Spring Isolators, Restraints and Field Measurement¶
Summary¶
This chapter completes installation. It covers spring isolators (open, housed, restrained, and seismic), floor mounted and ceiling hung isolation, direct mounted isolators, isolation rails, mounting angles, snubbers, and thrust restraints that resist the fan's pressure thrust. It also covers shaft and coupling alignment, anti-backspin devices, and measuring airflow in the field with piezometer rings and pressure transmitters. After completing it, readers can specify isolation hardware and explain how installed airflow is verified.
Concepts Covered¶
This chapter covers the following 16 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Shaft and Coupling Alignment | 1 |
| Spring Isolator | 10 |
| Snubber | 1 |
| Airflow Measurement | 1 |
| Pressure Transmitter | 1 |
| Direct Mounted Isolators | 1 |
| Isolator Mounting Angle | 1 |
| Floor Mounted Isolation | 1 |
| Ceiling Hung Isolation | 1 |
| Thrust Restraint | 1 |
| Isolation Rails | 1 |
| Open Spring Isolator | 4 |
| Housed Spring Isolator | 1 |
| Restrained Spring Isolator | 2 |
| Anti-Backspin Device | 16 |
| Seismic Spring Isolator | 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 11: Drives and Standard Fan Arrangements
- Chapter 12: Discharge Positions and Rotation
- Chapter 15: Motor Ratings, Speed Control and Positioners
- Chapter 16: Bearings, Shafts and Shaft Seals
- Chapter 22: Fan Bases and Vibration Isolation
The Last Few Feet of the Job
A well-selected fan can still disappoint if it rocks on the wrong springs, walks away from its duct at start-up, or spins backward on a windy night. This chapter covers the hardware and checks that make an installation quiet, stable and verifiable. By the end you can read an isolation line on a specification and tell the customer how their airflow will be proven. Let's follow the airflow.
Chapter 22 explained why fans are isolated, what deflection means, and how unitary, isolation and inertia bases support a fan. This chapter turns to the specific hardware on the order: the four kinds of spring isolator, the ways isolators attach to a fan, the snubbers and thrust restraints that limit motion, and three field topics that decide whether the installed fan runs as rated: shaft and coupling alignment, anti-backspin devices, and airflow measurement. The product statements come from the FanPedia vibration isolation pages (pp. 133–135), the piezometer ring pages (pp. 158–160) and the technical descriptions (pp. 162–197).
Spring Isolators¶
A spring isolator is a steel coil spring, with its top and bottom load plates, placed between a fan (or its base) and the supporting structure. Under the fan's weight the spring compresses; that compression is the isolator's static deflection. A soft spring that compresses a lot lets the fan bounce at a low natural frequency, and a fan that runs well above that frequency passes very little of its vibrating force into the floor. Springs reach much larger deflections than rubber-in-shear mounts or neoprene pads, which is why they are the usual choice when isolation really matters.
FanPedia (p. 134) states that most isolators Twin City Fan uses have deflections under 1 inch (25.4 mm), 1 inch, or 2 inches (50.8 mm). Higher deflection is available at additional cost after engineering review. Those three values are the vocabulary of a spring isolator line item: a specification that asks for "2-inch deflection springs" is asking for the high end of the standard range.
Two relationships connect a spring to the fan it carries. The first converts load into deflection.
Static Deflection of a Spring¶
where:
- \( \delta \) is the static deflection of each isolator, in inches
- \( W \) is the total weight carried by the isolators (fan, motor, drive and base), in pounds
- \( n \) is the number of isolators, assuming the load is shared equally
- \( k \) is the spring rate of one isolator, in pounds per inch of compression
The second, a standard result for an undamped spring and mass, gives the natural frequency of the fan bouncing on its springs.
Natural Frequency of a Spring-Mounted Fan¶
where:
- \( f_n \) is the natural frequency of the mounted assembly, in hertz (cycles per second)
- \( \delta \) is the static deflection, in inches
- 3.13 comes from \( \frac{1}{2\pi}\sqrt{g} \) with \( g = 386 \) in/s²
Isolation begins only when the fan's running frequency is more than \( \sqrt{2} \), about 1.41, times the natural frequency. Below that ratio the springs amplify vibration instead of blocking it.
Softer Is Lower, Lower Is Better
Notice that deflection sits under a square root: doubling deflection lowers the natural frequency only by a factor of 1.41. That is why the jump from 1-inch to 2-inch springs matters most on slower fans, whose running frequency sits closest to the springs' bounce.
Worked example (illustrative numbers). A fan, motor and isolation base weigh 2,400 lb and sit on four spring isolators, each with a spring rate of 600 lb/in. The fan runs at 1,180 rpm.
- Load per isolator: \( 2{,}400 / 4 = 600 \) lb.
- Static deflection: \( \delta = 600 / 600 = 1.00 \) in, a standard 1-inch spring.
- Natural frequency: \( f_n = 3.13 / \sqrt{1.00} = 3.13 \) Hz.
- Running frequency: \( 1{,}180 / 60 = 19.67 \) Hz.
- Frequency ratio: \( r = 19.67 / 3.13 = 6.28 \), far above 1.41, so the springs isolate. The undamped transmissibility \( T = 1/(r^2 - 1) = 0.026 \): about 2.6% of the vibrating force reaches the floor, an isolation efficiency of about 97.4%.
Real bases rarely load four corners equally, because the motor side is heavier. Isolator suppliers then choose a different spring for each corner so that every corner deflects the same amount; the calculation per corner is the same.
Diagram: Spring Isolator Deflection Calculator¶
Run Spring Isolator Deflection Calculator Fullscreen
Spring Isolator Deflection Calculator
Type: microsim
sim-id: ch23-spring-isolator-deflection-calculator
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate the static deflection of equally loaded spring isolators from total weight, isolator count and spring rate, to within 0.02 in.
Prerequisites: spring isolator, static deflection, spring rate, natural frequency, running frequency, frequency ratio, transmissibility (all defined in the section "Spring Isolators" above).
Evidence of Mastery: For three challenge cases, the learner types the deflection in inches before the sim shows it. An answer is correct when it is within ±0.02 in of the model value. Mastery is 3 of 3 correct. Free exploration of the controls afterward is not evidence.
Misconceptions: (1) Doubling deflection halves the natural frequency (it divides it by 1.41). (2) Any spring isolates, whatever the fan speed (below a ratio of 1.41 the springs amplify). (3) Adding isolators adds deflection (more isolators share the load, so each deflects less).
Instructional Rationale: Calculation at the Apply level needs practice with a check, so the learner commits a number before the answer appears. The exploration mode then shows how deflection, natural frequency and isolation respond to each input, which confronts the three misconceptions with computed values.
Content:
Adjustable quantities:
| Quantity | Min | Max | Step | Default | Unit |
|---|---|---|---|---|---|
| Total supported weight W | 400 | 8000 | 100 | 2400 | lb |
| Number of isolators n | 4 | 8 | 2 | 4 | count |
| Spring rate k (each) | 100 | 3000 | 50 | 600 | lb/in |
| Fan speed | 300 | 3600 | 10 | 1180 | rpm |
Displayed results: load per isolator (lb), static deflection (in, 2 decimals), natural frequency (Hz, 2 decimals), running frequency (Hz, 2 decimals), frequency ratio (2 decimals), transmissibility (3 decimals), isolation efficiency (%, 1 decimal), and a status line.
Challenge cases, in this order:
| # | W (lb) | n | k (lb/in) | Model deflection (in) | Feedback when wrong |
|---|---|---|---|---|---|
| 1 | 2400 | 4 | 600 | 1.00 | Divide the weight among the isolators first: 2,400 / 4 = 600 lb, then 600 / 600 = 1.00 in. |
| 2 | 3600 | 6 | 300 | 2.00 | Six isolators share 3,600 lb, so each carries 600 lb; 600 / 300 = 2.00 in. |
| 3 | 1600 | 4 | 1600 | 0.25 | Each isolator carries 400 lb; a stiff 1,600 lb/in spring compresses only 400 / 1,600 = 0.25 in. |
All values are labeled "illustrative" on screen.
Provenance: Equations and the 1.41 threshold are standard vibration-isolation results stated in the chapter section "Spring Isolators". The typical deflections (under 1 in, 1 in, 2 in; higher by engineering review) are from FanPedia p. 134. Challenge values are illustrative.
Rules:
- Load per isolator = W / n. Deflection δ = (W / n) / k.
- f_n = 3.13 / √δ (Hz). Running frequency f = rpm / 60. Ratio r = f / f_n.
- If r > 1.41: T = 1 / (r² − 1), efficiency = (1 − T) × 100%, status "Isolating".
- If r <= 1.41 and r is not within ±0.02 of 1.00: T = 1 / |1 − r²|, efficiency is shown as "none", status "Amplifying: springs too stiff for this speed".
- If |r − 1.00| <= 0.02: status "Resonance", T shown as "very large".
- If δ > 2.00 in: add the note "Above the typical 2-inch deflection; higher deflection requires engineering review (FanPedia p. 134)."
Learner Activity:
- The learner reads challenge 1 (W, n, k) and types a deflection.
- The learner presses Check. The sim shows the model deflection, the spring compressed to scale beside an uncompressed spring, and the derived natural frequency.
- After all three challenges, exploration unlocks with the default values. Changing any input updates every result at once.
- The learner should notice that raising n lowers deflection, that raising fan speed raises the ratio, and that a 0.25-in spring on a 300-rpm fan reaches the Amplifying status.
Feedback: Three challenges, fixed order, two attempts each. Correct: "Correct: δ =
Starting State: Challenge 1 with an empty answer box, an uncompressed spring drawing, and the question "How far will each spring compress?"
Chapter Anchors: Worked example: 2,400 lb on four isolators at 600 lb/in gives 600 lb each, 1.00 in deflection, 3.13 Hz natural frequency; 1,180 rpm is 19.67 Hz; ratio 6.28; transmissibility 0.026; efficiency about 97.4%. Isolation begins above a ratio of 1.41. Typical deflections under 1 in, 1 in or 2 in.
Open, Housed, Restrained and Seismic Springs¶
FanPedia (pp. 134, 196) names four spring isolator types. They share the same coil spring; what differs is how much of the spring is enclosed and how much motion the enclosure allows.
An open spring isolator is the simplest spring mount: a bare coil between two load plates. It does not offer any restriction of the motion caused by aerodynamic forces, so a fan on open springs is free to shift when its airstream pushes on it. Twin City Fan typically does not use floor mounted open springs. Ceiling hung open springs are mounted in a formed metal surround, and the typical deflection is 1 inch.
A housed spring isolator works the same way as an open spring but sits inside an enclosed housing. The housing can hold one or more springs, depending on the load and application. Typical deflection is 1 or 2 inches.
A restrained spring isolator is the same design as an open spring with a housing or frame added to restrain the vertical and/or horizontal motion of the spring. The restraint matters when a fan must not drift, rock or lift as its load and aerodynamic forces change. Typical deflection is 1 or 2 inches.
A seismic spring isolator is similar to a restrained spring, except that the housing nearly surrounds the entire spring so that it can withstand the loads generated during a seismic event such as an earthquake. Typical deflection is 1 or 2 inches. Seismic springs are the isolator-level answer to the seismic restraint requirement introduced in Chapter 22.
The table below lines the four types up against each other.
| Spring type | Enclosure | Limits motion? | Typical deflection (FanPedia p. 134) |
|---|---|---|---|
| Open | None (ceiling hung: formed metal surround) | No | 1 in |
| Housed | Enclosed housing, one or more springs | No more than an open spring | 1 or 2 in |
| Restrained | Housing or frame | Yes, vertical and/or horizontal | 1 or 2 in |
| Seismic | Housing nearly surrounds the spring | Yes, built for seismic loads | 1 or 2 in |
Where the Isolators Go¶
The second half of an isolation specification says how the isolators meet the fan and the structure. FanPedia (p. 133) groups the choices into five mounting styles. Two of them describe which way the load hangs: from the floor up or from the structure down.
Floor mounted isolation places isolators beneath the fan or its base, bearing on the floor, a housekeeping pad or a support frame. It can use springs, rubber-in-shear (RIS) mounts or neoprene pads, and it includes three of the styles below: directly under the fan, isolation rails and mounting angles, plus the isolation base from Chapter 22. FanPedia p. 131 calls for spring or RIS isolators under an isolation base, and p. 133 also lists neoprene pads. Twin City Fan engineering confirms neoprene pads can be used under an isolation base, though it is uncommon; springs or RIS isolators are the usual choice. RIS mounts are themselves a pad-type isolator, and are most often used under isolation rails, with the fan sitting on the rails.
Ceiling hung isolation suspends the fan from an overhead structure, such as a ceiling or roof steel, with the isolators between that structure and the fan. FanPedia calls these isolation hangers, and they can be spring or RIS hangers. The threaded rod that carries them is supplied by others, and ceiling hung springs and RIS mounts sit in a formed metal surround.
Direct mounted isolators sit directly under the foundation holes in the fan's own structure, with no base between. They can be springs, RIS pads or neoprene pads. FanPedia limits this style, usually, to fans with impellers of 36.5 inches in diameter or less. The exception is swingout housed centrifugal fans of any impeller size, with flat pads only. Some arrangements cannot take isolators directly under the fan at all: Arrangement 3F is typically not suitable (p. 163), and Arrangement 9F is not suitable; FanPedia's remedy for 9F is an Arrangement 1 fan on a separate isolation base (p. 166).
Isolation rails put the isolators between two pieces of flat bar. The top bar bolts to the fan's foundation holes and the bottom bar attaches to the floor or other mounting structure. Springs or RIS pads can be used. Rails spread the fan's foundation pattern onto a pair of bars, which gives the isolators a clean place to sit.
An isolator mounting angle is a structural angle mounted between the fan and its isolators. It is usually used when the fan's inlet side has no structural support of its own for an isolator to bear on. Springs or RIS pads can be used.
The list below restates the choices as questions to ask when you read a drawing.
- Does the fan hang from structure? Use isolation hangers (ceiling hung).
- Does the fan structure have usable foundation holes and an impeller of 36.5 in or less? Isolators can mount directly under the fan.
- Do the foundation holes need a continuous member to bolt to? Use isolation rails.
- Is one side of the fan, usually the inlet side, without structural support? Add a mounting angle.
- Must fan, motor and drive move together? Use an isolation base (Chapter 22).
Diagram: Isolation Hardware Sorter¶
Run Isolation Hardware Sorter Fullscreen
Isolation Hardware Sorter
Type: microsim
sim-id: ch23-isolation-hardware-sorter
Library: p5.js
Status: Specified
Bloom Level: Understand
Bloom Verb: classify
Learning Objective: The learner will classify eleven job-site descriptions by the isolation hardware FanPedia calls for, with at least 9 of 11 correct on the first attempt.
Prerequisites: open, housed, restrained and seismic spring isolators; ceiling hung isolation; direct mounted isolators; isolation rails; isolator mounting angle; snubber; thrust restraint (all defined in the chapter above this block, the last two in "Snubbers and Thrust Restraints").
Evidence of Mastery: The learner assigns each of the 11 descriptions to one of 10 bins. An assignment is correct when it matches the Correct bin column. Mastery is 9 of 11 correct on the first attempt.
Misconceptions: (1) A snubber and a seismic spring are the same device. (2) Any spring restrains a fan against airflow forces (open springs do not). (3) Every fan can sit on isolators bolted directly under it.
Instructional Rationale: Classifying realistic descriptions makes the learner map features (enclosure, direction of support, where brackets attach) onto hardware names, which is the Understand-level skill used when reading a specification.
Content:
Bins (10): Open spring isolator; Housed spring isolator; Restrained spring isolator; Seismic spring isolator; Snubber; Thrust restraint; Isolation hangers (ceiling hung); Isolators directly under fan; Isolation rails; Isolator mounting angle.
| # | Description shown to the learner | Correct bin | Why (shown as feedback) |
|---|---|---|---|
| 1 | The fan hangs from roof steel; the isolators go between the overhead structure and the fan on threaded rod by others. | Isolation hangers (ceiling hung) | A fan suspended from structure with isolators between structure and fan uses isolation hangers. |
| 2 | A horizontal-discharge fan on springs slides back at every start; the spec asks for two devices, 180° apart, tying the fan discharge to the discharge duct with threaded rod. | Thrust restraint | Thrust restraints are ceiling hung type isolators attached to fan discharge and duct, two per fan, 180° apart. |
| 3 | The inlet side of the fan has no structural member for an isolator to bear on. | Isolator mounting angle | A mounting angle is used when the fan inlet does not have a structural support. |
| 4 | Isolators sit between two flat bars; the top bar bolts to the fan's foundation holes and the bottom bar anchors to the floor. | Isolation rails | That is FanPedia's definition of isolation rails. |
| 5 | A fan with a 30-inch impeller (illustrative) gets springs bolted straight under the foundation holes in its own structure, with no base. | Isolators directly under fan | Direct mounting is usually limited to impellers of 36.5 in or less; 30 in qualifies. |
| 6 | A swingout housed centrifugal fan with a large impeller sits on flat pads bolted directly under its own structure. | Isolators directly under fan | Swingout housed centrifugal fans are the exception: any impeller size, flat pads only. |
| 7 | The spec calls for spring mounts whose housings nearly surround the entire spring so they withstand earthquake loads. | Seismic spring isolator | A housing that nearly surrounds the spring for seismic loads defines a seismic spring. |
| 8 | A separate device, one bracket on the base and one on the floor with elastomeric material between them, limits movement in any direction; it is used along with the isolators. | Snubber | A snubber works with isolators as a shock absorber; the elastomer prevents metal-to-metal contact. |
| 9 | Springs with a frame added to limit vertical and horizontal travel; no seismic requirement is stated. | Restrained spring isolator | A housing or frame that restrains vertical and/or horizontal motion defines a restrained spring. |
| 10 | A bare coil spring in a formed metal surround on a ceiling hung fan; it does nothing to resist aerodynamic forces. | Open spring isolator | Open springs offer no restriction of aerodynamic motion; ceiling hung types sit in a formed metal surround. |
| 11 | An enclosed housing holds two springs that act exactly like open springs. | Housed spring isolator | Housed springs work like open springs inside an enclosed housing that can hold one or more springs. |
Provenance: FanPedia pp. 133–135 and 196–197 (all definitions, the 36.5-inch limit, the swingout exception, thrust restraint details). The 30-inch impeller in item 5 is illustrative and is labeled so on screen.
Rules: Each description has exactly one correct bin. Bins can receive more than one description (item 5 and item 6 share a bin). No partial credit.
Learner Activity:
- The learner reads one description and places it in a bin.
- The sim marks the placement correct or incorrect at once and shows the "Why" text.
- An incorrect placement returns the card for a second try; the first-attempt result is what counts toward mastery.
- After all 11, the sim shows the first-attempt score and lists the descriptions missed with their reasons.
Feedback: 11 items in shuffled order, two attempts each. Correct: "Correct.
Starting State: Ten empty labeled bins and the first card face up, with the prompt "Which hardware does this job call for?"
Chapter Anchors: Four spring types; five mounting styles; direct mounting usually limited to impellers of 36.5 in or less; swingout housed centrifugal fans excepted with flat pads only; two thrust restraints per fan, 180° apart.
Snubbers and Thrust Restraints¶
Springs let a fan move. Two devices limit how far.
A snubber is a stop used together with the isolators on a fan or base. It serves as a shock absorber that prevents excessive movement of the fan or base in any direction, and it is often used when the fan must withstand seismic loading (FanPedia p. 135). A snubber is two brackets, one attached to the fan or base and one attached to the floor, with elastomeric (rubberized) material between them that prevents metal-to-metal contact. In normal running the fan floats on its springs; the snubber engages only when an unusual load, such as an earthquake, tries to move the fan too far.
A thrust restraint prevents excessive motion caused by aerodynamic force. Any fan that pushes air feels a reaction force, roughly its pressure rise times its outlet area, in the direction opposite the flow. For an illustrative outlet of 4 ft² at 4 in. wg, with 1 in. wg equal to 5.2 lb/ft², that is about \( 4 \times 5.2 \times 4 = 83 \) lb pushing the fan backward each time it starts. On stiff mounts the force is lost in friction; on soft springs the fan moves, and the flex connector takes the strain.
FanPedia (p. 197) describes the standard thrust restraint: ceiling hung type isolators (springs or spring/RIS pad combinations) attached to both the fan discharge and the discharge duct with threaded rod, adjusted to prevent horizontal motion. Two restraints are used per fan, mounted 180° apart, and FanPedia's drawing shows them for horizontal airflow in either direction.
Diagram: Snubbers and Thrust Restraints¶
Run Snubbers and Thrust Restraints Fullscreen
Snubbers and Thrust Restraints
sim-id: ch23-snubber-thrust-restraint-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the nine labeled parts of a snubber on an isolation base and of a thrust restraint across a fan-to-duct joint, given each part's function or location.
Content: 9 numbered markers: Snubber bracket (attached to the fan or base), Snubber bracket (attached to the floor), Elastomeric (rubberized) material, Isolators, Discharge duct, Flex connector, Fan discharge, Ceiling hung type isolator, Threaded rod
Provenance: FanPedia page 135, "Vibration Isolation: Isolator Types, snubbers on an isolation base (left, with detail inset) and thrust restraints (right)", with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 135.
Open Springs Do Not Hold a Fan in Place
A common trap is quoting open springs on a high-pressure fan and assuming the flex connector will keep it in place. Open springs offer no restriction of aerodynamic motion, so the fan shifts at every start and the connector tears over time. When pressure is high and discharge is horizontal, add thrust restraints or use restrained springs.
Shaft and Coupling Alignment¶
A coupling connects the fan shaft to the motor shaft, or one fan shaft to another in a twin fan assembly (FanPedia p. 129). Shaft and coupling alignment is the process of positioning the motor and the fan bearings so that the two shaft centerlines run as one straight line through the coupling. Misalignment comes in two forms: offset, where the centerlines are parallel but displaced, and angular, where they meet at an angle. Either form loads the coupling and the bearings on every revolution and shows up as vibration at running speed or twice running speed.
FanPedia hardware exists to make alignment repeatable. Motor positioners, also called motor alignment jacking screws, move the motor in small steps (p. 148). Bearing dowel pins hold a bearing's position once it is aligned (p. 176). A machined pedestal top gives the bearings and motor a flat reference surface (p. 130). Inertia bases are used on longer direct drive fans because their mass and stiffness keep the motor, coupling, shaft and bearings aligned (p. 175).
Align Last, Then Check Again
Do the final alignment after the isolators are adjusted to their working deflection and the duct and flex connectors are attached. Then recheck it after the first run, because a base that settles on its springs can carry the motor out of line.
Anti-Backspin Devices¶
An anti-backspin device, also called an anti-rotation device or anti-rotation clutch (FanPedia p. 125), prevents the rotor from freewheeling in reverse when the fan is not running (p. 162). It is a one-way clutch on the shaft: the fan turns freely in its correct rotation, and the device locks against rotation the other way.
Reverse freewheeling happens when air flows backward through an idle fan. The common causes are a second fan in parallel pushing air back through the stopped one, stack draft in a tall exhaust system, and wind on a roof or wall discharge. A fan spinning backward causes two problems. First, when its motor starts it must stop the rotor and then accelerate it the other way, which lengthens the high-current start and loads belts, couplings and shafts. Second, a rotor that can turn on its own is a hazard for anyone working on a fan that is supposed to be stopped. A zero speed switch, from Chapter 22, can report the motion; an anti-backspin device prevents it.
Where the device mounts depends on how the impeller is supported (p. 162):
- Overhung impeller designs, where the impeller sits at the end of the shaft (Arrangements 1 and 9, for example): mounted between the bearings.
- Center hung impeller designs, where the impeller sits between the bearings (Arrangement 3, for example): mounted on a shaft extension on the non-drive end.
Worked example. A plant runs two identical exhaust fans in parallel into one stack. Fan A is an Arrangement 9 SWSI belt drive; Fan B, the standby, is an Arrangement 3 SWSI belt drive. The operator wants to run one fan at a time.
- Is reverse flow possible? Yes. When Fan A runs, the shared stack connection is at a higher pressure than Fan B's inlet, so air flows backward through idle Fan B. The same happens to Fan A when Fan B runs.
- Which fans need the device? Both, because each will be the idle fan at some point.
- Fan A, Arrangement 9: the impeller is overhung, so the device mounts between the two bearings on the pedestal.
- Fan B, Arrangement 3: the impeller is center hung, so the shaft must have an extension on the non-drive end to carry the device. That extension is a shaft detail, so it belongs on the order, not as a field add-on.
- Line items: anti-backspin device on both fans, plus the non-drive-end shaft extension on Fan B. If the customer also wants a start interlock, add a zero speed switch.
Measuring Airflow in the Field¶
Airflow measurement is the determination of the volume of air a fan actually delivers in its installed system, in cfm. A catalog rating describes the fan on a test stand; a field measurement tells the owner what the system is getting, which is how system effect and duct errors are found. A traditional method is a pitot tube traverse across a duct. The method FanPedia describes uses the fan itself.
The piezometer ring, introduced in Chapter 22, uses the fan's inlet funnel as a flow nozzle (FanPedia p. 158). A tap on the face of the funnel connects to the high-pressure side of a pressure-measuring device; tubing around the throat of the funnel, the ring, connects to the low-pressure side. The air speeds up as it enters the throat, so the throat pressure falls, and that pressure difference rises with the square of the airflow.
Diagram: Piezometer Ring Components¶
Run Piezometer Ring Components Fullscreen
Piezometer Ring Components
sim-id: ch23-piezometer-ring-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the ten labeled components of a piezometer ring airflow measuring system on a fan inlet funnel, given each component's function or location.
Content: 10 numbered markers: Inlet funnel, Tubing around the throat, Couplings with "T" fittings, Union tee, Tubing to low pressure tap, Tubing to high pressure tap, Low pressure tap fitting, High pressure tap fitting, Hanger clip, Rivnut
Provenance: FanPedia page 158, "Piezometer Rings: Overview, inlet funnel components (main drawing) with the union 'T' fitting, fitting tap and coupling close-ups", with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 158.
Flow Through the Inlet Funnel¶
where:
- \( Q \) is the airflow, in cfm
- \( K \) is the flow coefficient for that fan's inlet funnel, supplied by the fan manufacturer, in cfm per \( \sqrt{\text{in. wg}} \)
- \( \Delta p \) is the pressure difference between the funnel face tap and the throat ring, in in. wg
With an illustrative \( K = 10{,}000 \), a reading of 1.44 in. wg gives \( 10{,}000 \times 1.2 = 12{,}000 \) cfm. If the reading falls to 1.00 in. wg, the flow is 10,000 cfm: a 31% drop in the reading is only a 17% drop in airflow. Twin City Fan's installation manual IM-105 carries the detailed piezometer information for each fan type (p. 160).
A pressure transmitter is the device that turns that pressure difference into a signal a controls system can use, typically an electrical output for the building automation system, with or without a local display. FanPedia (p. 160) lists two optional versions a customer can buy from Twin City Fan: a pressure transmitter without display, and a pressure transmitter/transducer with display, shown in regular and NEMA 4 enclosures. Either can be mounted on the fan or remotely by others. Transmitters must be sized based on the application and fan performance: the transmitter's range has to cover the pressure difference the fan will produce at its highest expected airflow, without being so wide that normal readings sit at the bottom of the scale.
Key Takeaways¶
- A spring isolator compresses under load; deflection = load per isolator divided by spring rate, and natural frequency = 3.13 / √deflection (Hz, inches). Isolation needs a running-to-natural frequency ratio above 1.41. Twin City Fan's typical deflections are under 1 in, 1 in and 2 in.
- Open springs restrain nothing; housed springs enclose one or more springs; restrained springs add a housing or frame to limit vertical and/or horizontal motion; seismic springs have a housing that nearly surrounds the spring for earthquake loads.
- Floor mounted isolation supports the fan from below; ceiling hung isolation suspends it on isolation hangers. Direct mounted isolators sit under the fan's foundation holes (usually impellers of 36.5 in or less; swingout housed centrifugal fans excepted with flat pads). Isolation rails and isolator mounting angles give isolators a place to bear.
- Snubbers limit motion in any direction, often for seismic loading; thrust restraints are paired ceiling hung isolators, 180° apart, between fan discharge and duct that resist aerodynamic thrust.
- Shaft and coupling alignment makes the motor and fan shaft centerlines one line; positioners, dowel pins, machined tops and inertia bases help hold it.
- An anti-backspin device stops an idle fan from freewheeling in reverse: between the bearings on overhung designs, on a non-drive-end shaft extension on center hung designs.
- Airflow measurement with a piezometer ring reads the pressure drop across the inlet funnel; airflow varies with its square root. A pressure transmitter, sized to the application, sends that reading to the controls.
Isolation Specified, Airflow Proven
You can now calculate spring deflection, name the right spring and mounting style for a job, explain why thrust restraints and anti-backspin devices go on the order, and describe how a piezometer ring proves installed airflow. That completes the path from a selected fan to a running, verified installation.