GB1602976A - Natural ventilators - Google Patents

Description

(54) NATURAL VENTILATORS (71) We, H. H. ROBERTSON (U.K.) LIMITED, a British Company of 27 Newgate Street, Chester CHi 1DE, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to ventilation systems of the so-called "natural" type where air flow through a building is at least principally controlled by the heat generated in the building, i.e. is not powered by fans associated with the ventilation system.

Such ventilating system require as ventilators large, usually elongate, air-exit structures or heads above wide vents or "chimneys" through the roofs of buildings, often referred to as ridge ventilators, in order to give satisfactory ventilation in view of the large volume of air flow required at the low speeds at which it will take place.

The size of the ventilator is at least partly dictated by the width of the vent or vents and the necessity to provide for weather protection, particularly from ingress of rain water. Hitherto, natural ventilators have had an elongate central ridged portion disposed above the vent or chimney and extending downwardly on each side to positions beyond but above the sides of the vent or chimney. This ridged portion is normally located between spaced side walls that extend outwardly and upwardly, i.e., divergently, from the sides of the vent or chimney and then curve or return inwardly about and above the edges of the ridged portion.

Such a ventilator structure is not very efficient in relation to air-flow and detailed research using models showed turbulence by vortex shedding at the vent or chimney exit and at edges of the ridged portion and also the collection of air under the ridged portion.

This inefficiency has been compounded where, in order to achieve a desired exit aperture between the upper edges of the side walls, it has often been necessary to provide for further weather protection by way of additional inclined shielding walls fixedly positioned between the ridged portion and the upper parts of the side walls or below the edges of the ridged portion.

Based on test results, our copending apapplication no. 52139/77 (Serial no. 1602190) proposes an elongate ventilator structure having a central longitudinal inner portion spaced from the outer side walls and of upwardly divergent sided form from a centrally interconnected longitudinal position so as to aid guidance of exit air in passageways between outer structure walls and the inner portion.In addition provision was envisaged for exit-air guiding means contributing to reducing the incidence of turbulence by elongate curving guides at the mouth of the duct or passage to be served and at free edges of the inner portion, contributing to aiding desired directions of air-flow and smooth transitions thereof by curved baffling within the transition from the mouth of the duct or passage to the space or airway between the divergent inner portion walls and outer side walls and within the space or airway between the edges of the inner portion and the outer side walls. In scaling up to full size ventilator structures from the orig inal models it was, of course, found that very significant improvements were obtained but we suspected that certain of the features might have greater effects than others and that subcombinations thereof might be particularly advantageous.

This has indeed been found to be the case, the most important exit-air guidance features being the curved guides at the mouth of the duct to be served and the curved baffling between the edges of the inner portion and outer side walls. These full scale tests and experiments also confirm not only the importance of streamlining flow at air speeds realisable in natural ventilators and hitherto thought to ;be worthwhile applying only to forced flow ventilators, but also the effect on the coefficient of discharge of the Reynolds number of the ventilator, especially in testing smaller scale models to apply test results to full scale ventilators and previously thought to be of little significance in relation to natural ventilators.In particular, the Reynolds number being dependent on the pro duct of air velocity and throat diameter leads to higher speeds for smaller throats and thus non-comparable turbulance. The discrepancies between models and full scale ventilators, though to be expected, did produce the somewhat surprising outcome as to feature importance, virtually eliminating both throat baffles and reducing, but not eliminating, the contribution of the curved guides at the inner member edges. This is considered to be a valuable feature of the ventilators herein especially in relation to the different relative significances of airway edge guiding and in airway baffle guidance at the input and towards the output side of the ventilator which, we feel, is a surprising result.Thus airway edge guidance makes much the greatest contribution at the duct or ventilator throat, whereas in airway baffling makes the greatest contribution at and past edges of an inner duct or throat covering portion.

However, detailed study and evaluation of smoke test films has led us to further surprising and important developments. Firstly, the unimportance of the throat baffle was deduced to be because of the air flow, even at such low speeds as 1 to 4.5 metres/second, being trained by uppermost curvature of the throat edge guides thereby drawing the air flow into the passageways past the inner portion without the need of further baffle assistance. In considering this result, we came to the idea that a reduction of drag at the diverging sides of the inner portion might further improve performance. Th its turn, this led us to the concept of a flexible boundary of the passage instead of the rigid sides of the inner portion.Believing that gas-togas contact would be the ultimate in flexible boundaries, we introduced an air entrapment member instead of the divergent inner portion, effectively reverting to the originally discarded cap, and, indeed found a significant improvement in discharge coefficient.

The essential feature is, however, that the duct-forming configuration accommodate more air than the desired air passageway width and thereby slow or entrap additional air to give the desired flexibility of interface to the main air exit streams. An arched or domed structure will clearly suffice for entrapment and facilitates drainage.

According to one aspect of the invention, then, a natural ventilator comprises an elongate structure associated with a buildingcommunicating duct or passage extending over at least part of a roof, the structure comprises2 upwardly extending outer lonal- tudinal side walls that initially diverge from adjacent sides of said duct or passage and then converge to positions spaced from each other, a central longitudinal inner part spaced from the outer side walls and of an arched or domed configuration facing the duct or passage that offers accommodation for air additional to the minimum actual cross-section of exit air flow from the duct through the structure so as to entrap such additional air as a cushion of flexible interface to main exit air flow, curved baffling within the space or airway between edges of the inner part and the outer side walls at transitions of the side walls from divergent to convergent, and elongate outwardly flared lips at upper ends of an entry or throat to the structure for air passing upwardly via the duct.

As with our other copending application no. 52138/77 (Serial no. 1602975) we prefer that the curved baffling be pivotally mounted so as to be movable to close off the air passageways through the ventilator.

The second improvement also arises from the action of the curved throat guides in training air flow because, for a normal ridged roof installation, air will already have a converging movement in passing towards the entry throat of the ventilator and initially continues to converge in the throat before being spread by the ventilator resistance and becoming entrained in the curved throat guides at their upper parts. Clearly, the effective throat width is at the position of maximum convergence of the air flow and thus is less than the actual ventilator throat width.

What we therefore propose is that the or further throat guides be additionally curved at or beyond lower positions of the throat, preferably as continuously though not necessarily constantly curving guides to the above mentioned curved guides, i.e. over their height, so as to entrain the air prior to entry into the throat proper. This is normally readily achieved within the thickness of a normal roof, especially one clad both internally and externallv, and, in effect, results in a flared throat entry. This aspect of the invention also has aeneral ability.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing showing a cross-section through a natural ventilator.

As discussed in relation to Figures 1 and 2 of our copendina application no. 52139/77 (Serial no. 1602190), natural ventilation systems are of substantial size compared with the building served and are usually mounted in a wide gap along the centre of a ridged roof. Louvred air entries at the lower parts of the building walls allow the heat generated within the building to "power" the ventilation system with exit air guided by the roof slopes to the ventilator structure proper, which will usually extend at least most of the length of the building and be constructed in modular sections ioined together at respective adjacent end frames.

Referring now specifically to the drawing, a preferred ventilator structure has a throat 20, typically of 1 to 5 metres width, fitting to or in a roof duct to be serviced. From the throat 20 outer casing walls 30 extend upwards in a generally convex shape on each side terminating at spaced positions affording a wide elongate exit mouth 32. Suitable provision is made for weathering side walls 34 of the elongate throat to the roof of the building to be served, and there will be a space between those side walls 34 and the lower edges of the outer casing walls 30 to allow fall-through of rain water.

The throat side walls 34 extend into the outer casing and have outwardly curved terminations or lips 36 to assist in reducing any turbulence in exit air as it divides into two streams in airways 38 defined between the casing walls 30 and edges 28 of a ridged throat weathering cap-like guard 40 as a central longitudinal inner part presenting downwardly divergent panels or walls 42 from a centrally connected position 44 central above the throat to positions spaced from the casing walls 30 at or close to the level at which they begin curving towards each other. The edges 28 of the panels or walls 42 could have curved terminations or guides, but a simple angled drain gutter for rainwater is satisfactory.

The curved guide lips 36 are cffective at the low air speeds envisaged and mentioned above to train air flow side-ways into the airways 38 and the cap-like guard 14 entraps air rising from the throat and provides a flexible interface 26 of no structural definition to main airflow with and through airways 38.

In order to assist streamlining airflow through the airways 38, curved baffles 50 are provided at the region of and above the ends of the cap panels 42. It will be noted that the baffles 50 have straight parts that are convergent above the panel edge terminations 28 and convergent, and curved parts from just below the panel edge terminations 28 and of greater curvature than the outer side walls 30. The normal means of shutting off the ventilator is as with our copending application no. 52138/77 (Serial no. 1602975), by pivotting of the baffles 50 at 54.Preferably their upper straight parts are moved away from each other so that their lower ends 56 overlap the panel terminations 28 and their upper ends 58 meet the outer casina V walls 30. so as to give good weathering protection, the lower ends 56 then being still at a greater spacing than the throat side walls 34.

It is also noted that, the casing walls 30 resemble a goblet shape with their lower parts having a substantial curvature to guard against splashing of rain water into the throat. Centre points for the various curvatures are as indicated and serve in achieving optimum flow, especially with adjust ruent of the baffles.

Details are shown of one form of end frame to which the panels constituting walls 30, 34 and 42 are affixed and the pivots 52 and 54 mounted. Specifically, spaced vertical frame members 60 define the throat 20 and help support a horizontal member or beam 62 itself supporting frame members 64 of the cap guard 40 that are secured by mounts 74, on upwardly divergent frame members 76 extending from aclJacent lower casing wall securements 78 on the throat verticals 60 to a top horizontal frame member or beam 86 at the ventilator mouth. The mounts 74 also carry upwardly extending slightly convergent struts or braces 80 that terminate on mounting plates 82 on the top frame member or beam 86. The resulting frame has the desired rigidity and the braces 82 carry mounts 88 for the upper baffle pivots 54.

Other satisfactorily rigid frame structures may be used, but the provision of a top horizontal member 86 is particularly advantageous in allowing the mounting thereon of linear electric motor means for controlling pivotting of the upper baffles 50 via suitable linkages as described in our copending application no. 52138/77 (Serial no. 1602975), thereby giving ready access for maintenance purposes via a catwalk or the like.

Such a structure is most usually a module of desired length allowing end-to-end and frame-to-frame interconnection in order to make up any desired size of ventilator.

For a 1 metre throat width a 2.5 metre long module most satisfactory results were obtained with a range of Reynolds members of from 1.90x104 to 3x105 corresponding to throat air velocities of 2.8 metres/second to 4.5 metres/second. The pressure loss coefficients (k) were about 2 and the coefficient of discharge was as high as 0.66 to 0.71 which is significantly in excess of the general level of 0.5 to 0.55 for prior ventilators and even compare favourably with values of about 0.62 for the inverted cap model of our above copending application no. 52139/77 (Serial no. 1602190).

Such test results are extremely encouraging as, even though of necessity at this stage, conducted in a test rig rather than in a full practice installation the measure of coefficients of discharge exceed 0.60 which has hitherto been quoted in the literature as a maximum for a square hole with no weathering provision at all, and even approaches if not exceeds that for an elongated hole.

Lower parts 90 of the throat side walls are shown curvingly divergent to assist in entraining air prior to entry into the throat proper. Dashed lines are used to show preferred continuous curving, but within the supports 60, whereas it would be equally feasible to have their closest spacing at the brackets 78. The closer that the throat inlet and outlet curving can approximate at normal air speeds to a shape conforming to the vena contracta effect an air passing through an opening corresponding to the throat size, the better will be the results. However, any significant throat entry training curvature' will assist in making fuller use of the actual throat width dimension in reducing the extent of detachment of the air stream from the throat walls at medial heights thereof.

The height of the throat baffles should, of course, be sufficient to give good protection against rain ingress, despite the upper outward flaring thereof, and the end framed structure herein proposed permits achievement of a satisfactorily rigid structure without any requirement for top support of the outer side walls at edge-most positions where substantial reductions of discharge coefficient are then likely to result.

WHAT WE CLAIM IS: 1. A natural ventilator comprising an elongate structure associated with a buildingcommunicating duct or passage extending over at least part of a roof, the structure comprising upwardly extending outer longitudinal side walls that initially diverge from adjacent sides of said duct or passage and then converge to positions spaced from each other, a central longitudinal inner part spaced from the outer side walls and of an arched or domed configuration facing the duct or passage that offers accommodation for air additional to the minimum actual cross-section of exit air flow from the duct through the structure so as to entrap such additional air as a cushion of flexible interface to main exit air flow, curved baffling within the space or airway between edges of the inner part and the outer side walls at transitions of the side walls from divergent to convergent and elongate outwardly flared lips at upper ends of an entry or throat to the structure for air passing upwardly via the duct.

2. A natural ventilator according to claim 1, wherein the inner part has its edges lowest and straddling the width dimension of the duct or passage.

3. A natural ventilator according to claim 2, wherein the curved baffling flattens its curvature above the edges of the inner part and extends substantially to the upper level of the outer side walls.

4. A natural ventilator according to any preceding claim, wherein the curved baffling is adjustable to close ofi the ventilator by obstructing airways between the inner part and the outer side walls.

5. A natural ventilator according to claim 4, wherein the curved baffling comprises elongate pivotted baffles one to each side of the inner part and pivotable to engage the nearest side wall and also underlap the edges of the inner part.

6. A natural ventilator according to claim 5, wherein the baffles are dimensioned and pivotted so that their lower edges are always spaced by more than the duct width even when underlapping the inner part edges.

7. A natural ventilator according to any preceding claim, wherein the or further throat guides are curved at or beyond lower positions of the throat to entrain air prior to entry into the throat proper.

8. A natural ventilator according to claim 7, wherein, at each side of the throat is a continuously curved guide affording air training at its lower part prior to entry into the throat proper and at its upper part into the ventilator structure proper between its said outer side walk.

9. A natural ventilator arranged and adapted to operate substantially as herein described with reference to and as shown in the drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. feasible to have their closest spacing at the brackets 78. The closer that the throat inlet and outlet curving can approximate at normal air speeds to a shape conforming to the vena contracta effect an air passing through an opening corresponding to the throat size, the better will be the results. However, any significant throat entry training curvature' will assist in making fuller use of the actual throat width dimension in reducing the extent of detachment of the air stream from the throat walls at medial heights thereof. The height of the throat baffles should, of course, be sufficient to give good protection against rain ingress, despite the upper outward flaring thereof, and the end framed structure herein proposed permits achievement of a satisfactorily rigid structure without any requirement for top support of the outer side walls at edge-most positions where substantial reductions of discharge coefficient are then likely to result. WHAT WE CLAIM IS:

1. A natural ventilator comprising an elongate structure associated with a buildingcommunicating duct or passage extending over at least part of a roof, the structure comprising upwardly extending outer longitudinal side walls that initially diverge from adjacent sides of said duct or passage and then converge to positions spaced from each other, a central longitudinal inner part spaced from the outer side walls and of an arched or domed configuration facing the duct or passage that offers accommodation for air additional to the minimum actual cross-section of exit air flow from the duct through the structure so as to entrap such additional air as a cushion of flexible interface to main exit air flow, curved baffling within the space or airway between edges of the inner part and the outer side walls at transitions of the side walls from divergent to convergent and elongate outwardly flared lips at upper ends of an entry or throat to the structure for air passing upwardly via the duct.

2. A natural ventilator according to claim 1, wherein the inner part has its edges lowest and straddling the width dimension of the duct or passage.

3. A natural ventilator according to claim 2, wherein the curved baffling flattens its curvature above the edges of the inner part and extends substantially to the upper level of the outer side walls.

4. A natural ventilator according to any preceding claim, wherein the curved baffling is adjustable to close ofi the ventilator by obstructing airways between the inner part and the outer side walls.

5. A natural ventilator according to claim 4, wherein the curved baffling comprises elongate pivotted baffles one to each side of the inner part and pivotable to engage the nearest side wall and also underlap the edges of the inner part.

6. A natural ventilator according to claim 5, wherein the baffles are dimensioned and pivotted so that their lower edges are always spaced by more than the duct width even when underlapping the inner part edges.

7. A natural ventilator according to any preceding claim, wherein the or further throat guides are curved at or beyond lower positions of the throat to entrain air prior to entry into the throat proper.

8. A natural ventilator according to claim 7, wherein, at each side of the throat is a continuously curved guide affording air training at its lower part prior to entry into the throat proper and at its upper part into the ventilator structure proper between its said outer side walk.

9. A natural ventilator arranged and adapted to operate substantially as herein described with reference to and as shown in the drawings.