A typical problem with industrial and institutional buildings is negative pressure. As noted in an earlier post, the remedy for this is Make-up Air, simply replacing each cubic foot of air that is exhausted. This can be a challenge if the load varies, for instance if there are several exhaust fans that may stage on and off at various times, if independent paint booths are operating, or if loading doors are opened and closed, thus changing the space pressure. The obvious result of any of the above scenarios will be positive space pressure. Positive pressure can be as problematic as negative pressure, albeit in a different way. In most cases the goal is to keep the space pressure as close to neutral as possible.
One way to accomplish this balance is through varying the make-up air supply. This can be done with either a variable frequency drive (VFD) on the supply fan motor, or by varying the position of an integral discharge damper. To control for this, a space pressure sensor of some sort is utilized. This sensor will sample the operating space and compare that to a balanced space, typically outside the building. The sensor will either be configured as a sensor/controller utilizing floating point or analog signal to address the VFD or discharge damper, or it will be a separate sensor and a manual, operator-controlled potentiometer. Using a potentiometer requires a person to monitor the sensor and adjust manually for balance. The sensor/controller, on the other hand, will always seek zero and will act to either increase or decrease airflow in the space by varying fan speed via VFD or discharge damper opening.
Further, there are specific design issues to be considered, such as the supply fan curve, the degree of anticipated pressure drop in the space, and number and volume of supply units, etc…. If the drop is larger than the unit fan curve will allow, multiple units may have to be utilized and staged according to demand. Although often overlooked, utilizing a variable air configuration on make-up air units is practical and affordable, and should be considered in any application with varying discharge demands.
A second option for balance is to maintain constant airflow from the make-up air unit and vary discharge fan speed and operation. In some industrial finishing and product application cases this option is preferable as it maintains a constant airflow across the worker, as defined in the Industrial Ventilation Guide and/or associated codes. In most of these applications the sequencing will be the same, utilizing space-pressure controls and VFD drives for the exhaust fans.
The best approach to address negative buildings is to have a qualified specialist take a look at all concerns and options and offer solutions as needed. RJ Owen Associates is a full service representative operating in the industrial heating and ventilation market. We can help with design, installation, commissioning, operation, and maintenance of industrial ventilation systems.