Scale Models A New Tool For Chemical Engineers WILLARD ALLPHIN Dewey and Almy Chemical Co. Cambridge B, Mass.
ONCE a chemical process has reached the stage when it must leave the laboratory, the flexibility of research methods disappears and new problems develop. Instead of the improvisations and easy modifications of the laboratory, there must be the more or less permanent installation of either pilot plant or fullfledged manufacturing equipment. In recent months we have made use of a model-making technique as a preliminary and accompaniment to actual building construction and have found it remarkably helpful. The technique has produced direct savings in cost and time, plus intangible but nonetheless vital benefits in the form of more convenient operations. Above all, use of the models has forestalled two of the bugbears of chemical plant construction: the necessity for making expensive changes during or immediately after erection, or the alternative of living with an inconvenient installation. Either alternative means the loss of valuable production time. One of the principal advantages in models is the aid they afford in visualizing the completed plant, with the resulting opportunity to make adjustments in structure and location of equipment for more convenience in operation. Although some architects and a few engineers have
Left above. same plant.
Schematic piping of a pilot plant. Above. The finished model of the Left below. Very simple tools such as these are used in model making.
used models in the past, their work has usually involved a literal representation of appearance. Our models, while made to scale and exact in size, are transparent, with the specific purpose of offering an aid to visualization rather than an exact duplication of exterior surfaces. Modeling a chemical plant falls into four divisions: making the equipment, erecting the structure, locating the equipment, and installing piping. Until the final locations of equipment are decided,
there will be many changes i n location and presumably some changes in. the structure itself. In fact, this i s one o f the principal advantages in model makings Those who have to do with the new plaint are always warned not to become so enamored of the model that they cannot bear* to change it. If no changes were made, model making would be of doubtful value. After the equipment locations are fixed, the piping is installed andfc>estlocations for pipe runs are obvious from the model,
whereas they would be difficult to see in drawings. Recent Dewey and Almy installations have been made without any piping drawings, the model having been taken to the job and the pipe fitters having worked directly from it. Use of Plastics The first Dewey and Almy model was done entirely from a piping standpoint, with blocks of wood to represent equipment and bent wires t o represent pipes. It was in an attempt to find something which would be more convenient for representing piping that we came to use transparent plastics. From this it was an easy step to making practically all of the equipment from plastics, and it was found that considerable detail could be shown without greatly increasing the cost. Transparent cellulose acetate is used wherever possible, but some of the rod and tube stock is only obtainable in cellulose nitrate. This does not amount to enough in the total model to cause an excessive fire hazard. The technique involved in working with cellulose acetate is simple, because two pieces can be cemented together almost instantly with acetone. "Welded" joints are made by brushing on a solution of cellulose acetate in acetone. This hardens quickly and forms a strong joint. We use sheets of 0.005, 0.015, 0.025, 0.050, 0.125, and 0.25 inch thickness, and rods and tubes in all available sizes. Sheets up to 0.025 inch are cut with scissors, 0.050 inch with tin snips, and 0.125 and 0.25 inch on a jig saw. The 0.050-inch stock is cut on a jig saw when making up structural steel shapes such as "I" beams, in order to get a square cut. On account of the shortage of structural steel, recent new buildings have been framed in wood, and wood was arbitrarily used in the models t o represent wood, whereas plastic material was used to represent metal. The plastic could be painted with opaque dyes, but it has been found preferable to leave it transparent, so that jacketed kettles are visible, steam coils and mirrors can be seen inside reactors, and piping can be seen behind various equipment. Thus a sacrifice of absolutely literal appearance is made in favor of easy visualization. . Valves are made from plastic wood, a piece of toothpick, and half of an ordinary snap fastener. They may seem needlessly elaborate, but are actually easy to make. One man can make about three dozen in an hour. It is always a question as to how much detail is justified in this type of work, but we believe that a considerable amount of detail is desirable, since it all adds up to easier visualization. When a dozen men, representing various departments and viewpoints, are sitting around a model, discussing the size of the building or the location of its equipment, a man should not have t o stop and think, "This is a reactor", or "That is a pump". The object should be detailed enough to be
realized almost unconsciously, so that he can be free to think of the real problems involved. Each run of pipe i s marked with a number to indicate the size. Pipe diameters in the model are only approximately to scale. Thus, if it is decided to change a pipe size, it is only necessary to change the number tab. Figures at the top and center of the opposite page show two stages of the modeling of a pilot plant. In the first figure, what might be called a threedimensional schematic has been used; that is, narrow strips of cellulose acetate in various colors have been run from one point to another to show the piping without reference to the best location of the pipes. In the center picture, the permanent piping has been installed, and it will be noted that structural and equipment changes have also been made. Two ladders have been removed, a stairway added, platforms enlarged, and equipment relocated. Figure at the top of this page shows the model of a plant with almost no piping. In this case, the principal value of the model was in determining the size of the building and the locations of the equipment. When the model was apparently complete, it was seen that walkways and working areas would be too crowded. It was then decided to make the central bay of the building 4 feet wider. This change has been incorporated in the model shown. The scale of all our models is 1 inch to the foot, and the model workman is scaled as 5 feet 10 inches. All models too wide to pass through an ordinary door are split, as shown in picture at top of the following page. Advantages of Models Each project brings its own special features in which model making is a helpful tool. In the case of an entirely new process, much time will be spent on the actual piping and connections, and many changes will be made. In the case of a new and larger plant for an existing process, the emphasis will be put on profiting by previous experience, and we find it very beneficial to have the foremen and some of the workmen from the plant come to the model shop and carefully criticize the job. They are frequently able to point out where a machine could be located to better advantage or where a valve should be placed for easier operation. As a sidelight on the method, stereoscopic photographs are made of each
Reading from top to bottom. Simple model with almost no piping; plant for which the center picture on opposite page was pilot set up in construction shed on the job; another recently completed plant; a close-up view of plant directly above it.
of the ways in which modeling makes for direct saving of money: 1. Reduced drafting time on structure, o n account of making changes in the model instead of on the drawing board. 2. Elimination of piping drawings. 3. Saving of pipe fitters' time, because following a model is easier than following a blueprint. 4. Saving in materials because of fewer changes on actual job. Our engineering department feels that these direct savings pay the entire cost of model making, but beyond these are the intangible benefits involved in the production of a more convenient and satisfactory plant. The Chief Engineer says: Scale models, when properly detailed and checked, present the job in a manner not possible by two-dimensional drawings. They save a considerable amount of supervisory and checking time in the
Above. This model is being assembled on the job for pipe-fitting use. Right. A plant under construction for which the structure in picture below was a model.
model for record purposes. These pictures, with their three-dimensional effect, are particularly useful if the model is to be discussed by someone in another city. It is not necessary to have a special camera for stereoscopic pictures of any still-life subject; one simply takes a picture, shifts the camera 2»/ie inches, and takes another picture. For best results, the pictures should be viewed through lenses of the same focal length as the camera which took them. On the type of plants shown in photographs, model-making costs, including materials and labor, run about 1 per cent of the total project cost. The following are some conduct of the work and prevent costly errors which in working from drawings may not be discovered in time. I have been very much interested to find that all of the foremen and workmen concerned prefer models to drawings because they are so much easier to follow. Even the draftsmen, who might have been expected to look askance at models, have been boosters for them. Most of the advantages which have been mentioned are of particular help in the work of the Engineering Department, but there is one more which is by no means the least important. When planning a new plant, the Engineering Department must present its ideas to other departments, the members of which may not b e expert at blueprint reading. A model is the ideal liaison tool. In short, our experience has shown that if a new plant is to be built or extensive changes are to be made in an old one, money can b e saved and a more convenient plant will result if you model it to scale and in sufficient detail for easy visualization. 558
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