Air is a non-condensable gas and the presence of excess air in a condenser will significantly degrade condenser performance and lower power plant efficiency. One of the major concerns in improving condenser performance and design is the efficient removal of air from a condenser and keeping air inventory inside a condenser to a minimum.
During steady-state power plant operation, the air leakage rate into a condenser can be measured via a rotameter, but the air inventory inside of the condenser cannot. The amount of air inside a condenser directly affects power plant condenser performance and is made up of two components, namely, air in-leakage and un-evacuated air pockets inside a tube bundle. The formation of air pockets is due to both poor design of tube bundle or bundles and inappropriate location of air off-take pipe or pipes, says Dr. William T. Sha, chairman of Sha & Associates, Inc., a consulting company to the power industry.
The current design of condenser air removal systems is based on both the effective steam flow at each main exhaust opening and the total number of exhaust openings without knowing pressure distribution inside condensers. Consequently, the air off-take piping and/or shrouding may be located in a high-pressure zone in the shell side. Steam and air, of course, tend to move from higher pressure to lower pressure zones and thus diminish air accumulation in the vicinity of the air off-take pipe. Accordingly, the efficiency of the air removal system is lowered and air pockets or bubbles are formed.
To minimize air inventory inside a condenser, 3-D modeling using computational fluid dynamics (CFD) for the condenser with air removal system is recommended, according to Sha and Philip Yakimow, principal engineer for Xcel Energy say. “Considering the strong thermal interaction between the steam and air mixture in shell side and the cooling water in the tube side, and the complex geometry and irregular structures involved in modeling of a condenser with air removal system, the porous media formulation is more suitable and advantageous to attack this problem,” says Sha.
Sha has developed a novel porous media formulation, rigorously derived through the averaging process of conservation equation of mass, momentum and energy. The formulation uses the concept of volume porosity, directional surface porosity, directional distributed resistance and distributed heat source or sink. “The concept of directional surface porosity greatly improves the resolution of a complex physical system and facilitates accurate computation of velocity and temperature field in anisotropic media,” says Sha. “The conservation of mass, momentum, energy and turbulent transport equations are solved as a boundary value problem in space and an initial value problem in time.
Sha and Yakimow explain that modeling via CFD produces 3D distributions of pressure, temperature, velocity and air and total air inventory of a condenser. “Based on the results of such modeling, many performance improvement and design insights may be gained leading to modifications such as strategically adding flow baffles, removing tubes, widening steam lanes, and so forth,” says Sha. Such modifications will enhance the flow field, which will minimize air inventory and improve condenser performance.
Sha identifies three fundamental principles for improving performance and design of a condenser with air removal system.
1. Enthalpy of steam entering a condenser should be kept to an optimum minimum based on overall power plant efficiency.
2. Pressure drop should be kept to a minimum for the steam or steam/air mixture traveling from any point in the shell side to air off-take pipe with perforated opening or openings so that back pressure can be minimized. The air off-take pipe should be located in the lowest pressure zone in shell side to minimize air pocket formation in tube bundle or air inventory in a condenser.
3. Heat transfer between steam on the shell side and cooling water on the tube side is optimized under all operating conditions of a condenser by minimizing fouling and air leakage into a condenser; eliminating or minimizing inundation; promoting vapor shear; and optimizing flow field to minimize air inventory and to enhance local heat transfer rate inside a condenser.
“The first principle is commonly accepted as the design standard,” says Sha. “The second principle addresses the design of air removal system as an integral part of the condenser design. In order to satisfy this principle, knowledge of the 3-D pressure and air distribution inside a condenser is required.
He says the third principle requires optimization of the 3-D flow fields inside a condenser via CFD to eliminate or minimize inundation, promote vapor shear, minimize air inventory and enhance local heat transfer rate inside a condenser. Flow fields should be considered explicitly when trying to improve performance and design of a condenser with air removal system and may also be employed as a diagnostic tool when troubleshooting an under-performing condenser. Yakimow notes that an increasing number of CFD models have been completed recently, whose results clearly validate the second and the third principles. p