Roof, Wall and Specialty Exhaust Fans¶
Summary¶
This chapter surveys the exhaust and ventilation products that sit on roofs and walls or serve a specific application: kitchen exhaust fans, smoke and heat exhaust fans, fiberglass fans for corrosive service, mancoolers, housed centrifugal and plenum fans, square inline and tubeaxial fans, wall exhausters, gravity relief ventilators, and hooded, louvered, and low-profile roof units. It explains upblast versus downblast discharge and how entrained flow works in lab exhaust. After completing it, readers can match a common building ventilation need to the product line built for it.
Concepts Covered¶
This chapter covers the following 17 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Kitchen Exhaust Fan | 3 |
| Smoke and Heat Exhaust Fan | 3 |
| Fiberglass Fan | 3 |
| Mancooler | 1 |
| Upblast Discharge | 3 |
| Downblast Discharge | 2 |
| Housed Centrifugal Fan | 91 |
| Plenum Fan | 79 |
| Square Inline Centrifugal Fan | 2 |
| Tubeaxial Fan | 28 |
| Centrifugal Wall Exhauster | 1 |
| Gravity Relief Ventilator | 1 |
| Cabinet Ventilator | 1 |
| Entrained Flow | 6 |
| Hooded Roof Exhauster | 1 |
| Louvered Roof Exhauster | 1 |
| Low Profile Roof Ventilator | 1 |
Prerequisites¶
This chapter builds on concepts from:
- Chapter 1: Air, Pressure and the Fan
- Chapter 2: Airstreams, Total Pressure and Standard Air
- Chapter 3: Basic Fan Anatomy
- Chapter 4: Centrifugal, Axial and Mixed Flow Fans
From Fan Families to Product Lines¶
Chapter 4 sorted fans into three families by the path air takes through the impeller. A Twin City Fan catalog, however, is not organized by family alone. It is organized by the job: a restaurant hood, an atrium that must be cleared of smoke, a wastewater plant full of corrosive vapor, a hot foundry floor. Each job produced a product line whose housing, discharge, materials and listings were shaped around that job's air and that job's mounting location.
Every Building Has an Exhaust Story
Walk across almost any commercial roof and you will see a dozen fans, each one chosen for a reason. By the end of this chapter you will be able to read that roof like a parts list and name the product line a customer's job calls for. Let's follow the airflow.
The chapter moves in four steps. It starts with the four general-purpose fan styles that appear inside buildings and ductwork, moves up to the roof and the wall to compare discharge directions, then covers the application specialists, and finishes with entrained flow, the trick that lets a laboratory exhaust fan dilute and launch its plume.
General-Purpose Building Fans¶
Four fan styles do most of the air moving inside a building's duct system and air-handling equipment. Two are centrifugal and differ in whether they have a housing; two sit in line with the duct.
Housed Centrifugal Fan¶
A housed centrifugal fan is a centrifugal fan whose impeller turns inside a scroll housing. FanPedia (p. 4) describes the type this way: air is drawn in through one side of the housing (single inlet) or both sides (double inlet) and is discharged at a right angle to the fan shaft. The scroll collects the air thrown off the impeller tips, converts part of its velocity into static pressure, and delivers it through a single rectangular outlet that bolts to a duct.
Two width-and-inlet combinations cover most catalog listings:
- Single width, single inlet (SWSI): one inlet and an impeller of standard width, as in the BC-SW.
- Double width, double inlet (DWDI): two inlets and a double-width impeller, which roughly doubles the volume a given impeller diameter can move, as in the BC-DW.
The same housed construction accepts backward inclined, airfoil, backward curved, radial tip, forward curved and radial bladed wheels, and it is the base for packaged configurations on FanPedia p. 4: roof-mounted filtered supply units, enclosed duct fans, and the lab exhaust fans covered at the end of this chapter.
Worked example: what leaves the outlet. A customer's drawing shows an SWSI housed fan delivering 6,000 CFM through an outlet that measures 24 in. by 18 in. How fast does the air leave the scroll (values illustrative, standard air)?
- Outlet area: \( A = 2.0 \text{ ft} \times 1.5 \text{ ft} = 3.0 \text{ ft}^2 \).
- Outlet velocity: \( V = Q / A = 6{,}000 / 3.0 = 2{,}000 \) fpm.
- Velocity pressure at standard air, using the relation from Chapter 2:
where \( VP \) is velocity pressure in inches of water gauge and \( V \) is air velocity in feet per minute.
A DWDI fan handling the same 6,000 CFM would split it, about 3,000 CFM per inlet. Either way, a housed fan concentrates all of its air into one outlet at a known velocity, so the duct connection there matters. Chapter 12 shows how that one outlet can be pointed in many directions.
Plenum Fan¶
A plenum fan is an unhoused centrifugal fan that runs inside an air plenum. FanPedia (p. 5) defines a plenum as an air-filled space within a structure, and the plenum fan's job is to pressurize that space. With no scroll, the impeller discharges air radially in all directions, 360 degrees around the wheel, and the surrounding cabinet does the job a housing would do: it collects the air and sends it to whatever outlets the cabinet has. The fan has an inlet funnel on one side and a frame that carries the impeller, shaft, bearings and drive.
The current Twin City Fan plenum fan line (E-Series EPF, EPFN, EPQ and EPQN, plus commercial, modular and EC versions) uses 9- or 12-blade airfoil impellers for general HVAC duty: large volumes of clean air at low to moderate pressure. A close relative, the plug fan, mounts its panel on the outside wall of the plenum with the impeller inside; Chapter 6 covers it.
The Cabinet Is the Housing
Notice what disappeared when the scroll went away: the fan no longer decides where the air goes, the plenum does. That is why a plenum fan can feed a top opening, two side openings and a bottom opening at once, while a housed fan feeds exactly one outlet.
Worked example: housed or plenum in an air handler. An air-handling unit manufacturer is laying out a cabinet that must deliver supply air through three duct connections: one on top and one on each side. The unit serves a pharmaceutical cleanroom, and the owner wants as little maintenance in the airstream as possible.
- Count the outlets. A housed fan would need a discharge plenum and transitions after its one outlet to feed three connections. A plenum fan pressurizes the cabinet section, so all three openings draw from the same space.
- Check the air. Clean air at low to moderate pressure is the duty the airfoil plenum line is built for.
- Choose the drive. FanPedia (p. 170) states that direct drive Arrangement 4 plenum fans mount the impeller on the motor shaft, eliminate belt residue and need less maintenance, and are therefore widely used in cleanroom and pharmaceutical applications.
- Conclusion. A direct drive plenum fan fits. A housed fan stays the better choice for a single ducted outlet or a free-standing fan. Chapter 11 explains arrangement numbers in full.
Square Inline Centrifugal Fan¶
A square inline centrifugal fan puts a centrifugal impeller inside a square housing with removable side panels. FanPedia (p. 6) notes that it can move air straight through the fan or turn it 90 degrees, depending on how the ductwork connects. The square shape lets the fan drop into rectangular duct runs and above-ceiling spaces, and the removable panels give service access without taking the fan out of the duct. Twin City Fan's square inline line includes the belt-driven BSI and direct-drive DSI with backward inclined wheels, plus a DSI mixed flow version. Choose it when the duct runs through a tight space, the air is relatively clean, and the installer wants one compact box that can absorb a duct turn.
Tubeaxial Fan¶
A tubeaxial fan is an axial fan in a plain cylindrical housing with no guide vanes. Air enters one end of the tube, passes through the impeller, and leaves the other end moving in the same straight line, carrying some rotational swirl because nothing straightens it. A vaneaxial fan adds a vane section that helps straighten the airflow (FanPedia p. 7). The current catalog positions tubeaxial fans for general ventilation through process air supply, in direct and belt driven models with fixed or adjustable pitch impellers; vaneaxials are aimed at moderate to high pressures.
The tubeaxial is also the base model for roof-mounted axial fans, which add a hood or a stack cap (FanPedia p. 7), such as the TUB upblast and THB hooded roof ventilators. In a belt-driven tubeaxial (FanPedia p. 15), the motor sits on a motor base saddle outside the tube, and a belt tube, also called a belt fairing, shields the belt where it crosses the airstream.
Worked example: duct velocity and duty. A contractor needs 8,000 CFM of exhaust from a warehouse process area through a straight, round duct, and asks whether a 24-in. tubeaxial is a sensible starting point (values illustrative).
- Tube area: \( A = \pi r^2 = \pi (1.0 \text{ ft})^2 = 3.14 \text{ ft}^2 \).
- Air velocity through the tube: \( V = 8{,}000 / 3.14 = 2{,}546 \) fpm, an ordinary duct velocity for general exhaust.
- Duty: a short, straight duct at low pressure points to a tubeaxial. A long duct with filters and higher pressure would point to a vaneaxial.
- Drive: if the exhaust carries heat or fumes the motor should not breathe, the belt-driven version keeps the motor outside the tube.
The answer is yes, confirmed against the catalog performance curve (Chapter 10).
The four general-purpose styles compare as follows.
| Fan style | Housing | Air path | Typical place in a building |
|---|---|---|---|
| Housed centrifugal | Scroll | In through the side, out at 90 degrees through one outlet | Ducted supply and exhaust, utility duty |
| Plenum | None; the cabinet acts as the housing | In through the funnel, out radially into the plenum | Inside air-handling units |
| Square inline centrifugal | Square box, removable panels | Straight through or a 90-degree turn | Rectangular duct, above ceilings |
| Tubeaxial | Plain tube, no vanes | Straight through, with swirl | Short ducted runs, roof ventilators |
On the Roof and Through the Wall¶
Most exhaust air ends up leaving through the roof or an exterior wall, and the defining choice there is the direction the air leaves the unit. FanPedia (p. 5) describes the centrifugal power roof ventilator family by that direction. Upblast and side wall configurations exhaust air in a straight line through the fan. Downblast, louvered and hooded configurations move air up through the fan and then deflect it down and outward.
Upblast Discharge¶
Upblast discharge sends exhaust air straight up, away from the roof surface. The impeller turns on a vertical shaft, the air rises through the wheel and leaves through the top of the unit, and the plume carries contaminants, heat and grease clear of the roof and of nearby air intakes. Because the outlet faces the sky, upblast units carry a weather defense: FanPedia's technical descriptions (p. 181) note that butterfly-type gravity dampers, built into roof-mounted upblast fans, prevent outside air from back-drafting in and provide weather protection when the fan is off. Twin City Fan's VCU, VCUB and BCRU upblast roof exhausters, and the kitchen and smoke versions covered below, all use this discharge.
Downblast Discharge¶
Downblast discharge turns the air leaving the impeller back down toward the roof, out and around the unit. The classic downblast centrifugal roof exhauster has a mushroom-shaped motor cover over a spun aluminum shroud; air rises through the wheel, strikes the inside of the cover, and spills out the bottom edge in all directions. The low silhouette and built-in weather protection suit clean air: restroom, general office and storage exhaust. The current catalog lists the VC and DCRD direct-drive and BCRD belt-driven downblast models. FanPedia's exploded view (p. 19) names the parts common to both roof exhauster types: motor cover, motor housing, motor, impeller, shroud, skirt, bird screen, curb cap and roof curb.
Diagram: Centrifugal Roof Exhauster Parts¶
Run Centrifugal Roof Exhauster Parts Fullscreen
Centrifugal Roof Exhauster Parts
sim-id: ch05-roof-exhauster-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the nine labeled parts of a centrifugal roof exhauster on FanPedia's upblast and downblast exploded views, given each part's function.
Content: 9 numbered markers: Motor cover, Motor housing, Motor, Bird screen, Impeller, Shroud, Skirt, Curb cap, Roof curb.
Provenance: FanPedia page 19, "Exploded Views: Centrifugal Roof Exhausters, upblast (left column) and downblast (right column)", with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 19.
Downblast Means Clean Air Only
A common trap is quoting a downblast exhauster for kitchen, fume or hot exhaust because it is cheaper and lower. A downblast unit lays its discharge back onto the roof, where grease collects and fumes find the nearest outdoor-air intake, so for anything but relatively clean air, switch the quote to an upblast unit.
Hooded, Louvered and Low-Profile Roof Units¶
Three more roof configurations differ mainly in how they shelter the discharge. The hooded and louvered exhausters share the "up, then down and out" path of the downblast; the low profile ventilator comes in both upblast and hooded forms.
A hooded roof exhauster is a centrifugal roof exhauster topped by a weatherhood. Air passes up through the impeller and the hood turns it down and outward around the base, giving full rain and snow protection over the opening. The current catalog lists the DCLH (direct drive) and BCLH (belt driven) models. The same hood idea appears on axial roof units, where the hood deflects exhaust downward or, on a supply unit, admits air from underneath.
A louvered roof exhauster replaces the hood with a louvered penthouse: a box with fixed weather louvers on its sides. Air rises through the impeller into the penthouse and is deflected down and outward through the louvers, which shed rain while keeping the unit's profile boxy and compact. The current catalog lists the DCLP (direct drive) and BCLP (belt driven) models.
A low profile roof ventilator is a propeller-type roof ventilator built to sit close to the roof deck, for roofs where a tall unit would be visible from the street or would catch the wind. The roof ventilator line offers low-profile versions in both discharge styles: the LUD and LUB upblast units (plus the LUBMO, with its motor out of the airstream) and the LHD and LHB hooded units, some of which add filters for supply duty. Like the rest of the roof ventilator line, these use cast aluminum adjustable-pitch or fabricated steel fixed-pitch axial impellers rather than centrifugal wheels.
Centrifugal Wall Exhauster¶
A centrifugal wall exhauster is the side-wall version of the centrifugal roof exhauster: the same backward inclined, non-overloading wheel and motor package, mounted on an exterior wall so the shaft is horizontal and air leaves straight out, away from the building face. It serves buildings without convenient roof access and duct runs that end at a wall. Several Twin City Fan models do double duty on a roof or a wall (VCU and VCUB); the BCRW is a dedicated belt-driven wall unit, the BCRWR is its kitchen version, and the fiberglass line includes the HA and HAB wall exhausters.
Gravity Relief Ventilator¶
A gravity relief ventilator has no fan at all. It is a weather-protected roof opening that lets air leave (relief) or enter (intake) a building under natural pressure differences: warm air rising inside the building, wind across the roof, or the slight positive pressure created when supply fans run. Twin City Fan offers them as spun aluminum (GRV), modular hooded (MGR relief, MGI intake), louvered (TEL relief, TIL intake) and fiberglass (MA) units. They are often specified beside powered fans: the supply fan pushes air in and the gravity relief ventilator lets it out without a second motor.
Cabinet Ventilator¶
A cabinet ventilator is a small centrifugal blower, usually with a forward curved wheel, packaged in a sheet-metal cabinet that mounts in or above a ceiling, on a wall, or inline in a duct. It is the quiet, continuous-duty fan for bathrooms, small offices, generator rooms and general HVAC exhaust. In Twin City Fan's current catalog the group is called "Ceiling & Cabinet Ventilators" and lists duct blowers (DBS forward curved inline, DBT twin forward curved inline) beside ceiling and inline ventilators (T ceiling ventilator, TL inline/ceiling ventilator). It is a close relative of the ceiling ventilator introduced in Chapter 4: the same quiet, indoor, clean-air duty, but packaged in a cabinet that also mounts on a wall or inline in a duct.
Before the next diagram, here is how the roof and wall configurations line up by airflow path.
| Configuration | Air path out of the unit | Fan inside | Typical use |
|---|---|---|---|
| Upblast roof exhauster | Straight up, away from the roof | Centrifugal | Grease, fumes, heat, contaminated air |
| Downblast roof exhauster | Up, then down and out over the roof | Centrifugal | Clean general exhaust |
| Hooded roof exhauster | Up, then down and out under a hood | Centrifugal | Clean exhaust with full weather cover |
| Louvered roof exhauster | Up, then down and out through side louvers | Centrifugal | Clean exhaust, compact penthouse look |
| Low profile roof ventilator | Up (upblast) or down and out (hooded), close to the deck | Axial propeller | General exhaust or supply on low-profile roofs |
| Centrifugal wall exhauster | Straight out, away from the wall | Centrifugal | Exhaust where the duct ends at a wall |
| Gravity relief ventilator | Out or in by natural pressure | None | Relief or intake without fan energy |
Diagram: Roof and Wall Exhaust Path Explorer¶
Run Roof and Wall Exhaust Path Explorer Fullscreen
Roof and Wall Exhaust Path Explorer
Type: microsim
sim-id: ch05-roof-wall-exhaust-path-explorer
Library: p5.js
Status: Specified
Bloom Level: Understand
Bloom Verb: classify
Learning Objective: The learner will classify seven roof and wall ventilation configurations by the path the exhaust air takes as it leaves the unit, given a cutaway of each unit with its discharge hidden.
Prerequisites: upblast discharge, downblast discharge, hooded roof exhauster, louvered roof exhauster, low profile roof ventilator, centrifugal wall exhauster, gravity relief ventilator, roof curb, gravity damper (all defined in the section "On the Roof and Through the Wall").
Evidence of Mastery: For each of the seven configurations, shown one at a time in shuffled order, the learner commits a prediction of the discharge path by choosing one of three paths: "Straight up", "Up, then down and out", "Straight out from the wall". The gravity relief ventilator is asked as a separate yes/no question: "Does this unit need a fan to move air?" A prediction is correct when it matches the Correct path column below. Mastery is 6 of 7 path predictions correct plus the gravity question correct, on first attempts. Clicking parts of a cutaway to read labels is exploration, not evidence.
Misconceptions: (1) Every roof exhauster blows air straight up. (2) A hood or louvers change the fan type inside the unit. (3) Downblast units are suitable for any exhaust because the air still leaves the building. (4) A gravity relief ventilator contains a small fan.
Instructional Rationale: Classifying by predicted airflow path before the path is animated forces the learner to reason from the unit's construction (open top, cover, hood, louvers, wall mount) rather than memorize model names, which is the Understand-level skill the chapter's matching task depends on.
Content:
| # | Configuration | Parts shown in the cutaway (each clickable, with the definition below) | Correct path | Feedback (why) |
|---|---|---|---|---|
| 1 | Upblast centrifugal roof exhauster | impeller, motor, open top with butterfly gravity damper, roof curb | Straight up | The outlet faces the sky, so the plume goes up and away from the roof surface. |
| 2 | Downblast centrifugal roof exhauster | impeller, motor, motor cover, shroud, skirt, bird screen, curb cap, roof curb | Up, then down and out | The motor cover turns the rising air back down so it spills out around the base. |
| 3 | Hooded roof exhauster | impeller, motor, weatherhood, roof curb | Up, then down and out | The hood deflects the air downward and outward, as the downblast cover does. |
| 4 | Louvered roof exhauster | impeller, motor, louvered penthouse, roof curb | Up, then down and out | The air rises into the penthouse and the louvers deflect it down and outward; louvers change the look, not the path. |
| 5 | Low profile roof ventilator, upblast version | axial propeller impeller, motor, short stack, roof curb | Straight up | It is an upblast unit built close to the deck; the profile changes, not the path. |
| 6 | Centrifugal wall exhauster | impeller, motor, wall mounting plate | Straight out from the wall | It is the side-wall version of the upblast exhauster, so air leaves in a straight line away from the wall. |
| 7 | Gravity relief ventilator | weather hood, opening, roof curb, no impeller | Up, then down and out | Relief air rises through the opening and the hood turns it down and out; the gravity question asks whether a fan moves it. |
Gravity question (asked after item 7): "Does this unit need a fan to move air?" Correct answer: No. Feedback: "Correct: a gravity relief ventilator relies on natural pressure differences, such as warm air rising or supply fans pressurizing the building." Incorrect feedback: "Look for the impeller: there is none. A gravity relief ventilator moves air only by natural pressure differences."
Part definitions revealed on click:
- Impeller: the rotating wheel that moves the air (centrifugal wheel in items 1-4 and 6, axial propeller in item 5).
- Motor: drives the impeller, directly or through a belt.
- Motor cover: the dome over the motor that also turns the discharge downward on a downblast unit.
- Shroud: the spun aluminum surround that guides air into and around the impeller.
- Skirt: the lower edge of the downblast unit, around which the air spills out.
- Bird screen: a mesh that keeps birds and debris out of the discharge.
- Curb cap: the base plate that covers the roof curb and carries the fan.
- Roof curb: the raised frame on the roof that supports the unit above the roof surface.
- Butterfly gravity damper: hinged blades that open when the fan runs and close by gravity when it stops, blocking backdraft and weather.
- Weatherhood: a hood over the discharge that turns air downward and sheds rain and snow.
- Louvered penthouse: a box with fixed weather louvers on its sides.
- Short stack: a low discharge collar on a low profile unit.
- Wall mounting plate: the plate that fixes the exhauster to an exterior wall.
- Opening (gravity unit): the weather-protected roof penetration that air passes through without a fan.
Provenance: Airflow paths for upblast, side wall, downblast, louvered and hooded configurations: FanPedia p. 5 (Centrifugal Power Roof Ventilator Exhaust Fans). Part names for roof exhausters: FanPedia p. 19 exploded view. Butterfly gravity damper: FanPedia p. 181. Low profile units and their impellers: tcf.com Roof Ventilators product page (LUD, LUB, LHD, LHB). Gravity relief ventilators: tcf.com Gravity Relief Ventilators product page.
Rules: Seven path items, shuffled once per session, then the gravity question. Path choices are the same three for every item. The cutaway shows parts but hides the discharge arrows until the learner commits. After a commit the sim reveals the correct path with arrows showing air entering the unit and leaving it.
Learner Activity:
- The learner sees a cutaway with the discharge hidden and may click any part to read its definition.
- The learner chooses one of the three paths and commits.
- The sim reveals the true path with flow arrows and shows the feedback text. The learner should notice that the parts above the impeller (open top, cover, hood, louvers) decide the path.
- After item 7 the learner answers the gravity question.
- A summary table shows all seven configurations with their paths, and the learner can reopen any item to view its path again without changing the score.
Feedback: Seven path items plus one yes/no item, one attempt each, no re-answering. Correct: "Correct:
Starting State: The first shuffled cutaway, discharge hidden, with the question "Where does the exhaust air go when it leaves this unit?"
Chapter Anchors: Seven configurations; upblast and side wall move air in a straight line; downblast, louvered and hooded move air up and then down and outward (FanPedia p. 5); low profile roof ventilators use axial propeller impellers.
Application Specialists¶
The next four product lines are defined less by airflow path than by what is in the air, or by where the people are.
Kitchen Exhaust Fan¶
A kitchen exhaust fan removes the hot, moist, smoky and grease-laden air that ovens, stoves, grills and dishwashers produce, and discharges it away from building surfaces. Grease is the deciding factor: it coats any surface it lands on and is a fire hazard, so kitchen fans use upblast or wall discharge that throws the plume clear of the roof, and they carry a safety listing for grease-laden air. Twin City Fan's current catalog lists its kitchen exhaust fans to UL 705 Supplement SC (formerly UL 762) and pairs them with grease collection systems. The line includes the BCRUR upblast roof and BCRWR wall exhausters, the VCU and VCUB roof/wall units, a BCVR kitchen utility set, and QCLBR and QSLR mixed flow inline fans.
Smoke and Heat Exhaust Fan¶
A smoke and heat exhaust fan is built to keep running while it moves smoke and hot gas out of a burning building, so occupants can escape and firefighters can enter. Its motor, bearings and materials are selected to survive a temperature that would stop an ordinary fan. Twin City Fan's current catalog rates its smoke and heat fans to withstand a minimum of 500°F for four hours or 1,000°F for 15 minutes. The line spans spun aluminum roof exhausters (BCRUSH), utility sets (BCVSH), mixed flow fans (QCLBSH, QSLSH), roof ventilators (LUBSH, TUBSH) and axial fans (TBSH, TCTSSH, TCVSSH); inline versions can be roof-mounted with a stack cap whose butterfly dampers are held by a fusible link designed to stay open in a fire. High-temperature construction in general is covered in Chapter 21.
Fiberglass Fan¶
A fiberglass fan has its housing, and often its impeller, molded from fiberglass-reinforced plastic (FRP) so it can exhaust moisture-laden, corrosive or chemically contaminated air that would eat through carbon steel. The current catalog describes FRP as a more economical answer than stainless steel or other exotic alloys, with molded housings that resist denting and cracking. Options let the resin and surface match the chemistry: vinyl ester resin, fire retardant resin, a synthetic surface veil, static grounding, and a stainless steel shaft. Twin City Fan builds fiberglass versions of nearly every fan style in this chapter, from housed centrifugal (BAFF, BCSF, RBOF) and inline centrifugal (ILCF) to tubeaxial (TAHF, TAMF), roof exhausters (WA, WAB), wall exhausters (HA, HAB) and panel fans (TCWPF). Wastewater treatment, plating and chemical process exhaust are typical jobs.
Mancooler¶
A mancooler is an axial fan packaged to blow air directly on workers in a hot industrial space. Twin City Fan's current catalog describes two forms: mounted units on a movable base whose swivel allows a full 360-degree rotation, and stationary units with a predrilled base plate for wall, column, ceiling or floor mounting (TCSM). Both have inlet and outlet screens as standard, fabricated to meet OSHA safety requirements, because the fan runs within reach of people. A mancooler does not exhaust anything; it moves air across the body to speed evaporative and convective cooling at a foundry pour station or a loading dock.
Ask What Is in the Air First
Here is a shortcut for any exhaust inquiry: ask "what is in the air?" before "how much air?" Grease points to a listed kitchen fan, fire duty to a smoke and heat fan, corrosive vapor to fiberglass, and clean air leaves every option open.
Entrained Flow in Laboratory Exhaust¶
Fume hood exhaust can be toxic, corrosive or explosive, and it must not re-enter the building through a nearby intake. Chapter 4 introduced the lab exhaust fan and its bypass air plenum; this section adds the second dilution mechanism FanPedia names (pp. 4 and 6), entrained flow.
Four flows need names before the arithmetic:
- Contaminated air is the exhaust drawn from the fume hoods.
- Bypass flow is fresh outside air drawn in through the bypass air plenum and mixed with the contaminated air before it reaches the fan.
- Fan flow is everything that passes through the impeller: contaminated air plus bypass air.
- Entrained flow is outside air pulled into the discharge plume by the high-velocity jet leaving the fan's nozzle or windband. It never passes through the impeller; the jet drags it along.
The plume leaving the stack carries the total flow, the fan flow plus the entrained flow:
where each \( Q \) is a volume flow rate in CFM. Because the contaminant amount stays the same while the air volume grows, the concentration in the plume falls in proportion:
where \( C_{\text{hood}} \) is the concentration in the hood exhaust and \( C_{\text{plume}} \) is the concentration leaving the stack, in the same units (ppm here).
Free Dilution Air
Think of the nozzle as a pump with no moving parts. Bypass air costs fan capacity because it goes through the impeller; entrained air rides along for free because the jet's own momentum pulls it in, adding mass to the plume and helping it climb.
Worked example: diluting a fume hood plume. A lab exhaust fan serves fume hoods that exhaust 8,000 CFM of contaminated air at an illustrative 100 ppm. The bypass damper admits 2,000 CFM of outside air, and the nozzle entrains 7,000 CFM more (values illustrative; real entrainment is a catalog-rated value for a specific model and nozzle).
- Fan flow: \( Q_{\text{fan}} = 8{,}000 + 2{,}000 = 10{,}000 \) CFM. The fan must be selected for this volume.
- Total flow: \( Q_{\text{total}} = 10{,}000 + 7{,}000 = 17{,}000 \) CFM leaves the stack.
- Plume concentration: \( C_{\text{plume}} = 100 \times 8{,}000 / 17{,}000 = 47.1 \) ppm.
- Interpretation: the fan moves 10,000 CFM but the plume carries 17,000 CFM, so entrainment more than halves the hood concentration at no extra fan volume. The larger, faster plume also rises higher, which is why the current catalog stresses discharge velocity and plume height for lab exhaust.
Twin City Fan's current lab exhaust line includes induced flow mixed flow fans (TVIFE, QIFE), induced flow centrifugal fans (BAIFE, BCIFE), and venturi nozzle fans (QFE, TFE). Chapter 6 returns to bypass flow, plume dilution and lab exhaust total flow.
Diagram: Lab Exhaust Entrained Flow Calculator¶
Run Lab Exhaust Entrained Flow Calculator Fullscreen
Lab Exhaust Entrained Flow Calculator
Type: microsim
sim-id: ch05-lab-exhaust-entrained-flow-calculator
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate the fan flow, total flow and plume concentration of a laboratory exhaust fan from its contaminated, bypass and entrained flows, with flows exact and concentration within 0.5 ppm.
Prerequisites: contaminated air, bypass flow, fan flow, entrained flow, total flow, concentration in ppm, and the three equations in the section "Entrained Flow in Laboratory Exhaust".
Evidence of Mastery: For three challenge cases, the learner types fan flow, total flow and plume concentration before the sim shows the answer. Flows are correct when they equal the model value exactly; concentration is correct when it is within ±0.5 ppm of the model value. Mastery is all nine values correct across the three cases, with at most two attempts per value. Moving the inputs in free-exploration mode is not evidence.
Misconceptions: (1) The fan must be sized for the total flow, including entrained air. (2) Entrained air passes through the impeller. (3) Bypass air and entrained air dilute in different ways, so only one of them reduces plume concentration. (4) Doubling the entrained flow halves the plume concentration.
Instructional Rationale: Apply-level calculation needs committed answers with a check. Separating fan flow from total flow in every case makes the learner state which air passes through the impeller, which is the idea the misconceptions get wrong.
Content:
Quantities in free-exploration mode:
| Quantity | Min | Max | Step | Default | Unit |
|---|---|---|---|---|---|
| Contaminated air from fume hoods | 2,000 | 20,000 | 500 | 8,000 | CFM |
| Bypass flow | 0 | 10,000 | 500 | 2,000 | CFM |
| Entrained flow | 0 | 20,000 | 500 | 7,000 | CFM |
| Hood exhaust concentration | 10 | 500 | 10 | 100 | ppm |
Challenge cases, in this order (hood concentration 100 ppm in every case):
| # | Contaminated (CFM) | Bypass (CFM) | Entrained (CFM) | Fan flow (CFM) | Total flow (CFM) | Plume concentration (ppm) | Feedback when wrong |
|---|---|---|---|---|---|---|---|
| 1 | 8,000 | 2,000 | 7,000 | 10,000 | 17,000 | 47.1 | Fan flow is contaminated plus bypass: 8,000 + 2,000 = 10,000. Total adds entrained: 17,000. Plume: 100 × 8,000 / 17,000 = 47.1 ppm. |
| 2 | 6,000 | 0 | 9,000 | 6,000 | 15,000 | 40.0 | With no bypass, fan flow equals the contaminated air, 6,000. Entrained air does not pass through the fan. Total 15,000; plume 100 × 6,000 / 15,000 = 40.0 ppm. |
| 3 | 12,000 | 4,000 | 0 | 16,000 | 16,000 | 75.0 | With no entrainment, total flow equals fan flow, 16,000. Bypass air alone still dilutes: 100 × 12,000 / 16,000 = 75.0 ppm. |
The sim labels all challenge and default values "illustrative".
Provenance: Flow definitions (bypass flow through a bypass air plenum, entrained flow through the nozzle or windband, fan flow, contaminated air, total flow): FanPedia pp. 4 and 6. Numeric values are illustrative and match the chapter's worked example (case 1).
Rules: Q_fan = Q_contaminated + Q_bypass. Q_total = Q_fan + Q_entrained. C_plume = C_hood × Q_contaminated / Q_total, rounded to 0.1 ppm. Q_total is never zero because Q_contaminated >= 2,000 CFM. Concentration is correct when |answer − model| <= 0.5 ppm.
Learner Activity:
- Case 1 is shown as a labeled flow diagram: fume hood air entering, bypass air joining before the fan, the fan, the nozzle, and entrained air joining the plume. The three answer boxes are empty.
- The learner types fan flow, total flow and plume concentration and presses Check. Each value is marked correct or incorrect; the flow arrows then fill in with their values.
- Cases 2 and 3 follow the same way. The learner should notice in case 2 that fan flow equals the hood air when bypass is zero, and in case 3 that total equals fan flow when entrainment is zero.
- After case 3, free exploration unlocks: the learner changes any quantity, and the fan flow, total flow and plume concentration update at once. The first time the learner doubles the entrained flow, the sim shows the before and after concentrations so the learner sees that the concentration does not halve.
Feedback: Three cases, fixed order, three values each, two attempts per value. Correct: "Correct:
Starting State: Case 1 with empty answer boxes and the question "How much air must the fan move, and how much leaves the stack?"
Chapter Anchors: Worked example: 8,000 CFM contaminated at 100 ppm, 2,000 CFM bypass, 7,000 CFM entrained, fan flow 10,000 CFM, total flow 17,000 CFM, plume concentration 47.1 ppm. All values illustrative.
Matching the Job to the Product Line¶
Everything in this chapter comes together when a customer describes a job and expects a product line in return. Work the description from the air outward: what the air carries, where it must go, where the fan can sit, and whether people stand in the airflow.
The sorter below gives you twelve such descriptions. If the product names still blur together, use the two comparison tables above as a checklist and work the sorter a second time.
Diagram: Ventilation Job Matcher¶
Run Ventilation Job Matcher Fullscreen
Ventilation Job Matcher
Type: microsim
sim-id: ch05-ventilation-job-matcher
Library: p5.js
Status: Specified
Bloom Level: Evaluate
Bloom Verb: recommend
Learning Objective: The learner will recommend the product line that fits each of twelve building ventilation jobs, judged by the air contents, discharge path, mounting location and occupant exposure described in the job.
Prerequisites: housed centrifugal fan, plenum fan, square inline centrifugal fan, tubeaxial fan, upblast discharge, downblast discharge, centrifugal wall exhauster, gravity relief ventilator, cabinet ventilator, kitchen exhaust fan, smoke and heat exhaust fan, fiberglass fan, mancooler, entrained flow (all defined earlier in this chapter).
Evidence of Mastery: The learner assigns each of the twelve job cards to one of twelve product lines and, for each, selects the one deciding factor from four choices (air contents, discharge path, mounting location, occupant exposure). A card is correct when both the product line and the deciding factor match the Content table. Mastery is 10 of 12 product lines correct on the first attempt.
Misconceptions: (1) Any roof exhauster can handle any exhaust air. (2) A smoke exhaust job only needs a large ordinary fan. (3) Stainless steel is the only answer for corrosive air. (4) A mancooler is an exhaust fan. (5) Every relief opening needs a powered fan.
Instructional Rationale: Recommending a product line from a job description is an Evaluate-level judgment against criteria. Requiring the learner to also name the deciding factor makes the reasoning visible, so a correct guess for the wrong reason is caught.
Content:
Product lines offered (all twelve shown on every card): Kitchen exhaust fan; Smoke and heat exhaust fan; Fiberglass fan; Mancooler; Downblast centrifugal roof exhauster; Laboratory exhaust fan with entrained flow; Plenum fan; Cabinet ventilator; Gravity relief ventilator; Square inline centrifugal fan; Centrifugal wall exhauster; Tubeaxial fan.
| # | Job card shown to the learner | Correct product line | Deciding factor | Why (shown as feedback) |
|---|---|---|---|---|
| 1 | A restaurant's grill hood exhaust must leave through the roof. | Kitchen exhaust fan | Air contents | Grease-laden air needs a listed kitchen fan (UL 705 Supplement SC) with upblast discharge away from building surfaces. |
| 2 | An atrium must be cleared of smoke during a fire, with the fan still running at high temperature. | Smoke and heat exhaust fan | Air contents | Smoke and heat fans are rated for 500°F for four hours or 1,000°F for 15 minutes; an ordinary fan is not. |
| 3 | A wastewater treatment building exhausts moist, corrosive air. | Fiberglass fan | Air contents | FRP housings resist moisture-laden and corrosive air at lower cost than stainless steel or exotic alloys. |
| 4 | Workers at a foundry pour station need a movable fan blowing air on them. | Mancooler | Occupant exposure | A mancooler is an axial fan on a 360-degree swivel base with OSHA-compliant inlet and outlet screens; it cools people rather than exhausting air. |
| 5 | Restroom exhaust on a flat roof where a low, weather-protected unit is preferred. | Downblast centrifugal roof exhauster | Discharge path | Clean air may be laid back onto the roof, and the downblast cover gives a low profile with built-in weather protection. |
| 6 | Fume hood exhaust must be diluted and thrown high above a research building's roof. | Laboratory exhaust fan with entrained flow | Discharge path | A nozzle or windband entrains outside air into a high-velocity plume, diluting it and raising plume height. |
| 7 | A cleanroom air-handling unit must supply three duct openings from one cabinet with minimal maintenance. | Plenum fan | Mounting location | The plenum fan pressurizes the cabinet so every opening is fed; direct drive Arrangement 4 avoids belt residue. |
| 8 | Quiet, continuous exhaust from an office bathroom, mounted above the ceiling. | Cabinet ventilator | Mounting location | A small forward curved blower in a cabinet is built for quiet continuous duty in ceilings. |
| 9 | A warehouse needs a relief opening that lets air out when the supply fans run, with no second motor. | Gravity relief ventilator | Discharge path | Supply-fan pressure pushes air out through the weather-protected opening; no fan is needed. |
| 10 | Clean exhaust duct runs through a tight above-ceiling space and must turn 90 degrees at the fan. | Square inline centrifugal fan | Mounting location | Its square housing fits rectangular duct and can pass air straight through or through a 90-degree turn. |
| 11 | A single-story shop with no roof access needs exhaust where the duct ends at an exterior wall. | Centrifugal wall exhauster | Mounting location | The side-wall exhauster discharges straight out from the wall, away from the building face. |
| 12 | Short, straight round duct carrying general process exhaust at low pressure. | Tubeaxial fan | Discharge path | A plain-tube axial fan moves air straight through at low pressure in a short duct run. |
Provenance: Kitchen listing, smoke and heat ratings, fiberglass description, mancooler description, cabinet ventilator applications and gravity relief ventilator description: tcf.com product category pages (Kitchen & Restaurant Exhaust Fans; Smoke & Heat Exhaust Fans; Fiberglass Fans; Mancoolers; Ceiling & Cabinet Ventilators; Gravity Relief Ventilators). Downblast and upblast airflow paths: FanPedia p. 5. Plenum fan Arrangement 4: FanPedia p. 170. Square inline centrifugal: FanPedia p. 6. Entrained flow: FanPedia pp. 4 and 6. Job cards are illustrative scenarios written for this chapter.
Rules: Twelve cards, shuffled once per session. Each card has exactly one correct product line and one correct deciding factor, as listed. Product line and factor are scored separately; the mastery count uses product lines only.
Learner Activity:
- The learner reads a job card and chooses a product line from the twelve.
- The learner chooses the deciding factor from the four and commits both.
- The sim marks each choice and shows the Why text. The learner should notice that air contents decide cards 1-3, while cards 7, 8, 10 and 11 turn on where the fan can sit.
- After all twelve cards, a summary lists each job, the learner's choice and the correct choice, grouped by deciding factor.
Feedback: Twelve cards, shuffled, one attempt each, no re-answering. Correct line: "Correct:
Starting State: The first shuffled job card with all twelve product lines and four factors available, and the question "Which product line was built for this job, and why?"
Chapter Anchors: Twelve jobs and twelve product lines. Smoke and heat fans withstand 500°F for four hours or 1,000°F for 15 minutes. Kitchen fans are listed to UL 705 Supplement SC (formerly UL 762). Mancoolers swivel a full 360 degrees and have OSHA-compliant screens.
When a real inquiry arrives, the same four questions apply, and the current catalog page for the chosen line supplies the sizes and ratings to quote.
You Can Read a Roof Now
You can now tell an upblast from a downblast at a glance, explain why a plenum fan needs no scroll, and work out how much air a lab exhaust plume really carries. That is the vocabulary behind most of the ventilation inquiries you will ever quote.
Key Takeaways¶
- A housed centrifugal fan delivers all its air through one scroll outlet at a right angle to the shaft; a plenum fan has no scroll and pressurizes the cabinet around it, so the cabinet decides where the air goes.
- A square inline centrifugal fan fits rectangular duct and can turn the air 90 degrees; a tubeaxial fan is a plain-tube axial fan and the base for roof-mounted axial units.
- Upblast discharge throws air up and away from the roof, for grease, fumes and heat; downblast discharge lays it back over the roof and is for clean air only. Hooded and louvered roof exhausters shelter a down-and-out discharge, and low profile roof ventilators keep axial units close to the deck.
- A centrifugal wall exhauster discharges straight out from a wall, a gravity relief ventilator moves air with no fan, and a cabinet ventilator is a quiet blower for ceiling or inline duty.
- Kitchen, smoke and heat, and fiberglass fans are defined by what is in the air: grease, fire, or corrosive vapor. A mancooler blows air on people.
- Entrained flow joins the lab exhaust plume without passing through the fan, so total flow exceeds fan flow and plume concentration falls in proportion.