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Understanding Stills: Condensers

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  • Harry
    Micro-Distillery Hardware - Condensers by Harry Jackson Condensation of vapour occurs in a variety of engineering applications. For example, when a
    Message 1 of 2 , Jul 9, 2009

       

       

      Micro-Distillery Hardware - Condensers

      by Harry Jackson

       

      Condensation of vapour occurs in a variety of engineering applications. For example, when a vapour is cooled below its saturation temperature, or when a vapour-gas mixture is cooled below its dew point, homogeneous condensation occurs as a fog or cloud of microscopic droplets.

      Condensation also occurs when vapour comes in direct contact with a subcooled liquid, such as spraying a fine mist of subcooled liquid droplets into a vapour space, or injecting vapour bubbles into a pool of subcooled liquid. The most common type of condensation occurs when a cooled surface, at a temperature less than the local saturation temperature of the vapour, is placed in contact with the vapour.  Vapour molecules that strike this cooled surface may stick to it and condense into liquid. [1]

      The above description of condensation infers that some exchange of heat is taking place in this process.  Of course this is correct, and the equipment tused for such actions is called a "heat exchanger".

      To understand condensation and condensers in distilling, it is necessary to first understand exactly what a vapour is. A vapour is a visible suspension in the air of particles of some substance.  It is a substance in the gas phase at a temperature lower than its critical temperature.  This means that the vapour can be condensed to a liquid or to a solid by increasing its pressure, without reducing the temperature [2].  At first this may seem confusing, but read on...

      Latent Heat (enthalpy) is the "hidden" heat when a substance absorbs or releases heat without producing a change in the temperature of the substance, eg, during a change of state.[3]
      To bring a liquid mixture to boil, energy in the form of heat must be transferred into the mixture.  Once the liquid is at boiling point, still more heat must be applied to bring the now boiling mixture to a rapidly vapourizing condition (a "phase change").  Here is a graphic description of the process, showing the two phase changes that occur when taking a solid to liquid, and then to the gaseous state or phase:

      Heating Curves [3]

      A substance is heated at a uniform rate:

      • Temperature of the solid rises uniformly until the melting point is reached.

      • At the melting point heat is absorbed and used to melt the solid without any temperature change (latent heat of fusion), all the energy is going into weakening the intermolecular forces between the particles in the solid.

      • When all the solid has melted to a liquid, the temperature starts to increase uniformly again until the boiling point is reached.

      • At the boiling point heat is absorbed without any change in temperature (latent heat of vaporization), all the energy absorbed is being used to overcome the intermolecular forces between the particles in the liquid.

      • When all the liquid has been vaporized to gas the temperature will once again increase.

      Condensers are a specific form of heat exchanger. Condensers convert the alcohol vapours that were produced by heating the wash, into liquid form.  They do this conversion by removing an amount of heat called latent heat, from the vapours.  Note there's no mention of removing any more heat than necessary to achieve the phase change. If more heat is removed, then the condensate becomes subcooled.  In heat transfer terminology, this heat is often referred to as the Latent Heat of vapourization, but in this instance because we are talking about condensers and condensation, it would be more correct to call it the Latent Heat of Condensation.  The term vapourization implies putting heat in, whereas condensation is the reverse, i.e. taking heat out.  However both terms are acceptable and are used interchangeably.

       

      The Condensing Process

       The condensing process starts with heat transfer, because heat is transferred from the hot vapours, through a dividing barrier (usually a copper tube wall) and absorbed by a coolant fluid on the other side of the barrier.  Heat energy always flows directionally from the hotter to the cooler substance, and temperature will always try to equalise between adjacent items of different temperature..

      The heat transfer reduces the temperature of the vapours to the dew point and sometimes below (subcooling; depends on operator and/or processing efficiency).  Thus the vapour collapses back to a liquid, the primary product of the process.  The newly warmed coolant is continuously removed from the heat exchanger, making room for fresh coolant to absorb heat and continue the process.  The primary product, liquid ethanol is  recovered, but may be further split into two streams or fractions;  one for product and another to be sent back to the system for re-vapourization and re-condensation.  This second stream is known as Reflux, and this return process is called refluxing (similar concept to transistor amplification feedback in electronics).    This has the benefit of further increasing the strength and purity of the primary product in a single processing run.

      During condensation, the liquid collects in one of two ways, depending on whether it wets the cold surface or not. If the liquid condensate wets the surface, a continuous film will collect, and this is referred to as filmwise condensation. If the liquid does not wet the surface, it will form into numerous discrete droplets, referred to as dropwise condensation. All surface condensers today are designed to operate in the filmwise mode, since long-term dropwise conditions have not been successfully sustained. [1]

      Dropwise condensation is a complex phenomenon that has been studied for over sixty years. It involves a series of randomly occurring subprocesses as droplets grow, coalesce, and depart from a cold surface. The sequence of these subprocesses forms a dynamic "life cycle." The cycle begins with the formation of microscopic droplets that grow very rapidly due to condensation of vapor on them and merge with neighboring droplets. Therefore, they are constantly shifting in position. As a result, rapid surface temperature fluctuations under these droplets occur. This active growth and coalescence continues until larger drops are formed. Although inactive due to condensation, these drops continue to grow due to coalescence with neighboring smaller droplets. Eventually, these large, so-called "dead" drops merge to form a drop that is large enough so that adhesive forces due to surface tension are overcome either by gravity or vapor shear. This very large drop then departs from the surface, sweeping away all condensate in its path, allowing fresh microscopic droplets to begin to grow again and start another cycle. [1]

       

      References:

      1.

      Handbook of heat transfer / editors, W.M. Rohsenow, J.P. Hartnett, Y.I. Cho. m 3rd ed.
      Ch.14 "CONDENSATION" by P. J. Marto, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, California

       2.  

      http://en.wikipedia.org/wiki/Vapour

       

       

      3.

       

      http://www.ausetute.com.au/heatlatent.html

       

       

      Enjoy

      Slainte!
      regards Harry

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