A heat exchanger is a device used to transfer heat from a fluid (liquid or gas) to another fluid
where the two fluids are physically separated.
|Shell and tube heat exchanger
for solar water heating systems
Solar water heating systems use heat exchangers to transfer solar energy absorbed in solar collectors
to the liquid or air used to heat water or a space. Heat exchangers can
be made of steel, copper, bronze, stainless steel, aluminum, or cast iron.
Solar heating systems usually use copper, because it is a good thermal conductor
and has greater resistance to corrosion.
Types of heat
Solar water heating systems use three types of heat exchangers:
This heat exchanger uses a heat-transfer fluid that circulates through
the solar collector, absorbs heat, and then flows through a heat exchanger
to transfer its heat to water in a storage tank. Heat-transfer fluids,
such as antifreeze, protect the solar collector from freezing in cold
weather. Liquid-to-liquid heat exchangers have either one or two barriers
(single wall or double wall) between the heat-transfer fluid and the
domestic water supply.
A single-wall heat exchanger is a pipe or tube surrounded by a fluid.
Either the fluid passing through the tubing or the fluid surrounding
the tubing can be the heat-transfer fluid, while the other fluid is
the potable water. Double-wall heat exchangers have two walls between
the two fluids. Two walls are often used when the heat-transfer fluid
is toxic, such as ethylene glycol (antifreeze). Double walls are often
required as a safety measure in case of leaks, helping ensure that the
antifreeze does not mix with the potable water supply. An example of
a double-wall, liquid-to-liquid heat exchanger is the "wrap-around heat
exchanger," in which a tube is wrapped around and bonded to the outside
of a hot water tank. The tube must be adequately insulated to reduce
While double-wall heat exchangers increase safety, they are less efficient
because heat must transfer through two surfaces rather than one. To
transfer the same amount of heat, a double-wall heat exchanger must
be larger than a single-wall exchanger.
Solar heating systems with air heater collectors usually do not need
a heat exchanger between the solar collector and the air distribution
system. Those systems with air heater collectors that heat water use
air-to-liquid heat exchangers, which are similar to liquid-to-air heat
Heat exchanger designs
There are many heat exchanger designs. Here are some common ones:
The heat exchanger is a coil of tubing in the storage tank. It can be
a single tube (single-wall heat exchanger) or the thickness of two tubes
(double-wall heat exchanger). A less efficient alternative is to place
the coil on the outside of the collector tank with a cover of insulation.
The heat exchanger is separate from (external to) the storage tank.
It has two separate fluid loops inside a case or shell. The fluids flow
in opposite directions to each other through the heat exchanger, maximizing
heat transfer. In one loop, the fluid to be heated (such as potable
water) circulates through the inner tubes. In the second loop, the heat-transfer
fluid flows between the shell and the tubes of water. The tubes and
shell should be made of the same material. When the collector or heat-transfer
fluid is toxic, double-wall tubes are used, and a non-toxic intermediary
transfer fluid is placed between the outer and inner walls of the tubes.
In this very efficient design, the tubes of water and the heat-transfer
fluid are in direct thermal contact with each other. The water and the
heat-transfer fluid flow in opposite directions to each other. This
type of heat exchanger has two loops similar to those described in the
shell-and-tube heat exchanger.
A heat exchanger must be sized correctly to be effective. There are many
factors to consider for proper sizing, including the following:
Usually, manufacturers will supply heat transfer ratings for their heat
exchangers (in Btu/hour) for various fluid temperatures and flow rates.
Also, the size of a heat exchanger's surface area affects its speed and
efficiency: a large surface area transfers heat faster and more efficiently.
- Type of heat exchanger
- Characteristics of the heat-transfer fluid (specific heat, viscosity,
- Flow rate
- Inlet and outlet temperatures for each fluid
For the best performance, always follow the manufacturer's installation
recommendations for the heat exchanger. Be sure to choose a heat-transfer
fluid that is compatible with the type of heat exchanger you will be using.
If you want to build your own heat exchanger, be aware that using different
metals in heat exchanger construction may cause corrosion. Also, because
dissimilar metals have different thermal expansion and contraction characteristics,
leaks or cracks may develop. Either of these conditions may reduce the life
span of your heat exchanger.
For more information about residential solar water heating systems, see: