Patent Description:
Vehicles such as aircraft generally utilize either a combination of recycled cabin air and exterior air, or exterior air exclusively to maintain interior air quality when the aircraft is in operation. Many aircraft have an environmental control system (ECS) that typically maintains the interior air quality by supplying air, thermal controls, and ventilation for maintaining cabin pressure for the crew and passengers as well as provides cooling methods for the avionics.

As technology continues to grow and expand, the capability and electronic functionality of aircraft avionics and systems ever increases. As a result of this expanded functionality, there is an increase of heat produced by this equipment and therefore, larger heat loads into the surrounding aircraft environment. To provide a comfort level for passengers and crew aboard the aircraft, increased cooling capacity is needed. Unfortunately, many conventional environmental control systems for aircraft are not designed to meet such needs for increased cooling capacity. This issue can be particularly exacerbated when an aircraft is on the ground, for example before take-off or after landing, on a hot day when the ground ambient air temperature is significantly hotter than the ambient air temperature at higher altitudes when the aircraft is in flight.

Accordingly, it is desirable to provide environmental control systems that address one or more of the foregoing issues, vehicles including such environmental control systems, and methods for operating such environmental control systems. Furthermore, other desirable features and characteristics of the various embodiments described herein will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.

<CIT> relates to a compressor temperature control system and method for an aircraft air cycle machine.

Various non-limiting embodiments of a vehicle, an environmental control system for a vehicle having an interior, and a method for operating an environmental control system for a vehicle are provided herein.

In a first aspect, the present invention relates to an environmental control system (ECS) as defined in claim <NUM>.

In a second aspect, the present invention relates to a method as defined in claim <NUM>.

In a third aspect, the present invention relates to an aircraft as defined in claim <NUM>.

The following Detailed Description is merely exemplary in nature and is not intended to limit the various embodiments or the application and uses thereof. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.

The exemplary embodiments taught herein provide a vehicle, specifically an aircraft. The vehicle includes a vehicle structure, such as, for example, an aircraft structure, fuselage, or the like, at least partially surrounding an interior containing interior air. A hot air source, such as, for example, an engine(s), an auxiliary power unit (APU), a dedicated electrically- or mechanically-powered compressor, or the like, is supported by the vehicle structure. The hot air source produces hot air that is extracted from the hot air source as hot bleed air.

An environmental control system (ECS) is carried or otherwise supported by the vehicle structure. The environmental control system is in fluid communication with the interior of the vehicle. The environmental control system includes an ECS refrigeration unit (also referred to as an ECS pack) and a low limit valve control. As used herein, the phrase "refrigeration unit" refers to a unit including one or more equipment items that cooperate to cool a fluid (e.g., air) stream(s). Equipment items can include one or more heat exchangers, compressors, valves, controllers, turbines, fans, sensors, conduits/pipes/tubes/ducts, and the like. During operation, the ECS refrigeration unit receives ambient air from outside the vehicle and a first portion and a second portion of the hot bleed air from the aircraft engine bleed air source(s). Within the ECS refrigeration unit, several thermodynamic or unit operations are performed. Heat is indirectly exchanged between the first portion of the hot bleed air and the ambient air to form a partially cooled, hot air stream. The partially cooled, hot air stream is subsequently compressed, further indirect heat exchanged one or more times, and expanded to form a cooled and expanded air stream. The cooled and expanded air stream has a temperature of less than -<NUM> (<NUM>°F).

The ECS refrigeration unit includes a low limit valve that introduces the second portion of the hot bleed air to the cooled and expanded air stream. A low limit valve control, which may be mounted on or otherwise form part of the ECS refrigeration unit or may be separate therefrom, regulates the low limit valve, for example via a low limit valve torque motor, to control the rate of introduction of the second portion of the hot bleed air to the cooled and expanded air stream to form a combined air stream that when exiting the ECS refrigeration unit is a sub-freezing air stream. The sub-freezing air stream has a temperature of less than <NUM> (<NUM>°F) but greater than the cooled and expanded air stream.

Forming the sub-freezing air stream having a temperature of less than <NUM> (<NUM>°F) but greater than the cooled and expanded air stream when exiting the turbine portion of the ECS refrigeration unit advantageously provides an output air stream that is at a lower temperature than the output air stream from conventional ECS packs. As such, the ECS refrigeration unit provides enhanced cooling capacity. Further, in an exemplary embodiment, the sub-freezing air stream is advanced downstream and combined with additional hot bleed air to form the mixed air stream having a temperature of from about <NUM> to about <NUM> (<NUM>°F to about <NUM>°F) which is introduced into the interior cabin area to provide a comfort level for passengers and crew aboard the vehicle, as well as cooling for the avionics.

Additionally, in an exemplary embodiment, the ECS refrigeration unit may be controlled to provide two different modes of cooling, an enhanced cooling capacity mode as discussed above, and a normal cooling capacity mode where the low limit valve control regulates the low limit valve to control the rate of introduction of the second portion of the hot bleed air to the cooled and expanded air stream to form a combined air stream that when exiting the ECS refrigeration unit has a temperature above freezing, such as a temperature of greater than about <NUM> (<NUM>°F), such as from about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F), for example about <NUM> (<NUM>°F). In this embodiment, the ECS refrigeration unit can be selectively operated in the enhanced cooling capacity mode, for example, when the vehicle or aircraft is on the ground on a relatively hot and/or humid day when increased cooling demand is required, and otherwise be selectively operated in the normal cooling capacity mode when cooling demand is more normal or typical, for example, when the vehicle or aircraft is in flight or on the ground on a relatively cool and/or relatively low humidity day.

<FIG> illustrates a schematic view of a vehicle <NUM> including an environmental control system (ECS) <NUM> with ECS refrigeration units <NUM> and <NUM>' in accordance with an exemplary embodiment. The vehicle <NUM> includes a vehicle structure <NUM> at least partially surrounding an interior <NUM> of the vehicle <NUM> and containing interior air <NUM>. As illustrated, the vehicle <NUM> is an aircraft and the vehicle structure <NUM> is an aircraft structure such as, for example, a fuselage. The interior <NUM> may include one or more interior areas <NUM> such as a cabin area, a lavatory area, a cockpit area, and/or the like.

The vehicle <NUM> includes engines <NUM> and <NUM>' and auxiliary engine in the form of auxiliary power unit (APU) <NUM> coupled to the vehicle structure <NUM> and in fluid communication with the environmental control system <NUM>. As illustrated, the vehicle <NUM> includes two engines <NUM> and <NUM>', but it is to be understood that various alternate embodiments of the vehicle <NUM> can include a single engine <NUM> or more than two engines <NUM> and <NUM>'. Additionally, for purposes of simplicity, further discussions of the engines <NUM> and <NUM>', the ECS refrigeration units <NUM> and <NUM>", and their associated equipment items will be described in terms of engine <NUM> and ECS refrigeration unit <NUM>, but it is the understood that these components and their associated equipment items are essentially identical to the engine <NUM>', the ECS refrigeration unit <NUM>', and their associated equipment items.

The engine <NUM> is, for example, a turbine engine such as, for example, a turbofan engine. The engine <NUM> includes one or more compressors <NUM> (<NUM>') and takes in ambient air <NUM> and pressurizes and combusts at least a portion of the ambient air <NUM> with fuel in a burner to drive one or more turbines to produce thrust <NUM> (<NUM>') that propels the vehicle <NUM> in a general forward direction <NUM>.

In an exemplary embodiment, when the engine <NUM> is running, for example during flight, hot bleed air is extracted from the engine <NUM> via line <NUM> (<NUM>') and/or line <NUM> (<NUM>') and fluidly communicated to the environmental control system <NUM> via lines <NUM> (<NUM>'), <NUM> (<NUM>'), <NUM> (<NUM>'), and <NUM> (<NUM>'). The hot bleed air may be extracted from any of various compressor stage ports of the compressor(s) <NUM> of the engine <NUM>. For example, hot bleed air extracted via line <NUM> corresponds to a lower compressor stage while hot bleed air extracted via line <NUM> corresponds to a higher compressor stage. When extracted, the hot bleed air is at increased pressure and temperature compared to the ambient air <NUM>. For example, at an altitude of about <NUM>,<NUM> feet, the ambient air <NUM> is at a temperature of about -<NUM> (-<NUM>°F) and a pressure of about <NUM> kPa (<NUM> psia) psia and the hot bleed air <NUM> has a temperature of from about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F) and a pressure of from about <NUM> to about <NUM> MPa (about <NUM> to <NUM> psig) when extracted from a low stage of the compressor <NUM> and a temperature of from about <NUM> to about <NUM> (about <NUM>°F to about <NUM>°F) and a pressure of from about <NUM> MPa to about <NUM> MPa (<NUM> psig to about <NUM> psig) when extracted from a high stage of the compressor <NUM> (<NUM> psig ≈ <NUM>.

As discussed above, the hot bleed air is extracted either from a lower compressor stage via line <NUM> or a higher compressor stage via line <NUM>. In an exemplary embodiment, a check valve <NUM> (<NUM>') moderates the flow of the hot bleed air when extracted via line <NUM> and a control valve <NUM> (<NUM>') moderates the flow of the hot bleed air when extracted via line <NUM>. The check valve <NUM> also prevents reverse flow between the higher compressor stage and the lower compressor stage. In an exemplary embodiment, hot bleed air from the higher compressor stage may be routed from the engine <NUM> via line <NUM> that includes the control valve <NUM> for regulating the flow rate of bleed air from the engine <NUM> for operating cases where lower stage extraction via line <NUM> would be insufficient for hot bleed air system demand. Hot bleed air extracted via line <NUM> and/or hot bleed air extracted via line <NUM> is routed to a heat exchanger <NUM> via lines <NUM> and <NUM>. A control valve <NUM> (<NUM>'), which is disposed between lines <NUM> and <NUM>, regulates the pressure of hot bleed air to the heat exchanger <NUM> (<NUM>'). In an exemplary embodiment, the regulated pressure from the control valve <NUM> to line <NUM> is from about <NUM> to about <NUM> psig depending on phase of flight of vehicle <NUM> and power setting of engine <NUM> and <NUM>'.

In an exemplary embodiment, the heat exchanger <NUM> is a single pass cross flow plate/fin stack heat exchanger <NUM>. The heat exchanger <NUM> provides the environmental control system <NUM> with the hot bleed air at a regulated temperature and pressure. For example, the hot bleed air communicated via lines <NUM> and <NUM> to the heat exchanger <NUM> is cooled by the heat exchanger <NUM> to a temperature of about <NUM>°F under normal dual engine bleed operation, or <NUM>°F during certain single engine bleed conditions and is then passed along to the environmental control system via line <NUM>. In an exemplary embodiment, the engine <NUM> includes a fan <NUM> (<NUM>') and engine fan air is extracted via line <NUM> (<NUM>') and fluidly communicated to the heat exchanger <NUM> for cooling the hot bleed air supplied from control valve <NUM>. The heat exchanger <NUM> may be operably coupled to a control valve <NUM> (<NUM>') that regulates the flow rate of engine fan air through the heat exchanger <NUM> with the exchanged fan air being exhausted overboard. Additionally, and in accordance with an exemplary embodiment, the heat exchanger <NUM> may include additional valves, controls, and or sensors to assist in regulating the temperature and pressure of the hot bleed air for fluid communication to the environmental control system <NUM>.

As such, in an exemplary embodiment, when the engine <NUM> is running, for example during flight, the engine <NUM> is a hot air source (e.g., an aircraft engine bleed air source) that produces hot air that is extracted as the hot bleed air, which is fluidly communicated to the environmental control system <NUM> as discussed above. Alternatively, the hot bleed air can be extracted from another hot air source. In an exemplary embodiment, the vehicle <NUM> includes the APU <NUM> that is coupled to the vehicle structure <NUM> and that is configured as an engine (e.g., another aircraft engine bleed air source) that burns fuel to generate electricity and pneumatic pressure for the vehicle <NUM> and produce hot air. For example, when the vehicle <NUM> is on the ground preparing for a departure or having recently landed, the APU may be running to supply electricity to the vehicle <NUM> and can be used instead as the hot air source to supply hot bleed air that is fluidly communicated to the environmental control system <NUM> via lines <NUM>, <NUM>, and <NUM>. In an exemplary embodiment, when the hot bleed air is extracted from the APU <NUM>, the hot bleed air has a temperature of from about <NUM> to about <NUM> (about <NUM>°F to about <NUM>°F) and a pressure of from about <NUM> MPa to about <NUM> MPa (about <NUM> to about <NUM> psig).

As briefly discussed above, the environmental control system <NUM> includes the ECS refrigeration unit <NUM>, and further includes manifolds <NUM> and <NUM> and a vehicle supply duct <NUM>. In an exemplary embodiment, a portion(s) of the hot bleed air advanced along line <NUM> is introduced to the ECS refrigeration unit <NUM>, and another portion of the hot bleed air is introduced to the manifold <NUM> via line <NUM>. Further, ambient air <NUM> is introduced to the ECS refrigeration unit <NUM> via line <NUM>.

<FIG> is a schematic view of the ECS refrigeration unit <NUM> in accordance with an exemplary embodiment. As discussed above, the ECS refrigeration unit <NUM> includes one or more equipment items, such as one or more heat exchangers, compressors, valves, controllers, turbines, fans, sensors, conduits/pipes/tubes/ducts, and the like, which cooperate to cool a fluid (e.g., air) stream(s). Referring to <FIG>, the ECS refrigeration unit <NUM> receives the ambient air <NUM> via line <NUM> and a first portion <NUM> and a second portion <NUM> of the hot bleed air via lines 38a and 38b, respectively. In an exemplary embodiment, the ECS refrigeration unit <NUM> includes heat exchangers <NUM>, <NUM>, <NUM>, and <NUM>, a compressor <NUM>, a turbine <NUM>, a check valve <NUM>, and a low limit valve <NUM> that are in fluid communication. The heat exchangers <NUM>, <NUM>, <NUM>, and <NUM> are each configured as a counterflow heat exchanger configured for indirect heat exchange (e.g., heat exchange between two adjacent, counterflowing fluid streams that are separated from each other (not in direct contact with each other) by a heat transfer medium, such as a plate/fins, wall, or the like). As illustrated, in the heat exchanger <NUM>, heat is indirectly exchanged between the ambient air <NUM> and the hotter, first portion <NUM> of the hot bleed air to partially cool the first portion <NUM> of the hot bleed air and form a partially cooled, hot air stream <NUM>.

The ECS refrigeration unit <NUM> is further operable to compress, further indirect heat exchange, and expand the partially cooled, hot air stream <NUM> to form a cooled and expanded air stream <NUM>. In an exemplary embodiment, the cooled and expanded air stream <NUM> has a temperature of less than about -<NUM> (<NUM>°F), for example from about -<NUM> to about -<NUM> (<NUM>°F to about <NUM>°F) and a pressure of from about <NUM> MPa to about <NUM> MPa (<NUM> psig to about <NUM> psig).

In particular and as illustrated, during steady state operation of the ECS refrigeration unit <NUM>, the check valve <NUM> is in the closed position, and the compressor <NUM> receives and compresses the partially cooled, hot air stream <NUM> to form a compressed, hot air stream <NUM>. The compressed, hot air stream <NUM> is passed along to the heat exchanger <NUM> for indirect heat exchange with the ambient air <NUM> to partially cool the compressed, hot air stream <NUM> and form a first partially cooled, compressed, hot air stream <NUM>. The first partially cool, compressed, hot air stream <NUM> is passed along to the heat exchanger <NUM> that receives and partially cools the first partially cooled, compressed, hot air stream <NUM> to form a second partially cooled, compressed, hot air stream <NUM>. The heat exchanger <NUM> receives and partially cools the second partially cooled, compressed, hot air stream <NUM> to form an additionally partially cooled, compressed, hot air stream <NUM> that is looped around and passes through a water separator <NUM> for water removal and the heat exchanger <NUM> for indirect heat exchange with the first partially cooled, compressed, hot air stream <NUM>. As illustrated, water removed by the water separator <NUM> is passed along line <NUM> to nozzle <NUM> sprays the water into the incoming ambient air just upstream of the heat exchanger <NUM> to facilitate cooling. The turbine <NUM> is in fluid communication with the heat exchanger <NUM> and receives, expands and cools the additionally partially cooled, compressed, hot air stream <NUM> to form the cooled and expanded air stream <NUM>. Further, during operation, the turbine <NUM> drives the compressor <NUM> via shaft <NUM>. As will be appreciated by those skilled in the art, temporarily during start-up of the ECS refrigeration unit <NUM>, the check valve <NUM> is in the open position and the partially cooled, hot air stream <NUM> bypasses the compressor <NUM> and rather is advanced through the heat exchanges <NUM>, <NUM> and <NUM> to the turbine <NUM>, to drive the turbine, which drives the compressor <NUM> via the shaft <NUM>, to pressurize the unit <NUM>, thereby closing the check valve <NUM> and transitioning the unit <NUM> to steady state operation.

Referring also to <FIG>, the second portion <NUM> of the hot bleed air is passed through the low limit valve <NUM> and introduced to the cooled and expanded air stream <NUM>. A low limit valve control <NUM> regulates the low limit valve <NUM> to control the rate of introduction of the second portion <NUM> of the hot bleed air to the cooled and expanded air stream <NUM> to form a combined air stream <NUM>. The combined air stream <NUM> is a sub-freezing air stream <NUM> that when exiting the ECS refrigeration unit <NUM> has a temperature of less than about <NUM> (<NUM>°F) but greater than the cooled and expanded air stream <NUM>, such as from about -<NUM> to about -<NUM>, for example -<NUM> (about <NUM> to about <NUM>°F, for example about <NUM>°F).

In an exemplary embodiment, the heat exchanger <NUM> is in fluid communication with the turbine <NUM> and the low limit valve <NUM> to receive the combined air stream <NUM> that is passed through the heat exchanger <NUM> for indirect heat exchange with the second partially cooled, compressed, hot air stream <NUM> prior to exiting the ECS refrigeration unit <NUM> as the sub-freezing air stream <NUM>.

In an exemplary embodiment, the sub-freezing air stream <NUM> exits the ECS refrigeration unit <NUM> through an outlet <NUM> (<NUM>') and along line <NUM> (<NUM>') that is configured as a duct <NUM> that is disposed downstream from the ECS refrigeration unit <NUM> and that fluidly connects the ECS refrigeration unit <NUM> to the manifold <NUM>. The duct <NUM> is coupled to the outlet <NUM>. In an exemplary embodiment, a temperature sensor <NUM> is disposed in the duct <NUM>, for example coupled to the duct's <NUM> wall inner surface proximate to the exit of the ECS refrigeration unit <NUM>. The temperature sensor <NUM> measures a temperature of the sub-freezing air stream <NUM> and communicates with the low limit valve control <NUM> via line <NUM> to provide a signal indicative of the temperature of the sub-freezing air stream <NUM> to the low limit valve control <NUM>. In an exemplary embodiment, the low limit valve control <NUM> regulates the low limit valve <NUM> in response to the signal.

Referring also to <FIG>, the duct <NUM> has an inner surface <NUM> that has an ice-phobic treatment <NUM>. A surface having an ice-phobic treatment means that the surface substantially resists or prevents ice nucleation formation of a supercooled water droplet, water below the normal freezing temperature of <NUM> (<NUM>°F), on the surface. That is, water in its solid form is prevented or delayed in forming on such surfaces, or if formed, the rate of accumulation on the surface is significantly slowed down. Additionally, adhesion of ice to the surface is reduced, such that it can be easily removed. In an exemplary embodiment, the ice-phobic surface has an ice adhesion strength of less than about <NUM> kPa, for example less than about <NUM> kPa. Various treatments known to those skilled in the art for forming an ice-phobic surface may be used. Non-limiting examples of ice-phobic treatments for surfaces include forming microscale structures by laser ablation, surfaces with nanostructures formed by dry etching, micro-inset and nano-structured aluminum surfaces formed by etching and anodizing, multilayer spin-coated micro-sized PMMA spheres that are crosslinked by silica, spray coated organo-silane/attapulgite nanocomposites, or the like. As such, when the sub-freezing air stream <NUM> is received and advanced along the duct <NUM> disposed at line <NUM>, the duct <NUM> resists, delays, or prevents ice formation along its the inner surface <NUM>. Advantageously, this minimizes any chance of downstream equipment being damaged for example by pieces of ice that might otherwise form and accumulate along the inner surface <NUM> of the duct <NUM> and eventually break free.

Further, in an exemplary embodiment, portions of the ECS refrigeration unit <NUM> have inner surfaces that have an ice-phobic treatment(s) <NUM> to resist or prevent ice formation as discussed above. In particular, the ECS refrigeration unit <NUM> includes conduit sections <NUM> (e.g., pipe sections) that direct the flow of air though the ECS refrigeration unit <NUM> to the various unit operation components to cool the air and form the sub-freezing air stream <NUM>. As illustrated in <FIG>, one or more of the conduit sections <NUM> in regions <NUM> of the ECS refrigeration unit <NUM> have inner surfaces that have an ice-phobic treatment(s) <NUM>. For example, this includes, independently, one or more of the the conduit sections <NUM> downstream from heat exchangers <NUM> and/or <NUM>, the turbine <NUM>, the low limit valve <NUM>, and/or the water separator <NUM>.

Referring to <FIG>, as briefly discussed above, a portion of the hot bleed air is introduced to the manifold <NUM> via line <NUM>. The manifold <NUM> and the manifold <NUM> are cooperatively configured to supply the portion of the hot bleed air via line <NUM> and the sub-freezing air stream via line <NUM>, respectively, that are mixed or combined together downstream to form a mixed air stream at line <NUM> and advanced therefrom to the vehicle supply duct <NUM>. In an exemplary embodiment, along line <NUM> is a control valve <NUM> that regulates the flow rate of hot bleed air out of the manifold <NUM> for controlling the temperature of the mixed air stream (combined with the sub-freezing air stream) along line <NUM>. In an exemplary embodiment, the mixed air stream has a temperature of from about <NUM> to about <NUM> (<NUM> to about <NUM>°F) and a pressure of from about <NUM> kPa to about <NUM> kPa (<NUM> psia to about <NUM> psia).

The vehicle supply duct <NUM> supplies the interior <NUM> of the vehicle <NUM> with the mixed air stream. Although only a single vehicle supply duct <NUM> is shown, it is to be understood that the vehicle <NUM> may include more than one vehicle supply duct (e.g., multiple cabin interior zone ducts, for example, that optionally can be controlled independently) that supply the interior <NUM> of the vehicle <NUM> with the mixed air stream. A valve <NUM> is in fluid communication with the interior <NUM> and is configured to exhaust a portion of the interior air <NUM> out of the vehicle, for example, to control pressure of the interior air <NUM> remaining in the interior <NUM>.

Referring to <FIG>, a method <NUM> for operating an environmental control system (ECS) for a vehicle in accordance with an exemplary embodiment is provided. The method <NUM> includes introducing (STEP <NUM>) a first portion and a second portion of hot bleed air to an ECS refrigeration unit. Ambient air is introduced (STEP <NUM>) to the ECS refrigeration unit. Heat is indirectly exchanged (STEP <NUM>) between the first portion of the hot bleed air and the ambient air in the ECS refrigeration unit to form a partially cooled, hot air stream.

The method <NUM> further includes operating (STEP <NUM>) the ECS refrigeration unit to compress, further indirect heat exchange, and expand the partially cooled, hot air stream to form a cooled and expanded air stream having a temperature of less than -<NUM> (<NUM>°F). The second portion of hot bleed air is advanced (STEP <NUM>) through a low limit valve in the ECS refrigeration unit to introduce the second portion of the hot bleed air to the cooled and expanded air stream. The second portion regulates the low limit valve with a low limit valve control to control the rate of introduction of the second portion of the hot bleed air to the cooled and expanded air stream to form a combined air stream that when exiting the ECS refrigeration unit is a sub-freezing air stream having a temperature of less than <NUM> (<NUM>°F) but greater than the cooled and expanded air stream.

Claim 1:
An environmental control system (ECS, <NUM>) for an aircraft (<NUM>), the environmental control system (<NUM>) comprising:
an ECS refrigeration unit (<NUM>, <NUM>') configured to receive ambient air (<NUM>) and a first portion (<NUM>) and a second portion (<NUM>) of hot bleed air, wherein the ECS refrigeration unit (<NUM>, <NUM>') is operable to:
indirectly exchange heat between the first portion of the hot bleed air (<NUM>) and the ambient air (<NUM>) to form a partially cooled, hot air stream (<NUM>); and
compress, further indirect heat exchange, and expand the partially cooled, hot air stream (<NUM>) to form a cooled and expanded air stream (<NUM>) having a temperature of less than -<NUM> (<NUM>°F), the ECS refrigeration unit (<NUM>, <NUM>') comprising a low limit valve (<NUM>) configured to introduce the second portion of the hot bleed air (<NUM>) to the cooled and expanded air stream (<NUM>); and
characterized in that the ECS (<NUM>) comprises a low limit valve control (<NUM>) configured to regulate the low limit valve (<NUM>) to control the rate of introduction of the second portion of the hot bleed air (<NUM>) to the cooled and expanded air stream (<NUM>) to form a combined air stream (<NUM>) that when exiting the ECS refrigeration unit is a sub-freezing air stream (<NUM>) having a temperature of less than <NUM> (<NUM>°F) but greater than the cooled and expanded air stream (<NUM>),
and
a duct (<NUM>) that is disposed downstream from the ECS refrigeration unit (<NUM>, <NUM>') and that is configured to receive the sub-freezing air stream (<NUM>), and wherein the duct (<NUM>) has an inner surface (<NUM>) that comprises an ice-phobic treatment (<NUM>).