ASHRAE Journal - August 2009 - 12

Author Response, from Page 11 • Reference was made to oversizing the DOAS AHU, an excellent idea. I have continually recommended reserve capacity in all my DOAS lectures and short courses. As for reserve capacity: a. It is needed to accommodate unforeseen latent loads; b. It improves the total energy recovery (TER) effectiveness and provides overall energy use reduction; and c. It can expand the system free cooling capabilities. • Yes, buildings served by DOAS must be pressurized, as noted in the letters. As a result, the supply and return airflow rates are often unbalanced, lowering the TER effectiveness compared to balanced flow. However, by employing reserve capacity, excellent elevated TER effectiveness is achieved. Space here does not permit discussion of options that provide 24/7 building pressurization and balanced flow in the main DOAS AHU during occupancy. • Finally, water-side economizers are excellent where water-cooled chillers are used. Realistically, the vast majority of DOAS AHUs use packaged air-cooled DX equipment. A DOAS without a full economizer generally easily beats the energy cost budget of Standard 90.1. I have addressed the DOAS economizer issue in previous articles. Stanley A. Mumma, P.E., Ph.D., Fellow ASHRAE, Bellefonte, Pa. More on Spreadsheet Re: Equal Friction Solution for Duct Sizing The May 2009 article, “Equal Friction Solution for Duct Sizing,” by Fred W. Dougherty, P.E., has at least two errors. First, the article says that “duct friction and fitting dynamic losses…will make up the external static pressure on the airhandling unit.” The ASHRAE Handbook says, “An energy loss in a duct system can be defined only as a total pressure loss.” By using total pressure for both fan selection and air-distribution system design, the design engineer is assured that he is using the correct fundamentals. These fundamentals will apply equally well to both highand low-velocity systems. Second, the fitting dynamic losses for round and rectangular ducts are calculated on the basis of air velocity only in the equal friction round ducts. But, it is well known that the rectangular fitting dynamic losses must be calculated only on the basis of the air velocity in the rectangular ducts. This type of engineering should not have a place on the pages of ASHRAE Journal. For reference, see “Myth About the Fan Static Pressure” at my company’s Web site at http://mysite.verizon.net/restkgfn/ airpolcontrol. Alex London, P.E., Huntington Beach, Calif. The Author Responds These are very good points. However, manufacturer’s catalog air delivery performance data for air handlers is provided as a function of external static pressure. This is what is of interest to the designer of small systems, who wants to be sure that the air handler selected can deliver the design airflow. It is the sum of friction and dynamic losses from the fan outlet to the fan inlet as defined by the manufacturer. I admit to being unclear about Mr. London’s second point. Fitting loss coefficients represent dynamic loss, but this still contributes to the widely used “external static pressure” of the system. In my opinion, the simplifying assumption that calculated fitting inlet velocity represents “true” velocity introduces no significant error with low-pressure systems. I’m happy to have given Mr. London a chance to put in a plug for his company. Fred W. Dougherty, P.E., Life Member ASHRAE, Panacea, Fla. August 2009 www.info.hotims.com/25207-81 12 ASHRAE Journal
ASHRAE Journal - August 2009

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