Page 196 - Industrial Ventilation Design Guidebook

I S 8 CHAPTER 4 PHYSICAL FUNDAMENTALS

The operating cost is high due mainly to the energy used for the heat
source, cooling water, and the necessary chemical treatment. To reduce the
running costs, waste process heat or solar collectors are used.
Falling-film vertical evaporators, direct expansion systems, and vacuum
freezing techniques may also be used.

Reverse Osmosis. The process of osmosis is used by plants to obtain
food and moisture from the soil. The density of the sap in the roots of the
plant is greater than that of the soil water surrounding it. The root wall pro-
vides a semipermeable membrane, and the difference in suction across it is the
osmotic pressure.
In reverse osmosis, the osmotic pressure is increased manually to get the
water to flow from a high-density area through a semipermeable membrane to
the. lower-density weaker solution. The water will pass through the membrane
and leave the solids behind. A pressure of about 2.76 MPa will extract 90%
or more of the dissolved absorbed solids; further refinement may be achieved
through a base exchange process.

Magnetic Water Treatment. This method is used in marine engineering
and district heating networks in Russia. The hard water to be treated, either hot
or cold, flows first through a filter and then at high velocity through permanent
magnets. The magnetic field influences the nature of the crystallization of the
hardness salts. This results in numerous nuclei being formed in the solution, cre-
ating sludge instead of a hard scale, which is easily removed by blowdown.
Deoerot/on
The removal of all gases in the water by means of traps or chambers will
improve the pumping characteristics and reduce corrosion and noise.
In the case of hot water, the oxygen in the water becomes about twice as
corrosive for every 20 °C increase in temperature; hence, removal of the oxy-
gen is of prime importance. Oxygen is extremely corrosive in hot-water sys-
tems containing demineralized water
In groundwater, gases such as carbon dioxide, hydrogen sulfide, and ra-
don may be dissolved under pressure. For efficient removal, an intensive de-
gassing process (GDT, or Gas-Degas Technology) has been developed by the
GDT Corporation (USA). It consists of groundwater and air (or ozone) being
intensively mixed in a venturi injector, followed by optimum residence time in
a reactor vessel, and finally the efficient removal of unwanted stripped gases in
a centrifugal separator.

Oxygen Scavenging
By removing oxygen completely, corrosion by this gas is eliminated. It can
be achieved by the addition of sodium sulfite or hydrazine, which reacts with
oxygen. The reaction product will not normally cause any problems.

Scale Control
Chemicals such as disodium or the polyphosphates are used to precipitate
scale-forming solids in the water. If alkalinity control is required, caustic soda