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 4.7.4 FORCED AIR
4.7.4.1 Description
For turbines that do not exhibit negative pressures under the headcover or do not induce
sufficient air flow, a forced air system can be applied. Forced air refers to a system where a blower or
air compressor is used to move the desired airflow into the turbine water flow passages.
4.7.4.2 Design Methodology
The design for forced air systems is straight forward compared to turbine venting and auto-
venting since the air flow is not dependent on the subatmospheric pressure areas in the water flow.
Many successful applications have been sized and designed based on full scale tests using portable air
compressors connected to existing air passageways in the turbine. These tests are used to determine
the effect of various air flows on the turbine, pressure requirements and oxygen transfer efficiency
(Harshbarger, 1983).
Design considerations include; space restrictions in and around the powerhouse, availability of
electric power, equipment reliability and total dissolved gas (TDG) limitations. The shear size and
massive power requirements of forced air systems for large hydropower projects render some
applications unwieldy or undesirable.
4.7.4.3 Applications
The TVA has forced air installations at Tims Ford Dam and Nottely Dam. Each of these
projects have large air blowers on the main turbine and small air compressors on a small hydropower
turbine used to provide minimum flows. Both projects are remotely operated, and compressor
maintenance and operation has occasionally been a problem. Tims Ford is particularly susceptible to
TDG supersaturation since the forced air system is supplying up to 3 mg/L of DO uptake. In fact, the
tailwater TDG is routinely monitored and the aeration load is switched to oxygen diffuser systems on
both large and small units as TDG limits are approached.
4.7.4.4 Summary
Forced air is a relatively straight forward aeration method that can often be designed based on
full scale tests at the hydro project. Application may be limited due to TDG concerns or space and
power requirements. Summary information is presented in Table 4.7.3. Aeration experience in
retrofitting existing turbines is shown in Table 4.7.4.
4.7-9
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