Boeing Design of Ecs Essay Example
Boeing Design of Ecs Essay Example

Boeing Design of Ecs Essay Example

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  • Pages: 11 (2945 words)
  • Published: December 20, 2017
  • Type: Essay
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The text outlines the significance of survivability in contemporary Jet aircraft and how it is achieved through a complex SEC (environmental control and life support systems). These systems ensure human physiological integrity and comfort throughout the flight. Aerospace medical experts collaborate with SEC engineers to translate requirements from the aerospace medical community into system components. Figure 1 in the design incorporates data on human comfort factors. The text then delves into the primary elements of the SEC that maintain human life and comfort during flight, using an example of a continuous flow of air from outside to inside and out of the airplane cabin. Initially, this air volume has standard conditions for temperature and atmospheric pressure. The text mentions that the airplane engines are at low thrust, slowly moving the Boeing 767


along the taxiway, hinting at discussing different flight phases later on. Figure 1 illustrates a diagram of a modern turbofan engine known as the 4000 engine, which serves as a core component for the Boeing 767 airplane. This core generates power while combining filtered and outside air to produce main thrust through bypassing airflow — commonly referred to as a "fan" engine used in modern generation airplanes. The core comprises fifteen separate compressor stages, a burner section for fuel addition, and six turbine stages driving both impresser and fan functions.A small portion of the air that is taken in by the engine goes into the core, while most of it passes through the bypass to create thrust. Figure 2 shows a simplified representation of the main components of a P 4000 engine, which is commonly found on United Airlines' 767-RARER aircraft with Prat

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& Whitney (P) 4000 engines. It should be noted that permission from The Boeing Company is necessary to reproduce or modify any part of this paper. The Bleed System of the Whitney 4000 engine is an essential part of the Secondary Engine Control (SEC). As outside air enters the compressor stages, it gets compressed to a pressure level known as 32 SSI and reaches an OFF temperature. Some of this compressed air is then extracted from the engine core through one of two bleed port openings located on its side. The choice between these ports depends on how valves controlling them are positioned. The first bleed port, also called "high stage," can be found at the fifteenth compressor stage, while the second one, referred to as either "IoW" or "intermediate stage," can be found at the eighth compressor stage. Note that specific designations may vary depending on the type of engine used. The high stage provides the highest air pressure available from the engine's compressor. When operating at low power levels, only this high stage can supply air at sufficient pressure to meet the demands placed on it by the bleed system.The preschooler, a heat exchanger, is part of the bleed system which includes valves. Its purpose is to deliver air at appropriate temperature and pressure for various pneumatic services on the aircraft. These services include air-conditioning packs, cabin ventilation system, potable water presentation, wing and engine anti-ice protection, air-driven hydraulic pump, hydraulic reservoir presentation, cargo heat, and cabin presentation. The 767 aircraft's bleed system is mostly automated but does offer pilots the option to shut it down using the overhead panel in the flight

deck. Figure 3 provides a diagram of this system. During takeoff preparation, pilots increase engine thrust to takeoff power causing the high stage compressor to compress air beyond what is necessary for certain services such as air-conditioning packs. About half of this excess energy cannot be used so the bleed system automatically switches to the low stage port in order to conserve energy. However, since engine conditions can vary greatly based on altitude and location throughout different seasons worldwide ranging from ground level up to an altitude of 43,100 feet , there will rarely be an exact match between available high or low stage engine compressor air and pneumatic systems' needs resulting in excess energy being released as waste heatThe bleed system is constantly monitoring engine conditions and selecting the most efficient port to minimize waste. However, temperatures in the bleed system often exceed safe levels for downstream systems. Safe temperature levels are determined by preventing fuel from spontaneously igniting. The purpose of the bleed system is to release excess energy as waste heat into the atmosphere.

Ensuring that the temperature of the pneumatic manifold is always lower than the fuel ignition point is crucial due to rapidly changing external conditions. When reaching cruise altitude, the outside air temperature is turned off at an atmospheric pressure of 2.9 SSI with a partial pressure of oxygen at 0.SSI. These conditions are not survivable for humans without protection.

Before descending to LAX, the low compressor stage compresses cold outside air at low pressure to over 30 SSI and increases its temperature above OFF. This compression process occurs before adding fuel, which only happens after the air passes through the

engine core's compressor stages. Figure 4 shows how the temperature of air taken from the engine compressor and sent to the bleed system changes throughout HTH departure until arrival at LAX.

Throughout this entire journey, heated air supplied to the bleed system significantly exceeds what is needed to eliminate any microorganisms present in outside air.At high cruising altitudes, the concentration of biological particles in the outside air is significantly lower compared to lower altitudes. The air supplied to the reconditioning packs is made sterile by reaching high temperatures from the engine compressor. During cruise, air enters the pneumatic manifold through the bleed system with an OFF temperature and a pressure of 30 SSI. Before reaching the air-conditioning packs located under the wing, it passes through an ozone converter. After leaving the ozone converter, the air remains at OFF temperature and 30 SSI pressure. The ozone converter has an efficiency of 60% and its useful life is almost over. The concentration of ozone leaving the converter is approximately 0.25 pump SALE. Once converted, this air flows through the air-conditioning packs and is then supplied to the cabin. Ozone levels in the cabin can vary depending on factors like season, altitude, latitude, and weather systems. On average, there is about 0.09 pump of ozone concentration in the cabin; however, when flying at 39,000 feet several ozone plumes may be encountered with concentrations as high as 0.1 pump which exceeds Federal Aviation Administration's three-hour time-weighted average limit of 0.05 pump for ozone concentration.
The peak ozone concentration limit is set at 0.5 pump SALE, assuming the worst-case flight scenario in April when ozone concentrations are typically highest. If this concentration

of ozone enters the cabin, passengers and crew may experience symptoms such as chest pain, coughing, shortness of breath, fatigue, headaches, nasal congestion, and eye irritation commonly associated with high ozone exposure.

Atmospheric ozone is present during the engine's compressor stages, the ozone catalytic converter, and the air-conditioning packs. When ozone comes into contact with the airplane's ducting, interior surfaces, and recirculation system it further dissociates. A noble catalyst like palladium in the ozone converter converts approximately 95% of the entering ozone into oxygen molecules. The converter has a lifespan of about 12,000 flight hours.

Meanwhile, the air volume enters reconditioning packs that utilize an air-conditioning pack functioning as an air cycle refrigeration system using passing through and into the airplane as refrigerant.The system includes an air cycle machine, valves, and heat exchangers for controlling temperature and flow. The air-conditioning pack delivers conditioned air to the cabin at the correct temperature, flow rate, and pressure. It provides dry, sterile, and dust-free air for presentation and temperature control. Each passenger receives about 5 CFML of conditioned air. Backup is ensured with two reconditioning packs (sometimes three for larger planes) delivering a total of around 10 CFML per passenger. Filtered recirculated air is mixed with the conditioned air from the packs using automatic systems that monitor flight parameters, temperature selection or zones, and cabin temperature. This results in approximately 20 CFML per passenger and a complete air exchange in the cabin every two and a half minutes. The high quantity of supply air prevents stagnant cold areas, maintains air quality, and controls odors through the bleed system controls located on the overhead panel in flight gradients.Temperature control is necessary

for monitoring the compartment on a regular basis as it has a significant impact on the airflow requirements from outside. Flight attendants can adjust temperatures based on passengers' comfort levels, and pilots have different options to handle abnormal operational situations. Figure 6 illustrates a schematic of the SEC pack bays that contain the air-conditioning packs.

The air in the mix manifold is cooled by passing through the air-conditioning sacks and leaves at an OFF temperature and pressure of 1.1 SSI. The relative humidity is below 5% and ozone concentration is less than 0.25 pump. Carbon dioxide concentration remains unchanged from outside air, around 350 pump. The recalculated air entering the mix manifold is essentially sterile.

High-efficiency particulate air type filters (HEAP-type) eliminate approximately over 99% of bacteria and viruses produced by passengers. These filters are similar to those used in critical wards of hospitals and industrial clean rooms, commonly employed not only in the 767 airplane but also in other modern aircraft. They are more effective than filters found in public conveyances or office buildings. While their efficiency increases with prolonged use, regular replacement is still needed.

Figure 8 provides comparison data on filtration systems, showcasing different filter efficiencies; however, it's worth noting that these devices cannot filter out gases.To maintain control of gases within the cabin, a significant amount of outside airflow is introduced per cubic volume to dilute them. According to Figure 8, the cabin experiences approximately 12.5 air changes per hour. To ensure the desired supply air temperature in each seating zone, the exchange rate of outside air should be between 1 and 2.5 times per seating zone if necessary. Seating densities can cause variations

in the supply air temperature across different zones.

The cabin's ventilation system now receives incoming air volume through an overhead distribution network. This network supplies dust-free and sterile air with a relative humidity of 10% to 20%. The air is drawn from beneath the floor and directed towards the seating zones using dedicated ducting risers. Temperature control is implemented, while keeping carbon dioxide concentration at around 1,050 ppm. Trim air is added to increase the temperature within the risers.

Precise management of airflow patterns becomes crucial to provide comfort without causing drafts due to the large amount of circulated air compared to a typical building setting. Air enters through overhead distribution outlets, resulting in controlled circular airflow patterns that sweep across cabin walls and floor for optimal comfort. The direction of this airflow is adjusted so it does not directly blow onto exposed parts (such as arms, hands, face, neck, and legs) of seated passengers.However, the air in the cabin is moving at a sufficient speed to prevent stale air. The velocity of the air hitting a seated passenger should be between 20 and 70 FMP (Figure 9). The airflow follows a specific path in the cabin, circulating and mixing before being exhausted through return air grilles located on the sidewalls near the floor. These grilles cover the length of both sides of the cabin. While inside the cabin, human metabolism consumes about one-third of 1% of oxygen, which is replaced by an equal amount of carbon dioxide from passenger respiration. The return air also contains contaminants from passengers or the cabin itself. When someone coughs or sneezes, microorganisms can be carried in aerosol droplets and expelled

through these return air grilles. Approximately half of the return air is exhausted out of the cabin while the other half is filtered by HEAP-type filters in the recirculation system. The design of the cabin ventilation system ensures that air supplied to one seat row will exit at approximately that same seat row, minimizing airflow in fore and aft directions and reducing spreading passenger-generated contaminants. The panel for controlling cabin pressure is situated alongside other air-conditioning controls in pilot's overhead panel. Normally, this system operates automatically without intervention from pilots.Figure 1 illustrates the key elements of the cabin pressure control system. In the past, commercial Jet airliners did not have cabin air recirculation systems, although some turboprops did. This was because early turbojet engines were inefficient, with all air passing through the core and thrust generated by high velocity turbine exhaust. The fuel required for cabin air was minimal compared to overall fuel consumption. However, advancements in engine technology led to the development of turbofans with a bypass ratio of around 2 to 1, improving fuel economy. As a result, providing bleed air to the cabin became cost-effective and 100% bleed air was used for passenger cabins.

The Cabin Pressure Control System is shorter in length and continuously monitors various aspects such as ground and flight modes, altitude changes (climb, cruise or descent), and holding patterns at different altitudes. Based on this information, it adjusts the cabin pressure by modifying the outflow valve located in the lower aft fuselage.The position of the outflow valve is adjusted continuously to maintain cabin pressure close to sea level without exceeding a cabinet-outside pressure differential of 8.SSI (see Figure 10

for the 767 cabin altitude schedule). At a cruise altitude of 39,000 feet, the cabin pressure is equivalent to 6,900 feet or a pressure of 1.5 SSI (approximately 450 feet less than Mexico). This allows for controlled air escape as needed to keep the cabin at the same altitude as the airplane. Passenger comfort is taken into account by minimizing direct pressure changes. Additionally, this system helps reduce operating costs by consuming less fuel compared to current systems. The ascent and descent have normal pressure change rates of 0.6 SSI per minute and 0.16 SSI per minute respectively. Modern turbofan engines with high bypass ratios (5 to 1) enable these rates while consuming less fuel. However, increasing the bypass ratio leads to significantly higher fuel consumption relative to using bleed air, especially in comparison with an equivalently sized turbojet with the same amount of bleed air (as shown in figure 12).The elimination of recirculation systems in modern airplanes and the use of 100% outside air for the cabin would have resulted in wasting over 40 million gallons of fuel for the 767 fleet so far, with no tangible benefits. This would have required at least 14 additional super tankers to supply fuel for the fleet, considering the crude oil barrels needed to extract jet fuel. Efficient recirculation systems were developed based on studies conducted by NASA, McDonnell Douglas Aircraft Company, and an airline. These studies showed that significant fuel savings could be achieved without compromising cabin air quality, as shown in Figure 11. The cabin pressure control system and cabin air quality in airplanes have been extensively studied by reputable organizations such as the National

Academy of Sciences, United States Department of Transportation (DOT), National Institute for Occupational Safety and Health (NOSH), Centers for Disease Control and Prevention (CDC), independent research groups, and airplane manufacturers. Table 1 provides the results of these studies consistently demonstrating high air quality in passenger cabins of airplanes. This is not surprising given how airplane SEC functions. The CDC has recently investigated tuberculosis transmission (TAB) on commercial Jet airliners comprehensively.The CDC's report from March 1995 states that airplane filters effectively eliminate TAB bacteria from the recirculated air. The risk of transmission on airplanes is minimal and comparable to other forms of public transportation for similar durations. Figure 12 illustrates this information. Modern Jet engines now use bleed air in recirculation systems with efficient filters to improve fuel consumption, resulting in reduced bleed air usage. This filtered recirculation helps save energy. In contrast, office buildings typically recirculate a significant percentage (65% to 95%) of their air, depending on outdoor temperatures. However, compared to airplanes, buildings have less effective recirculation filters for various contaminants such as microbial aerosols, ozone, particulates, volatile organic compounds (VOCs), and non-ass volatile organic compounds. The average measurements for these contaminants are as follows: Item CA coax - 600-1,500; microbial aerosols - 0.6/1.4; ozone - very low; particulates - 0.02; non-ass volatile organics (RSVP) - 40/175; TTS - very low; AGGIE (porn) - very low; 25 AGGIE - 5,000; RSVP - 0.1; 32 AGGIE -10,000 ; SHARE (porn) -1,000 . The SHARE value serves as an indicator of body odor and is equal to or lower than levels found in nonsmoking/smoking areas of common homes except during food service when ethanol (alcohol) was

servedRSVP (resalable suspended particulate) and TTS (total suspended particulate) are measurements used to assess air quality. RSVA values represent the average levels of these particles over an 8-hour workday and 40-hour workweek. SHARE represents the American Society of Heating, Refrigerating and Air-conditioning Engineers, while AGGIE represents the American Conference of Governmental Industrial Hygienists. The information is referenced from a report by the United States Department of Transportation (DOT-P-1 5-89-5), which discusses contaminant measurements, health risks, and mitigation options in airliner cabins. Another source cited is a Health Hazard Evaluation Report by the National Institute for Occupational Safety and Health (HEAT 90-226-2281), specifically focusing on Alaska Airlines.

The conclusion drawn from these reports is that modern commercial jet airliners have significantly improved cabin air quality compared to earlier times. In the past, planes were not pressurized and had limited altitude capabilities. However, advancements in environmental control systems have created a comfortable and safe environment inside aircrafts today. Despite harsh conditions at high altitudes resembling extreme ocean depths, airplanes incorporate complex features that convert outside conditions into a pleasant atmosphere. This includes bringing cold and high-altitude air into the plane and transforming it into a life-supporting medium.
The purity of cabin air is a crucial aspect to address, as there are misconceptions surrounding it. However, it is essential to highlight that the processes involved in maintaining or restoring the quality of the air volume are of utmost importance. These processes ensure high-quality air from its entry into the aircraft's engines until it is discharged overboard.

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