Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
(OP)
I am obligated to put a VFD on a hot water pump in a situation where the application is bad. I personally don't think it is going to reduce speed very much at all, but like I said, I am obligated. Common sense is not part of this equation. Can you help me choose the best location for the DP sensor.
I have a hot water system pump that serves an air handler in the boiler room and VAV coils throughout the building. The air handler is piped direct-return, and the VAV's are piped reverse-return.
When I calculate head in the AHU loop alone, I come up with 33'. If I calculate the VAV loop alone (even with full flow through the AHU), I get only 15'. The the pump is sized for the total flow of 72 GPM at a head of 33'. Approximately 75% of the flow goes through the air handler right in the boiler room.
The pump curve is pretty flat. Since the air handler coil and immediately piping constitute almost all of the pressure drop, I was going to place the DP sensor in the boiler room somewhere around the AHU coil. With the flat curve, the pump won't slow down much at all.
The control contractor on the job has the "rule of thumb mentality" that the DP sensor needs to be placed 2/3 of the way down the piping. I don't see this as an option. If I placed the sensor out in the system and tried to maintain 10' (or whatever) across the VAV boxes, I would have no way to guarantee flow through the AHU coil, would I?
Any suggestions on DP sensor placement and setpoint would be appreciated. See attached.
I have a hot water system pump that serves an air handler in the boiler room and VAV coils throughout the building. The air handler is piped direct-return, and the VAV's are piped reverse-return.
When I calculate head in the AHU loop alone, I come up with 33'. If I calculate the VAV loop alone (even with full flow through the AHU), I get only 15'. The the pump is sized for the total flow of 72 GPM at a head of 33'. Approximately 75% of the flow goes through the air handler right in the boiler room.
The pump curve is pretty flat. Since the air handler coil and immediately piping constitute almost all of the pressure drop, I was going to place the DP sensor in the boiler room somewhere around the AHU coil. With the flat curve, the pump won't slow down much at all.
The control contractor on the job has the "rule of thumb mentality" that the DP sensor needs to be placed 2/3 of the way down the piping. I don't see this as an option. If I placed the sensor out in the system and tried to maintain 10' (or whatever) across the VAV boxes, I would have no way to guarantee flow through the AHU coil, would I?
Any suggestions on DP sensor placement and setpoint would be appreciated. See attached.





RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
There maybe different options of controlling the pump speed rather than a constant DP setting. You can do a reset based on AHU demand, or possibly use a sensorless type pump that can auto-adapt to the demand...maybe...that would require a bit of digging.
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
i assume these are 2-way valves?
With static pressure reset (based on valve position), you can measure pressure over the pump.
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
Unless you have flow control on your loops, those flows will not happen. The system will just hydraulically balance itself and the flow through the high DP section ( your AHU in the mech room) won't be 51, but something lower and the flow through the VAVs more than 21.
If your pump curve is flat, why does anyone think a VFD is going to help anything.
Does the system work well now? If not what is the problem?
If common sense isn't present then just place the DP sensor at the pump outlet and be done with it. What I really think you need are some flow regulators and a flow meter to use to control the VFD to your total of 72. Is the temperature control by on/off or by bypass of the heating elements?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
as much as I would like to redesign the whole concept, I don't have that possibility here. so I have to put in the vfd, even if it doesn't do a darn thing. There are circuit setters at each coil that I did not show, so we will be able to get an initial balance. I'm just trying to find the optimal place to control the vfd giving the circumstances
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
Your key pressure drop as far as I can figure out is the large unit, so make that the control point / DP. If you need 33ft to make it work, but the other units need less ( thought they will be more once you balance the system), then make that your control point.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
A full layout drawing showing where the flow control is and how it works is needed before anyone can say whether the differential pressure can change or not.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
Of course flow will be reduced in 2-way valves when valves are not fully open (partial load). What else do you think a throttling valve does?
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump
RE: Hydronic Pump VFD Differential Pressure Sensor location when large pressure drop is near the pump