Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
(OP)
Hi Guys, I am doing a energy conservation project and trying to add a VFD on the secondary and tertiary pumps in a primary/secondary chilled water system. But I am a little concerned about chiller low delta_T symptom when the cooling load is low in some cool days. Anyone has any experience on this? Thanks so much!





RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
There is a ream of information available on this from a variety of sources. Start with the ASHRAE systems and equipment handbook and keep on reading.
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
If your primary loop is constant flow with a constant discharge temperature, the Delta T is going to vary with the secondary load not just the flow.
If you can vary the primary loop flow to match your load, you should be able to reduce low delta T issues.
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
All Low Delta T means is that you are satisfying the load with a flow rate that is too high. A properly controlled hydronic chilled water system seldom if ever requires more than 50-60% of design coil flow to satisfy the system. I know of one project in particular that the owner paid millions of dollars to replace his chilled water plant and still ended up with Low Delta T (running too many chillers to satisfy the load). This is because he tried to solve the problem in the chiller plant and not at the air handlers. Fortunately for the owner, the project was a performance contract. The performance contractor was on the hook and ended up spending an extra million out of pocket to fix the problem.
It all boils down to proper control at the air handlers. Properly controlled, the air handler should never operate below it's design Delta T. Most systems operate below design Delta T because the control valve is not capable of stable control at less than design flow. It is not impossible. In fact, there are designs that guarantee solving Low Delta T in chilled water systems. I work with them everyday.
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
The question is whether you believe that such action itself will help low delta t, but as far as I can understand, you decided on VFD pumps for different reason and just want to check about low delta t.
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
I guess the question is how do you make sure that the chillers are staging correctly. I have seen flow meters in the bypass lines of primary secondary pumping. I think that when the bypass is running backwards you know that the field is asking for more chilled water than you are producing and its time to bring on another chiller. Does anyone know about this strategy and have a good reference describing how it works??
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
Good on ya,
Goober Dave
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RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom
if the secondary flow rate was less than primary, then a part of supply water will go back through common pipe and mix with return water at primary inlet and that would decrease the return water temperature
RE: Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom