Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1()6~
. :.
: ATMOSPHERIC-FLUID INDIRECT HEAT EXCHANGER
Technical Field
This invention relates to an atmospheric-fluid
heat exchanger of the type in which a fluid, which has-a
freezing temperature which may be higher than ambient tem-
peratures, flows in one path of a fluid-atmospheric air
;; heat exchanger, is cooled in the exchanger by indirect con-
tact with the air of the atmosphere.
,~:
The freezable fluid, considered here, is a fluid
0 which will reeze when the temperature of the air of the
; atmosphere is colder than the freezing temperature of the
fluid. The fluid may be water, for example, from cooling
condensers of electric power stations; steam, for example,
exhausting from the turbines of electric po~Jer stations
(the atmospheric coolers are in this case known as"aero-
condensers"), or any other liquid or gaseous fluid satis-
fylng the aforementioned criterium of solidification.
The heat exchanger comprises either tubes, which
may or may not be provided with surface extensions, such
20 ~as fins, or flat hollow panels, or any other elements which
~` ~ prevent the direct transfer of substance between the air of
~ the atmosphere and the fluid to be cooled, while ensuring
r`~ the transfer of heat therebetween. These tubes, panels and
material elements may be formed, for example, of metal or
plastic, or both. Such heat exchangers are usually termed
"dry exchangers".
' The air is caused to move through the cooler
~, ....
` ~ ~ either byfans which dra~ in or blow air-through the exchanger,
:~.-, . .
: ' ' `
:. ~ ~L
.: ' 7~
-. . -
614
-2-
. .
or by the draught of a chimney which may or may not be
assisted by fans.
When such a cooler is inoperative in freezing
weather, when the ambient temperature is lower than the
freezing point of the freezable fluid to be cooled, the
exchangers and the piping provided for the latter are
drained of their fluid. When it is desired to put the cooler
back into service and the ambient temperature is materially
- lower than the freezing temperature of the fluid, there is a
serious freezing problem.
The freezable fluid sent into the exchanger often
solidifies when it comes in contact with the cold walls of
the exchanger before the liquid to be cooled has time to
sufficiently heat the exchanger walls.
. .
The risk of freezing the exchanger is more critical
as the length of the cooling tubes increases. For example,
; water at 65C sent into the finned tubes whose temperature is
-20C can freeze at the end of a few metres of travel in the
` tubes. In respect of high-power coolers, for example, those
; 20 employed for an electric power station of the order of ~000
MW, tubes of 10 to 20 metres length are currently employed.
i~ The tubes may be disposed in series and result in tube
lengths of several tens of metres. Further, when in freez-
ing weather the thermal head of the cooler is low, it is often
necessary to partly drain the installation in order to avoid
freezing of the fluid in the exchangers. The temperature of
the fluid in the heat exchange tubes remaining in service
will raise to a value at which there is no longer danger of
freezing. When it is necessary to increase the thermal head
of the cooler, the same problem arises of sending the fluid
to be cooled into elements which may be at a very low tem-
perature, thereby freezing the fluid.
.
~: Background of the Prior Art
- A number of patents and published patent appli-
-~ cations exist which are concerned with the problem of freez-
''
, . .
.:
~.,,
,'.: , - . ,
, ": ~ ,, .
.. . .
.
: ~ '
6~4
.
-3-
ing of the fluids to be cooled in atmospheric type heat
exchangers.
French Patent 1 506 865; French Certificate of
Utility N 2 200 491; German Patent Application 1 551 402;
French Patent 1 458 535, British Patent 1 484 178 and
German Patent Applications 2 539 759; 1 962 061; 2 153 967;
and 1 501 346 are each concerned with the operation of
such coolers when the ambient temperature is lower than
the freezing point of thefluid to be cooled and thereby
; 10 liable to freeze in the exchangers.
More particularly, French Patent N 1 506 865,
the French Certificate of Utility N 2 200 491 and the
; published German Patent Application 1 551 402 concern
permanent static arrangements of aerocondenser/exchanger
elements which reduce the dangers of freezing in cold
weather, thereby permitting operating of the cooler at
ambient temperatures, under partial thermal head. The
;; publications contribute nothing to the solution of the
problem of starting up a heat-exchanger with empty ex-
changer elements whose temperature is distinctly lower
than the freezing point of the fluid.
French Patent 1 458 535 relates to a system
for protecting coolers against freezing by recycling of the
air from the bottom of the peripheral tubes of an aero-
condensation installation. Whereas, such protective means
'~ may be valid during operation, it does not exist during
start-up operations.
British Patent 1 484 178 relates to a system
for the inhibition of the formation of ice on the inner
walls of the exchangers of an atmospheric cooling tower
' by vibrating the tubular elements of the exchangers. Such
a method at first sight appears applicable to starting up
;~ in freezin~ weather although no reference is made thereto
, in the patent. However, the method af the patent has a
; 35 number of drawbacks, such as requiring complex equipment
such as electromagnetic vibrators operating at high valtage.
.. ' - .
. . .
,
~ ' ' ' :'. ' . ,' . '. '.'' :: ' -': ' '' '
... . . . - - .
,- , ~ ,.
.. . : . ~ , ' , . , :
,, ". . , : '' ~ .' '.. .
,: ,
614
--4--
Further subjecting the cooling installation to vibrations
result in material fatigue and then failure. Further, the
problem of starting up in the cold state is more difficult
than that of the prevention of freezing during operation
since it is necessary to overcome the freezing mass of the
structural equipment before meeting with a situation com-
parable to that of operation under low temperature condi-
tions. ~ ~3~ ~9
~ The published German Patent Application ~
;~ ~ relates to protection against freezing in aerocondensers by
reduction of the heat exchange capacity by introduction into
: the aerocondensers of non-condensable gases. Such a method
at first sight would seem to be useful as an aid in starting
up of aerocondensers in cold weather; however, the patent is
silent on this point and the extent of its effectiveness is
,; questionable. Moreover, the method of this German Appli-
, cation has the drawback of modifying the nature of the fluid
in circulation and of requiring an installation for intro-
ducing, discharging, controlling and regulating the non-
condensable gas.
; The German Patent Application 1 962 061 relates
to a device for reducing the heat exchange area of aeor-
condensers by means of flaps capable of reducing, or even
preventing the passage of air through a large part of the
~- 25 heat exchange area. Such a device may contribute to the
; starting up o cold exchangers, however, this problem is
- not considered in the publication.
~; The German Patent Application 2 153 967 relates
to control of the air flow of natural draught cooling towers
by means of movable panels which surround the air inlet
- opening and are intended to ensure, among other functions,
protection against freezing, thereby permitting operation
at lower ambient temperatures than without such devices.
However, the disclosed arrangement does not alone solve the
. .
problem of the starting up of coolers with empty and cold
heat exchange batteries.
German published Patent Application 1 501 346
~~ concerns aerocondensers in which there is obtained a vari-
'`:
:' ' '' '
'
.
' , . ' ~ '~ ,
',' ' : '
614
-5-
''
ation in the effective exchange area by division of the
inner space of the elements of the aerocondensers into three
compartments and by the selective admission of steam in one,
in two, or in the three compartments, which permits adapt-
ing the operation of the aerocondensers to the variationsin the thermal head and to variations in ambient tempera-
ture. Although such methods permit operating the aero-
condensers at low temperature, they do not solve the problem
of starting up in cold weather.
German Patent N 1 241 852; French Patent N
1 552 731 and German published Patent Application 2 250 058
are concerned with the draining of coolers having dry
exchangers to prevent freezing. The patents fail to dis-
close how the installations described could be put back
: 15 into operation, that is, filled and started up again under
- severe freezing conditions. German Patent N~ 1 241 852
merely teaches, for this purpose, heating of the water in
, the water tanks, which is insufficient in the case of
severe freezing conditions.
Lastly, French Patent N 1 386 231 relates to
the partial draining of the exchangers of a cooler in order
` to reduce the exchange area to avoid freezing. Such a
method permits maintaining the cooler in service when the
ambient temperature drops particularly low; however, the
patent does not disclose a start-up procedure for use at
very low temperatures.
Low ambient temperatures therefore create, for
atmospheric coolers having dry exchangers, problems of
operation in respect of which many solutions have been pro-
: 30 posed without however, providing the solution to the pro-
~ blem of starting up in freezing weather.
,.' ' ' .
Brief Summary of the Invention
An object of the present invention is to over-
come the beforementioned difficulties,and the invention is
characterized by means of an auxiliary circuit containing
:,
. .
.. . .
'., ' ' " '. '' :', ' ' - ;
'~,'.'' . ' ' '.. ,. .'. '... ' .. ' . : . '
.~' - ,. - .
.
614
--~ a fluid which will not freeze at low ambient temperatures and
a source of heat for heating the fluid and the heat exchanger.
This auxiliary circuit normally permanently contains its low
freezing temperature fluid, irrespective of the ambient tempera-
ture. When, in freezing weather, fluid must be supplied to the
main circuit, the auxiliary circuit is previously brought into
operation at its full thermal head to heat the main exchanger
through which the freezable fluid flows. The heat exchange
capacity of the exchanger, the input temperature of the unfreez-
able fluid and the rate of air flow are so chosen that the air
;~ issuing from the unfreezable fluid-air exchanger is at such atemperature that it heats the freezable fluid-air exchanger,
located downstream thereof, to a temperature at which the
freezable fluid would not freeze when injected into the fluid-air
~ heat exchanger.
',~ The cooler according to the invention is available on
demand and can be immediately put into operation without a pre-
heating delay in freezing weather.
In summary, therefore, the present invention may be
broadly seen as providing atmospheric cooler, of the type in which
:: `
~ a fluid which is freezable at low ambient temperatures flows in a
S~ freezable fluid-atmospheric air heat exchanger and is cooled in
;-~ indirect contact with a stream of air of the atmosphere,
characterized by auxiliary circuit of a fluid which is unfreezable
at the low ambient temperatures, the auxiliary circuit comprising
at least one source of heat for heating the unfreezable fluid and
an unfreezable fluid-atmospheric air heat exchanger disposed in
the air stream upstream of the freezable fluid-atmospheric air
exchanger for heating the air of the atmosphere before its passage
B
--6--
.
. .
~:" `':
.
` ~ :
61~
--~ in the freezable fluid-atmospheric air exchanger.
Brief Description of the Drawing
The invention will be described hereinafter in more
detail with reference to the accompanying drawing which illustrates
four embodiments, wherein:
Figure 1 is a diagrammatic elevational view of an
atmospheric cooler according to the invention, in which the
freezable fluid and the unfreezable fluid are liquids;
Figure 2 is a diagrammatic elevational view of a
second embodiment of the invention, in which the freezable
fluid to be cooled is steam and the unfreezable fluid is a
liquid;
.~ Figure 3 is a diagrammatic elevational view of a
.: third embodiment of the invention in which the freezable
and unfreezable fluids are liquids which are heated by an
outside source of heat; and
Figure 4 is a diagrammatic elevational view of a
, fourth embodiment of the invention in which the freezable
; fluid is a ~iquid and the unfreezable fluid is a condensable
~ 20 gas.
."' '~
:; . .
~;
,.:
... .
:
.~ ''~
.....
``: B ~ -6~-
.
,;: , . .. . . .
. . . ~ . .
: . . . ' , : ~
: - . . .
~;:, , . , . : : , .
.. .. . . . .
i14
"
--7--
Detailed Description of The Invention
With reference to Figure 1, a cooler or heat
exchange unit 1 comprises in succession, in the downstream
direction with reference to the direction of flow of the air
of the atmosphere indicated by the arrows A, an unfreezable,
at normally low ambient temperatures, liquid-atmospheric air
dry exchanger 2, a freezable liquid-atmospheric air dry ex-; .
changer 3, and a fan 4. The cooler 1 may be constituted either
by a single heat exchange unit or an assembly of similar units
disposed for example in line or on a circle inside a cooling
tower.
The main freezable liquid-atmospheric air ex-
changer 3 is disposed in a main liquid circuit 5 comprising,
on the upstream side, a pump 6 and a first cut-off valve 7
lS for the exchanger 3 and, on the downstream side, a second cut-
off valve 8 for the exchanger 3. A heat exchanger 9 i5 con-
; nected between the freezable liquid exchanger 3, and the un-
freezable liquid exchanger 2. The main circuit 5 is completed
by a gas blow-off valve 10 branch-connected between the valve
7 and the exchanger 3, a draining valve 11 branch-connected
between the exchanger 3 and the valve 8, and a by-pass pipe
; 12 having a valve 13 thereacross and extending between the
; inlet of the valve 7 and the outlet of the valve 8. The
liquid which is circulated in the main circuit 5 may be water,
for example that cooling an electric power station condenser,
"
; or any other liquid suitable for cooling and freezable at low
ambient temperatures.
The auxiliary circuit 14 in which the exchanger
2 is connected comprises, downstream of the latter, the
freezable liquid-unfreezable liquid heat exchanger 9 and a
circulating pump 15. The unfreezable liquid may be, for exam-
ple, a mixture of ethylene glycol and water or any other suit-
able liquid unfreezable at low ambient temperatures.
In the embodiment just described, only the cooler
~- 35 unit ~ and possibly the valves 10 and 11 are exposed to the
ambient air, the other elements of the main circuit S and
''.,'~ .
i -
.,
....
., ~ , .......
.
.,
.:' .
''"
:., ~ . :
" . . ~:
;' ' ' .
614
, .
~ -8-
: .
the auxiliary circuit 14 being assumed to be always pro-
tected from freezing temperatures, for example, in the
building of an electric power generation station.
When the cooler unit 1 is inoperative during
the cold season, either because the whol~ of the cooler has
been stopped, or because it is operating at a reduced ther-
mal head and there is a danger of freezing, the part of the
main circuit 5 exposed to the ambient air between the valves
7 and 8 is drained via valve 11. Let it now be assumed tha$
this cooler unit must be put back into service. This opera-
- tion will be carried out in accordance with the following
sequence of operations:
With valves 7 and 8 closed and the valve 13 open,
the pumps 6 and 15 are started up. The temperature of the
freezable main liquid must be relatively high, but the thermal
head of the circuit 5 ~heat extracted) is at this time low.
The freezable liquid passes round the main exchanger 3 through
the by-pass 12 and transmits its heat to the unfreezable li-
quid in the exchanger 9. The unfreezable liquid progressively
heats the unfreezable liquid-air exchanger 2. When the latter
is sufficiently hot, the fan 4 is started up at its low speed,
if there is more than one speed. This fan draws in the am-
bient air which passes through first exchanger 2 where the air
is heated,which then heats the main exchanger 3. The thermal
head of the main circuit 5 is then sufficiently high to enable
the exchanger 2 to operate at high temperature although very
cold air passes therethrough.
As soon as the main exchanger 3, which is still
empty, has reached a sufficient temperature relative to the
freezing point of the main liquid, the valves 7 and 8 are
opened, and the valve 13 is then closed. The non-condensable
air or gas (nitrogen, for example, employed to avoid corrosion)
contained in the exchanger 3 is then blown off by opening the
valve 10 until the main liquid starts to arrive. Immediately
before proceeding to fill the main exchanger 3, the following
combinations of temperatures may be, for example, observed:
::
~ . .
~; .
:,.
.: . . .. - .:
: . . - . :
. .
.'' ' , -, . ' ' . . . ' :
: `, -, ~ '
, . . . .
6~4
~ EXAMPL~ 1
':, ' .
Ambient air temperature:-15C. The fan rotates at its rated
speed
freezing temperature of the freezable fluid: O~C
freezable liquid input temperature in the exchanger 9: 65C
freezable liquid output temperature of the exchanger 9: 52~C
unfreezable liquid input temperature in the exchanger 2: 60C
unfreezable liquid output temperature in the exchanger 2: 41~C
; temperature of the air leaving the exchanger 2: + 1C
10 termperature of the air leaving the exchanger 2: - 1C
temperature of the body of the exchanger 3: - 4~C
"'
EXAMPLE 2
.,
Ambient air temperature:-25C. The fan rotates at half-speed
freezing temperature of the freezable fluid: OC
freezable liquid input temperature of the exchanger 9: 65C
freezable liquid output temperature of the exchanger 9: 55C
unfreezable liquid input temperature of the exchanger 2: 60C
unfreezable liquid output temperature of the exchanger 2: 45C
temperature of the air leaving the exchanger 2: +3.5C
temperature of the air leaving the exchanger 3: + 1C
temperature of the body of the exchanger 3: - 4C
It will be observed that, in both cases, the tempera-
ture of the empty exchanger was brought to -4C which, bearing
in mind the input temperature of the freezable liquid which is
65C, and the length of the exchanger of the examples, al-
-though below the freezable temperature of the freezable fluid
is not so low that freezing will occur in the exchanger during
filling. Of course, the temperature to which the exchanger
must be heated in order to avoid freezing of the freezable
:,..;:
- 30 liquid depends on the input temperature of this liquid and on
the length of the elements, tubes or the like of the exchanger.
As soon as the gas contained in the exchanger 3 has
been blown off, the valve 10 is closed and the cooler unit 1
operates in a conventional manner. The auxiliary circuit 14
can be maintained in operation, in which case the exchanger 2
indirectly extracts a part of the heat of the freezable fluid
. .
.'~ ' .
.
, . .
614
--10--
and increases the thermal head of the unit, or stopped by
stopping the pump 15 which reduces consumption of power.
Reference will now be made to Figure 2 which
shows a first modification in which the same reference numer-
als as those employed in Figure 1 are used but with the addi-
. tion of an inde~ _ for designating identical or similar parts.
In this cooler, the main freezable fluid is steamwhich is condensed in the exchanger 3a and is therefore an
aerocondenser. In the drawing, the part of the main circuit
5a in which the steam circulates has been represented in the
form of a large pipe and the part of this circuit in which the
condensed liquid circulates,in the form of a small pipe. The
exchanger 9_ in this form of the invention is a small condenser
cooled by the unfreezable liquid of the auxiliary circuit 14_
which may be, as in the preceding example, a mixture of ethy-
lene glycol and water. The exchanger 9a is placed in parallel
with the exchanger 3_ in the by-pass pipe 12a of the main cir-
cuit 5_, in contrast to the configuration represented in
Figure 1 in which this exchanger is in series, downstream of
the exchanger 3.
In the embodiment shown in Figure 2, the exchanger
: 3a and 9_ are condensers and the parallel arrangement is ne-
i cessary to ensure that the two condensers receive steam at
:. their inlets, whereas when the freezable fluid-unfreezable
. 25 fluid exchanger is a liquid-liquid exchanger, it may be con-
nec.ted either in parallel or in series and, in the latter
.~ case, either upstream or downstream of the main exchanger.
In the latter case, the main exchanger receives the freez-
'. able fluid at its maximum temperature whic.h is preferable
:: 30 freez.ing weather ~less risk of freezing) and in warm weather
(improved cooling because the relative flow of the air and
~ freezable fluid is more or less in counter-current form,
.: whereas it is more or less in the parallel current form when
`. positioned upstream of the exchanger 9 of the auxiliary cir- '
~.......... 35 cuit).
The Figure 2 form of the invention operates in
the same manner as the cooler of Figure 1 except that the
,
:
., '
.
~i . , . , ~ , ..... . .
' .: . ,: ., ,, '-:
" . . . . . . .
'.:' ' '' ' . , '. ~ : ,
':,, ' . ~ : '
~ 1 .
, :
` 111(~614
. ,~,
~11 -
. .
.
steam condenses in the exchangers 3_ and 9_ and the condensate,
in the aerocondenser 3_, as in the auxiliary condenser 9_,
flows first under the effect of gravi~y, into one or more con-
densate tanks (not shown), before being taken up by the pump
6a, which is placed at the outlet instead of the inlet of the
main circuit as shown in Figure 1. The use of the valve 13a,
in the by-pass pipe 12a, to which is also connected the ex-
changer 9a, is optional, but it may be preferable to provide
- such valve to assist in maintenance or to carry out repairs
; 10 on the exchanger 9_. This, moreover, constitutes an advan-
tage of the parallel arrangement over the series arrangement,
which does not permit putting the auxiliary exchanger out of
service unless there is provided a by-pass pipe with a valve
(not shown) in parallel with the freezable fluid-unfreezable
; 15 fluid exchanger.
In Figure 3, in which the reference numerals em-
ployed in Figure 1 carry the letter b to designate correspon-
ding elements, there is represented an installation similar
to that of Figure 1, but in which there is no freezable fluid-
unfreezable fluid exchanger. The source of heat 16 for theunfreezable fluid is of any type, but advantageously it is
; the same source as that of the main freezable fluid, for ex-
ample, in an electric power station, the source would be the
condenser for the steam exhausting from the turbine (not
shown) In this case, the turbine condenser would have two
cooling circuits, namely a main circuit and an auxiliary
; circuit, the latter probably operating as a surface condenser.
The operation of this form of the invention is moreover
`i basically identical to that of the cooler shown in Figure 1.
In the modification shown in Figure 3, taking
the starting up heat directly from an outside source of heat
~ and not from the freezable fluid-unfreezable fluid exchanger
"- avoids pressure drops in the freezable fluid to which the
fluid is subjected to in such an exchanger.
Figure 4, (in which the same reference numerals
as those of Figure 1 to which the letter _ has been added are
~` employed to designate corresponding elements), represents an
.~`'~ ,
:
.:
~:;
, .,
, .
111~614
-12-
installation similar to that shown in Figure 1, but in which
the unfreezable liquid is a gas which is condensable and un-
freezable at low ambient temperatures such as, for example
ammonia or certain chlorofluorinated derivatives of methane
or ethane. The exchanger 9_ is thus in the nature of a dis-
tiller or an evaporator, and the exchanger 2c is a condenser.
The part of the auxiliary circuit 14c conducting the steam
from the evaporator 9_ to the condenser 2_ is represented in
the form of a large pipe whereas the part of this circuit
returning the condensate from the condenser 2O to the evapor-
ator 9_ is represented in the form of a small pipe. The cir-
culation of the unfreezable fluid here occurs by the thermo-
siphon effect with no mechanical assistance, such as the pump
15, (Figure 1): the condensate, in flowing under the effect
of gravity in the aerocondenser 2c, creates a suction which
draws the steam through the upstream pipe of the circuit 14_.
:, . .
Statement of Industrial Application
The cooler according to the invention has the fol-
lowing advantages among others:
The system permits starting up of industrial heat
exchangers of the atmospheric cooler type having a dry ex-
~ changer in freezing weather.
j The use of an auxiliary heat exchange circuit as-
sists in the discharge of all of the heat to be discharged
without overdimensioning the overall installation.
When an installation is brought into service in
the winter time, the unfreezable fluid is immediately avail-
able for discharging the heat head before the filling of the
- main heat exchange circuit. The cooler is consequently ready
to operate at any moment with no pre-heating delay.
t~ With very low outside temperatures and when the
heat head is low, the auxiliary circuit will function to dis-
;~ charge the heat without filling the main circuit.
-; The existence of two separate circuits imparts an
additional safety in operation of the cooler.
In normal operation in the winter time, the air
,. :
~/ ~
. .~
:,:
, . . .
- . -
': ' '', . , ."' .
',: ' . :' . -' " . : . - -
1~ 6~4
13-
encountering the first sheet of tubes of the exchanger of
- the main circuit preheated by the exchanger of the auxiliary
circuit, reduces the danger of freezing in the tubes of the ex-
, changer of the main circuit.
The energy for preheating the elements may be
supplied by the energy to be dissipated, thus eliminating an
outside source of heat.
; The invention eliminates complex systems such as
movable slatted shutters and their driving and regulating
means, greatly facilitating access to the elements of the
cooler for periodic maintenance and inspection.
The pressure drop created by the auxiliary ex-
changer is not an additional pressure drop since this ex-
, .
- changer contributes to the discharge of heat and consequently
permits reducing the exchange area of the primary exchanger.
In natural draft cooling towers, the heating of
the auxiliary exchangers heat the chimney at its base through-
out its periphery and thus initiates the natural draught.
, Many modifications may be made in the embodiments
described without departing from the scope of the invention.
For example, the examples of the main and auxiliary circuits
described may be combined in different manners and there may
be employed, as the unfreezable fluid, an inert gas which is
non-condensable at the ambient temperatures at the pressures
of operation.
,~ ' .
,.~ ., ~
,.. ~ , .
... .
,: '
... j
~'; .
. ~
i ~
, ".-
-,
~,~; . .
!
