A heat exchanger is a device designed to efficiently transfer the heat from one medium to another. In the case of an outdoor corn boiler, these media would be air and water.

A typical domestic setup would include a water-to-water heat exchanger for hot water and a water-to-air heat exchanger for forced air home heat. Water-to-water heat exchangers are also used to heat hot tubs, swimming pools and the water for radiant baseboard or radiant in floor
heating systems.

Water-to-Water Heat Exchangers

The three most common types of water-to-water heat exchangers used with outdoor
corn boilers are: Sidearm, Shell and Tube, and Brazed Plate. What differentiates these heat exchangers, besides the cost, is the way they're designed to transfer heat from one medium to another and the method used to create turbulence.

A key component in the efficient transfer of heat between liquids is turbulence. The
more turbulent the flow of water through a heat exchanger, the more efficiently heat
is transferred.

Sidearm Heat Exchanger

The sidearm heat exchanger is a popular and inexpensive choice for heating
domestic hot water. It incorporates a pipe within a pipe design where the
water in the inner pipe (your hot water) is heated by hot water from the
boiler circulating through the outside pipe.

Turbulence is created by scrolling on the outer surface of the inside pipe.

This straightforward design prevents clogging by sediment and resists
scaling. One drawback of the sidearm heat exchanger is reported slow
recovery under heavy use. Cost: $130-$150.

Shell and Tube Heat Exchanger

Shell and tube heat exchangers are available in dozens of tube configurations and
sizes ranging from a few feet long to 50 feet or more for power plant steam
generation.

A variation on the shell and tube design is shell and coil where a helical (spiralling) coil
replaces the tubes.

No matter what the design or application, the basic principle is the same. The water to
be heated flows through tubes, and the heated boiler water, encased by the shell,
flows around the tubes.

Turbulence is created by the baffles holding the tubes together in what is called a tube bundle.

Shell and tube heat exchangers for non-chlorinated water
applications, such as domestic hot water and hydronic
heating, are usually constructed with a brass shell and
copper tubes.

For swimming pools and spas the shell should be PVC or stainless steel with stainless
steel tubes. 316L grade stainless steel is commonly used for this application.

Cost: $200-$600 depending on copper or stainless construction and the overall size based on the volume of water to be heated.

Brazed Plate Heat Exchanger

The brazed plate heat exchanger combines compact size with a highly efficient design to produce a device for heat transfer that is up to six times smaller than a shell and tube heat exchanger of similar capacity.

The key to this efficiency lies in their unique construction. Corrugated stainless
steel plates are brazed together (eliminates gaskets) with every
second plate turned 180 degrees. This design creates two highly
turbulent fluid channels that flow in opposite directions (counter flow)
over a massive surface area.

Cost: $100-$500 depending on capacity.

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