GB2358464A

GB2358464A - A cooling apparatus

Info

Publication number: GB2358464A
Application number: GB0101587A
Authority: GB
Inventors: Nicholas Foley; Gerard Macmanus; Bruce Fryers; Michael Tate
Original assignee: 3Com Corp
Current assignee: 3Com Corp
Priority date: 2000-01-21
Filing date: 2001-01-22
Publication date: 2001-07-25
Also published as: GB0001442D0; WO2001054169A2; GB0101587D0; GB2358463A; AU6713300A

Abstract

A cooling apparatus for cooling electrical devices W,X,Y,Z using a flow of coolant comprises a duct 35, 37 through which the coolant flows past the devices W, X, Y, Z. The duct 35, 37 includes a first inlet 45, 45' for a first stream of coolant and a second inlet 41, 43 for a second stream of coolant downstream of the first inlet 45, 45'. The cooling apparatus also has a deflector 26 downstream of the second inlet 41, 43. In an alternative embodiment the cooling apparatus comprises a first inlet for a first stream of coolant, a second inlet downstream of the first inlet for a second stream of coolant, wherein the width of the second inlet is less than 50% of the width of the duct at the second inlet. In a further embodiment the cooling apparatus comprises a baffle in a duct for restricting the flow of a first stream of coolant past a device in the duct and a channel for transporting a second stream of coolant past the device and into the duct. In another embodiment the cooling apparatus comprises a channel for transporting a flow of coolant, the channel having an outlet, such that the outlet is adjacent an electrical device.

Description

2358464 COOLING APPARATUS The present invention relates to cooling

apparatus, in particular, but not exclusively, cooling apparatus for cooling electrical or other devices. The invention further relates to electrical equipment comprising the cooling apparatus, and a method of cooling equipment.

The invention relates more particularly to electrical devices which perform a processing or memory function, such as any processor (for example a semiconductor or other logic device, an integrated circuit, a microprocessor and the like) or any storage device (for example a mass storage device). Such devices may be in the form of an integrated circuit, possibly mounted on a printed circuit board. Accordingly, the electrical equipment may typically be a computer, modem, switch, hub or like device.

Electrical equipment such as a computer includes various electrical devices which generate heat during use. During use, such devices become warm. It is undesirable for the device to become too hot because excessive heat can cause damage to components of the device. It is known to cool electrical devices using one or more fans and indeed a cooling fan is often provided in electrical equipment. In a conventional arrangement, electrical devices are arranged within a casing. The fan sucks air through one or more vents in the casing, the air passes over the devices and. out of the casing via the fan.

However, under many conditions, such arrangements have been found to be inefficient.

One problem of such arrangements is that the air which is sucked through the casing is heated as it passes over the electrical devices and the air reaching devices downstream can be considerably hotter than that at the inlet. Thus there is less cooling of devices downstream than those upstream. Previously, in an attempt to overcome that problem, the arrangement of devices in the apparatus could be designed so that those devices requiring the greatest cooling were relatively close to the inlet. Otherwise, a high fan speed was chosen so that sufficient cooling was obtained downstream, allowing for the increase in air temperature.

It is an object of the present invention to improve the cooling of the electrical device and/or to improve the efficiency of the cooling. One benefit of improving the efficiency of the cooling is that smaller and/or fewer fans could be used to obtain the desired cooling effect.

Furthermore, it has been realised pursuant to the present invention that, in many situations, some of the devices in electrical equipment require more cooling than other devices. In order to cool such devices sufficiently, a high air flow through the equipment is required. Other devices which require less or no cooling are also subject to the high air flow unnecessarily.

According to a broad aspect of the present invention, there is provided cooling apparatus for cooling an electrical -device using a flow of coolant, the apparatus comprising a duct for transporting the coolant past the device, the duct including a first inlet for a first stream of coolant, the duct further including a second inlet for a second stream of coolant.

In preferred embodiments of the invention, the second inlet is arranged such that it is downstream from the first inlet.

The first stream of coolant therefore passes through the duct and cools one or more devices located in the duct. As the first stream passes along the duct, however, its temperature increases as heat is transferred from the device(s). At the second inlet, an additional stream of coolant is introduced into the duct. Thus the rate of coolant flow downstream from the second inlet may be increased, the increased rate of flow giving greater cooling of the devices downstream of the second inlet than would have been obtainable by the first stream of coolant alone. Furthermore, the rate of flow of the first stream of coolant may therefore be decreased compared with an arrangement in which no second stream is introduced into the duct. Where the coolant is air and the air flow is effected using one or more fans, the capacity of the fans and/or the number of fans may therefore be reduced.

it is envisaged that part of the coolant flow could be removed from the duct downstream of the second inlet, but to improve the cooling downstream, preferably the first and second streams of coolant remain in the duct downstream of the second inlet.

According to an aspect of the invention, there is provided a cooling apparatus for cooling electrical devices using a flow of coolant, the apparatus comprising a duct for transporting the coolant past a device, the duct including a first inlet for a first stream of coolant and a second inlet, downstream of the first inlet for a second stream of coolant and a deflector downstream of the second inlet for directing the second stream of coolant.

By providing a deflector, the coolant entering the duct through the second inlet can be directed so as to give improved cooling of the device or devices. In preferred embodiments, the deflector is arranged to direct the second stream onto the surface of a device in the duct, preferably onto a hot area of the device. Thus better cooling can be achieved.

Preferably the deflector comprises a deflector surface for directing the second stream, the angle of the surface to the coolant flow and/or an upper surface of the device being from 30" to 600, preferably from 30' to 50", preferably about 45". By choosing the deflector angle, the cooling may be improved.

Preferably the deflector extends downstream of the second inlet. Thus the deflector may provide a surface of the duct. Preferably the deflector comprises formations for modifg the flow in the duct. For example, the shape of the deflector may be used to vary the cross sectional area or other feature of the duct, to improve cooling.

Preferably the apparatus further comprises a channel for transporting the second stream of coolant to the second inlet. In preferred embodiments, the second inlet is fed by a channel which extends to the outside of the apparatus.

Preferably, the second inlet is at the downstream end of the channel and preferably the deflector is spaced from the downstream end of the channel.

Preferably the channel has only one outlet to the duct. This can greatly improve the characteristics of the coolant flow in the apparatus and thus improve cooling. In embodiments described below, a channel supplies one second stream to each of two ducts.

The relative heights of the channel and the duct or ducts serving them will be chosen to provide the most desirable coolant flow having regard to other considerations, for example the allowed size of the coolant apparatus in the electrical equ ipment. Preferably, the height of the channel is between about 20 % to 60 %, preferably 3 0 % to 50 %, more preferably about 40 % of the height of the channel plus the height of the duct or ducts served by the channel. In a preferred embodiment described below, the channel is 40.8 % of the total height of the channel plus two ducts (including the wall thicknesses).

Where the channel is supplying more than one inlet, the deflector may also split the coolant stream into separate streams for the inlets.

Preferably the second stream of coolant entering the duct at the second inlet has a temperature which is less than that of the first coolant stream at that point. Thus the second stream can have the benefit of reducing the temperature of the coolant flow downstream from the second inlet, and thus improving cooling of devices downstream.

in a preferred embodiment of the invention, the second stream of coolant is at ambient temperature. Preferably the temperature of the second fluid stream at the second inlet is about the same as the temperature of the first fluid stream at the first inlet. Preferably the first and second fluid streams come from the same source.

In preferred embodiments of the invention, the coolant is air and a fan is used to move the air through the duct. Preferably the air is taken from outside the electrical equipment to the first inlet; preferably air is taken from outside the electrical equipment to the second inlet. Preferably, the air is taken directly from outside the electrical equipment to the inlets.

Preferably the coolant forming the second stream is not used to cool devices upstream of the second inlet. Thus, preferably for the second stream, no devices are located upstream of the second inlet. Where the second stream is supplied by a channel, preferably there are no devices in the channel stream of the second inlet, and preferably no devices in the channel at all.

Preferably, the second inlet is positioned to provide efficient cooling of the devices. Preferably, the second inlet is adapted to be adjacent a device in the duct. Thus if the electrical equipment includes a device that requires particular cooling, the second inlet can be positioned adjacent that device, preferably immediately upstream of the device.

Preferably the width of the second inlet is less than the width of the duct at the second inlet. Thus preferably the inlet is narrower than the duct. Thus the size of the inlet can be chosen having regard to the size of the deviceto be cooled to improve efficiency of cooling by the second stream. Also, by reducing the size of the inlet, the flow velocity of the second stream can be increased. Preferably the inlet comprises a nozzle, a section which reduces in cross section downstream from its inlet to outlet, which converts the pressure head of the second stream at the nozzle to velocity head, thus further improving the efficiency of the cooling. Preferably, the width of the second inlet is less than 50% of the width of the duct at the second inlet.

That feature is of particular importance and is provided separately. Thus a further aspect of the invention provides a cooling apparatus for cooling an electrical device using a flow of coolant, the apparatus comprising a duct for transporting the coolant past the device, the duct including a first inlet for a first stream of coolant, the duct further including a second inlet downstream of the first inlet for a second stream of coolant, wherein the width of the second inlet is less than 50 % of the width of the duct at the second inlet.

in an embodiment of the invention, the apparatus further includes a baffle in the duct 30 for restricting the flow of the first stream past a device in the duct. Where electrical equipment includes "hot" devices in the duct, it often not possible to locate such devices at the upstream end of the duct. Thus, the coolant stream which passes the hot device has often already passed several other devices upstream, and thus the b temperature of the coolant stream at the hot device is already raised compared with its inlet temperature and thus the cooling effect of the stream is reduced. In accordance with other aspects of the present invention, that air stream is mixed with a second stream of cool air to reduce its temperature and thus increase the cooling. An alternative arrangement is for the first stream in the duct not to mix with the second stream until after the second stream has been used to cool the hot device. The hot device may, therefore, in some cases benefit from the fresh coolant provided by the second stream but, since there is no mixing of the first and second airstreams at the device, the flow and thus cooling may be improved. The mixing of two streams of air can lead to a decrease in velocity of the flow in some arrangements. The second and first streams may be combined downstream of the hot device. Thus the hot device may be provided with exclusive cooling whilst still being arranged at the most advantageous part of the equipment.

This feature is of particular importance and is provided independently. Thus a further aspect of the invention provides a cooling apparatus for cooling electrical devices in a duct using a flow of coolant, the apparatus comprising a duct for transporting the coolant, the duct including a first inlet for a first stream of coolant and a baffle in the duct for restricting the flow of the first stream past a device in the duct and a channel for transporting a second stream of coolant past the device and into the duct.

Also provided is a cooling apparatus for cooling an electrical device using a flow of coolant, the apparatus comprising a duct for transporting the coolant, the duct including a first inlet for a first stream of coolant and a second inlet downstream of the first inlet for a second stream of coolant, the arrangement being such that a device in the duct is cooled substantially exclusively by the second stream of coolant.

Preferably the configuration of the second inlet is such that the second stream is angled with respect to a surface of a device. While it is envisaged that the flow of the second stream could be parallel to the upper surface of the device, in arrangements in which a device is adjacent the second inlet, the flow of the second stream may be at an angle to the upper surface of the device and it is thought that that could lead to improved cooling. Preferably the direction of flow of the second stream is at an angle of between about 30 degrees and about 50 degrees, preferably about 45 degrees, relative to the upper surface of the device. It is thought that if the angle were too great, the velocity of the coolant flow might be decreased too much on impact with the device; if the angle is too small the effect of improved cooling from the angled flow might not be achieved. It is thought that by arranging for the second stream to strike the upper surface of the device at an angle, turbulent flow over the device and thus cooling of the device can be increased, thus improving efficiency of cooling.

Preferably the apparatus further includes a formation for directing the second stream of 10 coolant. The formation may include a baffle at the inlet for deflecting the second stream. The deflector may be arranged to direct the flow towards a device. The directing formation may be provided by a wall of the inlet or may include a separate formation, which may located at the inlet.

To improve the cooling, heatsinks may be provided on a surface of the device to draw heat away from the device. Such heatsinks may comprise one or more formations attached to the upper surface of the device. The heatsink may comprise fins attached to the upper surface of the device.

Where the device includes a heatsink, for example on its upper surface, the formation may be arranged to direct the coolant over the heat sink in a desirable way. Usually, the formation will be arranged to direct the coolant towards a hot area of a device to be cooled.

Preferably, the second inlet ftu-ther includes a formation for increasing turbulence.

Turbulent flow gives improved heat transfer from the device and therefore increased cooling.

Preferably, the sectional area of the duct varies along its lengtiL Continuity of the flow 30 of the coolant through the duct requires that as the sectional area of the duct decreases, the velocity of the coolant flow increases. As the coolant moves to a region of the duct having a smaller sectional area, pressure head of the coolant is converted into velocity head. Thus the duct is advantageously arranged so that a region of the duct which is adjacent to a device to be cooled has a relatively small sectional area, thus giving a fast -8flow of the coolant. A faster flow of coolant provides greater cooling of the device.

Where the sectional area of the duct varies, preferably the duct further includes a diffuser at the outlet of the duct to recover the pressure head and thus improve efficiency. The diffuser is a passage which gradually increases in sectional area downstream and its function is to reduce the velocity of the coolant to recover its head, or at least to attempt to retain the head.

Preferably, the duct includes a formation for directing flow in the duct. The apparatus may include baffles inside duct to channel the coolant, for example, over a device. Thus a device to be cooled can experience the full flow of the second stream, rather than the second stream being spread across the full width of the duct. Thus the effect of the baffles may be to provide a section of reduced width in the region of a device.

preferably, the duct includes a formation for increasing turbulence in the duct. As indicated above, increasing turbulence can increase cooling of the device. Turbulence may be effected, for example, by increasing the velocity of the flow in the duct and/or by providing raised areas on surfaces of the duct. The turbulence may be localised in specific regions of the duct. The means for generating turbulent flow may be one or more protrusions.

Preferably the apparatus further includes a channel for the second stream upstream of the second inlet. Thus the channel can be used to direct the second stream to the inlet. In preferred embodiments of the invention, the channel is used to direct the second stream from outside the equipment to the second inlet.

In a first preferred embodiment of the invention the second inlet is at the downstream end the channel. The channel could be a tube or pipe running directly to the second inlet. Alternatively, the second inlet can be arranged upstream of the end of the channel. The channel can have a plurality of outlets, each feeding a stream of coolant into the duct. For example a single channel could feeding coolant to various inlets in the duct, and a device requiring particular cooling could be arranged at each inlet.

Preferably, the channel extends along a wall of the duct. Thus the coolant in the channel may flow along an external surface of the duct. Such an arrangement can save space in the apparatus compared with an arrangement in which the channel and the duct are separate.

In a first preferred embodiment of the invention, the channel extends above the duct. For example, the duct extends between the channel and the circuit board. In such, an arrangement, preferably the second stream flows along the external surfaces ofthe upper wall of the duct and the second inlet comprises an opening in an upper wall of the duct. Alternatively, or in addition, the channel extends along a side wall of the duct and the second inlet comprises an opening in a side wall of the duct. The arrangement chosen will depend on the shape and arrangement of devices and the size and shape of the electrical equipment.

in a preferred embodiment ofthe invention described herein, the apparatus comprises two 15 adjacent ducts and a channel extends between the ducts feeding additional coolant into both ducts. Thus, preferably, the apparatus includes a plurality of ducts and the channel feeds coolant to each of the plurality of ducts. Such an arrangement can save space in the overall equipment.

Preferably, the apparatus further comprises a plurality of inlets adapted to be downstream of the first inlet. Such an arrangement is preferably used where there is a plurality of "hot" devices to be cooled. Such an arrangement may include a single channel feeding coolant into the inlets, or may have separate channels, each feeding an inlet.

In a further aspect ofthe present invention there is provided cooling apparatus for cooling an electrical device using a flow of coolant, the apparatus comprising a channel for transporting a flow of coolant, the channel having an outlet, such that the outlet of the channel is adjacent the device.

The apparatus according to the further aspect of the invention may not necessarily include a duct. The channel alone may be used to direct the coolant to the device. In arrangements including a plurality of "hot" devices to be cooled, a channel may be provided directed to each device. Other devices requiring less cooling may receive sufficient cooling from the coolant expelled by the channels such that a channel to all of the devices ofthe equipment might not be required. The channels may comprise separate individual channels or may comprise channels having a plurality of outlets, or may comprise a branched network of channels. Where the coolant is air, for example, the one or more channels may be provided with one or more fans which may, for example, blow 5 air along the channel or channels.

Preferably, the apparatus further includes a fan. The fan may be any device for moving the coolant through the duct and/or the channel. The apparatus may include a plurality of fans.

In a preferred embodiment of the invention one fan is arranged to move the coolant through both the first and second inlets. The fan may comprise a plurality of individual fan units. In an alternative embodiment, separate fan arrangements are used to move the coolant through the duct and through the channel.

is Preferably the fan is arranged to suck the coolant through the inlet. It is thought that the sucking of coolant through the duct and/or channel gives better flow characteristics and better cooling than if the fan arrangement were arranged to blow air through the duct and/or channel.

Preferably the coolant is air. It is, however, envisaged that other fluids could be used. For example, the coolant could comprise water or benzene. A suitable apparatus for effecting the flow of coolant would be used.

Preferably the apparatus further comprises an electrical device, and preferably comprises a plurality of electrical devices.

Preferably the device is positioned adjacent the second inlet.

The invention further provides electrical equipment including cooling apparatus as described above.

The invention further provides a method of cooling an electrical device using a flow of coolant, using a cooling apparatus as described above.

- I I - The invention Rirther provides a method of cooling a device using a flow of coolant, the method comprising transporting a first stream of coolant past the device in a duct and introducing a second stream of coolant into the duct upstream of the device.

As used herein, it should be understood that the terms "duct" and "channel" should preferably be interpreted broadly to include any passage or channel along which the flow of the coolant may be directed. The term is preferably not restricted to fully enclosed structures (as for example a tube), but preferably also includes structures which are not fully enclosed. The flow of the coolant could be enclosed only on three sides or on two sides: the channel or duct may comprise three surfaces or only two surfaces. It is envisaged that the channel or duct may only comprise a single surface but that surface may serve to effect the desired flow of coolant.

Preferably the duct and/or the channel comprises an enclosed structure, for exarnple a tube or passageway. Preferably the flow of coolant is enclosed on at least three sides within the channel and/or the duct. In some preferred embodiments of the invention, it will be preferable for the flow of coolant to be enclosed on all sides within the channel and/or duct.

The term "sectional area' preferably refers to a transverse cross section of the channel and/or duct being substantially perpendicular to the direction ofthe flow ofco olant at that location. Furthermore, unless clear to the contrary from the context "length7' preferably refers to a distance substantially parallel to the direction of flow of the coolant, and width preferably refers to a dimension being substantially perpendicular to the "Iength" and parallel to the upper surface of the device, where appropriate.

Where reference is made herein to "upper", "lower"and "above" and similar expressions, preferably it refers to an arrangement in which a device is attached on the top surface of a circuit board and its upper surface is substantially horizontal. it will be appreciated that circuit boards can, in practice, be mounted "sideways" or "upside-down" and that in such arrangements and the above-mentioned terms should be construed accordingly. For example, the "upper" surface of a device may, in fact, be its lowest surface when the device is mounted "upside-down".

Apparatus features may be applied to the method features and vice versa. The various features of the different aspects of the invention may be applied interchangeably to other aspects of the invention. Furthermore, each feature disclosed in the description, the claims and/or the drawings may be provided independently or in any appropriate 5 combination.

Embodiments of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, of which:

Figure I shows a sectional side view of a pair of ducts for cooling electrical devices; Figure 2 shows a perspective view of a cooling apparatus according to an embodiment of the present invention; Figure 3 shows a sectional side view of the cooling apparatus of Figure 2; Figure 4 shows a sectional side view of a part of the cooling apparatus; Figure 5 shows a sectional side view of a further embodiment of a cooling apparatus; Figure 6 shows a part of the cooling apparatus of Figure 5; Figure 7 shows a perspective view of a ftirther embodiment of a cooling apparatus; Figure 8 shows a sectional plan view of a layout of devices in a duct; and Figure 9 shows a sectional side view of a further alternative embodiment.

Figure I shows two circuit boards 1, 3 on each of which are mounted two electrical devices A, B, C and D which are all semiconductor chips. The devices A, B, C and D 25 are all similar devices each of which have a power of about 12 W. Between the two circuit boards and extending the full length of the circuit boards is a spacer 5. The arrangement includes a pair of side walls (not shown) onto which the two circuit boards 1, 3 and the spacer 5 are attached. The space bounded by the upper surface of the spacer 5, the side walls and the circuit board I defines an upper duct 7 in which the devices C 30 and D are mounted. The space bounded by the lower surface of the spacer 5, the side walls and the circuit board 3 defines a lower duct 9 in which the devices A and B are mounted.

The whole apparatus is mounted in a casing (not shown).

A pair of fans I I (shown schematically) are provided at an open end of the ducts 7, 9 and are arranged to suck air through the two ducts. In the present example, the fans have a speed of 14.5 cfm and 15.6 cfin. The air drawn into the ducts 7, 9 is taken from outside the equipment and is drawn through a grille (not shown) immediately adjacent the inlets 5 of the ducts. The direction of air flow is shown 13.

The temperature of the air entering the ducts is, for this example, 55'C.

5'C represents a "worst case" external ambient temperature which is used for the testing of the cooling apparatus.

The devices A, B, C and D are activated and begin to generate heat. Table I below shows the measured temperatures of the devices themselves and the air temperatures measured directly above the device and below it under the circuit board.

The device temperature indicated in the table is the device junction temperature which is the temperature of the working silicon of the device. The junction temperature may be estimated, for example, from a knowledge of the thermal characteristics of the device (provided by the manufacturer of the device) and the case temperature of the device or the air temperature local to the device. An example of a method for estimating the 20 junction temperature is as follows:

Calculation of Estimated Junction TeMperature of Device The following theory allows for the estimated calculation of air temperatures (local air temperatures), the temperature generated by the working silicon and thus, junction 25 temperature of the device.

From a knowledge of the sectional area of the duct a particular position (for example in the region of an item of equipment), the average temperature at that device can be determined. From knowledge of the flow velocity in the duct, the flow velocity at the 30 centre of the device can be interpolated ("interpolate velocity"). From a consideration of the power of the device under consideration as

well as the power of devices upstream of the device, the local temperature at the device can be determined.

For example, for a model having the device of interest downstream from two components having a power of 0.3W and 2.3W respectively.

0.3W + 2.3W = mCp(AT) Where m = pvA; AT = TLOcAL- TrN Thus 2.6w = mCp(TLOcAL- TIN) TLOCAL =(2.6w)/(mCp) + TrN Where Cp is the specific heat capacity of air (or other fluid in the duct); m is the mass flow rate of air in the duct; p is the density. of air; v is the velocity of air in the duct (interpolated local velocity); and A is the local cross sectional area of the duct.

To estimate thejunction temperature of the device, the manufacturer's data forthe device is used to calculate the temperature rise of the device (TRIsF) TRISE = IOJA XPDOI Where6JAis the thermal resistance of the device CC/W) The junction temperature of the device can therefore be estimated T"CTION =TLOCAL -TRISE Table 1 - external ambient temperature 551C Device Device Junction Temp 'C Temp above 'C Temp below OC A 117.1 58 57 B 1)l R 79 64 c 123.4 76 63 D 120.4 59 57 It will be seen that the air temperature is raised slightly from the inlet of the ducts 7, 9 to the upstream devices A and D, but that at the downstream devices B and C, the temperature has risen to as much as 79'C as a result of the heat lost from devices A and D (and any other devices upstream). Thus the cooling of the downstream devices B and C is less and their temperatures are measured as being as much as 123'C. Such temperatures can be considered to be unacceptable, and in many cases an additional fan, or larger fans, would need to be used to increase the cooling of the downstream devices to acceptable levels.

Figures 2 to 4 show a first example of a cooling apparatus. Figure 2 shows the duct is arrangement and Figure 3 shows that arrangement with two circuit boards 21 and 23. As before, each circuit board 21, 23 has two devices W, X and Y, Z which are similar to devices A, B, C and D. In the place of the spacer 5 of the arrangement of Figure 1 is a channel 25 and a deflector 26. The channel 25 is defined by upper and lower walls 27, 29 which extend substantially parallel to the circuit boards 21, 23 and extend to about halfway along their length. As can be seen from Figure 2, the duct arrangement includes side walls 31, 33 which extend towards the circuit boards 21, 23 to define an upper duct 35 and a lower duct 37. The channel walls 27, 29 are attached to the side walls 31, 33.

Downstream of the channel 25 is the deflector 26 which extends between the side walls 31, 33 and is described in more detail below.

A fan arrangement 39 is provided at the downstream end of the ducts 35, 37. The fans have the same speed as those described in respect of Figure 1.

The arrangementis mounted in a casinghaving grilles atthe inlets ofthe ducts 35,37 and the channel 25. The inlet vent comprises a rectangular grill which has the same overall area as the entire inlet end of the duct.

Considering Figure 3, when the fan arrangement is activated, air is sucked through the upper duct 35, the lower duct 37 and the channel 25. There is a gap between the end of the channel 25 and the deflector 26. Thus the air from the channel 25 passes into the upper duct 35 and the lower duct 37, the space between the end of the upper wall 27 of the channel 25 and deflector 26 forming an upper second inlet 41 into the upper duct 35 and the space between the end of the lower wall 29 of the channel 25 and the deflector 26 forming a lower second inlet 43 into the lower duct 37.

Considering the upper duct 35 and the two devices Y and Z, with reference to Figures 2 and 3, a first strewn of air passes into the duct 35 via the first inlet 45. The first stream of air passes over device Z and the temperature of the first stream rises.

A second stream of air enters the channel 25 and through the second inlet 41 into the duct 35. The second stream of air is at ambient temperature and at a lower temperature than the first stream at the second inlet. The device Y is arranged immediately downstream of the second inlet 41. The second stream is deflected by the upstream surfaces of the deflector 26 directly towards device Y, thereby improving cooling.

it will be seen that similar considerations apply to the lower duct 37.

Figures 2 and 3 show that the upper and lower walls 27, 29 of the channel 25 are shaped so that the inlet of the upper duct 45 (and the inlet 45' of the lower duct) comprises a nozzle: the sectional area of the inlet decreases downstream. This improves the flow characteristics in the duct as pressure head is converted to velocity head at the nozzle.

Consideration is now made to the deflector 26 having particular reference to Figure 4.

Figure 4 shows a different arrangement in which several devices are arranged downstream of the second inlets 41, 43.

The upstream surfaces 46 of the deflector split the second stream emitted from the channel 25 and direct it into the ducts 35, 37 towards the devices 47, 49. The angle of deflection e of the upstream surfaces 46 is chosen to give the most desirable angle for the second stream to hit the upper surface of the device 47, 49. Preferred values of 0 are in the range of 30 to 50 degrees, preferably 45 degrees.

It will be seen that the devices 47, 49 are arranged so that the second stream of air will strike the upper surface of the devices 47, 49 downstream from the device edge. Thus the second stream strikes the device in a region where the device is particularly hot.

Furthermore, the height of the duct at the downstream end of the upstream surfaces 45 is reduced compared with the height of the duct adjacent the channel. That reduction in height of the duct increases the velocity of the air flow in that region, and thus the cooling.

immediately downstream of the upstream surfaces 45 of the deflector 26, the thickness of the deflector decreases and the height of the duct increases, recovering some of the pressure head.

The dimensions of the arrangement of Figure 4 are approximately as follows:

a= 18mm b = 16mm c = 35mm d = 35mm e = 10Omm f = 26Amm g = 34nim At the end of the duct, after the last device, the deflector tapers downstream, thus increasing the height of the duct at its downstream end, forming a diffuser 50 to recover as much of the pressure head as possible.

Figure 2 shows that the deflector may further include a pair of baffles 51 which contain the second stream as it passes over the device Y. Thus the section of duct through which the second stream of air passes over the device Y is reduced both in height and width, thus increasing the velocity of air over the device and thus the cooling of the device.

Thus, in the arrangement shown, additional cooling of device Y is obtained by a combination of the lower temperature of the second stream of air and the reduced sectional area of the duct giving a greater flow velocity and thus heat extraction from the - 18device.

In the arrangement shown in Figure 2, the first stream of air passes through the whole width of the duct in the region of the baffles 5 1, so that the overall pressure head of the 5 air flow is not reduced too much.

The surface of the deflector may have a plurality of protrusions or "bumps" to increase the turbulence in the duct.

The example below shows the cooling effect which can be achieved with the duct arrangement of Figures 2 and 3.

The devices W, X, Y and Z are activated and begin to generate heat. Table 2 below shows the measured temperatures of the devices themselves. The difference in temperature compared with the corresponding devices of Figure 1 is also given.

Table 2 - external ambient temperature 550C Device Device Junction Temp C Temp difference C W 114.7 -2.4 X 112.1 -10.4 Y 111.4 -12.0 z 115.6 -4.8 It can be seen that while a small improvement in the cooling of the upstream devices is achieved, a very large improvement is seen in the cooling of the downstream devices. All of the devices are now below 120 'C.

In the arrangement shown in Figure 4, is has been found that the temperature of the first 30 stream immediately upstream of the second inlet has risen by 15 'C compared with ambient temperature entering the inlet of the duct. A reduction in 15 'C of the temperature of the device 49 has been recorded by the inclusion of the second inlet and the decrease in the sectional area of the duct at the device.

The cooling apparatus has been described above in respect of a two circuit board arrangement. Such an arrangement is advantageous because a single channel is used for two ducts, thus saving space. It will be appreciated, however, that the arrangement can easily be adapted for the case in which only one circuit board is used, for example by 5 effectively using only the lower or upper half of the channelldeflector arrangement.

Figures 5 and 6 show a further example of a cooling apparatus mounted between two circuit boards 121, 123, similar to the arrangement of Figure 3.

10The cooling apparatus includes a channel 125 and a deflector, the channel 125 having a height Hc and being defined by upper and lower walls 12 7, 129, each having a thickness W. and W13, respectively. In this example, the thickness of the walls 127 and 129 are the same and the walls comprise plastics material.

15The walls 127, 129 define ducts 135 and 137 which have heights of HT and HB respectively. In this example, the heights of the ducts 135 and 137 are the same. HO is the overall height: the distance between the circuit boards 121 and 123.

As air is sucked through the apparatus by the fan 139, it is divided into three air streams A,, A2, A3which pass through the upper duct 135, the channel 125 and the lower duct 137, respectively.

The deflector 125 has two deflector surfaces D, and D2which are at angles of (),, and 02 from the general direction of flow through the device. The deflector surfaces split the 25 coolant flow A2into two streams which pass to the devices Y and X.

The dimensions of the example of Figures 5 and 6 were optimised for cooling a particular arrangement. The dimensions were as follows:

HO 34.1 min Hc 13.9 mm HT HB 8.1 mm WT W13 2.0 nim 01 62 450 In this case, using 2mm. plastic Hc HO X 0.408 HT/HB HO X 0.238 The dimensions were varied to determine the effect of variation of the heights of the channel and ducts. The results are shown below in Table 3. The temperatures are expressed as change in the device temperature compared with the arrangement of Figure 3 (the results of which are shown in Table 2).

Table 3 - external ambient temperature 551' HTand HB 11c mm Device W Device X Device Y Device Z Infil OC:o c OC OC 9.1 11.9 +0.1 +0.6 +1.8 +0.2 10.1 9.9 +1.1 +2.3 +1.9 +0.3 11.1 7.9 +1.2 +2.9 +2.7 +0.9 7.1 13.9 +0.1 +0.7 +0.9 +0.8 The effect of the deflector in the arrangement of Figures 5 and 6 was also investigated by 20 testing an arrangement such as Figure 5 with the deflector 126 having different angles. Where the deflector angles 0 are 90% the deflector is in the forTn of a wall between the two ducts. The results are shown below in Table 4. The temperatures are expressed as a change in the device temperature compared with the arrangement of Figure 3 (the results of which are shown in Table 2). 25 Table 4 - external ambient temperature 550C Deflector Device W =EOC Device X10C Device Y:PC Device Z -+OC e, = 62 = 900 +2.3 +6.2 +7.1 +2.2 01 = 02= 300 +1.1 +3.4 +3.7 +1.5 = 02= 60' +0.5 +2.1 +2.7 +1.0 As indicated above, for this arrangement, 01 02= 45' was found to be the optimum angle for the deflector. Although the deflector provides a relatively "sharp turn" of the air flow, it is thought that the "jetting" of the air onto the device surface appears to reduce the temperature.

Figure 7 shows a further example of a cooling apparatus. The apparatus is similar to that of Figure 2 except that the baffles 51 extend upstream of the inlet 41. The apparatus is arranged in use so that the device is adjacent to the deflector 26 and within the area defined within the curved baffles 5 P. In such an arrangement, it will be seen that the device within the baffles will be cooled only by the air from the channel 25 which flows through the inlet 4 1. The upstream surface 3 51 of the baffle 5 1' deflects the air stream in the duct around the baffles.

The arrangement is thought to have two main advantages, both leading to better cooling of the device shielded by the baffle. Firstly, the air used to cool the device is exclusively air which has not cooled any device upstream. Secondly, there is no mixing of airstreams from the channel and the duct at the second inlet 41, which is thought to increase the velocity of the airstream at the device; mixing airstreams can reduce flow velocity.

Figure 8 shows an example of an arrangement of devices for which the ducts described herein are used. Figure 8 shows a circuit board on which is mounted a variety of devices including twelve IC devices 62. The devices are all mounted in a duct having a front wall 64, a rear wall 65 and a first inlet 66. While the output of each IC device 62 is a relatively modest 4.6W, the combined output of all of the IC devices 62 leads to an elevated air temperature (direction of airflow 68).

While acceptable cooling of device W' may be obtained because it is close to the first inlet 66, if the duct were not to include a second inlet, the downstream device Xis likely to become unacceptably hot. For this arrangement, the duct is provided with a second inlet (not shown) just upstream of device X.

In an alternative arrangement, the channel and deflector is arranged adjacent a side wall of the duct. In such an arrangement, the channel and deflector can be located between two side-by-side ducts. Thus the channel can feed both ducts. The duct and channel may comprise injection mouldable material, for example plastics material, and it is thought that the arrangement may be easier to mould if the channel is provided on the side of the duct.

A plurality of ducts and channel arrangements can be arranged on a single circuit board.

Figure 9 shows an alternative arrangement where the electrical equipment comprises one circuit board 223. In this case, only one duct 237 is provided. The cooling apparatus can be thought of as a half of the apparatus used for the two circuit board arrangements and 10has a wall 229 defining the duct 237 and the channel 225. A fan 239 is provided which pulls air through the apparatus. A deflector 226 is Provided downstream to direct air exiting from the channel 225 onto a device.

It will be understood that the present invention has been described above purely by way is of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims

Claims:

1. Cooling apparatus for cooling electrical devices using a flow of coolant, the apparatus comprising:

- a duct for transporting the coolant past a device, the duct including a first inlet for a first stream of coolant and a second inlet, downstream of the first inlet for a second stream of coolant; and - a deflector downstream of the second inlet for directing the second stream of coolant.

2. Apparatus according to claim 1, wherein the deflector comprises a deflector surface for directing the second stream, the angle of the surface to the coolant flow being from 30' to 60', preferably from 30' to 50', preferably about 45".

3. Apparatus according to claim 1 or claim 2, wherein the deflector comprises formations for modifying the flow in the duct.

4. Apparatus according to any of claims 1 to 3, further comprising a channel for transporting the second stream of coolant to the second inlet.

5. Apparatus according to claim 4, wherein the second inlet is at the downstream end of the channel.

6. Apparatus according to claim 4 or claim 5, wherein the deflector is spaced from the downstream end of the channel.

7. Apparatus according to any of claims 4 to 6, wherein the channel has only one outlet to the duct.

8. Apparatus according to any of claims 4 to 7, wherein the height of the channel is between about 30% to 50%, preferably about 40% of the height of the channel plus the height of the duct or ducts served by the channel.

9. Apparatus according to any of claims 1 to 8, wherein the second stream of coolant -24is at ambient temperature.

10. Apparatus according to any one of claims 1 to 9, such that for the second stream, no devices are located upstream of the second inlet.

11. Apparatus according to any one of claims 1 to 10, wherein the second inlet is adapted to be adjacent a device in the duct.

12. Apparatus according to any one of claims 1 to 11, wherein the width of the second inlet is less than the width of the duct at the second inlet, and is preferably less than 50% of the width of the duct at the second inlet.

13. Cooling apparatus for cooling an electrical device using a flow of coolant, the apparatus comprising a duct for transporting the coolant past the device, the duct including a first inlet for a first stream of coolant, the duct further including a second inlet downstream of the first inlet for a second stream of coolant, wherein the width of the second inlet is less than 50% of the width of the duct at the second inlet.

14. Apparatus according to any of claims 1 to 13 further including a baffle in the duct for restricting the flow of the first stream past a device in the duct.

15. Cooling apparatus for cooling electrical devices in a duct using a flow of coolant, the apparatus comprising:

- a duct for transporting the coolant, the duct including a first inlet for a first stream of coolant; - a baffle in the duct for restricting the flow of the first stream past a device in the duct; and - a channel for transporting a second stream of coolant past the device and into the duct.

16. Cooling apparatus for cooling an electrical device using a flow of coolant, the apparatus comprising:

- a duct for transporting the coolant, the duct including a first inlet for a first stream of coolant and a second inlet downstream of the first inlet for a second stream of coolant, the arrangement being such that a device in the duct is cooled substantially exclusively by the second stream of coolant.

17.Apparatus according to any one of claims 1 to 16, wherein the inlet further includes a formation for increasing turbulence.

18. Apparatus according to any one of claims 1 to 17, wherein sectional area of the duct varies along its length.

19. Apparatus according to any one of claims 1 to 18, wherein the duct includes a formation for directing flow in the duct.

20. Apparatus according to any one of claims 1 to 19, wherein the duct includes a formation for increasing turbulence in the duct.

21. Apparatus according to any preceding claim when dependent on claim 4, wherein the channel extends along a wall of the duct.

22. Apparatus according to any one of claims 1 to 2 1, further comprising a plurality of inlets adapted to be downstream of the first inlet.

23. Apparatus according to any one of claims 1 to 22, further including a fan, and preferably the fan is arranged to move the coolant through both the first and second inlets.

24. Apparatus according to claim 23, wherein the fan is arranged to suck the coolant through the inlet, and preferably the coolant is air.

25. Apparatus according to any one of claims 1 to 24, further comprising an electrical device positioned adjacent the second inlet.

26. Cooling apparatus for cooling an electrical device using a flow of coolant, the apparatus comprising a duct for transporting the coolant past the device, the duct including a first inlet for a first stream of coolant, the duct further including a second inlet for a second stream of coolant.

27. Cooling apparatus for cooling an electrical device using a flow of coolant, the apparatus comprising a channel for transporting a flow of coolant, the channel having an outlet, such that the outlet of the channel is adjacent the device.

28. Electrical equipment including cooling apparatus according to any one of claims 1 to 27.

29. A method of cooling an electrical device using a flow of coolant, using a cooling apparatus according to any one of claims 1 to 27.

30. A method of cooling a device using a flow of coolant, the method comprising transporting a first stream of coolant past the device in a duct and introducing a second stream of coolant into the duct upstream of the device.

31. Cooling apparatus being substantially as herein described with reference to and as shown in any of Figures 2 to 9 of the accompanying drawings.

32. A method of cooling equipment, the method being substantially as herein described with reference to any of Figures 2 to 9 of the accompanying drawings.