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Quiz: Inlets, Outlets and Flow Control

Test your understanding of inlet and outlet hardware and the methods for controlling a fan's airflow with these review questions.

1. What is an evasé?

  1. A loose flange shipped with the fan that matches its bolt pattern for field attachment to the duct
  2. A sheet-metal box on the fan inlet that turns air entering from the side into the impeller
  3. A strip inside the housing near the inlet funnel that reduces turbulence
  4. A section of ductwork on the fan discharge, with an outlet larger than its inlet, that lowers outlet velocity and raises static pressure capability
Show Answer

The correct answer is D. FanPedia describes the evasé as ductwork attached to the fan discharge that expands the outlet area to reduce outlet velocity and increase static pressure capability. Slowing the air converts part of its velocity pressure into static pressure (static regain). Option A describes a companion flange, option B an inlet box, and option C a wiper bar.

Concept Tested: Evasé

See: Evasé

2. According to FanPedia, how does an opposed blade outlet damper differ from a parallel blade outlet damper?

  1. It costs about 10% more and is recommended for modulating airflow over the entire range
  2. It costs about 10% less and is recommended only from full open to about 75% open
  3. It is available only with Type 2 airfoil blades, while parallel blade dampers use flat blades
  4. It is rated to 750°F, while parallel blade dampers are limited to room temperature
Show Answer

The correct answer is A. FanPedia states that opposed blade dampers cost about 10% more and are recommended where volume is modulated over the entire range, because airflow follows control arm movement more closely. Option B describes the parallel blade damper's recommended range with the cost reversed. Both damper types come in Type 1 flat and Type 2 airfoil blades (C), and both are available to 750°F construction (D).

Concept Tested: Opposed Blade Damper

See: Opposed Blade Damper

3. Why does an inlet box damper save more power at reduced load than an outlet damper?

  1. It closes completely, so the fan stops moving air and draws no power
  2. It sits upstream of the impeller, where the air is cooler and denser
  3. Its tilted blades pre-spin the air in the direction of impeller rotation, so the fan produces a lower curve and the impeller does less work
  4. It adds more resistance than an outlet damper, which moves the operating point farther left
Show Answer

The correct answer is C. When partly closed, the inlet box damper's parallel blades turn the air in the direction of impeller rotation before it reaches the blades. The impeller then adds less spin, so the fan produces a lower curve and draws less power. That makes it a fan-curve method, not just added resistance. Adding resistance (D) is what an outlet damper does, and it saves little power. Options A and B are not the mechanism the chapter describes.

Concept Tested: Inlet Box Damper

See: Inlet Box Damper

4. During commissioning of an EC exhaust fan, why does the technician set the maximum speed on the GridPoint controller before switching the motor to 0-10V mode?

  1. Because the motor cannot start in 0-10V mode unless a maximum speed is stored
  2. So the building automation system can never drive the fan beyond the design condition
  3. So the controller can be left plugged in permanently to regulate the fan
  4. Because the 0-10V signal sets only the minimum speed of the motor
Show Answer

The correct answer is B. The controller lets a user view and set both the operating speed and the maximum speed, and switch between constant speed and 0-10V modes. Capping the maximum speed before handing control to the building automation system protects against over-exhaust and noise. The controller is plug and play, so it is unplugged and moved to the next fan (C is wrong). Options A and D are not functions the chapter describes.

Concept Tested: EC Motor Speed Controller

See: EC Motor Speed Controller

5. Why should an outlet silencer be placed after a short straight run of duct rather than bolted tight to the fan outlet?

  1. Air leaving the outlet has an uneven velocity profile, concentrated toward the outside of the scroll, which raises the silencer's pressure loss above its rating
  2. Bolting it to the outlet would block the access door on the housing
  3. The silencer's weight must never be carried by the fan outlet flange
  4. Noise is loudest a short distance from the fan, so the silencer works best there
Show Answer

The correct answer is A. Air leaving a fan outlet does not have a uniform velocity profile for some distance downstream. A silencer bolted directly to the outlet sees that uneven flow, and its pressure loss rises above its published rating, another form of system effect. A short straight duct, or the silencer manufacturer's spacing guidance, keeps the loss near its rating. Options B and D are not reasons the chapter gives, and the support leg FanPedia shows (C) belongs to the inlet silencer.

Concept Tested: Outlet Silencer

See: Outlet Silencer

6. A drawing shows a fan viewed from the inlet side, with the inlet box opening on the right and air entering horizontally. What is the inlet box position?

  1. 90, horizontal right intake
  2. 180, bottom up intake
  3. 270, horizontal left intake
  4. 315, angular down intake
Show Answer

The correct answer is C. Inlet box positions are determined from the drive side, on a dial with 360 at the top increasing clockwise. A view from the inlet side mirrors left and right, so an opening on the right seen from the inlet side is on the left seen from the drive side: position 270, horizontal left intake. Option A is the common error of reading the drawing as drawn. Options B and D place the opening at the bottom and upper left.

Concept Tested: Inlet Box

See: Inlet Box

7. A fan delivers 8,010 CFM through a 2.0 ft² outlet at standard air. An evasé expands the outlet to 4.0 ft². By about how much does the evasé reduce the velocity pressure leaving the system?

  1. 0.25 in. wg
  2. 0.75 in. wg
  3. 0.50 in. wg
  4. 1.00 in. wg
Show Answer

The correct answer is B. Velocity is V = Q/A and VP = (V/4005)². Without the evasé, V = 8,010 / 2.0 = 4,005 fpm, so VP = 1.00 in. wg. With it, V = 8,010 / 4.0 = 2,002.5 fpm, so VP = 0.25 in. wg. The reduction is 1.00 − 0.25 = 0.75 in. wg. Option A is the new velocity pressure, not the reduction, and D is the original value. Only part of the 0.75 in. wg is recovered as static pressure.

Concept Tested: Evasé

See: Evasé

8. A belt-driven fan uses 50 BHP at design. The owner slows it with a VFD to deliver 70% of design airflow on a system with no fixed static pressure. Using the fan laws, about how much power will it need?

  1. 35.0 BHP
  2. 24.5 BHP
  3. 43.0 BHP
  4. 17.2 BHP
Show Answer

The correct answer is D. Airflow is proportional to speed, so 70% airflow needs 70% speed, and power varies with the cube of speed: 0.7³ = 0.343, and 0.343 × 50 ≈ 17.2 BHP. Option A assumes power falls in proportion to airflow, and B uses the square, which is the pressure relationship. Option C is roughly what an outlet damper would need (86% of design in the chapter's illustrative comparison), since a damper leaves the fan at full speed.

Concept Tested: Speed Control

See: Speed Control

9. At 70% airflow, the chapter's illustrative comparison has an outlet damper saving about 14% of the power and speed control about two-thirds. What best explains the gap?

  1. The damper's blades leak, so part of the airflow bypasses the control
  2. The damper leaves the fan at full speed and only steepens the system curve, so the fan makes pressure the damper throws away; slowing the fan produces a smaller curve, and power falls with about the cube of speed
  3. Speed control changes the system curve while the damper changes the fan curve
  4. The damper only works on hot or dirty air, where fan efficiency is lower
Show Answer

The correct answer is B. A damper adds resistance, so the operating point slides left along the unchanged fan curve to a higher pressure that the damper then wastes. Speed control produces a smaller fan curve and the operating point moves down the unchanged system curve, so the pressure is never produced. Option C reverses the two mechanisms. Blade leakage (A) and airstream type (D) do not explain the difference.

Concept Tested: Flow Control

See: What Flow Control Means

10. Warehouse wall exhaust fans with gravity dampers will now run at reduced speed on VFDs for long periods. What problem should you expect, and what is the remedy?

  1. The dampers will stay fully closed; remove them and add a manual damper
  2. The dampers will slam shut repeatedly; replace them with butterfly gravity dampers
  3. The dampers will open only partway at low airflow and flutter; recommend a motorized damper whose actuator powers the blades open
  4. The fans will stall; add an opposed blade damper to raise the system resistance
Show Answer

The correct answer is C. A gravity damper relies on airflow to hold its blades open. At low airflow the blades open only partway and may flutter, adding noise. FanPedia notes that motorized dampers are furnished with an actuator that powers the blades open rather than relying on the airstream, which matters most in low-flow conditions. A manual damper (A) cannot close automatically when the fan stops. Butterfly dampers (B) are integral to roof upblast fans and also open by airflow, and option D adds resistance without solving the flutter.

Concept Tested: Motorized Damper

See: Motorized Damper