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[Art Montemayor]

Dar:

I’m very surprised - and perhaps alarmed – to read that a Chemical Engineering student doesn’t know the difference between the various Unit Operations cited. Here, I’m assuming you are at least a 3rd year university student (since you are reading Don Kern’s book) and have already taken at least a full year of slated ChemE courses such as Introduction to ChemE, Unit Operations, and other entry courses. But that OK, you may just be a 1st year student who’s interested in getting ahead so I’ll dedicate some time to your query. No question is a stupid question - if asked in a sincere effort to learn and advance in your knowledge.

Boiler = In the conventional sense of the word, this is an apparatus designed to produce water vapor – steam – at specified conditions under steady conditions and demand by burning a fuel.. A boiler can be either “fire-tube” (also called a “Scotch Marine” type) or “water-tube” type. A fire-tube model employs the produced fuel combustion products (flue gas) within a nest of tubes that are surrounded by a water bath. The water-tube model employs the combustion process outside of tubes that are filled with water. The fire-tube model is for small steam capacities and the water-tube variety is for large steam production. The steam produced in a water-tube model is usually saturated. The water-tube type can produce superheated steam if required.

Reboiler = This is a heat transfer apparatus that is usually employed within a distillation tower (or any other process) where heat input is need to produce vapors from the internal fluids. The reboiler can be in the shape of a conventional shell and tube design or can be a simple coil (or U-tube bundle) inserted into the bottom of a distillation tower. It serves the purpose of producing process vapors under equilibrium conditions (the vapors are saturated) in order to employ them within the process and allow the vapors to activate heat and mass transfer (stripping & rectification).

Evaporator = This is, as you have stated, a device that usually concentrates the solute(s) within a solution by employing heat and vaporizing a portion of the liquid solution. This is usually a shell and tube design, although a direct-fired model can also be applied. In other words, the heat addition can be done by a hot fluid or by combustion products.

However, in some specific and special applications (like a refrigeration cycle) the term “evaporator” is also used to describe the heat transfer apparatus that “evaporates” the cold, liquid refrigerant by transferring heat from a hot process fluid – and thereby cooling the transfer fluid.

Vaporizer = This is usually a generic term used to describe any apparatus that literally converts a liquid into a vapor. This could be for the purposes of humidifying air (using liquid water) or converting a process fluid into a process vapor in order to employ it downstream as a vapor. This is a simple heat transfer device that only adds latent heat (no super heat is added – as in a boiler) and therefore effects a phase change. Depending on the thermal conditions of the feed liquid, the vaporizer may or may not be required to add sensible heat prior to vaporizing the feed liquid.

I hope these very short and simple descriptions serve to explain the difference between each of the terms. All these explanations should be found in a good Unit Operations text book. You can also find detail explanations regarding these heat transfer units within Perry’s Handbook for Chemical Engineers – which you should already have or be able to read at your local university library.