GB2147663A - Cooling by a stream of filtered air or gas - Google Patents

Abstract

A device for cooling equipment by a stream of filtered air or gas has a casing 2 and 3 containing a filter 1, the interior of the filter being sealed from the exterior. Contained entirely within the filter is an ejector consisting of a nozzle element 12 and a combining tube 7, operated by a supply of externally filtered compressed air or gas introduced through an inlet port 5 in the cover 2. The ejector causes a reduction in pressure inside the filter and air or gas is drawn through the filter material and the combined flow of operating and induced air or gas is discharged through orifice 8 on to the equipment to be cooled. <IMAGE>

Description

SPECIFICATION Improvements in and relating to apparatus for cooling by the expansion of compressed air or gas This invention relates to a device for cooling mechanical or electrical equipment or other moving or stationary apparatus which generates heat or which has to operate in a hot environment, by means of the expansion of compressed air or gas, in situations in which the cooling air or gas has to be filtered to prevent dust, oil, or other particulate matter from contaminating the equipment being cooled.

The said cooling device incorporates an integral air flow augmentor by means of which the quantity of air supplied to operate the device is multiplied by a factor ranging from about ten to about twenty by ambient air from the surrounding atmosphere being drawn in through a filter by an internal reduction in pressure generated by the said air flow augmentor.

The inlet compressed air itself is cleaned by passing it through a filter of a fineness at least equal to that of the filter through which the additional air is drawn by the airflow augmentor.

The arrangement of the parts of the said cooling device is such that the air flow aug mentor is completely contained within the filter through which the additional air is drawn. By this arrangement of parts the space occupied by the said cooler in minimized, an important factor in many applications.

According to the invention, a device for cooling mechanical or electrical equipment or other moving or stationary apparatus which generates heat or which has to operate in a hot environment by means of the expansion of compressed air or gas, in applications in which the cooling air or gas has to be filtered before being used to cool the said equipment, comprises a pleated paper filter, as used in automobile airfilters, in the form of a short hollow cylinder clamped between two circular plates forming the top and bottom covers respectively of the filter casing so that the central axis of the said filter element coincides with the central axis of the said top and bottom covers, the said top cover having at its centre and projecting into the interior of the casing a circular nozzle element drilled with a plurality of holes, the configuration of which will be described below, the said nozzle element being supplied with compressed air or gas through a connection external to the said top cover, the central axis of the said nozzle element coinciding with the central axis of a circular tube attached to the internal side of the said bottom cover, the said bottom cover having a hole through it corresponding with the bore of the said tube, the said tube projecting through the circular internal bore of the said filter element to a certain distance from the face of the nozzle element so that a gap exists between the top of the said tube and the face of the said nozzle element, the exterior of the said lower cover having suitably tapped holes for attachment to the equipment to be cooled.

The said filter element is of the normal type of automobile filter, consisting of a paper or porous material strip folded in zig-zag fashion and formed into a circular shape, the top and bottom edges of the porous strip being bonded into rubber or elastic plastic end plates so that when the said filter is clamped between the flat surfaces on the insides of the said top and bottom covers, an air-tight joint is formed. The object of using an automobile filter element is that spare units are easily obtainable in local garages and automobile spare part shops.

The said top and bottom covers are clamped together by a plurality of spacing bolts around the periphery of the said covers outside the said filter element. Outside the said spacing bolts and clamped between the said covers is a circumferential grill or mesh to protect the said filter element.

When gas is discharged from an orifice it is completely turbulent and because of turbulent friction at the periphery of the jet the emerging jet becomes partly mixed with the surrounding fluid so that the mass flow increases in the downstream direction; at the same time the jet spreads out and its velocity decreases but the total momentum remains constant.

The quantity of entrained air is therefore proportional to the periphery of the jet. By having a plurality of jets instead of a single jet of the same crosssectional area, the proportion of jet periphery to area is increased. The total circumference of the smaller nozzles will be equal to the circumference of a single large nozzle of the same cross-sectional area as the sum of the cross-sectional areas of the smaller nozzles multiplied by the square root of the number of smaller nozzles.

There is a limit to the smallness of the nozzles, firstly from an aspect of ease of manufacture and secondly from the necessity of avoiding blocking from particulate matter in the air supply. Practical experience has indicated that holes of approximately 0.04 inches in diameter are satisfactory, giving a discharge of about 2 standard cubic feet per minute each with an inlet pressure of 100 p.s.i.

In general, the angle of divergence of a jet is observed to be about 10 . To avoid a number of jets discharged from the nozzles impinging on and mixing with one another at too early a stage downstream of the nozzle, so that the full circumference of each jet is available to entrain the surrounding air or gas by turbulent friction for as far downstream of the nozzle as possible, the axes of the nozzle holes in the nozzle element are made mutually divergent, at an angle greater than the angle of divergence of the jet.

The object of the above said circular tube attached to the inner side of the said bottom cover is to prevent the divergent jets dissipating their energy by spreading too widely and to control the augmented air flow directionally to issue perpendicularly from the centre of the said bottom cover.

If the nozzle holes in the said nozzle element are parallel holes as produced by a drill only and the inlet air or gas pressure. is 100 p.s.i., this being the usual nominal pressure in a factory air system, then the air or gas will expand in the nozzle to the 'Critical Pressure' only. In the case of air the critical pressure is 0.528 so that the pressure at the nozzle outlet will be 52.8 p.s.i. and the velocity of the jet will be equal to the speed of sound. At this critical pressure the mass flow of air is a maximum and even though the pressure on the downstream side of the outlet is atmospheric, no further increase in mass flow is possible.

If, however, the nozzle is extended in the form of a cone of the correct divergence as in the de Laval nozzle, the jet will attain a velocity greater than the acoustic velocity. Although the mass flow remains the same, the increase in velocity causes an increase in momentum of the jet. This additional momentum allows additional quantities of the surrounding air or gas to be entrained by turbulent friction. Therefore, the form of the nozzle holes should be similar to that of the de Laval nozzle, that is, a parallel portion with a a rounded entrance followed by a divergent portion with an included angle of about 10% An embodiment of the invention is illustrated by way of example in the accompanying drawings.

Figure 1 is a plan view looking down on the top cover Figure 2 is a cross-sectional elevation Figure 3 is an outside elevation of the unit mounted on an enclosure Figure 4 is an enlarged cross section of the nozzle element.

An automobile-type filter element 1 is clamped between a top cover 2 and a bottom cover 3. The said covers 2, 3, are clamped together by a plurality of bolts or studs 4. The said top cover 2 contains an inlet port 5 communicating with the nozzle element housing 6.

The said bottom cover 3 has an air tube 7 secured to the said cover by adhesive or other means, the said air tube 7 being of the same internal diameter as the outlet port 8 in the said bottom cover. Tapped holes 9, 9, are provided in the bottom boss of the said bottom cover 3 for mounting the unit on the apparatus to be cooled, by means of screws 10, 10.

The said nozzle element 11 is screwed into the said housing 6 in the top cover 2, as shown in Figures 2 and 4. The said nozzle element contains a plurality of nozzle holes 12, 12, equally spaced on a circular pitch. The said nozzle holes 12, 12 are drily wed or otherwise formed on axes diverging at an angle 6 from the axis of the said nozzle element. The said nozzle holes 12, 12, are formed with a rounded entrance, a parallel throat portion and a diverging conical section downstream of the throat.

A mesh guard 13 around the outside of the filter element 1 is also clamped between the said top and bottom covers 2, 3.

Claims (5)

1. A device for cooling any equipment by a stream of filtered air or gas, comprising a casing containing a filter, the exterior of which is sealed from the interior except for air or gas being drawn through the porous filter material by a reduction in pressure inside the filter caused by an ejector operated by externally4iltered air or gas, the said ejector being contained entirely within the interior of the filter, the combined flow of air or gas through the filter with the operating air or gas being discharged through an orifice on to the equipment to be cooled.

2. A cooling device as claimed in claim 1 wherein one cover contains an inlet passage for air or gas leading to a nozzle element containing a plurality of convergent-divergent nozzles equi-distantly disposed around a circle, the axes of the nozzles being inclined outwards from the central axis of the nozzle element at an angle not less than the angle of divergence of the fluid jets themselves at the maximum operating pressure.

3. A cooling device as claimed in claims 1 and 2 wherein a filter element of normal automobile air filter type is clamped between two covers by bolts so that the only passage of induced air or gas from the outside to the inside is through the porous filter material.

4. A cooling device as described in claims 2 and 3 in which the ejector comprises a nozzle element and a combining tube, the said nozzle element and combining tube being coaxial with the casing and filter element, with a gap between the face of the nozzle element and the inlet end of the combining tube.

5. A cooling device of the type referred to substantially as described with reference to the accompanying drawings.