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Quiz: Reading Catalogs and Performance Tables

Test your understanding of Twin City Fan catalogs, performance tables, rating conditions and the checks that turn a table reading into an order line.

1. In a Twin City Fan catalog, what does a fan size number identify?

  1. One impeller diameter within a particular model
  2. The outlet dimension of the housing, in inches
  3. The housing width, which sets the arrangement
  4. The inlet diameter, which sets the inlet duct size
Show Answer

The correct answer is A. Fan size is the number Twin City Fan uses to identify one impeller diameter within a model; every other dimension follows from it. It is not the outlet dimension, the inlet diameter or the housing width. In Catalog 902, for example, size 913 is a 22.625-inch impeller, so the digits after the 9 are not the diameter. Because numbering belongs to each model's catalog, always read the diameter from that model's own performance page.

Concept Tested: Fan Size

See: Fan Size

  1. A ducted inlet and a free outlet
  2. A free inlet and a free outlet
  3. A duct on the fan outlet, with the inlet either free or ducted
  4. A duct on the fan inlet, with the outlet either free or ducted
Show Answer

The correct answer is C. Type B is a free inlet with a ducted outlet and Type D is ducted on both sides, so a B and D rating assumes there is a duct on the outlet. The catalog wording is "Free or ducted inlet, ducted outlet." A fan discharging straight into a plenum room breaks this rating condition, because the outlet velocity pressure that a duct would partly recover is lost, and the fan will not deliver the tabulated static pressure at the tabulated speed.

Concept Tested: Rating Conditions

See: Rating Conditions

  1. Class 22
  2. Class 32
  3. Class 45
  4. Class 22 with a derated maximum speed
Show Answer

The correct answer is B. A class is defined by a maximum safe speed for the size, so the class comes from comparing the cell's RPM with the footer limits. 2,879 RPM is above the Class 22 limit of 2,812 and at or below the Class 32 limit of 3,337, so the fan must be Class 32. The static pressure column alone does not decide the class. Derating applies to high temperature and cannot raise a class limit.

Concept Tested: Reading a Performance Table

See: Reading a Performance Table

4. In the 12 in. SP column of the RBA 913 table, 3,720 CFM needs 2,128 RPM and 4,092 CFM needs 2,167 RPM. Using linear interpolation, about what speed is needed for 3,906 CFM at 12 in. SP?

  1. 2,128 RPM
  2. 2,157 RPM
  3. 2,167 RPM
  4. 2,148 RPM
Show Answer

The correct answer is D. The fraction is (3,906 − 3,720) ÷ (4,092 − 3,720) = 186 ÷ 372 = 0.5, so the speed is 2,128 + 0.5 × (2,167 − 2,128) = 2,147.5, about 2,148 RPM. Options A and C take the nearest printed cell instead of interpolating between the two bracketing rows. Option B is the chapter's answer for 4,000 CFM, which uses a fraction of 0.753. For a quotation, confirm the final speed in Fan Selector.

Concept Tested: Interpolation

See: Interpolation

5. A job needs 3.0 in. wg static pressure at operating conditions where the ERG-100 correction factor is 0.75. At what static pressure should you enter the standard-air performance table?

  1. 2.25 in. wg
  2. 3.0 in. wg
  3. 4.0 in. wg
  4. 5.33 in. wg
Show Answer

The correct answer is C. Tables are at standard air, so the required static pressure is divided by the correction factor before entering the table: 3.0 ÷ 0.75 = 4.0 in. wg. Airflow is not corrected. Option A multiplies instead of dividing, option B skips the correction, and option D divides by the factor twice. After reading the table, the RPM applies as read and the BHP is multiplied by the factor to get power at operating conditions.

Concept Tested: Actual Operating Conditions

See: Actual Operating Conditions

6. Why does the class shading in a catalog performance table change from cell to cell instead of following straight columns of static pressure?

  1. Because each class boundary is a maximum speed, and a cell's RPM depends on both its airflow and its pressure
  2. Because each class is defined by a maximum static pressure that varies with outlet velocity
  3. Because the shading marks the cells with the highest static efficiency for each class
  4. Because each class is defined by a range of outlet velocities printed in the OV column
Show Answer

The correct answer is A. A class is defined by a maximum safe speed for the size, so the class boundary in a table is really a speed boundary. RPM rises with both airflow and pressure, so the boundary does not follow a straight row or column. On RBA 913, 32 in. SP is Class 32 at 4,464 CFM (3,321 RPM) but Class 45 one row lower (3,340 RPM). Maximum static efficiency is marked by underlining, not shading.

Concept Tested: Class Shading in Tables

See: Class Shading in Tables

7. On the RBA 913 table at 4,836 CFM, static efficiency is about 68% at 28 in. SP (the underlined cell) but only about 48% at 4 in. SP. What does this tell you about a duty of 4,836 CFM at 4 in. wg?

  1. The RBA 913 is the right choice, because it delivers the required airflow and pressure
  2. The fan should be run faster at the 4 in. duty to move it closer to peak efficiency
  3. The efficiency difference comes from a class change between the two cells
  4. The duty lies far from this size's best efficiency, so another size or family should be compared
Show Answer

The correct answer is D. A size can deliver a duty point and still be the wrong size. The fan selection range is the part of the table where the fan runs near its best efficiency, which Catalog 902 marks by underlining. At 4 in. wg the RBA 913 works at less than half the static efficiency it reaches at 28 in., because it is built for higher pressure. Comparing other sizes or families at the same duty, in the tables or in Fan Selector, finds a better fit.

Concept Tested: Fan Selection Range

See: Fan Selection Range

8. Catalog 902's availability table lists these arrangements for RBA 913: Class 22, Arrangements 1, 4, 8, 10, 4VI, 9A, 9B and 9F; Class 32, Arrangements 1, 4, 8, 9, 4VI and 9F; Class 45, Arrangements 1, 8, 9 and 9F. A customer orders an RBA 913, Class 45, Arrangement 4. What is the correct response?

  1. Accept it, because Arrangement 4 is offered for RBA 913 in other classes
  2. Decline it, because Class 45 lists only Arrangements 1, 8, 9 and 9F; offer one of those
  3. Accept it after changing the class to 32, because Class 32 lists Arrangement 4
  4. Decline it, because size 913 is built only in Class 22
Show Answer

The correct answer is B. Model, size, class and arrangement must be checked together against the availability table. Arrangement 4 is not in the Class 45 list, so this designation is not buildable; a Class 45 RBA 913 needs its own shaft and bearings, as in Arrangements 1, 8, 9 or 9F. Option A checks the arrangement one field at a time. Option C ignores the fact that the duty may need Class 45 speed. Size 913 is offered in Classes 22, 32 and 45, so D is wrong.

Concept Tested: Model Code

See: Model Code

9. A hot fan absorbs 8.96 BHP at its 300°F operating condition but 12.87 BHP when it starts in 70°F air at the site. The belt drive loses an illustrative 4 percent. Which motor choice is best justified?

  1. 10 HP, because the hot BHP plus drive loss is only 9.33 hp
  2. 10 HP, because the motor's service factor will carry the brief cold start
  3. 15 HP, because the cold BHP plus drive loss is about 13.41 hp
  4. 20 HP, because drives must be rated for 150 percent of motor horsepower
Show Answer

The correct answer is C. The motor must cover the densest air the fan will start in, plus transmission losses: 12.87 ÷ 0.96 = 13.41 hp, so the next standard rating is 15 HP. Option A sizes from the hot condition and would overload on every cold start. Service factor is margin, not capacity (B). The 150 percent rule (D) in Catalog 300 applies to drives and belts, not to the motor rating. A smaller motor is justified only by a deliberate cold-start control.

Concept Tested: Motor Selection

See: Motor Selection

10. Fan Selector returns a BC-SW size 365 at 1,150 RPM for a standard-air duty. Catalog 300 gives size 365 maximum speeds of 995 RPM for Class I and 1,283 RPM for Class II. Which order line is composed correctly?

  1. BC-SW 365, Class II, 1,150 RPM, in an arrangement and temperature Catalog 300 permits, with drive loss and accessory effects added before the motor is chosen
  2. BC-SW 365, Class I, 1,150 RPM, because Fan Selector has already chosen the class, with the motor sized from the BHP Fan Selector reports
  3. BC-SW 365, Class III, 1,150 RPM, because any selection above 1,000 RPM needs the heavier class, with the motor sized from the BHP Fan Selector reports
  4. BC-SW 365, Class II, 1,150 RPM, with the motor sized directly from the catalog BHP, because catalog BHP already includes drive losses
Show Answer

The correct answer is A. Since 995 < 1,150 ≤ 1,283, the fan must be ordered Class II, and the catalog must confirm that the arrangement and temperature are allowed. Catalog BHP excludes transmission losses and appurtenance effects, so both are added before choosing the motor, which rules out D. Class I (B) is below the selection speed, and nothing in the catalog requires Class III (C). The software computes the point; the catalog decides whether it is allowed.

Concept Tested: Fan Selector Software

See: Fan Selector Software