An Article by Chris Hecht – Sister Bay, WI

Your department has made the decision to invest in a CAF system on your next apparatus purchase. Here are a few things your department may want to consider adding or asking about that might turn a good CAFS unit into a great one.

But first the basics; there are several major manufactures of CAF systems each with there own individual subtleties. However the basic principle of every CAF systems is the same. Each unit takes water and mixes in a foam concentrate to create a foam solution air is then injected into the foam solution to create the finished foam.

This article will break each of the three major components of the systems (Water, foam and air) down and discuss some things departments may want to consider prior to purchasing a CAFS unit.

Water System

Water system; typically the water system consists of a standard fire pump receiving water for an onboard tank, positive pressure source or from draft. Some of the issues a department might want to consider or discuss:

High intake pressures

-Departments that routinely see high intake pressures may have some difficulties in creating high volumes of compressed air due to the pumping engine not turning at an increased RPM. A low engine RPM typically reduces the compressors ability to generate the high volumes of air needed for CAFS.


Gating the intake – This is the method many departments have used to control there intake pressures with or without CAF System, and continues to work well, however this can be labor intensive as the pump operator must continually monitor the intake and discharge pressure and make adjustments as flow rates change.
Direct Tank fills – allows the pump operator to continuously work from the tank allowing for higher engine RPM’s as the pump is working with no incoming pressure. Draw backs to direct tank fills are limited GPM flows if the situation escalates.
Direct tank fills off intake valves – While not a standard from most apparatus manufactures, several departments have had success with this method. To use this system a “T” is placed on the plumbing feeding the intake relief valve on the hydrant side of the intake valve. This “T” can then feed a direct tank fill using either a manually controlled valve or an auto tank fill system. If the auto fill system is not capable of meeting the flow demands of the situation, the pump operator simply opens the main intake valve to allow higher flows into the pump. This system is now being offered as a packaged setup from a component manufacture.
Pressure reducing intake valves – Pressure reducing valves allow water to pass through without restricting flow while lowering the intake pressure. Valves are typically available in two flow ranges, a low flow of 25 to 500 gpm range or a higher flow valve with a 125 to 1500 gpm rating. Valves maybe pre-plumbed into dedicated intake or be carried in compartments as a portable device to be placed in line as needed. Valves are adjustable to allow for the intake pressure to be controlled and may be run in the bypass mode allowing full flow with no pressure reduction. . Draw backs – flows at or below the rated capacity of the valve may result in a hunting or surging effect in the intake pressure. It is not possible to draft through a pressure reducing valve.

Discharge Plumbing

Many departments request a deck gun or LDH discharges to be foam or CAF capable; while this does increase the operational abilities of the unit a department must keep in mind the potential issues with these additional capabilities. With the majority of foam systems currently being placed in fire apparatus today, the water flow being supplied to all foam capable discharges is measured at a single point in the plumbing. This point in the foam manifold is usually the smallest point in the manifold.  There are several ways apparatus manufactures may provide these capabilities. Each presents operational differences a department needs to be aware of.


1.) All foam capable discharges including deck gun and/or LDH discharge plumbed only off the foam manifold. This option presents this simplest and cheapest method of providing foam to the desired discharges. However it also provides the greatest limitations to the department. This option will provide foam to all discharges from the foam manifold as specified by the department. However when flowing foam all discharges plumbed to the foam manifold will only be able to flow foam. This design would prevent a department from running a clear water deck gun if needed while foam was being used. Departments requesting a CAF capable deck gun should strongly consider adding a static mixing device to provide proper agitation of the CAFS prior to discharge of the finished foam. These devices work well while flowing CAF, but when trying to flow water only create significant pressure drop. A single point of plumbing as described in this option will require higher master pump discharge pressures when flowing high volumes to overcome either the restriction of the narrowing of the foam manifold or the static mixing devices.

2.) All foam capable discharges plumbed off the foam manifold with the deck gun and/or LDH discharge being dual plumbed off both the foam manifold and the clear water manifold. This option provides the department with the ability to flow foam or clear water out of their high flow discharges to meet their need.  Each discharge is provided with two sets of discharge valves and pressure gauges; one set plumbed from foam manifold and the second set from clear water manifold. This option does provide increased operation abilities for the department achieved at a higher cost due to additional plumbing, valves and gauges, as well as additional pump panel space.

Foam Systems

The foam system typicality consists of two main components:

The foam manifold where the water flow is measured, the foam is injected and the foam solution is delivered to the department specified discharges.

The foam pump delivers the proper amount of foam concentrate from the foam tank to the foam manifold .

Foam manifolds:

As discussed above most manufactures use a foam manifold that uses a single measuring point and foam concentrate injection point. These manifolds are available pre-manufactured in several sizes or may be custom built by the apparatus manufacture to meet the requirements. Departments should be aware of the size of the manifold and be aware of the possible restriction or limitation the manifold may place on the performance of the apparatus.
Manifolds that are too small in size will ultimately limit the department’s ability to flow higher volumes of water due to the restrictions in pipe size.  Manifolds that are to large in size do not accurately measure lower water flows resulting in poor quality finished foam due to lack of concentrate. Dual plumbing of some discharges as described above may allow the department the added flexibility to change the manifold size to best meet their needs.  Departments need to work with the apparatus manufacture of their choice to determine what will best meet their needs and be aware of the limitations of the system.

Foam pumps:

Foam pumps are available in many sizes and configuration. Power or drive options include electric, hydraulic or PTO.  Foam pump capabilities should be matched to the department’s needs. Similar to foam manifolds each size has its positives and negatives. Things a department might consider or questions to ask:

1.) What is the flow range the proposed foam pump can meet? In a CAF application, percentage of concentrate used will typically range from a low of 0.3%- 0.5% when flowing a wet type CAF to a to a high of 0.7%- 1.0% when flowing a fluid or dryer shaving cream type foam. These percentages equate to relatively low flows of foam concentrate in a class A application. Departments that require a class A and class B system will need to determine what their flow requirements for each application are. The injection flow rates for Class A and B are significantly different and a foam pump that meets all your class A requirements may not come close to meeting your class B needs.

2.) Does the foam pump start at a high flow and then reduce its injection rate to meet the desired flow rates?  Some foam pumps will start their injection cycles at a high flow and then slowly reduce the inject rate to meet the selected flow. This initial high flow of foam concentrate may result in the increased use of foam concentrates.

3.) Does the foam pump have an optional auto on feature? Some foam pump manufactures are providing their systems with a feature that automatically powers the unit up into it’s inject mode with the engagement of the pump. In this mode, as soon as the pump operator flows water, the foam pump begins injecting concentrate. Benefits of this option include one less operational step for the operator.  Negative of the option is the possible unwanted use of foam concentrate.

4.) Does the foam pump adjust for differences in foam concentrates and water?  Some systems work volumetrically and inject concentrate in proportion by volume.  Other systems use conductivity to maintain the correct proportion.  These systems compensate for variations in concentrate and water quality.

Additional considerations for foam systems.

Overboard auxiliary foam pick up

Departments that don’t wish to have both a dedicated class A and Class B foam systems on their apparatus but who may want the additional capabilities’ may consider an auxiliary foam pickup. Typically this is accomplished through the additional of a standard Class A/B selector valve assembly with the Class B side of the selector plumbed to a simple overboard pick up located as desired by the department. This system allows the department the option to use Class B foam from pails through their plumbed foam system. Other benefits include the ability to try different types or brands of foam concentrates through the overboard pickup without the need to dump or flush the foam tank.  Drawbacks to the overboard systems – some foam pumps have difficulty in drawing the foam concentrate out of a pail as they typically operate from a flooded state with the foam coming from a foam tank located above the foam pump. Costs while not as expensive as a dedicated Class B system the overboard pickup will add additional expense to the apparatus.

Air System

The air systems main component include the air compressor and its drive systems; the air to water pressure balancing system;  and air delivery devices including air valves and check valves.

Air compressor:

Typical fire apparatus mounted CAF systems use a compressor rated anywhere from 80 to 200 CFM. These compressors typically have four different drive options: 1. PTO driven from the tuck transmission 2. driven from the fire pump 3. Hydraulically driven   4.  powered by an auxiliary engine. Departments should explore their needs and determine what drive system works best for their needs:

PTO Driven – A PTO driven compressor may provide the department with some additional operational options that are desirable, the main benefit of a PTO is the ability to run the compressor without running the fire pump. This can be useful if the unit needs to supply large quantities of air for needs outside of the standard CAF fire fighting operations. PTO driven compressor are also frequently used for the retrofitting of CAF systems to in service apparatus. PTO driven compressor may also provide apparatus builders with additional mounting locations and options for apparatus that are short on pump compartment space. A department wishing to operate a compressor without running the fire pump will need some form of auxiliary cooling as discussed further below. Departments who maybe concerned about running their compressor without the cooling of the fire pump may specify an interlock device that prevents compressor engagement without the fire pump being engaged as well.

Pump Driven Compressor – A typical pump driven compressor receives its power through the pump. In this system the compressor is only powered if the fire pump is engaged and turning. Benefits to this system include a complete package mounted in the pump compartment, this package is usually completely assembled tested and calibrated by the pump manufacture. These systems are more cost effective and simpler to install for the apparatus manufacture. It is impossible to run the compressor without running the pump, reducing the chances of overheating the compressor. Drawbacks include no ability to run the compressor without running the pump, use of the compressor will always require use of the pump. May require additional space in the pump compartment.

Hydraulic driven compressor – In this system a hydraulic pump is driven off a transmission PTO.  Hydraulic fluid is then used to operate a hydraulic motor which drives the compressor.  The benefit is the flexibility of compressor location.  It can be mounted where space is available and hydraulic lines run to it.  The downside is the additional cost and complexity of the hydraulic drive system.

Auxiliary engine driven – This system has a separate engine which powers both the water pump and compressor.  The benefit is the option of true pump and roll capabilities because the CAFS is not connected to the truck power train.  The downside is increased cost and weight, the spaced needed for the system, and the maintenance of a second engine.

Compressor engagement – Both the PTO driven and pump driven compressors are capable of being equipped with auto on features. The auto on feature allows the compressor to come on line and be operational any time the pump is placed in gear. If the operation calls for a non CAF mode the operator simply disengages the compressor and proceeds with a water only or foam solution mode.  Benefits to this include one less step for the pump operator, reduces the chance that a key step will be forgotten or missed.  Drawbacks include possible unnecessary running time on the compressor

Auxiliary cooling:
A compressor that is allowed to run without adequate cooling will quickly over heat and do damage to the compressor. The overheating situation may be due to the pump operator failing to circulate water through the compressor cooler, or the result of debris plugging the oil cooler strainer. Departments who are concerned with over heating due to plugged strainers, operator errors, or PTO driven compressors that need the ability to run without the fire pumps should consider some form of additional compressor oil cooling. This cooling is usually provided through the use of an air to oil cooler. The air to oil cooler acts as a radiator for the compressor oil allowing the oil to be cooled independently of the water pump circulation. Most CAF installations provided with air to oil coolers will continue to use the standard fire pump water cooler as well. With both systems operating an added level of protection for the compressor is provided.

Auxiliary air outlets:
Departments specifying a CAF system often overlook the capabilities the compressor has to provide air for auxiliary uses. When specifying a CAF system, departments who may need high volumes of air should consider including the following. High flow air pressure regulators, air dryers and delivery devices all designed to flow the capacity of the compressor. These items are much easier to design into the system in the beginning then trying to add them at a later date.

Any additional cooling provided to the compressor will also act as a safe guard for the compressor should the water flow to the standard cooler be restricted at any time. Drawbacks to PTO systems – some additional cost if a department wishes to equip their apparatus with a additional cooling.


There are some simple things to consider that may improve your department’s abilities to maintain the CAF system;

Discharge outlet locations: Many departments have all their hose connections buried in or under hose beds. Connections located where they may be accessed easily without disturbing the hose loads will allow the operator to disconnect the hose loads and flow air from the discharges. The ability to flow air on a regular basis from a CAF system is important for the maintenance of the system. This air flow exercises all the key components in the systems and allows the operator to exercise their ability to run and understand the system. A hose load configuration that requires the unloading of a hose bed to flow air may result in an incomplete system check.

Strainer locations – Most CAF systems have several simple strainers built into them, these strainers need to be checked on a regular basis as part of a preventive maintenance program. These strainers include a foam concentrate strainer and a strainer in the water line feeding the compressor oil cooler. The frequency of the checks to these strainers will depend on several factors including hours of use, quality of foam, or clarity or amount of debris in the water an apparatus pumps. Departments that depend on drafting from lakes or ponds may have to check and clean their strainers more frequently then a department that works only from a hydrant system. The location and access to these strainers is important. Strainers that are difficult to access or check will result in a reduced number of inspections and may lead to a system that does not perform as expected.

Other components – Departments should work with apparatus manufactures so that all items that need frequent or even periodic checks or maintenance are easily accessible. These items should include strainers, fluid level sight glasses, valves, and filters. A CAF system that is maintained well will operate much more reliably and accurately.

Check Valves – A typical CAF system uses Check valves to keep the water, foam and air moving in the correct direction without one product backing into another system and contaminating it. Departments that have hard water or water that contains many minerals may find their check valves sticking. In many cases these check valves will stick in the closed position and will not allow product out. Preconstruction discussions with your apparatus builder may eliminate these issues by just confirming the placement of the check valves to allow water to naturally drain away from the valve.

This discussion is not designed to be a how to or a must have or a CAF system. It is intended to help generate discussion within fire department and between fire departments and fire apparatus manufactures as to what are the options and how to improve departments’ abilities and capabilities with their systems.