3 replies to this topic

[asmsyn]

we have a centrifugal blower of capacity 500 m3/hr at 350 mmWC(as per vendor document) which as installed long back.Now we need to check the performance of this blower.Can somebody help me in knowing what is the standard procedure to confirm the actual performance of blower in terms of flow and Static pressure.

To know flow we measured velocity used an anemometer.

[breizh]

Hi,

You may consider theses resources to support your query.
you need to have the curve ( DP vs Flow) of your blower .

Hope this helps
Breizh

Edited by breizh, 04 August 2011 - 03:18 AM.

[kkala]

You have to measure flow and ΔP of the blower, then place the data on its curve, to see whether (and how much) it is run down. In addition to the information by breizh, following may be useful.
A1. Performance data of two blowers (each ~90000 m3/h, 400 mm WC) was measured in the past (1980) precisely enough by using
(α) a thin (dia~3/8") pitot tube
(β) a differential manometer (DM), consisting of two long water tubes (dia~3/8") connected at their bottom by a rubber tube and filled with water.
DM was either attached to pitot tube, or to two duct holes upstream & downstream of the blower. Single gauge pressure upstream & downstream of blower (in mm WC) can be measured by same DM, if more convenient.
A2. Duct section was divided into 2 annuli and a central circle (totally 9 points of measurement). Four local velocities were measured on each annulus, using the pitot tube. Total flow: area of annulus1 x average velocity (of corresponding 4 velocities) + area of annulus2 x average velocity (of corresponding 4 velocities) + area of internal cirle x velocity at center.
A3. Measurements on two rather distant duct sections (e.g. suction and discharge) are recommended, to assess the difference. Avoid places of swirling flow on the duct (when pitot tube ΔP increases by turning it, so that it does not "face" the duct longitudinal axis). Selected duct section location had better follow the rules of "smooth" flow applicable for instruments.
A4. Measurements were repeated at reduced flows by closing the discharge dumper. Measured points were plotted versus the blower curve. As a result, one of the blowers was replaced, the other remained.
B. In the present case, measurements to verify blower curve can be similar, with following differences.
B1. Flowrate is much lower, duct is probably of (say) 4" dia. Avoid having pitot measurement only at the center, try to have 5 pitot measurements on one section; so try to divide duct section in one annulus and central circle, as indicated in the attached “section.xls”.
B2. Flow pattern may be influenced by the insertion of pitot tube and the holes. Plug the holes with a piece of cork (even when pitot tube is inserted through one of them).
B3. Look into alternatives as below, limited to flow measurement (ΔP has to be measured).
C. Alternatives for B.
C1. A hot wire anemometer will make flow measurement easier (but not more precise). Ours was destroyed in a month by the hot humid gases measured. Dimensions of the device (say) 6x6 cm2, thus it may be used only at the end of gases to atmoshere, if gases really go to atmosphere.
C2. A venturi meter can be placed for continuous flow monitoring. This device operated rather satisfactorily on a pilot kiln flue gases, without calibration. If the gas exits to atmosphere, a hand made venturi meter of very thin steel sheet (plus steel screws) can be placed there, to do the job temporarily.
C3. Placing more sophisticated fluid meters can also do the job, but it may not be worth the trouble.

Attached Files

•  section.xls   14.5KB   64 downloads

Edited by kkala, 10 August 2011 - 11:32 AM.

[kkala]

Some additional notes on the gas flow measurements using pitot tube and needing patience.
1. Differential manometer (DM) indications are continuously oscilating during measurement. Try to record the average, after observing the indications in the two DM legs.
2. Longer DM legs are expected to facilitate readings (shorter oscillations due to more liquid mass content in the tubes). Hg used as liquid in DM will shorten oscillation but also reduce scale of reading.
3. A good practice would be to measure a known flow rate of a fan in advance. This will get the correct familiarization. We had not callibrated the pitot tube, ordered from a specialized intrument company. DMs can be locally constructed, even in the maintenance shop (or can be also ordered).
4. The pitot tube should be of stainless steel to resist corrosion and plugging. It can be washed after the measurement.
Note: A hot wire anemometer was more expensive and sensitive to hot humid gases. It measures local velocity, though its dimensions (say 6x6 cm2) do not permit measurements for local points close to each other. In the considered case (duct of 4" dia), it could take only one measurement (on the gas exit to atmosphere, if any).