By Brett Wise
It’s a complicated world out there. As technology advances, we are bombarded with more and more jargon that sometimes is hard to comprehend. “Well, the new iPhone doesn’t have a higher pixel rating than the Samsung Galaxy, but it has more GB of memory and its processor speed is way faster!” Ok, if you say so.
Some of this has crept into the vacuum excavation world, as trucks use technology to become more efficient, easier to maintain, and less complicated to run.
One piece of the vacuum excavation world that hasn’t changed much over the years is the positive displacement (PD) blower. The technology used in these blowers has been around for decades in a wide array of applications, but there has been an increased focus on blowers with the rise of hydro excavation.
Why? Generally speaking, a PD blower is better in applications where the machine operator is vacuuming heavier material at longer distances where airflow isn’t at a premium. Simply put, while operating a hydro excavator with a PD blower, you can “set it and forget it”, meaning you can leave the suction nozzle in the muck and it will keep chugging the mud and sand into the debris body without issue or adjustment.
(For more on the differences between PD blowers and fans for Vactor applications, check out The Great Debate: Centrifugal Fan vs. Positive Displacement)
How does the PD blower work? Here is an illustration showing what is going on inside to create the airflow and vacuum:
Pretty simple, right? But there is some confusion when it comes to blower ratings. Here are some common terms used when discussing blower ratings:
Inches of Mercury (Hg): Shown as something like 18″Hg, this is a rating for how powerful the vacuum is that can be produced by the blower. 30″Hg or so is what is considered “pure vacuum” and can only be produced in laboratories or completely sealed environments. The higher the Hg rating, the larger the column of mud or material is that can be pulled up the suction tube in our vacuum excavation application.
Inches of Water (Sometimes referred to as Water Column): Similar to Inches of Mercury above, but since mercury is denser and heavier than water, the Inches of Water rating will always be a higher number. For example, 16″Hg is about 215″ of Water. These are two ways to communicate the same information.
CFM: Cubic Feet per Minute. How much air is the blower moving every minute? For vacuum excavation applications, this can be as low as 800CFM for vacuum trailers to 6,000+CFM for the big trucks working in the Canadian oilfields. Quite the range!
So what is best? It depends on your particular application, and it is all a balancing act. If you are using your vacuum excavator to clean out valve boxes and excavate 12″ holes to 15 feet, a 15″Hg 2,200CFM blower will do just fine. The larger the job, the larger the blower that will be required.
UPDATE: You could have the biggest, baddest blower on the market, and it might not perform as well as a smaller blower. Airflow on both the inlet and outlet side play a huge part in overall vacuum performance. An example I often use is that you can have a 1,000 horsepower engine in a drag racer, but if you are only getting 200 horsepower at the wheels that powerful engine doesn’t mean a whole lot!