| Heat exchangers are devices that transfer heat
from one fluid (liquid or gas) to another while keeping them
separate. A hot fluid flows on one side of a heat
exchanger’s metal partition, transferring its heat to a lower
temperature fluid flowing on the other side. The size of a heat
exchanger’s surface area affects it speed and efficiency: a
large surface area transfers heat faster and more efficiently.
Many machines and household appliances contain heat exchangers.
A car radiator is a heat exchanger: it transfers heat from the
engine cooling system to the outside air, preventing the engine
from overheating. Refrigerators and air conditioners have heat
exchangers to dissipate heat; furnaces have heat exchangers to
heat the cool air that enters the furnace. Solar systems also use
heat exchangers to transfer solar energy absorbed in solar
collectors to the liquid or air that is used to heat the water or
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 Exchangers
Solar heating systems use three types of heat exchangers:
liquid-to-liquid, liquid-to-air, and air-to-liquid. Many liquid
solar energy systems use a liquid-to liquid heat exchanger, which
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. They are often used when the
heat transfer fluid is toxic, such as ethylene glycol
(antifreeze). They 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 heat
losses.
While double-wall heat exchangers increase safety, they are
less efficient, because heat must transfer through two surfaces
rather than one. In order to transfer the same amount of heat, a
double-wall heat exchanger must be larger than a single-wall
exchanger.
Liquid-to-air heat exchangers transfer heat from a liquid solar
heating system to a home’s forced air duct system. The most
common type is the finned-tube heat exchanger, which is a finned
water coil inserted in the air duct. Supply air from the fan blows
over the coil and absorbs heat from it. The heated air is then
distributed throughout the house.
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. These are similar to
liquid-to-air heat exchangers.
Heat Exchanger Designs
There are many heat exchanger designs. One commonly used design
in solar domestic water heater systems is the coil-in-tank design.
The heat exchanger is a coil of tubing in the storage tank. It can
be a single tube (single-wall heat exchanger) or two thickness’
of tube (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.
Another popular design is the shell-and-tube heat exchanger.
The heat exchanger is separate from 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 to-be-heated fluid
(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.
A very efficient type of heat exchanger is the tube-in-tube
design. In this 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.
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