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MIL-HDBK-1003/13A
Water/antifreeze solutions are most commonly used because they are not overly
expensive.  They range from $5-$10 per gallon including inhibitors.  Ethylene
and propylene glycol are the two most commonly used antifreezes.  A 50-50
water/glycol solution will provide freeze protection down to about -30 deg.
F, and will also raise the boiling point to about 230 deg. F.
The use of water/glycol solution presents an additional corrosion problem.
Water glycol systems will corrode galvanized pipe.  At high temperatures
glycols may break down to form glycolic acid.  This breakdown may occur as
low as 180 deg. F and accelerate at 200 deg. F.  This acid corrodes most all
metals including copper, aluminum, and steel.  The rate of glycol
decomposition at different temperatures is still a subject of uncertainty.
The decomposition rate of glycol varies according to the degree of aeration
and the service life of the solution.  Most water/glycol solutions require
periodic monitoring of the pH level and the corrosion inhibitors.  The pH
should be maintained between 6.5 and 8.0.  Replacement of the water/glycol
solution may be as often as every 12-24 months or even sooner in high
temperature systems.  If these solutions are used in the collector loop, the
seller should specify the expected life of the solution and the amount of
monitoring required.  The cost of periodic fluid replacement and monitoring
should be considered in the economic analysis.
Since glycol-water mixtures do require a lot of maintenance (and since users
can be quite negligent) it is recommended that glycols not be used in family
housing solar heating and DHW systems, and that glycol-water solutions be
reserved for use in large-scale installations which have regular maintenance
schedules and where the high cost of silicone oils would be prohibitive.
2.1.8 Collector connections.  Water flow through nonhorizontal collectors
should always be against gravity, except in trickle-type collectors.  Usually
this means water inlet to the collector at the bottom, and outlet at the top.
Care must be taken so that equal flow goes to all collectors.  If the pipe
manifold pressure drop is large, then end collectors will get little flow
(see Section 2.9.1).  The design most usually used is one in which the
collectors are connected in parallel.  This results in low pressure drop and
high efficiency of each collector.  A series hookup results in the highest
temperature and the highest pressure drop but lowest collector efficiency.
Higher temperatures than in the parallel arrangement may be obtained with
parallel-series connections, but at the expense of reduced efficiency and
greater cost.  These high temperatures are not usually required for hot water
and space heating.  Figure 2-6 shows different connection configurations.
All collector systems should be installed using a reverse-return (Z flow)
piping layout as shown in figure 2-6a.  Up to about 12 collectors in a row
can be accommodated.  Very large installations may merit computer simulations
to optimize the flow balance of each stage.
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