chapter 7_measurement of ventilation system

EVALUATION OF LEV PERFORMANCE

Learning outcomes

• Describe methods for inspection, testing and evaluation of LEV performance.

• Conduct inspection, testing and evaluation methods for assessing the performance of LEV.

• Analyze and present the results of inspection, testing and evaluation methods of LEV performance to enable management for complying with relevant legislation.

Topics

• Reason • Inspection of LEV system• Testing and Examination of LEV

system• Precaution • Instrumentation • Procedure• Determination of capture or

face velocity• Determination of hood static

pressure• Types of losses• Determination of static

pressure

• Determination of hood velocity pressure & transport velocity

• Determination of air cleaners efficiency

• Determination of face efficency

• Instrumentation for fan inspection

• Type of fan• Inspection of fan• Testing of fan

RECAP…

What are the components of LEV

systems?

FUNCTIONS?

1.0 Reasons

1. Commissioning2. Proof of performance3. Balancing ventilation systems4. Baseline maintenance5. Troubleshooting ventilation systems6. Change of management

Inspection of LEV system

a) Inspection of physical condition of all component of local exhaust ventilation system

b) Observation of how the work carried out in relation to the utilization of local exhaust ventilation system

c) Smoke tube tracer test

d) Identify any thing that can be obstruction of flow

e) Observation of condition surrounding and near the hood

f) Inspection on the air cleaner device

g) Maintenance of the fan’s motor.

Testing and Examination of LEV system

At least every 12 months or at shorter interval as specifed by the designer.

Reasons:1. Commissioning2. Proof of performance3. Balancing ventilation systems4. Baseline maintenance5. Troubleshooting ventilation systems6. Change of management

a) Inspection of LEV system

b) Determination of capture or face velocity;

c) Determination of hood static pressure;

d) Determination of duct static pressure along the ducting system;

e) Determination of hood velocity pressure and transport velocity;

f) Determination of air cleaner’s efficiency; and

g) Determination of fan’s capacity and efficiency.

Testing and Examination of LEV system

Area that typically should be monitored frequently

1. Pressure drop across the air cleaner

2. Static pressure at the hoods

3. Static pressure at or across sites of frequent plugging or other problems

For hood that prevent high exposure to hazardous airborne contaminants, the hood static pressure should be measured at

least monthly.

Recommended maximum inspection intervals

Month SPh T SPend Fan inlet

Across air cleaning device

Pitot tranverse

1 X X

12 X X X X

24 X X X X X X

Not include for the system frequently have problems or controlling highly hazardous materials.

Precaution During Inspection, Testing and Examination

Inspection, testing and examination on:

1. Inside of ducts

2. Motors

3. Passages and scaffolds

4. Electrical parts

Instrument use for inspection

1) Inspection, Testing and Examination Procedure

A) Hood

1. Physical appearance of the hood

2. Obstruction of flow

3. Direction and size of opening area for exterior hoods

4. Velocity

5. Environmental monitoring

6. Hood static pressure

B) Duct system

1. Physical condition of duct outer surface

2. Physical condition of duct inner surface

3. Loose joint

4. Condition of inspection holes

5. Static pressure

6. Duct velocity

7. Temperature in duct

Inspection, Testing and Examination Procedure

C) Damper

1. Condition of damper

2. Static pressure


D) Fan

1. Condition of outer casing surface

2. Condition of inner surface of casings, impellers and guide vanes

3. Condition of belts

4. Rotating direction of fan

5. Condition of bearings


E) Air cleaner

1. Physical appearance

2. Static pressure


Testing and Examination of

LEV system

Airflow and static pressure measurement

• Change in pressure = change air flow

• Too little airflow = inadequate control by hood

• So excessive airflow = waste energy

2. Determination of capture or face velocity

- Use anemometer- Use smoke tube: show dispersion and show capture

distance (Capture velocity)- Air that enters a LEV system enters at the hood face

(the cross-sectional area of the hood at its opening) = face velocity.

Capture velocity = velocity that is necessary to capture the contaminant at its farthest possible distance from the hood.

Estimating airflow from hood static pressure

Measuring face velocities:

1. Divide an enclosing hood face into equal areas and sample the centre points of each area

2. Measure all velocities in the same plane

3. Accuracy = take velocities at each point then repeat every reading

4. Variability at a point, place the probe in position and record several reading (e.g. 20 readings at one second intervals)

5. Measured air velocity must be within 10 percent of design value of standard.

Face velocity for enclosing hood

Operation Face velocity (ft / min)Welding 150Paint spray booth 100 - 200Laboratory hood 100 – 500Abbrasive blasting room 500Belt conveyer enclosure 150 – 200Bin or hooper conveyer 150 - 200Mixer 100 – 200METALLIZ ENCLOSURE*Toxic 200 (with respirator)* Non – toxic 125 Melting furnace 150 – 200 + product of combustion, if anySwing frame grinder booth 150 Machining toxic materials 300

Measuring capture velocity:

- At nip point of the source of contaminants.

- The dotted mark shows the nip point

3. Determination of hood static pressure

Hood Static Pressure: Method of estimating air flow into an exhaust hood or duct. Method is quick, simple and practical.

Procedure of Measurement: U-tube manometer at one or more holes. The manometer is connected to each hole in turn by means of

a thick walled soft rubber tube. The difference in height of the water columns is read in inches. After hood static pressure (SPh) is known, the volumetric flow

rate is determined

Testing of Ventilation Systems 28

Points to Remember while Measuring Pressure:

Avoid pressure measurement at the heel of an elbow or other location.

Drill 2-4 pressure holes at uniform distances around the duct and obtain the average.

Tapered hood-a hole of 1/16 to 1/8 in diameter is drill on the duct at about one duct diameter away from the hood. (Vp , the average duct velocity pressure, can also be measured at the same location)

Hole should be drilled not punched.

When in use, the instrument must be pointed upstream and parallel to the duct for accurate measurement.

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30(c)d. jeff burton

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| SPh | = VP + he

SPh = – (VP + he)US Unit = inch w.g, SI unit: mw.g

Relationship between SP, VP, and Losses

Types of losses

• Entry loss• Friction loss• Loss due to elbow• Loss due to branch entry• Loss due to enlargement and contraction• Loss due to special fitting

At the hood, all available static pressure is converted to velocity pressure and hood entry loss.

Entry loss (he) : sum total of all the losses from the hood face to the point of measurement in the duct.

K = loss factor of hood; determine by manufacturer

SPh = VP + he

where he = K • VP • d (28)

SPh = VP + K•VP•d = (1+K• d) • VP

• Vena contracta = As air enters the hood, the cross-sectional area of flow contracts and forms a stream with cross-sectional area less than that of the duct.

• During this contraction, the velocity increases. The velocity pressure must also increase.

• It is clear that some static pressure must be converted to velocity pressure. During this conversion of static pressure to velocity, a loss of energy results: he

Hood efficiency:

If all SP change to VP, he = 0

Example 6-4 The hood static pressure is measured at SPh = -29.0 mm w.g. and the average duct velocity pressure is measured to be VP = 8.5 mm w.g. What is the Coefficient of Entry, Ce?

Ce = (VP/|SPh|)0.5 = (8.5/29.0) = 0.54 (unitless)

The hood provides 54% of the ideal flow rate.

4. Determination of static pressure along the duct system

Static Pressure:

Pressure which tends to burst or collapse a duct

Positive when > atmospheric

Negative when < atmospheric


Pressure Measurement:

At any point in the exhaust system, three air pressures exist

TP = SP + VP

Where:

TP = Total Pressure in “wg or pascal/psiSP = Static Pressure in “wg or pascal/psiVP =Velocity Pressure in “wg or pascal/psi


Instruments Used for Measuring Pressure:

Simple Piezometer Pressure sensor( U-tube Manometer, Inclined

manometer, portable digital manometer, aneroid gauges

Water gauge Reading pressure gauge Pitot tube

Measure the static pressure in the duct at the inspection holes - the duct before and after the position where dust tend to be piled up

The holes should be at least 7.5 duct diameters down stream from any disturbance.

If not possible, then four holes should be drilled 90o apart around the duct and static pressure are measure at each holes and averaged

SP points measurement

• Before and after elbow• Before branch entry • Before and after air cleaner• Before and after fan• Along long duct

Measure velocity pressure in the duct with a traverse pitot tube connected to a manometer

Velocity pressure can be converted to velocity by using conversion formula

5. Determination hood velocity pressure and transport velocity

At STP condition

When the contaminant is captured by the hood system and enters the ductwork, a minimum transport velocity must be maintained:

- to keep the contaminant from settling out of the gas flow stream and building up deposits in the ductwork.

Declines in gas flow rates could indicate that the minimum transport velocity is not being maintained.

Minimum transport velocities for different types of particulate matter:Type of pollutants Recommended transport

velocity (ft/min)Gas 1000 – 2000Light particulate loading 3000 – 3500Normal particulate loading

3500 – 4000

Location of Measuring Point- depend on shape and size

Velocity distribution is not uniform within the duct

For round ducts : Two traverses at right angle should be made.

6’’ and smaller - minimum 6 traverse points. 6’’ and larger - minimum 10 traverse points.

For round duct

For rectangular :

- The cross-section is divided ducts into equal areas

- A reading is taken at the centre of each area.

- At least 16 readings should be taken,

- Distance between measuring points = not exceed 6 inches.

- Best location to perform a traverse is at least 7.5 dia. downstream from any major disturbance = laminar flow

- If it less than 7.5 diameter, another traverse has to be made at a second location and compare the results.

- within 10% of range, the results are acceptable.

- Velocity will increase with distance from inner surface to a maximum value at the centre of duct

Essential Data to Be Collected: Area of the duct at the traverse location. Velocity pressure at each point in the traverse. Temperature of the air stream

Determination of transport velocity:

The minimum transport velocity is important to prevent settling of particles within the duct.

this minimum transport velocity affects only the large particles since smaII particles fol low the air movement and do not settle.

Use anemometer

6. Determination of air cleaners efficiency

Will learn in air pollution technology subject….

Next semester…..

7. Determination of fan efficiency

The fan speed, expressed as revolutions per minute (rpm)

Relationship between fan speed and air flow rate

Fan speed (rpm) Air flow rate (acfm)800 16 000900 18 000

1000 20 0001100 22 0001200 24 000

First fan law:

Fan Static Pressure:FSP = Fan TP – VP out

= SP out – SP in – VP in

The air stream moving through the fan experiences a static pressure rise due to the mechanical energy expended by the rotating fan wheel. The static pressure at the outlet is always higher than the static pressure at the inlet.

The fan SP is related to the square of the fan speed as indicated in the second fan law shown below. The fan static pressure rise is usually expressed in units of inches of water column.

Second Fan Law

The Fan Laws can be applied when:

• The fans have the same design and geometric shape.

• The fans have not been altered in shape or form. • The system characteristic curve has not changed.

A portion of a ventilation system is shown in Figure 2. At a fan speed of 900 rpm, the fan static pressure rise is 16.5 in. W.C. and the gas flow rate is 8,000 ACFM. Suppose the fan speed changes while the rest of the system remains the same. Estimate the new fan static pressure rise if the flow rate increases to 12,000 ACFM.

Solution: 1. Calculate the new fan speed, rpm2, when the flow rate is increased from 8,000 to 12,000 ACFM.

2. Calculate the new fan static pressure rise, Fan SP2, due to the higher fan speed.

Third Fan Law

Fan static pressure:

1. Must be sufficient to accelerate the air entering the hoods and,

2. To overcome the flow resistances of the hoods, ductwork, air pollution control systems, and stack at the prescribed hood, ductwork, and air pollution control system airflow velocities.

Estimate new operating conditions for the system if the system is changed as shown:

rpm Q (cfm) FTP (in w.g) AHP

Initial 1000 5000 8 6.29

1 1100

2 6000

3 10.0

Fan total pressure:FTP = Tp outlet – TP inlet

= Sp out + Vp out – Sp in –VP in

Fan Power:

SHP = BHP X KDL

RATED HP = SHP X SF

AHP = air horsepowerBHP = Break horsepowerSHP = Shaft horsepowerME = Mechanical efficiency (Usually 0.6)KDL = Drive Loss Factor (1.15 Pulley, 1.05 Direct)SF = Safety Factor (1.1)

Exercise:Estimate the AHP, BHP, SHP & the

rated HP motor you would choose for the following system:

FTP: 10.00 in w.gQ: 5000 cfmME: 0.65KDL:1.1

Static Pressure Profile of the System

The changes in the air stream static pressure from the point of entry into the hood to the fan.

The overall static pressure drop across each component of the overall system is related to the square of the airflow rates.

The multi-rating data used to select the fan represented a subset of the total data set that defines this fan characteristic curve.

There is a specific fan characteristic curve for each fan model, model size, and speed.

The intersection of the fan characteristic curve and the system characteristic curve is illustrated as Point A.

Instrument for fan inspection and testing

1. Smoke tube: Detect leaks

2. Tachometer: Measure Fan rpm

3. Pitot tube and Manometer: Measure Vp and Sp

4. Volt meter and Amp meter: BHP determination

5. Thermometer

Inspection of fan

1. Direction of fan rotation2. Blade and casing condition3. Belt condition4. Excessive vibration of bearing or housing5. Correct coupling alignment6. Proper alignment of fan impeller7. Safety devices

Testing of fan

1. Get fan rating curve2. Check fan speed3. Detect spinning flow4. Measure fan inlet Vp

Limitations

Monitoring = consistent behavior by systems moving air through hoods.

Do not directly predict hood effectiveness in controlling exposures

Environmental and exposure sampling should be conducted

Services done by qualified IH

Conclusion

Inspection, Testing and examination of LEV system is needed to ensure the effectiveness of LEV performance

chapter 7_measurement of ventilation system

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