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Professional carpet cleaners and restoration contractors will argue endlessly about which truckmount is the best. Which one is the most dependable? Which one is the hottest? Which one has the best vacuum?

In many cases, the discussion of vacuum will centre on blower specifications. Is there a foolproof way of determining which vacuum blower is right for your truckmount? Is bigger always better? What is air-flow? What is lift? Which is more important?

While it is impossible to fully answer these questions (and the numerous questions that would surely follow) in this article, we can take a look at what a blower is, how it works, and what factors help to determine selection and performance.

Blowing air

The blowers that are most commonly used for truckmounts are called positive displacement blowers. In positive displacement (PD) blowers, a definite amount of air is displaced (moved) through the blower with each revolution.

This amount varies little throughout the designed operating speed range of the blower.

One good way to look at it is that every time the blower turns one rotation, it “gulps” a certain amount of air, moves it through the casing of the blower, and discharges it from the other side. This is accomplished by two “lobed” rotors rotating in opposite directions.

With each revolution, a pocket of air is trapped between each of the two rotors and the casing. This pocket of air is pushed around the inside of the casing and forced out the other side.

The rotors operate within extremely close tolerances, but never touch, so no internal lubrication is necessary for the lobes. The air that goes in doesn’t pick up any contaminants, such as grease or oil, from within the blower, and is discharged in pretty much the same condition as it entered. There is no increase in pressure within the blower. Compression only occurs after the air has left the blower and encounters resistance in the system on the discharge side.

Does size matter?

Positive displacement blower sizes are usually designated by gear diameter and rotor length. Without going into too much detail about the differences between major manufacturers, the sizes and specifications of blowers vary only slightly between them.

So a “45” blower, a “4005” blower, and a “4M” blower are all very close in size, weight and rated performance. The first number (in this case “4”) refers to the diameter of the timing gears that transfer power and control timing between the rotors. This number approximates the distance from the centre of one rotor to the centre of the other, in inches.

The additional number(s) or letter(s) refer to the length of the rotors (front to back).

So, viewed from the front, a 45 and a 47 blower would look the same. But from the side, you would see that the 47 is about 2.5 inches longer than the 45.

In general, the larger the gear diameter, the larger the lobes are on each rotor. The larger the lobes, the more air that is transferred with each revolution. The longer the lobes, front to back, the more air that is transferred with each revolution. So from this it is easy to see that a 4007 blower would move more air than a 4005, with each revolution.

Speed and the laws of physics

A blower has a maximum safe speed at which it may be operated. If you know the quantity of air that is displaced (moved) through the blower with each complete revolution, and what the maximum speed is, you can figure out the theoretical maximum airflow in cfm (cubic feet per minute).

For instance, Blower A has a gear diameter of 5 inches and a rotor length of 6 inches. With each revolution, this blower displaces .210cfm. The blower is rated to operate at a maximum of 2,850rpm. 2,850 x .210 = 598cfm.

But the laws of physics place certain restrictions on how much air will actually pass through a blower once it is placed into a system to work.

In truckmounts, this work is to move air and water through your hoses and into your waste tank. As the blower draws air on the intake side, a partial vacuum (reduction in atmospheric pressure) occurs within the vacuum hose.

This creates a pressure differential between the air inside the hose, and the air outside.

High pressure flows to low pressure, so air begins to flow through the hose. The greater the pressure differential, the faster the air will move through the hose. This difference in pressure is referred to as “lift”, and it is a basic indicator of how much potential force the blower has with which to move air.

Test for yourself

If you take your hand and lay two fingers across the end of the vacuum hose, you will notice that the hose pulls tightly against your fingers. If you add another finger, you notice that it pulls even more tightly.

If you block the hose completely with your palm, it pulls harder still. What you are feeling is lift. The more lift you have, the better your blower will overcome restrictions in the system. If you put your finger inside the end of the hose, you will feel air rushing past it. The faster the blower turns, the more air you will feel. That is air flow, which is measured in cubic feet per minute (cfm), as mentioned earlier.

Real application

Let’s say you have a 5M (56/5006) blower with a theoretical maximum air flow of 598cfm. However, the published ratings are usually based on a system with some restriction placed on it. When we look at the performance chart of this particular blower, we see that this model is rated at 542cfm when operated at six inches Hg of lift. If we increase the restriction on the intake side, lift increases.

At the maximum rated lift of 16 inches Hg, the same blower is rated at 477cfm. So if you place a restriction on the intake side, thereby increasing lift, the blower loses some cfm.

This loss of cfm is referred to as “slip”, and is a measurement of the air slipping past the lobes rather than through the system with each revolution. Slippage increases as vacuum resistance on the inlet side increases.

Slippage increases when you use your wand, pulling air and water through a one-fourth inch by 12-inch slot that is pressed down to carpet, 25ft of 1.5-inch hose and 400ft of 2-inch hose (as one example). This creates a higher amount of vacuum on the inlet side, but you also experience more slippage.

This reduces the actual amount of air flowing through the system. Pressure created on the outlet side, through silencers, mufflers, reduction in pipe diameters and even bends can also increase slippage.

Keep in mind

There are other factors, such as fuel consumption, vehicle capacity and cost to be considered. Bigger is not always better.

If you opt for a much larger blower than you need, you will end up wasting all of that excess capacity in the form of higher fuel costs to run the equipment.

In the end, you have to make decisions on what the primary function of your machine will be, and choose accordingly.

Bruce DeLoatch, Cleaners Coach, USA

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