Feedwater heat recovery: economizer, condensate, then HP-1

The order to investigate for a fired boiler (biomass such as rice
husk, or LPG): first a flue-gas economizer, second as much clean, hot
condensate back to T-1 as possible, third the heat pump HP-1 where the
press return has enough heat and the combined heating and cooling
economics justify it.

- Diagram: new ECON-1 (option, fired boilers only) between the feed
  pumps and the boiler manifold, with a bypass for an electric bank and
  the flue-gas path from the boilers; feed in 90 to 95 C, outlet and
  stack temperature are vendor figures. HP-1 is now drawn as an option
  and labelled the third measure. AT conductivity on the condensate
  return to T-1; off-spec condensate goes to the warm tank.
- Found in Newbilt's offer 240620-04: steam and cooling water share the
  mould channels, with one steam/water outlet valve and steam trap per
  mould, so the condensate is only as clean as the cooling loop. Noted
  in the T-1 detail, sustainability.md and as a Newbilt RFQ question.
- With most condensate returned (about 15 % flashes at atmospheric
  pressure, estimate) the feed tank needs only a fraction of the ~70 kW
  that heating all make-up takes, so HP-1 may not pay.
- RFQ: economizer outlet feed and stack temperature, corrosion limit,
  pressure drop; rice husk ash is high in silica and abrasive, so
  fouling, cleaning, wear protection and a bypass.
- sustainability.md: the order, before the exclusions. biomass.md: new
  "Recovering heat at the boiler".

Checked headless (scripts/pw_sc_check.py): 33 boxes, 20 tags, no
overlap or overflow, no unstyled pipes, no card or page overflow at
1600, 1258 and 390 px in all four cases, every box opens its detail, no
errors.

docs/biomass.md

@@ -27,6 +27,18 @@ load and realistic duty cycles:
   is what the fuel store and the monsoon
   stock have to be sized for (estimate)
 
+### Recovering heat at the boiler
+
+A fired boiler loses heat up its stack, so an economizer that preheats the
+boiler feedwater with the flue gas is the first heat-recovery measure to
+investigate, ahead of condensate return and the feedwater heat pump
+([sustainability](sustainability.md#energy-efficiency-architecture-recover-before-generating)).
+How far the flue gas can be cooled is the vendor's figure: it depends on
+the flue-gas temperature, the corrosion limit and the fuel. Rice husk ash
+is high in silica and abrasive, so the RFQ asks for fouling and cleaning
+provisions, wear protection and a bypass rather than a clean-gas
+economizer ([steam and cooling](steam-cooling.html)).
+
 ## The 60 rai: own-grown fuelwood
 
 60 rai is 9.6 hectares. Managed as short-rotation coppice (planted once,