Skip links

The Great Debate: Centrifugal Fan vs. Positive Displacement

The Great Debate

It’s a common question:  “What’s better, fan or PD?” For many a sewer cleaning professional, the answer might be what they are familiar with, or what they’ve used in the past. But do you really know the difference in operation and performance?

First, let’s look at how each of these units work. All combination trucks rely on air conveyance to move the material from the sewer line to the debris body. The two methods of air conveyance are the fan, which is most commonly used, or the positive displacement pump.

A fan is just that, air is pulled into the center, compressed as it works through the vanes, and released at the outer tip of the fan. The key for the most efficient operation is to gain the most compression (hence Vactor’s closed-faced fan) and create as much tip speed as possible to move the maximum amount of air. This is the reason that Vactor uses 38″ fans – it is just a given that the larger the diameter of the fan, the more air it moves. Take as an example the household fan; the 24″ box fan moves more air than the 10″ desk fan.

The positive displacement pump, better known as a “PD”, takes air from the inlet side, compresses the air by taking large gulps and moves it from one side to the other at a very fast rate (see the illustration). The PD actually moves air, but also creates a vacuum if the airflow is blocked. The PD will continue to take gulps of air from within the debris body, pickup tube, and filters which creates negative air pressure within the system. A PD will continue to move water and material even if the pickup tube is buried (a fan unit requires airflow; so if you bury the pickup tube with a fan, it will not move material or air).

Fans work best with light and loose materials, dry to wet, short to moderate pulls, with low to moderate flowing lines. PDs on the other hand work best with short to moderate pulls, heavy, wet and dense materials, with moderate to high flowing lines.

The rule to consider for choosing a fan or PD is the 80/15/5 rule; what will be the use of the truck 80% of the time, then 15%, then 5%. It is always best to configure your truck for 80% usage. In other words, if you use the truck for jetting and cleaning sewer lines 80% of the time, then a fan would work wonderfully as most sewer lines are low to moderate flow with light and loose material. If you also used the truck 15% of the time for hydro-excavating, once again, the fan would work great because of the type of debris you are working with. But if your 80% use is in high flowing lines with heavy debris and long pulls or hydro-excavating, then the PD might be the best choice.

Another factor that should be considered when choosing between a PD over a fan unit is that the fan unit in its design is far more forgiving if material should enter into the housing. With tolerances between the vanes as close as .002″ in a PD, material entering into a PD pump can cause damage and possibly expensive repairs down the road.

When considering a new combination truck, it is wise to be open-minded, and look at all the possibilities that are available to you. What works best; fan or PD? Your application will help make that decision easier for you and your Vactor representative can assist you in making the right choices for your application.
The more we know, the more we can grow. Good luck, and be safe!

Leave a comment