Hose and Vacuum ramblings

One of the more common pitfalls that one sees in the vacuum truck and hydro-vac industry is the incorrect vacuum conveyance hose selection. Seems like a simple thing right?? Well it is… BUT as history teaches us there is no task so simple that it can’t be done incorrectly. 

Basically any “vacuum” truck that uses a positive displacement blower (PD for short) or Centrifugal Compressor (Fan) isn’t really a “vacuum” at all, come to think of it neither is the “vacuum” you use to clean your floors at home. 

What is it then? Well, its really just an Air Conveyance Machine!

Now, I realize that its not as catchy and you are probably are not going to ask your kids to clean the floors of their rooms with the “ Air Conveyance Machine” but it’s useful to take pause and consider what a machine is and why it does what it does. Basically the only purpose of any of these devices is to convey material and the way in which these machines do that is by moving air. 

The “vacuum source” be it a centrifugal compressor or positive displacement blower compresses and exhausts air from the debris body or tank as the case may be. This process of removing air from the tank creates a lower atmospheric pressure inside the tank than the outside atmosphere. Air rushes in from your hose because nature hates pressure differentials. This air rushing in is what creates air flow and consequently material flow.

So what does this mean for my operation? Well, one common mistake in the industry is to reduce hose size in the mistaken belief that decreasing hose size increases performance by increasing the velocity of the air as its being pulled through a smaller opening. While yes, it increases the speed of the air it only does it in the smaller section of hose and the air returns to normal speed when it reaches the larger section of hose. This generally has a negative impact on the amount of material that the machine is able to move because it is not able to move as much air ergo reduces the amount of material it can move. Material often builds up at the transition point further reducing performance. 

 A very wise man by the name of Gary W. Toothe who is the corporate trainer for Federal Signal (Vactor and Guzzler) and former contractor used this analogy to explain it in simple terms relative to material movement.

“ You have to drive 40 people to the airport and can choose one of two vehicles to do the job. One vehicle is a two seater sports car and the other is a 48 passenger bus. The sports car has a top speed of 200 KM/H and the bus 90 KM/H. Despite the fact that the car is more than twice as fast. The bus is the fastest way to move 40 people”. 

Think of this the next time you hook a 4” hose onto your 8” vacuum hose to move a pile of material, stop to consider that it makes for fast air not fast material movement because it has a substantial choking effect on the airflow.

Reduce airflow and you reduce the amount of material you can move.