Coefficient of utilization (cu)

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Coefficient of utilization (cu)

  1. 1. AWP x EWP No. of = luminaires (lumens/lamp) x (lamps/luminaires) x CU x LLFTOTAL The coefficient of utilization is a factor used to determine the efficiency of a lighting fixture in delivering light for a specific application The coefficient of utilization is determined as a ratio of light output from the luminaire that reaches the workplane to the light output of the lamps alone Coefficient of Utilization (CU)
  2. 2. Coefficient of Utilization (CU) Factors influencing coefficient of utilization: The efficiency of the luminaire The luminaire distribution The geometry of the space The reflectance's of the room surface Each luminaire has its own CU table specific to that luminaire’s light distribution and efficiency. CU values are listed in tables for different room geometries and room surface reflectance.
  3. 3. For a given room, the cavity ratios are in direct proportion to their respective cavity heights. For the case where the luminaires are mounted on the surface of the ceiling or are recessed into the ceiling, the ceiling cavity ratio is zero. It is necessary to convert the actual ceiling reflectance into an effective ceiling cavity reflectance (pCC) and the actual floor reflectance must be converted to an effective floor cavity reflectance (pFC).
  4. 4. CU Determination Step 1. Determine the room cavity ratio using the equation below Room cavity height (hRC) = Luminaire height – Workplane height RCR = 5 x Room cavity height (Length + Width) Length x Width
  5. 5. Step 2. Since the Lumen Method considers what occurs only within the room cavity, the ceiling and floor reflection factor are replaced with their effective reflectances. Ceiling reflection factor: 70% For very light colours and white 50% For light colours 30% for medium shades Wall reflection factor: 50% For light colours and white 30% For medium shades 10% For dark shades
  6. 6. To find the effective reflectance of a floor or ceiling cavity, find the floor cavity ratio and ceiling cavity ratio using the equations below
  7. 7. • Step 3. Find the effective cavity reflectances using cavity surface reflectances.
  8. 8. Step 4. Once all room cavity reflectances and the room cavity ratio are known, the CU value can be determined by selecting the appropriate value from the luminaire’s CU table.
  9. 9. Example 1 Classroom 20’ x 27’ x 12’ E=50 fc WP= 2’-6” AFF ρc= 80% hcc= 0.0’ ρw= 50% hrc= 9.5’ ρf= 20% hfc= 2.5’ fixture: fluorescent (#38) maintenance: yearly replacement: on burnout voltages & ballast: normal environment: medium clean
  10. 10. Example 1 Confirm fixture data S: T.15.1 p. 641
  11. 11. Example 1 Complete #1-6
  12. 12. Example 1 7. Determine lumens per luminaire Obtain lamp lumens from manufacturer’s data (or see Stein: Chapter 12) S: T. 12.5 p. 546
  13. 13. Lumen Flux Method 8. Record dimensional data 20’ 27’ 0’ 9.5’ 2.5’ ρc= 80% ρw= 50% ρf= 20%
  14. 14. Coefficient of Utilization Factor(CU) Calculation 9. Calculate Cavity Ratios
  15. 15. Example 1: Cavity Ratios CR = 5 H x (L+W)/(L x W) RCR = 5 Hrc x (L+W)/(LxW) = 4.1 CCR = 5 Hcc x (L+W)/(LxW) = 0 FCR = 5 Hfc x (L+W)/(LxW) = 1.1
  16. 16. Coefficient of Utilization Factor(CU) Calculation 10. Calculate Effective Ceiling Reflectance
  17. 17. 3. Obtain effective ceiling reflectance: Example 1: Coefficient of Utilization (CU) S: T.15.2 p. 667
  18. 18. Example 1 11. Calculate Effective Floor Reflectance Stein: T.15.2 P. 666
  19. 19. 3. Obtain effective ceiling reflectance: Example 1: Coefficient of Utilization (CU) S: T.15.2 p. 667 CU= 0.19  0.20
  20. 20. Example 1 12. Select CU from mfr’s data or see
  21. 21. CU=0.32 Example 1: Coefficient of Utilization (CU) RCR CU 4.0 0.39 4.1 X 5.0 0.35 CU= 0.386 S: T.15.1 p. 641

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