21 Air Conditioning

TTP	B1-L3	ATA21
Beech 90 Series	B2-L3	Air Conditioning

LJ-1 through LJ-501 consists of an 80,000 BTU combustion heater and a 16,000 BTU refrigerative air cooler working through a cabin temperature control box maintain cabin comfort. Cabin heat may be supplemented by electric heat. The air conditioner unit is electrically driven, has a capacity of 16,000 BTU and uses a refrigerant gas. LJ-1063 and after, and LA-2 and after share the same environmental design, control and operation.

Air Conditioning

The   vapor-cycle   air-conditioning   system   is in the nose of the aircraft. The compressor is motor-driven through two V-belts in the left side of the nose. Refrig- erant R-134a is used on aircraft from LJ-1417, and refrigerant R-12 is used on aircraft through LJ-1416 and all E90s and F90s. However, many of the earlier R-12 equipped aircraft have been converted to R-134a. The servicing ports are in the nose wheel well. The R-12 system is a screw- on, Schrader valve type. The R-134a ports are a quick-connect snap coupling.

A cooling fan on the compressor motor shaft provides airflow to remove heat from the condenser. During ground operations, air is drawn through a grill in the right side of the aircraft nose, passes through the condenser cooling fins, flows past the motor-fan assembly, and exits out a grill on the left side of the nose. In flight, airflow through the condenser is accomplished with ram air although the fan still turns whenever the compressor motor is running.

 The refrigerant is  compressed and sent to the condenser where either fan air or ram air cools the refrigerant, which condenses into a liquid. As a liquid, it moves to the receiver-dryer on the left sidewall of the nose wheel well. The receiver-dryer contains bags of silica gel desiccant to absorb any system moisture.

 The liquid refrigerant is then passed through a modulating expansion valve. The pressure drop across the valve allows the liquid to vaporize back into a gas as it passes through the evaporator. This evaporation process absorbs heat from the surrounding air as it passes by the evaporator cooling fins. The expansion valve and the evaporator are in an evaporator housing below the left avionics bay and just forward of the forward pressure bulkhead. The  valve  modulates  the  refrigerant flow to keep the temperature in the evaporator at a value above about 34°F. Keeping the temperature above 32°F (0°C) prevents any ice from forming on the outside of the evaporator. A 33°F evaporator sense switch is on the line downstream of the evaporator. If it senses a temperature of 33°F or less, power to the air-conditioner compressor is interrupted.

 

The air-conditioning circuitry is designed so the air conditioner is not allowed to turn on, in the automatic  mode, until  the left bypass valve is in the fully opened position. The right bypass valve is open, also, since the cool command starts on the right and then sequences to the left valve. In the automatic mode only, a microswitch on the left bypass valve interrupts power to the compressor if the left valve, driven by a heat command, closes more than 30° of travel. Additionally, 50°F ambient air-sensing switches, one for the manual cool circuit and another for the automatic cool circuit, normally interrupts power if the ambient air temperature goes below 50°F. The switches are in the nose of the aircraft just to the left side of the condenser.

 

The air-conditioning motor is rated at 130 amps. Unless somehow regulated, the starting amperage would spike to a level that could adversely effect the aircraft electrical system. An air-conditioner start control, in the aft of the nose air-conditioning bay, provides protection. When the air conditioner first starts, a relay in the start control closes and current flow is sent through a large resistor limiting any amperage peak. This allows the motor to slowly come to speed. After approximately 3/4 of a second, a continuous duty relay closes and the current is routed through a bus bar to the motor. The intermittent relay to the resistor then relaxes, and the entire motor load is carried through the bus bar. There is a small overlap, when both relays are closed, that prevents any arcing or heating in the relay contacts.

Cabin Airflow

The two-speed vent blower in the left keel area under the pilot rudder pedals picks up cabin air. From the vent blower, air passes through a door in the aft side of air conditioner evaporator housing. If the air conditioner is operating, the air is cooled as it goes though the evaporator. The air then moves forward in the pressurized ducting to the crossover duct. This duct takes the air over the top of the nose wheel well and moves it from the left to the right side of the aircraft. The air ducting  then  drops  down, starts  rearward,  and divides into two ducts. The smaller of the two ducts runs slightly inboard and below the larger electric heat duct. The smaller duct again divides in two, runs aft under the floorboards, and up both sides of the fuselage into the cockpit and cabin overhead vents. If the air was air conditioned, the overhead vents puts out the cooled air. The larger duct feeds air into the electric heater. The air then moves back into the mixing plenum, where it is mixed with bleed air from the flow control units. The flow-control bleed air comes forward from the flapper valves and tee, is reversed 180°, and is mixed with air coming aft from the electric heat duct. This combined air then moves aft and is distributed out through the cockpit, the defrost, the floor, and the aft cabin outlets.

 

Cabin air can be conditioned in several ways:

 

1.   HEATED  BLEED  AIR—The  vent  blower picks up existing cabin air and sends it through the nonworking air-conditioning evaporator and electric  heater.  In the  mixing plenum, this air is mixed with the bleed air  that  has  been  temperature  adjusted  by the bypass valves. Existing cabin air comes through the overhead ducts and heated and pressurized air comes out of the floor registers.

2.   AIR    CONDITIONED    AIR—The    vent blower picks up existing cabin air and sends it through the functioning air-conditioning evaporator and nonworking electric heater duct. In the mixing plenum, the air from the electric heat duct is mixed with the bleed air that has been passed through the fully-opened air-to-air heat exchangers (automatic mode). Air-conditioned air comes through the over- head ducts, and pressurized bleed air mixed with cooled air, comes out of the floor registers.

3.   ELECTRIC  HEAT AIR—The  vent  blower picks up existing cabin air and sends it through the non-working air-conditioning evaporator and the functioning electric  heater.  During ground maximum operation, all eight of the electrical heat elements are in operation. During flight operations, only normal electric heat is allowed, and four of the electrical heat elements are powered. In the mixing plenum, the electrically heated air is mixed with the bleed air from the bypass valves. Existing cabin air comes through the overhead ducts and electrically heated and pressurized air comes out of the floor registers 

CONTROLS AND INDICATIONS

 

PILOT SUBPANELS 

Left

PILOT   AIR–PULL   ON—Pulling   this   knob increases airflow to the cockpit and decreases air-

flow to the cabin.

 

Right

 

DEFROST AIR–PULL ON—Pulling on this knob allows air to flow to the windshield defroster from a valve on the crew heat duct

COPILOT SUBPANELS 

Left

 MANUAL TEMP Switch

INCR–DECR—When a manual mode is selected, moving this switch repositions the bypass valves in the bleed-air lines.

 

VENT BLOWER Switch

HI—Vent blower operates at high speed regard- less of the CABIN TEMP MODE position.

 

LO—Vent blower operates at low speed regardless of the CABIN TEMP MODE position.

 

AUTO—Vent blower operates at low speed if the CABIN TEMP MODE is in any position except OFF. If the CABIN TEMP MODE is in OFF, the vent blower does not operate.

 

If electric heat is selected, the vent blower operates at high speed.

 

 

LEFT–RIGHT BLEED AIR VALVES Switches

OPEN—The flow control unit is turned on and P3 is ducted to the cabin/cockpit for heating on the ground and heating and pressurization in the air.

 

CLOSED—The flow control unit is turned off. (Bleed air to the pneumatic system is on continu- ously and is not effected by this switch.)

 

CABIN TEMP Selector

In the automatic mode, this knob is used to set the cabin/cockpit temperature.

 

CABIN TEMP MODE Selector

MAN COOL—Cabin temperature is set by using the MANUAL TEMP switch to position the bypass valves and the air conditioner is enabled.

 

MAN HEAT—Cabin temperature is set by using the MANUAL TEMP switch to position the bypass valves.

OFF—The bypass valves remain in its last set- ting, which determines the amount of heat sent to the cabin/cockpit. (The vent blower will be off if it is set to the AUTO position.)

 

AUTO—The bypass valves positions automatically and the air conditioner turns on if required. (Electric heat can be turned on in the AUTO mode.)

 

ELEC HEAT Switch

NORM—Four electric heat elements are turned on and the vent blower operates at high speed. The electrical heat system operates independently of the bleed-air system.

 

GND MAX—Along with the four normal heat elements, an additional four, for a total of eight heat elements, are turned on. The switch is solenoid latched  in the ground maximum position and, on takeoff, the switch unlatches and physically drops to the NORM position.

 

OFF—All electric heat is off. Heating is available from the bleed-air heating system.

 

CABIN AIR–PULL DECREASE Knob

Pulling this knob decreases air to the cabin and increases air to the cockpit.

 

Right

 

COPILOT AIR–PULL ON Knob

Pulling this knob increases air to the cockpit and decreases air to the cabin.

FLOW CONTROL UNIT

LJ-502 AND AFTER LW-1 AND AFTER

 

Each flow control unit consists of an ejector and an integral bleed air modulating valve, firewall shutoff valve, ambient air modulating valve, and a check valve that prevents the bleed air from escaping  through  the  ambient  air  intake.  The flow of bleed air through the flow control unit is controlled as a function of atmospheric pressure and temperature. Ambient air flow is controlled as a function of temperature only. When the bleed air valve switches on the copilot left subpanel are turned on, a bleed air shutoff electric solenoid valve on each flow control unit opens to allow the bleed air into the unit. As the bleed air enters the flow control unit, it passes through a filter before going  to  the  reference  pressure  regulator. The regulator will reduce the pressure to a constant value (18 to 20 psi). This reference pressure is then directed to the various components within the flow control unit that regulate the output to the cabin. One reference pressure line is routed to the firewall shutoff valve located downstream of the ejector. A restrictor is placed in the line immediately before the shutoff valve to provide a controlled opening rate. At the same time, the reference pressure is directed to the ambient air modulating valve located upstream of the ejec- tor and to the ejector flow control actuator. A pneumatic thermostat with a variable orifice is connected to the modulating valve. The pneumatic thermostat (pneumostat) is located on the lower aft side of the fireseal forward of the firewall. The bimetallic  sensing discs of the thermostat are inserted into the cowling intake. These discs sense ambient temperature and regulate the size of the thermostat orifices. Warm air will open the orifice and cold will restrict it until, at -30ºF, the orifice will completely close. When the variable orifice is closed, the pressure buildup will cause the modulating valve to close off the ambient air source. An ambient air shutoff valve, located in the line to the pneumatic thermostat is wired to the LH landing gear safety switch. When the aircraft is on the ground, this solenoid valve is closed, thereby directing the pressure to the modulating valve, causing it to shut off the ambient air source. The exclusion of ambient air allows faster cabin warmup during cold weather operation. An electric circuit containing a time delay relay is wired to the above mentioned solenoid valves to allow the LH valve to operate 2 to 3 seconds before the RH valve. This precludes the simultaneous opening of the modulating valves and a sudden pressure  surge  into  the  cabin. A  check  valve, located downstream from the modulation valve, prevents the loss of bleed air through the ambient air intake. The ejector flow control actuator is connected to another variable orifice of the pneumatic thermostat and a variable orifice controlled by an isobaric aneroid. The pneumostat orifice is restricted by decreasing ambient temperature and the isobaric aneroid orifice is restricted by decreasing ambient pressure. The restriction of either orifice will cause a pressure buildup on the ejector flow control actuator, permitting more bleed air to enter the ejector.

TEMPERATURE CONTROL

 

LJ-1 THROUGH LJ-501

 

A four-position mode switch controls both the King Air combustion heater and the freon type air cooler. When switched to the AUTO position, both heating and cooling are automatically controlled through the cabin temperature control box. Adjust the comfort level  with the temperature controls in the cabin and on the instrument panel.

 

In either the MANUAL HEAT or MANUAL COOL position, the automatic temperature controls no longer work. The heater is cycled by a 225ºF thermal switch in the heater air outlet duct while the cooling mode is cycled by a 35ºF thermal switch on the evaporator.

 

To select automatic operation, turn the MODE switch to the AUTO position and adjust the cockpit temperature control for the desired temperature. Small temperature adjustments may be made with the cabin temperature control, but the main control is in the cockpit. The cabin temperature control box then selects either heating or cooling mode automatically as need to maintain the selected temperature. A 4ºF dead zone between heating and cooling modes prevents unneeded cycling between the two.

SYSTEM OPERATION

 

Fresh air for the air-conditioning system comes from  two  sources  either  ram  air  through  the nose  air  intake  during  non-pressurized  flight or supercharger air during pressurization or SCHGR-VENT operation. Air from either source enters the evaporator plenum chamber and is distributed through two sets of ducts. One set sup- plies the overhead eyeball outlets which furnish a constant source of high pressure cool air. The floor level outlets supply conditioned air to the cabin from the other set. Windshield defroster air comes from the conditioned air duct.

For best air circulation during either heating or cooling in LJ-3 through LJ-56, except LJ- 5, have the ventilation blower operating in the LO posi- tion.  During  ground  operations  (LJ-3  through LJ-69,  except  LJ-5)  the  landing  gear  safety switch automatically turns the ventilation blower on to the LO position. After takeoff, the landing gear switch opens and the Vent Blower switch then  controls  the  ventilation  blower.  On  LJ-l, LJ-2, LJ-5, LJ-23, LJ- 69 and after, and aircraft reworked to service letter 66-4, the ventilation blower turns on to the LO position anytime the air conditioner system is operating. For increased air circulation the vent blower switch is turned to HI.

Sometimes,  during  cooling  mode  of  operation on LJ-3 through LJ-97, except LJ-5 and LJ-23, the ventilation blower may stop operating. This is caused by the thermal overheat switch on the cooling modes compressor motor switching off the  blower  whenever  the  motor  overheats.  On LJ-l, LJ-2, LJ-5, LJ-23, and LJ-69 and after, the thermal overheat switch turns off the compressor motor itself whenever the motor overheats. As soon as the motor cools, the thermal overheat switch turns either the blower or motor back on.

To increase air conditioner cooling effectivity, open all eyeball outlets to minimize back pressure on the evaporator blower and close the pilot air outlet to prevent sub cooled air from circulating through the evaporator and choking the evaporator capacity.

 Manual Heat

 

When the cabin temperature control switch is placed in the manual heat position, electrical power is supplied to the normally closed 225ºF manual cycle switch opens the fuel solenoid valve completing  the heater  operation. The heater  is cycled by the 225ºF cycle switch operating the fuel solenoid valve. In the event of an overtemperture, the normally open 300ºF thermal switch will close, shorting the circuit to ground, causing the fuse to blow and the heater will stop, due to the loss of power to the fuel valve and fuel pump. All other will continue to operate. LJ-396 and after the basic circuit has been altered to incorporate the addition of a Heater Out Time-Delay Relay, Heat Out Annunciator Light, Heater Purge Relay,

125ºF Purge Switch, and a Heater Shut-DownRelay. With the Mode switch in Manual Heat, power is supplied to the Heat Out Time-Delay Relay (6 minute timer) which has a ground through the de-energized contacts of the Heater Purge Relay. If the heater fails to light off within

6 minutes the Heater Out Light will illuminate. Power also goes to the Combustion Blower Relay which gets its primary ground through the 25 ohm Heater Shut-Down resistor. With the Blower Relay energized, power is taken from the 15 amp circuit breaker to run the Combustion Air Blower, and simultaneously puts power through the amp fuse across two pressure switches to energize the Heater Shut-Down Relay. When the Shut-Down Relay energizes, this puts power to the Ignitor and Fuel Pump, and also to the 225ºF cycling switch to energize the fuel valve solenoid in the nose wheel well. As heater temperature rises to

125ºF, the contacts of the Heater Purge switch will close. This allows power to the Purge Relay which energizes to remove ground to the 6 minute Time-Delay Relay. Power from the 125ºF Purge Switch is sent to the vent blower relay to provide a holding lock on the relay during shut-down of the heater. The heater is cycled by the 225ºF cycle switch operating the fuel valve solenoid. All other units continue to operate.

 

Auto Mode Heat

 

The main difference in automatic versus manual heat is that power is applied to an automatic control box which will determine what mode will function. The combustion air blower relay will electrically “latch” closed by power from the circuit breaker through the relay coil and a diode, maintaining current flow through the relay coil. This means that when the automatic temperature control box arrives at the desired temperature level, electrical power will be removed from the Heat wire output to the fuel solenoid valve and fuel pump. The combustion air blower and ignition will continue to operate, thus clearing the heater. This is one advantage in using the automatic mode. The combustion blower and ignition will continue to operate until the freon system comes on or the mode switch is moved out of automatic. LJ-396 and after the automatic mode is generally the same as described for the early aircraft. The main difference is that when the temperature control box removes power to the heater wire, only power to the fuel solenoid valve is removed. The ignitor, fuel pump, combustion air blower, and the vent blower continue to operate.

 

Manual Cool

 

When the cabin temperature mode switch is placed in the manual cool position, power is sup- plied  to the vent blower and a normally  35°F thermal switch located on the face of the evaporator. The switch opens below 35°F to prevent ice from forming in the ducts. Power from the 35°F switch continues on to actuate the combustion air blower shut-off valve and the freon power relay. The freon compressor motor will come on, receiving power from a 150 amp circuit breaker.

Auto Mode Cool

 

As in auto heat, power is supplied to the automatic  temperature  control  box. The  automatic temperature control box takes information from a balanced Wheatstone bridge circuit made up of a cabin air sensor (thermistor blower), outside air temperature sensor, heater discharge sensor, and two series connected potentiometers. The control  box  will  then  apply  power  out  of  the Heat or the Cool wire or remain open. LJ-76, LJ-114 and after when a cool demand follows the heater operation the Combustion Air Blower (and ignition) relay must be unlatched. This is done through the time delay relay. The power output from the control box is applied to the heating element of the time delay relay and at the same time will go through the diode to the 25 ohm resistor, through the temperature mode switch and to the output of the combustion air blower relay. This creates the same voltage on each side of the relay coil leaving no voltage drop across the coil and the relay unlatches. The time delay relay closes after 5 seconds actuating the freon system as in manual cool. The time delay relay will cause the freon system to delay 5 seconds when first turned on in auto, if a freon demand is presented by the automatic system.

HEATER

 

LJ-1 THROUGH LJ-501

 

When the heater is in operation, a combustion blower furnishes air to mix with the fuel for combustion. If air flow through the heater is too low to support combustion, a differential pressure switch senses this and shuts off fuel flow to the heater until air flow is restored. The ignitor is on continuously during heater operation with temperature controlled by turning the fuel supply on and off.

PTP  Beech 90 Series	B1	LOC	FOT	SGH		MEL	TS
	B2	LOC	FOT	SGH		MEL	TS