An aircraft air-conditioning cart may be an electric or diesel-powered mobile cooling unit. An electrical cart has the advantage of lower noise and pollution levels on the ramp. However, a diesel-powered air-conditioning cart is useful if the electrical power is unreliable or lacking at the airport.
Parked Aircraft Ventilation
When the aircraft is on the ground with its engines shutdown, the aircraft needs air-conditioned air from an external source. This is provided by specially designed movable air-conditioning cart also known as PCA (Pre-Conditioned Air) system. This packaged unit contains a complete air conditioning system and air delivery source. They are used to pump fresh conditioned air into an aircraft cabin while the aircraft is parked at a gate or stationed at a maintenance site. It is necessary to keep a convenient air temperature for the comfort of the passengers as there may be harsh ambient conditions outside the aircraft. The air is either cooled or heated, filtered and moisture is removed from it, as necessary then pumped into the aircraft. The output air from the aircraft air conditioning unit travels through hoses that are connected to the aircraft underbelly. The conditioned air is then is distributed in the aircraft through the aircraft’s cabin ventilation system.
Use of APU
Air conditioning carts eliminate the use of the aircraft’s Auxiliary Power Unit (APU) and its related noise and exhaust during boarding and deplaning while the aircraft is at the gate. This results in significant cost savings to airlines as jet fuel consumption is reduced. Not using the APU also results in reduced maintenance costs. Due to all these benefits air-conditioning carts are regularly used in place of APUs for aircraft cooling on the ground. An air conditioning cart is designed to provide the same ambient conditions as the aircraft’s built-in climate control system; therefore, it must match with equal performance.
Design of Air-conditioning Carts
Air conditioning carts are generally designed to be more robust and efficient as compared to conventional portable air-conditioning units. Due to the tough requirements of portable air-conditioning units for the aircraft, there are many challenges in the design of the air-conditioning carts as compared to conventional air-conditioning units. Commercial air conditioning units recirculate the air in a confined space and do not require more than 15°–20°F of temperature reduction, whereas an air-conditioning cart unit has to take the air from the ramp area, as it does not recirculates the air and has to filter and often cool it by 60°–70° or even more. In the heating mode sometimes a temperature increase of over 100° or more is required. Heating is provided by the electric heaters in the unit.
Connection to the Aircraft
Aircraft manufacturers provide ducts of standard sizes in their aircraft as well as the external connection point for this purpose. Air-conditioning carts are designed to deliver air at a specified air volume and static pressure since the distribution duct size is already determined. Outside air density, humidity, temperature, etc. all play a role in controlling the delivery of the rated volume and condition of the discharge air to the cabin.
Ancillary factors must also be considered as pumping the specified volume of air through a small duct is already a big challenge. The duct systems are designed to minimize drastic directional changes and long distances which may impede airflow. Every aircraft type may have a different hose connection location, which requires varying lengths of hose to get to the connection point. Air conditioning carts have a built-in hose retrieval system which is designed to only dispense the exact length of hose required for the type of aircraft at the gate.
A well-designed control scheme and some standard functions are necessary for proper control of the refrigeration components. All aircraft have unique air volume requirements as specified by their respective designers based on the relative size and passenger capacity of the aircraft.
To have better temperature control, the temperature sensing probe of the cabin air is connected to the air-conditioning cart. The air-conditioning cart’s control system then adjusts the air temperature based on the current cabin temperature and the set point.
An air-conditioning cart is also often used to quickly heat or cool the air bridge connected to the aircraft for boarding the passengers. A dedicated hose and control functions are built into the air-conditioning cart as an option for this purpose. However many airbridges have dedicated HVAC units mounted on their rooftops which can do the same job more efficiently.
Maintenance of Air-conditioning Carts
Regular maintenance of the air-conditioning cart is necessary to achieve optimal performance. It is necessary to clean the evaporator coils, filters, and condensers, etc. regularly to keep the units running efficiently. Air-conditioning carts are designed with advanced refrigeration technology and require the services of a trained technician with a high level of HVAC understanding, to effectively maintain, diagnose, and repair these systems.
e2b calibration is an ISO/IEC 17025:2017 accredited laboratory. We specialize in cost-effective repair and maintenance services of all types of ground support equipment. Please contact e2b calibration for more information.