A Combined Cooling Solution
May 8, 2006
The Nestle Research and Development Center in Marysville, Ohio, was developing a process in which cold air was needed to chill stainless steel components that also had to remain frost-free during the process. The facility enlisted Bry-Air Inc. of Sunbury, Ohio, a manufacturer of dry, desiccant-based dehumidification systems, to find a solution to its problem.
The process air had to be provided to an existing system at a -45oF (-43oC) dewpoint. The air then would be further cooled prior to being used in the process. In this application, cooling alone would not effectively achieve the desired dewpoint requirements because of the need to maintain cold coil temperatures without causing condensation that would freeze on the coil itself. A desiccant wheel would effectively dry the air but would not have the ability to achieve the desired dewpoint due to the entering air conditions.
Lowering the air temperature decreases the air's ability to hold moisture. However, cooling the air simply to remove moisture typically is not a practical approach. Cooling-based dehumidification will provide air that is slightly dryer but still close to saturation at a lower temperature. To remove large amounts of moisture from the air by cooling, over-cooling and subsequent reheating are required. Such procedures typically have problems associated with operation and maintenance, as well as cycle and control, and are unsuitable for producing large quantities of dry air.
Another limitation of mechanical cooling is the freezing point of water. When air is dried using refrigeration, the cooling surfaces can reach subfreezing temperatures, causing frost or ice to form on the coil and reducing cooling system efficiency. As a result, complex anti-icing devices or dual systems and defrost cycles might be needed. Cooling systems are simply not practical when attempting to dry air below a -45oF dewpoint.
A Dryer SolutionDesiccants can adsorb the additional moisture from an airstream based on the vapor pressure difference between the airstream and the desiccant. The greater the difference in vapor pressure, the more efficient the desiccant will be. For this reason, an airstream can pass through a desiccant and become significantly dryer without elaborate cooling, compression or other complex controls. After the drying task is complete, the desiccant is regenerated with heat and is ready to dry more air. Desiccants are equally effective for extremely low to very high levels of humidity.
In the Nestle facility, Bry-Air Inc. installed a desiccant dehumidifier with a chilled water pre-cooling coil. If the air is pre-cooled prior to entering the desiccant wheel, it will emerge from the cooling coil colder, as well as near its saturation point. The air leaving the cooling coil will have a high vapor pressure. Following a reactivation cycle, the desiccant wheel will be dry and the vapor pressure will be low. Because the cooled air is nearly saturated and desiccant is dry, the system creates a large difference in the vapor pressure, which will allow the desiccant to adsorb an even greater amount of moisture from the process airstream.
The driving force in this application was not necessarily to provide dry air, but to provide cold air to the process and eliminate the possibility of condensation and frost forming on the cooling coil or equipment. By combining mechanical cooling and desiccants, Bry-Air Inc. was able to provide the facility with the required process air at a -45oF dewpoint.