Three-dimensional field studies of thermal backwashing plumes

Abstract

/ Using specially designed temperature profiling equipment, two surveys were conducted during thermal backwashing operations at Pilgrim Nuclear Power Station to determine the spatial and temporal extent of temperature rises above ambient. Thermal backwashing is a process where biofouling is combated by a heat treatment procedure. Backwashing formed a thermal plume about 5-to 6-ft thick (1.5-to 1.8-m) in front of the intake screenwall. Maximum observed surface temperatures were 101.0~ (38.3~ representing a rise (AT) of about 43.4~ (24.1~ above ambient. The frontal zone of the plume spread gradually seaward at about 0.2 kn. Its outer edge became thinner and rapidly cooled, presumably by advection and turbulent diffusion associated with currents from the reverse pumping and local changes from dissipation to the atmosphere. Along the intake shoreline, the plume was often less than 1 ft (0.3 m) thick. Most of the hot water was dissipated within several hundred feet of the intake with ~Ts of about 10.0 to 15.0~ (5.6 to 8.3~ above ambient. Under the influence of 15 mph southwesterly winds during the second survey, some warmed water was apparently carried beyond the outer breakwaters into Cape Cod Bay. These surveys provided real-time data indicating that the backwashing operation caused a relatively thin thermal plume, which spread rapidly from the intake out across the study area and along the seaward breakwater. Within a few hours these backwash thermal plumes were completely dissipated.The electric utility industry places emphasis on improving the efficiency of electrical generation. One area that is receiving significant attention is the control of biofouling within operating condensers. Biofouling, which occurs on the water side of operating condensers, contributes to the decrease in circulating (cooling) water flow rate and decreases the heat transfer efficiency of the condenser. These problems lead to an overall decrease in operating efficiency of the entire energy production system.Biofouling is a process which goes through various stages. There are two major types of biofouling: macrofouling (associated wiih setting and growth of shellfish within the condenser tubes) and microfouling (associated with the growth ofa bio-film). Cooling water flowing past the tube surface represents a nearly endless supply of a large variety of seed organisms (for example, mussel larvae), nutrients, and constituents for the growth process. The entire biofouling process is quite complex and involves physical, chemical, and biological reactions.The control of biofouling is quite site-specific. At Pilgrim Station, biofouling control is based upon the use of chlorine and thermal backwashes. The efficiency of chemical use is typically low because of the short residence time and the tendency toward secondary chemical