Heat exchange

How to improve waste water treatment

All EVALED® product lines are equipped with heat exchangers that optimise treatment potential and offer opportunities to reduce energy consumption.

Heat (energy) is exchanged between two systems having different temperatures. If heat is created during this process, the amount of heat given up by the first system is equal to the amount of heat received by the second (principle of conservation of energy).

Heat exchange can take place in three ways:

  • Conduction: when a temperature gradient is present in a stable medium, solid or liquid
  • Convection: when heat exchange occurs between a surface and a moving fluid, both having different temperatures
  • Irradiation: occurs between two surfaces having different temperatures, through the emission of energy in the form of electromagnetic waves and in the absence of an interposed medium (e.g. radiation from the sun on the earth).


Heat exchange is related to the amplitude of the thermal gradient (delta T) and the exchange surface, based on a constant that depends, in turn, on the material, how the surface is made and how dirty it is, according to the formula:

Q = K x S x ∆T / s

Heat exchange by conduction occurs between objects having different temperatures, without an exchange of matter.

In the case of an object with a surface (S) and a thickness (s) at an internal temperature (T1) and a lower external temperature (T2), the intensity of heat transfer is proportional to the temperature difference (∆T = T1-T2) between the two wall surfaces, and depends on the latter's characteristics, according to a heat transfer constant (K).

The flux (Q) will be higher the more:

  • the larger the surface area
  • the lesser the thickness
  • the higher the constant K

This means that more heat will be transferred if the greater the temperature difference, the larger the heat exchange surface, the smaller its thickness, and the higher the transmission coefficient or thermal conductivity. The latter (K) depends on the type of material, shape and fouling.



To bring water (1 kg) from room temperature to boiling temperature and to allow the transition from the liquid to the vapour state, at atmospheric pressure (100 kPa), the system requires approximately 2500 kJ (approximately corresponding to 700 Wh).

This energy is irretrievably lost to the atmosphere if the latent heat of condensation is not recovered.

There are technologies that can recover this energy and reuse it within the cycle, allowing less energy to be consumed for water evaporation:

  • Heat pump vacuum evaporation
  • Multiple-effect evaporation
  • Mechanical vapour recompression evaporation

By using such technologies, energy consumption can be reduced by up to 25 times.