The Australian Wine Research Institute

Key Points

• A feedwater economiser reduces steam boiler fuel requirements by transferring heat from the flue gas to incoming feedwater.
• A feedwater economiser is appropriate when insufficient heat transfer surface area exists within the boiler to remove combustion heat.
• Boilers that exceed 980 kWth and that are significantly loaded throughout the year are potential candidates for an economiser retrofit.

Background

Boiler flue gases are often rejected to the stack at temperatures in excess of 100°C higher than the temperature of the generated water or steam. In some cases, boiler efficiency can be increased by 1% for every 20°C reduction in flue gas temperature. In such cases, by recovering flue waste heat an economiser can reduce fuel requirements by as much as 5-10% and pay for itself in less than 4 years.

Example

A 78% efficient hot water boiler generates 24000 kg/hr of hot water at 95°C by burning natural gas. Initial average feedwater temperature is measured at 23.4°C, with a stack temperature of 250°C. Determine the annual energy savings that would be achieved by installing an economiser given 1500 hours of boiler operation per year at a cost of $0.025/MJ

From the steam tables, the following enthalpy values are available:

For feedwater at 23.4°C and 1 atm: 98 kJ/kg
For water at 95°C and 1 atm: 398 kJ/kg

Boiler heat output
= 24000 kg/hr x (1/3600) hr/s x (398-98) kJ/kg
= 2000 kW

From the table above, the recoverable heat is 165 kW (165 kJ/s)

Annual savings
= (165 kJ/s x 3600 s/hr x 1500 hr x (1/1000) MJ/kJ x 0.025 $/MJ) / 0.78
= $28,557

Assuming a simple payback of 4 years gives a capital threshold of $114k.

For additional examples refer to the spreadsheet Boiler Economisers (Microsoft Excel file, 28kb).

Adapted from US Dept of Energy Steam Tip Sheet #3, January 2006