Desiccant and evaporative cooling open cycles: standard and novel systems


Standard desiccant systems are used to produce conditioned fresh air directly. Therefore, they can be used only if the air-conditioning system includes some equipment to remove the surplus internal loads by supplying conditioned ventilation air to the building. In the majority of cases this air-flow consists of ambient air, which needs to be cooled and dehumidified in order to meet the required supply air conditions. In desiccant cooling systems employing sorptive rotors, the components installed in the air-handling unit are activated according to the operation mode of the air-conditioning system.

These operation modes implement different physical processes for air treatment, depending on the load and the outdoor air conditions.

DEC systems are based on the physical principle of desiccant and evaporative cooling. Unsaturated air is able to take up water until a state of equilibrium, namely saturation has been achieved.

The evaporative cooling process uses the evaporation of liquid water to cool an air stream.

The lower the relative humidity of the air, the higher is the potential for evaporative cooling. In this type of air conditioning systems the dehumidification effect is used for two purposes: to enhance the combined evaporative cooling potential at given environmental conditions and to control the humidity of ventilation air. However, a standard desiccant cycle is not able to provide desired supply air temperature and humidity states under all conditions. Particularly in hot — Figure 1-Psychometric chart representing humid climates the desiccant cooling cycle has the standard DEC pr°cess path

limitations. Therefore in those cases employing

standard components such as a rotary dehumidifier, a combination of a desiccant cooling air handling unit with a cold backup system is needed. The ECOS provides a valuable option for air-conditioning applications in hot-humid climates without the need of a back-up system. Moreover the new concept can be implemented for small capacity plants (about 200 m3/h) overcoming a traditional restriction of standard DEC plants.