Patent US4126269 - Ventilated enclosure

VENTILATED ENCLOSURE
BACKGROUND OF THE INVENTION

1. Field of the Invention 5 The present invention relates to ventilated enclosures

in which the temperature must be maintained at a substantially constant level or must not rise above (or fall below) a predetermined limiting level.

2. Description of the Prior Art 10 Such enclosures are generally formed either by

rooms into which hot or cold air is pumped by means of fans, or by cabinets within which a flow of hot or cold air, which is produced by fans, possibly associated with heating or refrigerating means, is used to heat or cool IS articles in the cabinets. More particularly, these cabinets may be of the kind which are used to hold the electrical or electronic circuits of a control or regulating installation or some other kind of installation, which circuits dissipate heat in the course of operation and conse- 20 quently need to be cooled if they are to be capable of operating properly.

In order to allow effective passage for the flow of air produced by the fans, each enclosure thus takes broadly the form of an air duct, one of whose ends is closed off 25 by a partition or a plate pierced with ventilation apertures, and whose other end is closed off by a rigid wall provided with openings which are designed to provide access for the flow of air caused by fans fixed to this wall. An enclosure of this kind may be formed, for 30 example, by a box for connecting printed-circuit boards of the type which is described and illustrated in U.S. Pat. Nos. 3,699,396 and 3,868,158. Advantageously, such an enclosure is provided with at least two fans, which enables ventilation to be continued even when 35 one of the fans breaks down. In addition, the use of a plurality of fans ensures more satisfactory distribution of the circulated air within the enclosure.

In the majority of cases, for example in cases where the enclosure is formed by a cabinet containing elec- 40 tronic circuits, or again in cases where the enclosure is intended to provide a controlled environment for items which do not easily withstand heat or cold, it is also essential for the temperature within the enclosure to remain below or above a predetermined limiting value. 45 This requirement may be met by placing within the enclosure a thermal detection device which generates a signal as soon as its temperature becomes higher than or lower than the predetermined limiting threshold. However, in cases where the enclosure contains electronic 50 components, a detector of this kind often proves unsatisfactory because it fails to detect abnormal rises in temperature in time to protect the components against catostrophic changes in temperature. In effect, the detector, positioned as it is in the stream of air at the out- 55 let, fails to produce a signal until after the air has heated up abnormally from contact with the components which has ceased to be cooled as a result of the stoppage of the fan. The result is that the components may have already been damaged at the time when the signal 60 produced by the detector appears.

To overcome this drawback, it has been proposed to mount on each fan a device for detecting inadequate ventilation which is designed to produce a signal when the speed of rotation of the associated fan falls below a 65 predetermined limiting value. Such a detection device may be formed for example by a dynamo which is coupled to the drive-shaft of the fan and which produces an

electromotive force whose magnitude is proportional to the speed of rotation of the shaft and can be measured by apparatus of a known kind. However, this solution has the disadvantage that it involves a relatively high initial cost and that it fails to reveal the reductions in air flow which may occur, even if the fans are turning at their normal speed, when the air-inlet openings opposite which the fans are mounted become partially or totally obstructed as a result of an accumulation of dust or of accidental blockage by an object such as a sheet of paper, for example. In this case, the fans circulate hot air around inside the enclosure.

SUMMARY OF THE INVENTION

The present invention substantially reduces or overcomes these drawbacks and provides a ventilated enclosure which is traversed by a flow of air produced by at least two fans, and which is fitted with relatively inexpensive detection means which are capable of detecting the inadequate output or flow of air from or in the enclosure which results from a ventillating fan for the enclosure stopping or from the air inlets of the enclosure becoming partially or totally blocked.

One aspect of the invention relates to a ventilated enclosure which is formed by a duct, one of whose ends is closed off by a partition pierced with ventilation apertures and whose other end is closed off by a wall provided with openings which are designed to provide passage for the air which is drawn into the interior of the enclosure from its exterior by at least two fans which are attached to this wall, the said enclosure being characterized in that each fan is provided with a detection device for detecting inadequate ventilation, the detection device is advantageously positioned in the area which is swept by the stream of air which is produced by the associated fan when in operation, and generates a signal when the output of air from this fan falls below a predetermined limiting value. Each detection device includes a thermal detection element which is designed to emit a signal when its temperature becomes equal to a predetermined temperature threshold and an air non-return casing. The air non-return casing contains the thermal detection element and is arranged to have passed through it only a fraction of the air output from the associated fan. This fraction of air output is such that, when the output from the fan is equal to the said predetermined limiting value, the temperature which prevails within the casing is equal to the said predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent from the following description, which is given by way of non-limiting example and refers to the accompanying drawings, which show:

FIG. 1 is a perspective view showing a group of ventilated enclosures formed by assembling boxes for connecting printed-circuit boards in a support rack;

FIG. 2 is a partial cross-sectional view of the assembly shown in FIG. 1 taken on the chain line looking in the direction of arrows 2—2, with the lower drawer covers removed for clarity to show the filters, fan elements and associated passages in full;

FIG. 3 is a partially cut-away perspective view of a device for detecting inadequate ventilation such as is fitted to each of the fans in the assembly shown in FIG. i;

FIG. 4 is a partial cross-sectional view showing the produced by both the fans mounted in an associated

composition of the thermal detection element of the drawer. This being the case, if only a single connecting

detection device shown in FIG. 3; box, such as box CI, is positioned just above a drawer

FIGS. 5 and 6 are two views partially in cross sec- such as 23, this box acts as a ventilated enclosure in

tion, diagrammatically illustrating the function of the 5 which the stream of air produced by the fans VI and V2

air non-return casing when ventilation is normal and housed in the drawer is channelled by the air duct

when the ventilation stops, and formed by the plates 11,12,13 and 14 of the box. Simi

FIG. 7 is a circuit diagram showing the manner of larly, in cases where a plurality of boxes are superim

connection of the detection devices fitted to the fans in posed above the same set of fans in columnar fashion

the assembly shown in FIG. 1. 10 (such as for example, by boxes CI and C3 which in FIG.

DESCRIPTION OF THEPREFERRED ...

EMBODIMENTS* such boxes of identicai dimensions forms a ventilated

The assembly which is shown in FIG. 1 includes a enclosure in which the stream of air produced by fans

support rack 10 in which is housed a plurality of con- 15 VI and V2 is channeled by the single air duct created by

necting boxes or housings CI, C2, C3, etc. Each of these lining up the air ducts of individual boxes end-to end.

boxes conveniently supports electrical circuits which In the embodiment shown in FIG. 1, it can thus be

are grouped together in the form of printed-circuit seen that the boxes CI and C3 which are superimposed

boards. For details of the construction of these boxes, just above the fans VI and V2 housed in drawer 23 form

reference may be made to U.S. Pat. No. 3,699,395, the 20 a first ventilated enclosure traversed by the stream of air

subject matter of which is incorporated by reference from these fans, while the boxes C2 and C4 which are

herein. superimposed just above the fans housed in drawer 24

Without repeating the entire explanation given in this form a second ventilated enclosure traversed by the

patent, it will suffice for purposes of explanation to state stream of air from these latter fans,

that each of the boxes is broadly in the shape of a parel- 25 The term ventilated enclosure is defined for purposes

lelepiped, formed, as shown in FIG. 1, by a back plate of this specification is including any air duct which is

11, two side plates 12 and 13 and a front plate 14, and of designed to have indrawn air pass through it, preferably

which the upper and lower faces are formed, as can be from the exterior to the interior of the duct, by fans

seen in FIGS. 1 and 2, by two gratings 15 and 16 which which are attached or supported to a wall arranged at

are suitably pierced so as to provide ventilation aper- 30 one end of the duct. Thus, such a ventilation enclosure

tures 17. The printed circuit boards contained in these could equally well be formed, for example, by a cabinet

boxes are designated in FIG. 2 by reference numeral containing electrical or electronic circuits which heat

CT. It can thus be seen that, in each box, the cards CT up in operation and therefore need to be cooled in order

are arranged parallel to one another and to the side to operate correctly, or again by a room in which air

plates 12 and 13 of the box. In the embodiment shown in 35 conditioning is provided by blowing in hot or cold air.

FIGS. 1 and 2, the connecting boxes CI, C2, C3, etc., The printed circuit boards CT, instead of being

are of identical dimensions and are so arranged in the mounted in connecting boxes as in the embodiment

support rack 10 as to form superimposed rows, the shown in FIGS. 1 and 2, could be arranged for support

boxes being placed beside one another in each of the within a single enclosure or air duct which could be

rows to form adjacent vertical columns. 40 produced by withdrawing the connecting boxes from

Referring to FIGS. 1 and 2, it can be seen that the the support rack 10 and attaching a plate similar to the

support rack 10 is formed by two side plates 18 and 19 back plate 20 to the front bars BV of the support rack in

which are connected together by a set of horizontally such a way as to form a vertical air duct through which

disposed front bars BV and a set of horizontally dis- the flow produced by fans VI, V2, V3 and V4 would

posed rear bars BR and by a back plate 20 which is 45 pass, this air duct being closed off at its upper end by the

attached in a known manner to the rear bars BR. The partition 21 which contains the apertures 22 and at its

upper part of the support rack, is completed by a parti- lower end by the support plates 25 to which the fans are

tion 21 which is provided with ventilation apertures 22; attached.

the partition 21 having been omitted from FIG. 1 for the FIG. 2 also shows that each fan is mounted within a

sake of clarity. 50 circular shroud 30 which in turn is mounted on one of

As shown in FIGS. 1 and 2, the circuits on the boards the support plates 25. Across this shroud is fitted a filter

CT which are contained within the connecting boxes 31 which is intended to trap the dust which is entrained

which have been placed in the support rack 10 are in the air drawn in by the fan. Also, in cases where hot

adapted to be cooled by small fans VI, V2, V3, etc., air has to be pumped into the enclosure, the fans are

which are housed in drawers such as 23 and 24 arranged 55 normally associated with one or more heating members,

in the lower part of the support rack and which when which heating members may, for example, each be

energized cause air to circulate through the boxes formed by an electrical resistor,

mounted in the support rack. The air passes over the In accordance with the present invention, each fan is

board CT as shown in FIG. 2 and through the ventila- equipped with a device for detecting a state of inade

tion apertures 17 and 22 mentioned above. 60 quate ventilation. As shown in FIG. 2, each detection

FIGS. 1 and 2 show that, in the embodiment being device is designated by one of the references Dl, D2,

described, each drawer disposed below the columnar D3, etc., with device Dl being associated with fan VI,

arranged boxes is formed by an upper support plate 25 device D2 with fan V2 and so on. It can be seen in FIG.

to which the fans are mounted. The plate 25 is provided 2 that each of the detection devices is fixed inside a

with openings 26 arranged to allow free passage to the 65 shroud 30 of the fan with which it is associated, that is

air drawn in by the two fans attached to the support to say in the area which is swept away by the stream of

plate. The connecting boxes forming a column are so air which the fan produces when it is operating. Each

positioned that each box is traversed by the stream of air detection device is intended to generate a signal when

the output of air from the fan with which it is associated falls below a predetermined limiting value, either as a result of an accidental blockage of the opening or openings across which the filter of the fan is positioned, or as a result of the fan stopping or merely of its speed of 5 rotation dropping considerably. In the embodiment illustrated and described, in which the air output of each fan is of the order of 50 liters per second when operating normally, this limiting value is selected to be 30 liters a second, which represents 60% of normal 10 output.

There will now be described in greater detail, with reference to FIG. 3, the constitution of a device for detecting inadequate ventilation which is produced in accordance with the invention. This detection device 15 comprises, as shown in FIG. 3, a thermal detection element 40 of a known kind, which is shown in schematic cross-section in FIG. 4, and an air non-return casing 41 in which the thermal detection element is housed. 20

FIG. 4 shows that the thermal detection element 40 is formed by a capsule 142 of cylindrical shape around which are arranged, concentric with the capsule, two permanently magnetized rings 143 and 144 and a ferrite ring 145. Ferrite ring 145 is positioned between rings 25 143 and 144. An electrical contact or switch 146 is housed in the capsule 142 at right angles to the ferrite ring 145 and along the longitudinal axis of rings 143,144 and 145. Switch 146 is constructed such that the contact arms have mechanical resilience biasing it towards the 30 open position; however, the switch contacts are normally held closed by the magnetic forces generated by rings 143 and 144. The lines of magnetic force of rings 143 and 144 are concentrated by ferrite ring 145 into a restricted region of space around contact 146. 35

If, for any reason, the temperature of the ferrite ring 145 rises and reaches a value corresponding to the Curie point of the ferrite, the ring 145 loses its magnetic properties and thus, brings about a change in the path of the lines of force and consequently causes the switch 40 contacts to open as a result of the reduced concentration of these lines in the region where the contacts are situated. The Curie point depends on the composition of the ferrite from which ring 145 is formed. In the example described, the composition has been so selected that 45 the Curie point of the ring 145 is 70" C.

It should be readily apparent that the type of thermal detection element which has just been described is in no way limiting, and that it could be replaced, to suit particular cases and applications, by any other type of 50 element of preselected characteristics which is capable of generating a signal when its temperature reaches a predetermined limiting threshold. Thus, for example, the detection element could be formed by an element of a kind similar to that which has just been described, but 55 in which the contact is normally open when the temperature of the ferrite ring is lower than its Curie point and which closes when the temperature of the ring exceeds the Curie point. Similarly, rings 143,144 and 145 could be replaced by three parallelepipeds arranged side-by- 60 side, that is to say, two permanently magnetized parallelepipeds enclosing a ferrite parallelepiped.

The air non-return casing 41 which encloses the thermal detection element 40 just described is, so produced that only a part of the stream of air produced by the 65 associated fan passes through it. As shown in FIGS. 3, 4 and 5, this air flows in the direction which is indicated by arrows F. Thus, in the embodiment being described,

the casing 41 is in the shape of a right-angled parallelepiped having two base-walls 42 and 43 and four side walls 44, 45, 46 and 47. The thermal detection element 40 extends parallel to base walls 42,43 and to two of the side walls 44 and 46. Casing 41 is mounted in the shroud 30 of the associated fan in such a way that its base wall 43, which will be referred to in the remainder of the text as its front wall, is orientated in a plane normal to the stream of air produced by the fan. In other words, this front wall 43 is perpendicular to the arrows F which indicate the direction of movement of air caused by the fan and the side walls 44, 45, 46 and 47 of the casing 41 are orientated parallel to the flow of air.

FIGS. 3 and 4 show that the front wall 43 and the side walls 44 and 46 of the casing are provided with orifices. The orifices formed in the front wall 43 are identified by reference character A and the orifices formed in the side-walls 44 and 46 are identified by reference character E. Inasmuch as orifices A face into the stream of air from the associated fan and that the orifices E are orientated tangentially to this flow, a part of the air blown by the fan is able to penetrate into casing 41 through orifices A, as shown by FIG. 5, and to leave it through orifices E. Depending upon whether the temperature of the air drawn in by the associated fan is higher or lower than that which would prevail within the enclosure if there were no ventilation, the air which passes through casing 41 either heats or cools the thermal detection element 40 situated in the casing.

In cases where the enclosure is intended to be cooled, the number and size of the orifices A and E are determined by experiment, in relation to the temperature of the air driven into the enclosure, in such a way that the temperature 6d to which the thermal detection element rises when the flow of air from the associated fan is equal to 60% of its normal output, is equal to or, failing this, is slightly higher than, the Curie point 6c of the ferrite forming the ring 145 in the detection element. However, with the device for detecting inadequate ventilation which has just been described, this requirement can only be met if the Curie point 6c of the ferrite is lower than the limiting temperature 61 which should not be exceeded in the enclosure. If, on the other hand, the Curie point 6c is higher than the limiting value 61, then it is necessary to add to the thermal detection element contained in the casing an additional heating element which enables the detection element to be raised to the Curie point temperature 6c when the output of air from the associated fan falls to 60% of its normal value. Under these conditions, when the output from the fan is normal, the temperature of the detection element is lower than, but nevertheless close to, the Curie point temperature 6c.

In the embodiment shown in FIG. 3, the additional heating element is formed by an electrical resistor R which is produced by depositing a thick film of an electrically resistive material on a flat support. Resistor R is arranged close to the detection element 40. The value of this resistor is selected, as a function of the intensity of the current which it carries, in such a way that the power which the resistor dissipates is sufficient to raise the detection element to its Curie point in a time of not more than two minutes from the time when the output of the associated fan falls to 60% of its normal value.

In the embodiment described, the detection element is approximately 15 mm long and approximately 6.5 mm in diameter. The resistor R is in the shape of a rectangle the length of which is substantially equal to that of the

7 8

detection element and the width of which is substan- raised, soon after fan V3 stops, to a temperature higher

tially equal to the diameter of the element. The resistor than the Curie point of its ring 145, thus causing its

dissipates a power of the order of 1.5 watts in the form contact 146 to open.

of heat energy and it is arranged, as shown in FIG. 3, FIG. 7 illustrates the manner in which the several

parallel to the detection element 40. The gap between 5 detection devices Dl, D2, D3 and D4 associated with

the resistor and the element 40 is of the order of a frac- fans VI, V2, V3 and V4 are connected in the embodi

tion of a millimeter. The length of the casing 41 in ment being described. In the figure, the thermal detec

which the resistor R and element 40 are enclosed is of tion elements which form part of devices Dl, D2, D3

the order of 3 cm, while its width and depth, are substan- and D4 respectively are referenced 401, 402, 403, 404

tially equal to 1 cm. The walls 43, 44 and 46 of the 10 and the switch contacts contained in these elements are

housing are each provided, in the embodiment de- referenced 461,462,463 and 464 respectively. Similarly,

scribed, with four orifices each of which is of the order the resistors associated with detection elements 401,

of 1.5 mm in diameter. 402, 403 and 404 are referenced Rl, R2, R3 and R4

Under these conditions, if the air output from the respectively. These resistors are connected in parallel

associated fan is normal, i.e. in the example described, is IS across the terminals of a current source 100 which is

of the order of 50 liters of air per second, and if the formed, in the embodiment being described, by a source

temperature of this air is of the order of 20° C, the of alternating current. Contacts 461,462,463 and 464 on

detection element 40 is raised to a temperature close to the other hand are connected in series, as shown in FIG.

60° C, that is to say, a temperature lower than the Curie 7, between the positive terminal (+) of a source of

point of the ferrite. Because of this, contact 146 of the 20 direct current and an alarm device which is formed, in

element is closed. If the output subsequently falls to the embodiment being described, by a relay coil B. The

60% of its normal value, that is to say to a level of 30 contact CB which is controlled by coil B is connected

liters per second, the detection element 40 is raised, between the positive terminal (+) and an indicator light

within the space of approximately 30 seconds, to a tern- V. The resistors Rl to R4 being supplied in this fashion,

perature slightly higher than 70° C. Ring 145, whose 25 it can be seen that, as long as the air outputs of fans VI

Curie point is 70° C, then loses its magnetic properties to V4 remain at their normal value, contacts 461 to 464

and contact 146 therefore opens and a signal is gener- will remain closed. Under these conditions, a direct

ated, in a manner which will be described below, as a current flows from the positive terminal (+) through

consequence of the break in the electrical circuit in the closed contacts 461 to 464 and holds coil B ener

which contact 146 is connected. 30 gized. Contact CB is arranged to remain open as long as

In cases where the enclosure is intended to be heated, coil B is energized and thus prevents indicator V from

on the other hand, the number and size of the orifices A being lit. Conversely, as soon as any one of the above air

and E mentioned above are so selected, in relation to outputs drops to the predetermined limiting value (i.e.

the temperature of the air driven into the enclosure, that 60% of its normal value), the contact of the correspond

the temperature 0d to which the detection element is 35 ing detecting element opens and thus de-energizes coil

brought when the flow of air produced by the associ- B. Upon B being de-energized, contacts CB close and as

ated fan is equal to 60% of its normal value is slightly a result indicator V lights and indicates that the output

lower than the Curie point of the ferrite. The result is of one of the fans VI to V4 has fallen to a level which

that, when the output of the fan is normal, the detection is inadequate for proper cooling of the circuits fitted to

element is raised to a temperature higher than the Curie 40 the boards CT.

point and contacts of switch 146 thus remain open. If The casing in which the thermal detection element is

the output then falls to 60% of its normal value, the enclosed may be of a shape other than that described

detection element 40 is brought to a temperature lower above, provided that when it is mounted on the shroud

than that of the Curie point. Because of this, contacts of of a fan, the casing has a front wall which faces into the

switch 146 close, thus causing a signal to appear in the 45 stream of air output by the fan, and has at least one side

electrical circuit in which the switch is incorporated. wall which is orientated tangentially to this stream.

In FIG. 2, the arrows indicate the the path which is Thus, the casing could, for example, take the form of a followed by the air drawn into the support rack 10 closed cylindrical box, this box being mounted in such a described in detail above, in that situation where fans way that its axis is orientated parallel to the stream of air VI, V2, and V4 are rotating normally and fan V3 is 50 produced by the associated fan, the front wall and the stopped. Since the air is under pressure in the lower part side wall of the box being provided with orifices which of the support rack contained between the fans and the allow only this flow of air to pass through it. connection boxes, a return flow of air takes place In cases where the Curie point of the ferrite forming through the fan V3 which is not operating. Also, be- the ring in the thermal detection element differs considcause this fan is stopped, the circuits on the boards 55 erably from that which normally prevails in the enclolocated above it receive scarcely any cool air and are sure, the casing in which the detecting element is placed thus no longer cooled. It can be seen from FIG. 2 that is made from a thermally insulating material. Thus, in the air which emerges from the support rack by. passing the embodiment described, the casing is made from a through the fan V3 which is stopped, travels in the plastic material which is preferably formed by a polyopposite direction from that in which it would travel if 60 carbonate.

the fan were operating normally. The detection device Although the description and the accompanying

D3 which is associated with fan V3 is in the path of this drawings relate to a preferred embodiment of the inven

return flow of air but, since top wall 42 of the casing of tion, it is understood that various modifications, addi

the device D3 contains no orifices, this flow of air, tions or substitutions which do not exceed the scope of

which is identified in FIG. 6 by arrows G, cannot reach 65 the said invention could be made, to suit individual

the inside of the casing. The thermal detection element application and cases, without the principle of the in

40 housed in the casing consequently is not cooled by vention being in any way affected thereby,

this air flow and it is therefore heated by resistor R and I claim:

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Ventilated enclosure