When the HVAC industry started broadly applying variable speed drives to pumps, application guidance on control valves and balancing wasn’t as well understood as it is today. The additional controls and function of a pump’s speed drive subtly changed control system operation, making attached heating and cooling systems less controllable—and often negatively impacting their energy efficiency.
The Danfoss AB-QM control valve solves these problems and helps substantially restore control, comfort and energy efficiency back to variable speed, variable-flow pumping systems used in HVAC.
Historically, conventional control and balancing valves relied on a number of design calculations applied by engineers. For example, when control valves operated, reducing flow, the pumps that provided the required water flow either stayed at the same pressure or increased slightly.
To an engineer, examining the pump characteristic curve implied how the control valves worked and interacted. Wide application of variable speed drives on pumps in the 1990s led to a number of technical debates about system operation, with many opinions on hydronic system operation. It was clear that things didn’t work the way they used to— before the drive was applied.
As control valves reduce flow, the drive slows the pump speed, lowering system pressure as a result. That’s opposite of the former pump and valve operation. The change in system pressure alters how a temperature controller regulates the valve.
In constant-speed pumps with standard valves, the controller “thinks” that as it opens and closes the valve, it always has a linear proportion of heating or cooling. Fifty percent open would mean 50 percent of the total capacity for heating or cooling the “old” way.
When a variable speed drive is connected to the pump, the amount of heat transfer controlled through the valve changes for each pump-speed adjustment. The temperature controller needs extra functionality to restore the heat transfer response to the same level it was in the constant-speed pump.
The AB-QM solves this problem by intergrating a differential pressure control regulator with a temperature control valve in the same body. This simple feature stabilizes pressure to the control valve, restoring the predictable nature back to he valve, regardless of pump speed. The pump only needs to produce the minimum required pressure for the valve to operate, just as pumps are currently controlled with variable speed drives.
In addition to allowing the temperature controller to work as intended, the unique user adjustment of the ABQM valve allows it to control the exact flow required for each individual coil. This is the balancing feature that requires a separate balancing valve in other system designs.While the AB-QM replaces the balancing valve, thereby reducing system cost, it provides differential pressure control—something the separate balancing valve cannot provide. The AB-QM also enhances functionality. Because it works over a specific flow range, it’s easy for commissioning personnel to reset the valve flow rate. It’s also much easier for commissioning personnel to tune their temperature controls as they know exactly what flow they’ll get for every volt of their output control signal.
Bottom line, a control valve is a simple device that requires a lot of engineering skill to be properly applied. To an owner or an occupant, all they want is to remain comfortable—and keep their utility bills as low as possible.
Using an AB-QM temperature control valve gives designers the assurance and functionality they need to make their system perform in the most optimal way. That makes a hydronic system using the valve, coupled with variable speed pumps using Danfoss drives, a tremendous technique for saving energy and being green. Owners will be happy, too, because the “green” they’ll see is the color of money saved.