Note: Descriptions are shown in the official language in which they were submitted.
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WETTING OF EVAPORATIVE COOLER PADS
Field of the Invention
This invention relates to evaporative air coolers used for the comfort cooling
of
building space. In particular, the invention relates to method and means for
wetting evaporative pads of evaporative coolers and the provision of
evaporative
coolers in a more compact configuration.
Description of the Prior Art
Throughout this description and the claims which follow, unless the context
requires otherwise, the word "comprise', or variations such as "comprises" or
"comprising", will be understood to imply the inclusion of a stated integer or
step
or group of integers or steps.
The reference to any prior art in this specification is not, and should not be
taken
as, an acknowledgement or any form of suggestion that that prior art forms
part of
the common general knowledge in Australia.
An evaporative air cooler essentially comprises a fan, evaporative pads, a
pump,
water distribution means and a cabinet to contain these components which also
incorporates a water reservoir. The evaporative pads are kept wet by pumping
water from the water reservoir via a water distribution system. In operation
outside
air is drawn through these wetted evaporative pads and is cooled by the
evaporation of water from within the evaporative pads. The cooled air then
passes
through the fan to ducting for distributing air to the space to be kept cool.
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The evaporative pads are made from a media which is easily wetted while
allowing air to pass through the media and interacting with the wetted
surfaces
within the media. The evaporative media has the characteristics of high
internal
surface area, good retention of water, good wicking characteristics and
structural
integrity. Traditionally, shredded woodwool contained within an open frame has
been used as an evaporative medium. More recently, manufactured media made
from resin impregnated corrugated paper has become almost universally popular.
The corrugated paper media has all the desired characteristics as well as its
own
structural integrity when formed into an evaporative pad without necessarily
requiring it to be contained in a frame.
Traditionally the evaporative media has been kept continually wetted while the
evaporative cooler is in operation. This requires water to flow continuously
through the media. A percentage of the water flow is evaporated into the
airstream
passing through the media, generally orthogonal to the direction of water flow
while any water not evaporated runs out of the bottom of the media and returns
to
the water reservoir. The rate of evaporation of water is determined by the
psychrometric properties of the air entering the evaporative pads and the rate
of air
flow. The total water flow rate circulating through the pads is determined by
the
characteristics of the pump and water distribution means.
All types of evaporative media pads have a limiting velocity of air flow which
can
be passed through the media without undesirable consequences. A most important
consequence to be avoided is for free water within the media to detach and
become entrained in the air flow stream as droplets. This tendency to detach
is
determined by the type of media, the rate of water flow in excess of that
required
for evaporation and the velocity of air through the media. Consequences of
water
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inclusion in the delivered air stream include wetting and leaking from
ductwork,
accelerated corrosion of ducting and accumulation of salt deposits (from salts
dissolved in the water) inside the conditioned space.
The water flow rate in excess of evaporation is determined at the design
stage. It
must be sufficient to ensure complete wetting of the evaporative media under
the
most adverse weather conditions, and enough to ensure dirt and salt deposits
within the media are flushed to the reservoir. In practice, the water flow
rate is
usually set between 5 and 10 times the evaporation rate under design
conditions.
This ensures there is still sufficient excess water under the most adverse
weather
conditions.
With water flow rates within the usual limits, the limiting air velocity
through the
evaporative media pads has been found by experiment to be about 2.5 m/s for
corrugated paper media and a little higher at around 3 m/sec for woodwool
media.
The design of evaporative coolers is based on these limitations of airflow
velocity
through evaporative pads. The area of evaporative pad required is determined
by
the design total air delivery rate of an evaporative cooler, divided by the
allowable
air flow velocity. This determines the fundamental dimensions of an
evaporative
cooler, setting the boundaries for the balance of a cooler design.
It would be commercially advantageous if the allowable airflow through the
evaporative media could be increased. This would allow reduction of the area
of
evaporative pad required to achieve the design performance and a more compact
overall design could be possible. A side benefit could be an evaporative
cooler
with a lower height thereby reducing the aesthetic intrusion of the cooler on
the
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roofline of the building. An overall smaller evaporative cooler for equivalent
total
airflow delivery will have obvious advantages in manufacturing cost.
The air velocity through the evaporative media could be increased considerably
if
the media was intermittently wetted but had no additional water flow through
it
during most of its operation. Under these circumstances, there would be no
free
water on the surfaces within the evaporative media and therefore no tendency
for
water to become entrained in the airflow across the surfaces. There will
remain a
requirement that all surfaces within the evaporative media always remain wet.
This requirement can be met by constructing the media from material which can
retain substantial volumes of water within itself through a porous
characteristic,
and with the capacity to readily distribute water to all surfaces through
internal
wicking. Typically, the corrugated paper media currently available satisfies
these
requirements.
Summary of the Invention
In a first aspect, there is described a method of controlling the operation of
an
evaporative air cooler, said method comprising intermittently wetting an
evaporative pad of the cooler with an amount of water in excess of the
capacity of
the pad to absorb and retain during each wetting operation of the pad, varying
the
airflow through the pad during the intermittent wetting to a velocity so as to
not
entrain substantial quantities of water in the airflow during the wetting, and
increasing the velocity of the airflow through the pad after each intermittent
wetting so as to raise the level of cooling output of the cooler between each
intermittent wetting; wherein airflow speed through the pad is reduced before
a
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wetting is commenced and increasing the airflow speed after a predetermined
period after wetting has ceased.
Preferably, the method of this invention is applied to an evaporative cooler
comprising a plurality of separate evaporative pad sets and wherein each pad
set
includes an associated separately controllable fan for varying the airflow
therethrough.
In another aspect, there is described a control system for an evaporative
cooler for
controlling the velocity of airflow through evaporative pads of the cooler in
association with intermittent wetting of the pads with quantities of water in
excess
of the capacity of the pads to absorb and retain during the wetting, said
control
system determining the velocity of airflow through the pads and being adapted
to
vary that velocity in dependence upon a period of wetting of the pads and
another
period of operation of the cooler apart from each period of wetting;
wherein the control system is configured to reduce the velocity of airflow
through
the pads before wetting is commenced and then increase the velocity of airflow
through the pads a predetermined period after wetting has ceased.
In a preferred embodiment the control system employs a static pressure
transducer
downstream of the airflow through each pad whereby a pressure differential
between each respective downstream transducer and ambient atmospheric pressure
provides a measure of the airflow velocity through the pad.
In a further preferred embodiment the control system employs static pressure
transducers upstream and downstream of the airflow through each pad whereby a
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pressure differential between each respective upstream and downstream
transducer
provides a measure of the airflow velocity through the pad.
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Still further embodiments of the control system employ a hot wire or rotating
vane
anemometer in the downstream airflow from each pad to provide a measure of the
velocity of the airflow through the pad.
Brief Description of the Drawings
The present invention will now be described by way of example with reference
to
the accompanying drawings, in which:
Figure 1 schematically shows a typical prior art evaporative air cooler with
its
component parts;
Figure 2 shows a section through an evaporative pad of the cooler of Figure 1
illustrating water entrainment in the airflow;
Figure 3 is a section view similar to Figure 2 showing controlled water
circulation
and differential pressure monitoring across the evaporative pad in
accordance with an embodiment of the present invention; and
Figure 4 shows a multi-fan evaporative cooler adapted to operate in accordance
with the present invention
Description of Embodiments
In a typical prior art evaporative air cooler of Figure 1, water from
reservoir 8 is
moved by pump 6 to water distribution system 4 which distributes water to the
top
of evaporative pads 2. Water travels through the evaporative pads 2 under
gravity
with excess water 14 returning from the underside of pads 2 to reservoir 8.
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A fan 10 draws air 20 into the evaporative pads 2 thereby cooling the air by
evaporation. After passing through the evaporative pads 2 and then fan 10,
cooled
air 22 is delivered to the ducting 24 for distribution to a conditioned space.
The combination of total air delivery (cooled air 22) and the total face area
of
evaporative pads 2 subjected to incoming air 20 determines the velocity of air
entering the evaporative media of evaporative pads 2.
Figure 2 schematically shows the effects of an evaporative pad when subjected
to
excess water. Water droplets from within the evaporative media can be
entrained
in the air flow as it passes through the evaporative pad. Water 30 from water
distribution system 4 is distributed to the top of evaporative pad 2. As this
water
flows down through evaporative pad 2 in a generally vertical direction under
gravity, air flow 20 (which will be hot dry air under normal operating
conditions)
enters the evaporative pad orthogonal to the water flow down through the pad.
As
the air flows through the pad, a proportion of the water wetting the internal
surfaces of the evaporative pad will evaporate into the air flow. This
evaporation
will cool the internal surfaces of the pad media, which will then cool the
temperature of the air flowing through the pad. Air flow 32 exiting the
evaporative
pad will therefore be cooler and have a higher humidity than the airflow 20
entering the evaporative pad. The rate of evaporation is dependent on the
psychrometric properties of the air 20 entering the pad and the velocity of
air flow
20.
Any water flowing through the evaporative pad in excess of the evaporation
rate
into the airflow passing through leaves the evaporative pad from the bottom
and is
returned to reservoir 8. This excess water is depicted as water 34 in Figure
2.
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This water in excess of evaporation requirements means that there will always
be
free water on the internal surfaces of the evaporative media. If the airflow
across
this free surface water is high enough, water will be displaced from the
surfaces
.. and become entrained in the airflow leaving the evaporative pad. The
entrained
water is shown as droplets 36 in Figure 2. Entrained water in the airflow of
an
evaporative cooler is an undesirable condition. The airflow required to cause
free
surface water within the evaporative media to become entrained in the airflow
is
dependent on the degree of excess water available and the velocity of air
entering
the evaporative pad.
An embodiment of the present invention in Figure 3 incorporates a control
system
with inputs from static pressure transducer 40 located on a side of the
evaporative
pad external to the evaporative cooler, and a static pressure transducer 42
located
on the inside of the evaporative cooler downstream of the evaporative pads.
The
difference in outputs of transducers 40 and 42 measures the static pressure
differential across the evaporative pad induced by the airflow through the
pad.
This static pressure differential has a direct relationship to the air
velocity through
the evaporative pad and correlates to a measure of that velocity.
The control system of the evaporative cooler is programmed to use this measure
of
air velocity to control the wetting of the evaporative pads. During a wetting
sequence, the control system reduces the speed of fan 10 until the air
velocity
through the evaporative pad is below the velocity known to result in water
entrainment in the airflow through the pad. Water is then applied via the pump
and
water distribution system for a period of time to achieve complete wetting of
the
evaporative media and the flushing of any dirt or contaminants caught in the
evaporative pad back into the reservoir.
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At the end of the time required for wetting, the water distribution system is
turned
off (generally by simply turning off the pump or switching a valve), and any
free
water within the evaporative media is allowed to flow back into the reservoir.
At
this time, the speed of fan 10 is increased resulting in an increase in the
airflow
through the evaporative pads. Since there is now no free water on the internal
surfaces of the evaporative media, there will be no water available to be
entrained
in the airflow. It is found that the airflow can be increased substantially
above the
prior art method of operation without any significant risk of water
entrainment.
When the evaporative cooler is operated without constantly flowing water,
evaporation and cooling of the airflow still occurs but from water stored
within the
bulk of the material from which the evaporative media pads are manufactured.
The
evaporative media pads are manufactured from materials with the property of
retaining relatively large quantities of water within the material and the
property of
readily wicking the stored water to all surfaces within the media. It is found
from
experience that the evaporative media can continue to cool and humidify the
air
passing through it for a considerable period of time before it is necessary to
wet
the pads again using the watering sequence described above.
With this method of operation, the airflow through the evaporative cooler can
increased considerably over the airflow in an equivalent prior art cooler at
all
times except during the wetting sequence. Since the wetting sequence only
requires a small percentage of the operating time (typically 10%-20%), an
evaporative cooler using the method of the present invention can deliver
considerably more cooled air on average than the equivalent prior art cooler.
Alternatively, an evaporative cooler of equivalent capacity to prior art
coolers
could be designed to be considerably smaller than such prior art coolers since
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much less surface area of evaporative pad is required to achieve the same
performance.
It will be appreciated that there are many methods by which the velocity of
airflow
through evaporative pads can be measured for use in controlling a cooler in
accordance with the present invention. It would be possible to use only a
single
pressure transducer to estimate the pressure differential since the reference
pressure outside of the evaporative cooler is the nominal ambient atmospheric
pressure. Alternatively, the air velocity through the pads could be measured
using
velocity transducers such as hot wire or rotating vane anemometers. This
method
of control can be applied by using any means of estimating air velocity.
The maximum time between wetting sequences to ensure continuous cooling of
the air delivered by the evaporative cooler will be very dependent on the
psychrometric condition of the air entering the pads. This period could be
shortened considerably in very hot and dry environments. In a simple control
system based on the current invention, fixed timers could be used to control
both
the wetting sequence and the time between wetting sequences. The time
intervals
could be based on experimental results for the normal design weather
conditions,
with an index factor applied to cover the worst anticipated weather condition.
Such a control would work satisfactorily in terms of continuously delivering
cooled air under all weather conditions, but would be wetting the pads more
frequently than necessary under milder weather conditions.
As a further refinement in another embodiment of the current invention, a
transducer is added which gives a continuous measurement of the psychrometric
conditions of the incoming air. Measurement of air temperature and relative
humidity is sufficient to calculate all the necessary properties of the air.
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these measurements, the control system is able to estimate the rate of water
evaporation from the pads during operation between wetting sequences and
adjust
the time interval between wetting sequences accordingly. Such an arrangement
would maximise the air delivery by reducing the number of wetting sequences in
mild weather with consequent increases in average performance.
Figure 4 shows a multi-fan evaporative cooler operating with a control system
in
accordance with an embodiment of the present invention. In operation, wetting
sequences are undertaken sequentially between the evaporative pads influenced
by
each of the fans. The sequence begins with a first fan 51and its immediately
adjacent evaporative pads 61. This fan 51 is reduced in speed until the
velocity
through the evaporative pads 61 is below the critical velocity for water
entrainment as measured by transducers 55 installed to deduce that velocity. A
wetting sequence is then undertaken on this evaporative pad. At the conclusion
of
the wetting sequence, fan 51 is returned to normal operating speed and a
wetting
sequence of slowing the fan and then wetting the pad is commenced for pads 62
adjacent fan 52. This sequence is repeated until all evaporative pads have
been
wetted (for example, in Figure 4, followed by the combination of fan 53 and
evaporative pads 63). Each fan and evaporative pad combination wetting
sequence
is monitored by respective pressure transducer pairs 55, 56, 57. An entire
wetting
sequence is again commenced when the controller determines that it is time to
re-
commence the wetting sequences to ensure that continuously cooled air is
delivered.
By application of this invention, compact, economical evaporative air coolers
can
be constructed which are smaller and less expensive than prior art coolers
while
delivering the cooling capacity of those larger prior art coolers.
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