ASHRAE Journal - February 2009 - 48

ASHRAE 19 50 59–2009 300 3400 8800 11500 8600 4100 36400 2600 39000 0.78 13.4 3200 5500 10000 8600 4100 31400 2600 34000 0.78 13.4 YE A R S JOURNAL Face Velocity, FPM OA Louvers & Dampers AHU FC + PW oc (10 Years) Filters Cooling Coils Casing Fan Subtotal Cost of Space Total AHU PD, "WG KWH/Yr x 103 OA Louvers & Dampers AHU FC + PWoc (5 Years) Filters Cooling Coils Casing Fan Sub Total Cost of Space Total AHU PD, "WG KWH/Yr x 103 250 3800 8600 11500 9500 4100 37500 3000 40500 0.65 11.2 4300 5800 11000 9500 4100 347000 3000 37700 0.65 11.2 400 2900 9400 12500 7400 4300 36500 2200 38700 1.25 21.5 2700 5400 10500 7400 4300 30300 2200 32500 1.25 21.5 500 2700 10200 15000 6800 4300 39000 2000 41000 1.70 29.3 2400 5600 12500 6800 4300 30600 2000 32600 1.70 29.3 600 2600 11500 17500 6400 4400 42400 1800 44200 2.42 41.7 2200 6300 12500 6400 4400 31800 1800 33600 2.42 41.7 Optimum 2600 (600) 8800 (300) 11500 (300) 8600 (300) 4100 35600 2600 38200 0.85 14.6 2200 (600) 5400 (400) 10500 (400) 7400 (400) 4300 29800 2200 32000 1.32 22.7 Table 2 Owning cost index for the components for incremental steps of face velocity based on 3120 hours of operating per year. 3120 Hours/Year Operation decreases—and therefore the cost—of their encasement. The casing consists of factory fabricated, field erected panels with a double facing of galvanized sheet metal enclosing 4 in. of glass fiber blanket insulation. Three feet of access space between components has been considered except between the coil and fan where 3 ft or 1/2 face area—1.5, in feet (to obtain a 45° maximum angle between coil extremities and the fan inlet), whichever is greater, is allowed. A casing aspect ratio of 1:1 assumed. Walk-in access doors between components, flexible connections and baffling between casing and component, where applicable, are included as first cost items. Fig. 5 diagrams the physical arrangement of the air handling unit and depicts the applicable cost curve with respect to face velocity. FAN Fan selection for this draw–through arrangement has been based upon 3.5 in. w.g. external static pressure. The air handling unit pressure loss, being dependent upon the face velocity, can affect the bhp and motor size as well as the possible class of construction and wheel diameter. Within the range of 250 and 600 fpm component face velocities, the total fan static pressure 4.15 to 5.92 in. w.g. can be accommodated with Class 2 air foil construction. A 49 in. wheel diameter (single width) is also satisfactory for the full range of duty. Motors are selected so that fan horsepowers, excluding drive losses, do not exceed 0.9 of the nameplate rating. For the sake of comparison, 75% static efficiency is assumed for all fan selection points to determine fan horsepower. A constant motor efficiency is also assumed. The initial cost of the a s h r a e. o rg fan includes an adjustable V belt drive, motor, slide rail base, belt guard and spring rail isolation base. The fan’s first cost is considered to be its owning cost index since maintenance cost— especially as influenced by the face velocity of the other air handler components— is rather insignificant. Conclusions: Table 2 tabulates the owning cost index for the components for incremental steps of face velocity based on 3120 hours of operating per year. To the total air handling cost has been added the cost of housing space in the equipment room. Floor area, rather than volume, is the basis since the height of even the largest 30000 cfm unit would probably fit in most mechanical equipment rooms. Twenty dollars per square foot was assigned as a reasonable incremental cost for this type space. It is not necessary to select all Fe b r u a r y 2 0 0 9 48 ASHRAE Journal
ASHRAE Journal - February 2009

Table of Contents for the Digital Edition of ASHRAE Journal - February 2009
