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Case Studies & White Papers:
Demand Based Static Pressure Reset Control for Laboratories, page 2

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The savings that can be accomplished using Demand Based Static Pressure Control compared to a fixed static pressure setpoint will vary based on a number of conditions including:
  • Lowest operating pressure drop of the valve
  • Duct conditions such as straight runs into the valve
  • Whether the valve actually measures airflow
  • Electric utility rates
  • CFM of the system
  • Efficiency of the selected fan system
Lowest operating pressure drop of the valve
An airflow valve that is capable of operating at a lower static pressure gives the opportunity for the largest energy savings. Dual blade dampers such as the AccuValve™ and bladder type dampers such as the Pneumavalve offer the lowest pressure drops of any critical environment airflow control valve on the market. Venturi valves can easily require an additional 0.75" to 1.25" to operate safely in a lab.

Duct Conditions
While a number of valves on the market state that no straight runs of ductwork upstream and downstream of the valve are required, it is important for the design engineer to attempt to use best engineering practices in locating airflow control valves in order to reduce operating pressure.

There is a penalty incurred when mounting any valve in ducts right after an elbow or similar conditions. System effect of the valve being located in these types of locations cause static pressure losses. This is true for any type of airflow control valve. When mounted in these locations the system will need to operate at higher system static pressures which of course relates to higher operating costs.

Where possible, straight duct runs should be designed into the ductwork before and after the airflow control valve to minimize system effect and static pressure losses. Where that is not possible, the inherent design of the AccuValve™ which does not require straight runs and operates at a much lower pressure than the venturi valve is the best engineering design choice.

True Airflow Measurement
While it would not appear that measuring airflow would have an impact on static pressure in a duct system, there are hidden costs of not measuring airflow. Through the measurement of true airflow, the valve will go to whatever position is necessary to achieve the airflow that is required.

This will occur regardless of the static pressure in the duct. On the other hand, a venturi valve which relies on a calibrated spring and plunger only "knows" to drive to a specific valve position and relies on the spring to compensate for pressure changes. If the pressure is too low, the spring will seat against a stop and will no longer control. If the pressure is low and the shaft/plunger has accumulated particulate, the increase in friction may not allow the spring/plunger to overcome the static pressure changes and the valve will not control properly. Because of these issues with the venturi design, static pressures in the duct are required to be higher than in a duct where true airflow measurement is the basis of control. In addition, where safety is important, true airflow measurement gives the user the knowledge that their critical environment is being controlled and monitored for "actual" conditions, not "assumed" conditions.

Calculating Static Pressure & Cost Savings
Savings based on static pressure in the system is based on the following equation:


To determine the savings that can be seen for this system the following equation would be


Based on the above equation, the building owner would see a savings of $178,939 over the 20 year life of the system for the supply fan. He could expect similar savings for the exhaust, giving a savings of $357,878 for the entire system.

These savings are quite significant. Designing a laboratory as VAV gives the owner savings by reducing the CFM thereby reducing operating cost and lessening the impact on the environment. It would be counter productive to give some of that energy savings back by using a high pressure duct device such as a venturi valve instead of a low pressure device such as the AccuValve™.

With the lower pressure drop afforded by using Demand Based Static Pressure Reset Control and low pressure drop airflow control valves, it is possible that silencers can be eliminated thereby reducing the static pressure in the system even further giving even greater savings in energy.

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