Key Takeaways
- Duct vent selection in industrial scope is a function decision. For a PP duct system, duct vent covers the terminal-and-control layer: volume dampers, backdraft dampers and rain caps — chosen by what each position must do.
- Manual, electric and backdraft answer different questions. Hand balancing, automated control and one-way protection after shutdown can coexist in the same system.
- Match the accessory to the duct, not the reverse. Read the main run diameter first, choose the matching standard step, and keep joints welded so the system stays one sealed circuit.
- Leakage class is a smoke-damper concept. Class I and II belong to the certified smoke-control domain; for industrial PP valves, verify pressure drop and limits from the datasheet.
- Quote with all nine fields. Diameter, quantity, grade, color, airflow direction, actuator requirements, media and temperature, connection and drawings decide whether the quote fits.
Specifying airflow accessories for an industrial PP exhaust system sounds simple until the quotes come back wrong: a position meant for one-way protection ends up with a manual valve, or a terminal is left open to weather. Much of that confusion starts with vocabulary. Searches for a duct vent usually surface residential registers and metal dampers, and buyers familiar with galvanized sheet-metal practice often copy those habits straight into corrosive industrial duty — where diameters, materials, joints and verification all work differently. Sorted by function first, the same decision becomes manageable: for duct runs inside the φ20–600 mm main range, you will be able to sort each position by what it must do, match diameters and welded joints, choose between PP, PVC and flame-retardant grades, verify operating limits, and turn the result into a quote the supplier can price first time.
Duct Vent vs. Duct Damper: Terms That Decide What You Actually Order
In an industrial PP exhaust system, the duct carries the air, the vent is where air leaves the process envelope, and the accessories — dampers and caps — are the parts you actually select. Sorting out which term names which part is the first decision, because every later choice hangs on it.
| Term | What it is in this guide | What you decide |
|---|---|---|
| Duct | The sealed channel that moves air between process points | Its diameter, material and how the run is routed |
| Vent | The outlet or opening where air leaves the system envelope | Where the system discharges and what protects that point |
| Volume damper | The executing part that adjusts flow by hand or by actuator | How much control the position needs — manual or electric |
| Backdraft damper | The one-way part that closes when the fan stops | Whether reverse flow at that position must be blocked |
| Rain cap | The terminal guard over the discharge point | Whether the outlet needs weather and debris protection |
Supply, Return and Exhaust Duty
Supply air pushes into the space, return air carries it back to the handler, and exhaust air pulls contaminants out. The direction of that work determines which side of each fitting faces the fan and which faces the opening. A fitting that works as a flow control on a supply branch behaves very differently when the same duct runs under exhaust, where pressure pushes outward and any leak discharges toward the room or the environment.
Because of that, the duty tells you which function a position owes the system before you compare products. An exhaust position that discharges outdoors may owe the system a one-way close after shutdown plus protection at the terminal, while an interior branch point owes a settable flow. Settle the duty first, and the fitting type follows.
Why Industrial Scope Changes the Answer
In residential practice the duct vent is an air grille or register — a finished terminal chosen largely for appearance and throw — and that vocabulary deserves no more than this sentence. Industrial work answers a different question: the concern is corrosive duty, fume duty and continuous exhaust of a process stream, not decorative throw.
In an industrial exhaust system the vent is the discharge point of that process stream, and the parts around it — plastic duct fittings, volume dampers, backdraft dampers and rain caps — must survive the media, hold the welded joint and keep doing their job with the fan off. Metal-damper habits about mounting and leakage do not transfer into this scope automatically. Name the duty — supply, return or exhaust — and decide what the fitting at that position must do before you choose a part number.
Airflow Accessory Function Matrix: Control, One-Way Flow and Terminal Protection
Every airflow accessory answers one of three questions: how much flow to set, which way air may travel once the fan stops, and what shields the outlet. The matrix below maps each function to its mounting position, action, product class and the decision it forces.
| Function | Typical mounting position | How it acts | Product class | What it decides for you |
|---|---|---|---|---|
| Manual volume control | Reachable branch or run | Blade set by hand | Manual volume damper, Ø63–500 mm, 12 standard steps | How much flow the position owes |
| Automated volume control | Position a controller must drive | Electric actuator opens, closes or modulates | Electric volume damper, Ø110–550 mm, 10 standard steps | Whether the position needs remote or automatic control |
| One-way backflow protection | After the fan, before the outlet, on branch lines, at outdoor terminals | Opens with forward airflow, closes when flow stops | Backdraft damper, Ø110–600 mm, 11 standard steps | Whether reverse flow must be blocked |
| Terminal weather protection | The run’s discharge point | Shields the outlet; welded on last | Rain cap, Ø110–600 mm, 11 standard steps | Whether the outlet needs weather and debris protection |
| Flange and coupling interface | Every accessory-to-duct joint | Socket reinforcement inside the hot air weld | Flanges and couplings of the same welded system | How the fitting joins the sealed circuit |
Read the matrix by position, not by product name. The three diameter ranges follow the three jobs: the manual volume damper steps through Ø63–500 mm in 12 standard steps, the electric volume damper through Ø110–550 mm in 10, and the backdraft damper and rain cap share Ø110–600 mm in 11 — custom diameters on order. For example, a small branch takes a manual valve, an outdoor terminal its cap or one-way part.
Volume Control
Volume control is the function most positions owe the system: set the flow once and keep it there. A manual volume damper does it with a blade the technician sets by hand at the valve, stepping through Ø63–500 mm in 12 standard steps to match the branch. It needs no power and no signal — only access for the balancing pass.
An electric volume damper covers positions a controller must drive: the actuator moves the blade for remote, automatic or interlocked operation, across Ø110–550 mm in 10 standard steps. The decision table later in this guide works through the fork between hand-set and actuator-driven control.
One-Way Backflow Protection
The backdraft damper is the one-way member of the duct damper family: it opens with forward airflow and closes on its own when the flow stops — no actuator, no wiring — so reverse flow at shutdown meets a sealed path. Confirm the airflow direction before mounting.
Four mounting points cover where reverse flow begins: after the fan, before the outlet, on branch lines, and at outdoor terminals. Each point decides whether that position needs a one-way part from the Ø110–600 mm series in 11 standard steps.
Terminal Weather Protection
At the discharge point, the rain cap shrouds the outlet against rain, snow and debris. It is welded on as the last part of the run, so the terminal closes the sealed circuit, and it leaves the internal air balance untouched.
The cap answers weather; it does not answer reverse flow. Where the outlet must also stop backflow, the backdraft damper and rain cap pair up — the cap shields the opening, the valve seals the path. Decide what each position owes the system first, then choose and specify the accessory class to match.
Manual vs. Electric vs. Backdraft: Choosing the Control Function
A manual damper does hand balancing, an electric damper does controller-driven control, and a backdraft damper does one-way protection after shutdown — while the three coexist in one system. The table below matches each field situation to its function.
| Field situation | Function required | Select |
|---|---|---|
| A technician balances the branch on site, at the duct | Volume control, set by hand | Manual volume damper — Ø63–500 mm, 12 standard steps |
| A remote, automatic or interlocked command must drive it (fume hood interlock, room pressure) | Automated control | Electric volume damper — Ø110–550 mm, 10 standard steps; actuator supply, signal and mounting confirmed at project level |
| Reverse flow must stop once the fan shuts down | One-way protection | Backdraft damper — self-closing at shutdown, no actuator |
| One system, several duties at different positions | Control, one-way and terminal protection combined | All three coexist on one run |
When Hand Adjustment at the Duct Is Enough
Most branches need a flow balanced once and then left alone. At commissioning the technician walks the run, turns each manual air damper valve until the branch carries its share, and the setting holds until the process changes. No wiring, no controller — the control logic lives in the balancing pass itself.
Check the duty before settling for hand control. Injection-molded PP or PVC bodies carry media from pH 1–14 and ambient conditions inside −15 °C to 80 °C, confirmed against the operating conditions at quotation. Where media, temperature and access for balancing line up, the manual valve is the complete answer.
When a Controller Must Drive the Damper
A fume hood exhaust interlocked with sash position, fans coordinating with standby units, a room held under negative pressure — all need a blade that responds to a controller. An electric volume damper serves these duties: the actuator opens, closes or modulates the blade on command, so the position follows the control logic, not the last hand at the valve.
Set the actuator boundary at project level, not from a catalog page. Supply, signal, torque and protection ratings are never catalog figures — the selection notes promise remote, automatic and interlocked capability; the actuator scheme is engineered against the project’s control logic. State the duty in the RFQ; hardware follows at design stage.
Worked example: an electroplating shop exhausts acid mist from a plating-line branch — media at pH 1–14, ambient duty inside the −15 °C to 80 °C envelope, confirmed at quotation, a single fan, discharge to atmosphere. The branch takes a manual volume damper for balancing, a backdraft damper for one-way protection, and a rain cap over the outlet — the three coexisting on one run.
Change the conditions and the selection shifts. Where two fans share a common exhaust duct, each fan gets its own backdraft damper — field practice from a national laboratory guideline, a residential-derived principle applied by industrial analogy — so a stopped fan is not back-fed through the running one. Where the process must interlock with a fume hood or a room-pressure controller, the branch upgrades to an electric damper with the actuator scheme confirmed at project level.
Decide the function per position, choose the matching class, and specify the operating conditions in the RFQ while the drawings are still open.
Size and Interface Matching: Diameters, Shape and Welded Joints
Match the accessory to the duct, not the other way round: read the main run’s diameter off the drawing, pick the accessory step that equals it, and keep the joint welded so the system stays one sealed circuit. Sizing is the layer that sits underneath the function decision made above — every accessory exists in fixed standard steps built around the round PP duct family. The table below compresses those steps for a line-by-line check against the order form.
| Accessory | Standard diameter range | Standard steps | Custom |
|---|---|---|---|
| Manual volume damper | Ø63–500 mm | 12 | On order |
| Electric volume damper | Ø110–550 mm | 10 | On order |
| Backdraft damper | Ø110–600 mm | 11 | On order |
| Rain cap | Ø110–600 mm | 11 | On order |
All four ranges sit inside the φ20–600 mm round duct family, so the accessories step along the same diameter ladder as the pipe itself instead of introducing a parallel sizing system. One observation follows from the step lists: small branches below Ø110 mm are served by the manual series only, which is rarely a constraint because hand balancing is the function a small line usually owes the system. Where a position needs electric control or one-way protection, the relevant step list begins at Ø110 mm.
Matching the Round PP Duct Range
The round PP duct series is the reference for every interface decision on the system. Fix each run’s diameter from the design first, then take the accessory in the equal standard step — a volume damper bought a step off the duct forces an adapter into a joint that should never have existed. The same logic covers the PP flange and coupling positions: the interface parts belong to the same welded system, so the connection never breaks the fusing logic.
Where the design is still open, the ventilation duct sizing guide shows how run diameters and airflow figures are read off the drawings, so the branch sizes the accessories must match are settled before any part is quoted. Order the sizing work early and the damper and cap steps fall out of the drawing instead of being negotiated after delivery.
Hot Air Welding Keeps One Sealed System
An accessory is not bolted onto the circuit — it is fused into it. Per DVS-based hot-gas welding practice, the hand-welding parameters are 305–315 °C measured 5 mm from the nozzle centre and a travel speed of 60–85 mm/min with a 3 mm rod; the rod is the same material as the parts being joined, and the settings are confirmed by trial welds before production.
The joint construction is part of the specification: each accessory connects through a hot air weld with socket reinforcement, the same joint the pipe spools use, so fitting and run become one continuous sealed circuit with no mixed joining methods at the accessory position. The installation guide covers the full welding procedure step by step; what belongs in the RFQ is the joint itself, written down like the diameter. Fix the main diameter first, choose the matching step from this table, and decide the custom options at quote time — specify the welded joint while the drawing is still open.
Material Selection: PP, PVC and Flame-Retardant Grades for Corrosive Duty
For corrosive duty the material grade is the specification: PP and PVC carry the acid-and-alkali envelope, and flame-retardant PPs is ordered explicitly when the duty demands it. The table below puts the three grades side by side so the media list can be walked line by line.
| Grade | Chemical resistance | Temperature window | Flame-retardant option | Typical duty |
|---|---|---|---|---|
| PP | pH 1–14, acid and alkali | −15 °C to 80 °C | Standard grade | General corrosive exhaust |
| PVC | Same pH 1–14 acid-and-alkali envelope | −15 °C to 80 °C | Confirmed per order | Non-rated fume duty |
| FR PPs | Same pH 1–14 acid-and-alkali envelope | −15 °C to 80 °C | Specified at quotation | Fire-sensitive zones |
Every row carries the same boundary: the pH 1–14 chemical resistance and the −15 °C to 80 °C window are confirmed against the media, concentration and temperature list at quotation, per material grade — working envelopes to verify, not unconditional values. It is the same envelope the manual and electric damper series quote against, so a valve chosen in the control section and a cap chosen at the terminal share one material story. The certifications named on the fitting pages are ISO 9001, ISO 14001 and RoHS, listed as printed; the grade decision itself still rests on the media and temperature columns.
When PP Beats Metal in the Accessory Position
Where the duty is corrosive rather than hot, PP earns the accessory position on more than chemical resistance alone. It stands up to the same acid-and-alkali spectrum that attacks galvanized sheet from the first season of service, and its poor heat conduction limits the condensation that gathers at the fitting itself, where a cold metal blade would collect moisture straight out of the air stream.
Handling and consistency carry the rest. A moulded PP fitting weighs little enough for one technician to position and weld on site, and moulding holds dimensions uniform across batches, so a coupling ordered this year seats on a run installed the year before. Both are site-level advantages — no datasheet line, but the difference between a clean installation and a day of improvisation.
When Flame-Retardant PPs Is the Safer Specification
Where the exhaust serves a fire-sensitive zone, the standard grade gives way to flame-retardant PPs. The backdraft damper and rain cap pages list injection-moulded PP or flame-retardant PPs as the two material options, and the flame-retardant grade is a quotation-time specification — it is never assumed from the standard listing. Name the fire requirement in the RFQ, and the grade follows the order rather than a default.
The same pages carry the one life figure that belongs here: a service life of up to 50 years under specified operating conditions. Read it as the manufacturer’s upper bound under the conditions the quotation confirms — not as a promise for any media, any temperature, any duty. A grade walked against the table above is what makes the figure worth quoting.
Walk your media and temperature list against this table, decide the grade for each position, choose the flame-retardant option only where the zone demands it, and specify both in the RFQ.
Leakage Class and Operating Limits: What to Verify Before You Buy
Leakage class is a certified smoke-damper concept, not a brochure line: in the UL 555S domain, Class II means under 20 cfm/ft² at 4.0 in. wg, while for industrial PP valves the verifiable numbers live in the manufacturer’s datasheet. The table keeps them separate.
| Basis | What it defines | What it means for your purchase |
|---|---|---|
| Smoke-damper domain (UL 555S) | Class II leakage — below 20 cfm/ft² at 4.0 in. wg (= 102 l/s/m² at 1.0 kPa), the minimum for most applications | Check the code requirement before quoting |
| Smoke-damper domain (UL 555S) | Class I leakage — below 8 cfm/ft² at 4.0 in. wg (= 40 l/s/m² at 1.0 kPa), required in hospitals, schools and stairwells | Expect it in the specification, not offered |
| UL 555S minimum listing conditions | 2,000 fpm (10.2 m/s) and 4.0 in. wg (1 kPa) at 250 °F (121 °C); the damper label carries the maximum static pressure and airflow it is approved to open and close under | Check it against the fan duty there |
| NFPA 90A damper-wall matching | Penetrations in barriers rated below 3 hours get 1.5-hour dampers; barriers rated 3 hours or more get 3-hour dampers | Match damper rating to barrier rating |
| Industrial PP valve datasheet basis | Leakage, pressure drop, maximum static pressure and maximum velocity — from the manufacturer’s datasheet or submittal; no published class table | Require those fields in the RFQ |
For smoke dampers the path runs from local codes first, then national codes; where neither specifies a minimum, the designer decides how tightly the penetration is sealed during a fire event. The last row is where an industrial PP purchase gets its numbers.
What Leakage Class Means in the Smoke-Damper Domain
Both classes describe tested fittings in the certified smoke-damper domain, not brochure claims. Class II is the floor for most applications; Class I — approved to leak less than 8 cfm/ft² at 4.0 in. wg (40 l/s/m² at 1.0 kPa) — serves hospitals, schools and stairwells, per AMCA’s damper guidance.
The label ends where the PP valve begins. XICHENG PP valves are not UL 555S-listed fittings; acceptance criteria for leakage and pressure drop come from the product datasheet. Those classes benchmark the life-safety domain, not a PP-valve grade.
Pressure Drop, Maximum Pressure and Velocity
Every damper works inside limits: a maximum static pressure and a maximum air velocity it can close against. Push past either and the blade can fail to close at shutdown, letting one fault cascade across the run. Both limits are checked in the submittal.
Pressure drop is the second figure to demand: the resistance an open damper adds comes from submittal data for the volume damper in that position, and the fan selection should absorb it before commissioning. Ask for the drop and the limits together.
Toxic Gas Duty: Risk Review Before Damper Selection
One duty changes the order of decisions. NFPA guidance, as quoted by fire-damper manufacturers, states that fire dampers shall not be installed if the material being exhausted is toxic and if a risk evaluation indicates that the toxic hazard is greater than the fire hazard.
The industrial reading is procedural: for toxic or highly corrosive streams, run the risk review before fixing damper positions, then decide whether a standard fire or smoke fitting belongs there. Decide which domain your project lives in, and specify the datasheet fields your purchase order accepts.
Terminal Protection: Rain Caps, Backflow and Outlet Combinations
At the outlet, two protections answer different threats: the rain cap answers weather, the backdraft damper answers reverse flow — and on a shared exhaust duct each fan needs its own damper. What you order follows from the outlet itself: where it discharges, how it is oriented, and how many fans share the path. The combinations below cover the four cases an industrial PP system presents.
| Outlet | Configuration | What each part does |
|---|---|---|
| Vertical roof outlet, simple duty | Rain cap alone | Shields the outlet against rain, snow and debris; welded on last; leaves the internal balance untouched |
| Roof or wall outlet where reverse flow matters | Rain cap and backdraft damper together | The cap shields the opening, the valve seals the path — it opens with forward airflow, closes when flow stops, no actuator; confirm airflow direction before mounting |
| Horizontal wall discharge | Cap matched to the outlet orientation, Ø110–600 mm 11-step series | Weather protection for a horizontal discharge, on the run’s own diameter step |
| Shared common exhaust duct, several fans | One backdraft damper per fan, plus a motorized damper near the roof outlet in humid duty | Each fan’s valve seals its own path at shutdown; the motorized unit holds the outlet against moist air drift |
One sizing fact ties the table together: the rain cap and the backdraft damper share the Ø110–600 mm range in 11 standard steps, so cap and damper can be ordered on the same step — read the run diameter once and both part numbers follow. The three functions keep their separate roles, as the decision table showed.
Where the Rain Cap Goes
Sequencing is the practical point: the run is welded first, and the cap goes over the outlet end as the final weld of the job. Because it is ordered on the same diameter step as the run, it arrives with the pipe and the installation never waits on a missing terminal.
The cap is a shroud over the end of the duct, not a restriction inside it. Because it does not affect the airflow balance inside the duct, the balancing pass completed earlier stays valid once the terminal goes on.
Shared Exhaust Ducts: One Damper per Fan
Where several fans discharge into one common exhaust duct, the guideline position — documented in a national laboratory publication, residential in origin and applied here by industrial analogy — is one backdraft damper per fan. Without it, a running fan can push air backwards through a unit that is stopped, and the idle branch becomes a return path into the process space.
Humid duty adds a second fitting. In moist conditions the same guideline calls for a motorized damper near the roof outlet, closing the top of the stack against moist air that drifts in while the system rests; it works alongside the per-fan valves, not instead of them, and joins the fittings the maintenance walk should check for free movement.
Decide what each outlet owes the system — weather, reverse flow or both — choose the configuration from the table, and specify the pairing on one quote line.
Placement and Balancing: Where Each Fitting Goes in the Run
Placement is a sequencing decision: the backdraft damper sits where reverse flow begins, the control valves sit where they can be reached, and every adjustable fitting stays reachable for the balancing pass. The sequence matters because each of these duct accessories is welded into the run — a fitting set in the wrong position is a cut-and-reweld job, not a readjustment.
| Position | Why there | What to mount | Watch-outs |
|---|---|---|---|
| After the fan | Stops backflow through the idle fan | Backdraft damper | Confirm the flow direction before welding |
| Branch takeoffs | The point each branch owes its share | Manual volume damper | Keep the valve reachable for the balancing pass |
| Near terminal outlets | Fine-trim what the branch delivers | Manual or electric volume damper — actuator type matched to the control logic: spring-return, floating or modulating | Where electric, leave access for actuator service |
| The discharge outlet | Weather and one-way closure | Rain cap, or cap and backdraft damper paired | The cap goes on last, as the terminal-protection section covered |
| Fire and smoke boundaries, where the building design requires them | Code-mandated isolation | Certified life-safety dampers, per the fire design | Not an accessory substitution — the fire designer owns this position |
Balancing is the reason placement matters. At commissioning the technician walks the run and sets each volume damper until every branch carries the share the zoning assigned it, and those settings are revisited at seasonal changes, when drift in the system shifts the airflow. Where a branch serves a zone with its own schedule, its setting is checked against the design airflow at each revisit, so the zone keeps the share it was sized for. The pass only works if each valve can be reached and read on site — reachability is a placement outcome, not a fitting feature.
Adjustments follow the original zoning logic. Throttling a branch beyond its share builds pressure back up against the rest of the run, and a blade forced away from its intended setting quietly cancels the control the system was designed around. Electric positions carry the same duty: the actuator type was matched to the control logic at selection, and placement must preserve the access that logic needs for service. When a fitting starts to misbehave, the maintenance and troubleshooting guide covers the symptoms and fixes.
Decide where each fitting goes while the layout is still on paper, choose the access every adjustable part needs, and specify those positions with the order — placement errors are the cheapest to correct before the first weld.
FAQ: Choosing Duct Vent & Airflow Accessories
A quote that comes back right the first time carries nine fields — diameter, quantity, material grade, color, airflow direction, actuator requirements, media and temperature, end connection, and drawings. The checklist below makes each field quotable.
| Quote field | Why it matters | Example entry |
|---|---|---|
| Diameter | Pick the equal standard step from the accessory tables above | Ø250 mm |
| Quantity | Parts per position, plus spares for the balancing pass | One per branch position |
| Material grade | PP, PVC or flame-retardant PPs — grade specified at quotation | Flame-retardant PPs |
| Color | Beige or grey, fixed at order | Grey |
| Airflow direction | Confirmed before mounting — one-way parts seal a single direction | Exhaust, fan to outlet |
| Actuator requirements | Supply, signal and mounting confirmed at project level — electric positions only | Project-level confirmation |
| Media and temperature | pH envelope and temperature window as confirmed-at-quotation values | pH 1–14, −15 °C to 80 °C — confirmed at quotation |
| End connection | Hot air weld with socket reinforcement keeps one sealed circuit | Hot air weld, socket reinforcement |
| Drawings | Duct schedule or drawings attach the duty to the numbers | Duct schedule attached |
The questions below compress what the modules established. Treat them as a handover to purchasing.
| Question | One-line answer |
|---|---|
| Manual or electric damper? | Manual where a technician balances by hand; electric where a controller must drive the blade. |
| Rain cap and backdraft damper together? | Yes — the cap keeps weather out of the outlet, the damper seals reverse flow. |
| What standard diameters exist? | Three step lists across the four series, custom diameters on order. |
| How is a flame-retardant grade ordered? | Explicitly, at quotation — it is never a default. |
| What does a quote need? | The nine checklist fields, with boundary conditions written in. |
Manual or electric damper — how do I choose?
The criterion is the control duty, not product preference. Choose manual where a technician balances at the duct, and electric where a remote, automatic or interlocked command must drive the blade. State the duty in the RFQ, and the actuator scheme follows at project level.
Can a rain cap and a backdraft damper be installed together?
Yes — they answer different threats at the same outlet. The cap shrouds the outlet against rain and debris; the damper seals the path when the fan stops, one valve per fan on a shared duct. Check the airflow direction before mounting, then order both parts on one quote line.
What standard diameters are available?
Standard steps run Ø63–500 mm for manual volume dampers, Ø110–550 mm for electric, and Ø110–600 mm for backdraft dampers and rain caps — all inside the φ20–600 mm main duct range, with custom diameters on order. Read the run diameter off the drawing, then check it against this list before quoting.
How do I specify a flame-retardant grade?
The backdraft damper and rain cap lines come in injection-moulded PP or flame-retardant PPs — the flame-retardant grade is a quotation-time specification, never a default. Name the fire requirement and the duty in the RFQ. Confirm the operating envelope per grade when the quotation returns.
What information does a quote need?
The nine fields of the checklist above, with their boundary conditions written beside them — copy the table into the RFQ. Attach the duct schedule or drawings so the duty travels with the numbers. A quote built this way comes back right the first time.
The selection compresses to four moves: decide the function each position owes the system, choose the matching standard step, fix the material grade against the media list, and verify the operating fields before ordering. For the hand-balanced series, the manual volume damper page carries the standard step list for the manual series. Specify the nine fields and their boundaries in the RFQ, and the quotation decides the project on its merits, not on follow-up questions.
