Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

ineffective, the velocity was so low that lint
from the dryer collected in the duct, largely
clogging it and creating a fire hazard. Long
air-duct runs—if used—need to be carefully
engineered to ensure proper operation.

BLOWER CONFIGURATIONS ORTYPES As a
general rule, the most efficient and most
common types of blowers are squirrel-cage
blowers (centrifugal blowers). Axial blowers
(helicoidal blowers) look like “fans.” Though
in smaller sizes and less-expensive units, ax-
ial blowers aren’t usually as effective, in
larger sizes they can work quite well.
It can be difficult to locate blowers of any
type larger than 4 inches (10.2 cm) in diame-
ter. Two sources for large marine blowers
are Delta-T Systems of Jupiter, Florida
(www.deltatsystems.com), and Gianneschi &
Ramacciotti blowers, available in the United
States through Cole Marine Distributing, Ft.
Lauderdale, Florida (www.colemarine.com).
Figure 14-11 shows an axial blower from Delta-
T Systems, which is available from 12 to 36
inches diameter, with capacities ranging from
800 to over 25,000 cfm (1,360 m^3 /hr. to over
42,500 m^3 /hr.). Moving this much air takes a
great deal of power. Large blowers moving
over 1,200 cfm (2,000 m^3 /hr.) or so need to be
powered off the AC electric system. Up to
about 1,200 cfm (2,000 m^3 /hr.), you can find
blowers that will run off 12- or 24-volt DC.

CONTROLLINGNOISE Air flowing too fast
through a duct or louvers causes unaccept-
able noise. To keep this under control, the
cross-section area has to be large enough to
keep velocity to under 600 feet per minute
(fpm) or 3 m/sec. in sleeping spaces and
under 800 fpm (4 m/sec.) in other passenger
spaces. Machinery spaces can accept up to
about 2,000 fpm (10 m/sec.), since duct noise
here isn’t a factor. Exceed this air velocity in
passenger spaces, and you’ll need to line or
jacket the air ducts with sound insulation. In
fact, some home and industrial systems use
just this approach—small-diameter, high-
velocity air ducts with heavy sound insula-
tion. This reduces the amount of space such
ducting takes up, but few such systems are
available for marine use.
Example: We can find the velocity in
fpm or m/sec. for our cabin air duct with
blower as follows:
From Table 14-3, a 3-inch duct has a sec-
tion area of 7.1 sq. in.

7.1 sq in. ÷ 144 sq. in./sq. ft. =0.049 sq. ft.

131 cfm ÷ 0.049 sq. ft. =2,673 fpm; this is too

fast; need a larger duct

Required duct section for 600 fpm:

131 cfm ÷ 600 fpm max. =0.218 sq. ft.

0.218 sq. ft.×144 sq. in./sq. ft. =31.4 sq. in.

use a standard 6-in. dia. duct to keep noise

down

Or

From Table 14-3, a 7.6-cm duct has a section

area of 45.6 cm^2 :

45.6 cm^2 ÷ 10,000 cm^2 /m^2 =0.0045 m^2

221 m^3 /hr. ÷ 0.0045 m^2 =49,111 m/hr.

49,111 m/hr. ÷ 3,600 sec./hr. =13.6 m/sec.;

too fast; need a larger duct

Required duct section for 3 m/sec.:

221 m^3 /hr. ÷ 3,600 sec./hr. =0.0614 m^3 /sec.

0.0614 m^3 /sec. ÷ 3 m/sec. max. =0.02 m^2

0.02 m^2 ×10,000 cm^2 /m^2 =200 cm^2

use a standard 150 mm dia. duct to keep

noise down

The on/off switch ideally would be

located next to the main overhead cabin light

by the stateroom door and clearly labeled as

“Ventilator.”

Note that the 3-inch (7.6 cm) squirrel-

cage blower we specified for this vent duct is

Dia.=× 2 200 cm^2 ÷ =π 15 9. cm

Dia. =× 2 314 ..sq. in.÷ =π 63 in.

Chapter 14:Ventilation of Passenger and Storage Areas

Figure 14-11.

Axial blower

(Courtesy Delta-T

Systems)