Environmental Systems

seat row may be equipped with a separate self-contained generating unit. There are several important disadvantages to chemical generator systems, the first being that once generators are activated there is no way to turn off the flow of oxygen. Units continue to supply oxygen until all of the chemicals are expended. Another disadvantage is that the chemical reaction required to produce oxygen generates heat, posing the risk of passengers burning themselves if the generating unit is touched.

Environmental systems on large turbine-powered aircraft are complex, compared with the light aircraft systems that you may have experienced in the past. An airline aircraft is so large that balancing temperatures throughout the vehicle can be challenging. Besides, these aircraftoperate through such a huge range of temperatures that their systems must be quite versatile. A given airplane might sit for two hours on a 105°F ramp, then experience temperatures of –40°F in flight.

In piston aircraft, cabin heat is generated through combustion heaters or engine exhaust heat. Turbine environmental systems, however, get their heat from engine bleed air. Modified bleed air is introduced into the aircraft for pressurization and heating, and in many cases, for cooling.

Proper operation of environmental systems is rather critical in pressurized turbine aircraft. Since pressurization air is high-pressure bleed air (read “hot”), all environmental air must be cooled, to some degree, before it’s put into the cabin. You may be used to thinking of air conditioning as a luxury in aircraft, but in many turbine aircraft out-of-service environmental cooling equipment means “no-go.”

A combination of heat exchangers, air cycle machines, and/or vapor cycle machines are used to modify and control cabin temperatures in turbine airplanes.

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hot cold

Heat Exchangers

Heat exchangers are simple, passive devices that transfer heat between two different fluids (Figure 5.13). Your car radiator is an excellent example of a heat exchanger. Air passing through the radiator from the grill absorbs heat from the engine coolant pumped through the radiator core. In aircraft, heat exchangers are used to absorb and remove heat in a variety of applications within the environmental system and elsewhere.

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heat is transferred from hotter fluid (gas) to cooler one

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Heat exchangers: When two fluids (including gases) of different temperatures come in contact, heat is transferred from the hotter fluid to the cooler one. Heat exchangers use this principle for temperature control in many turbine aircraft applications, including bleed and environmental systems, oil coolers, and fuel heaters. cold

Air and Vapor Cycle Machines

Since bleed air coming from the engines is already hot, the environmental system’s challenge is to cool it. The two devices commonly used for this purpose, air cycle machines (ACMs) and vapor cycle machines (VCMs or “Freon units”), are related in that each works on similar physical principles. When a gas is compressed, it gets hot. When expanded, gas cools, meaning that it transfers heat to the surrounding air. The amount of heating or cooling is proportional to the change in volume of the gas (see Figure 5.14).

If you start with a liter of gas at a given temperature and compress it to a smaller volume, the compressed gas will be hotter than it was originally. Now if you remove some of the heat from that compressed gas by blowing some cool air past it (say, through a heat exchanger) and then expand it back to its original volume again, it will be cooler than it was to begin with. This is the basic operating principle of both air and vapor cycle machines.

Air Cycle Machine (ACM)

In air cycle machines, high-pressure bleed air from the engines is first passed through a compressor, further squeezing the already hot gas. It is then routed through a heat exchanger or two to remove heat. The now cooler but still highly compressed air then passes through an expansion turbine into a larger chamber. The combined effects of driving the turbine and expanding into a larger chamber dramatically cools the air (usually down close to freezing; water traps are critical in the system to prevent freeze-up).

The expansion turbine is connected by shaft to the ACM’s compressor, so expanding air works to compress upstream bleed air similar to the way a turbine engine or a piston engine turbocharger works. This cycle may be repeated several times, with the end result that system air temperature is cooled far below ambient temperature (Figure 5.15).

Vapor Cycle Machine (VCM)

A vapor cycle machine, when installed in your car or home, is otherwise known as an air conditioner. There are several key differences between an ACM and a VCM. One is in what material is

ACM

air cycle machine 1. Warm air enters unit. 2. Air is compressed, generating heat. 3. Heat is removed from air.

HEAT 4. Air is expanded, now cooler than it was upon entering unit.

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VCM

vapor cycle machine 1. Warm refrigerant gas enters unit. When gas is compressed, heat is produced.

2. Gas is compressed, generating heat. Fan or ram air carries away heat

When gas is expanded, heat is absorbed from the environment.

3. Heat is removed from refrigerant, causing it to condense. 4. Refrigerant absorbs heat and evaporates, now cooler than it was upon entering unit.

compressed and expanded to do the cooling. While air cycle machines use air for this purpose, vapor cycle machines use refrigerants specially selected for cooling capacity. (Refrigerants have higher thermal capacities than air, so they transfer more heat on each cycle.)

The most important difference is that VCMs take advantage of another physical property that greatly adds to their efficiency. As you may remember from high school physics, a great deal of energy is absorbed when a substance changes phase from liquid to gas. Refrigerants (such as Freon) are designed to undergo phase changes with every cycle of temperature, compression, and expansion. Refrigerant gas is compressed in a VCM’s compressor. It is then run through a special heat exchanger, known as a condenser, where heat is removed. As the gas cools under pressure, it condenses into a liquid (hence the name “condenser”). The liquified refrigerant continues on its journey to another heat exchanger, the evaporator, which interacts with cabin air. As the name implies, the refrigerant is allowed to drop in pressure in the evaporator. As it evaporates (another phase change), the refrigerant absorbs a tremendous amount of heat from the passing cabin air. The cooled air is returned to the cabin, while for the refrigerant it’s off to the compressor again to start a new cycle. (See Figure 5.16.)

Why two different types of machines? Air cycle machines are ideally suited for turbine aircraft due to the supply of (already) compressed bleed air, reasonably simple systems, and no need for special coolants. On the other hand, ACMs require significant volumes of bleed air, and turbine components make ACMs relatively expensive. Large aircraft always have ACMs installed because of their economy of use, hefty pressurized air (bleed) sources, and the need to process large volumes of air.

VCMs, on the other hand, are efficient, significantly less expensive, and are well suited to aircraft with limited engine bleed capacity. A vapor cycle machine does, however, require a separate mechanical compressor, which adds complexity and weight.

Small turboprops and corporate jets, in many cases, have vapor cycle systems installed. This is particularly true of older corporate aircraft, which tended to have less available power and, therefore, less bleed capacity. Since newer turbine aircraft generally have more powerful engines, and ACM technology has developed rapidly for smaller aircraft, ACMs are more common in newer models.

2. Compressed bleed air is cooled by outside air via heat exchanger. ram air cool air to cabin

heat exchanger

1. Hot, high-pressure bleed air is further compressed.

high-pressure bleed air compressor expansion turbine

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air cycle machine

Note “family resemblance” of ACM compressor/expansion turbine assembly to turbine engine (and to piston engine turbocharger).

FIGURE 5.15 | Air cycle machine (ACM).

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recirculated cabin air

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3. Air expanding to ambient pressure turns expansion turbine, is greatly cooled, and drives compressor.