At thirteen years old, I didn’t realize polystyrene would melt in the microwave. So, I expected my foam bowl to hold up as I heated my instant mac and cheese. Of course, I was disappointed in the result, if not a bit hangry as I avoided what plastic-tainted noodles I could as I ate supper. Similarly, operations personnel too often choose drag conveyors unsuited for the application in which the conveyor will perform, especially applications that involve materials with elevated temperatures. Personnel may want or even expect cheap chain conveyors to perform well in hot conditions, but what they want or expect doesn’t matter. The laws of physics will ensure their mild-steel-and-paint conveyors will corrode quickly for a low return on investment, just as my foam bowl melted.
When preparing a project involving hot material transfer, personnel must first resign to the fact the conveyors they choose cannot be the “foam bowl” kind, i.e., they cannot be cheap. Whatever solution they choose must withstand the operating conditions in which they work long enough to justify their cost. Painted mild steel—the basic structural material and finish of most drag conveyors—will not last long enough in high-temperature applications to justify their expense. And the hotter, more acidic, and more abrasive the material, the lower their ROI.
We’ve seen what happens when manufacturers attempt to save money with cheap conveyors when handling materials with elevated temperatures. They end up replacing the conveyor. Quickly. In one instance, we bid a stainless-steel SMART Conveyor™ to a client who needed to convey hot pellets. The company went with a lower-cost, mild-steel option, however, and in six months the machine had rusted so badly the company had to replace it. Ironically, by trying to save money by purchasing the wrong machine, the manufacturer had to pay for the machine twice over.
Manufacturers who convey hot materials cannot choose mild steel and paint and expect good results. They need conveyors with a finish and components that can withstand the application. In applications that involve nonabrasive materials with a sustained temperature below 400°F (204°C), a conveyor with a galvanized finish on mild steel may be suitable. When a material has a sustained temperature higher than 400°F (204°C), such as hot ash, mild steel is not a viable option. Manufacturers must go with stainless steel conveyors in such conditions. For sustained temperatures up to 797°F (425°C), manufacturers may choose Grade 304 stainless steel. Hotter temperatures up to a sustained 1,900°F (1,038°C) require Grade 330 stainless steel. Applications with higher sustained temperatures call for a superalloy like Inconel®, which is designed to function in extreme temperatures.
If the material is abrasive, this, too, affects what finish is suitable for the application. A galvanized finish won’t hold up if the material is abrasive, for example. The zinc layer that forms the galvanized finish, while harder than mild steel, is very thin, and abrasion will remove it, thereby exposing the mild steel beneath. If the material is abrasive, the mild-steel conveyor handling it therefore requires an abrasion-resistant liner like AR steel or UHMW to protect against wear. (See Wear-Resistant Materials for Conveyors for more information on liners.) When the material is both abrasive and hot, conveyors perform best with stainless steel construction. (Depending on the application, a manufacturer may get by with a conveyor that incorporates stainless steel components; the entire conveyor need not always be stainless.)
Highly acidic or alkaline characteristics will also make a galvanized finish unsuitable. The American Galvanizers Association states zinc performs best in a pH range between 5.5 and 12. The galvanized layer will continue to protect the mild steel in pH levels between 3.5 and 5.5 and between 12 and 13.5, but it will not last nearly as long. Outside this range, the zinc’s resistance to corrosion decreases precipitously.
The design of the conveyor also plays into whether a galvanized finish will be practical or not. Traditional drag-chain conveyors run the chain and paddle along the conveyor’s bottom, wearing both the bottom pan and the paddles. Paddles in these conveyors also commonly scrape the sides of the conveyor. Due to this, a galvanized coating will not provide adequate protection against corrosion even when conveying nonabrasive materials because internal friction in the conveyor will itself wear away the zinc layer.
Wear is one of the reasons Biomass Engineering & Equipment designed its SMART Conveyors™ the way it did. The chains in SMART Conveyors™ run in channels outside the material path, so they do not wear the bottom pans. The chains also hold the paddles off the floor and sidewalls of the conveyors. Thus, when we galvanize our conveyors, the internal components do not wear through the finish to expose the mild steel underneath.
BE&E has manufactured both galvanized and stainless-steel conveyors for materials with abrasive and corrosive properties. We’re committed to providing best-in-class performance with the right design for the application—you’ll get no recommendations from us for a cheap solution so we can make a sale. We’ve seen what happens when you microwave foam, and we’ve seen what happens when you install a cheap conveyor. We want you to spare you the result.