Common sources of waste heat include:

  • high and low temperature industrial heating systems (furnaces, kilns, ovens, dryers and boilers) and their exhaust gases
  • industrial technologies such as compressed air systems
  • ventilation system extracts in buildings
  • refrigeration systems.

Waste heat minimisation and recovery begins with determining the type of heat being wasted, the temperature at which it is being wasted, the amount of heat recoverable and the means by which it can be reused. Table 1 identifies waste heat sources in relation to their level of quality. These areas present a number of opportunities for cost-effective heat loss minimisation and recovery.

Table 1: Waste heat sources and quality

Waste heat source Quality of waste heat
1 Heat in flue and exhaust gases The higher the temperature, the greater the potential value for heat recovery.
2 Heat in vapour streams As above, but when condensed latent heat also is recoverable.
3 Convective and radiant heat lost from exterior of equipment Lower grade if collected may be used for space heating or air preheats.
4 Heat losses in cooling water Lower grade, but useful energy savings if heated water is used elsewhere to transfer heat or in commercial or district residential heating; the lower grade heat can be supplemented by the use of heat pumps to raise the quality.
5 Heat losses in providing chilled water or in the disposal of chilled water High grade if it can be used to reduce demand for refrigeration; Low grade if refrigeration unit used as a form of heat pump.
6 Heat stored in products leaving the process may be significant Quality depends on temperature.
7 Heat in gaseous and liquid effluents leaving process Poor if heavily contaminated and thus requiring alloy heat exchanger.
8 Waste heat from air compressors Low or high grade depending on compressor type.
9 Factory/warehouse exhaust ventilation Low grade for preheating replacement fresh air; efficiency can be up to 75% depending on type of heat exchanger used.

Source: Carbon Trust UK HQ (2011) Heat Recovery: A Guide to Key Systems and Applications (Opens in a new window) PDF 4.0 MB

Heat loss minimisation

Energy efficiency improvements can be achieved with existing industrial furnaces, ovens, boilers and other heated systems by reducing heat loss due to flue/exhaust gas releases, air infiltration, wall conduction and radiation.1

Figure 1: Heat losses in a standard industrial heating process.

Source: US Department of Energy (2004) A Best Practices Process Heating Technical Brief: Waste Heat Reduction and Recovery for Improving Furnace Efficiency Productivity and Emissions Performance (Opens in a new window) PDF 539 KB

Heat losses can also be minimised in buildings by installing effective insulation, double-glazing or low-emissivity windows as well as insulation of ventilation and duct systems.

Waste heat recovery

Once heat losses are minimised, investing in waste heat recovery can yield significant energy savings. The higher the temperature, the higher the quality and the more cost-effective the heat recovery will be.

Recovered heat can also be improved so it is more effective. For example, a heat pump can use low temperature waste heat to produce higher temperatures using relatively little electricity (the geothermal or heat pump is an example of this). Generating electricity on-site or replacing electric motors with gas engines can increase the amount of recovered heat available while improving full-cycle energy efficiency relative to grid electricity.

The heat recovery technologies that are most appropriate and cost effective for a particular enterprise also need to be considered. The most cost-effective heat recovery technologies tend to improve the energy efficiency of heat generating processes themselves. These heat recovery opportunities include:

  • pre-heating of combustion air for boilers, ovens, furnaces, kilns and other industrial process pre-heating
  • hot water generation, including pre-heating of boiler feed water
  • space heating of buildings through heat recovery in ventilation systems
  • power generation using cogeneration and trigeneration systems.

There are numerous technologies for heat recovery procedures that can improve efficiencies and reduce costs. Table 2 provides more detailed analysis of which heat exchange or cogeneration technology to use for each specific waste heat recovery opportunity.

Table 2:  Sample of waste heat recovery strategies and enabling technologies  

High and medium temperature industrial and manufacturing processes Temperature (Celsius) Common waste heat recovery strategies (general) Heat recovery technological options
Furnaces (metals)

600–1650

– Steam generation for process heating or for mechanical/electrical work

– Combustion air preheat

– Furnace/kiln/oven load
pre-heating

– Transfer heat to medium/low temperature processes

– Pre-heating boiler feedwater

– Using waste heat boilers for electricity generation

– Pre-heating recycled feed materials before they are melted in furnaces

Cogeneration and trigeneration (containing heat exchangers, prime movers and electric generator technologies) as well as heat exchange technologies including the following:

– Recuperators
– Regenerators
– Heat wheels
– Heat pipes
– Ebullient cooling systems
– Forced circulation systems
(cooling system)
– Economisers
– Deep economisers
– Waste heat recovery boilers

Cement kiln (dry process) 620–730
Glass melting furnace 1000–1550
Solid waste incinerators 650–1000
Fume incinerators 650–1450
Steam boiler exhausts 230–480
Gas turbine and exhausts 370–540
Drying and baking ovens 230–600

Cooling water and steam production from furnaces, ovens, kilns, hot processed liquids and solids

27–260

– Pre-heating boiler feedwater

– Steam generation for process heating or for mechanical/electrical work

– Waste heat recovery boilers

– Cogeneration and trigeneration (containing heat exchangers, prime movers and electric generator technologies)

Building ventilation systems 20–25

– Heat exchange to the incoming cool fresh air by the hot outgoing exhaust air

– Thermal wheels

– Plate heat exchangers

– Heat pipes

Refrigeration plant or water chillers

20–90

– Recovery of superheat and hot water to be
re-used

– Refrigerant-to-water heat exchanger (de-superheater)

Source: Adapted by Smith M, from US Department of Energy (2008)2 and UK Carbon Trust (2011)3

For more information

  • Waste Heat Recovery: Technology and Opportunities in US Industry 2008 (Opens in a new window)
    • US Department of Energy and BCS, Incorporated
    • PDF 4.7 MB

    This report provides an extensive overview of conventional and developing heat recovery technologies in the United States and abroad. It evaluates key industrial waste heat sources, describes current practices and identifies barriers to waste heat recovery. It also suggests research, development and demonstration efforts that can further promote heat recovery practices.

  • Heat Recovery: A Guide to Key Systems and Applications 2011 (Opens in a new window)

    The application of heat recovery techniques can significantly reduce energy consumption, running costs and carbon emissions. This technology guide outlines the basic principles of heat recovery as well as some of the common terminology. It looks at applying heat recovery to various systems and processes within buildings. This guide is divided into six sections by heat recovery sector, namely the basics, boilers, refrigeration, ventilation, industrial processes and next steps.

There are numerous heat recovery technologies available which can assist businesses in finding cost-effective ways to recover heat, improve efficiencies and reduce costs (see Table 3).

The higher the temperature of the excess heat, the more likely it is that recovery will be cost effective; many low-temperature heat recovery options exist in industry and commercial buildings. For example, a heat pump can compress low-temperature ambient or waste heat to generate higher temperatures using relatively little electricity (the geothermal heat pump is an example of this).

Table 3: Examples of waste heat recovery strategies and enabling technologies

Heat recovery technology

Temperature range

Typical sources of waste heat

Typical uses

Type of heat exchange

Cogeneration and trigeneration

low–high

Exhaust from gas turbines, reciprocating engines, incinerators and furnaces, cement kilns

Electricity and steam generation

gas–liquid

Radiation recuperator high

Soaking or annealing ovens, melting furnaces, incinerators, radiant-tube burners, reheat furnace

Combustion air preheat

gas–gas

Convection recuperator

medium–high

Soaking or annealing ovens, melting furnaces, incinerators, radiant-tube burners, reheat furnace

Combustion air preheat

gas–gas

Furnace regenerators high

Melting furnaces, reheat furnaces

Combustion air preheat

gas–gas

Burner regenerators high

Radiant-tube burners

Combustion air preheat

gas–gas

Rotary regenerators high

Exhaust ventilation, exhaust from boilers, high temperature furnaces such as aluminium

Combustion air preheat

gas–gas

Metallic heat wheel

low–medium

Boiler exhaust, curing and drying ovens

Combustion air preheat, space heat

gas–gas

Hygroscopic heat wheel

medium

Boiler exhaust, curing and drying ovens

Combustion air preheat, space heat

gas–gas

Ceramic heat wheel

medium–high

Large boilers, incinerator exhaust, melting furnaces

Combustion air preheat

gas–gas

Plate-type heat exchanger

low–medium

Exhaust from boilers, incinerators & turbines

Drying, curing, and baking ovens

Combustion air preheat, space heat

gas–gas, gas–liquid

Heat pipe low–high

Waste steam, air dryers, kilns (secondary recovery), reverberatory furnaces (secondary recovery)

Drying, curing & baking ovens

Combustion air preheat, boiler makeup water preheat, domestic hot water, space heat

Drying, curing & baking ovens

gas–gas, gas–liquid

Finned tube heat exchanger

low–medium

Boiler exhaust

Boiler feedwater preheat

gas–liquid

Waste heat boilers

low–high

Exhaust from gas turbines, reciprocating engines, incinerators, furnaces

Hot water or steam generation

gas–liquid

Tube shell and tube exchanger

low–medium

Refrigeration condensates, waste steam distillation condensates, coolants from engines, requiring heating air compressors, bearings & lubricants

Lliquid feed flows requiring heating

gas–liquid, liquid–liquid

Deep economizers

low

Exhaust gases from gas turbines, reciprocating engines, incinerators and furnaces, cement kilns

gas–gas

Indirect contact condensation recovery

medium–high

Exhaust from gas turbines, reciprocating engines, incinerators and furnaces, cement kilns

Water is run across systems with hot gases which cools the gases (past condensation point) whilst turning the water into steam [what does the above mean?]

gas–liquid

Direct contact condensation recovery

medium–high

Exhaust from gas turbines, reciprocating engines, incinerators and furnaces, cement kilns

Water is run across systems with hot gases which cools the gases (past condensation point) whilst turning the water into steam

gas–liquid

Transport membrane condenser

high–low

Exhaust gases from gas turbines, reciprocating engines, incinerators and furnaces, cement kilns

Boiler feedwater makeup

gas–liquid

Heat pumps (upgrading low temperature waste heat)

low–high

Low temperature product streams found in process industries including chemicals, petroleum refining, pulp and paper, and food processing.

Upgrading  waste heat to a higher temperature, or using waste heat as an energy input for driving an absorption cooling system

liquid–gas
Thermal wheels low Building ventilation exhaust streams

Heat exchange to the incoming cool fresh air by the hot outgoing exhaust air

gas–gas

Plate heat exchangers

low Building ventilation exhaust streams

Heat exchange to the incoming cool fresh air by the hot outgoing exhaust air

gas–gas
Heat pipes low Building ventilation exhaust streams

Heat exchange to the incoming cool fresh air by the hot outgoing exhaust air

gas–gas

Source: Adapted by Smith M, from US Department of Energy (2008) and UK Carbon Trust (2011)