The sub-zero bootstrap ECS pack is a normal part of a modern civil transport aircraft. The CAU section of the pack is normally capable of delivering air at -20oC, although this is too low a temperature for delivery to the cabin. However, the low temperature performance results in a smaller pack size with corresponding weight reduction. To counteract the low temperature the hot air by-pass may be used to blend with the very cold air, or re-circulated cabin air may be used for the same purpose.
The objective of the assignment is to provide design calculations that will provide preferred parameter values satisfying extremes of ECS requirements, for a 490 seat, ten-abreast, long range civil transport.
The following parameters should be evaluated:
1) the pack mass flow rate,
2) the pack outlet temperature,
3) the percentage re-circulation rate.
Any other parameter values used in the calculations should also be reported.
The ECS pack must be capable of providing passenger comfort and safety in cruise conditions and under the following two extreme conditions, both referred to a prolonged ground-holding period at an airport.
a) very hot climate, with and without strong winds, air temperature, ISA +25oC.
b) very cold climate, with and without strong winds, air temperature, ISA - 40oC.
It is expected that you will provide calculations of skin temperatures, ECS mass flow rates and recirculation flow rates. You should provide reasoned arguments for any parameter values selected, and use accepted targets of cabin temperatures and fresh air requirements.
Cruise conditions considered should be relevant to a typical aircraft role, and should be stated clearly in your report. However, typically, the cruise Mach number will be M = 0.85, with cruise in the stratosphere.
A fuselage length in the region of 70m is practical. You may select an existing aircraft type in order to derive cruise data and fuselage length. A table of atmospheric properties is provided.
You should state any assumptions made, provide a discussion of the results you obtain, and identify any design compromises and worst case conditions.
Standard project report structure should be used. Marks will be awarded for clarity of style, application of design method, accuracy of calculations, demonstration of physical insight, and standard of presentation.
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Target fresh air = 0.005793 kg/s
Hence, our first estimate is not sufficient. Reduce Tecs to -20oC using a sub-zero bootstrap system, to give
mecs/Np = ( + 307.25 )/(1005*(22 + 20) = 0.00728 kg/s/Pax
This is bigger than minimum requirements but must remain as Tecs is low enough.
Use re-circ to reduce cold air shock to passengers – raise Tin to 5oC.
mr Tc + mecs Tecs = ( mr + mecs ) Tin
mr 22 + 0.00728*(- 20) = ( mr + 0.00728 ) 5
mr = 0.01 kg/s
re-circ = 0.01/( 0.01 + 0.00728 ) = 0.59 = 59 %
This is satisfactory for a short period.
If we reduce re-circ to 50%, Tin = 1oC - this is too low....
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