123 Air operated power brake equipment

12.3.1 Air dryer (Bendix) (Fig. 12.7(a and b)) Generally, atmospheric air contains water vapour which will precipitate if the temperature falls low enough. The amount of water vapour content of the air is measured in terms of relative humidity. A relative humidity of 100% implies that the air is saturated so that there will be a tendency for the air to condensate. The air temperature and pressure

(a) Charging cycle

(a) Charging cycle

determines the proportion of water vapour retained in the air and the amount which condenses.

If the saturation of air at atmospheric pressure occurs when the relative humidity is 100% and the output air pressure from the compressor is 8 bar, that is eight times atmospheric pressure (a typical working pressure), then the compressed air will have a much lower saturation relative humidity equal to 100 = 12.5%.

Comparing this 12.5% saturation relative humidity, when the air has been compressed, to the normal midday humidity, which can range from 60% in the summer to over 90% in the winter, it can be seen that the air will be in a state of permanent saturation.

However, the increase in air temperature which will take place when the air pressure rises will raise the relative humidity somewhat before the air actually becomes saturated, but not sufficiently to counteract the lowering of the saturation relative humidity when air is compressed.

The compressed air output from the compressor will always be saturated with water vapour. A safeguard against water condensate damaging the air brake equipment is obtained by installing an air dryer between the compressor and the first reservoir.

The air dryer unit cools, filters and dries all the air supplied to the braking system. The drying process takes place inside a desiccant cartridge which consists of many thousands of small microcrystalline pellets. The water vapour is collected in the pores of these pellets. This process is known as absorption. There is no chemical change as the pellets absorb and release water so that, provided that the pores do not become clogged with oil or other foreign matter, the pellets have an unlimited life. The total surface area of the pellets is about 464 000 m2. This is because each pellet has many minute pores which considerably increase the total surface area of these pellets. Dry, clean air is advantageous because:

1 the absence of moisture prevents any lubricant in the air valves and actuators from being washed away,

2 the absence of moisture reduces the risk of the brake system freezing,

3 the absence of oil vapour in the airstream caused by the compressor's pumping action extends the life of components such as rubber diaphragms, hoses and 'O' rings,

4 the absence of water and oil vapour prevents sludge forming and material accumulating in the pipe line and restricting the air flow.

Charge cycle (Fig. 12.7(a)) Air from the compressor is pumped to the air dryer inlet port where it flows downwards between the dryer body and the cartridge wall containing the desiccant. This cools the widely but thinly spread air, causing it to condense onto the steel walls and drip to the bottom of the dryer as a mixture of water and oil (lubricating oil from the compressor cylinder walls). Any carbon and foreign matter will also settle out in this phase. The cooled air charge now changes its direction and rises, passing through the oil filter and leaving behind most of the water droplets and oil which were still suspended in the air. Any carbon and dirt which has remained with the air is now separated by the filter.

The air will now pass through the desiccant so that any water vapour present in the air is progressively absorbed into the microcrystalline pellet matrix. The dried air then flows up through both the check valve and purge vent into the purge air chamber. The dryness of the air at this stage will permit the air to be cooled at least 17° C before any more condensation is produced. Finally the air now filling the purge chamber passes out to the check valve and outlet port on its way to the brake system's reservoirs.

Regeneration cycle (Fig. 12.7(b)) Eventually the accumulated moisture will saturate the desiccant, rendering it useless unless the microcrystalline pellets are dried. Therefore, to enable the pellets to be continuously regenerated, a reverse flow of dry air from the purge air chamber is made to occur periodically by the cut-out and cut-in pressure cycle provided by the governor action.

When the reservoir air pressure reaches the maximum cut-out pressure, the governor inlet valve opens, allowing pressurized air to be transferred to the unloader plunger in the compressor cylinder head. At the same time, this pressure signal is transmitted to the purge valve relay piston which immediately opens the purge valve. The accumulated condensation and dirt in the base of the dryer is then rapidly expelled due to the existing air pressure in the lower part of the dryer. The sudden drop in air pressure in the desiccant cartridge chamber allows the upper purge chamber to discharge dry air back through the purge vent into the desiccant cartridge, downwards through the oil filter, finally escaping through the open purge valve into the atmosphere.

During the reverse air flow process, the expanding dry air moves through the desiccant and effectively absorbs the moisture from the crystals on its way out into the atmosphere. Once the dryer has been purged of condensation and moisture, the purge valve will remain open until the cylinder head unloader air circuit is permitted to exhaust and the compressor begins to recharge the reservoir. At this point the trapped air above the purge relay piston also exhausts, allowing the purge valve to close. Thus with the continuous rise and fall of air pressure the charge and regeneration cycles will be similarly repeated.

A 60 W electric heater is installed in the base of the dryer to prevent the condensation freezing during cold weather.

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