# Thread: VFD on condensor pumps

1. Originally Posted by waterworld
...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 don't have to have turbulent flow to have efficient heat transfer...efficiency is a matter of in's and out's. i will agree that higher water flows generally gain you more efficient heat transfer on the water side, however, at what cost? more pump electricity? maybe, maybe not...it depends on the EXACT situation and we just don't know enough about THIS OP's situation to make a well informed thought. there may be times (lower loads) that 'high efficiency' heat transfer is not necessary. we also can not control the refrigerant flow rate on the refrigerant side of the tube...as the chiller unloads and slows down the refrigerant flow rate, there is going to be less heat transfer on that side of the tube as well...if the heat doesn't get into the copper of the copper tube, then the water can not remove it no matter how perfect the water flow rate is.

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.....
please be careful in stating generalized terms such as this. i have a chiller whos design evaporator is 566 gpm. subtracting 20% gives me a flow rate of 453 gpm...the minimum allowable gpm is 173. 453 is >250% better than the allowable low end...i would not think that i would have laminar flow if the flow dropped to 453 gpm...more laminar than 566? sure, but at what point do you cut things off? it depends on the design. lets keep the generalizations to the politicians.

Originally Posted by waterworld
...if it were water it would 'rifle' and resistance from the 'rifling' would create friction.....
water in a non-rifled tube creates friction too.

since it seems that you have quite a bit of education and experience AND that you are going to be an engineer of some kind, please make sure that EVERY job has at least 10 pipe diameters of straight pipe before entering the chiller bundle...if you do, you will be one of the only ones that will do that and for that i thank you.

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using a vfd on a condenser pump typically eliminates the triple duty valve( pressure drop) and allows use of only a check valve. Setting a pump to run a sixty hertz, then neutering it back with a triple duty is less effiicent than running the vfd at the correct frequency to maintain proper flow. its a fact that has somehow missed your schooling. Its why every utility and their mom here is giving rebates for them.

As far as variable flow through a condenser, thats a whole other discussion, but lets say for example, you have three 150 ton compressors, and one is running. why is it imperative to run full desgin flow through there? you arent gaining anything. your head will only get to a certain point, as determined by desing approach, cleanliness, and water temps. its not like it will drop off lower if you run three times the water flow through the condenser.

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Originally Posted by flange
using a vfd on a condenser pump typically eliminates the triple duty valve( pressure drop) and allows use of only a check valve. Setting a pump to run a sixty hertz, then neutering it back with a triple duty is less effiicent than running the vfd at the correct frequency to maintain proper flow. its a fact that has somehow missed your schooling. Its why every utility and their mom here is giving rebates for them.

As far as variable flow through a condenser, thats a whole other discussion, but lets say for example, you have three 150 ton compressors, and one is running. why is it imperative to run full desgin flow through there? you arent gaining anything. your head will only get to a certain point, as determined by desing approach, cleanliness, and water temps. its not like it will drop off lower if you run three times the water flow through the condenser.
No disrespect to any of you at all... but I really don't like shots being taken at my 'schooling'. I didn't bring that up until Knew York called me a Know it All. Anyhow, you don't have to believe me; it doesn't change my life one bit. Facts are facts and you can choose to believe them or not. I didn't make this stuff up; it is standard heat exchange physics.

And why does it matter if only one compressor is running? Because even though only 1/3 of the load is being utilized your condenser remains the same size at all times. In theory, if you could restrict the size of the condenser tubes to correlate with the lower flow rates then fine... but you can't. Once you vary the flow rate low enough you drop to laminar and you inhibit heat transfer. Heat transfer is not linear, you may vary the flow by 40% or whatever but that doesn't equal a 40% less efficient heat transfer. It likely equals a 70 to 80% less efficient heat transfer. And pressures are not 'missing from my schooling'. The more turbulent the flow the greater the pressure drop through the exchanger or condenser. The more laminar the less the pressure drop... and yes the more the pressure drop the more the pumping power required. But what do you think the electricity cost is when you have a fouled condenser?

Read this document titled The Fundamentals of Heat Exchangers and argue with that. Again, I didn't make this up and I'm not devising this as I write. You can choose what facts to believe all you want.

And, I don't care what they give rebates for. It just show the lack of understanding that the municipalities have for HVAC systems. Not only that, but this discussion has been limited to flow through a condenser and hasn't included flow through the tower. Tower's require a minimum flow in order to reject the appropriate amount of heat as well. Often those minimum's are ignored when a VFD is utilized.

Don't let the facts get in the way of the truth.

jayguy: Turbulent flow is the most efficient form of heat transfer. That's not debatable. Look it up or read the documents I attached. I didn't write them. Laminar flow relies completely on thermal conductance to transfer heat while turbulent is well mixed and distributes the collected heat within the stream.

4. Originally Posted by waterworld
No disrespect to any of you at all... but I really don't like shots being taken at my 'schooling'....
i did not mean any disrespect to your schooling.

Originally Posted by waterworld
....jayguy: Turbulent flow is the most efficient form of heat transfer. That's not debatable. Look it up or read the documents I attached. I didn't write them. Laminar flow relies completely on thermal conductance to transfer heat while turbulent is well mixed and distributes the collected heat within the stream.
i completely agree with you here. i did not disagree with you on the turbulent flow being the most efficient form of heat transfer...it IS the most efficient...plain and simple. my argument was with your statement of: 'you MUST have turbulent flow in order to have efficient heat transfer'. while this may generally be true, it isn't always NECESSARY (or possible, like low load situations) to have efficient heat transfer.

i think that water flow and its associated uses is a ridiculously simple concept and it is not that difficult to understand...to engineer is different...but to understand and use (already engineered products) seems quite simple to me.

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so please, explain why setting a pump speed to to exact frequency required for a given flow is less efficient than setting it at 60htz and then choking back flow with a balancing valve? I am trained t o think the opposite is more correct. hell, make me a beliver, and it will save me tens of thousands on my next design build project by eliminating vfd's and control wiring.

6. The soft start option a VFD gives you compared to across the line is the only advantage I can think of on a system that uses constant flow.

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soft start is great, as it reduces inrush, and helps to prolong the life of components. the real benefit is that a properly selected pump/vfd combo will see greater hydrauic efficiency at lower speeds in many instances, thereby reducing actual horsepower used dramatically depending upon nameplate horsepower. the larger the motor, the greater the savings. this is true, in general, down to about fifty percent of design flow.

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Originally Posted by flange
so please, explain why setting a pump speed to to exact frequency required for a given flow is less efficient than setting it at 60htz and then choking back flow with a balancing valve?
I'm not sure to whom this question is directed, but I am in your camp with a VSD on a pump set to the correct frequency to maintain a desired flow. Many people don't understand the savings of a VSD is a cube function. A 100 HP motor running at 30 Hz is equivalently 12.5 HP, run at 45 Hz and the equivalent is 42 HP. There are many advantages to eliminating the triple duty valve and controlling flow with a VSD controlled pump.

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waterworld, who sees no value in vfd's on a condenser pump.

10. There seem to be a lot of generalizations being proposed here. All systems have common tenants of operation, but like some have pointed out, every system has differences in the equipment used and the way it is installed. Enough straight pipe before a condenser or an evap to generate concentric flow changes a host of other variables that improves performance. Poor design/installation is often masked by increasing flows/HP needed.

The high MHZ choppers in the newer drives can produce a "designer waveform" in response to the "stored software profile" of the motor that was produced when "auto tune" was run.

JMO.....

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I realize this thread is old, but I would like to correct some of the misinformation you have received. Most centrifugal chillers have no issue with variable condenser water flow. I recommend that you contact your chiller sales engineer about your chiller's specific requirements. The chiller O&M manual or design manual usually provides the minimum or maximum allowable flows. These limits are based on tube velocity.

The sequence of operation for the condenser water pump VFD can be designed to maintain the chiller's design condenser water temperature range or it can be designed to maintain a specific refrigerant head. The sequence needs to include the minimum and maximum chiller flow limits as required by the manufacturer.

To determine the savings associated with lower condenser water flow, I recommend you calculate the energy consumed by the pump, chiller and cooling tower under different conditions (temperature and flow). The most efficient operation will be the one with the lowest combined energy of all three components. Systems that have relatively long distance between the chiller and the cooling tower or systems with more than two chillers will benefit the most from the use of variable condenser water flow.

Regarding cooling tower temperature control, the best sequence again should consider the energy consumed by the pump, cooling tower and chiller combined. Setting the temperature at a constant 80 deg F is the least efficient method. Setting the fan speed to achieve the lowest allowable temperature is better, but not the best. The best method is to reset the condenser water supply setpoint based on the wet bulb temperature (WBT) plus the approach temp. The approach temp will vary with the WBT and load, so there should be a reset schedule for the approach. For example, the approach may be 6 deg F when the WBT is 78 deg F and the load is 100%, but it could be 10 deg F when the WBT is 60°F at 100% load.

Hope this helps.

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The guy who asked the question hasn't been back since a day after he asked the question over a year ago.

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That means we answer all his questions

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