Vacuum Cleaners
How a Vacuum Cleaner Moves Dirt
Have you ever used a leaf blower or watched one in action? The blower moves
debris through the action of moving air – the movement of the air causes an aerodynamic
drag force that pulls the debris along with the air. This drag force increases with higher
wind speeds and, hence, can move heavier objects at these higher speeds. This action of
moving a solid object by entraining it in moving air is known as "pneumatic transport."
Pneumatic transport is widely used in industry to move solid particles from one point to
another. It is how piles of grain are transported between containers. It is how they get the
wheat flour into the bags you buy at the store. It is how they get the cereal in the box. It is
one way they move coal at a power plant. It is how a vacuum cleaner moves dirt.
So, if you move air past a solid object, a drag force is set-up. This force is
proportional to the square of the velocity times the frontal area of the solid. If the force is
high enough to overcome the static frictional and gravity forces that hold the object in
place, the object will move and, if higher still, the object will become suspended
(entrained) in the air and move with the air.
A vacuum cleaner operates using this principle of pneumatic transport. The
objective is to move dirt from the floor or furniture to a collection bag within the vacuum
cleaner. To do this, the vacuum cleaner moves air and entrained dirt through itself. It
contains a blower, a cleaning nozzle, connecting tubes, and a breathable collection filter
bag (or in some designs a collection chamber). Because it uses pneumatic transport
principles, the more air the vacuum moves, the better its performance. So one goal in
designing a vacuum cleaner is to generate airflow. A key to this end is the design of the
blower and connecting tubes.
Vacuum Cleaner Design
A blower is used to move the air in a vacuum cleaner. Typically, a centrifugal
(often called radial) blower is used (see Figure 1 below), which is turned by an electric
motor. In such a blower, a rotating impeller will bring about a pressure difference: low
pressure or suction at the inlet end and high pressure at the exit. The pressure difference
causes air to move. The higher this pressure difference, the higher the air flow rate. Fluid
enters the blower along its centerline parallel to the rotational axis. It is turned 90 o within
the impeller to exit flowing radially outwards through its volute and diffuser. The
impeller (Figures 1 and 2) contains blades that act as small wings to create the pressure
difference. Generally, the more blades, the higher the pressure difference that a particular
blower can generate and the lower noise it produces. The impeller is encased within a
casing. The closer the fit between the impeller and the casing, the higher the pressure and
flow rate it can attain without significant leakage.
Figure 1. Cut-away View of Centrifugal Blower (also known as a Radial Blower).
Figure 2 Cut-away views of a typical impeller used in a centrifugal blower.
The nozzle is designed to create a high airflow near the surface being cleaned to
allow dirt to be swept up by the moving air and entrained into the vacuum cleaner. A
rotating beater bar is often used to beat the carpet to stir up trapped dust and to sweep up
larger debris. Once stirred, the dust is entrained by the moving air.
Connecting tubing becomes the vessel for pneumatic transport of dirt within the
vacuum cleaner. The connecting tubing connects the nozzle to the blower or the cleaning
tools to the blower. Some tubing may be visible on the outside of the vacuum cleaner, but
there are also tubing passages internal to the device, as well. The tubing design is very
important. Short, large diameter tubing is preferred over long and/or small diameter
tubing. This is because flow resistance in a tube is proportional to the tube length and is
inversely proportional to the tube diameter to the fifth power (flow losses  L/d5)! So the
distance from the nozzle to the blower is preferably short and spacious. It is common to
find connecting tubing as small in diameter as 1.25 inches, which is okay for short runs,
but 2 inch diameter or more are preferred to maintain higher flow rates.
Vacuum cleaner blower designs fall into one of two categories: Dirty blower
designs and clean blower designs. In a dirty blower design, the dirt moves through the
blower and is dumped directly or indirectly through connecting tubing into a bag. In a
clean blower design, the dirt is transported directly or indirectly by tubing to a bag, with
the blower on the “clean” side of the bag – the bag filters the air before it enters the
blower.
The dirty blower design is common in older designs and still used in lower end
products. It has some packaging advantages, including very short connecting tubing.
However, consider the task of a dirty blower design – not only will the blower see the
transported dirt pass through itself but also that occasional penny or screw that gets swept
up. So to prevent internal blockage and breakage, the blower design tends to have fewer
and thicker blades, wide clearances between the impeller and casing, and a less exact
aerodynamic design. These result in poorer air handling performance. Note that it is
possible to create a good performing vacuum cleaner using the dirty blower design, but it
is more challenging and rarely cost competitive.
The clean blower design is common to newer and mid- to high-end units. Because
it sees only clean air, design tolerances can be tighter and blade design can be more exact.
However, in upright cleaners packaging can be more of a challenge. Often, the
connecting tubing can become long – degrading performance. Better machines use large
diameter internal connecting tubing between the nozzle and the filter bag.
Vacuum Cleaner Performance
Typical blower-system performance curves for several vacuum cleaners are
shown in Figure 3. These are plotted as “pressure” or static (suction) head versus “air
flow rate”. The static head tends to be high at low flow rates and falls to zero at the
maximum flow rate. The blower will actually operate at the point on the curve where the
blower curve matches the system flow resistance pressure. Normal operating condition is
somewhere in the middle of the curve but varies with the specific cleaning chore (plush
carpets impart a higher pressure resistance then bare wood floors, etc.). The vast majority
of vacuum cleaners today do not utilize a speed controller on the blower. This means that
the blower rotating speed changes with cleaning chore. The more air that flows through
the blower, the slower it turns, because of the increased load of accelerating the larger
mass of air. A typical blower turns at 30,000 down to 13,000 RPM and can vary by 
8,000 RPM during normal cleaning chores.
Figure 3. Typical Flow Curves for Commercial Vacuum Cleaners
Lets think about the blower curves in Figure 3: If you block the end of the nozzle,
the flow falls to zero – the static head is highest (hence, high suction and referred to as
the seal suction). The RPM rises since there is no air flow load on the impeller. As you
vacuum a carpet, the nozzle is partially blocked – this is normal loading and somewhere
in the middle of the curves. In fact, the low plush carpet condition is labeled with a ‘star’
on the curves in Figure 3. The RPM is somewhere near its designed operating value so it
is likely to give its best performance here. If you place nozzle in the open air, the only
“blockage” is the losses in the vacuum lines, giving maximum flow rate at lowest suction
and lowest RPM. The best cleaning vacuum cleaner will tend to have higher suction at
higher flow rates because this gives best pneumatic conveying capability. In fact, the
product of static head and flow rate is a measure of cleaning capability. The differences
in the curves shown reflect design differences in the different devices. All of these units
have very good cleaning performance.
The concept of seal suction brings up an interesting pet peeve. A typical sales
pitch is to feel the suction of a vacuum cleaner when it is totally blocked. The salesperson
has you place your hand over the end or something similar: Strong suction yet no air
movement – hence, no pneumatic transport and no cleaning. So this test tells little about
how well a particular vacuum will clean. A better test is to know air flow rate under a
partial blockage. Seal suction is useful, but remember air flow is what it takes to clean.
The motor used to turn the impeller plays an important role. The blower-motor
combination is a team. Both must be well matched: The motor must be sized large
enough to turn the blower under load and to turn it at the blowers design speed. A larger
motor will generally draw a higher amperage. But the current draw of the motor has
almost no bearing on cleaning performance. An inefficient blower-motor combination
can draw as much current as an efficient top performing combination.
The design of the vacuum cleaner package greatly affects blower performance
and this is where the engineer is challenged. As noted earlier, a goal is to minimize
connecting tubing losses. The nozzle design effects its cleaning effectiveness. The beater
bar design, if used, affects cleaning effectiveness. The filter bag affects performance, too,
as it causes a pressure loss and reduces flow rate. Some newer designs avoid the bag
altogether to maintain a higher flow rate. The whole package is usually made of injection
molded plastic panels with some metal parts. This makes the unit light weight and easily
mass produced. But injection molding limitation can limit design.
Also, consumer desires are strongly reflected in design. Consumers want a
lightweight, high performance unit with a good feel and balance with easy filter change.
The move to plastic devices serves this well. They do not want a real loud device but shy
away from very quiet units because they do not sound powerful. Hence, the exhaust ports
are tuned to give an acceptable sound. They want every device possible in easy reach, so
many units carry these tools onboard. They want a reasonable price, which is relative, so
there is a market for everyone. Generally, a very good unit can be had for about $150 –
220 (year 2000 prices). The $69 discount store unit will make noise, look like a real
vacuum cleaner and pick up gross debris but not deep dust. The $1200 units are for those
who do not know any better.