Chiller Secondary Loop VFD Control
I have a secondary loop VFD controlled with 10+ 2-way modulating valves and two 3 way valves. The VFD control is currently set to maintain a specific dp. In the winter time, the flow that this loop maintains is beyond what one chiller with a constant flow pump can provide (actual load in tons is ok for one chiller) so I'm forced to run two chillers and get a low delta T due a large amount recircing in the bypass line.
I am thinking about trying an alternate control method with the vfd secondary loop. Instead of maintaining a constant dp could I not use the actual % open of the valves? My control system would constantly be looking at the open % of each valve and attempt to drive the most open to 90% and stay there (by reducing VFD speed and total flow). If it gets above 90% then it would speed back up to maintain 90%. If another valve would open past 90% then that would become the new PV. Thoughts?
I'm not sure if the system would hunt more or less with your scheme. You could try it and see. The problem is that during part load the valves and VFD start trying to accomplish the same end. When the valve closes down 25% and the pressure goes down the flow changes twice dropping the flow from say an expected 50% thru the valve to an unexpected 40% and the valve tries to compensate by re-opening then the pressure changes again...
If there ever comes time to change out your valves, you ought to look at pressure independent control valves. You set the system to provide a minimum of 1 psig at the most remote location and the valve uses that 1 psig pressure to obtain any flow within its range. So as valves close down the VFD just tracks that most remote valve and drops or raises the pressure to hold 1 psig at that valve. The valve then, because the pressures are all regulated down to 1 psig at the outlet of the balancing section, can give the coil optimum flow and not just what passes through because of pressure variations.
The other thing I might suggest (and it too is an equipment change) is adding a buffer tank between the primary and secondary loops and let the secondary side operate as it wants and control the chiller from the tank temperature, but I would have to know more about your system to know if that could help you or not.
Check the manufacturer Of the chillers Integrated Part load values, You may be running a lot more efficient than you think When running both machines
I know the guy who know's the "Chiller Whisperer"
Sorry boss, I did not realize I was in the AOP
I know the guy who know's the "Chiller Whisperer"
Are you sure you have the proper flow on the primary loop? What is design flow and delta T vs actual flow and full load delta T of the chiller? You may be able to increase the primary flow and play around with it. Also, on your two 3 way valves, are there isolation valves on your bypass lines? Although you should have 3 way valves on your farthest AHU's to keep the water flowing in the loop on low load days, you don't necessarily need the bypass isolation valves wide open. Remember, on 3 way valves, you have full flow flowing when the valves are closed (actually more flow because there is less restriction than flowing through the coil). In the wintertime, if the modulating 3 way valves are mostly closed off, closing back (but not completely closing) the bypass isolation valves will slow down the flow in the secondary loop while maintain a low flow to keep the chilled water lines purged.
As was mentioned above, check you need to check the overall power used per ton with a single chiller running full, compared to 2 running at part load. Ideally, kW/ton for the plant should be calculated continually - using vfd kW readings compared to tonnage out of the plant. In my plant, total plant kW/ton with a single chiller maxed runs about 0.70-0.75, but upshifting to 2 chillers at 50% yields a drop in usage down to about 0.60-0.63 kW/ton (humidity variable).
That said, controlling off of valve position is not necessarily a bad thing, but you may want to consider a few other options. For starters, you may want to look at whether the higher chilled water temp is actually a problem. I'm not sure what type of facility you are talking about, and what kind of humidities you are dealing with, but if your chilled water valves are cut back on just about every air handler, than it is likely your chilled water supply temp is actually lower than it needs to be - in which case, it could be walked up.
My plant is not primary secondary, but variable primary - but here is how we reset everything:
Zones report to air handlers on whether or not they need colder air (this is based on reheat valve position, and humidity if it is a humidity controlled zone).
If no zones are calling for colder air, the air handler incrementally increases supply temp setpoint (and conversely, if a zone IS calling for colder air, it lowers setpoint - all within fixed limits)
Air handlers report to the chiller plant whether they need colder water. This is based on chilled water valve position.
If no air handler is >90% chilled water valve position, it will incrementally increase chilled water supply setpoint (and conversely, if an air handler IS above 90%, it will lower chilled water temp setpoint - again, within fixed limits)
I find that altering chilled water temp has much greater effect on overall efficiency than altering building d/p.
Using valve position to control the VFD will work and could reduce the overall flow. However, you will likely still have too much CHW flow for one chiller during the winter time. I recommend you close the bypass on the 3-way valves if you have a minimum flow bypass in your system that will insure you will have the minimum chiller evaporator flow required. If the 3-way valves are larger than 2", you may need to replace them because they may not hold when you close the bypass side. Pressure independent valves are best as vangoghsear suggested. You don't necessarily need flow at all times at the end of the system. If there is a load, the valves will open and you will get flow no matter what.
If you do change the control of the VFD to be based on valve position, make sure you set dP limits to prevent a faulty thermostat from asking the pump to run at 100% when it doesn't need to.
I agree with CrazziFuzzy that running two chillers may require less overall energy than one chiller, but make sure you account for the energy consumed by the chiller, pumps and cooling tower combined.
If not limited by humidity requirements, resetting the CHWS temp higher will save energy, but will also increase the CHW flow required which brings you back to the high flow concerns you have during the winter time operation. Each degree that you raise the CHW temp will save about 1% of the chiller energy.