reading blower performance curve

[RV]

When a blower in a vacuum application also has piping on the discharge side, resulting in positive pressure, how do you determine the flow, temp rise, input power etc from the performance curves? Most blower vendors' literature includes separate vacuum and pressure curves for flow, temp rise, and input power. Here's my actual example: the blower moves 420 scfm at 60" WC vacuum and 640 scfm at 8" WC pressure according to the charts. We have pressure indicators on either side but no flow indication. How do you estimate actual flow using the curves? Is the process the same for estimating input power, temperature rise etc. off of separate vacuum and pressure curves? Thanks for your help!

 

[ChasBean1]

RV, it sounds like you have two data points on a performance curve, unless I understood wrong. The performance of the unit per its curve should be the difference between the inlet and outlet conditions. It should not be based on the condition of one side OR the other.

 

[RV]

ChasBean1,

Thanks for your reply, but I am still confused. What do you mean the performance should be the difference between the inlet and outlet conditions? Maybe using the real numbers can help us communicate better so here is the data:

There is a pressure indicator on both the suction and pressure side of the blower. While operating, the suction side indicates 60" WC gauge vacuum and the pressure side indicates 8" WC gauge pressure. Now, looking at the SEPARATE vacuum and pressure curves for this blower, I read the following information:

Vacuum curve (at 60" WC vac gauge)
Flow = 420 scfm
Power input = 9500 watts
Air temp rise = 34°C

Pressure curve (at 8" WC gauge)
Flow = 640 scfm
Power input = 5000 watts
Air temp rise = 10° C

Converting the flow in each case to scfm at standard atmospheric pressure (14.7 psi abs) results in two different flows, we have two different values for power input, and we have two different values for air temp rise.

So at these conditions, how do you determine the actual flow, power input, and temperature rise for this blower using the information from these two performance curves?

I hope this helps clarify my question.

 

[ChasBean1]

Thanks - I think I understand better now. In a regular fan application I would take the 60" vacuum and add it at 8" pressure (i.e., the total difference), and look the 68" up on the one manufacturer's performance curve. Here they gave you two curves, indicating to me that the performance characteristics are different if we're operating at a suction shutoff head versus a discharge shutoff head. Or it could be that the manufacturer is trying to simplify an application concept by maybe mistakenly making it more complex. Can you humor us by posting, 1) the performance data at 8" vacuum and 2) the performance data at 60" pressure? Tx, -CB

 

[RV]

ChasBean1:

Here is the manufacturer's spec sheet for the particular blower with the vacuum and pressure curves. I'm sure this will help!

http://www.rotronindustrial.com/pdf/EN_CP909.pdf

 

[ChasBean1]

RV,

Here's how I read this. This unit can be used to provide (A) a certain pressure at a certain flow, or (B) a certain vacuum at a certain flow. Manufacturer test data indicates different performance characteristics for each application. I think curve A reflects close to zero suction pressure and curve B reflects close to zero outlet pressure.

Since you are using this in a vacuum application, use the vacuum curve for data analysis. Because of the outlet backpressure, use 68 in. w.c. on the "suction" curve. This equates to about 380 cfm.

If you used the "pressure" curve, this would be about 480 cfm.

I imagine the truth here lies somewhere inbetween, but about 13% away from the suction curve while 87% away from the pressure curve (from the 60 in. suction, 8 in. discharge ratio). This is based on linear interpolation of a phenomenon that is likely to be nonlinear.

As a result, I would estimate your flow to be 393 cfm.

Now of course, the manufacturer produced this data and may say otherwise because their facts differ from what I've interpreted...

Note that regardless of the curve data, I'll still be confident in using a differential across the unit to analyze anything the unit is doing rather than using two separate conditions (suction versus discharge) to determine operation. -CB

 

[chicopee]

The answer to your flow question is simple; whatever scfm you have at the vacuum readings, those same scfm will be on the outlet side. of course the acfm will be based on temperature and pressure conditions on the outlet side and can be calculated w/ the appropriate temp & pressure correction factors .

 

[ChasBean1]

Good point - the curves might end up the same based on SCFM versus ACFM temp corrections. I ran through some calculations to check this and proved that 1 does in fact equal 1.

(you laugh now, but this will be cut & paste into future thermo books as ChasBean1's First Law... we'll see who has the last laugh then.)


;-)

 

[RV]

Thanks for your replies, but I think my question remains the same. Please note, I understand that scfm in should equal scfm out. However, how do you recommend I calculate/convert to scfm? I ask because before posting this question, I ran through the calcs myself to convert the flow reported by each curve to scfm (convert to 14.7 psi abs atm pressure and 520°R abs temp).

Formula used:
CFM1 = CFM2 x (Pabs2/Pabs1) x (Tabs1/Tabs2)

When you convert 420 cfm at 60" WC vac (from the suction curve) to scfm you get a different value than by converting 640 cfm at 8" WC (from the pressure curve) to scfm. And therein lies my question. Which value do I use? I once asked a blower vendor this question and they said to use a weighted average:

(vacuum pressure/total diff pressure) x (flow from suction chart at vacuum pressure) + (discharge pressure/total diff pressure) x (flow from pressure chart at discharge pressure)

In this case:
(60" WC/68" WC) x (420 cfm) + (8" WC/68" WC) x (640 cfm) = 446 cfm.

From my experience testing these blowers in instances where flow meters are installed, the results do not concur with the above calc.

Also, Chicopee stated that scfm in = scfm out, but that doesn't address how to use the separte suction and pressure curves for power input and blower temperature rise to determine what these values will be.

Perhaps I am just overthinking the issue and getting myself confused! If so, please help me figure out where I am getting off track! Thanks again!

 

[TD2K]

I'd do this a little different.

Blowers are essentially PD machines so the suction pressure is the critical parameter in the scfm flow rate through the unit. The additional 8" WC discharge pressure you are operating at (versus the 0 shown on the curves) will tend to increase the internal slip in the machine, decrease the flow rate and increase discharge temperature and Hp.

Back to basics, If you take 420 SCFM at 60" WC vacuum, that works out to 492 ACFM entering the blower suction.

Now, run the blower with atmospheric suction pressure but a discharge pressure of 8" WC and theoretically, you should still have 492 ACFM entering the blower. But, since this is at atmospheric pressure, it's also equal to 492 SCFM. The vendor's data says however at this condition, the blower moves 640 scfm.

There are two reasons for this. At the vacuum condition, the dP across the blower is 60" WC versus 8" for the second case. That is going to affect the volumetric efficiency for the blower AND it's going to possibly affect the internal slippage. I think that is what is causing the difference.

Since you are actually running 60" WC, I'd use that curve (at 0 "WC discharge pressure). I'd then use the formulas for a recip compressor (they are in the GPSA handbook and a lot of other soruces) and adjust the volumetric and mechanical/thermodynamic efficiencies to match the curve. Then, change the discharge pressure to 8" WC from atmospheric and see how the volumetric efficiency changes (my guess for about a 10% increase, not much) and you can see the increase in discharge temperature.

 

[saxon]

RV, This may be an oversimplification of your problem, but why don't you contact the Manu'f. Engineering or Technical Sales group, give them the parameters the blower is operating at (inlet and outlet press/temp. and motor amp draw) and let them develop a curve based on that data and the equip. model.

Just an idea.
saxon

 

[ChasBean1]

This is true Saxon - we can speculate but until the producers of these curves better define how they collected and present this data it could be a moot point. I started messing with the curve data in a spreadsheet and adjusted the vacuum readings and pressure readings to standard pressure by correcting for density and the curves almost 'zipped' together perfectly, until higher pressures/vacuums, where they tailed off. I was thinking the "blower performance corrected to standard conditions" curves may have been adjusted for temperature, but not the density difference at the range of pressures... I went on the notion that mass flow in equals mass flow out, versus the commonly misconstrued notion that volume flow in equals volume flow out (this should hold true if SCFM adjustments account for all varying parameters). RV, I want to thank you for letting me add to the confusion while still leaving you hanging with your question. Please contact the guru at the manufacturer (there's usually one who's always on vacation or at lunch) and post what he says. Thanks. -CB

 

[ccfowler]

Sorry about coming late to this discussion--I've been distracted lately.

These are regenerative turbine machines and very much NOT positive displacement machines even though they have very usefully steep pressure vs. flow curves. From experience, I've found that this type of machine works most predictably (and best operationally) in mid-range flow conditions.

The manufacturer's (speaking generically, not just this manufacturer) curves usually seem to reflect the actual performance of the integrated package (induction motor, blower, and both inlet and discharge silencers). Being a form of centrifugal machine, the ACTUAL shaft speed is of vital importance in predicting or evaluating performance. The performance estimation work gets messy at very low flows (and high differential pressures) due to the induction motor slip rate and the substantial temperature rise through the blower. I believe that this accounts for the data fit issues that ChasBean1 discussed.

At excessively low flow, high differential conditions, blower temperature rise can get to be operationally troublesome. When operated reasonably well within their normal rated operating range, these can be very useful and reliable machines. The manufacturer's use of scfm can be handy, but it is important to keep in mind that the machine always works on acfm under all conditions.

 

[ChasBean1]

RV, I think as a result of all this discussion you should first 1) Contact the manufacturer with your specific question. Even direct him to this thread. If flow is critical you can 2) install an inline rotameter. Correct the rotameter indication based on the temperature/pressure conditions at the point which it is installed. -CB

 

[TD2K]

And, let us know what you find out!!!

 

[RV]

Thanks for all of your input. Hopefully I can address recent postings.

The reason I didn't want to ask the vendor to produce a curve just for this one instance is because I have run into this problem multiple times and would like a solution that I can transfer comfortably to different blowers and applications.

I did indeed contact the "guru" at the manufacturer and he said that the rule of thumb he had previously provided me (see my April 21 posting) is still the best way he knows how to approach this. The rule of thumb is to use a weighted average:

(vacuum pressure/total diff pressure) x (flow from suction chart at vacuum pressure) + (discharge pressure/total diff pressure) x (flow from pressure chart at discharge pressure)

The vendor was very helpful, but said he does not have a better means than this for calculating the flow. To paraphrase he said that the blower behaves differently under pure vacuum or pure pressure and that the curves are published in scfm.

So I think the best approach will be to convert the flow from both suction and pressure charts to true standard conditions, and if these numbers disagree, use the weighted average. After that all I can do is check against an in-line flowmeter and cross my fingers that the reading agrees with the calculation!

 

[ChasBean1]

RV, I respectfully disagree.

With zero backpressure and at 60 in. w.c. vacuum, the unit supplies 420 cfm. By your correction, increasing the backpressure to 8 in. w.c. by restricting the outlet will cause a 26 cfm flow increase. This is not logical.

See my 4/18 post. You should instead look at total DP on each curve and do a weighted average:

380+[(8/68)*(480-380)] = 392 cfm

380 = suction curve flow at 68" vacuum
480 = pressure curve flow at 68" pressure

 

[TD2K]

ChasBean1, you don't believe in miracles?

 

[pmover]

RV,

a suggestion:

think about developing your own curve for subject blower(s). may take a little while in preparing, but will be useful in future analysis. measure the following parameters:
establish steady-state conditions
inlet P & T
outlet P & T
compressor wheel and motor rpm
motor Volt and amps
measure inlet flow (if possible). if not, extrapolate flow.

determine compressor efficiency, compressor head, motor power, assume reasonable mechanical losses (i.e. 2-5%), and you should be able to generate a decent curve and compare with mfg.
repeat steps for each data point at various flows.
good luck!
-pmover

 

[ChasBean1]

TD2K, what comes first, the chicken or the egg? My 10-year-old niece told me that the egg does. You have eggs for breakfast and chicken for dinner. Also, Bill Cosby says the glass is half-empty when you're drinking and half-full when you're pouring. And per previous poster CHD01 I agree that the more you learn, the less you’re certain of. So I think in the end of our lives these truths we know are small pieces of the puzzle that is life. Miracles make up the remainder… ;-)

CB