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Chapter 6 Environmental Control System

1. What are the heat sources that give rise to cooling problem of aircraft?

Kinetic Heating: a/c skin heats do to friction,

Solar Heating: effects canopy and widescreen from military a/c

Avionics Heat Load: avionics equipment and digital electronics increase heat loadAirframe Systems Heat Loads: produced by ECS itself, hydraulic, engines, ect

Equipment tolerenace should be -40C to +90C

2. For an aircraft at Mach 0.8 at sea level on a 40C day, estimate the ram air temperature and the

temperature of unconditioned equipment bays.

Trecovery= Tamb(1+0.18M2)

Tram = Tamb(1+0.2M2

) IN KELVINS, M= MACH NUMBER

leadings edge heating (ram temp)

all other surfaces away from leading edge have slightly lower temperature, (recovery temp)

3. What are the two heat sinks used for airborne aircraft cooling?

Outside Air

Internal Fuel

If heat sinks are too hot, some sort of heat pump is used

4. Discuss ram air cooling and fuel cooling processes in comparison with each other.

RAM AIR COOLING

Rejecting aircraft heat load to the air flowing round the aircraft can be done byscoping air close to

boundary layer t

Bad because increases drag cause of scoop, pipework and heat exchanger matrix slow down ram airflow

Ram air temp soon exceeds temp required for cabin cooling when up to speed.

At high altitude, density becomes low, reducing ram air mass flow

Fan is used to enchance ram airflow during low taxi-ing and low speed flight

ram air not sufficient for on groud conditions

FUEL COOLING

Higher heat capacity, higher heat transfer coefficient,

cools engine oil, hydraulic oil, gearbox oil

5. What is the main source of conditioning air for aircraft?

engine bleed is main source of cooling for a/c

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6. Discuss open loop and closed loop bleed air systems in comparison with each other.

Open Loop Continuously bleed large amount air from engine,refrigerator,coolppl before dumping

air

Closed Looop, collect air, refrigerate, and recycle it to use again

7. Describe how bleed air is extracted from a turbofan engine.

*Air is taken from intermediate/high pressure stage of engine compressor

(Depending on power setting)

*At lower power setting air is extracted from high pressure section of compressor,

-at high power settings, air is extracted from the intermediate compressor stage

*depending on Throttle settings, ameliorates to some degree in air pressure and temp

*Charge air pressure needs to be reduced asap to required working pressure for safety

*pressure reducing shut off valve( PSOV) regulates outlet air pressure to constant, regardless

upstream pressure*Air temp needs to be reduced to 100*C for de-icing or demisting

*

8. What is the operation principle of air cycle refrigeration systems?

*Source of high pressure air cooled by ram air and expanded across turbine to reduce temp further

*Turbine drives fan or compress which acts like brake

9. The following figure shows a three-wheel bootstrap air conditioning system. Identify the major

components and describe the function of each component identified. Where does the power that

drives the motion of the system come from?

*Consists of cold air unit, heat exchanger

*Turbine of cold air units drives a shaft compressor. Both are mounted on common shaft.

* Compressor is used to increase air pressure with corresponding increase in temperature

*Temperature is reduced in ram air cooled heat exchanger. May lead to water being condensed

from air

*

*

10. Discuss vapor refrigeration systems in comparison with air cycle refrigeration systems.

*Closed loop where heat load is absorbed by evaporation of liquid refriderant*When refridgerant passes through a compressor with corresponding increase in pressure and

temp, before being cooled in a conderser where heat is rejected into heat sink. Ferron flows back to

expansion valve

*They have limited temperature ranger compared to air cycle systems

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11. Discuss the inefficiency of current environmental control systems.

In cooling down the engine bleed air, a great deal of heat and therefore potentially useful energy is

rejected to ram air.

In addition, further engine power is required to overcome the drag caused by the ram air heat

exchangers.

The more avionics, the heavier the aircraft, not only due to the avionic equipment weight itself, but

also due to the weight of the environmental control system equipment and the air distribution

pipework.

Furthermore, additional engine bleed air is required as the avionic heat load increases, but bleeding

more air off the engine is detrimental to engine performance.

The increasing avionic heat load on military aircraft may lead to further developments of closed-loop

environmental control systems in the future, since there is potential to vastly reduce the amount ofengine bleed required, and thus overcome the problem of detrimental effects of open loop systems on

engine performance.

12. State four methods by which avionics equipment can be cooled using conditioning air. Which

method is the most efficient way? Which is the least efficient?

In cooling down the engine bleed air, a great deal of heat and therefore potentially useful energy is

rejected to ram air.

In addition, further engine power is required to overcome the drag caused by the ram air heat

exchangers.

The more avionics, the heavier the aircraft, not only due to the avionic equipment weight itself, but

also due to the weight of the environmental control system equipment and the air distribution

pipework.

Furthermore, additional engine bleed air is required as the avionic heat load increases, but bleeding

more air off the engine is detrimental to engine performance.

The increasing avionic heat load on military aircraft may lead to further developments of closed-loopenvironmental control systems in the future, since there is potential to vastly reduce the amount of

engine bleed required, and thus overcome the problem of detrimental effects of open loop systems on

engine performance.

In civil aircraft it is often sufficient to draw cabin ambient air over the avionic equipment racks using

fans.

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This will have the effect of increasing the overall cabin temperature but, since the total avionic heat

load is not massive, the environmental control system has sufficient capacity to maintain cabin

temperatures at acceptable levels.

In civil aircraft it is often sufficient to draw cabin ambient air over the avionic equipment racks using

fans.

This will have the effect of increasing the overall cabin temperature but, since the total avionic heat

load is not massive, the environmental control system has sufficient capacity to maintain cabin

temperatures at acceptable levels.

On a military aircraft with a high avionic heat load, only a few of the avionic equipments are located in

the cabin.

The majority are located in either conditioned or non-conditioned equipment bays, an installation

decision which is made by taking into consideration such criteria as the effect of temperature on

equipment reliability or damage, and the amount of engine bleed available for air conditioning.

13. Discuss how aircraft humidity control is accomplished?

Passenger comfort is achieved not only by overcoming the problems of cooling and cabin

pressurization, but also by controlling humidity in the passenger cabin.

This is only a problem on the ground and at low altitudes, since the amount of moisture in the air

decreases with increasing altitude.

Humidity levels must be controlled to prevent damage to electrical and electronic equipment due to

excessive condensation.

Humidity control also reduces the need for windscreen and window de-misting and anti-misting

systems.