MEASURING AIR FLOW. Ventilation (Outside Air) Calculations. Derive % of OA as part of total air flow using air temperatures:

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Adrian Gregory King
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1 MEASURING AIR FLOW 1 Ventilation (Outside Air) Calculations Derive % of OA as part of total air flow using air temperatures: % Outside Air (OSA) = [(Return Air T Mixed Air T) / (Return Air T Outside Air T)] x 100 Or [(Mixed Air T Return Air T) / (Outside Air T Return Air T )] x 100 We need to know the temperatures in the AHU/RTU! From BMS From Measurement Once you know the percentage, you also need to know the total air flow in system! 1

Total Air Flow in System Get specifications of total air flow from schedules (Note that the schedules will provide the design air flow which may be different from the

) Or Measure total air flow in supply duct Pitot tubes to derive velocity pressure and then calculate velocity Direct velocity measurement with anemometer Remember

2 Total Air Flow in System Get specifications of total air flow from schedules (Note that the schedules will provide the design air flow which may be different from the actual air flow.) Or Measure total air flow in supply duct Pitot tubes to derive velocity pressure and then calculate velocity Direct velocity measurement with anemometer Remember Traverse requirements Calculate duct air flow from velocity FPM x duct cross sectional area (in ft 2 ) = CFM, total air flow Then Total Air Flow in CFM x % OA = OA Ventilation CFM Total Air Flow From Schedules Find any mechanical schedules Remember, though, this is design Distinguish outside ventilation air from the total air flow 2

3 Measuring Air Flow in Ducts Velocity Pressure ( W.C.) = Total Pressure Static Pressure Flow Velocity (Feet per Minute) = 4005 (constant) X Sqrt of Velocity Press CFM = Velocity (FPM) x duct cross sectional area (in Feet) FPM Feet per Minute or (Feet per second (measured) x 60 seconds per minute) 5 OA Air Quantity Problem DATA Pitot Tube Measurements:.8 Total.4 Static Supply Air Duct Size: 24 x 36 Outside Air Temperature: 60 Return Air Temperature: 75 Mixed Air Temperature: 70 Fan Total Flow = 4,806 CFM 3

Solution Velocity Pressure ( W.C.) = Total Pressure Static Pressure Velocity Pressure =.8.4 =.4 Velocity (Feet per Minute) = 4005 (constant) X Sqrt of Press Velocity = 4005 x.4 = 4005 x.

4 Solution Velocity Pressure ( W.C.) = Total Pressure Static Pressure Velocity Pressure =.8.4 =.4 Velocity (Feet per Minute) = 4005 (constant) X Sqrt of Press Velocity = 4005 x.4 = 4005 x.2 = 801 fpm CFM = Velocity (FPM) x duct cross sectional area (in Feet) Flow = 801 x (24 /12 x 36 /12 ) = 801 x 2 x 3 = 4,806 CFM Deriving Outside Air (OA) Quantity from Air Temperature Readings % OSA = (Return Air T - Mixed Air T) / (Return Air T - Outside Air T) X = / X 100 Qty OSA = Tot CFM X % OSA 1,202 = 4,806 X 25% 4

5 HVAC Problems A building has a volume of 240,000 cubic feet. Assume that the ventilation rate provides 1 air change per hour (ACH). How many CFM of ventilation air is this? Is this sufficient for 250 occupants based on the ASHRAE 62.2 general Standard for Ventilation on an office building? CFM = CF (ft 3 ) X (ACH / [60 Minutes/Hr]) 4,000 CFM = 240,000 ft 3 X (1 ACH / [60 Minutes/Hr]) 250 people X 20 CFM per person = 5,000 CFM (required) 9 HVAC Problems A building has a volume of 240,000 cubic feet. Assume that the overall ventilation rate is 6,000 CFM. How many air changes per hour (ACH) is this? How many occupants will this cover? ACH = (CFM X 60 Minutes/Hr) / CF (ft 3 ) of Blg volume 1.5 ACH = (6,000 CFM X 60 Min/Hr) / 240,000 ft 3 6,000 CFM / (20 CFM/Person) = 300 people 10 5

6 Ventilation Problem An AHU serves a space designed for 200 people. It is equipped with a variable speed fan controlled by a demand controlled ventilation system. The AHU control panel display shows the fan speed in terms of Herz (HZ), where 60 Hz equal full fan output. At the control panel display you are also able to read Return Air temperature, Mixed Air temperature, and Outside Air temperature and total air flow (supply air). Applying the general guideline from ASHRAE 62.1, approximately how much outside air should be supplied in CFM when the space is at full capacity? 200 People X 20 CFM/Person = 4,000 CFM of outside air For the Same AHU When there are 50 people in the space, what fan speed read out should you expect to see in the AHU control panel display (Based on anticipated CO2 or cooling load and related fan speed adjustment)? 50 / 200 (People load ratio) X 4,000 CFM (Full Output) = 1,000 CFM 50 / 200 (People load ratio) X 60 Hz (Full Fan Output) = 15 Hz 6

7 For the Same AHU At a particular time you read from the control panel that you are delivering 2,000 CFM with the following air temperatures: Return Air: 78 F Mixed Air: 66 F Outside Air: 60 F. How would you calculate the percentage and the quantity of Outside Air that is being provided? %O/A = (RAT MAT) / (RAT OAT) X 100 %O/A = ( ) / ( ) X % = ( 12 ) / ( 18 ) X 100 2,000 CFM (Total Air) X 66.7% = 1334 CFM (O/A) Another AHU Problem An AHU is delivering 10,000 CFM with 20% outside air to a space that is 100 X 50 X 8 ceiling height. How would you calculate the number of ventilation air changes per hour (ACH) (fresh air only)? 10,000 CFM X 20% OA = 2,000 CFM of OA X 60 MIN/H = 120,000 CF/H 100 X 50 X 8 = 40,000 CF volume of space 120,000 CF/H X 1 AC/40,000 CF = 3 ACH 7

8 More Ventilation Problems In a rectangular 12 x 24 duct we measure an average air velocity of 4 feet per second (FPS). How would you calculate the air flow through across the coil in CFM? CFM = FPM X Duct cross sectional area But what about compatible units of measure? Velocity needs to be in Feet per Minute and duct cross sectional area must be in square feet duct cross sectional area in feet = 1 X 2 = 2 ft CF/M = (4 Ft/Sec X 60 Sec/Min) X 2 ft 2 8

HAP e-help. Obtaining Consistent Results Using HAP and the ASHRAE 62MZ Ventilation Rate Procedure Spreadsheet. Introduction

HAP e-help. Obtaining Consistent Results Using HAP and the ASHRAE 62MZ Ventilation Rate Procedure Spreadsheet. Introduction Introduction A key task in commercial building HVAC design is determining outdoor ventilation airflow rates. In most jurisdictions in the United States, ventilation airflow rates must comply with local