Tube Velocity In Heat Exchangers.pdf 2c5t3j

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-, u-tubes)

Effects of Excessive Tube Velocity • Erosion (abrasion) • Corrosion • Pressure drop • Vibration

Generally Acceptable Guidelines for Tube Velocities Based on Tube Material • iralty (general service) up to 6 feet per second • iralty (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 deg 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