A debate has ensued for years in the fire service concerning the comparative advantages and benefits of smooth-bore and combination nozzles. In this article, the first in a two-part series, we will modernize some outdated information and interject genuine facts into this age-old debate, first by comparing both streams via visual observation, and then by examining in greater depth the reach, impact force, nozzle reaction, air movement and extinguishment capabilities of each, as well as the effects of pressure and turbulence on the streams.

It seems that more and more articles are appearing that state the smooth-bore nozzle must be the “nozzle of choice” for interior operations because it perceivably offers greater penetration and knockdown abilities. Historically, much of the information concerning these nozzles came from sources — such as the IFSTA and the NFPA — that provided excellent information — indeed, information that was the best 
available at the time of publication — but which could benefit from an update, especially concerning currently available equipment. For instance, it wasn’t until the late 1990s that the NFPA changed Standard 1964 to recognize the option of low-pressure combination nozzles, compared with the previous standards that dictated a pressure of 100 psi. Further, much of the information that has been passed on through the years concerning nozzles is subjective in nature and has little or no scientific evidence to support it.

Indeed, many departments are unfamiliar with the newer, low-pressure combination nozzles and lack experience regarding valid test methods. Some departments test by setting their pumps at a fixed pressure and placing a variety of nozzles on the end of the hose. This practice does not take into account friction loss in plumbing, the designed flows and nozzle pressures, or the corresponding hose-friction losses. All of these variables can result in both confusion and misleading conclusions.

Accurate testing requires the use of calibrated flow meters, line gauges and pitot tubes, as well as a basic understanding of physical science. To accurately assess the similarities or differences of a smooth-bore nozzle compared with a combination nozzle, one must first start with nozzles that have the same flow at equal nozzle pressures, i.e., apples to apples.

Fixed Gallonage

With the advent of today’s low-pressure nozzles, there are a variety of combination nozzles that are capable of producing the same flows at the same pressures as those provided by a smooth-bore tip. This article will examine a “fixed-gallonage” nozzle, as it operates most like a smooth-bore — flow increases as pressure increases; conversely, flow decreases as pressure decreases. Nozzle manufacturers offer a variety of different flow rates on their low-pressure nozzles that often can be modified simply by replacing the stem or a disk.

The first requirement is to decide upon a minimum flow. NFPA 1410 outlines a minimum flow requirement of 300 gpm; 100 gpm on the initial attack line and 200 gpm on the back up. With today’s increased fire loads, the consensus among firefighters in the field tends toward boosting the minimum flow on the initial attack line to 150 gpm. The closest nozzle to this would be a 
7/8-inch smooth-bore with a flow of about 160 gpm.

With that in mind, let us examine a fixed-gallonage, 200 gpm (at 75 psi) nozzle and a 7/8-inch smooth-bore tip. Both will deliver approximately 160 gpm when pumped to 50 psi at the tip and about 200 gpm when pumped to 75 psi at the tip. The tip pressures are only manufacturer’s recommendations. You can under pump or over pump a nozzle to meet your needs, as long as the nozzle is tested to ensure that the stream quality, range and the desired gallonage is achieved.

Most combination nozzles will produce effective streams at well below the manufacturer’s rated operating pressure. In this case, we under pumped the 
75 psi, low-pressure nozzle to 50 psi. This resulted in the same operating pressure as the smooth-bore. At this pressure, the flows are equal, at about 160 gpm. When we pump the low-pressure nozzle at its rated pressure of 75 psi and over pump the smooth-bore to 75 psi, we find that both produce flows of about 200 gpm. If the gallonage available at 50 psi is not doing the job, the firefighters can request the higher pressure, which also will increase the flow.

Fluid dynamics are said to be empirical in nature, which means that much of the knowledge is gained by observation. Look at the pictures on the left. The first shows both streams side by side, while the second provides a closer view of the tops of the streams. Both streams are flowing equal gallons at equal inlet pressures. Which one is from the smooth-bore? (If you are not sure, that’s OK; many firefighters who saw these in person guessed incorrectly.)

For the tests, deluge gun bases were used to ensure equal angles when comparing the streams vertically — roughly 32° for optimum reach. The second image shows vertical streams that were done for comparison purposes. Both streams had the same height and nearly identical stream quality and appearance. The stream on the left is from the combination nozzle.

Fogging the Issue

One of the ongoing debates within our industry concerns the notion that the fog nozzle produces water in small drops. However, it is somewhat misleading to call them “fog” nozzles, because that is only one setting on a combination nozzle. The fact of the matter is that both combination and smooth-bore nozzles deliver water in droplet form. A true solid stream, which is termed a “laminar flow”, would be glass-like and only is attainable at very low pressures (less than 
5 psi). This would produce a stream that has no practical reach. In addition, such a stream would not have the surface area to absorb heat and convert water to steam. You often see streams of this nature at places like the Bellagio in Las Vegas or the Crystal Gardens at Navy Pier in Chicago. While they are interesting to look at, the stream would not provide much in terms of surface area, would not quench much fire, and thus would be ineffective for firefighting purposes. Water absorbs the greatest amount of heat when actually converted to steam (Over 
8,000 BTUs per gallon). While the stream from a smooth-bore nozzle looks solid to the naked eye, high-speed photography reveals that it actually consists of water droplets, as is the stream from a combination nozzle when used in the straight-stream setting.

The pressure in a flowing hose line is directly related to velocity. When nozzle pressure is increased in a flowing stream from a fixed-gallonage nozzle, water velocity is increased as well. And, as water velocity increases, so too does turbulence. Firefighting streams are classified as “turbulent.” In fluid dynamics, the degree of turbulence is identified via a “Reynolds” number. A Reynolds number greater than 3,000 indicates a turbulent stream, while a Reynolds number greater than 5,000 guarantees a stream that entrains air. Generally, firefighting streams have a Reynolds that is greater than 5,000. The molecules at the center of the hose are moving at one speed and the molecules near the edge are being forced to slow down, a phenomenon known as friction loss. In simple terms, the turbulence forces the molecules of water to begin moving in many different directions.

As the molecules that already are moving in many different trajectories leave the nozzle, they are further agitated by friction with the air. This friction — both at the edges of the stream and in front — further break the stream into smaller droplets until it finally loses the battle and falls to the ground in a shower of droplets. So, the higher the hose-line pressure and water velocity, the greater the water turbulence and, consequently, the smaller the droplet size exiting the nozzle. This, along with lower-volume flows compared with traditional smooth bores, gave the old 100-psi combination nozzles smaller droplets with less firefighting knockdown power. The smaller the droplet size, the greater the effects of air on the stream and the less its reach will be.

The truth of the matter is that to a certain degree, the nozzle type has less to do with the formation and size of the droplets than the nozzle pressure and resulting velocities. So, regardless of whether a smooth-bore or combination nozzle is used, if the nozzle pressures and flows are equal, the stream quality tends to be equal as well. In other words, at equal nozzle pressures and flows, the stream from a smooth-bore and the stream from a combination nozzle set to straight stream, consist of equally sized droplets and neither is a truly solid stream. Water drop-off tests, where containers are placed at intervals below the streams in order to measure any water that may drop off, have shown that at equal flows and pressures, the containers captured equal amounts of water. This indicates, assuming that reach is equal, that both streams are of fairly equal droplet size and are equally disrupted by air.

Angles and Exits

Some engineers feel that the change in direction that a combination nozzle must make, along with the straight forward thrust from the barrel, helps re-focus the stream and its water molecules. This may explain why some had a slightly greater reach at equal pressure and flows. Now we also can understand why a stream straightener (which is nothing more than some thin veins) helps the reach of a smooth-bore nozzle by re-focusing some of the agitated water molecules. Conversely, the stream straightener does not appear to affect the combination nozzles significantly because the nozzle design is already performing that task.

If we compare streams, we see more coning or expansion of the stream as it exits the tip of a smooth-bore nozzle. This is due to the molecules being agitated by turbulence, which in turn causes them to travel in many different trajectories. Some are traveling at angles away from the center axis of the stream; as a result, the instant they exit the smooth-bore tip they travel in an outward direction. In addition, as the water molecules are forced through the tapered opening of the smooth-bore tip, the molecules at the center of the stream are traveling at a greater velocity than those at the sides, which further contributes to a separation of the water molecules as they exit the nozzle. In comparison, water from the combination nozzle exits at a uniform velocity. In either case, both nozzles demonstrate a reach that is pretty similar. Moreover, with a reach well in excess of 100 feet, how important is this for interior firefighting?

As we examine the combination nozzle set to a fog pattern, the water movement takes on a trajectory that is away from the center axis, and more droplets become visible to the naked eye. In addition, the water begins to engage the nozzle teeth, which further impacts the droplets by adding rotation to the water molecules, which further fractures the stream. This can be a benefit in situations where the firefighter is facing possible electrical hazards or flammable liquid fires, when he needs hydraulic ventilation, or needs to achieve a greater rate of conversion and/or increased aeration during a foam application.

Is it Hollow?

Some might argue that the stream from the combination nozzle set to a straight stream is not a solid stream but rather a hollow stream. This is where physics come into play. As noted above, both streams are in droplet form due to the turbulence formed by the pressures needed for firefighting purposes. The stream from a combination nozzle in a straight stream setting exits the nozzle in an annular pattern, like a donut, and the result is a hollow stream as the water works its way past the baffle. However, this is only for a few feet and acts as an advantage in that the negative pressure in the center of the stream draws the water back together.

Water exhibits a property called “cohesion,” which means that the molecules are highly attracted to each other, so they come together or combine to form a stream. However, due to turbulence and forces of the air, the droplets do not come together to form a truly laminar stream but rather, just as occurs with the smooth-bore nozzle, a stream of larger droplets is formed. What we see is that the combination stream is tighter and more focused than the smooth-bore stream for the first three to four feet, but after that they flow in an equal fashion. In fact, a pitot reading taken a few feet out from the tip of either nozzle produced similar readings in both streams.

Next: Part two of this article will discuss reach, impact force, nozzle reaction and the knockdown ability of a smooth-bore compared to a straight stream.

Ron Eilken is a 29-year veteran of the fire service and is a deputy chief with the Des Plaines (Ill.) Fire Department. His past duties included service in the department’s training division and engineering program. He has been an instructor at the fire academy, FDIC and various fire colleges and fire departments across the country.

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