One part of checking out the status of a chiller system is to review the chiller flows and primary vs. secondary flow (& decoupler flow).
Often, CH systems do not have gpm meters (oh, so fun).
Calculating water flows from field readings is part art and part science, which results in an estimate. Best to get a couple of 'votes' and take the average - which mean different ways to calculate flows (it's time consuming).
I've seen & done three ways:
1). Take pumps delta-P readings and using the pump curve.
2) Take delta-P reading across the CH evap bundle and use the factory bundle curve. (see attached PDF).
3) Take current readings of the pump motor. Use the motor nameplate data and do some 'back calc' math to get GPM.
kVA = 1.73 x kV x Amps
kW = kVA x PF
Calc. HP = kW x Motor Eff.
Actual GPM = (Calc. HP/nameplate HP)^1/3 x design gpm
Appreciate any comments or other methods to get CH gpm. Even if the BMS graphics displays flow - do you trust it when checkin' out the water balancing?
The most reliable indication of water flow thru the chiller is with chiller pressure drop. Test the pump shut-off head and compare it to the pump curve. Measured shut-off head should be within 5 to 7 feet of that shown on the pump curve. Set the flow thru the chiller by chiller pressure drop and then measure pump delta P and compare with operating point on the pump curve. That gives you a pretty good baseline for primary flow. Secondary flow is not as cut and dried because the flow varies and efficiency varies with it. My preference is to measure pump shut-off head and then establish water flow as close as possible to design with pump delta P. Measure pump KW and compare against where it should be at design. Back that up with reading flow indicated by the triple duty valve and compare with pump test results. Compare measured temperatures to determine direction of flow thru the bridge and approximate the quantity. If the bridge is blind the difference in readings is about as close as you can get to bridge flow. It takes all of that as far as I'm concerned to get close enough to have confidence in the final results. If tested with low secondary flows the results in my opinion will be less than satisfactory.
I also prefer chiller pressure drop.
Just remember that tube cleanliness and tube obstructions can affect that. Usually only have to worry about that on open systems, but always best to keep it in mind.
Excellent point Nuclchiller,
My response only addressed water flow measurement. In my opinion the best indication of how efficient the system is operating is the secondary return water temperature. Low return temperature indicates excess secondary water flow and wasted pumping horsepower. The closer that temperature is to design the better.
Great replies. Sometimes it's not 'learning', but confirming.
One of seasoned "NEBB" guys did take readings at the Triple Duty Valve.
So, what if CHWS has a field installed CHW flow meters that transmits back to the BAS?
I've seen many - clamp on ultrasonic (Siemens Sitrans series); turbine; vortex (Onicon).
Do you use it? Trust it? Like the saying goes, 'trust everybody, but cut the cards"
If the plant has put in a Hartman or OE CH optimization 'program' - they use very accurate flow meters - they put them on each chiller and the on the main's. Why? Because the 'black box' needs to calculate accurate BTUs (flow & DT). Seems reasonable to use that flow data.
I would research the quality and accuracy of the flow meter including allowable turn down ratio, verify it had been sized correctly and accurately calibrated. If satisfied with all that I would trust the BAS because it basically regurgitates what it is told. When I did design and needed an accurate flow meter for variable flow I only trusted the magnetic type. That was a while back and there now may be others.
As a chiller tech, I recall one customer who contracted with JCI to handle all their controls. Their plants all had flow meters that appeared to work very well. I couldn't begin to tell you what brand they were, but I do recall they had a local digital display that I almost always compared my pressure gauge measurements against. I ALWAYS measured with a gauge, but curiosity compelled me to look at the flow meter readings. I don't remember seeing appreciable differences.
However, I have seen some that I didn't trust. Ironically, among the worst were early versions supplied by Trane, installed at the factory on their centrifugal chillers. I used to work for Trane. I typically set the chiller panel to ignore them. Every one I saw was part of an option package, where flow measurement wasn't even being used. This was back in the 90's, I have heard they use a much improved flow meter now.
Record the model and the serial number of the chiller. Contact the manufacturer's sales representative. They may be able to provide you with the chiller selection information. This document will provide you the design pressure drop and design flow. If you measure the pressure drop accross the chiller's barrell, you can calculate the flow.
Good reply - that's exactly what I had to do on a recent new 40T chiller install. There was a big 'bugaboo'
regarding getting good flow values between the DDC tech (on a MetaSys/JCI system) and the tech setting up the Acronis flow meter.
So I put in my Chiller BTU Station (see Energy Section - 'CH BTU Station'). Was suspect since it was low flow system (40T unit).
The chiller was located in Monterey, CA. The OAT weather in late September was cold, so the chiller didn't run to ofter, when it did it was for a short duration. Cool weather meant low flow. Had to purchase Pete's Plugs adapters to interface the 1/4" NPT connectors on my pressure sensors.
Tried to used the factory curves from the Aqua Snap manual - but it didn't work out. So used the "square-root' method as you pointed out. The trick was using the delta P value at Zero GPM. Real world is different that the 'paper world'. Got some reasonable flow results - wish I had warmer weather (another 'real world' situation). In the end we were basically satisfied that the Acronis/DDC flow setup was ok and could trust the gpm values reported on the MetaSys front-end. Having good flow #'s means good chiller BTU calculation. We also trended the chiller amps to get kW/Ton.
Deux 1,
The discussion is primarily about how to measure flow on existing systems. The fixed plate orifice would have to be installed and only has a turn down ratio of 10 to 1. If I was going to do that I would install a magnetic flow meter that has a turn down ratio of 30 to 1 and an accuracy of better than 1%. The mag meter also requires a lot less undisturbed flow than the orifice plate. The mag meter also costs a lot more money.
No man can be both ignorant and free.
Thomas Jefferson
10:1 turn down for an FPO? That's pretty good - maybe there's a brand out there. I'm heard that FPOs typically have a turn down between 3:1 to 5:1. A mag flow meter has a great turn down, but maybe not needed to cost-effectively monitor chiller flow. Typical chillers operate above 40%. Ok, lets say above 30%. For a 1000 Ton chiller @ 2.4gpm/ton the max flow rate is 2,400 gpm (10F design drop). 30% of 2,400 gpm = 720gpm. The turn down ratio = 2400/720 = 3.3:1. In my IMHO, I wouldn't use an FPO, especially if you have to modify the piping. Just slap in a pair of reliable pressure transducers to trend the pressure drop acorss the chiller evap barrel. Of course to get GPMs, you'll need the chiller factory pressure drop -gpm data.
you are on target tridiumtech. The point someone made about fouled tubes is also true but if you use the pump, chiller pressure drop and triple duty valve for comparison you can tell if the tubes are fouled. Mag meters are very expensive and cost prohibitive in most cases but can read to 0.5% accuracy with ideal flow and around 2% with very little undisturbed flow. If you want to add the economical best choice I think I would go with the ventura meter. It requires only about 5/2 pipe diameters up and down stream for good accuracy and cost moderately more than an orifice flange.
When I did balance work I had to take readings for flow meter calibration and you would be surprised at how many that had to track variable flow couldn't be set up for full range accurate operation. That is one reason I like chiller pressure drop.
No man can be both ignorant and free.
Thomas Jefferson
Determining flow from the pressure drop on the pump assumes that the impeller is correct or there's little wear.
When doing chiller system performance analysis the president of the energy company I work for likes to do all three:
1) CH evap barrel, 2) drop on pump and 3) drop on the triple-duty. And we also do a 4th: monitor the pump amps and
use the pump laws + the design flow rate (again assumes proper impeller). It's time consuming, especially doing several
spot readings during the day (rather than using trending loggers). The benefit is that you can look the client 'in the eye' with
your flow #'s. When you tell the client the flow is low due to the installed pumps didn't match the design prints - emotions can
run 'high'. I've seen plant techs get pretty heated when they're told the real condition of their plant and why its not meeting the design capacity. System performance and energy efficiency go hand-in-hand.
The pump impeller can be verified by running a shut off head test. You need a pump curve to match the results with. It takes some experience working with pumps to determine if the impeller is ok because in 20 years of testing water systems I only saw two pumps that followed the chiller exactly. I don't know about you guys but I found the triple duty valve to be the least reliable but close enough for comparison.
The pump power is a good point. Since I did TAB I had to do a full work up and always had that information. I found that application of the pump laws was a lot more reliable than application of the fan laws because pressure readings on water systems is a lot more accurate than air systems. The occasional unstable system though drove me crazy.
No man can be both ignorant and free.
Thomas Jefferson
I've only done a few shut off head tests, or I should say, been around when they were being done. Maybe a bit of 'black art', but hands-on experience is very important. At a pharmaceutical plant job, a NEBB co-worker was troubleshooting why a new pump, at lower elevation, could get enough gpm up to an AHU. So he started the pump to get it up to full flow. Then he slowly closed the discharge valve until it was fully closed, then read the supply and discharge pressure. The pressure readings were compared to the max. pressure found on the pump curve. We found out that the new pump was too small. Ah, sure, you do the same test to see if you impeller is worn (or sized incorrectly).
I went to the "Engineering Toolbox" and found this equation: head (ft) = [(d x n)/1840]^2. d = impeller outside diameter (inches), n = wheel velocity (rpm). Do you ever get that 'technical' and use this formula? Thank for the reply Wayne.
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