Recuperating Waste Heat with Heat Exchangers

by Marjorie Steele, Editor for IQS

With all the “green” alternative energy, recycling and sustainable manufacturing facilities popping up across the U.S., it seems only fitting to talk about heat exchangers. Recycling re-usable materials like aluminum, steel, HDPE, rubber and pulp products is a topic of frequent discussion in both consumer and manufacturing industries, but what about energy recycling? Industrial facilities and process manufacturers in pharmaceutical, bio-diesel, pulp and food process industries have been using the heat exchanger – a rather basic design concept – for decades to transfer heating and cooling. In recent years engineers have been tweaking the design, replacing typical coolants with gases or liquids which need to be heated anyway, allowing facilities to recycle their own energy. Automotive radiators, heater cores and evaporators work this way, with tubes of liquid coolant absorbing excess energy from the engine, which is then blown by fans into the car interior as heating.

Industrial process facilities recover, or “recuperate” otherwise wasted heat energy using heat exchangers, but on a much larger scale. Typical shell and tube heat exchangers use a “bundle” of tubes encased in a shell, in which heat energy is transferred from hot liquids or gases flowing through in through the tubes to liquid or coolant which flows over and around the tubes within the shell, capturing heat energy and flowing back out. Plate and flat plate heat exchangers work similarly with hot and cold liquid chambers separated by metal plates. Refrigerators use flat plate heat exchangers to create cool air. In industrial manufacturing, these same heat exchangers can be used not only to cool liquids and air which have been heated by processing, but to input a portion of that heat energy back into manufacturing processes.

Plate heat exchanger

Let’s look at a tomato paste manufacturing facility, for example. A large vat is heated over a series of hot water and electric coils to cook tomatoes down to a paste. During this process, large amounts of hot gas and water steam radiate from the product. This hot gas must be released somehow – either into the facility or into the atmosphere, and if it goes unrecuperated is considered waste. A recuperative heat exchange system would capture the hot gas and steam through an overhead duct and carry it to an air-to-liquid shell and tube heat exchanger. The extra water vapor would condense and collect for reuse within the heat exchanger, transferring heat energy to the liquid inside the pipes. The pipes in turn flow to the coils below the vat, using the energy that was released from the vat to continue heating the tomato paste.

Recirculating air heat image by Exothermics, Inc.

Recuperating wasted process energy can result in significant savings for facilities across the manufacturing spectrum, especially in high energy processes such as metal treating, oil refining, chemical processing and power generation. Energy can be transferred from air to air, air to liquid or liquid to liquid in any manufacturing processes which uses or requires heat. Capturing process heat to warm facilities during cold weather can save enormously on facility heating costs, causing a well-build, efficient heat exchange system to pay for itself, sometimes within a few months.

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