Page 100 - Membranes for Industrial Wastewater Recovery and Re-Use

80 Membranes for Industrial Wastewater Recoverg and Re-use

3.1.3 Overview of boiler feedwater systems
An integral component of thermoelectric power production is the use of boilers to
vaporise water into high-purity steam that is used to drive turbines. The operating
pressure of a boiler influences the heat transfer rates, with higher pressures yielding
rates of over 21 kJ m-l h-l. Materials used for boilers include mild steel, copper
alloys (admiralty brass or copper-nickel), or stainless steel (Kemmer, 1988).
Cooling water either flows through the tubes (tubeside cooling) or through the shell
and around the tubes. The two major types of boiler designs that are in widespread
use include firetube and watertube systems. The firetube boilers tend to be used in
boilers that generate less than 68 000 of steam per hour, whereas the watertube
boilers tend to have a higher output. The overall efficiency of a boiler is related to the
effectiveness ofcontacting water with the source ofheat (Le. coal, gas, oil, etc.).
Firetube boilers consist of a bundle of tubes set into a water drum. Water
circulates around the outside of the tubes where it changes to steam and rises to
the top of the drum and exits through a steam header. In watertube boilers,
water circulates inside the tubes and the flame and hot combustion gases flow
across the outside of the tubes. Steam forms on the heated side of the tubes
producing a steam-water mixture that is less dense than the water on the
unheated sides. Water circulation occurs either through density differentials or
is controlled using a pump. The use of mechanical circulation provides more
consistent control, particularly at high pressures where the density of steam and
water are similar (Kemmer, 1988).

3.1.4 Sources of water used for cooling operations and boiler feedwater
Water used for cooling operations can come from a variety of sources including
surface water (streams, lakes, or impoundments), groundwater, seawater,
estuarine water, or reclaimed water. In selecting a water source, the major
requirement is that an adequate supply of water is available to accomplish
cooling. The water quality of the source dictates the need for pre-treatment and
the feasibility of recycling the water with or without sidestream treatment.
Water quality also influences the economic feasibility of using the water source
for boiler feedwater or closed recirculating systems (Baron et al., 2000:
Wijesingheetal., 1996).
Because of the wide range of water sources that can be employed for cooling
operations, water quality guidelines have been developed (Crook et al., 1994). A
summary of these guidelines is given in Table 3.2 for cooling and boiler water.
Water quality guidelines are based on prevention or minimisation of corrosion,
scale or fouling. Boiler water quality requirements are more stringent than those
for cooling water due to the relatively higher temperature and pressure conditions
needed for steam production. In general, boilers that operate under higher
temperature and pressure conditions require higher quality feedwater,
particularly if the heat source is in direct contact with the tube, as in watertube
designs. Toprevent operational problems it is important to control the salt content
of boiler water, as generally indicated by the conductivity, which then generally