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Learn more about Heat Exchangers
Heat exchangers are used to transfer heat from one material to another. Often, but not always, between liquids. They can be used for heating or cooling.
Finned tubes can be used to transfer the heat of the product to the ambient air. The product passes through an uninsulated tube to which fins have been attached to increase the surface area. Ducting and fans may be used to increase airflow over the fins for greater efficiency.
Tube-in-tube heat exchangers surround the product tube with an outer tube through which a heating medium such as steam or a cooling medium such as chilled water, is passed. Some systems use 3 tubes to greater heat exchange.
Shell and tube heat exchangers consist of a shell with a number of tubes passing through it. Liquid is fed into one end of the shell and out the other. Some designs incorporate a baffle in one end. The liquid makes two passes through the shell exiting on the same end as the inlet.Other types of heat exchangers are available and should be carefully matched to the process.
Tube-in-Tube heat exchangers are used to continuously cool or heat liquids. They come in 2 styles: double tube also known as tube-in-tube and triple tube or tube-in-tube-in-tube.
Double tube heat exchangers surround the center tube with an outer tube. Product flows through the center tube and is heated or cooled by the medium in the outer tube.
Some liquids, particularly viscous products, will not flow evenly. That is, the flow velocity will be slower at the center increasing toward the outer diameter. Other liquids such as blood, ketchup, honey, cornstarch are non-Newtonian meaning that they can act as solids under strain and liquids when the strain is removed. These too may not flow uniformly.
Triple tube heat exchangers add a center tube, also containing the heating or cooling medium. This helps assure more consistent heat transfer by working from both sides of the liquid. The larger annular diameter also assure helps more uniform flow.
Heating/cooling mediums can include steam, hot water, chilled water or refrigerant. Normally heat exchangers are counterflow with the hot or cold medium introduced at the discharge. This maintains the maximum temperature differential throughout, facilitating heat transfer. Temperature sensors monitor the process liquid discharge and control the medium flow to maintain a consistent temperature.
An advantage of Tube-in-Tube heat exchangers is that they are long, with a lengthy contact time when compared to other types. This long contact time allows for more gradual heating and cooling. Product flow rates need to be carefully calculated and controlled to avoid laminar flow which will impede uniform heat transfer. Turbulent flow is generally desired and corrugations or baffles in the tubing or baffles may be included to help assure it.
Tube-in-Tube heat exchangers are typically stainless steel though other materials may be used. Pharmaceutical heat exchangers are usually made from type 316 or 316-L. This type of stainless steel is especially susceptible to chloride stress corrosion. This is a type of corrosion that causes cracking by weakening the bonds between the metallic crystals. Even minute amounts of chlorine, such as the chlorine in normal potable water, can cause this. It can happen shockingly fast, in a matter of hours. It is critical to assure that any cooling/heating water be chlorine free.
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