Why PCA Unit is needed when AGPU is already there?

Another important question is why the aircraft does not use the power of the AGPU to run the air conditioning system when the aircraft ground power unit (AGPU) is already connected to the aircraft. Why is there a separate machine, the Preconditioned Air PCA unit for air conditioning?

The answer lies in studying how the aircraft's own air conditioning system works.We also discuss the aircraft's interior air conditioning system in our post on Air Starting Units (ASU), which we recommend you read to learn more about this matter.In short, Aircraft's interior air conditioning systems do not run on electricity like air conditioning systems for residential applications.The aircraft's on-board air conditioning is controlled by the air conditioning "package". Airliners typically have two of these "packs" for air conditioning requirements and redundancy.A "package" is a package of parts that make up an aircraft air conditioning unit. The unit includes a heat exchanger, air compressor and expansion turbine, collectively referred to as an "air cycle machine".Air cycle machines do not run on electricity.Therefore, it cannot be powered by the power it receives from the AGPU. Air cycle machines are responsible for converting the air received from the source into conditioned air that is ultimately supplied to the passenger cabin.Interestingly, the source air that the air cycler becomes the conditioned air is not actually the outside air received at the altitude of the aircraft.The source of this air is the jet engine of the aircraft and is called bleed air,which is the air that is expelled from the jet engine and supplied to the air cycle machine.At first glance,it might seem counterintuitive that the air that needs to be supplied to passengers actually comes from jet engines, which require high temperatures, combustion, and thrust. However, it is true.The air conditioning package (or air cycle machine) receives bleed air from any one of the jet engines of the aircraft or its auxiliary power unit (APU). This air has high temperature and pressure and cannot be used in air conditioning in its original form.Air cycle machines take this air through different stages of heat exchange and expansion,bringing its temperature down and making it suitable for air conditioning. 

Types of Preconditioned Air (PCA) Units Based on Power Source

Now that we have developed an idea of what preconditioned air is and why we need a preconditioned air (PCA) unit, let's take a look at the different types of preconditioned air (PCA) units available in the market.

Pre-conditioned air (PCA) units are divided into two types according to the power source,

1. Engine driven preconditioned air (PCA) unit.

2. Electric pre-air conditioning (PCA) unit.

Let's briefly look at the two types.

Engine driven preconditioned air (PCA) unit

Engine-driven preconditioned air (PCA) units are powered by an internal combustion engine, usually a diesel engine.The engine drives a generator to generate electricity to power the HAVC refrigeration compressors of the PCA units.These devices are long-established routines used at airports. The more advanced technology in the engine-driven PCA unit is the direct drive technology, the engine directly drives the refrigeration compressor, and the generator is removed in the middle.

Electric pre-conditioning air conditioning (PCA) unit

Electric preconditioned air (PCA) units operate on mains power. It receives electrical power as input from the airport building power supply.All of its compressors and internal circuits are powered by this source.Electric preconditioned air (PCA) units have no emissions on slopes and are generally more efficient in terms of energy consumption than engine driven preconditioned air (PCA) units. In addition, they are quieter to operate, so they are superior to engine-driven preconditioned air (PCA) units.

Based on installed preconditioned air (PCA) unit type

In addition to power supply, preconditioned air (PCA) units can be classified into three types according to the type of installation,

1. Bridge outfit.

2. Fixed to the ground (also called apron mount).

3. Mobile PCA unit.

Bridge Preconditioned Air (PCA) Units.

As the name suggests, bridge PCA units are mounted on jet bridges.They are suspended under a telescopic tunnel of the bridge with the aid of special mounting frames or brackets.Some configurations also allow the PCA unit to be mounted on the bridge instead of below it.Typically, bridge-mounted PCA units are electric PCA units. Engine-driven PCA units were not mounted under the bridge as they tended to generate more vibration,which was undesirable for the integrity of the jet bridge.

Fixed floor preconditioned air (PCA) units

Fix the PCA unit to the ground if it is not installed under a bridge. Such PCA units are mounted on aircraft supports (ramps) and are usually electrically powered.The supply air reaches the aircraft from the PCA unit by any of the following means,

1. Underground piping leads to the utility pit system to deliver conditioned air from the PCA unit to the aircraft via piping installed in the apron trench.

2. Ducts or ducts installed under the jet bridge receive conditioned air from the PCA unit, run it the entire length of the jet bridge tunnel and terminate in a flexible hose connected to the aircraft.

Mobile Preconditioning Air Conditioning (PCA) Units

The mobile PCA unit is a trolley mounted PCA unit that can be towed to any desired location or mounted on the chassis of a truck that can go anywhere. All components of these units are assembled on a platform mounted on a trolley or truck chassis with a tray for securing the PCA hose.

Energy and Environmental Considerations

Energy efficiency and environmental considerations play a key role in deciding whether PCA units used at airports are diesel engine driven or electric. In response to stringent aviation environmental protection regulations, electrically powered PCA units are becoming more common.Furthermore,electric PCA units are also more efficient from an energy efficiency point of view. Producing energy in bulk is always cheaper than producing localized energy on-site for a specific device. As a result, electric PCA units utilize electricity from the building rather than generating electricity on site like diesel engine powered PCA units.

Space Preservation & Ramp Safety

Space savings are also an important aspect when choosing the type of PCA unit to install on an aircraft stand, as the apron becomes a busy place during aircraft operations for various ground support operations.In this regard, bridge-mounted PCA units are preferred as they do not have any footprint on the ground. By freeing up space on the ramp, the space not occupied by the PCA unit can be used for movement of other ground support equipment.Freeing up space is important not only from an operational efficiency perspective, but also from a safety perspective. The more people and machines on a ramp, the greater the chance of an accident. By making ramps cleaner, ramp operations are also made safer.A fixed ground PCA unit can also be beneficial in this area if used with a utility pit system. The utility pit system allows the PCA hose to be ejected from the ground near the aircraft, rather than laying on a ramp where vehicles could accidentally run it over.Mobile PCA units are least efficient in this area as they must be towed by tractor to the apron and towed away from the apron when the flight takes off, creating the greatest movement. Also, they remain close to the aircraft, increasing crowds and vehicles on the tarmac.