There are generally four extreme climatic conditions that may involve an airconditioning system.

1. Hot and Humid (e.g. Miami) where both dry bulb temperature and humidity ratio is always high. Here the air-conditioning system is typically composed of a cooling coil with a surface temperature below the dew point temperature. That way, the system can achieve cooling and dehumidification.

2. Hot and Dry (e.g. Phoenix) where dry bulb temperature is high but the humidity ratio is low. Here the evaporative coolers will be most advantageous.

3. Cold and Dry (e.g., Minneapolis), the environment is typically very dry. In this case, the air-conditioning system usually is composed of a heating coil and a humidifying device, with the former being located upstream of the latter. The humidifying device may be a spray washer composed of a spray chamber in which a number of spray nozzles and risers are installed. Spray washers are typically used in industrial applications where the device performs the dual function of air humidification and cleaning.

4. Cold and Humid (e.g. Seattle), the outdoor air is cool but humid. Here, there may not be a need to humidify the air and the air-conditioning system typically is composed of a heating coil only. However, because the humidity ratio at low temperatures is low even though the relative humidity may be high, some humidification may be required when the dew point is low during the winter.

In addition to these four extreme climatic conditions, the air-conditioning system may need to operate in the fall and spring. This mixed-mode operation usually requires switching between heating and cooling based on the value of the outdoor temperature. However, because of the potential energy waste associated with this mode of operation, the system usually is set to operate only if the outdoor temperature goes outside of a pre-specified wide band (to minimize the frequency of cycling between the heating and cooling modes).

To provide the reader with an idea of the psychrometric analyses that need to be performed on an air-conditioning system, the cooling and dehumidification process is described in more detail below. The cool, humid mode of operation is identical to the cold, dry mode (except in the absence of the humidifying device).