Coolant tube (chemistry)

A cooling tube or condenser is a laboratory apparatus, made of glass, that is used to condense the vapors that emerge from the distillation flask, by means of a cooling liquid that circulates through it, usually water.

Structure

It consists of two cylindrical tubes. The ends of the inner glass tube are generally provided with ground glass joints, so that they can be easily fitted with other glass items. The upper end can be left open to the atmosphere, or vented through a bubbler, or a drying tube to prevent the entry of water or oxygen. The outer glass tube usually has two connections where neoprene or rubber hoses fit, inlet and outlet of the coolant (usually tap water or water cooled with an antifreeze mixture) that passes through it. For maximum efficiency, and to maintain a smooth and properly directed thermal gradient that minimizes the risk of thermal shock of the inner glass tube, the coolant will usually (but not necessarily; see "Allihn condenser" more below) enters through the lower connection, and exits through the upper connection. Maintaining a correct thermal gradient (i.e., coolant entering at the coldest point) is the critical factor. Multiple capacitors can be connected in series.

The refrigerant liquid, usually water, has to be constantly circulating in the opposite direction to the gases that circulate through the condenser; in order to have a temperature at which the vapor can condense into liquid. Normally, a high coolant flow rate is not necessary to maintain proper cooling.

The equation that allows to calculate the heat exchanged between the water the steam to condense is given by the principle of the mixtures: ${displaystyle Q{cedido} = -Q{ganado}}$

and each of these terms follows the definition

${displaystyle Q = mcdot c_{e}cdot (t_{f} - t_{i})}$

where Q= heat exchanged; m = mass; c_e= specific heat; t_f = final temperature; t_i= initial temperature.

Applications

Condensers are often used in reflux, where hot vapors of a liquid solvent, which is being heated in the flask, are cooled in the condenser and allowed to drip. This reduces solvent loss and allows the mixture to be heated for long periods.

Condensers are used in distillation to cool hot vapors, liquefying them in the condensation liquid for selective collection. For fractional distillation, an air or Vigreux condenser is generally used to slow the rate at which hot vapors rise, giving better separation between the different components in the distillate.

For microscale distillation, there are commercially available apparatus that include the heating vessel and condenser cast in one piece. This reduces retention volume, avoiding the need for ground glass gaskets and preventing contamination from grease and air leaks.

Air-cooled condensers

Air condenser

The air condenser is the simplest type of condenser. There is only one tube, and the heat of the liquid diffuses through the glass, being cooled by the outside air. It is related to the models used by alchemists. The air condenser is generally used for fractional distillation and high temperature condensation, and can be filled with some materials, such as glass beads, metal pieces, or Raschig rings to increase the effective number of plates.

In a standard Liebig condenser the refrigerant liquid can often be replaced by forced air circulation.

Vigreux Capacitor

A Vigreux condenser is a modification of the air condenser. It is usually used as a fractionation column for fractional distillation. Unlike straight-walled columns, a Vigreux column has a series of downward notches on the inside that serve to significantly increase the condensation surface without increasing the length of the condenser. Because of that added complexity, Vigreux columns also tend to be considerably more expensive than traditional straight-wall designs. They are named after the chemist Henri Vigreux.

Water cooled condensers

Liebig capacitor

The Liebig condenser is the simplest design of water-cooled condenser. The inside of the tube is straight, so it is cheaper to manufacture. Although they are named after a German chemist, Baron Justus von Liebig, he cannot be credited with inventing it, because it was already known for some time before. However, it seems that he was the one who popularized the device.

The inventors in fact, all of them independently, were the German chemist Christian Ehrenfried Weigel in 1771, the French scientist, Pierre-Isaac Poissonnier in 1779, and the Finnish chemist Johan Gadolin in 1791.

Liebig himself mistakenly attributed the design to a German pharmacist Johann Friedrich August Göttling who had made improvements in 1794 based on Weigel's design.

The Liebig condenser is much more efficient than a simple retort because it uses liquid cooling. Water can absorb much more heat than the same volume of air, and its constant circulation through the outer water jacket keeps the temperature of the condenser constant. Therefore, a much larger flow rate of inlet vapor can be condensed in a Liebig condenser than in an air condenser or retort.

Graham Condenser

A Graham condenser or spiral condenser has a spiral coil that runs the entire length of the condenser. There are two possible configurations for a Graham condenser. In the first, rarely used, the spiral conducts the refrigerant, and condensation takes place on the outside of the spiral. This configuration maximizes flow capacity as vapors can flow over and around the coil.

In the second configuration, the outer tube contains the refrigerant, and condensation takes place inside the coil. This configuration maximizes condensate collection, since all vapors flow through the entire length of the spiral, therefore having prolonged contact with the refrigerant liquid.

Dimroth Capacitor

A Dimroth capacitor, named after Otto Dimroth, is somewhat similar to the Graham capacitor. It has a double internal spiral for the cooling medium so that both the coolant inlet and outlet are at the top. The vapors flow through the external tube from bottom to top. Dimroth capacitors are more efficient than conventional coil capacitors. They are often found in rotary evaporators.

Allihn Capacitor

The Allihn condenser or bulb condenser or simply reflux condenser is named after Félix Richard Allihn.

The Allihn condenser consists of a long glass tube with a jacket of water that circulates through the outer tube. It has a series of constrictions and bulges in the inner tube that increase the surface area on which the vapor components can condense. It is ideal for laboratory scale reflow.

Once again, it can be easily replaced by the straight model, taking care to introduce the coolant at the coldest point to maintain a correct thermal gradient, that is, through the lower inlet in this case.

Friedrichs condenser

A Friedrichs condenser (sometimes called Friedrich condenser), also known as spiral condenser or coil, consists of a large cold finger-type internal spiral arranged within a larger diameter cylindrical capsule. The refrigerant flows through the inner tube, consequently, the increased circulating vapors may condense on the inner tube as they cool. Compared to a Graham of similar dimension, which also includes an internal spiral tube, the Friedrich condenser often provides more efficient condensation because the Friedrich condenser provides a greater effective surface area for cooling. That is, the vapors are cooled not only by the refrigerant flowing through the inner tube, but also through the outer cylindrical wall.

The coil or spiral condenser is known as a Friedrichs condenser because it was invented by Fritz Walter Paul Friedrichs, who published a design for this type of condenser in 1912.

Other refrigerants

Apart from water, condensers can be cooled by other refrigerant fluids, such as refrigerated ethanol which can be cooled thermostatically in a recycle pump unit. The use of liquids other than water allows the condenser to cool below 0°C. This is necessary if the liquid being condensed boils at a temperature below 0°C (for example, dimethyl ether, which boils at -23.6°C).

Solids (dry ice) or mixtures (acetone/dry ice) can also be used as refrigerants, which can be used in cold finger type condensers. As with other alternative refrigerants, these do not allow cooling below 0°C.

Gallery

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  One of the simplest types — the Liebig capacitor.

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  The two configurations of the Graham capacitor.

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  The Dimroth capacitor.

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  The Allihn capacitor.

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  The Friedrich capacitor.

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  Two Liebig capacitors and an Allihn capacitor

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  The Graham capacitor

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  Cold fingers