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how to measure static pressure of AHU

how to measure static pressure of AHU

how to measure static pressure of AHU

(OP)
May I ask how to measure the static pressure values of an AHU? Thanks

External pressure of AHU


Total pressure of AHU or static pressure of fan unit


Static pressure of fan inlet


Static pressure of fan outlet


Static pressure of upstream of filter


Static pressure of downstream of filter

RE: how to measure static pressure of AHU

(OP)
Am I correct?

RE: how to measure static pressure of AHU

Nice graphics! What was the question? Static pressure can be measured in any section of an AHU.

RE: how to measure static pressure of AHU

(OP)
sorry, the picture cannot be shown in above post. my question is that, if i use an manometer to measure the static pressure at the position shown in the picture, is it right? the picture is re-posted in the below. sorry and thanks

External pressure of AHU

link: https://lh5.googleusercontent.com/R2zXGkAU_7Xwk9Nv...

Total pressure of AHU or static pressure of fan unit

link: https://lh5.googleusercontent.com/rhsyn7ftRnmzR200...

Static pressure of fan inlet

link: https://lh6.googleusercontent.com/_gaNpS0JKjKGgChs...

Static pressure of fan outlet

link: https://lh4.googleusercontent.com/8Vg0o0JhVC82yrM8...

Static pressure of upstream of filter

link: https://lh6.googleusercontent.com/H-gEBRG7h82csKF1...

Static pressure of downstream of filter

link: https://lh3.googleusercontent.com/CBPNGzRbr3Zj29Fn...

If the pictures cannnot be shown, please click the links below the pictures. I don't know why sometime they can be shown, sometime cannot. Sorry.

RE: how to measure static pressure of AHU

All your links are error 403 for me. It's very simple. The measured static pressure at the fan outlet is the ESP (external static pressure). This is the pressure in the system downstream of the fan. The difference in pressure between the immediate inlet and outlet of the fan is the TSP (total static pressure). The TSP is all that matters when sizing fans.

RE: how to measure static pressure of AHU

Actually, the total pressure is all that is important in sizing fans.

RE: how to measure static pressure of AHU

(OP)

Sorry, again the pictures cannot be shown. however, I have changed the hyperlink to anther account. I think this time the link should be valid and accessible. the pictures show how I use a manometer to measure different static values. I want to confirm whether I use the manometer correctly (both connecting to the manometer and the point I was measuring) Thanks.


External pressure of AHU

link: https://drive.google.com/file/d/0B6nZP28lo52eVktBN...

Total pressure of AHU or static pressure of fan unit

link: https://drive.google.com/file/d/0B6nZP28lo52ea3k5Z...

Static pressure of fan inlet

link: https://drive.google.com/file/d/0B6nZP28lo52eWU54T...

Static pressure of fan outlet

link: https://drive.google.com/file/d/0B6nZP28lo52eSXNTS...

Static pressure of upstream of filter

link: https://drive.google.com/file/d/0B6nZP28lo52eZzF5e...

Static pressure of downstream of filter

link: https://drive.google.com/file/d/0B6nZP28lo52eUDNkM...

RE: how to measure static pressure of AHU

Peter ,

You have got it right except maybe for the second which should be total pressure of fan not AHU.Willard is right. SP is irrelevant as there are both static and dynamic losses in a ductwork so we should only be looking at total pressure.I remember we have had this discussion on the forum before.The attached ASHRAE article will hopefully clear things up a bit?

RE: how to measure static pressure of AHU

(OP)
thanks all

RE: how to measure static pressure of AHU

Ha, Nice point Willard, sometimes that is lost... Fans/pumps produce energy. What we do w/it (flow or potential is up to us)...

RE: how to measure static pressure of AHU

SAK9/Willard. Sorry guys. TSP for fans is by difference in static only, nothing at all to do with velocity pressure.

I will prove it if you want to continue this post.

RE: how to measure static pressure of AHU

CB,I am keen to hear why only SP needs to be considered.

RE: how to measure static pressure of AHU

(OP)
chasbean, sure and please tell more....thanks!

RE: how to measure static pressure of AHU

Chasbean,

Does that mean static regain is not correct?

RE: how to measure static pressure of AHU

@Chas, I totally agree with you if you mean this: Fan static pressure is the difference between Fan total and Fan velocity pressures.JMTC

RE: how to measure static pressure of AHU

What is the shape of a fan curve? Why does it end by touching the X axis? What does the Y axis mean?

RE: how to measure static pressure of AHU

I don’t mean to be elusive — I’m saying that as fan flow increases the velocity pressure increases so any fan curve would not curl down toward zero as flow reaches maximum. The performance of any fan is simply based on the static pressure at the inlet vs. the static pressure at the outlet. This is the TSP. The velocity pressure is not a component of fan curves.

Back to academia, total pressure IS static plus velocity pressure. TSP on a fan curve however does not have a velocity pressure component.

RE: how to measure static pressure of AHU

(OP)
may i ask another question?

i know cfm varies as rpm and sp varies as (rpm)^2

if i measure my ahu and find the following data,

original rpm=1805
new rpm =1355
original cfm= 8.1
new cfm=6.1
original sp= 1635
but new sp=730

for the rpm and cfm, it obeys the rule; however, sp breaks the rule

"Measure wrongly" is the only reason for getting such data?

any other reason?

thanks

RE: how to measure static pressure of AHU

(OP)
furthermore, if i find that all the three parameter do not obey the fan rule,

can i draw the conclusion : the existing motor and fan may be changed by someone, that's why all the three parameters i measured do not match with the nameplate stick on the AHU?

thanks

RE: how to measure static pressure of AHU

(OP)
anyone can help? thanks

RE: how to measure static pressure of AHU

peterchan222


cfm varies as rpm and sp varies as (rpm)^2
I think these rules is just theoretical rules for a theoretical specific design of the forward blade fan
so there are many factors cause difference in values between theoretical case and actual one
for example of these factors
number of blades
blades orientation (forward blade or backward blade)
curvature of blades
thickness of blades.....etc

RE: how to measure static pressure of AHU


peterchan222

But to be honest I think that all previous factors may affect on flow rate and power but The strange thing for me is to be the difference in static pressure
may be the point you measured sp in the AHU is wrong

RE: how to measure static pressure of AHU

(OP)
hatem, however, i have already followed the measuring points shown on the pictures above.

RE: how to measure static pressure of AHU

yes, I have some notes on pictures
first, when you measure pressure difference you should take two points in two ducts have the same size to avoid the effect of dynamic pressure
second, all points must be not in turbulence flow area, so your point in mixing box as shown in pictures is wrong because it makes false reading

RE: how to measure static pressure of AHU

peterchan22, fan laws do say new SP is 921 and not 730 as you noted. But as previously said, fan laws are an approximation...

RE: how to measure static pressure of AHU

(OP)
hatem,i can understand that the flow in the chamber is turbulent, however, where should i measure if i want to obtain the same value i need?

RE: how to measure static pressure of AHU

you can measure at point in that straight portion of AHU which is directly before filters. You may find a somewhat laminar flow in this region
important note: Don't forget that you should take points in the same duct size (the same cross section area of flow ) when measuring static pressure difference to avoid the effect of dynamic pressure

RE: how to measure static pressure of AHU

I have a 3 ton package heat pump system and have a question about External Static Pressure. The system list a max ESP 0.5 iwc. My measured ESP is .42 iwc, which includes 0.25 of the return filter. The thing I'm finding strange is the entire ESP is in the return duct, with the supply duct barely registering unless I start closing down registers. Is this an issue and why?

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