Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
3~ 5 D- 5 5 ~ 2
BACKGROUND OF THE: INVEN :[ ION
___ _ __ _
The recent United States Govexnment ban on -the
use of polychlorinated biphenols as coolants for medium and
power transformers necessitates Ihe use of expensive silicone
based oils for transformer cooling purposes. Since the ~uantity
of oil required for total immers:ion cooling i6 in the order of
hundreds of gallons, alternate means for cooling transformers
have been proposed. One efficient me-thod comprises the use of
a condensable fluid and utilizes the vaporiza-tion and condensatlon
cycle of the fluid to remove the heat from the transformer
surface during the vaporiæation portion of the cycle and
transferring the heat via a heat exchanger during the
condensation portion of the cycle. U.S. Patent 3,024,298
issùed March 6, 1962 to Goltsos et al discloses an evaporative-
gravity cooling system using a condensable fluorochemical as
a coolant for electronic devices.
Another prior art method for cooling electronic
devices utilizes liquid film cooling to take heat from the
electrical device during operation and transfer the heat -to a
heat exchanger medium~ U.S. Patent 2,924,635 issued February
9, 1969 to Narkut discloses electrical apparatus utilizing a
fluid dielectric atmosphere for both electrical insulation and
as a cooling mechanism for dissipating heat developed during
operation of the apparatus.
The purpose of this invention is to propose a novel
pPrcolation method for vapor cooling transformers without the
requirement for~a liquid distribution pump within the liquid
coolant reservoir and wi~hout the requirement for force cooling
the heat exchanger as described earlier for the prior art.
: ~ , . . .... . .
~: 3 0 SU~RY OF TH ` INVENTION
The invention comprises a self-propelled vapor cooling
system for electrical apparatus containing a noncondensable gas
~: C à~
'.
~19~2 5D~5542
and a condensable refrigerant fluid, a heat exchanger, and
a novel thermal pump arranyement for percolation cooling the
electrical apparatus. The invention includes a molecular
sieve vapor trap for removing und.esirable vapor phase substances
that may contaminate the refrigerant.
BRIEF DESCRIPTION OF THE DRAWING
FIGURE 1 is a perspective view i.n parti~l section of
one cool.ing apparatus oE the prior art;
FIGURE 2 is a front view in partial section of a
further cooling apparatus of the prior art;
FIGURE 3 is a ront perspective view in partial
section of the cooling apparatus accordi.ng to the invention;
FIGURE 4 is an enlarged side view in partial section
oE the molecular sieve trap assembly of the apparatus o
FIGURE 3;
FIGURE 4A is an alternate arrangement oE the molecular
sieve txap assembly~
FIGURE 5 is a sectional view of the novel thermal
pump for use with the apparatu.s of FIGURE 3;
FIGURE 6 is a graphic representation of the thermal
distribution profile within the cooling ducts of transformers
for various cooling mediums;
FIGURE 7 is a graphic representation of the cooling
rate as a function of liquid cooling level for the percolation
cooling system of this invention;
FIGURE 8 is a cross-sectional view of a further
embodiment of the thermal pump of FIGURE 5;
FIGURE 9 is an enlarged cross-sectional view of a
further embodirnent of the device of FIGURE 8;
~30 FIGURE~10 is a top perspective view of a transformer
for use with the novel percolation cooling system of this
invention;
:
- 2 -
. ~ ,
~ 5D~55~2
FIGURE 11 is a side view oE a further embodimen-t of
the device of FIGURE 3; and
FIGUR~ 12 is a side view of another embodiment of
the system of FIGURE 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows one type of prior art device 10 containing
a fluid condensable coolant 11 within a tank which is in contact
with a transformer 13 to be cooled. The transformer 13 becomes
cool by heating the coolant 11 to its vaporization temperature
and cau~es the coolant 11 to transfer into a heat exchanyer 14
which is forced-cooled. The coolant 11 readily condenses
within the heat exchanger 14 and gives up its energy as heat
of condensation.
FIG. 2 shows a liquid cooling means 15 containing a
tank 16 and a reservoir 17 containing a liquid coolant 18. The
- device 15 further contains a pump 19 and a plurality of nozzles
9 for spraying the coolant 18 in liquid droplet form over the
surface of a transformer 20. The coolant 18 forms a thin liquid
film over the surface of the transformer 20 and the liquid film
upon contacting the hot transformer surface readily evaporates
and becomes transported in a gaseous state into a condenser 21
for transferring the heat obtained from the transformer 20 to
the condenser 21 by the heat of condensation. An expansion
tank 22 containing a noncondensable gas is often employed with
systems of this kind and an auxiliary heat exchanger 23 may be
required~
FIGURE 3 shows one embodiment of the novel percolation
cooling apparatus employing the principles of this invention.
The apparatus 24 includes a boiler 25 consisting of a tank 26,
a piece of electrical apparatus such as a transformer 27 which
.
.' :
~ ~ 3 -
~. .
"
~'','
3;~ rl D 5 5 '~ 2
generates heat during opera~i.oJI, and a con~ensab:le coolant
medlum 28. Sys~em 24 further includes a heat exch.lng~r 29
having an upper mani:fold 30, a lower maniEold 31, para3.lel
headers 40, and a plurali~y of interconnecting cooling tubes
32. An expansion tank 33 contain:ing a noncondensable gas i.s
often employed when the coolant Z8 provides the dllal function
of percolation cooling and acts as a dielecrr:ic for the
electrical apparatus employed. The noncolldensable gas provides
the dielectric medium for electrically insulating the electrical
apparatus when the tempera~ure is insuficient to cause the
coolant 28 to beconle vaporized and to completely cover the
electrical apparatus 27. The expansion tank 33 is connected
to the heat exchanger 29 at both the upper and lower manifolds
30, 31, and is connected to the boiler 25 by means of duct 34.
Duct 34 houses a container 35 of a molecular sieve substance 36
the purpose of which will be described ~elow. I'ransormer 27
has a feed-through bushing 5~ attached for making electrical
contact with the transformer 27 as is common with medium and
~ power transformer assembliés.
;: 20 The duct 34 is shown in enlarged detail i~ FXGUR~ 4
where the top of tank 26 is connecked to lower manifold 3i by
means of duct 34. The container 35 of molecular sie~e m~terial
36 contains a contaminant trap 37 at the bottom section thereof~
The vapor transmission path for the coolant 28 is indicated by
arrows and proceeds as follows. - Upon becomlng heated, and
vaporizing the coolant 28 enters into the inlet 38 to within.
i: :
the top:portion 39 of the duct 34 down through the molecular
sieve:~material 36;into the lower manifold 31 by means of
mani~fold inlet 31. The coolant 28 CoDsists of a vaporizable
chlorof;luorocarbon~such as trichlorotrifluoroethane which may
: -4-
:
~ )S5~
react ~ith water vapo-r ~Ind 'become disas~;oci,~ d (l~ in~-J
opera-tion. The type of t-ransformeI ~,o he oooled genera'~l,y
contains a wrapp:ing of paper in,llLa~ion which at t'he
operating temperatur~s involved may yield an amount ~f'
water vapor which canno~ be completely removed during the
initial thermal treating process. The purpose o the
molecular sieve material 36 therefore is ~o remove any water
vapor or other harm~'ul l:i,quid substaIlces that ma~ be evolved
from the trans~ormer paper and pi,,ked up ky the coolant
material 28 during its vapor transi,tion process. A partieularly
effective granular molecular sieve material 36 is a zeolite
type 4A made by the Linde Division o the IJn:ion Carbide Corpo-
ration. It has been determined that the Linde molecular sieve
material 36 effectively removes all traces o~ water vapor from
the coolant 28 and that other liquid contaminants remain in
the trap 37 provided at the bottom section o~ the duct
34. In the absence of the molecular sieve material 36, the
- coolant material 28 ~trichlorotrifluoroethane) may become
cloudy. The coolant 28 when used with the sieve material 36
~ 20 remains clear during con-tinuous operation. Ater passing
- through the molecular sieve material 36 the ~aporized coolant
28 transmits into the lower manifold 31 by means o~ manifcld
inlet 31' and from there out to the cooling tubes 32 by means
of the headers 40. The vaporized coolant 28 readily cond~nses
25`; ~ within the tubes 32 and returns in liquid orm by means of a
' separate return through the lower maniold 31 back into the
~ank 26 via condensate return pipe 41.
An alternate arrangement of the molecular sieve 36 is
~ .
~i shown in FIGVRE 4A. The duct 34 shown in FIGURE 4 has been
.j
` 30 eliminated and the molecular sieve 35 is included in the lower
.~
~ manifold 31. The vapor transmission p~th for the coolant 28
. .
, . . .
.. , . . . . ............. : , . ..... . . .
: ' '
6~2 51) r~ 5 /t 2
is indicated by arrows and proceeds as follows - llpOn b~com:ing
heated and vaporizing the cool.ant 28 cnters into the inlet 38
inside the bottorn manifold 31 ancl down -through -che molecular
sieve material 36 into headers 40 ~nd -then into cooli-n~ tubes
32. The vaporized coolan~ 28 readily condenses within the
tubes 32 and returns in liquid for:m by rn~ans of a separate
return pipe 41 into the tank 26. The re-turn pipe 41 ext~nds
above the bottom of manifold .;1 so that a trap 37 i.s provided
~or other liquid contaminants.
In FIGUR~S 4 and 4A the co~ld.erlsate returTI 41 excends
below the level oE the coolant 28 in the kank 26 so that the
vapor phase of coolant 28 must enter ~he inlet pipe 38 (above
the liquid level) through the moleculaT sieve 36. The molecular
sieve 36 can also be effective by proper sizing of the pipes
41 and 38. With ~he return pipe 41 above the liquid level of
coolant 28 the vapor phase of coolant 28 enters bo~h the
return pipe 41~and t~e inlet pipe 38. By proper sizing o:E the
pipes 41 and 38, as is well known in the art of fluid mechanics,
sufficient vapor can be made to flow through inlet.pipe 38 and
sie~e 36 to remove water. Since vapor is continuously formed,
condensed, and regenerated,it is not necessary to provide
100 per cent vapor flow through sieve 36. Also, by proper
. sizing of pipe 41 the upward vapor flow through pipe 41 will
~; :: not interfere with the downward condensate return through
~1.
The novel heat pipe arrangement 42 of FIGURE 5 is
described as follows. The tank 26 containing the transformer
27 and li~uid coolant 28 having a liquid le~el 43 is particN-
larly arranged ]n the following manner. The trans~ormer 27
:contains a plurality of transformer windings 45 with a series
.
~ ~ ~ o~f;passages SUC}l as cooling ducts 44 extending from the trans-
~; ~
,:
' '
: . . -6-
.
~ 5l)55~
.
f~rmer bottom 46 to the t-ransformer -top 47 and a corlcen-trically
located core member 48. The transFormer thermal pumpirl~
assembly 42 is designed to transfer the coolant 28 ~ron the
transformer bot~om sectlon 46 through the ducts 44 by rneans
of the thermal gradien~ existing with:in the plurality of
ducts 44. The cooling ducts 44 provide a conduit for the
cool~nt 28 which becomes heated in transit and cools the
transformer 27 by the change of state from a liquid to a
vapor. The temperature distribllliorl profile within the
cooling ducts 44 for a liquid level ~3 as shown is 5uch that
the temperature of the coolan~ 28 at the transformer bottom
section 46 is lower than the temperature at a point P
corresponding to the liquld level 43 since the heating
mechanism ;s the wattage generated by the transformer 27 and
the point P is subjected to a smaller trans~ormer cooling
surface than for example point P'in the vicinity o~ the trans-
~. .
former bot~om section 46. The temperature at point P is alsohigher than the temperature at point P" at the transormer
top section 47 for the heat pump of this invention to be
operative. Point P" is a lower temperature than polnt P
since point P" is subjected to a greater cooling su.~ace than
; ~ point P,provided the proper liquid level is maintained. When
power is applied to the transformer 27 a plurali~y of bubbles
; 49 begin to move in an upward direction within the cooling
ducts 44 and become heated to a greater degree as they proceed
~: :
further within the trans~ormer 27 since the region at point P
has a higher temperature as described earlier. As the bubbles
49 contlnue to proceed to the vicinity o the transformer top
; ;s~ection 47 they acquiTe enough thermal energy to leave the
; transformer 27 at: the vicinity of point P" as vapor droplets
:
~ 49' in the direcl:îon indicated by the directional arrows.
,
: .
: ::~:
~ ~ -7-
~J)5S~2
Since the droplets 49' foree coolcln~ 28 through the ~ucts ~4
to the transformer top section ~7, the top sectiorl ~7 becomes
cooled by the process of evapora~ion of coolant 28. Further
bubbles 49 enter into the cooling ducts 44 and proceed
through the regions indicaced at points P', and P" ln a
continuous proc~ss during the time the tr~lnsformer is operating.
This process is somewhat similar to the perco~ 'cion effect
wsed for continuously redistributirlg water in a col~Eee
percolator.
The temperature distribution within the cooling ducts
44 for transormer 27 is shown in FIGURB 6. The temperature
in degrees C is plotted as a function of the relative length
of the ducts 44 for the same transformer i~ it were air-cooled
A, oil-cooled Bl and percolation-cooled C. The air-cooled
temperature gradient A shows that the temperature continuously
increases from the bottom section of the transformer through
the center sec~ion -to the top of the transormer since the
transformer itself is its own source o heat and the air-cooling
mechanism of heat transer is insufficient to cool the entire
transformer uniformly. The temperature gradient for an oil-
~ cooled transformer ~ shows tha~ the temperature gradient rom
`- the bottom to the top of the transormer continuously increases at a slower rate than that for the aiT~cooled transformer A.
,
The temp~ra~ures ob~ained with oil or air cooling for ~his
~, ~
2S transformer are greater than permissable with the insulations
;
commonly used. In order to use oil or air cooling, additional
cooling ducts must be provided to lower the temperatures. Thus,
additional conductor material is required for oil or air cooling
as compared with percolation cooling. The temperature gradient
, 30 for the percolation-cooled transformer C is as follo~s.
:',
:' , ': ,
~ 55~2
At the bottom regi.on of the transformer 27 a~ po:int P~
the ends of the windings are exposed to the coolant 28 which
is vaporized due to the heat generaked by the winding
conductors, thus, cooling ~he bo~tom o-f the winding 450 rrhe
coolant 28 enters the ducts 44 and vaporiæes due to contact
with the part of the liinding 45 next to the duct 44. ~ince the
bottom of the winding 45 has a greater surface area exposed
to the coolant it will be at a J.ower temperature t!lan -the
inner parts of the winding 45. Thc inner parts are cnoled by
vaporization of the coolant 28 i.n ducts 44 and by thermal
; conduction to thè cooler ends of the winding 45. Upon
vapori~a~ion the coolant 28 forms bubbles 49 which rise
rapidly upward through the ducts 44 forcing some of the liquid
phase of coolant 28 to the top of the ducts 44 and then onto
the top end of the winding 45. The liquid coolant 28 is
vaporized upon contact ~ith the upper surface o-f ducts 44
~1 and the upper end of the winding 45. It was, therefore,
:: ~ determined that for the mechanism of percolation cooling,
that is, when coolant 28 is provided within cooling ducts 44,
2a and evaporation rapidly takes place at the transformer upper
~ surface 47, the rate of heat transfer away from the transformer
-; 27 is su~ficient to cool the top surface 47, of the trans~ormer
~ 27 at a rate that is equal to the rate at which the transformer
': : : 27 becomes heated during normal operating conditions. The
, ~ . .
2~5 temperature at point P":at the top surface 47 can be as low as
. the temperature~indicated at point P' at the transformer
..:
: bottom 46 depending upon the boiling point of the coolant 28,
: the liquid le~el 43, and the dimensions of the cooling ducts 44.
~ The cooling rate for a plurality of heat pumps 42 having
¦ ~ 3Gl :~differing liquid l~evels 43 for a fixed coolant composition is
, 1
;
~: : .. . . 9
.. ~ ~ . .
,, ~. . . . . . .
3~
r)r)5s~,2
shown in FIGURE 7. Xt was therl determi.rled. tha~ -tile l:iqllid
level ~3 expressed in per cent height re:lat:ive ~o the
transformer top section 47 in FIG[JR~ 5 could be dec7ea:;.rd
without seriously efecting ~he e:ffiçiency of the trarls:former
cooling rate for liquid levels down to less than 75% of the
maximum dimension indicated. The cooling rate profile R
remains relatively steady down -to a 75~ liquid Jev~l as
indicated in therma} units per unit time and begi.ns to decrease
for liquid levels less ~han approximately 60%. P'ol li..rluid
levels between 60 and 50% the cool.ing rate decreased sub
stantially and below 50 per cent the cooling rate was not
~dequate. This phenomenon is not as yet well understood, but
is believed ~o depend upon the heat trans~er characteris~ics
for the coolant 28~ as well as the geometry and numbèr o~
transformer cooling ducts, and the power rating of the trans-.
former. FIGURE 6 indicates that the temperature at any distance
~ within the percolation-cooled device C is lower than that
; within the oil-cooled transformer B and the air-cooled trans-
former A.
: FIGURE 8 is a further embodiment of the transformer
.
~.. .. heat pump 42 o~ FIGURE 5. In the embodiment of FIGURE 8 the
, , ,
` transformer 27 containing the plurality of cooling ducts 44
and vapor bubbles 49 is modified by duct extensicns 50
coincident with th ends of the cooling.ducts 44 at the ~ 25 trans~ormer top sur~ace 47. The extensions 50 carry the
vapor bubbles 49 to an extended height h abvve the top
.
: surface 47 and deposits the liquid 28 within a specially
d~esigned distribution tray 51 having a plurality of spaced
perforations 52. The embodiment of FIGURE 8 combines the
percolation cooling me-hod of this invention with the liquid
' , ~ ,
'
.~ . ,
1 0 -
.
~ - " ' " . . .
~ 32 SD5S42
sur:face film e-vaporat:ion o:~ the ~r:ior art to furth~r increase
the coollng efficiency. The distribu-tor tray 51. cul1ects the
coolant 28 in liquid form ~nd rerl:is~ributes the ooolant 28
in the form of drople~s 49' which subsequently drop throu~h
the perforations 52 onto the ~rans:former top surface 47. To
keep a continuous ~low of coolant 28 OTltO the top surface of
the transformer coils 45 an upwardly extending dam member 53
is provided at the top surface 47 of the transformer 27 as
indicated in FIGURE 8.
FIGURE 9 shows a further use ~or the extension 50 o~
the cooling duct 44, In this embodiment the extension S0
. is brought up into close contact with a busbar 54 so that
the liquid droplets 49' can contact and cool busbar 54.
The transformer 27 for providing the heat pump 42 of
this invention is shown in enlarged top perspective view in
FIGURE lO. The ducts 44 between ~he transformer coils 45 at
the transforme~r top section 47 have a wldth t~WI~ and a length
. "L" as indicated. The:required number.of ducts 44 are used
. in order to maintain the operating temperature below the
maximum permissable value. The width "W" is approximately
3/16 of an inch, and the length "L" is approximately l-l/2
inches. The core 48 is centrally located as is common with
; transformers of the medium and power type. The dimensions~
: number, ~nd location of the tr~nsformer cooling ducts 44 are
.~ 25 determinative o:E the quantity of coolant used to provide the
heat pump of this invention and it is anticipated tha~ a
substantial savings of transformer coolant fluid can be
realized by a p-roper heat pump design. For oil-cooled trans-
formers having the tempera~ure gradient indicated at B in
FIGURE 6 approximately 155 gallons of coolant oil is generally
required whereas for the same size transformer 50 gallons of
: ~ :
, ~:
.,
I
' ' ' ' - 1 1 , ,
' ~ .. : . .
~ 5~)5S~2
coo.Lant are recluired to pro~ide the terrlperature gradient
indicated .for the percolation-cooled device at C. Ihe hea~
pump device of this in~ention, there:fore~, pro-vides bf.~tter
cooling e-ffi.ciency than standard oil-cooled total immersion
prior art systems at d subs~antial savings in materials costs.
Further embodiments o~ the percolat.ion cooled trans-
formers of this invention are S}IOW]l in FIGIIR~S 11 AND 12. As
described earli~r a noncondensable gas such as n:atrogen, C2~6,
C2ClF5. SF6 is requently employed as a dielectr.ic for providing
insulation between the transformel windings when the -transformer
liquid coolant is at a low temperature. The noncondensable ga~
pressure also determines the boiling temperature o:~ the con-
densable coolant and is adjusted so that the condensable coolant
boils within the operating range o~ the transformer temperature.
For the trichlorotrifluoroethane coolantg Freon 113 such as the
type manufactured by DuPont for example~ can be used within
the heat pump 42 of FIGURE 5. A medium transformer rated at
18000 watts three phase operated continuously at a boiling
temperature of 67~C when the nitrogen fill pressure was ad-
justed to g.ive a condensable vapor pressure o 12 P.S.I.G.
: It is anticipated that the operating temperature character~
~ . istics can be accuratel~ controlled by the duct dimens:lolls
; ànd the coolant boiling temperature~ as deter~ined by the
- nitrogen ill pressure, provided a quantity of coolant
:: ~25 always remains in the li~ui~ ~ha~e. If the entire quantity of
coolant vaporized the pressure within the system would behave
as an ideal gas and increase in proportion to temperature.
:~ The use:of the noncondensable gas generally requires an upper
: ~ man:ifold 30 and an expansion tank 33 as shown in FIGURES 3 and
,~ : 30 11 for the following reasons. The expansion tank 33 provides
. -12- .
~ , , ' .
~ 5D55~2
a receptacle .Eor -~he noncondensable gas a-~ter ~he cl~ndellsable
gas has become sufficiently vaporized to d:isplace the non-
cond~nsable gtas and ~o expel it ~rom the vi.cinity o t~le
transformer windings. The upper manifold 30 is generally
requlred when the liquid co.olant 28 is heat treated to outgas
any residual noncondensable gases such as air absorbed by the
coolant during transportation and storage. In order to outgas
the coolant 28 the trans:~ormer is short circuited to cause
the trans~ormer to become heated and to separate the absorbed
air from coolant 28. The heated air separates from the coolant
28 in the outgassing -process and ente-rs the expansion tank ~3
after passing through condenser tubes 32. The air then
readi.ly transmits into upper mani~old 30 which is connected
to the expansion tank 33 by means of connecting pipe 55 and
connecting valve 56. Once the transformer coolant has been
completely outgassed from residual air and the outgassed air
is contained wlthin expansion tank 33 the upper manifold 30
is isolated from the expansion tank 33 by closing valve S6.
' The air is then removed from the expansion tank 33 by evacu-
ation with a vacuum pump. A known quantity of the desired
:~; noncondensable gas nitrogen, SF6, C2F6, or C2ClFs is then
:~ added to the expansion tank 33 through a ~illing valve S~.
; ~ The valves 56 and 58 are then opened and the noncondensable
~ gas is allowed to flow throughout the system. During operation
,, . ~ .
25 the pipe 57 serves to return any condensate of coolant 28
~; from the expansion tank to the main liquid supply. Condensate
of coolant 28 may form in expansion tank 33 due to 1uctuations
n ambi:ent temperature. Pipe S7 is connected to the lowest
, .. .
point of expansion tank 33 to mlnimize condensate hold up in
~he expansion tank.
i
13-
., :
~ .
: .
5~55~
The embodiment oE E'IGURE 12 is similar -to -the er~odiment
of FIGURE 11 except that the upper manifold 30, -the connec-ting
pipe 55, the connecting valve 56 and drain valve 58 can ba
dispensed with. It has been discovered that the transformer
coolant can be pre-evacuated by heating and outgassiny the
coolant prior to filling within the transformer such that the
steps indicated earlier for FIGURE 11 are no longer required.
Once the transformsr coolant is completely outgassed particular
care is taken to insure that the outgassed coolant does not
reabsorb air in the final transformer filling stage.
Althou~h the percolation cooling system employing the
heat pump of this invention is directed to applications
involving medium and power transformers, this is by way of
example only. The method and apparatus employed within the
examples shown can be applied to cool other type of electrical
apparatus providing the heat pump trans~er design of this
invention can be incorporated within the electrical apparatus
involved.
It should also be readily apparent that ~or certain
ranges of am~ient temperatures and condensable coolants the
noncondensable gas may be omitted wi-thout effecting -the
operation of thé apparatus as described.
,
',~
:
- 14 -
,
