www . refrigerationandaircon . co . za RACA Journal I October 2023 41
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8 . Paddlewheel anemometers : Consisting of a rotating wheel placed in the duct , paddlewheel anemometers measure air velocity . The rotation speed is proportional to the air velocity , providing a simple and cost-effective method for air flow measurements .
For duct flow measurements , instruments like those listed in items 1 and 2 above provide readings at predetermined positions using traverse measurement . Air velocity measurements are taken at multiple predetermined points across a duct crosssection , and these measurements are used to calculate the average air velocity and air flow rate .
When conducting air flow measurements , it ' s crucial to choose the appropriate method ( s ) based on the system ' s characteristics , where the air flows are being measured ( in ducts , at inlets and outlets , etc .), the required accuracy , and the available equipment . Additionally , proper calibration and adherence to best practices are essential for obtaining accurate and reliable measurements .
For air flow measurement protocols in HVAC systems , there are several widely referenced standards and guidelines : 1 . SANS 10173 : This manual , focused on duct leakage testing , provides useful guidelines and methods related to air flow measurements in duct systems .
2 . ASHRAE Standard 111 : This standard outlines procedures for measuring , testing , adjusting , and balancing building HVAC systems , including air flow measurements in ducts and air distribution systems .
3 . ASHRAE Standard 129 : Although not specific to air flow measurement , this standard includes requirements and guidelines for thermal comfort , which may involve assessing air velocity and movement in occupied spaces .
4 . ISO 5167-1 : This ISO standard provides principles and requirements for flow measurement using pressure differential devices , which can be applicable to certain air flow measurement techniques like orifice plates .
5 . ISO 5801 : This standard specifies methods for testing the performance of industrial fans , including air flow measurement procedures under standardised conditions .
6 . ISO 12569 : While primarily concerned with air permeability , this standard includes procedures using fan pressurisation methods , which can be relevant to air flow measurement protocols in building envelopes .
7 . NEBB Procedural Standards : The National Environmental Balancing Bureau ( NEBB ) provides guidelines for testing , adjusting , and balancing environmental systems , including air flow measurement procedures .
8 . CIBSE TM23 : The Chartered Institution of Building Services Engineers ( CIBSE ) offers technical guidance for testing buildings for air leakage , which is relevant to air flow measurement in identifying and quantifying air leakage rates .
9 . EN 12599 : This European standard outlines specifications and requirements for instruments used to measure air flow and volumetric flow rate in ventilation systems .
10 . SMACNA HVAC Air Duct Leakage Test Manual : Although more focused on duct leakage testing , this manual also provides valuable guidelines and methods related to air flow measurements in duct systems .
Keep in mind that some of these standards may be region or organisation-specific , so the applicable standards could vary based on your location and project requirements . Always refer to the latest versions of these standards for the most up-to-date information .
Before concluding , I want to highlight a critical aspect often overlooked in air flow measurements – the impact of atmospheric pressure . Changes in atmospheric pressure can significantly affect air flow measurements , especially when they occur . Air flow measurements are usually referenced to a standard condition , typically at sea level where atmospheric pressure remains relatively constant . When atmospheric pressure deviates from this standard condition , the measurements can be affected in the following ways : 1 . Density changes : Air density is directly related to atmospheric pressure . As pressure increases , the air becomes denser , and vice versa . These density changes directly influence the volumetric flow rate . A decrease in atmospheric pressure results in less dense air , leading to a decrease in volumetric flow rate for a given mass flow rate . Conversely , an increase in atmospheric pressure causes denser air , leading to a higher volumetric flow rate for the same mass flow rate .
2 . Instrument calibration : Many air flow measurement devices , such as pitot tubes , anemometers , and flow sensors , are calibrated under standard atmospheric conditions . When atmospheric pressure changes , the calibration of these devices may no longer be accurate , resulting in measurement errors .
3 . Pressure drop : Air flow measurements using pressure differential devices , like orifice plates or venturi meters , are affected by atmospheric pressure changes . The pressure drop across these devices is proportional to the square of the air velocity . If atmospheric pressure changes , the pressure drop may deviate from the expected values , leading to inaccurate flow measurements .
4 . Fan performance : HVAC systems with fans may experience changes in fan performance due to varying atmospheric pressure . The fan ' s ability to deliver the desired air flow rate can be affected , resulting in discrepancies between the specified and actual air flow rates .
5 . Leakage rates : Changes in atmospheric pressure can impact the air leakage rates in the building envelope and ductwork . Pressure differences across the building
www . refrigerationandaircon . co . za RACA Journal I October 2023 41