Quiz: Air, Pressure and the Fan¶
Test your understanding of airflow, pressure, fan speed, horsepower and air density with these review questions.
1. What does airflow volume measure?¶
- The weight of the air contained in a duct, in pounds
- The volume of air passing a point in a unit of time, in cubic feet per minute
- The speed of the air at one point in a duct, in feet per minute
- The total volume of air inside a room, in cubic feet
Show Answer
The correct answer is B. Airflow volume is a rate: not how much air is in the duct, but how much air passes a point each minute. In the United States it is stated in cubic feet per minute (CFM). Weight in pounds is mass, which depends on density as well as volume. Speed in feet per minute is air velocity. A room's volume in cubic feet is a quantity, not a rate.
Concept Tested: Airflow Volume
2. Which statement correctly defines one inch of water gauge (in. wg)?¶
- The pressure that holds up a column of mercury one inch tall
- The pressure needed to move one cubic foot of water per minute
- About one twenty-seventh of atmospheric pressure at sea level
- The pressure that holds up a column of water one inch tall
Show Answer
The correct answer is D. An inch of water gauge is the pressure that holds up a column of water one inch tall, read on a U-tube manometer as the difference between the two water levels. It is a very small pressure: atmospheric pressure at sea level is about 407 in. wg, and 1 psi is 27.7 in. wg, so option C confuses the psi conversion with the atmosphere. Inches of mercury are used for barometric pressure, not for fan pressures.
Concept Tested: Inches of Water Gauge
3. A supply fan draws outdoor air through a louver, a filter and a coil before the air reaches the fan inlet. What is the static pressure at the fan inlet relative to the room?¶
- Negative (suction), because the inlet-side components have each used up some pressure
- Positive, because the fan pushes on the air in both directions
- Zero, because static pressure exists only where the air is not moving
- Equal to the outlet static pressure, because the airflow is the same on both sides
Show Answer
The correct answer is A. Static pressure is negative (suction) on the inlet side of a fan and positive on the discharge side. In the chapter's example, the louver, filter and coil drops of 0.15, 0.50 and 0.75 in. wg add up, so the fan inlet sits at −1.40 in. wg. Static pressure acts whether or not the air is moving, and the fan raises it from the inlet value to the outlet value rather than keeping it equal.
Concept Tested: Static Pressure
See: Static Pressure
4. Why do fan catalogs treat the airflow entering a fan as equal to the airflow leaving it?¶
- Fans are rated with sealed housings, so no air can leak out between inlet and outlet
- Static pressure and velocity pressure cancel each other inside the housing
- A fan raises the pressure only a few percent above atmospheric, so the air's volume barely changes
- Catalogs rate fans at standard air, a condition at which air cannot be compressed
Show Answer
The correct answer is C. What separates a fan from a compressor is how little it raises the pressure. Even a heavy-duty fan at 10 in. wg adds only about 2.5 percent to the 407 in. wg of the atmosphere, and a typical HVAC fan at 2 in. wg adds about 0.5 percent. The air is squeezed so little that inlet and outlet CFM are taken as the same number, and the fan is rated at its inlet. Standard air can still be compressed; it simply fixes the density used for ratings.
Concept Tested: Fan
See: The Fan
5. A duct 18 in. by 12 in. carries 3,000 CFM. What is the average air velocity in the duct?¶
- 2,000 feet per minute
- 13.9 feet per minute
- 1,500 feet per minute
- 4,500 feet per minute
Show Answer
The correct answer is A. First convert the duct size to square feet: (18 × 12) ÷ 144 = 1.5 ft². Then V = Q ÷ A = 3,000 ÷ 1.5 = 2,000 feet per minute. Option B comes from dividing by the duct dimensions in square inches without converting, the slip the chapter warns about. Option D multiplies by the area instead of dividing, and option C uses the area as if it were 2 ft².
Concept Tested: Air Velocity
See: Air Velocity
6. Which horsepower value appears in a catalog performance table as the power that must be delivered to the fan shaft?¶
- Air horsepower
- Brake horsepower
- Motor nameplate horsepower
- Synchronous horsepower
Show Answer
The correct answer is B. Brake horsepower (BHP) is the power the fan shaft needs, and it is the number in a catalog performance table. Air horsepower is the smaller useful power delivered to the air, since no fan is perfectly efficient. Motor horsepower is the nameplate rating of the standard-size motor chosen to supply the brake horsepower, ideally loaded between 50 and 100 percent. "Synchronous" describes a motor speed, not a horsepower.
Concept Tested: Horsepower
See: Horsepower
7. If a fan's speed is increased and nothing else changes, how do its airflow, pressure and power respond?¶
- All three rise in direct proportion to the speed
- Airflow rises with the square of speed, pressure in proportion to speed, and power with the cube
- Airflow stays the same, while pressure and power rise with the square of speed
- Airflow rises in proportion to speed, pressure with its square, and power with its cube
Show Answer
The correct answer is D. Fan speed is the most powerful lever on fan performance: airflow rises in proportion to speed, pressure with the square of speed, and power with the cube. That is why the chapter's belt-drive example warns that a 10 percent speed increase raises power by about a third (1.1³ = 1.33), so the motor must be rechecked. Chapter 25 develops these relationships as the fan laws.
Concept Tested: Fan Speed
See: Fan Speed
8. A plant at 5,000 ft elevation (pressure ratio 0.832) needs 2.5 in. wg of actual static pressure from a fan handling 70°F air. At what static pressure should the fan be looked up in a standard-air catalog?¶
- 2.08 in. wg
- 2.50 in. wg
- 3.00 in. wg
- 3.33 in. wg
Show Answer
The correct answer is C. At 70°F the temperature factor is 1.000, so the density ratio is 0.832. A catalog is printed at standard air, so convert the requirement to catalog conditions by dividing: 2.5 ÷ 0.832 = 3.00 in. wg. Multiplying (option A) flips the rule and undersizes the fan, and selecting at 2.50 in. wg (option B) would leave the fan about 17 percent short of the pressure the system needs.
Concept Tested: Altitude
See: Altitude
9. A fan rated at standard air runs at its catalog speed but handles 400°F air at sea level. How do its airflow, static pressure and brake horsepower compare with the catalog values?¶
- CFM falls by the density ratio; static pressure and brake horsepower are unchanged
- CFM is unchanged; static pressure and brake horsepower are multiplied by the density ratio
- CFM, static pressure and brake horsepower all fall by the density ratio
- CFM rises to make up for the thinner air; brake horsepower is unchanged
Show Answer
The correct answer is B. Each revolution of the impeller sweeps out about the same volume of air, so at a fixed speed the fan delivers the same CFM in hot air. Hot air is thinner (a density ratio of 0.616 at 400°F), so the static pressure developed and the horsepower drawn both scale with that ratio. Option A reverses the rule. Option C wrongly reduces the volume flow, and option D invents an airflow increase.
Concept Tested: Air Temperature
See: Air Temperature
10. Recirculated air inside a 400°F drying oven carries no dust, vapor or moisture. How should this airstream be classified for fan selection, and why?¶
- Clean air with its own temperature concerns, because heat alone does not make air contaminated
- Contaminated, because any airstream hotter than 130°F counts as contaminated
- Contaminated, because hot air attacks fan metal in the same way that acid mist does
- Clean air with no special concerns, because only solids in the air affect a fan
Show Answer
The correct answer is A. A contaminated airstream carries something other than clean air: solids, moisture, corrosive gases, flammable vapors or dusts, or hazardous fumes. Clean air at an extreme temperature is not contaminated by itself, but it raises its own questions about density, materials, bearings and seals. The 130°F figure is the limit for standard double-width fans with bearings in the airstream, not a definition of contamination, and corrosion comes from chemicals rather than heat alone.
Concept Tested: Contaminated Airstream