Heat added by pump 3

[asv80]

Hello,

How to determine the heat added by a pump of given horsepower to the fluid being pumped?

Take, fluid - water (20°C), motor - 100 HP

For an approximate estimate I thought, 100 HP is the motor rating and maybe the maximum possible heat that can be added to the fluid would be equivalent of the brake HP. So take about 90% of BHP as the heat added to the fluid.

Any thoughts on this?

Thanks

 

[BigInch]

Way less than that. Hopefully you will be in the range of 60-80% pump efficiency, therefore you only need to convert 20 to 40% of the pump shaft horsepower to heating. Check your pump curve for the exact efficiency at your operating flowrate.

If you're less than 60% eff at your op flowrate, get a new pump. It should pay for itself very quickly.

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[asv80]

BigInch,

Thanks for your reply.

I do not have the pump curve, but the pump is supposed to be a recent installation and hopefully it is working at the rated efficiency.

In our case temperature is critical and we think the pump is adding the heat into the process. We are planning to install a cooler after the pump to remove the heat so, we need to know how much heat the pump will add at the maximum.

 

[Warpspeed]

It should be possible to directly measure the temperature before and after the pump, and draw some conclusions from that?

If you also know the mass flow, Kw to produce that temperature rise can be calculated. That, and the pump curve efficiency estimation method mentioned above by BigInch, should pretty much tie all the figures together.

 

[quark]

In most of the cases, during normal operation the temperature fise is insignificant, particularly for water and sensors may not correctly indicate the rise in temperature. There is a long thread about the same topic about 2 years back (if my memory is correct, it was started by electricpete)

The temperature rise due to inefficiency of pump is given by

deg.F = [(BHP-WHP) x 42.41]/(lb/min x specific heat)

Water HP, WHP = gpm x 8.33 x sg x head (ft)/33000

or

⁰C = [(BKW-WKW) x 14.34]/(ltrs/min x specific heat)

Water kW, WKW = head (m) x flow (ltrs/min)/6000

Suppose, in your case if the head is 100 mtrs and assuming a motor efficiency of 90% and that of pump as 70%, the BHP is 90 and WHP is 63 (100*0.9*0.7).

Flow can, approximately, be 63*33000/(8.33*100)= 2496gpm
density of water at 20C is 62.3lb/cu.ft and 1 cu.ft is 7.48 gallons. So, 2496 gpm is 20792 lb/min.

So, the temperature rise will be (90-63)*42.41/(20792) = 0.055F

the link below gives you the concept of heat due to friction in pumps,

http://www.mcnallyinstitute.com/01-html/1-04.html

 

[BigInch]

As asked by Warp above, what's the inlet and outlet temps?
That should tell you right there.

If not, Is this a centrifugal pump? What's the rated BEP flow. What fluid?

If the pump is running at rated "BEP" flow, the heat rise should be miniminal and probably not exceeding 1-3 ºC.

Most pumps will have minimal heat outputs until you get down towards 25%-30% BEP where heat starts increasing. If you try to run at 10% or less of BEP flowrate, you can get very high heating rates.

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[iainuts]

Inefficiency alone isn't responsible for temperature rise. A perfectly efficient pump results in isentropic compression which will still result in a temperature rise in the fluid as given by the first law:
dU = 0 = Hin - Hout + Win
so Hout = Hin + Win

All the work goes into the fluid which results in an increase in enthalpy, regardless of how efficient the pump is.

If you know Hin (which you do because you know the pressure and temp of the fluid) and if you know Win (which you can make a very good aproximation of since you know the motor amps and therefore the power - assume some motor efficiency and losses for gearbox, etc) then you know the Hout. Since you can easily measure the outlet pressure, outlet temperature follows.

In other words, there's no HEAT added to the fluid, but there is WORK added to the fluid as it goes through a pump, and regardless of how efficient the pump is, the WORK added to the fluid is equal to the work used to compress it. For a 100 hp motor, you might assume 5 - 10% inefficiency in the electrical windings, some additional loss for power transmission, and the remainder of that work goes directly into compressing the fluid which will also result in the temperature rise.

 

[BigInch]

Almost, except in actuallity, its adiabatic.

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[25362]

Pump overheat, seizing, or both, can be attributed to a variety of factors. From a list offered by Sam Yedidiah in his Centrifugal Pump User's Handbook (Chapman and Hall):

1. low flow or dry running as explained above
2. vapor or air pockets
3. poorly matched parallel pumps
4. internal misalignment, from pipe strain or poor
foundation
5. rubbing of parts
6. worn or damaged bearings
7. poor lubrication
8. wrong galling-prone metallurgy of wearing (stationary
and rotating) rings

Pick out the relevant cause.

 

[BigInch]

Root cause of those are,

1. low efficiency due to poor internal Euler vectors.
2. low efficiency, high energy required to compress air in relation to a fluids relative incompressibility, pump's don't work well with air.
3. low efficiencies as both pumps track each other in areas deviating from BEP, see #1
4. casues friction between internal rubbing parts that have been misaligned by distorted pump casings due to high pipe forces or foundation sinking or soil swelling due vibratory settlements or to high moisture content.
5. friction
6. friction
7. friction
8. friction

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[asv80]

The product is passes through many stages and in each stage temperature needs to be constant for the reaction to happen.

Properties are very similar to water. Sometimes the viscosity is higher by upto 10 centipoise more than water.

Flow rate required ranges from 800 gpm to 1400 gpm in different systems.

We may be able to measure temperature in and out of the pump but we want to get a general expression we can use to determine how much temperature rise will happen when the process changes.

The temp rise we consider may happen is about 0.5 deg F, which affects the reaction which is why we think about having a pump and a cooler in series.

Also, what is a "BEP" flow?

thanks

 

[25362]

To BigInch, I'd add, with your approval, the word "abnormal" to "friction".

 

[BigInch]

BEP = rated best efficiency point.

All centrifugal pumps have one flow in their entire operating envelope where they run at maximum efficiency (minimum_power_consumed/unit_mass_moved). This is due to internal fluid flow angles and clearances for any given speed.

You will usually find these somewhere on the pump's nameplate in the form of a rated flow and a corresponding rated differential head for a particular speed of rotation of the impeller inside the pump.

"Pump curves" are usually available for any serious pump that shows these characteristics and mark the BEP point.

If I can draw a digital representation of such a curve


Diff
Head
^
|......... BEP
| .... /
|<--------------*...
| | ..
| | .
| | .
+---------------o--------.----->
Flow (increasing)

If you have this curve, looking down from the marked BEP point you will find the flow, looking to left, you will find the differential head.

You will also notice that maximum differential Head (d pressure) is produced at 0 flow. Or you could move to high flow and low pressure, or eventually arrive at maximum flow, but at zero pressure. Everything is always a trade-off in life, heh?

YES 25362,
Thanks, make it so.

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[BigInch]

Well, I guess I can't draw good digital diagrams.

Its not WYSIWYG. It looked good before I hit the post button.

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[BigInch]

Well your last info changes thing some. If you want to control temperatures within 0.5ºF, I would not try to do that by roughly estimating the heat input from a pump. IMO, you will need to do an accurate calculation, starting from an accurate power-efficiency curve and detailed knowledge of the adiabatic bulk modulus and the specific heat of your fluid. No rough estimate will get you in that ball park.

You'd probably be much better off by pumping out at some nearby temperature, then fine tuning the final temperature of the fluid delivered to the reactor with some kind of a constant temperature heating (or cooling) exchanger.

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