Introduction

Choosing the right compressor for your soil displacement hammer is a crucial step. We've highlighted the main differences between the 2 most common type of compressor, piston vs screw, and show you why you should choose one instead of the other.

The piston / reciprocating compressor

Piston compressors or reciprocating compressors are still the most common type found across all industries due to their relative low cost. An engine, petrol or diesel, drives an air compression unit via belt. The principle of this air compression unit is very similar to the internal combustion engine that's in your car. As it spins, the ambient air is sucked in the piston chambers, compressed and expelled to the air tank using a system of tuned cams and valves. Therefore it involves a lot of wear and tear down the track due to friction and operation heat which means more oil carry-over and more maintenance cost.

The oil carry-over effect is when the pistons, cylinder, valves, etc wear, the oil in the crank case gets pass the piston rings and is carried over in the air line making a poor air quality. This "normal" wear and tear decrease the efficiency over time, increase the maintenance schedules and therefore the running cost.

The rotary screw compressor

These units are usually sound proofed and Diesel powered making the initial cost higher than piston compressors. The screws are driven directly by the engine making a more rigid unit. the screws have special shape, one 'male' and one 'female' compressing the incoming air at a smooth and pulse-free pace. No need for storage tank, the air is delivered constantly at every stage. The rotary screw compressors are fluid cooled which provide many advantages :

• it lubricates the bearings
• cleans the air (better air quality means a longer air tool life)
• create a non-wearing seal between the screws and the casing (meaning lower maintenance cost down the track and always very efficient)
• cool the compressed air down significantly and easily

Piston compressors have usually internal working temperature of about 200°C compare to 70~80°C for screw compressors.

A cooler operation temperature means a much easier way to cool, remove the moisture and clean the outlet air. Again, screw compressors will deliver a much better quality air and will therefore contribute to the life of your air tools.

IMPORTANT : The duty cycle

A rotary screw compressor has a 100% allowable duty cycle and operate continuously if the need arises.

Piston compressors operate intermittently in order to cool down between 2 tank fill. They have an allowable duty cycle of about 60 to 70%. For this reason, they need to be oversized to compensate their periodical 'shut-down'. Even with large storage tank, the lack of constant compressed air can be a problem for the good operation of a soil displacement hammer. The soil displacement hammer can lock itself in the ground if there is not enough air to keep the piston hitting the barrel. That's one of the reasons why we recommend screw type compressor to use with our Soil Displacement Hammer NODIG Mole.

Cost & maintenance

As said earlier, the piston compressors have low initial costs but high maintenance cost. knowing that you need to oversize it to suit your need. Not only that, but poor air quality will damage your air tools and/or will slow your operations down when the compressor needs repairing. Your air tools will also need more maintenance or replacing. All this contribute to a higher cost down the track.

As opposed, a screw type compressor has a higher initial cost but lower maintenance cost (over the years) and will deliver a good quality air which is crucial for the long life of your air tools. It will also keep your crew working as the clean air will be available when they need it. These compressors do not need to be oversized to compensate for limited duty cycle and are more efficient. For example, a 7.5Hp rotary will do the job of a 10Hp piston. Less energy is required and therefore operating cost reduced.

Recommended compressor for a soil displacement hammer

We recommend the rotary screw DENYO DIS-55LB compressor for our NODIG Mole ranging from 45mm diameter to 76mm diameter. It can be easily carried on the back of a ute. It is perfectly suited for any NBN, fiber optic installation work.

Even if you need less air for the NODIG Mole 45mm for example, having a bigger compressor can only be beneficial. It will allow you to use it with many other air tools such as a jackhammer for example, in order to break out a bit of concrete or rock when digging the launching pit. The NODIG Mole will only take the air volume it can take and the screw compressor will adjust itself automatically, using flow control valves.

DENYO is a very well known, high quality Japanese compressor brand, reputed for their reliability and sturdiness over the years. Shop online with us for FREE delivery to Perth, Sydney, Canberra, Melbourne, Brisbane and Adelaide. Other destination, please contact us. +61 8 9493 0642. email us here.