AIR VELOCITY MEASUREMENT

AIR VELOCITY MEASUREMENT

Purpose:

To provide a procedure to measure for airflow velocity and uniformity of velocity and supply airflow rate in clean room and clean zone.

Scope:

Applicable to all Air Handling Units system, Forced Air Ventilation system, Laminar Air Flow system(Unidirectional airflow system), Dynamic pass boxes, Garment Cubicles, Air showers, Air duct, Non unidirectional airflow installation(Diffuser, Grills, Risers).

Responsibility:

All HVAC Engineers.

Definition:

Uniformity of air flow: Unidirectional air flow pattern in which the point-to-point readings of velocities are within the defined percentage of the average airflow velocity.

Unidirectional airflow: Controlled air flow through entire cross section of clean zone with steady velocity and approximately parallel streamlines.

Non-Unidirectional airflow: Air distribution where the supply air entering the clean zone mixes with the internal air by means of induction.

Laminar Air flow: Control airflow through the entire cross section of a clean zone with a steady velocity and approximately parallel streamline.

Installation: Clean room or one or more clean zones, together with all associated structures, air handling systems, services and utilities.

Air change rate: Rate of air change expressed as air change per unit time and calculated by dividing the volume of air delivered in the time unit by the volume of the space.

PROCEDURE:

AIR VELOCITY MEASUREMENT IN CLEAN ROOM HAVING NON UNIDIRECTIONAL AIR FLOW SYSTEM USING ANEMOMETER(VANE TYPE/HOT WIRE ANEMOMETER)

1.      Switch ON the system/equipment of which air velocity measurement to be done.

2.      Test to be conducted atleast 30 minutes after HVAC system has been operated and the conditions have been stabilized.

3.      Remove the diffuser/Grill before taking reading if applicable.

4.      Define the measuring plane perpendicular to the supply airflow and divide the measuring plane into grid cells of equal area. Individual area should not exceed approximately 0.4 sq.mt(4 sq.ft).

5.      The number of measuring points should be the square root of 10 times of area in square meters but no less than 4 points and at least one point for each filter.(Refer-ISO 14644-3).

6.      Measurements should be taken at the center of each grid cell.

7.      Hold the anemometer(Vane type anemometer or Hot wire anemometer) about 150 mm to 300 mm from filter face for measuring the filter uniformity of velocity.(Ref- ISO 14644-3).

8.      Switch ON the anemometer.

9.      Hold the fan or probe of anemometer at each position to ensure a repeatable reading and allow at least 5 second for each measurement. Note down the average reading at each position during that period for multiple locations.

10.  Note down the time required for stabilize or repeatable reading.

11.  Note down the air velocity readings at supply and return airflow.

12.  Switch OFF the anemometer.

13.  Switch OFF the equipment. If required.

14.  The total supply air flow rate should be evaluated as:

        Q=€(Ac X Vc)

Whre, Q=Total airflow rate,

            Ac=Area of each cell

           Vc=Airflow velocity at each cell center.

AIR VELOCITY MEASUREMNT IN CLEANROOM HAVING NON-UNIDIRECTIONAL AIRFLOW USING FLOW HOOD:

1.      Start the AHU/FAV of which air quantity measurement to be done.

2.      Test to be conducted at least 30. Min. after HVAC system has been operated and the conditions have been stabilized.

3.      Assemble the hood by fixing four supports pole in the four corner of the flexible fabric frame.

4.      Press the ON/OFF key on the display board to turn ON the power.

5.      Hold the top of the instrument along the perimeter edge of the diffuser or grill so as to form a seal. Ensure that there should not be any air leakage or by pass of air from the perimeter edge of the diffuser or grill.

6.      Ensure that the arrow marking on the display is showing correct direction of air flow.

7.      When the side sealing along the perimeter is proper and arrow is correct, press the sample button on the top of the right handle and wait till beep tone comes.

8.      The display will show the value of total air quantity.

9.      Note down the air quantity through the grill or diffuser.

10.  Air velocity through the grill or diffuser can be calculated as total air quantity divided by filter area.

11.  For measuring velocities of Diffuser/Grills, LAF/RLAF above 600 X 600 mm size Capture Hood/Vane type/Hot wire anemometer to be used.

SUPPLY AIR VELOCITY FLOW EATE IN AIR DUCTS:

Where anemometer is used, for a rectangular duct, the measuring plane in the duct should be divided into grid cells of equal area of minimum of 9 and maximum of 16 and each cell dimensions should be approximately 0.4 Sq.mt(4 Sq.ft) or smaller then the airflow velocity should be measured at the center of each grid cell. Measuring time at each position should be ar least 5 second and ensure for a repeatable reading. The average values during that period should be recorded.

The total supply airflow rate should be evaluated as:

Q=€(Ac X Vc)

Whre, Q=Total airflow rate,

            Ac=Area of each cell

           Vc=Airflow velocity at each cell center.

AIR CHANGE RATE:

1.      Start the AHU.

2.      Take mean velocity reading of each supply grill/diffuser and calculate cfm using following formula.

CFM=Velocity(ft/min)XArea of grill(ft²).

3.      Calculate total cfm of grill/ diffuser by adding cfm of each grill/ diffuser respectively.

4.      Calculate volume(ft³) of the area.

5.      Calculate the number of air changes using following formula.

                                                      Total cfm

     No. of air changes/hr=---------------------------------------X60

Total volume of the area

      6.   Air changes should be more than 6 for Area ISO 8, And should be more than 30 for area ISO 5, ISO 6, And should be more than 20 for areas ISO 7.

AIR VELOCITY MEASUREMENT OF UNIDIRECTIONAL AIR FLOW SYSTEM USING ANEMOMETER (VANE TYPE/HOT WIRE ANEMOMETER)

1.      Switch ON the system/equipment of which air velocity measurement to be done. Let the equipment run for 30 minutes before to stable the system.

2.      Check the zero setting of Manometer across HEPA filter, adjust if required.

3.      If the pressure drop is not within the limit, then Switch OFF the system and clean the pre filter and intermediate filter as per company sop and ensure for the rectification.

4.      Check the differential pressure across HEPA filter and the Pre filter.

5.      The number of measuring points should be the square root of 10 times of area in square meters but not less than 4 points.

6.      Define the measuring plane perpendicular to the supply airflow and divide the measuring plane into grid cells of equal area. Individual area should not exceed approximately 0.4 sq.mt(4 sq.ft).

7.      The total filtration area to be considered as one plane and not the individual filters.

8.      Hold the anemometer (Vane type anemometer or Hot wire anemometer) about 150 mm to 300 mm from filter uniformity of velocity.(Ref- ISO 14644-3)

9.      Measurements should be taken at the centre of each grid cell.

10.  Switch ON the anemometer.

11.  Hold the anemometer (Vane type anemometer or Hot wire anemometer)  at each cell till the anemometer reading stabilizes or ensures the repeatable reading and at least 5 second for each measurement.

12.   Note down the time required for stabilize or repeatable reading.

13.  Note down the air velocity readings and their corresponding grid identification on a diagram of clean room or clean zone.

14.  Switch OFF the anemometer.

15.  Switch OFF the equipment. If required.

16.  Calculate the arithmetic mean and the standard deviation(Uniformity) of the airflow velocity, expressed as a percentage of the mean, as follows:

                                                    Standard deviation

Relative standard deviation=--------------------------(100%)

                                                    Average velocity

ACCEPTANCE CRITERIA:

1.      Airflow velocity (Homogenous air speed) should be within the range of 72 to 107 FPM OR 0.36 to 0.54 m/s for Laminar air flow system(unidirectional airflow system) as per EU guideline.

2.      For uniformity testing the relative standard deviation should be less than 15%.

3.      If the velocity readings within the required limit are not observed then open the damper gradually so as to get desired mean air velocity. Even after opening the damper velocity is not maintained then an investigation should include review of status of blower, re filter & HEPA filter motor, damper, etc.,

FREQUENCY FOR AIR VELOCITY MEASUREMENT:

1.      For LAF, Dynamic pass box, Unidirectional Air flow installation Garment cubicle filter face velocity and uniformity to be measured quarterly or if any modification or as per company norms..

2.      For AHU of Grade B(ISO 5) to be measured quarterly or earlier if any modification or as per company norms.

3.      For AHU of Grade C(ISO 7) to be measured six monthly or earlier if any modifications or as per company norms.

4.      For AHU of Grade D and ISO 8, to be measured yearly or earlier if any modification or as per company norms.

References:

ISO 14644-3: Clean rooms and associated controlled environments-Part 3 Metrology and test methods.

ISO 14644-4: Clean rooms and associated controlled environments-Part 4 Design to Good Manufacturing Practice.