Refrigeration – Processes – Reducing pressure on compressed gas
Reexamination Certificate
2001-03-02
2002-10-08
Doerrler, William C. (Department: 3743)
Refrigeration
Processes
Reducing pressure on compressed gas
C062S402000, C062S415000, C062S275000
Reexamination Certificate
active
06460353
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to environmental control systems and, in particular, to aircraft air conditioning and thermal management.
An aircraft environmental control system typically consists of an engine bleed air-driven high pressure air cycle system which provides conditioned, temperature-controlled, dehumidified air for cockpit and crew member cooling, pressurization, cooling of air-cooled avionics, cooling of liquid-cooled equipment (such as radar) and various other pneumatic utility subsystems. An example of a conventional prior art environmental control system adapted for military fighter aircraft use is depicted in FIG.
1
. The air cycle refrigeration system
10
includes an air cycle machine
24
, primary heat exchanger (not shown), secondary heat exchanger
13
, condenser heat exchanger
15
, water extractor
16
, reheater
14
, liquid-to-air load heat exchanger
18
, together with sensors, valves and associated controls (omitted for clarity).
In operation, the compressor
12
driven by turbine
17
compresses preconditioned engine bleed air
11
with the heat of compression subsequently rejected to ambient air through the secondary heat exchanger
13
. The cooling turbine
17
portion of the air cycle machine
11
extracts energy from the preconditioned bleed air
11
and chills the air to typically subfreezing temperatures (e.g., ~40 deg F.). This air is delivered to the hot side inlet of the condensing heat exchanger
15
which cools a cross-stream airflow output from the reheater heat exchanger
14
, condensing entrained moisture into droplets which are removed in the water extractor
16
. This airflow then passes through the reheater
14
and is delivered to the cooling turbine
17
inlet where the air is expanded through the turbine
17
, giving up energy and is cooled in the process. This cooled air is further temperature regulated by the addition of hot bypass air through a temperature control valve (not shown) to provide temperature-controlled, dehumidified conditioned air
19
.
A complication arises during operation when moisture is present as the air is cooled to below subfreezing temperatures since entrained water present in the air stream condenses into a fine spray of ice crystals. The ice entrained in the air stream will begin to accrete on downstream surfaces and, in particular, on the cold side inlet face of the condensing heat exchanger
15
and, if left unchecked, will back-pressure the turbine
17
and choke off flow.
To prevent excessive ice accumulation, anti-ice provisions, such as internal hot-air hot bars
23
incorporated into the condenser
15
face, are typically employed. Commonly, de-ice provisions are also provided consisting of a hot air bypass or anti-ice valve
22
to allow the hot bleed air
11
to bypass the cooling turbine
17
to melt any accumulated ice once a preset temperature and/or pressure drop is exceeded. The addition of heat to melt accumulated ice substantially reduces the available cooling capacity of the refrigeration unit and it is therefore desirable to minimize the add-heat function to the extent practicable.
Cooling of liquid-cooled loads is typically accomplished as shown in
FIG. 1
by means of an added thermal transport loop connected to the refrigeration unit
10
by coolant lines
20
and
21
with suitable pumping means (not shown) to a remotely located liquid-cooled load(s). Waste heat from liquid-cooled load(s) such as radar is rejected to the liquid-to-air load heat exchanger
18
disposed either upstream or downstream of the condenser
15
in a series arrangement. This additional heat often presents a severe performance penalty and must be carefully considered in the design to avoid degrading condenser operation and, hence, water removal and to avoid undercooling of the cockpit or air-cooled equipment.
The series arrangement of condenser
15
in front of the liquid-to-air load heat exchanger
18
as shown in
FIG. 1
creates two main performance problems. First, the coldest inlet air the downstream heat exchanger
18
experiences is limited by the minimum outlet temperature of the upstream heat exchanger
15
. As a result, the performance of the downstream heat exchanger
18
is often less than optimal or desired, resulting in elevated liquid supply temperatures (e.g., in excess of 110 deg F. for a ~9 kw load in a typical case). For refrigeration packs with the condenser
15
located upstream of the liquid-to-air load heat exchanger
18
, the liquid heat load that can be rejected is limited by the maximum air temperature of cooling air delivered to the cockpit and/or air-cooled avionics equipment. In the case of a liquid-to-air load heat exchanger
18
located upstream of the condenser
15
, condensing operation may be degraded to an unacceptable degree as a result of high cold side inlet temperatures such that efficient condenser
15
operation no longer occurs. This, in turn, results in excessive humidity and moisture delivered to the cockpit and/or air-cooled equipment and increasing sensible heat.
The second performance problem associated with prior art environmental control systems is inadequate ice control and removal. The prior art approach in high pressure air cycle systems is to reduce the amount of entrained moisture entering the cooling turbine
17
by means of a condenser
15
and a swirl type inertial water extractor
16
. The amount of water removed is dependent on the internal surface metal temperature of the condenser
15
, i.e., the lower the temperature, the larger the condensed droplets. Not all of the water is necessarily removed, however, particularly under low altitude, moist, tropical day conditions. This entrained moisture condenses into ice crystals as the air is expanded through the cooling turbine
17
to below freezing temperatures. The resultant ice discharged from the cooling turbine
17
tends to accrete on chilled surfaces of downstream ducting and the inlet face of the downstream heat exchanger
15
, eventually freezing over the heat exchanger inlet and interrupting airflow unless de-ice or anti-ice control provisions are incorporated. Operation of de-ice or anti-ice controls, however, directly subtracts from the inherent refrigeration capacity of the cooling turbine
17
. Use of hot air de-ice should therefore be minimized to avoid excessive conditioned air supply temperatures. A further difficulty arises when ice that is allowed to accumulate and then melt as the resultant slug of liquid water is introduced in the conditioned air stream, necessitating additional drainage provisions.
As may be seen from the foregoing discussion, there is a need for an environmental control system that provides improved efficiency and anti-ice control.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a heat exchanger subsystem for an environmental control system comprises a heat exchanger array having a plurality of heat exchanger elements that operate in parallel to an inlet airflow to the heat exchanger array such that each heat exchanger element is thermally connected to a separate load that is thermally independent or isolated from other loads. In particular, an environmental control system is disclosed comprising a heat exchanger subsystem downstream of an air cycle machine cooling turbine, with the subsystem having an air-to-air heat exchanger and a liquid-to-air heat exchanger which operate in parallel with each other when connected to an inlet airflow to the heat exchanger subsystem. The heat exchanger subsystem enables accommodation of relatively large liquid-cooled loads without degradation of cooling or interaction with air-cooled loads.
In another aspect of the invention, an anti-ice control subsystem having an array of parallel heat exchange elements or hot bars is disclosed for use in conjunction with a heat exchanger of relatively narrow coldside air passage fin spacing such as a liquid-air heat exchanger external to and upstream of the liquid-air core. The anti-ice control subsystem enables subfreezing operation of the liqu
Brager Ron
Reed Larry
Udobot Aniekan
Doerrler William C.
Zak, Jr. Esq. William J.
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