Fractionating tower separates a gaseous phase into fractions. The tower operates by condensing the vapor into liquid. Towers are generally filled with packing. The fractionating tower contains horizontal steel trays for separating and collecting the liquids. At each tray, vapors from below enter perforations under the bubble caps. The latter permit the vapors to bubble through the liquid on the tray, causing some condensation at the temperature of that tray. An overflow pipe drains the condensed liquids from each tray back to the tray below, where the higher temperature causes re-evaporation. The evaporation, condensing and scrubbing operation is repeated many times until the desired degree of product purity is reached. Then side streams from certain trays are taken off to obtain the desired fractions. Products ranging from uncondensed fixed gases at the top to heavy fuel oils at the bottom can be taken continuously from a fractionating tower. Steam is often used in towers to lower the vapor pressure and create a partial vacuum.