Air cooler & Air conditioner?
Air has capacity to hold water, up to a certain level , known as saturation point, in vapour form. Also energy (or heat) is required when water evaporates from liquid state to vapour state, which is termed as latent heat of vaporization. So, if air can host this water to steam transformation, its temperature can be brought down. How much temperature can be reduced is depending on how much water can be transformed and settled inside. This ability of air to reduce temperature by absorbing water is changing with the existing level of water vapour inside it. Hence dry air(low humidity) has more ability to absorb water than humid air which is available near coastal area.
Air cooler working on above principle consists of a fan which drives area through a wet medium( say a honeycomb type filter, continuously wetted by a water from a tank.)
Maximum cooling possible with this method is limited to the saturation point(100% humidity)
When temperature comes down humidity goes up. Air cooler is not comfortable in highly humid area, say places close to sea.
A straight forward reduction of temperature when it passes through a cooling coil. Cold medium inside the coil is either a refrigerant( say R22 , R410A etc) or a chilled medium( say chilled water from a chiller). This is generally possible through a vapour compression device. A simple air-conditioner consists of a compressor compressing refrigerant gas to high pressure(automatically high temperature) then cooled at high pressure in a condenser coil to liquid state. High pressure liquid is expanded in a expansion device which results in liquid which thirsts to get converted to gaseous state. This is accomplished in the cooling coil, refrigerant absorbs heat from air surrounding the coil, to change itself from liquid to gas.