Mechanical Vapour Re-Compression System

There is a law in Physics which states that no one can create or destroy energy. You can only convert one form of energy into another. Mechanical vapour recompression system is one such energy recovery process.

Usually, you find substantial wastage of energy in industries. One of the prime examples is the wastage of steam through evaporation. Steam has tremendous power. James Watt realised the potential of steam to invent the steam engine. Therefore, wastage of steam is a criminal waste of energy. You have methods to recycle this steam energy to do various kinds of work. The mechanical vapour re-compression system works on this principle.

Mechanical Vapour Re-compressor (MVR) – The Concept

MVR involves taking low-pressure water vapour and compressing it to generate energy. The result is that you have a small volume of vapour at high temperature and pressure. One can use this energy to do useful work. It is possible to use this energy as a heating medium thereby reducing the operating cost of the mechanical setup.

Mechanical Vapour Re-compressor – The Working

Usually, steam escapes out of the evaporator and dissipates into the atmosphere. It is wastage of energy in many ways. There are ways to channelize this energy and reuse it. The MVR works on this concept.

It is a system where one uses the compressor to recompress the vapours generated by the evaporator at high pressure using electrical energy. On compressing these vapours, you reduce the volume but generate energy. You use this converted energy as a heating medium thereby reducing the need for fresh steam or energy to heat the furnaces.

Of course, you use electrical power to recompress the vapours in the MVR, but the cost evens out over a period. It is because the cost of steam is increasing steadily. Therefore, the MVR becomes a more economical option as compared to the conventional multiple effect evaporators.

There is a similar option in the form of Thermal Vapour Re-compressor (TVR) system where you use thermal energy to recompress the vapours instead of mechanical and electrical power. The MVR is an advantageous system when compared to the TVR.

Mechanical Vapour Re-compressor – The advantages over the TVR

  • The MVR system results in a reduction in steam consumption as it uses electrical power for recompressing the vapours. It automatically results in the decrease in operating costs.
  • .The evaporators need a continuous supply of cold water. The MVR envisages a reduction in the consumption of cold water thereby decreasing the overall costs.
  • The MVR can work with a lower capacity steam boiler as compared to the TVR system.
  • Similarly, you need a cooling tower of lower capacity.
  • Therefore, the MVR needs less space as compared to the TVR.
  • The TVR uses multiple effect evaporators whereas a single effect evaporator can do the job in the MVR system.
  • The reduction in the overall costs reduces the payback period simultaneously.

This table will make things clear. We compare the MVR with the TVR and see how the MVR reduces the payback period.

We make the following assumptions to facilitate the calculations.

  • Steam Cost – ₹ 2.50 per kg
  • Power Cost – ₹ 5.00 per KW
  • Cooling water cost – ₹ 1 per cubic metre per hour
  • Operating hours – 20 hours a day and 300 working days in a year
Triple Effect Evaporator with TVRSingle Effect Evaporator with MVR
Steam Consumption570 kg/hr400 kg/hr – for 3 hours
Cooling water consumption51 cubic metres/hour35 cubic metres/hour – 3 hours
Power Requirement66 HP270 HP
Steam Cost₹ 85 lakhs₹ 4.50 lakhs
Cooling water cost₹ 3.05 lakhs₹ 16,000
Power Cost₹ 10.34 lakhs₹ 42.30 lakhs
Total Cost₹ 98.40 lakhs₹ 46.96 lakhs

Therefore you save ₹ 51.44 lakhs per year. You have to factor the difference in the capital cost. It is around ₹ 40 lakhs. Consequently, you can see that the MVR is beneficial over the TVR as it comes with a payback period of 10 months.


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