Why is airflow always measured?

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The science describing how all vacuum cleaners clean flooring, regardless of who designed and manufactured them, has been known for over 250 years and is (relatively) easy to understand. As a scientist with knowledge in this area, I’ve decided to produce this summary to complement the full video lecture already available online to help people who genuinely want to understand. The commonly seen measurements of airflow and suction, particularly at an open hose or equivalent, do not directly inform cleaning capability on carpet. The reason for this requires an understanding of this known science, and a brief and hopefully accessible explanation is given below. I appreciate this post might not be for everyone.

The science:
To accelerate a particle out of a carpet, an aerodynamic force needs to be imparted by a moving fluid (air in this case). This aerodynamic drag force (equation 1, figure 1) scales with the square of the airspeed and therefore dominates all other terms. Airspeed is the single most directly influential aerodynamic parameter in determining the applied force.

Figure 1.jpg

There are three other factors that affect either the airspeed or the drag force:
1. Cleaner head
2. Dirt properties
3. Carpet properties

The cleaner head is particularly important (point 1. above). It’s designed to be fully sealed to the floor, as this makes use of a well-understood physical phenomenon first described by Daniel Bernoulli, with a special case described by Giovanni Venturi. The edges of the cleaner head create a constriction in bounded fluid flow that results in a substantial increase in the speed of the air on entry—airspeed being the most important parameter, as above in equation 1. Equation 2 in figure 1 quantifies the Venturi effect and shows that the pressure difference inside and outside the cleaner head (i.e. suction) drives the airspeed increase. Equation 3 in figure 1 combines the earlier two equations into a single expression to quantify the acceleration of a dirt particle from a carpet. It is NOT a function of the total magnitude of airflow, but of suction pressure within the head and some other terms that relate to the dirt particles themselves. This relates to point 2, above: at any given suction pressure, particles with a greater drag coefficient—like fluff, or that are larger—like hair relative to microscopic dust, are easier to remove, whereas heavier particles, like sand, rice, and grit, are tougher to remove. The other influence of the cleaner head is the brush bar. This directly grabs loose surface dirt but also can locally separate pile to increase airspeed at greater depth to better accelerate particles out. It also agitates—which doesn’t mean makes particles visually bounce outside the head, which is literally irrelevant since they’re not going anywhere—but repeatedly jostles fibres under the head where the aerodynamic drag force is applied and freeing any that become netted by the fibres to be accelerated in the flow.

Carpet properties (point 3 above) also affect airspeed as a function of pile depth and is a bit more complicated to consider. Most modern fitted carpets have liquid-impermeable, non-porous backing, which reduces the airspeed to zero as you approach the base (see figure 2), preventing particles from being easily removed at the base on deeper pile carpets. Particles rarely naturally get to that depth on such flooring since deeper carpet is actually a filter that physically blocks them; almost all dirt remains in the upper regions with basic housekeeping (plenty of evidence of that here). Loose shaggy rugs often have porous, permeable backing allowing relatively higher airspeed at the base, but typically relatively lower airspeed towards the surface, for a given cleaner and air power—see figure 2, which is illustrative.

Figure 2.jpg

To understand this and a number of other points I’ll come onto, you need to understand vacuum dynamics (or basic electrical circuit theory) to help appreciate the physical relevance of suction, airspeed, and air power, and the relationship between them. Suction, airflow, and air power are all functions of the air resistance in the air circuit and the maximum suction pressure the motor can produce, as derived in equations 4–6 in figure 3, where their normalised relationships are also plotted.

Figure 3.jpg

There are two important things to understand from this:

1. The type of carpet affects the air resistance, R. Loose shaggy rugs reduce it whereas fitted carpets with impermeable backing increase it, and the plot shows how suction, airflow, and air power respond accordingly.

2. Motors that can provide a greater air power (under resistive load) generate a greater maximum suction pressure (equation 6). This allows suction to be sustained in the presence of higher leakage airflow into the cleaner head on flooring that offers reduced air resistance (equation 4), thereby maintaining airspeed and dirt removal rate.

To expand on these two points, the equations and plot illustrate why a larger airflow into a cleaner head, such as occurs on loose shaggy rugs or with a poor suction seal, is bad. It neutralises suction pressure and cleaning performance and requires more air watts and greater maximum suction pressure to compensate. The total magnitude of airflow increases, but its speed decreases. Furthermore, the equations show that as the air resistance of flooring increases, you need a higher-pressure motor to compensate and sustain suction pressure (equation 4) and maintain airspeed. What this means is that cleaners with low suction motors (e.g. the old-fashioned ‘dirty fan’ machines) don’t have a problem on carpets with permeable backing since there’s little resistance for them to overcome. But as air resistance increases, they can’t draw as strong a vacuum under the cleaner head to maintain airspeed, affecting cleaning performance. There is test data to demonstrate this in this video. There are lots of tricks of the trade to cover up this shortcoming, and they’re discussed in the lecture.

When you understand this science, it’s natural to cringe when you see measurements of suction and airflow from an open hose (or equivalent)—which isn’t representative of a cleaner head that’s well sealed to carpet. What should be measured instead is actual dirt extraction rate under representative, real-world conditions in a way that shows understanding of statistics. Dirt removal from carpets is stochastic, since it’s a first-order system, and any measurements need to capture this and look at removal rates from a carefully controlled initial condition, following a precise real-world representative and reproducible methodology. An example of the data you get by doing this that allows fair relative performance comparisons, and without the aid of a laboratory and robots, can be found in figure 4 and any of my more recent reviews.

Figure 4.jpg

Hopefully this clarifies the science and helps people to now spot common errors. The full treatment of the science can be found in this video lecture. I can also clarify any questions here.
 
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So with Ametek/Lamb motors at least, you get a chart like this:
1752066172357.png

The orifice size is just a standardized way of created a repeatable restriction.

From my understanding (and I think this agrees with what you have described) the goal should be to have the maximum air wattage for the restriction of your particular system. That would be the combined restriction created by your cleaner head (and its interface with the surface being cleaned), hose, filter, and exhaust.

The question would be how to relate the orifice diameters to the restriction of an actual system. For central vacuum systems, I believe Ametek/Lamb recommends looking at the 5/8" rating for most installations.
 
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From my understanding (and I think this agrees with what you have described) the goal should be to have the maximum air wattage for the restriction of your particular system. That would be the combined restriction created by your cleaner head (and its interface with the surface being cleaned), hose, filter, and exhaust.
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The orifice diameter plot you show from one of many industry standards is specifically for the aerodynamic flow properties through a pipe as a function of resistance (diameter size). While the relationship is valid, only the resultant suction level within the cleaner head is relevant for dirt extraction from carpets that the head is well sealed to. That is variable in use and dependent on the flooring type and quality of suction seal. The cleaner head is the only relevant location when considering dust extraction from carpets.

The substantial ducting, typical of central vac systems, regardless of its properties, is all downstream of the cleaner head and has the effect of simply adding additional air resistance to the air system. In this case, in figure 3, the first equation for total resistance would get another R term to account for the additional pipe-related air resistance that becomes non-trivial in central vac systems. This highlights one of the weaknesses of central vac systems—this increased pipe resistance requires a much higher-pressure motor (or multiple in series) and more air watts (more energy consumption) to increase maximum motor suction pressure and compensate for the increased resistive losses. Without that boost, the increase in total resistance would reduce the net suction pressure in the cleaner head, diminishing performance, as clear from equation 4. So, the reason why companies may recommend a 5/8" rating, for example, for most installations from the standard is because it minimises this additional energy loss but has nothing directly to do with resulting cleaning performance, which is exclusively driven by the net pressure at the cleaner head.

This nicely shows again why measurements in other parts of the air system are not directly relevant to actual dirt extraction rates. Focus should only be on the cleaner head. Suction and airflow are also not worth measuring at the head, really, since pickup performance is also determined by how effectively the head utilises aerodynamic resources and maximises air speed locally a depth within the pile via various design features discussed above. Again, the only metric worthwhile is the actual measured dirt extraction rate trends as outlined above, which automatically fully accounts for all these other unknown and hard-to-measure variables.
 
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I think you could get a decent approximation of an equivalent orifice size for a system with just a pressure gauge. Set everything up, including the cleaning head to cleaning surface interface, so that it closely replicates your normal use. Turn the vacuum on, and measure vacuum pressure as close to the motor as possible.

Then remove the cleaning head and as much of the hose as possible and install an orifice plate and gradually increase the size until your get the same vacuum pressure.
 
It's a bit round the houses, and unfortunately, approximating an equivalent orifice size offers as little value as measuring airflow at an open hose for the reasons explained here.

Some of the science can be hard to understand but once you do, it's clear what to measure and why. Hopefully it's provided fairly accessibly in the original post and happy to clarify anything further.
 
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It's a bit round the houses, and unfortunately, approximating an equivalent orifice size offers as little value as measuring airflow at an open hose for the reasons explained here.

Some of the science can be hard to understand but once you do, it's clear what to measure and why. Hopefully it's provided fairly accessibly in the original post and happy to clarify anything further.
The value, in my mind, would be in selecting a replacement motor. Obviously a little late if you don't have a currently working motor like me, but if you were able to see that your system presents a resistance that is equivalent to, for example, a 5/8" orifice plate, then when when looking for a new motor (or entire vacuum), you could compare based on which are rated for the most air watts when tested with a 5/8" orifice plate.

I think it would be generally accurate to say that for a given cleaning head, dirt extraction will be increased if there is an increase in air watts for the system configuration. So if you were able to characterize a system in this way, you would be able to get more performance by choosing a motor that provides that higher air watts in the chosen configuration.
 
The value, in my mind, would be in selecting a replacement motor. Obviously a little late if you don't have a currently working motor like me, but if you were able to see that your system presents a resistance that is equivalent to, for example, a 5/8" orifice plate, then when when looking for a new motor (or entire vacuum), you could compare based on which are rated for the most air watts when tested with a 5/8" orifice plate.
Oh, I see. Well that's not what this post is about really; it was more about measuring metrics (like open hose airflow) which have no direct bearing on cleaning performance, yet are anyway. Tuning a motor for a central vac system is another thread entirely, in which case, yes, you'd follow the industry standards to minimise energy losses in an already very lossy air system.

I think it would be generally accurate to say that for a given cleaning head, dirt extraction will be increased if there is an increase in air watts for the system configuration. So if you were able to characterize a system in this way, you would be able to get more performance by choosing a motor that provides that higher air watts in the chosen configuration.
Yes, that's right. Increasing air power (under resistive load) means you increase the maximum suction pressure the motor could provide and thus the net suction at the cleaner head (equations 4 and 6). But what happens at the head (which is highly variable in use) modifies what happens in the piping downstream and so tuning based on the cleaner head is required rather than the pipe. Finding a coupled relationship between the two is complex and unique to each specific sytem anyway, and only what happens at the head determines cleaning performance. It's worth appreciating that the 'peak' airpower in figure 3, or your figure above, will rarely be achieved since head pressure and leakage airflow flucuate widely during use of any machine, based on floor type, quality of seal with the floor, mass extraction rate of dust etc. The goal should be to achieve the optimum head suction pressure for most situations to achieve best cleaning performance. You also don't want the pressure under the cleaner head to drop too low just by increasing air power, otherwise you get clamping. Finding the most energetically efficient way of achieving not just optimum cleaner head pressure, but high airspeed deeper in the pile, without affecting filtration performance, is continually researched, and I'm aware of a roadmap.

Central vac systems are wrought with fundamental problems, since they introduce this extra resistance from the lengthy ducting and are hugely energetically expensive. A better design of cleaner at a fundamental level can completely sweep away the weaknesses and achieve high levels of cleaning performance far more efficiently. The best cleaning performance I'm aware of can be achieved with just several hundred Watts. Anything more is evidence of relatively poor technology. Research continues to find how to get the best cleaning performance using minimum energy. Some people do buy into central vacs and like them regardless, and each to their own of course.
 
Yea central vacs win me over for the external exhaust option if nothing else. But the motor technology definitely does seem to have lagged behind more portable options, probably because you don't have the pressures of weight reduction or energy efficiency driving improvement.

I wish there were more good comparisons of central vacuum cleaning heads in the style of some of the videos you have. I agree that differences there can easily overshadow so many other aspects of the system overall.
 
Never been too impressed with machines that just exhaust dirt to the atmosphere. Always seemed like a copout and evasion of properly handling dust. Might as well just empty the vacuum bin out of the window as well like how the Victorians disposed of their waste. It's even less respectable at the moment, as there are some incredibly impressive filtration technologies being researched and developed for vacuum cleaners that eliminate cyclones—one of which I'll hopefully produce a new video about this weekend. Other than as a fun mancave project, I've never understood what people see in central vacs, at least technologically, but that's a conversation for a different thread I'd enjoy.

Back on original topic though, the reason why filters need to be rinsed in any vacuum (or bags replaced) is because, similar to a central vac's additional R term for total air resistance due to unavoidable huge resistance from all the ducting, a clogged filter/bag also adds a non-trivial R term, reducing head suction pressure and thus entry airspeed, impairing cleaning performance (figure 3).
 
Central vacuums can have HEPA level filtration, and plenty of installations have interior exhaust. I'd say the level of filtration needed depends on what you are vacuuming and your proximity to neighbors. I personally don't have any nearby neighbors, so I wouldn't mind the minimum filtration necessary to prevent impeller damage, except that I have synthetic carpets and I occasionally use my central vacuum in the workshop. Both of those result in vacuuming up debris that would fairly universally be considered pollutants if discharged outside. Thankfully, the debris from both is pretty much entirely captured by a basic paper bag, so less waste generated from the filtration effort itself. Everything else that might be exhausted is pretty much just going back to where it came from.

On your point though, that's the reason for wanting the minimum filtration level necessary to sufficiently clean the exhaust air - less total system restriction. With interior exhaust, you are generally going to want better air cleaning (and therefore more restriction, all else equal.)

I'm curious though how much the ducting of a central system impacts performance. One thing some probably see in central vacuums is the higher potential cleaning performance, although I would be curious to see how the hose-end air wattage compares. 600-700 air watts is pretty common for central vacuum motors - with 1.9" ID smooth bore pipes, I wonder if that would be enough to bring them down to portable vacuum levels.
 
The losses from ducting in central vacuums are significant and are dominated by frictional pressure loss, elbows, tees, and reducers, and possibly narrow or undersized piping. Losses scale with length, air velocity, and the number of fittings. This is completely clear from the motor power source. In more energy efficient and fundamentally different approaches to vacuum cleaning, the optimal properties at the cleaner head, giving measured industry-leading cleaning performance, can be achieved with just a few hundred Watts of electrical power (reducing all the time). And very high cleaning performance is now being achieved in 'default modes' of leading battery-powered machines consuming around 250 W. Contrast this with a central vac motor that continually burns through, 1.5–4.5 kW depending on system size. This indicates a serious inefficiency in its fundamental nature. Ducting losses can account for 40–70% of total system losses, in contrast to 20–40% from filtration. Micro-tweaking to minimise filtration losses is to ironically overlook the fundamental problem with central vac systems and that primary losses unavoidably come from their very nature and design. I could do an entire video on this, but the only one I've done on central vacs reaches the same conclusion.

Air watts are nothing more than a measure of above floor cleaning potential and do not scale for cleaner heads on sealed carpet due to the detailed science given above. There should be no fixation on such numbers as it only reveals a lack of understanding of the relevant science, which is now made available since I recognised it was sorely absent. As above, only actual cleaning performance dirt removal trends following carefully conducted and statistically significant testing should be measured; other system numbers are not indicative and reveal lack of understanding.
 
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Definitely going to have higher losses in a central vacuum, but those losses decrease as air speed decreases. As you mentioned, peak airflow rates on a wide open system aren't a very good indicator of actual cleaning performance. I'd be surprised if, given the same cleaning head and method of use, a non-central vacuum were able to outperform a central one since the central one's more powerful motor should be more than enough to compensate for those losses.

I think you might be misunderstanding my point about air watts. For a given cleaning head, achieving a higher measure of air watts necessarily means more airflow AND more vacuum pressure, since air watts are defined by those two values. Really, you could just measure one of those two component measurements and get an equally valid comparison since the restriction of the system is fixed (or at least, close enough to it for the ranges we are discussing). That is one thing that would be interesting to measure to compare cleaning potential of two systems.

Measuring air watts for an open hose end as I mentioned (or one of the component measurements) would be specifically for comparing system losses not related to the cleaning head. The goal of that wouldn't be (directly) for comparing cleaning potential, but for understanding those system losses better.
 
Yes, there’s no evidence for the cleaning performance of central vacs that I’m aware of. That in itself is quite astonishing and says so much. There’s plenty of data for other formfactors that’s independently verified to formal international industry test standards, whose manufacturers are quite keen to shout about—and why wouldn’t you if you had nothing to hide. Measured cleaning performance counts over other indirect metrics, but it’s the only thing manufacturers of central vacs don’t provide—along with their motor’s power consumption in a clear and plain-as-day front end way. Draw your own conclusions, given it’s quite easy to provide this data if a manufacturer really cares about their customers. Even for the sake of being hypothetical, it’s also very far from respectable or desirable if a product achieves decent performance by utterly brute forcing in an incredibly wasteful and tremendously inefficient manner that costs more. That’s not to say people can’t like central vacs, but they are terribly wasteful and it’s reasonable to suspect they don’t perform the best relative to other approaches given the lack of a single piece of reputable data after all this time from all these manufacturers and champions, given every opportunity to do so, as others have.

I understand your point about Air watts, but it is definitely a misunderstanding on your part, which I can help clarify again. While it’s true to claim that more airpower gives better cleaning performance for cleaner heads sealed to the floor, there are other important physical limitations that kick in preventing these benefits from being realised, and this is widely misunderstood still—despite being publicly available. Too much air power lowers the resulting sealed head pressure too much, causing clamping making it too hard to push, brush bar strain, enhanced wear, and reduced lifetime, amongst other problems. In fact, the most powerful and efficient battery-operated devices on the market actually reduce their operating air power when used on carpet, relative to above floor cleaning, for exactly these reasons, yet achieve exceptional cleaning performance, as I’ve shown in the product reviews. Brute forcing airpower is NOT a design goal.

Again, the skill in vacuum cleaner head design, is to use the minimum electrical input energy to generate optimally low head pressure, maximise airspeed at depth in pile, as well as loosening trapped dirt, so it can be better accelerated away. The best approaches and products available can achieve this with just a few hundred Watts of total power consumption—and this is reducing all the time, contrasting terribly with the fundamentally wasteful approach of central vacuums whose resources are utterly squandered in the most appalling and unrespectable way—although again, I recognise and understand the man-cave hobbyist tinkering that goes on amongst a niche crowd; nothing wrong with that.

The science above and in the associated lecture cover all this and is now publicly available in an accessible way, essentially preventing any reason for this not to be appreciated by all audiences going forward. I intend to do updated revisions of that lecture over time (it’s a lot of work).
 
It seems like you are saying that central vacuums are bad because they have too little power because of system losses... but they are also bad because too much power reduces performance.

Regarding motor information - I'm always a fan of companies being more transparent, but in this case I think the information provided on a spec sheet like Ametek's is pretty good. What more would you hope for? The motor (or more to the point, the power unit) doesn't come packaged with a certain length and type of piping, a specific number of elbows, or necessarily a particular hose and cleaning head. About the best they can do is tell you performance at a give system restriction, which is exactly what is provided.

The ideal amount of power will depending on the specific scenario, as you indicated, and thankfully that's fairly easily adjusted on most central vacuums as well. Regarding vacuum heads, I'd say that's another benefit of central vacuums - you can swap them out in seconds. There are plenty of great options out there, although I would expect that the best portable vacuum cleaning head is slightly better than the best central one, if only because of the pressures of a limited power budget. That said, central vacuum cleaning heads can afford to be a bit more specialized. I used a different one on carpets than I do on hard floors.

I do agree with you about efficiency, at least as far as the motors go (I don't think system losses are as big of a factor as you say). Would be nice to see some good brushless options. Unfortunately the demand just isn't there it seems, since brushless motors are more expensive than their universal motor counterparts, and a lot of their other benefits are a bit diminished in this type of application.

It might be interesting for you to test out a decent CV. I'd never used (or really knew much about) them until I moved into my current home, which has a 50 year old NuTone. Tested it out, and it was immediately clear that it cleaned much better than any of the portable ones I've used. There's a lot that can be gathered from theory and device specifications, but ultimately, it's real world performance that matters here.
 
Not quite; I'm saying they're bad because they consume (waste) excessive electrical power and that too much airpower causes new problems (clamping) such that theoretical extra cleaning benefits can't be realised practically. Claims that being able to produce high air watts is a virtue is not true above a limit; it's wasteful as discussed.

I'm sure some companies may very well state clearly how much power their motors consume in operation. Some don't as discussed here and obfuscate with only voltage and current draw information instead, which is interesting to say the least.

Having to swap heads out for different floor types is no virtue; it's a hassle and evidence of poor design since we know from leading products that it's possible to cater for all floor types with a single, well-designed head. Unless you mean change heads to above floor tools, in which case, unless they're stored on board with you, you've then got the unnecessary extra hassle of having to go get them and then put them away afterwards—extra avoidable hassle I and many others certainly wouldn't want to adopt.

Unfortunately, the nature of a central vac means I'm never going to be able to get one to test in my home—again reflecting on their fundamentally poor inherent design. The big white elephant in the room always ignored—other than the stunningly excessive cost relative to other vacuum cleaning technology approaches—remains their sheer, unavoidable energy consumption to achieve what we know is possible with a few hundred Watts, and the complete lack of reputable, independently verified evidence from any manufacturer or champion of them of their cleaning performance despite unlimited opportunity. Until that's provided—and I've no confidence it ever will be—there's literally nothing to go on.

Edit: I should add that not everyone likes central vacs out there.
 
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