Construction Classes, Materials and Spark Resistance¶
Summary¶
This chapter explains how a fan's construction is matched to its service. It covers construction class, construction materials (stainless steel, aluminum, Corten steel, coatings, and shaft and impeller materials), and spark-resistant construction, including non-ferrous materials, Type C, rub rings and rub plates, and grounding devices. It introduces high-temperature construction (maximum operating temperature, shaft coolers, high temperature grease), high-moisture construction with drains and weep holes, insulated fans, and flue gas recirculation. After completing it, readers can choose a construction class and material for a stated airstream.
Concepts Covered¶
This chapter covers the following 27 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Construction Class | 237 |
| Housing Drain | 17 |
| Construction Material | 40 |
| Spark Resistant Construction | 49 |
| High Temperature Construction | 69 |
| High Moisture Construction | 14 |
| Weep Hole | 1 |
| Stainless Steel Construction | 2 |
| Aluminum Construction | 17 |
| Coatings | 14 |
| Spark Resistant Type C | 3 |
| Fan Grounding Device | 1 |
| Maximum Operating Temperature | 6 |
| Shaft Cooler | 5 |
| Insulated Fan | 21 |
| Flue Gas Recirculation | 1 |
| Impeller Material | 3 |
| Shaft Material Grades | 2 |
| Corten Steel | 1 |
| Non-Ferrous Material | 16 |
| Rub Ring | 1 |
| Rub Plate | 1 |
| Double Wall Construction | 14 |
| Insulated Jacket | 1 |
| Insulated Inlet Box | 1 |
| High Temperature Grease | 3 |
| Forged Shaft | 1 |
Prerequisites¶
This chapter builds on concepts from:
- Chapter 2: Airstreams, Total Pressure and Standard Air
- Chapter 3: Basic Fan Anatomy
- Chapter 11: Drives and Standard Fan Arrangements
- Chapter 16: Bearings, Shafts and Shaft Seals
- Chapter 18: Inlets, Outlets and Flow Control
Welcome to Construction Classes
A fan that is perfectly sized can still fail in a month if it is built from the wrong stuff for the air it moves. This chapter teaches you to read an airstream description and turn it into the construction line on the order: the material, the spark-resistant type, the temperature package, the drains and the insulation. Get that line right and the fan survives the job it was sold for. Let's follow the airflow.
Construction Class: Matching the Fan to Its Airstream¶
Chapter 2 sorted airstreams into clean, particulate-laden, hot, wet, corrosive and explosive. Chapter 3 named the parts the air touches: the housing, the impeller, the inlet funnel and the shaft where it passes through the housing. This chapter joins the two ideas. Every part in the airstream is exposed to whatever the airstream carries, so the way those parts are built has to answer the airstream's demands.
Construction class is this book's name for a fan's build designation: the set of construction choices, beyond size, arrangement, rotation and discharge, that make the housing and impeller fit for the airstream they will see. Standard construction suits clean air near room temperature. When the air is hot, wet, corrosive or explosive, or must not leak out of the fan, a special construction replaces or adds to the standard one. FanPedia's Fan Construction section (pp. 104-123) lists the special constructions Twin City Fan builds: spark resistant (Types A, B and C), high temperature, insulated, nominally leak-tight, regenerative thermal oxidizer (RTO), high moisture, and swingout and clamshell designs that open the fan for cleaning. Underneath every one of them sits a choice of construction material, the metal or other material the airstream parts are made from.
Construction class is not the same thing as fan class. Fan class (Class I, II, III and IV, defined by AMCA Standard 99-2408 and covered in Chapter 10) is a pressure and speed rating: it decides how fast a fan size may safely run. Construction class decides what the fan is built from and how. The two answers are independent, and both go on the order. A Class II fan can be built in standard steel or in spark-resistant construction. If a document ties "class" to a maximum speed, it means fan class.
A Stack of Answers, Not One Checkbox
Think of construction class as a stack of answers to separate questions: what is it made of, can it strike a spark, how hot does it run, does water collect, does the surface need covering, and must it seal? Most jobs answer "standard" to most of them. The skill is noticing the one or two questions where the airstream says otherwise, and checking that the answers in the stack do not contradict each other.
The table below organizes the airstream conditions from Chapter 2 against the construction each one calls for. Every row is developed later in this chapter or in Chapter 21.
| Airstream condition | Construction to consider | What it changes on the fan | Where it is covered |
|---|---|---|---|
| Clean air near room temperature | Standard construction | Nothing beyond the catalog's standard material and finish | This chapter |
| Corrosive | Construction material and coatings | The material or surface of the airstream parts | This chapter |
| Explosive | Spark resistant Type A, B or C | Which parts are non-ferrous; rub plates and rub rings | This chapter (Types A and B in detail in Chapter 21) |
| Hot | High temperature construction, by package | Materials, shaft cooler, grease, bearings, pedestal | This chapter and Chapter 21 |
| Wet (steam or condensation) | High moisture construction; drains | Shaft seals, cover plates, drains | This chapter |
| Hot surface, condensation or noise | Insulated fan | An outer shell, cladding or jacket | This chapter and Chapter 21 |
| Hazardous gas that must stay in the duct | Nominally leak-tight or RTO construction | Welds, gaskets, shaft seals | Chapter 21 |
Because the answers stack, some combinations are ruled out or constrained. FanPedia states these limits directly:
- High moisture construction cannot be used together with high temperature construction (FanPedia p. 122).
- Nominally leak-tight construction is not recommended for applications over 600°F (FanPedia p. 120).
- In high temperature construction for Arrangement 4 centrifugal fans and Arrangement 4 plug fans, aluminum is allowed for rotating parts only up to 250°F (FanPedia pp. 111, 114). Twin City Fan engineering notes that current practice does not offer Arrangement 4 for high-temperature applications, so treat this as FanPedia's rule for the construction it describes.
- For Type C spark-resistant centrifugal fans at 501°F and above, the aluminum inlet funnel is replaced by a steel funnel used with a rubbing band and rubbing bars (FanPedia p. 106).
- In all spark-resistant types, bearings are not allowed in the airstream (FanPedia pp. 104-109).
Worked Example: Building the Construction Line¶
An inquiry reads: "Arrangement 8 centrifugal fan exhausting a dryer. Exhaust contains solvent vapors. Operating temperature 550°F. Specification calls for spark-resistant Type C. Fan stands beside an operator platform." Work through the stack one question at a time.
- How hot? 550°F is far above room temperature, so the fan needs high temperature construction. Arrangement 8 is one of the arrangements (1, 8, 9, 9F and 10) with temperature packages, and 550°F falls in the 501°F-600°F package (FanPedia p. 110). That package uses mild steel and Corten or stainless steel, high temperature grease, and fixed and floating bearings.
- Can it strike a spark? The solvent vapors make the airstream explosive, and the specification names Type C. At 550°F, which is at or above 501°F, Type C uses a steel inlet funnel with a rubbing band (naval brass or Monel) and rubbing bars (Monel), plus an aluminum rub plate where the shaft passes through the housing (FanPedia p. 106).
- Is it wet? Nothing in the inquiry says so. That is fortunate, because high moisture construction could not be combined with the high temperature package anyway.
- Is the surface a hazard? A 550°F housing beside an operator platform is a burn risk, so insulated construction belongs on the quote, with the type chosen later in this chapter.
- Must it seal? The inquiry is silent, so ask. If the answer is yes, 550°F does not exceed 600°F, so temperature alone does not rule out nominally leak-tight construction.
The result is a construction line with four entries (501°F-600°F high temperature package, steel or stainless steel airstream parts, Type C with steel funnel, insulated construction) and one open question for the customer. Each entry traces to a sourced rule, which is what lets the quote survive an engineering review.
Construction Materials¶
A construction material is the metal, alloy or other material that a fan part is made from. For the parts in the airstream, it is the first line of defense against corrosion, heat, abrasion and sparks. Mild steel is the default: FanPedia notes that shafts are mild steel unless otherwise specified (p. 80), and most housings and impellers in the catalogs are welded steel. The current catalogs also offer housings in lock-seam galvanized steel, aluminum and stainless steel for some lines, as Chapter 6 showed for utility sets. A material change is made when the airstream would attack mild steel, when the fan must not strike sparks, or when the temperature is beyond what a lighter material can carry.
Materials also have to be joined. When a customer specifies that a corrosion-resistant housing, such as stainless steel, must not be welded to a mild steel pedestal, Twin City Fan uses buffer strips between the two, and a bolt-on, non-rotatable pedestal is available for dissimilar metals such as a stainless steel or aluminum housing on a mild steel pedestal (FanPedia p. 85). The shaft is handled in a similar way when it is large.
Worked Example: A Stainless Steel Airstream on a Large Fan¶
A customer specifies stainless steel for every airstream surface on a large Arrangement 3 fan with a shaft of about 10 inches in diameter. A solid stainless steel shaft of that size is expensive. FanPedia describes cladded shafting for exactly this case: when a shaft is large (9 inches in diameter or more) and a special material such as stainless steel is required in the airstream, the shaft and hub are mild steel, and only the sections in the airstream are clad with thin-gauge or thin-wall material matching the airstream material (p. 81). Common base grades for cladded shafts are 1018 and 8620.
- Check the size: 10 inches is at least 9 inches, so cladding is an option.
- Check what the airstream touches: the shaft between the seals and the hub. Those sections get stainless cladding; the sections at the bearings and coupling stay bare mild steel.
- Check the joint to the pedestal: if the specification forbids welding stainless steel to mild steel, add buffer strips or use a bolt-on pedestal.
The fan meets the stainless steel requirement wherever the air can reach, at a lower cost than an all-stainless rotor.
Stainless Steel Construction¶
Stainless steel construction means building the airstream parts (and, where specified, the shaft) from stainless steel, an iron-based alloy whose chromium content gives it corrosion resistance. It is chosen for corrosive or washdown airstreams and where cleanliness matters. For shafts, FanPedia lists Types 304 and 316, and Type 2205 Duplex when the customer specifies it (p. 80). Two cautions follow from the material itself. Stainless steel is iron-based, so it is not a non-ferrous material for spark resistance. And when a stainless housing meets a mild steel pedestal, the joint needs the buffer strip or bolt-on treatment described above.
Aluminum Construction¶
Aluminum construction means using aluminum for fan parts, either for the whole airstream or for selected parts. Aluminum is light, resists many corrosive atmospheres, and is non-ferrous, so it is the most common material in spark-resistant construction. Twin City Fan uses it widely: standard axial impellers are aluminum, the F4/F6 axial impellers have an aluminum hub (FanPedia p. 34), Type B spark-resistant fans typically use an aluminum impeller and rub plate, Type C centrifugal fans use an aluminum inlet funnel and rub plate, and shaft coolers are cast aluminum.
Aluminum's weakness is temperature, because it loses strength sooner than steel as it heats. FanPedia sets these limits:
- Aluminum rotating parts in high temperature Arrangement 4 centrifugal fans and Arrangement 4 plug fans: up to 250°F (pp. 111, 114).
- Standard aluminum impeller on high temperature axial fans: up to 275°F (p. 116).
- Air kits: aluminum is listed for non-rotating parts only (p. 115).
Each limit is a check of the form
where \( T_{\text{operating}} \) is the highest airstream temperature the fan will see and \( T_{\text{limit}} \) is the source's limit for that part.
Worked Example: Can the Impeller Stay Aluminum?¶
Three inquiries each ask for an aluminum impeller.
- Arrangement 4 plug fan, oven recirculation at 230°F. The limit is 250°F, and 230 ≤ 250, so aluminum rotating parts are allowed. The fan falls in the 181°F-300°F package, which FanPedia offers on select models (p. 114).
- The same fan at 270°F. Now 270 > 250, so the aluminum impeller is not allowed. The rotating parts must be mild steel and Corten or stainless steel.
- Arrangement 9 axial fan at 260°F. The standard aluminum impeller is limited to 275°F, and 260 ≤ 275, so the impeller material passes. The axial page adds other requirements at this temperature: a fixed (not adjustable-pitch) impeller is required over 250°F, and the Arrangement 9 packages run 201°F-250°F and 201°F-300°F on select fan models (p. 116). The material is acceptable; the impeller style and model still need checking.
The same word, aluminum, gets three different answers because each limit belongs to a specific part on a specific fan type.
Corten Steel¶
Corten steel (from the trade name COR-TEN) is a weathering steel: a low-alloy steel whose small additions of elements such as copper, chromium and nickel let it form a tight, adherent oxide layer that slows further corrosion. In Twin City Fan literature it appears in one place consistently, the high temperature material lists, where FanPedia names "Mild Steel and Corten" together with stainless steel for every centrifugal high temperature package and for air kits (pp. 110-115). Treat it as a steel option for hot airstreams, chosen alongside mild steel, and take the exact choice from the current catalog and the engineering review.
Coatings¶
A coating is a layer applied to the surface of a fan part, such as paint or a specialty protective coating, to protect the base material from corrosion or abrasion, or to give it a finish. A coating is usually the lowest-cost way to protect mild steel from a mildly corrosive airstream, which is why most fans leave the factory painted. FanPedia notes that housings get weep holes partly so manufacturing can drain wash water out before painting (p. 147). Twin City Fan publishes the details of its coatings in the Manuals & Coatings Sheets library on tcf.com.
The trade-off against a material change is simple to state. A coating protects only where it is intact. If the airstream abrades it, if it is scratched at installation, or if the airstream is hotter than the coating can take, the base metal is exposed. A stainless steel, aluminum or fiberglass part has no surface layer to lose. So the coating decision is really a temperature and abrasion decision.
Worked Example: Checking a Coating Against the Airstream¶
A process exhaust runs at 120°F with a mildly corrosive vapor. Once a week the line is cleaned with a hot bake-out at 300°F. The customer asks for coated mild steel to save cost. Suppose the coatings sheet for the proposed coating lists a maximum service temperature of 250°F (an illustrative value; read the real one from the current sheet).
- Normal operation: 120 ≤ 250, so the coating is fine.
- Upset operation: the bake-out reaches 300°F, and 300 > 250, so the coating would be exposed beyond its limit every week.
- Decision: either choose a coating whose listed limit is at least 300°F, or move the airstream parts to a material that needs no coating.
The lesson is that the design temperature for a coating is the highest temperature the fan will see, including cleaning cycles and upsets, not the normal running temperature.
Impeller Material and Shaft Material¶
Impeller material is the material the impeller's blades, hub and plates are made from. Centrifugal impellers are usually welded steel, with stainless steel or aluminum when the airstream calls for it. Axial impellers are commonly aluminum, and FanPedia shows fiberglass impellers (the TF and FG7) and fiberglass hubs (the FG4/FG6) for corrosive service (p. 34). The impeller material controls three things at once: corrosion resistance, the temperature limit (aluminum's limits above), and spark resistance (Type B requires a non-ferrous impeller).
Shaft material grades are the specific steel or alloy specifications a fan shaft is made from. FanPedia lists them (p. 80):
- Mild steel by default: Grade 1045 TG&P (turned, ground and polished) or Grade 4140 TG&P (hot rolled annealed), with hot rolled stress relieved on special request.
- Stainless steel when specified: Types 304 and 316, and Type 2205 Duplex as specified by the customer.
- AMPCO-45, a nickel-aluminum bronze, on axial fans when AMCA A sparkproof construction is required in marine applications.
- Monel for fans with AMCA A sparkproof construction where a specialty shaft seal is required.
- Titanium on special request.
A forged shaft is a shaft made by working hot metal (at approximately 2,200°F) into shape by impact or pressure in hammers, upsetters, presses or rolls, then machined in rough and final stages to Twin City Fan's specifications by outside vendors (FanPedia p. 82). Forged shafts are used in industries with very large fans, such as API, cement and power, usually on Arrangement 3DI and 3SI fans with a large major diameter and several turndowns. Forging reduces machining, but forging, heat treating and machining may involve several vendors and a long lead time, so forged shafts are usually supplied at the customer's request.
The table below summarizes the materials covered so far.
| Material | Typical use on a fan | Main limitation |
|---|---|---|
| Mild steel | Default housing, impeller and shaft | Corrodes in aggressive airstreams unless coated; ferrous |
| Corten steel | Steel option in high temperature packages | Ferrous; choose with the engineering review |
| Stainless steel | Corrosive or washdown airstreams; shafts (304, 316, 2205) | Cost; ferrous, so not spark resistant; joint to mild steel |
| Aluminum | Axial impellers, Type B impellers, Type C funnels and rub plates, shaft coolers | Temperature: 250°F rotating (Arr. 4 high temperature), 275°F axial impeller |
| Fiberglass | Axial impellers and hubs for corrosive service | Not covered by FanPedia's grounding device page, which excludes fiberglass |
| Coating on steel | Low-cost corrosion protection | Protects only where intact; has its own temperature limit |
Spark Resistant Construction¶
Spark resistant construction is construction that lowers the chance that the fan itself strikes a spark, by making key airstream parts non-ferrous and by arranging the parts so that rotating and stationary components are unlikely to touch. It is used where the airstream may carry flammable vapors, gases or combustible dusts. Twin City Fan builds three types, which follow the AMCA spark-resistant classification (FanPedia's shaft page refers to "AMCA A Sparkproof construction," p. 80). Chapter 2 summarized them; here they are as the order sees them:
- Type A gives the highest degree of spark resistance: every fan component in the airstream is non-ferrous, assembled to reduce the possibility of contact between stationary and rotating parts. Bearings are restrained (FanPedia p. 104).
- Type B requires a non-ferrous impeller, with the airstream parts assembled to reduce the possibility of contact. Typically this is an aluminum impeller and an aluminum rub plate; if the fan fails mechanically, the aluminum impeller contacts a steel inlet cone (p. 105).
- Type C offers a minimal level of spark resistance: it only reduces the possibility of contact, by putting non-ferrous parts at the points where contact is most likely (p. 106).
Types A and B are developed further in Chapter 21; this chapter covers Type C in detail and the parts all three share.
Non-Ferrous Material¶
A non-ferrous material is a metal or alloy that is not based on iron. The reason it matters is the spark itself: a steel part striking another steel part can throw hot fragments that ignite a flammable atmosphere, while a non-ferrous part rubbing steel is far less likely to do so. FanPedia lists the typical non-ferrous materials for spark-resistant construction as aluminum, aluminum/nickel/bronze, Monel, copper, brass and bronze (pp. 104-109). Monel is a nickel-copper alloy, and AMPCO-45, named on the shaft page, is a nickel-aluminum bronze.
Worked Example: Sorting a Parts List¶
A purchase order for a Type B fan lists these airstream part materials. Sort each one before checking whether the impeller qualifies.
- Impeller: aluminum. Non-ferrous, so the impeller meets Type B.
- Rub plate: aluminum. Non-ferrous.
- Inlet cone: mild steel. Ferrous, and that is allowed for Type B: the steel inlet cone is the expected contact point for the aluminum impeller (p. 105).
- Rubbing band: Monel. Non-ferrous (nickel-copper).
- Hub set screw: Type 316 stainless steel. Ferrous, because stainless steel is iron-based.
- Housing: Corten steel. Ferrous, which Type B allows.
- Shaft seal ring: brass. Non-ferrous.
The order meets Type B because the impeller is non-ferrous. It would not meet Type A, which requires every airstream component to be non-ferrous; there, FanPedia's Type A list calls for a non-ferrous housing and inlet funnel and specifies how the hub set screws are installed (stainless steel, flush with the hub) (p. 104).
Stainless Is Not Non-Ferrous
A common trap is reading "stainless steel" on a drawing and assuming it satisfies a non-ferrous requirement. Stainless steel is an iron-based alloy, so it is ferrous. When a spark-resistant type calls for non-ferrous parts, look for one of the six materials on FanPedia's list, and ask before substituting.
Spark Resistant Type C, Rub Plates and Rub Rings¶
Spark Resistant Type C is the minimal spark-resistant construction: the impeller and housing stay steel, and non-ferrous parts are placed at the points where a failing rotor would touch the housing first. Two parts do most of that work.
A rub plate is a non-ferrous plate, typically aluminum, placed at the opening of the housing where the shaft passes through. It protects against contact between the steel shaft and the steel housing. FanPedia lists its other names: shaft seal, enclosure plate, striker plate, spark plate and ash tray (pp. 105-106). On a Type B fan with an outside protruding hub, a rub plate is not required (p. 105).
A rub ring is a non-ferrous ring mounted where the impeller could shift into a stationary part, so that the ring, not the steel housing or plate, is touched first. Radial bladed fans and pressure blowers use an inlet rub ring and a Monel rub ring, and pressure blowers add a Monel rub strip (pp. 107-108).
The parts change with the fan type, which is why FanPedia gives Type C its own page for each family.
| Fan type | Type C non-ferrous parts | Source |
|---|---|---|
| Centrifugal fans | Aluminum inlet funnel and aluminum rub plate. At 501°F and above: steel funnel with a rubbing band (naval brass or Monel) and rubbing bars (Monel) | FanPedia p. 106 |
| Radial bladed fans | Inlet plate, rub plate, inner drive plate, Monel rub ring | FanPedia p. 107 |
| Pressure blowers | Aluminum inlet rub ring, rub plate, Monel rub strip, Monel rub ring | FanPedia p. 108 |
| Axial fans | Rub plate | FanPedia p. 109 |
Two notes on those pages decide many orders. First, bearings are not allowed in the airstream in any spark-resistant type. Second, if a radial bladed fan or pressure blower has a non-ferrous impeller as standard, the order should be Type B rather than Type C (pp. 107-108), because the impeller already meets the stricter requirement.
Just as important is what spark-resistant construction does not do. No type makes the fan explosion-proof. Type C leaves a steel impeller turning inside a steel housing; it only protects the likely first contact points. None of the types stops tramp metal or other debris that the airstream carries from striking the impeller. And the motor is a separate decision, made by hazardous-location rating, as Chapter 2 explained. Static electricity is also a separate hazard, handled by grounding.
The interactive matrix below lets you test the differences among the three types on a centrifugal fan before checking FanPedia's answer.
Diagram: Spark Resistance Parts Matrix¶
Run Spark Resistance Parts Matrix Fullscreen
Spark Resistance Parts Matrix
Type: diagram
sim-id: ch20-spark-resistance-parts-matrix
Library: html
Status: Specified
Bloom Level: Analyze
Bloom Verb: differentiate
Learning Objective: The learner will differentiate spark-resistant Types A, B and C on a centrifugal fan by predicting, for six fan components, whether each type makes the component non-ferrous, leaves it steel, or does not list it, with at least 15 of 18 cells correct on the first attempt.
Prerequisites: spark resistant construction, Types A, B and C, non-ferrous material, rub plate, inlet funnel, pedestal (all defined in the section "Spark Resistant Construction" above and in Chapter 3).
Evidence of Mastery: The learner sets each of the 18 cells (6 components × 3 types) to one of three states, "Non-ferrous", "Steel" or "Not listed", and presses Check. A cell is correct when it matches the answer table in Content. Mastery is 15 of 18 correct on the first Check. Opening a component's description is exploration, not evidence.
Misconceptions: (1) Type C makes the impeller non-ferrous (it does not; the impeller stays steel). (2) Type B makes the whole airstream non-ferrous (only the impeller and rub plate are). (3) Spark-resistant construction makes the shaft non-ferrous (the shaft is steel in all three types).
Instructional Rationale: Analyze-level differentiation requires the learner to compare the three types part by part, so the learner commits a prediction for every cell before the answers are revealed. Checking a whole matrix at once shows the pattern (A: almost everything; B: impeller; C: contact points) rather than isolated facts.
Content:
Rows (components) with the description shown when a row label is opened:
- Housing / frame: the scroll and sides that enclose the impeller.
- Impeller: the rotating wheel, with its welded hub.
- Inlet funnel: the cone that guides air into the impeller eye.
- Rub plate: the plate at the housing opening where the shaft passes through.
- Shaft: the steel or alloy shaft carrying the impeller.
- Pedestal: the structure that carries the bearings, outside the airstream.
Answer table (column headings "Type A", "Type B", "Type C"):
| Component | Type A | Type B | Type C | Reason shown as feedback |
|---|---|---|---|---|
| Housing / frame | Non-ferrous | Steel | Steel | Only Type A makes the whole airstream non-ferrous; B and C keep a steel housing. |
| Impeller | Non-ferrous | Non-ferrous | Steel | Type B is defined by a non-ferrous impeller; Type C leaves the impeller steel. |
| Inlet funnel | Non-ferrous | Steel | Non-ferrous | In Type B the steel inlet cone is the contact point for the aluminum impeller; in Type C the aluminum funnel is the first contact point. |
| Rub plate | Not listed | Non-ferrous | Non-ferrous | Type A's list has no rub plate because the airstream parts are already non-ferrous; B and C use an aluminum rub plate at the shaft opening. |
| Shaft | Steel | Steel | Steel | The shaft is steel construction in all three types. |
| Pedestal | Steel | Steel | Steel | The pedestal is outside the airstream and is steel in all three types (bolted on for Type A). |
A note shown under the matrix after Check: "Bearings are not allowed in the airstream in any of the three types. Type C at 501°F and above uses a steel funnel with a rubbing band and rubbing bars instead of the aluminum funnel."
Provenance: FanPedia, Spark Resistant Construction, Centrifugal Fans, pp. 104 (Type A), 105 (Type B) and 106 (Type C), https://tcf.com/fanpedia/. Component lists are taken from each page's "Non-Ferrous Construction" and "Steel Construction" lists.
Rules: Each cell has exactly one correct state from the answer table. All cells start empty. Check is available only when all 18 cells are set; otherwise the learner is told how many cells remain empty. Score = number of cells matching the answer table.
Learner Activity:
- The learner opens any row label to read what the component is.
- The learner sets each of the 18 cells to Non-ferrous, Steel or Not listed.
- The learner presses Check. Each cell is marked correct or incorrect, and each row with an incorrect cell shows a hint naming the question to ask for that component; no cell's answer and no reason is shown yet.
- The learner may press Try Again, which clears only the incorrect cells and keeps the correct ones; the first-Check score is kept as the recorded score.
- After a second Check, or on pressing Show Answers, the full answer table, each row's reason and the note are revealed.
Feedback: One matrix of 18 cells in fixed row order. First Check: "You matched n of 18 cells." Correct cells are confirmed. Hint after a first wrong attempt: each row with an incorrect cell shows a hint that names the question to ask for that component (for example Inlet funnel: "Think about where the rotating impeller could first touch a stationary part in each type"), never a cell's state. After the final attempt: each row shows its "Reason shown as feedback". At 15 or more: "You can tell the three types apart part by part." Below 15: "Look at the Impeller and Inlet funnel rows first; they carry the main difference between B and C." Answers are revealed after the second Check or on request.
Starting State: An empty 6 × 3 matrix with the question "Which parts does each type make non-ferrous?"
Chapter Anchors: Six components and three types (18 cells); mastery 15 of 18; Type C steel funnel at 501°F and above; bearings are not allowed in the airstream in any type; the chapter states that Type B typically uses an aluminum impeller and an aluminum rub plate with a steel inlet cone, and that Type C typically uses an aluminum inlet funnel and an aluminum rub plate.
Fan Grounding Device¶
A fan grounding device is a stud, lug or pad on the fan housing to which a grounding conductor is connected, so that the fan is bonded to the plant's electrical ground and static charge from the moving airstream cannot build up on it. FanPedia shows three standard forms (p. 140): a 3/8-inch grounding stud (stainless steel stud standard), a 3/8-inch grounding stud with lug (stainless steel stud and nuts, aluminum lugs standard), and a grounding pad with a clearance hole (stainless steel standard). Options include a threaded hole with stud, copper, and a two-hole lug. Standard locations are on the drive side or the inlet side, and the page applies to all materials except fiberglass. FanPedia also notes that a lug is commonly mistaken for a grounding stud. Grounding is often specified together with spark-resistant construction, because it handles the static-discharge hazard that non-ferrous parts do not.
High Temperature Construction¶
High temperature construction is the special construction for fans whose airstream is hot enough to weaken materials, expand parts, and cook bearings. FanPedia describes industrial processes that need fans for airstreams from 276°F to over 1,000°F on centrifugal general construction (p. 110), and from 181°F for Arrangement 4 fans (p. 111). Twin City Fan engineering (2026-10-09) advises that Arrangement 4 is not used for high-temperature applications today: its standard ranges run up to about 180°F (for example EPQN 0-180°F, QSL 0-160°F, TCTA 0-105°F, from Twin City Fan's model temperature tables), so FanPedia's Arrangement 4 packages are historical descriptions and a hot job belongs on Arrangements 1, 8, 9 or 10. Typical uses are recirculating air in kilns, dryers, industrial ovens and furnaces, exhausting hot gases and fumes, and supplying air for heating and drying systems.
Twin City Fan organizes high temperature construction into packages, each a set of construction details for one temperature band. For centrifugal fans in Arrangements 1, 8, 9, 9F and 10, the packages are 301°F-500°F, 501°F-600°F, 601°F-800°F, 801°F-1,000°F, and 1,001°F and over, which requires engineering review (p. 110). Arrangement 4 fans have a 181°F-300°F package, with 301°F and over requiring engineering review (pp. 111, 114). Every centrifugal package lists mild steel and Corten or stainless steel among its materials (Arrangement 4 fans also allow aluminum rotating parts up to 250°F), and the packages for fans with their own bearings add the bearing rules: high temperature grease, and fixed and floating bearings to allow for shaft growth. FanPedia refers readers to Fan Engineering Letter FE-3200 for the full treatment. Cooler boxes, heat shields, high temperature pedestals and the axial packages are covered in Chapter 21.
Worked Example: Selecting a Package¶
An Arrangement 1 centrifugal fan will recirculate furnace air at 720°F.
- Find the band: 720°F lies between 601°F and 800°F, so the fan needs the 601°F-800°F package.
- Materials: mild steel and Corten, or stainless steel. Aluminum is not used in rotating parts at this temperature.
- Bearings: high temperature grease, fixed and floating bearings.
- Shaft heat: the temperature is over 300°F, so a shaft cooler is recommended (FanPedia p. 192).
- Pedestal: FanPedia shows a separated pedestal design for Arrangements 1, 8 and 9F at 601°F and above, which provides up to a 3-inch gap between the housing and the pedestal (p. 112).
Had the temperature been 1,050°F, the band would be "1,001°F and over," and the order would go to engineering review before a price is given.
Maximum Operating Temperature¶
The maximum operating temperature is the highest airstream temperature at which a fan, or one of its parts, packages or materials, is rated to run. Every element on the construction line has one: the package band, the impeller material, a coating, a leak-tight construction. The fan as a whole is limited by the lowest of them, the weakest link:
and the selection is acceptable only if \( T_{\text{design}} \le T_{\max,\text{fan}} \), where \( T_{\text{design}} \) is the highest temperature the airstream will reach, including upsets and cleaning cycles.
Worked Example: Finding the Weakest Link¶
A centrifugal fan is quoted with the 301°F-500°F package, a coating whose sheet lists 400°F (illustrative), and nominally leak-tight construction. The customer's design temperature is 450°F.
- Package: upper bound 500°F.
- Coating: 400°F (illustrative).
- Nominally leak-tight construction: not recommended over 600°F.
- The fan's limit is the minimum: \( \min(500, 400, 600) = 400 \)°F.
- Check: 450 > 400, so the selection fails. The coating, not the package, is the weak link.
Changing the coating to one rated at least 450°F, or removing it in favor of a material that needs none, raises the fan's limit to 500°F, and 450 ≤ 500 passes.
The Fan Is Only as Hot-Rated as Its Weakest Part
Notice that nobody rates "the fan" for temperature in one number; each part carries its own limit, and the lowest one wins. When a hot job fails a review, list every part's limit side by side. The culprit is usually a small item, such as a coating, a grease or an aluminum part, not the package itself.
Shaft Cooler and High Temperature Grease¶
A shaft cooler is a cast aluminum part mounted on the shaft outside the housing, between the shaft seal and the bearings. Heat travels from the hot airstream along the steel shaft toward the bearings; the cooler dissipates that heat into the surrounding air before it reaches them, protecting the bearings. FanPedia's technical descriptions recommend shaft coolers for applications over 300°F and note that the bore size is needed when one is ordered as a stand-alone part (p. 192). On the high temperature pages it appears next to the shaft seal on every centrifugal arrangement (pp. 110-115).
High temperature grease is a bearing lubricant formulated to keep its consistency and lubricating film at elevated bearing temperatures, where ordinary grease would thin, run out or break down. The centrifugal high temperature packages for fans with their own bearings require it (pp. 110, 112, 113), as do air kits (p. 115), and high temperature axial fans require it over 275°F (p. 116). Grease selection and relubrication are covered in Chapter 17.
The chart below collects the temperature limits from this chapter on one scale so you can test a design temperature against them.
Diagram: Fan Temperature Limits Ruler¶
Run Fan Temperature Limits Ruler Fullscreen
Fan Temperature Limits Ruler
Type: chart
sim-id: ch20-fan-temperature-limits-ruler
Library: Chart.js
Status: Specified
Bloom Level: Apply
Bloom Verb: use
Learning Objective: The learner will use FanPedia's temperature limits to decide, for five stated fans and design temperatures, which package, material or construction rule applies, answering at least 4 of 5 correctly.
Prerequisites: high temperature construction, temperature package, maximum operating temperature, aluminum construction, shaft cooler, high temperature grease, spark resistant Type C, nominally leak-tight construction (all introduced in this chapter above this block).
Evidence of Mastery: The learner answers five multiple-choice questions, each about one fan at one design temperature, before the answer is revealed. An answer is correct when it matches the correct option in Content. Mastery is 4 of 5 correct on the first attempt. Moving the temperature marker is exploration, not evidence.
Misconceptions: (1) One aluminum limit applies to every fan type (the centrifugal rotating-part limit is 250°F; the axial impeller limit is 275°F). (2) A temperature exactly at a boundary belongs to the lower band in every case (bands are inclusive as written; 600°F is not "over 600°F"). (3) If the package fits, nothing else needs checking (Type C funnels, leak-tight construction and shaft coolers have their own thresholds).
Instructional Rationale: Apply-level use of limits needs practice placing a specific temperature against specific thresholds, so each question asks the learner to commit an answer, then shows the marker on the chart at that temperature as confirmation. The chart makes the overlapping limits visible at once, which exposes misconception 1.
Content:
The chart is a horizontal range chart on a temperature axis from 0°F to 1,200°F. Each row is one limit, drawn as a range:
| # | Row label | Range shown (°F) | Source |
|---|---|---|---|
| 1 | Aluminum rotating parts, Arr. 4 centrifugal and Arr. 4 plug fans | 0 to 250 | FanPedia pp. 111, 114 |
| 2 | Standard aluminum axial impeller | 0 to 275 | FanPedia p. 116 |
| 3 | High temperature grease required, axial fans | over 275 to 1,200 (open-ended) | FanPedia p. 116 |
| 4 | Shaft cooler recommended | over 300 to 1,200 (open-ended) | FanPedia p. 192 |
| 5 | Arr. 4 centrifugal package | 181 to 300 | FanPedia p. 111 |
| 6 | Centrifugal package (Arr. 1, 8, 9, 9F, 10) | 301 to 500 | FanPedia p. 110 |
| 7 | Centrifugal package (Arr. 1, 8, 9, 9F, 10) | 501 to 600 | FanPedia p. 110 |
| 8 | Centrifugal package (Arr. 1, 8, 9, 9F, 10) | 601 to 800 | FanPedia p. 110 |
| 9 | Centrifugal package (Arr. 1, 8, 9, 9F, 10) | 801 to 1,000 | FanPedia p. 110 |
| 10 | Centrifugal package, engineering review | 1,001 to 1,200 (open-ended) | FanPedia p. 110 |
| 11 | Type C centrifugal: steel funnel with rubbing band and rubbing bars | 501 to 1,200 (open-ended) | FanPedia p. 106 |
| 12 | Nominally leak-tight construction recommended | 0 to 600 | FanPedia p. 120 |
Hovering or tapping a row shows its label, range and source. Open-ended rows are labeled "and above".
Questions, in fixed order:
| # | Question | Options | Correct | Feedback (reason) |
|---|---|---|---|---|
| 1 | Arrangement 4 centrifugal fan at 240°F. May the impeller be aluminum? | Yes; No | Yes | 240 ≤ 250, the limit for aluminum rotating parts in Arr. 4 high temperature construction. |
| 2 | Arrangement 9 axial fan at 290°F with a standard aluminum impeller. Is the impeller within its limit? | Yes; No | No | 290 > 275, the limit for the standard aluminum axial impeller; this temperature also requires high temperature grease. |
| 3 | Arrangement 8 centrifugal fan, spark-resistant Type C, at 550°F. Which inlet funnel? | Aluminum funnel; Steel funnel with rubbing band and rubbing bars; No funnel change needed because Type C only adds a rub plate | Steel funnel with rubbing band and rubbing bars | 550 ≥ 501, where FanPedia replaces the aluminum funnel with a steel funnel, rubbing band and rubbing bars. |
| 4 | Arrangement 1 centrifugal fan at 650°F, customer asks for nominally leak-tight construction. Is it recommended? | Yes; No | No | 650 > 600; FanPedia does not recommend nominally leak-tight construction over 600°F. |
| 5 | Arrangement 1 centrifugal fan at 320°F. Which package, and is a shaft cooler recommended? | 181°F-300°F package, no cooler; 301°F-500°F package, cooler recommended; 301°F-500°F package, no cooler; 501°F-600°F package, cooler recommended | 301°F-500°F package, cooler recommended | 320 lies in 301-500, and 320 > 300, where FanPedia recommends a shaft cooler. |
Provenance: FanPedia pages listed per row, https://tcf.com/fanpedia/. Question temperatures are illustrative design points chosen for practice and must be labeled "Illustrative design temperatures."
Rules: Temperature marker: min 0, max 1,200, step 10, default 70, unit °F. A row is "active" at marker temperature T when T lies within its range: rows with "to" ranges include both ends (for example 301 <= T <= 500); "over 275" means T > 275; "over 300" means T > 300; open-ended rows have no upper bound. Active rows are marked as active, inactive rows are dimmed but still readable. When a question is answered, the marker moves to that question's temperature.
Learner Activity:
- The learner moves the temperature marker and sees which limits are active at each temperature.
- The learner opens question 1, chooses an option and submits.
- The marker jumps to the question's temperature, the relevant row is highlighted, and the feedback appears.
- The learner continues through questions 2 to 5 in order.
- After question 5, a summary shows the score and each question's reason.
Feedback: Five questions, fixed order, one attempt each. Correct: "Correct." followed by the reason. Incorrect: "Not quite." followed by the reason. The correct option is revealed immediately after each answer. A running score "n of 5" is shown; at 4 or more the summary says "You can place a design temperature against the catalog limits."
Starting State: The marker sits at 70°F with only rows 1, 2 and 12 active, and the prompt "Move the marker to your job's temperature. Which limits does it cross?"
Chapter Anchors: 250°F aluminum rotating-part limit (Arr. 4); 275°F standard aluminum axial impeller and high temperature grease over 275°F; shaft cooler recommended over 300°F; packages 181°F-300°F (Arr. 4) and 301°F-500°F, 501°F-600°F, 601°F-800°F, 801°F-1,000°F, 1,001°F and over (Arr. 1, 8, 9, 9F, 10); Type C steel funnel at 501°F and above; nominally leak-tight not recommended over 600°F.
Moisture: Weep Holes, Drains and High Moisture Construction¶
Hot air is not the only airstream that needs special handling. Water that condenses or collects in a housing corrodes the scroll, can be thrown around by the impeller, and can freeze. Three features deal with it: the weep hole, the housing drain and high moisture construction.
A weep hole is a small hole at the lowest point of a fan housing. FanPedia says most housings come with one as standard (p. 147). Manufacturing uses it to drain wash water out of the housing before painting, and afterward it lets moisture drain from the housing once the fan is installed (pp. 147, 197). A weep hole is a simple opening; when the water must be piped away or shut off, the housing needs a drain instead.
Housing Drain¶
A housing drain is a connection at the lowest point of the housing that lets collected moisture be piped away or closed off. FanPedia describes the standard drain as a half coupling of pipe welded to the housing scroll, with a diameter that varies by fan model; special diameters can be provided, and a plug or valve can be added to close it (p. 147). Inlet boxes may also get drains, as a feature or an added accessory. The specialty drains FanPedia lists are a drain extended from the inlet side, a flanged drain, a drain with a valve to open and close it, and drains from the fan and the inlet box piped to a common point.
Worked Example: Placing the Drain on an Insulated Fan¶
An insulated centrifugal fan with a detached inlet box handles humid process air. The drain line on the order is built up as follows.
- Lowest point: the drain goes at the lowest point of the scroll for the fan's discharge position, because water runs downhill.
- Access: the fan is insulated, so a drain flush with the housing would be buried. FanPedia's insulated fan pages say the housing drain usually extends from the inlet end of the housing for accessibility (pp. 117-118).
- Inlet box: humid air condenses in the inlet box too, so add an inlet box drain and pipe the two to a common point.
- Closure: the customer wants to drain only during shutdowns, so specify a valve rather than a plug.
The line reads: "Housing drain extended from inlet side, inlet box drain, piped to a common point, with valve."
Ask Where the Water Goes
Here's a quick check for any wet or insulated job: ask the customer where the drain line will be piped. If the answer is "nowhere," a weep hole may do; if they need to collect or control the water, specify a drain with a plug or valve, and extend it if the fan is insulated.
High Moisture Construction¶
High moisture construction is a modification for applications where steam or condensation may collect in the fan housing (FanPedia p. 122). It is used on Arrangement 9 axial fans only. The modification seals the path between the wet airstream and the drive: FanPedia's drawing shows a shaft seal with seal material and cover plates at the impeller end and the drive end. And it carries the combination rule from the start of this chapter: high moisture construction cannot be used together with high temperature construction.
Worked Example: Three Wet Inquiries¶
- Arrangement 9 axial fan exhausting steam from a washdown area at near-ambient temperature. High moisture construction applies: the fan is an Arrangement 9 axial, steam is present, and no high temperature construction is needed. Add a drain at the lowest point.
- The same fan, but the exhaust reaches 220°F. Arrangement 9 axial fans at that temperature fall in a high temperature package (201°F-250°F, p. 116), and high moisture construction cannot be combined with high temperature construction. The inquiry goes to engineering review rather than being quoted with both options.
- Arrangement 1 centrifugal fan with condensation in the housing. High moisture construction is not offered for this fan type. The tools are a housing drain at the lowest point and, if condensation on the outer surface is the problem, insulated construction.
Insulated Fans¶
An insulated fan is a fan whose housing is covered with insulation and an outer skin. FanPedia gives the same three purposes for every insulated construction: to insulate the fan surface from high temperature, from condensation, or from sound (pp. 117-119). Twin City Fan offers three constructions. Aluminum clad construction, in which an outside vendor insulates the fan and builds a stucco-embossed aluminum skin around it, is covered in Chapter 21. The other two are described here.
Double wall construction (FanPedia calls it steel/double wall construction) is built by Twin City Fan as a second structural outer housing, the outer shell, around the inner housing, with insulation in between (p. 118). The outer shell is at least 14 gauge mild steel, or stainless steel if requested. The insulation is fiberglass or mineral wool unless otherwise specified, and a structural fabricated angle between the inner and outer housings holds it in place; on fans with impellers 40 inches and larger, that angle is scalloped to reduce heat transfer. Because the shell adds thickness, the access door is usually raised 2 inches above the insulation, the housing drain usually extends from the inlet end, the inlet funnel is modified to extend through the insulation, the inlet, outlet and centerline height are extended or raised if required, split bars on split housings protrude 2 inches past the insulation, and the shaft cooler housing is embedded in an insulation cavity.
An insulated jacket is a removable insulating blanket wrapped around the fan. FanPedia describes a 2-inch-thick jacket around the entire housing, including the housing surface inside the pedestal, with vendor-supplied Type "E" and low-density fiberglass insulation alone or in combination (p. 119). The jacket is removed in labeled pieces for shipment and can be opened or removed to reach access doors, drains, housing splits, lifting lugs, shaft and bearing guards, pedestals, inlet boxes, frame angles and shaft seals. On high temperature applications, the jacket wraps around a cooler box. FanPedia notes that jackets can also be used as a safety device to protect personnel from injury.
The table compares the three insulated constructions just described.
| Construction | Who builds the insulated skin | Skin | Insulation | Best fit |
|---|---|---|---|---|
| Aluminum clad (Chapter 21) | Outside vendor | 0.040-inch (minimum) stucco-embossed aluminum | 3-lb density fiberglass or mineral wool unless specified | A vendor-built aluminum skin over the finished fan |
| Steel / double wall | Twin City Fan | 14 gauge (minimum) mild steel or stainless steel outer shell | Fiberglass or mineral wool unless specified | A rugged, structural, factory-built insulated housing |
| Insulated jacket | Vendor-supplied insulation | Removable 2-inch jacket | Type "E" and low-density fiberglass | Frequent access to accessories; personnel protection |
An insulated inlet box is an inlet box built with insulation, used for high temperature or sound applications, with the same insulated constructions as the fan housing (FanPedia p. 145). When the fan is insulated and has an inlet box, the box usually needs insulating too, or it becomes the hot, noisy or sweating surface the fan insulation was meant to eliminate.
Worked Example: Choosing an Insulated Construction¶
Return to the 550°F dryer exhaust fan from the start of the chapter, which stands beside an operator platform. Maintenance staff open its access door and drain monthly.
- Purpose: high temperature surface and personnel protection. All three constructions insulate; FanPedia names personnel protection explicitly for the jacket.
- Access: monthly door and drain access favors a construction that opens easily. The jacket is made to be opened or removed to reach access doors and drains.
- Temperature: at 550°F a cooler box is used, and the jacket wraps around it.
A jacket fits this job. Had the customer asked for a permanent, structural skin with no field removal, double wall construction would be the choice, with its raised access door and extended drain.
Flue Gas Recirculation¶
Flue gas recirculation (FGR) is the practice of returning part of a combustion system's exhaust gas to the combustion air stream; in boiler and burner systems it is commonly used to lower flame temperature and control emissions. For the fan, FGR means a hot, recirculated gas enters the inlet. Twin City Fan provides an FGR connection for this: a special flange connection on the front of the inlet box that allows air from the whole system to be recirculated through the fan (FanPedia p. 145, where FGR is expanded as "flue gas recirculation or recovery"). Because the recirculated gas is hot, an FGR job usually also brings high temperature construction and an insulated inlet box onto the construction line.
Putting It Together¶
You now have every answer in the construction stack: materials and coatings, spark resistance, temperature, moisture, insulation, and the FGR connection. The MicroSim below gives six inquiries, each with one decision that a construction class line must get right.
Diagram: Construction Class Advisor¶
Run Construction Class Advisor Fullscreen
Construction Class Advisor
Type: microsim
sim-id: ch20-construction-class-advisor
Library: p5.js
Status: Specified
Bloom Level: Evaluate
Bloom Verb: recommend
Learning Objective: The learner will recommend the construction choice for six stated airstream inquiries, consistent with the FanPedia rules in this chapter, with at least 5 of 6 correct on the first attempt.
Prerequisites: construction class, construction material, stainless steel construction, buffer strips, spark resistant Types A, B and C, non-ferrous material, high temperature construction and packages, high moisture construction, housing drain, insulated fan, insulated jacket, double wall construction, flue gas recirculation, inlet box (all defined earlier in this chapter or in Chapter 18).
Evidence of Mastery: For each of six inquiries, the learner selects one recommendation from four options and commits it before the reason is shown. A recommendation is correct when it matches the correct option in Content. Mastery is 5 of 6 correct on the first attempt. Reading the inquiry's airstream tags is exploration, not evidence.
Misconceptions: (1) High moisture and high temperature construction can be stacked. (2) Type C is always the lowest-cost answer, even when the impeller is already non-ferrous. (3) Insulation is only for heat (it also addresses condensation, sound and personnel protection). (4) A stainless housing can always be welded to a mild steel pedestal. (5) An FGR job is just a normal inlet box.
Instructional Rationale: Evaluate-level recommendation requires judging competing options against stated rules, so each inquiry offers four plausible constructions, three of which break a specific rule from the chapter. Committing before the reason appears produces evidence of judgment rather than recognition.
Content:
Each inquiry shows a short description and tags for the airstream conditions present (hot, wet, corrosive, explosive, personnel exposure, recirculated gas).
| # | Inquiry shown to the learner | Options (A-D) | Correct | Reason (shown as feedback) |
|---|---|---|---|---|
| 1 | Arrangement 9 axial fan exhausting steam from a washdown area at near-ambient temperature. | A: High moisture construction with a drain at the lowest point. B: High temperature package. C: Spark resistant Type C. D: Standard construction; steam is just air. | A | High moisture construction is for steam or condensation in the housing and is offered on Arrangement 9 axial fans; a drain removes the water. |
| 2 | The same Arrangement 9 axial fan, but the steam exhaust reaches 220°F. | A: High moisture construction plus a high temperature package. B: Refer to engineering review; high moisture and high temperature construction cannot be combined. C: High moisture construction only; 220°F is not hot. D: Insulated jacket only. | B | FanPedia states that high moisture construction cannot be used with high temperature construction, and 220°F falls in an axial high temperature package. |
| 3 | Radial bladed fan; specification calls for spark-resistant Type C; this fan's standard impeller is non-ferrous. | A: Type C with rub plate and Monel rub ring. B: Type B. C: Type A. D: No spark resistance needed. | B | FanPedia notes that a fan with a non-ferrous impeller as standard should use Type B. |
| 4 | Arrangement 1 centrifugal fan recirculating kiln air at 700°F; a walkway passes beside the fan and workers can touch the housing. | A: Standard construction with a heat-resistant coating. B: 601°F-800°F package, no insulation. C: 601°F-800°F package with insulated construction (for example an insulated jacket). D: 801°F-1,000°F package. | C | 700°F lies in the 601°F-800°F band, and a touchable hot housing calls for insulation; jackets are also named as personnel protection. |
| 5 | Forced draft fan with an inlet box; the boiler system returns part of its hot exhaust to the fan inlet. | A: FGR connection on the inlet box, with an insulated inlet box for the hot gas. B: High moisture construction. C: Spark resistant Type C. D: Weep hole only. | A | Recirculated flue gas enters through an FGR connection on the front of the inlet box, and the hot gas makes an insulated inlet box appropriate. |
| 6 | Stainless steel housing on a mild steel pedestal; the specification forbids welding stainless steel to mild steel. | A: Weld the housing to the pedestal anyway. B: Buffer strips between housing and pedestal, or a bolt-on pedestal. C: Coat the pedestal. D: Aluminum pedestal welded to the housing. | B | FanPedia uses buffer strips when stainless must not be welded to mild steel, and a bolt-on pedestal for dissimilar metals. |
Provenance: FanPedia, https://tcf.com/fanpedia/: high moisture construction p. 122; axial high temperature packages p. 116; spark resistant radial bladed fans p. 107 (note 2); centrifugal high temperature packages p. 110; insulated jackets p. 119; inlet box FGR connection and insulated inlet box p. 145; buffer strips and bolt-on pedestals p. 85. The inquiries are illustrative and must be labeled "Illustrative inquiries."
Rules: One correct option per inquiry, as in the Correct column. Option order is fixed as listed (A-D) so feedback can refer to letters. Score = number correct on first attempt.
Learner Activity:
- The learner reads inquiry 1 and its airstream tags.
- The learner selects one option and presses Recommend.
- The sim marks the choice correct or incorrect and shows the reason; if incorrect, the learner may choose again once for practice, but the first answer is the one scored.
- The learner moves to the next inquiry, through all six in fixed order.
- A summary lists each inquiry, the first answer, the correct answer and the reason, and builds the construction line for inquiry 4 as an example: "601°F-800°F package; mild steel and Corten or stainless steel; insulated jacket."
Feedback: Six inquiries, fixed order, first attempt scored, one extra practice attempt per inquiry. Correct: "Recommended construction is sound." plus the reason. Hint after a first wrong attempt: "That breaks a rule." plus a hint chosen by the wrong option that names the rule it breaks without naming the correct option (for example inquiry 2, option A: "Check whether high moisture and high temperature construction can be used together on one fan"). After the final attempt (the practice attempt, or moving on without one): "That breaks a rule." plus the reason, and the correct option is revealed; a correct first attempt shows the reason and the correct option at once. Final score "n of 6"; at 5 or more: "You can turn an airstream into a construction line."
Starting State: Inquiry 1 with its tags (wet) and the four options, with the prompt "What should the construction line say?"
Chapter Anchors: Six inquiries; mastery 5 of 6; high moisture construction is for Arrangement 9 axial fans only and cannot be combined with high temperature construction; 220°F falls in an axial high temperature package (201°F-250°F); non-ferrous standard impeller means Type B; 700°F falls in the 601°F-800°F package; jackets can protect personnel; FGR connection on the front of the inlet box; buffer strips or bolt-on pedestal for stainless on mild steel.
Key Takeaways¶
- Construction class is the build designation that matches the housing and impeller to the airstream: standard construction, or special constructions such as spark resistant, high temperature, insulated, nominally leak-tight, RTO and high moisture, all resting on a construction material. It is separate from fan class, which is a pressure and speed rating.
- Construction answers stack, and some combinations are ruled out: high moisture cannot combine with high temperature; nominally leak-tight is not recommended over 600°F; aluminum rotating parts in Arrangement 4 high temperature fans stop at 250°F.
- Materials: mild steel is the default; stainless steel (304, 316, 2205) resists corrosion but is ferrous; aluminum is light and non-ferrous but temperature-limited; Corten appears with mild steel in high temperature packages; coatings protect only where intact and have their own temperature limit. Large stainless airstream shafts can be cladded; forged shafts serve very large fans.
- Spark resistant construction has three types: A (all airstream parts non-ferrous), B (non-ferrous impeller), C (non-ferrous parts at the likely contact points). Type C uses rub plates and rub rings, and switches to a steel funnel with rubbing band and bars at 501°F and above. No type makes a fan explosion-proof, and bearings never sit in the airstream.
- Non-ferrous materials on FanPedia's list are aluminum, aluminum/nickel/bronze, Monel, copper, brass and bronze. A grounding device (stud, lug or pad) bonds the fan to ground for static.
- High temperature construction is sold in packages by temperature band. The fan's maximum operating temperature is the lowest limit of any part on the construction line. Shaft coolers are recommended over 300°F, and high temperature grease is required wherever the fan or kit has its own bearings (centrifugal packages for fans with bearings, air kits, and axial fans over 275°F), not in every package.
- Weep holes drain wash water and moisture; housing drains can be plugged, valved, flanged, extended and piped to a common point. High moisture construction is for Arrangement 9 axial fans only.
- Insulated fans address heat, condensation and sound through aluminum clad, double wall or jacket construction; insulate the inlet box too. An FGR connection on the inlet box brings recirculated flue gas into the fan.
You Can Write the Construction Line
You just learned to turn an airstream description into a construction line: material, spark-resistant type, temperature package, drains and insulation, with every entry traced to a rule. That is the part of the order that keeps a fan alive on the job. Chapter 21 takes the hottest and tightest of these constructions further.