Heat Exchanger

Heat always flows from a warmer fluid to a cooler one. A heat exchanger puts two fluids close together, separated by a metal wall such as a tube or a plate, so heat passes through the wall while the fluids stay apart. The larger the surface area and the bigger the temperature difference, the more heat moves.

Most heat exchangers in buildings use counterflow, where the two fluids move in opposite directions. This lets the fluid being heated leave at a temperature closer to the entering temperature of the hot fluid than a parallel flow design allows. The remaining temperature gap is called the approach, and a small approach means an effective heat exchanger.

Heat exchangers in buildings fall into a few families:

• Shell-and-tube: one fluid flows through a bundle of tubes inside a larger shell that holds the other fluid. Used in chiller evaporators and condensers and in steam-to-water converters.
• Plate (plate-and-frame or brazed plate): thin corrugated plates stacked together, giving a large surface in a small footprint. Common for waterside economizers and for isolating one water loop from another.
• Finned-tube coils: water or refrigerant inside tubes, with air passing over fins. The heating and cooling coils in air handlers and fan coil units are heat exchangers.
• Air-to-air: energy recovery wheels, fixed plate exchangers, heat pipes and run-around loops that transfer heat between exhaust air and incoming outdoor air.
• Double-wall exchangers: used where a leak could contaminate potable water, such as some domestic water heating applications.

Almost every HVAC and plumbing system contains at least one heat exchanger. Common examples include:

• Chillers, which have two: the evaporator and the condenser
• Boilers and furnaces, where hot combustion gases heat water or air
• Plate heat exchangers between cooling tower water and chilled water, for free cooling in cold weather
• Steam-to-hot-water converters and district energy connections
• Domestic water heaters and heat recovery from condenser water
• Energy recovery units on ventilation systems

The main enemy of a heat exchanger is fouling: scale, biological growth, sediment or dirt that coats the heat transfer surface and acts as insulation. Fouling shows up as a growing approach temperature and a higher pressure drop across the exchanger. Plate units can also leak at their gaskets, and tube bundles can corrode or crack.

Routine maintenance includes water treatment, cleaning strainers, monitoring temperatures and pressures on both sides, cleaning tubes or plates on a schedule, and pressure testing when a leak is suspected. In furnaces, a cracked heat exchanger is a safety issue because it can let combustion gases, including carbon monoxide, into the airstream.

Teams track entering and leaving temperatures on both sides, pressure drop, cleaning dates, gasket replacements and the design conditions from the submittal. In a digital twin such as Syyclops, those readings and work orders can be attached to the heat exchanger as an asset in the building model.