Handbook of air conditioning and refrigeration / Shan K

Design Interface
AIR SYSTEMS: AIR DUCT DESIGN 17.73
ing information layers are grouped by supply, return, and exhaust ducts and subdivided into ducts,
diffusers, and fittings. For each of these duct components, layers can be single-line, 2D or 3D construction
with tags, labels, dimensions, and accessories. Plan types similar to the drawing information
layers include 2D or 3D supply, return, and exhaust ducts. Besides, there are also floor, interior,
exterior, and bird’s-eye view plans.
A level layer indicates its attribute characteristic and the vertical elevation. Each layer can be
edited, modified, and displayed.
Computer-aided duct design software either must provide interfaces with available duct sizing programs
or itself must have the software for duct sizing. It is preferable that the duct design software
size the ducts by using equal friction, static regain, constant-velocity, and even cost optimiztion T
methods. The duct design software extracts system geometry and flow rates from the AutoCAD
drawing, exports the information to the selected duct sizing program, and uses the sizes calculated
to produce a double-line or 3D drawing to replace the centerline drawing.
Running Processes
Drawing a Centerline Layout. Designers are required to enter a fan and centerline layout in AutoCAD.
They have the option to add terminals, diffusers, necessary dampers, fittings, duct sections,
and related data to the drawing. Terminal text, line color, and fitting text are interpreted as the airflow,
duct shape, and fitting configurations. An ASHRAE Duct Fittings Database with local loss coefficients
of 225 types of fittings provides default fitting types and coefficients.
Analyzing the Duct System. With a completed centerline layout, designers are asked to choose
the duct sizing software. Default values that are used for the centerline layout, such as duct shape,
and airflow rates are passed to the duct-sizing software.
The designer is asked to select the fan location. The computer program connects all lines to the
the fan in an attempt to connect each line to the end of the preceding line. The computer program
tests the continuity of the duct system by checking that all selected components are connected and
all junctions can be found. All lines must have a fan, terminal, or diffuser, or another centerline at
each end. The computer program generates nodes and inserts a node tag for the fan, main duct section,
and branch takeoff endpoints. Node shape is determined according to the duct shape and class
of node. Duct section information is stored in the upstream node. Each main duct node and branch
duct node are set with default attributes required by the duct-sizing program.
Editing Default Attributes and Running Duct-Sizing Program. If the designer is satified with all
default values, he or she can proceed to run the duct-sizing computer programs. If the designer needs to
change any of the default values, she or he can use AutoCAD to edit duct-sizing input default attributes
and visually pick the duct sections from the layout for editing before running the duct-sizing program.
When the editing work is done, modified node data are used to update input data for the ductsizing
program. The designer uses the command to run the chosen duct-sizing program.
Converting from Centerline to Double-Line or 3D. Centerline layout or centerline 3D layout can
be input into sizing calculations, and the computer program will produce a double-line layout or a
3D duct system layout. The conversion begins with the fan and produces the layout in the following
order: fittings, transitions, reducers, duct segments, and dampers.
Total Pressure Loss and Fan-Sizing Calculations. Some computer-aided duct design computer
programs have the ability to determine an existing duct layout shape and size in the CAD environment
and load it into the duct system total pressure loss and fan sizing calculations.