14 June 2002
e. Heat Exchangers
(1) Purpose. Heat exchangers are used to transfer thermal energy
between fluids while keeping them separate to prevent mixing or to maintain a pressure
difference between fluid loops.
(2) Types. Heat exchangers are available in a wide variety of sizes and
configurations. The primary concern is the chemical composition of the fluids used in
the heat exchanger. The fluid determines whether a single- or double-isolation heat
exchanger will be necessary. Double-isolation heat exchangers are required whenever
there is possible contamination of the potable water supply by a toxic collector loop
fluid. Also important is the heat exchanger location with regard to the storage tank.
Immersion-type heat exchangers are located within the storage tank and operate by
isolation external heat exchangers are separate from the tank and require two pumps to
circulate the fluid on both the hot and the cold side. For solar systems, increased
performance due to forced convection heat transfer in external heat exchangers usually
offsets the additional cost of operating a second pump. For this reason, external,
forced convection heat exchangers are usually used for systems designed under this
(3) Configurations. Of the many configurations of heat exchangers
possible, two have found widespread use with liquid-based solar systems. The most
common heat exchanger used in past solar projects is the shell-and-tube configuration,
in which an outer casing or shell encloses a tube bundle. These units are usually
thermally efficient, compact, reliable and easy to maintain and clean. Shell-and-tube
exchangers typically provide only single isolation. The other commonly used heat
exchanger is the plate or plate-and-frame type. This type of exchanger is becoming
increasingly popular with designers and contractors. It can afford single- or double-wall
protection, provide high performance, use superior materials, have low volume and
surface area, and be easily enlarged or reduced if the system size is changed. Most
heat exchangers are available with copper fluid passages, and many plate-type
exchangers have stainless steel passages.
(4) Heat Exchanger Performance. A common measure of heat exchanger
performance is its effectiveness. Effectiveness is defined as the ratio of the actual rate
of heat transfer to the maximum possible. Two common problems, fouling and
freezing, can decrease heat exchanger effectiveness. Fouling is the term used for
scale and corrosion that collects in the passageways. Fouling decreases the amount of
energy transferred and is often taken into account in heat exchanger analysis. The
amount and rate of fouling to be expected depend on the fluids and materials used.
Heat exchangers can freeze in systems containing antifreeze due to reverse
thermosiphoning or improper control.
(5) Effect on System Performance. The use of heat exchangers can only
serve to degrade the performance of the solar energy system. However, since they are
required for most systems, their impact on performance should be understood.