Consulting-Specifying Engineer - March 2009 - (Page 50) Damper leakage limits free cooling BY DAVID SELLERS, PE, Facility Dynamics Engineering, Portland, Ore. Most people familiar with HVAC equipment would be quick to recognize the benefits afforded by low-leakage dampers applied to mixed-air plenums in the context of the outdoor air damper. After all, a leaky outdoor air damper in a cold or hot and humid environment can lead to energy waste by imposing an unnecessary outdoor air conditioning load on the system, either when it is on minimum outdoor air or during unoccupied hours. Furthermore, the leakage in a cold environment can lead to frozen coils and subsequent water damage issues and repairs. However, another factor comes into play when you consider the economizer dampers in a mild environment and look at what happens if the return damper does not seal well. Economizer-equipped systems in mild environments tend to spend a lot of time near or at the 100% outdoor air position. This is because the ambient temperatures are frequently in the range of the required discharge temperature. For example, in San Francisco, there are 7,896 hours per year (out of a total of 8,760 hours) where the temperature is 50 to 75 F and the enthalpy is low enough to make it advantageous to cool a 100% outdoor airstream versus a recirculated airstream with minimum outdoor air. In a situation like this, outdoor air damper leakage, while a problem in theory, is not much of an issue from a practical standpoint. But a return damper leaking 10% to 15% when it is supposed to be fully closed, compromises the ability of the economizer to provide free cooling because it elevates the return temperature above the current outdoor air temperature. For example, on a 55 F day, with 75 F return air and a 55 F discharge temperature requirement, a system leaking 10% of its return air when it is supposed to be on 100% outdoor air is delivering a 57 F mixed air temperature (not a 55 F mixed air temperature). As a result, many control systems would stage on mechanical cooling when it is not actually necessary if the damper quality was improved. One new and “chilling” development is that on Dec. 11, 2008, the U.S. Environmental Protection Agency issued a proposed rule to ban the sale or distribution of air conditioning and refrigeration products containing HCFC-22, HCFC-142b, or blends containing one or both of these substances, starting Jan. 1, 2010. According to a transcript of a hearing held to obtain verbal comments from industry stakeholders, the proposed rule, once finalized, would “restrict the sale and distribution in interstate commerce, including the export and import of appliances that have been precharged with HCFC-22, HCFC-142B, or a mixture of those substances.” The EPA currently is considering legislation to further regulate HCFC-22 regarding the shipping of pre-charged equipment. Stay tuned to the EPA for the announcement of further regulatory changes. No matter how the HCFC-22 phaseout is eventually structured, effective Jan. 1, 2030, no one may produce any HCFC-22 (or any other Class II substance). The takeaway lesson here is: Keep one eye on the prices of 410A refrigerant and equipment and the other on the price of HCFC-22. 4.50 4.00 3.50 Low velocity High velocity Pressure Drop, inches w.c. Flow Obstruction Dimension change made in a turn with an expanding elbow 3.00 2.50 2.00 1.50 1.00 Flow 0.50 0.00 -0.50 0 1,000 2,000 3,000 4,000 5,000 Velocity, feet per minute Normal application range Obstruction Dimension change made by expanding after the elbow Figure 1: Pressure drop with two configurations of ductwork is shown. The method with the expansion after the elbow has a much lower pressure drop, which would be quieter and save energy. Source: David Sellers, PE, Facility Dynamics Engineering, based on ASHRAE data 50 Consulting-Specifying Engineer • MARCH 2009
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