|
|
||
 A diffuser system that can accommodate the required air flow rate is then sized based on the
volume, depth, air flow, and time to destratify. The diffuser system consists of a perforated (1-mm-
diam holes located 0.3 m on center) linear system of pipe anchored to the bottom of the lake. The
recommended minimum limit of perforated pipe length is 50 m (Davis 1980).
The compressor must be sized to provide the required air flow at a pressure sufficient to
overcome the back-pressure and losses in the delivery and diffuser system, which include the
hydrostatic pressure head, pressure loss due to friction and at the bends, and excess pressure at the end
of the pipe. Davis (1980) provides the necessary equations to calculate all of the required pressures.
The free air flow through one diffuser hole is determined using a relationship between the ratio
of absolute hydraulic pressure (press due to the water depth and atmospheric pressure) to the mean
internal pressure (pressure at the end of the diffuser pipe plus the absolute hydraulic pressure). This
value is multiplied by the number of holes in the diffuser pipe. The result should be equal to or greater
than the air flow required. If not, the length of pipe must is increased and the pressure requirements a
recalculated. Several iterations through the pressure and length calculation and checks may be required
to arrive at a satisfactory solution.
To complete the system, an anchoring system that is capable of keeping the diffuser pipe
submerged must be designed.
According to the design criteria (Davis 1980), the system should be activated at the beginning
of the stratified period or when the oxygen content of the hypolimnion falls to 50 percent of the
saturation level. Operation should continue throughout the stratification season to maintain isothermal
conditions or the desired DO level.
4.6.1.4 Applications of Pneumatic Destratification
A number of pneumatic destratification systems have been installed in the United States
England, Australia, and Europe. Summarized in Table 4.6.1 are reservoir characteristics and the
pneumatic destratification system parameters for several systems that have been installed and tested.
Early equipment design and testing were conducted by the US Army Engineer District,
Savannah, on the destratification system installed and operated in Allatoona Lake, Georgia (USAED,
Savannah 1973). This configuration used a multiple-port diffuser system consisting of five cross-type
diffusers (approximately 19.8 m across) clustered about 2,000 ft upstream of the dam face. Each
diffuser was supplied with a 250-cfm compressor.
This system was operated continuously from
May through September 1968 and from mid-March through mid-September
4.6-3
|
||
|
||
