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Gas Fired Industrial Makeup Air Units

Date Added: February 01, 2013 02:02:22 AM
Author: Oleg Tchetchel
Category: Business & Economy: Consultancy

Gas Fired Pressurization Units move large quantities of air at low temperature differentials (usually 50° or less), which is a strategy to minimize temperature stratification in the large spaces they are employed to heat. The equipment is typically mounted on the roof of the facility or at grade on elevated supports to ensure that the supply air is delivered high. The high air delivery allows for a longer “throw” of the air, thereby requiring less equipment to cover the required floor area. The most common fuel is natural gas, however, units can be converted to burn propane. There are two primary categories of gas-fired AHUs: indirect-fired and direct-fired. Indirect-fired units burn the fuel and air mixture inside of a heat exchanger while the air traveling to the space passes over the outside of the heat exchanger. In this design, the products of combustion travel through a vent to the outside of the building. Ventilating Fan Co. ia an engineering representative and developer of various types of gas-fired air makeup units for industrial building ventilation. Direct fired units utilize air that will be sent to the heated space for combustion without use of a heat exchanger. The products of combustion are mixed directly with large volumes of outdoor air. Such mixing is considered safe because of the high dilution ratio. In addition, thorough burning of the natural gas takes place so that no harmful products of combustion enter the airstream. One product of combustion is water vapor, which can be problematic with very tight building construction due to the potential for condensation in colder climates. For tight buildings, it is best to consider the use of indirect-fired equipment. There are several common configurations of direct-fired units. The 80/20 design can vary the quantity of outside air from 100% down to 20%. The burner has a high turndown ratio and only outside air should be drawn across the burner. The discharge air volume is fixed and the quantity of outside air can be adjusted to maintain building pressurization. The makeup air unit configuration is ideal for supplying large quantities of replacement air for facility exhaust systems. Such systems can include paint spray booths and other industrial exhaust applications. These units supply a fixed amount of 100% outside air. Air Recirculation units recirculate a fixed 80% of the air while bringing in 20% outside air that passes over the burner. They supply a minimum amount of ventilation to spaces that do not require large quantities of outside air to maintain building pressurization. Variable Air Volume VAV units have a varying supply air volume and work similar to the 80/20 design, however, the recirculated air passes through a bypass section instead of through the facility. This type of unit is used in applications where building pressurization is desired and where contaminants located in the space cannot be recirculated. Facilities that have indoor vehicle operation frequently accumulate carbon monoxide and associated noxious fumes. For such instances, a ventilation sequence can be instituted to limit this potentially harmful buildup. The pressurization units for these buildings could be fitted with a carbon monoxide detector with an initial setpoint of, for example, 50 ppm that would trigger an alarm and energize a time-delay relay. If the condition still exists after, say, five minutes, then the return air damper would modulate closed and the outside air damper would modulate to the 100% open position. A second setpoint of 100 ppm would initiate an alarm and eventually de-energize the burner. The dampers would then be positioned to enable the exhaust mode. After the carbon monoxide returns to a safe level, the sensor could reset the unit to normal operation. In addition to emergency ventilation to purge carbon monoxide, the pressurization unit can be used to provide a source of ventilation during warm and hot months of the year. A reverse acting thermostat would modulate the outside air and return air dampers to bring in more outside air to maintain the indoor setpoint. These units can also be fitted with either chilled water or DX cooling coils to provide tempered or conditioned air to the space during the summer. Direct fired pressurization units are approximately 93% efficient, with much of the available energy in the burned natural gas being delivered to the space in the form of heat. The supply airstream absorbs the heat that would be normally lost through a flue or vent pipe as in the indirect-fired configuration. Additional information can be found at the Ventilating Fan Co. web site http://ventilating.com/index.html Oleg Tchechel Industrial Air Make-Up Systems Designer Ventilating Fan Co. http://ventilating.com/products/heat_exchangers.html http://www.ventilating.com/request.html
 
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