The primary difference in larger portable equipment is the possibility of adversely affecting building pressurization by ducting air into and out of the building.Positive pressurization results when more air is being delivered to a building than is being exhausted out by the same or another piece of equipment.This positive pressurization can be done with blowers, LGR’s, desiccant dehumidifiers, heating and/or cooling equipment.An example would be a desiccant dehumidifier sitting outside with only the process air being fed to the building.This would result in positive pressurization because the unit is drawing outside air into the building and there is no return line to the desiccant or another ventilation system removing air from elsewhere in the building.There are many times when a slight positive pressurization is a good setup.A slight positive pressurization is when most, but not all, of the air being processed is returned to the unit and a slight amount of air is made up or introduced into the equipment on the intake side resulting in more CFM coming into the building than leaving.This is beneficial because a slight positive pressurization will force air up flue vents, out cracks around doors and windows, and into wall cavities though electrical and plumbing fixture if allowed.This slight positive pressure will also help eliminate the possibility of moisture in the outside air from infiltrating into the building and slowing our drying time in humid climates.If climate (ambient air) control is needed on the unaffected area, it is recommended that a slightly positive pressurization is created in the unaffected area so as to reduce the chance for cross contamination and loss of environmental controls.Also, when using considerable positive pressurization, always track the weather as changes in temperature and moisture will continually change the quality of the air entering the building through your unit, unless it is set up to accommodate such changes.Neutral Pressurization is somewhat of an oxymoron in that if there is truly a neutral condition, there is no pressurization. Roughly translated this means bringing in the same amount of air as you are removing from the building or the unit is inside re-circulating the air and there is no exhaust to the outside of the building.If climate (ambient air) control is needed on the unaffected area, it is recommended that a slightly positive pressurization is created in the unaffected area so as to reduce the chance for cross contamination and loss of environmental controls.Also, when using considerable positive pressurization, always track the weather as changes in temperature and moisture will continually change the quality of the air entering the building through your unit, unless it is set up to accommodate such changes.Neutral Pressurization is somewhat of an oxymoron in that if there is truly a neutral condition, there is no pressurization.
Roughly translated this means bringing in the same amount of air as you are removing from the building or the unit is inside re-circulating the air and there is no exhaust to the outside of the building.This neutral (recirculation) or a slight positive is the best way to dry when the outside environment is more humid than inside, or the outdoor temperature is unacceptable.Negative Pressurization is produced when more air is leaving the building than entering it though your system.This occurs most commonly when a desiccant dehumidifier is configured inside a building with the reactivation air exhausted outside of the building.This configuration creates a negative pressure inside the building. This draws outside air into the structure though cracks, vents, flue stacks, windows, and doorways.This is usually an unacceptable situation unless called for by contamination removal criteria in which case engineering controls need to be set up to avoid combustion gasses, moisture, or unwanted temperature from entering the building.Pressurizations within large structures can play havoc on the Specialist ‘s drying strategy through unexpected processes within the building’s structure.In a high-rise tower, one must not forget the volume of air that can be pumped through the elevator shaft through the mere movement of the elevators themselves.These shafts can act as enormous pistons that will pump and draw air from one environment (floor) to the next.This phenomenon must be recognized in the creation of a responsible drying strategy