Energy Conservation

How much evaporation can be realized from each BTU of heating medium? There is, of course, no single answer to this question.  Each evaporation assignment requires its own answer.  Relative fouling characteristics and boiling point rise are two of the many basic physical considerations that relate to potential energy economy.  The ultimate practicality of any energy conservation approach can only be determined after all economic factors are considered.

Multiple effect, thermal vapor recompression and mechanical vapor recompression are the three basic methods for realizing more from each BTU of heating medium.  As the following discussion indicates, in each case, the heat contained in the exhaust vapor is saved to be utilized again.

 

Multiple Effect

In this approach, two or more evaporator effects are arranged in series such that the vapor from the preceding is used in the steam chest of the following effect.  The result is a reduction in the use of steam in proportion to the number of effects - i.e. when compared to a single effect, two effects cut the steam requirement to about 1/2, three effects to about 1/3, four effects to about 1/4, etc.  Water consumption is also reduced.  Teach effect can utilize a different type of evaporator approach depending on the product being processed and the results desired.  

 

 

Thermal Vapor Recompression

In thermal vapor recompression, relatively high pressure steam (usually above 75 pounds gauge) is introduced at high velocities through one or more nozzle into the inlet chamber of a thermal recompressor booster.  There motive steam picks up or entrains a quantity of lower pressure vapor from the vapor/liquid separator.  The mixture of steam and vapor is then passed through a vanturi at sonic velocities to convert velocity energy to pressure energy at the booster discharge (an absolute pressure somewhere between that of the motive steam and the process vapor pressure).  The ratio of vapor pic-up per pound of motive steam varies with motive steam pressure and the compression ratio selected.  

If the product has a relatively low boiling point rise and is comparatively non-fouling, thermal vapor recompression can prove to be an economical approach to energy conservation - effectively converting a single effect evaporator to a double effect efficiency.  Added to a double effect evaporator, an efficiency comparable to to three effects can be realized.  Greater energy economy can be realized by designing a multi-effect system (6 effects of more) to operate with a low compression ratio and relatively low temperature difference.  This approach can increase energy efficiency to a magnitude of over 10lbs of evaporation per pound of motive steam but requires larger surface areas. 

Although thermal vapor recompression is the most economical approach to recompression, requiring the lowest equipment and installation cost, careful evaluation of each system is essential to determining the correct point of application.  Sometimes, too, when retrofitting, the operation economies can be accompanies by a significant increase in evaporation capacity. 

Mechanical Vapor Recompression

In this approach, evaporated vapor is ducted to a suitable mechanical compressor normally driven by an electric motor.  A steam turbine drive can be employed when high pressure steam is available and low pressure exhaust steam is desired.  The compressed vapor (now at higher pressure and temperature) is then used as the heating medium in the evaporator to again evaporate vapor from the product. 

Mechanical vapor recompression can be applied in either single or multi effect operations concentrating liquids with low boiling point rises.  Low compression ratios are desirable in order to realize the greatest economy bat the the expense of requiring more evaporator surface area.  Compression ratios selected are influenced by energy costs as well as by the materials of construction for the evaporator.  Usually a single effect mechanical recompression system will operate at an economy equivalent to a steam system having 14 to 19 effects, or up to 50 theoretical effects.  Depending upon the cost of electricity, the operation of the mechanical unit may cost between 1/4 to 1/2 what a steam heated quadruple effect evaporator would require. 

Many factors must be considered when evaluating units.  In addition to energy costs, utility available and amortization of capital cost must be studies as well as adaptability to existing plant conditions.  Buflovak can assist in making these studies and comparison.