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Quiz: Airstreams, Total Pressure and Standard Air

Test your understanding of velocity pressure, total pressure, standard air and the airstream conditions fans handle with these review questions.

1. What is velocity pressure?

  1. The pressure air exerts equally in all directions on the walls of a duct
  2. The pressure lost across a filter or coil at the design airflow
  3. The pressure that would be produced if a moving airstream were brought to a stop
  4. The absolute pressure of the atmosphere at the job site
Show Answer

The correct answer is C. Velocity pressure (VP) is the kinetic energy of moving air expressed as a pressure: the push you feel on a flat hand held against a strong wind. It exists only because the air is moving, acts only in the direction of motion and is always positive. Option A describes static pressure, option B a pressure drop, and option D barometric pressure.

Concept Tested: Velocity Pressure

See: Velocity Pressure

2. How does the fan industry define standard air?

  1. Air with a density of 0.075 lb/ft³, very nearly dry air at 70°F and 29.92 in. Hg
  2. Air at 32°F and sea level, the coldest condition a fan normally starts in
  3. Air at 70°F at the job site's actual elevation and barometric pressure
  4. Air at the fan's operating temperature, with all moisture removed
Show Answer

The correct answer is A. Standard air is air with a density of 0.075 lb/ft³. Dry air at 70°F and a barometric pressure of 29.92 in. Hg (sea level) has very nearly this density, so it is often described as "70°F at sea level." Catalog static pressures and brake horsepower are standard-air values unless a table says otherwise, and the 4005 constant in the velocity pressure equation also assumes standard air. Job-site conditions are corrected to it with the density ratio.

Concept Tested: Standard Air

See: Standard Air

3. Which statement about total pressure along a duct system is correct?

  1. Total pressure can rise at a duct expansion, where the air slows down
  2. Total pressure equals static pressure minus velocity pressure
  3. Total pressure is always positive on the inlet side of a fan
  4. Total pressure falls steadily in the direction of flow and rises only where a fan adds energy
Show Answer

The correct answer is D. Total pressure (TP = SP + VP) is all the energy the air carries at a point. Friction and fittings turn some of it into heat, so it falls steadily along the duct, and it never rises unless a fan adds energy. At an expansion, static pressure can rise as velocity pressure converts back, but total pressure does not. On the inlet side, total pressure is usually negative, because the static pressure there is negative.

Concept Tested: Total Pressure

See: Total Pressure

4. A fan's inlet reads −1.00 in. wg static pressure and 0.30 in. wg velocity pressure. Its outlet reads +1.50 in. wg static pressure and 0.60 in. wg velocity pressure. What is the fan total pressure?

  1. 2.50 in. wg
  2. 2.80 in. wg
  3. 1.40 in. wg
  4. 3.40 in. wg
Show Answer

The correct answer is B. Find the total pressure at each connection first: inlet TP = −1.00 + 0.30 = −0.70 in. wg, and outlet TP = 1.50 + 0.60 = 2.10 in. wg. Fan total pressure is outlet TP minus inlet TP: 2.10 − (−0.70) = 2.80 in. wg. Option A ignores velocity pressure, option C drops the inlet sign so the inlet subtracts instead of adds, and option D adds both velocity pressures instead of subtracting the inlet total.

Concept Tested: Pressure Rise

See: Pressure Rise and Air Power

5. A 10,000 CFM airstream passes from a large duct into a smaller duct with no branches or leaks. What happens to the airflow and the velocity?

  1. The airflow falls in proportion to the area ratio, and the velocity stays the same
  2. The airflow and the velocity both fall, because the smaller duct adds resistance
  3. The velocity stays the same, and the static pressure doubles
  4. The airflow stays the same, and the velocity rises in proportion to the area ratio
Show Answer

The correct answer is D. The airflow continuity equation, Q = A × V, says that air entering a duct with no branches or leaks must come out the other end, so the same CFM passes every cross-section. With a smaller area, the velocity must rise in proportion. In the chapter's example, 10,000 CFM moves at 2,500 fpm in a 24 in. by 24 in. duct and at 3,600 fpm in a 20 in. by 20 in. duct.

Concept Tested: Airflow Continuity Equation

See: Velocity, Area and Continuity

6. A fixed duct system requires 2.0 in. wg at 8,000 CFM. The owner wants 10,000 CFM through the same ductwork. About how much pressure will the system require?

  1. 2.50 in. wg
  2. 3.91 in. wg
  3. 3.13 in. wg
  4. 1.60 in. wg
Show Answer

The correct answer is C. For a fixed duct system, the pressure required rises with the square of the airflow. The airflow ratio is 10,000 ÷ 8,000 = 1.25, so the pressure ratio is 1.25² = 1.5625, and 2.0 × 1.5625 = 3.13 in. wg. Option A scales pressure linearly, option B uses the cube (which applies to air power), and option D divides instead of multiplying.

Concept Tested: Duct System

See: The Duct System

7. An order asks for an "explosion-proof fan" to exhaust a room where flammable solvent vapors are present and the motor sits in the same room. Which response matches the chapter?

  1. Specify spark-resistant construction of the type the specification calls for, and separately a motor rated for the hazardous location
  2. Specify Type C spark-resistant construction, because it makes the whole fan explosion-proof
  3. Specify an explosion-proof motor only, because the airstream parts cannot cause ignition
  4. Specify Type A construction, and have the fan supplier assign the motor's class, division and group
Show Answer

The correct answer is A. Spark-resistant construction (Types A, B and C) lowers the chance that the fan's moving parts strike a spark; it does not make the fan explosion-proof. The motor is a separate decision: an explosion-proof motor rated under the National Electrical Code by class, division and group, which the owner or authority having jurisdiction determines, not the fan supplier. Type C is the minimal level of spark resistance, and a steel impeller rubbing a steel inlet can throw sparks.

Concept Tested: Explosive Atmosphere

See: Explosive Atmospheres

8. Which material does Twin City Fan's current catalog describe as a more economical answer than stainless steel for moisture-laden, corrosive air?

  1. Painted mild steel
  2. Fiberglass-reinforced plastic (FRP)
  3. Cast aluminum
  4. Non-ferrous spark-resistant alloy
Show Answer

The correct answer is B. The current catalog states that its fiberglass fans are designed for moisture-laden, corrosive or chemically contaminated air, and that FRP offers a more economical solution than stainless steel or other exotic alloys. Mild steel is the standard material and can lose metal quickly in corrosive service. Aluminum and non-ferrous construction address spark resistance, not corrosion. The final material is always confirmed against the chemicals, temperature and current catalog limits.

Concept Tested: Corrosive Airstream

See: Corrosive Airstreams

9. A fan selected for 300°F oven exhaust at sea level draws 6.97 bhp at its operating temperature, but 10.0 bhp at the catalog point. Why must the motor be checked against the 10.0 bhp figure?

  1. Hot air carries more moisture, which adds load to the impeller during operation
  2. The fan moves more CFM in cold air, so it needs more power to start
  3. When the oven starts cold, the air is near standard density and the fan draws the full catalog horsepower
  4. A motor loses part of its rated horsepower when it runs in a hot airstream
Show Answer

The correct answer is C. Power scales with density. At 300°F the density ratio is 0.697, so the fan draws only 10.0 × 0.697 = 6.97 bhp in service. When the oven starts cold, the air is near 70°F and standard density, so the fan draws the full 10.0 bhp until the air heats up. A motor sized only for the hot condition would be overloaded at every cold start. CFM at a given speed does not change with density.

Concept Tested: Hot Airstream

See: Hot Airstreams

10. A commercial laundry exhaust carries warm, steam-laden air with lint, and the fan sits at the low point of the duct. Which set of conditions and provisions fits this airstream?

  1. Hot only: high-temperature construction with a shaft cooler and heat shield
  2. Corrosive only: a fiberglass fan with a stainless steel shaft
  3. Wet only: a standard weep hole and an open drip-proof motor
  4. Wet and particulate-laden: a piped housing drain, an enclosed motor, and lint noted on the order
Show Answer

The correct answer is D. Steam-laden air at the duct's low point will condense and collect in the fan, so a housing drain at the lowest point, piped to a floor drain, is better than relying on a weep hole. An open drip-proof motor should not be used in wet atmospheres. Laundry exhaust also carries lint, which makes the airstream particulate-laden as well. Real airstreams often combine conditions, so each one is identified and noted separately.

Concept Tested: Wet Airstream

See: Wet Airstreams