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Author: Oleg Tchechel
Date Created: Mar 27, 2009 - 3:13:14 PM
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NISCO Co. have completed a major re-design of its line of industrial air makeup units for building ventilation. In industrial plants where there is a high density of employees and many manual operations involved, an improvement in the environment can produce substantial benefits in terms of increased production, reduced errors, and a decline in complaints and absenteeism among employees.
A Direct Gas-Fired Air-Handling Unit AHU may be used as a primary or secondary source to heat a building space. When used for this application, a room thermostat controls unit outlet air temperature.
On a call for heat, a space-heating unit will provide air at the desired room temperature. An airhandler may also be designed to increase outlet temperature when room temperature is low. A duct stat or probe at the outlet of the unit prevents outlet temperatures from exceeding the maximum allowable value.
When a unit blower runs continuously, room air is monitored and the outlet air temperature is modulated to maintain the temperature.
Systems are usually designed for either Space Heating or for Make-Up Air use. However, standard Make-up Air Units may be provided with a room thermostat that monitors indoor minimum set point, a signal is sent to the unit to override outlet temperature set point and turn the unit to full fire. When minimum room temperature is satisfied, unit returns to outlet air temperature control mode.
Return Air Units are a design that includes a damper to bring room air back into the unit. These re-circulate indoor air through a unit with a minimum of outside air added. This type of unit is able to provide air movement for indoor comfort and reduced temperature stratification, but with a minimum of heat added to the building.
Return Air Units are widely used for direct fire space heating applications. They can be supplied with a maximum 80% return / 20% fresh air, per ANSI standards. Other options include a 50/50 arrangement. “80/20” Units are configured to modulate from the 80% return/20% fresh air to up to 100% outside or fresh air. 50/50 units are generally designed to switch from 50% return / 50% fresh to 100% fresh, with no modulation between.
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.
All return air should be brought in downstream of the burner and not recirculated through the burner. This is to prevent buildup of waste products from the combustion process within the building. It also prevents contaminants originating within the building environment from being passed through the direct fire burners. The combustion of unknown contaminants can be a serious Safety or Indoor Air Quality problem, and so must be avoided.
Additional information can be found at the NISCO web site http://www.nisco.net/airhandler.html.
Oleg Tchechel
Industrial Air Make-Up Systems Engineer
NISCO
http://www.nisco.net/aircurtain.html
http://www.nisco.net/fanblower.html
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