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System Design with Automatic Balancing

There are two major differences in overall system design when using automatic instead of manual balancing. A system with automatic flow regulators requires fewer balancing devices than a manually balanced system. With automatic balancing any overheading is distributed to the AutoFlow devices at the terminal units; in a manually balanced system the overheading appears at the triple-duty valve at the pump discharge.

Manual System

A correctly designed manual system requires that balancing devices be placed in series at each of the subsystems; i.e. terminals, branches, risers, mains and the pump discharge. The system cannot be proportionally balanced without metering and throttling devices at each of these points. Final system balance is obtained by throttling the valve at the pump discharge and/or trimming the pump impeller to reduce the flow to the design point.

Automatic System

In a correctly designed automatically balanced system, automatic regulators are placed only at the terminal equipment such as fan coils and air handlers. They are not required, nor are they recommended, for branches, risers, mains or at the pump discharge. AutoFlow valves can be placed in series with each chiller or boiler to prevent overflow.

It is not necessary to throttle the valve at the pump discharge as with manual balancing because each AutoFlow valve has dynamically absorbed the overheading.

To measure this overheading, determine the differential pressure at the most remote AutoFlow valve and subtract the five feet required for its operation. This excess pressure head can be reduced by trimming the pump impeller or reducing the pump RPM in variable speed systems.

 
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Balancing Valve on Coil Return
As recommended by ASHRAE
Required Balancing Devices in a Properly Balanced System

In accordance with standard recommendations.

 

* Except for Primary / Secondary or Modular Chiller

At The Terminal

The preferred location for manual or automatic balancing valves is the return side of the coil, as shown in the detail in the Balancing Valve on Coil Return diagram above. The return side is recommended by ASHRAE and is preferred by many engineers because it will:

  • Minimize air entrapment
  • Reduce noise problems
  • Decrease the possibility of ATC valve cavitation

Refer to Appendix D of the "Coil Hook-Up Design Guide" for more details.