Economisers: Enhancing Efficiency, Reducing Consumption

An economiser is a heat-recovery heat exchanger. In a boiler system, it transfers heat from hot flue gas to boiler feedwater, reducing the fuel required to raise that water to steam-producing temperature.

• How an economiser works
• The different types of economiser
• Where economisers are commonly used
• The advantages, limitations and maintenance requirements of economisers

So What is an Economiser?

An economiser, in mechanical engineering and thermodynamics, recovers heat from a hot stream and transfers it to a cooler process stream that would otherwise need direct heating.

In a boiler system without an economiser, hot flue gas leaves through the stack. A boiler economiser is installed in that flue-gas path, usually as a bank of tubes carrying boiler feedwater. Heat passes from the gas outside the tubes to the cooler water inside before the water enters the boiler.

The recoverable duty is calculated from measured flow and temperature data:

$$Q={\dot{m}}_w{c}_{p,w}(T_{w,out}-T_{w,in})$$

where $Q$ is recovered heat rate, ${\dot{m}}_w$ is water mass flow rate, $c_{p,w}$ is water specific heat capacity, and $T_{w,in}$ and $T_{w,out}$ are economiser water inlet and outlet temperatures. Fuel avoided is approximately $Q/{\eta }_b$, where ${\eta }_b$ is boiler efficiency at the relevant operating condition.

For natural-gas-fired boilers, the US Department of Energy states that boiler efficiency generally increases by about 1% for every 40°F (22°C) reduction in flue-gas temperature. Its feedwater-economiser guidance gives typical fuel reductions of 5–10% where adequate heat recovery is available and the final stack temperature is suitable. Source: US Department of Energy

How Does an Economiser Work?

Heat transfers from higher-temperature flue gas to lower-temperature feedwater through the economiser tube wall. The heated feedwater enters the boiler, while cooled flue gas passes to downstream equipment or the stack.

Economiser performance is established from heat balance and operating measurements, not stack temperature alone. Relevant inputs include:

• Flue-gas temperature, composition, flow rate and excess-air level
• Feedwater temperature, flow rate and required outlet temperature
• Available heat-transfer surface area and cleanliness
• Gas-side and water-side pressure drop
• Fuel type, sulphur content and particulate loading
• The minimum permissible gas outlet temperature for the selected materials and operating regime

The temperature difference between flue gas and feedwater provides the heat-transfer driving force. The design must balance recovered duty against condensation, corrosion, fouling and fan-power penalties.

Types of Economisers

The two principal boiler economiser types are non-condensing and condensing economisers.

Non-Condensing Economisers

Non-condensing economisers keep flue gas above the applicable dew point, preventing condensate formation in the heat exchanger and downstream ductwork.

The limiting temperature is fuel- and composition-specific. Natural-gas systems can often be designed around a final stack temperature of 250°F (121°C) in DOE’s example, which assumes 15% excess air; this is not a universal set point. Sulphur-bearing fuels require assessment of acid-dew-point corrosion risk from the actual fuel analysis and flue-gas composition. Source: US Department of Energy

These economisers are used where feedwater preheating is required but condensing operation is not justified, or where carbon-steel construction is retained.

Condensing Economisers

Condensing economisers cool flue gas below its water-vapour dew point, recovering sensible heat and latent heat released by condensation.

They require a sufficiently cool and dependable heat sink, such as makeup water, return water or a process-water circuit. They also require corrosion-resistant gas-side and condensate-contact materials, condensate drainage and, where necessary, neutralisation. Natural-gas condensing economisers commonly use stainless-steel gas-side components; fuel switching or sulphur-bearing fuels require separate corrosion assessment. Example equipment design

The appropriate type depends on fuel, flue-gas composition, feedwater or return-water temperature, operating conditions, space, maintenance capability and the required payback period.

Applications of Economisers

Boiler economisers are used where a consistent flue-gas heat source and a useful water or process-fluid heat sink exist.

One common application is in power plants, where economisers preheat boiler feedwater using flue-gas heat before it enters the steam generator.

Economisers are also used in commercial and industrial boiler plants, including hospitals, universities, manufacturing sites, food-processing facilities and district-heating schemes. They can be installed on new boilers or considered as a retrofit where adequate flue-gas temperature, space, draft capacity and water demand are available.

Economisers are also used in industrial processes. Steel mills, chemical plants, refineries and other heat-intensive facilities can recover exhaust-stream heat to preheat feedwater, combustion air, process fluids or incoming materials.

Selecting and Designing an Economiser

A successful economiser installation begins with a site-specific assessment. Hot flue gas alone does not guarantee that heat recovery will be practical or economical.

Key design considerations include:

• Heat source: Record flue-gas inlet temperature, mass flow, oxygen or excess-air level, fuel analysis and operating hours at minimum, normal and maximum load.
• Heat sink: Establish water or process-fluid flow, inlet temperature, required outlet temperature and availability at each boiler load. Size duty from $Q=\dot{m}{c}_p\Delta T$, then verify it against a gas-side heat balance.
• Temperature data: Specify gas inlet and outlet temperatures, water inlet and outlet temperatures, design approach temperature and allowable variation during turndown, starts and low-load operation.
• Dew-point protection: For non-condensing designs, set the minimum gas outlet temperature from the actual fuel, excess air, moisture and acid-dew-point assessment rather than applying a generic temperature. Use a bypass, recirculation or control valve if low feedwater temperature can drive the cold end below that limit.
• Materials and condensate: Use materials compatible with the predicted gas and condensate chemistry. Condensing sections require corrosion-resistant gas-side surfaces, sloped drainage, accessible traps and a defined condensate-disposal or neutralisation arrangement.
• Pressure drop: Define permissible gas-side pressure drop from the induced- or forced-draught fan curve at maximum gas flow, including the required stack-draft margin. Calculate added fan power as $P\approx \Delta p\dot{V}/{\eta }_{\mathrm{fan}}$. Set water-side pressure drop from the available pump head and required minimum water flow; verify both at clean and fouled conditions.
• Fouling allowance: Select tube geometry, gas velocity, soot-blowing or washing provision and access doors for the fuel’s ash and particulate loading. Include clean and fouled heat-transfer and pressure-drop cases in the duty calculation.
• Water quality: Confirm feedwater treatment and chemistry limits to prevent scale and oxygen-related corrosion on the water side.
• Controls and protection: Provide temperature, pressure-drop and flow measurements; alarms for abnormal outlet temperatures or pressure loss; freeze protection where relevant; and a bypass or isolation arrangement for cold starts, low-load operation and maintenance.

For retrofit projects, engineers should also check available space in the flue path, structural support, stack-draft capacity, drainage, maintenance access and integration with existing boiler controls.

Advantages of Using Economisers

Economiser benefits should be assessed from measured duty, annual operating hours and fuel cost.

Improved Efficiency

For a natural-gas-fired boiler, DOE guidance estimates roughly a 1% efficiency improvement for each 40°F (22°C) reduction in flue-gas temperature, subject to combustion conditions and the minimum safe stack temperature. This is a screening estimate; project calculations should use measured gas flow, temperatures and boiler efficiency. Source: US Department of Energy

Reduced Energy Consumption

DOE reports that feedwater economisers can often reduce fuel requirements by 5–10%. Its example for a 50 MMBtu/h natural-gas boiler with a 500°F (260°C) initial stack temperature estimates 4.6 MMBtu/h of recoverable heat when the final stack temperature is 250°F (121°C). Source: US Department of Energy

Cost Savings

Annual fuel-cost saving can be estimated as:

$$\text{Annual saving}=\frac{Q_{\mathrm{recovered}}\times \text{operating hours}\times \text{fuel price}}{{\eta }_b}$$

Using DOE’s 4.6 MMBtu/h example, 8,400 operating hours per year, $8/MMBtu fuel and 80% boiler efficiency gives an annual saving of $386,400 before allowing for fan power, pumping, maintenance and capital cost. Source: US Department of Energy

Lower Emissions

Where recovered heat displaces fuel combustion, direct combustion emissions fall in proportion to verified fuel reduction. Calculate the reduction by multiplying avoided fuel use by the applicable site or regulated fuel-emission factor; do not assume a fixed emissions saving from economiser installation alone.

Better Use of Existing Plant

An economiser can recover heat from an existing boiler or process where the flue-gas profile, heat sink, draft capacity and corrosion controls support the calculated duty.

Limitations and Operational Risks

Corrosion is a principal concern. Cooling flue gas below the applicable acid dew point can form corrosive condensate on tubes, ductwork and downstream equipment. Condensing economisers are designed for this operating mode but require compatible materials, drainage and condensate management.

Soot, ash and particulate deposits on the gas side, and scale on the water side, reduce heat transfer and increase pressure drop. DOE notes that a 1/32-inch soot layer can reduce efficiency by 2.5%, while scale-related efficiency losses can range from 1% to 7%; the effect depends on the deposit and equipment. Source: US Department of Energy

Low water flow, tube leakage, loss of draft margin and freezing in exposed installations are further risks. Controls and inspection should detect these conditions before they cause equipment damage.

Maintenance Requirements for an Economiser

Regular maintenance preserves designed heat duty and pressure-drop margin.

A typical maintenance programme should include:

• Inspecting tubes, headers, seals and supports for leaks, wear and corrosion
• Cleaning soot, ash and other gas-side deposits where required
• Monitoring feedwater quality to limit internal scale formation
• Trending flue-gas and water-side pressure drop against clean baselines
• Reviewing inlet and outlet temperatures, water flow and calculated heat duty
• Inspecting condensate drains and neutralisation systems on condensing units
• Checking controls, bypass dampers, sensors and alarms

The maintenance interval depends on fuel type, operating hours, flue-gas cleanliness and economiser design. A unit burning clean natural gas will generally require different cleaning arrangements from one installed on solid-fuel or oil-fired equipment.

Conclusion

A boiler economiser transfers flue-gas heat to feedwater or another usable liquid stream. Its value is determined by calculated recoverable duty, annual operating hours, fuel price, boiler efficiency and the added cost of fan power, pumping, maintenance and capital.

For natural-gas-fired boilers, a 40°F (22°C) reduction in flue-gas temperature is a useful screening indicator of approximately 1% efficiency improvement, but the final stack temperature must remain compatible with the fuel, gas composition and selected materials. Condensing designs can recover additional latent heat when a sufficiently cool heat sink and suitable condensate-corrosion controls are provided.