Piping commercial pressure glycol or drainback solar projects

It may seem that piping a large number of solar collectors would be easy – just connect one to another, right? However, there are multiple factors to consider. For example:
• The maximum number of collectors in one string is eight.
This could also help you determine the number of collectors to use on your project. If your calculations say a project will require 11 SS-40 (4×10’) collectors, you couldn’t install them all on one string, or row. You could create two strings, one of five and the other six collectors. Alternatively, you might do two rows of five, for a total of 10 collectors, or two rows of six, totaling 12. If you have uneven numbers of collectors in each string, you will need to balance the flow so that each collector receives about 1 to 1.25 GPM.
If you place multiple 4-foot-wide collectors together, remember that the actual measurement of each collector, with connections, will be 50.25″. For example, if you are placing the maximum number in a row of eight collectors, 8 x 50.25 = 402 inches, or 33.5 feet.

• It is not advisable to do reverse return plumbing on multiple strings of solar collectors.
Reverse-return plumbing eats up more copper piping than needed and, as a result, raises costs. It also leaves more tubing exposed, resulting in greater heat loses because fewer BTUs are delivered to the tank.
Also remember that collectors are not perfectly tuned when assembled. If you have two strings of eight collectors each, one string would most likely flow slightly more easily than the other, causing reverse return piping to not function correctly.
If the collector array has only two strings and the feed and return can enter the center of the array, this may be close enough for self-balancing.

If you have two strings of eight collectors and the plumbing comes in from the right or the left, the first string of collectors will take the majority of the flow; that means the second string, further down the plumbing, will get cheated. Use balancing flow meters to solve this discrepancy.

Array of 16 flat-plate collectors sloping to right

An example of piping from the right or left with balancing valves. The first string will rob flow the second array.

Piping with naturally balanced flow.

An example of piping for a naturally balanced flow.

• Size the plumbing for the flow.
Consider telescoping the plumbing in larger installations using multiple strings of collectors. Or, in other words, size the plumbing for the flow. For example, when the last string of eight collectors only needs to flow at 8 GPM, there are 33 feet that can use 1″ copper. And we all know how expensive copper is.
Consider these drawing for actual projects when sizing your next solar system:

Example of telescopic piping

Example of ‘telescopic’ piping, in which the pipe size corresponds with the flow.

Example of telescopic piping

Another example of telescopic piping in which the pipe size is reduced to meet or match the flow.

• Use balancing flow meters.

These are readily available valves that are designed for balancing flow. When you need to balance multiple strings of collectors, these instructions may be handy.
Consider a drainback system on a sloped roof with one row of collectors higher than the other. If correctly set, flow balancing valves will even out the flow through the lower and the higher rows on a linear flow, even when the controller speeds up and slows down the pump.
Never use a ball valve for flow restriction.

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