The air control circuit controls the compressor air pressure. It can be divided into four sub circuits. The circuit components include the air inlet valve, auto sync control, minimum pressure valve, piloted balance valve, PMC valve, and the shuttle valve. The circuit can operate in different modes as determined by the Auto Sync switches. The air control circuit components should be adjusted by NO ONE other than an F-7 Certified Mechanic.
A breakdown of these components follows:
Air Inlet Valve (AIV)
The AIV is mounted on the inlet to the air compressor. It serves as the throttle controlling the compressor discharge pressure. Unlike the throttle on an engine which is an adjustable stationary valve the AIV is a modulating valve which is constantly opening and closing. It controls the air entering the compressor. More air allowed into the compressor equals more discharge pressure. The AIV is controlled by the air control circuit which has three different valves allowing for operation the three different modes.
When a CAF system is in operation, it sounds as though the system is rapidly throttling up and down on a constant or periodic cycle. What you are hearing is actually the AIV opening and closing to meet the air intake demands.
The AIV device has no firefighter adjustments. This unit should only be maintained by a qualified F-7 EVT Certified mechanic.
PMC Valve
The PMC Valve sets the system’s maximum pressure. The setting is factory set (generally around 150 psi). The PMC device is not to be adjusted by firefighters. This unit should only be maintained by a qualified F-7 EVT Certified mechanic.
Shuttle Valve
When the compressor system is shut down, all pressure in the system is bled off. The bleed off occurs when the shuttle valve opens. Bleed off can take 1-several minutes to occur. When the system starts, the shuttle valve closes.
Pilot Balance Valve (PBV)
Prior to the early 1990’s, to make CAF work, the pump and compressor each had to have a separate engine driver. The Solution (W + Con) and air (+A) must be equal for both to enter the discharge hose (+A – Agitation) to make foam. As the system would fluctuate, one pressure would go higher than the other, interrupting the smooth flow of a Sol + Air mixture. The system operator was required to maintain vigilance and make constant throttle adjustments on both engines.
In the early 1990’s, the pilot balance valve (PBV) was introduced. The PBV is a double diaphragm device that monitors the water pressure and automatically adjusts the air intake to match the water pressure. CAF systems could now be built with a shared driver and one throttle that requires no adjustment and delivers a smooth flow of Sol + Air (W + Con + A) mixture.
Minimum Pressure Valve (MPV)
The MPV is a spring-loaded device just downline from the Air Oil Separator (AOS – see Hydraulic Components). The MPV is factory set usually at around 30-40 psi. The system will use this minimum pressure to push the hydraulic oil through the compressor lubricating and cooling system. At minimum pressure, there is no air available to mix with the solution in the hose. To make CAF, the pressure must be above the minimum.
The MPV REQUIRES regular operation to keep the spring and plunger from seizing. It will generally seize in the minimum pressure position. Enough air will pass to push the hydraulic oil, but the seized MPV will not allow any additional air to pass to make CAF. The most visible symptom of this issue is noticing no air/solution mix in the hose. Solution only will flow, yet all the pressure gauges are functioning properly, including showing pressure above the minimum. It is imperative that the system be operated regularly (according to manufacturer’s instructions). USE IT OR LOSE IT!!! A seized MPV is not fixable in the field and will require your F-7 Certified EVT mechanic.
Auto Sync Control
The auto sync control is the operator-controlled portion of the air control circuit. The auto sync system can have either manual or electric switches that direct pressurized air to control the CAF system modes. The operator-controlled modes are:
- Unload
- Auto/Run
- Fixed/Run
Unload Mode – whereby no air is requested for compression. (used for NAF or flushing).
Auto/Run Mode – Whereby air compression is requested to match the solution pressure (For making CAF)
Fixed/Run Mode – whereby air compression is requested at the fixed or maximum pressure (generally factory set at 150 psi) regardless of the solution pressure. (used for air only applications including special air powered tools).
The air control circuit also has automatic functions including monitoring the water pressure in the PBV and in the operation of the AIV to bring air into the compressor.
Air Gauge
CAF systems have two pressure gauges on the pump panel. One for water pressure, and one for air pressure. The air pressure gauge allows the operator to monitor the air pressure for the different operational modes. An additional option is a double gauge whereby the air & water are attached to one gauge that incorporates 2 needles. One needle for the water, one for the air.
Air Manifold
The air manifold is a manifold that takes in air coming from the MPV and distributes the air to the individual discharge outlet valves where the air will mix with the solution heading into the fire hose to agitate and form CAF.
Air Only Outlet
Some CAF systems are equipped with an air only outlet, generally mounted on or near the pump panel. This outlet is useful for air only needs where foam solution would be undesirable. This outlet can be used to air up tires, recharge extinguishers, other air tools, or just as an air blower for cleaning air filters, blowing dust from a cab or blowing leaves, and as the air outlet necessary for the CAF system Warm Up Check. Also referred to as Auxiliary Air Outlet.
Jet Primer
The jet primer is a water pump primer used with CAF Systems to pull a draft. The jet primer either incorporates manual or electric switches. The system utilizes compressor air to create a vacuum to pull a draft. This is done using compressor air flowing through a venturi to create a low pressure. If all leaks are sealed in the hard suction lines and connections, the jet primer can pull a draft in very short order, however it is less efficient at high altitude due to decreased air pressure.
