TM 9-4940-444-14 & P
EXHAUST PIPING INSTRUCTIONS - cont
2. Header Caps.
The ends of header pipe shall terminate in a cleanout and inspection hole. The cap shall fit tightly into the pipe to
prevent air leakage.
G. Damper and Gates.
1. Damper or gates shall never be permitted in an exhaust system unless provided for the specific purpose of
balancing the system and then permanently fastened against further manipulation. However, they (the gates or
dampers) may be used for the specific purpose of Switching the air from one branch system to another, where
branches are not intended to be operated simultaneously.
NOTE: When dampers are used in an exhaust system and one or more dampers are shut off, the system is
deprived of the volume of air for which it was designed, resulting in much lower velocities in the headers,
causing the dust and dirt to settle in the pipes and plug up the system.
H. Pipe Reinforcements.
Pipe and duct work shall not be reinforced for any reason internally. If reinforcement is necessary, it shall be located
on the outside of the piping either longitudinally or about the girth.
Location of Discharge.
All types of exhaust systems discharging out-of-doors shall have the outlet stack at least ten feet above the roof line of
any adjacent buildings.
J. Weather Cap.
Stack and vents shall discharge upwards vertically. Weather caps shall be used only on such outlets where it is
necessary to protect equipment below from the weather. A resistance pressure of 20% of the velocity pressure shall
be added to the total pressure of system when weather caps are used.
Weather caps shall not be used on stacks connected directly to wet collectors or other equipment that will not be
damaged from moisture. Where there is a possibility of rain or snow entering the system through the stack outlet, a
drain pipe shall be located in the bottom of the fan housing to drain the accumulation of moisture, or a drain may be
put in the under side of an elbow. Drain pipes so installed may be water sealed to obtain automatic functioning.