Note: Descriptions are shown in the official language in which they were submitted.
CA 02313788 2000-07-12
Air-conditioning system for airplane cabins
This invention relates to an air-conditioning system for conditioning moisture-
containing, pressurized air for air-conditioning a room, in particular for air-
condi-
tinning airplane cabins, and to a corresponding method.
Fresh air for air-conditioning airplane cabins is conditioned from the air
(known as bleed) bled off the engine at high pressure and high temperature.
Air-
conditioning systems draw the necessary cooling power out of the pressure and
tem-
perature potential of the engine air. In the course of the fresh-air
conditioning proc-
ess the bleed is cooled, dehumidified and expanded to the cabin pressure of 1
bar in
ground operation or about 0.8 bars in flight operation. Special value is
attached in
fresh-air conditioning to dehumidification in order to prevent icing of
individual
components of the air-conditioning system and ice crystallization in the fresh
air to
be conditioned. The necessity of dehumidification exists mainly in ground
operation,
however, because in flight operation, i.e. at high altitudes, ambient air and
thus the
bled-off engine air is already extremely dry.
With reference to Fig. 4 an air-conditioning system will be described in the
following as is used in present-day Airbus and Boeing passenger airplanes, for
ex-
ample the A330/340 and Boe 757/767.
Via flow control valve FCYthe amount of bleed required for supplying fresh
air to the cabin is bled off an engine at about 2 bars and 200°C. In
ground operation
bleed is withdrawn from an auxiliary engine at about 3 bars. The bleed is
first passed
through primary heat exchanger PHX and cooled to about 100°C. Then the
bleed is
compressed further in compressor C to about 4.5 bars and 160°C and
cooled again to
about 45°C in main heat exchangerMHX. The high pressure of 4.5 bars is
necessary
to be able to realize a high degree of dehumidification in the following water
extrac-
tion cycle. This air cycle system is therefore also known as a "high-pressure
water
extraction cycle".
The high-pressure water extraction cycle comprises condenser CON, as pro-
posed in EP 0 019 492 A3, and water extractor WE following condenser CON.
Compressed, cooled bleed is cooled in condenser CON by about dT = -15K, con-
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densed water is then extracted in water extractor WE, and the thus
dehumidified air
is subsequently expanded in turbine T to the cabin pressure of about 1 bar,
the tem-
perature at the turbine outlet being about -30°C. Thus conditioned
bleed, before be-
ing mixed as fresh air with recirculated cabin air in a mixing chamber, is
passed
through condenser CON of the high-pressure water extraction cycle in heat-
exchang-
ing fashion in order to cool the compressed, cooled bleed to the temperature
neces-
sary for water extraction in water extractor WE. Air expanded in turbine T and
cooled is thereby accordingly heated again by dT = +15K to about -15°C.
The conditioned air is then mixed with recirculated cabin air in a mixing
chamber (not shown). Temperature control valve TCYcan be used to increase the
temperature at the turbine outlet to obtain an optimum mixing temperature with
the
admixed, recirculated cabin air. For this purpose part of the bleed precooled
in pri-
mary heat exchanger PHX is branched off and resupplied to the conditioned air
stream after turbine T.
The high-pressure water extraction cycle has, in addition to condenser CON,
heat exchanger REH (reheater) preceding condenser CON. Compressed, cooled
bleed is first passed through heat exchanger REH before entering condenser
CON,
and subsequently the dehumidified air is passed through heat exchanger REH
before
entering turbine T. Heat exchanger REH has substantially the function of
heating the
dehumidified air by about dT = SK and vaporizing residual moisture while
simulta-
neously recovering energy before air enters the turbine. Residual moisture in
the
form of fine droplets can destroy the turbine surfaces since air almost
reaches the
speed of sound in turbine T. A second function of heat exchanger REH is to
relieve
condenser CON by cooling compressed, cooled bleed before it enters condenser
CON by dT = -SK.
It is typical of such an air-conditioning system that the energy gained in
turbine
T is used to drive compressor C, on the one hand, and fan F, on the other. All
three
wheels, that is turbine/compressor/fan, are disposed on a common shaft and
form air
cycle machine ACM, also known as a three-wheel machine Fan F conveys a cooling
air stream branched off from ambient air through a cooling shaft in which
primary
and main heat exchangers PHX, MHX are disposed. Fan F must be driven actively
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by turbine T in particular in ground operation. In flight operation ram air
suffices, it
being optionally throttled by a valve at the cooling shaft inlet.
The overall system is designed for ground operation at an ambient temperature
of 38°C. In order to optimize the effectiveness of the heat-exchange
process in the
cooling shaft, water gained in the high-pressure water extraction cycle is
supplied at
a temperature of about T = 20°C and a pressure of 3.5 bars in the
cooling shaft inlet
in fine droplets to be vaporized therein, thereby improving the effectiveness
of the
heat exchangers.
In case air cycle machine ACM fails completely, for example because the nec-
essary mass flow rate of compressed air is not attainable for fulfilling the
parameters
necessary for the system to work, bypass valve BPV is provided for bypassing
tur-
bine T. In this case check valve CV opens automatically since an overpressure
trig-
gering check valve CV builds up before compressor C as turbine T is not
driven. The
opening of check valve CV causes compressor C to be bypassed or "short-
circuited".
In this state, fresh air is supplied directly through primary and main heat
exchangers
PHX, MHX to the mixing chamber following the air-conditioning system to be
mixed with recirculated cabin air.
As mentioned at the outset, icing in the conditioned fresh air is a problem.
In
order to avoid icing, anti-icing valve AIV is provided for directly branching
off part
of the air bled off the engine and resupplying it to the conditioned air
stream after
turbine T. A further way of avoiding ice is to design the turbine such that no
tem-
peratures below 0°C occur at the turbine outlet. However, this latter
variant requires
much more energy if the same cooling power is to be reached. Therefore, it is
pref
erable to supply hot air at the turbine outlet.
An improved variant of this air-conditioning system provides that air cycle ma-
chine ACM is extended by a second turbine. This makes the three-wheel machine,
turbine/compressor/fan, into a four-wheel machine,
turbine/turbine/compressor/fan
(US 5,086,622). The second turbine is disposed on a common shaft with the
other
wheels in order to recycle the energy gained by the turbines into the air-
conditioning
system, as in the conventional three-wheel system. The second turbine
supplements
the first turbine such that air dehumidified in the high-pressure water
extraction cy-
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cle is expanded in two stages, the condenser of the high-pressure water
extraction
cycle being disposed with the air pipe between the two turbines in heat-
exchanging
fashion. This is more favorable energetically than the conventional structure
of the
air-conditioning system because air exiting the first turbine is comparatively
warm,
preferably above 0°C to avoid ice, and this air is heated in condenser
CON by for
example dT = +15 Kelvin to a comparatively high energy level, so that the
second
turbine can utilize this high energy level to gain energy which gets lost in
the con-
ventional system. This system is known in expert circles as a "condensing
cycle".
A development of the four-wheel machine is described in WO 99/24318 and
generally designated a 2+2-wheel system. The two turbines are accordingly
disposed
on separate shafts, the first turbine with the compressor and the second
turbine with
the fan being on a common shaft in each case.
The problem of the present invention is to adapt the above-described air-condi-
tioning system or method so that it can be designed more flexibly and the
overall
aff ciency optimized more easily, in particular to make it adaptable to the
particular
system requirements more flexibly and therefore better energetically through a
greater number of freely selectable system parameters.
A further problem is to provide an air-conditioning system and method with
which one can reduce icing during air conditioning.
A further problem is to improve overall eff ciency over known systems and
methods.
Yet a further problem is to be seen in increasing overall efficiency in
particular
in flight operation.
These problems are solved by the air-conditioning system and method having
the features stated in the independent patent claims and claims dependent
thereon.
It is essential to the invention that compression of bleed is effected in two
stages. One of the two compression stages procures the energy required for
compres-
sion in conventional fashion by regenerative utilization of energy gained
during ex-
pansion of dehumidified air. For this purpose one of the two compressor wheels
is
disposed on a common shaft with a turbine wheel, for example, so that
compressor
wheel and turbine wheel, optionally with a fan wheel in addition, form a
(first) two-
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or three-wheel air cycle machine. The compressor wheel of the first
compression
stage is preferably disposed on a common shaft with the turbine wheel, but it
can
also be the compressor wheel of the second compressor stage. The other
compressor
wheel can be driven with energy external to the system for example. This makes
it
possible to design the (first) air cycle machine such that the compressor and
turbine
disposed on a common shaft have comparatively high efficiency. This initially
re-
sults in the compressor power of the air cycle machine being below the
compressor
power necessary for bringing the engine air to be conditioned to the pressure
neces-
sary for air dehumidification. The lacking compressor power is therefore
provided
by the additional compressor stage. This permits the air-conditioning system
to be
designed flexibly and the overall e~ciency optimized easily.
Due to the second compressor stage it is in particular possible to produce ice-
free conditioned air. The invention exploits the fact that at a given
temperature the
amount of water condensing out of air increases with rising pressure. Since
the tem-
perature can only be influenced within limits due to the system, in particular
because
compressed engine air cooled in the main heat exchanger cannot be cooled below
ambient temperature in the cooling shaft (designed for 38°C ambient
temperature), a
comparatively high compression pressure of >_ 4.6 bars can be produced with
the
additional compressor stage to reach the desired high degree of condensation
in the
high-pressure water extraction cycle. Freedom from ice is reached e.g. at -
10°C and
1 bar with < 1.8 g of water per kilogram of dry air.
Instead of having a power source external to the system for the additional com-
pressor, one can also operate the latter regeneratively by effecting not only
compres-
sion of bleed but also expansion of dehumidified air in two stages, for
example in
two separate turbines, and utilizing the energy delivered by the turbines for
the first
compressor stage, on the one hand, and for the second compressor stage, on the
other. The air-conditioning system then comprises two machines each having at
least
a compressor wheel and a turbine wheel on a common shaft. Additionally the fan
can be disposed on one shaft and a motor on the other shaft, whereby the motor
can
also be designed as a generator.
Disposing the compressor and turbine wheels on two separate shafts or in two
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separate machines permits much more flexible design of the overall system than
conventional air-conditioning systems. One attains an optimum design in
particular
of compressor and turbine.
Freedom from ice can be obtained without any problem in particular when not
only compression of bleed but also condensation of moisture contained in the
air is
effected in two stages in the high-pressure water extraction cycle. For this
purpose a
first condenser of the high-pressure water extraction cycle is disposed for
heat ex-
change with dehumidified air before the turbine inlet, in case of two-stage
expansion
before the second turbine inlet, and a second condenser of the high-pressure
water
extraction cycle for heat exchange with dehumidified and expanded air after
the tur-
bine outlet, compressed air being passed through said condensers in heat-
exchanging
fashion in order to condense water and then extract it. Effectiveness of
dehumidifi-
cation is increased substantially by two-stage condensation. This holds in
particular
when expansion is also effected in two turbine stages.
When small amounts of ice in the cooling air are no great problem and high ef
ficiency of the overall system is important, it is advantageous to combine two-
stage
compression with two-stage expansion, compressed air being passed in heat-
exchanging fashion through a condenser disposed between first and second
turbines
to extract moisture. Efficiency can be fiu~ther improved if dehumidified air
is guided
past compressed, not yet dehumidified air in heat-exchanging fashion in a
reheater
before entering the first turbine stage. This relieves the condenser, on the
one hand,
since compressed air is precooled before entering the condenser. On the other
hand,
any residual moisture contained in the dehumidified air is vaporized before
the first
turbine inlet so that the turbine surfaces are protected from being destroyed
by water
drops. In terms of efficiency this variant is to be ranked the most favorable.
The invention offers the further advantage that it is possible to switch off
the
additional compressor stage and optionally the turbine stage driving said
compressor
stage by means of suitable bypass circuits. This is useful in particular in
flight op-
eration, when air moisture and freedom from ice of the cooling air play no
part so
that high compression for the high-pressure water extraction cycle is
unnecessary. In
flight operation one can completely switch off one of the two machines by
opening
CA 02313788 2000-07-12
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and/or closing valves, thereby avoiding unnecessary losses and therefore
increasing
efficiency in flight operation.
Designing the air-conditioning system with two separate machines each com-
prising compressor and turbine wheel on a common shaft, one of which can be
switched off in flight operation, offers further advantages resulting from the
fact that
a greater pressure ratio is available in ground operation than in flight
operation due
to the system. This makes it energetically favorable to provide a relatively
small tur-
bine nozzle (baffle screen cross section) in ground operation. Said small
nozzle is
realized by connecting the two turbine stages in series, resulting in a "total
nozzle"
smaller than each individual nozzle. In flight operation about the same volume
flow
is required for air-conditioning the airplane cabin despite a lower available
pressure
ratio, however, so that in flight operation a large nozzle would be necessary
for
about the same air flow. Since one machine and thus one turbine stage is
turned off
for the overall system in flight operation, a large nozzle results by reason
of the sole
remaining turbine of the second turbine stage for the overall system. One can
thus
increase efficiency in flight operation. This gain in efficiency is preferably
utilized
for designing the primary and main heat exchangers with minimal overall size
and
weight, under the constraint that the necessary volume flow rate is just met.
In the
final effect one can thus achieve a smaller overall size and thus lower total
weight of
the air-conditioning system by the measure of providing two machines instead
of
only one machine.
A further advantage resulting when the air-conditioning system has two sepa-
rate machines each with a coupled compressor and turbine is that if one
machine
fails at least the other machine still works and the air-conditioning system
can be
operated further without restriction. With the redundancy required for
aircraft, this
means that one fewer air conditioner or "pack" per aircraft is necessary, for
example
only two packs instead of three. As a consequence, the accordingly lower
number of
components decreases weight, increases the reliability of the installation and
reduces
expense for maintenance and repair.
Finally, it is to be ascertained that both in ground operation with two
machines
and in flight operation with one switched-off machine the energy gained during
ex-
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pansion in the turbine or turbines is largely recovered via the two compressor
stages
(ground operation) or the sole remaining compressor stage (flight operation).
In the following the invention will be described by way of example with refer-
ence to Figures 1 to 3, in which:
Figure 1 shows a diagram of an inventive air-conditioning system,
Figure 2 shows a diagram of an improved embodiment of the system of Fig-
ure 1, in particular for producing ice-free conditioned air,
Figure 3 shows a diagram of an improved embodiment of the system of Fig-
ure 1 with improved efficiency, and
Figure 4 shows an air-conditioning system according to the prior art.
Figure 1 shows an air-conditioning system differing from the air-conditioning
system described in Figure 4 with respect to the prior art substantially in
that two
compressors C1 and C2 are provided in order to bring bleed cooled in primary
heat
exchanger PHX to the pressure necessary for high-pressure water extraction.
Com-
pressors C1 and C2 are to be designed depending on whether freedom from ice or
high efficiency of the air-conditioning system is more important. In Figure 1,
com-
pressor C1 of the first compressor stage together with turbine T and fan F
form
three-wheel machine ACM. That is, compressor C 1 and fan F are driven regenera-
tively by energy gained in turbine T. Compressor C2 of the second compression
stage is operated by separate motor M, i.e. by external energy. Check valve
CYZ
opens automatically when compressor C2 is blocked or when motor M of compres-
sor C2 is not switched off in flight operation for example. Check valve CVl
opens
automatically when air cycle machine ACM is blocked or bypass valve BPY2 is ac-
tively opened.
The air-conditioning system schematically shown in Figure 1 otherwise corre-
sponds fundamentally in structure and function to the system of Figure 4,
whereby it
should be taken into account that the reheater is not absolutely necessary but
of great
advantage in particular in case absolute freedom from ice is to be achieved.
Figure 2 shows a further development of the invention. In the air-conditioning
system shown schematically therein, dehumidified air is expanded in two stages
via
turbines Tl and T2. Energy gained during expansion in turbine Tl is utilized
regen-
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eratively to drive compressor C2, while energy delivered by turbine T2 is
utilized
regeneratively by compressor Cl, as before. In addition to condenser CONl in
the
high pressure dewatering cycle, through which condensed bleed is guided in
heat-
exchanging fashion past dehumidified air expanded in turbine Tl, second
condenser
CON2 is provided through which air precooled in condenser CONI is guided in
heat-exchanging fashion past air expanded by turbine T2. Condensers CONI and
CON2 are especially advantageous when conditioned air is to be free from ice.
Oth-
erwise one can do without condenser CON2, which one does in particular when
high
efficiency of the overall system is to be achieved.
Before air expanded in the first turbine stage enters condenser CONl, water
extractor WE2 is advantageously provided in addition to water extractor WEl
pro-
vided in the high-pressure water extraction cycle. Extracted water is supplied
to ram-
air heat exchangers MHXlPHX to be vaporized therein. Water extractor WE2 is ad-
vantageous in particular when air cycle machine ACM is blocked, since the
effec-
tiveness of first water extractor WEl is greatly restricted here.
Further, one can open economy valve ECV to switch off the high-pressure wa-
ter extraction cycle, which is useful in particular when air cycle machine ACM
fails
and not enough pressure is available for energetically suitable utilization of
the high-
pressure water extraction cycle. Water extraction is then effected at low
pressure by
water extractor WE2. Condensers CONI and CONZ are inoperative in this case.
As in the air-conditioning system described above, one can switch off the ma-
chine comprising turbine Tl and compressor C2 in particular for flight
operation by
opening bypass valve BPYI. By opening bypass valve BPY2 one can also bypass
air
cycle machine ACM, in particular if it fails.
Figure 2 shows optional motor M, with which the efficiency of the system can
be optimized, by dotted lines on the shaft interconnecting turbine Tl and
compressor
C2. One can either make additional energy available to the system. Or, and in
par-
ticular, one can utilize the motor as a generator in order to supply surplus
energy to
the board wiring.
While the air-conditioning system shown in Figure 2 is in particular suitable
for providing ice-free conditioned air, Figure 3 schematically shows an air-
condi-
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tinning system having especially favorable e~ciency. As described with respect
to
Figure 4 (prior art), reheater REH is disposed before turbine Tl and condenser
CON
after turbine Tl in heat-exchanging fashion for compressed air to flow through
and
for condensation of moisture contained therein. Reheater REH can fundamentally
be
omitted, but is advantageous for the reasons stated above. Moisture contained
in
compressed air is condensed in condenser CON at a comparatively high energy
level, in contrast to the prior art described in Figure 4, whereby this energy
can be
utilized in turbine T2 as fundamentally proposed in US 5,086,622. However, in
US
5,086,622 turbines Tl and T2 are disposed jointly with compressor C1 and fan F
on
a common shaft in a single air cycle machine ACM. Since according to the
invention
compression is divided into two stages, and turbine Tl plus compressor C2 and
tur-
bine T2 plus compressor C1 each form separate machines, efficiency can be in-
creased further because the design of the air-conditioning system is
altogether more
variable.
As described in Figure 2, economy valves ECVl and ECVZ serve optionally to
bypass the high-pressure water extraction cycle. By opening bypass valve BPVl
one
bypasses the machine comprising turbine Tl and compressor C2 in flight
operation.
Bypass valve BPY2 accordingly serves to bypass air cycle machine ACM if it
fails.
Both bypass valves can also be used optionally as temperature control valves.
Tem-
perature control valve TCV2 is likewise optional, while temperature control
valve
TCV4 should preferably be provided in the air-conditioning system. As
mentioned
above, one can actually omit reheater REH, but it is advantageous for the
reasons
stated at the outset.
Depending on the system requirement and/or to simplify the system, individual
valves can be omitted, as mentioned above, or they can be partly combined. In
par-
ticular one can for example combine valves ECYI, BPVl and ECYZ into one line
with only one valve, resulting in a less complex system altogether. The
installation is
then optimized for flight operation with a switched-off machine (turbine
Tl/com-
pressor C2).
