Economizer Selection and Payback Calculation

When assessing an economizer investment, consider where the recovered heat will be used as well as how much heat is available in the flue. Without a useful heat demand, theoretical capacity may not translate into the expected economic benefit. Correct selection matches the hot gas source with a suitable heat consumer.

Economizer systems can transfer flue gas heat to boiler feedwater or suitable process water. There is no universal saving percentage or payback period. This guide explains quotation inputs and a preliminary calculation method.

Data needed for economizer selection

• Fuel type, boiler operating loads and flue gas temperature.
• Flue gas flow, composition and fouling potential.
• Water flow and inlet and target outlet temperatures.
• Actual annual operating hours and continuity of heat demand.
• Allowable gas-side pressure drop and existing fan information.
• Installation space, connections and maintenance access.

Measurements only at maximum load are insufficient if the plant usually operates at part load. Reflect the actual load profile in the investment assessment. Check whether production and hot water demand occur at the same times.

Condensing and non-condensing designs

Cooling flue gas requires assessment of condensation limits and corrosion conditions. In a condensing system, materials, condensate removal and suitability of the entire flue are separate design considerations. Simply forcing existing equipment to a lower temperature does not make it a suitable condensing system.

The US Department of Energy’s condensing economizer guide also emphasises useful heat demand and site-specific engineering. The quotation should state the operating regime for which the project is designed.

Simple payback calculation

A preliminary assessment uses:

Simple payback period = Total installed investment / Annual net savings

Annual net savings are the avoided energy cost minus additional electricity, maintenance and other operating expenses. The investment should include installation, pipework, controls and connections, not only the economizer.

A wholly hypothetical example

Assume average useful heat recovery of 100 kW and 4,000 useful operating hours per year. Annual useful heat is then 400,000 kWh. If the cost of the displaced useful heat is assumed to be TRY 2/kWh, the gross benefit is TRY 800,000 per year.

Assuming additional annual expenses of TRY 100,000 gives net savings of TRY 700,000 per year. With a total installed investment of TRY 1,400,000, simple payback is two years. These figures are not market prices, product performance data or a HEATMEK quotation; they only illustrate the calculation method.

When using fuel cost in a real calculation, account separately for boiler efficiency and unit conversions. Useful heat cost is different from the purchase price per unit of fuel.

What should be compared in quotations?

Ask for the same gas and water conditions, annual operating profile and scope of supply in every quotation. Check pressure drop, maintenance method, material suitability and the measurement plan alongside thermal duty.

Measuring temperature, flow and energy consumption at comparable loads after commissioning helps assess the expected benefit. Evaluate the whole system rather than inferring savings only from a drop in flue gas temperature.

Frequently asked questions

Can every boiler be retrofitted with an economizer?

Suitability depends on the flue, fan, fuel, water circuit and layout. A site assessment may be required.

Is the payback period fixed?

No. Operating hours, energy prices, load profile and maintenance expenses affect the result.

Prepare your site data and request a technical assessment and quotation from HEATMEK for your economizer project.