Silicones have many of the characteristics of synthetic oils. Differences include an
even lower surface tension, so they are even harder to confine. Thread sealants
must be fluorosilicone-based, or joint failure is inevitable. Silicone oil is very
expensive, typically 3 to 10 times the cost of other fluids.
Silicone oils must never be mixed with water, and must always be introduced into
clean, dry piping systems. Water must never be used for pressure testing or
2.7.5 Heat Exchangers, The purpose of a solar heat exchanger is to transfer
the collected solar heat from a non-freezing fluid into the water in the storage tank.
Heat exchangers used in solar heating systems are typically one of three types:
o Coil-in- tank
o Shell and tube
Tube-in-tube heat exchangers are typically used on smaller systems (20 sq. ft. to
600 sq. ft. of collector area). As their name implies, they consist of a tube within a
tube. One fluid moves through the innermost tube, and the other fluid moves in the
opposite direction through the space between the outer and inner tubes.
In many cases, two tube walls are between the fluids, affording double wall
protection to the water being heated. In most double wall exchangers of this type,
small passageways between the two walls provide leak detection and prevent any
possibility of contamination.
Some designers choose to have solar fluid in the innermost tube, and others prefer
to use it for water. Be sure you know which design is used for a particular system
before undertaking repairs.
Most manufacturers add small fins to the wetted surfaces of tube-in-tube heat
exchangers. This increases the surface area and keeps the fluid in turbulent flow,
resulting in improved heat transfer rates.
2.7 COMPONENT OPERATION