COOLING OIL

Background of the invention

The invention relates to an oil cooler comprising a first oil cooler part having a first part cooling capacity and a second oil cooler part having a second part cooling capacity. The invention also relates to a wind turbine comprising an oil cooler and a method for cooling oil by way of an oil cooler.

Description of the Related Art

It is known to cool cooling liquids by means of ambient air because the ambient air provides a free and readily available cooling source. And particularly in relation to wind turbines it is known to cool cooling liquids by guiding the liquid through a cooler (also sometime referred to as a radiator, a heat exchanger, heat sink or other) placed outside the nacelle of the wind turbine, in that when heat generating components in the wind turbine is strained the most - because of high wind speeds - the more cooling effect the wind will have on the cooler.

An external passive cooler is normally designed to warm ambient conditions and relatively low wind speed, since this is the conditions, where it requires the largest cooler area to remove the heat from the cooling liquid. However, a consequence is that at lower temperatures and higher wind speeds, the cooling capacity of the cooler is much higher. At cold ambient temperatures, for example below 0° C and high wind speeds (e.g., more than 11 m/s) the cooler will be so efficient that the temperature of the cooling liquid sent through the cooler will be reduced to ambient temperature.

It is therefore not possible to cool e.g., the gearbox oil of a wind turbine gearbox by running it directly through an external passive oil cooler in that oil reduced to temperatures below e.g., 0° C to -10° C, will be very viscous and therefore have great difficulties in flowing through the cooler. At temperatures below e.g., -20° C, the oil can get so viscous that the cooler clogs completely.

Thus, from the US patent US 7,832,467 B2 it is known to form an oil cooler with a bypass tube and a blocking element arranged to enable flow through the bypass tube if the oil clogs the cooler or the temperature of the oil drops below a certain level. However, this cooler design is not particularly cost-efficient.

An object of the invention is therefore to provide for a more cost-efficient oil cooling technique and provide a wind turbine with a more cost-efficient oil cooling technique.

The invention

The invention provides for an oil cooler comprising a first oil cooler part having a first part cooling capacity and a second oil cooler part having a second part cooling capacity, wherein the second part cooling capacity is greater than the first part cooling capacity. The oil cooler also comprises a bypass conduit arranged to guide oil from the first oil cooler part past the second oil cooler part, and valve means arranged to control flow through the bypass conduit, wherein the valve means are controlled based on at least one characteristic of oil flowing through the oil cooler.

Providing the cooler with valve means controlling the flow through a bypass conduit directing the oil from the first oil cooler part around the second oil cooler part in response to one or more qualities of the oil flow is advantageous in that it is hereby possible the adjust the overall cooling capacity of the oil cooler in a simple manner, and thereby adapt the overall cooling capacity of the oil cooler to specific conditions - such as wind speed, ambient temperature, wind direction or other - in a simple and effective manner. Forming the cooler with a first oil cooler part and a second oil cooler part so that the cooling capacity of the second oil cooler part is higher than the cooling capacity of the first part cooling capacity is advantageous in that the oil cooler hereby becomes more versatile in that the first oil cooler part can be designed for cold ambient temperature operation only and therefore has to have a relatively low cooling capacity. The second oil cooler part however, can be designed to operate in higher ambient temperatures and therefore has to have a relatively high cooling capacity so that the oil cooler in more efficient over the entire ambient temperature range at which the oil cooler operates.

Furthermore, the present oil cooler design ensures that the first oil cooler part is not bypassed when ambient conditions become warmer and can hence be added to the complete cooler area for warm ambient conditions, when heat rejection is critical whereby the cooler becomes more cost-efficient.

It should be emphasised that by the term ''cooler'' is to be understood as any kind of arrangement capable of cooling oil - such as any kind of radiator - e.g. an oil-air convective cooling radiator -, a heat exchanger - such as an oil-air heat exchanger - or a similar arrangement. The cooler may or may not comprise a fan or a similar arrangement arranged to generate or increase the flow through the cooler of the medium to which the oil delivers heat when passing through the cooler.

Furthermore, in this context the term “valve means’" is to be understood as any kind of valve capable of controlling the flow through a bypass conduit in an oil cooler. I.e., the term includes any kind of spring-loaded valve, motor actuated valve, thermo actuated valve, pilot pressure or pressure actuated valve or any other kind of valve.

In an aspect of the invention, the valve means comprise a spring-loaded valve. A spring-loaded valve - also called a spring check valve - is a simple and effective way to form a mechanical valve which will enable flow through the bypass conduit when the pressure on the valve exceeds the closing force of the spring.

In an aspect of the invention, the valve means comprise a motor actuated valve, a thermo actuated valve or a pilot pressure actuated valve.

A motor actuated valve enables more precise control of the valve e.g. in response to a temperature sensor, a pressure sensor or other detecting properties of the oil being cooled in the oil cooler.

A thermo actuated valve is advantageous in that it enables precise operation in response to the temperature of the oil at the valve without the need of additional sensors.

A pilot pressure actuated valve is advantageous in that it enables precise operation of the valve according to the pressure of the oil at the valve or distant to the valve without the need of pressure sensors or the like.

In an aspect of the invention, the characteristic of the oil flowing through the oil cooler includes a temperature of the oil.

Arranging the valve means to control flow through the bypass conduit based on the temperature of the oil in the oil cooler is advantageous in that the lower the temperature of the oil becomes the more viscous the oil becomes - i.e. the risk of reduced flow or even clogging increases. And the point of the cooler is to cool the oil down below a specific temperature and if this temperature has already been reached after passing the oil though the first oil cooler part there is no point in leading it through the second oil cooler part. In an aspect of the invention, the characteristic of the oil flowing through the oil cooler includes a pressure of the oil.

If the ambient temperatures are too low, the oil becomes too viscous to flow freely through the second oil cooler part and the pressure inside the oil cooler will thereby increase. Thus, it is advantageous to control flow through the bypass conduit based on the pressure of the oil inside the oil cooler.

In an aspect of the invention, the characteristic of the oil flowing through the oil cooler includes a viscosity of the oil.

If the oil becomes too viscous it cannot flow freely through the second oil cooler part and the efficiency of the oil cooler is reduced and the risk of clogging increases. Thus, it is advantageous to control flow through the bypass conduit based on the viscosity of the oil inside the oil cooler.

In an aspect of the invention, the valve means are arranged to enable flow through the bypass conduit if a pressure inside the oil cooler is between 1.2 - 30 Bar, preferably between 1.4 - 20 Bar, and most preferred between 1.6 - 10 Bar.

If the pressure at which the valve means will enable flow through the bypass conduit is too low the oil is not sufficiently cooled and if the pressure is too high the oil is cooled too much and the oil risks clogging the oil cooler. Thus, the present pressure limits present advantageous limits in relation to efficiency of the cooler.

In an aspect of the invention, the second part cooling capacity is greater than the first part cooling capacity in that first part cooling channels through the first oil cooler part are shorter than second part cooling channels through the second oil cooler part. Forming the cooling channels - through which the oil flows through the oil cooler parts - shorter in the first oil cooler part than in the second oil cooler part is a simple and efficient way of ensuring that the second part cooling capacity is greater than the first part cooling capacity. Furthermore, this enables that the first part cooling channels second part cooling channels and the second part cooling channels can be formed by the same tubing, pipes or similar which enables low manufacturing costs.

In an aspect of the invention, the second part cooling capacity is greater than the first part cooling capacity in that the smallest cross-sectional area of first part cooling channels of the first oil cooler part is bigger than the smallest cross-sectional area of second part cooling channels of the second oil cooler part.

Forming the smallest cross-sectional area of the first part cooling channels bigger than the smallest cross-sectional area of the second part cooling channels is a simple and efficient way of ensuring that the second part cooling capacity is greater than the first part cooling capacity. Furthermore, the larger cross-sectional area of the first part cooling channels reduces the risk of the oil clogging in the first oil cooler part in extreme conditions.

In an aspect of the invention, the first oil cooler part and the second oil cooler part are formed as a single contiguous unit.

Forming the first oil cooler part and the second oil cooler part as a single contiguous coherent unit is advantageous in that this enables easy handling and fitting of the oil cooler.

In an aspect of the invention, the first oil cooler part and the second oil cooler part are connected by a common oil conduit arranged so that oil flowing through the first oil cooler part is exciting in the common oil conduit and so that oil entering the second oil cooler part is entering from the common oil conduit. Forming the oil cooler with a common oil conduit arranged between the first oil cooler part and the second oil cooler part is advantageous in that this enables a simple, compact, and efficient oil cooler design.

In an aspect of the invention, the bypass conduit is fluidly connected to the common oil conduit.

Connecting the bypass conduit to the common oil conduit is a simple and efficient way of ensuring that the bypass conduit can guide the oil past the second oil cooler part when needed.

In an aspect of the invention, the smallest cross-sectional area of the bypass conduit is greater than the smallest cross-sectional area of second part cooling channels of the second oil cooler part.

Forming the cross-sectional area of the bypass conduit greater than the cross- sectional area of the second part cooling channels of the second oil cooler part is advantageous in that it hereby is ensured that the oil can flow more freely through the bypass conduit when the viscosity of the oil hinders free easy flow through the second oil cooler part. Thereby is an efficient oil cooler design ensured.

Further, the invention provides for wind turbine comprising an oil cooler according to any of the previously discussed oil coolers, wherein a wind turbine gearbox is arranged inside a nacelle of the wind turbine and wherein the oil cooler is arranged to cool oil from the wind turbine gearbox by means of air ambient to the nacelle.

Using the oil cooler according to the present invention for cooling gearbox oil in a wind turbine is advantageous in that the oil of a wind turbine gearbox needs much cooling and in a wind turbine the wind outside the nacelle is a readily available and efficient cooling source. Furthermore, the higher the wind speed, the more cooling is needed and the more efficient the oil cooler will cool the oil when using ambient air.

The invention also provides for a method for cooling oil by way of an oil cooler, the method comprising the steps of:

• guiding oil through a first oil cooler part of the oil cooler, the first oil cooler part having a first part cooling capacity,

• guiding oil exiting the first oil cooler part to a second oil cooler part of the oil cooler, the second oil cooler part having a second part cooling capacity, wherein the second part cooling capacity is greater than the first part cooling capacity, and

• controlling flow through a bypass conduit by way of valve means based on at least one characteristic of the oil flowing through the oil cooler, wherein the bypass conduit is guiding at least a part of the oil exiting the first oil cooler part past the second oil cooler part.

Forming the oil cooler so that the oil to be cooled is first guided through a first oil cooler part having a relatively low cooling capacity and through the bypass conduit past the second oil cooler part is advantageous in that the oil cooler is hereby capable of cooling the oil sufficiently even in very low ambient temperatures without risking that the oil clogs the oil cooler. And by guiding the oil also through the second oil cooler part - having a relatively higher cooling capacity - it is ensured that the oil cooler is capable of cooling the oil sufficiently even in high ambient temperatures. And by controlling flow through the bypass conduit by means of valve means based on at least one characteristic of the oil, the most efficient cooling of the oil is achieved without risking clogging of the oil cooler in that the valve means can be arranged to enable flow based on oil temperature, oil pressure and/or other. Hereby is also ensured that the cooling capacity of the first oil cooler part is utilised at high ambient temperatures to enable a cost-efficient method.

In an aspect of the invention, the flow through the bypass conduit is controlled based on a temperature of the oil.

Operating the oil cooler so that the flow through the bypass conduit is controlled by the valve means based on the temperature of the oil is advantageous in that it hereby is possible to guide at least some of the oil through the bypass conduit if the temperature of the oil is below a certain predefined level after having passed through the first oil cooler part. Hereby the overall efficiency of the oil cooler is increased.

In an aspect of the invention, the flow through the bypass conduit is controlled based on a pressure of the oil.

Operating the oil cooler so that the flow through the bypass conduit is controlled by the valve means based on the pressure of the oil is advantageous in that it hereby is possible to guide at least some of the oil through the bypass conduit if the pressure of the oil at or near the entrance of the second oil cooler part is above a certain predefined level. Hereby the overall efficiency of the oil cooler is increased.

In an aspect of the invention, the flow through the bypass conduit is controlled based on a viscosity of the oil.

Operating the oil cooler so that the flow through the bypass conduit is controlled by the valve means based on the viscosity of the oil is advantageous in that it hereby is possible to guide at least some of the oil through the bypass conduit if the viscosity of the oil at or near the entrance of the second oil cooler part is above a certain predefined level. Hereby the overall efficiency of the oil cooler is increased. In an aspect of the invention, the valve means enable flow through the bypass conduit if a pressure inside the oil cooler is between 1.2 - 30 Bar, preferably between 1.4 - 20 Bar, and most preferred between 1.6 - 10 Bar.

If the pressure at which the valve means will enable flow through the bypass conduit is too low, the oil is not sufficiently cooled and if the pressure is too high the oil is cooled too much and the oil risks clogging the oil cooler. Thus, the present pressure limits present advantageous limits in relation to efficiency of the cooler.

In an aspect of the invention, the oil cooler is passively cooled.

Passively cooling the oil - i.e. by allowing unforced air to flow freely through the cooler to exchange heat with the oil without the air being forced by a fan, ventilator or similar - is advantageous in that this enables a simple and inexpensive cooling process.

In an aspect of the invention, the previously discussed method is for cooling oil by way of an oil cooler according to any of the previously discussed oil coolers.

Hereby is achieved an advantageous embodiment of the invention.

Figures

The invention will be described in the following with reference to the figures in which fig. 1 illustrates a large modem wind turbine as known in the art, fig. 2 illustrates a simplified cross section of a nacelle comprising an oil cooler, as seen from the side, fig. 3 illustrates an oil cooler with oil flowing through the first and second oil cooler parts, as seen from the front, fig. 4 illustrates an oil cooler with oil flowing through the first and second oil cooler parts and a bypass conduit, as seen from the front, fig. 5 illustrates an oil cooler with oil flowing through the first oil cooler part and a bypass conduit, as seen from the front, fig. 6 illustrates an oil cooler with a motor actuated valve, as seen from the front, fig. 7 illustrates an oil cooler with a pilot pressure actuated valve, as seen from the front, and fig. 8 illustrates cooling channels of an oil cooler, as seen from the front.

Detailed description of related art

Fig. 1 illustrates a large modern wind turbine 1 as known in the art, comprising a tower 2 and a wind turbine nacelle 3 positioned on top of the tower 2. The wind turbine rotor 4 comprises three wind turbine blades 5 mounted on a common hub 6 which is connected to the nacelle 3 through the low speed shaft extending out of the nacelle 3 front. In another embodiment the wind turbine rotor 4 could comprise another number of blades 5 such as one, two, four, five or more. Fig. 2 illustrates a simplified cross section of a nacelle 3 of a wind turbine 1, as seen from the side. Nacelles 3 exist in a multitude of variations and configurations but in most cases the drive train in the nacelle 3 almost always comprises one or more of the following components: a gearbox 15, a coupling (not shown), some sort of breaking system 16 and a generator 17. A nacelle 3 of a modem wind turbine 1 can also include a converter 18 (also called an inverter) and additional peripheral equipment such as further power handling equipment, control cabinets, hydraulic systems, and more.

The weight of the entire nacelle 3 including the nacelle components 15, 16, 17, 18 is in this embodiment carried by a nacelle structure 19. The components 15, 16, 17, 18 are usually placed on and/or connected to this common load carrying nacelle structure 19.

Most of the components in the nacelle 3 are heat generating components 6 in that they are electrically and/or mechanically active at least at some time during idling or normal operation of the wind turbine 1. In this embodiment the heat generating components 6 are the gearbox 15, generator 17, electrical power handling equipment such as the converter 18 and control cabinets (not shown) but in another embodiment the heat generating components 6 could further include bearings, lubrication systems, yaw or pitch motors and other motors.

In this embodiment the gearbox oil is cooled by a cooling circuit comprising an oil cooler 7 - to be discussed in detail in the following - placed outside on top of the nacelle 3 so that the air may flow freely through the oil cooler 7 to passively cool the oil flowing through the oil cooler 7. However, in another embodiment the system could also comprise a fan located in front of the oil cooler 7 to actively cool the oil cooler 7. Also, in another embodiment the oil cooler 7 could be partly or entirely placed inside the nacelle 3 so that ambient ait would be guided through the oil cooler 7 inside the nacelle 3. The wind turbine 1 is provided with a yaw arrangement ensuring that the rotor 4 is always facing the wind during normal operation of the wind turbine 1 and by placing the oil cooler 7 outside the nacelle 3 facing the rotor 4, the oil cooler will also always be facing the wind, thus ensuring efficient cooling of the oil cooler 7. In this embodiment the circuit further comprises a pump 21 for circulating the oil in the cooling circuit between the oil cooler 7 and the gearbox 15.

In another embodiment the oil cooler 7 could also or instead be used for cooling insulating oil in the generator 17, the converter 18, in a transformer (not shown) or other electrical power handling equipment or heat generating equipment in the wind turbine 1 or the oil cooler 7 could be used for other purposes such as cooling motor oil in a combustible engine, for cooling oil in gearboxes used for other purposes or in relation to any other system where cooling of oil is needed.

Fig. 3 illustrates an oil cooler 7 with oil flowing through the first and second oil cooler parts 8, 9, as seen from the front. The oil flow path is illustrated by the arrows.

In this embodiment the oil cooler 7 comprises a first oil cooler part 8 and a second oil cooler part 9 arranged on top of each other and separated by a common oil conduit 20. I.e. in this embodiment the oil enters the oil cooler through an oil inlet 22 from where it flows down through the first part cooling channels 13 of the first oil cooler part 8 down to the common oil conduit 20 from where it flows down through second part cooling channels 14 of the second oil cooler part 9 until it finally exits the oil cooler 7 through an oil outlet 23. I.e. in this embodiment the oil is arranged to flow down through the oil cooler 7 during normal operation but in another embodiment the oil cooler 7 could be arranged to make the oil flow upwards, sideways, in any other direction or any combination thereof through the first oil cooler part 8 and/or the second oil cooler part 9 during normal operation of the oil cooler 7.

In this embodiment the common oil conduit 20 is an integrated chamber formed between the exit of the first oil cooler part 8 and the entrance of second oil cooler part 9 but in another embodiment the common oil conduit 20 could be formed differently by e.g. comprising an oil reservoir (not shown), by being formed by one or more hoses or tubes or other.

In this embodiment the first part cooling channels 13 has a first part cooling capacity and the second oil cooler part 9 has a second part cooling capacity and in this embodiment the second part cooling capacity is greater than the first part cooling capacity in that the second part cooling channels 14 are substantially longer than the first part cooling channels 13. Given that the number of first part cooling channels 13 and second part cooling channels 14 is the same and the first part cooling channels 13 and second part cooling channels 14 are identical in design (same cross-sectional area, same turbulator design etc.) the second part cooling capacity is approximately 3.5 times greater than the first part cooling capacity, in that the second part cooling channels 14 are substantially 3.5 times longer than the first part cooling channels 13. However, in another embodiment the second part cooling capacity would only be 1.1, 1.5, 2, 2.5, or 3 times greater than the first part cooling capacity or the second part cooling capacity would be 4, 5, 6, 7 or 10 times greater than the first part cooling capacity e.g., depending on the specific oil to be cooled, the specific use and/or location of the oil cooler, or other considerations. However, as will be discussed in the following the relatively larger cooling capacity of the second oil cooler part 9 in relation to the first oil cooler part 8 can also or instead be obtained by other means.

In this embodiment the oil cooler 7 is also provided with a bypass conduit 10 arranged to guide oil exiting the first oil cooler part 8 past the second oil cooler part 9 under certain circumstances to be discussed in the following. In this embodiment the bypass conduit 10 is fluidly connected to the common oil conduit 20 but in another embodiment the oil cooler 7 would be formed without a common oil conduit 20 and the bypass conduit 10 could then be connected directly to the exit of the first oil cooler part 8, to the entrance of the second oil cooler part 9 or another location between the first oil cooler part 8 and the second oil cooler part 9 e.g. by means of one or more hoses or tubes.

In this embodiment the valve means 11, in the form of a spring-loaded check valve 12, is arranged in the bypass conduit 10 to control flow through the bypass conduit 10. In this embodiment the spring-loaded check valve 12 is provided with a predefined setting so that the spring-loaded check valve 12 will start opening if the pressure in front of the valve means 11 exceeds 2 Bar. Thus, if the pressure pressure drop across the second oil cooler part 9 is higher than 2 Bar, the spring-loaded check valve 12 will open and allow flow of oil through the bypass conduit 10 and directly down to the oil outlet 23 past the second oil cooler part 9. However, in another embodiment the valve means 11 could be arranged to start opening when the pressure is lower, such as when the pressure exceeds 1.8, 1.5, 1.3 Bar or even lower or the valve means 11 could be arranged to start opening when the pressure is higher, such as when the pressure exceeds 3, 5, 7, 10 Bar or even higher. Thus, in this embodiment the valve means 11 are controlled based on one characteristic of oil flowing through the oil cooler 7 in the form of the oil pressure in front of the valve means 11. However, in another embodiment the characteristic of oil flowing through the oil cooler 7 based on which the valve means 11 are controlled could also or instead include an oil temperature, an oil viscosity, flow speed of the oil through the first oil cooler part 8 and/or other.

Fig. 4 illustrates an oil cooler 7 with oil flowing through the first and second oil cooler parts 8, 9 and a bypass conduit 10, as seen from the front.

In this embodiment the characteristic of oil flowing through the oil cooler 7 has changed in relation to the embodiment disclosed in fig. 3 so that the valve means 11 has opened and allowed at least some of the oil to run through the bypass conduit 10. I.e. in this embodiment the ambient temperature has dropped and/or the wind speed has increased so that the viscosity of the oil running through the second oil cooler parts 9 has also increased whereby the pressure in front of the valve means 11 has increased to push the valve means 11 open.

Fig. 5 illustrates an oil cooler 7 with oil flowing through the first oil cooler part 8 and a bypass conduit 10, as seen from the front.

In this embodiment the ambient temperature has dropped even further and/or the wind speed has increased even further so that the viscosity of the oil after having passed through the first oil cooler part 8 has increased so much that it cannot pass through the cooling channels 14 of the second oil cooler parts 9 or at least so much that the oil will flow more freely through the bypass conduit 10. The pressure in front of the valve means 11 has therefore increased considerably to push the valve means 11 fully open. And since the cross-sectional area of the bypass conduit 10 is considerably bigger than the cross-sectional area of each of the second part cooling channels 11 of the second oil cooler part 9 the oil can still flow through the bypass conduit 10 and out of the oil cooler 7 through the oil outlet 23 even though the oil is very viscous.

Fig. 6 illustrates an oil cooler 7 with a motor actuated valve 24, as seen from the front.

In this embodiment the valve means 11 are formed as a motor actuated valve 24 operating according to input from a sensor 26, which in this case is located at the end of the bypass conduit 10. However, in another embodiment the valve means 11 could be connected to more than one sensors 26 - such as two, three, four or even more - and/or the sensor 26 could be located differently in the oil cooler 7 - such as another location in the bypass conduit 10, it could be integrated with the valve means 11, it could be located in the first oil cooler part 8 or in the second oil cooler parts 9 or another location. In this embodiment the sensor 26 is a pressure sensor but in another embodiment the sensor 26 could also or instead be a temperature sensor, a viscosity sensor, a flow speed sensor or other.

Fig. 7 illustrates an oil cooler 7 with a pilot pressure actuated valve 25, as seen from the front.

In this embodiment the valve means 11 are formed as a pilot pressure actuated valve 25 so that the valve means 11 is operated in response to a pilot pressure at the end of the bypass conduit 10. However, in another embodiment the pilot pressure line could be connected to another place in the oil cooler 7 - such as another place in the bypass conduit 10, right in front of the valve means 11, it could be located in the first oil cooler part 8 or in the second oil cooler parts 9 or another location.

However, in another embodiment the valve means 11 could also be formed as a thermo actuated valve (not shown) or another type of valve suited for enabling flow through the bypass conduit 10 in response to at least one characteristic of the oil flowing through the oil cooler 7.

In all the embodiments disclosed in figs. 3-7 the first oil cooler part 8, the common oil conduit 20, the bypass conduit 10, and the second oil cooler part 9 are formed as a single contiguous and coherent unit in that all the parts are interconnected to form a single unit that can easily be moved around and mounted. However, in another embodiment the bypass conduit 10 could also or instead be formed as a pipe or a hose guiding the bypass oil directly to an oil reservoir (not shown), to the oil outlet 23 or other and/or the first oil cooler part 8 and the second oil cooler part 9 could be arranged separate from each other and then be connected by the common oil conduit 20 which in turn could at least partly be formed by a pipe or a hose. Fig. 8 illustrates cooling channels 13, 14 of an oil cooler 7, as seen from the front.

Note, that fig. 8 only discloses a cut out portion of the oil cooler 7.

In this embodiment the second part cooling capacity is greater than the first part cooling capacity in that the smallest cross-sectional area of first part cooling channels 13 of the first oil cooler part 8 is bigger than the smallest cross-sectional area of second part cooling channels 14 of the second oil cooler part 9 whereby the pressure drop across the first oil cooler part 8 is considerably lower than the pressure drop across the second oil cooler part 9 even if the flow capacity of the first oil cooler part 8 is the same as the flow capacity of the second oil cooler part 9. However, in another embodiment the first part cooling channels 13 and/or the second part cooling channels 14 could also or instead be provided with internal turbulators (not shown) that will turn laminar flow into turbulent flow and thereby also increase the heat exchange with the passing air outside the cooling channels 13, 14. Thus, by only providing turbulators in the second part cooling channels 14, the second part cooling capacity can be greater than the first part cooling capacity without otherwise changing the design of the cooling channels 13, 14. And/or turbulators in the first part cooling channels 13 could be formed different from the turbulators in the second part cooling channels 14 e.g. by forming the turbulators in the first part cooling channels 13 thinner or having a larger pitch or other which would reduce the pressure drop through the first oil cooler part 8 compared to the second oil cooler part 9. And/or the difference in cooling capacity could be enabled through differences in the heat sink design on the outside of the cooling channels 13, 14.

The invention has been exemplified above with reference to specific examples of first and second oil cooler parts 8, 9, valve means 11, cooling channels 13, 14 and other. However, it should be understood that the invention is not limited to the particular examples described above but may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims. List

1. Wind turbine

2. Tower

3. Nacelle

4. Rotor

5. Blade

6. Heat generating component

7. Oil cooler

8. First oil cooler part

9. Second oil cooler part

10. Bypass conduit

11. Valve means

12. Spring-loaded valve

13. First part cooling channels

14. Second part cooling channels

15. Gearbox

16. Braking system

17. Generator

18. Converter

19. Nacelle structure

20. Oil conduit

21. Pump

22. Oil inlet

23. Oil outlet

24. Motor actuated valve

25. Pilot pressure actuated valve

26. Sensor