By Selele Mashilo
Air conditioning for comfort is required in aeroplanes at various altitudes especially for temperature and cabin pressure control.
Additional equipment requirements must be met, over and above conventional air conditioning systems. The equipment must be compact, lightweight, easily accessible for maintenance, vibration free and unaffected by aeroplane vibration and landing impacts.
Design conditions vary according to altitude for hot, standard and cold days. We are used to heat load calculations based on a particular area for summer and winter conditions. The difference with aeroplanes is that they experience various temperatures and pressures at various altitudes within short spaces of time. Normal temperature rate of change during climb and descent can be about 0.09⁰C/s and might be as high as 0.28⁰C/s. Moisture control is critically important for the occupants of the cabin and crew areas. Remember also that the moisture can add to aeroplane weight. In a pressurised aeroplane, the amount of air flowing into the cabin should be equal to air leakages arising from structural leakages like door seals, skin joints, toilets and electronic equipment vents.
Cooling and heating loads are determined by a heat transfer study and analysis of the solar and the internal heat transfer from occupants and electrical equipment. Air film coefficients which vary with altitude must also be taken into account considered. It needs to be considered that the higher you go the colder it gets and therefore the lower the atmospheric pressure design conditions become.
Cabin air sources and distribution
Air entering the cabin is for pressurisation and ventilation. The air is from compressors which are engine or air turbine driven. Where gas turbines engines are used, air may be bled directly from engine compressors. The recommended ventilation rate is 2.4ℓ/s per passenger and 14.2ℓ/s for cockpit crew members.
Refrigeration systems of aeroplanes use air as the refrigerant. Commonly used are air cycle systems, vapour cycle systems or a combination of the two.
Air cycle system:
Refrigeration in a simple air cycle is achieved by three steps:
- heat transfer
- expansion by the turbine
Air is first compressed by the compressor to a pressure higher than the space to be cooled. The heat of compression is removed from the air by a heat exchanger rejecting it to a suitable heat sink such as air. In the third step, air is cooled by extracting work from it as it expands through a turbine.