Foam is a great tool for improving our firefighting capabilities: It’s extremely effective in extinguishing multiple types of fires and does so in a shorter timeframe; it makes water more efficient, providing more firefighting capability from the same volume of water; it’s a cost-effective investment; and it’s simple to operate. But many people don’t understand how it works, and aren’t familiar with some of the tools that help simplify foam use. In this article, I’ll discuss how foam is made using aspirating nozzles, as well as one important method firefighters can employ when using foam on the fireground.
Foam Basics
Foam is made by first mixing foam concentrate with water to create a foam solution. This can be done via a wide variety of foam proportioners now available on the market.
Once the solution is made, it must then be combined with air, and the mixture must be agitated to create finished foam bubbles. There are several commonly used methods of agitation, the most common of which is to pump foam solution out of a fog nozzle, which allows agitation to occur when the product hits its target. The second method involves a compressed air foam system (CAFS) in which air is injected into the foam solution at the discharge. Agitation occurs as the mixture rubs against the inside of the hose. The third method is to use an aspirating foam nozzle, which creates agitation and forms the foam bubbles within the nozzle.
Why Use It?
Aspirating nozzles have several key qualities that make them an optimal choice when using foam:
- They are a low-energy system, meaning that the only energy available to produce bubbles in the nozzle comes from the water pump.
- They’re easy to make, but there’s also a wide variety of aspirating nozzles on the market. Some are fixed tubes with no adjustment. Others are adjustable usually by changing the stream pattern. Each nozzle manufacturer also makes clip-on aspirating nozzles, which attach to the bumper of a fog nozzle when needed.
- They produce a wide variety of foam types and volumes, and can be used with both Class A and B concentrates.
- They can be designed to bring the air into either the front or the back of the nozzle. Air is drawn into the nozzle through the Venturi effect. As the foam solution passes through the center of the nozzle, a low level of pressure is created, which allows the air to enter the nozzle. This process consumes energy. The more air that’s drawn in, the more energy is consumed. This causes a reduction in stream reach.
Expansion Ratios & Nozzles
One term that’s commonly used when referring to aspirating nozzles is “expansion ratio,” which determines the difference between the volume of foam solution pumped into the nozzle and the volume of finished foam bubbles exiting the nozzle. For example, if one gallon of foam solution enters the nozzle and 50 gallons of bubbles exit the nozzle, the expansion ratio is 50/1. Expansion ratios are broken into three categories: low, medium and high. Low expansion starts at 1/1 and goes up to 20/1. Medium expansion starts at 21/1 and goes up to 200/1. High expansion begins at 201/1 and can go as high as 1,000/1.
Low-Expansion Nozzles
Low-expansion nozzles are typically fixed tubes with no adjustment. They produce a wet foam, which is appropriate for many tactical applications, such as fire attack and mop-up. These nozzles typically operate at 80 to 100 psi nozzle pressure and a Class A foam percentage of 0.5%.
Medium-Expansion Nozzles
Medium-expansion nozzles are typically adjustable, allowing variations in foam volume and foam consistency. Tactical applications include wildland fireline work and mop-up and overhaul of vehicle fires. Operating pressure for these nozzles is typically 60 psi. This lower pressure is necessary because as the bubble size increases, the bubbles become more fragile. Higher pressure will cause the bubbles to break, reducing the effective production of the nozzle. Larger bubbles also require more structure, which comes from an increase in foam percentage, usually 0.5% to 0.7%.
High-Expansion Nozzles
High-expansion nozzles produce a large volume of dry foam due to the large volume of air taken in and the low water content. As the bubbles become even bigger, the same principles discussed for medium-expansion nozzles apply; nozzle pressures drop to around 40 psi and the foam percentage must be increased to the range of 0.7% to 1.0%, which is the maximum percentage for Class A foams. High-expansion foams are typically used on compartment fires or to fill void spaces.
Keep in mind that as the expansion ratio increases, the stream reach of the nozzle will decrease. Again, this is due to the entrainment of more air consuming more energy and the lower nozzle pressure that’s needed to prevent destruction of the bubbles in the nozzle.
Medium Is Best
The medium-expansion nozzles are the most effective because they’re the most versatile. These nozzles can be used for everything from initial attack, to mop up and overhaul, to wildland fire breaks and pretreatment of fuels. Every foam-capable apparatus, including CAFS-equipped rigs, should be equipped with an adjustable, medium-expansion nozzle.
A Final Note
Although some departments might be hesitant to use foam, there are a multitude of products on the market today that both expedite extinguishment and make it easier to use and maintain foam. One such product is the aspirating foam nozzle. All you have to do is mix concentrate with water–the nozzle does the rest.
