Bristol’s Six Principles for Good Solar Hydronic Design
#47: Piping the solar collectors with flexible tubing
By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.
The best innovations in solar heating systems in recent years have evolved by combining pre-engineered and pre-assembled components. When multiple components are matched and assembled during manufacture, it eliminates the labor of assembly and the high potential for error in the field. In solar home heating systems, the use of modular heat distribution manifolds and pre-engineered pump stations, for example, has gained in popularity because of the speed of installation and the increased reliability of the results.
Another area where modular equipment is now available is in the solar connective piping itself. The insulated (metal) supply and return piping that connects the solar collectors to the boiler room must be resistant to very high temperatures. The outdoor runs must be resistant to weather and sunlight. The path of this piping often runs through tight spaces with awkward angles and bends. Traditionally, this piping has typically been done with copper tubing in a meticulous three-step process involving (1) soldering copper pipe together with couplings and elbows, (2) installing the insulation and (3) finishing with a weatherproof cover. The use of pre-insulated flexible piping can eliminate most of that work in the field, saving many hours of installation.
Flexible stainless steel insulated pipe
Figure 47-1 shows the construction detail of a stainless steel solar supply and return pipe assembly using an example available from Caleffi Solar. It consists of two corrugated stainless steel pipes inside an EPDM closed-cell foam insulation jacket with an integrated sensor wire built in. It is most commonly packaged in 50-foot coils. The insulation is coated with a copolymer foil that protects against damage from solar UV radiation. It comes in three common sizes, ½”, ¾” and 1” nominal pipe sizes. This modular tubing is lightweight and easy to bend, with a springy feel to it. The two corrugated pipes are made of 316L stainless steel with a bending radius of 5 inches, a working pressure rating of 150 psi and a maximum fluid temperature of 350 F.
The stainless tube can be cut using a tubing cutter, the same as that used with copper tubing. It requires special adapters at each end to allow connection to standard U.S. pipe thread. Adapters are available from suppliers, and it is always a good idea to order a few extra to have on hand in the field. The adapters that I have used so far are surprisingly fast and easy to install and “act like” a threaded brass union with a flat gasket when making the final pipe connection. Short pieces can be cut and fitted with adapters to make other connections to the glycol system or to other boiler-fluid piping.
The pressure drop
Because the tubing is corrugated, the inside surface has a rough finish, which results in a higher pump resistance than smooth copper tubing. This must be taken into account when designing the solar glycol loop and the glycol pump to go with it. At common glycol flow rates for solar collectors, the difference is in the range of 20 – 30 percent higher pump resistance than in comparable smooth pipe. Figure 47-2 shows how the corrugated stainless compares to copper tube, using the Aeroline products, for example. Aeroline makes pre-insulated smooth copper tubing, assembled in the same way as their stainless steel product, so a direct comparison can be easily made.
For example, on Figure 47-2, a flow rate of 5 gpm through a pair of ¾” tubes 50 feet long requires about 10 feet of head to pump through copper (seen as a red line and labeled “CU”). The same flow rate through “Aerorapid” corrugated ¾” pipe requires over 12 feet of heat (seen as a blue line). If 1” corrugated pipe is used with the same flow rate, less than 4 feet of head is required.
Figure 47-3 shows the pump head available from a common Grundfos 3-Speed circulator. Notice that, for this example, at 5 gpm the 1” corrugated could be pumped at low speed and the ¾” copper could be pumped at medium speed, while the ¾” corrugated may require high speed.
Remember that the discussion above has been limited to the pump resistance due only to the supply/return piping. Other plumbing components, such as collectors, heat exchanger, shut off valves, elbows and other fittings that provide additional resistance must also be included to obtain the total pressure drop for the entire glycol loop. The circulator pump must be sized to overcome the total resistance to flow.
The glycol mixture and its average temperature have a significant effect on the pump resistance. Figure 47-2 covers a typical mixture and temperature (as labeled), but colder temperatures or thicker mixtures (for example) will have different results.
Figure 47-2 presents the flow through a pair of pipes, one supply and one return. This means that a 50-foot long sample actually contains 100 linear feet of pipe. Sometimes you have to look at the labels or fine print carefully to avoid comparing a single pipe to a pair of pipes when looking at the flow resistance of different products or brands.
These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned only to provide examples for illustration and discussion and do not constitute recommendation or endorsement.
Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.