# Thread: Manual D equivalent length

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## Manual D equivalent length

I am curious how correct is it to use equivalent lengths for the fittings?
If you wanna be precise, equivalent length would depend on a duct size and velocity, so you cant use the same value for different diameter ducts. The difference in actual pressure drop would be huge.

Also I couldn’t find any good data for register boots. Pressure drop will obviously depend on an aspect ratio as well as ratio of a rectangular to round transition. How can they suggest to use the same equivalent length?

And what’s the deal with no pressure loss data for an angle with only outside corner rounded? This is the mostly used fitting!

I have a feeling that manual D is just a step up from a rule of thumb but nothing close to the real world and a real engineering.

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## Depth of understanding dP loss calcs ... see ASHRAE

Originally Posted by serik
I am curious how correct is it to use equivalent lengths for the fittings?

If you wanna be precise, equivalent length would depend on a duct size and velocity, so you cant use the same value for different diameter ducts.

Also I couldn’t find any good data for register boots.

And what’s the deal with no pressure loss data for an angle with only outside corner rounded?

I have a feeling that manual D is just a step up from a rule of thumb but nothing close to the real world and a real engineering.
ACCA MANUAL D IS a very nice 'cookbook method' based on Real engineering principles that allows one to consistenly determine acceptable duct layouts.

I F YOU WISH to study further, ASHRAE has many guidelines and books which are a little more complex to implement in a calculation.

Selection of fitting equivalent length is the responsibility of the user.
Many fittings ( unique boots , ...) are not addressed with specifc L/D factors. However, with experience you may select the appropriate dP values that fit Your applications.
If not you can test and "back-fit" into an updated analysis to develop your own L/D factors that you can use in the future.

One needs to study system pressure drop, definition and use of L/D's (equivalent length) to have a real appreciation for guidelines such as Manual D.

http://www.ashrae.org/education/page/1470

https://eweb.ashrae.org/eweb/Dynamic...&#39;%25146%25')

xx

3. In this energy conservation age, why does ACCA feel they need big money for any information they provide.

Every working Tech & Contractor needs all the information they can get access to. Money should not be a barrier to much needed Tech information.

However, ACCA copyrights everything, even though most of it is general math, physics & science knowledge that should be taught on every level of education throughout America.

In this age of electronic communications, Internet & broadcasting, there is no need for ignorance where knowledge aught be instantly available & free to all.

When user's realize there HVAC systems' have problems, contractors would get called & result in more work during the seasonal slack times.

Many HVAC system user's don't know, due to many factors, that their 5-ton system may only be delivering 3.5-Ton or less of cooling.
(But Complain, it's Not Big Enough!)
Many times it's due to lack of airflow due to ill designed duct systems.
(Along with duct hot air leakage into Returns &/or Supply leakage)

Ductwork systems & airflow are a most critical area toward full nominal performance & efficient operating conditions. The other area of neglect is the refrigerant system.
- Darrell

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Well, in a first place, the correct definition of L/D is an equivalent length of a fitting in pipe diameters, not just an equivalent length.

I have an ASHRAE fundamental handbook and most tables for Co include such things as Area and Flow ratios for Ts and Wyes and reducers. What I am trying to say there is no way to do a correct estimation by using only L/D and not accounting for those other things.

Originally Posted by dan sw fl
If not you can test and "back-fit" into an updated analysis to develop your own L/D factors that you can use in the future.
How many HVAC techs have you seen even using manuals? Now I am trying to imagine them doing field tests and measuring pressure drops and analyzing data, i.e. performing aerodynamics research...

Common! Most of them are high school dropouts and have no basic knowledge or understanding of any engineering principles what’s so ever. All they have is a surface knowledge of how to do thing A and thing B without understanding what's behind. And even that took them dozen times to get it right, and some still did not get it.

I agree to udarrell, all those acca manuals are way overpriced for no reason. We are complaining about health care costs, what about cost of education? No book should cost more than 5-10 dollars. Its all part of the same scam as lawers, insurances, liability, HOAs etc... Designed to suck money out of people and spend on the things they dont need.

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## The brains in the field ...

Originally Posted by serik

How many HVAC techs have you seen even using manuals? Now I am trying to imagine them doing field tests and measuring pressure drops and analyzing data, i.e. performing aerodynamics research...
AND there are also 10s of thousands of Air Balancers (i.e, former techs) who do KNOW exactly what affects air flow ! !!

6. Originally Posted by dan sw fl
AND there are also 10s of thousands of Air Balancers (i.e, former techs) who do KNOW exactly what affects air flow ! !!
The Air Balancer Engineers primarily work in the coml arena, I've never seen one in residential, & we need them there, too.

We appreciate you Dan, however, we have a chaotic mess in residential with very few doing squat toward correcting all the horrendous duct system & airflow problems
. - Darrell

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Originally Posted by serik
I am curious how correct is it to use equivalent lengths for the fittings?
If you wanna be precise, equivalent length would depend on a duct size and velocity, so you cant use the same value for different diameter ducts. The difference in actual pressure drop would be huge.

Also I couldn’t find any good data for register boots. Pressure drop will obviously depend on an aspect ratio as well as ratio of a rectangular to round transition. How can they suggest to use the same equivalent length?

And what’s the deal with no pressure loss data for an angle with only outside corner rounded? This is the mostly used fitting!

I have a feeling that manual D is just a step up from a rule of thumb but nothing close to the real world and a real engineering.
Your statements show your lack of real field experience. So, you feel you have the right to judge techs in this industry based on your test scores for school? Manual D cost about 65 bucks which could EASILY be recooped on one job. Get off of your soapbox and get your hands dirty.

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I am not a pro and dont have much experience with HVAC, but I understand engineering and science behind it. I never saw an actual manual D except for a few pages I could find online. So my statements are based on the limited information I have. And I believe they are right when it comes to the fact, that manuals are incomplete and lack many fittings, use too many approximations and ignore some details. How am I supposed to model a pressure drop across the different aspect ratio register boots??? Do they expect me to do a CFD on it? or field measurements on the design stage? when nothing is in place yet?

Hi five to udarerll!

You are absolutely correct, average residential construction in US, is of unacceptable quality. That includes not just HVAC but everything from structural to electric. I am so pissed off because I realized how badly my house is built, considering how much money I paid to the builder...

We need a lot better codes and better enforcement, seems like inspectors dont have much time and just assume that licensed contractor did it right. Thats all they care about, if he has a license. And they built it to look nice on the outside, but inside is always a mess and tons of flaws.

And lets not even talk about labor rates all the contractors charge. I don’t see why somebody without a college degree should be charging more money per hour than a PhD is making.

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## Resource search

Originally Posted by serik
I am not a pro and dont have much experience with HVAC, but I understand engineering and science behind it.

I never saw an actual manual D except for a few pages I could find online.

So my statements are based on the limited information I have. And I believe they are right when it comes to the fact, that manuals are incomplete and lack many fittings, use too many approximations and ignore some details.

How am I supposed to model a pressure drop across the different aspect ratio register boots??? Do they expect me to do a CFD on it? or field measurements on the design stage? when nothing is in place yet?
How many manufacturers manuals, ACCA and ASHRAE handbooks/ publications/ documents have you reviewed IN DETAIL ?

http://www.engineeringtoolbox.com/du...ss-d_1122.html

http://www.engineeringtoolbox.com/du...oss-d_444.html

http://www.hvactools.net/products/as...-8910-08.shtml

http://www.mcgillairflow.com/textDocs/productsPG.htm

Please name more than another dozen references to indicate that you are at least making a small effort to avail yourself on these resources ! ?!
Last edited by dan sw fl; 07-31-2009 at 12:45 PM.

10. Originally Posted by serik
I am not a pro and dont have much experience with HVAC, but I understand engineering and science behind it. I never saw an actual manual D except for a few pages I could find online. So my statements are based on the limited information I have.
You read a couple pages of a book. Have no experience, But decided the book is wrong.

Go out and buy a manometer and probes. Then do some PD testing, and tell us how wrong you are.

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I read a few books including ASHRAE fundamentals, Andel HVAC Fundamentals, Handbook of air conditioning, heating, and ventilating; SMACNA etc.

Those are all good resources, I am not opposing, but my main concern is that there is no universal book or manual which would cover all the scenarios.
So its needed to combine info from multiple books and multiple editions just to get all the required data for all the fittings. Form this experience, I am sure that even 200 dollar CD with 200 fittings will not have all the fittings I need, which are basic fittings by the way, nothing fancy. I dont understand why is it so hard to find any data on register boots for example.

More to it, each book uses different definitions, and many omit important details, like units. And the fact that you need to read 10+ books to figure out simple things, proves that there is no good book or manual which would cover all the aspects of the matter.

I am a perfectionist and want to build the system which would perform to its highest possible potential. So making an assumption, even for a single fitting does not sound acceptable to me.

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## Realism is needed to intregrate components into a system and knowing

Originally Posted by serik
I read a few books including ASHRAE fundamentals, Andel HVAC Fundamentals, Handbook of air conditioning, heating, and ventilating; SMACNA etc.

Those are all good resources, I am not opposing, but my main concern is that there is no universal book or manual which would cover all the scenarios.

So its needed to combine info from multiple books and multiple editions just to get all the required data for all the fittings. From this experience, I am sure that even 200 dollar CD with 200 fittings will not have all the fittings I need, which are basic fittings by the way, nothing fancy. I dont understand why is it so hard to find any data on register boots for example.

More to it, each book uses different definitions, and many omit important details, like units. And the fact that you need to read 10+ books to figure out simple things, proves that there is no good book or manual which would cover all the aspects of the matter.

I am a perfectionist and want to build the system which would perform to its highest possible potential. So making an assumption, even for a single fitting does not sound acceptable to me.
that A calculation is simply a guide on how to design in all fields.

dP for Register boots are even provided on a TRANE (or other) ductulator which is a Very common HVAC tool.

components ...
http://www.hartandcooley.com/grd/all_grd.htm

Inability to formulate a fairly accurate, reasonable calculation indicates lack of judgment, inability to match fitting K values to various components and inadequate comprehesion of basic flow principles. If a flow calculation is off by 15% and one knows how to design a system through good air distribution and appropriate equipment selection, the system will function properly.

Perfectionism may belong in the science fields, however engineering based on simple physical princples remains an art form in the design world.

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My copy of Manual D has 15 boot fittings with equivalent lengths.

Manual D was written before the laptop computer was available. Fittings were given equivalent lengths for ease of computation. Exactness is less critical for residential because of the low velocity airflows.

Different organizations spend money to characterize duct fittings. It does take time and money to do that. That is why it takes a Manual D, SMACNA, ASHRAE, ACCA, etc., to build a library.

The reason that the references are expensive is that they are printed in low run production. There isn't the capture of overhead that happens with a best seller of a million copies.

I calculate fittings with either equivalent length or coefficent C. My computer calculates velocities and quantity of air based on the fan.

If it is assumed that Manual D follows it's own recommendations for air velocity, then a coefficient C can be derived from the equivalent lengths. Then the calculation can be done for any velocity.

This is not something that is calculable perfection. Upstream fittings affect turbulent airflow for downstream fittings. There is no 5.31 inch diameter round duct to achieve perfect velocity and quantity of airflow. The standard of performance is 15%.

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