      
                                     MEMORANDUM
      
      
      Tetra Tech, Inc.
      10306 Eaton Place, Suite 340
      Fairfax, VA 22030
      phone	703-385-6000
      fax	703-385-6007
      
      TO:		Paul Shriner and Jan Matuszko
      FROM:		Tim Havey, Tetra Tech, Inc.
      DATE: 		July 30, 2008
      
      SUBJECT:	Maximum Wedgewire Screen Slot Size
      
Summary
For reasons discussed below, establishing a numeric maximum slot size for pre-approved wedgewire screen applications is unnecessary and would not eliminate certain site-specific design considerations. Unrelated factors at existing facilities naturally limit the maximum slot size for any screening application, including wedgewire. Excessively prescriptive conditions might require a facility to install a system that is over-engineered to meet the performance standard, or unintentionally restrict a permitting authority's desire to adjust design requirements that account for site-specific factors. Because biological factors can vary significantly from one site to another, a single numeric limit could simultaneously be overly restrictive for one facility and insufficiently restrictive for another.
Discussion
Wedgewire screen performance is a function of two exclusion principles -- physical and hydraulic. Leaving aside hydraulic considerations for now, this discussion focuses on the main factors influencing screen slot size -- the means by which physical exclusion is accomplished. (It is noted that slot size also indirectly influences hydraulic exclusion. Ultimately, the optimal slot size will balance these competing factors and allow for the smallest/fewest screen assemblies while minimizing impingement and/or entrainment.)
Historically, intake screen slot sizes were derived from the condenser tube diameter. The condenser tube is the narrowest point in the cooling water pathway and as such, is most susceptible to clogging from debris. As a general rule of thumb, the maximum screen slot size is never larger than 1/2 the condenser tube diameter, which is why a large number of facilities have intake screens with 3/8-inch mesh panels (corresponding to condenser tubes that are 3/4 or 7/8 inches in diameter). 
Slot sizes for conventional traveling screens typically refer to a square opening (3/8" x 3/8") that is punched or woven into the screen face. Wedgewire screens are constructed differently, however, with the slot size referring to the maximum distance between longitudinally adjacent wires. Because wedgewire screen slots are oblong, the slot size approximation method noted above should be conservatively modified to select slot sizes that are smaller than (1/2) the condenser tube diameter.
As with any screening technology wedgewire screens function best when kept free of debris and intake flows are uninterrupted. As a passive technology, however, wedgewire screens are designed to be somewhat "self-cleaning" by minimizing debris accumulation on the screen face in the first place instead of incorporating a robust cleaning mechanism (air-burst cleanings are essential, but are meant to be used infrequently and for brief durations). Minimizing debris accumulation is best accomplished by maximizing the screens' through-slot area (larger slot size), which corresponds to a lower velocity ratio and smaller flow field. The tradeoff is a diminished ability to exclude smaller organisms. In general, decreasing the slot size increases the probability that debris (including fish) will be impinged. 
Adding biological criteria to design parameters changes the slot size considerations. The goal of protecting smaller organisms -- namely eggs and larvae -- may require using smaller slot sizes than would be required otherwise but the selection basis is still comparable: choose the slot size that will exclude the target species and life stages while maximizing the screen open area. This allows for the smallest flow field and reduces the number and size of the screen assemblies while still being protective enough. 
                                 Slot Size Tradeoffs
      
      
      
      
      
      Smaller slot size					Larger slot size
      
      Greater exclusion					Lower exclusion
      Larger flow field					Smaller flow field
      Higher velocity ratio					Lower velocity ratio
      Higher impingement probability			Lower impingement probability
      Larger/more screen assemblies				Smaller/fewer screen assemblies 

