the prior art "extended side bypass" unit when one considers that for this solution one needs to provide a greater distance from the filter 5 to bypass channel 7, and bypass channel 7 to fan inlet 3. As a design rule of thumb, components should be distanced so that the air flow does not make turns steeper than 45^° as a maximum and 30^° as a nominal from the mean flow direction through the unit, parallel to the fans centreline (cl), figs 1 b and 1 e. Turns steeper than this create unnecessary flow losses due to the abrupt flow turns. The space required for the good installation of the rotor between the end of the filter 5 and the fan wall 17 is shown as (xFF) in Fig 4b for the extended side bypass, Fig 1 b for the invention and Fig2b for the prior art AHU without a bypass. By comparing (xFF) one can see that this length is increased similarly for the invention and the extended side bypass over that for the prior art unit without bypass. Considering that the width (W) of the unit is increased with the extended side bypass the overall increase in the envelope of the AHU required by the invention is less than that required for the prior art extended side bypass design when following good design practice in ensuring flow paths that are at nominal flow angles. Another advantage of the invention, Figs 1 e and 1 b, is that the off set of bypass channel 7 to the fan inlets centreline (cl) shown by the distance (xo) is less than that required for the extended side bypass design shown by distance (xo) in Fig 4b which results in less flow disturbance and hence noise at the fan inlet than the prior art extended side bypass design.

In order to determine the relationship of bypass performance versus rotor angles, a series of computation fluid dynamics simulations were made with the bypass valve 10 fully open. Figure 5 shows the results from these simulations of performance of an AHU of the invention with a range of rotor angles for a centrifugal plenum fan of diameter 250mm delivering 500 l/s. The following assumptions where used when creating the simulation model, see figure 6:

(i) Rotor width 200mm (an industry standard width)

(ii) Rotor face diameter 650 mm

(iii) Unit internal height 400 mm

(iv) Extra distance required for practical rotor 6 installation.

(v) Extra thickness of the rotor casing for the inclusion of the rotor sealing device.

(vi) Minimum extra length of the rotor housing across the rotors diameter.

(vii) Possible bypass channel 7 width

The rotor angle (ra) was varied from 5 to 55^° whilst all other dimensions were kept constant such that smaller rotor angles gave larger bypass opening. In fig 5a, Curve (i) shows the variation of the rotor angle (ra), fig 6, with the width (vii) of the bypass channel 7 either expressed as ratio of the internal width of the unit (iii) or as a percentage of the rotor area. Curve (ii) shows the ratio of open bypass pressure drop normalised by the pressure drop across the rotor 8 with the bypass closed against the size of the bypass channel (vii) either expressed as ratio of the internal width of the unit (iii) or as a ratio of the rotor area.

Figure 5b shows the additional pressure losses over the prior art perpendicular rotor of the same type and dimensions that result with decreasing rotor angle (ra) relative to the flow direction through the unit. From fig 5b the rotor angle needs to be above 18^° to avoid excessive pressure losses, over 20% of the rotor pressure drop, when the bypass valve is closed. From fig 5a the rotor angle needs to be less than 45^° to ensure a reasonable sized bypass channel 7 with an area over 15% of the rotor 6 area and providing a pressure reduction ratio less than 0.6 of the rotor pressure drop when the bypass valve is in the fully open position. From this analysis it is considered that the preferable rotor angle (ra) is between 18° to 45° but most preferably between 25° to 40°.

It has been found that for the air flow side where the bypass valve is located the furthest away from the fan wall 17, fig 1 b, the distance marked "x" can be reduced to zero such that the corner of the rotor 6 can make contact with the fan wall 17. In such a manner the overall increase in length causes by the angling of the rotor 6 is decreased to reduce to unit and LCC costs.

One embodiment of the bypass means is shown in figure 1 , with the bypass means located on one side of the rotor. This would allow both bypass dampers to be connected together and operated by one motor. Advantageously the bypass damper 10 acts as a guide to the bypass airflow towards the inlet of the fan. Figs c and 1 d show a close up of the bypass damper 10 which is made from a bypass blade 15 which is mounted on a bypass blade shaft 16. In fig 1 c the bypass damper 10 would be perpendicular to the flow direction through the AHU when in the closed position and would open in a clockwise manner so that the damper blade 15 directs the flow towards the inlet of the fan 13, as in the partially open case position shown in fig 1 d. Fully open the damper blade 15 would be between 80^° to 90^° to the vertical. To maintain the correct pressure balance over the rotor 6 with the bypass valve 10 partially open and fully open regulation both bypass dampers 10 and 10' would need to open to the same degree. This means that the bypass dampers 10 would need to be connected. In order for the damper blades 15 to rotate correctly, they need to turn in opposite directions to each other. This can be achieved by connecting the two blades of the bypass damper 10 with damper blades shafts 16 via two equally sized cog wheels. This means that the dampers for each of the air sides would be connected such that they turn simultaneously. Alternatively bypass dampers 10 can be placed at both sides of the rotor 6. The preferred method of opening these would be the same as for the single side bypass design in that they rotate to direct flow towards the inlet of the fan 13.

As with all the bypass options the inclusion of a bypass leads to increased noise and flow turbulence which leads to some instability in the fan operation when the bypass valve 10 is open. This is due to the offset of the bypass channel 7, from the centre line of the inlet of the fan 8, marked as (xo) in fig 1 b and fig 1e, which causes turbulence and swirl in the inlet of the fan 13. The increase in noise levels can be quite substantial. A flow

straightening structure may be added to the fans inlet as used in the patent application WO 2010085197.

A flow straightening structure 9, fig. 7a, is placed at the fan inlet 13, fig 7b. The flow straightening structure 9 is a matrix of short length passageways parallel to the fans centreline (cl) which allows the air to pass through. This has the effect of straightening the swirl in the air flow through the inlet plenum 12 and in reducing the turbulent structures in the flow. The combined effect of this is that the fan noise is significantly reduced. In a preferred embodiment of the invention the cell structure flow straightening structure has hydraulic diameters (D_cen) in the range from 2 to 20mm with a cell length L=2 to 30^*D_cen with a maximum length of 0.2 x fan diameter or 100mm, whichever is the shortest. In a preferred embodiment of this invention the flow straightening structure 9 is a honeycomb cell structure. Preferably the cell structure has a cell size of 6.4mm fig 7a (i) and cell length of 30mm fig 7a (ii). Fig. 7c shows sound test results for the prototype invention delivering 400 l/s supplying a duct delivery pressure of 200Pa with the bypass damper 10 fully open and sound intensity values measured at a 1 m distance from the air outlet. Fig 7c, curve (i) shows the sound levels without the flow straightening structure 9. Fig 7c, curve (ii) shows significant sound improvements to the sound values when a honeycomb flow straightening structure 9 is added onto the fans inlet 13 having a cell structure with a cell size of 6.4mm fig 7a (i) and cell length of 30mm fig 7a (ii) The AHU as presented herein will typically provide a balanced decentralised ventilation solution to a building or part thereof. The aim is to provide an AHU specifically designed to have an optimised ventilation solution both in terms of LCC: with the outer dimensions of the unit being optimised so as to reduce capital costs; with a heat exchanger bypass means so that fan energy is saved when the full energy recovery is not required. This objective is achieved by combining a centrifugal plenum fan 8 of either backwards curved or airfoil type with a rotary heat exchanger 6 that is tilted at an angle between 18° to 45^° to the flow direction through the unit in either of the horizontal or vertical planes. Angling the rotor at an angle between 18° and 45^° enables a bypass channel 7 area of between 15% to 40% of the original cross sectional area without the need to extend the dimensions neither (W nor H) of the AHU's cross section. The length of the unit is increased to accommodate the angled rotor. For comparison Fig. 2a shows a general plan view of an AHU of the prior art AHU.

Claims

An Air Handling Unit (AHU) comprising at least one rotary heat exchanger (6) tilted at an angle, one or more air inlets (2), air outlets (3), air filters (5), fan enclosures (4), and one or more centrifugal plenum fans (8) producing an air flow through the AHU, characterized in that the AHU further includes one or more bypass channels (7), with bypass dampers (10) being placed on one or both sides of the rotary heat exchanger (6), said bypass dampers (10) when arranged in an open, partially closed or completely closed position directs the airflow produced by the centrifugal plenum fans (8) to pass through either the bypass channel (7), or the rotary heat exchanger (6), or both.

The AHU according to claim 1 , wherein a flow straightening structure (9) is placed at an inlet (13) to the fan (8).

The AHU with bypass according to claim 2, wherein the flow straightening structure (9) has a cell structure with a hydraulic diameter (D_cen) from 2 to 20mm and a cell length (T) of 2 to 30*D_cen, the maximum length of the cell length (T) being 0.2 x fan diameter or 100mm whichever is the shortest.

The AHU with bypass according to claim 2, wherein the flow straightening structure (9) has a honeycomb cell structure (17).

The AHU according to any one of claims 1 -4, wherein the at least one rotary heat exchanger (6) is tilted at an angle of between 18 to 45° to the air flow direction through the unit.

The AHU according to any one of claims 1 -4, wherein the at least one rotary heat exchanger (6) is tilted at an angle of between 25 to 40° to the air flow direction through the unit.