Direct-Fired Industrial Makeup Air Units

NISCO Co. ia an engineering representative and developer of various types of gas-fired air makeup units for industrial building ventilation.

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.

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.

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.

Direct-fired pressurization equipment is capable of delivering plenty of ventilation to a facility when it is configured as the 80/20 system described previously. Infrared systems, however, may need to be supplemented with a separate ventilation system. With certain classes of facilities, the infrared system would not require additional equipment for ventilation, since large structures with many doors would provide enough natural ventilation through leakage.

Gas-fired pressurization systems each offer good solutions to heating large structures such as warehouses, distribution centers, aircraft hangers, and manufacturing facilities. In very cold climates, it might be beneficial to go with a hybrid solution using radiant heat at the perimeter as the primary heating source and placing pressurization units at the center of the facility for ventilation and for pressurization when doors are open.