Bleed Line Calculations - BAC PCT Series Operation & Maintenance Manual

Bleed Line Calculations

Bleed rate is determined by the following formula;
Bleed Rate = B = E/(n-1) Where:
B = Bleed Rate (L/S)
E* = Evaporation Rate (L/S)
Q = Process Fluid Flow Rate (L/S)
R = Range = Entering – Leaving Fluid Temperature (°C)
n = Number of Cycles of Concentration = CR/CM
CR = Concentration in Re-circulating Water
CM = Concentration in Make-Up Water
*The evaporation rate (E) can be determined by any one of the following methods: 1. The
evaporation rate is approximately 1.5 L/hr for each kW of operating Total Heat Rejection
(THR). 2. The evaporation rate (L/hr) is = 6.3 x R (°C) x water flow.
The following example illustrates a bleed rate calculation:
Closed Circuit Cooling Tower Process Fluid Flow Rate = 50 L/S
Range = 5.5°C
Maximum Allowable Chloride Concentration = 250 ppm
Concentration of Chlorides in Make-Up Water = 45 ppm
Solution:
E = 6.3 x R x flow = 6.3 x 5.5 x 50 = 1732.5 L/hr
n = CR/CM = 250/45 ppm = 5.55
Bleed Rate = B = E/(n-1) = 1732.5/(5.55-1) = 381 L/hr
Therefore in this case we must bleed 0.159 L/S 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.
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