High Temperature, Leak-Tight and RTO Construction¶
Summary¶
This chapter covers the most demanding construction options, starting with high temperature: cooler boxes and guards, heat shields, high temperature pedestals, thermal expansion and high expansion bearings, and high temperature packages for centrifugal and axial fans. It completes spark-resistant construction with Types A and B, then covers nominally leak-tight fans, fan leakage, gasketing, continuous seal welds, and the soap bubble test. It closes with regenerative thermal oxidizer (RTO) service conditions. After completing it, readers can specify construction for hot, leak-tight, or RTO duty.
Concepts Covered¶
This chapter covers the following 22 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Cooler Guard | 1 |
| Cooler Box | 2 |
| Insulation Pins | 2 |
| Spark Resistant Type A | 15 |
| Heat Shield | 15 |
| Thermal Expansion | 2 |
| Nominally Leak-Tight Fan | 47 |
| High Temperature Package | 2 |
| Aluminum Clad Construction | 1 |
| Recess Cone | 1 |
| Buffer Strip | 1 |
| Spark Resistant Type B | 14 |
| High Temperature Pedestal | 14 |
| Fan Leakage | 2 |
| Gasketing | 14 |
| Regenerative Thermal Oxidizer | 15 |
| High Expansion Bearing | 1 |
| Axial High Temperature Package | 1 |
| Continuous Seal Weld | 1 |
| Centering Plate | 1 |
| Soap Bubble Test | 1 |
| RTO Service Conditions | 14 |
Prerequisites¶
This chapter builds on concepts from:
- Chapter 2: Airstreams, Total Pressure and Standard Air
- Chapter 3: Basic Fan Anatomy
- Chapter 4: Centrifugal, Axial and Mixed Flow Fans
- Chapter 6: Industrial Fans and Application Product Lines
- Chapter 15: Motor Ratings, Speed Control and Positioners
- Chapter 16: Bearings, Shafts and Shaft Seals
- Chapter 17: Bearing Details, Lubrication and Pedestals
- Chapter 20: Construction Classes, Materials and Spark Resistance
When the Airstream Fights Back
Some jobs hand you an airstream that is hot enough to cook a bearing, dirty enough that no gas may escape, or both at once. This chapter shows you how to read those jobs and turn them into a construction list a factory reviewer will sign off on. Let's follow the airflow, and the heat that rides along with it.
Most fans in this book could be built from a standard catalog configuration. The fans in this chapter cannot. A kiln recirculation fan, a fan pulling solvent-laden air out of an automotive paint booth, and a fan feeding a regenerative thermal oxidizer all start from a catalog model, but each one needs special construction that changes the housing, the pedestal, the bearings and the seals. Chapter 20 introduced maximum operating temperature, shaft coolers, high temperature grease and spark-resistant Type C. This chapter finishes those threads and adds two new duties: keeping gas inside the fan, and serving an RTO.
All Twin City Fan construction details in this chapter come from the Fan Construction section of FanPedia (pp. 103-121). FanPedia notes on nearly every page that construction varies by model, so treat each list here as the starting point for a conversation with the factory, not a substitute for the current catalog or an engineering review.
How Heat Moves Through a Fan¶
A high temperature fan has one structural problem and one bearing problem. The structural problem is that hot steel grows. The bearing problem is that heat travels from the airstream into the shaft and the pedestal, and the bearings sit at the end of both paths. Every high temperature feature in this chapter does one of three jobs: it lets parts grow without binding, it sheds heat before it reaches the bearings, or it blocks the path the heat would take.
Thermal Expansion¶
Thermal expansion is the increase in a part's length (and every other dimension) as its temperature rises. For a straight part such as a fan shaft, the growth is proportional to its length and to its temperature rise.
Linear Thermal Expansion¶
where:
- \( \Delta L \) is the change in length (in.)
- \( \alpha \) is the material's coefficient of linear expansion (in./in./°F); for carbon steel a typical textbook value is about \( 6.5 \times 10^{-6} \)
- \( L \) is the original length (in.)
- \( \Delta T \) is the temperature rise (°F)
Example (illustrative). A steel shaft has 60 in. between bearing centers and warms by 300°F in service. Its growth is \( 6.5 \times 10^{-6} \times 60 \times 300 \approx 0.12 \) in. That is about one-eighth of an inch, and two bearings that both clamp the shaft axially would have to absorb it as thrust. This is why FanPedia's high temperature packages for pedestal-mounted fans, plug fans, air kits and axial fans call for fixed and floating bearings: the fixed bearing locates the shaft and the floating bearing lets it grow, as Chapter 17 explains.
The housing grows too. That is one reason flanged joints, split housings and pedestal attachments on hot fans get special treatment later in this chapter.
The High Expansion Bearing¶
Some fans need more axial freedom than a standard floating bearing allows. FanPedia's bearing modifications page (p. 101) shows a high expansion bearing and shaft modification, offered for air kits only, and the air kit high temperature page (p. 115) lists "high heat modified bearings" as an option. Read the name literally: it is a bearing arrangement modified to accept more thermal growth of the shaft than the standard bearing does. FanPedia's before-and-after drawing shows the change includes the shaft and its locking hardware (a socket head screw ground flat to fit the keyway), so plan it at order time. FanPedia does not say why it is offered for air kits only, so treat the restriction as a catalog fact and ask the factory if another fan type needs it.
Shedding and Blocking Heat at the Shaft¶
Heat reaches the bearings two ways: along the shaft, and through the housing into the pedestal. The shaft path is handled first, with a short family of parts that appear on almost every high temperature drawing in FanPedia.
The shaft cooler (also called a heat flinger, heat slinger or cooling impeller) is a cast aluminum wheel clamped on the shaft between the housing and the inboard bearing. FanPedia's technical description (p. 192) says it dissipates the heat transferred to the shaft from the airstream, protecting the bearings, and recommends it for applications over 300°F. The cooler only works if it has air to throw heat into, and it only stays safe if nobody can touch it. Three parts look after that:
- A cooler guard is the guard that encloses the spinning shaft cooler so that hands and tools cannot reach it, while still letting air move across the cooler's fins. FanPedia shows a cooler guard on Arrangement 4 fans, plug fans, air kits and insulated fans (pp. 111-119), and lists the shaft cooler and cooler guard together as a common accessory (p. 127).
- A cooler box is an uninsulated open area built around the shaft cooler so it can dissipate heat. FanPedia (p. 110) makes it standard on fans with aluminum clad insulation and on housings with insulation pins, because without it the insulation would bury the cooler and the cooler would have no cool air to work with. On insulated jackets, the jacket wraps around the cooler box (p. 119).
- Insulation pins are pins attached to the outside of the housing that hold field- or vendor-applied insulation in place. They are optional on high temperature centrifugal fans (p. 110) and are installed by the insulating vendor on aluminum clad fans (p. 117).
Diagram: High Temperature Fan Construction¶
Run High Temperature Fan Construction Fullscreen
High Temperature Fan Construction
sim-id: ch21-high-temp-construction-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the seven labeled high temperature construction features of a centrifugal fan on FanPedia's drawing, given each feature's job.
Content: Seven numbered markers: shaft seal, bearing and shaft cooler (in the top drive-end close-up), insulation pins and raised access door (on the main drawing), and safety screen and cooler box (in the bottom close-up).
Provenance: FanPedia page 110, "Special Construction: High Temperature, Centrifugal Fans (General Construction)", Arrangements 1, 8, 9, 9F, 10, with the high temp style pedestal insets, with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 110.
Block, Shed or Allow
Every hot-fan part you will meet in this chapter answers one question: does it block heat, shed heat, or allow growth? A shaft cooler sheds, a separated pedestal blocks, and a floating bearing allows. When a drawing shows a part you don't recognize, ask which of the three it does and you will usually know why it is there.
Aluminum Clad Construction¶
Aluminum clad construction is an insulated fan in which an outside vendor insulates the housing and builds an aluminum skin around it. FanPedia (p. 117) gives its purpose as insulating the fan surface from high temperature, condensation or sound. The cladding is stucco-embossed aluminum at least 0.040 in. thick over 3-lb-density fiberglass or mineral wool unless another type is specified, and the fan ships from the vendor or back to TCF. Because the insulation adds thickness, the access door is raised (usually 2 in. above the insulation), split bars protrude 2 in. past it, the drain is extended, and inlet, outlet and centerline height are increased if required. On high temperature applications it gets a cooler box. Chapter 20 compares it with steel double wall construction and insulated jackets.
Recess Cone¶
A plug fan or air kit mounts its bearings and drive on the outside of an insulated wall, so the shaft cooler has to sit somewhere inside the wall's thickness. A recess cone (also called a drive cone) is, as FanPedia draws it, a cone-shaped pocket formed into the insulated plug or wall that gives the shaft cooler and shaft seal a recessed space with access to outside air. FanPedia shows it as "Design #2" for Arrangement 9 plug fans, where "Design #1" leaves a plain gap for the shaft cooler instead (p. 113), and on Arrangement 3 air kits (p. 115). Arrangement 4 plug fans use a recess cavity for the same purpose (p. 114). The wall opening around a plug fan is filled by an insulated plug; on Arrangement 9 high temperature plug fans it is optional up to 500°F where the plug or wall is less than 4 in. thick, and required at 301°F and above where it is 4 in. or more (p. 113).
Diagram: High Temperature Fan Heat Path¶
Run High Temperature Fan Heat Path Fullscreen
High Temperature Fan Heat Path
Type: infographic
sim-id: ch21-high-temp-heat-path
Library: html
Status: Specified
Bloom Level: Understand
Bloom Verb: classify
Learning Objective: The learner will classify ten high temperature fan parts by whether each one blocks heat, sheds heat, or allows thermal growth, with at least 8 of 10 correct on the first attempt.
Prerequisites: thermal expansion, fixed and floating bearings, shaft cooler, cooler box, insulation pins, recess cone, insulated plug, high temperature pedestal, separated pedestal, heat shield (all defined in this chapter or Chapter 17 before this block).
Evidence of Mastery: For each of the 10 labeled parts on a side view of a high temperature fan, the learner commits a job (Block, Shed or Allow) before the part's description is revealed. A choice is correct when it matches the "Job" column. Mastery is 8 of 10 correct on the first attempt. Opening a description after answering is exploration, not evidence.
Misconceptions: (1) A cooler box is an insulated enclosure (it is deliberately left uninsulated so the cooler can shed heat). (2) Insulation pins are themselves insulation. (3) Every high temperature part is about keeping heat out; the floating bearing instead lets the shaft grow.
Instructional Rationale: Classifying each part by its job forces the learner to explain why it is on the drawing, which is the Understand-level skill a reviewer needs; committing before the reveal turns each label into a prediction rather than a lookup.
Content: Two side views, labeled with the ten parts below: a belt-driven centrifugal fan on a high temperature style pedestal (parts 1 and 3-8, 10) and an Arrangement 9 plug fan through an insulated wall (parts 2 and 9). Three job choices are offered for every part: "Blocks heat from reaching the bearings", "Sheds heat that has already reached the shaft", "Allows parts to grow without binding".
| # | Part label | Job | Description shown after answering (feedback) |
|---|---|---|---|
| 1 | Shaft cooler | Shed | A cast aluminum wheel on the shaft that throws off heat conducted from the airstream; recommended over 300°F. |
| 2 | Recess cone | Shed | A cone-shaped pocket in the insulated plug or wall that gives the shaft cooler room and outside air. |
| 3 | Cooler box | Shed | An uninsulated open area around the shaft cooler, so insulation does not smother it. |
| 4 | Insulation pins | Block | Pins that hold the housing insulation, which blocks heat leaving the housing surface. |
| 5 | Floating bearing | Allow | The non-locating bearing, which lets the shaft grow axially as it heats. |
| 6 | Separated pedestal | Block | A gap of up to 3 in. between housing and pedestal reduces heat conducted into the pedestal and bearings. |
| 7 | High temp angle | Block | An angle (also called heat angle, inverted angle or spacer angle) that spaces the pedestal from the housing on a high temperature style pedestal. |
| 8 | Motor heat shield | Block | A shield on the pedestal between the hot housing and the motor. |
| 9 | Insulated plug | Block | Insulation in the plug or wall that limits heat passing from the hot side to the drive side. |
| 10 | Fixed bearing | Allow | The locating bearing; it fixes one end so all growth goes toward the floating bearing. |
Provenance: Part names and jobs from FanPedia pp. 85, 90, 110-113 and 192, and Chapter 17 of this book (fixed and floating bearings). The fan drawing is a generic illustration and must be labeled "illustrative".
Rules: Each part takes one answer. Correctness is an exact match with the Job column. Part 10 is scored Allow because fixing one end is what lets all growth go to the floating bearing; its feedback says so.
Learner Activity:
- The learner selects a labeled part; the three job choices appear.
- The learner commits a choice. The part is marked correct or incorrect and its description is shown.
- The learner continues until all 10 parts are answered; the parts may be answered in any order.
- After all 10, the learner can filter the drawings to show only Block, only Shed or only Allow parts, and should notice that 5 parts block, 3 shed, and only the 2 bearings allow.
Feedback: 10 items, any order, one attempt each, no re-answering. Correct: "Correct:
Starting State: The full drawing with all ten labels unanswered and the question "What job does each part do: block, shed or allow?"
Chapter Anchors: Shaft cooler recommended over 300°F; separated pedestal gap up to 3 in.; ten parts (5 Block, 3 Shed, 2 Allow); three jobs named Block, Shed and Allow.
High Temperature Pedestals and Heat Shields¶
The second heat path runs from the hot housing, through the pedestal, into the bearings and motor. The answer is to change the pedestal itself.
A high temperature pedestal is a fan pedestal built to limit the heat conducted from the housing into the bearings and motor. FanPedia shows three forms. The high temperature style pedestal adds a high temp angle (also called a heat angle, inverted angle or spacer angle) between housing and pedestal, with a standard shaft seal and shaft cooler (p. 90). The pedestal with insulated panel adds an insulated panel on the pedestal facing the housing. The separated pedestal design leaves a gap of up to 3 in. between housing and pedestal, which reduces conducted heat and makes room for insulation (pp. 86, 112). FanPedia ties each form to arrangements and temperatures:
| Pedestal form (p. 112) | Arrangements | Temperature limit shown |
|---|---|---|
| Pedestal with motor heat shield | 10 | Up to 600°F |
| Pedestal with insulated panel | 10 | Up to 600°F |
| Pedestal with motor heat shield | 9 (multiple variations) | Up to 600°F |
| Separated pedestal design | 1, 8 and 9F | 601°F and above |
One dimension note comes with the separated design: for Arrangements 1 and 9F, the overall pedestal length is shortened by the short leg of the angle (or the width of the channel) that forms the gap.
A heat shield is a plate mounted between a hot surface and a component it protects, so radiant and convected heat from the housing does not reach that component directly. FanPedia lists it as a pedestal feature "used on high temperature applications" (p. 85). The most common form is the motor heat shield on Arrangement 9 and 10 pedestals (p. 112), where the motor sits on the pedestal close to the housing. Axial fans use one too: the Arrangement 9 axial high temperature package shows a heat shield next to the belt tube (p. 116).
Worked example: choosing a pedestal (illustrative job). An oven builder needs an Arrangement 10 recirculation fan for 550°F air.
- Temperature band. 550°F falls in the 501°F-600°F centrifugal package band, covered in the next section.
- Pedestal. Arrangement 10 puts the motor inside the pedestal envelope, so heat reaching the motor is the concern. FanPedia offers two Arrangement 10 forms up to 600°F: the pedestal with motor heat shield and the pedestal with insulated panel. Either is in range.
- Bearings. At 301°F and above, one fixed and one floating bearing, with high temperature grease. (These are airstream bands. Twin City Fan engineering letter FE-1300 recommends special high temperature grease when the bearing temperature exceeds 160°F, which is a different temperature from the airstream; see Chapter 17.)
Now the builder raises the setpoint to 700°F. Both Arrangement 10 pedestal forms stop at 600°F, and FanPedia's separated pedestal design is drawn for Arrangements 1, 8 and 9F at 601°F and above. The right response is not to quote Arrangement 10 anyway; it is to propose an Arrangement 1, 8 or 9F fan with a separated pedestal, or to send the Arrangement 10 request for engineering review.
The Limit Lives on the Pedestal, Too
It is easy to check the airstream temperature against the housing and impeller materials and stop there. The pedestal form, the arrangement and any aluminum rotating parts each carry their own temperature limit, so check all of them and quote the lowest one as the fan's limit.
High Temperature Packages¶
A high temperature package is a set of construction details that FanPedia groups by an airstream temperature band. The band, not the exact temperature, decides the materials, the pedestal, the shaft cooling and the bearing requirements. Industrial uses listed on every package page are recirculating air in kilns, dryers, industrial ovens and furnaces; exhausting gases and fumes from industrial processes; and supplying air for heating and drying systems. Every page refers to Twin City Fan's Fan Engineering Letter FE-3200 for details.
The bands differ by fan family, which is why a salesperson needs to know the family before the temperature means anything:
| Fan family (FanPedia page) | Package bands | Notes |
|---|---|---|
| Centrifugal, Arr. 1, 8, 9, 9F, 10 (p. 110) | 301-500°F, 501-600°F, 601-800°F, 801-1,000°F, 1,001°F and over | Over 1,000°F requires engineering review; mild steel, Corten or stainless steel |
| Centrifugal, Arr. 4 (p. 111) | 181-300°F; 301°F and over | 301°F and over requires engineering review; aluminum rotating parts only up to 250°F |
| Arr. 9 plug fans (p. 113) | Same five bands as Arr. 1, 8, 9, 9F, 10 | Insulated plug and recess cone or gap for the cooler |
| Arr. 4 plug fans (p. 114) | 181-300°F (select models); 301°F and over | 301°F and over requires engineering review |
| Air kits, Arr. 3 (p. 115) | Same five bands, 301°F to 1,001°F and over | Enclosure and insulation by others in the field; high heat modified bearings optional |
| Axial fans (p. 116) | See below | Separate axial package |
Arrangement 4 and current practice. The Arrangement 4 rows above are FanPedia's package descriptions. Per Twin City Fan engineering (2026-10-09), Arrangement 4 (direct drive, impeller on the motor shaft) is not used for high-temperature applications today. Its standard ranges are typically up to about 180°F, for example 0-180°F for the EPQN plenum fan, 0-160°F for the QSL mixed flow fan and 0-105°F for the TCTA tubeaxial fan (Twin City Fan's model temperature tables). Other ranges are special order, so confirm any Arrangement 4 job above its standard range with the factory. For a hot airstream, the packages in this chapter apply to belt driven or pedestal-mounted construction such as Arrangements 1, 8, 9 and 10.
The axial high temperature package is narrower because axial fans keep the motor or bearings inside the airstream's housing and typically use aluminum impellers. FanPedia (p. 116) describes axial high temperature service from 106°F to 600°F, but the bands it lists stop at 300°F: Arrangement 4 axial bands of 106-150°F, 151-195°F and 196-240°F (the upper two requiring engineering review), and Arrangement 9 bands of 201-250°F and 201-300°F (select models). The standard aluminum impeller is good to 275°F; the impeller must be on the outlet end of the housing, a fixed impeller is required over 250°F, high temperature grease is used over 275°F airstream (not the 160°F bearing temperature in FE-1300; see Chapter 17), lube lines are metal, and flange bearings need scalloped bearing plates. On Arrangement 9 axial fans, cool air is pulled into the bearing housing through the belt tube, and a heat shield, retaining washer and safety wire hardware appear on select models. The current axial catalogs are narrower than FanPedia's 600°F headline and differ by product line. The aluminum-impeller TCTA tubeaxial and TCVA vaneaxial catalogs list high temperature construction only up to 250°F (TCTA: "200°F-250°F, belt driven only"). The steel-impeller TCTS tubeaxial is "good for operation to 200°F" as standard and can be customized for up to 300°F, and the Arrangement 9 TCVS vaneaxial can be customized for up to 500°F (all catalogs and product pages from tcf.com). No axial catalog or product page found lists 600°F, so quote anything above those limits only after the factory confirms it. A high temperature axial job is therefore belt-driven (Arrangement 9) with the motor out of the airstream, and its limit depends on the model. Twin City Fan engineering confirms that Arrangement 4 is not used for high-temperature applications, so the catalog limits above (TCTA and TCVA to 250°F, TCTS customizable to 300°F, TCVS Arrangement 9 to 500°F) apply to belt driven Arrangement 9 construction, not to direct drive; the standard Arrangement 4 TCTA is rated 0-105°F.
Diagram: High Temperature Package Finder¶
Run High Temperature Package Finder Fullscreen
High Temperature Package Finder
Type: microsim
sim-id: ch21-high-temp-package-finder
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: determine
Learning Objective: The learner will determine the FanPedia high temperature package band, and whether engineering review is required, for a stated fan family and airstream temperature, with at least 7 of 8 jobs correct.
Prerequisites: high temperature package, axial high temperature package, fan arrangement, plug fan, air kit (defined in this chapter and Chapters 4 and 11).
Evidence of Mastery: For 8 job tickets, the learner selects a band from the list for that ticket's fan family and marks "Engineering review required: yes/no" before checking. A ticket is correct when both the band and the yes/no match the answer table. Mastery is 7 of 8 correct.
Misconceptions: (1) All fans share one set of temperature bands. (2) An Arrangement 4 centrifugal at 400°F has a standard package (it requires engineering review). (3) Axial packages reach the same temperatures as centrifugal packages.
Source note: The Arrangement 4 bands are FanPedia's (pp. 111, 114, 116). Per Twin City Fan engineering (2026-10-09), current practice does not offer Arrangement 4 for high-temperature applications, so the Arrangement 4 tickets practice reading FanPedia's tables, not a current ordering path.
Instructional Rationale: Applying a lookup rule to new tickets, with the family chosen first, builds the habit of reading the family before the temperature; immediate checking with the rule shown corrects the "one table for all fans" misconception.
Content:
Band lists by family (shown once the learner opens a ticket):
| Family | Bands offered | Engineering review |
|---|---|---|
| Centrifugal Arr. 1, 8, 9, 9F, 10 | 301-500°F; 501-600°F; 601-800°F; 801-1,000°F; 1,001°F and over | 1,001°F and over |
| Centrifugal Arr. 4 | 181-300°F; 301°F and over | 301°F and over |
| Arr. 9 plug fan | 301-500°F; 501-600°F; 601-800°F; 801-1,000°F; 1,001°F and over | 1,001°F and over |
| Arr. 4 plug fan | 181-300°F (select models); 301°F and over | 301°F and over |
| Air kit, Arr. 3 | 301-500°F; 501-600°F; 601-800°F; 801-1,000°F; 1,001°F and over | 1,001°F and over |
| Axial Arr. 4 | 106-150°F; 151-195°F; 196-240°F | 151-195°F and 196-240°F |
| Axial Arr. 9 | 201-250°F; 201-300°F (select models) | none stated |
Job tickets, in this order:
| # | Ticket | Correct band | Review? | Feedback reason |
|---|---|---|---|---|
| 1 | Arr. 1 centrifugal, kiln exhaust, 450°F | 301-500°F | No | 450°F falls in the first Arr. 1 band. |
| 2 | Arr. 4 centrifugal, dryer, 250°F | 181-300°F | No | Arr. 4 has its own lower band; aluminum rotating parts are limited to 250°F. |
| 3 | Arr. 4 centrifugal, oven, 400°F | 301°F and over | Yes | Arr. 4 above 300°F always goes to engineering review. |
| 4 | Arr. 9 plug fan, oven, 650°F | 601-800°F | No | Plug fans use the same five bands as Arr. 1, 8, 9, 9F, 10. |
| 5 | Air kit Arr. 3, furnace, 1,100°F | 1,001°F and over | Yes | The top band on every centrifugal and air kit page requires engineering review. |
| 6 | Axial Arr. 4, warm exhaust, 180°F | 151-195°F | Yes | FanPedia marks the 151-195°F axial band as requiring engineering review. |
| 7 | Axial Arr. 9, drying air supply, 240°F | 201-250°F | No | 240°F is inside the 201-250°F band offered on Arr. 9 axial fans. |
| 8 | Arr. 10 centrifugal, process fume exhaust, 520°F | 501-600°F | No | 520°F is above 500°F, so it moves up to the second band. |
Provenance: FanPedia pp. 110, 111, 113, 114, 115 and 116 (2021 edition). Job tickets are illustrative and must be labeled "illustrative jobs"; their applications are drawn from FanPedia's list of common high temperature fan uses (recirculating air in kilns, dryers, ovens and furnaces; exhausting process gases and fumes; supplying air for heating and drying systems, pp. 110-115).
Rules: A temperature belongs to a band when lower bound <= temperature <= upper bound. "And over" bands have no upper bound. For ticket 7, 240°F lies in both Arr. 9 axial bands; the correct answer is 201-250°F because the 201-300°F band is limited to select models, and the sim accepts 201-300°F as incorrect with the reason "select models only; start from the general band". Review is "Yes" exactly when the band is listed in the Engineering review column.
Learner Activity:
- The learner reads a ticket showing family, application and temperature.
- The learner opens the band list for that family and selects one band.
- The learner marks review yes or no and presses Check.
- If both halves are right, the sim shows the answer and the reason and the next ticket unlocks. If not, a hint appears with no band or review answer and no per-half marks, the band chart stays closed, and the learner tries again; after the second attempt each half is marked correct or incorrect, the answer and reason are shown, and the next ticket unlocks.
- After ticket 8, the learner can enter any family and temperature from 100°F to 1,200°F in 10°F steps (default 300°F) and see the band and review flag, and should notice that the listed axial bands stop at 300°F.
Feedback: 8 tickets, fixed order, two attempts each. Correct: "Correct:
Starting State: Ticket 1 shown with the band list closed and the question "Which package band covers this job?"
Chapter Anchors: All bands, review requirements and the 250°F aluminum limit as stated in the chapter's package table and axial paragraph; 8 tickets; mastery 7 of 8.
Spark-Resistant Construction, Types A and B¶
Spark-resistant construction keeps a fan from striking a spark when a rotating part rubs a stationary one, which matters when the airstream can carry flammable vapor or dust. The A/B/C letters follow AMCA Standard 99-0401, and FanPedia illustrates Twin City Fan's construction for each. Chapter 20 covered Type C, the minimal level, which only reduces the chance of contact (typically with an aluminum inlet cone and an aluminum rub plate). Types A and B go further by changing what the parts are made of. In all three types, bearings are not allowed in the airstream. The non-ferrous materials FanPedia lists are aluminum, aluminum/nickel/bronze, Monel, copper, brass and bronze.
Type A¶
Spark Resistant Type A provides the highest degree of spark resistance. It requires that all fan components in the airstream be made of non-ferrous material, and that they be assembled to reduce the chance of contact between any stationary and rotating component (p. 104). On a centrifugal fan that means a non-ferrous housing or frame, impeller with welded hub, inlet funnel, shaft sleeve, interior fasteners (hub set screws in stainless steel, flush with the hub) and hub cap with a non-ferrous bolt. Parts outside the airstream stay steel: a bolted-on pedestal, the shaft and the shaft locking collars. Type A also restrains the bearings with bearing dowel pins (the first choice) or bearing stop blocks, so a bearing cannot shift and bring the wheel into the housing. On axial fans (p. 109) the housing, impeller, fasteners and sleeve are non-ferrous, the shaft is stainless steel, and the motor mount plate, weather cover and belt guard are mild steel, with restrained bearings (slotted steel spring pins or stop blocks).
Worked example (illustrative job). A specification for a solvent recovery exhaust calls for "AMCA Type A spark-resistant construction" on an Arrangement 1 centrifugal fan. Walk the airstream from inlet to outlet:
- Inlet funnel, impeller and housing: all non-ferrous; aluminum is the usual choice.
- Where the shaft enters the airstream: a non-ferrous sleeve covers it, and the hub cap uses a non-ferrous bolt.
- Hardware inside the housing: non-ferrous, with stainless set screws ground flush in the hub.
- Outside the airstream: steel shaft and collars, and a steel pedestal bolted on, since aluminum and steel are not welded together.
- Bearings: restrained with dowel pins, and outside the airstream.
Type B¶
Spark Resistant Type B requires that the impeller be non-ferrous and that the airstream parts be assembled to reduce the chance of stationary-to-rotating contact (p. 105). In practice that is an aluminum impeller (welded hub) and an aluminum rub plate, the plate at the housing's shaft opening that FanPedia also calls a shaft seal, enclosure plate, striker plate, spark plate or ash tray. The housing, frame, fasteners, pedestal, inlet funnel, shaft and collars stay steel. If the fan has an outside protruding hub, no rub plate is needed. FanPedia states the residual risk honestly: after a mechanical failure, the aluminum impeller will contact a steel inlet cone. Axial Type B makes the impeller non-ferrous and restrains the bearings (p. 109), and for radial bladed fans and pressure blowers FanPedia adds a rule: if the fan already has a non-ferrous impeller as standard, use Type B (pp. 107-108).
Worked example (illustrative job). A grain-handling customer asks for "spark-resistant" without a type, on a pressure blower that already has a cast aluminum impeller as standard. FanPedia's rule points straight to Type B, which the standard impeller nearly satisfies already. Add the rub plate at the shaft opening, keep bearings outside the airstream, and confirm the type in writing on the order, because "spark-resistant" alone does not tell the factory which type to build.
The three types compare as follows:
| Type A | Type B | Type C | |
|---|---|---|---|
| Non-ferrous parts | Everything in the airstream | Impeller (plus rub plate) | Inlet cone or rub ring, rub plate |
| Housing | Non-ferrous | Steel | Steel |
| Remaining risk | Lowest | Aluminum wheel can strike the steel inlet cone | Steel wheel; aluminum inlet cone and rub plate are the first contact points |
| Bearings | Restrained; not in airstream | Not in airstream (restrained on axials) | Not in airstream |
Ask Two Questions Before Quoting Spark Resistance
Ask "which type?" and "how hot?" in the same breath. Types A and B lean on aluminum, and FanPedia limits aluminum rotating parts to 250°F on Arrangement 4 fans and the standard aluminum axial impeller to 275°F, so a hot spark-resistant job needs the factory's review before you quote it.
Nominally Leak-Tight Fans¶
The next two duties have nothing to do with sparks and everything to do with where the gas goes.
Fan Leakage¶
Fan leakage is air or another gas mixture leaking into the fan housing or out of it. Which way it moves depends on pressure: where the housing is above atmospheric pressure, gas leaks out; where it is below, room air leaks in. FanPedia (p. 120) names the consequences. When the gas carries hazardous contaminants, leakage can be dangerous; excessive leakage can also waste energy, create an environmental or safety hazard, damage the fan bearings, or make excessive noise.
What "Nominally Leak-Tight" Means¶
FanPedia is direct about the limits: a fan generally cannot be built totally leak-tight. A nominally leak-tight fan is one built to reduce leakage to a level agreed with the customer, verified by shop testing, and recorded on the inspection form (p. 120). The word nominally is a promise about the method and the agreed level, not a promise of zero leakage.
You Are Selling a Leakage Level, Not a Seal
Notice that the customer, not the catalog, sets the acceptable leakage. That makes the first question on a leak-tight job "how much leakage is acceptable, and of what gas?" rather than "which seal?"
FanPedia's construction list for nominally leak-tight fans is short, and every line closes a leak path:
- Arrangements 1, 8 and 9 only, with a solid drive side on the housing (no drive plates to leak around).
- Not recommended over 600°F, and split housings not recommended.
- Close bolt spacing. Bolted connections use close-centered hole patterns on 3 in. to 4 in. centers: inlet and outlet flanges, access doors, cover plates, inlet funnels and split housings. An ANSI flange hole pattern is sufficient if it is standard for the fan.
- Gasketing on all connections, with double gaskets.
- Continuous welds instead of intermittent welds (locations vary by model).
- Shaft seal of the lip type, Graphoil stuffing box type, or double ring mechanical (double carbon ring) type, sealing on the fan shaft itself; do not use a shaft sleeve and cap.
- Drain of the pipe coupling type with a plug.
FanPedia refers to Fan Engineering Letter FE-700 for details.
Worked example (illustrative job). A chemical plant requests a fan for a process vent containing a toxic vapor, at 350°F, and suggests an Arrangement 4 fan with a split housing to make maintenance easier. Check the request against the list:
- Arrangement 4 is not one of the allowed arrangements (1, 8 or 9). Propose an Arrangement 1 or 8 fan; Arrangement 8 keeps the direct-drive feel of Arrangement 4.
- A split housing is not recommended. If maintenance access truly requires one, it will need centering plates and double gaskets, and the customer should know it adds leak paths.
- 350°F is under the 600°F limit, so leak-tight construction is possible. The high temperature package for the band still applies, so the order carries both.
- Choose a seal from the three allowed types, and ask the customer to state the acceptable leakage that the soap bubble test will be judged against.
Gasketing¶
Gasketing is the compressible sealing material placed in a bolted joint so the joint seals when the bolts are tightened. On a leak-tight fan, FanPedia requires gasketing on all connections, including inlet and outlet flanges, the funnel, inlet plate, access doors and split housing, and requires double gaskets (p. 120). RTO fans call for silicone sponge gasket and silicone caulk, with an alternative to silicone on automotive jobs (p. 121). A gasket only seals as well as the clamping force on it, which is why gasketing comes paired with close-centered bolt patterns: widely spaced bolts let the flange bow open between them.
Worked example (illustrative). A leak-tight fan has an access door, a two-flange connection (inlet and outlet), and a bolted inlet funnel. Count the gasketed joints: inlet flange, outlet flange, inlet funnel and access door make four joints, each with a double gasket, so the bill of materials carries eight gasket pieces before any cover plates are added. Then confirm each joint's bolt pattern is on 3 in. to 4 in. centers, or that the flange uses the fan's standard ANSI pattern; a joint that fails that check will leak whatever gasket it uses.
Continuous Seal Welds, Centering Plates and the Soap Bubble Test¶
Three smaller items complete a leak-tight fan.
A continuous seal weld is a weld run without gaps along the full length of a seam, so the joint itself is gas-tight. Standard housings may use intermittent (stitch) welds, which are strong but leave gaps between beads. FanPedia's leak-tight and RTO drawings call out continuous welds, and the RTO drawing adds seal welds, with locations that vary by model (pp. 120-121).
A centering plate is a plate fitted where a split housing's joint line crosses an opening, sealing the open areas around the shaft seal and around the inlet funnel or inlet plate. A split line through a round opening leaves small gaps that a flat gasket cannot close; the centering plate closes them. FanPedia requires centering plates on any leak-tight or RTO fan built with a split housing.
The soap bubble test is the shop leak check. In the usual method the housing is put under pressure, a soap solution is applied to seams, joints and seals, and bubbles mark any leak. FanPedia (pp. 120-121) says leak-tight and RTO fans are tested this way at the shop, leaks are fixed if needed, and the result is recorded on the inspection form.
Regenerative Thermal Oxidizer (RTO) Service¶
A regenerative thermal oxidizer (RTO) is an industrial air pollution control process that destroys pollutants in process exhaust by oxidizing them at high temperature. FanPedia (p. 121) lists typical sources: industrial process ventilation such as automotive paint booths, printing, and paper mills. In general terms, an RTO passes the exhaust through beds of ceramic media that store heat, burns the organic pollutants in a combustion chamber, and periodically reverses the flow so the outgoing hot gas reheats the media for the next cycle. The fan that pushes or pulls the exhaust through that system sees contaminated gas, pressure swings when the flow switches, and occasional high temperature, which is why RTO duty has its own construction.
Much of RTO construction matches the leak-tight list: a solid drive side, close-centered bolt patterns on 3 in. to 4 in. centers, a 600°F recommended limit, gasketing on all connections, centering plates on split housings, continuous and seal welds, and the soap bubble test. The differences are what make it its own option:
- Gasketing material: silicone sponge and silicone caulk, but automotive jobs require an alternative to silicone. (Silicone contamination is a known cause of paint finish defects, which is the practical reason a paint shop forbids it.)
- Shaft seals: friction type, single carbon ring type, or a commercially available carbon ring or other seal.
- Dampers, if required, must have stuffing boxes on their shafts.
- Structure: FanPedia's drawing shows pedestal foot gussets and a vane section.
Two Lists That Look Alike
If leak-tight and RTO construction blur together, that is normal; FanPedia's two pages share most of their lines. Learn the shared core once, then memorize only the differences: arrangements and drain for leak-tight, silicone rules, seal types and dampers for RTO.
RTO Service Conditions¶
RTO service conditions are the operating facts the fan must survive that only the customer or the RTO builder knows. FanPedia lists them as special requirements that sales must specify (p. 121):
| Item sales must specify | Why the factory needs it |
|---|---|
| Pedestal: concrete requirements | Whether the pedestal is filled or set on concrete changes its construction |
| Fan base: how the fan will be mounted in the field | The base is built to suit that mounting |
| Fan operation: VFD, cycling, bake-out conditions | Speed range, repeated starts and temperature excursions change the fan's mechanical design |
A bake-out is a periodic high temperature cycle that burns accumulated organic deposits out of the RTO's media; for the fan, it is an excursion above normal operating temperature that must be known in advance.
Worked example (illustrative job). An RTO builder sends an inquiry for an automotive paint shop: a fan on a VFD, mounted on a structural steel skid, with normal gas at 300°F and a weekly bake-out that raises the fan inlet to 650°F for several hours. Read it against FanPedia:
- Temperature. RTO construction is not recommended over 600°F. The 650°F bake-out exceeds that, so the job needs engineering review before it is quoted, even though normal operation is well inside the limit. The high temperature package must be chosen for the bake-out temperature, not the normal one.
- Gasketing. Automotive job: no silicone. The order must name the alternative gasket and caulk.
- Operation. VFD and cycling go on the order as service conditions.
- Base and pedestal. Skid mounting goes on the order as the field mounting; ask whether the pedestal needs concrete.
- Seals and testing. Choose a friction or carbon ring seal; the fan will be soap bubble tested at the shop.
Diagram: Leak-Tight Versus RTO Requirement Sorter¶
Run Leak-Tight Versus RTO Requirement Sorter Fullscreen
Leak-Tight Versus RTO Requirement Sorter
Type: microsim
sim-id: ch21-leak-tight-rto-sorter
Library: p5.js
Status: Specified
Bloom Level: Analyze
Bloom Verb: differentiate
Learning Objective: The learner will differentiate FanPedia's nominally leak-tight and RTO construction requirements by sorting 12 requirements into Leak-Tight Only, RTO Only, or Both, with at least 10 of 12 correct on the first attempt.
Prerequisites: fan leakage, nominally leak-tight fan, gasketing, continuous seal weld, centering plate, soap bubble test, regenerative thermal oxidizer, RTO service conditions (all defined in this chapter above this block).
Evidence of Mastery: The learner places each of 12 requirement cards in one of three bins and locks each placement. A placement is correct when it matches the "Bin" column. Mastery is 10 of 12 correct on the first attempt.
Misconceptions: (1) RTO construction is just leak-tight construction under another name. (2) Leak-tight fans may be any arrangement. (3) Silicone gasketing is acceptable on every RTO job.
Instructional Rationale: Sorting near-identical requirement lists into shared and distinct bins makes the learner analyze the structure of the two options, which is the comparison a salesperson performs when a customer says "RTO" or "leak-tight".
Content:
| # | Requirement card | Bin | Why (feedback) |
|---|---|---|---|
| 1 | Solid drive side on housing (no drive plates) | Both | Both pages list it; drive plates are a leak path. |
| 2 | Not recommended for applications over 600°F | Both | Both pages state the same 600°F limit. |
| 3 | Bolted connections on 3 in. to 4 in. centers | Both | Close bolt spacing keeps gaskets clamped on both options. |
| 4 | Centering plates if the housing is split | Both | Both pages require them to seal the open areas at seals and inlet. |
| 5 | Soap bubble test at the shop, recorded on the inspection form | Both | Both pages describe the same shop test. |
| 6 | Arrangements 1, 8 and 9 only | Leak-Tight Only | The arrangement limit appears only on the leak-tight page. |
| 7 | Shaft seal: lip, Graphoil stuffing box, or double carbon ring | Leak-Tight Only | These three seal types are the leak-tight list. |
| 8 | Drain: pipe coupling type with plug | Leak-Tight Only | The plugged pipe-coupling drain is called out on the leak-tight page. |
| 9 | Silicone sponge gasket and silicone caulk, with an alternative on automotive jobs | RTO Only | The silicone rule belongs to RTO duty. |
| 10 | Dampers, if required, must have stuffing boxes | RTO Only | Only the RTO page sets a damper requirement. |
| 11 | Sales must specify VFD, cycling and bake-out conditions | RTO Only | These are the RTO service conditions. |
| 12 | Shaft seal: friction, single carbon ring, or commercial carbon ring | RTO Only | These are the RTO seal types. |
Provenance: FanPedia p. 120 (nominally leak-tight) and p. 121 (RTO), 2021 edition.
Rules: One bin per card. Correctness is an exact match with the Bin column. Cards are dealt in a shuffled order seeded by 21 so every learner sees the same order.
Learner Activity:
- The learner reads a card and predicts its bin by placing it.
- The learner locks the placement; the card shows correct or incorrect with the reason.
- After all 12 are locked, the learner can view the two FanPedia lists side by side with shared lines linked, and should notice that 5 of the 12 requirements are shared.
Feedback: 12 cards, shuffled with seed 21, one attempt each, no re-placement after locking. Correct: "Correct:
Starting State: Three empty bins labeled Leak-Tight Only, Both, and RTO Only, the first card face up, and the question "Which construction calls for this?"
Chapter Anchors: 600°F limit; 3 in. to 4 in. bolt centers; Arrangements 1, 8 and 9; three leak-tight seal types; silicone alternative on automotive jobs; 12 cards; 5 shared requirements.
Buffer Strips¶
One last pedestal item often travels with hot, corrosive or leak-tight jobs because those jobs tend to use stainless steel housings. A buffer strip is a strip placed between a corrosion-resistant housing and a mild steel pedestal when the customer specifies that the two may not be welded together (p. 85). It lets a stainless steel housing sit on a less expensive mild steel pedestal without a dissimilar-metal weld; the alternative FanPedia offers is a non-rotatable bolt-on pedestal, described in Chapter 17. When a specification forbids welding stainless to carbon steel, one of the two must appear on the order.
Key Takeaways¶
- Hot fans need parts that allow growth (fixed and floating bearings, the air kit high expansion bearing), shed heat (shaft cooler, cooler box) or block it (separated pedestal, high temp angle, heat shield, insulation).
- \( \Delta L = \alpha L \Delta T \): a steel shaft can grow about an eighth of an inch over a few hundred degrees, which a single floating bearing absorbs.
- High temperature packages are temperature bands that depend on the fan family. Centrifugal Arr. 1, 8, 9, 9F, 10, Arr. 9 plug fans and air kits share five bands up to 1,001°F and over; FanPedia reviews Arrangement 4 above 300°F, but Twin City Fan engineering does not use Arrangement 4 for high-temperature applications (standard ranges up to about 180°F); the listed axial bands stop at 300°F.
- High temperature pedestal forms carry their own limits: Arr. 9 and 10 heat shield and insulated panel pedestals are shown up to 600°F; the separated pedestal (up to 3 in. gap) is for Arr. 1, 8 and 9F at 601°F and above.
- Spark Resistant Type A makes everything in the airstream non-ferrous and restrains the bearings; Type B makes the impeller non-ferrous and adds a rub plate. Check aluminum temperature limits on hot spark-resistant jobs.
- A nominally leak-tight fan reduces leakage to a level agreed with the customer: Arr. 1, 8 or 9, solid drive side, double gaskets, 3-4 in. bolt centers, continuous welds, an approved shaft seal, centering plates on split housings, and a soap bubble test, up to 600°F.
- RTO fans share that core and add silicone rules (none on automotive jobs), their own seal types, stuffing boxes on dampers, and service conditions that sales must specify: concrete, field mounting, VFD, cycling and bake-out.
Hot, Tight and Ready to Quote
You can now take a hot, hazardous or RTO inquiry and turn it into a construction list where every line traces to a reason: a package band, a pedestal limit, a spark-resistant type, a leak path closed. That is the hardest construction reading in the book, and you just did it.