Economizer (boiler feedwater economizer)

01

Estimate recoverable heat and savings

The US Department of Energy's Steam Tip Sheet #3 (Use Feedwater Economizers for Waste Heat Recovery) explains that "boiler flue gases are often rejected to the stack at temperatures more than 100°F to 150°F higher than the" steam generated, that "boiler efficiency can be increased by 1% for every 40°F reduction in flue gas temperature", and that "an economizer can often reduce fuel requirements by 5% to 10% and pay for itself in less than 2 years". Its table, based on natural gas, 15% excess air and a final stack temperature of 250 °F, gives recoverable heat of, for example, 4.6 MMBtu/h for a 50 MMBtu/h boiler with a 500 °F stack, and 26.1 MMBtu/h for a 200 MMBtu/h boiler at 600 °F. [2]

Its worked example: an 80% efficient gas-fired boiler making 45,000 lb/h of 150 psig steam from 117 °F feedwater has an output of about 50 MMBtu/h; with a 500 °F stack the economizer recovers 4.6 MMBtu/h, saving 4.6 × $8.00 × 8,400 h / 0.80 = $386,400 a year at $8.00/MMBtu. [2]

Editorial calculation: cooling the stack from 500 °F to the 250 °F limit for natural gas is a 250 °F drop, about 6% efficiency by the 1%-per-40 °F rule of thumb; check the result against the table for your boiler size. [2]

Give the supplier the boiler type, capacity and pressure, fuel and excess air, flue gas flow and temperature after tuning, feedwater flow and temperature, operating hours and load profile, available space and duct connections, allowed flue gas and water-side pressure drop, materials and cleaning access, and the code the pressure parts must meet.

02

Check stack limits, candidates and design data

The tip sheet sets the lowest flue gas temperatures to avoid condensation and stack corrosion at 250 °F for natural gas, 300 °F for coal and low-sulphur fuel oils, and 350 °F for high-sulphur fuel oils. It says an economizer is appropriate when the boiler lacks enough heat transfer surface to remove combustion heat, and that boilers above 100 boiler horsepower, operating at 75 psig or more and significantly loaded all year are excellent retrofit candidates; it advises measuring stack temperature after tuning the boiler to the maker's specifications with clean surfaces and near-optimum excess air. [2]

DOE's steam system sourcebook distinguishes non-condensing economizers, which must run reasonably above the flue gas dew point (for natural gas, above about 250 °F), from condensing economizers, which are designed to let flue gas components condense, and says "these economizers typically extract more energy than do noncondensing economizers" and that "often, special materials are required, such as specialty stainless steels". [1]

Editorial note: these limits are in °F; 250, 300 and 350 °F are about 121, 149 and 177 °C. Ask the supplier how its design keeps tube metal above the acid dew point at low load, and for flue- and water-side pressure drop at full and part load.

Economizer maker E-Tech, which also owns the Kentube brand, offers custom box economizers with vertical or horizontal gas flow for larger boilers, at capacities it gives as 10,000 to more than 250,000 pph (lb/h), with single-casing, removable-end-panel and two-stage condensing versions. For stacks with no room for a separate unit it builds the Economite in-stack economizer to fit an existing stack of any diameter, with baffles and shells to direct gas flow and solid, serrated or longitudinal fins. Both pages repeat the rule of a 1% boiler-efficiency gain for each 40 °F drop in flue gas temperature that DOE gives. [5] [6] [2]

03

Fix tube materials and test the case for a condensing stage

A 2014 article in AIChE's Chemical Engineering Progress (CEP), written by Daniel Skiles of Cleaver-Brooks' engineered boiler systems group, describes the boiler feedwater economizer as a rectangular finned-tube layout with feedwater headers inside the casing and single inlet and outlet connections, with welded return bends and header connections to reduce the chance of leaks. It names the tube grades: carbon steel for feedwater economizers, as electric-resistance-welded SA-178-A or seamless SA-192, and stainless steel for condensing economizers, citing Type 316, to prevent cold-end corrosion from condensation collecting on the tubes. [3]

The article's worked example is a 50,000 lb/h natural-gas boiler at 150 psig with 228 °F feedwater from the deaerator and 80% make-up water. A condensing economizer placed after the standard economizer cools the flue gas from 300 °F to 131 °F and heats make-up water from 65 °F to 133 °F, recovering 2.6 MMBtu/h and raising efficiency (HHV basis) from 83.5% to 88.0%. CEP puts the dewpoint of water, "for a typical application with a moisture content of 17–18%", at 130–140 °F. [3]

CEP says a closed system that returns its condensate needs very little make-up water, so a condensing economizer gives little benefit there, and says "typically, a make-up water flow of 50–100% of the total feedwater flow to the boiler is needed for a condensing economizer". It adds that the small make-up quantities of high-pressure boilers make a condensing economizer impractical. Ask the supplier for the tube specification and material of each stage, the design flue gas exit temperature and the make-up water fraction assumed. [3]

E-Tech's condensing economizer page quantifies the latent heat a condensing stage targets: it states that natural-gas combustion products contain 11% to 12% moisture, representing up to 9% of the fuel's energy, and that cooling the gas below its dew point with feedwater-cooled finned tubing releases about 1,000 Btu per pound of condensate, with one cubic foot less CO₂ for each cubic foot of natural gas saved; it claims efficiencies upward of 96% for its condensing units. [4] Editorial calculation: at about 1,000 Btu/lb, condensing 1,000 lb/h of water from the flue gas returns roughly 1 MMBtu/h, about 2% of a 50 MMBtu/h boiler's input; ask the supplier for the condensate rate it guarantees at your return-water temperature. [4]

04

Trace the economizer parts

This is a functional reference built from the sources cited; it is not one maker's design.

05

Fill the specification fields with values for your boiler

An economizer quotation needs values for your boiler. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the boiler, fuel and flue gas data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Boiler: [type; capacity; pressure]
Fuel: [gas; oil; coal; sulphur]
Flue gas: [flow; temperature after tuning]
Feedwater: [flow; temperature]
Operation: [hours; load profile]
Limits: [stack temperature; pressure drops]
Installation: [space; ducts; code]

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