Handbook of air conditioning and refrigeration / Shan K

15.10, the cooling coil face velocity should not exceed 500 fpm (2.5 m/s) for a smooth fin coil and
550 fpm (2.75 m/s) for corrugated fins. The lower limit of the coil face velocity depends mainly on
the initial and energy cost analysis, including cost of the AHU, fan room size, the total number of
annual operating hours, and the unit rate of electric power. A lower limit of face velocities between
400 and 450 fpm (2 and 2.25 m/s) at design conditions may be considered appropriate under many
specific circumstances.
For a fan room of adequate head room, it is preferable to use a high coil to reduce the face velocity
and the pressure drop of coils and filters. Use of a high coil has little influence on the floor area of
the fan room except that when a coil is higher than 42 in. (1070 mm), a cooling coil should split into
two coils vertically and use two separate condensate pans vertically to prevent condensate carryover.
16.3 SELECTION OF AIR-HANDLING UNITS
Table 16.1 lists general data of the supply fan and coil of a typical horizontal draw-through AHU; fan B
means a class II fan. In Table 16.2, the volume flow–fan static pressure performance of this horizontal
draw-through modular AHU (unit size 30) with inlet vanes is presented. A backward-inclined centrifu-
1 gal fan of 22 �4-in.
(565-mm) diameter is used. In Table 16.2, the following items are also listed:
● Supply volume flow rate , in cfm of standard air.
● Air velocity at fan outlet, fpm.
AIR SYSTEMS: EQUIPMENT—AIR-HANDLING UNITS AND PACKAGED UNITS 16.9
● Fan static pressure, in in. WC. Fan total pressure can be obtained by adding the fan velocity pressure
to the fan static pressure; velocity pressure pv, in in. WC, can be calculated by
pv � (vout /4005) here vout indicates the fan outlet velocity, in fpm.
2
● Revolutions per minute (rpm) of fan impeller.
● Brake horsepower (bhp) input to fan shaft.
V˙ s
The following are recommendations for selection of AHUs from a manufacturer’s catalog:
● Draw-through AHUs are widely used. For a small AHU, a vertical unit saves floor space if the
headroom of the fan room is sufficient except the AHU is ceiling-hung. For a large AHU, a horizontal
unit is often the right choice.
● The size of the AHU is selected so that the face velocity of the cooling coil vcoil is optimum. For
corrugated fins, maximum vcoil should not exceed 550 fpm (2.75m/s).
● For large AHUs, choose a backward-curved centrifugal fan with airfoil blades or a backwardinclined
fan for higher efficiency. Select the rpm of the supply fan or supply and return fans in
order to meet the required system total pressure loss, i.e., external total pressure plus the total
pressure loss of the AHU.
● For VAV systems, an adjustable-frequency variable-speed drive should be compared with inlet
vanes via life-cycle cost analysis. Energy consumption also should be analyzed at part-load
operation. For a large AHU, an adjustable-frequency variable-speed drive may be cost-effective.
When an AHU is equipped with a small airfoil or backward-inclined centrifugal fan, inlet
vanes for capacity control are not recommended because of the extremely high air velocity at the
fan inlet.
● The required coil load is met through the variation of the number of rows of coil and the fin spacing.
An even number of rows are often used so that the inlet and the outlet of the coils are on the
same side. Four-row coils are often used for a mixing AHU, and a makeup AHU seldom uses a
coil that exceeds eight rows.
● In locations where outdoor air temperatures go below 32°F (0°C), coil freeze-up protection
attained by installing a preheating coil and the improvement of the mixing of airstreams in the
mixing box should be considered.