Why Does Fluid Velocity Matter In Heat Exchangers?

One of the core devices in any fluid temperature control system is a heat exchanger. Many factors can affect the efficacy and effectiveness of the heat exchanger, including the transfer of thermal load, operating pressure, and chemical compatibility that play a part in determining the suitable materials used in the final product of the heat exchanger. However, the velocity of fluids in the heat exchanger is often one of the most overlooked factors that play a critical role in the overall performance of the fluid temperature control system. In this article, we dissect the reason why.

The Significance of Velocity

In terms of heat exchangers, velocity refers to the rate and motion of the fluid flow. Often, many heat exchanges designs permit low fluid velocity, which can be problematic in the performance of the temperature control system. The reduction in velocity can cause the dispensed fluid (a mixture of solids and liquids) to settle easily, as it cannot agitate the fluid sufficiently to keep the solids such as fillers and pigments suspended within the liquid.

Therefore, this would usually result in the system underperforming below optimum levels as the solids narrow the passage of the tubes in the heat exchangers. This occurs because the solids would amalgamate and form into a mass, and when it is pulled along by the liquid, the loose clumps will cause the nozzle to clog. Unfortunately, in many cases, this issue would go undetected and causes an increase in the heat exchanger’s back pressure, putting immense stress on the system. The result? An increase in downtime and the frequent replacement of nozzles and guns contribute to the overall cost.

Why Higher Fluid Velocity is Optimal

The reason why higher fluid velocity is pretty simple – it helps significantly with thermal transfer in the heat exchanger. Breaking this logic down: the higher the velocity of the fluid, the higher the turbulence and agitation, the faster the thermal transfer. With higher turbulence in the fluid, this will cause the fluid to move against the walls, increasing the amount of friction that generates heat, in other words – thermal energy.

The more fluid volume comes in contact with the walls, the more effective the thermal transfer becomes. On the other hand, the opposite is a laminar flow, considered a linear and smooth flow along the passage. Therefore, as the fluid turbulence reduces, the film temperature increases, resulting in a decrease of thermal energy transfer.

Taking into Account Fluid Velocity

Measured in Reynold Numbers, a higher turbulent flow is usually indicated by a higher unit number, while a lower unit number characterizes a smoother and laminar flow. You will want to achieve a higher Reynold number to increase thermal transfer. However, it is worth noting that when there is an increase in friction caused by turbulence, the amount of pressure drops throughout the heat exchanger system. Therefore, as the fluid viscosity increases with more turbulence, you will need higher velocity to increase the Reynold Number and prevent a laminar flow. This can cause the overall pressure to drop throughout the heat exchanger and reduce thermal transfer, which can cause the settling of fluids. Therefore, it is important to consider fluid velocity when designing heat exchangers.

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