Redesign the
Bennett Hall Basement HVAC System

Introduction: In 2001 a climate control system was installed to serve the basement research laboratories and shop of Bennett Hall to provide ventilation air amounts to meet current OSHA standards of 6 air changes per hour for laboratories where chemicals are used, and a comfortable working environment year around. The system provides the ventilation air and keeps the environment at a comfortable temperature, but doesn’t provide low enough humidity on days with high dew points of outside air.

Goal: Redesign the system to provide good humidity control at ≤ ? % with comfortable working temperatures. The term “< ?” means you have to determine what is an acceptable value to design for by talking with Drs Hess and Smith for whom low humidity is important for proper functioning of their research equipment.

Background: The figure below shows a schematic of the existing system. Ventilation air for the entire basement is brought in and cooled (or heated depending on the season) in a central air handler, and is then sent to the individual room unit coolers which mix re-circulated room air with the ventilation air and provide additional cooling in summer if needed. The problem with the system is that it doesn't remove enough water vapor from the ventilation air in summer to always keep the humidity low enough. The humidity is high enough sometimes, for rust to form on some equipment, and some laboratories need humidity < ? %, 40 % has been mentioned as a preferred level, to prevent condensation on cryogenic equipment or electrical leakage from very high voltage sources.

Background: The figure above shows a schematic of the existing system. Ventilation air for the entire basement is brought in and cooled or heated depending on the season in a central air handler, and is then sent to the individual room unit coolers which mix re-circulated room air with the ventilation air and provide additional cooling in summer if needed. The problem with the system is that it doesn't remove enough water vapor from the ventilation air in summer to always keep the humidity low enough. The humidity is high enough sometimes that rust forms on some equipment, and some laboratories need humidity less than 50 %, 40 % is preferable, to prevent condensation on cryogenic equipment or electrical leakage from high voltage sources.

There are basically two alternative solutions to the lack of proper humidity control; additional moisture can be removed by

·         cooling the air to a lower temperature than at present or

·         passing the air through a desiccant dehumidifier (hygroscopic material).

Cooling the ventilation air further would result in an uncomfortably cold environment unless the very cool dry air were subsequently reheated somewhat.

Reheating can be done in many ways; the most common are a hot water heating coil or an electric heater in the air duct. Fuel combustion reheating can be done but tends to be more expensive and result in larger temperature swings about the desired temperature than the hot water coil or electric heater methods.

A desiccant dehumidifier requires the absorbed water to be driven out of the desiccant so it can be re-used over and over; this is usually done in a continuous process by passing the dry desiccant material through the cool humid air to absorb the moisture and then passing the desiccant material through a heated air stream to remove moisture from the desiccant and then dumping the heated humid air outside the air conditioned space.

Both processes require more energy than the present system.

There are economic tradeoffs between these alternative methods of dehumidification:

The ventilation air escapes the basement through existing exhaust air hoods, cracks around doors and other ventilation passages.

Milestones:

Return to project list