Installation manual

ManualsBrandsApplied Air ManualsHeaterGAS-FIRED HEATERS

Example 1

A heater rated at 200,000 BTU’s is currently

supplying 3000 CFM of air with a 62° F

temperature rise. The heater needs adjustment to

supply 4500 CFM of air. What is the temperature

rise for this heater (assuming 200,000 maximum

available BTU’s)?

Using the equation to determine the temperature

rise:

∆ T = 200,000/(4500 x 1.08)

∆ T=41°

Static Pressure vs. HP

Construction of the ductwork connected to a

heater unit is another element in understanding

heater operation. Some of the essential elements

in understanding ductwork are static pressure,

blower curves and motor amperage. Static

pressure is the pressure created in the duct by the

flow of air. As air is forced through a section of

duct, it exerts forces on the walls of the ductwork

containing it. The measurement of this force is

static pressure and is commonly measured in

inches water column using a device called a

manometer. As a rule, air travels easily in a

straight line and does not like to turn. When we force

air in a duct to turn or transition, the air exerts even

more force on the walls of the duct and we create

more static pressure. As the static pressure

increases, the blower moves less air due to the

resistance in pushing (or pulling) the air through the

duct. This decreases the amount of current that the

motor is using to turn the blower wheel (lowers the

motor amperage). This relation can be seen on a

blower curve, which plots CFM’s vs. static pressure as

a function of blower RPM and motor horsepower.

Looking at the blower curve, pick a given CFM and

static pressure to determine the blower RPMs and

motor horsepower. Example 2 illustrates the use of

the blower curve.

Example 2

A unit with a 15” wheel is set to run at 4000 CFM with

0.125” of static pressure. What was the factory RPM

setting and motor HP? During the installation,

however, turns were added to the ductwork to avoid

interference with some of the building structure,

bringing the static pressure up to 0.25”. What do the

new RPM’s need to be, and do we need a different

motor?