Environmental Systems Flashcards
What is a heat exchanger?
A heat exchanger is simple, passive devices that transfers heat between two different fluids. 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.
What is the basic operating principle behind both air cycle machines / ACMs and vapor cycle machines / VCMs?
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.
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.
If you remove some of the heat from that compressed gas by blowing cool air past it - with 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.
What is an air cycle machine, and how does it work?
Air cycle machines cool hot, bleed air.
High-pressure bleed air from the engines is first passed through a compressor, further squeezing the already hot gas.
It is then routed through at least one heat exchanger and maybe more 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 in the system to prevent freezing.
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.
What is a vapor cycle machine?
In a home or car, a vapor cycle machine is called an air conditioner.
It’s different from an ACM in that…
1. A refrigerant gas such as freon is used instead of air to do the compressing and expansion. Refrigerants have higher thermal capacities than air, so they transfer more heat on each cycle.
- Refrigerants are designed to undergo phase changes with every cycle of temperature, compression, and expansion. That absorbs a lot of energy.
Refrigerant gas is compressed in the 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 then goes 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 the refrigerant returns to the compressor again to start a new cycle.
What are the pros and cons of air cycle machines / ACMs and vapor cycle machines / VCMs?
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 lots of bleed air, and turbine components make ACMs relatively expensive.
Large aircraft always have ACMs installed because of their economy of use, hefty pressurized bleed air sources, and the need to process large volumes of air.
VCMs, on the other hand, are efficient, a lot 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.
One other advantage of a VCM is that it can be set up to provide cooling on the ground, without an operating engine, APU, or external high-pressure air source.
While many VCM compressors are engine-driven, they can also be set up to operate from electric motors. That way, a crew sitting on a hot ramp can plug in ground power and cool down the passenger cabin before start-up.
Given the different efficiencies and benefits of ACMs and VCMs, many aircraft have both systems installed.
What is a PACK?
In large aircraft, the whole environmental heating/cooling system is bundled together, including ACM / air cycle machine, bleed heat source, VCM / vapor cycle machine (if installed), and mixing valves.
This package is commonly referred to as a “PACK.” Normally two are installed for capacity and redundancy.
