Energy Efficiency Measures

COMMERCIAL: HEAT RECOVERY

There are many areas in such buildings as hospitals, manufacturing facilities requiring clean rooms, and laboratories that must be zoned as “once-through” systems in which the air that heats, cools, and ventilates is used only once. However, much of this HVAC energy can be recovered before it exits the building by installing heat-recovery coils in the exhaust air handlers. This heat can then be used to precondition the outside air coming into the building. Energy can be recovered without risk of contamination.

Waste heat recovery on boiler stacks can be used to preheat boiler makeup water, thereby improving overall energy efficiency quite substantially. Heat recovery from stacks in heat treating furnaces is frequently used to preheat combustion air, thereby achieving savings of well over 50%.

Water-to-water heat exchangers are quite useful in a range of applications, from dyeing operations (where energy from a depleted batch of hot dye water is used to pre-heat the next batch) to various operations in chemical plants.

Heat exchangers allow for the transfer of heat from one fluid to another (including air) without the contents of one stream polluting those of the other. When the requirement for ensuring that absolutely no transfer of contents is high (e.g., exhaust air from hospitals), double-wall heat exchangers are used.

Heat exchangers are frequently employed in industry to save energy and enhance the performance of both batch and continuous processes. For example, a plant that uses large quantities of steam to heat batches of dye can install a heat exchanger to preheat the water for fresh batches of dye by using the waste water from an old batch. This both increases the speed of heating the new water and lowers energy requirements precipitously, while retaining good quality control over colors. When water or steam is involved in such heat transfer functions, “counter flow” shell-and-tube or plate-type heat exchangers are routinely employed. These result in good heat transfer coefficients at minimal risk of cross contamination.

Air-to-air heat exchangers are widely employed in processes which require heating materials to high temperatures over long periods of time, such as in ceramics or heat treating applications. Instead of allowing the hot combustion air to be vented directly to exhaust stacks, heat exchangers recover as much as 80% or more of the heat from the exhaust stream and use it to pre-heat combustion air. This can save well over half of the primary energy used in such facilities.

Other examples of the use of heat exchangers include:

• Condensing steam from a boiler to produce hot water for service hot water or other processes;
• Isolating two systems which operate at different pressures while extracting heat from the higher temperature system;
• Moving heat or cool in various refrigerator cycles that may include changing of state from liquids to gases in the heat exchanger; and
• Moving heat into and out of thermal storage containers.

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