# Thread: VFD on condensor pumps

1. Lower condenser flow you have greater chance of fouling up condenser.

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Originally Posted by commtech77
If you saw a dc square wave on a oscilloscope it would have the same electrical properties as its AC counter part. The waveform just looks different. The motor is still a AC motor.
So what you are saying is that a PWM square wave DC waveform is running an AC motor? PWM square wave almost seems like an oxymoron to me. Put your Fluke 87 on the DC scale and measure T1-T2, T2-T3 and T1-T3 when the VSD is up and running. Tell me the values you read. An AC motor needs AC voltage to run it. Now put your meter on the AC scale and do the same test. I'll wait for your answer.

3. Originally Posted by KnewYork
So what you are saying is that a PWM square wave DC waveform is running an AC motor? PWM square wave almost seems like an oxymoron to me. Put your Fluke 87 on the DC scale and measure T1-T2, T2-T3 and T1-T3 when the VSD is up and running. Tell me the values you read. An AC motor needs AC voltage to run it. Now put your meter on the AC scale and do the same test. I'll wait for your answer.
maybe a week or two before I can get access to one of my drives so Ill try again because its hard to visualize the wave form without access to a oscilloscope.

We'll use just 120 as an example. Just one waveform instead of 3 superimposed on top of each other 120 degrees out of phase. ( 1.0 pf )

That ac sine wave changes direction and polarity from peak to peak 60 times a sec.

A 120 DC modulated square wave would act similar with exception to the peak duration. Same amplitude or peak value just a change in frequency.

It is a constructed psuedo ac output from a drive but the motir sees the current wave form asbeinsinusoidal. It doeant know the difference

4. Its not practicle to change the frequency of a pure ac sign wave because that would mean taking a generator out of synchronous speed. ( Tested 1.2 to 1.7 megawatts generators for a year ).

So " building " the sign wave ( not really a sign wave when you alter frequency constantly ) with DC currnet that swithes polarity just like a ac sign wave is more practicle with better control.

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The original question was about installing vfd's on tower pumps. the short answer is Yes, it can save you money. tpyically the flow is neutered to desing conditions by using a triple duty valve and spinning the pump at sixty htz. By opening up, or better yet, removing those triple doodies, you can then set flow with a vfd. for example, you can do both soft start, and then ramp the drive to a preset frequency of say fifty htz, as determined by the flow through your chiller. This will save you dough rae me. just how much depends on a lot of factors.

some newer chillers can handle variable condenser flow and temps. We have some turbocore chillers that can handle down to fifty five degrees, and roughly half flow at part load, to produce kw/ton in the range of .3 it isnt sales pitch, its real. We have had engineers stand there and verfiy performance several times.

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If you halve the water flow of a condenser, you will foul the tubes and are approaching laminar flow as well, depending on design.
Any baby chiller can do .3 with 55 degree entering water, but not for long with scaled and dirty tubes however. Repost in a year the results of those tubes, and get those engineers to help clean them.

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we got turbocore machines running this way for years, nothing new. on conventional chillers different story, I agree. First one went in in 2003 and is still purring away, but what do i know?

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Let me guess... you have turbocore's made by Mag Lev...

You can't escape physics my friend. It's heat transfer 101, you MUST have turbulent flow in order to have efficient heat transfer. You may think you are saving energy on your condenser pumps by using a VFD, but you are using even more electricity on your compressors by doing this.

A centrifugal chiller must have 3 gpm per max design ton in the condenser. If you have less, by even 20%, no matter what your load is, then you will create laminar flow. Laminar flow is a condition where the center of the stream is faster than the rest of the stream because you are experiencing friction at the tube wall. Friction, which creates heat, will inhibit heat transfer. Essentially, you are creating heat where heat should be absorbed.

What may confuse some is that you will still get a 10 degree split from entering to leaving temps. That is normal under laminar conditions, however the skin temperature of the tubes will be around a 30-40 degrees of split, meaning the skin temp of the tubes closest to the entering water side would be about 100 degrees or so because as the flow splits from the 8" or 10" or whatever size condenser line into the smaller condenser tubes it becomes momentarily turbulent, however at low flow rates, (less than 3 gpm per max ton) the skin temp at the leaving side will be around 135. This is bad... it means the head pressure of the chiller is higher than it should be and you are wasting electricity.

Again, everything I said is provable and not theoretical. Next time your chiller is running at 50% load and your VFD is at 60% or whatever, take a look at the head pressure; then turn the VFD to 100% and take a look again... it will have gone down. So will your electricity bill.

The reason I mentioned Mag Lev is that they have a real dip **** of a sales person in the US who is using the VFD's on condenser pumps as a sales pitch to save electricity. He's alone in this endeavor, but I'm sure his commissions are great.

Also, when people talk about fouling of the condenser because of low flow they are 100% correct; however it doesn't always happen. The reason is that scale is a function of pH and temperature. pH is not an issue here however even if the temperature of the tube skin reaches 135 - 140 degrees, then you may not scale if your chemical guy has the tower running at low cycles. If that is the case, then your condenser would be clean, but you are still wasting electricity and now you are wasting water as well. A good chemical guy will run the LSI of a tower around 2.4 to 2.5 in order to maximize water conservation as well as minimize chemical usage. If that is the case, then you would scale at that high of a skin temp at the low flow of a VFD. No chemical or gadget can stop that.

VFD's on condenser water pumps is a huge issue in the water treatment community right now. It severely limits what the chemical treatment program can do and will likely scale the condenser or at a minimum force the water treatment vendor to lower the cycles of concentration severely in order to avoid scaling... which mean wasting a ton of water costing you even more money in water usage and sewer charges.

Like I said, were all prisoners of physics, no matter what the Mag Lev guy says.

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Originally Posted by waterworld
A centrifugal chiller must have 3 gpm per max design ton in the condenser. If you have less, by even 20%, no matter what your load is, then you will create laminar flow.
This statement cannot be made unless you know the characteristics of the tube. Most manufacturers (if not all) are using enhanced tubes that promote turbulence of the water inside the tube. The manufacturer I worked for published minimum and maximum flows through the vessels based on feet per second. I believe the water treatment community has an issue with proper treatment if the flow drops below 3'/sec. That threshold isn't reached in most condenser selections at 3 GPM per ton. Your claim of a drop in 20% of rated flow will cause laminar flow just isn't true, not with tubes that have internal enhancements.

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Originally Posted by KnewYork
This statement cannot be made unless you know the characteristics of the tube. Most manufacturers (if not all) are using enhanced tubes that promote turbulence of the water inside the tube. The manufacturer I worked for published minimum and maximum flows through the vessels based on feet per second. I believe the water treatment community has an issue with proper treatment if the flow drops below 3'/sec. That threshold isn't reached in most condenser selections at 3 GPM per ton. Your claim of a drop in 20% of rated flow will cause laminar flow just isn't true, not with tubes that have internal enhancements.
The only thing you are right about is that the 3 gpm per ton is a thumb rule. Everything else is, well wrong. Enhanced tubes create turbulence? Have you ever shot a rifle? Enhanced tubes don't do anything to create turbulence, if anything they would further the laminar condition.

Enhanced tubes were designed to keep a rigid structure surrounding the tube wall while have lower depressions to allow for increased heat transfer. They don't do anything for turbulence.

Turbulence is caused by a rapid variation of velocity and pressure in a fixed space and within a fixed time. Basically shoving 12 lbs. of **** into a 5 lb bag in a second... enhanced tubes don't contribute to any of those factors.

Oh and as for feet per second... I understand what you mean and i've not heard of feet per second being used for condensers in HVAC applications. That doesn't mean your wrong I've just no heard of it. I can say that no matter the min/max flow recommendation for a condenser in gpm/ton or ft/sec will correlate. Is has to.

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Originally Posted by waterworld
The only thing you are right about is that the 3 gpm per ton is a thumb rule. Everything else is, well wrong. Enhanced tubes create turbulence? Have you ever shot a rifle? Enhanced tubes don't do anything to create turbulence, if anything they would further the laminar condition.
Another know-it-all. Use an analogy that supports my argument. The rifling inside a barrel puts a "spin" on the projectile, which is exactly what a rifled or an enhanced tube does to the fluid inside of it. Without rifling inside a rifle barrel the projectile would not spin just like laminar flow, no? Internally enhanced tubes create two benefits...enhanced heat transfer and the opposite of laminar flow. Call it what you will...turbulence, non-laminar...whatever. HRS heat transfer has a good video on YouTube if you care to watch it.

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Originally Posted by KnewYork
Another know-it-all. Use an analogy that supports my argument. The rifling inside a barrel puts a "spin" on the projectile, which is exactly what a rifled or an enhanced tube does to the fluid inside of it. Without rifling inside a rifle barrel the projectile would not spin just like laminar flow, no? Internally enhanced tubes create two benefits...enhanced heat transfer and the opposite of laminar flow. Call it what you will...turbulence, non-laminar...whatever. HRS heat transfer has a good video on YouTube if you care to watch it.
Wow... that's not really fair. I have a Master's in Chemistry and I am preparing to take my exam for my P.E. in chemical engineering and I have been in the water treatment industry for many years. I think that gives me some standing to give an opinion even if it does not agree with yours. Furthermore your video is an illusion. It shows a corrugated tube diverting air bubbles. There is no flow of water, only air bubbles. What you aren't understanding is that if it were water it would 'rifle' and resistance from the 'rifling' would create friction and therefore would tend to create a laminar condition, not turbulent.

And the spin you mention, laminar flow is where the center of the stream is traveling faster than the outside of the stream. Exactly what happens when the flow if water is spinning in an enhanced tube. The outside of the stream is being slowed down by the resistance created at the grooves and forced to spin while the center of the flow remains uninhibited by resistance and flows freely at a faster level. The heat transfer is inhibited by the thermal conductivity created by the resistance at the spirals. Heat transfer 101. My analogy has nothing to do with yours. The rifling of a gun stabilized the trajectory by centering the gravity of the bullet down the center of the rifle.... like LAMINAR FLOW.

Don't like what I have to say? Read this document by about 6 chemical and mechanical engineers stating that enhanced tubes do not induce turbulent flow but tend to create friction and laminar flow and inhibit heat transfer. Maybe they are know-it-all's as well...

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