ASHRAE Journal - February 2009 - 50

ASHRAE 19 Face Velocity, FPM OA Louvers Filters Cooling Coils Total Add 3.5” System P.D. Fan BHP Motor HP *Based on 3120 hours/yr 10 year operating cost. 250 0.05 0.40 0.20 0.65 4.15 26.1 30 300 0.03 0.40 0.30 0.73 4.23 26.6 30 50 59–2009 400 0.05 0.60 0.60 1.25 4.75 29.8 40 YE A R S JOURNAL 500 0.08 0.60 1.02 1.70 5.20 32.7 40 600 0.12 0.80 1.50 2.42 5.92 37.2 50 Optimum* 0.12 0.40 0.30 0.82 4.32 27.2 30 Table 4 Pressure drop and power summary. unit is about 20% less than a field erected unit. Assuming equal fan efficiency, the operating costs of both constructions will be essentially equal. Therefore, the optimum face velocity will occur at a slightly lower value— in the range of 250 to 300 fpm based upon 10-year payback for 3,120 hours of operation per year. • Lower Capacity Units—The operating cost per cfm is essentially constant for a given face velocity. However, the initial cost per cfm increases as the unit becomes smaller. A limited study based on a field erected unit of 15000 cfm capacity for 10-year payback and 3,120 annual operating hours indicates an optimum face velocity of 400 fpm versus 300 fpm for one twice the size. Face velocity for a factory built unit of 15000 cfm should optimize about the same value as for the 30000 cfm field erected unit. • Higher Capacity Units—The initial cost per cfm decreases as the sizes increases, but the rate of change is not great. Therefore, the optimum face velocity can be expected to be close to the values indicated in the study. • Different Filter Types—Both initial and operating costs of the filters will, of course, vary depending upon type. About 15 to 40% of the Owning Cost Index is attributable to filters. Those with lower pressure drop characteristics and replacement costs should shift the optimum face velocities somewhat upward. Further study on this subject must await expanded data correlating face velocity and pressure loss to filter life. However, no drastic shift in the conclusions of this study, because of the stabilizing effects of the other unit components, is anticipated. BIBLIOGRAPHY “Techniques of Life–Cycle Costing.” C.W. Griffin Jr. Construction Specifier, October, 1973. “How to Calculate Life Cycle Cost.” Air Conditioning and Refrigeration Business, September, 1973. “Here’s How to Calculate Payback Time.” Air Conditioning & Refrigeration Business, May, 1974. “Evaluation Criteria for Resource Optimization.” J.W. Griffith. BRI Fall Conferences, November, 1973. “Air Filtration: Resistances, Energy and Services Life, in. Robert H. Avery. Heating, Piping & Air Conditioning, December, 1973. “The Cost of Clean Air.” Morton Bell. Air Conditioning, Heating & Ventilating, July, 1968. “Characteristics and Application of High Performance Dry Filters.” P.M. Engle Jr., C.J. Bauder. ASHRAE Journal, May, 1964. “Filter Operation at Other than Nominal Capacities.” Cambridge Filter Corporation. “Energy Consideration in Selection and Use of Air Filters.” Robert H. Avery (unpublished). summary With professional judgment, it appears that this study does support a general conclusion that, with reasonably pragmatic economic objectives in mind, face velocities for the filter and chilled water coil components of air handling units should be selected in the 300 to 400 fpm range. High performance louvers should be selected at a face velocity roughly 200 fpm higher than the other components. Note that this study is predicated on meeting economic objectives. Energy conservation is achievable as a by–product of meeting those objectives. If energy is priced realistically—not depressed as a political palliative—investment cost objectives can be a major stimulus toward energy conservation in air conditioning system design. 50 ASHRAE Journal a s h r a e. o rg Fe b r u a r y 2 0 0 9

ASHRAE Journal - February 2009

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