GB2136313A - Air Intake Equipment - Google Patents

Abstract

The equipment, e.g. for gas turbine engines, and usually of the three-stage sort, comprises a first vane stage 5 for removing the bulk of the water. Preferably a second coalescer stage 8 for coalescing - small droplets passing through the first vane stage, and a third vane stage 14 for removing the coalesced droplets are included. To provide anti-icing facilities, it is proposed to heat the first stage 5 of such equipment by using a self- regulating polymer in the form of strips or a coating, the polymer being such as to become non-conductive upon reaching a predetermined temperature. The coalescer stage 8 may also be heated. The first vane stage 5 may be preceded by a sea or fresh water scrubbing spray. <IMAGE>

Description

SPECIFICATION Air Intake Equipment This invention relates to air intake equipment, more particularly but not solely, for gas turbine engines for use in a marine or other adverse environment, to remove moisture, and particularly salt water, from the air stream entering the intake.

While the invention is described primarily in connection with gas turbine engines for use in a marine environment, it is not limited to use in such environments. In particular, where the atmosphere is polluted or dusty, as in industrial or desert locations, the air may need to be scrubbed, possibly in brackish or other low grade water, prior to use, and the air intake equipment may be needed to eliminate scrubbing spray droplets from the scrubbed air. Also, the invention may be applied to the intake equipment for Diesel engines or for heating and/or ventilating plant in marine or other adverse environments.

Vane type moisture separation systems have been widely used over many years in the natural gas, petrochemical, nuclear, steam generation and marine industries, wherever high efficiency removal of liquids from a gas stream has been required. In more recent times, with the advent of gas turbines as major power sources on ships and offshore oil and gas platforms, it has been a natural development to use these systems to remove salt water entrained in the air streams entering such engines.

Gas turbines used in marine or coastal installations (i.e., installations in areas where the atmosphere is likely to contain brine droplets) are subject to salt ingestion via their combustion air intakes, and even under the calmest weather conditions the salt content in the ambient air is likely to exceed the extremely low level necessary for a satisfactory engine life. It is also to be remembered that gas turbine air intakes situated in close proximity to the sea will not only be subjected to air-borne salt in spray or aerosol droplet form, but they will also experience massive water loadings, often of green sea intensity, when weather conditions are bad.

Specialised intake protection systems therefore became necessary to reduce the salt-inair content to the acceptable level of no more than 0.01 ppm by weight demanded by gas turbine manufacturers, under any environmental condition and, furthermore, it was vital that this duty should be performed by equipment with a low pressure drop, compact dimensions, low weight, high strength and prolonged resistance to atmospheric attack.

In order to meet these exacting requirements, resort is normally had to a three-stage air intake equipment. This type of equipment has been in regular use for a number of years, and consists of three distinct separate elements placed in series.

The first and third stages are each a developed form of vane type separator which consists of a number of parallel vanes mounted vertically, with air flow at right angles to the face of the vane unit The air flows through a tortuous path between the separator vanes resulting in impingement of moisture droplets on the vane bodies. Each vane has multiple changes of direction and multiple catchment pockets to trap the moisture droplets separated by this impingement. These droplets drain vertically downward to a catchment trough underneath the vane section. Many different vane profiles are now available.

Boxing or framing is built around the outsides of the vane bank to hold the vanes in place as well as to provide "cover-up" to prevent by-passing of air around the vane eges or tops. Water is removed from the bottom drain trough by way of auxiliary drains.

In the three-stage system, a filter/coalescer, constituting the second stage, is situated between the first and third stage vane elements and this serves the purpose of coalescing any fine aerosol droplets which may have passed through the first-stage into larger sizes which are then easily removed by the vane type separation elements of the third-stage.

The purpose of the first stage is to remove all large water loads from 'green sea' intensity down through the sprays and mists to aerosol droplets of, typically, approximately 13 microns in diameter. It is necessary for the first stage to be extremely efficient in the removal of these entrainments in order to prevent excessive loading of the second stage, formed by a coalescer pad or other element.

The second stage acts as a filter/coalescer serving to catch and coalesce the smaller aerosol droplets which may have passed through the first stage. These enlarged coalesced droplets will then either drain off the second stage itself, or be re-entrained into the airstream and carried on to the third stage.

The third stage of the system is the final separation stage and removes from the airstream those relatively large coalesced droplets which are re-entrained off the second stage. It also serves to protect the airstream against the reentrainment of highly concentrated droplets of brine solution which are generated on the second stage pad or element after the system has been operated at relative humidities below approximately 70%. In this environmental condition the second stage acts as a filter and captures dry or partially dry salt particles. These collect in solid form on the filter/coalescer medium, but when the relative humidity of the environment rises above 70% the hygroscopic nature of the salt will cause condensation to occur along with some re-entrainment of brine droplets, which contain high concentrations of salts.The third stage serves to remove these droplets from the airstream entering the gas turbine.

When operating under conditions of extreme cold, all such equipment may be subject to icing up, thus partially or completely blocking the air intake. Such icing may for instance be due to frozen spray or snow accumulating on the first stage vanes or to freezing fog penetrating the first stage and freezing in the coalescer. The third stage vanes are not normally prone to icing.

The conventional approach to the problem of icing is to heat the incoming air as it approaches the intake equipment along a duct, e.g. by providing a bank of perforate tubes fed with hot air from a convenient source. An alternative is to place a finned tube heat exchanger in the inlet duct, this heat exchanger deriving its heat source from engine exhaust gases. In either case the equipment involved for prevention of icing is expensive capital equipment which takes up a large amount of space and is relatively heavy.

These latter factors may be bearable for some applications, but make shipboard use of such equipment, e.g. on warships fitted with gas turbine engines, virtually impossible.

It has also been previously proposed to provide heating of the vanes of a vane type separator by coating the vanes with a layer of electrically resistant metal. However, this presents problems of control.

In accordance with a first aspect of the invention, there is provided air intake equipment for the removal of moisture from an air stream comprising one or more stages, namely at least one stage consisting of a vane type moisture separator and, in an arrangement of more than one stage, one stage consisting of a coalescer, in which the vane type separator is provided with heating means to heat the surface to be contacted by the air stream to minimise the formation of ice, wherein the heating means is formed by an electrically resistant heater applied to the vanes of the vane type separator, the resistant heater being self-controlling so as to switch itself off when the temperature reaches a predetermined level.

In accordance with a second aspect of the invention, there is provided air intake equipment for the removal of moisture from an air stream comprising three stages, namely first and third stages consisting of vane type moisture separators and an intermediate second stage coalescer, in which the first stage vane type separator is provided with heating means to heat the surface to be contacted by the air stream to minimise the formation of ice, wherein the heating means is formed by an electrically resistant heater applied to the vanes of the vane type separator, the resistant heater being selfcontrolling so as to switch itself off when the temperature reaches a predetermined level.

The electrical energy for heating may be derived from a generator driven by the engine to which the intake equipment is attached.

The vanes of the vane type separator may be covered with a suitable polymer which acts as an electrically conductive coating, and is connected up to a source of electrical supply, to act as a resistance heating element. The polymer is such as to be self-regulating, i.e. such as to interrupt the current when its temperature reaches a predetermined limit.

Alternatively, the separator may be heated by discrete strips of conductive polymer mounted on or embedded in the material, normally metal, of the vanes.

So far as the coalescer is concerned, a similar coating may be applied to some or all of the yarns or fibres making up the woven or non-woven coalescent pad or other element, or to the supporting wire mesh or other support for the element. In the alternative, a proportion of electrically resistant thread may be incorporated, e.g. woven in to provide a distributed heater.

A coalescer of the single or multi-layer bag type might be used rather than the corrugated woven or non-woven element type described. A bag type coalescer could be heated in a similar manner to the pad type.

The drainage components of the separator would normally additionally be heated by electrical self limiting trace heating or otherwise.

Additionally, some systems may be in four stage form, with an additional or initial stage comprising a sea or fresh water scrubbing spray.

In such a case the vane becomes the second stage, the coalescer the third, and the final set of vanes the fourth. The incorporation of a scrubbing spray would not affect the type of anti-icing system used.

The invention will be further described with reference to the accompanying drawings, in which: Figure 1 is a cut away and partly exploded perspective view of a three-stage air intake equipment to which the present invention may be applied; Figure 2 is a sectional view of a detail of a first stage with heating means; and Figure 3 is a diagrammatic sectional view of a second stage adapted for heating.

Turning first to Figure 1, this shows a typical form of three-stage air intake equipment. The incident air is illustrated by the arrow 1, and the discharge air by arrow 2. The equipment is mounted in a frame 3 having a mounting flange 4.

As explained above, first stage vanes 5 present a tortuous path to the air entering the equipment so that the moisture content of the air is trapped on the vanes, runs down drainage channels (not shown) formed on the vanes and is discharged via a drain aperture 6. Equipment of this type is normally expected to remove droplets down to aerosol sizes of typically 13 microns diameter, so that all that enters the second or coalescer stage of the equipment is air with these fine droplets entrained. The coalescer consists of a frame 7 supporting a coalescing medium in the form of a pad or other element 8, which is shown as being maintained in a corrugated form by support wires 9. The frame 7 defines a panel which is removable for maintenance or replacement and an access cover 11 and seal 12 are illustrated as being provided to close the mounting frames securely and retain the coalescer in position.

The frame 7 is also shown as being provided with a seal 13 which co-operates with the third stage supports and prevents by-passing of the air around the second stage.

The third stage vanes are illustrated by the reference numeral 14, and are generally similar to those employed in the first stage. The function of the third stage is primarily to remove the larger droplets formed from the fine droplets in the coalescer and re-entrained in the airstream, but it also acts to remove high salt content droplets which may be formed after operation in relative humidities below about 70%.

Turning now to Figure 2, this shows of a form of first stage vane which consists of a single corrugation having an incident portion 21, leading to a drain channel 22. A downstream portion 23 of the corrugation leads to a further drain channel 24, and the corrugation is shown as terminating in a straightening tail 25. The material of the vane is usually marine grade aluminium, but could be stainless steel or a suitable plastics material. As so far described, the equipment is as normally used.

Electrical heating for anti-icing is applied by coating all or part of the surface of the vanes with a self-regulating polymer coating. Particularly useful for this purpose is a polymer sold by the United States firm Raychem Inc. which becomes non-conductive on reaching a predetermined temperature, thereby switching itself off.

In particular, the electrical heating may be obtained by applying discrete strips of the selfregulating polymer to the surfaces of of the vane, separated therefrom by insulating layers. If the conditions are appropriate, the strips may be embedded in the material of the vanes.

Turning now to Figure 3, this shows one arrangement for providing heating for the coalescer. Instead of the support wires 9, the frame is traversed by a series of tubes 28, e.g. of elongate section as illustrated in Figure 3, and these tubes serve to support the element 8 of the coalescer, which may for instance be a woven or non woven material, and maintain it in the corrugated form which is conventional in coalescers. In addition, the outer surface of the ducts may be heated by resistance strips or a coating of the self-regulating polymer. To obtain best heat transfer characteristics, the tubes are preferably of extruded aluminium.

As an alternative, some or all of the fibres or yarns of the element may be coated with a resistance coating, such as that mentioned above, which is connected up to form a heater, or as a further alternative, the element may include a proportion of resistance thread, also of selfregulating polymer and connected to a suitable electrical source to act as a heater.

In a non-woven element, a heating element may be formed by one or more strips of selfregulating polymer embedded in the element.

Since the coalescer needs to be removable from the assembly, it needs to be provided with a self-closing, weatherproof electrical connector.

It will be appreciated that the vane type separator may in some circumstances be used on its own, without a coalescer to form a single stage separator. Two stage arrangements consisting of a vane type separator and a coalescer, normally upstream of the separator are also used.

Attention is drawn to our co-pending application No. 8406894 filed simultaneously herewith, entitled "Coalescer" and claiming priority from the same application No. 8307420.

Various modifications may be made within the scope of the invention. For example, the corrugations of the coalescer element may be deeper than illustrated in Figure 3.

Claims (7)

1. Air intake equipment for the removal of moisture from an air stream comprising one or more stages, namely at least one stage consisting of a vane type moisture separator and, in an arrangement of more than one stage, one stage consisting of a coalescer, in which the vane type separator is provided with heating means to heat the surface to be contacted by the air stream to minimise the formation of ice, wherein the heating means is formed by an electrically resistant heater applied to the vanes of the vane type separator, the resistant heater being selfcontrolling so as to switch itself off when the temperature reaches a predetermined level.

2. Air intake equipment for the removal of moisture from an air stream comprising three stages, namely first and third stages consisting of vane type moisture separators and an intermediate second stage coalescer, in which the first stage vane type separator is provided with heating means to heat the surface to be contacted by the air stream to minimise the formation of ice, wherein the heating means is formed by an electrically resistant heater applied to the vanes of the vane type separator, the resistant heater being self-controlling so as to switch itself off when the temperature reaches a predetermined level.

3. Air intake equipment as claimed in claim 1 or 2, for an engine, in which the electrical energy for heating is derived from a generator driven by the engine.

4. Air intake equipment as claimed in claim 1, 2 or 3, in which the resistant heating is provided by discrete strips of resistant material applied to, or embedded in, the separator.

5. Air intake equipment as claimed in claim 1, 2, 3 or 4, in which an electrically heating element is applied to the coalescing medium of the coalescer for anti-icing.

6. Air intake equipment as claimed in any of the preceding claims, in which the coalescing medium of the coalescer includes a proportion of electrical resistance thread to act as a heating element.

7. Air intake equipment including anti-icing facilities substantially as hereinbefore described with reference to the accompanying drawings.