The heart of the mechanical air system, air conditioning/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 a more detailed family. Similar in dimension and performance, the concept box can be swapped out during the detail design period for a more detailed manufacturer or "construction-issue" family, or even set of families if the AC/AHU unit has been constructed from its manufacturer's component parts (see Figure 10.4).

Figure 10.4. Basic AHU and type parameters

Placing the majority of AC/AHUs generally happens on the level that they are inserted on, with no offset. This can depend upon whether the unit is mounted on rails and whether those rails are part of the AC/AHU family (see Figure 10.5).

Figure 10.5. Floor-mounted and skid-mounted AC/AHU

Generally created as nonhosted families, VAV 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. One of the nice enhancements in Revit 2011 is that with the persistent properties box, the offset can be predefined, prior to the VAV (in this case) being placed. This makes for a much better workflow than previous releases where most objects were placed on the reference level and then subsequently moved to the correct invert level (see Figure 10.6).

Figure 10.6. Changing the invert level

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).

Figure 10.7. Typical mechanical and electrical connections to mechanical equipment

Although it may be tempting to create types such as Extract 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 the duct is. Although using a name that specifies Supply or Extract may be initially attractive, doing so can lead to ambiguity and misunderstanding 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 type and system types you are likely to use.

Real World Scenario: Schematic Layouts

Mike has been given the task of preparing a schematic ductwork layout for mechanical engineer Steve. Because the design architect has yet to supply a BIM model, he has some initial 2D AutoCAD plans and sections. Steve completes a rough design on paper along with calculations for duct sizes. When Mike starts the job, he links the AutoCAD section into a new section view. From here he can create building levels based on the section. With this done, floor plans are created, and the AutoCAD plans are linked into the relevant levels. Mike can now create a single-line, schematic layout for the project. At this stage, floor offsets, downstand beams, and coordination between services almost can be ignored, because the duct is being shown only as a single line. The main benefit of this is that the bulk of modeling for the duct system can be achieved at an early stage of the project and retained/modified as the project progresses.

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 10.13) for default duct types, offsets, and length of flex duct, because these are used during the routing process.

Now you're ready to use automatic duct routing. Here's how:

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 adequately high enough above the air terminals and associated equipment to generate the layout; otherwise, the layout tool will crash.

Figure 10.21. Completed duct layout

For the experienced design drafter, using the automatic tools may seem to be too limiting; however, through use, 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:

Figure 10.23. Trimming the duct

Figure 10.24. Completed duct

When creating ducts that set down/up or rise/drop, it is much easier to model these in a sectional or elevational view (as in Figures 10.23 and 10.24). 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 being worked on — is modified, and the view is closed.)

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:

Figure 10.28. Duct Sizing dialog box

Friction and Velocity can be used independently of each other, when using the Only method. Alternatively, they can be used in conjunction with each other with the And and Or functions.

These 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 sizes of ducts used in the actual sizing calculations (Figure 10.29 and Figure 10.30).

Figure 10.29. Mechanical Settings dialog box, air

Figure 10.30. Mechanical Settings dialog box, duct sizes

During the sizing process, you can also apply constraints to the branch parts of the run. These are defined as follows: Calculated Size Only, Match Connector Size, and Larger Of Connector And Calculated (Figure 10.31). Depending on the stage of the design or your actual role (such as consultant), you could choose to select Calculated Size Only, because the design is still in early stages and equipment is Generic.

Figure 10.31. Duct sizing constraints

However, later in the project or as a contractor, when the equipment has been specified, 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, reducing your manufacturing costs.

This 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 where you know access is a potential issue.

When designing your system, you should be aware that Revit does not take into consideration external or internal insulation in sizing calculations or in clash detection. Duct insulation does not form part of any interference checking, so although you can see the clash, it does not get reported. In addition, duct lining can be shown on the plan but does not affect the airway size. These factors may be important, especially in areas where accessibility is an issue; the designer should be extra vigilant in these situations.

Lining and Insulation can be added only as instance parameters, so if the system depends upon a lined or insulated duct, the user will have to select all ducts for the system and then specify the insulation or lining required. This would also have to be done to elbows, tees, and so on. One tip here is to apply the required insulation in a schedule (Figure 10.32).

Figure 10.32. Scheduling duct

In this example, there are two schedules, one showing the system you are modifying, with all the ducts (Figure 10.33). The other is a duplicate but doesn't show All Instances (Figure 10.34). When you type in the required insulation thickness, rather than it being applied to only one section of duct, it applies to all ducts of the same type in that system.

Figure 10.33. Scheduling duct

Duct sizing will work 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. If you are using Revit to do your documentation process without using the analysis and design tools, then all duct sizing will need to be done manually.

Figure 10.34. Applying insulation

Real World Scenarl: Ducting and Coordination

Multinational Consulting and Engineering company Beca Carter Hollings and Ferner, based in Auckland, New Zealand, planned to roll out Revit MEP to their engineers and drafting staff. Although there is a strict implementation process, offices around the country and farther afield take hold of the reins and give them a good shake. While the engineers are getting familiar with the design tools, Beca decides to develop Revit to provide documentation for projects and train its drafting staff to achieve this end.

With Revit Architecture rapidly becoming the documentation and virtualization tool of choice, the next logical step is for building services. With this process relatively new to Beca, the decision was made to complete documentation in new projects using Revit MEP in areas where a high level of coordination is required. In circulation and office areas where coordination, although important, is not critical for the consulting engineer, traditional 2D drafting methods are still employed.

On a new hospital project, Beca uses this technique to construct major plant areas. Very quickly the company discovers the value of this high level of coordination as the complex plant areas become extremely crowded. The image shown here indicates only the ductwork in this area (for a larger, color image, refer to the website file cs1001.jpg). Interference checking is used within Revit during the documentation process to ensure services are missing not only each other but also architecture and structure elements.

With design still being done in other software, modeling the plant areas with Revit MEP enabled Beca to concentrate on utilizing new software to its optimum without committing additional untrained staff to an untested environment.