Condensers

The purpose of a condenser is to extract heat from a vapor so it condenses into a liquid. It's as simple as that. Or is it?

Supplying energy to something can be as easy as striking a match. Removing it can be much more difficult. When we discussed vaporization, we showed that it takes a lot of energy for molecules to move from the liquid to the vapor phase, and vice versa. When a molecule goes from the vapor to the liquid phase, this same energy is given up, and it must go somewhere. This is a great deal of energy, particularly in the case of water, so any method we use must be very efficient if the condenser is going to be a reasonable size.

Factors Affecting Condenser Efficiency Surface Area and Temperature Differential

Transfer of heat is directly proportional to the amount of area available for heat to pass through, and to the difference in temperature between the coolant and the substance to be cooled. If you double the area, you double the heat transfer. If you double the difference in temperature, you double the heat transfer. The basic design of any condenser is maximize the surface area for heat transfer.

Thermal Resistance and Thickness

The type of material used to make a condenser is very important. Most handbooks detail the "thermal conductivity" of materials, but we think it's simpler and more useful to think in terms of "thermal resistance". The numbers are easier to comprehend, and thinking about how well materials hinder the flow of heat can lead to better insights in condenser design. Since the materials we use to make a condenser form a barrier between the coolant and the vapor we want to condense, it's helpful to know how high those barriers are.

On the scale below, copper (which is an excellent heat conductor) is given a resistance rating of 1. It's 3 times more difficult for heat to flow through brass, 490 times for glass and 13,000 times for air. In fact, air is a very good heat insulator.

Substance Thermal Resistance

This table compares these materials at equal thickness. If the thickness is halved, heat transfer will double, and if thickness is doubled, heat transfer will be cut in half.

If heat transfer were the only consideration, copper would clearly be the best material to use. So why are laboratory condensers made of glass when it has such high thermal resistance? There are three main reasons:

• glass is an inert material that can withstand the corrosive effects of most chemicals,

• skilled glassblowers can easily fashion intricate designs,

• chemists can see the results of reactions, like changes in color.

In a laboratory, these factors outweigh the high thermal resistance and inefficiency.

Air Cork

13,000 7,800 2,354 2,000

Liquid ethanol

Wood (dry woods, average)

Water

Glass

Plastics (PTFE, polypropylene, etc)

Stainless Steel - 321

Stainless Steel - 410

Brass

Aluminum

Copper

In the distillation of ethanol, we're not dealing with corrosive materials, we don't need intricate designs, and we don't need to see what is happening, so we can use materials like copper and brass.

The thermal resistance of the material used for a condenser is very important, but it doesn't tell the whole story. When a vapor condenses on a cool surface, it coats that surface with a thin film of liquid. Compare the thermal resistance of liquid ethanol and cork. As soon as ethanol condenses on a surface, it forms a layer of heat insulation that drastically reduces the efficiency of the condenser! It pays to ensure that the distillate runs off the cooling surface rapidly and is not held up in any way. Smooth surfaces are best because they don't hold up the condensate. In the same way, if you use fins or spines, they should be oriented in the direction that condensate will flow.

Turbulence

A vapor molecule will only give up its heat when it actually contacts a cool surface. A vapor molecule 3mm (V8 inch) away from the cooling surface sees no cooling effect. The normal motion of the vapor molecules will ensure that contact is made eventually, but increasing the vapor's motion will increase efficiency (just like blowing on a spoon of hot soup). Anything that increases turbulence in the vapor increases efficiency, and even a slight disturbance of the vapor flow is enough. Cooling vanes often have small twists and bumps for this purpose.

This principle applies to liquids as well as gases. The more turbulent the flow of coolant, the better the heat transfer. Static (unmoving) water is a reasonably good heat insulator, which is why a wetsuit can keep you comfortably warm while swimming in cold waters.

Flow Direction

Heat exchangers are usually far more efficient if the cooling liquid and the substance being cooled flow in opposite directions. The simple explanation for this is that it keeps a larger temperature difference over more of the heat transfer surface. Many models exist to describe this behavior, and the deeper you look, the more complex they get. Fluid thermodynamics is beyond the scope of this book, but you can do the simple experiment to see how it works in your condenser.

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Responses

  • mirin grant
    What size condensor for 2" column?
    7 years ago

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