The inlet and outlet conditions at an exhaust fan of a local exhaust ventilation system are given below: Industrial Ventilation Homework, NUS, Singapore

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University National University of Singapore (NUS)
Subject industrial ventilation

Fans & Motors

 Question 1: The inlet and outlet conditions at an exhaust fan of a local exhaust ventilation system are given below:

Static pressure at the fan inlet (SPin) = – 4.00” w.g. (-102 mm w.g. or -999 Pa)

Static pressure at the fan outlet (SPout) = + 0.80” w.g. (+20 mm w.g. or +196 Pa)

Velocity pressure at the fan inlet (VPin) = = 0.50” w.g. (13 mm w.g. or 127 Pa)

Velocity pressure at the fan outlet (VPout) = 0.50” w.g. (13 mm w.g. or 127 Pa)

Determine

a)     the fan total pressure (FTP)

b)     the fan static pressure (FSP)

(Answer in any unit)

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Question 2

a.Given the following design parameters of a local exhaust ventilation system at STP.

Air flow rate (Q) = 5,000 scfm (2.36 scms)

Fan total pressure (FTP) = 10.0” w.g. (254 mm w.g. or 2489 Pa)

Mechanical efficiency (ME) of fan = 0.65

Drive loss factor (Kdl) = 1.15

  1. What is the required air horsepower (AHP) and air kilowatt (AkW)?
  2. What is the required brake horsepower (BHP) and brake kilowatt (BkW)?
  3. What is the required shaft horsepower (SHP) and shaft kilowatt (SkW)?
  4. What rated horsepower (RHP) and rated kilowatt (RkW) would you choose?

B. The air flow rate in a local exhaust ventilation system is 0.25 cms. The static pressure at the fan inlet is -1,500 Pa and the static pressure at the fan outlet is +100 Pa. Assume STP.

  1. a) Determine the fan total pressure (in Pa) and the air kilowatt (AkW).
  2. b) If the air flowrate is increased to 0.3 cms, what will be the fan total pressure (in Pa) and the air kilowatt (AkW)?

Question 3

A. A fan operating at a rotating speed of 1,250 rpm delivers 240 cmm (8,469 cfm) of air flowrate. What rotating speed is required to increase the air flowrate to 290 cmm (10,233 cfm)? Assume STP.

B. A fan when operating at a rotating speed of 1,800 rpm develops a static pressure of 127 mm w.g. (5” w.g. or 1245 Pa). If the fan speed is increased to 2,000 rpm, what would be the new static pressure? Assume STP.

C. A fan operating at 830 rpm delivers 8,000 cfm (3.78 cms) at 6.0” w.g. (152 mm w.g. or 1494 Pa) static pressure and requires 11.5 horsepower (8.6 kW). It is desired to increase the flow rate to 10,000 cfm (4.72 cms) in the same system. What would be the horsepower HP or kilowatt kW required?  Assume STP.

 Question 4

A. Determine the fan speed in rpm that will deliver 6,300 cfm at 6.5” w.g. static pressure (SP) from the following fan rating table. Assume STP.

Flowrate6” w.g. SP7” w.g. SP
cfmrpmBHPrpmBHP
554010688.6011459.93
604010889.50116010.88
6550109510.50117111.98
7060112511.60118813.06

B. Using the four sets of fan curves (fan total pressure vs air flowrate) provided below, find the most appropriate fan (i.e. state the model no – Buffalo XXX BL) with an appropriate rpm for a system exhausting 15,000 cfm and at a FTP = 2.0” w.g. Assume STP.

Question 5

A local exhaust ventilation system is to be installed at 2,000 m (6,560 ft) above sea level and operated at 15 °C (59 °F). The volume flow rate for the system is 2.13 cms (4,500  cfm). The duct system is designed as if at standard conditions; the fan total pressure is 55.88 mm w.g. (2.20”w.g.), and the shaft kiloWatt is 2.33 kW (3.12 HP).

  1. What flowrate (value of Q) should be selected when choosing the fan?
  2. What shaft horsepower (actual SHP) should be used in the selection of the motor?
  3. What fan total pressure would be expected to measure after installation?

 Ventilation in Confined Spaces

Question 1

A. An average carbon monoxide (CO) emission rate per vehicle is 10 liters/km or 10,000 cc/km. Assume 500 vehicles passing through a 2 km tunnel every hour and a forced or dilution ventilation is provided at 90 m3/s (cms) in the tunnel by air blowers located within the tunnel, what is the steady state concentration of CO inside the tunnel? Assume perfect mixing (dilution or mixin. In Question A above, what is the steady state concentration of CO if the mixing or dilution factor k is 4?

Question 2

A. The vapour pressure of gasoline is 62 kPa at 25°C and 1 atm (101.3 kPa). Its lower explosive limit (LEL) is 1.2 %vol and upper explosive limit (UEL) is 7.1 %vol. Determine the saturation or maximum concentration of gasoline vapors in air at 25°C. Is the saturation concentration within the flammable range? Why?

B. The concentration of gasoline vapors in a 220 liters (0.22 m3) oil drum measured with a flammable gas meter at 25°C is 5 %vol. Determine the mass (in g) of gasoline vapors within the drum. Given: average MW of gasoline = 95.

Question 3

Methane (natural gas) is infiltrating into a 10 m3 manhole. Given: the normal oxygen level in air is 20.9 %vol. Determine the volume of methane gas (in m3) required to deplete the oxygen level to

(a) 19.5 %vol and

(b) 10 %vol.

Question 4

A. A 5 m3 manhole is contaminated with carbon monoxide (CO) generated by an internal combustion engine. If the concentration of CO is 25,000 ppm and the dilution air flowrate is 20 cmm, how long will it take to dilute the CO concentration to its permissible exposure limit (PEL) of 25 ppm? Assume perfect mixing (k=1).

B. In Question A above, what is the time required to dilute the CO concentration to 25 ppm if the dilution factor k is 4?

 Question 5

A. A 10 m3 sewage manhole is contaminated with 5,000 ppm of hydrogen sulfide (H2S).  Determine the dilution air flow rate Q required to dilute the H2S to its permissible exposure limit (PEL) of 10 ppm in 5 minutes’ time. Assume perfect mixing (k=1).

B. In Question A above, what is the dilution air flow rate Q required if the dilution or mixing factor is 6?

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