Note: Descriptions are shown in the official language in which they were submitted.
~L~r916~3
IMPROVED APPARATUS AND METHOD OF
AIR-CONDITIONING PARKED AIRCRAFT
Background of the Invention
This invention relates to aircraft air conditioning
systems and, more particularly, to apparatus and method of cooling
the passer.ger cabins of parked aircraft.
A number of systems have been developed over the years
to satisfy the requirement of maintaining the temperature of the
passenger cabin of modern day aircraft at a level comfortable to
the passengers (the ASHRAE - defined personal comfort zone~ during
the time the aircraft is parked. In such aircraft, the high
density of passengersr the interior lighting, the large number of
windowsr and the heavily insulated fuselage all contribute to
raising the temperature of the cabin of the parked aircraft to
uncomfortable levels. Accordingly, it has been found necessary to
provide a cooling system to lower the aircraft cabin temperature,
even when the aircraft is parked in locations with relatively cold
outside ambient temperatures.
C
1~9i~3
--3--
One type of prior art system for cooling
the cabin of a parked aircraft utilizes an on-board
auxiliary power unit which is generally a small,
jet-fueled turbine. The turbine, which is operated
when the aircraft is parked, i5 used to power the
on-board cooling system. This same cooling system is
powered by the main engines during flight.
Another type of prior art system for
cooling the cabin of a parked aircraft utilizes one
or more on-board air cycle machines which are
special-purpose heat pumps. These machines cool the
cabin air when they are supplied with a source of
high-pressure, high temperature air. During flight,
the source of air is an on-board compressor driven by
the main engines. When the aircraft is parked, a
ground-based air compressor is connected to the
airplane to drive the heat pumps~ This connection is
made using a hose which links the compressor to a
heat pump connector provided on the outer surface of
the aircraft fuselag0.
Yet another type of system for cooling the
cabin of a parked aircraft utilizes a ground-based
air conditioner unit which provides cool air under
pressure directly into the cabin air-conditioning
duct system. This ground-based air conditioner,
1;?~91643
which may be fixed in location or portable, is
connected to the parked airplane using a flexible
hose. This hose links the air conditioner to a
connector, provided on the exterior of the fuselage,
which communicates directly with the cabin ducts. In
this mechanization, there is no need to operate the
on-board cooling system when the aircraft is parked.
Of the previously described types of
cooling systems, the ground-based air conditioner
unit is generally recognized as being the most energy
efficient. Typically, prior art ground-based air
conditioner units require from one-fifth to one-tenth
the energy of those systems employing on-board
auxiliary power units, and from one-half to
one-fourth the energy of those systems employing
on-board air cycle machines.
Even though ground-based air conditioner
systems are generally more efficient than many other
types of cooling systems, they still require large
amounts of power for their operation. For example,
electrically operated air conditioner systems for
large commercial jet aircraft may require in excess
of three hundred and fifty kilowatts of power for
their operation.
1~91643
- 5 - 28101-1
Further, prior art ground-based air conditioner systems
typically employ very large and powerful blowers in order to
generate sufficient air flow to maintain the desired cabin
temperature. These blowers, some of which may be rated in excess
of one hundred and fifty brake horsepower, generate substantial
levels of noise adjacent the air conditioner unit. In addition,
these large blowers cause the cool air to exit the cabin air ducts
at sufficiently high velocities to produce noticeable cabin noise.
Yet another drawback to prior art ground-base air
conditioner systems is the high moisture content of the cool air
delivered to the cabin air ducts. This moisture increases cabin
humidity, causing mist formation and contributing to passenger
discomfort.
Summary of the Invention
Accordingly, it is an aspect of the present invention
to provide an apparatus and method for cooling the cabins of
parked aircraft.
It is another aspect of the present invention to reduce
the power consumption of ground-based aircraft air conditioning
systems.
Thus, the invention in one aspect provides a ground-
based air conditioning system for parked aircraft which departs
from conventional mechanizations of such air conditioning systems
by supplying to the aircraft air which is cooled below the freez-
ing point of water.
For air conditioning purposes, the prior art has for
~ 643 28101-1
the most part rejected the use of air cooled below forty degrees
Fahrenheit on the basis that such cold air results in an in-
efficient system and in user discomfort. However, the present
invention now provides an air conditioning configuration, which
cools the air entering the aircraft below forty degrees Fahrenheit
(preferably below thirty-two degrees Fahrenheit) to provide a
system for cooling parked aircraft which consumes less power,
produces less outside and cabin noise, and provides a drier and
more comfortable cabin environment than prior art systems of
comparable cooling capacity.
Air is provided at sub-freezing temperatures while the
problem of excess frost buildup on the cooling elements is sub-
stantially prevented.
Accordingly, in one aspect of the invention there is
provided a method of maintaining the temperature and preferably
also the hwnidity of the passenger cabin of a parked aircraft at
a level comfortable to passengers, the temperature being prefer-
ably held at approximately 75F while the aircraft is parked,
which method has the steps of: reducing the temperature of a
portion of the ambient air external to the aircraft to a
temperature below the freezing point of water or by using a heat
transfer fluid having a temperature which is below the freezing
point of water, using cooling means located external to the
aircraft; providing the cooled air to the cabin to reach and
maintain the comfortable level.
In a preferred embodiment of this method, the
D
~?,9~ 3
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temperature of air derived from outside the aircraft is first
reduced to a temperature which is near, but above the freezing
point of water, thus condensing a substantial quantity of water
from the air and the air is subsequently cooled to a temperature
which is below the freezing point of water and preferably at
least 5F below the freezing point of water, thus removing a
substantial quantity of the remaining water in the air, using
cooling means external to the aircraft.
The cooled air is suitably delivered through air trans-
port means to an external air inlet of the aircraft and throughair ductways to the passenger cabin for a time sufficient to
reach and maintain the comfortable level and to preferably remove
heat from the air transport means and the passenger cabin combined
at a rate of more than 10 BTU per pound of air delivered.
The cooled air may also be provided at an airflow rate
established by an electric motor driven blower such that the ratio
of the cabin heat flow rate, in units of BTUs per hour, to the
cooling airflow rate, in units of pounds per minute, is greater
than 600, and where the power consumed by the electric motor
driven blower is less than one half the power required by the cool-
ing apparatus~
In another preferred embodiment, the portion of the
ambient air is cooled using electrically powered vapour compression
cycle cooling apparatus located external to the airplane, and the
air cooled by the cooling apparatus is provided to the aircraft
external air inlet at a level of temperature which is below the
~?,916~3
- 7a - 28101-1
freezing point of water.
In yet another preferred embodiment, the step of
reducing the temperature of the portion of the ambient air includes
using first heat exchanger means and the step of delivering the
cooled air to the passenger cabin includes using secondary heat
exchanger means and periodically interrupting the flow of a chilled
heat transfer fluid flowing through the heat exchanger means in
order to melt ice accumulations.
The heat exchanger means is preferably a fluid-to-air
heat exchanger means and preferably includes first and second
sections of coiled tubing.
In a further preferred embodiment of the method, the
step of reducing the temperature of the portion of the ambient air
includes the steps of providing a heat exchanger having first and
second sections of coiled tubing, circulating a fluid, preferably
a compressible heat transfer fluid, through the second and first
sections, respectively, to the heat exchanger, cooling the fluid
entering the second heat exchanger section below the freezing point
of water, forcing the portion of ambient air past the first and
second heat exchanger sections, respectively, where the air exit-
ing the first heat exchanger section is slightly above the freez-
ing point of water and has most of its moisture removed, and the
air exiting the second heat exchanger section, which is at the
second level of temperature, is below the freezing point of water.
The step of circulating the fluid preferably comprises,
compressing the fluid exiting the first heat exchanger section,
1)
1~,9164~3
- 7b - 28101~1
condensing the compressed fluid into a liquid, expanding the
liquid as it flows into the inlet of the second heat exchanger
section, and circulating the compressed, but not condensed or
expanded, fluid through at least the second heat exchanger section
for a predetermined interval of time on a periodic basis to
enable the compressed fluid to defrost the second heat exchanger
section.
The invention provides in another aspect a cooling
system for maintaining the temperature of the passenger cabin of
a parked aircraft at a comfortable level and preferably at approxi-
mately 75F, the aircraft having external air inlet means and air
ductways means communicating with the passenger cabin, the system
comprising cooling means for reducing the temperature of a por-
tion of the ambient air external to the aircraft to below the
freezing point of water, and transport means for providing the
cooled air to the aircraft external air inlet means.
The cooling means may be an electrically powered vapour
compression cycle cooling means preferably a fluid-to-air heat
exchanger means located external to the airplane~ which preferably
reduces the temperature of the portion of the ambient air to a
temperature at least 5F below the freezing point of water.
Alternatively, the system includes a fluid-to-air
heat exchanger means and associated control means for cooling air
from outside said aircraft with a heat transfer fluid having a
temperature which is below the freezing point of water.
The transport means preferably provides the air cooled
lZ,91G~3
- 7c - 28101-1
by the cooling apparatus to the aircraft external air inlet at a
second level of temperature which is below the freezing point of
water and at a cooling airflow rate established by an electric
motor driven blower.
The cooling air is preferably supplied at an airflow
rate such that the ratio of the cabin heat flow rate, in units of
BTUs per hour, to the cooling airflow rate, in units of pounds
per minute, is greater than 600.
The power consumed by the electric motor driven blower
is preferably less than one half the power acquired by the cooling
apparatus and such that the ratio of the cabin heat flow rate in
units of BTU per hour, to the blower power requirement, in kilo-
watts, is greater than 400 and preferably greater than 7500.
In still another preferred embodiment, the system for
maintaining the temperature of the passenger cabin of a parked
aircraft includes first fluid to air heat exchanger means and
associated control means for cooling air derived from outside the
aircraft to a temperature which is near, but above the freezing
point of water, usin.g a chilled heat transfer fluid to absorb
heat from the air; means for removing water condensing from the
air in the first heat exchanger means; second fluid-to-air heat
exchanger means and associate control means for cooling further
the same air to a temperature which is below the freezing point
of water, preferably by using chilled heat transfer fluid at a
temperature sufficiently low to form ice on the surface of the
second heat exchanger means; means for periodically interrupting
.~
1~16~
- 7d - 28101-1
the flow of chilled heat transfer fluid through the second heat
exchanger to permit ice accumulations to melt; means for removing
water formed during the periodic interruptions of flow; and
means for delivering cooled air through the air delivery means,
the air inlet, and the air ductway means to the passenger cabin
in amounts sufficient to remove heat from the air transport means
and the passenger cabin combined at a rate of more than 10 BTU
per pound of air and to reach and maintain the comfortable level.
Other features, aspects, and advantages of the inven-
tion will become apparent from a reading of the specification
taken in conjunction with the drawings, in which like reference
numerals refer to like elements in the several views.
Brief Description of the Drawings
Figure 1 is a block diagram of a ground-based air
conditioning system of the type used in the present invention
for cooling a parked aircraft;
Figure 2 is a schematic diagram of a direct expansion
type of cooling configuration which may be used in the air
conditioning system of Figure 1 to practice the present invention;
6~3
Figure 3 is a schematic diagram o a chilled liquid type
of cooling configuration which may be used in the air conditioning
system of Figure 1 to practice the present invention; and
Figure 4 is a schematic diagram of a combination chilled
liquid and direct expansion type of cooling configuration which
may be used in the air conditioning system of Eigure 1 to practice
the present invention.
Description of the Preferred Embodlment
Referring to Figure 1, there is shown a block diagram of
ground based air conditioning system 10 used to cool -the cabin of
a parked aircraft 12. The system 10 includes a cooling unit 14
which provides a source of cold fluid which is circulated through
a heat exchanger 16. A blower 18 is used to force ambient air,
entering through inlet 20, over the surfaces of the heat exchanger
16, whereby the air is cooled to the desired temperature when it
exits from outlet 22. The heat exchanger 16 and the blower 18 are
generally housed in a common enclosure which is located ad~acent
the airplane parking area, for example on an aircraEt passenger-
loading bridge. This enclosure may be Eixed in location or may be
portable. The cooling unit 14 may be located in that
C
~L~,91~L3
same area or may be remotely located as part of a
central airport cooling system. In such instance,
the cooling unit 14 is connected to the heat
exchanger 16 using suitable piping. Electrical power
to operate the blower 18 and the cooling unit 14 may
be provided by a portable generator ~typically driven
by a diesel engine), or by the airport power utility
using fixed power lines.
The air conditioning system air outlet 22
communicates with one end of a flexible hose 24. The
other end of the hose 24 is attached to a connector
26 provided on the outside surface of the airplane
12. The connector 26j in turn, communicates with a
network of air conditioning ducts 28 distributed
throughout the passenger cabin of the aircraft 12.
In order to provide a comfortable
environment in the passenger cabin of the parked
aircraft, it is desirable to maintain cabin
temperature at a nominal seventy-five degrees
Fahrenheit, even under worst case conditions of
maximum cabin heat load. Factors influencing this
heat load are outside ambient temperature, sunlight
radiating through the windcws, interior cabin lights,
and the number of passengers in the cabin.
1 ~?.,9~643
--10--
For the most part, prior art air
conditioning systems for parked aircraft accomplish
the cooling task by providing air at the heat
exchanger outlet 22 which has been cooled to between
forty and fifty degrees Fahrenheit. Typically, the
air flowing through the flexible hose 24 undergoes a
temperature rîse of about five degrees Fahrenheit.
Accordingly, the temperature of the air entering the
airplane at the connector 26 is between forty-five
and fifty-five degrees Fahrenheit in prior art
systems. The blower 18 is sized to provide a
sufficient rate of cool air flow to maintain the
desired cab n temperature under conditions of maximum
cabin heat load.
~ ntil the present invention, those skilled
in the art had for the most part rejected the concept
of providing air at the heat exchanger outlet 22
which is colder than forty degrees Fahrenheit. These
beliefs are based in part on the technical
information accumulated over the years relating to
the air conditioning of residential and commercial
buildings.
~?~9~64~3
Building air conditioning systems are
characterized by the employment of very large air
ducts having a low pressure drop. Air is cooled by
cooling units to between forty and fifty-five degrees
Fahrenheit and is distributed through the building
duct system at very low flow rates. Generally, air
pressure of only one to three inches of water is
needed to distribute the cool air throughout the
building. Accordingly, the power consumed by the
blowers employed to move the air is quite small
compared to the power needed to cool the air.
Further, the very low air pressure used in these
systems means that the blowers do not contribute
significant heat to the a1r as the result of air
compression.
In general, air cooling apparatus is less
efficient when called upon to cool the air below
forty degrees Fahrenheit. Further, air at these cold
temperatures results in an uncomfortably chilly
atmosphere in a building environment. It has also
been found that air at these cold temperatures is
sufficiently low in humidity that it causes an
uncomfortably dry environment for the building
occupants. Another factor to be considered is that
the cooling of air below approximately thirty-five
~?,,g~43
degrees Fahrenheit requires the use of heat
exchangers having sub-freezing surfaces temperatures.
These sub-freezing temperatures cause icing and frost
buildup problems which do not occur in the prior art
air-conditioning systems. Thus, the prior art has
generally rejected the use of air cooled below forty
degrees Fahrenheit in air conditioning systems, based
on energy inefficiency, hardware mechanization
problems, and on user discomfort.
In contrast to the prior art teachings, the
present invention employs air temperatures below
forty degrees Fahrenheit (preferably below the
freezing point of water) in ground-based air
conditioner systems for parked aircraft with a
resultant decrease in power consumption and an
increase in passenger comfort. This surprising
result may be shown to be related to a combination of
factors which are peculiar to aircrat air
conditioning systems, as outlined below.
~ / Due to space limitations, aircraft cabin
air conditioning ducts are necessarily small in cross
section. Further, the high passenger density in
modern day aircraft, in conjunction with other heat
producing factors previously mentioned, result in a
heat load which requires a substantial rate of
~9~6~3
cooling air flow to maintain the desired cabin
temperature. The combined requirement of high air
flow rate and small duct cross-section results in the
need for substantial air pressure to force the cool
air into the cabin at the required flow rate.
The need for substantial air pressure is
met in prior art systems by employing large and
powerful blowers which consume a substantial portion
of the overall energy consumed by the system. By way
of example, in some prior art parked aircraft air
conditioning systems, the blower consumes in excess
of forty percent of the overall system power
consumption. This is as opposed to building air
conditioning systems where the blower represents only
a few percent of the overall power consumption.
The need for large blowers to produce the
required air flow rates further contributes to system
inefficiency because these blowers add heat to the
air. This heat, which must be removed by the cooling
system, results from the compression of the air which
occurs when it is pressurized.
In the present invention, it has been found
that a reduction in the temperature of the air
delivered to the airplane from that of prior art
systems enables a reduction in the air flow rate
*?,9~6~3
-14-
necessary to maintain the desired cabin temperature.
Such a temperature reduction (typically, to a
temperature level below the freezing point of water)
does result in a reduction in the efficiency of
operation of the cooling apparatus. However, the
ability to reduce the air flow rate provides major
system power savings because it both reduces the size
of the blower and reduces the cooling requirements of
the cooling apparatus. An overall power savings in
excess of thirty percent is not uncommon for parked
aircraft air conditioning systems built in accordance
with the teachings of the present invention. The
following examples are presented to more fully
illustrate the features of the invention.
EXAMPLE I
This example compares the per~ormance of
prior art electrically operated ground-based air
conditioner systems with a comparable system built in
accordance with the teachings of the present
invention when used to cool the cabin of a narrow
body jet aircraft such as the Boeing Aircraft Co.
Model 727-200.
~9164~3
-15-
Under the conditions of an outside ambient
temperature of one hundred degrees Fahrenheit, bright
sunlight, the interior cabin lights turned on, and
134 occupants on board, the cabin heat load for the
above model aircraft is approximately 85,940 BTU per
hour. The goal of the air conditioning system is to
maintain the cabin temperature at seventy-five
degrees Fahrenheit at this heat load.
Referring briefly to Figure 1, prior art
ground-based air conditioning systems generally
provide air at no lower than forty degrees Fahrenheit
at the outlet 22 of the heat exchanger. A typical
five degree temperature rise in the hose 24 results
in a minimum air temperature of forty-five degrees
Fahrenheit at the airplane connector 26. Using the
above figures, it may be shown that the cooling air
flow rate required to achieve the desired cooling is
about 200 pounds per minute. At this flow rate, the
blower 18 must develop air pressure of about
thirty-four inches of water. Of this pressure, about
five inches of water are attributable to the
restriction in the heat exchanger 16 and related
manifolds, about ten inches of water are attributable
1?,9~ 3
-16-
to the restriction imposed by the hose 24 and its
related connections, and the remaining nineteen
inches of water are attributable to the restrictions
imposed by the cabin ducting 28.
Assuming a typical blower efficiency of
sixty-two percent and a blower motor efficiency of
ninety percent, it may be shown that a blower rated
at twenty-three brake horsepower is required to
provide the necessary air pressure. A blower of this
size consumes about 19.2 kilowatts of electric power.
It may also be shown that the cooling unit 14 must be
sized to provide cooling at the rate of 327,600
BTU per hour (about 27.3 tons) in order to process
200 pounds per minute of air and achieve the
necessary temperature reduction to forty degrees
Fahrenheit at the heat exchanger outlet 22. Further,
the cooling unit 14 must alsc remove the heat
contributed by the blower 18 in compressing the air.
The twenty-three brake horsepower blower
contributes heat at the rate of about 58,512 BTU per
hour (4.9 tons). Accordingly, the cooling unit 14
must be sized to supply a total of 32.2 tons of
cooling. A typical cooling unit 14 might employ
direct expansion type refrigeration apparatus. A
well designed electrically operated air cooled
~;?,9~6qL3
refrigeration unit of this type which is configured
to cool air to forty degrees Fahrenheit may be
expected to consume about 1.6 kilowatts per ton of
cooling capacity. Accordingly, the prior art cooling
unit described above would consume about 51.5
kilowatts which, when combined with the blower power
consumption, yields a total prior art system power
consumption of 70.7 kilowatts.
We turn now to a system configured in
accordance with the teachings of the present
invention to provide the~same cabin temperature of
seventy five degrees Fahrenheit under the same heat
load of 85,940 BTU per hour. The present system is
designed to provide air at the heat exchanger outlet
22 at a temperture of twenty-five degrees Fahrenheit,
which is well below the freezing point of water
(thirty-two degrees Fahrenheit). Allowing for a five
degree Fahrenheit rise in the hose 24, the air
temperature at the connector 26 is thirty degrees
Fahrenheit.
. Using the above figures, it may be shown
that the cooliny air flow rate requirement to achieve
the desired cooling is only 133 pounds per minute,
which is 33.5 percent less than the comparable prior
art system air flow rate. At this lower flow rate,
1?,9~ 3
-18-
the blower 18 need only develop air pressure of about
thirteen inches of water. Of this pressure, about
three inches of water are attributable to the
restriction in the heat exchanger 16 and related
manifolds, about five inches of water are
attributable to the restriction imposed by the hose
24 and its related connections, and the remaining
five inches of water are attributable to the
restrictions imposed by the cabin ducting 28.
Assuming the same blower efficiencies as in
the prior art system description above, it may be
shown that the blower in. the present system need only
be rated at 5.9 brake horsepower. Such a blower
typically consumes 4.9 kilowatts of powerl which is
abcut seventy five percent less power than that
consumed by the blower in the prior art system.
The reduced air flow rate results in a
reduction in the cooling requirements of the cooling
unit 14 to 268,128 BTU per hour (about 22.3 tons).
The additional heat contributed by the smaller
blower is only 15,010 BTU per hour (1.3 tons).
Accordingly, the cooling unit 14 in the present
~. .
~?.J916~3
--19--
system need only be sized to supply 23.6 tons of
ccoling, which is about twenty-seven percent less
than the cooling requirement for the prior art
system.
As explained earlier, cooling units such as
the direct expansion type refrigeration unit are less
energy efficient when designed to provide cooling
below about forty degrees Fahrenheit, which is the
case for the present system. Such a cooling unit may
be expected to consume on the order of 1.85 kilowatts
per ton of cooling capacity, which is about sixteen
percent less efficient than the prior art cooling
unit. Accordingly, the sub-freezing cooling unit
would consume about 43.7 kilowatts which, when
combined with the blower power consumption, yields a
total system power consumption of 48.6 kilowatts.
Comparing the total power consumption
figures for the two previously described systems, it
will be appreciated that, in spite of the less
efficient cooling unit, the system of the present
invention consumes 22.1 kilowatts less than
comparable prior art systems, resulting in a power
savings of over 31 percent.
~,9~643
-20-
EXAMPLE II
This second example compares the
performance of prior art electrically operated
ground-based air conditioner systems with a
comparable system built in accordance with the
teachings of the present invention when used to cool
the cabin of a wide body jet aircraft such as the
Boeing Aircraft Co. Model 747-200.
Under the conditions of an outside ambient
temperature of one hundred degrees Fahrenheit, bright
sunlight, the interior cabin lights turned on, and
511 occupants on board, the cabin heat load for the
above model aircraft is approximately 305,305 BTU per
hour. The goal of the air conditioning system is to
maintain the cabin temperature at seventy-five
degrees Fahrenheit at this heat load.
The prior art ground-based air conditioning
system provides air at forty degrees Fahrenheit at
the outlet 22 of the heat exchanger. A typical five
degree temperature rise in the hose 24 results in an
air temperature of forty-five degrees Fahrenheit at
the airplane connector 26. Using the above figures,
it may be shown that the cooling air flow rate
required to achieve the desired cooling is about 706
pounds per minute. At this flow rate, the blower 18
.~"
~ ~,9~6~3
must develop air pressure of about seventy-three
inches of water. Of this pressure, about five inches
of water are attributable to the restriction in the
heat exchanger 16 and related manifolds, about
fifteeen inches of water are attributable to the
restriction imposed by the hose 24 and its related
connections, and the remaining fifty-three inches of
water are attributable to the restriction imposed by
the cabin ducting 28.
Assuming a typical blower efficiency of
sixty percent and a blower motor efficiency of ninety
percent, it may be shown that a blower rated at one
hundred and eighty brake horsepower is required to
provide the necessary air pressure. A blower of this
size consumes about one hundred and fifty kilowatts
of electric power. The cooling unit 14 must be sized
to provide cooling at the rate of 1,156,428 BTU per
hour (about 96.4 tons) in order to process 706 pounds
per minute of air and achieve the necessary
temperature of forty degrees Fahrenheit at the heat
exchanger outlet 22. Further, the cooling unit 14
must also remove the heat contributed by the blower
18 in compressing the air.
~?,9~6~3
-22-
The one hundred and eighty brake horsepower
blower contributes heat at the rate of about 457,920
BTU per hour (38.2 tons). Accordingly, the cooling
unit must be sized to supply a total of 134.6 tons of
cooling. As described in the previous example, a
direct expansion refrigeration unit configured to
cool air to forty degrees Fahrenheit may be expected
to consume on the of order of 1.6 kilowatts per ton
of cooling capacity. Accordingly, the prior art
cooling unit described above would consume about
215.4 kilowatts which, when combined with the blower
power consumption, yields a total prior art system
power consumption of 365.4 kilowatts.
We turn now to a system configured in
accordance with the teachings of the present
invention to provide the same cabin temperature of
seventy-five degrees Fahrenheit under the same heat
load of 305,305 BTU per hour. The present system is
designed to provide air at the heat exchanger outlet
22 at a temperature of twenty-five degrees
Fahrenheit. Allowing for a five degree Fahrenheit
rise in the hose 24, the air temperature at the
connector 26 is thirty degrees Fahrenheit.
~?,9~6~3
-23-
Using the above figures, it may be shown
that the air flow rate requirement to achieve the
desired cooling is only 471 pounds per minute, which
is 33.3 percent less than the comparable prior art
system air flow rate. At this lower flow rate, the
blower 18 need only develop air pressure of about
forty-six inches of water. Of this pressure, about
four inches of water are attributable to the
restriction in the heat exchanger 16 and related
manifolds, about ten inches of water are attributable
to the restriction imposed by the hose 24 and its
related connections, and the remaining thirty-two
inches of water are attributable to the restriction
imposed by the cabin ducting 28.
Assuming the same blower efficiencies as in
the prior art system described above, it may be shown
that the blower in the present system need only be
rated at 75 brake horsepower. Such a blower
typically consumes 62.5 kilowatts of power, which is
about fifty-eight percent less power than that
consumed by the blower in the prior art system.
The reduced air flow rate results in a
reduction in the cooling requirements of the cooling
unit 14 to 949,536 BTU per hour (about 79.1 tons).
The additional heat contributed by the smaller blower
~?.916~3
-24-
is only 190,800 BTU per hour (15.9 tons).
Accordingly, the cooling unit 14 in the present
system need only be sized to supply 95 tons of
cooling, which is about twenty-nine percent less than
the cooling requirement for the prior art system.
Using the energy consumption factor of 1.85
kilowatts per ton of cooling capacity established
earlier, the sub-freezing cooling unit would consume
about 175.8 kilowatts which, when combined with the
blower power consumption, yields a total system power
consumption of 238.3 kilowatts.
Comparing the total power consumption
figures for the two previously described systems for
wide-body aircraft, it will be appreciated that in
spite of the less efficient cooling unit, the system
of the present invention consumes 127~1 kilowatts
less than comparable prior art systems, resulting in
a power savings of about thirty-five percent.
In addition to power savings, air
conditioning systems constructed in accordance with
the teachings of the present invention offer other
advantages over prior art systems. For example, the
air cooled to forty-five degrees Fahrenheit at the
aircraft connector in prior art sytems is generally
extremely high in moisture content, resulting in a
.
~,9~643
relative humidity in the aircraft cabin in the range
of 70 to 80 percent, which is considered
uncomfortable. On the other hand, in the present
invention, by cooling the air to sub-freezing
temperatures, a great deal of moisture is removed by
condensation. Accordingly, the air entering the
cabin is much lower in humidity than in prior art
systems and a cabin relative humidity of less than
fifty percent may be expected.
It will be appreciated that the air
entering the cabin in the present system is colder
than that of prior art systems. From the prior art
teachings reiated to building air conditioning, one
might expect passenger discomfort at these lower
temperatures. On the contrary, it has been found
that the high cabin heat load, the high density of
passengers, the relatively short time during which
the airplane is parked with passengers aboard, and
the rapid mixing of air due to its relatively high
duct exit velocity, all contribute to obviate
passenger discomfort.
It is interesting to note that the low
humidity of the cabin air produced using the present
invention could enable the cabin temperature to be
increased several degrees above the commonly selected
~.,9~643
seventy-five degrees Fahrenheit, while maintaining
the same or greater level of passenger perceived
comfort. It is well known to those skilled in the
art that such perceived comfort is a function both of
temperature and humidity.
Another feature of the present invention is
the reduction of cabin noise. ~sing the present
invention, air is delivered into the cabin at a flow
rate which is about thirty-three percent less than
that of prior art systems. This reduced air flow
rate decreases wind noise to léss than half that of
prior art systems. Noise produced outside the
airplane in the vicinity of the blower 18 is also
reduced by practicing the present invention. The use
of smaller, less powerful blowers results in blower
noise which is about six to ten decibels less than
that of prior art systems.
Installation and operating costs of systems
constructed in accordance with the teachings of the
present invention are greatly reduced over these same
costs for prior art systems. For example, the
reduction in system electrical power enables a
reduction in airport wiring costs for fixed
~?,9~6~3
-27-
installation ground-based systems. Since an air
conditioning system is generaly provided at each gate
in an airport, this installation savings is
significant.
Returning to Figure 1, the cooling of air
to sub-freezing temperatures in the present invention
presents the problem of frost buildup on the surfaces
of the heat exchan~er 16. It will be appreciated
that to achieve the low air temperatures used in the
present invention, it is necessary to cool at least
some portion of the surfaces of the heat exchanger 16
belowe the freezing point of water. The buildup of
frost on these surfaces is the result of the freezing
of moisture which has been condensed from the air.
Excess frost accumulation can block the heat
exchanger air flow passages and reduce system cooling
efficiency. Figures 2 through 4 show various air
conditioning configurations which have been developed
as part of the present invention to overcome the
frost problem.
Figure 2 is a schematic diagram of a
ground-based air conditioning system 10 employing a
direct expansion type cooling unit 14. The heat
exchanger 16 includes first and second sections 30
and 32, respectively, which are typically formed of
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-28-
coiled copper tubing. The section 30 is positioned
closest the blower 18, while the section 32 is
positioned adjacent the heat exchanger outlet 22.
The sections 30 and 32 are interconnected so that a
compressible fluid refrigerant, such as Freon, may
flow through the section 32 from an inlet 34 to an
outlet 36 and then through section 30 from an inlet
38 to an outlet 40. It will be appreciated that the
flow of fluid is from a location closest the outlet
22 to a location closest the blower 18 in the heat
exchanger 16. This flow pattern establishes a
temperature gradient in the heat exchanger 16 whereby
the air flowing from the blower 18 to the outlet 22
is made progressively colder.
The outlet 40 is connected to an inlet of a
compressor 42 used to compress the fluid. An outlet
of the compressor 42 is connected to an inlet of a
condenser 44 which may be air cooled by a fan 46 in
order to li~uify the compressed fluid. An outlet of
condenser 44 is connected to an inlet of an expansion
valve 48, the outlet of which is connected to the
inlet 34 of the section 32. The expansion valve 48
is used to cause the fluid to expand and evaporate in
the coils of the sections 30 and 32, thus lowering
the temperature of the heat exchanger surfaces. An
~?,J9~6~3
-29-
electrically operated bypass valve 50 is connected
between the outlet of the compressor 42 and the inlet
34 of the heat exchanger section 32. The normally
open valve 50 is closed in response to an electrical
signal applied on line 52.
The sections 30 and 32 are sized so that
when the blower 18 forces ambient air over these
sections, the section 30 reduces the temperature of
the air until it is only a few degrees above
freezing. At this temperature, a large percentage of
the moisture in the air is removed without any
freezing of this moisture occurring on the surfaces
of the section 30. The condensed moisture which
drips from the surfaces of the section 30 is removed
from the exchanger 16 using a suitable drain 54. The
chilled air then passes over the surfaces of the
second section 32 where it is cooled to the desired
sub-freezing temperature. Because most of the
moisture has already been removed by section 30 (only
about 0.004 pounds of moisture remain per pound of
dry air), the rate of ice formation on the surfaces
of section 32 is relatively slow.
~?.,9~6~3
-30-
The ice which does form on the surfaces of
the section 32 is periodically removed by opening the
bypass valve 50. Opening this valve 50 causes hot
refrigerant gas (at approximately 150 to 200 degrees
Fahrenheit) from the compressor 42 to enter the
section 32 tubing and rapidly melt the ice. The
melted ice is drained away using drain 56. The valve
50 may be operated on a periodic basis by an
electrical timer to automatically defrost the section
32. It has been found that operation of the valve 50
for about two minutes every twenty minutes is
adequate to prevent excess frost buildup.
During this short defrost cycle, the
temperature of the air exiting the exchanger 16 will
rise to about sixty degrees Fahrenheit. However, the
large thermal mass provided by the hose 24 and the
duct system 28 acts to maintain the temperature of
the air entering the cabin at very nearly the desired
temperature during this time. Accordingly, the
defrost cycle has negligible effect on the overall
cabin temperature.
Figure 3 is a schematic diagram of a
ground-based air conditioning system employing a
chilled liquid type cooling unit 14. The heat
exchanger 16 includes first and second sections 58
~1~9~ 3
and 60, respectively, which are typically formed of
copper tubing. The section 58 is positioned closest
the blower 18, while the section 60 is positioned
adjacent the heat exchanger outlet 22. The sections
58 and 60 are interconnected so that a liquid having
a freezing point below that of water may flow through
the section 60 from an inlet 62 to an outlet 6~ and
then through section 58 from an inlet 66 to an outlet
68. It will be appreciated that the flow of liquid
(which may be a mixture of glycol and water) is from
a location closest the outlet 22 to a location
closest the blower 18 in the heat exchanger 16.
The outlet 68 is connected to an inlet of a
pump 70 used to pump the liquid through the heat
exchanger. An outlet of the pump 70 is connected to
an inlet of a chiller 72, and an outlet of the
chiller is connected to the inlet 62 of the section
60. The chiller 72 may be any one of a number of
apparatus configurations which is able to cool the
liquid to a temperature below the freezing point of
water (for example, to twenty degrees Fahrenheit).
The sections 58 and 60 are sized so that
when the blower 18 forces ambient air over these
sections, the section 58 reduces the temperature of
the air until it is only a few degrees above
,
~Z,9~3
-32-
freezing. As in the previously described
configuration, a large percentage of mbisture is
removed without any frost occurring on the surfaces
of the section 58. Condensed moisture drains via
drain 74. The chilled air passes from the section 58
to the section 60, where it is cooled to the desired
sub-freezing temperature, and the air then exits
through the outlet 22. The rate of ice formation on
the surfaces of the section 60 is relatively slow,
because most of the moisture in the air has already
been removed in the section 58. It should be noted
that the flow of chilled liquid through the heat
exchanger 16 from the outlet 22 toward the blower 18
aids in establishing the section 60 as the coldest of
the two sections and provides the desired cooling
gradient.
The frost which does form on the surfaces
of the section 60 is periodically removed by stopping
the pump 70, which stops the flow of cold liquid
through the heat exchanger 16. The warm ambient air
being drawn in by the blower 18 and forced over the
heat exchanger surfaces acts to quickly defrost the
section 60. Condensed moisture drains from the
section 60 via drain 76. It has been found that by
stopping the pump 70 for two minutes every twenty
,. .
~,9~6~3
minutes, the section 60 is adequately defrosted. As
in the previous system description, the short defrost
interval does not produce any significant increase in
airplane cabin temperature because of the large
thermal mass of the hose 24 and duct system 28.
Figure 4 is a block diagram of a
ground-based air conditioning system employing both
chilled liquid and direct expansion cooling. The
heat exchanger 16 includes first and second sections
78 and 80, respectively, which may be formed of
copper tubing or the like. The section 78 is
positioned closest the blower 18, and includes an
inlet 82 and outlet 84.
The section 78 is connected as part of a
chilled liquid system comprising a pump 86 and a
chiller 88. The pump 86 circulates a liquid, which
may be water, through the chiller 88 and through the
section 78 from the inlet 82 to the outlet 84.
Connected between the pump 86 and the chiller 88 is a
heat exchancler coil 90 which is part of a condenser
92 further described below.
The section 78 is designed to cool the
incoming ambient air to a temperature slightly above
the freezing point of water. Accordingly, the liquid
flowing through the section 78 need not be chilled
~,9~6~3
-3~-
below the freezing point of water. In fact, the
chiller 88 may be part of a central airport cooling
system for generating cold water, and a portion of
this water can be routed to flow through the section
78. Condensed moisture collecting on the surfaces of
this section is drained via drain 94.
The section 80 is designed to cool the
already chilled air from the section 78 to the
sub-freezing temperature desired at the outlet 22.
The section 80 is connected as part of a direct
expansion refrigeration system similar in operation
to that described above for the mechanization shown
in Figure 2.
A compressible fluid refrigerant, such as
Freon, flows through the section 80 from an inlet 96
to an outlet 98. The outlet 98 is connected to an
inlet of a compressor 100 designed to compress the
fluid. An outlet of the compressor 100 is connected
to an inlet of the condenser 92 which is designed to
condense the fluid to a liquid. The heat exchanger
90, which is connected as part of the chilled water
system described above, provides cooling for the
condenser 92 in lieu of air cooling. An outlet of
the condenser 92 is connected to an inlet of an
expansion valve 102, and an outlet of the expansion
~ ~?,9~6~3
valve 102 is connected to the inlet 96 of the section
80. The expansion valve 102 causes the fluid to
expand and evaporate in the coils of the section 80,
thus lowering the temperature of the heat exchanger
surfaces to achieve the desired air temperature.
An electrically operated bypass valve 104
is connected between the outlet of the compressor 100
and the inlet 96 of the section 80. The valve 104 is
operated periodically to provide hot compressed fluid
to the section 80 in order to~defrost the surfaces of
this section. As in the previous mechanizations, a
two minute defrost interval every twenty minutes has
been found to be adequate. Melted ice is drained
from the section 80 using drain 106.
While there have been shown and described
preferred embodiments of the invention, it is to be
understood that various other adaptations and
modifications may be made within the spirit and scope
of the invention. It is thus intended that the
invention be limited in scope only by the appended
claims.
WHAT IS CLAIMED IS:
