Simple airflow question.

[mingmongmang]

Hi,
If I've chosen the wrong forum please advise. My question regards forced air cooling of some heat sources in an enclosure. An illustration of system is attached and is also here.

OK..
The system pulls air in through a filter, around some heat sources, through some ports into a plenum and is then exhausted via a blower.

My questions are :
1, how to measure the static pressure of the system so that I can find airflow using the blower's CFM/pressure graph?

2, How can I measure/calculate how much airflow I'm losing due to the ports entering the plenum?

I'm totally new to this field so any extra insight will be much appreciated.

Thanks in advance,
Jon.

 

[btrueblood]

Look up the term "manometer", good sources will also discuss the slightly more complex "slant tube manometer". Very simple item to build.

 

[chicopee]

For a quick estimate, get a long piece of tygon tubing(3/8" dia or thereabout);partially fill it with colored water; stick one end not too close to the fan intake and perpendicular to the air flow and the other end outside the building (but not at the intake louver) from which the fan is drawing its air; with the fan shutt off mark the water levels on both ends of the tube and make sure that there is enough room in the tube at the fan intake to allow for the water colum to rise without spilling the water out of the tube; turn the fan on and measure the increase in height at the fan intake and the decrease in height on the tube end outside the room; both measurements shoud be the same value; compare this value to your fan characteristic curve for the CFM.
Regarding your second question,there should not be any CFM loss, just some pressure drop and for that you can again use the same tygon tubing for a second test.

 

[ione]

Further to your second point, use a Pitot-tube to have an indirect measure of the fluid velocity. Then dividing the velocity by an average section area you’ll get an average flow rate.

 

[mingmongmang]

chicopee: Regarding your second question,there should not be any CFM loss, just some pressure drop...

But the ports must be restricting the total flow of the system. Can I learn how much by measuring the pressure drop across the ports?

 

[mingmongmang]

I guess I should give a little more background. The illustration I gave is a simplified version of a system we have on our test floor right now. It's struggling to stay cool and I've decided my first approach will be to try to increase airflow using the existing blower.
Just from looking at the unit my gut feeling is the ports entering the plenum are a major restriction but I'd like to get some empirical data before making changes.

I've got hold of a 0-5" of water 'differential pressure gage' so I'll be able to measure the static pressure per chicopee's advice and I'll also measure the drop across the plenum ports and post results here.

 

[chicopee]

If the ports are increased in size, the pressure drop across them will be lowered and consequently the total pressure drop(intake filters, plenum ports and fan outlet louver) of the entire system will be lowered and the fan flow rate will increase as will be evident in the fan characteristic curve. Note CFM thru filter=CFM thru plenum ports=CFM thru fan louver since at these relatively low pressures and temperatures, air is treated as an incompressible fluid.
when you install your manometer between the equipment room and plenum be sure that the set up is not influence by the plenum ports, so stay clear for about a couple of feet.

 

[IRstuff]

You must measure the fan pressure and use its fan curve to determine whether there's anything to be done to improve things, i.e., if your fan is already operating at its capacity, changing the ports will make little difference.

However, your simplified schematic leaves out significant information about how the airflow is actually traversing your heat sources. Is the air really reaching your equipment and actually removing the expected amount of heat? You say there's difficulty in cooling, but where is that occuring? Perhaps some simple diverters are needed to change the airflow to be more directly on the heat exchanging surfaces of your equipment. Some temperature measurements are in order, as are some velocity measurements near the heat exchangers. Which of the two boxes is having problems, or are both having problems?

A brute force approach would be to use a smoke test to actually visualize where the air is flowing in your system.

TTFN

FAQ731-376

 

[mingmongmang]

chicopee:when you install your manometer between the equipment room and plenum be sure that the set up is not influence by the plenum ports, so stay clear for about a couple of feet.
I don't have 2 feet of space unfortunately, that plenum is 8" high. When you say 'influenced by plenum ports', do you mean venturi effect acting across the end of my gage tubing?

 

[mingmongmang]

IRstuff: You must measure the fan pressure and use its fan curve to determine whether there's anything to be done to improve things, i.e., if your fan is already operating at its capacity, changing the ports will make little difference.

Well I'm suspicious that the restriction of the ports themselves may be taking the fan beyond capacity. Yes I need to address the flow paths over the heat sources but first I want to be sure I'm not losing flow unnecessarily at the plenum/exhaust end of the system (hence my initial question 2).

Also I'm still not sure how to measure the fan pressure. If I just put a manometer tube in the blower's inlet stream, won't the reading be lowered by venturi effect?

 

[IRstuff]

People have been successfully measuring airspeeds with pitot tubes for nearly 100 years; I'm sure someone must have figured out how to get them to work

TTFN

FAQ731-376

 

[mingmongmang]

IRstuff, care to explain that post?

 

[chicopee]

Yes venturi and turbulance around corners.

Pitot tube is a good instrument but you have to several readings for an average value and depending where those reading are taken some fan may push air in the opposite direction of the normal air flow if the readings are too close to the fan.

If you can get a fan characteristic curve and get your delta P
as mentioned above with a home made or shop made manometer,you can get your CFM.

 

[James51]

Ming,

From your sketch it looks like your fan is inside the plenun - wondering if this is an axial fan? Also from your sketch it looks like you may not have the proper room or straight run of ductwork to do a Pitot Traverse - there are no dimensions on your sketch. design looks like you are creating a lot of difficulty for the fan to operate effectively.

It might be also helpful if you provided fan data i.e. point of rating on the fan pressure and flow and HP.

As indicated by others above - If you have the fan curve you will see the increased pressure means less flow. If you reduce your pressure you will need to be sure not to free wheel the fan / over amp the motor and cause it to fail. Your limit on fan air flow is fan motor amps - if you can provide a bigger openning i.e. reduce pressure drop and have a means to close off a protion of flow if amps get too high - slide gate or damper - you can maximise fan output that way.

Any air flows you get using a pitot tube or measureing DP will be at least 10% off - using fan motor amps might be a better gage for you.