TAB Journal Spring 2014 - (Page 3)

Indirectly Determining AIR VOLUME Dustin Fielden Systems Commissioning & Testing, Inc. W hat can be done if an airflow measurement is needed, but there is no way to directly take a reading? Occasionally, there are situations that arise where it is necessary to determine creative ways to measure air volume. The following situation is a recent example where this was encountered. accurate place to traverse the second floor drop. Balancing the first floor drop was conducted using the branch dampers that were installed to 1690 CFM. The second floor drop airflow was determined by subtraction to be: This scenario involves a 2 story, 72-room long-term care/rehab facility. Each of the 72 patient rooms had a split heat pump unit (HP) above the ceiling with no access to the ductwork. The outside air to the 72 units was supplied by two ERV's (energy recovery ventilator), one serving the west side and one serving the east side of the building. Each ERV was specified to provide 90 CFM (cubic feet per minute) to 36 (typical) split HP units with two supply outlets and one return inlet; 18 units per floor (3240 CFM total for each ERV). Now that it was confirmed the system was supplying equal air to each floor, an accurate place to traverse one of the terminals was sought. A section of hard duct near the end of the run on the second floor was traversed and recorded 80 CFM. Next, using a velgrid, an average air velocity of 185 ft/min was recorded through the return inlet for the split HP. The supply totals had already been balanced for the split units and this particular unit total was 690 CFM. The actual return total was 610 CFM: Occasionally, there are situations that arise where it is necessary to determine creative ways to measure air volume. During the initial walk through of the jobsite, it was observed that each outside air duct was run using 6" round flexible duct. The flex duct was used because there was very limited space above the ceiling grid for the ductwork. Since flexible ducts are difficult to accurately traverse, the solution would have to be somewhat inventive to determine the air volume at each outlet. To begin, for consistency, it was verified that all 36 split units had their fans running. Both the supply and exhaust fans for the ERV were running, with the exhaust already balanced. Exposed ductwork on the roof was reviewed for traverse locations. The main supply duct was then traversed, with a recorded CFM of 3310. The first floor drop was traversed; however, there was no TAB Journal Spring 2014 1620 CFM (3310 CFM - 1690 CFM = 1620 CFM) (690 CFM - 80 CFM = 610 CFM) From that, an Ak of 3.3 was calculated for the return inlet from the following equation: (610/185 = 3.3 or 185 x 3.3 = 610) For the second ERV the same method above was used and found a slightly lower Ak of 3.2. For the remainder of the project the systems were balanced to the designed criteria using the following equations: The units supplied by ERV-1: Outside Air = Supply total of the split HP - (Return grill velocity x 3.3) The units supplied by ERV-2: Outside Air = Supply total of the split HP - (Return grill velocity x 3.2) This method made what seemed like a difficult situation manageable after all. Often, changes are made to ductwork during construction that can make it almost impossible to determine the air volume. Sometimes, it is possible to indirectly determine air volume using some simple calculations. 3

Table of Contents for the Digital Edition of TAB Journal Spring 2014

Indirectly Determining Air Volume
Calibration and Balancing of Phoenix Valves in Laboratories
The Trouble with Canned Specs
Environmental Considerations of the Southwest
Airflow Measuring Station Considerations
Putting the Proper "SPIN" on Things
Chasing the Air

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