**NISTIR 6967**

**LoopDA -- Natural Ventilation Design and\
Analysis Software**

W. Stuart Dols

Steven J. Emmerich

![](media/image1.png){width="4.114583333333333in" height="4.65625in"}

![](media/image2.png){width="3.6805555555555554in"
height="0.8381944444444445in"}

**NISTIR 6967**

**LoopDA -- Natural Ventilation Design and\
Analysis Software**

W. Stuart Dols

Steven J. Emmerich

*Building Environment Division\
Building and Fire Research Laboratory\
National Institute of Standards and Technology\
Gaithersburg, MD 20899-8633*

Prepared for:

Architectural Energy Corporation\
Boulder, Colorado

and

U.S. Department of Energy

April 2003

![](media/image3.wmf)

**U.S. Department of Commerce**

Donald L. Evans, *Secretary*

**Technology Administration**

Phillip J. Bond, *Undersecretary of Commerce for Technology*

**National Institute of Standards and Technology**

Arden L. Bement, Jr., *Director*

# Abstract

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. This report describes a computer program 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 CONTAMW. 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. This report describes the first version of the LoopDA
program, provides an example of its application to the design of a
naturally ventilated building and describes needs for future
enhancements to the tool to increase its usefulness within the design
community.

**Key Words:** airflow analysis; building technology; computer program;
design tool; indoor air quality; multizone analysis; natural ventilation

# Software Disclaimer

This software was developed at the National Institute of Standards and
Technology by employees of the Federal Government in the course of their
official duties. Pursuant to title 17 Section 105 of the United States
Code this software is not subject to copyright protection and is in the
public domain. LoopDA is an experimental system. NIST assumes no
responsibility whatsoever for its use by other parties, and makes no
guarantees, expressed or implied, about its quality, reliability, or any
other characteristic. We would appreciate acknowledgement if the
software is used.

This software can be redistributed and/or modified freely provided that
any derivative works bear some notice that they are derived from it, and
any modified versions bear some notice that they have been modified.

Certain trade names or company products are mentioned in the text to
specify adequately the experimental procedure and equipment used. In no
case does such identification imply recommendation or endorsement by the
National Institute of Standards and Technology, nor does it imply that
the equipment is the best available for the purpose.

**Table of Contents**

Abstract [iii](#abstract)

Software Disclaimer [iv](#software-disclaimer)

1 Introduction [1](#introduction)

2 Loop Equation Design Method [2](#loop-equation-design-method)

3 The LoopDA Software Tool [3](#the-loopda-software-tool)

3.1 Installing and Running LoopDA [4](#installing-and-running-loopda)

3.2 Basic Approach to Design Using LoopDA
[5](#basic-approach-to-design-using-loopda)

3.3 Working with LoopDA [6](#working-with-loopda)

4 Design Example using LoopDA [18](#design-example-using-loopda)

4.1 Sample Building [18](#sample-building)

4.2 Design Conditions and Criteria [18](#design-conditions-and-criteria)

4.3 Pressure Loops [20](#pressure-loops)

5 Conclusion [29](#conclusion)

6 References [30](#references)

7 Acknowledgments [31](#acknowledgments)

# Introduction

NIST is investigating the application of state-of-the-art natural
ventilation concepts in small commercial buildings in California and
developing design methods for natural ventilation in new and retrofit
applications. An earlier report \[1\] reviewed the application of
natural ventilation in commercial buildings including the associated
technology, potential advantages of natural ventilation and issues that
need to be addressed. The report also addressed opportunities and issues
specific to the application of natural ventilation to commercial
buildings in California including analysis of climate suitability via a
new ventilative cooling metric, consideration of ambient air quality,
and discussion of relevant codes and standards. It also reviewed current
design and analysis processes and tools and described a plan for the
development of new design and analysis guidance and tools.

The key recommendation of the earlier report was to develop a software
tool to implement the *Loop Equation Design Method* within the existing
multi-zone modeling environment of CONTAM \[2\] to gain the advantages
of the existing user interface, airflow element base, and capability to
perform detailed analysis after completing the natural ventilation
system design. The *Loop Equation Design Method* (presented in detail by
Emmerich et al. \[1\]) is a method for sizing of natural ventilation
components that is general and practical and allows direct consideration
of stack and wind-driven airflow that is critical in addressing natural
ventilation airflow. The tool should assist the designer in performing
the needed steps of developing the natural ventilation system design
including: establishing the global geometry and topology of zones and
interconnecting flow paths, establishing design conditions, setting up
and solving the representative loop equations based on the "design" form
of the airflow components, and analyzing operation under a variety of
conditions.

This report describes the first version of the LoopDA program (for
**Loop** **D**esign and **A**nalysis tool), which was developed as a
version of CONTAM. The report is organized into three main sections --
Loop Equation Design Method (a brief review of the method implemented in
LoopDA), the LoopDA Software Tool (a description of the program and how
to use it), and Design Example Using LoopDA (an example application of
LoopDA to the design of a natural ventilation system for a small
commercial building).

# Loop Equation Design Method

The Loop Equation Design Method that is proposed for the Design
Development stage of the overall design process of a natural ventilation
system is described in detail by Emmerich, Dols, and Axley\[1\]. (Note:
The other design stages -- Conceptual Design and Design Performance
Evaluation -- both precede and follow the Design Development Stage and
are also described in \[1\]. Detailed theory of the method is presented
in \[3\] and \[4\].). 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.

As detailed in the following section, the user will perform some of
these steps explicitly (e.g., Step1). Other steps are performed
implicitly by LoopDA without user interaction (e.g., Step 5). Still
others are accomplished by a combination of explicit and implicit
actions (e.g., Step 3).

# The LoopDA Software Tool

LoopDA was developed as a means to perform the eight steps of the *Loop
Equation Design Method* presented in the previous section. 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.

## Installing and Running LoopDA

LoopDA runs on Windows 95/98/NT/XP/2000. There are three basic
components to LoopDA software system: the main Windows executable
program *LoopDA.EXE*, the spreadsheet template *LOOPDATEMPLATE.XLS*, and
the analysis or simulation engine *CONTAMX2L.EXE*.

The setup files are contained in the self-extracting archive file
\"LoopDAz.exe.\" Simply execute the self-extracter to decompress the
setup files. Extract the files to the subdirectory of your choice, or
simply select the default location. Once you have extracted the setup
files, you can run the setup program, \"setup.exe.\" This will prompt
you to perform an automatic installation of the program. Read the
instructions to complete the installation.

-   Files Installed

The following table lists the files installed by the setup program. For
each file, the directory to which it is installed, the name and a brief
description are given. The \<program\> directory is that selected by you
when you install the program. The default is C:\\Program Files\\LoopDA.
The \<font\> directory is that of the operating system fonts. The
default is \<windows\>/FONTS, where \<windows\> depends on the operating
system you are using, e.g., Windows 2000 \<windows\> = WINNT and Windows
95/98 \<windows\> = WINDOWS.

  ---------------------- ----------------------- ---------------------------
  **Directory**          **File Name**           **Description**

  \<program\>            LoopDA.exe              User interface

                         Contamx2l.exe           Solver

                         cwhelp2.hlp\            CONTAMW help files
                         cwhelp2.cnt             

                         olch2d32.dll\           Charting and help display
                         roboex32.dll            dynamic link libraries

                         LoopDA.doc              LoopDA documentation to be
                                                 used in conjunction with
                                                 CONTAMW help files

  \<program\>\\samples   \*.prj                  Sample project files

                         \*.xls                  Spreadsheet files

  \<font\>               walton##.fnt            SketchPad fonts where \##
                                                 ranges from 01 to 16 for
                                                 different SketchPad
                                                 resolutions
  ---------------------- ----------------------- ---------------------------

-   Uninstalling LoopDA

The LoopDA setup program will also provide you with an uninstall
feature. You uninstall LoopDA much as you would a typical Windows
program. Access the **Control Panel** from the
**[S]{.underline}ettings** selection of the **Start** menu. Select
Add/Remove Programs from the Control Panel. Select LoopDA from the list
of installed programs and click the \"**Add/[R]{.underline}emove...**\"
button to uninstall LoopDA.

-   Running LoopDA

After you install LoopDA, you can run it by selecting LoopDA from the
NIST program group of the Start menu.

## Basic Approach to Design Using LoopDA

This section present a brief over of the basic steps to take to
implement the Loop Equation Design Method using LoopDA to size the
airflow components of a natural ventilation system. Ideally you would
perform steps a -- d once for a given model of a building and iterate
through steps e and f. The details of each of these steps are presented
in the following section *Working with LoopDA*. Step g is the process of
performing simulations based on your design.

a.  Define ambient design conditions (see Working with Weather and
    Wind)\
    Set temperature, wind speed and wind direction

b.  Draw the elevation view of the building (see Working with Walls and
    Zones)\
    Draw the walls\
    Define the zones

c.  Draw ventilation airflow and duct flow paths to be sized (see
    Drawing Airflow Paths and Drawing Ducts)

d.  Define airflow and duct flow elements (see Working with Inverse
    Airflow Elements and Working with Inverse Duct Flow Elements)\
    Select inverse element type\
    Set design airflow rate Q~des~\
    Set design temperature T~des\
    ~Set wind pressure coefficient C~p~()

e.  Draw loops (see Working with Pressure Loops)\
    View loop data\
    Export loop data

f.  Import into spreadsheet (see Working with the LoopDA Spreadsheet)

g.  Perform analysis\
    Set the forward element properties based on design values.\
    Switch to *analysis mode* and run simulation as with CONTAMW.

## Working with LoopDA

This section serves as a user guide for LoopDA. Using LoopDA is very
similar to using CONTAMW, so the information contained in this section
is presented as a supplement to the CONTAMW 2.0 User Manual \[2\] that
is available while running LoopDA as an online help system. References
to sections of the CONTAMW User Manual / online help will be given in
the following sections as {CW: Section} and the areas in which the two
programs differ will be presented in detail.

### Working with the SketchPad

When working with LoopDA, a major portion of your interaction will be
with the SketchPad or drawing region of the interface to develop a
schematic representation of your building. The basic use of the
SketchPad is very similar to CONTAM, i.e., drawing, selecting and
editing building component data. {CW: Working with the SketchPad} The
fundamental difference is that LoopDA presents an elevation view of a
vertical section of a building, whereas CONTAMW presents plan views of
individual levels of a building. In CONTAMW there are level commands and
toolbar buttons that provide access to different building levels. These
commands are not available in LoopDA.

Because LoopDA is meant to be both a "true" design tool as well as an
analysis tool, it has an additional mode of operation that is not in
CONTAMW, i.e., the *Design mode*. You size ventilation components in the
design mode and therefore an analysis would not be appropriate due to
undefined components. The analysis mode is intended to enable the
evaluation of a given design once all of the components have been
defined or sized. While in the analysis mode, the drawing tools will be
deactivated so that you can't modify your design. You set the mode of
operation via the analysis and design toolbar buttons shown below in
Figure 1.

![](media/image4.png){width="4.155555555555556in"
height="0.4166666666666667in"}

Figure 1 -- LoopDA toolbar highlighting Design/Analyses mode buttons

### Working with Weather and Wind

The weather and wind features are used to establish the design ambient
conditions for both design and analysis modes of operation. However,
only the Steady State weather data is relevant to the design mode of
operation. Set the ambient temperature, wind speed and direction of the
Steady State weather data of the Weather and Wind Properties prior to
working with your airflow loops as this data is used in forming the
pressure loop equations. Note that because LoopDA presents an elevation
view, the wind direction should be from either the left or right side of
your drawing. This corresponds to either a direction of 0º or 180º
respectively, based on the default *relative north* value of -90º
(CONTAMW defaults to 0º) as also established in the Weather and Wind
Properties. Also, set the reference wind speed data to obtain the
desired *wind speed modifier* for your building location {CW: Defining
Steady State Weather and Wind}

The CONTAMW User Manual contains a detailed presentation of how weather
and wind are handled by CONTAMW and LoopDA in the analysis mode. {CW:
Working with Weather and Wind} As presented in that section, both steady
state and transient weather data can be implemented depending on the
type of simulation you are performing.

### Working with Walls and Zones

The mechanics of drawing walls to form zones are exactly the same as in
CONTAMW. The main difference between LoopDA and CONTAMW is that you are
drawing elevation views of a building as opposed to plan views. Each
drawing that you create should represent a vertical section of the
building in question. To represent multiple sections, you could either
draw them individually on a single SketchPad, or you can create
individual LoopDA project files for each building section. {CW: Drawing
Walls, Ducts and Controls; CW: Working with Walls}

Walls must be drawn so that fully bounded regions are created that make
up the *zones* of your building representation. The properties of each
zone must then be defined by a zone icon as typical for building
components. {CW: Working with Zones} The zone properties are not needed
in the design process, but are required during the analysis phase.

### Working with Airflow Paths

In LoopDA, airflow paths are the openings between zones, including the
ambient, that form direct connections between the zones (as opposed to
ducted connections discussed below). In the design mode, you first lay
out (draw) the location of the natural ventilation airflow paths that
will make up the ventilation flow loops and then define their design
properties.

#### Drawing Airflow Paths

You draw airflow paths in LoopDA exactly as done in CONTAMW. However, in
LoopDA, you can only place airflow paths directly on walls. {CW:
Creating Airflow Paths}

#### Defining Airflow Paths

Once a flow path is drawn you must define its design properties in order
to use the path in a ventilation airflow loop. The following items are
relevant to the *design mode* of operation, i.e., to form *Loop Design
Equations*.

Airflow Element -- *Airflow elements* describe the mathematical
relationship between the flow through an airflow path and the pressure
drop across the path. You must select an airflow element type prior to
setting the other parameters. As presented in the following subsection,
the design mode is only concerned with *Inverse Airflow Element* types.
LoopDA currently supports only two types of inverse airflow elements,
but all types of CONTAMW airflow elements are supported in the *analysis
mode*. You should create an airflow element for each flow path you want
to design in order to maintain a unique set of parameters for each path.

Elevation -- Set the elevation of each airflow path relative to the
reference height of zero for the bottom of the building. This value will
be used in calculating stack pressures in both the design and analysis
modes.

Wind Pressure -- Set the wind pressure to be variable and define a wind
pressure coefficient profile. You can simply set this to be a constant
profile or to vary according to the angle of incidence of the wind on
the wall or flow path. {CW: Airflow Path -- Wind Properties} Care should
be taken when defining flow paths on horizontal walls, as they will
default to have azimuth angles that might not make sense. Be sure to set
the reference wind speed modifier to the desire value either
individually for each flow path or globally via the Weather data that
defaults to provide a modifier of 1.0.

Detailed information on the analysis parameters are provided in {CW:
Working with Airflow Paths}, and details of working with the Inverse
Airflow Element data are provided below.

### Working with Inverse Airflow Elements

Basically, an *inverse airflow element* is one that can be sized using
the loop equation design method. In CONTAM, airflow component equations
are formulated in the so-called "forward" form while the loop method
uses these same equations in "inverse form" -- a form relating the
pressure drop across components to the volumetric (or mass) flow rate
and characteristic design parameters (CDP), e.g., an area for an orifice
element. A detailed discussion of inverse airflow elements is presented
in Axley \[3\] and \[4\].

This version of LoopDA supports two types of inverse airflow elements:
orifice and general power law. The mathematical relationships for each
are presented here in forward and inverse form.

-   Orifice Element

Forward form: ![](media/image5.wmf)

Inverse form: ![](media/image6.wmf)

Characteristic Design Parameter: *A~orfc~*

Corresponding Analysis Parameter: *Cross sectional area*

-   General Power Law Element

Note: This element can be used to represent a range of flow paths,
including a self-regulating vent when a relatively large value of the
exponent *n* is specified \[2\].

Forward form: ![](media/image7.wmf)

Inverse form: ![](media/image8.wmf)

Characteristic Design Parameter: *C*

Corresponding Analysis Parameter: *Flow coefficient (C)*

#### Creating Inverse Airflow Elements

To create an inverse airflow element you select the "New Element" button
on the Flow Element page of the Airflow Paths Property Sheet that is
displayed when defining an airflow path. This will display a list of all
available types of *forward airflow elements*. Select either the general
*power law element*, represented in the list as *Q = C(dP)\^n*, or the
*orifice element* from the list of elements on the Airflow Element
Models dialog box. This will reveal the Airflow Element Properties sheet
that contains two pages of parameters: one that contains the analysis
parameters specific to the type of model selected and one containing the
design parameters for the inverse element.

#### Defining Inverse Airflow Elements

Figure 2 shows the Airflow Element Properties sheet for an orifice
element and the design parameters associated with the element. The
parameters on the Design page are those used in formulating the pressure
loop equations along with the previously mentioned airflow path
properties.

![](media/image9.png){width="3.0743055555555556in"
height="3.8965277777777776in"}

Figure 2 -- Airflow Element Properties sheet displaying design data for\
an inverse orifice element

For each of these inverse elements the design page displays information
that is specific to the element for which the properties are being
displayed. The following is a description of each of the properties
displayed.

-   Name: This is the airflow element name you provide for this element.
    This name is used for both the forward and inverse flow element.

-   Flow Rate: Specify the design flow rate for this element. It is your
    responsibility to ensure that design flows do not violate continuity
    when multiple loops pass through common flow paths, as is the case
    for Loops 2 and 3 in Figure 9 of the example presented later.

-   CDP Design: Once you have selected a value for the *CDP*
    (characteristic design parameter), enter it here and select
    *Defined* as described below. The type of parameter this value
    represents will depend on the type of inverse element you are
    sizing.

-   Temperature: Set the design air temperature for this element.

-   Defined/Undefined: Select *Undefined* when the element has not yet
    been sized. As you select sizes for elements, you set their *CDP
    Design* values and set them to be *Defined*.

-   CDP: This is the name given by LoopDA for this particular element.
    This will be either *A~orfc~* or *C~e~* corresponding to the
    *orifice* and *power law* elements respectively. When you view and
    export loop data, a distinct area or diameter identifier will be
    provided for the element. This identifier will include the path
    number assigned by LoopDA.

-   CDP Order: This is the order of the exponent of the design parameter
    as it would appear in the loop design equation as detailed by Axley
    \[3\] and \[4\]. This will be either 2.0 or 1/n corresponding to an
    area (orifice) or flow exponent (general power law) respectively.

### Working with Ducts 

You can use LoopDA to size duct segments and inlet and outlet terminals
that represent stacks or connections between zones. In the design mode,
you first draw the individual duct segments then define their design
properties. LoopDA implements the *Darcy-Colebrook* duct flow element.
This element provides for the sizing of both *duct flow segments* based
on a friction factor and *fittings* (inlets and outlets) based on a loss
coefficient.

#### Drawing Ducts

You draw ducts in LoopDA exactly as done in CONTAMW. However, in LoopDA,
you are working in the elevation view. {CW: Working with Ducts} You
should [draw duct segments for each inlet, airflow segment and outlet of
a duct]{.underline} as shown in the figure below. This will provide a
separate term in the loop equation for each component of the duct
system. The details of defining duct segments and terminals will be
presented in the following sections.

![](media/image10.png){width="6.613888888888889in"
height="4.561805555555556in"}

Figure 3 -- Detail for drawing ducts for loop analysis

#### Defining Ducts

Once a duct is drawn you must define its design properties in order to
use it in a ventilation airflow loop. The following items are relevant
to the *design mode* of operation, i.e., to form *Loop Design
Equations*.

Duct Flow Element -- *Duct flow elements* describe the mathematical
relationship between the flow through a duct segment and the pressure
drop across the path. You must select a duct flow element type prior to
setting the other parameters. As presented in the following subsection,
the design mode is only concerned with *Inverse Duct Flow Element*
types. LoopDA currently supports only one type of inverse duct flow
element, but all types of CONTAMW duct flow elements are supported in
the *analysis mode*.

Select whether you want to create an *Airflow Segment* or
*Terminal/Fitting* (use this for creating *inlets* and *outlets* that
you want to size based on a loss coefficient). This will enable/disable
and zero-out various input parameters for the duct to help insure that
you define only those that are relevant to the type of segment as
described below.

![](media/image11.png){width="3.7909722222222224in"
height="2.4652777777777777in"}Figure 4 -- Duct segment/fittings
properties sheet

Segment Length -- If you are defining a segment icon to be a duct
segment (as opposed to a terminal), provide a value for the *Duct
segment length* and make sure the *Sum of loss coefficients* is set to
zero.

Friction Factor -- If you are defining a segment icon to be an *airflow
segment*, provide a value for the *Friction factor*. You can use the
friction factor calculator provided on the Duct Airflow Element
Properties input dialog box to help determine this value and the
*Roughness* value for analysis.

Segment Sum of Loss Coefficients -- If you are defining a segment icon
to be a duct *Terminal*/Fitting (inlet or outlet), then provide a value
for the *Sum of loss coefficients* and make sure the *Duct segment
length* is set to zero.

Elevation -- Set the *elevation* of each *duct junction* and *terminal
icon* relative to the reference height of zero for the bottom of the
building. This value will be used in calculating stack pressures in both
the design and analysis modes. Note that the difference in elevations of
adjacent junctions is not necessarily the same as the length of the
segment between them.

Wind Pressure -- See previous Defining Airflow Paths section.

#### Working with Inverse Duct Flow Elements

Inverse duct flow elements are similar in nature to airflow elements.
This version of LoopDA supports only the Darcy-Colebrook type of inverse
duct flow element. However, this element can be used to size either a
*Duct component* (*Airflow segment*) or *Duct fitting component*
(*Fitting/Terminal*) as described by Axley \[2\]. The forward form shown
below is that implemented in CONTAM. This form can be split into two
terms: one containing the loss due to the friction factor *f* (and the
duct dimensions) and one containing the losses due to fittings *C~d~*.

-   Darcy-Colebrook Element

Forward form: ![](media/image12.wmf)

Inverse forms:

Duct component ![](media/image13.wmf)

> Characteristic Design Parameter: Equivalent duct diameter *D~e~* which
> simplifies to the actual diameter *D* for circular ducts (See Chapter
> 34 of \[5\] for a discussion of hydraulic, actual and equivalent duct
> diameters.)

Corresponding Analysis Parameter: *Duct shape and dimensions*

> Relevant Parameters: Set segment *Sum of loss coefficients* *C~l~* to
> 0.0. Design value for friction factor *f* and *Duct Shape*.

Duct fitting component ![](media/image14.wmf)

Characteristic Design Parameter: Opening area *A*

Corresponding Analysis Parameter: *Duct shape and dimensions*

> Relevant Analysis Parameter: Set *Duct segment length* *L* to 0.0\
> Note: *friction factor* and *duct shape and dimensions* are not
> relevant to fittings

#### Creating Inverse Duct Flow Elements

To create an inverse duct flow element you select the "New Element"
button on the Flow Element page of the Duct Segment/Fittings Properties
sheet that is displayed when defining a duct flow path. This will reveal
the Duct Airflow Element Properties sheet that contains three pages of
parameters: one that contains the analysis parameters specific to the
Darcy-Colebrook duct model, one containing *Shape, Size and Leakage*
data and one containing the *Design* parameters for the inverse element.

#### Defining Inverse Duct Flow Elements

Figure 5 shows the Duct Airflow Element Properties sheet for a
Darcy-Colebrook element and the design parameters associated with the
element. The parameters on the *Design* page are those used in
formulating the pressure loop equations along with the previously
mentioned duct flow path properties.

![](media/image15.png){width="5.302083333333333in"
height="3.9895833333333335in"}

Figure 5 -- Duct Airflow Element Properties sheet displaying design
data\
for a Darcy-Colebrook element (Fitting/Terminal)

For each of these inverse elements the design page displays information
that is specific to the element for which the properties are being
displayed. The following is a description of each of the properties
displayed.

The Name, Flow Rate, Temperature and Defined/Undefined items are the
same as described in the case of inverse airflow elements.

-   Segment Type: This indicated whether the duct segment icon
    represents an Airflow segment or a Fitting/Terminal segment as
    selected when defining the Duct Segment/Fitting Properties (see
    Figure 4).

-   CDP Design: Once you have selected a value for the *CDP*
    (characteristic design parameter), enter it here and set the element
    to be "defined." The type of parameter this value represents will
    depend on the type of inverse element you are sizing. This will be
    either an *equivalent duct diameter D~e~* or opening area *A*
    depending on whether you are sizing a duct or terminal,
    respectively. Further, *D~e~* is the actual diameter when dealing
    with circular ducts as described in Chapter 34 of the ASHRAE
    Fundamentals \[5\].

-   Friction Factor: This value is only required for *airflow segments*
    and is ignored for *fittings/terminals*. It is dependant on the
    roughness and Reynolds number. Therefore, it is indeterminable at
    design time, because the flow, and hence the velocity, in the duct
    is not known. You can use the right side of the page to estimate a
    design friction factor. Details on friction losses are presented in
    Chapter 34 of the ASHRAE Fundamentals \[5\].

-   CDP: This is the name given by LoopDA for this particular element.
    This will display "A" or "De" to indicate either area or diameter
    for *fitting/terminal* segment or *airflow segment* respectively.
    However, when you view and export loop data, a distinct area or
    diameter identifier will be provided for the element. This
    identifier will include the duct segment number assigned by LoopDA.

-   CDP Order: This is the order of the exponent of the design parameter
    as it would appear in the loop design equation as detailed by Axley
    \[3\] and \[4\]. This will be either 2.0 or 5.0 corresponding to an
    area (*fitting/terminal*) or diameter (*airflow segment*)
    respectively.

### Working with Pressure Loops

Once you have defined the paths and ducts that you want to size, you can
now form pressure loops that define the desired airflow paths of
ventilation air through the building. The basic method consists of
drawing loops by connecting design flow paths and ducts on the SketchPad
in the direction that air should flow through them; selecting the loop
to generate a loop equation; reviewing loop properties within LoopDA;
exporting loop data to a spreadsheet template to view loop equation
details, review asymptotic plots and apply design constraints to select
component sizes. Once you decide on the size of an element in a loop,
you return to LoopDA and set that element's CDP and regenerate the loop
equation so that the defined element is "moved to the right hand side"
of the loop equation. Continue this process until all elements in a loop
are sized and for each loop under consideration.

#### Drawing Loops

Drawing loops is very similar to drawing walls with the wall drawing
tool {CW: Drawing Walls}. Select the loop drawing tool from the toolbar
as shown highlighted in Figure 6.

![](media/image16.png){width="4.155555555555556in"
height="0.4166666666666667in"}

Figure 6 -- LoopDA toolbar highlighting the Loop Drawing Tool button

The following rules apply when drawing loops:

-   You must begin drawing loops in the ambient zone.

-   Draw loops in the direction you want air to flow through the
    building.

-   Each loop must end where it began (LoopDA will automatically
    complete the loop when you reach the beginning cell on the
    SketchPad).

-   Each loop must contain exactly two flow paths that are connected to
    the ambient, i.e., an inlet into the building and an outlet from the
    building.

-   Loops are allowed to cross over and share common sections with the
    exception of the loop direction/selection arrow.

-   Loops may not cross over themselves.

#### Selecting Loops

To select a loop once it is drawn click on its direction arrow. This
will highlight the loop in orange as shown later in the description of
the pressure loops of the design example (see Figure 9).

#### Deleting Loops and Loop Elements

To delete a loop, select it to then click the Delete key on the keyboard
or select Edit→Delete from the menu. You should always delete loops that
pass through any airflow or duct flow paths before you move or delete
the paths or ducts through which a loop passes.

#### Viewing and Exporting Loop Data

To access loop properties either select the loop then press the Enter
key on the keyboard or double-click on the loop selection arrow. This
will display the Pressure Loop Properties dialog box (see Figure 11
presented later in the Pressure Loops section of the design example).
This dialog box displays the following information:

-   Loop Number: This is an identification number assigned by LoopDA.

-   Name: You can enter an optional name for this loop to help you
    identify it when you export it to a spreadsheet.

-   Description: You can enter an optional detailed description for this
    loop to help you further identify it when you export it to a
    spreadsheet. This can be very useful as you progress through the
    iterative stage of sizing elements in that same loop.

-   Number of Loop Nodes: This is the number of airflow paths, duct
    junctions, terminals and segments through which the loop passes.

-   Number of Equation Terms: This is the number of terms that would
    appear on the left hand side of the pressure loop equation. There
    will be an equation term for each airflow path and duct segment
    through which the loop passes, i.e., the sizable components, whether
    defined or undefined.

-   Driving Pressures: These are the pressure differences induced by the
    wind and buoyancy-driven airflows for the selected loop. In terms of
    the loop equation, these are the values that appear on the
    right-hand-side of the equation.

> **Wind:** This is the sum of the wind pressures acting on the two
> external openings of the loop in consideration.

![](media/image17.wmf)

*C~p~* is the algebraic sum of pressure coefficients. It is summed
positively when traversing the loop from the exterior to a wall surface
and negatively otherwise.

*~o~* is the density of the outdoor air calculated by LoopDA based on
the ambient temperature as entered in LoopDA (and CONTAMW) under Weather
and Wind Parameters.

*U~met~* is the design wind speed at the meteorological station as
entered in LoopDA (and CONTAMW) under Weather and Wind Parameters.

*C~h~* is the wind speed modifier determined based on terrain and
elevation effects in LoopDA (and CONTAMW) under Weather and Wind
Parameters.

> **Stack:** This is the sum of the buoyancy-induced stack pressure
> differences determined by traversing the loop systematically in the
> order of airflow accounting for differences in density and elevation
> along the way.

![](media/image18.wmf)

*g* is the acceleration of gravity (9.81 m/s^2^).

*~ij~* is the density of air within the flow element based on the design
air temperature of the inverse element.

*z~ij~* is the difference in elevation between two nodes in the loop. It
is positive for drops and negative for rises in elevation along the
loop.

> **Total Pressure Drop (RHS):** This is the total pressure drop
> available to drive airflow through the undefined airflow components.
> This accounts for any defined Flow Element Terms within the loop in
> question and is the value that would appear on the right-hand-side
> (RHS) of the loop equation.

![](media/image19.wmf)

-   Flow Element Terms: This is a detailed accounting of each term that
    would appear on the left-hand-side of the loop equation. It includes
    the *Type* of component (path or duct), the component number *\#*
    (as assigned by LoopDA), the *Name* of the characteristic design
    parameter including the appended component number, the *Order* of
    the exponent of the CDP as it would appear in the loop equation, the
    value of the element-specific *Numerator* that would be associated
    with this component in the loop equation, the *Asymptote* or minimum
    value of the CDP that would provide the total pressure drop
    equivalent to the RHS value above, whether or not the item is
    *Defined* or has been sized and should be accounted for on the RHS
    of the loop equation, and the *Value* of the CDP if it has been
    defined.

The Element Term that is highlighted in this list will be that against
which all others will be plotted when the asymptotic plots are generated
by the spreadsheet once the loop data is imported into the LoopDA
spreadsheet template. You can select the component against which you
would like all others to be plotted. By default the CDP with the highest
order is highlighted unless it is already defined. If all undefined
elements have the same order, then the first element in the list is
highlighted.

-   Export Loop Button: Click this button to export loop data. You will
    be prompted to give the file a name (having a default extension of
    .txt). This will create a tab-delimited ASCII file that can be
    easily imported into the LoopDA spreadsheet template. Make note of
    the name and location of the file you save, so you can locate it for
    importing into a spreadsheet program.

### Working with the LoopDA Spreadsheet

This version of LoopDA takes advantage of the powerful features of
spreadsheet software by exporting loop data in a format compatible with
typical spreadsheet programs. A spreadsheet template is provided with
the software that simplifies the presentation of the loop equation data
generated by LoopDA. Along with the LoopDA project file, the spreadsheet
also serves as a means of documenting the steps you go through in
performing your design analysis.

The sections immediately following provide the mechanics of using the
LoopDA spreadsheet, whereas the practical use of the spreadsheet in the
Pressure Loop Sizing Method is presented further along in the Design
Example.

#### LoopDA Spreadsheet Template

The LoopDA spreadsheet template is a simple spreadsheet that contains
headings for the loop data, a section that calculates plot ranges for
the CDPs, and a chart for each set of loop data you import that
automatically plots the imported data.

The exported data provide much more detailed information than is
presented in the *Pressure Loop Properties* dialog box. The data is
subdivided into the following sections:

-   File Information

-   Loop Information

-   Outdoors

-   Wind Pressures

-   Stack Pressures

-   Loop Flow Elements

-   CDP Asymptotic Values

-   Chart Data

#### Saving Loop Spreadsheets

Before you begin working with the LoopDA spreadsheet template, you
should save it under a new name specific to your current project. It's
up to you as to how you manage your loop spreadsheets. One way is to
maintain a separate spreadsheet for each loop that contains the various
stages of undefined elements. The template is preset with four sets of
data pages and asymptotic plot pages. You can easily increase the number
of pages by simply copying the existing ones.

#### Importing Loop Data

To import a LoopDA-generated loop file, open the file using your
spreadsheet software and parse it as a tab-delimited file. Then simply
select the top left cell of the desired sheet in the template (typically
A1) and paste the data (values only using the Paste Special... command
for example).

#### Plotting Loop Data

Data will be plotted automatically for up to five elements (four terms
vs. one). You can expand this by simply copying the cells located in the
bottom of column E to the column(s) on the right of E.

### Performing Analysis

Be sure to set the forward element parameters based on your design
values.

# Design Example using LoopDA

## Sample Building

The use of LoopDA to design a natural ventilation system is demonstrated
using an example from Axley \[3\]. The sample building is based
conceptually on the Inland Revenue Building in England (see \[6\] for
more detail on this building). It is worthwhile to note that by using
this sample building the Conceptual Building phase mentioned above is
already completed -- allowing us to focus on the Design Development
phase.

Figure 7 shows an elevation section view of this building model. As seen
in the figure, the building is three stories tall with the 1^st^ and
2^nd^ floors being ventilated by air flowing in through inlet *orifices
a* and *c* and out through a common exhaust stack represented by *zone
s* and *opening h*. The 3^rd^ floor is ventilated by air flowing in
through two *self-regulating inlet vents e* and *f* (see \[3\] for a
detailed description of these vents) and out through another exhaust
stack represented by *opening g*. Each zone of the building has a volume
of 800 m^3^.

![](media/image20.wmf)

Figure 7 -- Global geometry, topology, and pressure nodes for the
ventilation flow loops of a building model based on the Inland Revenue
Building, England\
(repeated from Figure 3.16 of Axley 2001)

## Design Conditions and Criteria

In addition to the global geometry presented in Figure 7, the designer
must select indoor temperatures, outdoor temperature, wind speed, wind
pressure coefficients, and ventilation flow rates to perform the design
analysis. It is not obvious what values to select for many of these
parameters. For example, it may not be sensible to select extremes
(e.g., very low wind speed) for the design ambient conditions. Such
extremes may rarely occur or may prove too severe a challenge for a pure
natural ventilation system. For many U.S. climates, it may be necessary
to either tolerate limited periods during which indoor environmental
conditions will be outside design conditions considered typical for
conventional, mechanically conditioned buildings or to provide some
supplemental mechanical systems (i.e., a hybrid ventilation system) for
such periods. That said, the procedure followed below provides a
realistic example problem only and is not being proposed as formal
guidance for determining design conditions for natural ventilation
systems. Emmerich, Dols, and Axley \[1\] described a simple method to
evaluate the suitability of a given climate to cool a commercial
building with natural ventilation and went on to apply that method to
ten California locations. That analysis indicated that the coastal
climates of California are well suited to natural ventilation with
respect to climatic considerations. Therefore, the building described
above will be situated in San Francisco for the purposes of this sample
application of LoopDA. Per the earlier climate suitability analysis, a
natural ventilation system could potentially cool a commercial building
with combined solar and internal heat gains of 10 W/m^2^ to 80 W/m^2^
greater than 90 % of the hours of the year with a supplemental night
cooling system expected to be effective for any days that would be
overheated based on direct ventilative cooling alone. Assuming a heat
gain rate of 20 W/m^2^ (a low but achievable design target), the
required average ventilation rate is 2.2 h^-1^ with a standard deviation
of 2.6 h^-1^. Therefore, a design ventilation rate of 5 h^-1^ (or about
one standard deviation above the average) is a reasonable design
objective for summer cooling. The coincident design indoor temperature
for this summer condition may be selected as 26 ºC (based on the high
end of ASHRAE's summer thermal comfort zone \[5\]). shows the hourly
outdoor temperatures and coincident wind speeds for the San Francisco
TMY2 weather file \[7\]. From Figure 8, it appears conservative to
choose an outdoor temperature of 20 ºC with a wind speed of 2 m/s for
the design conditions as hours with both higher temperatures and lower
wind speeds are rare as shown by the dashed box.

Although it is typical to use a wind pressure profile that is dependent
on wind direction (and possibly relative location of an opening on a
wall), a single design value will be chosen for this example assuming
the building will be oriented to take advantage of prevailing wind
direction. Per Figure 16.7 of the ASHRAE Handbook of Fundamentals \[5\],
a surface average wind pressure coefficient of 0.3 will be selected for
the ventilation openings in the walls. Per Figure 16.9 of the ASHRAE
Handbook of Fundamentals \[5\], a single value of -0.5 will be used for
the wind pressure coefficient ![](media/image21.wmf){width="8.375in"
height="3.848611111111111in"}on the exhaust stacks.

Figure 8 -- Wind speeds vs. outdoor temperatures for San Francisco TMY2
weather file

## Pressure Loops

With the building geometry and design conditions established, one may
use LoopDA to form the loop equations and size the various natural
ventilation system components. Figure 9 shows the LoopDA Sketchpad
representation of the building elevation with the four pressure loops
that must be analyzed. Loops 1 and 4 are equivalent loops following
nodes 13-14-15-18-20-19-13 and nodes 21-17-16-18-20-21 serving *zone 3*
(per Figure 7). Loop 2 follows nodes 8-9-10-11-12-6-7-22-8 serving *zone
2* and Loop 3 follows nodes 1-2-3-4-5-6-7-22-1 serving *zone 1*.

![](media/image1.png){width="5.839583333333334in"
height="6.647222222222222in"}9 -- LoopDA Sketchpad representation of
natural ventilation design sample problem

### Loops 1 & 4

Loops 1 and 4 are equivalent loops each providing 0.55 m^3^/s (half of
the design 5 h^-1^) to *zone 3* via self-regulating inlet vents. Both
loops exhaust through the *stack terminal g* on Figure 7 for a total
design flow of 1.1 m^3^/s through the stack terminal. The
self-regulating inlet vents are modeled in LoopDA as power law flow
elements with an exponent of 0.1 (see Axley 2001 for more information)
and an undetermined characteristic design parameter (CDP) *C~e~* (see
Figure 10). The stack terminal is modeled as an orifice with the area
*A~orfc,g~* (referred to as *Aorfc1* by LoopDA) as the CDP. The complete
pressure loop properties from LoopDA are shown in Figure 11.

![](media/image22.png){width="2.8986111111111112in"
height="3.670138888888889in"}
![](media/image23.png){width="3.926388888888889in"
height="3.720833333333333in"}

Figure 10 -- Airflow element design window Figure 11 -- Pressure loop
properties window for\
for the self-regulating inlet vent of Loop 1 Loop 1

After entering the information for Loop 1, the loop data are exported to
a tab-delimited file that is pasted into the LoopDA spreadsheet template
(click the icon below to view the Loop 1 information as it appears after
being pasted into the LoopDA template).

![](media/image24.wmf)

Since Loop 1 has only two elements with undetermined CDPs, the LoopDA
template plot is simple and shows the sizing relationship between *C~e~*
and *A~orfc,g~* (see Figure 12). From Figure 12, the CDP for either
element may be chosen (based on other considerations such as available
component sizes from catalog data) and the minimum required size for the
other CDP is fixed. For this example, selecting a value of 0.8 for
*C~e~* dictates a minimum orifice area of 0.85 m^2^ which might be
rounded up to 1.0 m^2^ based on availability.

![](media/image25.wmf){width="6.009722222222222in"
height="3.7416666666666667in"}Figure 12 -- Asymptotic relationship
between CDP of self-regulating inlet vent Ce (Ce2) and\
CDP of stack terminal A~orfc,g~ (Aorfc1) for Loop 1

### Loop 2

Loop 2 supplies 1.1 m^3^/s (i.e., 5 h^-1^) to *zone 2* via *inlet c* on
Figure 7. The flow exhausts through *outlet d* into the stack zone s and
out the *stack terminal h* on Figure 7. Note that the stack must also
exhaust the ventilation flow from *zone 1* for a total design flow of
2.2 m^3^/s through the stack. The inlet and outlet vents are modeled in
LoopDA as orifice flow elements with *A~orfc,c~* (*Aorfc5*) and
*A~orfc,d~* (*Aorfc4*) as the undetermined CDPs. The stack is modeled
using a Darcy-Colebrook duct model with an orifice as the inlet (the CDP
is the area *A3*), an orifice as the outlet (the CDP is *A1*), and a 1m
long round duct segment (the CDP is the duct diameter *D3*). The
complete pressure loop properties from LoopDA are shown in Figure 13.
Note that, if desired, the user must impose the condition that the stack
orifice areas be based on the duct diameter during the sizing process as
LoopDA treats them as independent CDPs.

![](media/image26.png){width="4.738888888888889in"
height="4.477083333333334in"}

Figure 13 -- Pressure loop properties window for Loop 2

![](media/image27.wmf)After entering the information for Loop 2, the
loop data are exported to a tab-delimited file which is pasted into the
LoopDA spreadsheet template (click the icon below to view the Loop 2
information as it appears after being pasted into the LoopDA template).

Loop 2 has only five elements with undetermined CDPs (although *D2*,
*A1*, and *A3* are related). Figure 14 from the LoopDA template plot
shows the sizing relationship between *D2* and all other CDPs. From
Figure 14, *D2* will be set at 1.2 m. Basing the stack inlet and outlet
areas (*A1* and *A3*) on this diameter sets these areas at 1.1 m^2^
(which is larger than the minimum requirements for these areas per the
Loop 2 worksheet and Figure 14).

![](media/image28.wmf){width="6.832638888888889in"
height="3.625in"}Figure 14 -- Asymptotic relationships for Loop 2

Unlike Loop 1, Loop 2 still has undetermined CDPs. Returning to the
LoopDA sketchpad, entering the parameters established for the stack duct
and terminals (*D2*, *A1* and *A3*), and exporting Loop 2 (see Figure 15
for the pressure loop properties) for a second time yields a second
worksheet for this loop (click the icon below Figure 15 to view).

![](media/image29.png){width="4.388194444444444in"
height="4.1402777777777775in"}

Figure 15 -- Loop 2 pressure loop properties after setting the sizes of
D2, A1 and A3

![](media/image30.wmf)As seen in the worksheet and Figure 15,
*A~orfc,c~* (*Aorfc5*) and *A~orfc,d~* (*Aorfc4*) remain to be selected.
Figure 16 shows the asymptotic relationship between these remaining
undetermined CDPs. From Figure 16, the inlet *Aorfc5* will be selected
as 2.0 m^2^, which sets the minimum outlet *Aorfc4* also at
approximately 2.0 m^2^.

![](media/image31.wmf){width="5.829861111111111in"
height="3.7729166666666667in"}Figure 16 -- Asymptotic relationship of
remaining undetermined CDPs for Loop 2

### Loop 3 

Loop 3 consists of the same type of elements as Loop 2 -- *inlet orifice
a* (*Aorfc7*), *outlet orifice b* (*Aorfc6*), and the shared *stack
exhaust g*. As seen in Figure 17, the previously defined stack CDPs are
retained leaving only the inlet and outlet of *zone 1* to be defined.
The LoopDA spreadsheet is similar to the second Loop 2 worksheet (click
below to view). The asymptotic relationship between the undetermined
CDPs *Aorfc6* and *Aorfc7* is shown in Figure 18. Selecting the outlet
to be the same as for *zone 2* at 2.0 m^2^ sets the minimum size for the
inlet *Aorfc7* at approximately 1.3 m^2^.

![](media/image32.wmf)

![](media/image33.png){width="4.738888888888889in"
height="4.477083333333334in"}

Figure 17 -- Pressure loop properties window for Loop 3

![](media/image34.wmf){width="5.829861111111111in"
height="3.795138888888889in"}Figure 18 -- Asymptotic relationships for
Loop 3

### Design Summary and Analysis

The completed design of the four ventilation loops is summarized below
in Table 1.

  ---------- ----------------------------- ---------------- ------------ ----------
  **Loop**   **Element**                   **CDP**                       **Size**

                                           Figure 7         LoopDA       

  1          Self-regulating inlet vent    C~e~             Ce2          0.80

  1          Stack terminal                A~orfc,g~        Aorfc1       1.00

  2          Inlet                         A~orfc,c~        Aorfc5       2.00

  2          Outlet                        A~orfc,d~        Aorfc4       2.00

  2          Stack inlet                   A~stack\ in~     A3           1.13

  2          Stack diameter                D~stack\ duct~   D2           1.20

  2          Stack outlet                  A~stack\ term~   A1           1.13

  3          Inlet                         A~orfc,a~        Aorfc7       1.30

  3          Outlet                        A~orfc,b~        Aorfc6       2.00

  3          Stack inlet                   A~stack\ in~     A3           1.13

  3          Stack diameter                D~stack\ duct~   D2           1.20

  3          Stack outlet                  A~stack\ term~   A1           1.13

  4          Self-regulating inlet vent    C~e~             Ce3          0.80

  4          Stack terminal                A~orfc,g~        Aorfc1       1.00
  ---------- ----------------------------- ---------------- ------------ ----------

Table 1 -- Design values of sized components

LoopDA may now be used to analyze the completed design by performing
simulations ranging from steady state under non-design conditions to a
seasonal or annual analysis using TMY2 or other weather data for the
location. One condition that could be analyzed is the potential for
excess outdoor airflow under a winter condition. Referring back to
Figure 8, an outdoor temperature of 5 ºC and a wind speed of 8 m/s will
be used. Also, the wall opening wind pressure coefficients will be
increased to 0.6 to increase the impact of wind for this case. The
indoor temperature is set to 20 ºC for this case.

![](media/image35.png){width="6.794444444444444in"
height="5.175694444444445in"}

Figure 19 -- Simulation results for winter condition analysis. Blue
lines indicate direction and relative magnitude of airflow (note: status
bar indicates a flow of 1.21 sm^3^/s\
for the self-regulating inlet vent)..

The results of the design analysis of a winter condition are shown in
Figure 19. This analysis shows the risk of greatly over ventilating the
space if no provision is made for altering the ventilation openings
under a winter operating condition as the flow into zones 1 and 2
increases to 3 to 4 times the design flow. The self-regulating inlets
perform better but the outdoor airflow still doubles from the design
case. For a real building, the designer would likely specify a desired
minimum winter ventilation flow and then use LoopDA in design mode to
determine desired vent opening sizes in a winter mode. An operational
strategy could then be established whereby, for example, half of the
openings are used during winter operation.

#  {#section .unnumbered}

# Conclusion

This version of LoopDA was developed to demonstrate the implementation
of the *Loop Equation Design Method* within an existing multi-zone
modeling environment. It will serve to provide designers with a
framework for implementing the design method in order to become familiar
with the method, to explore the practicality of using this method within
the overall natural ventilation system design process, and to provide
feedback to the developers as to the potential for this tool for use by
the design and analysis community.

As this is a preliminary version, it is reasonable that the potential
for enhancements exists. Some potential enhancements have already been
identified including: the implementation of a more diverse set of
inverse flow component types, more robust user interface features and
consideration for the design of mechanical components of hybrid
ventilation systems. In general, inverse airflow elements are relatively
easy to add to the existing environment. However, depending on the
element type, the inverse solution to sizing some elements could prove
to be more difficult than others, e.g., a polynomial fan curve. The user
interface could perhaps be improved by eliminating the need for the
external spreadsheet; providing automated population of analysis
components based on design component sizes; verifying continuity of flow
through components serving multiple loops; generation of reports
summarizing design conditions and system design, enabling the
application of a more diverse set of design constraints, e.g., requiring
duct inlet, outlet and segments of the same size or providing a direct
means to consider infiltration.

# References

1.  Emmerich, S.J., W.S. Dols, and J.W. Axley. *Natural Ventilation
    Review and Plan for Design and Analysis Tools*. NISTIR 6781,
    National Institute of Standards and Technology. 2001.

2.  Dols, W.S. and G.N. Walton. *CONTAMW 2.0 User Manual*. National
    Institute of Standards and Technology. DRAFT 2002.

3.  Axley, J.W. *Application of Natural Ventilation for U.S. Commercial
    Buildings*. GCR-01-820 NISTIR 6781, National Institute of Standards
    and Technology. 2001.

4.  Axley, J.W. *Residential Passive Ventilation Systems: Evaluation and
    Design*. AIVC Technical Note 54. Coventry, AIVC.

5.  ASHRAE 2001. *ASHRAE Handbook - 2001 Fundamentals*, Atlanta, GA.

6.  Irving, S. and E. Uys. *CIBSE Applications Manual: Natural
    Ventilation in Non-domestic Buildings*. 1997, CIBSE: London.

7.  Marion, W. and K. Urban. *User's Manual for TMY2s*. National
    Renewable Energy Laboratory, 1995.

# Acknowledgments

The California Energy Commission through the Architectural Energy
Corporation under Interagency Agreement #00-OA-045 and the U.S.
Department of Energy under Interagency Agreement #DE-AI01-01EE27615
supported this effort. The authors would also like to acknowledge the
contribution of Vern Smith of the Architectural Energy Corporation and
Andrew Persily of NIST as well as James W. Axley for his work in the
area of the design of natural ventilation systems and George N. Walton
for the development of the multi-zone modeling analysis tool CONTAM upon
which the tool developed herein is based.