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A Review of Mechanical Seals Heat Transfer Augmentation Techniques
Article in Recent Patents on Mechanical Engineering · May 2013
DOI: 10.2174/2212797611306020001
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Recent Patents on Mechanical Engineering 2013, 6, 000-000 1
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A Review of Mechanical Seals Heat Transfer Augmentation Techniques
Nian Xiao and M. M. Khonsari*
Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803, USA
Received: November 01, 2012; Accepted: January 08, 2013; Revised: January 01, 2013
Abstract: A mechanical face seal is one of most widely used components in rotating machinery such as pumps, mixers,
blowers, and compressors. The primary function of a mechanical seal is to prevent leakage of the process fluid from the
pump housing and shaft to the environment. Excessive heat generated at the face seal interface has been recognized as one
of main causes of failure of mechanical seals. In the past few decades various efforts have been attempted to remove heat
from the interface uniformly in order to reduce the interfacial temperature, eliminate thermally-induced failure, and thus
increase the life of a mechanical seal. In this paper, a class of the pertinent patents on mechanical seals that use heat trans-
fer augmentation techniques are reviewed and discussed. Also presented are several new approaches recently proposed by
the authors specifically developed for enhancing heat transfer augmentation in mechanical face seal.
Keywords: Heat exchanger, heat transfer augmentation, improved fluid circulation, integrated assembly, mechanical seal, mi- cro-topography, no flush flow, surface textural side-wall.
1. INTRODUCTION
A mechanical seal consists of a primary ring that rotates with the shaft and a mating ring affixed to the housing. Dur- ing operation, the primary ring rubs against the mating ring where contact between the surfaces is maintained by springs attached to the primary ring; see Fig. (1). A thin film of fluid in the gap between the primary and mating rings must be maintained to hydrodynamically lubricate the faces and sepa- rate the ring surfaces so as to avoid direct rubbing that causes wear. However, the majority of seals fail long before the seal face wears out. The primary cause of failure in the majority of applications is excessive heat.
As pointed out by Lebeck [1], performance of a seal de- pends largely on three factors related to the interface tem- perature. First, when the interfacial temperature is too high the thin film of lubricating fluid across the gap- typically on the order of a few micrometers -may flash to vapor. Anno [2] reported that, for face seals containing micro-asperities, the film had a thickness varying from about 0.4 m to 0.2 m as the applied pressure varied from 1,000 to 70,000kg/m
2 . Pape
[3] analytically and experimentally studied the lubricant film and concluded that at the lowest film thickness, the coeffi- cient of friction is higher than the theoretical prediction be- cause of insufficient lubrication of the faces. The lubricating film is often so thin that excessive frictional heat generation can result in significant rise in temperature once a part or the whole liquid film is vaporized. As a result, the seal can be quickly destroyed. Second, many face seal materials simply cannot withstand exposure to high levels of temperature for a prolong length of time. Also, operation at very high
*Address for correspondence to these authors at the Department of Me-
chanical Engineering, 1419A Patrick Taylor Hall, Louisiana State Univer-
sity, Baton Rouge, LA 70803; Tel: (225) 578-9192; Fax: (225) 578-5924; E-mail: [email protected] and [email protected]
temperatures can result in failure of other sealing component such as the O-rings. Another problem is associated with non- uniform heating of seal rings known to cause distortion of the surfaces and alteration of the interface shape. With non- uniform heating, wear progressively changes the face profile to compensate for the thermal distortion and affects face surface finish. Li [4] analyzed the mechanical and thermal deformations of the seal components and concluded that thermal deformation is more significant than the mechanical deformation, for it can distort the originally flat sealing sur- face into slightly convex surface. Doust and Parmar [5] reached a similar conclusion. They measured the pressure and thermal distortion of seal components and demonstrated the influence of interface temperature rise on the fluid film geometry. They also showed that thermal distortion is greater than that caused by pressure, particularly in pump seals that run on hydrocarbons. The frictional heat can result in thermoelastic deformation which alters the contact pres- sure distribution. These seals when operate above a certain critical speed, enter into a condition known as thermoelastic instability (TEI) that causes localization of pressure and ex- cessive heat generation that manifest themselves in the form of hot spots at the sliding interface [6]. Jang and Khonsari [7] developed a model to study thermoelastic instability, and found that the surface roughness and the hydrodynamic lu- brication play important roles on the instability. Thus, heat transfer is of paramount importance on the operational per- formance of a mechanical seal.
The source of heat generation is the sliding friction at the seal interface and viscous heating of the fluid film between faces and around the rotating ring. Clearly, for low viscosity fluids, heat is mostly generated from the former. The heat generated at interface conducts through the seal ring materi- als and this heat must be carried away by the cooling fluid by convection. The cooling fluid is either from the circulated
2 Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 Xiao and Khonsari
pumped working fluid or from a pressurized external barrier fluid reservoir. Shirazi et al. [8] numerically predicted the fluid temperature within the seal housing and concluded that the fluid temperature distribution inside the seal housing is nearly uniform if a relatively large radial clearance exists between seal rings and housing wall.
The sealing flush flow rate has a significant effect on the fluid and solid temperature, but the direction of flush flow has little effect on the maximum fluid and solid temperature. Merati et al. [9] experimentally and numerically investigated the thermal behavior of a mechanical seal. They reported the heat generated at the seal face is mostly convected to fluid near the interface of the primary and mating rings and the highest temperatures occur near the inner diameter of the seal face. Doane et al. [10] also showed that large tempera- ture gradients only occur in the solid region near interface. Phillips et al. [11] experimentally determined the thermal characteristic of a mechanical seal and showed that the fluid velocities in the gap above the mating ring wear nose be- tween the mating and primary rings are less than those of the bulk flow field. This impedes heat transfer from the mating ring in the region of the heat generation at the seal interface.
Important factors in the conventional flush cooling at- tributed to enhancing heat transfer are: higher flush flow rate and larger radial clearance (gap) between seal and housing. Hence, heat transfer augmentation devices are often applied close to seal interface to promote mixing.
This paper presents a review of patents on the heat trans- fer augmentation techniques applied in the mechanical face seals. We focus our attention to issued or published US pat- ents over a 20-year period of 1989 to date. Two major cool- ing methods investigated here are: heat exchangers applied in the seal assembly and micro-topography on the seal face.
2. MECHANICAL SEAL WITH HEAT EXCHANGER
A heat exchanger is a device for improving heat transfer from one medium to another. The media may be separated by a solid wall or may be in direct contact. Heat exchangers
have been used for the purpose of cooling mechanical seals for a long time, but most of them are not installed close enough to the face surfaces and need complicated heat trans- fer paths and a large volume of coolant. An effective heat exchanger should be in close proximity to the seal interface. In what follows, four types of mechanical seal with heat ex- changer designs are discussed. They are seals with no flush flow, seals with an integrated heat exchanger, seals that offer improved fluid circulation and those that incorporate surface texture into the design.
2.1. Mechanical Seal with no Flush Flow
Most seal flush plans involve recirculation of the fluid being sealed from a pump into seal housing. The concern is that higher pressure fluid- like the fluid from the discharge side of a pump -increases the sealed fluid pressure and re- sults in increasing the interfacial heat generation, and endan- ger flashing of the liquid across the face. To reduce the inter- facial heat, Drumm [12] patented a mechanical seal with heat exchanger in 1989. The invention is a design of the annular heat exchanger 12 as shown in Fig. (2). The heat exchanger is a hollow body made of high thermal conductivity material. It comprises of inner and outer cylindrical walls 43 and 44, end walls 45 and 46, and fluid inlet 52 and outlet 54. A se- ries of annual ribs 48 and 50 are fabricated on both of oppos- ing surfaces to enhance heat transfer. Rotating seal ring 28 is made of a wear resistant material such as silicon carbide and stationary seal ring 32 is made of softer material such as car- bon graphite. Both rings are completely surrounded by the annular heat exchanger. Note that there is a thin annular fluid zone between sealing rings and heat exchanger. However, there is no direct flush flow towards the seal faces in this design. The fluid movement is limited to flow around the seal rings and, therefore, the convective heat transfer near the interface is low. The generated heat at the interface is dissi- pated through the thin fluid zone into heat exchanger. For heavily viscous or other thermo-sensitive fluids as the sealed fluid, this heat exchanger can either introduce heating or cooling into the seal chamber.
Fig. (1). Mechanical seal major components.
Heat Transfer Augmentation in Mechanical Seals Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 3
Fig. (2). A shaft mounted rotary end face seal incorporating a heat
exchanger (for the complete description of numbers see [12]).
Murray and Smith [13] patented a fluid cooled seal ar- rangement named circumferential flow channel for carbon seal runner cooling for a gas turbine engine in 1997. 22 is the stationary sealing element made of carbon, and 23 is the ro- tating sealing element. The cooling fluid, generally engine oil for high temperature operation, is pressurized to flow through passageway 29 and released through a series of cir- cumferentially spaced openings 39. The cooling fluid is in- jected radially into channel 45 by centrifugal forces, which is fabricated in the rotating sealing element 23. When the cool- ing fluid completely runs through the channel 45, it flows across the outwardly tapered surface and back to the oil lu- brication system, see Figs. (3 & 4). The inventors claim that this arrangement provides a uniform film of cooling fluid, resulting in a more uniform heat transfer from the rotating sealing element.
Fig. (3). A partial fragmentary view of the fluid cooled seal ar-
rangement (for the complete description of numbers see [13]).
Fig. (4). An illustrative end view of fluid passage way and flow
channel (for the complete description of numbers see [13]).
Hwang and Pope [14] patented an expanding carbon
rings circumferential seal system. Compared to conventional
contracting designs, the carbon rings are placed inside the rotating sealing element (runner) rather than outside. They
claim that the oil can be directly impinged on the runner and
have better cooling effect. Gockel and Robinson [15] de- signed an internally cooled seal housing for turbine engine.
The housing has an enclosed annular groove fabricated in the
wall. The cooling fluid is circulated through the groove to cool down the stationary carbon seal ring and housing.
Takahashi [16] invented an “outside style” mechanical seal in 2011 specially suitable for high temperature sealed
fluids, like those used in thermal power and chemical plants.
The inventor claims no-flush fluid and no-cooler design is needed even when very high temperature fluid is sealed. The
seal unit 1 is installed in a seal cover 5 outside stuffing box
9, see Figs. (5 & 6). The inventor recommends using silicon carbide in both stationary ring 7 and rotating ring 8. A cool-
ing jacket 11 is installed inside the stuffing box 9. The sealed
high-temperature fluid is passed through the gap between cooling jacket and shaft 3. The gap is set to between 0.1 and
0.2mm. The high temperature fluid in the extremely small
size gap is carried away through the cooling jacket, which is cooled by water through cooling feed opening 28. The heat
generated at seal interface is dissipated by air or an inert gas
flow through quenching opening 16, and reduced tempera- ture sealed fluid. This mechanical seal design is more com-
pact in size than the conventional one, since no flushing lines
and cooler are used.
2.2. Integrated Mechanical Seal Assembly
This kind of mechanical seal has a heat transfer unit inte- grated into the stationary seal ring to enhance heat transfer. Winkler et al. [17] designed a mechanical seal assembly with integrated heat transfer unit in 2011. The unit fitted in
4 Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 Xiao and Khonsari
Fig. (5). A front sectional view of entire mechanical seal (for the
complete description of numbers see [16]).
Fig. (6). Enlarged view of relevant parts (for the complete descrip-
tion of numbers see [16]).
the interior of the stationary seal ring 2 has a porous interior structure, see Fig. (7). The porous structure 2b is preferably made of silicon carbide. The coolant is directly flushed through the porous structure and discharged. The inventors claim that the heat transfer unit enables a direct dissipation of heat generated at the seal interface, and thermal distor- tions are avoided. The sealing gap temperature can be ad- justed to a defined temperature by controlling the coolant temperature.
Fig. (7). A schematic view of a mechanical seal assembly (for the
complete description of numbers see [17]).
A mechanical seal having a double-tier mating ring was designed Khonsari and Somanchi in 2005 [18]. First station- ary ring 4 and second stationary ring 10 in a double-tier ring structure 2 is illustrated in Fig. (8). The flush coolant is di- rected to the first ring 4, which has a series of radial holes for coolant passing through, see Fig. (9). Second ring 10 with a circumferential diverter 12 induces coolant mixing, see Fig. (10). The inventors claim that the presence of coolant in the interior of stationary ring is effective in reducing heat gen- eration at the seal faces, and shows no evidence of surface distress.
Fig. (8). A schematic diagram of the double-tier stationary seal ring
(for the complete description of numbers see [18]).
Heat Transfer Augmentation in Mechanical Seals Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 5
Fig. (9). First stationary ring (for the complete description of num-
bers see [18]).
Fig. (10). Second stationary ring (for the complete description of
numbers see [18]).
2.3. Mechanical Seal with Improved Fluid Circulation
During the mechanical face seal operation, the highest interface temperature generally occurs near the inner diame- ter of the stationary ring. The cooling fluid is manipulated to circulate around that area so as to achieve optimal cooling result. Khonsari and Somanchi designed a stationary seal ring with a plurality of through-channels to solve the prob- lem in 2007 [19]. Fig. (11) is a side view of the stationary ring. The inventors claim this design is highly effective in removing heat from seal faces by channeling the coolant adjacent to the interior surface of seal face. The radial slots on the stationary ring can minimize clogging and fouling of coolant passages caused by debris and other contaminants contained in the coolant.
Fig. (11). Aside view of a single-piece, perforated stationary ring
(for the complete description of numbers see [19]).
Young [20] invented a mechanical seal assembly with improved fluid circulation in 2001. The inventor claims that the design can circulate the fluid more thoroughly within the housing, and thus leads to reduced face temperature, thereby eliminating coking. The closed loop fluid path 44, a continu- ous groove, is milled into the shaft sleeve 42, see Figs. (12 & 13). The rotational motion of the sleeve and the closed loop fluid path create a radial circulation about the shaft as to en- hance the heat transfer and lower the seal face temperature.
Fig. (12). A vertical pump gear box seal with oil (for the complete
description of numbers see [20]).
In 1995, Wasser [21] designed a dual mechanical face seal having an improved mating ring. The surface 78 and axis of the shaft form a frusto-conical space 80, which is
6 Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 Xiao and Khonsari
Fig. (13). A plan view of the sleeve with continuous groove (for the
complete description of numbers see [20]).
preferably at an angle of about 15 o , see Figs. (14 & 15). The
frusto-conical space 80 coupled with opening 67 provides air flow path and circulation into and out of space 80 and cavity 82, where is between the primary ring 146 and shaft sleeve. The air flow circulation is driven by means of a specially designed drive collar 90. The inventor claims that this kind of arrangement provides additional cooling to the sealing rings and can effectively lower the temperature of at least the outboard seal of a dual mechanical face seal.
Azibert and Clark [22] patented a stationary seal ring with inclined inner surface relative to the outer surface of shaft sleeve. They claim that their design promotes the fluid circulation and effectively removes heat. Hornsby [23] de- signed a plurality of tapered axial notches on the shaft sleeve underneath the stationary seal ring to enhance pump effect and achieve a better circulation of barrier fluid.
Fig. (14). A partial cross-sectional view of the dual seal system (for
the complete description of numbers see [21]).
Fig. (15). A detailed cross-sectional and side view of sealing ring
(for the complete description of numbers see [21]).
2.4. Surface Textural Side-wall of the Stationary Ring
(Surface Heat Exchanger)
Given that the available space for heat transfer inside a seal chamber is limited, one may take advantage of surface heat transfer augmentation by means of texturing the sur- faces to increase the effective area for convective heat trans- fer. The common textures include surface roughness, grooves, dimples and ribs. A seal ring with textural surface can also increase mixing around the seal faces since the maximum heat flux occurs on the rotating ring surface near the interface between the rotating and stationary rings [9]. Another advantage of surface texture is that this technique can be applied in the conventional flushing seal chamber. The surface texturing is preferably used for seal mating ring, which is made of hard material. The primary ring is gener- ally made of a softer material. One of the designs has been by Khonsari and Gidden in 2008 [24]. Single circumferential groove 5 is shown in Fig. (16) and double grooves 5 and 7 are shown in Fig. (17). A series of subdivided fins 9 are shown in Fig. (18). Experimental tests were conducted using both conventional stationary ring and the textural ring. Un- der identical condition the interface temperature of the tex- tural ring was reduced from 49
o C to 43.5
o C.
Fig. (16). A stationary ring with a single circumferential groove (for the complete description of numbers see [24]).
Fig. (17). A stationary ring with double circumferential grooves
(for the complete description of numbers see [24]).
Heat Transfer Augmentation in Mechanical Seals Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 7
Fig. (18). A stationary ring with groove and subdivided fins (for the complete description of numbers see [24]).
Surface texturing simply in the form of dimples have been widely used in electronics cooling, heat exchangers, turbine blade internal cooling passages, etc. Dimpled surface can increase surface area and generate vortices to enhance mixing. It also has less pressure drop than most of other tex- tures, which is important to avoid fluid vaporizing due to significant pressure drop. Recently, Khonsari and Xiao [25] designed a stationary ring with dimpled sidewall in 2012. Cylindrical dimples were fabricated by a laser machine. A dimpled surface stationary ring is illustrated in Fig. (19). The diameter of each dimple is 0.9mm, and the depth is roughly 0.12mm. Dimples are in cylindrical shape and in a staggered arrangement, the total number is 480. The test results show that this technique can lower the total interface temperature by 10%.
Fig. (19). Dimpled surface stationary ring (for the complete de-
scription see [25]).
3. MICRO-TOPOGRAPHY RING SURFACE
The sealing gap between seal faces must be maintained very narrow to prevent flow leakage through the gap. Heat generation and wear increase if the gap is too small. Re- search shows that micro-topography produced directly on the surface of the mating ring provides some fluid pressure in the gap and effectively separates the sealing faces. The total applied separating load (i.e., the opening force) is supported
by the hydrostatic and hydrodynamic fluid pressure. How- ever, proper balance is needed since too much fluid pressure between the mating and primary ring can result in excessive leakage. Thus, seal gap must be carefully designed to pro- vide enough fluid pressure and minimize leakage. Extensive research efforts have been directed to the studies of micro- topography on the seal faces in the last decade. The partial surface texturing can introduce hydrostatic pressure build up in the sealing dam. Etsion and Halperin [26] theoretically optimized the face pattern parameter to obtain maximum hydrostatic pressure effect. Yu et al. [27] measured the inter- face temperature rise using a seal face with micro-pores pat- tern and a conventional seal, and found that the temperature rise of a laser-patterned seal face is about 30% less than that of the conventional seal. The result showed that the hydro- dynamic pressure is built up in the micro-pores to separate the rings surface and reduce heat generation. Grooves, such as spiral grooves and T-grooves, are typical hydrodynamic face patterns. Lai [28] designed a face seal with double spiral grooves in 1993. The inventor claims that the double spiral pumping groove configuration, made on either the primary or the mating ring face, achieves the desired gap between the seal faces and substantially minimize the fluid leakage be- tween the faces. The grooves on the primary ring face were produced by etching on the primary ring face, and their depth is preferably from about 5 m to about 8 m. The rela- tive lengths of radial projection of the spiral grooves 71, 73 are expressed by the ratios:
3 d2/d1 1/3
1 d3/d1 0
The downstream pumping grooves 71 pump the high pressure fluid from housing 65 in between seal faces provide the desired gap. Upstream grooves 73 pump fluid back to the housing so that fluid leakage is minimized, see Fig. (20).
Fig. (20). A seal ring face provided with one preferred embodiment
of spiral pumping grooves construction (for the complete descrip- tion of numbers see [28]).
8 Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 Xiao and Khonsari
Muller [29] patented several seal face patterns in 1996. The patterns are formed by means of laser beams on the slid- ing surface of ceramic sealing rings and their depth is from 1 micrometer to a few micrometers. One of the inventions in- cludes stepped hollows in the rotating ring seal face to create a hydrodynamic load support, see Fig. (21). Fluid enters the additional depression 51, which is open towards the seal ring edge, by fluid pressure. The fluid is out of depression 51 into the sealing gap as the rotating ring slides over the stationary ring. Then, the fluid entrains into a cascade hollows from shortest 424 to longest 421. As the fluid accumulates inside the hollows, the hydrodynamic pressure is built up at the shallowest end 420 of 421. When the entrained fluid pressure is higher than outside fluid pressure, the fluid entrained in- side the hollows is ejected back to the seal housing. Many other patents using the hydrodynamic patterns were also is- sued in past 20 years and most of them were applied in the gas seals; some examples are in [30-39].
Fig. (21). The flow path through hollow shaped depressions and
additional depression(for the complete description of numbers see
[29]).
Lebeck and Young [40] were issued a patent for wavy- tilt-dam seal ring in 1989. The inventors claim that the type of face shape of the ring can promote hydrostatic and hydro- dynamic lift and minimize leakage. The wavy-tilt-dam shape was ground or machine contoured onto the primary ring made of tungsten carbide, silicon carbide or stainless steel. The number of waves is determined by the stiffness or con- formability of the primary ring or opposing seal ring. The most preferably amplitude of the waviness is about 2.5 m. The waves 19 are formed in a circumferential direction on the face the ring. The waves 19 and tilts 20 are acted to en- hance lubrication and reduce friction. The dams 21, the flat areas, tend to minimize leakage, see Figs. (22 & 23).
Today, most face patterns are fabricated by laser beam. Young [41] developed a method for applying micro- topography to mechanical seal faces using an Excimer laser in 2012. The wave and other hydrodynamic face geometries are applied on the seal face of mechanical seals for rotary rock bits, examples are in [42, 43].
Fig. (22). Nine waves on the face of the ring (for the complete de-
scription of numbers see [40]).
Fig. (23). Contact of the mating ring with primary ring (for the
complete description of numbers see [40]).
The use of MEMS based micro-structures or micro- roughness methods can enhance lubrication and heat transfer in bearings and mechanical seals. Stephens and Kelly pat- ented [44] a high aspect ratio microstructures (HARMs) em- ployed to cover load-bearing surface of mechanical seals or bearings, see Fig. (24). The microstructure can be micro- channels or microposts. They claim the HARMs microstruc- ture, used as heat sink and fluid/solid lubricant storage and distribution, can effectively enhance heat transfer and pro- vide lubricant to the load-bearing surface. The HARMs mi- crostructure is fabricated using LIGA process on a nickel alloy or a ceramic. The aspect ratio for mechanical seals is- recommended to be between 0.5 and 50, preferably between 1 and 10. Another patent of HARMs is in [45]. Lebeck [46] patented a seal interface having area different in roughness. The inventor claims that this design can supply asperity tip load support to opposing interface.
Heat Transfer Augmentation in Mechanical Seals Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 9
Fig. (24). A field of square nickel posts fabricated with the LIGA
process (for the complete description see [44]).
4. CONCLUSION
Heat removal is an important factor in design of me- chanical face seal. The review of mechanical seal cooling techniques in this article have shown different approaches to deal with heat generation at the seal faces. This review is based on patents issued or published over the twenty-year span (1989 to date). The patents issued and patent applica- tions range from simple heat transfer devices add-on in the mechanical seal assembly to more radical changes in the conventional mechanical seal arrangement. It is the authors’ hope that this review provides pertinent information and in- sight to the reader on the recent innovations on mechanical seals with superior thermal performance. Ideally, a good design of mechanical seal unit should be easy to fabricate, install, dramatically increase the seal service life and cost- effective. To this end, recent developments on micro- topography of surfaces can be combined with heat transfer augmentation methodologies to enhance thermal perform- ance and reduce wear.
5. CURRENT AND FUTURE DEVELOPMENT
The need for mechanical face seals is high in industry. It is very important to improve the wear problem caused by heat generation at the seal faces. The commercial potential for such techniques is enormous. The authors have also de- signed a stationary seal ring using surface texturing tech- nique. Given that a lot of heat is dissipated from the rotating ring, the same technique can be applied the rotating seal ring. Also, an integrated heat transfer unit is under development, since the unit can directly transfer heat from seal rings inter- face and dissipate to outside the seal chamber. Both concep- tual designs are simple and easy to make, also no need to change original flush arrangement.
CONFLICT OF INTEREST
The authors confirm that the contents of this article have no conflicts of interest.
ACKNOWLEDGEMENTS
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hanced with microstructures. US6280090 (2001). [45] Stephens, L.S., Kelly, K.W. Mechanical seals enhanced with mi-
crostructures. US6149160 (2000). [46] Lebeck, A.O. Differential surface roughness dynamic seals and
bearings. US4834400 (1989).
A Review of Mechanical Seals Heat Transfer Augmentation Techniques
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A Review of Mechanical Seals Heat Transfer Augmentation Techniques
Nian Xiao and M. M. Khonsari*
Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803, USA
Received: November 01, 2012; Accepted: January 08, 2013; Revised: January 01, 2013
Abstract: A mechanical face seal is one of most widely used components in rotating machinery such as pumps, mixers,
blowers, and compressors. The primary function of a mechanical seal is to prevent leakage of the process fluid from the
pump housing and shaft to the environment. Excessive heat generated at the face seal interface has been recognized as one
of main causes of failure of mechanical seals. In the past few decades various efforts have been attempted to remove heat
from the interface uniformly in order to reduce the interfacial temperature, eliminate thermally-induced failure, and thus
increase the life of a mechanical seal. In this paper, a class of the pertinent patents on mechanical seals that use heat trans-
fer augmentation techniques are reviewed and discussed. Also presented are several new approaches recently proposed by
the authors specifically developed for enhancing heat transfer augmentation in mechanical face seal.
Keywords: Heat exchanger, heat transfer augmentation, improved fluid circulation, integrated assembly, mechanical seal, mi- cro-topography, no flush flow, surface textural side-wall.
1. INTRODUCTION
A mechanical seal consists of a primary ring that rotates with the shaft and a mating ring affixed to the housing. Dur- ing operation, the primary ring rubs against the mating ring where contact between the surfaces is maintained by springs attached to the primary ring; see Fig. (1). A thin film of fluid in the gap between the primary and mating rings must be maintained to hydrodynamically lubricate the faces and sepa- rate the ring surfaces so as to avoid direct rubbing that causes wear. However, the majority of seals fail long before the seal face wears out. The primary cause of failure in the majority of applications is excessive heat.
As pointed out by Lebeck [1], performance of a seal de- pends largely on three factors related to the interface tem- perature. First, when the interfacial temperature is too high the thin film of lubricating fluid across the gap- typically on the order of a few micrometers -may flash to vapor. Anno [2] reported that, for face seals containing micro-asperities, the film had a thickness varying from about 0.4 m to 0.2 m as the applied pressure varied from 1,000 to 70,000kg/m
2 . Pape
[3] analytically and experimentally studied the lubricant film and concluded that at the lowest film thickness, the coeffi- cient of friction is higher than the theoretical prediction be- cause of insufficient lubrication of the faces. The lubricating film is often so thin that excessive frictional heat generation can result in significant rise in temperature once a part or the whole liquid film is vaporized. As a result, the seal can be quickly destroyed. Second, many face seal materials simply cannot withstand exposure to high levels of temperature for a prolong length of time. Also, operation at very high
*Address for correspondence to these authors at the Department of Me-
chanical Engineering, 1419A Patrick Taylor Hall, Louisiana State Univer-
sity, Baton Rouge, LA 70803; Tel: (225) 578-9192; Fax: (225) 578-5924; E-mail: [email protected] and [email protected]
temperatures can result in failure of other sealing component such as the O-rings. Another problem is associated with non- uniform heating of seal rings known to cause distortion of the surfaces and alteration of the interface shape. With non- uniform heating, wear progressively changes the face profile to compensate for the thermal distortion and affects face surface finish. Li [4] analyzed the mechanical and thermal deformations of the seal components and concluded that thermal deformation is more significant than the mechanical deformation, for it can distort the originally flat sealing sur- face into slightly convex surface. Doust and Parmar [5] reached a similar conclusion. They measured the pressure and thermal distortion of seal components and demonstrated the influence of interface temperature rise on the fluid film geometry. They also showed that thermal distortion is greater than that caused by pressure, particularly in pump seals that run on hydrocarbons. The frictional heat can result in thermoelastic deformation which alters the contact pres- sure distribution. These seals when operate above a certain critical speed, enter into a condition known as thermoelastic instability (TEI) that causes localization of pressure and ex- cessive heat generation that manifest themselves in the form of hot spots at the sliding interface [6]. Jang and Khonsari [7] developed a model to study thermoelastic instability, and found that the surface roughness and the hydrodynamic lu- brication play important roles on the instability. Thus, heat transfer is of paramount importance on the operational per- formance of a mechanical seal.
The source of heat generation is the sliding friction at the seal interface and viscous heating of the fluid film between faces and around the rotating ring. Clearly, for low viscosity fluids, heat is mostly generated from the former. The heat generated at interface conducts through the seal ring materi- als and this heat must be carried away by the cooling fluid by convection. The cooling fluid is either from the circulated
2 Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 Xiao and Khonsari
pumped working fluid or from a pressurized external barrier fluid reservoir. Shirazi et al. [8] numerically predicted the fluid temperature within the seal housing and concluded that the fluid temperature distribution inside the seal housing is nearly uniform if a relatively large radial clearance exists between seal rings and housing wall.
The sealing flush flow rate has a significant effect on the fluid and solid temperature, but the direction of flush flow has little effect on the maximum fluid and solid temperature. Merati et al. [9] experimentally and numerically investigated the thermal behavior of a mechanical seal. They reported the heat generated at the seal face is mostly convected to fluid near the interface of the primary and mating rings and the highest temperatures occur near the inner diameter of the seal face. Doane et al. [10] also showed that large tempera- ture gradients only occur in the solid region near interface. Phillips et al. [11] experimentally determined the thermal characteristic of a mechanical seal and showed that the fluid velocities in the gap above the mating ring wear nose be- tween the mating and primary rings are less than those of the bulk flow field. This impedes heat transfer from the mating ring in the region of the heat generation at the seal interface.
Important factors in the conventional flush cooling at- tributed to enhancing heat transfer are: higher flush flow rate and larger radial clearance (gap) between seal and housing. Hence, heat transfer augmentation devices are often applied close to seal interface to promote mixing.
This paper presents a review of patents on the heat trans- fer augmentation techniques applied in the mechanical face seals. We focus our attention to issued or published US pat- ents over a 20-year period of 1989 to date. Two major cool- ing methods investigated here are: heat exchangers applied in the seal assembly and micro-topography on the seal face.
2. MECHANICAL SEAL WITH HEAT EXCHANGER
A heat exchanger is a device for improving heat transfer from one medium to another. The media may be separated by a solid wall or may be in direct contact. Heat exchangers
have been used for the purpose of cooling mechanical seals for a long time, but most of them are not installed close enough to the face surfaces and need complicated heat trans- fer paths and a large volume of coolant. An effective heat exchanger should be in close proximity to the seal interface. In what follows, four types of mechanical seal with heat ex- changer designs are discussed. They are seals with no flush flow, seals with an integrated heat exchanger, seals that offer improved fluid circulation and those that incorporate surface texture into the design.
2.1. Mechanical Seal with no Flush Flow
Most seal flush plans involve recirculation of the fluid being sealed from a pump into seal housing. The concern is that higher pressure fluid- like the fluid from the discharge side of a pump -increases the sealed fluid pressure and re- sults in increasing the interfacial heat generation, and endan- ger flashing of the liquid across the face. To reduce the inter- facial heat, Drumm [12] patented a mechanical seal with heat exchanger in 1989. The invention is a design of the annular heat exchanger 12 as shown in Fig. (2). The heat exchanger is a hollow body made of high thermal conductivity material. It comprises of inner and outer cylindrical walls 43 and 44, end walls 45 and 46, and fluid inlet 52 and outlet 54. A se- ries of annual ribs 48 and 50 are fabricated on both of oppos- ing surfaces to enhance heat transfer. Rotating seal ring 28 is made of a wear resistant material such as silicon carbide and stationary seal ring 32 is made of softer material such as car- bon graphite. Both rings are completely surrounded by the annular heat exchanger. Note that there is a thin annular fluid zone between sealing rings and heat exchanger. However, there is no direct flush flow towards the seal faces in this design. The fluid movement is limited to flow around the seal rings and, therefore, the convective heat transfer near the interface is low. The generated heat at the interface is dissi- pated through the thin fluid zone into heat exchanger. For heavily viscous or other thermo-sensitive fluids as the sealed fluid, this heat exchanger can either introduce heating or cooling into the seal chamber.
Fig. (1). Mechanical seal major components.
Heat Transfer Augmentation in Mechanical Seals Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 3
Fig. (2). A shaft mounted rotary end face seal incorporating a heat
exchanger (for the complete description of numbers see [12]).
Murray and Smith [13] patented a fluid cooled seal ar- rangement named circumferential flow channel for carbon seal runner cooling for a gas turbine engine in 1997. 22 is the stationary sealing element made of carbon, and 23 is the ro- tating sealing element. The cooling fluid, generally engine oil for high temperature operation, is pressurized to flow through passageway 29 and released through a series of cir- cumferentially spaced openings 39. The cooling fluid is in- jected radially into channel 45 by centrifugal forces, which is fabricated in the rotating sealing element 23. When the cool- ing fluid completely runs through the channel 45, it flows across the outwardly tapered surface and back to the oil lu- brication system, see Figs. (3 & 4). The inventors claim that this arrangement provides a uniform film of cooling fluid, resulting in a more uniform heat transfer from the rotating sealing element.
Fig. (3). A partial fragmentary view of the fluid cooled seal ar-
rangement (for the complete description of numbers see [13]).
Fig. (4). An illustrative end view of fluid passage way and flow
channel (for the complete description of numbers see [13]).
Hwang and Pope [14] patented an expanding carbon
rings circumferential seal system. Compared to conventional
contracting designs, the carbon rings are placed inside the rotating sealing element (runner) rather than outside. They
claim that the oil can be directly impinged on the runner and
have better cooling effect. Gockel and Robinson [15] de- signed an internally cooled seal housing for turbine engine.
The housing has an enclosed annular groove fabricated in the
wall. The cooling fluid is circulated through the groove to cool down the stationary carbon seal ring and housing.
Takahashi [16] invented an “outside style” mechanical seal in 2011 specially suitable for high temperature sealed
fluids, like those used in thermal power and chemical plants.
The inventor claims no-flush fluid and no-cooler design is needed even when very high temperature fluid is sealed. The
seal unit 1 is installed in a seal cover 5 outside stuffing box
9, see Figs. (5 & 6). The inventor recommends using silicon carbide in both stationary ring 7 and rotating ring 8. A cool-
ing jacket 11 is installed inside the stuffing box 9. The sealed
high-temperature fluid is passed through the gap between cooling jacket and shaft 3. The gap is set to between 0.1 and
0.2mm. The high temperature fluid in the extremely small
size gap is carried away through the cooling jacket, which is cooled by water through cooling feed opening 28. The heat
generated at seal interface is dissipated by air or an inert gas
flow through quenching opening 16, and reduced tempera- ture sealed fluid. This mechanical seal design is more com-
pact in size than the conventional one, since no flushing lines
and cooler are used.
2.2. Integrated Mechanical Seal Assembly
This kind of mechanical seal has a heat transfer unit inte- grated into the stationary seal ring to enhance heat transfer. Winkler et al. [17] designed a mechanical seal assembly with integrated heat transfer unit in 2011. The unit fitted in
4 Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 Xiao and Khonsari
Fig. (5). A front sectional view of entire mechanical seal (for the
complete description of numbers see [16]).
Fig. (6). Enlarged view of relevant parts (for the complete descrip-
tion of numbers see [16]).
the interior of the stationary seal ring 2 has a porous interior structure, see Fig. (7). The porous structure 2b is preferably made of silicon carbide. The coolant is directly flushed through the porous structure and discharged. The inventors claim that the heat transfer unit enables a direct dissipation of heat generated at the seal interface, and thermal distor- tions are avoided. The sealing gap temperature can be ad- justed to a defined temperature by controlling the coolant temperature.
Fig. (7). A schematic view of a mechanical seal assembly (for the
complete description of numbers see [17]).
A mechanical seal having a double-tier mating ring was designed Khonsari and Somanchi in 2005 [18]. First station- ary ring 4 and second stationary ring 10 in a double-tier ring structure 2 is illustrated in Fig. (8). The flush coolant is di- rected to the first ring 4, which has a series of radial holes for coolant passing through, see Fig. (9). Second ring 10 with a circumferential diverter 12 induces coolant mixing, see Fig. (10). The inventors claim that the presence of coolant in the interior of stationary ring is effective in reducing heat gen- eration at the seal faces, and shows no evidence of surface distress.
Fig. (8). A schematic diagram of the double-tier stationary seal ring
(for the complete description of numbers see [18]).
Heat Transfer Augmentation in Mechanical Seals Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 5
Fig. (9). First stationary ring (for the complete description of num-
bers see [18]).
Fig. (10). Second stationary ring (for the complete description of
numbers see [18]).
2.3. Mechanical Seal with Improved Fluid Circulation
During the mechanical face seal operation, the highest interface temperature generally occurs near the inner diame- ter of the stationary ring. The cooling fluid is manipulated to circulate around that area so as to achieve optimal cooling result. Khonsari and Somanchi designed a stationary seal ring with a plurality of through-channels to solve the prob- lem in 2007 [19]. Fig. (11) is a side view of the stationary ring. The inventors claim this design is highly effective in removing heat from seal faces by channeling the coolant adjacent to the interior surface of seal face. The radial slots on the stationary ring can minimize clogging and fouling of coolant passages caused by debris and other contaminants contained in the coolant.
Fig. (11). Aside view of a single-piece, perforated stationary ring
(for the complete description of numbers see [19]).
Young [20] invented a mechanical seal assembly with improved fluid circulation in 2001. The inventor claims that the design can circulate the fluid more thoroughly within the housing, and thus leads to reduced face temperature, thereby eliminating coking. The closed loop fluid path 44, a continu- ous groove, is milled into the shaft sleeve 42, see Figs. (12 & 13). The rotational motion of the sleeve and the closed loop fluid path create a radial circulation about the shaft as to en- hance the heat transfer and lower the seal face temperature.
Fig. (12). A vertical pump gear box seal with oil (for the complete
description of numbers see [20]).
In 1995, Wasser [21] designed a dual mechanical face seal having an improved mating ring. The surface 78 and axis of the shaft form a frusto-conical space 80, which is
6 Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 Xiao and Khonsari
Fig. (13). A plan view of the sleeve with continuous groove (for the
complete description of numbers see [20]).
preferably at an angle of about 15 o , see Figs. (14 & 15). The
frusto-conical space 80 coupled with opening 67 provides air flow path and circulation into and out of space 80 and cavity 82, where is between the primary ring 146 and shaft sleeve. The air flow circulation is driven by means of a specially designed drive collar 90. The inventor claims that this kind of arrangement provides additional cooling to the sealing rings and can effectively lower the temperature of at least the outboard seal of a dual mechanical face seal.
Azibert and Clark [22] patented a stationary seal ring with inclined inner surface relative to the outer surface of shaft sleeve. They claim that their design promotes the fluid circulation and effectively removes heat. Hornsby [23] de- signed a plurality of tapered axial notches on the shaft sleeve underneath the stationary seal ring to enhance pump effect and achieve a better circulation of barrier fluid.
Fig. (14). A partial cross-sectional view of the dual seal system (for
the complete description of numbers see [21]).
Fig. (15). A detailed cross-sectional and side view of sealing ring
(for the complete description of numbers see [21]).
2.4. Surface Textural Side-wall of the Stationary Ring
(Surface Heat Exchanger)
Given that the available space for heat transfer inside a seal chamber is limited, one may take advantage of surface heat transfer augmentation by means of texturing the sur- faces to increase the effective area for convective heat trans- fer. The common textures include surface roughness, grooves, dimples and ribs. A seal ring with textural surface can also increase mixing around the seal faces since the maximum heat flux occurs on the rotating ring surface near the interface between the rotating and stationary rings [9]. Another advantage of surface texture is that this technique can be applied in the conventional flushing seal chamber. The surface texturing is preferably used for seal mating ring, which is made of hard material. The primary ring is gener- ally made of a softer material. One of the designs has been by Khonsari and Gidden in 2008 [24]. Single circumferential groove 5 is shown in Fig. (16) and double grooves 5 and 7 are shown in Fig. (17). A series of subdivided fins 9 are shown in Fig. (18). Experimental tests were conducted using both conventional stationary ring and the textural ring. Un- der identical condition the interface temperature of the tex- tural ring was reduced from 49
o C to 43.5
o C.
Fig. (16). A stationary ring with a single circumferential groove (for the complete description of numbers see [24]).
Fig. (17). A stationary ring with double circumferential grooves
(for the complete description of numbers see [24]).
Heat Transfer Augmentation in Mechanical Seals Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 7
Fig. (18). A stationary ring with groove and subdivided fins (for the complete description of numbers see [24]).
Surface texturing simply in the form of dimples have been widely used in electronics cooling, heat exchangers, turbine blade internal cooling passages, etc. Dimpled surface can increase surface area and generate vortices to enhance mixing. It also has less pressure drop than most of other tex- tures, which is important to avoid fluid vaporizing due to significant pressure drop. Recently, Khonsari and Xiao [25] designed a stationary ring with dimpled sidewall in 2012. Cylindrical dimples were fabricated by a laser machine. A dimpled surface stationary ring is illustrated in Fig. (19). The diameter of each dimple is 0.9mm, and the depth is roughly 0.12mm. Dimples are in cylindrical shape and in a staggered arrangement, the total number is 480. The test results show that this technique can lower the total interface temperature by 10%.
Fig. (19). Dimpled surface stationary ring (for the complete de-
scription see [25]).
3. MICRO-TOPOGRAPHY RING SURFACE
The sealing gap between seal faces must be maintained very narrow to prevent flow leakage through the gap. Heat generation and wear increase if the gap is too small. Re- search shows that micro-topography produced directly on the surface of the mating ring provides some fluid pressure in the gap and effectively separates the sealing faces. The total applied separating load (i.e., the opening force) is supported
by the hydrostatic and hydrodynamic fluid pressure. How- ever, proper balance is needed since too much fluid pressure between the mating and primary ring can result in excessive leakage. Thus, seal gap must be carefully designed to pro- vide enough fluid pressure and minimize leakage. Extensive research efforts have been directed to the studies of micro- topography on the seal faces in the last decade. The partial surface texturing can introduce hydrostatic pressure build up in the sealing dam. Etsion and Halperin [26] theoretically optimized the face pattern parameter to obtain maximum hydrostatic pressure effect. Yu et al. [27] measured the inter- face temperature rise using a seal face with micro-pores pat- tern and a conventional seal, and found that the temperature rise of a laser-patterned seal face is about 30% less than that of the conventional seal. The result showed that the hydro- dynamic pressure is built up in the micro-pores to separate the rings surface and reduce heat generation. Grooves, such as spiral grooves and T-grooves, are typical hydrodynamic face patterns. Lai [28] designed a face seal with double spiral grooves in 1993. The inventor claims that the double spiral pumping groove configuration, made on either the primary or the mating ring face, achieves the desired gap between the seal faces and substantially minimize the fluid leakage be- tween the faces. The grooves on the primary ring face were produced by etching on the primary ring face, and their depth is preferably from about 5 m to about 8 m. The rela- tive lengths of radial projection of the spiral grooves 71, 73 are expressed by the ratios:
3 d2/d1 1/3
1 d3/d1 0
The downstream pumping grooves 71 pump the high pressure fluid from housing 65 in between seal faces provide the desired gap. Upstream grooves 73 pump fluid back to the housing so that fluid leakage is minimized, see Fig. (20).
Fig. (20). A seal ring face provided with one preferred embodiment
of spiral pumping grooves construction (for the complete descrip- tion of numbers see [28]).
8 Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 Xiao and Khonsari
Muller [29] patented several seal face patterns in 1996. The patterns are formed by means of laser beams on the slid- ing surface of ceramic sealing rings and their depth is from 1 micrometer to a few micrometers. One of the inventions in- cludes stepped hollows in the rotating ring seal face to create a hydrodynamic load support, see Fig. (21). Fluid enters the additional depression 51, which is open towards the seal ring edge, by fluid pressure. The fluid is out of depression 51 into the sealing gap as the rotating ring slides over the stationary ring. Then, the fluid entrains into a cascade hollows from shortest 424 to longest 421. As the fluid accumulates inside the hollows, the hydrodynamic pressure is built up at the shallowest end 420 of 421. When the entrained fluid pressure is higher than outside fluid pressure, the fluid entrained in- side the hollows is ejected back to the seal housing. Many other patents using the hydrodynamic patterns were also is- sued in past 20 years and most of them were applied in the gas seals; some examples are in [30-39].
Fig. (21). The flow path through hollow shaped depressions and
additional depression(for the complete description of numbers see
[29]).
Lebeck and Young [40] were issued a patent for wavy- tilt-dam seal ring in 1989. The inventors claim that the type of face shape of the ring can promote hydrostatic and hydro- dynamic lift and minimize leakage. The wavy-tilt-dam shape was ground or machine contoured onto the primary ring made of tungsten carbide, silicon carbide or stainless steel. The number of waves is determined by the stiffness or con- formability of the primary ring or opposing seal ring. The most preferably amplitude of the waviness is about 2.5 m. The waves 19 are formed in a circumferential direction on the face the ring. The waves 19 and tilts 20 are acted to en- hance lubrication and reduce friction. The dams 21, the flat areas, tend to minimize leakage, see Figs. (22 & 23).
Today, most face patterns are fabricated by laser beam. Young [41] developed a method for applying micro- topography to mechanical seal faces using an Excimer laser in 2012. The wave and other hydrodynamic face geometries are applied on the seal face of mechanical seals for rotary rock bits, examples are in [42, 43].
Fig. (22). Nine waves on the face of the ring (for the complete de-
scription of numbers see [40]).
Fig. (23). Contact of the mating ring with primary ring (for the
complete description of numbers see [40]).
The use of MEMS based micro-structures or micro- roughness methods can enhance lubrication and heat transfer in bearings and mechanical seals. Stephens and Kelly pat- ented [44] a high aspect ratio microstructures (HARMs) em- ployed to cover load-bearing surface of mechanical seals or bearings, see Fig. (24). The microstructure can be micro- channels or microposts. They claim the HARMs microstruc- ture, used as heat sink and fluid/solid lubricant storage and distribution, can effectively enhance heat transfer and pro- vide lubricant to the load-bearing surface. The HARMs mi- crostructure is fabricated using LIGA process on a nickel alloy or a ceramic. The aspect ratio for mechanical seals is- recommended to be between 0.5 and 50, preferably between 1 and 10. Another patent of HARMs is in [45]. Lebeck [46] patented a seal interface having area different in roughness. The inventor claims that this design can supply asperity tip load support to opposing interface.
Heat Transfer Augmentation in Mechanical Seals Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 2 9
Fig. (24). A field of square nickel posts fabricated with the LIGA
process (for the complete description see [44]).
4. CONCLUSION
Heat removal is an important factor in design of me- chanical face seal. The review of mechanical seal cooling techniques in this article have shown different approaches to deal with heat generation at the seal faces. This review is based on patents issued or published over the twenty-year span (1989 to date). The patents issued and patent applica- tions range from simple heat transfer devices add-on in the mechanical seal assembly to more radical changes in the conventional mechanical seal arrangement. It is the authors’ hope that this review provides pertinent information and in- sight to the reader on the recent innovations on mechanical seals with superior thermal performance. Ideally, a good design of mechanical seal unit should be easy to fabricate, install, dramatically increase the seal service life and cost- effective. To this end, recent developments on micro- topography of surfaces can be combined with heat transfer augmentation methodologies to enhance thermal perform- ance and reduce wear.
5. CURRENT AND FUTURE DEVELOPMENT
The need for mechanical face seals is high in industry. It is very important to improve the wear problem caused by heat generation at the seal faces. The commercial potential for such techniques is enormous. The authors have also de- signed a stationary seal ring using surface texturing tech- nique. Given that a lot of heat is dissipated from the rotating ring, the same technique can be applied the rotating seal ring. Also, an integrated heat transfer unit is under development, since the unit can directly transfer heat from seal rings inter- face and dissipate to outside the seal chamber. Both concep- tual designs are simple and easy to make, also no need to change original flush arrangement.
CONFLICT OF INTEREST
The authors confirm that the contents of this article have no conflicts of interest.
ACKNOWLEDGEMENTS
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