MIL-HDBK-1003/13A
b.
Same as above except collectors in a double row series-parallel
arrangement (see Figure 2-6).
Up to 5 collector branches
-
1/2-inch headers
6 to 10 collector branches
-
3/4-inch headers
11 to 15 collector branches
-
1-inch headers
16 to 22 collector branches
-
1-1/4-inch headers
More than 23 collectors
-
1-1/2-inch or larger (size
for each design)
2.9.2 Pumps and collector flowrate. Pumps are sized in accordance with
recognized practices also. Since solar systems are nothing more than a
combination of pipes, valves, and fittings it is possible to do a heat loss
calculation to determine the system head. Charts are available in standard
fluid flow handbooks that give the friction losses or "equivalent length of
feet in pipe" for various fittings and valves. These are merely summed for
the entire system.
The flowrate through the collector loop is determined by the maximum amount
of energy which must be removed from the collector. This maximum is about
225 Btu/ft2/hr. Often a manufacturer will specify the flowrate through his
collector and this value should be used. If not, an estimate can be made by
determining the flowrate necessary to remove the maximum amount of energy
while minimizing the collector inlet temperature (to maintain high collector
efficiency). The rule of thumb for this calculation is 0.015 to 0.020 gpm
for each square foot of collector area for water. For other fluids this can
be scaled by the value of the specific heat of the fluid as compared to water
(Cp = 1 Btu/lbm-deg. F).
Now that head loss and flowrate are determined, a pump can be selected by
using the manufacturers' standard tables and graphs. For typical domestic
hot water systems and space heating systems for a house for a family of four,
the pumps are quite small, averaging 1/12 to 1/20 hp and costing about 5
each. In some systems, like a drain down system, pump sizes can be much
larger due to the higher vertical "head" requirements.
If the water in the system is open to the atmosphere or if the water is to be
used for drinking the pump should be made of bronze or stainless steel on all
water-wetted surfaces to minimize corrosion. Pumps will have longer life if
they are placed in low temperature parts of the water circuits. Pumps can be
"staged" to give more flow or head. Two pumps in series will give the same
flow against twice the head. Two pumps in parallel will give twice the flow
at the same head. Two or more small circulator-type pumps are often cheaper
than a single larger pump.
2.9.3 Valves. Valves, other than seasonal or emergency shut-off valves,
should be electrically operated and located out of the weather or well
protected. A vent must be provided at the high point in liquid systems to
eliminate entrapped air and it should also serve as a vacuum breaker to allow
draining of the system. To avoid multiple venting, systems should be piped
to
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