BRITISH leasing company Angel Trains has developed a refrigerant-free air-conditioning system in co-operation with Liebherr Verkehrstechnik of Germany, and is considering a range of applications on new or refurbished rolling stock.
Since air-conditioning was first introduced on British trains in 1971, most systems have relied on vapour compression cycle technology, using chemical refrigerants. In recent years there has been pressure to reduce or eliminate CFC refrigerants which damage the environment. However, alternative refrigerants proved costly and reduced equipment reliability. As a result, Liebherr has been developing air-cycle air-conditioning for German Railway, using aerospace technology.
The heart of Liebherr’s open-loop sub-pressure unit is a Motor Air Machine: a combined motor - turbine - compressor operating at up to 32000 rev/min. The MAM drive shaft is carried by active magnetic bearings, which require no lubrication and have a design life of over 30 years. An electric coil maintains the air gap at 0·8mm, and roller bearings are provided to ’catch’ the shaft in the event of any power loss. The drive has been tested for impact loads up to 6 g.
The turbine draws in fresh air at atmospheric pressure, and then reduces the pressure to lower the temperature, typically to around 2íC. This cold air is used as a refrigerant, passing through a heat exchanger that cools the air from the vehicle interior. The process air returns to the MAM, where it is compressed to above atmospheric pressure and expelled from the system.
Angel and Liebherr agreed in mid-1999 to fund jointly a trial of air-cycle equipment on two MkIII open standard HST trailer coaches leased to First Great Western. A third coach with vapour-cycle equipment was included as a ’reference’ vehicle, and all three were fitted with data logging equipment, GPS location finders and GSM telephone communications. Performance was monitored remotely, giving a very good insight into actual service conditions.
Because the HST auxiliary supply is tapped from the main generator, the three-phase supply can vary between 260V at 32Hz and the full 415V at 50Hz, with the available power varying from 14 to 26 kW against a design air-conditioning requirement of 25 kW. Monitoring of the HST on London - South Wales services found that the train was only providing full power for around 20% of the time. Even though the air-cycle unit has a coefficient of performance close to 1, compared to 2 for a vapour-cycle unit, the Liebherr equipment required no more power over a full year than the reference vehicle’s unit.
During the six-month trial, availability exceeded 99%, and no maintenance was required beyond the routine changing of air filters. Cooling, heating and ventilation performance matched that of the reference vehicle, with no increase in power consumption and a reduction in noise and vibration. Direct comparisons between two cars in the same HST set showed better control of temperature, with a reduction of peaks and troughs around the target figure. Subjective views from passengers and train crew were that the air-cycle coaches were particularly comfortable during summer peak temperatures.
CAPTION: By monitoring the power supply to the air-conditioning in real time, Angel Trains and First Great Western were able to track the performance of the test HST units on the London - Bristol / South Wales corridor