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installation. Direct fired heaters are designed to operate in a fresh flowing air stream. If the airflow stops or is different from the factory settings, the unit will shut down or perform below its design capability. It is important to follow the installation and start-up procedures to maximize the heater’s performance. The manufacturer has designed a unit that is easy to install, start up and service. If you have any questions, call the Service

Department at 1-800-334-9256.


II. INTRODUCTION AND OVERVIEW toward less user interaction with the heater units.

Most customers prefer the unit to sit on the roof, turn on, and run by itself with no user interaction

(temperature selection, summer-winter selection, etc.). This means that the field installation has to be done properly; no user interaction can cover up installation problems. Now, more than ever, installation and start up is critical to customer satisfaction and product operation.

There are many types of heaters available for commercial use. We will concern ourselves with

100% fresh air (make up air) Direct Fired Gas units for commercial kitchen and industrial use. The advantages of direct fired gas heaters include:

Energy (heat) must be added to a specific volume of air in order to change its temperature. Direct fired gas heaters create heat by burning gas. Heat is measured in BTU’s (British Thermal Units), which is a measure of heat, not temperature.

Low cost per BTU of heating

Readily available

Most efficient heat transfer

There are two general types of gas-fired heaters, direct and indirect. Indirect fired heaters, like a residential furnace, burn gas inside a metal tube called a heat exchanger. The air that is used to heat the application is heated indirectly when it is passed around the tubes. This air never comes in contact with the flame. The combustion products from the flame inside the tubes are exhausted through a flue pipe to the atmosphere. A direct

fired heater burns the gas directly in the air stream.

The products of combustion are included in the air that is used to heat the application.


Temperature vs. Heat

The temperature changes when the energy of a specific volume of air increases or decreases. Energy

(in the form of heat) is added to a volume of air and changes the temperature. When this energy is added to different volumes of air, different temperatures are achieved. To further illustrate this point, let’s look at how to calculate temperature rise, i.e., the difference between the air temperature after it is heated and before it is heated.


T = --------------

CFM X 1.08

This equation shows that if the CFM’s increase and the BTU’s remain constant, the temperature rise will

decrease. Conversely, if the CFM’s decrease and the BTU’s remain constant, the temperature rise will

increase. (See following examples).

These direct fired heaters are well suited for a wide range of uses in the commercial and industrial field. For kitchen ventilation, where heated make up air is needed in large volume in cool climates, the direct fired unit is a prime choice.

It is able to handle large volumes of air with substantial temperature rise at minimal expense.

The units are reliable, and most replacement parts are readily available in the field.

Industrial plants and processes where there is a large amount of air being exhausted are excellent candidates for direct fired units. Where welding hoods and plasma tables are used, the direct fired unit can provide fresh outside air at a comfortable temperature for occupants. Today the trend is


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?



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