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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 B
TUs. The
larger the heat exchanger (length x width), the higher the Btu output. Sizes
range from 50,000 B
TU (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.