Explanations: Cabin Atmosphere Control Systems

Question 1

What is the turbine engine compressor?

Answer 1

It increases the pressure of he air to provide conditions favorable for combustion and expansion of the hot gases through the turbine.

Question 2

When does the refrigeration process take place in an air cycle cooling system?

Answer 2

as the compressed air expands through the turbine wheel of the air-expansion turbine.

Expansion of the air reduces both its temperature and its pressure.

Question 3

What type of air passes through the condenser coils of a freon vapor-cycle cooling system?

Answer 3

It is ambient, or outside, air and it removes the heat from the heated and pressurized refrigerant. The loss of this heat causes the refrigerant to condense from a vapor into a liquid.

Question 4

Where does ventilating air get its warmth?

Answer 4

It picks up heat from the heater and carries it to places in the aircraft where it is needed.

Question 5

What is an expansion turbine?

Answer 5

A centrifugal or axial-flow turbine, through which a high-pressure gas is expanded to produce work that is often used to drive a compressor or generator.

Question 6

Where is combustion air for each cabin heater received?

Answer 6

through either the main air intake or a separate outside air scoop.

To prevent too much air from entering the heaters as the air pressure increases, either a combustion-air-relief valve or a differential-pressure regulator is provided.

Question 7

Where is the combustion air relief valve located?

Answer 7

in the line leading from the ram intake air duct and is spring-loaded to dump excess air into the cabin-heater exhaust-gas stream.

Question 8

Where is the differential pressure regulator located?

Answer 8

Also the combustion-air intake line. however, it controls the amount of air reaching the combustion heater in a slightly different manner to the combustion air relief valve.

Question 9

What is a combustion heater?

Answer 9

A combustion heater is a heater that burns stuff to make heat. The “stuff” is generally natural gas or propane, but oil burners are also combustion heaters. So, too, are pellet stoves and coal-burning fireplaces. Electric heaters are NOT combustion heaters; nor are heat pumps combustion appliances.

Question 10

What is a combustion air system?

Answer 10

combustion air systems provide a continuous and controlled stream of intake air into the mechanical space containing the combustion heating system. They help furnaces or boilers function by ensuring they are supplied with the correct amount of fresh air to support complete combustion.

Question 11

What controls the amount of air leaving the cabin?

Answer 11

The cabin altitude controller controls the outflow valves that regulate the amount of air that leaves the cabin.

Question 12

What does the turbine in an air-cycle cooling system do?

Answer 12

It extracts energy from the compressed air and expands it to drop its temperature.

Question 13

When does the dump valve relieve all positive pressure from the cabin?

Answer 13

When the aircraft is on the ground.

Question 14

What happens if liquid refrigerant is introduced into the low side of a vapor-cycle cooling system?

Answer 14

It will likely not change into a vapor before it gets into the compressor and cause the reed valves in the compressor to be damaged.

Question 15

What is the low side of a vapor cycle cooling system?

Answer 15

The Low Side: Absorbing the heat. Cold, low pressure refrigerant leaves the expansion valve and enters the evaporator coil. The blower fan moves warm indoor air over the coil, and the refrigerant absorbs heat from the air. As it does so, it turns into gas form.

Question 16

Where is the sight glass in a vapor-cycle cooling system?

Answer 16

in the reciever-dryer.

Question 17

When charging a vapor-cycle air cooling system after it has been evacuated, refrigerant is put into the high side of the system.

If the compound gauge, connected to the low side, fails to come out of a vacuum, there is a blockage in the system. The low side is not getting any refrigerant.

Answer 17

Obviously, you dumb fuck.

Question 18

What does it mean to evacuate a vapor-cycle air cooling system?

Answer 18

A vapor-cycle cooling system is evacuated to remove any water that may be present. A vacuum pump is attached to the system and the pressure is lowered. This lowers the boiling point of any moisture, which then changes into a vapor and is removed by the vacuum pump.

Question 19

What does it mean to charge a vapor-cycle cooling system?

Answer 19

When charging a vapor-cycle air cooling system after it has been evacuated, refrigerant is put into the high side of the system.

Question 20

what is a compound gauge in a vapor cycle cooling system?

Answer 20

(low side) All air conditioning systems can, under certain conditions, drop from a pressure into a vacuum on the low side. It is necessary that a gauge be used that will show either pressure (psi and kPa) or inches of mercury vacuum (Hg.).

Question 21

What is the receiver-dryer in a vapor cycle cooling system?

Answer 21

The receiver/drier functions as the filter for the A/C system, removing contaminants and moisture from the refrigerant. Receiver/driers are always found on A/C systems which have an expansion valve and are located on the high side of the system, usually between the condenser and the internal expansion valve.

Question 22

What is an expansion valve in a vapor cycle air cooling system?

Answer 22

The expansion valve removes pressure from the liquid refrigerant to allow expansion or change of state from a liquid to a vapor in the evaporator.

Question 23

What can stop the flow of refrigerant through the expansion valve?

Answer 23

It only takes a ting particle of dirt or other contaminant to stop the flow of refrigerant through the tiny orifice in this valve.

Question 24

What is a hydrostatic test of a high pressure oxygen cylinder?

Answer 24

the cylinder is filled with water and pressurized to 5/3 of their working pressure.

Question 25

How often should lightweight cylinders be hydrostatically tested?

Answer 25

every 3 years.

Question 26

What happens during the hold and exhalation periods of breathing through a demand-flow system?

Answer 26

the oxygen supply is stopped. Thus, the duration of the oxygen supply is prolonged as none is wasted.

Demand-flow systems are used most frequently by the crew on high performance and air transport category aircraft.

Question 27

How is the selected cabin altitude maintained in a pressurized aircraft?

Answer 27

by the controller modulating (opening or closing as needed) the outflow valve.

The safety valve is open when the aircraft is on the ground and is closed in flight unless the cabin pressure exceeds a preset value.

Question 28

Explain hot compressor bleed air or i’ll kick your ass.

Answer 28

Hot compressor bleed air loses some of its heat energy as it flows through a primary heat exchanger. This slightly cooled air then flows into the air-cycle machine where it is compressed and its temperature and pressure are both increased. This heated air then passes through a secondary heat exchanger, where it gives up some of its heat energy, and then it flows into the air-cycle expansion turbine. here it uses up a large amount of its heat energy in driving the compressor. The air expands as it leaves the turbine and becomes cold.

Wtf was all that nonsense?

Question 29

Explain an air cycle cooling system again. You gotta get this shit.

Answer 29

In an air-cycle air conditioning system, hot compressor bleed air mixes with bleed air that has been cooled in the primary heat exchanger. The air leaving the mixer is held at a constant temp of 300 degrees F.

This 300 degree F is compressed by the refrigeration unit compressor, then flows through the secondary heat exchanger for its intitial cooling.

After giving up some of its heat in the secondary heat exchanger, it flows through the expansion turbine where is gives up more heat energy for its last stage of cooling.

From the expansion turbine, the cold air is mixed with just enough 300 degree F air to maintain its temperature at the level called for by the cabin temperature regulator.

Question 30

Now explain a vapor cycle cooling system.

Answer 30

Refrigerant in a vapor cycle cooling system enters the expansion valve as a high-pressure liquid and sprays out into the evaporator coils as tiny droplets of low pressure liquid refrigerant.

Heat from the cabin enters the refrigerant in the evaporator coils and changes the liquid into vapors.

The refrigerant vapors are compressed in the compressor where their temperature and pressure are both increased. The hot, high-pressure refrigerant gas leaves the compressor and flows into the condenser.

In the condenser, ambient air blowing over the coils absorbs heat from the refrigerant and it changes back into a liquid. After passing through the receiver-dryer to filter out moisture and any contaminants, it is ready to begin its trip through the system again to pick up more heat from the cabin.

Question 31

How is cabin pressurization restricted when the aircraft is on the ground with the cabin closed?

Answer 31

the safety dump valve is held open by vacuum controlled by dump valve solenoid, which is actuated through the landing gear safety, or squat, switch.

As soon as the aircraft is airborne and the weight is off the landing gear, the switch allows the dump solenoid to close the safety dump salve, allowing the cabin to be pressurized.

Question 32

What does the mixing valve in an air-cycle air conditioning system do?

Answer 32

It mixes air from three sources to get cabin pressurizing air of the desired temperature.

Question 33

What are the three sources of air in an air-cycle air conditioning system?

Answer 33

hot air before it passes through the primary heat exchanger.
cool air after it has passed through the secondary heat exchanger.
cold air after it passes through the air-cycle machine.

Question 34

What should be done when bubbles can be seen in the sight glass of a vapor cycle cooling system?

Answer 34

the system should be operated for a period of time to allow the system to stabilize, then the sight glass rechecked to see if it still indicates low refrigerant charge.

Question 35

If the outflow valve is not adjusted to close faster during a high rate of climb, what is at risk of occuring?

Answer 35

The cabin altitude may change too fast.

Question 36

What is up with the thermostatic expansion valve?

Answer 36

It is an adjustable restrictor. It controls the fluid that flows into the evaporator in a central cooling system or heat pump. This enables the system to control the coolant flow rate and pressure in the compressor.

Question 37

Okay google. That was all well and good, but what is the deal with the thermostatic expansion valve according to the study guide?

Answer 37

The position of the thermostatic expansion valve is controlled by the temperature of the refrigerant as it leaves the evaporator. This temperature is sensed by the sensing bulb of the expansion valve.

An externally equalized thermal expansion valve also sense the pressure at the outlet of the evaporator to compensate for the pressure drop across a large evaporator.

Question 38

What is a jet pump in a pressurization and air conditioning system?

Answer 38

A special venturi in a line carrying air from certain areas in an aircraft that need an augmentated airflow through them. Jet pumps are often used in the lines that pull air through galleys and toilet areas.

A nozzle blows a stream of air into the throat of the venturi. This increases the velocity of the air flowing through the venturi and produces a low pressure that pulls air from the compartment to which it is connected.

Question 39

What are the isobaric, differential, and unpressurized, cabin pressurization modes of operation

Answer 39

isobaric - the pressure inside the cabin remains constant.

differential - a constant difference is maintained between the pressure inside the cabin and the pressure of the outside air.

unpressurized is not pressurized. duh.

Question 40

what does a pressurization controller use barometric pressure, cabin altitude, and cabin rate of change for?

Answer 40

It uses cabin altitude for isobaric control, barometric pressure for the differential range of control, and the cabin rate of climb controls the rate of pressure change inside the cabin.

Question 41

How does a vapor-cycle cooling system remove moisture?

Answer 41

a vacuum pump is attached to the system and the pressure is lowered. This lowers the boiling point of any moisture, which then changes into a vapor and is removed by the vacuum pump.

Question 42

How must freon be added to a vapor cycle cooling system?

Answer 42

Care must be used when adding liquid freon. The outside air temperature must be high enough for all of the liquid to change into a vapor before it reaches the compressor.

Question 43

Where is the sub cooler in some vapor cycle cooling systems?

Answer 43

between the receiver dryer and the expansion valve.

Question 44

What pressure should oxygen bottles never be allowed to drop below?

Answer 44

50 psi, because this risks moisture to collect and further risk corrosion.