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by Fred W. Arndt, He'll Process Equipment Corp.
Some Economic Materials of Construction for Corrosive Fume Handling Glass fiber-reinforced plastics, polyethylene, and PVC find increasing use in ducts URING the past few years, newer D materials of construction have come into prominent use in the field of moderate temperature corrosive fume handling. In particular, structural plastic fume ducts have demonstrated a noteworthy position on an economic basis. F u m e ducts, formerly constructed of expensive alloys or steel, with expensive sheet rubber linings, have been fabricated of plastic and operate at equal anticipated service life. Several types of plastics have been foremost in size and n u m b e r of installations completed. Probably the greatest proportion of these plastic fume-handling systems have been of the glass fiber—reinforced type. Industrial installations have been made in rigid P V C [(polyTable 1.
Tensile strength, lb./sq. inch Compressive strength Impact, ft.-lb. (unnotched) Flezural strength, lb./sq. inch Specific gravity Coefficient of expansion/" F. Useful temp, range, ° F.
vinyl chloride) ] and polyethylene. T h e physical and chemical properties of each of these materials are covered by numerous reviews and manufacturers' bulletins. For most acid services, the physical rather than chemical properties are often the determining factor in selection of the proper plastic. T h e chief physical properties (approxim a t e values) of glass-reinforced plastics, P V C , and polyethylene are shown in T a b l e I to illustrate their limitations from this standpoint. If temperatures near the boiling point of water are encountered in the gas stream, glass-reinforced plastics are more satisfactory, even though in general their corrosion resistance m a y not be so great. T h e polyester types of glass-rein-
Physical Properties
Glass-Reinforced Thermosetting Plastics 9,000 25,100 12.45 22,000 1.33 1 X 10 To 250-300
PVC 9,000 10,000 0.6 16,000 1.45 0.4 X 10 To 150
Polyethylene 1,700 to 2,000
forced plastics are not generally considered to have good alkali resistance. Regardless of the relative merit of each type of plastic, the glassreinforced type is now becoming commonplace in large industrial installations. An approximate cost comparison of various materials of construction for typical ducts is shown in T a b l e I I . T h e weights give some indication of ease of handling the duct work upon installation. Solid plastic fans are also slowly finding acceptance in this field. Although their sizes are not large as compared to metal fans, they offer several advantages where severe corrosive fumes must be removed. T h e cost of these plastic fans is somewhat less than for a comparable fan of stainless steel, Monel, or sheet rubber-lined steel construction. T h e y are more costly than a coated steel fan but do not have the disad-
300 16
1,500 to 1,700 0.92 9 X 10 To 130
Typical centrifugal and a x i a l types of fan made of p o l y v i n y l chloride). Blades are of solid plastic, designed so that fumes d o not come in contact with metal parts
Stack installed by large phosphate fertilizer manufacturer. Constructed of glass-reinforced polyester of a high heat distortion g r a d e , it is 2.5 feet in diameter and 60 feet high and weighs less than 1000 pounds. All interior surfaces are reinforced with Dacron cloth to prevent hydrofluoric acid attack VOL. 49, NO. 4
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APRIL 1957
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I/EM
EQUIPMENT AND DESIGN
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A Workbook
Feature
Tank cover and vent stack, typical application for plastic construction
vantage of possible failure d u e to pinholes in the coating of the fan blades. Coated fans usually fail because of chipping or eroding of the coating at the blade edges or tips. Inherently the coatings at these edges or tips are thinner than on flat surfaces. Some industrial applications of these structural plastic m a t e r i a l s . . .
A large phosphate fertilizer manufacturer had the problem of exhausting gases containing hydrofluoric acid fumes as the chief cor-
rosive. These gases reach temperatures as high as 250° F. This stack was installed over a year ago. O n e chemical manufacturer installed a tank cover over a 16-footdiameter tank containing potassium chloride brine at a p H of 2.5 to 5.5 saturated with chlorine. Air, water, and some hydrochloric acid are also present in the fumes. T h e temperature of the tank varies from 122° to 170° F. T h e cover is entirely selfsupporting and because of its size was fabricated in flanged sections, shipped separately, and bolted together at installation.
Table II. Approximate Costs of 1 2-Inch Diameter Ducts Material Cost/Ft." Lb./Ft. Plain steel, galvanized 16 ga. 3.00 to 3.50 8.5 Coated 8.5 4.50 to 5.00 Rubber-lined .0.00 to 13.00 10.0 Stainless steel 8.5 8.50 to 10.00 Rigid poly(vinyl chloride) 3.0 6.00 to 9.00 (solid plastic) Glass reinforced polyester 6.00 to 9.00 3.0 Polyethylene 3.0 6.00 to 9.00 Cement-asbestos composition 2.35 to 2.50 18.0 Stoneware 7.00 to 7.50 37.0 Rectangular equivalent, Vî-inch 6.5 2.00 to 3.00 marine plywood, good 2 sides, AA exterior type with several coats of proper paint • F.O.B. various points, not including installation. Table III.
Relative Costs and Service Life of Fans Static Pressure Range Fraction of Inch to 1 Inch 1 to 3 Inch Service Service Cost" Cost» life life position position position position Type Type Tube axial-coated 1 1 2 Centrifugal-coated 3 Centrifugal-coated 2 2 1 2 Solid plastic 3 3 3 Tube axial1 Vane axial, coated solid plastic 4 Tube axial-alloy Centrifugal rubber1 lined and alloy Ejectors 1 5 Solid plastic Rubber-lined • Lowest cost is lowest number. Numbers do not indicate magnitude of cost but only sequence of price within pressure range indicated. For different makes, models, and accessories the order may shift somewhat.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Brass strip pickling line
T h e cover was designed to support not only its own weight plus the weight of a vent stack, but also a snow load of 20 pounds per sq. foot. T h e plastic selected was a glass-reinforced epoxy laminate on the under surfaces of the cover and the inside surfaces of the vent stack, flanges, a n d outlets. T h e remainder of the cover was made of glass-reinforced polyester because of its lower cost. T h e cover can be considered as being essentially of two-ply construction: the epoxy glass portion for the desired corrosion-resisting properties and the upper or outer layer of the polyester glass for maxim u m strength and low cost consistent with satisfactory corrosion resistance for the external conditions. In the processing of metals, pickling solutions have always presented a problem in corrosive fume handling. In the brass strip pickling line illustrated, the hot sulfuric acid fumes are removed through the glass-reinforced polyester duct system and cleaned in a solid plastic fume scrubber before being discharged to out-of-doors through the plastic stack. Spray piping in the fume scrubber is of extruded polyv i n y l chloride) and the spray nozzles are of solid Teflon construction. T h e entire unit is without exposed metal parts. This exhaust system has now been in operation for a p proximately 1 year with no maintenance whatsoever. T h e cost of this equipment in all-plastic construction was less than the cost of identical equipment fabricated of rubberlined steel. Additional advantages of plastic construction include ease of installation, facility of field repair, and maintenance-free exterior.
