May 23, 2008

Waste Heat Recovery - Consider Exergy

Waste Heat - Every process engineer is aware of this word. But do you know how much % of total energy requirement in your plant is going as waste heat?....Don't Know......It can be as high as 50%.....!!!!......

Yes, it can be. So what is waste heat...Simply from language it is the heat wasted in the process. So it is not just flue gases which are carrying this waste heat but to your surprise the biggest carrier of waste heat, usually, is cooling water. YES.

How to minimize it.....???.

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Generally the following are the sources of waste heat.

  1. Flue gases from Fired Furnaces

  2. Distillation Column condensers

  3. Power Generation Turbine Condesers

  4. Steam Condensate

  5. Process Coolers & Heaters

  6. Other Sources
You can list out other items in this list for sharing, if you have it.

We all plan & wish to recover this heat to the maximum possible extent & thus do lot of work on analysing different combinations, their technical evaluation, Feasibility etc. For simplest example, All industries where steam is used have lot of flue gases at ~180-200°C. I have already mentioned one article for recovering heat from flue gases which can reduce the flue gas temperature up to 100°C. But the question remains - is it possible? Which stream should I use for heating & finally resulting it in useful work.

The objective is not just heating of any colder stream. The objective is to finally convert it in useful heat or work. Here comes the role of Exergy.

Exergy can be defined in simple terms as - The potential of any stream to produce useful work. So,

Energy is efficiently used when the quality of the source is matched to the quality demanded by the task. Thus, electricity is a thermodynamically sound way to drive the motor that agitates the clothes in the washing machine. It is not a thermodynamically sound way to heat up the washing machine's water. By thermodynamically matching sources to tasks, we can avoid the enormous waste of using high quality energy for low quality tasks, and minimize the growing social and economic costs of energy production

Now let us take an example of heat recovery from LP steam for easy calculations. Suppose steam is available at 140°C saturation. So its Enthalpy is 653.3 Kcal/Kg and its entropy is 1.656 Kcal/Kg/K. Now suppose you use it for heating of cold water from 40°C in an exchanger with steam exit at 102°C. In this case Enthalpy is 640.2 & entropy is 1.752 in same units.

So for this process based on exergy formula (I am not putting them here, you can find them anywhere or in my upcoming article on exergy basics)

E = T0 x (S2 - S1) - (H2 - H1)
= 298.15 x (1.752 - 1.656) - (640.2 - 653.3)
= 28.6 + 13.1
= 41.7 Kcal/Kg

This is positive so we will get work out of it. This is the maximum amount of work which can be extracted from hot stream however what we are getting is only enthalpy gain in cold water i.e. 653 - 640 = 13 Kcal/Kg, so only 30% energy is used.

Compare it with hot water heating you will find that it will be practically 95% efficient & that is why we should consider exergy approach for any heating or cooling system design.

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