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Outdoor Corn Boiler & Wood Furnace Heat Exchangers
A heat exchanger is a device designed to efficiently transfer the heat from one
medium to another. In the case of either an outdoor corn boiler or outdoor wood
furnace, the heat exchange media is air or 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.
Sidearm heat exchanger
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 as shown in the
picture below of a partially constructed tube bundle.
Shell and tube heat exchanger
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 tubing. Grade 316L 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.
Brazed plate heat exchanger
The key to this efficiency lies in their unique construction. The
corrugated stainless steel plates are brazed together (eliminates
gaskets) with every second plate turned 180 degrees. This unique
design creates two highly turbulent fluid channels that flow in
opposite directions (counter flow) over a massive surface area.
Cost: $100-$500 according to capacity.
Water to Air Heat Exchanger
Water-to-water heat exchangers are also used to heat hot tubs, swimming pools, and  
radiant baseboard or radiant infloor 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 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.
Water to Air Heat Exchangers
A water to air heat exchanger for forced air home heat
replaces the gas or oil burner in your furnace as the heat
source. The heat exchanger is installed in the supply plenum
of your furnace, usually with 90 degree brackets. (The supply
plenum directs heated air from the furnace into the ductwork)
While hot water from the outdoor furnace or boiler circulates through rows of copper
tubing in the heat exchanger, the fan on your existing furnace, controlled by a
separate thermostat, blows air up and through the heat exchanger and circulates the
heated air throughout the house.

The heat output of a water to air heat exchanger is stated in Btus. The larger the heat
exchanger (length x width), the higher the Btu output. Sizes range from 50,000 Btu
(14"x 16") for small homes to 200,000 Btu (24"x 28") for commercial applications.  

Water to air heat exchangers are also used in air handlers for geothermal systems
and garage heaters. Cost: $150 - $400.