High efficiency environmental control systems and methods

A high efficiency environmental control system uses air exhausted from a pressurized cabin to cool compressed air entering the cabin. Air exhausted from the cabin may flow directly from the cabin through a heat exchanger which cools the compressed air entering the cabin. Alternatively, air exiting the cabin may flow through a turbine which expands and cools the air prior to passing through the heat exchanger. The turbine may be used to drive a compressor to provide the compressed air to the cabin. Supplemental power to drive the compressor may be provided by a second turbine driven by bleed air from one or more turbine engines, or by an electric motor. Bleed air may also be mixed with air from the compressor to provide fresh air to the cabin. To further increase efficiency and meet the cooling requirements of the cabin on the ground and at altitude, switch dampers may be employed to selectably vary the flow path of air supplied to the cabin.

TECHNICAL FIELD 
The present invention relates to high efficiency environmental control 
systems for vehicles powered by turbine engines which recover energy from 
air exiting a controlled pressurized space. 
BACKGROUND ART 
A common approach to environmental control system design for vehicles using 
turbo machines is based on the air cycle machine or "pack". The pack 
outputs cool air at a pressure sufficient to move the air through 
distribution ducts and at a rate sufficient to satisfy the fresh air needs 
of the occupants (while also compensating for intentional and 
unintentional outward leaks). The air cycle machine is said to emulate the 
"air-standard refrigeration cycle" found in thermodynamic texts. However, 
differences exist which provide the opportunity for increasing efficiency 
of the traditional air cycle machine approach. 
The typical air cycle machine extracts an amount of "bleed air" from the 
engine core compressor ports, such as the eighth stage (low stage bleed 
air) or fourteenth stage (high stage bleed air). The bleed air is 
compressed or pressurized and as a result heated to a very high 
temperature relative to ambient. The bleed air is then passed through a 
compressor which further increases the temperature and pressure. Ram air 
flow is used to cool the compressed bleed air before it is expanded and 
further cooled in a turbine to an appropriate temperature and pressure to 
operate a water separateor and cool the controlled space, generally 
referred to herein as the cabin. 
In terms of engine fuel consumption, bleed air is very expensive. For 
example, in a typical aircraft application cruising at 35,000 feet, one 
pound per second (pps) of low stage bleed air costs the same as 158 
kilowatts (211 hp) of shaft power extracted from a gearbox coupled to the 
engine: 1.2% specific fuel consumption (SFC). Furthermore, fan air used to 
precool the bleed air before passing it to the compressor may add an 
additional 0.52% SFC. As such, it is desirable to reduce or eliminate the 
use of bleed air to increase efficiency of the environmental control 
system. Alternatively, it is desirable to fully utilize the energy of the 
bleed air. 
For aircraft applications, cabin or fuselage pressure is regulated by 
restricting outflow of air through an outflow valve. The outflow valve has 
an open area modulated to provide a desired pressure. To maintain a 
constant cabin pressure level, the rate of air supplied must equal the 
rate of air leaked plus the rate of air exhausted through the outflow 
valve. In an attempt to recover energy from the outflow air by converting 
it to forward thrust, the outflow valve is carefully shaped to form a 
converging-diverging supersonic nozzle since the pressure ratio allows an 
exit Mach number of about 1.6 in a single process. The actual savings in % 
SFC are difficult to isolate and measure due to various factors, such as 
the possibility of creating an adverse yaw moment which requires rudder 
compensation thereby leading to additional aerodynamic drag. As such, it 
is desirable to provide an alternative approach to energy recovery to 
improve the efficiency of the environmental control system. 
DISCLOSURE OF THE INVENTION 
A general object of the present invention is to improve the efficiency of 
an environmental control system for a pressurized cabin, such as those 
typical of aircraft. This object is accomplished by recovering energy from 
the air exhausted from the cabin using a variety of alternative 
environmental control system configurations. 
In carrying out the above object and other objects, features, and 
advantages of the present invention, a high efficiency environmental 
control system is disclosed. The system includes a heat exchanger which 
uses air exhausted from the cabin to cool air supplied to the cabin. The 
system may be configured in any of a number of alternative embodiments to 
realize the features and advantages of the present invention to recover 
energy from air exhausted from a pressurized space. Various embodiments 
employ alternative configurations which are selectively utilized based on 
current cooling demands to further improve system efficiency. In one 
embodiment, a heat exchanger having a first inlet and corresponding first 
outlet is placed in fluid communication with the outflow port of the 
cabin. The heat exchanger includes a second inlet and corresponding second 
outlet in fluid communication with the inflow port of the cabin. As such, 
air exhausted from the cabin passes through the first inlet and outlet of 
the heat exchanger to cool air supplied to the cabin through the second 
inlet and outlet to increase efficiency of the system. When cabin pressure 
is insufficient to attain a desired cooling level of air supplied to the 
inflow port of the cabin, a fan interposed the first inlet of the heat 
exchanger and the outflow port of the cabin may be used for moving air 
through the heat exchanger. 
In a number of embodiments, air exiting the outflow port is passed through 
a turbine interposed the cabin and the heat exchanger such that the 
turbine expands and cools the air exhausted from the cabin to further cool 
the air supplied to the cabin through the second inlet and outlet of the 
heat exchanger. The turbine may be used to drive a compressor which is 
mechanically coupled to the turbine. The compressor receives ram air and 
compresses the ram air (relative to ambient) to provide compressed air to 
the compressor outlet. The compressed air is passed through the heat 
exchanger where it is cooled prior to entering the controlled space or 
cabin. To supply additional power for driving the compressor, one 
embodiment uses a second turbine which is mechanically coupled to the 
compressor. The second turbine receives bleed air from at least one engine 
and has an outlet in fluid communication with the heat exchanger. 
Alternatively, a motor may be coupled to the compressor for providing 
energy to drive the compressor. The motor receives electrical power from a 
generator coupled to one of the engines. 
A number of advantages are associated with the present invention. For 
example, the present invention recovers energy from air exhausted from the 
cabin to improve efficiency of the environmental control system. The 
present invention provides a number of alternatives to reduce or eliminate 
the use of bleed air from one or more turbine engines. While particularly 
suited for aircraft applications, the present invention may be utilized 
for any application having a pressurized space or cabin. 
While embodiments of this invention are illustrated and disclosed, these 
embodiments should not be construed to limit the claims. It is anticipated 
that various modifications and alternative designs may be made without 
departing from the scope of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1, an environmental control system (ECS), indicated 
generally by reference numeral 10, is shown. ECS 10 provides fresh, 
pressurized or compressed air to a controlled space such as cabin 12. As 
used herein, cabin refers to the entire volume within the pressurized 
shell of an enclosed space such as the fuselage of an aircraft. In 
general, cabin 12 includes one or more inflow ports, indicated generally 
by reference numeral 14, for supplying fresh, pressurized air to cabin 12. 
The pressure within cabin 12 may be regulated by restricting the air 
exiting cabin 12 through an outflow port, indicated generally by reference 
numeral 16. Preferably, air passing through outflow port 16 is restricted 
by varying turbine parameters, as described in greater detail herein. 
However, an outflow valve may be used where required in combination with 
varying turbine inlet geometry. At a constant cabin pressure, the rate of 
air supplied through inflow port 14 must be equal to the air exhausted 
from cabin 12 through outflow port 16 in addition to air lost through 
leaks, indicated generally by reference numeral 18. The leaks represent 
both intentional and unintentional outward flow of air such as may occur 
through door seals, or pressure shell penetrations by tubes, wires, and 
the like. Throughout the drawings, double arrows are used to indicate air 
exhausted to atmosphere, or ambient. 
ECS 10 is powered by at least one turbine engine 20. In one embodiment of 
the present invention, three such turbine engines are used to power an 
aircraft. Ram air, indicated generally by reference numeral 22, enters 
engine 20 through fan 24 before passing through multi-stage compressor 26. 
The compressed air is fed to a combustion chamber 28 where it is mixed 
with fuel from a fuel source 30. The combustion process is used to drive 
turbine 32 which is mechanically coupled by shaft 34 to compressor 26. The 
jet exhaust then passes to atmosphere as indicated generally by reference 
numeral 36. 
A transmission or gear box 38 is mechanically coupled to shaft 34 to 
selectively drive a generator 40 which generates electrical power, 
generally indicated by reference numeral 42, to meet the electrical 
demands of the vehicle. 
Air may be extracted from engine 20 from fan 24 as indicated by reference 
numeral 44 or from a bleed port 46. Reference numeral 46 represents both 
high stage, and low stage bleed air. Typically, low stage bleed air is 
extracted from the eighth stage of multi-stage compressor 26 whereas high 
stage bleed air is extracted from the fourteenth stage of multi-stage 
compressor 26. The embodiment illustrated in FIG. 1 may be referred to as 
a minimum bleed configuration since the bleed air extracted is reduced 
from the prior art requirement of about 7.5 pounds (mass) per second (pps) 
to about 4 pps. 
Bleed air (high stage or low stage) is extracted from bleed port 46 and 
passes through a pre-cooler 48 where it is cooled by fan air 44. 
Preferably, pre-cooler 44 is a cross-flow, non-mixing, air-to-air heat 
exchanger which exhausts the cross-flow air to atmosphere as indicated by 
reference numeral 50. Due to the reduced flow requirements, pre-cooler 48 
need reject only about 345 kw as opposed to the prior art design which 
requires heat rejection of about 640 kw. As such, less air is required 
from fan 24 to achieve the required cooling. This may contribute to 
savings in percent SFC as described herein. 
A portion of the air exiting pre-cooler 48 is diverted as indicated by 
reference numeral 52 to provide anti-ice functions for the aircraft. The 
remaining air flow is directed into one or more packs 54 for cooling prior 
to being supplied to cabin 12. Depending upon the particular application, 
an aircraft may have one pack associated with each engine, multiple 
engines associated with a single pack, or multiple packs associated with 
one or more engines. For purposes of describing the present invention, 
pack 54 represents one or more packs which satisfy the cooling and fresh 
air requirements for cabin 12. Air entering pack 54 may be routed through 
a first path, indicated generally by reference numeral 56, or a second 
path, indicated generally by reference numeral 58, depending upon the 
particular cooling demand and pressure differential between cabin 12 and 
ambient. The path indicated by solid lines 56 represents the path used to 
provide maximum cooling to an aircraft on a hot, humid day during a sea 
level climb. 
As indicated in FIG. 1, path 56 routes bleed air from pre-cooler 48 to heat 
exchanger 60. Heat exchanger 60 includes a first inlet and corresponding 
first outlet in fluid communication with outflow port 16 of cabin 12. Heat 
exchanger 60 also includes a second inlet and corresponding second outlet 
in fluid communication with inflow port 14 of cabin 12. As such, air 
exhausted from cabin 12 passes through the first inlet and outlet of heat 
exchanger 60 to cool air supplied to cabin 12 through the second inlet and 
outlet to increase the efficiency of the system. 
As indicated in FIG. 1, heat exchanger 60 may be alternatively placed in 
fluid communication with inflow port 14 of cabin 12 through water 
separator 62 either directly, as indicated by dashed line 64, or through 
turbine 66 depending upon the cooling demand of cabin 12. 
A compressor 68 receives ram air 22 through a compressor inlet to provide 
compressed air at the compressor outlet which is mixed with bleed air from 
engine 20 before entering second inlet of heat exchanger 60. Compressor 68 
is mechanically coupled to and driven by turbine 66. Compressor 68 may 
also be driven by a second turbine 70 which is mechanically coupled 
thereto via shaft 72 when operating in the high efficiency mode, such as 
during cruising at altitude. Second turbine 70 is driven by bleed air from 
engine 20 as indicated generally by reference numeral 58. As such, turbine 
70 is selectively mechanically coupled to compressor 68 when functioning 
in the maximum efficiency or economy mode. The outlet of turbine 70 is 
then in fluid communication with the outlet of compressor 68 where air is 
mixed prior to entering the second inlet of heat exchanger 60. 
To supply additional air flow through the first and second inlet and outlet 
of heat exchanger 60, ram air may be provided during economy mode through 
path 74. For maximum cooling, a low pressure fan 76 may be used to extract 
available cooling effect from air exiting cabin 12 through outflow port 
16. In general, on a hot, humid day, the air exiting cabin 12 contains a 
significant amount of cooling effect which may be exploited via heat 
exchanger 60. Low pressure fan 76 is used to provide a sufficient air flow 
when a reduced pressure ratio exists between cabin 12 and atmosphere or 
ambient, such as typically occurs at or near sea level. Air exiting the 
first outlet of heat exchanger 60 is exhausted to ambient as indicated 
generally by reference numeral 78. 
The minimum bleed configuration illustrated in FIG. 1 is based on providing 
the majority, if not all, of the cabin air from compression of ram air 
through compressor 68. The duty of such a compressor is constant in mass 
flow rate and requires pressure rises far smaller than those produced at 
the bleed stage of the engines. This approach recovers as much energy as 
possible from air exiting cabin 12 through outflow port 16. In the maximum 
economy mode, this air is passed through turbine 66 whose power is used to 
partially supply the energy to drive compressor 68 via shaft 80. As stated 
above, the outflow air has considerable available energy with respect to 
the outside ambient in most flight regimes other than at low altitudes. 
For example, considering a typical application at 35,000 feet altitude, a 
turbine 66 having an adiabatic efficiency of 80% would be able to recover 
29.5 kw for each pps of air flow. For an inflow rate of about 6 pps at 
35,000 feet, and a leak rate of about 1 pps, the energy available from the 
outflow air of about 5 pps can be recovered which amounts to around 150 
kw. Use of turbine 66 also provides another serendipitous effect: the air 
exiting turbine 66 is significantly cooler than ambient air such that it 
serves as an excellent heat sink for the heat exchange process of pack 54. 
The additional power to drive compressor 68 may be provided by the 
selectably engageable turbine 70 or alternatively by a motor, as 
illustrated and described herein. 
For the configuration illustrated in FIG. 1, representative calculations 
result in a required effectiveness for heat exchanger 60 of about 0.74 as 
opposed to the required effectiveness of the prior art configuration which 
exceeds 0.90. As such, heat exchanger 60 can be smaller than the existing 
heat exchanger resulting in attendant savings in manufacturing costs and 
fuel cost during operation. 
At low altitudes in hot conditions, the high efficiency configuration 
indicated by dashed lines in FIG. 1 is modified to the maximum cooling 
configuration as indicated by the double solid lines of FIG. 1. Under 
these conditions, the exhaust air from the cabin still provides a better 
heat sink than the ambient air so low pressure fan 76 is included to move 
the exhaust air through heat exchanger 60. To achieve the low temperatures 
needed to dehumidify and handle the cooling loads, ram air is compressed 
by compressor 68 up to the bleed pressure and merged with bleed air prior 
to being cooled in heat exchanger 60. Turbine 66 is then used to expand 
and cool the supply air prior to entering water separator 62 and 
distribution ducts within cabin 12. 
Computer simulations have been performed assuming isentropic efficiency of 
80% for all turbo machines and 90% for the combined efficiency of gear box 
38 and generator 40. The parameters (heat rate, effectiveness, and 
log-mean temperature difference) for heat exchanger 60 operations were 
determined and operating conditions adjusted as required to match 
predetermined boundary conditions. In this embodiment, a model CF6-80 jet 
engine manufactured by General Electric was utilized. Other assumptions 
used for the simulation were that the fan bleed generated by fan 24 for 
pre-cooler 48 was reduced proportional to the reduction in the rate of 
bleed air exiting port 46 and that a constant electric load of 165 kw was 
provided for miscellaneous electrical loads. This configuration was 
analyzed for climb and descent flight regimes at sea level, 10K, 20K, 30K, 
and 40K feet and cruise at 43K feet. The computer simulations indicate a 
potential fuel savings of 3.31% SFC at cruise levels. 
Referring now to FIGS. 2 and 3, another embodiment of an environmental 
control system according to the present invention is shown. This 
embodiment may be referred to as an "all-electric" configuration since the 
use of bleed air from engine 20 has been eliminated. FIG. 2 illustrates 
the all-electric configuration having flows selectively arranged for 
maximum economy, while FIG. 3 illustrates the flows to achieve maximum 
cooling. Likewise, FIG. 2 is described using representative calculations 
for a 35,000 foot cruise, while FIG. 3 uses representative calculations 
for a hot day sea level climb. Like reference numerals indicate similarity 
of function for components illustrated and described with reference to 
FIG. 1. One of ordinary skill in the art will recognize that various flow 
rates, heat exchanger parameters, and other such considerations will vary 
depending upon the particular application. As such, the representative 
numbers found throughout the specification are provided to establish the 
viability of the various configurations under different operating 
conditions. The actual numbers may be ascertained for each particular 
application by one of ordinary skill in the art. For example, while 
pre-cooler 48 of FIGS. 2 and 3 appears schematically identical to 
pre-cooler 48 of FIG. 1, the pre-cooler of FIGS. 2 and 3 must provide less 
heat rejection since the bleed air has been eliminated from the ECS 
function. As such, bleed air through pre-cooler 48 of FIGS. 2 and 3 is 
provided only for the anti-ice function, indicated generally by reference 
numeral 52. 
The all-electric configuration illustrated in FIGS. 2 and 3 may require a 
larger gear box 38 and generator 40 than the corresponding components of 
FIG. 1 to accommodate the increased electrical demand. For the maximum 
economy mode illustrated in FIG. 2, generator 40 must provide 165 kw of 
electrical power to cabin 12 in addition to approximately 143 kw of 
electrical power for motor 100 which is mechanically coupled to compressor 
68 via shaft 72. Due to the approximate conversion efficiency of 90% used 
for motor 100, the 143 kw of electrical power is converted to about 129 kw 
of mechanical power transmitted to compressor 68 via shaft 72. 
In the configuration of FIG. 2, compressor 68 pressurizes ram air 22 which 
passes through heat exchanger 102 where it is cooled using air exhausted 
from cabin 12 through outflow port 16. Turbine 66 is mechanically coupled 
to compressor 68 via shaft 80. Turbine 66 includes an inlet in fluid 
communication with outflow port 16 of cabin 12 and an outlet in fluid 
communication with a first inlet and corresponding first outlet of heat 
exchanger 102 to cool the air exiting compressor 68 and passing through a 
second inlet and corresponding second outlet of heat exchanger 102. 
Turbine 66 expands and cools air exhausted from cabin 12 to generate about 
156 kw of mechanical power to drive compressor 68 via shaft 80. If 
required, additional ram air 22 may be provided via path 74 to the first 
inlet and outlet of heat exchanger 102 to increase the heat rejection. For 
this configuration, heat exchanger 102 rejects about 293 kw of thermal 
energy but requires an effectiveness of about 0.93 to accommodate the 
maximum cooling regime illustrated in FIG. 3. 
To provide maximum cooling on a hot day at low altitude, the configuration 
of FIG. 2 is modified via appropriate mechanization to achieve the 
configuration of FIG. 3. Under these conditions, generator 40 must provide 
about 235 kw of electrical power to motor 100 in addition to the 165 kw of 
electrical power required for various equipment within cabin 12. As such, 
generator 40, motor 100, and gear box 38 should be sized accordingly. In 
this configuration, compressor 68 receives ram air 22 and delivers 
pressurized air to second inlet and corresponding second outlet of heat 
exchanger 102, similar to the configuration of FIG. 2. However, the second 
outlet of heat exchanger 102 is in fluid communication with the inlet of 
turbine 66 which expands and cools the air prior to entering water 
separator 62 and passing through inflow port 14 of cabin 12. Fan 76 is 
used to provide additional air flow through first inlet and outlet of heat 
exchanger 102 since the pressure differential between cabin 12 and 
atmosphere is insufficient to provide the necessary heat rejection, 
approximately 370 kw, by heat exchanger 102. Motor 100 provides about 211 
kw of mechanical power to drive compressor 68 via shaft 72. Compressor 68 
is also driven via shaft 80 by turbine 66 which provides an additional 181 
kw of mechanical power. Fan 76 requires about 2 kw of electrical power 
which is incorporated into the power demand of the cabin equipment. 
As such, the environmental control system illustrated in FIGS. 2 and 3 
includes compressor 68 which receives ram air 22 through a compressor 
inlet to provide compressed air at the compressor outlet. Heat exchanger 
102 includes a first inlet and corresponding first outlet for cooling air 
from compressor 68 which passes through a second inlet and corresponding 
second outlet. The first inlet of heat exchanger 102 is in fluid 
communication with outflow port 16 of cabin 12 when the system is in the 
maximum cooling mode while being in communication with the outlet of 
turbine 66 when the system is in the maximum economy mode. Turbine 66 is 
mechanically coupled to compressor 68 and includes an inlet in fluid 
communication with the second outlet port of heat exchanger 102 when the 
system is in the maximum cooling mode and in fluid communication with 
outflow port 16 when the system is in the maximum economy mode. Turbine 66 
also includes an outlet in communication with inflow port 14 of cabin 12 
when the system is in the maximum cooling mode, but in fluid communication 
with the second inlet of heat exchanger 102 when the system is in the 
economy mode. As such, heat exchanger 102 recovers available energy from 
air exhausted through outflow port 16 to cool air supplied to cabin 12 
through inflow port 14. This configuration includes motor 100 which is 
mechanically coupled to compressor 68 for driving the compressor while 
functioning in both the maximum cooling mode and the economy mode. 
Referring now to FIG. 4, another embodiment of an environmental control 
system according to the present invention is shown. The hexagons of FIG. 4 
are used to identify representative values for temperatures and pressures 
of air at various points throughout the system as summarized in Table 1 
below. 
TABLE 1 
______________________________________ 
FULL BLEED FLOW CONFIGURATION 
43,000 FEET CRUISE 
Symbol P (psia) 
T (.degree.R) 
______________________________________ 
1 29.3 856.9 
2 13.7 725.4 
3 3.38 402.1 
4 2.35 649.1 
A 2.35 416.3 
C 10.92 535.0 
D 11.45 520.0 
P.sub.1 4.78 503.9 
P.sub.2 4.67 929.8 
V 10.85 520.0 
______________________________________ 
As indicated in Table 1, this configuration may be referred to as a "full 
bleed flow configuration". This arrangement provides electrical recovery 
of both outflow air and bleed air energy which is eventually converted 
back to mechanical energy provided to engine 20 through a motor 110. In 
this configuration, a first turbine 112 is interposed cabin 12 and heat 
exchanger 114 such that the inlet of turbine 112 is in fluid communication 
with outflow port 16 of cabin 12, while the outlet of turbine 112 is in 
fluid communication with the first inlet and corresponding first outlet of 
heat exchanger 114. Turbine 112 expands and cools air exhausted from cabin 
12 to further cool air passing through second inlet and corresponding 
second outlet of heat exchanger 114 prior to entering inflow port 14 of 
cabin 12. Turbine 112 is mechanically coupled to an electric generator 116 
via shaft 118 to convert about 145 kw of mechanical power to about 131 kw 
of electrical power, assuming an efficiency of about 90%. The electrical 
power is used to supply the power requirements for cabin 12 while excess 
power is fed to motor 110 to help drive compressor 26 via shaft 34 and 
gear box 38. A second turbine 120 receives bleed air from bleed port 46 of 
engine 20. Turbine 120 is mechanically coupled to generator 122 to convert 
mechanical energy from the turbine to electrical energy for cabin 12 
and/or motor 110. Assuming the representative numbers provided in Table 1, 
turbine 120 provides approximately 200 kw of mechanical power to generator 
122 via shaft 124 which is converted to about 180 kw of electrical power. 
Air exiting the outlet of turbine 120 passes through the second inlet and 
corresponding second outlet of heat exchanger 114 prior to entering cabin 
12 through inflow port 14. Based on the representative numbers provided in 
Table 1, the effectiveness of the heat exchanger 114 should be about 0.76. 
As with the embodiments illustrated in FIGS. 1-3, heat exchanger 114 is 
preferably a cross-flow, non-mixing, air-to-air heat exchanger. While not 
specifically illustrated, this embodiment also includes a water separator 
placed between heat exchanger 114 and inflow port 14 of cabin 12. 
Based on the representative numbers of Table 1, the configuration of FIG. 4 
indicates a potential fuel savings of about 0.91% SFC for a typical 
application cruising at 43,000 feet altitude. 
Referring now to FIG. 5, another embodiment of an environmental control 
system according to the present invention is shown. The configuration of 
FIG. 5 is an alternate all-electric configuration which does not use bleed 
air for the ECS function. While pre-cooler 48 of FIG. 4 appears 
schematically identical to pre-cooler 48 of FIGS. 1-4, the pre-cooler of 
FIG. 5 provides less heat rejection since the bleed air has been 
eliminated from the ECS function. As such, bleed air through pre-cooler 48 
of FIG. 5 is provided only for the anti-ice function, indicated generally 
by reference numeral 52. Generator 130 converts about 445 kw of mechanical 
power to about 400 kw of electrical power to supply 165 kw of electrical 
power to cabin 12 and about 235 kw of electrical power to motor 132. 
Turbine 70 is mechanically coupled and driven by motor 132 via shaft 134. 
Turbine 70 is also mechanically coupled to compressor 68 via shaft 72. A 
first directional valve 136 operates in conjunction with a second 
directional valve 138 to selectively place turbine 70 in fluid 
communication with heat exchanger 142. This provides additional cooling in 
a maximum cooling mode where energy recovery from the outflow air is 
insufficient to meet the cooling requirements for cabin 12. 
As also illustrated in FIG. 5, a mixing valve 140 is provided to supply ram 
air via path 144 to heat exchanger 142 where necessary. The ram air is 
combined with air exhausted from the outlet of turbine 66 prior to passing 
through heat exchanger 142. To improve efficiency of the ECS when cooling 
demand is lower, directional control valves 136 and 138 are actuated to 
remove turbine 70 from the flow path. As such, heat exchanger 142 is in 
fluid communication with inflow port 14 of cabin 12 through a water 
separator (not specifically illustrated). As with FIG. 4, representative 
numbers for the various states indicated by the hexagons of FIG. 5 are 
summarized in Table 2 below. 
TABLE 2 
______________________________________ 
ALL ELECTRIC CONFIGURATION 
SEA LEVEL CLIMB 43,000 FEET CRUISE 
Symbol P (psia) 
T (.degree.R) 
P (psia) 
T (.degree.R) 
______________________________________ 
1 37.0 588 -- -- 
2 17.1 495 -- -- 
3 38.0 778.0 13.22 734.2 
4 38.0 778.0 13.22 734.2 
5 572.4 371.8 
6 572.4 371.8 
7 762.8 665.6 
A 14.7 562.7 2.35 416.3 
C 14.7 535 10.92 535 
D 17.1 495 12.22 483.6 
P 125.7 1119.1 34.82 999.5 
P.sub.1 19.27 640.0 4.70 503.9 
P.sub.2 17.14 754.2 4.67 929.8 
R 16.22 578.8 3.66 472.7 
V 14.63 570 
______________________________________ 
Referring now to FIGS. 6 and 7, another embodiment of an environmental 
control system according to the present invention is shown. FIG. 6 
illustrates the flow path for a maximum cooling mode of operation while 
FIG. 7 illustrates the flow path for a maximum economy mode of operation. 
The flow path is altered using one or more switch dampers such as first 
switch damper 150, second switch damper 152, and third switch damper 154. 
Representative numbers for pressures and temperatures throughout the 
system are indicated by the hexagons in the Figures and summarized below 
in Table 3 for various operating conditions. 
TABLE 3 
______________________________________ 
MINIMUM BLEED AIR CONFIGURATION PRESSURES AND 
TEMPERATURES 
SEA 43,000 FEET CRUISE 
43,000 
LEVEL CLIMB MAX ECONOMY FEET CRUISE 
Symbol 
P (psia) 
T (.degree. R) 
P (psia) 
T (.degree. R) 
P (psia) 
T (.degree.R) 
______________________________________ 
1 56.6 642.7 29.26 856.9 34.82 999.5 
2 17.3 495.0 13.22 717.7 13.22 806.2 
3 57.6 894.4 13.22 734.2 13.22 734.2 
4 57.6 842.6 728.2 13.22 13.22 757.5 
5 14.9 572.4 2.55 372.8 2.55 371.8 
6 14.9 572.4 2.55 371.8 2.55 371.8 
7 772.8 666.1 657.5 
A 14.7 562.6 2.35 416.3 2.35 416.3 
B 57.60 798.4 29.26 856.9 34.82 999.5 
C 14.78 535 10.92 535 10.92 535 
D 17.10 41.07 12.22 483.6 12.24 520 
P 125.7 1119.1 34.82 999.5 34.82 999.5 
P.sub.1 
19.27 640.0 4.78 503.9 4.78 503.9 
P.sub.2 
17.14 754.2 4.67 929.8 4.67 92.95 
R 16.22 578.8 3.66 472.7 3.665 472.6 
V 14.63 570 10.85 510 10.85 5.0 
______________________________________ 
As illustrated in FIGS. 6 and 7, switch dampers 150, 152 and 154 are used 
to selectively couple the inlet of turbine 70 either directly to the bleed 
air from engine 20 or to the second outlet of heat exchanger 160. At the 
same time, the switch dampers also selectively couple the outlet of 
turbine 70 to either inflow port 14 of cabin 12 or the second inlet port 
of heat exchanger 160. Otherwise, the configuration illustrated in FIGS. 6 
and 7 is similar to the minimum bleed configuration illustrated in FIG. 1. 
The switch dampers function to reroute the flows such that, when needed, 
the bleed and compressed ram flows can be combined and then pre-cooled in 
heat exchanger 160. Further expansion through turbine 70 brings the 
temperature down to the 35.degree. F. needed to support the water 
separation function performed by a water separator (not specifically 
illustrated). The switch dampers are needed only for the infrequent 
low-altitude and ground-cooling modes such that they are designed to be 
low loss devices when in the "straight-through" mode illustrated in FIG. 
7. 
Fan 76 is shown in parallel with a check valve 162 to accommodate those 
cases where the cabin pressure is insufficient to support the exhaust of 
conditioned air from cabin 12. Depending upon the particular application, 
the air driven by fan 76 may bypass turbine 66. 
For all the embodiments illustrated and described which utilize a turbine 
in fluid communication with the outflow port 16 of cabin 12, every attempt 
should be made to make the turbine variable or controllable to provide a 
means of regulating the outflow rate at a particular pressure condition so 
as to regulate the rate of change of cabin pressure for a given inflow 
rate from the one or more packs. This may be accomplished by using 
adjustable or controllable inlet guide vanes or first-stage stator blades 
in the turbine. In an ideal case, such an arrangement makes an outflow 
valve unnecessary. However, an outflow valve may be provided in parallel 
to pass a small, variable amount of air to the ambient to accomplish cabin 
pressure control. Use of the outflow valve should be minimized, however, 
since any air exhausted through the outflow valve represents lost work. 
While embodiments of the invention have been illustrated and described, it 
is not intended that such disclosure illustrate and describe all possible 
forms of the invention. It is intended that the following claims cover all 
modifications and alternative designs, and all equivalents, that fall 
within the spirit and scope of this invention.