A.O. Smith TC-099 - Manual (Page 13)

VF BOILER SERVICE MANUAL
AOS WPC - Tech Training 12 of 72 Ashland City, TN © 2007
Servicing should only be performed by a Qualified Service Agent
PRINCIPLE OF OPERATION (CONT)
Venturi Operation - Gas Train
A Venturi is connected to the inlet side of the combustion blower on VF Boilers. The gas line
from the outlet of the gas valve connects to the Venturi. The Venturi contains a cone shaped
restrictor that constricts the air passage to the blower. As air enters the constriction point it’s
velocity increases. A pressure drop occurs at this point and creates a negative (vacuum)
pressure in the cavity between the cone shaped restrictor and the Venturi housing. This
negative pressure “pulls” gas from the outlet of the gas valve into the blower where it is
mixed with combustion air and then supplied to the burner.
As the blower speed is increased by the MCB (page 25) working in conjunction with the VFD
(page 11) the velocity of air flowing through the Venturi is also increased. This further
increases the vacuum created by the Venturi and more fuel gas is pulled from the gas valve.
This increases the firing rate (input Btu/hr) of the VF boiler. As the blower speed is
decreased less fuel gas is supplied and the firing rate is reduced.
The MCB controls blower speed according to temperature demand. When system
temperature sensed at the Controlling Probe (pages 62 and 63) is well below the VF Boiler’s
Operating Set Point (pages 8 and 63) the MCB will run the blower at maximum speed =
100% firing rate. As system temperature approaches the VF Boiler’s Operating Set Point the
MCB will modulate the blower at slower speeds which subsequently reduces the firing rate.
VF boilers are designed to modulate between 25% and 100%; a 4 to 1 turn down rate. See
the illustration below.
Combustion Air
Fuel
Gas
Velocity Increases
At Constriction Point
Pressure Drops
Negative Pressure Area
Negative Pressure Area
Gas Valve
Venturi Blower
Burner
Blower Inlet
Gas Valve
Venturi
Blower
Burner Flange
Combustion
Air Inlet
VF Boiler Gas Train
Blower Outlet