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27
Bleed Line Calculations
: Bleed rate is determined by the following formula:
B = E
Where
: B = Bleed Rate (USGPM)
E = Evaporation Rate (USGPM) = Q (USGPM) x R (°F) x 0.001
Q = Process Fluid Flow Rate (USGPM)
R = Range
n = Number of Cycles of Concentration = CR/CM
CR = Concentration in Recirculating Water
CM = Concentration in Make-up Water
Given
:
• 3299C Cooling Tower
• Process Fluid Flow Rate = 800 USGPM
• Maximum Allowable Chloride Concentration = 250 ppm
• Concentration of Chlorides in Make-up Water = 45 ppm
• Range = 10°F
Find
: Bleed Rate
Solution
: So in this case,
E = Q * R * 0.001 = 800 * 10 * 0.001 = 8 USGPM
n = CR = 250 ppm = 5.55
B = E = 8 USGPM = 1.75 USGPM
Therefore, in this case we must bleed approximately 1.75 USGPM to limit the
concentration of impurities.
This example focuses on a single parameter (chloride concentration) of water only. The
bleed rate required for a system (when evaluating more than one parameter) is the highest
bleed rate required to keep all parameters within recommended limits.
NOTE:
The evaporation rate (E) can
be determined by any one of the
following methods:
•
The evaporation rate is
approximately 2 USGPM per 1
million BTUH of heat rejection.
•
The evaporation rate is
approximately 3 USGPM per 100
tons of refrigeration.
•
Evaporation Rate =
Q (USGPM) * R * 0.001.
(n-1)
NOTE:
Evaporation is proportional
to the load and will vary seasonally.
BAC recommends the use of a
conductivity meter to maximize
water conservation.
CM
45 ppm
(n-1)
(5.55-1)
Bleed Rate
Bleed Rate