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

wedgewire screen applications is unnecessary and would not eliminate certain site

design considerations. Unrelated 

slot size for any screening application

conditions might require a facility to install a system that is over

performance standard, or unintentionally restrict a permitting authority’s desire to adjust 

design requirements that account for site

vary significantly from one site to another

overly restrictive for one facility and insufficiently restrictive for another.

Discussion

Wedgewire screen performance is a function of two exclusion principles

hydraulic. Leaving aside hydraulic cons

main factors influencing screen slot size

accomplished. (It is noted that slot size also indirectly influences hydraulic exclusion. 

Ultimately, the optimal slot siz

smallest/fewest screen assemblies while minimizing 

Historically, intake screen slot size

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

that are 3/4 or 7/8 inches in diameter). 

MEMORANDUM

Paul Shriner and Jan Matuszko

Tim Havey, Tetra Tech, Inc.

Maximum Wedgewire Screen Slot Size

For reasons discussed below, establishing a numeric maximum slot size for preations is unnecessary and would not eliminate certain site

. Unrelated factors at existing facilities naturally limit the maximum 

slot size for any screening application, including wedgewire. Excessively prescriptive 

a facility to install a system that is over-engineered to meet the 

, 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 

for one facility and insufficiently restrictive for another.

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

screen slot sizes were derived from the condenser tube diameter. Th

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 

have intake screens with 3/8-inch mesh panels (corresponding to condenser tubes 

that are 3/4 or 7/8 inches in diameter). 

- 1 -

-approved 

ations is unnecessary and would not eliminate certain site-specific 

at existing facilities naturally limit the maximum 

prescriptive 

to meet the 

, or unintentionally restrict a permitting authority’s desire to adjust 

Because biological factors can 

gle numeric limit could simultaneously be 

physical and 

iderations for now, this discussion focuses on the 

the means by which physical exclusion is 

accomplished. (It is noted that slot size also indirectly influences hydraulic exclusion. 

e will balance these competing factors and allow for the 

impingement and/or entrainment.)

the condenser tube diameter. 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 

inch mesh panels (corresponding to condenser tubes 
- 2 -

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’ throughslot 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