Editor’s Note: No two geothermal loop systems are the same, which is why constant education is a requirement for geothermal drillers. In this new department, we will explore the variety of loop systems, installation methods, drilling approaches and unique product offerings in the residential geothermal heat pump market. Take it away, Brandon.
Solid bedrock is often buried under the ground with overburden lying on top. As discussed in part one of this series, drilling with augers through the overburden can be very efficient if the drill rig is equipped with enough torque. Once the bedrock has been reached, the string of augers is removed, temporary PVC casing is inserted if necessary and a down-the-hole hammer (or DTH hammer) completes the rest of the borehole.
Air circulation can be used for clearing cuttings from standard rotary drill bits, however, the most common method of air rotary drilling is with a DTH hammer. This method is also the fastest way to drill into rock. The principals behind DTH hammers are similar to the way a hammer drill makes a hole in a brick wall. An air compressor is coupled to the drill rig to provide power for the hammer. This generates a rapid succession of impacts, causing the rock to break.
The bit itself is not driven up and down; rather, the bit is turned slowly while a heavy piston is pushed up and down in a cylinder behind the bit. The bit rotates so that the next blow occurs on a different part of the bottom on the hole. The air-driven piston hammers against the back of the bit at the bottom of each stroke and the exhaust from the hammer flushes the cuttings up and out of the bottom of the hole, allowing penetration rates of up to 150 ft per hour with ideal conditions and equipment.
Some drill rigs have onboard air compressors, while others utilize portable air compressors. The advantages of a portable compressor become clear on limited space residential sites and retro-fits. A compact track mounted drill can easily access areas that a larger truck mounted drill with an onboard compressor cannot. The compressor must supply adequate air pressure (PSI) and volume of air (CFM). The required pressure and volume is determined by the bore diameter, drill rod diameter, bore hole depth and amount of water down the hole. Air pressure is crucial to the performance of the hammer. High pressure air drives the piston faster, producing more energy per blow and more blows per minute.
Button bits (made up of tungsten carbide inserts or teeth) drill very efficiently as the force of the blow is distributed evenly on the bottom of the hole. The shape of the drill bit face can be flat, convex or concave to suit different drilling requirements. Various button patterns are available to suit the rock and hole size. The inserts or buttons are available in different diameters to break out hard rock. Alternatively, smaller ballistic insert bits can be used for softer formations. See your drilling rig or DTH hammer manufacturer for more information.
Lubrication and Dust Suppression
Just like any other piece of precision machinery, the DTH hammer must be lubricated. The hammer manufacturer recommends the correct grade of lubricating oil to be used. Many drill rigs have an onboard lubricator that automatically injects oil into the air stream while the hammer is operating.
To suppress the dust from the drill cuttings, a water injector can be fitted to the drill rig. This simple pump injects a controlled amount of water into the air stream which carries the water to the bottom of the bore hole. This keeps the drill site free of dust. Since DTH hammers do not require as much down force or torque as mud rotary drilling, the system can be used on lighter, less expensive and more mobile drill rigs. A rig with sensitive down-feed control and the ability to “hold back” a portion of the drill strings weight ensures that the optimum weight is applied to the drill bit. DTH hammer drilling delivers impressive penetration rates in almost all rock formations with straighter, cleaner holes compared with mud rotary.
Of course, soil conditions can change from drill site to drill site, so which system is right for you: mud or air? This all-important question will be discussed in the fourth part of this series.
Brandon Wronski is an equipment specialist with RigKits LLC, based in Charlotte, N.C.