Part 8: The Downcomer

This section discusses how I constructed the downcomer, which carries the exhaust gases from the top of the furnace to the dust collector.

On every post-WWII blast furnace I've seen, the downcomer has three identifiable sections: (1) the top, at the uptakes; (2) a center piece which guides the gas away [usually at an angle]; and (3) a vertical bottom segment going straight into the dust catcher. The center length is joined to the top and bottom sections by a 45-degree elbow at each end. Sometimes each of the three sections is nearly identical in length - the Walthers furnace seems to reflect this design. Other structures have a long center section with stubby ends adjoining the uptakes and the dust collector. I chose to model the latter arrangement.

Materials Used

Now I'm going to show a different approach to modeling industrial piping elbows. Earlier chapters had diagrams showing how to cut straight pipe segments at various angles; for the downcomer I decided to try using PVC pipe fittings from my local hardware supplier.

Here is a tabular list of the raw materials I purchased from a nearby Home Depot^® store:

Qty Description Store Inventory Description Size (if applicable) Notes

1 Tubing, 11/4" Outer Diameter Schedule-40 1-inch PVC Pipe Minimum 4ft length

1 Tubing, 1" Outer Diameter Schedule-40 3/4-inch PVC Pipe Minimum 1ft length Used as [inner] dowel tubing

2 45-degree Elbow, 11/4" Outer Diameter Schedule-40 45^O Elbow, 3/4-inch

Enhancing the Elbows

This might be considered by some to be an optional step; the Walthers furnace kit has smoothly rounded curves on all the large piping. But as for me, I prefer to replicate the appearance of the prototype as much as possible. One of the ways I do this is to make the curves look 'jagged,' i.e., like straight pieces of pipe cut at various angles and welded together. [Pipe manufacturers don't produce large elbows in real life because (1) they would be extremely difficult to transport, and (2) the market for large elbows is very small compared to others. Segmented curves accomplish the same end at a much lower cost.]

A newly purchased PVC elbow is, like its Walthers or Plastruct counterpart, smoothly rounded. So the first thing I did was to scribe three circumferential lines in the vicinity of the bend (see diagram). These would resemble seams as they appear on the prototype. Then, using the freshly-etched lines as boundaries, I filed-down the round bulge of the bend to make it resemble two joined straight segments (see diagram). This, combined with the etched lines, produced a more typical-looking downcomer elbow.

Cutting and Joining

Having obtained reasonable-looking elbows, I now needed to cut the adjoining segments of 11/4"-diameter tubing to complete the downcomer. Since I wanted to have a longer "central" section, I cut a piece roughly 9 inches in length; then I cut another 2-inch stub for the bottom end that would fit into the dust collector. [I already had the top section, having produced that earlier.]

Next, I needed a way to join the elbows to the straight sections. To accomplish this, I cut four pieces of 1"-diameter pipe, each about 1 inch in length, and used these as dowels inside the larger pipe (see assembly diagram). Since the interior of the stright piping was slightly narrower than that of the elbows, some sanding was required on one end of each dowel tube to make them fit. Since the joints had to swivel freely to allow the piping to be positioned correctly with the components on either end, I glued only one end of each dowel tube. One additional operation I performed was to drill two tiny holes in the lower end of the upper elbow, then tap these out for set screws. Why? Because after several instances where the downcomer came crashing down while I adjusted the lower end, I realized that I needed an adhesive-free method for securing the top end. [The set screws are so tiny that, once painted, you will not see them if you don't already know they exist.]

The photo below shows the center straight section and the lower elbow.

NOTE: I eventually had to slightly angle the bottom end of the center section in order to locate my dust collector in the desired spot.

Click here to view a closeup photo of the completed downcomer.

-KJL 07/25/02

Qty	Description	Store Inventory Description	Size (if applicable)	Notes
1	Tubing, 11/4" Outer Diameter	Schedule-40 1-inch PVC Pipe	Minimum 4ft length
1	Tubing, 1" Outer Diameter	Schedule-40 3/4-inch PVC Pipe	Minimum 1ft length	Used as [inner] dowel tubing
2	45-degree Elbow, 11/4" Outer Diameter	Schedule-40 45^O Elbow, 3/4-inch