| THE MAIN AIR SYSTEM The compressed Air System on a Class 60 is designed by Westinghouse. The compressed air system is divided into three different working pressures:- 1) Main Air - 10 Bar. 2) Main Reservoir supply and pipe - 7 Bar. 3) Air Brake pipe - 5 Bar. (1 Bar = 14.5 PSI). System Description:  The locomotive has two 3 phase AC compressors, each compressor is
fitted with an air after-cooler. Air is drawn into each compressor
through its associated air intake filter and flexible hose. This air is
then delivered into the main reservoir via a high pressure flexible hose and check
valve. This check valve prevents a loss of air in the event of a
burst delivery hose or a failed compressor. The compressor is
governed in the normal way (even though it is an AC machine ). The governor pressures are nominally set to 8.5 Bar to start the compressors & 10 Bar to stop the compressors. In the event of a governor failure it can be isolated by operating the governor isolating cock. The system will then be regulated at 12 bar by a safety valve. A Metcalf/Salam air dryer, located in the main reservoir air line, ensuing that only dry air reaches all air operated equipment by removing moisture condensate from the compressed air supply. Further air filtration takes place before the air enters the main reservoirs. The compressed air system on the class 60 locomotive has three main air reservoirs, giving a maximum total capacity of 1000 litres. Each of these reservoirs is fitted with its own individual automatic drain and a manually operated drain cock. A low main air pressure governor is fitted to the system to remove traction power and apply the brakes should the pressure fall to 4.5 bar. This governor also ensures that the locomotive becomes /remains inoperative until the main air system has been charged sufficiently to ensure satisfactory operation of the brakes. Two brake supply reservoirs, one for each bogie, are fed via a strainer, check valve and choke valve. The brake supply reservoirs can also be supplied with air at 5 bar from the brake pipe via a similar strainer, check valve and choke valve. The brake supply reservoirs feed the distributors, each distributor having connections to :- 1) A control reservoir fitted with a release valve. 2) The brake pipe. 3) A brake selector cock giving Goods/Passenger timings. 4) The brake cylinders via isolating cocks, flexible hoses and double check valves. An air supply of 10 bar is also taken to the automatic air brake controller, the locomotive main reservoir pipe, and to each straight (locomotive onlu) air brake controller for operation of the straight air brakes. The cab gauges are also supplied to give an indication of the main reservoir pipe pressure. Metcalf - Salem "Twin Tower" Air Dryer: The dryer has been developed especially to suit traction requirements. The dryer removes moisture from the compressed air supply and ensures that only clean, dry air reaches all air operated equipment. This ensures that all air operated equipment functions more efficiently and reliably. The twin tower system includes coalescers desiccants. The coalescers remove impurities from the compressed air supply, and the desiccants, which are a type of molecular sieve and are located in the air dryer canisters, remove the moisture. After removing the moisture the desiccants are regenerated (restored to their original dryness) during the operating cycle. The two air dryers are connected by a common inlet manifold with integral pre - colescer, and a common outlet manifold. Also included is a memory type electronic timer which controls the system cycle and allows the unit to regenerate only when a pre - determined pressure (6.7 bar) is reached and the compressor is operating. Until such time both dryers allow a maximum flow of air to pass and charge the entire system. Air enters the inlet manifold and flows into the pre - coalescer where it passes through the coalescer element, this removes microscopic oil aerosols, a desiccant in the air drier then removes water vapour from the compressed air. With the compressor pumping and the pre - determined pressure reached, the R.H. side of the dryer begins to regenerate while the solenoid valve is energised allowing air to pass and to close the inlet check valve, at the same time the spring loaded outlet check valve closes, when both the inlet and outlet check valves are closed and the air dryer is isolated, the sump mounted purge valve opens and there is a sudden drop in air pressure, the drop in pressure forces the collected impurities from the air dryer to atmosphere whilst the moisture absorbed by the desiccant beads is released to the surface of the beads, the moisture released to the surface of the beads is picked up by the dry air and exhausted through the open purge valve at a level slightly above atmospheric pressure, at the same time any collected impurities are exhausted from the pre - coalescar to atmosphere. While the R.H. side is in the regenerating phase the L. H. side is in the dehydrating phase. This reverses every 60 seconds. Main Reservoir Supply and Pipe (7.0 bar system): The main reservoir pipe is coupled between locomotives coupled in multiple and between all vehicles in the train working on the two pipe system. The system is fed from the 10 bar main air supply via a combined filter/regulator, (which reduces the pressure to 7 bar) and a duplex check valve ( to prevent main air falling below 5 bar should the reservoir pipe rupture). The main reservoir pipe feeds a pressure gauge in each cab, also the brake pipe pressure control unit, the sanding valves and the warning horns.  The Air Brake System (5 Bar System):The automatic air brake on the locomotive is the traditional type, where the pressure is varied in the brake pipe. The pipe being continuous throughout the train is charged to 5 bar to release the brake and reduced accordingly to apply the brake. Each vehicle is fitted with a distributor which responds to the pressure in the brake pipe to apply or release the brake. On the locomotive each distributor (of which there is one for each bogie) is connected to:- 1) A control reservoir fitted with a release valve. 2) The brake pipe. 3) A brake selector cock giving Goods / Passenger brake release timings. 4) The brake cylinders via an isolating cock, flexible hose and double check valve. The brake pipe pressure can be directly vented from the cabs by operation of the drivers emergency valve. A switch is included in the valve which is connected to the electrical control circuits, and to the brake control unit which also vents air train pipe when de-energised. To reset the emergency valve the knob requires pulling fully out. An interlock valve arrangement is provided between the parking brake and the automatic air brake which comprises of:- 1. An emergency application valve. 2. A vent valve. 3. An interlock magnetic valve. If an attempt is made to move the locomotive with the parking brake applied, air train pipe pressure is automatically vented when the locomotive speed reaches 6 mph, thus applying the brakes. The Straight Air Brake: The straight air brake provides a means of applying and releasing the locomotive air brake independently of the train brake. A straight air brake valve is provided in each cab for the control of this brake. Each valve has a control handle with three dented operating pistons corresponding to APPLY and RELEASE and an unmarked VERTICAL position. The inlet port of each valve is coupled to the 10 bar air system and the outlet port is coupled via a double check valve to a common delivery pipe which then branches into two, to the individual brake cylinders, via a protection choke, pressure regulator and an isolating cock. The brake is applied by moving the brake valve forward to the APPLY position, this allows air to flow from the 10 bar supply to the brake cylinders, when the required brake cylinder pressure is reached the handle is moved to the VERTICAL position cutting of the supply. If required, the brake valve can be left in the APPLY position, the brake cylinder pressure being limited to 4.25 bar. Similarly the brake cylinder pressure can be released by moving the brake valve handle backwards to the RELEASE position, this releases the air from the brake cylinders to atmosphere, when the brake cylinder pressure has been reduced to the required level the brake handle is moved back to the VERTICAL position preventing any further loss of pressure. In running it is essential that the brake valve in the rear cab is in the RELEASE position. Page Last Updated: 3rd December 2008. |