The condensate-and-flash lessons harvested the steam system's own waste. This lesson widens the lens: the heat leaving the plant by every other exit — the stack, the blowdown drain, the compressor cooling circuit, the humid dryer exhaust — and the recovery equipment that brings each one home. The method throughout is one rule, applied four times: match the waste stream's temperature to a duty that wants exactly that grade of heat (lesson A11's ΔT logic, run as a treasure hunt).
The stack: the biggest single exit
Lesson B4 showed the stack carrying 15–20% of the fuel (dry gas plus fuel moisture); the stack is therefore recovery's first address. The economiser — a finned coil in the flue warming feedwater — is the classic: every 20 °C shaved off the stack is roughly 1% of fuel, and warming feed is the perfect matched duty (B5's arithmetic, fed free). On gas firing, condensing recovery goes further: cool the flue below the water dewpoint (~55–60 °C for natural gas) and the latent heat of the combustion moisture pays out — several extra percent — into any cool duty like make-up or process water. The honest boundary is acid dewpoint: sulphur-bearing fuels (coal, FO, and husk's flue chemistry) form corrosive condensate on surfaces that run too cold, so solid-fuel economisers keep respectful exit temperatures and corrosion-resistant design; condensing territory belongs to clean gas. Every stack project starts with two numbers: exit temperature and fuel chemistry.
The boiler-house pair: blowdown and vents
Already covered in their home lessons, listed here because the audit sees them together: blowdown heat recovery (B6's flash-plus-coil pair, keeping 80–90% of the bleed's heat) and vent condensing — a feed-tank or deaerator vent that steams hard all shift can pay for a small vent condenser warming make-up water. Small, boring, quick paybacks; the classic second-year projects after F2's big three are done.
The utility-room finds: compressors and exhausts
Air compressors convert ~90% of their motor power to heat and hand it to cooling air or water at 35–70 °C — a steady, clean, utterly reliable warm stream that most plants blow at the sky. Matches: wash water, boiler make-up preheat, space heating, or the ideal heat-pump source (F6's list, first entry). Humid dryer exhausts carry double cargo — sensible heat plus the latent heat of every kilogram of moisture dried — and give it up generously to incoming cold air across a recuperator. Their tax is fouling: dust and condensing vapours coat surfaces, so the equipment that survives is the cleanable kind (plate recuperators with access doors, run-around coil pairs keeping the two air streams safely apart). Design for the dirt on day one or the project dies at its first blocked core.
Grade discipline: the one rule that ranks everything
Waste heat is only worth what a matched duty will pay for it. High-grade waste (stack) deserves high-grade duties (feedwater); low-grade waste (compressor water) deserves low-grade duties (make-up preheat) — and crossing the grades wastes the project: spending stack heat on warm wash water squanders exergy, while pushing 40 °C water at a 90 °C duty needs F6's machinery. Rank candidate projects by the same ₹/1000 kcal displaced (B2) that ranks everything else in this faculty, and let the audit's register (F3) hold them against the steam-side wastes — the second harvest queues behind the first exactly as far as the paybacks say, no further.
- Take the stack temperature at full fire and compare with commissioning records — every 20 °C of drift is 1% of fuel, recoverable or fixable (B4 vs F7).
- Walk the roof and drains with the IR gun: every stream leaving above ~40 °C goes on the treasure list with its temperature and schedule.
- Compressor house first — it is the easiest win on the list: steady, clean, and already piped.
- For any dryer-exhaust project, ask one question before the thermal sums: how will it be cleaned, how often, by whom?
- The second harvest: stack, blowdown, vents, compressors, exhausts — heat already bought, leaving by other doors.
- Economiser rule: 20 °C off the stack ≈ 1% of fuel. Acid dewpoint sets the floor; condensing is clean-gas territory.
- Match grade to grade; never spend high-grade waste on a low-grade duty.
- Design exhaust recovery for the dirt, or don't build it.
- One ranking currency for all projects: ₹/1000 kcal displaced.
The economiser is nearly as old as the boiler itself: Edward Green patented his cast-iron feedwater economiser in 1845, and "Green's economiser" became so universal in British mill practice that the word outlived the brand. Mill engineers of the 1860s could recite this lesson's stack rule from experience alone — proof that waste heat recovery is not a modern virtue bolted onto steam engineering, but one of its founding instincts: the fuel was always too expensive to send up the chimney twice.
FAQ
How much fuel does an economiser actually save?
The working rule: every 20 °C shaved off the stack temperature is roughly 1% of fuel. With flue gas leaving at 180–250 °C carrying 15–20% of the fuel's heat, the stack is recovery's first address — and feedwater is its perfect matched duty, because the boiler already pays to heat it (B5).
Why can't I fit a condensing economiser on my husk or coal boiler?
Acid dewpoint. Sulphur-bearing fuels form corrosive condensate on any surface that runs too cold, so solid-fuel economisers must keep respectful exit temperatures and corrosion-resistant design. Cooling the flue below the ~55–60 °C water dewpoint to harvest the latent heat of combustion moisture is clean-gas territory. Every stack project starts with two numbers: exit temperature and fuel chemistry.
What is the easiest waste-heat win in most plants?
The air-compressor house. Compressors turn about 90% of their motor power into heat and hand it over at 35–70 °C — steady, clean, already piped, and usually blown at the sky. Match it to wash water, make-up preheat or space heating, or use it as the ideal heat-pump source (F6).
Why do dryer-exhaust recovery projects so often fail?
Fouling, not thermodynamics. The exhaust is latent-rich and generous across a recuperator, but dust and condensing vapours coat every surface. The equipment that survives is the cleanable kind — plate recuperators with access doors, or run-around coils keeping the streams apart. Ask "how will it be cleaned, how often, by whom?" before doing any thermal sums.
What does "match grade to grade" mean in waste-heat recovery?
Give each waste stream a duty that wants exactly its temperature. Stack heat deserves feedwater; 40 °C compressor water deserves make-up preheat. Crossing the grades wastes the project — high-grade heat on a warm-wash duty squanders exergy, and low-grade heat pushed at a 90 °C duty needs heat-pump machinery. Rank every candidate in ₹/1000 kcal displaced, the faculty's one currency.
Check yourself
Five quick questions on this lesson. No marks, no records — the score is for you.