3 replies to this topic

[curious_cat]

When designing an agitator for a stirred tank, I calculate my HP requirement based on correlations for blend time, Power Number, Flow number etc.. What are further adjustments before I can arrive at the motor electrical HP? required?

I'm assuming there's at least two loss components:

(1) the electrical -> mechanical conversion loss of the motor and
(2) the gearbox, seal, bearing etc. loss.

What are typical loss coeffients or are there any tables to estimate these?

My calculations were estimating approximately 1 hP / 1000 L (delivered to the liq). This is a 12,000 L tank with a ~150 rpm turbine agitator. Two phase L-L duty. I'm assuming 12 hP will be underrated but not sure what's a more realistic assessment.

Also, just to confirm motor rated hp is on the electrical input basis and not mechanical power delivered, right?

PS. Apologies for the typo in the Title.

Edited by curious_cat, 02 August 2012 - 06:07 AM.

[Steve Hall]

The two-phase liquid-liquid duty is interesting, especially since you say you used blend time correlations. Having said that, a "rule of thumb" for power is 0.2 to 0.5 hp per 1000 gallons, so your 1 hp per 1000 liters sounds like a lot. (Rule of thumb for liquid-liquid mixtures is 5 hp per 1000 gal, more in line with your statement).

The motor efficiency is provided by the motor vendor, based on your specifications, but should typically be in the 90% to 95% range. Drive train losses depend on the design also.

The agitator vendor will determine what losses to include in the overall calculation. Based on your limited information, I would expect to see a 20 hp motor quoted, but wouldn't be surprised to see 25 hp. Yes, motor is rated on electrical input basis.

[curious_cat]

Hmm....Now I'm wondering if my calculations are off. e.g. I was using "Engineering Design" by Sinnot and his Table 15.5 shows


Table 15.5 Power Requirements—Baffled AgitatedTanks

Agitation Applications ==========Power, kW/m3

Mild Blending, mixing ==========0.04–0.10
Homogeneous reactions========0.01–0.03
Medium Heattransfer=========== 0.03–1.0
Liquid-liquid mixing ============1.0–1.5
Violent Fineslurrysuspension ====>2.0

That's one of the things I was basing my 1 hp/1000 L on.

Any critiques?

[breizh]

http://www.haywardgo...undamentals.pdf

Hope these resources can support your query

Breizh