First, there is the thermal load that must be transferred. Next are working pressure, flow rate, and chemical compatibility requirements. Coupled with the thermal transfer, these determine the configuration (gasketed plate, brazed plate, shell & tube, etc.) and the materials of construction (carbon steel, stainless steel, aluminum, titanium, etc.).
One of the least understood and rarely considered parameters, however, is the velocity of the fluids in the heat exchanger, yet this can play a critical role in the performance of the system.
The Impact of Low Velocity
Velocity is, of course, a function of the flow rate of the fluid and the volume of the path that the fluid must follow. Probably the most common error in heat exchanger design is to allow too low a fluid velocity. It turns out that this can create multiple issues with system performance.
If the velocity of the fluid is not sufficient to create enough turbulence to maintain the solids in suspension, settling will occur. In systems with a low velocity — generally less than 1.0 fps (feet per second) — the pigments and other fillers will start to settle out in the heat exchanger (or in the piping for that matter).
Usually this goes undetected — that is, until, in the extreme, the passages start to plug and the backpressure in the heat exchanger increases, indicating that it requires service.
In the meantime, however, as the passages narrow, the velocity increases, creating stress on the surface of the settled solids.
You might think that these solids would just rejoin the coating as it passes by, but it’s not that simple. As they settle, these solids agglomerate into a mass. When this mass is “pulled-on” by the fluid going by, pieces break loose and clumps travel through the system to the point of dispense causing nozzle clogs.
This results in downtime and maintenance — often the gun or nozzle is replaced — even though it was really not the root cause of the problem — which represents an excess overhead cost.
More often than not, however, especially in the case of paints and coatings, multiple colors are run through the system. These settled pigments and solids now appear as contamination in the new color — and as any painter will tell you, it doesn’t take very much red to turn a white pink!
Again, this creates quality issues and excess costs, all caused by not considering the velocity in the heat exchanger.
Velocity Affects Thermal Transfer
Often less understood, is the effect of velocity on the thermal transfer characteristics of the heat exchanger: It turns out that the higher the velocity through the heat exchanger, the higher the turbulence — and the higher the turbulence, the more efficient the thermal transfer.
You may be wondering how that can be. The answer lies in the turbulence of the flow.
You’ll recall that turbulent flow creates a great deal of disturbance along the flow path. This turbulence forces the fluid against the walls of the heat exchanger and increases the friction between the fluid and the surface.
The turbulence constantly pushes fresh fluid against the metal wall, and the greater the volume of fluid in contact with the walls of the heat exchanger, the greater the thermal transfer.
The opposite of turbulent flow is “laminar flow,” which you may also recall is defined as a smooth, linear flow through the system. As turbulence falls, the film on the walls of the heat exchanger increases. Called the “film coefficient,” as it increases, the thermal transfer efficiency is reduced.
Reynolds Numbers and Pressure Drop
Turbulence is measured in “Reynolds Numbers.” Turbulent flow is marked by higher Reynolds Numbers and laminar flow is marked by lower Reynolds Numbers.
While increasing Reynolds Numbers positively influence thermal transfer, the increase in friction also increases the pressure drop through the heat exchanger, whereas the smooth flow through the heat exchanger associated with laminar flow produces a lower pressure drop, but also a lower thermal efficiency.
It is interesting to note that, as the viscosity of the fluid being dispensed increases, the greater the velocity required to drive the Reynolds Numbers up and prevent laminar flow — which has an even greater impact on pressure drop through the heat exchanger.
Because it is essential to take pressure drop into account when designing a dispensing system, this can be a vicious circle trying to balance thermal transfer with pressure drop — especially in high pressure systems such as those associated with high-viscosity sealers and adhesives.
You must always consider fluid velocity in your temperature control system design.
EDITOR'S NOTE: This post was originally published in February 2014, but due to its popularity and relevance, we're sharing it again for anyone who may not have seen it.
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