1.5.2.Waste heat recovery systems

Commercial waste heat recovery systems may be divided into passive systems and active systems. The passive systems do not require significant mechanical or electrical consumption for their operation as the heat is transferred from a higher temperature source to a lower temperature stream. these systems are used in recycling energy back into the same industrial process or recovering energy for other on-site uses. The active systems require mechanical or electrical energy for their operation and they are used to increase the waste heat temperature or to generate electricity. Examples of active systems are: cogeneration systems, heat pumps, absorption systems.

Table XXVIII shows examples of the waste-heat recovery systems used for the various heat sources from the industrial process heating systems [3].

28. Waste Heat Recovery Systems [3].

Waste heat ecovery equipment	Temperature range (°C)	Waste heat sources	Uses
Radiation recuperator	High	Boiler flue gas	Air preheating
Convective recuperator	Medium-high	Soaking/annealing oven, melting furnace, afterburners, reheat furnace	Air preheating
Furnace	High	Glass/steel-melting	Air preheating
Metallic heat wheel	Low-medium	Curing/drying oven, boiler	Air preheating, space heating
Ceramic heat wheel	Medium-high	Large boiler flue gas	Air preheating
Passive regenerator	Low-high	Drying/curring/baking oven	Air preheating, space heating
Finned-tube regenerator	Low-medium	Boiler flue gas	Make-up water preheating
Heat pipes	Low-medium	Drying/curring/baking oven/waste steam/ air dryers/kilns/recerberator furnace	Air preheating, make-up water preheating, space heating
Gas/steam turbines	Medium-high	High-pressure stream reduced for low-pressure application, waste steam	Generation of mechanical/electrical power

The components of a waste heat recovery system are [4]:

• for low to high temperature heat sources:

• heat exchangers;

• power generation equipment (ex. ORC turbine, steam turbine);

• auxiliary equipment (ex. pumps and fans);

• equipment for handling difficult waste heat sources.

• for low temperature heat sources: heat pumps and absorption units.

Heat exchangers

Heat exchangers are the devices that provide efficient heat transfer between two or more fluids at different temperatures. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact [6].

Heat exchangers are available in many configurations and designs to meet the different needs, materials, temperatures and operating conditions.

The heat exchangers can be classified as follows:

• by physical configuration:

  • shell-and-tube;

  • concentric tube;

  • plate-and-frame;

  • finned-tube;

  • spiral;

  • heat wheels;

  • heat pipes;

  • waste heat boilers.

• by fluid streams between which heat is transferred:

  • gas-to-gas;

  • gas-to-liquid;

  • liquid-to-liquid.

• by heat transfer mechanisms:

  • single phase;

  • two phase.

• by flow arrangements:

  • parallel flow;

  • counter flow;

  • cross flow.

According to the operating principle, heat exchangers can be classified as:

• recuperative exchangers;

• regenerative exchangers;

• run-around coils;

• direct contact heat exchangers.

In recuperative heat exchangers hot and cold fluids circulate through the device simultaneously, and heat is transferred through the wall separating the fluids. In the regenerative exchangers, the same heat exchange surface is exposed alternately to both hot and cold fluid; the heat taken from the hot fluid is accumulated in the walls of the device and then transferred to the cold fluid. In a run-around coil, an independent circulating fluid is used to transport heat between the hot and cold streams. In direct contact heat exchangers, heat transfer process is accomplished through direct contact and mixing the hot and cold fluids

According to the working regime, heat exchangers are divided into two categories:

• continuous flow heat exchangers (recuperative exchangers with or without heat agents mixture), where heat exchange occurs under steady state;

• discontinuous flow heat exchangers, where heat transfer occurs intermittently (accumulator exchangers in which heat is stored when available and then delivered on demand);

• periodic flow heat exchangers (regenerative exchangers, where cyclically, the heat carried away by the hot fluid accumulates in an inert material which does not react chemically with heat carriers and then is transferred to the cold fluid).

Another classification of heat exchangers is by typical use. These types can have various physical configurations such as:

• recuperators which are typically used to recover heat from flue gases to heat air;

• economizers which are typically used to recover heat from flue gases to heat boiler make-up water;

• regenerators used to recover heat from exhaust to preheat air using thermal mass accumulator;

• heat recovery steam generators (HRSG) used to recover heat for steam generation;

• waste heat boilers used to recover heat for water heating or steam generation.

The characteristics of commercial heat exchangers used in waste heat recovery application are given in Tab. V.