As discussed in last month’s article “Proper Grouting Protects Your Loop, Safeguards Groundwater,” grouting a geothermal borehole serves three vital purposes: to seal the hole/protect the ground loop, to protect groundwater from possible contamination and to improve thermal conductivity.
Since each project is unique and, as there are so many products that can be used, it is important that the heating and cooling requirements of the project and ground conditions including soil conductivity and groundwater chemistry are all taken into consideration by the engineer when selecting grouting material.
Bentonite grout is often mixed with an additive like silica sand, carbon or graphite to increase its heat transfer abilities. Herein lies the challenge—as additives are mixed and grout thermal conductivity increases, so does the difficulty to mix and pump it. It is no wonder that many contractors charge more for grout mixes with higher thermal conductivity. Not only is the material more expensive, but so are the labor costs. The good news is that selecting the right grout mixing and pumping equipment can make life a lot easier. What equipment options are available and how will they affect grouting productivity? Although various mixers and pumps are commonly used, let’s briefly look at a couple of the most popular.
Paddle vs. Colloidal
Paddle mixers (classified as low-shear mixers) are engineered with blades positioned and angled onto a rotating shaft. They are designed to scoop, lift and tumble materials in a gentle, but thorough mixing action. While being mixed, the material travels in a figure-eight pattern with the material being drawn from the outsides of the chamber to the middle where the mixing is most aggressive. Paddle mixers allow easy access for cleaning between batches. They are simple to maintain, wear and tear is minimal, and energy consumption is low—thereby reducing operational costs.
Colloidal mixers (classified as high-shear mixers) are designed with a high-speed rotor (operating in the range of 2,000 rpm) in a close-fitting chamber. This creates a vortex of intense turbulence and high-shearing action. The circulating material spins the heavy, unmixed grout toward the outside walls of the mixing tank while the lighter material such as water and partially mixed grout, is sucked in toward the high-speed rotor at the throat of the mixing tank. The mixture becomes thicker and thicker as is passes through the rotor, until the entire mix is uniform.
A colloidal mixer for geothermal applications. This unit is powered by electric motors, but can also use diesel or gas engines. Source: RigKits LLC
The speed of the colloidal rotor is fixed for mixing so the amount of time the mix is left in the mixing chamber controls the amount of shear the product experiences. If you want the bentonite to hydrate in the bore, dump the product into the pumping tank once mixed. If you need it fully hydrated, let it spin in the mixing tank for a few extra minutes.
The high speed prevents solids from settling in the bottom of the tanks—reducing the risk of clogging, as is sometimes the case with paddle mixers. Mixing silica sand into the bentonite is done rapidly and easily without having to resort to finely crushed silica (which is expensive). The time taken mixing the product is measured in seconds, not minutes; it’s that fast. Dry product can be poured quickly into the mixing tank with little risk of clogging the system. A mix with 80 percent silica sand solids content is simple with a colloidal mixer, while with paddle mixers it is nearly impossible. A paddle mixer will require a higher percentage of bentonite, which affects the thermal conductivity of the grout compared with the higher silica content achievable with colloidal mixers. Speed of mixing and low risk of clogging are the main advantages of a colloidal mixer. This reduces cost in labor, materials and the bore lengths required.
Once the grout is mixed, the next step is pumping it into the bore. Most geothermal grout mixing units, both paddle and colloidal type, include a large capacity hopper and pump. It is desirable to have a two-tank system for continuous pumping: a mixing tank and a pumping tank. However, low-cost paddle mixers can get away with mixing tank only. This slows down the pumping and mixing operations because you can either pump or mix. You cannot do both at the same time, but this simplicity reduces the equipment costs.
The pump hopper agitates or recirculates the mix until it is pumped to prevent settling, and enables a new batch to be mixed while the previous is being pumped. For pumping the mixed grout, common designs are piston pumps and progressive-cavity pumps, as they are capable of pumping higher solids contents and higher pressures. The mixer, hopper and pump are a balanced system allowing the mixer to stay ahead of the pump, thereby displacing continuous flow. Options include twin mixing tanks for continuous pumping, multi-cylinder piston pumps with overlapping strokes that reduce pressure fluctuation and other bells and whistles.
The progressive-cavity pump consists of a helical rotor that resembles a corkscrew, which fits into a rubber stator. The rotary motion is off-center, which is why these pumps are also called eccentric screw pumps. The grout mix is forced along the cavities in a constant flow. A piston pump sucks the grout mix through the inlet check valve on the up-stroke and into the cylinder. On the down-stroke the outlet check valve opens, thereby displacing the mixture. Piston pumps are capable of higher pumping pressures.
The use of excessive water will negatively affect grout properties like permeability, durability and thermal conductivity. Therefore, it is fundamental that the necessary amount of water and sand is correctly measured. It is also critical that the sand is evenly suspended and distributed in the mixture to ensure proper heat transfer from the earth to the loop and vice versa.
In selecting any piece of equipment, upfront cost is not the only factor that influences cost effectiveness. Colloidal mixers, although costing more, achieve higher sand carrying capability, require less water and further improve grout properties. Depending on the product being pumped, a 5-inch outside diameter, 300-foot geothermal bore with 1-inch U-bend ground loop can be grouted in 6 to 7 minutes. Due to the speed of mix, ease of use, ability to use low cost coarse silica sand and the resultant higher efficiency of the geothermal bore, these units are very cost effective. While grouting the bore for many first-time installers can be a steep learning curve, selecting the right mixing and pumping equipment and using the correct gout ingredients can make life much easier for the geothermal loop installer.