91. Smooth Bore Nozzles vs. Combination Nozzles
How is a fire stream best formed?
Since the inception of the combination nozzle, the argument has existed of which nozzle makes the best straight stream; the smooth bore or the combination?
To better understand both sides of the argument, this section will show how each stream is formed.
Comparisons of velocity, impact force and reaction force will be made.
Side by side photos show the streams from a combination nozzle and from a smooth bore.

92. Smooth Bore Nozzles Advantages: Disadvantages: Simple in design
Inexpensive
Disadvantages:
Has only one correct operating pressure to develop optimum velocity and reach
Tip size must be changed to change flow
Must have variety of tips for different flows
No fog capabilities
Not effective for mechanical ventilation
Smooth Bore nozzles are inexpensive and simple in design.
A smooth bore nozzle is a restriction at the end of the hose, with a specific diameter hole.
To provide the optimum velocity and reach, only one pressure will work effectively, delivering a single flow.
To change flow, the tip size (hole size) needs to be changed.
Friction loss and pump pressure calculations must be made for each size tip, to provide proper flow and pressures.
To achieve multiple flows, several size tips need to be available.
Smooth bore has no fog capability and is not very effective for mechanical ventilation.

93. Unequal Velocity of Smooth Bore
Side Wall Friction
Center "core" at higher velocity
than outer circle of stream.
Stream expansion begins upon exiting tip
Greater velocity at center of stream
Lower velocity at outside of stream
Caused by side wall turbulence
Causes outside of stream to “peel” away
Less reach than straight stream from combination nozzle
Let’s compare how each stream is formed.
The smooth bore produces stream velocities that are not the same within the cross sectional area.
The center of the stream is moving faster than the outside of the stream.
This causes the outside to rapidly peel away as it leaves the nozzle.
As you can see in the photo on the lower right, the outer edges of the stream are starting to peel away very soon after exiting the nozzle.

94. Equal Velocity of Combination Nozzle
Partial Vacuum
Water exits nozzle at EQUAL velocity and pressure
Focused Point
Equal velocity across stream
Partial vacuum within the pattern focuses stream a short distance from the nozzle
Re-converged stream has a uniform cross-sectional velocity
Tighter stream with more reach than smooth bore
From a combination nozzle, the stream is shaped by deflecting off the baffle and stream shaper as it exits the nozzle.
This creates a partial vacuum in the center of the stream which helps keep the stream cohesive to a focal point in front of the nozzle.
The stream is moving at a uniform cross-sectional velocity.
This creates a tighter stream with more reach than a smooth bore.

95. Equal Velocity of Combination Nozzle
Partial Vacuum
Water exits nozzle at EQUAL velocity and pressure
Focused Point
As seen in the top photo, the stream from a combination nozzle draws back together once it leaves the nozzle.
There is less water peeling away from the outer edges of the stream, compared to the smooth bore stream.
Let’s look at the two nozzles and streams side by side.

96. At Equal Flows... Which stream has more reach?
The smooth bore and the combination nozzle are flowing equal flows of 125 GPM.
Look at the photo at upper right.
The combination nozzle’s stream is being drawn together at the focal point.
At about the same distance, the smooth bore’s stream is starting to break away from the outside.
In the bottom photo, the stream from the combination nozzle is still intact, many feet from the nozzle, while the smooth bore keeps getting wider and wider.
Less water is hitting the target from the smooth bore than from the combination nozzle.

97. At Equal Flows... Combination Nozzle Reaches Farther Than Smooth Bore Nozzle
In this photo from a different comparison, the same results are shown.
At equal flows, a combination nozzle will provide greater stream reach than a smooth bore.
The design of the nozzle and velocity of the stream are the major factors.

98. Straight Stream From Combination Nozzle...
Not hollow as seen by Pitot gauge pressure reading at focal point
Because of higher nozzle pressure & velocity than smooth bore, combination nozzle stream hits harder
It has been wrongly stated that the straight stream from a combination nozzle is hollow.
At the focal point, this clearly shows that the stream is not hollow, as shown by pressure reading on the Pitot gauge.

99. Impact Force of Stream F = M V Force = Mass times Velocity WV g
M = Mass of water /unit of time
V = Velocity of water /unit of time
W = Weight of water /unit of time
g = Gravitational constant 32.2 ft/sec2
F =
Scientifically, this will show the impact force of a 150 GPM stream at different pressures.
In order to determine the solution to the formula “Force equals Mass times Velocity”, we must first consider the weight of water in a 150GPM stream for one minute.
On this planet, the gravitational constant is 32.2 ft/sec2.

100. Impact Force of 150 GPM Stream...WV
32.2 ft./sec.2
W = 150 GPM x 8.34 lb./gal = 1251 lb./min.
W = 1251 lb./min. divided by 60 sec./min.
W = lb./sec.
V = 150 GPM
7/8” Smooth 44 psi = 80 ft./sec. (54.5 MPH)
Combination 75 psi = ft./sec. (72.9 MPH)
Combination 100 psi = 121 ft./sec. (82.5 MPH)
F =
Water weighs about 8.34 pounds per gallon.
150 GPM would be 1251 pounds per minute.
For the formula we need to convert this to pounds per second.
Velocity is typically measured in feet per second.
The formula for determining stream velocity is:
Velocity at 150 GPM is shown for combination nozzles and smooth bore nozzle.

101. Comparison of Velocity... @ 150 GPM
Nozzle Type Nozzle Pressure Velocity
7/8” Smooth Bore 44 psi mph
Combination Nozzle 75 psi mph
Combination Nozzle 100 psi mph
Velocity is the rate of motion of a particle in a specific direction.
The greater the velocity of the water when it leaves the nozzle, the farther it will reach before gravity pulls it to the ground.
So at equal flows a combination nozzle at 100 psi will reach farther than a smooth bore or a lower pressure combination nozzle.

102. Comparison of Impact Force... @ 150 GPM
7/8” Smooth 44 psi
20.85 lb./sec. x 80 ft./sec.
32.2 ft./sec.2
Combination 75 psi
20.85 lb./sec. x ft./sec.
Combination 100 psi
20.85 lb./sec. x 121 ft./sec.
F = = 51.8 lb. Force
F = = 69.2 lb. Force
F = = 78.3 lb. Force
How hard does a stream hit?
This is a comparison of the impact force of a 150GPM stream from each of these nozzles.
The 7/8” smooth bore hits with about 52 lb. Force
The 75 psi combination nozzle hits with about 69 lb. Force.
The 100 psi combination nozzle hits with about 78 lb. Force.
It is clear that a stream from a 100 psi combination nozzle hits with more force than the 75 psi combination nozzle or 7/8” smooth bore nozzle.

103. Comparison of Nozzle Reaction Force... @ 150 GPM
Nozzle Nozzle Pressure Reaction Force
7/8” Smooth 44 psi lb. force
Combination 75 psi lb. force
Combination 100 psi lb. force
This shouldn’t be a surprise to you (Sir Isaac Newton discovered this!)
The reaction force from a nozzle is equivalent to the impact force.
If you compare the numbers from the last slide on impact, to these numbers on reaction force, you will see they are very close.
Rounding off of numbers in the mathematical process has made the slight differences that are shown.

104. Impact Force & Velocity...
Combination nozzle has higher velocity
Smooth Bore has lower velocity
Higher velocity with same flow equals more force
Therefore, harder hitting stream is typically realized from combination nozzle at higher pressure
Combination nozzles have higher velocity, due to higher operating pressures.
A smooth bore has lower velocity, due to lower operating pressure.
Higher velocity with same flow equals more force.
Working within the safe design pressures for all nozzle types is recommended.

105. Stream Impact Testing...
Flow of 160 GPM (typical of smooth bore & combination nozzles)
Differing distances and target sizes used
Flows & Pressures closely monitored for valid test results
Actual impact testing was done to verify the results of the formulas.
Different distances and target sizes were used.
Flows and pressures and force measurements were closely monitored and recorded for valid test results.
Results: Harder hitting, farther reaching stream from combination nozzles because of higher stream velocity