Industrial Heat Exchangers
www.deltathx.com
Tube Velocity in Heat Exchangers
One of the variables that must be balanced in the design of heat exchangers is the fluid velocities. Not only does
fluid velocity have an impact on the heat transfer coefficients it also can impact; vibration, pressure drop,
propensity for fouling and overall service life of the heat exchanger. For most fluids, in most applications, a
higher tube velocity results in a higher inside heat transfer coefficient, however tubes have limits to the
maximum velocity they can handle. Heat exchangers also have limits to the minimum velocity. In addition to
make designs more complex, a heat exchanger can be designed with an ideal tube velocity for heat transfer and
longevity, however if the fluid flow is varied during the operational life of the exchanger it can have a great
impact on the service of that exchanger.
Maximum Recommended Tube Velocity:
Recommended maximum tube velocities will vary by source, not necessarily due to differing opinions, but
mostly due to the fact that; fluid temperature, chemistry and other factors will have an effect on the impact tube
velocity will make on a given material.
Factors Effecting Acceptable Tube Velocities
• Type of fluid and temperature
• Fluid make-up (suspended solids, treatment, chemistry)
• Tube material
• Heat Exchanger construction (multi-pass, u-tubes)
Effects of Excessive Tube Velocity
• Erosion (abrasion)
• Corrosion
• Pressure drop
• Vibration
Generally Acceptable Guidelines for Tube Velocities Based on Tube Material
• Admiralty (general service) up to 6 feet per second
• Admiralty (fresh water service) up to 8-9 feet per second
• Carbon Steel 6-9 up to feet per second
• Copper 6-7 up to feet per second
• Copper-Nickel (90/10) up to 9 feet per second
• Copper-Nickel (70/30) up to 15 feet per second
• Inconel 625 / Hastelloy C-276 maximum 20 feet per second
• Stainless Steel (304 or 316) up to 12 feet per second
• Stainless Steel (304L or 316L) up to 15 feet per second
Note: As described above, these guidelines are generalized and every application is unique.
Consideration to all the factors effecting tube velocity must be considered.
Low Tube Velocity:
For most fluids in most applications; a lower tube velocity means a lower inside heat transfer coefficient,
therefore the heat transfer is less effective, so thermal designers will typically try to keep the velocity as high as
the tube material and pressure drop will allow. Similar to high tube velocities, what is considered to be too low of
a tube velocity is also dependant on; the fluid, temperature, chemistry and construction.
For clean pure fluids, with little or no suspended solids and
are not prone to fouling; the goal is to avoid laminar flow. If
the inside heat transfer coefficient drops too low,
turbulators can be considered. If the tube velocity is below
three feet per consideration should be given to the fluid in
the tubes. For water applications, see the following section.
Effects of Too Low of a Tube Velocity
Fouling
Corrosion
Deposition
Low heat transfer coefficient (laminar flow)
Page 1 of 2
Industrial Heat Exchangers
www.deltathx.com
Water in Tubes
As far as common heat transfer fluids go, water presents interesting challenges because it is a moving target.
Whether cooling tower water, river water, lake water, sea water, well water or municipal water; the chemistry,
temperature, organic make-up and suspended solids often changes seasonally or more frequently. The changes
in the water source needs to be considered when designing a heat exchanger.
It is not an uncommon practice to throttle back cooling water when the demand is low or the water is particularly
cold (in many cooling applications these circumstances occur at the same time). Since we typically design
cooling systems for the most demanding condition, (highest load and warmest water) the water flow may be
able to be reduced significantly. The water flow may be reduced such that the tube velocity may end up in the
range where fouling or organic growth can occur even though the design tube velocity was in an appropriate
range. Operating under even short durations at low flow can have a long term effect on a heat exchanger.
Natural water sources, particularly river water, can bring mud, silt or other
suspended solid in a heat exchanger. Consideration should be given to
ensure the velocity throughout the heat exchanger, including the channels or
headers is high enough to carry the solids through the exchanger.
As with all heat transfer fluid, if the tube velocity in water applications is
below three feet per consideration should be give to the quality of the water
in the tubes. A design velocity below three feet per second may be suitable
for clean, treated water. If fouling is a concern, selecting a higher fouling
factor may be warranted however too high a fouling factor may result in more
tubes being required which, in turn, increases the flow area and lowers the
water velocity.
Conversely, operating at higher than recommended tube velocities
for even short durations can remove the oxide layer that protects
the inside of copper tubes making it susceptible to corrosion. The
propensity for erosion or tube abrasion can increase at higher tube
velocities, particularly at return bends. If the water has higher than
normal suspended solids, particularly sand or silt, a design velocity
below the recommended maximum for a tube material should be
considered.
Click Here for More Information on Fouling in Heat Exchangers
Click Here for More Information on Corrosion in Heat Exchangers
Page 2 of 2