14 June 2002
prevents reverse thermosiphoning, which can occur when the system is off and warm
fluid in the collector loop rises from the heat exchanger to the collector array and is
Pressure Relief Valves. A pressure relief valve is required in any line
containing a heat source that can be isolated (such as a collector row) and is also
typically provided between the heat exchanger and the suction side of the collector loop
pump. The latter pressure relief valve is provided in case of stagnation in the fully open
collector loop. This relief valve should open before those at the top of the loop due to
the elevation head experienced at the bottom of the loop. Pressure relief for solar
systems should be set at 125 psi (862 kPa) (maximum system design pressure). The
discharge from pressure relief valves will be either routed to an appropriate floor drain
or captured as required by either local or state regulatory requirements. The discharge
should be piped to avoid personnel injury from the hot fluid. Some means for
determining if fluid has discharged may be provided.
Temperature-Pressure Relief Valves. Temperature/pressure relief
valves are similar in operation to pressure relief valves, except they also contain a
temperature sensor to detect and relieve any temperature exceeding the design
temperature. They should be installed on the solar storage tank and set for 125 psi
(862 kPa) or 210 degrees F (99 degrees C).
Manual Air Vents. Manual air vents are recommended to purge trapped
air within the system. They should be located at the high point(s) of the system where
air will accumulate. Air can be present in the system from the initial charge or can be
drawn in at leaks in the system piping or components. Automatic air vents with air
separators have a tendency to fail when moisture condenses and freezes near the relief
port, and should thus be avoided.
Strainers. Standard plumbing practice recommends that a strainer be
located before the pump to test for system flush.
Operation. Circulation pumps are required in both the collector and
storage loops. Both pumps are activated simultaneously by the control sub-system
when it has been determined that net energy collection can occur.
Flow Path Pressure Drop. The pump size is based on the required flow
rate and the resistance to flow in the loop (at that flow rate). The total pressure loss to
be overcome by the pump is the sum of the individual component and piping pressure
losses around the loop. To calculate the pressure drop around the loop, the piping
layout must be determined, certain major components specified, and approximate pipe
lengths, fittings and diameters known. The pressure drop in the plumbing is calculated
by first determining a flow path length, which is equal to the length of all linear piping
plus the "equivalent lengths" of all valves and fittings. These equivalent lengths can be
found in most plumbing handbooks; or accounted for by multiplying the linear pipe