Case Study – Solutions that fix identified problems and deliver a lower cost outcome

Coachair provides a bus operator with a full audit of the fleet as part of our service offer to maintain their ACs. Amongst issues identified was a problem particular to one of the chassis / AC configurations that accounted for the majority of in-service failures and ad-hoc repairs.

Coachair established that the unit would not operate on days where the temperature was above 30C, and even though the compressors were regularly overhauled, they would not last longer than 24 months.

The Coachair solution:

A complete design analysis was performed on both the AC system and the application onto the bus chassis. Coachair identified that the system operated on R407c but was designed for R134a and therefore was imbalanced. Further, the screw compressor was directly mounted to the engine and effectively became a heat sink causing it to run at higher temperatures. The screw compressor was not designed for these extreme temperatures.

Coachair developed a cost-effective solution that provided a lower cost outcome and effectively eliminated the in-service failures. This solution provided the following:

1. Replacement of the screw compressor with the manufacturer’s recommended compressor for which the engine was designed to accommodate. This compressor was one-third of a new screw compressor’s cost and 20% cheaper than an overhauled screw compressor.
2. The new compressor incorporated an unloading feature. This feature, alongside the required Coachair control logic alterations, allowed the compressor to operate within the design conditions necessary for R407c to reduce the failures further.
3. An additional feature was High Ambient Unloading. Due to the system imbalance, the unit operating on R407c would always trip out at ambient temperatures above 30-35C. The Coachair control logic would unload the compressor to a reduced capacity if the pressures were too high. Cooling would still be provided for passenger comfort however would not trip the system into fault and become inoperable.

As a result of this program, buses were no longer pulled from service due to high-pressure trips. In addition, the compressor overhaul/replacement program increased from two to six years and the need for ad-hoc repairs was significantly decreased on these vehicles.