By Chad Edmondson (JMP) and Norman Hall (RLD)
Today we begin a series of blogs exploring modern variable speed pump control.
While we have written about variable speeding pumping in the past, we believe that the increasing presence of variable speed systems, along with the rapid evolution of control options, warrants a comprehensive look at variable speed pump control as a singular topic.
Variable speed pump control is particularly relevant today because of the increasing acceptance of ASHRAE 90.1-2010 and above into building codes all over the country. The map below shows how many states have adopted the standard into their commercial building codes as of February 16, 2017. In a matter of a year or two, we can assume that most of the country will be some shade of green, making variable speed pumping and control extremely important in modern mechanical system design.
ASHRAE 90.1-2010/13 and Pump Control
ASHRAE 90.1-2010 (and up) significantly ramp up the efficiency requirements on chilled water pumping systems. Most notably the standard sets a very low threshold of tolerance for constant speed pumping. It also gets very specific about how variable speed pump control shall be applied.
The standard includes three important statements about variable flow system design and control:
As shown above in red, any variable flow chilled water system (hot water does not apply) that utilizes motors of greater than 5 HP must have some sort of control that limits motor demand to no more than 30% of design wattage at 50% flow. At just 5 HP, this would apply to most any commercial chilled water system out there.
Furthermore, the standard states that the controls can operate as a function of either the desired flow OR the minimum required differential pressure at the heat exchanger requiring the greatest differential pressure, typically the most remote coil.
This last amendment is especially critical because it indicates that when differential pressure control is used and the building has a DDC operated BMS (building management system), then the controller must continually change or “reset” the differential setpoint downward until one 2-way valve is “nearly wide open.” (Note: We can assume that “nearly,” means not wide open, since there must be some degree of throttling on the valve to maintain correct flow.)
This is a game changer. Many engineers incorporate differential pressure setpoint control for variable speed pumping systems. Few systems currently reset that DP setting.
There are exceptions to the standard. The flow rate cannot be less than that required for proper operation of equipment. Also, if the total pumping horsepower of the system is less than 75 HP, this portion of the standard need not apply.
Control Curve vs. Control Area
With these changes, ASHRAE 90.1 – 2010 (and up) effectively distinguishes the two categories of variable speed pump control logic that exist today:
(1) Control that is based on the “Control Curve” of the system and the selected pump
(2) Control that is based on a “Control Area,” which also considers the diverse load conditions that are likely to exist within any building or system at a given time.
Next week we will review “Curve Control” and “Area Control” and outline the strategies for implementation.