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Slinky Coils

"Slinky" is the term commonly used rather than the imposing scientific name - curtate cycloid. When used with geothermal heat pump systems, the Slinky is a flattened, overlapped plastic pipe circular coiled ground loop heat exchanger. It concentrates the heat transfer surface into a smaller volume, requiring less land area and shorter trenching.

The compact slinky at 10 inch pitch is equivalent to 12 feet of pipe per foot of trench and will reduce trench length by about two-thirds compared to two-pipes at 4 and 6 foot depths.

An extended slinky at 56 inch pitch is equivalent to 4 feet of pipe per foot of trench and will reduce trench length by about one-third. Specific design lengths will vary with climate, soil and the geothermal heat pump's run fraction. Slinkys can be installed horizontally at the bottom of a 3 foot wide trench or vertically in a narrow trench.

Contractors have developed other Slinky configurations specifically to meet their installation needs. The coils are fabricated flat for transportation by tying the return pipe to the base of the coil. Before the coil is placed in the trench, the return pipe is cut free from the bottom and is installed on top of the coils. In this case, the spacing is about 20 inches between each loop and only intersect at the bottom.

In another method the coils are 30 to 32 inches in diameter making it easier to shape while the worker is standing up and the coils are tied together. Slinky assemblies can be fabricated on the job site, or pre-assembled in the shop on rainy days or slack periods using a simple fixture and later trucked to the job site.

In forming the slinky from a pipe roll, allow the pipe roll to remain in the same circular configuration as manufactured and shipped. A common mistake is to uncoil the pipe as if it were to be placed in a straight line. Begin with the outside coil of the pipe roll. The individual pipe coils are pulled from the roll to and through the fixture for tying, much the same way you would unroll a new garden hose.

Ties are applied with the coil in the fixture. They must be strong enough to hold the coil in position during fabrication, transportation and placement in the trench. After backfilling, the ties are of no value and can deteriorate with no adverse effect. Plastic wire tie wraps with metal catches is one alternative; re-bar wire ties is another. Some contractors prefer duct tape with a sufficient number of wraps. Duct tape secures the pipe over a wider area with less pressure and requires no special tools. The pipe is less likely to crimp and the cost is typically less.

Tie spacing is about 10 inches between each loop. The pipe does overlap and is tied where the loops intersect at top and bottom. With the extended slinky, the loops are tied where they intersect.

Evaluation tests conducted by Pennsylvania Power and Light Company indicated:

  • No significant difference was found in the performance of a vertically versus a horizontally installed Slinky
  • Proper backfilling is critical for good performance
  • Horizontal installations seemed easier to properly backfill
  • Loops for one circulator should use no more than 750 to 800 feet of pipe to minimize pressure losses and maintain turbulent flow

PP&L recommends for each ton of capacity a 100 foot trench, 3 feet wide and 6 feet deep in the Slinky area, 4 feet deep in the pipe header area, using 750 to 800 feet of ¾-inch pipe on a 17 inch pitch.

There are a number of new ways the "Slinky" coil concept is being applied. For example, a Georgia contractor innovated a way to install the coils vertically in shallower - 20 to 30 feet deep - but larger diameter boreholes. A series of plastic strips keep the coils properly spaced when the assembly goes into the ground. The saving is in the installation time. Other such innovative improvements will no doubt be made as more installations are completed.