CA1164360A - Method for removing moisture particles - Google Patents
Method for removing moisture particlesInfo
- Publication number
- CA1164360A CA1164360A CA000387029A CA387029A CA1164360A CA 1164360 A CA1164360 A CA 1164360A CA 000387029 A CA000387029 A CA 000387029A CA 387029 A CA387029 A CA 387029A CA 1164360 A CA1164360 A CA 1164360A
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Abstract
ABSTRACT OF THE DISCLOSURE
An improved method for removing particles en-trained in air, particularly moisture particles which may contain salt or other contaminating matter. The im-proved method is particularly useful in connection with marine applications for removing moisture particles from air being introduced to a gas turbine on a marine vehicle The improved method includes the steps of passing the air at a velocity greater than 20 ft. per second through an inertia separating device for inertially removing at least a portion of the larger sized particles in the air to provide partially processed air, and then passing the partially processed air at a velocity which is greater than a predetermined velocity through an impact filtering pad for removing particles entrained in partially processed air. The impact filtering pad comprises at least one layer of a plurality of fibers, each of the fibers having a diameter greater than .001 inches and less than .006 inches, and the ratio of total surface area of the fibers in the pad to the volume of the pad being greater than 45 ft.-1 and less than 1400 ft. -1. The predetermined velocity at or above which the partially processed air is passed through the impact filtering pad is greater than 20 ft. per second and is chosen according to the diameter of the fibers in the pad so that coalescense of moisture particles captured by the pad is minimized.
An improved method for removing particles en-trained in air, particularly moisture particles which may contain salt or other contaminating matter. The im-proved method is particularly useful in connection with marine applications for removing moisture particles from air being introduced to a gas turbine on a marine vehicle The improved method includes the steps of passing the air at a velocity greater than 20 ft. per second through an inertia separating device for inertially removing at least a portion of the larger sized particles in the air to provide partially processed air, and then passing the partially processed air at a velocity which is greater than a predetermined velocity through an impact filtering pad for removing particles entrained in partially processed air. The impact filtering pad comprises at least one layer of a plurality of fibers, each of the fibers having a diameter greater than .001 inches and less than .006 inches, and the ratio of total surface area of the fibers in the pad to the volume of the pad being greater than 45 ft.-1 and less than 1400 ft. -1. The predetermined velocity at or above which the partially processed air is passed through the impact filtering pad is greater than 20 ft. per second and is chosen according to the diameter of the fibers in the pad so that coalescense of moisture particles captured by the pad is minimized.
Description
~ 3 ~ 0 The presen-t i.nvention relates to separating dev:ices and methods, and more particularly to separa-ting devices and me-thods for gas turbines in environments having rnoisture par-ticles in the air such as gas turbines for marine applications. For e~ample, the method of the present inven-tion is particularly useful for removing moisture particles entrained in air entering the air in-take of a ~as turbine for ships.
Moisture separators are provided for gas turbines for marine applications as the moisture particles in the air generally contain sal-t which, if it should be in-troduced into the turbine, would deleteriously affect the component parts of the turbine, as for example, by chemical corrosion. Further, the dry particles entrained in the air, for example sand and/or salt crystals, can cause "pitting"
of the -turbine components if they are not removed. However, by far the greatest concern is the moisture particles containing salt.
One early prio~ ar-t device for removing moi~ture and other particles from the air introduced into -the tur-bine comprised a single stage wire mesh pad placed in the air intake duct. The mesh pad was generally comprised of a plurality of layers forming an approximately two inch thick pad wi-th each oE the layers in turn comprised of a plurality of .006 inch diame-ter wires knitted into a grid or screen having approximately five to six stitches per inch, i.e., there were approximately five wires per inch of length stitchecl or joined to~ether. The mois-ture particles were removed as a result of the par-ticles impact-ing acJainst and being cap-tured by the wires of the pad 1 3 6 ~
as -the air passed -through the device. Howe~er, such a wire mesh pad did not adequately and efEiciently remove all sizes of moisture particles entrained in -the air and thus, was not acceptable.
Another prior art device for removing moisture particles which was found to be more acceptable, comprised a three stage separa-tor in which the first and -third stages were iner-tia separating vanes and the second stage was a coalescer. The first and third stage vanes had generally similar performance characteristics, one o~ which was tha-t they exhibited very poor efficiency in removing smaller sized droplets, namely, eigh-t microns and below.
The reason for this is simply that inertia separa-tlng devices work on the principle that as -the air flow turns to bypass the vanes or other impacting medium the moisture - particles, being of larger size arld more mass t cannot make the turns and instead impinge or impact upon the impacting medium, thereby being removed from the air stream. However~
these conventiorlal inertia vanes do not prove efficient to remo~e lower size droplet particles since such particles, being of lighter mass, can make the turns to avoid impact-ing on the vanes. ~s a consequence, the prior art employed a second stage filter comprised generally of a plurality of polyester fibers of .001 inch diameter or smaller. The duty of the second stage was to cap-ture -the fine droplets by inertial impaction that had passed through the first stage.
Owing to their smaller diameter, the capture efficiency of the fibers was high so that some of the f:ibers would tend to collec-t several droplets which then coalesced or grouped together untll a drople-t was formed which was large enough -to be re-en-tralned by the aerodynami.c drag forces of the air passing therethrough. These re-entrained droplets were then captured by the third stage inertial device (either vanes or cyclones).
However, the a~dition of the third stage added grea-tly to -the overall size and weight of the moisture - separator, which already was quite large in order to aci~ieve the high mass flow rates needed by the gas turbines for ships (on the order of 2000 cubic feet per second).
Further still, the use of a polyester fiber pad having a plurality of closely spaced and dense fibers, each of a small diameter (on the order of .001 of an inch or smaller), tended to increase the flow resistance, and thus the pressure drop across the separator, for a given velocity of air flow. Thus, to achieve the desired flow rate, -the size of the polyester pad and -thus the size of the air duct in which the moisture separator was supported also had to be increased, thereby further adding to the -weight, bulkiness, cost, etc. of the separating device.
It should be noted in this connection that various types of other filtering or separating arrangements and, in particular filtering pad arrangements, not speci-fically directed for use in marine applications are known : in the prior art. For example, in U.S. Patent No. 4,086,070 to Argo et al, -there is di.sclosed a fiber bed separator and method for separation of aerosols from gases without re-entrainment. In the improved fiber bed separator of Argo et al., ~here is provided a pair of fiber beds each com~
prised of a plural.ity of fiber elements or wires. In this reference, the distinction is made between the amount 3 ~ 0 of wa-ter which is held by the bed after draining by gravity and that held against the drag of flowing gas or air there-through. More part:icularly, the fiber diameter and bed ~oidage for -the first or front bed in Argo et al. is selected such that at the design bed velocity and aerosol loading, the first bed will not be flooded and the ~sidual saturation thereof against gas drag on the liquid collected will be less than the residual saturation of the bed against gravity drainage of the liquid. In other words, the front or first bed of Eibers in Argo et al. is such that the collected liquid in the bed tha-t does not drain by gravity will be blown through to -the second ~iber bed. In the second fiber bed on the other hand, the fiber diameter and bed voidage are chosen so that the residual saturation thereof against gas drag will be greater than the residual saturation of the bed against gravity drainage so that the liquid collected by the second bed will drain of~
by gravity as oppo~ed to being re-entrained.
Thus, in accordance with -the ~rgo et al teachi~, a more dense fiber pad is used as a first stage i~ a filter-ing device in which -the residual saturation against gravity drainage is greater than the residual saturation against gas phase drag so tha-t liquid which does not drain from the first stage will be blown through to a second pad where the residual saturation against gas phase drag is greater than the residual saturation against gravit~
drainage so that liquid wi]l drain off rather than be re-entrained in the air. Accordingly, the Argo et al solu-tion to the problem of re-entrainment experienced in the prior art is to provide a second pad ha~ing specific charac-teristics chosen so that the pad has a greater tendency J 1 ~3~(~
-to dra:in Gaptured or collected l:Lquid by means o:f: gravity than to permit re-entrainment.
However, upon a deta:il.ed examination of the Argo et al system, it is seen that the flow veloci-ties through the fiber pads w:ith which the Argo et al ar-rangement is concerned are relatively low in comparison to the flow velocities through filtering arrangement used in connection with marine applications; i.e., the Argo et al arrangement is specifically designed to be utilized in connec-tion wi-th flow veloci-ties which are generally less than 10 feet per second, whereas in marine applications flow velocities of greater than 20 feet per second are expe-rienced. Furthermore, it is to be noted that ~rom an : examination of the Argo et al reference, in particular Figures 1 and 7, Argo et al suggests that the residual saturation of the second fiber bed against gas phase drag in virtually all instances decreases as the velocity through the bed increases, and that the upper limit of velocity through the bed is usually set hy re-entrainment problems. This may be due to the fact that the Argo et al reference teaches that the particles captured by the fiber beds coalesce and drain by gravity.
In this regard, it is to be noted that re-entrain-ment of any collected or coalesced liquid droplets would require an additional device downs-tream of the filter pad for the purpose of recapturing the re-entrained particles in order to prevent the introduction of such moisture particles which are re-entrained into -the turbine.
An~ther type of arrangement of the prior art~
again not specifically directed for use as a moisture separator in marine applica-tions, is disclosed :in U.S.
J ~ ~3~0 Paten-t No. 4,158,~-~49 to Sun et al. This patent discloses an in]et air cleaner assembly for -turbine engines which are mainly used in connection with agricultural aircraft for the application of chemicals over large and not readily accessible land areas. In the arrangement of -this reference, there is provided a first stage vortex air cleaner and a second stage mist eliminat,or assemb-y. The first stage vortex air cleaner operates generally on the principles of inertia separating devices for removing heavier and larger sized contaminate particles in the air to provide partially processed air. The second stage of the filtering arrangement comprises a plurality of superimposed knitted wire mesh sheets which define passages therethroug~
for removing light-weight or well disperse~ solid contami-nate particles and liquid drople-ts. The wire mesh sheets are comprised of a plurality of monofiliments which may have a diame~er ranging from .0005 inches to 0.035 inches and which are compressed substantially throughout the surface area. The monofiliments preferably are coated with oil or another nonvolatile liquid so that liquid which is removed by impingement on the oil coated wires of the mesh will tend to flow by gravity downwardly and collect in the bottom of the mist eliminator stage for subsequent removal.
Thus, it is apparent that even in this reference which is not mainly concerned or directed to a separator assembly for marine applications, there is the sugges-tion that liquid which is removed by the mist eliminator as-sembly coalesces and drains by gravity to a lower portîon of the assembly. As such the use of this asse~bly in a ~ ~ 6~3~0 marlne appllca-t:ion where rnois-ture particles are to be removed, would sugges-t -that a further separating device is required do~nstream thereo~ for capturing any coalesced particles which become re-entrained. That is, because of - the suggestion of coalescence occurring~ it would be expected, especially at the particularly hi.gh ~low velocities with which separating devices for marine applica-tions are concerned that -there is the possibili-ty of the coalesced particles becoming re-entrained in the air stream. Thus, it would be expected that such re-entrainment o* coalesced particles would create a problem, such as experienced in connection with some of the other prior art arrangements discussed hereinabove if the Sun et al arrange-ment were used, and thus necessitate the use of additional separator devices or means downstream of the filtering pad,.
In this regard, as noted above, coalescense would create a problem of re-entrainment at the relati~ely high flow veloci-ties such as experienced in connection with marine applications since the aerodynamic drag forces produced thereby have a greater tendency to cause re-entrainment of the enlarged coalesced droplets.
According to the presen-t invention, there is pro~ided a method of removing particles entrained in the air, the air including particles of moisture, the method comprising the steps of: passing the air at a velocity greater than 20 feet per second through an inertia separat-ing means for inertially removing a-t least a portion o~
the larger sized particles from the air to provide par tially processed air; and passi~g the part~ally processed air at a velocity which is greater than a predetermined velocity -through an impact fil-tering pad for removing o particles entrained in said partially processed air, said impact filtering pad compri~ing at least one layer of a plurali-ty of fibers, each of said fibers having a diameter greater -than .001 :inches and less -than .006 inches, and the ratio of total surface area of said fibers in said pad to the volume of said pad being greater than ~5 ft. l and less than 1400 ft. l, and said predetermine~ ~elo~ity being greater than 20 feet per second and chosen accordins~ to the diameter of said fibers of said impact filtering pad so that coalescence of moisture particles captured by said impact filtering pad is minimized.
In order that the invention may be fully under-stood, it will now be described with reference to the accompanying drawings in which:
Figure 1 is a perspective view, partially broken away, of a separa-tor device employed in accordance with the presen-t invention as a moisture separa-tor for the air intake to a gas turbine on a ship.
Figure 2 is a partial sectional view of the moisture separator illus-trated in Figure 1, taken along lines 2-2 of Figure 1.
Figure 3a is a plan view of a portion of one layer of the impact filtering pad of the present invention.
Figure 3b is a side view taken along lines 3b-3b of Figure 3a.
Figure 4 is a graphic representation illustrating the coalescense of moisture particles for -the impact filter-ing pad as a function of velocity through the pad and the fiber diameter of fibers comprising the pad, Turning now to the drawings in which like reference characters represent like elemen-ts, there is l :~ B~360 shown -in Figure 1 a mois-ture separating device 10 in accor-dance w:ith the presen-t invention in which the moi.s-ture separator 10 i5 p:Laced in the air inta~e duc-t 12 o~ a gas -turbine (not shown). As the separa-ting device 10 in accordance with the present invention is particularly use~ul ~or removing moisture par-ticles entrained in the air in which the moisture particles contain salt, the presen-t invention is particularly useful for use with gas turbines on ships or other amphibious vehicles. It should also be understood that -the separating device of the present inven-tion is also useful in removing dry particles entrained in the air, such as for example, sand, salt crystals, etc. which also have a tendency to damage turbine components. However, as -the separating device is primarily intended to remove moisture par-ticles e~-trained in air, the present invention will be describad with re~erence to such application.
It should be noted here that it is not the mois-ture per se which is undesirable but rather con-taminate particles such as salt and the like which are dissol~ed in or carried by the moisture particles which are un-desirable. However, as can be appreciated, the elimina-tlon.of the moisture particles from the air will also serve to eliminate these undesirable con-tamina-ted particles.
In gas turbines or marine applications, the air ducts are placed high up in the ship or the vehicle so that as lit-tle moisture as possible might be entrained in the air being in-troduced into the gas turbine, and in such instance, the moisture separators would be mounted at the entrance o~ the air duct. The re~ui.red air flow to be introduced to the -turbine is on th~ order of .
J :~ ~43~0 ~agnitude of 2000 cubic feet per second and greater. The mass fLow ra-te in turn is dependen-t upon the cross-sec-tional area of a duct through which the air is in-troduced and -the veloci-ty of the flow -therethrough For example, if the flow velocity through the duct can be increased, the cross-sectional.area of the duct (and thus the size of the duct) can be reduced subs-tantially. On the other hand, -the pressure drop across the moisture separator placed in the duct must be maintained at an acceptable level. As the pressure drop across the moisture separator is dependent on both the flow resistance pre~en~ed by the moisture separator and also the velocity ~-E the flow there-through, the pressure drop thus serves as a limi-t on the increase in flow velocity which can be obtained -to satisfy . the requirements for a given mass flow rate. Therefore, as - it is desirable to decrease the size of the air cl~ct ~to save on costs, weight, etc.), it is desirable to design the moisture separator so as to have as low a flow resis-tance as possible. At the same time, the reduction in flow resistance presented by the moisture separator must not be such as to impair the effici.ency to remove the molsture particles from the ai.r passing through -the separator.
With these principles in mind, the moisture separator in accordance with the present invention will now be discussed. As shown in Figure 1, the moisture separator 10 comprises a two stage separating device.
The first stage 14 of the separating device 10 comprises a conventional inertia device through which the air flow to be introduced to the turbine first passes. This inertia ~ :~ 6 ~
-Ll-device, in the preferred ernbodiment, co~prises, as is conventional, a plurali-ty of chevron or V-shaped vanes 16 vertically oriented and closely spaced to one another.
However, it should, of course, be understood that other types of inertia devices could be used, as for example, cyclone separators. The plurality of vanes 16 are supported in a support frame 17 which holds the uppe~ and lower ends of the vanes 16 in fixed position. ~ur-ther, a drain for mois-ture removed from the air is generally provided in the bottom of -the first stage 14, althou~h it has not been shown.
As best seen in Figure 2, the ~anes 16 of the first stage inertial device 14 each have three peaked sections 18. The peaked sections 18 of adjacent vanes 16 thus define a tortuous path for the air ~lowing there--through. In o-ther words, as the air flows t~rough the vanes 16, it must turn or bend several ~imes to follow the path be-tween the peaked sections 1~ of adjacent vanes 16. As the entrained particles, such as f~r examp7e, mois-ture particles containing salt, sand or other par~icularmatter, are generally of a larger mass than the air particles, the en-trained particles are thrown outwardly during the turns against the surfaces of the vanes 16 due to the centrifical force exerted thereon. That is 7 the lighter air particles are capable o~` making the turns through the series of peaked sections 1~, whereas the heavier mass particles are not, thereby resul-ting in the larger particles impacting on the surface of the vanes 16. Each of the peaks 20 of the vanes 16 in~clude st~ps Z2 for preventing the impac-ted par-ticles from sliding along -the surface of the vanes 16 and becoming re-entrained `J 1 ~
by virtLle of the aerodynamic drag force exerted by the air flow.
As can be apprecia-ted, the heavier the particles entrai.ned in the air, -the more likely such particles will impac-t on the vanes 16 and thus be removed from the air 10w. On the other hand, lighter particles may have a tendency to successfully follow the flow path and remain entrained in the air. For example, inertia separa-tor devices such as the stage 14 have been found to be effi-cient in removing moisture particles of a size over 8microns (25.4 microns approximately equals .OOl inches~ in .the range of the high flow velocities with which moisture separators for gas turbines are concerned, i.e., greater than 20 -feet per second. However, the plurality of vanes 16 have not exhibited a high efficiency for removing moisture particles in the lower size droplet range, nar~ely 8 microns and below. Thus, it is necessary to utilize further stages or devices for removing such lower sized particles.
Ater being partially processed by the first stage 14 of the moisture separator 10, the air is in-troduced through the second stage 24 which serves to further process the air to remove particles still en-trained therein. As best seen in Figures 2, 3a and 3b, the second stage of the separati.ng device comprises an impact separat-ing pad 26 comprised o at leas-t one layer 28 of a plurality of fibers 30. In the embodiment shown in Figures
Moisture separators are provided for gas turbines for marine applications as the moisture particles in the air generally contain sal-t which, if it should be in-troduced into the turbine, would deleteriously affect the component parts of the turbine, as for example, by chemical corrosion. Further, the dry particles entrained in the air, for example sand and/or salt crystals, can cause "pitting"
of the -turbine components if they are not removed. However, by far the greatest concern is the moisture particles containing salt.
One early prio~ ar-t device for removing moi~ture and other particles from the air introduced into -the tur-bine comprised a single stage wire mesh pad placed in the air intake duct. The mesh pad was generally comprised of a plurality of layers forming an approximately two inch thick pad wi-th each oE the layers in turn comprised of a plurality of .006 inch diame-ter wires knitted into a grid or screen having approximately five to six stitches per inch, i.e., there were approximately five wires per inch of length stitchecl or joined to~ether. The mois-ture particles were removed as a result of the par-ticles impact-ing acJainst and being cap-tured by the wires of the pad 1 3 6 ~
as -the air passed -through the device. Howe~er, such a wire mesh pad did not adequately and efEiciently remove all sizes of moisture particles entrained in -the air and thus, was not acceptable.
Another prior art device for removing moisture particles which was found to be more acceptable, comprised a three stage separa-tor in which the first and -third stages were iner-tia separating vanes and the second stage was a coalescer. The first and third stage vanes had generally similar performance characteristics, one o~ which was tha-t they exhibited very poor efficiency in removing smaller sized droplets, namely, eigh-t microns and below.
The reason for this is simply that inertia separa-tlng devices work on the principle that as -the air flow turns to bypass the vanes or other impacting medium the moisture - particles, being of larger size arld more mass t cannot make the turns and instead impinge or impact upon the impacting medium, thereby being removed from the air stream. However~
these conventiorlal inertia vanes do not prove efficient to remo~e lower size droplet particles since such particles, being of lighter mass, can make the turns to avoid impact-ing on the vanes. ~s a consequence, the prior art employed a second stage filter comprised generally of a plurality of polyester fibers of .001 inch diameter or smaller. The duty of the second stage was to cap-ture -the fine droplets by inertial impaction that had passed through the first stage.
Owing to their smaller diameter, the capture efficiency of the fibers was high so that some of the f:ibers would tend to collec-t several droplets which then coalesced or grouped together untll a drople-t was formed which was large enough -to be re-en-tralned by the aerodynami.c drag forces of the air passing therethrough. These re-entrained droplets were then captured by the third stage inertial device (either vanes or cyclones).
However, the a~dition of the third stage added grea-tly to -the overall size and weight of the moisture - separator, which already was quite large in order to aci~ieve the high mass flow rates needed by the gas turbines for ships (on the order of 2000 cubic feet per second).
Further still, the use of a polyester fiber pad having a plurality of closely spaced and dense fibers, each of a small diameter (on the order of .001 of an inch or smaller), tended to increase the flow resistance, and thus the pressure drop across the separator, for a given velocity of air flow. Thus, to achieve the desired flow rate, -the size of the polyester pad and -thus the size of the air duct in which the moisture separator was supported also had to be increased, thereby further adding to the -weight, bulkiness, cost, etc. of the separating device.
It should be noted in this connection that various types of other filtering or separating arrangements and, in particular filtering pad arrangements, not speci-fically directed for use in marine applications are known : in the prior art. For example, in U.S. Patent No. 4,086,070 to Argo et al, -there is di.sclosed a fiber bed separator and method for separation of aerosols from gases without re-entrainment. In the improved fiber bed separator of Argo et al., ~here is provided a pair of fiber beds each com~
prised of a plural.ity of fiber elements or wires. In this reference, the distinction is made between the amount 3 ~ 0 of wa-ter which is held by the bed after draining by gravity and that held against the drag of flowing gas or air there-through. More part:icularly, the fiber diameter and bed ~oidage for -the first or front bed in Argo et al. is selected such that at the design bed velocity and aerosol loading, the first bed will not be flooded and the ~sidual saturation thereof against gas drag on the liquid collected will be less than the residual saturation of the bed against gravity drainage of the liquid. In other words, the front or first bed of Eibers in Argo et al. is such that the collected liquid in the bed tha-t does not drain by gravity will be blown through to -the second ~iber bed. In the second fiber bed on the other hand, the fiber diameter and bed voidage are chosen so that the residual saturation thereof against gas drag will be greater than the residual saturation of the bed against gravity drainage so that the liquid collected by the second bed will drain of~
by gravity as oppo~ed to being re-entrained.
Thus, in accordance with -the ~rgo et al teachi~, a more dense fiber pad is used as a first stage i~ a filter-ing device in which -the residual saturation against gravity drainage is greater than the residual saturation against gas phase drag so tha-t liquid which does not drain from the first stage will be blown through to a second pad where the residual saturation against gas phase drag is greater than the residual saturation against gravit~
drainage so that liquid wi]l drain off rather than be re-entrained in the air. Accordingly, the Argo et al solu-tion to the problem of re-entrainment experienced in the prior art is to provide a second pad ha~ing specific charac-teristics chosen so that the pad has a greater tendency J 1 ~3~(~
-to dra:in Gaptured or collected l:Lquid by means o:f: gravity than to permit re-entrainment.
However, upon a deta:il.ed examination of the Argo et al system, it is seen that the flow veloci-ties through the fiber pads w:ith which the Argo et al ar-rangement is concerned are relatively low in comparison to the flow velocities through filtering arrangement used in connection with marine applications; i.e., the Argo et al arrangement is specifically designed to be utilized in connec-tion wi-th flow veloci-ties which are generally less than 10 feet per second, whereas in marine applications flow velocities of greater than 20 feet per second are expe-rienced. Furthermore, it is to be noted that ~rom an : examination of the Argo et al reference, in particular Figures 1 and 7, Argo et al suggests that the residual saturation of the second fiber bed against gas phase drag in virtually all instances decreases as the velocity through the bed increases, and that the upper limit of velocity through the bed is usually set hy re-entrainment problems. This may be due to the fact that the Argo et al reference teaches that the particles captured by the fiber beds coalesce and drain by gravity.
In this regard, it is to be noted that re-entrain-ment of any collected or coalesced liquid droplets would require an additional device downs-tream of the filter pad for the purpose of recapturing the re-entrained particles in order to prevent the introduction of such moisture particles which are re-entrained into -the turbine.
An~ther type of arrangement of the prior art~
again not specifically directed for use as a moisture separator in marine applica-tions, is disclosed :in U.S.
J ~ ~3~0 Paten-t No. 4,158,~-~49 to Sun et al. This patent discloses an in]et air cleaner assembly for -turbine engines which are mainly used in connection with agricultural aircraft for the application of chemicals over large and not readily accessible land areas. In the arrangement of -this reference, there is provided a first stage vortex air cleaner and a second stage mist eliminat,or assemb-y. The first stage vortex air cleaner operates generally on the principles of inertia separating devices for removing heavier and larger sized contaminate particles in the air to provide partially processed air. The second stage of the filtering arrangement comprises a plurality of superimposed knitted wire mesh sheets which define passages therethroug~
for removing light-weight or well disperse~ solid contami-nate particles and liquid drople-ts. The wire mesh sheets are comprised of a plurality of monofiliments which may have a diame~er ranging from .0005 inches to 0.035 inches and which are compressed substantially throughout the surface area. The monofiliments preferably are coated with oil or another nonvolatile liquid so that liquid which is removed by impingement on the oil coated wires of the mesh will tend to flow by gravity downwardly and collect in the bottom of the mist eliminator stage for subsequent removal.
Thus, it is apparent that even in this reference which is not mainly concerned or directed to a separator assembly for marine applications, there is the sugges-tion that liquid which is removed by the mist eliminator as-sembly coalesces and drains by gravity to a lower portîon of the assembly. As such the use of this asse~bly in a ~ ~ 6~3~0 marlne appllca-t:ion where rnois-ture particles are to be removed, would sugges-t -that a further separating device is required do~nstream thereo~ for capturing any coalesced particles which become re-entrained. That is, because of - the suggestion of coalescence occurring~ it would be expected, especially at the particularly hi.gh ~low velocities with which separating devices for marine applica-tions are concerned that -there is the possibili-ty of the coalesced particles becoming re-entrained in the air stream. Thus, it would be expected that such re-entrainment o* coalesced particles would create a problem, such as experienced in connection with some of the other prior art arrangements discussed hereinabove if the Sun et al arrange-ment were used, and thus necessitate the use of additional separator devices or means downstream of the filtering pad,.
In this regard, as noted above, coalescense would create a problem of re-entrainment at the relati~ely high flow veloci-ties such as experienced in connection with marine applications since the aerodynamic drag forces produced thereby have a greater tendency to cause re-entrainment of the enlarged coalesced droplets.
According to the presen-t invention, there is pro~ided a method of removing particles entrained in the air, the air including particles of moisture, the method comprising the steps of: passing the air at a velocity greater than 20 feet per second through an inertia separat-ing means for inertially removing a-t least a portion o~
the larger sized particles from the air to provide par tially processed air; and passi~g the part~ally processed air at a velocity which is greater than a predetermined velocity -through an impact fil-tering pad for removing o particles entrained in said partially processed air, said impact filtering pad compri~ing at least one layer of a plurali-ty of fibers, each of said fibers having a diameter greater -than .001 :inches and less -than .006 inches, and the ratio of total surface area of said fibers in said pad to the volume of said pad being greater than ~5 ft. l and less than 1400 ft. l, and said predetermine~ ~elo~ity being greater than 20 feet per second and chosen accordins~ to the diameter of said fibers of said impact filtering pad so that coalescence of moisture particles captured by said impact filtering pad is minimized.
In order that the invention may be fully under-stood, it will now be described with reference to the accompanying drawings in which:
Figure 1 is a perspective view, partially broken away, of a separa-tor device employed in accordance with the presen-t invention as a moisture separa-tor for the air intake to a gas turbine on a ship.
Figure 2 is a partial sectional view of the moisture separator illus-trated in Figure 1, taken along lines 2-2 of Figure 1.
Figure 3a is a plan view of a portion of one layer of the impact filtering pad of the present invention.
Figure 3b is a side view taken along lines 3b-3b of Figure 3a.
Figure 4 is a graphic representation illustrating the coalescense of moisture particles for -the impact filter-ing pad as a function of velocity through the pad and the fiber diameter of fibers comprising the pad, Turning now to the drawings in which like reference characters represent like elemen-ts, there is l :~ B~360 shown -in Figure 1 a mois-ture separating device 10 in accor-dance w:ith the presen-t invention in which the moi.s-ture separator 10 i5 p:Laced in the air inta~e duc-t 12 o~ a gas -turbine (not shown). As the separa-ting device 10 in accordance with the present invention is particularly use~ul ~or removing moisture par-ticles entrained in the air in which the moisture particles contain salt, the presen-t invention is particularly useful for use with gas turbines on ships or other amphibious vehicles. It should also be understood that -the separating device of the present inven-tion is also useful in removing dry particles entrained in the air, such as for example, sand, salt crystals, etc. which also have a tendency to damage turbine components. However, as -the separating device is primarily intended to remove moisture par-ticles e~-trained in air, the present invention will be describad with re~erence to such application.
It should be noted here that it is not the mois-ture per se which is undesirable but rather con-taminate particles such as salt and the like which are dissol~ed in or carried by the moisture particles which are un-desirable. However, as can be appreciated, the elimina-tlon.of the moisture particles from the air will also serve to eliminate these undesirable con-tamina-ted particles.
In gas turbines or marine applications, the air ducts are placed high up in the ship or the vehicle so that as lit-tle moisture as possible might be entrained in the air being in-troduced into the gas turbine, and in such instance, the moisture separators would be mounted at the entrance o~ the air duct. The re~ui.red air flow to be introduced to the -turbine is on th~ order of .
J :~ ~43~0 ~agnitude of 2000 cubic feet per second and greater. The mass fLow ra-te in turn is dependen-t upon the cross-sec-tional area of a duct through which the air is in-troduced and -the veloci-ty of the flow -therethrough For example, if the flow velocity through the duct can be increased, the cross-sectional.area of the duct (and thus the size of the duct) can be reduced subs-tantially. On the other hand, -the pressure drop across the moisture separator placed in the duct must be maintained at an acceptable level. As the pressure drop across the moisture separator is dependent on both the flow resistance pre~en~ed by the moisture separator and also the velocity ~-E the flow there-through, the pressure drop thus serves as a limi-t on the increase in flow velocity which can be obtained -to satisfy . the requirements for a given mass flow rate. Therefore, as - it is desirable to decrease the size of the air cl~ct ~to save on costs, weight, etc.), it is desirable to design the moisture separator so as to have as low a flow resis-tance as possible. At the same time, the reduction in flow resistance presented by the moisture separator must not be such as to impair the effici.ency to remove the molsture particles from the ai.r passing through -the separator.
With these principles in mind, the moisture separator in accordance with the present invention will now be discussed. As shown in Figure 1, the moisture separator 10 comprises a two stage separating device.
The first stage 14 of the separating device 10 comprises a conventional inertia device through which the air flow to be introduced to the turbine first passes. This inertia ~ :~ 6 ~
-Ll-device, in the preferred ernbodiment, co~prises, as is conventional, a plurali-ty of chevron or V-shaped vanes 16 vertically oriented and closely spaced to one another.
However, it should, of course, be understood that other types of inertia devices could be used, as for example, cyclone separators. The plurality of vanes 16 are supported in a support frame 17 which holds the uppe~ and lower ends of the vanes 16 in fixed position. ~ur-ther, a drain for mois-ture removed from the air is generally provided in the bottom of -the first stage 14, althou~h it has not been shown.
As best seen in Figure 2, the ~anes 16 of the first stage inertial device 14 each have three peaked sections 18. The peaked sections 18 of adjacent vanes 16 thus define a tortuous path for the air ~lowing there--through. In o-ther words, as the air flows t~rough the vanes 16, it must turn or bend several ~imes to follow the path be-tween the peaked sections 1~ of adjacent vanes 16. As the entrained particles, such as f~r examp7e, mois-ture particles containing salt, sand or other par~icularmatter, are generally of a larger mass than the air particles, the en-trained particles are thrown outwardly during the turns against the surfaces of the vanes 16 due to the centrifical force exerted thereon. That is 7 the lighter air particles are capable o~` making the turns through the series of peaked sections 1~, whereas the heavier mass particles are not, thereby resul-ting in the larger particles impacting on the surface of the vanes 16. Each of the peaks 20 of the vanes 16 in~clude st~ps Z2 for preventing the impac-ted par-ticles from sliding along -the surface of the vanes 16 and becoming re-entrained `J 1 ~
by virtLle of the aerodynamic drag force exerted by the air flow.
As can be apprecia-ted, the heavier the particles entrai.ned in the air, -the more likely such particles will impac-t on the vanes 16 and thus be removed from the air 10w. On the other hand, lighter particles may have a tendency to successfully follow the flow path and remain entrained in the air. For example, inertia separa-tor devices such as the stage 14 have been found to be effi-cient in removing moisture particles of a size over 8microns (25.4 microns approximately equals .OOl inches~ in .the range of the high flow velocities with which moisture separators for gas turbines are concerned, i.e., greater than 20 -feet per second. However, the plurality of vanes 16 have not exhibited a high efficiency for removing moisture particles in the lower size droplet range, nar~ely 8 microns and below. Thus, it is necessary to utilize further stages or devices for removing such lower sized particles.
Ater being partially processed by the first stage 14 of the moisture separator 10, the air is in-troduced through the second stage 24 which serves to further process the air to remove particles still en-trained therein. As best seen in Figures 2, 3a and 3b, the second stage of the separati.ng device comprises an impact separat-ing pad 26 comprised o at leas-t one layer 28 of a plurality of fibers 30. In the embodiment shown in Figures
2, 3a and 3b, the pad 26 is actually comprised of a plurality of layers 28 of kni-tted wettable fibers 30. The layers 28 of pad 26 are supported in a frame 32 which is afEixed to the downstream side of the firs-t sta~e 14.
~ :~ fi~6~) In the preEerrecl embodiment, the fibers 30 forrn-ing the l,ayers 28 of the pad 26 are comprised of monel wire. Monel is an alloy oon-tainirlg primarily nickel and copper, and other elements, and has been found to be extremely useful in marine type applications since it is especially resistant -to corrosion by salt entrained in the mois-ture of sea water.
As best seen in Figure 3a which is a plan view of a portion of one layer of the pad 26, the wire fibers 30 have been knitted -together to form a mesh or grid havin~
the wires 30 joined or tied together. Preferably, the diameter of the fibers or wires 30 is on the order of .002 inches, but the diameter can range be-tween .001 inches to .006 inches. With .002 inch diame-ter wire, it has been found with conventional knitting apparatus that there are approximately 10 to 12 stitches or joints per inch of mesh. That is, in a one inch length of the wire mesh 28, there are appro~imately lO -to 12 wires 30.
Also in the preferred embodiment, during the knitting operation, the wires 30 are crimped or ~ent slightly so that each of the layers 28 does not lie com-pletely flat. This is shown in Figure 3b. Thus, when the layers 28 are combined -to form the pad 26, the layers 28 will not lie flat against one another bu-t ins-tead will provide a cushioning or "lofty" medium. In the preferred embodiment, the crimp is sufficient to allow no more than 20 or 24 layers per inch of depth of -the pad 26. In this regard, it is to be noted that without -the crimping of the wires 30, it has been found tha-t there would be ap-proxima-tely 70 to 75 layers 28 of wires 30 per inch of pad depth. The reason that crimpin~ of the wires 30 is
~ :~ fi~6~) In the preEerrecl embodiment, the fibers 30 forrn-ing the l,ayers 28 of the pad 26 are comprised of monel wire. Monel is an alloy oon-tainirlg primarily nickel and copper, and other elements, and has been found to be extremely useful in marine type applications since it is especially resistant -to corrosion by salt entrained in the mois-ture of sea water.
As best seen in Figure 3a which is a plan view of a portion of one layer of the pad 26, the wire fibers 30 have been knitted -together to form a mesh or grid havin~
the wires 30 joined or tied together. Preferably, the diameter of the fibers or wires 30 is on the order of .002 inches, but the diameter can range be-tween .001 inches to .006 inches. With .002 inch diame-ter wire, it has been found with conventional knitting apparatus that there are approximately 10 to 12 stitches or joints per inch of mesh. That is, in a one inch length of the wire mesh 28, there are appro~imately lO -to 12 wires 30.
Also in the preferred embodiment, during the knitting operation, the wires 30 are crimped or ~ent slightly so that each of the layers 28 does not lie com-pletely flat. This is shown in Figure 3b. Thus, when the layers 28 are combined -to form the pad 26, the layers 28 will not lie flat against one another bu-t ins-tead will provide a cushioning or "lofty" medium. In the preferred embodiment, the crimp is sufficient to allow no more than 20 or 24 layers per inch of depth of -the pad 26. In this regard, it is to be noted that without -the crimping of the wires 30, it has been found tha-t there would be ap-proxima-tely 70 to 75 layers 28 of wires 30 per inch of pad depth. The reason that crimpin~ of the wires 30 is
3 ~ () desirable i.s that for a given number of layers 2~, the res:i.stance -to flow (which is propor-tional to the pressure drop across -the pad) wi.11 be lower if the distance between adjacent layers 28 is increased.
It should be notecl tha-t wire fibers 30 have been ~ound to be preferable to ~orm the layers 28 since the wires 30 will retain the crimp which is provided during the kni-tting or stitching opera-tion to form the layers 28.
Further, wires 30 provide a wettable surface which is most useful in capturing -the particles of mois-ture, which impact on the wires 30 as the air flows thereacross.
The mechanism by which the wire fiber pad 26 serves to remove particles from the air is inertial impac-tion. As the air "weaves" its way through the -thickness of pad 26, the particles entrained in the air (for example particles of moisture.containing salt, particles of sand, etc.) are not able to avoid the wires 30 and thus impact on the wires 30. With moisture particles, the surface tension or adhesion between the metallic wires 30 and the moisture particles which impact thereon is grea-t and thus, -the moisture particles are captured and removed from the stream of air. This would also be -true with respect to o-ther types of particulate matter en-trained in the air, such as for example, salt crystals ~produced when water of sea water.evaporates) or sand, but it is to be noted that the adhesion or surface tension between such par-ticles an~ the surface of the wires 30 is much less than with moisture particles. However, -this is acceptable since the ~reatest concern is the sea water containin~
~0 salt.
- It should be noted that the smaller -the slze of the fibers 30, the more effi.cient the fibers 30 are 3 ~ !) in capturing f:ine mois-tl.lre particles en-trairled in -the air. That is, small cl:iameter fibers have a relatively high captu~e effici.ency by impaction. However, with too small of a ~iber size, the chances are significantly in-creased that several drople-ts of moisture will be captured by the same fiber. When this occurs, the small droplets tend to coalesce together to form large droplets, such as for example, over 8 microns in size. Since the aero-dynamic drag force wi].l be greater on these coalesced droplets, there is a good chance that they will be re-entrained in the air.
This is the reason why, wi-th prior art separa-ting devices which used a fiberous pad having fiber ~iameters of .001 inch or smaller, it was necessary to provide a third stage for capturing the large coalesced droplets.
While it has been found -that the degree of coalescence of smaller size par-ticles, say in the range of 2 to ~
microns growing to 10 microns in size and above, is not great ~on the order of 3 to 5%~, it is significant enough that a third inertial impac-ting stage, such as for example, vanes or cyclones, was necessary to remove such coalesced particles from the air. However, in the present invention, this problem of coalescing is avoided b-y using a slightly larger size fiber 30, preferably having a diameter of .002 inches.
With respect to the various parameters for design-ing mois-ture separating devices for gas turbines, it is to be noted that the efficiency of a pad 26 for removing a particular size par-ticle from the air is expressed by the well-known rela-tionship for capture in a lattice ar-rangement:
~ 3 ~i~3~0 r~p = 1 _ e--2l2~f~t where:
np is -the eff-iciency for removing a particular size particle;
nf is the efficiency o-f a sing~Le ~fiber :For remov-ing such particular size particle;
~ is the ratio of the total sur*ace area o-the fibers o.E.the pad to the volume of the pad; a~d - t is the thickness of the pad.
Thus, as it is desirable to maintain -the efficiency as close to one as possible, it is desirable to increase -the product ~t. On the o-ther hand, the pressure drop across the pad is rela-ted to the quan-tity ~2-t. Thus, in-creasing ~ to too great a quantity, will cause the pressure to drop across the pad to increase, which as noted above, is undesirable. On the other hand, it should be n~ted that the -thickness of the pad 26 is one dimension which also goes into determining the volume of the pad. Thus~ the efec-t on the pressure drop by simply increasing the pad 20 thickness (but maintaining the number and ~ize of the wires 30 the same) actually serves to decrease the pressure drop.
In accordance with the.present invention, the values for ~ should range be-tween 45 ft. 1 to approximately 1400 ft~ 1. In this way, the efficiency can be maximized while at the ~ame tlme the pressure drop maintained at an acceptable level. More preferably, the value for ~ should range between 75 and 200 ft. 1, Also, it has been found desirable to maintain the thickness o:E the pad 26 between 1/2 inch and five inches, although of cour~e, a lesser thickness, e~en down to a single layer 28 of wire mesh, 3 ~ ~
could be utiLi~ed depending on the desired efficiency characteris-tics ancl pressure ~Irop characteristics for a par-ticular application. In the preferred embodiment, the thickness of the pad 26 is appro~imately three inches and comprises on the order of 70 layers 2~ of the .002 inch diame-ter wire 30 ~laving lO to 12 s-ti-tches per inch.
For such a pad 26, the value for ~ is approximately 84 ft. . If the pad thickness is compressed to I l/2 inches, the value for ~ would double to approxima-tely 168 -ft.
It is to be noted that wi-th respect to the polyes-ter pads utilized in the prior art, the value for ~ was on the order of 1800 ft. . Accordingly, for a given thickness of pad, the pressure drop across a polyester fiber pad of the prior art is significantly higher than is across the pad 26 according to the presen-t invention.
Therefore, -the velocity of the air flow through -the pad 26 of the present inven-tion can be increased over that which was possible with the polyester pad of the prior art. Thus, the siæe and weight of the moisture separator lO can be substantially reduced, while at the sam~ time maintaining an accep-table pressure drop level. Furthermore, since higher velocities for the air flow through the separator 20 can be attained with the pad 26 o-f the present invention, the particle separa-tion efficiency is also increased. This is the result of the fact tha-t droplet retention by impaction improves with an increase in velocity.
Moreover, owing to the larger diameter of the fibers over that of -the polyes-ter pad, the tendency of the drops to coalesce is elimina-ted. The reason ~or this is twofold. First, as the air velocity is increased, the ~ 1 6~ 3~ 0 ~18-ability of drople-ts to coalesce is impaired. Secondly, the aerodynamic drag forces a-t higher velocities is larger, and thus, the drople-ts are re-entrained in -the a;r at lower sizes thereby pre-venting the grow-th of droplets by coalescing to larger sizes, say in the 1~ to 13 micron range and abo~e. The re-entrained droplets can then be captured by another fiber. Since the moisture droplets are not coalesced, -there is no fur-ther need for a third stage inertia device in the moisture separator 10, which in the prior ar-t was only useful for recap-turing the coalesced rnoisture droplets. The elimination of the need for a -third stage filter results in substantial savings, not only in cost, but also in the weight and size of the moisture separator 10.
An important point to note in this regard is -the fact that for any given wire or fiber size in the impact filtering pad 26, there is a minimum flow velocity above which no coalescence will occur. I-t is thi~ realiza-tion which makes such an impact filtering pad 26 as con-templated by the present invent1on particular~y usefulfor the flow veloci-ty ranges incurred in marine applica-tions. That is, for wire sizes which are efficient in removing particulate matter from air, the flow velocity for marine applications can satisfactorily be chosen so that no coalescence will occur in the filtering pad 26, thus avoid-ing the necessity of any additional filtering means or - stage downstream of -the pad 26.
More particularly, Figure 4 shows the relation-ship be-tween the occurrence of coalescence and fiber diameter as a function of flow veloci-ty through -the impact f:i:L-tering pa~l 26. The abscissa represents a ~le~lsure of coalescence and khe ordina-te ir, ~i.gure ~ represen-ts f:lc~w velocity. Three curves are shown in r':igure ~ or three di*ferent fiber d:iaMeters. 'rhe lowermosk curve represents the relationship between coalescence and ~~I.ow ve~ocity for a filtering pad comprised of 0.001 inch d-lame~er polyester fibers; the ~iddle c~r~e is for a pad c~mprise~ of 0 002 inch diameter monel wires; and ~he uppermost cur~e is for a pad comprised oE 0.006 inch diameter monel wires.
In this regard, these curves were generated using me~s~red test data for lower flow velocities and impaction theory for higher flow velocikies.
It is to be noted that impaction theory su~ges-ts tha~ the capture efficiency for a given wire size increases with the square of the par-ticle diameter. There~ore, in the absence of coalescence, the efficiency of cap-tured drop~ets in the range of lO to 13 microns would be grea-ter than -tha~
in the 2 to 4 micron range. However, when coalescing occurs, some of the droplets in the 2 -to ¢ micron range are coalesced together and promoted -to the lO to 13 micron regime. This has the effect of improving the efficiency for the 2 to 4 micron droplets and lowering i~ for the 10 to ~3 micron droplets. Therefore, the d;.fference be-tween the efficiencies for removing 10 to 13 micron particles and for removing 2 to 4 micron parkicles represents a measure of coalescence, i.e.:
: n1o-13~ - n2_~ n where n10_l3~ is the ef~iciency for removin~ particles i.n the 10-13 micron range; n2_~ is the ~ffi~ency for removing particles in the 2-4 micron ran~e; and ~n is -the ~ ~ 64~
di*fererlce in such efficiencies and ~epre~ents a measure of coalascence. This difference in efficiences accordingly constitutes a measure of coalescence for the impact filter-ing pad. ~hus, for an greater than 0, no coalescence is occuring; however, if An is less than 0, then coalescence does occur.
Figure 4 illustrates the flow velocities which must be attained to eliminate coalescence with pads com-prised of 0.001 and 0.002 inch diameter fibers. For the 0.001 inch fiber pad, a flow velocity of 60 feet per second is needed, whereas for the 0.002 inch wire diame-ter pad only a flow velocity of 30 feet per second is required to eliminate coalescence. The 0.006 inch wire diameter pad never exhibits coalescence in the 2 to 13 micron range.
- Generally speaking, a flow veloci-ty o* 60 feet per second, required by the 0.001 inch fiber diameter pad to elimina-te coalescence, will lead to an unacceptably high pressure drop for marine gas turbine inlet separator applica-tions. This wire size for the impact filtering pad thus represents a lower limit on fiber diameter size for the impact filtering pad 26. On the other hand, -the 0.006 inch wire diameter pad, while it never e~hibits coalescence in the 2 to 13 micron range, has a poor capture efficiency for smaller droplets and a higher pressure drop. Therefore, this wire size represents an upper limit on wire or fiber diameter size for moisture separator applications. In terms of the preferred embodiment in which the irnpact fil-tering pad 26 is comprised of 0.002 inch diameter wires, this presents an optimum choice since only a flow velocity on the order to 30 fee-t per second ~ ~ ~4.~6~
is required to e].:iminate coalescence. Owing to thi..s rela-tively small wire s:ize, the capture eff:iciency for small droplets is rela-tively high and the pressure drop across the filtering pad is reLatively low.
Thus, it will be appreciated tha-t for a given fiber size there is a minimum flow velocity above which no coalescence will occur. This realization is most impor-tant since the absence of coalescence allows the elimina-tion of a third or additional stage downstream of the impact filtering pad-26. Fur-ther, it is the absence of this realization by any of the prior art which negates - any suggestion in such prior art to use an impact filtering pad in accordance with the principles of the present inven-tion in combination with an inertia separating device 14 for a moisture separator 10 for a gas turbine. In this regard, once the particular wire size is chosen so as to be in the desired range for providing a highly efficient pad for remo~ing par-ti.cles of all sizes, the size or cross sectional area of the pad 26 need only be chosen so as to provide a flow ~eloci-ty above the predetermine~ limit for -that particular chosen size of fibers so that no coales-cence will occur, taking into consideration of course - the mass flow velocity which is re~uired for turbine opera-tion.
In this regard, as has been noted hereinabove, the wire size for such an acceptable efficiency for par-ticles of all ranges is preferably be-tween 0.001 inches and 0.006 inches, and most preferabl~ is 0.002 inches.
The pad 26 is then constructed to provide a flo~ velocity greater than 20 feet per second and, accordin~ -to the 7 ~ & 0 fiber diameter size whi.ch has been chosen, -to insllre the absence of coalescence dur-iny operation. For instance, if 0.002 inch diameter wires or fihers 30 are u-tilized for the impact fil-terlng pad, the flow veloci-ty should be greater than 30 fee-t per second. In terms o~ gas tur-bines for marine applications, such a flow velo~ity is acceptable to provide a relatively low pressure drop across the moisture separa-tor 10. Consequently, for such an ar-rangement, a -two s-tage moisture separa-tor 10 may be uti-lized for efficiently removing particles from the air flowtherethrough, and in particular mois-ture particles there-from, without the need for additional separating stages downstream thereof.
. I-t is to be noted that the closer tha-t the diam-eter of each individual wire 30 is to .001 inch, the higher the required ve]ocity to eliminate coalescence as the wire has a grea-ter tendency to coalesce which/ as noted, is undesirable if the third s-tage of the prior art is to be eliminated. On the other hand, increasing ~he wire diameter -to almost .006 inches tends to reduce the effi-ciency of the pad 26 to remove the particles from the air and further serves to increase the resistance to flow and.thus, pressure drop across the pad. Accordingly, be-cause of these competing considerations 7 it has been found preferable that the diameter of the ~ibers 30 be on the order of .002 inches. This will lead to acceptable ranges of flow veloei-ty to eliminate or minimize coalescence while also providing an acceptable pressure drop.
Accord:ingly, it is seen that the -two stage separ-ating device 10 in accordance with -the present invention o results in signifi.can-t aclvan-tages over -the prior ar-t.
~he effi.ciency for removing par-ticles entraine~l in the air is maintained (if no-t increasecl) wh:ile reducing the number of s-tages that is necessary. This in turn results i.n a substantial savings in weight, cost, and size for the device 10. ~urthermore, the air flow v~locities through the air duct ean be increased wi-thou-t increasing the pres-sure drop across the moisture separator 10, thus allowing for a still further reduction in size of the overall as-sembly and a reduction in the chances o~ eoalescenee.
While the present invention has been describedmainly in the context of removing moisture particles en-- trained in air for the gas turbine for ships, it will of course be understood by persons skilled in the art~
that the filter pad 26 of the present invention can also be used for removing other types of particles. For example, the pad 26 is also effieient for removing sancl or salt erys-tals whieh may be entrained in the air.
Aceordingly, there is diselosed herein an im-proved method for removing partieles entrained in air, and in partieular for removing moisture particles entrained herein. In accordance with the improved method, the air is initially passed through an inertla separating means 14 at a flow velocity greater than 20 feet per second.
This flow velocity is chosen as such relativ~ly high flow velocities provide for more efficiency of the inertia separating device 1~ to separate partieles from the air.
The par-tially proeessed air from the inertia separating ~eans 14 is -then passed at a flow ve].oeity greater than 30 a predetermined flow veloeity to an impact fil-tering pad 26 for removal of par-ti.e:les entrained in the par-tially 3 fi O
processed air. The impact filteriny pad 26 comprises at leas-t one layer o:f a plurality of f:ibers 30, each of the f-ibers 30 having a diameter greater than .001 inches and less than .0~6 inches, and the ratio of tota] surface area of the :~ibers in the pad 26 to the volume of the pad 26 being greater than ~5ft. 1 and less than 1400 ft. 1, The predeterminèd flow veloci-ty is greater than 20 feet per second and is chosen according to the diameter of the fibers 30 in the impact fil-tering pad 26 so that coalescence of moisture particles captured by the impact filtering pad 26 is minimized. In this manner, further filtering stages or devices will not be required downstream of the impact filtering pad 26 for recapturing re-entrained particles. This is a result of the minimization of coalescence which might otherwise cause captured particles -- to be promoted to a larger size which have a greater pos-sibility of becoming re-entrained by the air flow passing through the impact filtering pad 26.
While the preferred embodiment of the present invention has been shown and described, it will be under-stood -that such is merely illustrative and -that changes may be made without departing from the scope of the inven- -tion as claimed~
It should be notecl tha-t wire fibers 30 have been ~ound to be preferable to ~orm the layers 28 since the wires 30 will retain the crimp which is provided during the kni-tting or stitching opera-tion to form the layers 28.
Further, wires 30 provide a wettable surface which is most useful in capturing -the particles of mois-ture, which impact on the wires 30 as the air flows thereacross.
The mechanism by which the wire fiber pad 26 serves to remove particles from the air is inertial impac-tion. As the air "weaves" its way through the -thickness of pad 26, the particles entrained in the air (for example particles of moisture.containing salt, particles of sand, etc.) are not able to avoid the wires 30 and thus impact on the wires 30. With moisture particles, the surface tension or adhesion between the metallic wires 30 and the moisture particles which impact thereon is grea-t and thus, -the moisture particles are captured and removed from the stream of air. This would also be -true with respect to o-ther types of particulate matter en-trained in the air, such as for example, salt crystals ~produced when water of sea water.evaporates) or sand, but it is to be noted that the adhesion or surface tension between such par-ticles an~ the surface of the wires 30 is much less than with moisture particles. However, -this is acceptable since the ~reatest concern is the sea water containin~
~0 salt.
- It should be noted that the smaller -the slze of the fibers 30, the more effi.cient the fibers 30 are 3 ~ !) in capturing f:ine mois-tl.lre particles en-trairled in -the air. That is, small cl:iameter fibers have a relatively high captu~e effici.ency by impaction. However, with too small of a ~iber size, the chances are significantly in-creased that several drople-ts of moisture will be captured by the same fiber. When this occurs, the small droplets tend to coalesce together to form large droplets, such as for example, over 8 microns in size. Since the aero-dynamic drag force wi].l be greater on these coalesced droplets, there is a good chance that they will be re-entrained in the air.
This is the reason why, wi-th prior art separa-ting devices which used a fiberous pad having fiber ~iameters of .001 inch or smaller, it was necessary to provide a third stage for capturing the large coalesced droplets.
While it has been found -that the degree of coalescence of smaller size par-ticles, say in the range of 2 to ~
microns growing to 10 microns in size and above, is not great ~on the order of 3 to 5%~, it is significant enough that a third inertial impac-ting stage, such as for example, vanes or cyclones, was necessary to remove such coalesced particles from the air. However, in the present invention, this problem of coalescing is avoided b-y using a slightly larger size fiber 30, preferably having a diameter of .002 inches.
With respect to the various parameters for design-ing mois-ture separating devices for gas turbines, it is to be noted that the efficiency of a pad 26 for removing a particular size par-ticle from the air is expressed by the well-known rela-tionship for capture in a lattice ar-rangement:
~ 3 ~i~3~0 r~p = 1 _ e--2l2~f~t where:
np is -the eff-iciency for removing a particular size particle;
nf is the efficiency o-f a sing~Le ~fiber :For remov-ing such particular size particle;
~ is the ratio of the total sur*ace area o-the fibers o.E.the pad to the volume of the pad; a~d - t is the thickness of the pad.
Thus, as it is desirable to maintain -the efficiency as close to one as possible, it is desirable to increase -the product ~t. On the o-ther hand, the pressure drop across the pad is rela-ted to the quan-tity ~2-t. Thus, in-creasing ~ to too great a quantity, will cause the pressure to drop across the pad to increase, which as noted above, is undesirable. On the other hand, it should be n~ted that the -thickness of the pad 26 is one dimension which also goes into determining the volume of the pad. Thus~ the efec-t on the pressure drop by simply increasing the pad 20 thickness (but maintaining the number and ~ize of the wires 30 the same) actually serves to decrease the pressure drop.
In accordance with the.present invention, the values for ~ should range be-tween 45 ft. 1 to approximately 1400 ft~ 1. In this way, the efficiency can be maximized while at the ~ame tlme the pressure drop maintained at an acceptable level. More preferably, the value for ~ should range between 75 and 200 ft. 1, Also, it has been found desirable to maintain the thickness o:E the pad 26 between 1/2 inch and five inches, although of cour~e, a lesser thickness, e~en down to a single layer 28 of wire mesh, 3 ~ ~
could be utiLi~ed depending on the desired efficiency characteris-tics ancl pressure ~Irop characteristics for a par-ticular application. In the preferred embodiment, the thickness of the pad 26 is appro~imately three inches and comprises on the order of 70 layers 2~ of the .002 inch diame-ter wire 30 ~laving lO to 12 s-ti-tches per inch.
For such a pad 26, the value for ~ is approximately 84 ft. . If the pad thickness is compressed to I l/2 inches, the value for ~ would double to approxima-tely 168 -ft.
It is to be noted that wi-th respect to the polyes-ter pads utilized in the prior art, the value for ~ was on the order of 1800 ft. . Accordingly, for a given thickness of pad, the pressure drop across a polyester fiber pad of the prior art is significantly higher than is across the pad 26 according to the presen-t invention.
Therefore, -the velocity of the air flow through -the pad 26 of the present inven-tion can be increased over that which was possible with the polyester pad of the prior art. Thus, the siæe and weight of the moisture separator lO can be substantially reduced, while at the sam~ time maintaining an accep-table pressure drop level. Furthermore, since higher velocities for the air flow through the separator 20 can be attained with the pad 26 o-f the present invention, the particle separa-tion efficiency is also increased. This is the result of the fact tha-t droplet retention by impaction improves with an increase in velocity.
Moreover, owing to the larger diameter of the fibers over that of -the polyes-ter pad, the tendency of the drops to coalesce is elimina-ted. The reason ~or this is twofold. First, as the air velocity is increased, the ~ 1 6~ 3~ 0 ~18-ability of drople-ts to coalesce is impaired. Secondly, the aerodynamic drag forces a-t higher velocities is larger, and thus, the drople-ts are re-entrained in -the a;r at lower sizes thereby pre-venting the grow-th of droplets by coalescing to larger sizes, say in the 1~ to 13 micron range and abo~e. The re-entrained droplets can then be captured by another fiber. Since the moisture droplets are not coalesced, -there is no fur-ther need for a third stage inertia device in the moisture separator 10, which in the prior ar-t was only useful for recap-turing the coalesced rnoisture droplets. The elimination of the need for a -third stage filter results in substantial savings, not only in cost, but also in the weight and size of the moisture separator 10.
An important point to note in this regard is -the fact that for any given wire or fiber size in the impact filtering pad 26, there is a minimum flow velocity above which no coalescence will occur. I-t is thi~ realiza-tion which makes such an impact filtering pad 26 as con-templated by the present invent1on particular~y usefulfor the flow veloci-ty ranges incurred in marine applica-tions. That is, for wire sizes which are efficient in removing particulate matter from air, the flow velocity for marine applications can satisfactorily be chosen so that no coalescence will occur in the filtering pad 26, thus avoid-ing the necessity of any additional filtering means or - stage downstream of -the pad 26.
More particularly, Figure 4 shows the relation-ship be-tween the occurrence of coalescence and fiber diameter as a function of flow veloci-ty through -the impact f:i:L-tering pa~l 26. The abscissa represents a ~le~lsure of coalescence and khe ordina-te ir, ~i.gure ~ represen-ts f:lc~w velocity. Three curves are shown in r':igure ~ or three di*ferent fiber d:iaMeters. 'rhe lowermosk curve represents the relationship between coalescence and ~~I.ow ve~ocity for a filtering pad comprised of 0.001 inch d-lame~er polyester fibers; the ~iddle c~r~e is for a pad c~mprise~ of 0 002 inch diameter monel wires; and ~he uppermost cur~e is for a pad comprised oE 0.006 inch diameter monel wires.
In this regard, these curves were generated using me~s~red test data for lower flow velocities and impaction theory for higher flow velocikies.
It is to be noted that impaction theory su~ges-ts tha~ the capture efficiency for a given wire size increases with the square of the par-ticle diameter. There~ore, in the absence of coalescence, the efficiency of cap-tured drop~ets in the range of lO to 13 microns would be grea-ter than -tha~
in the 2 to 4 micron range. However, when coalescing occurs, some of the droplets in the 2 -to ¢ micron range are coalesced together and promoted -to the lO to 13 micron regime. This has the effect of improving the efficiency for the 2 to 4 micron droplets and lowering i~ for the 10 to ~3 micron droplets. Therefore, the d;.fference be-tween the efficiencies for removing 10 to 13 micron particles and for removing 2 to 4 micron parkicles represents a measure of coalescence, i.e.:
: n1o-13~ - n2_~ n where n10_l3~ is the ef~iciency for removin~ particles i.n the 10-13 micron range; n2_~ is the ~ffi~ency for removing particles in the 2-4 micron ran~e; and ~n is -the ~ ~ 64~
di*fererlce in such efficiencies and ~epre~ents a measure of coalascence. This difference in efficiences accordingly constitutes a measure of coalescence for the impact filter-ing pad. ~hus, for an greater than 0, no coalescence is occuring; however, if An is less than 0, then coalescence does occur.
Figure 4 illustrates the flow velocities which must be attained to eliminate coalescence with pads com-prised of 0.001 and 0.002 inch diameter fibers. For the 0.001 inch fiber pad, a flow velocity of 60 feet per second is needed, whereas for the 0.002 inch wire diame-ter pad only a flow velocity of 30 feet per second is required to eliminate coalescence. The 0.006 inch wire diameter pad never exhibits coalescence in the 2 to 13 micron range.
- Generally speaking, a flow veloci-ty o* 60 feet per second, required by the 0.001 inch fiber diameter pad to elimina-te coalescence, will lead to an unacceptably high pressure drop for marine gas turbine inlet separator applica-tions. This wire size for the impact filtering pad thus represents a lower limit on fiber diameter size for the impact filtering pad 26. On the other hand, -the 0.006 inch wire diameter pad, while it never e~hibits coalescence in the 2 to 13 micron range, has a poor capture efficiency for smaller droplets and a higher pressure drop. Therefore, this wire size represents an upper limit on wire or fiber diameter size for moisture separator applications. In terms of the preferred embodiment in which the irnpact fil-tering pad 26 is comprised of 0.002 inch diameter wires, this presents an optimum choice since only a flow velocity on the order to 30 fee-t per second ~ ~ ~4.~6~
is required to e].:iminate coalescence. Owing to thi..s rela-tively small wire s:ize, the capture eff:iciency for small droplets is rela-tively high and the pressure drop across the filtering pad is reLatively low.
Thus, it will be appreciated tha-t for a given fiber size there is a minimum flow velocity above which no coalescence will occur. This realization is most impor-tant since the absence of coalescence allows the elimina-tion of a third or additional stage downstream of the impact filtering pad-26. Fur-ther, it is the absence of this realization by any of the prior art which negates - any suggestion in such prior art to use an impact filtering pad in accordance with the principles of the present inven-tion in combination with an inertia separating device 14 for a moisture separator 10 for a gas turbine. In this regard, once the particular wire size is chosen so as to be in the desired range for providing a highly efficient pad for remo~ing par-ti.cles of all sizes, the size or cross sectional area of the pad 26 need only be chosen so as to provide a flow ~eloci-ty above the predetermine~ limit for -that particular chosen size of fibers so that no coales-cence will occur, taking into consideration of course - the mass flow velocity which is re~uired for turbine opera-tion.
In this regard, as has been noted hereinabove, the wire size for such an acceptable efficiency for par-ticles of all ranges is preferably be-tween 0.001 inches and 0.006 inches, and most preferabl~ is 0.002 inches.
The pad 26 is then constructed to provide a flo~ velocity greater than 20 feet per second and, accordin~ -to the 7 ~ & 0 fiber diameter size whi.ch has been chosen, -to insllre the absence of coalescence dur-iny operation. For instance, if 0.002 inch diameter wires or fihers 30 are u-tilized for the impact fil-terlng pad, the flow veloci-ty should be greater than 30 fee-t per second. In terms o~ gas tur-bines for marine applications, such a flow velo~ity is acceptable to provide a relatively low pressure drop across the moisture separa-tor 10. Consequently, for such an ar-rangement, a -two s-tage moisture separa-tor 10 may be uti-lized for efficiently removing particles from the air flowtherethrough, and in particular mois-ture particles there-from, without the need for additional separating stages downstream thereof.
. I-t is to be noted that the closer tha-t the diam-eter of each individual wire 30 is to .001 inch, the higher the required ve]ocity to eliminate coalescence as the wire has a grea-ter tendency to coalesce which/ as noted, is undesirable if the third s-tage of the prior art is to be eliminated. On the other hand, increasing ~he wire diameter -to almost .006 inches tends to reduce the effi-ciency of the pad 26 to remove the particles from the air and further serves to increase the resistance to flow and.thus, pressure drop across the pad. Accordingly, be-cause of these competing considerations 7 it has been found preferable that the diameter of the ~ibers 30 be on the order of .002 inches. This will lead to acceptable ranges of flow veloei-ty to eliminate or minimize coalescence while also providing an acceptable pressure drop.
Accord:ingly, it is seen that the -two stage separ-ating device 10 in accordance with -the present invention o results in signifi.can-t aclvan-tages over -the prior ar-t.
~he effi.ciency for removing par-ticles entraine~l in the air is maintained (if no-t increasecl) wh:ile reducing the number of s-tages that is necessary. This in turn results i.n a substantial savings in weight, cost, and size for the device 10. ~urthermore, the air flow v~locities through the air duct ean be increased wi-thou-t increasing the pres-sure drop across the moisture separator 10, thus allowing for a still further reduction in size of the overall as-sembly and a reduction in the chances o~ eoalescenee.
While the present invention has been describedmainly in the context of removing moisture particles en-- trained in air for the gas turbine for ships, it will of course be understood by persons skilled in the art~
that the filter pad 26 of the present invention can also be used for removing other types of particles. For example, the pad 26 is also effieient for removing sancl or salt erys-tals whieh may be entrained in the air.
Aceordingly, there is diselosed herein an im-proved method for removing partieles entrained in air, and in partieular for removing moisture particles entrained herein. In accordance with the improved method, the air is initially passed through an inertla separating means 14 at a flow velocity greater than 20 feet per second.
This flow velocity is chosen as such relativ~ly high flow velocities provide for more efficiency of the inertia separating device 1~ to separate partieles from the air.
The par-tially proeessed air from the inertia separating ~eans 14 is -then passed at a flow ve].oeity greater than 30 a predetermined flow veloeity to an impact fil-tering pad 26 for removal of par-ti.e:les entrained in the par-tially 3 fi O
processed air. The impact filteriny pad 26 comprises at leas-t one layer o:f a plurality of f:ibers 30, each of the f-ibers 30 having a diameter greater than .001 inches and less than .0~6 inches, and the ratio of tota] surface area of the :~ibers in the pad 26 to the volume of the pad 26 being greater than ~5ft. 1 and less than 1400 ft. 1, The predeterminèd flow veloci-ty is greater than 20 feet per second and is chosen according to the diameter of the fibers 30 in the impact fil-tering pad 26 so that coalescence of moisture particles captured by the impact filtering pad 26 is minimized. In this manner, further filtering stages or devices will not be required downstream of the impact filtering pad 26 for recapturing re-entrained particles. This is a result of the minimization of coalescence which might otherwise cause captured particles -- to be promoted to a larger size which have a greater pos-sibility of becoming re-entrained by the air flow passing through the impact filtering pad 26.
While the preferred embodiment of the present invention has been shown and described, it will be under-stood -that such is merely illustrative and -that changes may be made without departing from the scope of the inven- -tion as claimed~
Claims (8)
1. A method of removing particles entrained in air, the air including particles of moisture, the method comprising the steps of:
passing the air at a velocity greater than 20 feet per second through an inertia separating means for inertially removing at least a portion of the larger sized particles from the air to provide partially processed air; and passing the partially processed air at a velocity which is greater than a predetermined velocity through an impact filtering pad for removing particles entrained in said partially processed air, said impact filtering pad comprising at least one layer of a plurality of fibers, each of said fibers having a diameter greater than .001 inches and less than .006 inches, and the ratio of total surface area of said fibers in said pad to volume of said pad being greater than 45 ft. 1 and less than 1400 ft.-1, and said predetermined velocity being greater than 20 feet per second and chosen according to the diameter of said fibers of said impact filtering pad so that coales-cence of moisture particles captured by said impact filter-ing pad is minimized.
passing the air at a velocity greater than 20 feet per second through an inertia separating means for inertially removing at least a portion of the larger sized particles from the air to provide partially processed air; and passing the partially processed air at a velocity which is greater than a predetermined velocity through an impact filtering pad for removing particles entrained in said partially processed air, said impact filtering pad comprising at least one layer of a plurality of fibers, each of said fibers having a diameter greater than .001 inches and less than .006 inches, and the ratio of total surface area of said fibers in said pad to volume of said pad being greater than 45 ft. 1 and less than 1400 ft.-1, and said predetermined velocity being greater than 20 feet per second and chosen according to the diameter of said fibers of said impact filtering pad so that coales-cence of moisture particles captured by said impact filter-ing pad is minimized.
2. The method of Claim 1 in which said impact filtering pad comprises a plurality of said layers of a plurality of fibers.
3. The method of Claim 2 in which each of said fibers has a diameter of .002 inches.
4. The method of Claim 3 in which said predeter-mined velocity is approximately 30 feet per second.
5. The method of Claim 2 in which the ratio of the total surface area of said fibers in said pad to the volume of said pad is greater than 75 ft.-1 and less than 200 ft.-1.
6. The method of Claim 3 in which the ratio of the total surface area of said fibers in said pad to the volume of said pad is greater than 75 ft.-1 and less than 200 ft.-1.
7. The method of Claim 5 in which each of said fibers comprises a metallic wire.
8. The method of Claim 7 in which said wires of each of said layers are crimped.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000387029A CA1164360A (en) | 1981-09-30 | 1981-09-30 | Method for removing moisture particles |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000387029A CA1164360A (en) | 1981-09-30 | 1981-09-30 | Method for removing moisture particles |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1164360A true CA1164360A (en) | 1984-03-27 |
Family
ID=4121061
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000387029A Expired CA1164360A (en) | 1981-09-30 | 1981-09-30 | Method for removing moisture particles |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1164360A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112107888A (en) * | 2020-08-07 | 2020-12-22 | 青岛科技大学 | High-efficiency liquid-liquid two-phase separator with rotational flow and phase separation coupling |
-
1981
- 1981-09-30 CA CA000387029A patent/CA1164360A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112107888A (en) * | 2020-08-07 | 2020-12-22 | 青岛科技大学 | High-efficiency liquid-liquid two-phase separator with rotational flow and phase separation coupling |
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