Quiz: Spring Isolators, Restraints and Field Measurement¶
Test your understanding of spring isolators, mounting styles, restraints, anti-backspin devices and field airflow measurement with these review questions.
1. According to FanPedia, what is the defining limitation of an open spring isolator?¶
- It can only be used with deflections under 1 in.
- It offers no restriction of the motion caused by aerodynamic forces
- It must be filled with elastomer to prevent metal-to-metal contact
- It can only be installed on seismic projects
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The correct answer is B. An open spring isolator is a bare coil between two load plates. FanPedia states that it does not offer any restriction of the motion caused by aerodynamic forces, so a fan on open springs can shift when its airstream pushes on it. Twin City Fan typically does not use floor mounted open springs. Ceiling hung open springs have a typical deflection of 1 in. (A). Elastomer between brackets describes a snubber (C). Seismic duty calls for seismic springs (D).
Concept Tested: Open Spring Isolator
2. What distinguishes a seismic spring isolator from a restrained spring isolator?¶
- It uses a rubber element loaded in shear instead of a coil spring
- It has no housing, so it can move freely during an earthquake
- It is always used with deflections greater than 2 in.
- Its housing nearly surrounds the entire spring so it can withstand seismic loads
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The correct answer is D. FanPedia describes the seismic spring as similar to a restrained spring, except that the housing nearly surrounds the entire spring so it can withstand the loads generated during a seismic event. Both types have typical deflections of 1 or 2 in., so option C is wrong. A rubber element in shear describes a rubber-in-shear isolator (A). An unhoused spring (B) is the open spring, which is the opposite of a seismic design.
Concept Tested: Seismic Spring Isolator
3. A fan, motor and isolation base weigh 3,200 lb and sit on four equally loaded spring isolators, each with a spring rate of 400 lb/in. What is the static deflection of each spring?¶
- 0.5 in.
- 8.0 in.
- 2.0 in.
- 1.0 in.
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The correct answer is C. Using \( \delta = (W/n)/k \), each isolator carries 3,200/4 = 800 lb, and 800/400 = 2.0 in. That is the high end of Twin City Fan's typical deflections. Option B divides the total weight by the spring rate without sharing it among the four isolators. Option A flips the division (400/800). Option D results from assuming each spring carries only 400 lb.
Concept Tested: Spring Isolator
See: Spring Isolators
4. Where does FanPedia mount an anti-backspin device on a center hung impeller design such as Arrangement 3?¶
- On a shaft extension on the non-drive end
- Between the two bearings on the pedestal
- Inside the impeller hub
- On the motor shaft, inside the motor housing
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The correct answer is A. FanPedia ties the mounting location to how the impeller is supported. On overhung designs such as Arrangements 1 and 9, the device mounts between the bearings (B). On center hung designs such as Arrangement 3, the impeller sits between the bearings, so the device goes on a shaft extension on the non-drive end. That extension is a shaft detail that belongs on the order. FanPedia gives no hub or motor-shaft location (C, D).
Concept Tested: Anti-Backspin Device
5. A plant runs two identical exhaust fans in parallel into one stack but operates only one at a time. Which fans need an anti-backspin device?¶
- Only the fan that runs most often
- Only the standby fan, because it is the only one that sits idle
- Neither fan, because a zero speed switch prevents reverse rotation
- Both fans, because each will be the idle fan at some point while air flows back through it
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The correct answer is D. When one fan runs, the shared stack connection is at a higher pressure than the idle fan's inlet, so air flows backward through the idle fan and can spin it in reverse. Each fan takes the idle role at some time, so both need the device. A zero speed switch can report the motion but does not prevent it (C). Options A and B overlook that the roles switch.
Concept Tested: Anti-Backspin Device
6. Which description matches FanPedia's standard thrust restraint?¶
- A single snubber bolted between the fan base and the floor
- Two ceiling hung type isolators, 180° apart, connecting the fan discharge to the discharge duct with threaded rod
- A rigid steel strap that replaces the flex connector at the fan outlet
- A seismic spring placed under the fan's inlet side
Show Answer
The correct answer is B. A thrust restraint prevents excessive motion caused by aerodynamic force. FanPedia's standard version uses ceiling hung type isolators, springs or spring/RIS combinations, attached to both the fan discharge and the discharge duct with threaded rod, adjusted to prevent horizontal motion. Two are used per fan, mounted 180° apart. A snubber (A) limits motion in any direction, and a rigid strap (C) would defeat isolation.
Concept Tested: Thrust Restraint
7. A fan's inlet funnel has a flow coefficient \( K = 8{,}000 \) cfm per \( \sqrt{\text{in. wg}} \). The piezometer reading is 2.25 in. wg. Using \( Q = K\sqrt{\Delta p} \), what is the airflow?¶
- 12,000 cfm
- 18,000 cfm
- 3,556 cfm
- 5,333 cfm
Show Answer
The correct answer is A. \( \sqrt{2.25} = 1.5 \), so \( Q = 8{,}000 \times 1.5 = 12{,}000 \) cfm. Option B multiplies K by the pressure difference directly instead of by its square root. Options C and D divide instead of multiply. The square-root relationship is why the chapter notes that a large drop in the reading corresponds to a much smaller drop in airflow.
Concept Tested: Airflow Measurement
8. A piezometer ring reading falls to one-quarter of its commissioning value. Assuming the same flow coefficient, what has happened to the airflow?¶
- It has fallen to one-quarter of the original value
- It has fallen to one-sixteenth of the original value
- It has fallen to one-half of the original value
- It is unchanged, because the ring measures pressure, not flow
Show Answer
The correct answer is C. Airflow varies with the square root of the pressure difference, \( Q = K\sqrt{\Delta p} \). A reading at one-quarter gives \( \sqrt{1/4} = 1/2 \), so airflow is half the original. Option A assumes a linear relationship. Option B squares the ratio instead of taking its root. Option D ignores that the ring's pressure difference is used precisely because it changes with the square of airflow.
Concept Tested: Airflow Measurement
9. What is a snubber, as FanPedia describes it?¶
- Two brackets, one on the fan or base and one on the floor, with elastomeric material between them, used with the isolators to prevent excessive movement in any direction
- A spring isolator whose housing nearly surrounds the coil
- A one-way clutch that stops the shaft from turning backward
- A flat bar that bolts to the fan's foundation holes to carry isolators
Show Answer
The correct answer is A. A snubber works alongside the isolators as a shock absorber that prevents excessive movement of the fan or base in any direction. It is often used when the fan must withstand seismic loading. Its elastomeric material prevents metal-to-metal contact, and in normal running the fan floats free of it. Option B is a seismic spring, option C is an anti-backspin device, and option D describes isolation rails.
Concept Tested: Snubber
10. A housed (non-swingout) centrifugal fan has a 44 in. impeller, and the specification asks for springs bolted directly under the fan's foundation holes. What is the best assessment?¶
- Acceptable, because direct mounting has no size limit when springs are used
- Acceptable, provided flat neoprene pads are used instead of springs
- Not usually acceptable, because direct mounting is generally limited to impellers of 36.5 in. or less; use another method such as an isolation base
- Not acceptable, because centrifugal fans can never be isolated
Show Answer
The correct answer is C. FanPedia usually limits isolators directly under the fan to impellers of 36.5 in. diameter or less. The exception is swingout housed centrifugal fans, which may be any size but only on flat pads. This fan is not a swingout, so option B misapplies the exception, and option A ignores the limit. Larger fans are isolated by other methods, such as rails, a mounting angle or an isolation base, so option D is wrong.
Concept Tested: Direct Mounted Isolators
11. When sizing a pressure transmitter for a piezometer ring, what does FanPedia's guidance require?¶
- Always choose the widest available range to avoid over-ranging
- Always choose a NEMA 4 enclosure with a display
- Mount it remotely, never on the fan
- Size it based on the application and fan performance, so its range covers the highest expected pressure difference without leaving normal readings at the bottom of the scale
Show Answer
The correct answer is D. FanPedia states that transmitters must be sized based on the application and fan performance. The range must cover the pressure difference at the highest expected airflow, yet not be so wide that normal readings sit at the bottom of the scale, which option A would cause. FanPedia offers versions with and without a display in regular and NEMA 4 enclosures (B). They can be mounted on the fan or remotely (C).
Concept Tested: Pressure Transmitter