Page 160 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 127

have the boiler at less than slightly negative pressure to reduce the nuisance of dust
leaking from the casing flanges and seams. If positive pressure is expected, casings
must be welded gastight.
Ash is collected in hoppers as flue gases make low-velocity turns within the
boiler. The sides of the hoppers must be steep to prevent ash buildup.

4.5.3 Watertube Boiler Design
Waste heat boilers are typically designed for natural circulation, although some
forced-circulation designs have been used. The fluid in the downcomers is mostly
water at or slightly below saturation temperature and is of greater density than the
water and steam mixture in the risers. Heat transfer depends on tube spacing and
diameter and the total heating surface. Tubes for waste heat boilers typically have
approximately a 50 mm (2 in) diameter, spaced approximately 100 mm (4 in) apart.
Some boilers also include special banks of tubes not used directly to produce
steam. A superheater is a section of tubes that carries only steam so that the heat
transferred raises the temperature of the steam to greater than saturation (superheat).
Superheated steam is used for steam turbines to prevent excessive amounts of mois-
ture (water droplets) from forming in the turbine. This increases the amount of useful
energy that can be taken from steam by the turbine before water creates operating
problems. An economizer is also used to increase the temperature of the feedwater
and is not designed to produce steam. Because the feedwater is at lower tempera-
tures than the boiling water (saturation temperature), the flue gas can be cooled to
lower temperatures for maximum energy recovery from the flue gas.
The usual arrangement for waste heat boilers is to include a superheater ahead
of the boiler and an economizer after the boiler. In some designs, the superheater sec-
tion is located downstream of the boiler inlet, nested in the middle of the boiler evap-
orator tubes.

4.5.4 Feedwater
Raw water cannot be used in a boiler without chemical treatment to remove scale-
forming materials, dissolved oxygen, and acids. Dissolved oxygen will attack boiler
steel at saturation temperature. Treatment requirements are sometimes dictated by
the ultimate use of the steam, such as when steam turbines are used at the facility.
Not all dissolved minerals in the water are removed by most water treatment
systems. A portion of the drum water is removed to reduce concentrated impurities
that tend to settle in the mud drum. Low point blowdown connections are provided
for periodic removal of settled solids. Dissolved solids and solids suspended in the