Hydronic System Balancing with Calibrated Venturis
ASHRAE recommends that manual balancing valves be placed on the coils, risers, branches, and pump discharge so that the system can be proportionally balanced. This method is preferred by most designers for constant speed systems. Proportional balancing strives to have all circuits with pressure drop equal to the index circuit which already has the most loss. In the typical layout, the index circuit is highlighted. When proportional balancing is complete, the design GPM is achieved by trimming the pump impeller or throttling the pump balancing valve.
As an illustration, one major circuit has piped direct return; the other reverse return. Reverse return provides equal length distribution piping for all coil circuits and, in theory, requires only minor adjustment of the manual balancing valves. It also requires about 30% more distribution pipe. The reverse return loop provides each circuit with an equal length of distribution pipe; therefore, the high drop AHU coil becomes the index circuit.
Caution: Direct return, proportionally balanced systems are not recommended for variable speed pumping systems. The reason is that flow deficits may occur in some of the interior circuits. (See Flow Design Brochure F010, page 12 for more information.)
Three Benefits Over Circuit Setters
A manual circuit balancing valve performs three functions: flow measurement, flow adjustment, and flow shut-off. Of these, flow measurement is the most important. The three major factors to consider when evaluating a flow measurement device are: accuracy, ease of use, and permanent pressure loss.