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LoopDA

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The Loop Equation Design Method has been proposed for sizing ventilation airflow components of natural and hybrid ventilation systems. While the approach has been demonstrated on a limited basis, the method has been automated in order to better evaluate its reliability under a more controlled, i.e., less error-prone, environment. A computer program was developed by NIST that implements the Loop Equation Design Method of sizing the openings of naturally ventilated buildings. The tool, referred to as LoopDA for Loop Design and Analysis , is integrated with an existing multi-zone analysis tool CONTAM. LoopDA provides the designer of natural ventilation systems with an environment in which to perform and document the process of designing the opening sizes of natural ventilation systems and analyzing the system behavior under a variety of operating conditions.

The Loop Equation Design Method consists of the following eight steps:

  1. Lay out the global geometry and multi-zone topology of the natural ventilation flow loops for each zone of the building.
  2. Identify an ambient pressure node and additional pressure nodes at entries and exits of each flow component along the loops.
  3. Establish design conditions: wind pressure coefficients for envelope flow components, ambient temperature, wind speed and direction, and interior temperatures; evaluate ambient and interior air densities.
  4. Establish first-order design criteria (i.e., a ventilation objective) and apply continuity to determine the objective design airflow rates required for each natural ventilation flow component.
  5. Form the forward loop equations for each loop established in step 1 above by systematically accounting for all pressure changes while traversing the loop.
  6. Determine the minimum feasible sizes for each of the flow components by evaluating asymptotic limits of the loop equation for the design conditions.
  7. Develop and apply a sufficient number of technical or non-technical design rules or constraints to transform the under-determined design problem defined by each loop equation into a determined problem.
  8. Develop an appropriate operational strategy to accommodate the regulation of the natural ventilation system for variations in design conditions.

LoopDA was developed as a means to perform the eight steps of the Loop Equation Design Method. While it does not fully automate all eight steps, it greatly simplifies and provides a means to manage the entire process. Specifically, the software accommodates each of the steps as follows:

  1. LoopDA provides a SketchPad interface that enables you to draw a schematic representation of the global geometry and multizone topology of the building and to draw the natural ventilation flow loops through the relevant airflow paths of the building.
  2. The SketchPad provides the ambient pressure node and keeps track of the pressure nodes associated with each of the airflow paths that you identify on the SkethPad. The direction in which you draw the loops establishes the intended direction of natural ventilation airflow for the purposes of design.
  3. LoopDA provides for the establishment of design conditions by allowing you full control in setting ambient conditions of temperature, wind speed and direction. It also enables you to set the design temperatures of all airflow paths and automatically calculates the air densities of each. The program also provides a means to input the wind pressure coefficient of all exterior openings.
  4. LoopDA provides a means for you to define the first-order design criteria for each airflow path to be sized, however, it is up to you to select the design criteria and to ensure that continuity is not violated in the event that an opening serves multiple flow loops.
  5. Once you have established the geometry, design conditions and criteria and drawn the flow/pressure loops, LoopDA will form the forward loop equations for each loop by traversing the loop in the established direction and accounting for pressure changes due to the pressure/flow relationships of the various flow components, wind and stack effects.
  6. LoopDA calculates the minimum feasible sizes of each unsized flow component in a loop by evaluating asymptotic limits of the loop equation for the design conditions.
  7. LoopDA provides the ability to export loop information to a spreadsheet template (provided with the program) that displays all the data associated with a given loop, generates asymptotic plots and thus provides a means to view relationships between the flow components of a loop. This aids the application of design constraints, selection of component sizes and documentation of the steps in designing the natural ventilation airflow paths.
  8. Having sized the natural ventilation airflow, you can then utilize LoopDA to analyze the building performance under varying conditions. LoopDA implements the established multizone building simulation capabilities of CONTAMW 2.0. You perform analysis to investigate the effects of unintentional air infiltration, non-design weather conditions, and forced-flow elements to simulate hybrid ventilation systems.