Economizer Justification for a Cycling Gas-Fired Boiler

Work in Btu per pound of fuel: the flue-gas mass scales with firing rate, so the percentage saving is independent of boiler load, which makes this a sound first-pass estimate.

1. Excess air: EA = 5.0 ÷ (20.9 − 5.0) = 31%, air factor 1.31.

2. Flue-gas mass per lb of fuel: stoichiometric air for natural gas ≈ 17.2 lb/lb; actual = 17.2 × 1.31 = 22.6 lb/lb; products ≈ 22.6 + 1.0 = 23.6 lb/lb fuel.

3. Heat recovered per lb of fuel: cp ≈ 0.25 Btu/lb·°F; q = 23.6 × 0.25 × (480 − 320) = 945 Btu/lb fuel.

4. As a fraction of input: 945 ÷ 23,000 ≈ 4.1% of HHV.

5. Current fuel input: steam duty = 25,000 × (1,196 − 73) = 28.1 MMBtu/h; at 80% boiler efficiency, input = 35.1 MMBtu/h, i.e. about 34,400 ft³/h at 1,020 Btu/ft³.

6. Savings: 0.041 × 35.1 = 1.44 MMBtu/h → $10.80/h; × 6,000 h = $64,800/yr.

7. Simple payback: 175,000 ÷ 64,800 = 2.7 years.

Assumptions that move the answer:

• The 4.1% holds only if excess air and flue-gas mass stay constant and the economizer achieves 320°F at all firing rates, including 4,000 lb/h. At low fire the flue gas is already cooler and the economizer saves proportionally less; at 4,000 lb/h the absolute saving is roughly a sixth of the figure above.
• 80% boiler efficiency is assumed, not measured. A measured efficiency (with unburned CO and radiation losses accounted for) is worth the $400 it costs.
• The 945 Btu/lb uses a constant cp; real flue gas cp rises with temperature and moisture, so the true figure is within about ±5%.
• No credit is taken for reduced stack draft or fan power, and no cost for the added flue-gas pressure drop (typically 1–2 in. w.c.), which the existing burner fan may not have margin to overcome. If a new fan is needed, add $20,000–40,000.

On the rotted economizer - he is right, and he has identified the real risk.

With 105°F feedwater entering a counterflow economizer and 320°F flue gas leaving, the tube-wall temperature at the water inlet end sits within a few degrees of 105°F. Natural-gas flue gas at 31% excess air contains about 15% water vapour by volume, giving a water dew point near 130°F. So the cold end of that economizer runs below the dew point continuously, in a wet, aerated environment; add any sulfur (odorant, trace H2S, or oil carryover from a dual-fuel burner) and you get dilute acid on the tubes. Three-year tube life is exactly the expected outcome, and the vendor's 320°F outlet figure says nothing about the metal temperature at the water inlet.

Fix before buying, not after:

• Raise the feedwater inlet temperature to at least 160–180°F. The cheapest route is the deaerator the plant should have anyway: a 5 psig deaerator delivers 228°F water, which puts the cold-end metal far above the dew point and solves the oxygen corrosion problem in the boiler.
• Specify a feedwater bypass with an automatic control that maintains a minimum economizer inlet temperature, or a recirculating loop.
• Use extended-surface tubes with a corrosion allowance and specify the tube material for the calculated metal temperature, not for the flue-gas temperature.

On short-cycling - also right, but it is a control problem, not a reason to reject.

At 4,000 lb/h the boiler is near its low-fire limit; cycling means the burner goes off and on, and each off-cycle the economizer cools, wets and then reheats. Thermal cycling plus a wet cold end is the corrosion-fatigue recipe. Mitigations: a properly sized bypass damper that protects the economizer during off cycles, an economizer that drains freely (no water pockets, no horizontal runs without slope), and fixing the cycling itself - the load is stable, so oversized firing or a bad modulation setting is the real cause.

Verdict: the objection is valid but the answer is "preheat the feedwater and fix the cycling," not "don't install an economizer."

Reject or defer on these grounds:

1. Cold-end corrosion cannot be engineered out within budget. If there is no room or budget for a deaerator, preheater or recirculation loop, the recession is not optional - a bare economizer at 105°F inlet will fail in the same three years as the last one, and the true payback including a tube replacement or a tube-sheet replacement at year three is roughly double the 2.7 years quoted. In that case either pair the economizer with a deaerator in one project or defer.
2. A near-term change in the boiler's duty. If the plant has an electrification, heat-pump or heat-recovery study that could retire this boiler in three to five years, a 2.7-year payback is not the deciding number - the economic life is. Anything with a payback longer than half the remaining plant life should be deferred.
3. Codes and pressure-part status. At 150 psig the economizer is a Section I pressure part. It needs a listed safety valve if it can be isolated from the boiler, its own design registration, and a way to be vented and drained for inspection. If the 20 ft of straight stack run cannot accommodate the economizer plus a bypass and access for tube cleaning, the project cost estimate is wrong.
4. Draft and fan margin. The extra 1–2 in. w.c. of flue resistance may exceed the existing burner fan's capability, particularly at high fire with a dirty economizer. If the fan must be replaced, the capital cost and the added electrical load both change, and the project is no longer a 2.7-year payback.
5. Water chemistry you cannot control. An economizer adds a large, low-velocity waterside surface - a magnet for deposition. If the plant has no deaerator, no oxygen control and a softener that is not continuously monitored, the economizer will fail waterside before it fails fireside. Fix chemistry first; it is cheaper than tube replacement and it is a prerequisite, not a parallel workstream.

How the decision changes: Items 1 and 5 turn the question from "should we buy an economizer?" into "should we buy a deaerator plus an economizer?" - a different, larger project that is usually justified on both fuel and boiler-life grounds. Present it that way rather than defending the vendor's 2.7-year figure.