State Industries SHE50-100PE - Manual (Page 24)

22 Servicing should only be performed by a Qualified Service Agent
venturi
A Venturi is mounted on the inlet of the Combustion Blower. All combustion air flows through the Venturi. The
outlet of the 24 Volt Gas Valve connects directly to the side of the Venturi. Fuel gas flows from the outlet of
the 24 Volt Gas Valve into the side of the Venturi directly.
Inside the Venturi there is a cone shaped restrictor that constricts the air passage to the Combustion Blower
inlet. As air enters the constriction point it’s velocity increases. A pressure drop occurs at this point and
creates a negative pressure in the cavity between the cone shaped restrictor and the Venturi housing. This
negative pressure “pulls” gas from the outlet of the 24 Volt Gas Valve into the Combustion Blower where it is
mixed with combustion air and then supplied to the Burner. See Figure 22 below.
This gas feed system does not produce a typical manifold gas pressure to the Burner. The manifold (offset)
gas pressure, gas pressure to the Burner, is very low. See Gas Pressure on page 25.
Negative Pressure
Negative Pressure
Velocity Increases
At Constriction Point
Pressure Drops
GAS VAL
VE
Combustion Air
VENTURI
VENTURI INLET
GAS INLET
24 VOLT GAS VALVE
BLOWER
BLOWER
SUPPLY GAS
BURNER
Figure 22