View Full Version : Relationship between static pressure & friction per 100'

gmacw

02-21-2005, 08:17 PM

I have a question that I hope someone out there can clarify.

Most job specifications state the static pressure that ductwork should be.

What is the relationship between static pressure & friction per 100' on a ductulator?

Let's say that the job calls for duct to be made for 2" static. Is there a correlation between staic pressure & friction as shown on a ductulator. Is there a formula? If so what is it?

For low pressure ducts we size ducts @ .08 to .1 or will use the fpm side of ductulator. What I don't know is if specifications call for a certain static, how do I use the ductluator to determine the duct sizing based on the given static pressure.

I'm sure this is an easy answer but appreciate any help in clarifying.

There is a formula and I'll try to post it tonight or by morning. Ashrae, developed a chart because friction does not increase in equal steps, there is a curve involved as in most cases even with fluids. The friction factor is based on the roughness of the pipe and reynolds number? I'm not sure why would design a system on the friction factor and I'm thinking you don't want to go backwards on this, the formula is quite complicated.

http://www.efunda.com/formulae/fluids/calc_reynolds.cfm

Here is a link to more on the reynolds number.

Be back later...

Correct, the static pressure is usually a big part of the system design (The static head is which is required to overcome the frictional resistance of the air against the surface of the duct.) It has to do with the recommended velocity, which is different for several applications; First things first, you have to know the CFM required for the rooms? Hypothetically, lets say the room requires 650 CFM, your designing the ductwork for .1 in static pressure, this scale is marked (friction per 100 feet of duct) To overcome the friction of 100 feet of duct and deliver 650 CFM at .1 inch static pressure your ductwork better be around 12 x 10. Now look at the other area where the CFM = FPM, this scenario pushes 650 cfm at a velocity of 850 fpm. Now take the 650 CFM (because the quantity needed for the room hasn’t changed) at .2 inches of static pressure, you can decrease the ductwork to about 10 x 9, now look what it does to your velocity? 650 cfm = 1150 fpm. Above what is typically recommended for residence.

Today I was reading an engineering systems mag. There was a good article on noise levels, much on how noise is measured, the higher the velocity, the higher the noise level…

If someone could expound on this more, it would be very much appreciated…I could use more discussion here?

Dusty Books. You can have all the information you really need, or don’t need, by taking a few minutes in the old bookstores. I picked up a couple of books published in the 50’s on HVAC, I purchased a book “Heating and Air Conditioning” from 1935, 400 pages of knowledge for $6.00, I also found the book “Air Conditioning and Engineering” edited and published by The American Blower Corporation 1955. I can’t remember the exact cost, but I’m thinking under 10.00? These are valuable resources for learning much of this and I’d like to learn more on airflow through discussion. Another book with a great section on airflow is the book titled “How to design heating cooling comfort systems” by Business News Publishing ISBN is 0-912524-36-7

I have several books and the advantage of this is, the way things are explained, for me it’s easier if I get a couple different explanations or view points on a particular subject

gmacw

02-26-2005, 08:52 PM

Bud, Thanks for the quick reply and the information. Let me ask you this... If the specifications call for duct to be 4" static & the unit is to deliver say 40,000 cfms, what would you slide the ductulator to if you wanted to find starting duct size? I mean It's not as simple as .1 is 1" of static.. Is it? That's what I mean by correlation between static pressure and a ductulator. :wink:

tnbndr

02-27-2005, 01:54 PM

gmacw:

There is no direct relationship between the static pressure and the friction loss on the ductulator. You need more information. The rated static pressure is what is needed to overcome restrictions in the duct design and layout. The static pressure when applied to the duct design manual (SMACNA) leads to how the duct must be constructed, gauge, stiffeners,locks, etc, to hold up to the designated static pressure.

If you are working with a system desinged by another engineer, ask them what their design parameters were, friction loss or velocity. You can then apply these to the ductulator. If you have a duct size and you know the cfm carried you can work backwards on the ductulator.

Some rough guidlines are <3"SP use .08-.1 friction loss and don't exceed 1000fpm. 3"&>SP use .20 friction loss and don't exceed 1800fpm. Special systems such as kitchen exhaust, fume exhaust have other parameters to watch for.

As Bud suggests old books or SMACNA manuals contain a lot of information.

Hope this helps. :)

It was my understanding (and correct me if I’m wrong) that the numbers such as .1, .08, .2, etc on the scale of “Friction per 100 feet of duct” is that of the static pressure necessary to over come the friction of 100 ft of duct?

As stated in the earlier post

(The static head is which is required to overcome the frictional resistance of the air against the surface of the duct.)

So the numbers are that of the “Static” and if you’re going to look into designing the system with 1 inch of static? You would dial to the .10 or 2 inches of static, you’d dial to .20?

bordontn

02-27-2005, 07:39 PM

Bud, may I comment again? We design our residential at .06 return and .08 for the supply. Actually called friction loss measured in inches of water guage.That would be .008 per foot of duct. If you add up the values of each fitting plus total length of duct you get your total S.P. F'rinstance: 90°ell = 10'/round take-off=35'/90°boot=30'

straight boot w/90°ell=15'.Anyway most package units are rated at 2"or better.If your unit won't produce, then you can increase your duct size; staighten some turns.

You can pick a few extra feet in your favor by using rect. to round take-offs.15' per..

bordontn

Thanks everyone, it is how we get smarter, what a great way to hone in on the skill. I spent the past 2 hours reading on duct desing (again) and forgot a couple fundamental principles on pressure drop or what is known on the ductulator as friction loss..:) Thanks for all the great input, it keeps the wheels rolling and our minds sharp.

DennisShannon

03-04-2005, 05:48 AM

Ok, let's go with this.

First you should remember that your duculator is specific to the length

(100') so the following formula's can apply which are quite simple

FL1, being the friction loss for 100' exactly on your duculator

FL2, being the total friction loss in all duct's accumulated

L, being the length of the duct input.

FL2= FL1x100divided by 100

or

FL1= FL2x100divided by the length

of course you would need to know the friction and that can be found

by the following

v which is the velocity of the air expressed in fpm = 4005 x the square

root of the vp or velocity pressure

and vp or velocity pressure can be found by this formula

vp = velocity divided by 4005 then squared

If you need more please call and I will fax you as much as you need.

Regard's,

Dennis Shannon

DennisShannon

03-04-2005, 06:03 AM

Ok, let's go with this.

First you should remember that your duculator is specific to the length

(100') so the following formula's can apply which are quite simple

FL1, being the friction loss for 100' exactly on your duculator

FL2, being the total friction loss in all duct's accumulated

L, being the length of the duct input.

FL2= FL1x100divided by 100

or

FL1= FL2x100divided by the length

of course you would need to know the friction and that can be found

by the following

v which is the velocity of the air expressed in fpm = 4005 x the square

root of the vp or velocity pressure

and vp or velocity pressure can be found by this formula

vp = velocity divided by 4005 then squared

If you need more please call and I will fax you as much as you need.

Regard's,

Dennis ShannonI was half asleep and made a mistake for formula

FL2 = FL1x length divided by 100

sorry I don't drink coffee in the morning

Dennis Shannon

DennisShannon

03-06-2005, 09:18 AM

Hello,

To follow up on the question's about duct static pressure please

see the following from SMACNA , first under HVAC duct construction

standard's metal,flexible 2nd ed. it states " each duct system shall

be constructed for the specific duct classification shown on the contract

drawing's,where no pressure classes are specified by the designer,

the 1" water gauge(250Pa) pressure class is the basis of compliance

with these standard's, regardless of velocity in the duct. Except when

the duct is variable volume. All variable volume duct upstream of VAV

boxes has a 2"(500Pa) basis of compliance when the designer does

not give a pressure class.

Also, for fibrous duct construction standards the maximum static

pressure should not exceed 2" w.g. (498Pa), positive or negative.

And the maximum air velocity should not exceed 2,400'/min(12m/sec)

I hope this can be of help to anyone that does not have this information

readily available.

Regard's,

Dennis Shannon

Thanks Dennis, it is a big help. How does this correlate with the dial type - duct calculator? Is this implying that the duct calculator shouldn’t be used? Can we dial to our necessary velocity (right side) locate our CFM and from the top see if we are in the static pressure range by the friction 100 feet per duct by the CFM relationship? Example, if we want 1000 fpm and a CFM of 1000, can we assume that our static pressure is slightly above .1 from the friction per 100 feet?

DennisShannon

03-07-2005, 08:34 AM

Good morning,in no way would I ever say that a ductulator should not be used in fact just the opposite, it,s just that many of us have used it to our own desire's,

meaning we can spin a ductulator any way we want, but our number's may not be the proper figures to be used,in your example you in some respect's would be going

backword's since you are trying to find static pressure through number's you are

indiscriminately giving to the ductulator . Alway's remember that we really need to

know either the static pressure which would be the expression in my previous formula's Vp, or the fpm as expressed by v and of course we also need to know the

length of the duct's themselves to be able to use the aforementioned formula's.

The reason I seem to have the answer's right now is because I am preparing to

take my mechanical contractor's test in the State of Florida and many of these

factor's are sitting right in front of me.

Regard,s

Dennis Shannon

EBEBRATT

04-30-2005, 06:39 PM

Designing ductwork- if the specifications call for a duct to be rated for 2" of static pressure this refers to the construction of the duct to withstand this pressure. It has nothing directly to do with the volume of air being moved. We hope that the designer has sized the system so that the required quantity of air can be moved without exceeding this static pressure at the fan outlet. A basketball filled with air is static pressure (air not moving)

Friction per/100 ft and the ductulator or calculations are used to size the duct for the air volume needed. The duct must be sized so the quantity of air can be moved without exceeding the design static. Velocity (noise ) usually determines the ducts minimum size.

Ed

bordontn

05-02-2005, 06:59 PM

The Ductulator takes the place of 3 or 4 pages of tables and charts published by ASHRAE years ago..Someone just re-invented the wheel ! Old timers probably remember the square root table. Nowdays you just plug your number into the calculator and hit the square root sign. Try to design a duct system using those old tables instead of the wheel. I think you'll learn to use it. :idea:

bordontn

russhardy

01-29-2006, 05:13 PM

How do you determine the pressure calssification table to use from the smacna manual? If the designer does not specify I know it says to use the 1" chart but how is that actually determined? Example: you have an existing rooftop hvac unit that you are designing and sizing the ductwork for.....you know the cfm for the rooms and you know the velocity you want to achieve.....what tells you which chart to base your guaging and joint reinforcement etc on? Do you add up all of the friction loss in the system and then if it is under .5" you would use the .5" chart or how is it done?

tanner

02-01-2006, 12:22 PM

If you are building ductwork for an existing unit or for one that has been specified then your best source of the fans ability is from the manufacture. They will provide you with a fan curve which is nothing more than the CFM the unit provides at a certain static pressure. If the equipment is small commercial or residential most times the maximum External Static Pressure (ESP) is around .5 for the full flow of air. For example a 5 ton unit moving 2000 CFM at .5 ESP with a direct drive motor may only move 1850 CFM at .7. So just because they may not want to be above .5 one too many offsets or elbows can cause it. Fans are usually selected after ductwork sizes are designed for air noise and flow then calculated for the total pressure loss and ordered with the correct motor and drives, not the case with a direct drive motor you are stuck with what ever the performance is. In that case you layout the ductwork single line and determine your longest run to include fitting equivalent lengths. This method is recommend for duct runs up to 900 FPM. Remember not to add parallel ductwork together only the longest supply run and the longest return run. If there is enough pressure to go from the unit to the longest equivalent run then there is enough pressure to go the shortest distance, this is because pressure expands in all directions equally and the reason for dampers is to create a pressure drop. Sort of like adding a long piece of ductwork. If in doubt of the pressure class to use, with most units under 15 tons use 2". The stiffness will guarantee no call back or issues to deal with.

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