Page 185 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.21

Temperature sensors sense the temperature on the tube side of the first effect and signal
the steam control valve to maintain the required temperature. This steam condenses on the
outside of the tubes of the first effect, giving up its latent heat of vaporization to the feed-
water inside of the tubes, causing the water to boil and generate vapor.
The pure steam generated in the first effect is introduced into the shell side of the sec-
ond effect. The pure steam condenses, producing distilled water, while giving up its latent
heat to the high-purity feedwater inside the second effect tubes, which in turn causes the
feedwater to boil and generate vapor. Each effect operates at a lower pressure than the pre-
vious effect in order to provide the temperature difference that allows the transfer of heat.
The pure steam generated in the tube side of the first effect by the condensing power steam
passes through the mist eliminator to remove any entrained water droplets. Feedwater from
the first effect passes through an orifice and into the tube side of the second effect. The first
effect pure steam enters the shell side of the second effect and is condensed on the outside
of the tubes.
The condensate (distilled water) passes through an orifice and enters the shell side of
the third effect. Feedwater in the second effect passes through an orifice and into the tube
side of the third effect.
After passing through the mist eliminator, the last effect pure steam enters the condenser
and condenses on the outside of the condenser coils. This distilled water from the last
effect and the distilled water from the previous effects is cooled by the cooling water of
the condenser. The distilled water exits the condenser and enters the distillate pump, where
it is pumped through the distillate control valve and through the storage-dump valve. The
condenser level controller senses the distillate level and signals the control valve to main-
tain the desired level.
Noncondensable gases in the condenser are vented to the atmosphere. The condenser
temperature is maintained at a predetermined level by the cooling water flow. The unit is
protected by pressure relief valves along with high- and low-level alarms.
Refer to Fig. 4.4 for a schematic diagram of a multieffect distillation unit.

FILTRATION

Deep Bed Sand Filtration
Deep bed filters are designed to remove coarse suspended particulates larger than 10 μm
in size. They are pressure-type filters that use either multigraded sand or multimedia in the
filter. Particulate removal on the order of 98 percent should be expected. For additional
discussion, refer to Granular (Sand) Filters in Chap. 3.
Sand-only filters for laboratory water systems should generally operate at a face
2
velocity of about 4 gpm/ft of cross-sectional bed area. Multimedia filters operate at
2
about 6 gpm/ft of cross-sectional bed area. Since these values are general in nature,
it is important to operate these units at the velocities recommended by the individual
manufacturers.
The multimedia filter achieves a more uniform distribution of filter media throughout
the bed and is considered a more effective type of filter. A typical multimedia filter for labo-
ratory use consists of a top layer of anthracite having a 1.1-mm grain size and 1.5 specific
gravity, a middle layer of sand having a 0.5-mm diameter grain size and a specific gravity
of 26, and a bottom layer of garnet having a 0.2-mm grain size and 4.2 specific gravity.
2
Normal operational flow rate is from 6 to 15 gpm/ft of bed area.
Backwashing is required to clean the filter, with the effluent discharged to the sanitary
2
drainage system. A backwash flow rate of 10 to 15 gpm/ft is generally required for effec-
tive cleaning.

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