Note: Descriptions are shown in the official language in which they were submitted.
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Cooling System
This invention relates to cooling systems, more
particularly to those including a closed circuit
evaporative heat exchanger of the forced draught
configuration.
Closed circuit evaporative heat exchangers are used
in a variety of industrial settings to provide cooling
or condensing of refrigerants. Very broadly, cooling is
provided by means of a cooling fluid which draws heat
from the area to be cooled and transports it to a heat
exchanger where the fluid is cooled again. In the case
25 of a refrigerant condensing system, as part of a
refrigeration process, refrigerant vapour enters the
heat exchanger where it is condensed and leaves the heat
exchanger as liquid. In both cases air is blown over
the heat exchanger coils to remove heat from the liquid
or vapour. The cooling process is enhanced by spraying
water onto the coils so that a proportion of the water
is evaporated by the air flow.
In such systems the majority of the water sprayed
onto the heat exchanger coils in the air distribution
plenum does not evaporate but drains off into a sump at
the bottom of the air distribution plenum. From there
it is pumped through a strainer back to the spray
nozzles to be recycled. Typically evaporative heat
exchanger products are designed to use parts which are
common to both closed circuit and open cooling towers.
The sump in conventional closed circuit towers is
therefore of a sufficiently large capacity to be able to
be used in an open tower as well as a closed circuit
configuration.
As mentioned above, the cooling provided by the
heat exchanger is enhanced by spraying water onto the
coils of the heat exchanger. However, such enhanced
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cooling is not always necessary. For example, during
the winter months sufficient cooling may be achieved
without the evaporative effect of the water, i.e. so
called "dry operation" is possible.
However, dry operation requires the sump to be
drained as the water therein would otherwise freeze and
cause damage to the system as a result of the forced
flow of cold air over it. This is problematic since the
processes of draining and replenishing the sump are time
consuming - typically taking several hours. Furthermore
it is usually necessary to shut off the cooling system
during at least some of the draining or replenishment
period, in order to prepare the sump for dry or wet
operation respectively by securing make-up valve floats,
level controls etc. Tt is not therefore considered
practically or economically feasible to drain and
replenish the sump every day. This means that dry
operation can only be carried out for a short proportion
of each year where even during the daytime, temperatures
are predictably low enough that "wet" operation will not
be required. It will be appreciated that the potential
savings of water and energy required to operate the
water pump and any sump heaters are seriously curtailed
as a result.
Zt is also known in some closed circuit evaporative
heat exchanger systems to provide a sump located
remotely from the air distribution plenum. Most of the
unevaporated water is either drained or.pumped
continuously to the remote sump during wet operation,
and is then pumped from the remote sump back to be
sprayed onto the heat exchanger Coils again. The
advantage of such a remote sump is that it is not
necessary to drain the whole body of water from a sump
in the air distribution plenum in order to achieve dry
operation since only a small amount of water remains in
the air distribution plenum arid this can be drained
relatively quickly. Water in the remote sump is riot
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subjected to the cold airflow in the air distribution
plenum and so may be prevented from freezing by suitable
heaters.
There are several disadvantages to a remote sump
however. Firstly, additional space is required to
accommodate it, which is generally expensive. Secondly,
more powerful pumps are required to account for the
additional static height through which the water must be
pumped. Thirdly, the overall number of components
required and the cost of installation is also increased.
Together these factors can more than outweigh any cost
saving in being able to operate the system more
efficiently with respect to water consumption and spray
pump energy. It may be however that a remote sump is
necessary to allow dry operation and prevent over-
cooling in some circumstances.
Another problem with conventional closed circuit
evaporative heat exchanger arrangements is that it is
necessary to halt operation of the system in order to
carry out routine maintenance such as inspection,
functional testing, cleaning etc. of the parts inside
the air distribution plenum. This is a particular
problem for conventional systems without a remote sump
since equipment in the sump and water make-up system
will also be affected. Such regular interruption of the
running of the system is obviously disruptive and
expensive.
It is an object of the present invention to provide
an evaporative heat exchanger in which the problems set
out above are at least partially alleviated.
When viewed from a first aspect the present
invention provides a closed circuit evaporative heat
exchanger comprising an air distribution plenum, means
for spraying water into said plenum and a collection
surface for collecting unevaporated water sprayed into
said plenum, such that said water is arranged to drain
into a sump within said plenum, substantially without
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remaining on the collection surface.
Thus it will be seen by those skilled in the art a
sump is still provided within the main air distribution
plenum, but the sump is not used to collect the
.5 unevaporated water but instead a collection surface
drains into the sump. This means that the sump may be
at least partially thermally isolated from the main part
of the plenum. This makes it possible to prevent the
water therein from freezing when ambient air
temperatures are below freezing point, whereas this is
not viable with the conventional sump arrangement which
is exposed to the airflow in the plenum. Such
arrangements have the advantage of substantial
flexibility in that they are able to be swapped between
wet and dry operation rapidly and as often as required,
but without the disadvantages of providing a remotely
located sump.
The sump is arranged such that water therein is
prevented from freezing during cold weather operation.
This can be achieved by ensuring a sufficient degree of
thermal isolation and providing heating means,
preferably thermostatically controlled. This allows
varying environmental temperatures to be accounted for
as well.
The collection surface could simply be arranged to
drain into the sump below it - i.e. the arrangement
could effectively be similar to a Conventional one but
with a lid or the like covering the sump and including a
drain hole or holes. In this case the upper surface of
the lid would form the collection surface.
It is preferred however that the drain interface
between the collection surface and the sump is arranged
to form a liquid lock between the two so that an uneven
air pressure may be maintained between them. The
benefit of this feature is that the sump may then be
maintained at substantially atmospheric pressure, whilst
the main~part of the plenum is at an elevated pressure
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resulting from the forced airflow. The physical
isolation of the interior of the sump from the interior
of the main air distribution plenum also avoids contact
with the water sprays. These two factors allow at least
the sump to be accessed for maintenance even when the
system is in operation with the associated fans running.
It will be appreciated that this capability gives a
significant advantage over prior art systems which have
to be taken out of operation for even routine
. 10 maintenance.
The amount of water used in the spray Cycling
system via the sump may, as in the prior art, be of a
volume similar to that associated with a sump used in an
open tower cooling system. However, the Applicants have
appreciated that since, in accordance with invention, a
new form of sump is contemplated, the minor benefit of
commonality between sump modules is forfeited, but that
this means that volume restriction imposed by using a
common sump need no longer apply and that in fact
additional benefit may be achieved by using less water.
Thus, in preferred embodiments, the evaporative
water spray system is arranged to operate with just
sufficient water for wet operation. In one exemplary
embodiment the system operates with approximately 90
litres of water per square metre of coil area. This
contrasts with a conventional system in which a volume
of approximately 240 L/m2 is used (which is consistent
with use of a standard sized sump).
Not only does the use of a significantly reduced
volume of water save water, but it also means that the
sump may be smaller than would otherwise be the case,
less powerful heaters are required to prevent it from
freezing, and less chemical water treatment is required,
all of which help to reduce costs.
The preferred embodiment. of the invention set out
above is understood to involve using the minimum water
volume needed for the evaporation process. In practice
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this minimum quantity is dependent upon the capacity of
the water distribution system including pipework, the
proportion of the water which is falling through the air
distribution plenum at any one time and the minimum
quantity of water required by the pumping system to
operate properly. This is to be contrasted with the
prior art in which significantly larger volumes than the
minimum for wet operation are used and indeed in which
consideration has not previously been given to this
minimum required quantity.
It will be appreciated that in practice in
accordance with the invention, means will be provided
for replenishing water lost through evaporation. Any
means well known in the art may be used such as a float-
operated valve, electronic sensor, optical sensor etc.
Such water replenishment may have some inherent
hysteresis such that the actual volume of the water in
the system at any one time may cycle,between a
predetermined maximum and minimum.
A preferred embodiment of the present invention
will now be described, by way of example only, with
reference to the accompanying drawings in which:
Figure 1 is a cut-away view of a conventional
closed circuit evaporative heat exchanger shown for the
purposes of reference only; and
Figures 2a and 2b are cut-away side and end views
respectively of a closed circuit evaporative heat
exchanger in accordance with the invention.
Turning firstly to Figure l, a prior art closed
circuit evaporative heat exchanger may be seen. A
sealed heat exchanger coil A through which coolant
liquid passes is provided in an air distribution plenum
B. A fan system C driven by a motor D is provided at
one end of the plenum B. At the top of plenum B is a
series of nozzles E which are arranged to spray water
over the heat exchanger coil A. A sump F is provided at
the bottom of the plenum B with a capacity of 240 litres
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per square metre of coil area and a pump G is provided
to pump water up from the sump F to the spray nozzles E.
A water replenishment system H using a float valve
ensures that a minimum quantity of water in the sump is
~5 maintained.
In operation coolant liquid or refrigerant vapour
is fed to the heat exchanger coil A where heat is
extracted from it to Cool or condense it before it is
returned, as is well known in the art. The fan C forces
a rapid flow of air over the heat exchanger coil A in
the air distribution plenum B to extracting heat from
the coolant fluid or vapour. Evaporative cooling is
provided by the water spray system which draws water
from the sump F using the pump G. Some of the water
sprayed from the nozzles E will evaporate. The
remainder of the water is collected in the sump F from
where it is recycled back up to the spray nozzles E.
Water lost through evaporation is replaced by the water
replenishment system H.
In order to obtain access to the sump F to carry
out inspection and maintenance it is necessary to shut
down the system and switch the fan C off, thereby
limiting the frequency with which this can be carried
out in practice. Furthermore, if the environmental
temperature is such that the additional cooling effect
of the water spray system is no longer required, all of
the water must be drained from the sump F in order to
prevent it freezing due to the cooling effect of the
forced Cold air flow. This is so time consuming that it
can only be carried out when the operator is confident
that the temperature will not rise sufficiently again to
require wet operation for a considerable time (i.e. over
a period of more than a day).
An embodiment of the present invention may now be
seen in Figures 2a and 2b. As in the apparatus
described with reference to Figure 1, the closed circuit
evaporative heat exchanger Comprises a heat exchanger
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coil 2 disposed in an air distribution plenum 4 and
conveying a coolant fluid or refrigerant to and from an
area to be cooled (not shown) via pipe couplings 6. At
one end of the air distribution plenum 4 is a fan 8
driven by a motor 10 via a belt 12. The blades of the
fan 8 are housed within a casing 14 and thus are not
visible in Figure 2a.
Unlike the system shown in Figure 1, there is no
open sump at the bottom of the air distribution plenum
4. Instead the sump 16 is defined in the lower part of
one end of the air distribution plenum 4 by a sloping
baffle wall 18. In this embodiment the sump has a
capacity of ninety litres per square metre of coil area,
but this is purely exemplary and this figure is
dependent e.g. upon the coil length. The baffle wall 18
depends downwardly from the rear end wall 20 of the
plenum 4. It terminates so as to leave a gap between
its end and the base of the sump 16. The baffle wall 18
extends between the two opposed side walls of the plenum
4 - in other words perpendicular to the plane of Fig. 2a
or left to right in Fig. 2b.
The area of the lower part of the air distribution
plenum 4 not taken up by the sump 16 is formed as a
sloping base 22. The base 22 slopes towards the baffle
wall 18, but stops just short of its so as to leave a
small gap 24 running from one side wall of the plenum 4
to the other. The base also extends between the two
side walls so that the gap 24 runs along the width of
the plenum 4. The baffle wall 18 and the base 22 each
form collection surfaces onto which water falling from
the heat exchanger coils 2 will land.
Within the sump 16 there is a float-operated valve
26 connected to an inlet spout 28 for maintaining a
minimum level of water in the sump 16. This level of
.water is set to be the minimum amount required for wet
operation of the heat exchanger (taking into account the
capacity of the pipes etc. in the rest of the system).
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At the base of the sump 16 is a strainer 30 through
which water from the sump may be drawn by a pump 32 and
pumped up a vertical pipe 34 on the outside of the rear
end 20 of the plenum, where it re-enters the plenum 4 to
feed a water distribution pipe 36.
A series of nozzles 38 is spaced along the water
distribution pipe 36 so that water is forced out in a
conical spray over the heat exchanger coils 2 under the
pressure imparted by the pump 32. One such nozzle 38
may be seen more clearly in the scrap detail view above
it. Above the water distribution pipe 36 is a series of
drift eliminators 40, one of which is also shown more
clearly in a scrap detail view. These separate water
droplets entrained in the airstream leaving the heat
exchanger and prevent those droplets being lost from the
system.
Finally, a pair of access doors 42 are provided in
the lower part of the end wall 20 of the plenum to allow
external access to the interior of the sump 16.
Operation of the apparatus will now be described.
As in the prior art system the fan 8 forces air to flow
over the heat exchanger coils 2 to extract heat from the
coolant fluid therein. When additional cooling is
required the pump 32 is operated to draw water from the
sump 16 through the strainer 30 and force it through the
nozzles 38 so as to form a fine spray over the heat
exchanger coils. A significant Cooling effect is
achieved by evaporation of some of the water. The
unevaporated water falls down towards the bottom of the
air distribution plenum 4 and onto the collection
surfaces formed by either the baffle wall 18 or the
sloping base 22. Water falling onto these parts does
not remain there but drains into the small gap 24
between them.
As may be appreciated from Figure 2a the level of
water in the sump is such that the gap 24 is at least
partially filled with water. This forms a water lock
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between the air distribution plenum 4 and the sump 16.
This water lock allows a differential air pressure to be
maintained between the main part of the plenum 4 and the
sump 16 so that access to the sump 16 may be obtained,
e.g. for inspection and maintenance, whilst the main fan
8 is still running and the system is operational.
During dry operation, the pump 32 is shut off and
the remaining water drains through the gap 24 into the
sump 16. Within sump 16 the water is no longer in
direct contact with the air stream generated by the fan
12. It will be appreciated therefore that since no
water remains in the main part of the air distribution
plenum 4, i.e. in contact with the cold air flow, the
likelihood of it being frozen is significantly reduced.
Although not shown in Figure 2a, thermostatically
controlled heaters are provided to maintain the
temperature of the water in the sump 16 above freezing.
However, since the sump 16 is relatively small compared
to the distribution plenum 4, and is separated from the
cold air stream by the baffle wall 18, the power
required for such heaters is relatively.low.
It will furthermore be appreciated that the
quantity of water in the sump 16 is significantly less
than in the sump F in Figure 1. Not only does this give
savings on the amount of water required to fill the
equipment, but also the cost of chemical treatment
needed and the amount of heat required to prevent it
from freezing.
The embodiment described above gives the overall
advantage that it is fully flexible in that it can be
operated in either wet or dry mode as required and
moreover can be switched very quickly between these
modes.
Example
A prototype apparatus similar to that described
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above with reference to Figs. 2a and 2b was constructed
and tested. The sump water volume of the test apparatus
was 860 litres and the fan gave an airflow of 27 cubic
metres per second over the heat exchanger coils.
However, a normal atmospheric pressure was maintained in
the sump interior by virtue of the water lock.
T~Ihen the pump of the evaporative cooling system was
switched off and the ambient temperature reduced to -10°
C, the sump and water lock remained completely free of
ice with a heat input from the sump heater of a modest 4
KW.
