32 
BULLETIN 1373, U. S. DEPARTMENT OF AGRICULTURE 
LOUVERS 
Louvers, either fixed or adjustable, may be used to provide ventila- 
tion where doors and windows are impracticable. They prevent any 
strong direct drafts across the area being ventilated and at the same 
time provide for the admission of fresh air. Except in places where 
windows are likely to be frequently broken, louvers are not so desir- 
able as windows. 
STATIONARY ROOF VENTILATORS 
Ventilation by the use of monitor roofs is an improvement over the 
ordinary skylight, because windows can be opened on two sides of 
the monitor, and the wind passing through takes out the warm air 
which has risen to the top of the building. The fixed-type venti- 
lator for installation on the roof (fig. 24) usually consists simply of 
a flat metal cone, supported slightly above or within an open metal 
cylinder somewhat larger than the 
ventilating pipe to which it is attached. 
Wind blowing from any direction will 
operate the ventilators, provided they 
are properly installed. The tendency 
of warm air to rise to the top of the 
building also contributes to their opera- 
tion. Thus their operation costs very 
little. With a wind velocity greater 
than 800 feet per minute, the velocity 
in the pipe of ventilators of the better 
class will be more than 50 per cent of 
the wind velocity. 
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REVOLVING ROOF VENTILATORS 
Fig. 24.— Ventilator 
The revolving or swinging hood (fig. 
25) operates on the same principle as 
the stationary roof ventilator, but it 
accomplishes its object in a somewhat 
different way. In the stationary type 
the wind currents are deflected past 
the ventilator opening in such a way 
that a low-pressure area is created at 
the outlet and a light suction is created 
in the ventilator piping. In the revolv- 
ing type the action of the wind on a vane attached to the hood 
turns the ventilator so that the opening is always on the side 
opposite to that from which the wind is blowing. The wind pass- 
ing the opening creates a low-pressure area and a light suction in 
the ventilator pipe. 
PRESSURE VENTILATORS 
As a rule the ventilators for bins, garners, and equipment (fig. 26) 
are very simple. The pipe from the bin, garner, or hopper is extended 
through the roof and curved or formed like a gooseneck to keep out 
rain, snow, etc. These ventilators depend upon increased pressure 
within the bin or hopper for their successful operation. Grain enter- 
ing closed bins, hoppers, or garners displaces the air already there, 
