Bleed Rate - BAC HXV Operation & Maintenance Manual

NOTE: The peak evaporation rate (E)
can be estimated 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.
NOTE: Evaporation is proportional
to the load and will vary seasonally.
BAC recommends the use of a
conductivity meter to maximize
water conservation.
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Bleed Line Calculations: Bleed rate is determined by the following formula:
B = E
(n-1)
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:
• Closed Circuit 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
CM 45 ppm
B = E = 8 USGPM
(n-1) (5.55-1)
Therefore, in this case we must bleed approximately 1.75 USGPM to limit the
concentration of impurities to an acceptable level.
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.
= 1.75 USGPM