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Mechanical Vapour Compression

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Overview

Evaporation and crystalization are common operations throughout many industries. Without these processes, some industries might not be able to achieve the quality and production results they need for either a concentrated product or a properly treated waste stream. A number of products and chemicals refined today would be unattainable, and a host of process by-products would be left unutilized if it were not for these processes, however, a significant obstacle is the energy use required.

To evaporate one kilogram of water at atmospheric conditions, about 2300 kJ of energy is required. If this energy were generated using a boiler fired by industrial fuel oil, it would require on the order of 70 liters of fuel oil for each ton of water evaporated, and result in some 194 kg of CO2 emissions. However, there are fortunately more efficient means of attaining the required energy.


The MVC Process

While there are several methods for generating the energy required of carrying out an evaporation or crystallization process, including Multiple Effect Evaporation (MEE) and Thermal Vapor Compression (TVC), typically none are as efficient as Mechanical Vapor Compression (MVC), and thus MVC is usually the preferred technology for most users. In the MVC process, the vapor compressor is the key component in providing the energy required for evaporation. In many cases, the vapor compressor is driven by means of electrical energy from the user’s electrical grid through a motor, although there are other suitable sources of energy such as lower grade plant steam through a turbine or lesser fuel quantities through an engine.

In most evaporator designs, the feed stream is evenly distributed onto heat transfer surfaces on the process side of the heat exchanger. Evaporation takes place when the feed stream is heated to boiling by a heat source on the opposite side of the heat exchanger. In the MVC cycle, it is the water vapor that is evaporated off of the feed stream that is pulled through the vapor compressor and thereby the temperature and pressure of the vapor is increased to be used as the heating medium on the opposite side of the heat exchanger by releasing its latent heat of vaporization. As the vapor loses its heat energy in the heat exchange, it condenses as distilled water on the distillate side of the heat exchanger. Likewise, as the feed stream loses water due to evaporation, concentrated dissolved solids of product or waste results on the process side.

In a waste water evaporator, for example, the concentrated solids may be dumped or dried while the condensate produced can be reused as process water. On the other hand, for someone producing a concentrated product such as milk powder, the product concentrate from the evaporator may be further processed and refined, while the condensate that results will be used for other plant purposes or disposed of.


 

Features

In development of present day Fläkt Woods Vapor Compressors the key design standpoints have been the cost-effectiveness in valuation of the investment cost, maintenance and operation costs over the Vapor Compressor’s whole life cycle. With drastically improved closer to centrifugal compressor performance - more than 8 C of delta t in a single stage and by combining two units in series temperature increase of more than 16 C can be achieved -, but with significantly lower investment costs and trouble-free operation, Fläkt Woods Vapor Compressors provide great value for operators of modern-day MVC plants

At Fläkt Woods, we understand the unique process, mechanical and aerodynamic performance requirements of vapor compression applications, which differ from most any other fan or compressor application.

We specifically design and fabricate our Vapor Compressors for each and every project and the unique requirements of the application. Our unique combination of impeller, bearing, and rotor dynamic technologies ensure excellent mechanical properties in terms of strength, rigidity, and load capability. Our rotor system is constructed to resist the demanding loads imparted in vapor compression applications due to process carryover, condensate and water spray, fouling, disrupted vapor flow, and temperature and chemistry challenges.

We will design most Vapor Compressors to operate direct coupled to the main driver, and usually eliminate the need for any gears, which reduces the amount of equipment and instrumentation, noise, and maintenance.

We manufacture from a variety of materials to meet specific process conditions, including high strength quench and tempered steels, Stainless and Duplex Stainless Steel Alloys.

All component and interface connections and running clearance areas are machined to ensure the best operational integrity.

Finally, besides in-process NDE and quality checks, and regardless of how large any unit may be, every fan is run tested at the factory before shipment to ensure the highest operational quality and reliability from the very start.