What a Heat Exchanger Does
A heat exchanger transfers heat from one fluid to another without letting the two fluids mix. A wall, usually metal, separates them. The hotter fluid warms the wall, and the wall warms the cooler fluid. That simple idea shows up almost everywhere in a plant: cooling hot oil with water, preheating feed water with exhaust steam, condensing vapor back to liquid, or warming a process stream before it enters a reactor.
Plants use heat exchangers mainly to heat something, cool something, or recover energy that would otherwise be wasted. Recovering heat is often the most attractive use, because the energy is already being produced and would otherwise leave through a stack or a cooling tower.
The Basic Physics in Plain Terms
Three things largely determine how much heat a given exchanger can move.
- Temperature difference. Heat flows from hot to cold, and a larger difference drives a faster transfer. As the two fluids approach the same temperature, the transfer slows.
- Surface area. More wall area between the fluids means more heat moved. Many designs are really clever ways of packing a large surface into a small space.
- Heat transfer resistance. The wall material, the thickness, the speed of the fluids and any deposits on the surfaces all change how easily heat passes through.
Fluid flow arrangement matters too. In parallel flow, both fluids travel in the same direction. In counterflow, they travel in opposite directions, which usually allows the cold fluid to leave closer to the hot fluid’s inlet temperature. For that reason counterflow is a common goal in design, although real units often use a mix of arrangements.
Shell-and-Tube Exchangers
The shell-and-tube design is probably the most recognizable. A bundle of tubes sits inside a cylindrical shell. One fluid flows through the tubes and the other flows around them in the shell. Baffles inside the shell steer the shell-side fluid back and forth across the tubes so it does not simply take the shortest path.
Typical strengths include tolerance for high pressures and temperatures, a long track record, and a wide range of materials. Tube bundles can sometimes be pulled out for cleaning or replacement, depending on the design. Typical drawbacks are size and weight, and the space needed to withdraw a bundle for service. Shell-and-tube units are common in refineries, chemical plants, power plants and large process facilities.
Plate Exchangers
Plate heat exchangers use a stack of thin corrugated plates with narrow channels between them. The two fluids flow through alternating channels. The corrugations create turbulence, which improves heat transfer, so these units are often compact for the duty they perform.
Gasketed plate designs can be opened and the plates cleaned or added, which suits food, beverage and utility service. Brazed and welded plate designs trade that serviceability for the ability to handle higher pressures or more aggressive fluids. A common limitation is that narrow passages can clog with solids or fibers, and gaskets limit the temperature and chemicals a gasketed unit can handle.
Other Common Designs
- Air-cooled exchangers pass process fluid through finned tubes while fans blow ambient air across them. They are used where water is scarce or expensive, though performance follows the outside air temperature.
- Double-pipe exchangers are a pipe within a pipe. They are simple and suit small duties or very high pressures.
- Spiral exchangers wind two channels into a coil, which can handle fluids carrying solids or sludge.
- Condensers and evaporators are heat exchangers built for a phase change, such as steam turning to water or a refrigerant turning to vapor. They appear in power and industrial refrigeration systems, and in process evaporators.
Where They Are Used
In power generation, condensers turn exhaust steam back into water, and feed-water heaters recover heat to improve plant efficiency. In chemical and petrochemical plants, exchangers control reaction temperatures and recover heat between process stages. In food and beverage plants, they pasteurize and cool products, usually with designs that are easy to clean. In buildings and campuses, they separate loops so that a boiler loop and a heating loop do not share the same water. They are also part of industrial cooling systems, where a closed loop is cooled by a separate water or air circuit.
What Affects Performance Over Time
Heat exchangers rarely fail dramatically. More often they slowly lose capacity. The usual cause is fouling, a layer of scale, corrosion products, biological growth or process residue that insulates the surface. Heat exchanger fouling raises energy use and can push process temperatures out of range, so it is worth tracking temperatures and pressure drop across a unit. Other problems include tube leaks, gasket leaks and erosion where fluid velocity is high. Good industrial water treatment on cooling and boiler water is one of the most practical ways to slow fouling and corrosion.
Questions to Ask When Choosing a Type
Selection is normally done with the equipment maker and a qualified engineer, but a few questions help frame it.
- What are the two fluids, and are they clean, corrosive or likely to carry solids?
- What pressures and temperatures will each side see?
- How much space is available, and how will the unit be cleaned or inspected?
- What happens if the barrier leaks? In some services, cross-contamination is a serious hazard and calls for a design with added protection.
- Is the heat being recovered for reuse, which may justify more capital cost?
Because these units hold hot or pressurized fluids, they are usually treated as pressure equipment, and local codes, inspection schedules and the manufacturer’s instructions apply. Work on them should be done by trained personnel following lockout tagout procedures and site rules. This article is general information, not engineering advice.