By Chad Edmondson
To control a variable speed pumping system, we must determine the minimum operational head of the system. Some refer to this value as constant head, minimum head, independent head, etc. Throughout this and in other JMP blogs, we refer to it as the “control head” as it is ultimately the basis for any control strategy that one chooses.
As quick review, control head is the minimum amount of head the variable speed pump(s) must produce to be able to instantly establish full flow through the critical coil(s). How we calculate the control head varies depending on whether we are using a curve control or area control strategy.
Control Head for Sensorless Curve Control
When it comes to sensorless pump control, or any of the three methods of curve control, we recommend basing the control head on the pressure drop across the entire critical zone. Otherwise, you increase your risk of experiencing a miss at this critical and typically most remote coil.
Keep in mind that when using sensorless pump control the default control head from the factory is typically set at 40% of the estimated pump head at design conditions. That and the fact that almost all pumps are over-headed to begin with makes meeting the ASHRAE 90.1 requirement for using no more than 30% design wattage at 50% of design flow a bit tricky. Engineers must be mindful of this and understand that any over-heading of the pump must be corrected during commissioning. The commissioning of any curve control system is critical to make sure we are operating efficiently while maintaining building comfort. Without commissioning, the system is also unlikely to meet the ASHRAE requirement.
We recommend using flow limiters or PIC valves in any variable speed system that uses curve control to help reduce the control area of the system. A JMP guideline to calculating the control head for curve control pump control (e.g. sensorless) is to specify it as the total pressure drop through the last zone, including the coil and piping and pressure loss between the pipe tees in that zone. This ensures sufficient pump head to provide full flow through the last two (2) zones if they are the only ones open. Even then we may still experience an occasional miss depending on the diversity of the system and the size of the control area. Flow limiters or PIC valves help reduce these misses and improve our chances of keeping the owner and occupants comfortable.
Control Head for Area Control
For systems that utilize an area control strategy, the control head should be calculated by adding the pressure drop of the coil, control valve, and flow limiter (if applicable) in the critical zone. Again, the critical zone is typically the coil that is furthest from the pump. By maintaining this differential pressure across the critical zone, you ensure that you can always provide full flow across this zone whenever it is needed.
For the fun of comparison, consider the difference in the following control heads for our example system with a design flow of 1000 GPM at 89 feet of head:
In this example it’s clear which control strategy will yield the most operational savings. By using area control with a DP sensor, we can substantially reduce the control head and thus the energy consumption of the pump(s).