# Thread: How are unit static pressure is REALLY selected?

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## How are unit static pressure is REALLY selected?

I'm just trying to get a general idea of how other people size their units, specifically the static pressure.

As a preface, I know Fluid Dynamics relatively well and understand what static pressure, velocity pressure and dynamic pressure are. What I'm wondering is how companies(building engineers, design build contractors and/or consultants) select their units and what kind of analysis is done.

What I think actually happens is people select units based on experience. They know the amount of ductwork involved and select a static pressure accordingly.

Does anyone actually calculate the pressure losses associated with the Reynolds Number, flow rate, friction factor, ductwork material, etc?

In general, how do you select your units static pressure?

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.08"-.1"

3. General considerations for selecting the static pressure of a duct design are :

- Application of equipment , office , factory , constant volume , variable volume , institutional , etc.

- Amount of space available for supply duct

- Noise considerations

- Length of run

- Type of duct material

- Location of duct in relation to conditioned space

- Type of supply air fan , centrifugal (forward curved , backward incline , airfoil) , tube or vane axial

- There are so many considerations , entire books have been written on the subject.

- For residential and light commercial , our prefered duct design static was .08 to .1 in wc. we prefer high volume , low velocity , low noise

Sometimes you need a good throw to the air from an outlet , maybe a sidewall grill in a large open space , there you may want to design to a higher pressure and velocity.

Basically , you can size and design your static pressure all the up to what the fan curve for that piece of equipment tells you. A particular blower may deliver its rated CFM up to a specified pressure level.

4. They usually imagineer it and then the air balance contractor finds the real static pressure required to make design airflow. It's called Value Engineering!

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Originally Posted by duke of earl
They usually imagineer it and then the air balance contractor finds the real static pressure required to make design airflow. It's called Value Engineering!
This is usually my experience! Guess work at best. Engineer's plan a design, it gets tweaked during construction, then commissioning happens. Then, its the T&B and the manufacturers problem!

6. Originally Posted by OVOleg
I'm just trying to get a general idea of how other people size their units, specifically the static pressure.

As a preface, I know Fluid Dynamics relatively well and understand what static pressure, velocity pressure and dynamic pressure are. What I'm wondering is how companies(building engineers, design build contractors and/or consultants) select their units and what kind of analysis is done.

What I think actually happens is people select units based on experience. They know the amount of ductwork involved and select a static pressure accordingly.

Does anyone actually calculate the pressure losses associated with the Reynolds Number, flow rate, friction factor, ductwork material, etc?

In general, how do you select your units static pressure?
The best answer would be to buy the Manual D book that specifies proper procedures for duct design. The standard max ESP(external static pressure) for psc blower motors is 0.5" w.c., ECM(electronically commutated motor) typically has max of 0.8" w.c. You'll find this info under the blower performance chart in product specs. Also don't get confused like a lot of guys due between friction rate and ESP. An acceptable friction rate is 0.08-0.16" w.c. Again, best and most thorough answer is to read the Manual D. As far as how contractors design duct, it varies. Some municipalities require a submittal of your Manual D work sheets or ACCA approved software like wrightsoft or elite. In my current area no one inspects duct design so contractors use a ductulator set at 0.1" w.c., usually they don't have problems because they keep the duct work simple and use dampers to avoid hot and cold spots. The software costs a very good chunk of change and to do it by hand takes 8-9 man hours, that's when you know what your doing, if noone else is doing it than you can lose money by not including the cost in your job or lose the job by having a higher price, plus they would have to take the time to actually learn how to do it.

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## It all depends!!!!

With residential and small commercial you have direct drive fan motors with a set amount of static pressure available for the quantity of air you want to move, so you size the duct so that the most difficult path the air travels will have enough pressure to get there.
with Commercial we do a reasonable design and then add up the pressure needed and order the unit with a fan motor big enough to do the job.

One of the interesting things about duct sizing/ design is that most people are focused on the duct material when it is the efficiency of the fittings that really matters. Also ther is a situation call system effect losses that cosider the first few feet off of the high velocity discharge of some airhandlers/ fans. If this first few feet to duct is not done right, 1/3 or more of fan HP can be lost.

Lots to understand, Oh and remember, there is a big difference in the fluid water vs air when you start to dig in

8. I always do the static pressure loss calculations by hand. I lay out the system, size the ducts, and then check for total loss. Then compare against mfr. literature to make sure I will get the air I need. After doing thousands over the year I can pretty much look at a design and tell you what it will be. But sure enough, the one time I don't check by doing the calc, my guess will be wrong.

And don't forget to include loss through filter, wet coil, and economizer, if you have one.

Years ago I made some excel spread sheets for hydronic loss and another for duct loss. That was the only time I ever payed attention to reynolds numbers.

I wish more people would learn how to properly size duct and total esp...the industry would be way better off...as well as our customers.....and it's really so simple even a caveman can do it.

So for me, I don't select the static pressure, it's a result of how I design it.

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Originally Posted by mason
The best answer would be to buy the Manual D book that specifies proper procedures for duct design. The standard max ESP(external static pressure) for psc blower motors is 0.5" w.c., ECM(electronically commutated motor) typically has max of 0.8" w.c. You'll find this info under the blower performance chart in product specs. Also don't get confused like a lot of guys due between friction rate and ESP. An acceptable friction rate is 0.08-0.16" w.c. Again, best and most thorough answer is to read the Manual D. As far as how contractors design duct, it varies. Some municipalities require a submittal of your Manual D work sheets or ACCA approved software like wrightsoft or elite. In my current area no one inspects duct design so contractors use a ductulator set at 0.1" w.c., usually they don't have problems because they keep the duct work simple and use dampers to avoid hot and cold spots. The software costs a very good chunk of change and to do it by hand takes 8-9 man hours, that's when you know what your doing, if noone else is doing it than you can lose money by not including the cost in your job or lose the job by having a higher price, plus they would have to take the time to actually learn how to do it.
Great advice thanks. Going to pick up a copy of that book soon

10. I agree with both the posts that say the fittings are very impotent and you can look at a design and know if it works. I also do my worksheets by hand and some of the other guys I've talked to or had class's with think I'm a crazy, good to know I'm not alone : ). To be honest even if you don't do or have any interest in design of ductwork you should still have an understanding of how it works. I've seen many techs stumped because they don't know what ESP is or how to use the return and supply w.c reading for diagnosing a problem, loose insulation anyone?

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Check out 2009 ASHRAE Fundamentals Chapter 21. Duct Design. That will point you in the right direction. Fan inlet and outlet conditions along with fittings are critical.

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Originally Posted by jb-eng
Check out 2009 ASHRAE Fundamentals Chapter 21. Duct Design. That will point you in the right direction. Fan inlet and outlet conditions along with fittings are critical.
Thanks let me check to see if my boss has it. Any good place to order the ASHRAE books? I have been looking for awhile.

Just got Manual D(pretty expensive but I'll read it either way).

Thanks!

13. ## Reality

Here is what happens in my world.

1. Engineer picks equipment that he has been using since the late 70's

2. Designs for cfm/sq.ft.....rule of thumb stuff........

3. Inserts duct at the same dim and velocity that his last job was.

4. He does not do 3d cad so all of his duct goes through sprinkler pipe, gas lines,Transformer and panel conduits,elevator shafts, structural beams

5. They know it isnt going to work but hey, they got the job done and turned in on schedule.

6. I redo the design but not enough to require re-engineering, but enough to deliver air to plan so the TAB comes in, then when I get get asked why I deviated from the plan I just ask "DO YOU REALLY WANT ME TO DO IT LIKE THIS!"

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