the combination of these has taken the simple condenser concept through simple changes (e.g., addition, in allihn-type condensers, of "bubbles" or undulations to the inner, straight vapor tube of the simple liebig design, so that diethyl ether ( 35 °c), could be accommodated), on to many unique condenser configurations, types of packings of the vapor space, and applied cooling media and mechanisms (see below). in this array of designs, the direction of vapor and condensate flows depends on the specific application (e.g., being "countercurrent" in reflux procedures, and concurrent in many simpler distillation procedures); the same is true with regard to the optimal flow direction for the cooling fluid (air, water, aqueous ethylene glycol co-solutions, etc.) relative to the direction of vapor flow. note, while the traditional coolants, air and chilled tap water, have often been used without recirculation (that is, allowed to exit to atmosphere or drain, respectively), larger scale operations and municipal and other regulations make engineered recirculation necessary, and it is always required for special cooling liquids, such as low temperature alcohols and cosolutions.