Hi Ask An Expert: February 2014

(Hi) Protect Fans From Wind Milling.

Fan wind milling is very common in our industry. When a motor is not operating, in many cases a fan will be rotating in the wind. Sometimes this is caused by the wind and other times by an updraft in the tower. The fan can turn forward and sometimes in reverse. This can be damaging to the fan and stack if the wind gusts are high enough to cause the stack to deflect and contact the fan. When fan blade tips contact stacks, there is usually damage to the blades and the stacks. Higher winds cause more stack deflection and more deflection causes more blade contact which causes more damage. Fans should be locked down and held stationary during high wind conditions; especially hurricanes. The best way to safely stop these fans is a braking system that can quickly lock-out the mechanical set during a plant lock-down prior to a hurricane. This lock-out system should be easy and take less than one minute per fan. The brake should be designed to not slip during high winds. It should not be capable of coming loose or disengaging during hurricane gusts. During high wind condition when the fan is stationary, the stack may still deflect, but there is no rubbing activity causing wear which will eventually damage the blades and stacks. With a brake there will be no fan damage and no stack damage.

Hi AMCA Green Tip #5: Minimize Air System Leakage.

System leakage is typically expressed as a percentage of fan airflow at maximum operating conditions. Measurements by Lawrence Berkeley National Laboratory (LBNL) of system leakage in large commercial buildings in California found that adding 15% leakage at operating conditions leads to a fan power increase of 25% to 35%. Simulations performed by LBNL also found that a leakage rate of 19% compared to a "tight" system (5% leakage) can cause a 40% to 50% increase in fan-energy usage, and an increase of 2% to 14% of the total annual HVAC energy consumption. The study reported that this amounts to $7,400 to $9,500 per VAV system per year in Year 2000 dollars and utility rates in California. Leakage-testing requirements are a growing trend in national and state codes and standards for energy efficiency and green buildings. ASHRAE, SMACNA (Sheet Metal and Air Conditioning National Association), SPIDA (Spiral Duct Association), and AMCA are developing new test procedures and product certifications around tighter air systems.

Look for a lot of new "system leakage" thinking and guidance in the 2012 ASHRAE Handbook: Systems & Equipment, which is just hitting the press.Consulting engineers and design-build contractors shoulder a large part of the responsibility for ensuring tight air systems at the design phase.

The first step is to work with the building owner to agree on the level of tightness (system leakage percentage) that the project merits, and then to specify air system components and sealants that will perform together as a system to meet the system leakage objective. Engineers also need to select a testing standard that contractors or commissioning providers will use to ensure the leakage objective is met.

28 Feb 2014

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27 Feb 2014

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