Duct Losses vs Static Pressure
Duct Losses vs Static Pressure
(OP)
I am new to the HVAC realm and just visiting it at that. I have a system that I am designing which is probably most similar to a car's. I need to spec an Air Handler and my biggest concern as opposed to cooling power is the flow rate coming out at the end ports. When I run through calculations using Equal Friction, I get worst case losses of just under 6 in H2O. All the air handlers I come across have static friction ratings of .5 most commonly and with some changes that can get into the 1.x's but now where near my 6inH2O.
Does this mean if I use one of these AHU's with my system that I will not get the flow I need? I dont really understand the implications of the static pressure of the AHU. Is it strictly a "must supply more static pressure" than losses? Or is there another way to look at it that will mean my system could work afterall?
Thanks for any help,
Signed,
"GOTNOCLUE"
Does this mean if I use one of these AHU's with my system that I will not get the flow I need? I dont really understand the implications of the static pressure of the AHU. Is it strictly a "must supply more static pressure" than losses? Or is there another way to look at it that will mean my system could work afterall?
Thanks for any help,
Signed,
"GOTNOCLUE"





RE: Duct Losses vs Static Pressure
As with pumping water, the system static losses will be the total of the worst case run.
This is usually the longest run, but not always, some shorter runs may have higher restriction fittings, etc...
Upsizing the duct, minimizing flex duct, full sweep elbows, turning vanes in square elbows, & more, can relieve losses.
The losses are based on a "per 100'" length.
a 50' duct at .08"/100' will be .5x.08= .04"WC lost in that portion of the duct.
Trying to find a blower to overcome 6" TSP will be an expensive task, & cost of operation of that blower will be expensive. It will also add to the load as the heat added to the airstream (if it is in the airstream) will need cooling as well.
How is this system similar to a cars? (Hot water/DX?)
RE: Duct Losses vs Static Pressure
Im trying to get a handle on the whole thing, if my blower TSP is too low, what happens? Does flow stall? Do I just get a lesser flow? I am sure there may be different results for different situations, but I am just curious in general.
I going to make sure I accounted properly for the "per 100'" definition. But anything else you can add would be greatly appreciated.
Thanks for the help, I really appreciate it
"GOTNOCLUE"
RE: Duct Losses vs Static Pressure
You may look at resizing the supply duct to the nozzle outlets to reduce your critical static pressure.
RE: Duct Losses vs Static Pressure
How many cfm do you figure for the system?
How many tons cooling?
RE: Duct Losses vs Static Pressure
Total cfm we come up with is around 1600, 23K BTU/hr. Largest duct is 4"dia with couple as small as 1.25"dia. If anything here looks too wacky, just let me know.
RE: Duct Losses vs Static Pressure
1600 cfm for 23000 btu/hr is ok. For 1600 cfm, with a 4" size duct, you should get ridiculously high pressure drops, many multiples of 6.
RE: Duct Losses vs Static Pressure
RE: Duct Losses vs Static Pressure
Thanks again for all the help, if I seem frustrated, it's all at the numbers, I appreciate all the input. And as you can see have a lot to understand with this stuff.
Thanks,
Mike
RE: Duct Losses vs Static Pressure
Anyway the major manufacturers should be able to accommodate you.
RE: Duct Losses vs Static Pressure
Your previous post said you had 10 outlets, correct?
Are you hoping to deliver similar amounts of air to each diffuser? Are there any balancing dampers on any of these branch ducts, or was this originally designed to deliver measured amounts based on the varying duct sizes/lengths?
(Figuring out how someone else designed an older installed system is like reading a dirty crystal ball...)
What is the reason for 4 Tons of airflow for 2 tons of cooling?
In my book 400 cfm per Ton is the rule. If you can get by with less air, you may be able to cut your TSP in half or more, & still deliver the conditioning required.
Even in high velocity, Space-Pak type systems, the airflow is lower (prx 250 cfm per ton I'm thinking) These system will run upward of 2" TSP & care less about velocity, static, & noise. That's why a typical high velocity system will have a 2HP blower for 2-1/2 tons vs. a 1/2HP blower in a conventional system.
What is the air velocity across the coil. If your moving the air too fast you'll loose the latent heat extraction & never control humidity. The coil will barely be wet & the drain won't be needed, conversly too slow a face velocity will lead to coil freeze up, no airflow & compressor shutdown on pressures etc...
good luck,
slj
RE: Duct Losses vs Static Pressure
As far as the coil velocity goes, that again, not something I have a good grasp on. I have a system that I am looking at that will provide the airflow and cooling I need (strictly speaking in cfm and wattage specs), but with the ESP issue, I get concerned about the effects; in my understanding, if we are overloading the systems's pressure, we will influence the flow across coils and begin to address your concern there. Maybe I'm wrong.
Can anyone give me an HVAC knowledge infusion?
Thanks 12000 BTU/hr again ... anything anyone can add is great
RE: Duct Losses vs Static Pressure
The duct size and volumes make a lot of my flags go up at 3" sp. 7' of 4" duct handling 250 cfm is about 0.35" wc duct loss only. Coil is likely 0.5-1.0" wc, so we're still way under 3" wc.
Does the system have a filter? What kind?
RE: Duct Losses vs Static Pressure
I am trying to basically "plug" an AHU in as long as it can give me what I need.
I hope Im not frustrating too many out there, bear with me.
RE: Duct Losses vs Static Pressure
TSP & ESP, That is the question!
TSP stands for Total Static Pressure.
ESP stands for External Static Pressure.
Problems arise in asking "External to What?"
Some Mfgrs regard it as external to the AHU/System,
others say external to the blower.
I've found it difficult to get a Mfgr to commit to the ESP question & I've debated & changed my mind a number of times on this issue.
Since some AHU's have coils & filters built in, they can be considered part of the unit/system (that the SP is external to)
Some will take the filter at .1-.25 inch (clean to dirty) & throw in another .1-.2 for a damp/clean coil.
I usually give the coil to the system, but I think the filter should always be considered external to the system, as it is one of the most restrictive components (except for small duct & bad fittings)& it's pressure loss against the blower slides back & forth based on it's condition.
If anyone else can explain TSP & ESP otherwise, I'm willing to change my mind again.
Again, Good Luck!
slj
RE: Duct Losses vs Static Pressure
If the AHU purveyor gives you some flack that his is better than the other's, guess what he's selling............
Physics is independent of the manufacturer.
RE: Duct Losses vs Static Pressure
1. 0.50" wg total external static pressure produces 1000 cfm
minus 0.10 " wg design supply side resistance (DUCT WORK)
=0.40 wg
2. -0.05 wg design return side resistance
= 0.35 wg static pressure remaining
3. -0.20 wg wetted coil resistance
= 0.15 wg static pressure remaining to produce 1000cfm
4. -0.05 wg resistance fo clean filter
= 0.10 wg remaining to produce 1000 cfm
5. -0.08 wg for dirty filter resistance
= 0.02 wg static pressure remaining for 1000cfm
Now if the external static pressure is exceeded because of poor installation, or other conditions then the cfm produced by the system will drop.
There are charts avialible to determine what the total pressure drop will be when you figure the total equivelent run of the longest supply and return duct.
Example: say on the return duct with a design pressure drop of 0.05 wg you determine that the total equivilent length(TEL) is 95-104 ft that is = to a pressure drop of 0.05
wg which is perfect, now for the supply duct, the design is 0.10 "wg so if the(TEL)is say130-150 ft the total pressure drop = 0.07 " wg
Rmember all lengths of duct and all fittings for turning , reductions, etc. havd a TEL of duct that must be added to the regular length of the duct run. hope this helps!
RE: Duct Losses vs Static Pressure
I'm printing a copy to hand out to a few I work with.
Not to pick nits with your example..
"return duct with a design pressure drop of 0.05 wg you determine that the total equivilent length(TEL) is 95-104 ft that is = to a pressure drop of 0.05"... supply duct, the design is 0.10 "wg so if the(TEL)is say 130-150 ft the total pressure drop = 0.07" wg"
The S.A. duct will have a pd of .13 to .15 call it .14,
added this to the R.A. pd giving a pd of .24 for duct system.
Unless these items have been included in the TEL calcs, you will have to add the pd's of the coils, filters, diffusers & grilles, dampers, etc... & TSP will be known. now you can select the blower.
Again certain mfgrs of equipment will rate the AHU with ESPs external to the built in components (coils & occasionally clean filters) & others will rate the blower alone.
good luck
slj
RE: Duct Losses vs Static Pressure
When you mentioned taking the worst-case run for determining system requirements, that was one of the things I understood, but with respect to that, I was looking at balancing the system which I have also come across and makes sense to me ... I think. What doesnt make sense is that I was just thinking about the whole "balancing act" (like that?) and from the way I unsderstand it, if your system is balanced, aren't ALL runs going to be approx equal? Again, not questioning anything, just trying to make sense of this.
thanks,
Mike
RE: Duct Losses vs Static Pressure
Balancing is done by introducing an artificial pressure drop with a damper in individual run-outs.
Balanced does not mean equal flow on every branch.
RE: Duct Losses vs Static Pressure
Please any and all correct my thought process if gone astray. Not challanging your point willard3, just trying to understand it.
Thanks for your help,
Mike
RE: Duct Losses vs Static Pressure
I kind of hear you there, but I wasnt meaning equal "flow" but nearly equivelant losses.
snip
This is equal friction design and is the basis of the Trane and etc wheels.
There are always physical constraints for ductwork size and aspect ratio. Air has lousy specific heat and it takes lots of it in big pipes to heat or cool. Ventilation also takes high air volumes.
HVAC systems are contained within other structures and all systems compete for volume.
Given the above, it's not always possible to have equal friction ductwork and compromises are made. It's the Engineer's job to make sure it works.
RE: Duct Losses vs Static Pressure
Appreciate it,
Mike