By Chad Edmondson
Think you know all there is to know about variable speed control strategies? Don’t be so sure. A new, patent pending technology from HYFAB called DemandSet brings another option to the table. It has the potential to yield pump energy savings that are even better than traditional curve and area pump control strategies.
What is DemandSet Control?
DemandSet control is a pump control solution that requires all control valves in the system to be full-stroke pressure independent control (PIC) valves. It controls the variable speed pump without remotely installed sensors. DemandSet controls the pump speed so that one PIC valve is always at the lowest end of its control range. It does not use control head (neither actual nor virtual). This eliminates control head creep and any programming in the field.
How Does It Work?
DemandSet control periodically reduces the pump speed, each time monitoring the pump flow for any resulting flow reduction. If no flow reduction is detected, the control maintains the new lower speed until the next “test." This strategy requires that all valves be PIC valves and that a single flow meter be located near the pump. The PIC valve must be full stroke (i.e. the actuator is driven through its full range regardless of valve design flow versus maximum flow) so that the valves are accurate at reduced flows.
Consider the following system installed with PIC valves and operated with DemandSet Control. In the example shown below, the PIC valves must see a minimum of 10 ft. of pressure across the valve to maintain control. Without this minimum pressure, the spring inside the PIC valves integral differential pressure regulator will become fully sprung (uncompressed) and will no longer have any authority over the flow.
Like differential setpoint control, we want to maintain the lowest possible pump speed and still maintain 10 feet of pressure at the critical valve. (Remember, PIC valves combine the work of a balance valve, control valve, and differential pressure regulator all in one, so no additional balancing devices are required.) It is critically important to understand that as long as we are in the control range of the PIC valve, a change in pressure will NOT result in a change in flow. Only a change in demand will result in a change in flow. That is the beauty of using PIC valves!
The following diagram shows what this DemandSet controlled system might look like at full flow conditions:
What Happens When Demand Drops?
Now let’s say demand drops and flow is half what it was before. We have 600 gpm total with 200 gpm to each circuit (see below) with the PIC valves throttling to reduce the flow. If the pump speed stays the same and the flow is reduced, the pressure drop across the valves increases as we ride to the left of the pump curve. Notice that the pressure drop across the PIC valve in Zone 3 is now 17.5 feet and is the lowest of all valves and we are still well above the required 10 feet for this valve to still be in control.
What will the controller do next?
So, the DemandSet controller goes thru its cycle and slows down the pump. Now we are down to 12.5’ at the critical zone which still happens to be Zone 3. The flow did not change and we still have at least 10 feet of pressure drop across all of the valves:
There is still a little room to play with, so the controller decreases pump speed a little more:
At this point we are still maintaining the required 200 gpm at each circuit, but we have reached the minimum pressure threshold of 10 feet for the PIC valve in Zone 3. If the controller reduces pump speed any further it will instantly sense a reduction in flow. At that point, it will increase the pump speed back to the last speed at which it was able to maintain enough pressure drop across the critical PIC valve to keep it in control of the flow.
This control cycle continues at a specific time interval with the control taking its cues from the flow sensor at the pump to determine whether it should increase or decrease the speed of the pump.
DemandSet control offers the highest possible pumping efficiency of any current strategy while eliminating the need for programmed setpoints and commissioning. We are able to keep the critical valve in the system almost wide open similar to the ASHRAE 90.1-2013 requirement for differential pressure reset but without looking at the valve position. It is also compatible with any brand of variable speed drive and pump.