Heat Exchangers (101 Series #3)

Written by guest author and ally Aaron Sawheb.

Ah, heat exchangers, these little bundles of thermodynamic magic exist on a multi-level spectrum; they can drive the optimization of a process or create a major bottleneck, (when purchased correctly) they can be accompanied by a dedicated service team or remain inoperable indefinitely with not-so-much as an email reply, they can make extraction a joy or a nightmare, and they can be made to any specification of quality. Clearly, the primary concerns for an extraction operation are that a heat exchanger is appropriately sized for a process, that they are built to a standard of quality that is equivalent to the end-product, and that they are purchased from a company that ensures timely service and troubleshooting assistance.

Of course, heat exchangers are also extremely complex from a technical perspective—engineers spend countless hours determining operational parameters, flow characteristics, correction factors, fouling resistance, chemical compatibilities, pressure drops, efficiencies…the list goes on. As an individual focused on extractions, it is important to sift through this mound of data and focus on the kernels of relevant information that will empower them to make the appropriate purchasing decision.

Preparing for Purchase

Before contacting a company for a quote, it is imperative that an individual determines several operating parameters that they foresee for their heat exchangers:

  • What is the temperature of the fluid entering the heat exchanger?
  • What temperature change does one wish to achieve with the heat exchanger?
  • What volume of process fluid does one wish to process per unit time?
  • At what pressure will the process fluid enter the heat exchanger?
  • What is the velocity of the process fluid entering the heat exchanger?
  • What is the viscosity of the process fluid entering the heat exchanger?
  • What amount of pressure drop is acceptable across the heat exchanger?
  • How much heat is required for the heat exchanger to successfully change the temperature of the process fluid?

Several of these questions have been answered in previous discussions, namely concerning temperatures and heat, but any further information that is indeterminable will be left to the engineers from the potential supplier. The more information provided to a manufacturer, the better the chance of procuring equipment that will benefit your process instead of hindering it. The purpose of the above discussion is simply a means of communicating that: the more one knows about the science of their process, the better one is equipped to interact with equipment manufacturers and suppliers. From this point on, the conversation will focus on the scientific understanding of this equipment and how this knowledge can translate to a net benefit for an extraction process.

Overall Heat Transfer Coefficient (U)

This quantity, which should not to be confused with a previously discussed “U” (Internal Energy), is a measure of the “overall heat transfer.” It is a function of the calculated heat requirement of the exchanger (Q), the surface area available for heat transfer (A) and the Log-Mean Temperature (TLM) of the process fluid. As the value of U increases, so does the ability of the heat exchanger to deliver its intended function:

Calculation of U is no simple matter, it heavily relies on the materials, geometry, convection and fouling resistance of a heat exchanger, and is a bit beyond the scope of this discussion, see Figure 1 for an example. Instead, the value of U can be calculated periodically to assess the overall well-being of a heat exchanger being used.

Over time, U will decrease in value and therefore decrease the effectiveness of the heat-exchanger. When U dips below an acceptable value, it generally indicates that it needs to be cleaned—an extremely important consideration for extraction operations. On the other hand, if a unit is cleaned too frequently it creates a bottleneck in the operation and can decrease profitability. However, the value of U is also intimately connected with the quality of process-fluid entering the heat exchanger, for one batch of crude material U can be extremely high and for another it can be extremely low—its important to be flexible with process requirements for U so as to not create an unrealistic expectation from suppliers.

Fortunately, the calculation of this quantity is based on fairly simple variables—Q and A can be assumed to remain somewhat constant (see “Fouling” below)—the remaining variable TLM is simply a measurement of the difference between the temperature of process fluid and cooling fluid entering and exiting the unit, and given its relative simplicity will be left to the reader to determine (which will save space for more important information).

In the context of making a purchasing decision, one should leverage historical information retained by the company, ask questions like: how quickly does the value of U decrease for this heat exchanger? At what value of U should a customer clean this equipment? Does the company provide instructions for efficiently cleaning this heat exchanger? If this heat exchanger cannot reach a certain value of U, will the company replace or service it?

These several questions alone will help buyers vet potential equipment suppliers to determine how well a company knows their product and to eliminate potentially troublesome suppliers.

Fouling

Fouling occurs when particulates in the heat-exchanging fluid or process fluid adhere to the surface of the heat exchanger. Over time, this particulate buildup reduces the flow of fluids through the equipment as well as the surface area (A) available for heat transfer and reduces the overall effectiveness of the unit. Unfortunately, and especially in the case of extraction, fouling is a major concern and should be considered before purchasing any heat exchanger.

In the opinion of the author, the first consideration in the context of fouling is controlling the input material to the heat exchanger. In the context of extraction, it becomes important to remove as many non-desirable constituents from the crude extract as possible; the removal of components such as lipids/waxes, filterable particulates and other easily seperable constituents can greatly increase the longevity of a heat exchanger. The second important factor for consideration is the material of construction of the heat exchanger. Heat-exchangers, no doubt come in many shapes and sizes and are made of any number of materials—it seems reasonable to model a process after analagous manufacturing processes to ensure that the appropriate decisions are made when selecting a heat exchanger.

Since the author is familiar with the pharmaceutical industry, it will be used as an example for the sake of discussion. Pharma processes often employ the use of stainless steel (SS) heat exchangers; SS is relatively resistant to chemical interactions, it is easily cleanable, it comes in a variety of material compositions and it can be polished to a finish that reduces the potential for fouling. SS, as mentioned, comes in a variety of material compositions (different percentages of elements are present in the final material) that increase in quality as follows: 304, 316, 316L. When selecting a material, an engineer will often prefer to choose 316L as it provides the largest protection against the above-mentioned issues. Moreover, manufacturers will often take the step of “electropolishing” the surface of the material to a certain roughness value (Ra). In the context of this discussion, SS is simply an example, what is more important is that the reader familiarize themselves with the various materials and material grades that can be selected from as it will allow them to make a more informed decision about purchasing.

Specifically, an individual could ask the following questions: what materials of construction are used for surfaces that directly contact the process fluid? Are several grades of the material avaialble for this heat exchanger? Does the manufacturer provide electropolish for this heat exchanger? Are material certificates and certificates of electropolish provided with this heat exchanger? Do these certificates match standardized requirements set forth by governing bodies (like ISO, ASME, etc)?

Conclusion

There are many concerns when purchasing heat exchangers, some are much more imperative to assess than others, but a rounded understanding of all factors is helpful in making the right process decisions. Make sure you ask as many questions as you can, and always remain vigilant when suppliers make outlandish promises—their motivation is to sell equipment, and sometimes they do so at the misfortune of a buyer. Ensure that you find an equipment supplier that takes the time to inform their customer, who provide accurate information when asked, and who aren’t afraid to tell you they aren’t sure.

As for an expansion of this conversation, there is much more to be discussed. Stay posted…

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