What is Static Pressure?

In dust collection, the collector must work to move large amounts of air at high speeds; capturing it at the tool, pulling it through the ductwork, spinning it into the cyclone, and finally pushing it back out through the filter – all without releasing any dust into the environment. To do this the dust collector needs to provide sufficient airflow (CFM) to each connected tool and must overcome several resisting forces.

Whether or not the air is clean or laden with dust, all air has mass. This means that as it moves, frictional forces slow it down. Making matters worse, friction increases as the air's mass and speed increase. Friction is created by the air interacting with other materials, including the dust within it, the components it moves through (e.g. hose, ductwork, etc.), and the air itself. The total degree of resistance within this airstream is referred to as static pressure (SP) and it is often measured in inches of water column pressure (WC).

How Does Static Pressure Affect Dust Collection Performance?

Every dust collector has an airflow rating, usually written in the form of x CFM at y WC, that indicates the total volume of air being moved through the dust collector against a specific resistance. When the collector turns on, its fan starts rapidly spinning and pushing air through the system, creating a negative pressure zone inside the collector. All of the air connected to the dust collector immediately starts moving towards this area to balance the pressure zones and reach equilibrium; this is what creates the "suction force" that pulls air from the tool and keeps it clean of dust and debris.

Working against this airflow is the positive pressure being created within the ductwork - commonly referred to as static pressure loss. Every component that the air moves through will increase the frictional forces acting upon the air and reduce the air's velocity. Certain components are more detrimental than others. In general, the more turbulence a component introduces to the airstream – whether by having a rougher surface area (like flex hose) or by joining multiple airstreams together (like wye joints) – the more resistance it creates.

See below for a data table of common pressure loss values for each ducting component:

Airflow through ductwork Diagram — Illustration of how airborne particles flow through various ducting components and how the shape of those components imposes greater frictional forces on the air, creating more resistance and slowing the airflow. Darker areas indicate areas of higher friction.

SHOW / HIDE DATA TABLE

Static Pressure (Air Friction) Losses Measured in Ducting

	3" Dia.	4" Dia.	5" Dia.	6" Dia.	7" Dia.	8" Dia.
5 ft of Rigid Pipe	0.355" WC	0.285" WC	0.230" WC	0.185" WC	0.145" WC	0.115" WC
1 ft of Flex Hose	0.352" WC	0.280" WC	0.225" WC	0.180" WC	0.141" WC	0.108" WC
90° Elbow	0.470" WC	0.450" WC	0.531" WC	0.564" WC	0.468" WC	0.405" WC
45° Elbow	0.235" WC	0.225" WC	0.266" WC	0.282" WC	0.234" WC	0.203" WC
45° Wye Branch	0.282" WC	0.375" WC	0.354" WC	0.329" WC	0.324" WC	0.297" WC
Static Pressure loss values are representative of airflow at sea level moving at 4000 FPM.

In ducting layouts, the pressure loss is cumulative along an entire run from the tool to the collector's inlet. If the total pressure loss in the system is too great, then the airflow will be insufficient to move the dust within it, leaving a mess on the tool and in the ductwork. For this reason, all Oneida Air Systems dust collectors include a fan performance curve which graphs the expected airflow at any static pressure. With a fan curve, shop owners can estimate performance once the system and ductwork are all assembled and design according to these limitations.

Does Altitude Affect Static Pressure Loss?

While air is indeed thinner at higher altitudes, it would be incorrect to say it negatively affects static pressure loss. Because the air is thinner, the dust collector's fan blower must be larger (both in terms of horsepower and impeller size) to move the same amount of air as it would at normal altitudes. However, the thinner air also means that the effects of positive pressure build-up within the ducting are lessened as well.

In other words, shop owners do not necessarily have to change their ducting plans to accommodate differences in altitude. They will, however, need a more powerful dust collector. Here is a table illustrating the CFM correction factor for increasing altitudes (planning for the worst case scenario).

For example, if you had shop that would normally be sized for a 2HP dust collector, but it's located at 9,000 ft in the Rocky Mountains, you would instead need a 3HP system to provide sufficient CFM to your tools (2HP x 1.48 = 2.96HP ≈ 3HP).

Altitude Above Sea Level	HP Correction Factor
1,000 ft	1.03×
2,000 ft	1.07×
3,000 ft	1.10×
4,000 ft	1.12×
5,000 ft	1.16×
6,000 ft	1.21×
7,000 ft	1.27×
8,000 ft	1.35×
9,000 ft	1.48×
10,000 ft	1.64×

How is Static Pressure Measured?

To find the static pressure of a dust collection system, we need to measure its negative pressure zone relative to the ambient air inside the shop – like how a tire's positive air pressure is rated relative to air outside its steel and rubber walls. And just as there are different scales to measure temperature by (e.g. Fahrenheit, Kelvin, etc.), there are also a variety of ways in which we can measure air pressure. In dust collection, since air pressures are still relatively low compared to what might be inside a car tire, we use what's called a manometer to find how the pressures affect a volume of water - giving us a reading shown as inches in water column (WC).

How Does A Manometer Work?

A manometer is a fairly simple tool: a U-shaped hose laid over an imperial ruler. The bottom of the "U" is filled with a small amount of colored water, while the hose ends are connected to the two different pressure zones. Where a dust collector is concerned, one end of the manometer is open to the air in the shop, while the other end is inserted into a length of straight pipe connected to the collector's inlet. The fan motor's negative pressure zone creates a vacuum inside the manometer, drawing the water inside the hose up one side of the "U" and down the other. The distance between the two heights of water in the hose shows the system's static pressure rating.

There are also digital manometers, which are much more compact, more precise, and can provide measurements on alternate pressure scales (e.g. psi, kPa, inHg, etc.). Oneida Air Systems uses a digital manometer with a pitot tube inserted into 10 feet of ducting connected to the collector's inlet to measure the air specifications for all of our systems.

1 psi = 2" Hg = 27.7" WC

Analog Manometer Diagram — Example of an analog manometer in use. Filled with water and colored with red dye to more readily show measurements on the ruler. Drawing not to scale.

Comments

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AJP

Good information for the average user. There are a few grammatical errors and typos. Those looking for professional systems designed and built by the best will notice them.

Oneida Air Systems

We're sorry to hear that AJ. We would hope to be judged on the performance of our products - not just the quality of our writing - but we'll take another pass through this article and see if we can improve it even further. Thanks for keeping an eye out for us!

Bill Wolverton

After reading this article, if I am understanding the process, in comparing dust collection systems, I should buy the system with a higher static pressure, considering all else is equal. Is my thinking correct? I have a small wood shop doing mainly personal work.

Oneida Air Systems

You're on the right path Bill - if you had two dust collection systems with comparable or equal CFM ratings, the tool with a higher static pressure will benefit you more overall. Higher SP will allow the dust collector to better overcome the losses from ducting and hose, and maintain a higher suction performance overall.

Charles Elkins

Thanks for the explanation. I can now make some sense of the airflow charts you provide on each piece of the equipment you offer.

Royd Roberts

Thanks for the explanations, you make it much more clear why I need static pressure or CFM to over come loss in the hose & pipe.

Crispin Spencer

Your level of detail and attempts to provide us with substantive information relative to your products is greatly appreciated. I do however, have a question. I don't understand the apparent disparities in your static pressure loss chart. Why does pressure rise in a 5" 90 degree elbow relative to a 4" 90 degree elbow? There are also confusing pressures on the 45 degree and Wye lines. Can you clarify. Thanks.

Oneida Air Systems

That's a great question! It's definitely counterintuitive, but the reason for the increase in static pressure losses among 5–6" elbows and wye joints specifically is because friction losses do not increase in a linear progression relative to the size of the duct. It's also important to consider that these measurements were taken empirically; Any chart on frictional losses in ductwork fittings varies based on the source i.e. the ambient air temperature, the air velocity, the pipe's surface material, the construction of the piping, etc. At best, charts like this are simply a rough guideline. Other information can be found in engineering industrial ventilation guides such as the American Conference of Governmental Industrial Hygienists (ACGIH).

Oneida Air Systems

We simplified some things for this article, but with regards to straight pipe, smaller diameters will have greater friction loss per linear foot compared to larger diameter piping because the cross-sectional area of the duct increases at a greater rate than the surface area of the wall. Within the pipe, the airstream is slowed by the friction exerted on it by the inner wall of the pipe (which is a factor of the pipe's circumference i.e. diameter × Π) and by the air particles themselves bumping against each other (which is a factor of the cross-sectional area of the pipe i.e. π × radius²). This means that the cross-sectional area increases quadratically while the wall area is increasingly linearly. However, the walls of the duct have a greater frictional force than the air within the duct, so as you can see there are a number of factors in play - all of which are dependent on the velocity of the air at any given time.

Mike Femia

I've seen some people encourage using 2 45-degree fittings in place of a 90. Does that actually have much of an impact?

Oneida Air Systems

We would recommend using a single 90-degree elbow whenever possible simply because with less fittings there's less chance of air leaks overall. Exceptions can be made when you need to have a wider sweeping turn in your layout, but in those instances a large radius elbow will always be better from a performance standpoint.

john

If you have a fan that has too much static pressure would it make it harder to clean down filters with pulse jet?

Oneida Air Systems

Provided that your fan has an adequate cyclone separator installed in front of it, your fan's static pressure should not have any noticeable impact on how easy it is to clean your filter with a pulse jet. With systems like our Supercell and Dust Cobra, the fan's airflow is actually reversed to free dust off of the filter.

mark

This is great information! I knew the flex hoses I've been using sucked... or didn't suck enough... anyway... How would I find that useful "x CFM at y WC" info for my trusty 20yo-workhorse-never-die-craftsman 16gal shopvac?

Oneida Air Systems

Unfortunately, it's going to be pretty tough to find that info for a shop vacuum - they're just not rated that way. Instead, you'll find that they'll usually have a "Water Lift" rating, which is the static pressure measured in inches of water column (WC). As far as the CFM goes, they're often going to range from 100-250 CFM for most standard vacuums.

Gove M Johnson

In reading through all your very helpful materials regarding designing a dust control system, I haven't found any info as to the static pressure loss contribution your many cyclone devices add to a system. I'm particularly interested in the static pressure loss I should factor in for the 5" Super Dust Deputy I'm considering for my system design. Where can this info be found?

Oneida Air Systems

Hi Gove, for any cyclone you typically want to budget 2" of static pressure loss for any cyclone installed into a ducting run. Actual losses could be much less than that, especially with our Super Dust Deputy's patented design features - but you should budget for more just in case. We're happy to hear that you're finding our blog articles helpful!

Jia Tai Tan

What prevents your dust collection system from sucking this thing dry if all of your blast gates are closed?

Oneida Air Systems

Jia Tai Tan, Any user should take care not to "dead head" their system but closing off all the blast gates while in use. It's highly possible that closing off the vacuum can cause damage to your hose, tools, and ducting due to the heavily increased pressure. In some cases this can even cause lengths of light-gauge ducting to collapse on themselves.

Doug S

I have a dust collection system using many of your components in my shop in CO. I have increased the motor size to 7.5 hp and use a VFD to control the motor RPM due to the altitude loss at 6300' elevation. I have installed a 0-25" WC magnehelic gauge, and I can pull up to -20" of static pressure with all gates closed. I have 20 gauge spiral ducting, as at this vacuum level I think I would collapse regular pipe. How does this compare with typical dust collection systems? Is -10" enough? -15"? I would like to know where to set the VFD to achieve the nominal values of static pressure, and I am curious as to the pressures your different systems obtain.

Julio

Is this the same static pressure thats measured through the filter? Im attempting to decrease the static pressure in my DC system to make my unit more effective; or maybe increasing my CFM???

Kyle

Thank you so much for the explanation. May I ask how the reading on the manometer, let's say resulted +3-2 (Manometer filled with water, not Hg), means how much pressure drop? I would like to learn about that to establish a reasonable filter bag replacement standard.

James Trabits

What it seems is never covered is the actual relation of velocity versus SP along with them independently. EG: Your V3000 being run with one 4" pipe open to collect dust. Opening another 4" whether or not it is connected to the tool will do what to the actual ability to collect fine dust? Velocity is a requirement for separation of fines. If one only had the same volume of air being moved through the cyclone - assume for a minute the cyclone only has a shop vac worth of volume/SP drawing air, then the fines may not accelerate to the required velocity to separate efficiently - right? Would they not be sucked into the filter since they do not ever achieve separation? SO what is the air flow required to separate fines? Should the entire available "suction" or "cyclone available air draw" be used at the tool IF possible even if only surrounding the sides of the tool in order to keep up velocity through the cyclone? It's time to address the simple items. How about your engineering staff's take [yes I am one too] on BEST practices while using the unit? I own a V3000 - just purchased last month, like it a lot, but if airflow [velocity specifically] is compromised - such as only having a 2" opening for a hand tool being used and all other flow shut off - then the limited flow will decrease fine separation - due to very slow gentle airflow - right? I mean, close off everything except for suction in a 1" hose connected to a hand orbital sander and the flow is slow enough that the some fines will remain suspended in the cyclone [I think a MAJORITY] and just go to the filter. Very nice having the HEPA on the unit to make up for not the most efficient use of the cyclone. Oneida - gives us your take, please. This is not a condemnation or complaint, it is a Best Practices use question I think has not been addressed and you are the best ones to tell us the answer. I think there is more to it than just using the right pipe - and we can not always use the best pipe for every tool, just the way it is. Thank you.

Todd

Regarding the table above "Static Pressure (Air Friction) Losses Measured in Ducting", I believe it has some errors. Reading across the table for rigid pipe and flex pipe, the static pressure loss starts high with the smallest diameter, and drops and the diameter of the pipe increases. However for the 45 and 90 degree elbows, the table shows an INCREASE is static pressure loss and the diameter increases: 90 elbow @ 3" (0.470) -> @4" (decrease to 0.450) ->@5" (INCREASE to 0.531) -> @6" (INCREASE to 0.564) -> @7" (large decrease to 0.468) -> @8" (0.405). The increases mid-way as the diameter increases and then the sudden decrease doesn't make much sense. Likewise for the 45 degree elbow.

Oneida Air Systems

Hi Todd, Thank you for reviewing this in such detail! While it may appear as an anomaly, this data is reflective of our testing at 4000 FPM. The relationship between the shape of the duct and the air duct is not truly linear so there are instances (such as 5"x90 degree elbow) that are in fact worse for air pressure at the given test conditions. In the instance of the elbows, the pressure loss is from the air molecules impacting the outer radius of the turn, creating friction and slowing down the air. But in larger diameter elbows, some of the air is able to still move along the inside radius and lessen this restriction.

Bob Kirkendall

In reading through these comments if find there are some good questions that I would like to see answered. Is there a forum where you provide answers to these questions? I would like to see responses to valid questions or an explanation of why a question is not important. Thanks much for your great products and supporting information.

Oneida Air Systems

The closest thing we have to a forum would be our FAQ section at oneida-air.com/faq. In addition to the growing library of articles in our Information Center, the FAQ addresses a lot more specifics topics.

Spindle

What is the static pressure drop (resistance) caused by the standard Dust Deputy 2" cyclonic separator at 50 to 150 CFM? This page provides information about ducts in general, but not about your very own products.