Section 1.2: Air Reservoirs and Moisture Control
Key Takeaways
- Air reservoirs are welded steel tanks that store compressed air, ensuring the vehicle has a sufficient volume of air for multiple emergency stops if the engine stalls.
- The supply reservoir (wet tank) receives air first, acting as a cooling chamber where water and oil vapors condense out of the air.
- Service reservoirs (primary and secondary dry tanks) store clean, dry air and distribute it to the rear and front brake circuits respectively.
- Daily draining of all reservoirs is a critical safety task to prevent water, oil, and sludge from freezing, rusting the tanks, or damaging valves.
- Air dryers use desiccant cartridges to trap moisture before it enters the tanks, while alcohol evaporators inject alcohol vapor to prevent freezing in extreme cold.
Air Reservoirs and Moisture Control
In an air brake system, compressed air must be stored in sufficient quantities to ensure reliable, instantaneous braking. This storage is provided by air reservoirs, which are heavy-duty welded steel tanks mounted to the vehicle's chassis. Because the process of compressing air concentrates moisture and oil, managing these contaminants inside the reservoirs is one of the most critical safety and maintenance duties of a commercial driver.
The Role and Types of Air Reservoirs
Air reservoirs serve a dual purpose: they store the high-pressure air required to actuate the foundation brakes, and they act as a safety reserve. If the engine stalls or the compressor fails, the air stored in these reservoirs allows the driver to make several full brake applications to bring the vehicle to a safe, controlled stop.
An air brake system uses two distinct classifications of reservoirs:
- Supply Reservoir (Wet Tank): The supply reservoir is the first tank in the system, receiving air directly from the compressor discharge line. It is commonly referred to as the wet tank because it acts as a cooling and condensation chamber. When hot, highly pressurized air leaves the compressor, it enters the wet tank, where it cools rapidly. As it cools, moisture (water vapor) and oil droplets (from the compressor lubrication system) condense and settle at the bottom of the tank.
- Service Reservoirs (Dry Tanks): Air flows from the supply reservoir into downstream service reservoirs, commonly called dry tanks. These tanks are designated as primary (which typically supplies air to the rear brakes) and secondary (which typically supplies air to the front brakes). Because the air has already cooled and deposited its moisture in the wet tank (and passed through an air dryer, if equipped), the air in the service reservoirs is clean and dry.
Moisture Accumulation: The Silent Hazard
As the compressor pumps air, it draws in atmospheric humidity. The compression process heats the air and concentrates the water vapor. When the air cools inside the tanks, this vapor condenses into liquid water. Along with water, small amounts of lubricating oil bypass the compressor's piston rings, creating a toxic mixture of water, oil, and carbon sludge at the bottom of the reservoirs.
If this mixture is not removed, it presents major safety hazards:
- Freezing: In Canadian winters, accumulated water in the air lines or valves can freeze. Ice blockages can completely plug lines, preventing air from reaching the brakes or preventing brakes from releasing, leading to wheel lockup or complete brake failure.
- Corrosion: Water accelerates rust and corrosion inside steel reservoirs. Rust flakes can break loose and clog the tiny ports of control valves.
- Component Damage: The acidic nature of engine oil combined with water degrades the rubber diaphragms, seals, and O-rings inside foot valves, quick release valves, and relay valves, leading to air leaks and sluggish brake response.
Draining Reservoirs: A Critical Daily Duty
To prevent the hazards of moisture accumulation, all air reservoirs must be drained daily. Under ICBC safety rules, draining the tanks is a mandatory part of the pre-trip inspection.
Manual vs. Automatic Drain Valves
Tanks are equipped with one of two types of drain valves at their lowest point:
- Manual Drain Valves (Petcocks or Pull-Cords): These require the driver to manually open the valve. Many vehicles feature pull-cords connected to spring-loaded valves, allowing the driver to drain the tanks from the side of the vehicle without crawling underneath. To drain the tanks completely, the driver must open the valve and keep it open until all water, oil, and sludge are expelled, and only clean, dry air escapes. Tanks should be drained in sequence, starting with the wet tank (supply reservoir) and then the dry tanks.
- Automatic Drain Valves: These valves are designed to automatically eject a small burst of moisture and air every time the governor cycles or pressure changes in the line. They are often equipped with electric heating elements to prevent freezing. However, drivers must not rely solely on automatic drains. During the daily inspection, the driver must manually operate the override or pull-cord to verify that the automatic valve is functioning and that no moisture remains in the tank.
Air Dryers and Alcohol Evaporators
Modern air brake systems are equipped with auxiliary devices to assist in moisture control, especially in cold climates.
Air Dryers
An air dryer is installed in the discharge line between the compressor and the supply reservoir. It removes moisture and oil vapor before the air ever reaches the tanks.
- Desiccant Cartridge: The air dryer contains a replaceable cartridge filled with highly porous desiccant beads (a molecular sieve). As compressed air passes through the cartridge, the desiccant adsorbs water vapor on its surface.
- Purge Cycle: When the governor reaches cut-out pressure, it sends a control signal to the air dryer. The dryer opens its purge valve, creating a sudden blast of exhaust air that expels the collected water and oil from the bottom of the dryer shell. At the same time, a small amount of dry air from a purge volume flows backward through the desiccant, stripping off the adsorbed moisture and preparing the cartridge for the next cycle.
Alcohol Evaporators
For vehicles operating in extreme cold where air dryers may become overwhelmed, an alcohol evaporator provides an effective line of defense.
- Operation: This device contains a reservoir filled with methyl alcohol (methanol). As air is drawn into the compressor or flows through the supply lines, it passes over the alcohol reservoir, vaporizing the alcohol and mixing it with the air stream.
- Function: The alcohol vapor mixes with any moisture present in the system. Because alcohol has a much lower freezing point than water, the mixture will not freeze, even at sub-zero temperatures, keeping the lines and valves free of ice blockages.
- Maintenance: Drivers must check the alcohol level daily during winter and top it up with approved air brake methyl alcohol. Using the wrong fluid can damage rubber seals.
Which reservoir receives air directly from the compressor first and is primarily responsible for collecting condensed water and oil?
Why must air reservoirs be drained daily?