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AST 473Cessna Citation IIAir Conditioning System Dr. Barnhart
Air Conditioning General • Compressor bleed air provides the air to operate the system • Mixed with ACM air for cooling • Fans circulate cabin air • Optional flood cooling system provides a rapid cool of the cabin.
Air Conditioning General • Crew has both auto and manual control • PRESS SOURCE selector selects the source of the bleed air • Ambient airflow from tail scoop is an option if unpressurized
Air Conditioning UNs 0482, 0485 and Subsequent • Bleed air passes through the (solenoid operated) flow control valves- 6ppm • Bleed from Rt. Engine branches in tail cone; one branch- normal flow cnt’l valve, other branch to motor operated gnd. Valve (larger air draw as described previously for ACM ejector)
Air Conditioning • PRESS SOURCE selector switch- tilt panel fwd. of throttle • NORMAL- flow from ea. Eng.- DC fail position • If rt. eng. > 72% N(2) pressure switch closes the gnd. Valve preventing too much airflow to the ACM- BLEED AIR GROUND light extinguishes- reverses if rt. Throttle below 70% • If primary pressure switch fails and rt. Eng. >74% N(2)- secondary switch closes gnd. Valve and illuminates ACM OVER PRESS light- gnd. Valve will remain closed until DC is interrupted
Air Conditioning • In EMER left eng. Bleed goes directly into cabin- EMER PRESS ON light illuminates • OFF- all valves closes- no pressurization available
Air Conditioning • Precooler- bleed air from eng. Passes through a pre-cooler in the ACM ambient air duct- air-to-air heat exchanger. • Cools bleed air prior to the ACM heat exchangers • ACM- after passing via the precooler in the ram air duct then air flows via the ACM primary heat exchanger then via the ACM compressor then secondary ht. exchanger and finally across the cooling turbine- this is the cooling process.
Air Conditioning • Water Separator- removes moisture from air prior to entering cabin through a check valve. This moisture is injected into air flowing over the heat exchangers to increase cooling efficiency
Air Conditioning • Temp. Control- via a temp. select rheostat or a manual HOT-MANUAL COLD switch • Temp. is controlled by allowing some of the bleed air to bypass ACM via the mixing valve. Valve is positioned by elec. Motor- valve opened (warmer) or closed (cooler) by either: • Automatic Control • Manual temp. control
Air Conditioning • Cabin temp. controlled automatically when temp. rheostat is in the automatic range. This temp. selector is a rheostat that sends a variable temp. setting to cabin temp. controller • Controller compares desired setting to actual cabin temp. via the cabin temp. sensor (in top of cabin btn. Main and emer. Exit. Also receives temp. input from supply duct temp sensor in tailcone; compares inputs and tells mixing valve motor to change temp
Air Conditioning • For example- if selector is rotated clockwise for warmer- a higher temp- controller receives this input, compares it to cabin and supply duct sensors causing the mixing valve to open allowing more bleed air to bypass the ACM, boosting the temp.
Air Conditioning • In Auto- system has a supply air low temp. Limit of 35 deg. F. to prevent ice in water separator • Input to controller for this is from supply duct temp. sensor downstream of the water separator in the supply duct.
Air Conditioning • With temp. rheostat in manual position, mixing valve is controlled manually by the MANUAL HOT-MANUAL-COLD switch which is spring loaded to OFF • This causes manual movement of mixing valve where it remains until changed- valve has a 10 second travel time- must be cautious to prevent water separator freeze in this mode
Air Conditioning • System Protection- If bleed air temp. btn. Compressor and turbine exceeds 435 deg. F. the overheat relay causes flow control and shutoff valves (or ground valve) to close and the EMER PRESS valve to open • In flight ACM shuts down and noise level in cabin goes up dramatically • If overheat lasts longer than 12s the emer. Lockout relay energizes and ACM remains inop. If less than 12s it reverts to previous setting and emer. Valve closes
Air Conditioning • To reset the system to normal after 12s shutdown- rotate the PRESS SOURCE selector to EMER position and then re-select a position other than GND or OFF
Air Conditioning • On ground when ACM overheats the emer. press. Valve does not open since it has been de-activated by left main squat switch but EMER PRESS light does come on- reset procedure is the same
Air Conditioning • In EMER mode- watch pulling the left throttle down too much for temp. control as cabin alt. may rise.
Air Conditioning • Air duct from ACM to cabin is protected from overheat by a duct overheat sensor. If temp. exceeds 157 deg. The AIR DUCT O’HEAT annunciator light illuminates • Most likely when heat is demanded and most bleed is bypassing the cooling process of ACM • Pilot should select MANUAL with rheostat and close mixing valve by holding MANUAL HOT- MANUAL-COLD switch to the COLD position- 10 sec. • Check TEMP circuit breaker in- no power means no temp. control in either mode.
Air Conditioning • DC failure- defaults as if in NORMAL position. Emer. Press. Valve fails closed • Also temp. control is lost- DC mixing valve fails as set at time of power loss
Air Conditioning • Air distribution system: • Overhead conditioned air ducts and outlets • Footrest and armrest warmer manifolds supplied by underfloor conditioned air duct which also supplies the windshield defog. Outlets • Conditioned air enters cabin via single duct then branches at a flow divider- cockpit + main and aux. plenums then to pax. Scn. • Air circulation enhanced by using overhead fan and defog. Fan.
Air Conditioning • Operation- with engines operating select source- air flows: • Water separator • Ducting • Check Valve (aft bulkhead)
Air Conditioning • Temp. in supply duct determines position of recirculating air inlet door. Below 100 Deg. F. door fully closed- conditioned air flows via both overhead and underfloor ducts. • Above 100 deg. F. door fully open- all hot air from ACM diverted to underfloor duct • Air from overhead ducts is now recirculated cabin air only-cooler than incoming air from ACM
Air Conditioning • Air flowing through the overhead ducting system is distributed and controlled by manipulation of the individual Wemac outlets • Increased airflow through these outlets can be obtained by selecting HI or LOW with the OVHD fan switch on co-pilot’s instrument panel
Air Conditioning • Air flowing to underfloor ducting is divided by the flow divider assembly; part to flight compartment part to pass. Footwarmer and armrest warmer manifolds. • Position of flow bias valve is determined by AIR FLOW DISTR selector.- 5 position selector- allows selection if incr. or decr. Flow to cabin or cockpit; CKPT- most flow to cockpit- CABIN- most flow to armrest and footwarmer manifolds
Air Conditioning • Using defog fan in conjunction with flow divider increases flow to cockpit • Max. flow- CKPT with AIR FLOW DISTR selector and selecting HI with defog fan switch (HI, LOW, OFF) • Pilot’s footwarmer must be closed for max. defog at windshields • When footwarmer outlet is closed, a side window defog valve is also closed- prevents miost air from getting btn. Side window panes and condensing as it meets cold soaked outside air.
Air Conditioning • Optional Flood Cooling- air outlet grill on upper aft press. Bulkhead- high volume of cool air from the ACM • Bypasses both overhead and underfoot duct systems- eliminates heating by hot-soaked ducting system • Use on ground and in-flight ONLY below 10,000 ft. • Powered by rt. Gen. directly via a 20 amp breaker- prevents use of flood fan on ground without engines running (battery depletion).
Flood Cooling • FLOOD COOLING switch- next to pressurization controller on center panel- 2 position- ON and OFF. • OFF- Airflow is via normal dist. System • ON- Normal dist. System blocked- all flow directed via the flood cooling duct • Within flood duct is a DC powered axial blower- increases airflow at low pwr. settings
Flood Cooling • At low pwr. Settings with Flood Cooling ON- ACM flow insufficient for blower thus ambient tailcone air is mixed with ACM air. • An ambient check valve closes at higher power settings blocking outside airflow