Ductwork, like pipework, is a system family through which the Autodesk® Revit® MEP platform can calculate airflow rates and pressure drops on any correctly defined system. Ductwork also provides the graphics for the traditional “drawn” documentation and the variety of ways this can be represented on a drawing sheet at different stages of a project, regardless of whether the duct is presented as a single line at concept design or as a fully coordinated double line for a construction issue.
The three main types of ducts are rectangular, round, and oval. They are the basis for your design and documentation. There are two ways of using ducts, either with the Duct tool or the Placeholder Duct tool. Placeholders allow the designer to rough in layouts in single-line mode.
Although at first glance this seems similar to a view that is set to a coarse level of detail, the placeholders display only a single line, regardless of the level of detail that is set. In addition, placeholders do not create fittings when a duct run is drawn, although they do show a symbolic representation where rises/drops occur. However, they still have the use of all the design tools that an engineer requires, allowing the duct to be sized based on airflow, elevation, and so on. All these duct types, whether placeholder or normal, connect to air terminals and a variety of mechanical equipment. They also host duct fittings and accessories. By using the fittings and accessories available with the standard installation, the user can create supply, return, and exhaust systems with very little additional thought.
During your implementation, however, you should also consider the benefits of creating additional duct types that suit the way you work and your company's standards.
In this chapter, you will learn to do the following:
Air distribution components come in many shapes and sizes. Depending on the design, they can be mounted in a variety of ways:
In each of these instances, the designer must decide whether to use hosted components (and if so, which type of hosting) or whether to just host to the active level/work plane. There are several ways of placing these objects, and some of these may be conflicting.
An example of this is an installation that has diffusers hosted in the ceiling as well as areas where the architect's design is for suspended fittings (see Figure 10.1). In this project, assume that there is an external architect and that the architectural model is being linked. This means straightaway that you cannot use ceiling-hosted air terminals. Although you can see the linked ceiling, Revit recognizes it only as a face. Because of this, the air terminals need to be created either as face-hosted families or families that are hosted to the level—that is, no physical hosting. Additionally, since Revit MEP 2014, there has been the ability to host an air terminal to a duct, regardless of how that particular family has been created. When placing a duct-hosted air terminal, click the Air Terminal On Duct button to access this feature.
The most important point here is that, after you commit to one type of family (hosted, level hosted, wall, and so on), you can exchange, for example, a ceiling-hosted air terminal only with a similarly hosted air terminal, not just those that are of the same category (see Figure 10.2).
The next problem is that the air terminals that are suspended are in fact the same type as those that are mounted within the ceiling tiles. The engineer wants to be able to schedule and filter them as one. How do you manage this?
The ability to copy/monitor air terminals (as well as lighting fixtures, mechanical equipment, and plumbing fixtures) means that the services engineers can monitor the locations of air terminals that the architect has placed because the architect is responsible for placing these objects. As with the other items that can be copied/monitored, the services designer can choose to copy the original family type from the linked file or “map” it to one of their own choosing. In Figure 10.3, you can see, however, that after the air terminals have been copied/monitored, regardless of the type of host association, this ceiling-hosted family has no host but is in the correct location specified by the architect.
Mechanical equipment comprises the components that make up the majority of large- to medium-size plant objects for the mechanical designer. From air conditioners (ACs) and air-handling units (AHUs) to air curtains and heat pumps, these all provide the geometry and parameters associated with HVAC design. As with all components, choosing the hosting type is important. A level-hosted object cannot be exchanged for a face-hosted or ceiling-hosted one.
The heart of the mechanical air system, the air-conditioning or air-handling units, can start life as generic “boxes” with intake and exhaust. Although basic in construction and with no manufacturer data attached, generic ACs/AHUs can have the same number of parameters as more detailed families. Similar in dimension and performance to generic ACs/AHUs, the concept box can be swapped out during the detail design period for a more detailed manufacturer or construction-issue family, or even for a set of families if the AC/AHU unit has been constructed from its manufacturer's component parts (see Figure 10.4). It is, however, entirely likely that the connectors in this generic unit will not be in the correct location when the more detailed manufacturer's family is loaded. This can mean some rework is necessary in order to produce a more accurate model.
The majority of large ACs/AHUs are generally placed on the level where they are inserted, while fan coil units (FCUs) or heat pumps are generally placed in a ceiling void, although they can also be in a plant room. The placement of these units can depend on whether the unit is mounted on rails, a plinth, or anti-vibration mounts and whether these mounting devices are part of the AC/AHU family (see Figure 10.5).
Generally created as level-hosted families, variable air volume (VAV) terminal boxes are usually mounted somewhere within the ceiling void, suspended from the underside of the slab above. In terms of placement, these are given an offset relative to the building level associated with the active view. Using the Properties palette, the offset can be predefined prior to the VAV being placed in this case. This makes for an easier workflow than in previous releases, in which most objects were placed on the reference level and then subsequently moved to the correct elevation (see Figure 10.6).
Connections to both AC units and VAV boxes can include heated- and chilled-water services and electrical for connecting to water and electrical systems (see Figure 10.7). Also note the connector labels in this view, which allow easier identification of connections and their flow direction, where applicable. These symbols also double as a shortcut to start creating ducts from the connector; just click the icon to start.
Heating coils, chilled-water coils, or reheat coils can be shown as part of the equipment or provided separately. They can also be duct mounted. Generally for FCUs, heat pumps, and most ACs/AHUs, the coil connections would be built into the equipment, but for a built-up AHU or for a VAV that may or may not include reheat, providing a coil separately can make it easier to swap out without changing the entire family. Duct-mounted coils need to be made adjustable to duct size and coil width (determined by the number of rows) and will need to have the pressure drop provided from the manufacturer's data.
Electric duct heaters can also be required. With these you need to ensure that you allow for the fire rating required., Building this into the family ensures that you are not relying on the users to remember it every time.
Ducts can be displayed in a variety of ways; they can be rectangular, round, oval, or as a placeholder, as shown in Figure 10.8.
Placeholder ducts, as you can see in Figure 10.8, are shown only as a single line, although they retain nearly all the characteristics and properties of a regular duct. In Figure 10.9, you can see the differences. Placeholder ducts do not have values for Justification, Insulation, or Lining, because they are not needed at the conceptual stage of the project when placeholders are being used.
Within Revit ducts are system families, and they are the glue that hold systems together. However, it also relies on component or loadable families to create a duct type, as described in the next section. Although ducts hold a huge amount of information, for the user there are several important considerations to note, such as the type of bends, transitions, and other fittings that make up the duct run. Also take into consideration lining and insulation, which can be applied to runs or parts of runs after placement.
Since Revit MEP 2014, we have had the ability to specify the type of system that a duct is associated with (exhaust, return, or supply), without actually being connected to any equipment. You will be able to change the system type of a duct, even after it is connected to equipment, but not the classification. For example, if your duct is connected to a supply terminal, then you can change your system type to Outside Air, but not to return or exhaust air, as the Oustide Air System type should have been created from the Supply Air System Classification. If your duct is not connected to any equipment, you can change it to any system you like, as shown in Figure 10.10.
When highlighted in the System Browser, both ducts and placeholder ducts are highlighted in the drawing area. If a duct is created in the wrong system type, simply select one section of the run and change it to the required system. All the connected ducts and fittings will also change. This ability can have several benefits, including these:
In Revit MEP 2015, all systems are named. Even though they may be default names, it is easy to change them on the fly. You also have the ability to divide a system. This is dependent on elements within the system not being physically connected and could be used as a means for breaking up large systems that may be slowing down the design process. To achieve this, select an appropriate system, as shown in Figure 10.11. Notice that although it is physically not connected with a duct, it is a single system. Also, with the supply air system selected, you can see the new Divide System button on the ribbon.
Once you click the Divide System button, a message similar to the one in Figure 10.12 appears. The number of systems depends on the number of physical breaks in your original system.
With the system now split, the original system and the new one (or however many have been created) are automatically sequentially numbered, as shown in Figure 10.13.
Interference checking in Revit 2015 is a comprehensive and accurate tool. Not only can you use the placeholder ducts (and pipes) in the interference check, but duct insulation can also be included as part of the process (see Figure 10.14).
Creating new duct types is a way of managing how your runs of ductwork and connect into each other, such as whether bends are mitered or radiused. Although you can change any of the fittings inserted retroactively, it is much easier to create a run in one go, using either the automatic or manual tools.
Although it may be tempting to create types with names such as Exhaust and Supply, try to keep these names more specific, such as Stainless-2D Radius/Taps or Galvanized-Mitered/Tees. This way, the user only has to think about what material was used for the duct. Using a name that specifies Supply or Extract may be initially attractive, but doing so can lead to ambiguity and misunderstandings among the modeler, designer, engineer, and potentially the client. Another downside to this is the need to create multiple types of bends, tees, crosses, and so on for all the different material types and system types you are likely to use.
To create additional duct types or to edit existing types, right-click one of the existing types in the Project Browser (as is typical for all Revit system families). To create a new type, click Duplicate. This copies an existing duct type and appends the number 2 to the end of the name (for example, Mitered Elbows/Tees 2). Right-click again to access the type properties. At this point, you can choose to rename the duct type and supply the necessary fittings. These fittings need to be preloaded to use them, but you can also reuse those already in use in the project/template.
When using the automatic routing tools, as a rule of thumb you should work on small sections—all the feeds to a VAV box is one good example. This means the computer has fewer objects to calculate and the routing suggestions have less room for error. Before even starting this process, check the options under Mechanical Settings (see Figure 10.15) for default Duct Type, Offset, and Maximum Flex Duct Length values because these are used during the routing process. Also note that these settings can be set for the different system types.
Now you're ready to use automatic duct routing. Here's how:
After you click the Duct button, the Create Duct System dialog box appears (Figure 10.18).
This allows you to edit the system name and, if necessary, open the new system in the System Editor to add/remove objects and select equipment. Objects that are not part of the current system appear as halftone in the drawing area. When they are selected to be added to the system, their appearance changes to full weight (see Figure 10.19).
With the system selected, you have several options; one of these is to create a layout. You can use either regular ducts or the placeholder option.
You have several options for automatically generating your duct layout. These include Network, Perimeter, and Intersection options. Each can give you several solutions that can vary depending on the predefined settings for the duct layout, which can also be accessed from the Options Bar. Figure 10.22 shows that you can click the Edit Layout button, which allows you to select the layout lines in order to make changes to the layout. Using a plan and a 3D view side by side is a great way to ensure that a layout will work before clicking Finish Layout.
Main duct runs are shown in blue, while branch runs are shown in green.
It's worth pointing out here that the sizes used for this layout are based on the connection sizes and the settings for duct sizes located in the Mechanical Settings dialog box. There is another important consideration at this point. The designer/drafter must ensure that the default settings for the main and branch ducts are high enough above the air terminals and associated equipment to generate the layout; otherwise, the Layout tool cannot generate a layout, even if the layout is created with placeholder ducts. This can be achieved in two ways: either by selecting the Settings button on the Options Bar (shown earlier in Figure 10.21) or by selecting the actual sketch lines of the solution, which allows the user to manually choose the offset.
Using the automatic tools may seem too limiting or even pointless to an experienced design drafter who already knows the sizes and routing that must be used. These tools are great for a quick mock-up or presentation of a design that is more conceptual. However, with experience, most users eventually settle for a variety, sometimes for no better reason than “a change is as good as a rest.” Sometimes, however, the manual tool is much more efficient, especially when connecting different areas into a system or laying out runs back to a rooftop AHU.
To begin manual duct routing, do the following:
Note the default settings in the Properties palette for constraints, including Justification, Reference Level, and Offset. The system type is undefined, and because the duct is unconnected, the airflow is 0.
You can adjust additional properties from the Options Bar and the Modify | Place Duct tab, shown in Figure 10.25.
You can also inherit the elevation and size of existing connectors, whether from ducts or families, when connecting to them. For round or oval ducts, you can also choose the Ignore Slope To Connect option, meaning that the duct will connect regardless of slope, or you can use the specified angle.
In Figure 10.26, the duct has been split and a section created. To create a duct set-down, you would hover over one end of the duct, right-click the connector, and select Draw Duct. The duct would then be drawn along the preferred route.
When creating ducts that set down/up or drop/rise, it can be much easier to model these in a section or elevation view, as shown in Figure 10.26 and Figure 10.27. To do this, first you need to create a suitable view. Quite often, individuals have a personal section that they move around the model. It can be opened when required, and then the duct (or whatever service is being worked on) is modified and the view is closed. This section, which should be named for the individual, can then be used to show a quick 3D view of the area using the Orient To View option.
Changing the justification of ducts is a relatively easy task, but it can take practice to actually master. Selecting a duct or a run of ducts/fittings gives the user the option to change justification. Select your duct run, and from the Modify tab, select Justify.
This opens the Justification Editor, shown in Figure 10.28. Here, the control point can be displayed, and the alignment along the selected route can be selected by using the Alignment Line button. You can choose justification by using one of the nine Justification control buttons. This can have significant effects on your layout, so it is best to use this tool in small, easy-to-view sections or 3D views.
You can also change an entire duct run from one type of duct to another. In previous versions, this involved multiple selections and filters. Now all the user has to do is select the duct run, as you can see in Figure 10.29. Here, the duct system has been selected, and the only proviso is that flexible ducts, air terminals, and mechanical equipment are not in the selection set.
This is a logical step, because the ducts and duct fittings in the selection set are usually also defined within the duct type. Therefore, when you select the objects and click the Change Type button, you are presented with a condensed Properties box that allows you to change the run type for others already in use in the project (see Figure 10.30).
In this next example, the design has gone through a few iterations, and changes have been made to various fittings and portions of the duct run. Since Revit MEP 2014, the Reapply Type option has been available. Selecting the entire run allows the user to reapply the original duct types to the objects selected, as shown in Figure 10.31.
The direction of flow and graphical warnings are really helpful for the designer. The direction of flow is a temporary display when you select any object that has a mechanical or piping connection. This is a great benefit for designers and drafters because they can visualize which way air (or gas or water) is flowing through an object. As you can see in Figure 10.32, the selected VAV box has two supply ducts: one entering from the right and the other exiting from the left. There is also a return-air and electrical connection.
Warnings related to MEP connections can be displayed as graphical notifications when there is a disconnect in the system. You can change the warnings to suit your current task from the Analyze tab. Select the Show Disconnects button and then select the options you require, as shown in Figure 10.33.
Since Revit MEP 2014 we have had the ability to cap open ends of a duct or pipe. The reason for doing this is that an open end in Revit (pipe or duct) results in incorrect calculations because Revit is expecting a contained system. To achieve this, either select a duct or create a selection set of ducts, and click the Cap Open Ends button displayed on the ribbon, as shown in Figure 10.34.
To extend an existing layout, as indicated in Figure 10.35, select one of the fittings (typically a bend or tee). Notice a small plus sign (+) adjacent to the side of the fitting that does not have a connector. Clicking this turns a bend into a tee and a tee into a cross, eliminating the need to delete a fitting and insert an appropriate one.
The most important factors to consider when using the Duct/Pipe Sizing tool is that the ducts form part of a system and that this system should have a name that suits your design (not a default), which is created at the same time as the system being created. The system must also have a valid airflow, so either you specify the airflow of the air terminals or that flow is specified from the space and volume calculations.
Use the Tab key to select your system, so all duct pieces in the system are highlighted (Figure 10.36).
The Duct/Pipe Sizing tool is located on the Modify | Multi-Select tab of the ribbon. This button becomes active when you select a single duct or duct run that is part of a fully enclosed system, whose components are connected properly in regard to flow direction. Clicking the button opens the Duct Sizing dialog box. Figure 10.37 shows its available options.
Although the sizing method you choose can be applied to an entire system, this is not necessarily the most efficient way to do things. If there are 5,257 objects in your system, it's probably not a great idea to ask your computer to process that amount of information—it may take a while! The logical choice is to split the task of duct sizing into manageable sections, such as a floor plan, a zone, or a group of air terminals fed from the same VAV box.
The various methods for sizing are as follows:
Friction and Velocity can be used independently of each other when using the Only option. Alternatively, they can be used in conjunction with each other with the And and Or functions.
These options allow the designer to force the sizing ducts to meet the parameters specified for both Velocity and Friction. With the Or method, the least restrictive of either of the parameters is used.
The Equal Friction and Static Regain methods use the ASHRAE duct fitting database.
Air properties are set in the Mechanical Settings dialog box, as are the available sizes of ducts used in the actual sizing calculations (see Figure 10.38 and Figure 10.39).
During the sizing process, you can also apply constraints to the branch parts of the run. These are defined as Calculated Size Only, Match Connector Size, and Larger Of Connector And Calculated (see Figure 10.40). Depending on the stage of the design or your actual role (such as consultant), you could choose Calculated Size Only, because the design is still in its early stages and the equipment is generic. These settings allow you to coordinate with other model components in areas where space is limited. For example, if you have only 2′-0″ (600 mm) of space above the ceiling, you could restrict the height of your duct to 20″ (500 mm).
However, later in the project when the equipment has been specified, as a contractor you may want to select Match Connector Size and Larger Of Connector And Calculated to reduce the number of duct sizes used on the project, thereby reducing your manufacturing costs.
This dialog box also allows you to specify a limit on the size of the ducts, which can further reduce costs or give you the ability to specify, say, a continuous duct height in places where you know access is a potential issue.
To add insulation and lining to a selection set that contains both ducts and duct fittings, simply create your selection set and choose the appropriate option, as shown in Figure 10.41.
Lining and insulation can have overrides or filters applied so that they display in the required manner, even to the extent of showing insulation as transparent with a hidden line style, as indicated in Figure 10.42.
Although lining and insulation can be added only as instance parameters, the workflow for using this tool is much easier than in most previous releases. Even so, if the entire system requires lined or insulated ducts, you have to select all ducts for the system and then specify the insulation or lining required.
Duct sizing works only where air terminals, ducts, duct fittings, and mechanical equipment are seamlessly connected with no gaps and where the equipment has a defined airflow. This airflow could be entered as part of the initial analysis or subsequent manipulation of the objects.
Once a system is created, you can use the analytical tools to inspect the duct system for airflow, pressure, and pressure loss. From the System Browser, select the system you want to analyze (as shown in Figure 10.43). Remember, you can also Tab-select the system. With the system selected, click System Inspector on the ribbon. If the Inspector tool isn't available, it's a clue that something is wrong.
This activates the System Inspector tool, shown in Figure 10.44. Click System Inspector again and pass your cursor over the elements in the system; notice that airflow and pressure loss are defined.
As with all calculations, you may want to keep a record of this analysis as part of your project documentation. Introduced in Revit MEP 2014 was the ability to create a duct (or pipe) pressure-loss report as discussed in Chapter 8, “HVAC Cooling and Heating Load Analysis.” This tool is located on the Analyze tab or on the Modify | Duct Systems tab. Here you have a choice: either select the system first or specify the system you want to create the report on in the Analyze tab. Once the system is selected, the dialog box shown in Figure 10.45 is displayed. In this dialog, you can specify the fields to use and save the report format for later use. Clicking Generate creates an HTML web page of your results.
Now that you have reviewed the process of using the duct routing tools, you will learn how to apply them in a simple exercise:
Chapter10
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