Installation manual

5
First we can determine what size motor and what
blower RPM has been set by the factory based on
the initial static pressure numbers. By looking on
the 15” Blower Curve, we pick the intersection of
0.125” (from the left axis) and 4000 CFM (from the
bottom axis). This point is just below the 700 RPM
line indicating a blower speed of approximately
690 RPM. This point is also to the left of the 1.5
horsepower line, indicating a 1.5 horsepower
motor was probably supplied. Next, we can
determine the new blower RPM and what motor
will be needed. By looking at the 15” Blower Curve
again, pick the intersection of the 0.25” static
pressure at 4000 CFM’s. This point is halfway
between the 700 and 800 RPM curves, indicating
that approximately 750 RPM’s will be required
from the blower. The correct motor for this
situation would NOT be the 1.5 horsepower,
because the point is to the right of the 1.5
horsepower curve. The correct choice would be a
2 horsepower (it is to the left of the 2 horsepower
line).
Gas Flow
Another critical element in understanding heater
operation is gas flow. Gas pressure changes
based on flow rates. A simple example of this is
shown in example 3, where a portion of the
plumbing is smaller in the middle. This could
represent a device in the line such as a gas valve.
Note in the first illustration, with no gas flow, there
is equal pressure in both sections of the pipe (P1
and P2). In the second illustration, where the gas is
flowing, the second section of pipe has much less
pressure than the first (P2 is less than P1).
This is important because the incoming gas pressure
is often measured when the heater is not running.
Even though the line may appear to have the
appropriate pressure, the pressure will drop when the
heater is operating. If the gas supply system is sized
properly, the upstream regulator will compensate for
the drop, and increase the pressure to the heater.
Another important part of heater performance relates
to the heating value of the fuel. Natural gas has
Methane as its main constituent, while LP consists of
Propane. The same heater is capable of burning both
of these fuels, however, the volume required for LP is
roughly half the volume required for natural gas for the
same BTU’s. This is because natural gas (-380,000
BTU/lbmol) has a heating value that is approximately
half the heating value of Propane (-955,000
BTU/lbmol).
V. OPERATION PRINCIPLES
The direct-fired heater is most easily understood
when broken down into smaller individual systems.
There are two main systems, a make-up air fan and a
heater. The make-up air fan consists of a heavy-duty
blower and motor. The heater may be further broken
down into two control systems, the Flame Safety
Control (FSC) and the Modulating Gas System
(MGS). The burner mixes air with the gas (Natural or
Propane) which heats the air.
Example 3
Illustration 2: Gas flow from left to right. P1 > P2
Illustration 1: No gas flow. P1 = P2
Fireye Flame Safety Control