Gas Turbine Handbook : Principles and Practices

Accessories 99
Coolers
Lube oil coolers are required to remove heat from the oil before
it is re-introduced into the gas turbine. The degree of cooling required
is a function of the friction heat generated in each bearing, the heat
transferred from the gas turbine to the oil by convection and conduction,
and heat transferred from the hot gas path through seal leakage.
The oil must be cooled to within acceptable limits, normally 120°F-
140°F (50°C-60°C). Cooler design and size is based on the viscosity
of the oil at the bearing operating temperature, the maximum allowable
bearing metal temperature, and the temperature and available
flow rate of the cooling media. To maximize heat transfer, fins are
installed on the outside of each tube and turbulators are placed inside
each cooling tube. The turbulators help transfer heat from the hot oil
to the inner wall of the cooling tubes and the fins help dissipate this
heat. When turbulators are used it is necessary to provide sufficient
maintenance pull space to facilitate removal and replacement of each
turbulator. The cross section design of the turbulators is a function of
oil flow rate and viscosity. Tubes with turbulators tend to foul faster
than tubes without turbulators. Tube fin construction may be either
rolled in, tension wound, or welded “L.” To be effective the fin must
be in continuous contact with the tube. U-bend tubes should not be
used as there is no way to effectively clean them.
The cooling media may be either air or a water/glycol mix.
The cooling media selection is primarily a function of location and
the availability of adequate utilities. For example air/oil coolers are
widely used in desert regions, while tube and shell coolers can be
found in the arctic regions and most coastal regions.
Air/Oil Coolers
Air/oil coolers utilize ambient air as the cooling media. Cooling
coils are arranged horizontally in the length and number of passes
necessary to satisfy the footprint limits and the cooling load (plus
a margin of safety of approximately 15% to 20%). Cooling fans are
usually electric motor driven, often with two speed motors. This arrangement
allows for high and low cooling flows. To closely match the
cooling flow to the required heat load, changeable pitch fan blades can
be provided. If the heat load changes over a period of time the blades
can be adjusted in the field to meet the new heat flow requirements.
When variable cooling control is necessary, an oil bypass control loop