DESIGNING MECHANICAL & ELECTRICAL SYSTEMS WITH CAD

DESIGNING MECHANICAL & ELECTRICAL SYSTEMS WITH CAD©
by Bill Smith, president of Elite Software

CAD has greatly benefitted today's sophisticated mechanical and electrical engineering designers. Although CAD certainly speeds drafting work, there are many other reasons why CAD has proven useful to designers. Some of the more commonly cited benefits include improved accuracy, easy interference checking, improved drawing aesthetics (better layout, labeling, and titling), reduced number of drawings (more can go on a single sheet), and automatic dimensioning. Other advanced benefits include the ability to integrate design calculations with the CAD process. Performing hydraulic calculations on a fire protection piping system and automatically calculating maximum fault currents from a one-line electrical drawing are just a few examples of how design calculations can be integrated with CAD.

Mechanical and electrical designers are now using many different systems to reap the benefits of CAD. Only a few short years ago, this was not the case. Early CAD systems required large, expensive, mini-computers or even main-frame computers. Personal computers were just coming on the market, and the early ones simply did not have the necessary computing power. Even if the first personal computers had been powerful enough, no CAD software existed for them anyway. Therefore, design firms who wanted to implement CAD back in the late seventies and early eighties were faced with few choices in vendors, and entry level prices ($75,000 to $100,000) that seem monumental by today's standards. Only the largest of firms considered CAD in those times.

However, personal computers evolved rapidly, especially after IBM entered the market, and PC based CAD software began to emerge. Although the early PC CAD software was very primitive in comparison to main-frame CAD software, it was much less expensive. Soon, CAD became affordable for everyone, and the PC based CAD software continued to improve rapidly. The easy affordability of PC based CAD systems has resulted in their overwhelming dominance of the market in terms of number of units installed.

The CAD world has now evolved to a point where some personal computers are as powerful as the larger mini and main-frame computers used in early CAD systems. In addition, PC based CAD software is nearly the equal of the main-frame CAD software. On the other hand, the main-frame computers and CAD software have dropped dramatically in price. From a very general perspective, it can be said that main-frame based CAD systems still hold a performance edge, but even with the latest price reductions, still cost significantly more than PC based CAD systems.

As both PC and main-frame based CAD systems evolve, the distinctions between the two types of systems are blurring. Personal computers are becoming as powerful as main-frame computers, and main-frame computers are being made as small as personal computers. Soon, computers for CAD systems will simply be evaluated on their capabilities and price with no reference whatsoever to whether its a PC or main-frame type computer.

No matter what CAD system is used, it must be very capable to handle the diverse needs of mechanical and electrical designers. Mechanical applications include HVAC equipment and piping, air conditioning duct systems, domestic water supply, sanitary drainage, and fire protection systems. Electrical design is primarily divided between power and lighting. However, there are many specialized aspects to basic electrical design including one-line diagrams, reflected ceiling plans, panel schedules, site plans showing incoming utilities, and circuit diagrams. A CAD package that does not have specific features for designing and drawing such systems is not much use to mechanical and electrical designers.

Many designers are surprised to discover that the basic CAD packages offered by most software vendors can only draw simple lines and arcs. Although anything that can be put on a drawing is nothing more than some combination of lines and arcs, that is little consolation to the designer who wants to draw a duct fitting or a circuit breaker. If a designer has only basic, primitive drawing commands available in his CAD system, manual drafting is actually faster.

What must be understood in purchasing a CAD system is that much more than a basic CAD package is needed to optimally use CAD. The key to making CAD truly productive is the use of quality "drawing aid" software that helps the basic CAD package perform application specific tasks. For example, if a designer wants to put a duct fitting in a drawing he shouldn't have to draw it from scratch. He should be able to "point" to a picture of the desired fitting and then indicate where it is to be inserted in the drawing. Similarly, he should have this capability for all mechanical and electrical devices.

Since drawing aid software is extremely important to the productive use of CAD, it must be evaluated as carefully as for a basic CAD package and omputer. Before PC based CAD was available, all CAD components including the computer, basic CAD package, and drawing aid software had to be purchased from one vendor to work properly. This simplified CAD system evaluations because each part of the CAD system did not have to be obtained from separate companies. However, this situation also tended to cause compromise decisions. If a designer liked the computer used by one vendor, but preferred the drawing aid software from another vendor, he could not mix components to form the best CAD system for his needs.

The advent of PC based CAD software not only made CAD more affordable, it also made it more versatile, but unfortunately more complex to understand. However, the cost savings and the availability of a huge library of drawing aid software has more than offset the complexity of putting together a PC based CAD system.

The most popular PC based CAD software is AutoCAD from AutoDesk, Inc. Other popular CAD packages include VersaCAD by VersaCAD Corp, CADVANCE by Isicad Inc., and CADKey by CadKey a division of Micro Control Systems. The main reason for AutoCAD's dominance is that there is much more drawing aid software available for it than any other basic CAD package. Although drawing aid software is available for virtually all basic CAD packages, no CAD package enjoys the level of third party support that AutoCAD does.

Regardless of the CAD package used, there are several levels at which CAD may be implemented. The most basic level involves standard 2-D drafting using symbol libraries which are the simplest of drawing aids. Symbol libraries are collections of common details and symbols used repeatedly in drawings. They are a must for all CAD users, both novices and experts. The only problem with symbol libraries is that every design firm uses slightly different variations on standard symbols. It is usually impossible to purchase a symbol library containing every symbol in the exact form desired. Fortunately, however, symbol libraries can be modified by the designer to exact requirements. The key is to purchase a symbols library that requires the least modification.

Symbol libraries are very inexpensive ($50-$150), and some are even free. Many manufacturers like Penn Ventilator, Anderson Windows, and others provide free symbol libraries that contain details of their products. Every year, more and more manufacturers are providing free symbol libraries in the hopes of winning customer good will and more specifications. It is also nteresting to note that these free libraries are being supplied almost exclusively for use with AutoCAD.

The next step beyond using symbol libraries is to employ a comprehensive drawing system that includes not only symbols, but also the ability to automatically create certain items. For example, a good mechanical drawing system allows a designer to draw an ordinary line that is automatically turned into a double line duct section complete with markings indicating whether the duct is round, rectangular, or flexible. Similarly, if a designer wants to specify a 45 degree elbow fitting, he shouldn't have to worry about the size of the duct section it's going on. The mechanical drawing system should automatically insert the correct size fitting.

Electrical drawing systems go beyond simple symbol use in several ways. For example, a ceiling grid and lighting plan can be automatically created by just entering the room area and the grid spacing. Another advanced feature of electrical drawing systems is the ability to make only one entry for the multiple insertion of light fixtures, outlets, and other electrical symbols. These features make creating high quality electrical drawings very fast and easy.

In essence, drawing systems are intelligent drawing aid software that is used with a basic CAD package to facilitate the drawing of specific applications like duct systems, piping networks, electrical one-line diagrams, and lighting plans.

Complete drawing systems are the ultimate in drawing aid software. They combine both symbol libraries and automatic drafting. Some even allow for the integration of design calculations right from the drawing file. For example, Carrier, Chase Systems, Intergraph, Softdesk and AutoDesk have mechanical drawing systems that allow HVAC duct sizing direct from a duct drawing file.

Electrical drawing systems have lagged somewhat behind mechanical drawing systems in terms of integration with calculation programs. Some electrical calculation programs can create reports such as panel schedules and coordination plots that are readily useable by CAD packages. However, most electrical designers want the ability to draw a one-line electrical diagram with CAD, and then have fault currents and voltage drops automatically calculated from the drawing. Unfortunately, this capability doesn't exist at the moment, but rapid progress is being made towards these goals by companies such as SKM Systems Analysis, Elite Software, Chase Systems, and Intergraph Corporation.

Drawing systems integrated with design programs are extremely beneficial because all the manual typing of input data into the design program is eliminated. As time passes, more and more mechanical and electrical design applications will be integrated with CAD. Hot and cold water service supply pipe sizing, sanitary drain and vent sizing, electrical component sizing and load flow analysis are example mechanical and electrical applications that will soon be integrated with CAD. Even HVAC load calculations will be routinely performed from CAD drawing files. Carrier already has a program called LOADLINK that aids in the calculation of HVAC loads from an AutoCAD drawing file. Ultimately, all mechanical and electrical design tasks will be integrated with CAD.

Engineers have been hearing about the wonders of CAD for many years now. However, many designers are unclear about how a CAD system works, and are skeptical in general about their usefulness. Although every firm implements CAD a little differently, there are some elements common to all CAD installations.

The first concept to understand with CAD is that all drawing information is stored in drawing files which are specific to the CAD package being used. A drawing file has graphic data organized by "layers". The designer usually selects a color for each layer and a short name descriptive of the layer function. The number of layers used in a drawing is up to the designer, but most designers use ten to twenty layers per drawing.

Layers in CAD drawings serve several important functions, but the main purpose is to separate the various kinds of graphic information. The ability to easily turn on and off layers with a CAD system is one of its major advantages. Everything pertinent to the task at hand can be viewed whenever desired.

Since each layer can show objects drawn with only one line type and one color, designers often find it necessary to create multiple layers for a single design aspect. For example, switch legs in an electrical drawing are normally part of the lighting control switches layer. If the designer wants to show switch legs using a center line type instead of a standard line type, he has to put switch legs on a separate layer. Although the new layer for switch legs uses a different line type, the designer would probably use the same color for the switch leg layer as for the lighting control switches. While the designer usually assigns a unique color to each layer, occasionally he does not do so if several layers relate to the same design function.

Lee Grosser of Lee Grosser and Associates in Cincinatti, Ohio is typical of many mechanical designers using CAD. For comprehensive mechanical drawings Grosser uses an average of fourteen layers with the following functions: 1) Sheet Borders, 2) Title Block, 3) Text, 4) Background (floor plan), 5) Electrical, 6) HVAC Equipment, 7) HVAC Controls, 8) Ductwork, 9) Chilled Water Piping, 10) Hot Water Piping, 11) Condensate Drain Piping, 12) Plumbing Water Supply Piping with Fixtures, 13) Sanitary Drains and Vents, and 14) Fire Protection Piping. Grosser varies the number of layers for each project depending on the size of the project and what it entails.

Jobs using packaged HVAC equipment have fewer layers because no chilled and hot water piping is required. Many small projects also omit HVAC controls and fire protection piping. Very complex jobs may use more than fourteen layers. For example, it is sometimes desirable to separate supply ductwork from return ductwork or sanitary drains from vent stacks. As drawings become very large and complex, it is desirable to use more specialized layers. On the most complex jobs, Grosser sometimes creates two mechanical drawings rather than one. This decreases the drawing file sizes, and keeps the number of layers per drawing manageable. In general, electrical design requires many more drawings than mechanical design. Whereas the entire mechanical design of many projects can be easily maintained in one CAD drawing, electrical design nearly always requires multiple CAD drawing files. Gerry Maloney of Maloney and Associates in Dallas, Texas is typical of many electrical designers in that he uses two primary electrical drawings, one for lighting and one for power.

Besides layers for sheet borders and a title block, a lighting drawing typically contains layers for the ceiling grid, lights, lighting control switches, lighting circuitry, and text. As mentioned previously, some designer's use an extra layer for switch legs in their lighting drawing so that the switch legs can be shown as a center line type. Power drawings have the usual sheet layers as well as layers for receptacles, conduit and wiring, panels and equipment, and underground conduit. The power drawing also typically contains a mechanical reference layer that the electrical designer uses to route conduit and wiring. In addition, most designers have miscellaneous layers for special notes, existing electrical equipment, hidden lines, leaders, and other such items.

Complex electrical design sometimes requires more than two electrical drawing types. Jose Villegas of Goetting Associates in San Antonio, Texas has done many large electrical designs, and he typically uses five different types of electrical drawings. Each of the drawing types contains from five to twelve layers. The drawing types with associated layers are described as follows.

1. Site Plan: This drawing concerns utilities and has layers for underground and overhead power, telephone, and communication lines. There are also layers for site plan notes and miscellaneous items.

2. Lighting: This drawing has up to twelve layers for lighting fixtures, emergency lights, lighting circuits, lighting text, power devices, power circuits, power notes, panel boards, fire alarm devices, communication devices, data devices, and miscellaneous items.

3. One-Line Diagrams: This drawing has layers for panels and equipment, conduit and wiring, underground conduit, miscellaneous items, and notes.

4. Panel Schedules: This drawing has layers for loads and descriptions, panel schedule formats, panel schedule text, and date/revision information.

5. Details: This drawing contains all the electrical details. There are layers for schedules, details, and schedule/detail text.

Many of the layers mentioned are for setup and reference purposes only. For example, once the sheet borders and title block are established the designer doesn't need to work on those layers anymore. Other layers, like the background layer are primarily reference layers, and are never altered by the designer. Some layers serve double duty as both a reference layer and an active work layer. For example, the mechanical designer actively works on the HVAC equipment layer to locate compressors, pumps, and air handlers. When complete, that same layer becomes a reference layer for the electrical designer laying out conduit and wiring. Reference layers are very important to have so that system layout and interference checking are easy to perform.

All CAD systems allow the designer to make changes in only one layer at a time, but the designer can specify any combination of layers to be visible as work is performed. For example, if a designer is working on the ductwork layer, he will probably have the background and HVAC equipment layers visible at the same time. As the ductwork progresses, the designer will also periodically turn on the chilled/hot water layers and electrical lighting layer so that interference with the ductwork can be checked.

Although CAD has many advantages over manual drafting, there are some problems unique to CAD. One of the common mistakes in using CAD is that objects appearing in sharp detail on the screen may appear as a dark "blob" when plotted on paper. This phenomenon can occur because an object that appears fine on the screen when "zoomed", may not show up well when plotted at the desired drawing scale. Thus, a designer using CAD must be very conscious about drawing small objects in too much detail.

Another common problem in using CAD is that associate designers must be very careful to coordinate their work on a drawing. Each designer must be sure to use the latest drawing file at the start of their work. If work is performed concurrently, the designers must be sure to consolidate their changes back to one master file. If due care is not exercised, the HVAC designer could layout a duct system that is not checked for interference with recent changes made by the electrical designer. Using a networked CAD system where it is easy for designers to reference the latest drawing file helps to alleviate the problem. However, as long as designers are aware of this potential problem, it can be easily avoided, even withnon-networked CAD systems.

The above mentioned CAD problems are really very minor. The greatest hurdle to overcome is the initial transition from a manual drafting office to a CAD based firm. All necessary hardware and software must be purchased, and extensive training must be scheduled. A complete new mind set is required, and everyone involved must have a positive attitude. However, if given a fair chance, CAD can be profitably used by even the smallest of firms.

Ideally, both designers and draftsmen should become CAD operators. As more and more mechanical and electrical design software becomes integrated with CAD, it becomes advantageous for designers to spend more time with the CAD system right at the beginning of every new job. If trends continue, both mechanical and electrical designers will soon be able to begin conceptual design work right on the CAD system, and then evolve that initial work right to the final contract drawings.

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