Flue gas heat recovery. Compressor heat recovery. Energy Trend

A plant may have high-temperature flue gases, hot water, heat from refrigeration systems and compressors, yet still lack a sound heat recovery project. The mere presence of a hot stream does not mean that it can be used at the right temperature, at the same time, and close enough to a heat sink for the investment to make economic sense.

That is why a heat recovery assessment should start with the source–sink pair. Only then should you select a heat exchanger, buffer tank, process-parameter change or industrial heat pump.

Heat recovery at a glance

Waste heat is a potential resource, not a guaranteed saving. You still need to find a heat sink that can actually use it.
The key factors are available capacity over time, temperature level, operating hours, simultaneity of source and sink, distance, and seasonality of demand.
First look for direct uses: heating water, air, raw material, process medium or another stage of the process.
A buffer tank helps bridge short time shifts, but it will not solve the absence of demand for several months of the year.
A heat pump makes sense when heat is available but its temperature is too low for the heat sink. However, the electricity needed to raise the temperature must be included in the calculation.
Economics should be based on the energy that will actually be displaced, not on the nominal capacity of the waste-heat source.

“Heat is available” and “heat can be used” are not the same thing

Industrial plants usually have more heat sources than are apparent at first glance. The U.S. Department of Energy estimates that in U.S. industry 20–50% of input energy may be lost as waste heat, including in flue gases, cooling water, hot equipment and products. This illustrates the scale of the phenomenon, but it is not a factor that can automatically be applied to a specific plant in Poland. Source: U.S. DOE.

In practice, it is useful to distinguish four levels of potential:

Level	What it means	Typical question
Heat is physically present	A medium or piece of equipment is at a higher temperature than its surroundings or another stream	Where is it generated and for how long is it available?
Heat can technically be recovered	It can be extracted safely without impairing the primary process	Can a heat exchanger, pipework or an additional circuit be installed?
Heat has a useful sink	There is a process, installation or building that needs this energy at the right time and temperature	What exactly will the recovered heat replace?
Heat recovery is economically viable	The value of avoided energy purchases exceeds the costs of investment, auxiliary energy and maintenance	How much energy will actually be used over a year, and what price is currently paid for it?

From an auditor’s perspective, I would look for the heat sink first and only then for the recovery equipment. The most common conceptual error is sizing an installation for the maximum heat source without checking whether the plant has simultaneous demand for that capacity and temperature.

Six parameters that determine the real potential for heat recovery

1. Capacity available over time

The rated power of a compressor, furnace or refrigeration unit is not the amount of heat that can be recovered continuously. Equipment may operate at part load, cyclically, with different production recipes, or only on one shift.

That is why a source profile is needed, not a single measurement point. In practice, temperature, load or flow are analysed over a typical day, week and season, and for batch processes also during individual phases of the cycle.

2. Temperature level

The higher the source temperature relative to the heat sink, the easier it is to use the heat without raising its temperature. However, knowing only the source temperature is not enough. You also need the return temperature or the temperature of the medium entering the heat sink, as well as the temperature that must be reached after heating.

For example, a source at a temperature close to the required heat-sink temperature may have high capacity but little value for a passive heat exchanger. Possible alternatives are preheating a colder stream, lowering the required system parameters, or using a heat pump.

3. Operating hours

A source operating 8,000 hours per year and one running for only a few hundred hours may have the same instantaneous capacity but completely different annual potential. Likewise, a heat sink may operate only during production hours, only in winter, or around the clock.

It is therefore worth comparing actual operating schedules, rather than assuming “the plant operates three shifts” when a specific machine or line has stoppages, changeovers and variable load.

4. Simultaneity of source and sink

Heat is easiest to use when it is generated at exactly the same time as another process needs it. This is why heat recovery within a single process is often more predictable than heating a building with heat from a year-round process.

If the source and sink are offset by several minutes or a few hours, a buffer tank may help. If the source produces heat in summer and the only sink is space heating, an ordinary storage tank will not solve the seasonal mismatch.

5. Distance

Every metre of pipework or ductwork adds cost, heat loss, flow resistance, pump or fan energy, support structures and future maintenance. Crossings between buildings, fire zones and parts of the plant that cannot easily be shut down for installation are particularly unfavourable.

For a low-temperature source, several hundred metres of transport can turn an attractive energy balance into a poor investment. With a very hot and stable source, a greater distance may be acceptable, but the infrastructure still has to be included in the calculation.

6. Seasonality of demand

Heat from refrigeration may be most available in summer, when demand for space heating is lowest. Compressor heat may be available all year, but demand from the central heating system will be high only during the heating season.

A process load, domestic hot water, preheating make-up water or another year-round stream may therefore be a better heat sink. Seasonality does not rule out a project, but it must be included in the energy balance from the outset.

Also consider: medium quality, fouling and safety

Two streams with identical capacity and temperature can require completely different solutions. Dust in flue gas, grease or moisture in exhaust air, aggressive condensate constituents, deposits in wastewater or hygiene requirements may require a different heat exchanger, an intermediate circuit, easy access for cleaning, or complete separation of the media.

Main sources of waste heat in industry

Source	Capacity over time and operating hours	Temperature level	Simultaneity of source and sink	Distance	Seasonality of demand
Compressors	the profile depends on actual load and cascade operation; do not rely on the sum of rated powers	usually a low or medium temperature level; the parameters of the available medium must be measured	the best arrangement is achieved when a process, domestic hot water or washing demand requires heat while the compressors are operating	the closer the heat sink is to the compressor room, the simpler the transfer and the lower the losses	space heating is seasonal, so a year-round process heat sink is usually more valuable
Refrigeration	the source follows the refrigeration load and often operates for many hours per year	usually a low or medium temperature level; a higher heat-recovery temperature should not be achieved at the expense of refrigeration-system performance	domestic hot water, washing and process loads that occur at the same time as cooling production are favourable	a long route may require an intermediate circuit and increases pump costs and heat losses	heat may also be available in summer, when building heating demand is low
Flue gas and boilers	capacity depends on burner load, operating hours and source modulation	often a high temperature level, but the practical heat-recovery range is limited, among other things, by the dew point and installation conditions	the most natural heat sink is a process load operating at the same time as the flue-gas source	duct length, space for the heat exchanger and the route to the heat sink can determine feasibility	the process may operate year-round, whereas space heating remains a seasonal heat sink
Furnaces, dryers and heating lines	the profile may be continuous or batch-based; the cycle schedule and duration of stream availability are what matter	often high or very high, but stream quality may constrain heat-exchanger selection	the best result comes from preheating the charge, air or the next stage in the same cycle	using the heat close to the line is usually simpler than transferring it to a distant boiler room	the main factors are production seasonality and batch schedules; building heating adds another seasonal constraint
Process operations	capacity is closely linked to the recipe, flow rate, production shift and line load	from low to high; temperature must be assessed together with the minimum parameter required by the heat sink	there is often natural simultaneity between successive process stages, but this should not be assumed without profiles	keeping the source and sink within one line usually limits transport costs; locating them in separate halls may reverse the economics	seasonality is driven mainly by the production programme and product demand
Hot water, condensate and wastewater	not only temperature but also flow rate and discharge pattern matter; short peaks may require a buffer tank	usually low or medium; often suitable for preheating cold water or as a heat-pump source	the heat sink should draw heat during discharge, or the system needs short-term storage	distance from the discharge point, sewer or wastewater treatment plant can be critical to installation cost	depends on the production programme; use for building heating remains seasonal
Ventilation and exhaust air	capacity depends on airflow, temperature and fan operating schedule	usually low; the easiest use is heat recovery to supply air or a low-temperature heat sink	supply and exhaust air often operate simultaneously, but this must be confirmed from schedules and controls	the location of air-handling units, exhausts and supply points affects the length of ductwork or glycol circuits	strongly dependent on climate and ventilation/heating needs; matching is often better in winter than in summer

Heat recovery from compressors and compressed-air systems

Compressors are among the most intuitive sources of waste heat. The U.S. DOE states that more than 80% of the input energy to a compressed-air system is ultimately dissipated as heat. This does not mean, however, that more than 80% of the electricity consumed by every compressor can be recovered economically. Some of the heat is difficult to access, some is generated in unfavourable locations, and actual heat uptake is limited by demand from the heating system or process. Source: DOE Better Plants.

In a compressor room, heat may be available, among other places, in cooling air, the oil circuit, intercoolers and aftercoolers. In practice, the viable option depends on compressor design and the manufacturer’s permitted configurations.

What needs to be checked:

the actual compressor load over time, rather than the sum of motor rated powers;
the temperature and stability of the available heat source;
the operating hours of each compressor and the compressor-room cascade control strategy;
whether the heat sink operates at the same time – for example washing, process-water preparation, domestic hot water or space heating;
the distance between the compressor room and the heat sink;
what happens to the heat in summer if the main heat sink is space heating.

The simplest option is to use warm cooling air to heat an adjacent zone. A more versatile option is a water circuit with a heat exchanger that can supply process-water preheating or another heat sink. A buffer tank makes sense when source and sink cycles are shifted by a short period. A heat pump becomes attractive when compressor heat is regularly available but its temperature is too low for the heat sink.

The order of measures also matters. Heat recovery should not be used to justify an oversized compressed-air system, excessive pressure or leaks. First, unnecessary energy use within the compressed-air system should be reduced; only then should heat recovery be designed around the energy that genuinely has to be consumed. We discuss diagnostics of the entire system in more detail on our page about compressed air system audits.

Heat recovery from industrial refrigeration

A refrigeration system removes heat from a cooled process or space and has to reject it on the condensing side. This means that chillers and industrial refrigeration systems can provide a useful source for heating water, domestic hot water, washing processes, space heating or other heat sinks.

In its industrial refrigeration documentation, Danfoss identifies heat-recovery systems as a way to use surplus heat, including for space heating and hot-water production. At the same time, the manufacturer stresses that the parameters of each application must be considered individually. Danfoss documentation.

For this source, simultaneity is particularly important. If a cold store, freezer facility or refrigeration process operates around the clock, the source can be very stable. The problem arises when the only intended heat sink is building heating: the highest refrigeration load often occurs when heat for space heating is not needed.

A year-round process heat sink may provide a better match. Heat can be used directly through a heat exchanger or desuperheater, via a dedicated water circuit, and, where time differences are short, with a buffer tank. If the heat sink requires a higher temperature, an industrial heat pump can be considered.

However, the entire refrigeration system must be assessed, not just the heat-recovery circuit. It makes little sense to deliberately worsen refrigeration operating conditions merely to obtain a higher recovered-water temperature. Changes in condensing conditions, additional pressure losses and controls must be included in the energy balance. We discuss improvements to the efficiency of the refrigeration source itself on our industrial refrigeration page.

Heat recovery from flue gas, boilers and stacks

Flue gas is one of the most important high-temperature heat sources. Depending on the process, it can be used to preheat feedwater, make-up water, combustion air, process air or another process medium. Where the temperature level is sufficient, hot water or steam can also be generated in a waste-heat boiler.

The BAT conclusions for the ferrous metals processing industry include, among other measures, using hot flue gases to preheat the charge and combustion air and in waste-heat boilers to produce steam or hot water. The same document highlights an important limitation: at an existing plant, implementation may be constrained by lack of space or by insufficient demand for steam or hot water. BAT conclusions.

Source capacity depends on instantaneous burner load, flue-gas flow, temperature and the boiler or furnace operating schedule. In a modulating system, the profile can be completely different from the rated output.

Temperature is usually an advantage, but flue gas cannot be cooled without limit. The dew point, condensation risk, chemical composition of the condensate, corrosion resistance of the heat exchanger and stack, fouling of heat-transfer surfaces and permissible flue-side pressure drop all have to be considered.

Sensible and condensing heat recovery

In conventional heat recovery, the flue-gas temperature is reduced without deliberately entering deep condensation. In condensing heat recovery, cooling is greater, allowing part of the energy associated with water-vapour condensation to be recovered as well. This requires suitable materials, condensate drainage and, where necessary, treatment, as well as verification of the entire flue-gas path.

High-temperature flue gas should usually be used directly first: to heat air, water or a process. A heat pump is more likely to appear later in the cascade, when a stable lower-temperature source remains after the first heat-recovery stage.

Furnaces, dryers and other high-temperature processes

In furnaces, dryers and heat-treatment lines, waste heat may be present in flue gas, process air, hot product, water-cooled components or equipment surfaces. The greatest potential does not always mean routing the heat to the central heating system.

A better option is often cascaded use of energy within the process: preheating combustion air, preheating the charge, drying feedstock, heating a process bath or supplying a stage with a lower temperature requirement. The U.S. DOE lists heat recovery, load preheating and combustion-air preheating as typical measures in process-heating systems. DOE materials on process heating.

For batch processes, the cycle profile is critical. For part of the time the source may have very high capacity and then virtually disappear. In this situation, a larger heat exchanger does not solve the timing problem. A buffer tank or another heat sink operating in step with the furnace cycle may be needed.

The effect of heat recovery on process quality, temperature control, gas flow and safety must also be considered. For dirty process gases, the design should provide access for cleaning, a heat-exchanger bypass and a way to keep the plant operating while the exchanger is being serviced.

Heat recovery from process operations

The most valuable sources are often located not in the boiler room but directly on the production line. They may include hot products, process baths, machine cooling fluid, hot condensate, warm process water or a stream leaving one stage of the process.

In such cases, the first option should be to check whether the heat can be transferred to another stage of the same process. The advantage is usually good simultaneity: if the hot and cold streams arise with production, their profiles are naturally linked.

The BAT conclusions for the textiles industry include, among other measures, reusing warm cooling water and warm process solutions and recovering heat from wastewater and waste gases through heat exchangers. BAT conclusions for the textiles industry.

However, two media cannot always be coupled directly. In the food, chemical or pharmaceutical industries, constraints may include hygiene, contamination risk, heat-exchanger material or required pressures. In such cases, a heat exchanger of suitable design or an intermediate circuit is used to physically separate the source from the heat sink.

If the source temperature is too low, first check whether it can preheat a colder incoming stream. Only if no such heat sink exists and a higher temperature is genuinely required does it make sense to assess a heat pump.

Hot water, condensate and process wastewater

Warm water is often overlooked because it appears less striking than hot flue gas. Yet plants with washing, rinsing, pasteurisation, wet processes or intensive cooling may have large and repeatable flows of warm water or wastewater.

The key question is: is there a cold stream at the same time that needs to be heated anyway? This could be make-up water, water for the next process stage, washing water or a medium returning to the circuit.

For water-based sources, plate heat exchangers work very well where medium quality permits. With wastewater and dirty media, deposits, clogging and cleaning must be considered, and an intermediate circuit is often needed. If discharge is short-lived while the heat sink operates more steadily, a buffer tank can increase the proportion of heat utilised.

Low-temperature water or wastewater can also serve as a potential heat source for an industrial heat pump. However, it is not enough to say that “the water is at 35–40°C”. Its flow rate, availability over time, minimum temperature after cooling and the actual heat sink on the heat-pump side must be known.

Heat recovery from ventilation and exhaust air

In ventilation systems, the source is air extracted from a production hall, warehouse, technical room or process. The simplest heat sink is supply air. Depending on whether the two air streams may come into contact, plate or rotary heat exchangers can be used, or an indirect system with fluid circulating between two air-handling units.

In industrial plants, air contamination is particularly important. Dust, grease, moisture or process substances can foul the heat exchanger or rule out solutions where there is a risk of transferring constituents between exhaust and supply air.

Fan energy must also be included. Every heat exchanger increases flow resistance. In its ventilation retrofit documentation, NREL notes that heat-recovery assessments should consider both heat-exchange effectiveness and additional pressure drop, as well as fouling issues in certain exhaust streams. NREL source.

Ventilation seasonality can be favourable when warm exhaust air is used in winter to heat supply air. If the objective is instead to use this source for a year-round process requiring a higher temperature, a heat pump may be needed.

Heat exchanger, buffer tank, parameter change or heat pump?

Technology is best selected in an order that gives priority to the simplest and least energy-intensive options.

1. Direct use and temperature cascading

The best option is achieved when heat from the source can be sent directly to the heat sink: hot air for space heating, a warm process stream to preheat feedstock, and flue gas to heat combustion air or process water.

It is worth thinking in cascades. A high-temperature source can first supply a process requiring higher parameters, with the remaining heat then used by a low-temperature heat sink.

2. Heat exchanger

A heat exchanger is the basic solution when source and sink operate simultaneously and there is a sufficient temperature difference. Its role is to transfer energy without mixing the media.

Selection should not be based solely on catalogue capacity. Inlet and outlet temperatures, flow rates, permissible pressure drops, fouling, materials, cleanability and part-load operation all need to be checked.

3. Buffer tank

A buffer tank stores energy for a limited time. It helps when a furnace operates in batches, a compressor cycles on and off, or hot water is discharged intermittently while the heat sink operates more steadily.

However, a buffer tank should not be treated as a seasonal solution. A tank intended to bridge one or several hours is a completely different matter from storing energy from summer to winter.

4. Changing heat-sink parameters

Sometimes the cheapest “heat-recovery technology” is to reorganise the system so that the heat sink can accept heat at a lower temperature. Examples include preheating cold make-up water instead of heating an already hot supply stream, connecting heat recovery to the system return, separating circuits with different temperatures, or using the heat for preheating rather than final heating.

Such a change can enable passive heat recovery and avoid the electricity consumption of a heat pump.

5. Industrial heat pump

A heat pump raises the temperature of a stable waste-heat source to a level useful for the heat sink. The DOE describes industrial heat pumps as active heat-recovery devices that are particularly useful when passive recovery is not possible. The economics depend, among other things, on whether the delivered heat replaces purchased fuel, steam or another source, and whether the cost of electricity used to drive the heat pump is lower than the value of the displaced energy. DOE source.

The key factor is the required temperature lift. The greater the difference between source temperature and the temperature required by the heat sink, the more demanding the solution becomes. Before selecting a heat pump, it is therefore worth checking whether the heat sink really needs to operate at its existing parameters.

We discuss this technology in more detail in a separate article on industrial heat pumps.

6. Using heat outside the plant

If a plant has a stable source but no sufficient internal heat sink, off-site users can be considered: another industrial installation, buildings, a district heating network or another local application.

This option, however, is much more dependent on distance, infrastructure, network parameters, contractual continuity and the seasonal profile of the heat user. The EED also requires distance, local demand and infrastructure costs to be considered in assessments of waste-heat utilisation. Directive (EU) 2023/1791.

How to assess the economics of heat recovery

The most important rule is: do not calculate savings from the source’s maximum capacity.

Suppose a source can provide 120 kW of heat at a given moment, but the nearest heat sink can simultaneously accept no more than 70 kW. In a system without storage, direct useful heat recovery cannot exceed 70 kW even before accounting for heat-exchanger losses, distribution losses and auxiliary equipment. A larger heat exchanger will not automatically turn the remaining 50 kW into savings.

The annual energy balance should then determine how many hours this level of simultaneous demand actually occurs. This is often where the difference between a “large heat source” and a “good investment project” disappears.

Benefits should primarily include

thermal energy whose purchase or generation will actually be reduced;
the parameters and efficiency of the source that currently supplies that energy;
the actual operating profile rather than the declared number of plant operating hours;
any additional process benefits, but only where they can be demonstrated reliably.

Costs must include more than the heat exchanger itself

the heat exchanger, heat pump or other heat-recovery device;
pipework, ductwork, pumps, fans, valves and controls;
buffer tank, insulation and support structures;
modifications to the source and heat sink;
electricity for auxiliary equipment;
cleaning, servicing and replacement of components exposed to corrosion or fouling;
shutdowns or installation work within the production system;
condensate treatment or disposal, where required.

In industrial projects, it is often more economical to use a smaller share of the available heat for many hours than to design the system around a short-duration peak.

What data should be collected before selecting a system?

For an initial reliable assessment, it is useful to have two data sets – one for the source and one for the heat sink.

For the source:

temperature and its variability;
load, flow rate or another parameter that allows heat availability to be determined;
operating schedule and process cycles;
type of medium, its cleanliness, pressure and material constraints;
the point at which heat can be extracted safely.

For the heat sink:

required capacity over time;
inlet temperature, required outlet temperature and permissible variability;
operating hours and seasonality;
the type of energy that recovered heat is intended to replace;
location relative to the source.

Only by comparing these profiles can you determine whether the project requires a simple heat exchanger, a buffer tank, a circuit modification or a heat pump.

Heat recovery, Polish white certificates and current regulations

As at 13 August 2026, the applicable consolidated text of the Polish Energy Efficiency Act is published in the Journal of Laws of 2025, item 711. The Act lists energy recovery, including in industrial processes, as a type of energy-efficiency improvement measure. It also refers to the use of waste heat from industrial installations for heating or cooling facilities. Polish Energy Efficiency Act – ELI.

This means that a heat-recovery installation can be assessed for eligibility for energy efficiency certificates, i.e. Poland’s white certificates, but this is not an automatic subsidy for every investment. The statutory conditions must be met and energy savings demonstrated. As a rule, a certificate is not available where the average annual saving is less than 10 toe.

Timing is also crucial. An application for a certificate is submitted by an entity planning to start the works, and the Polish Act defines the start of works, among other things, as the start of construction works related to the measure, the first legally binding commitment to order equipment, or another commitment that makes the measure irreversible – whichever occurs first. Eligibility must therefore be established before such a commitment arises; the title of a document or agreement alone does not determine its legal effect. Polish Energy Efficiency Act – ELI.

At EU level, Directive (EU) 2023/1791 on energy efficiency (EED) provides, among other things, for a cost-benefit analysis of waste-heat utilisation for newly planned or substantially refurbished industrial installations with an average annual total energy input exceeding 8 MW. This is, however, an obligation addressed to Member States as part of implementation of the Directive, not a requirement that should automatically be treated as a current standalone obligation for every Polish undertaking. EED.

DRAFT LEGISLATION — NOT YET IN FORCE. According to the Polish Council of Ministers’ legislative work programme, UC77, which is intended to implement further elements of Directive (EU) 2023/1791 into Polish law, remained a draft bill as at 13 August 2026. The planned date for adoption of the draft by the Council of Ministers was indicated as Q3 2026. The legal position should therefore be checked again before starting a specific investment. UC77 draft bill – gov.pl.

Common mistakes when assessing heat recovery

treating equipment rated capacity as constant recoverable heat capacity;
multiplying peak capacity by all plant operating hours without a load profile;
selecting a heat exchanger without checking the required temperature on the heat-sink side;
treating space heating as a year-round heat sink;
ignoring distance, distribution losses and pump or fan energy;
using a buffer tank to solve a seasonal mismatch rather than a short time shift;
selecting a heat pump without checking whether the heat sink can operate at a lower temperature;
failing to account for fouling, condensation, corrosion or hygiene requirements;
counting only the value of recovered heat without the cost of worsening the performance of the primary equipment;
starting the investment before checking eligibility for Poland’s white certificates.

FAQ – waste heat recovery in industry

Can all waste heat be recovered?

No. Heat may be physically present but technically inaccessible, at too low a temperature, available at a different time from demand, or too far from the heat sink. What ultimately matters is the amount of heat that can be used safely and economically.

Which waste-heat source is best?

There is no single best source. A good source is stable, has a useful temperature level and operates at the same time as a nearby heat sink. A compressor at a moderate temperature may therefore be a better source than very hot flue gas if its heat can be used throughout the year.

Is a heat exchanger always enough for heat recovery?

No. A heat exchanger is sufficient when the source is enough hotter than the heat sink to transfer the required capacity and both streams are available at the same time. If simultaneity is lacking, a buffer tank may be needed; if the source temperature is too low, the heat-sink parameters may need to be changed or a heat pump used.

When is a buffer tank needed?

When source and sink operate over a similar period but their short-term profiles do not coincide. A buffer tank can capture heat from a batch cycle and release it in the following hour, but it cannot replace a year-round heat sink for a source whose heat is needed only in winter.

When is an industrial heat pump worth using?

When the waste-heat source is stable and has sufficient capacity, but its temperature is too low for the heat sink. The required temperature lift, electricity consumption, operating hours and the value of the fuel or heat to be displaced by the heat pump then need to be compared.

Can heat recovery qualify for Polish white certificates?

Yes, if the measure meets the conditions of the Polish Energy Efficiency Act and an audit demonstrates the required energy savings. Eligibility must be checked before work starts, because beginning implementation before the application is submitted may prevent effective use of the certificate.

Where should a plant assessment begin?

Not with a heat-exchanger catalogue and not by asking about heat-pump capacity. First build a map of sources and heat sinks and compare their temperature and capacity profiles over time. Only then can you see which source–sink pairs make technical and economic sense.

If you want to verify the heat-recovery potential using data from a specific plant, contact us. The starting point should be a comparison of source and heat-sink profiles, followed only then by the selection of a heat exchanger, buffer tank, process-parameter change or heat pump.