Aqueous solutions of glycol are corrossive to metals and therefore it must be used with an
inhibitor
. This corrossion inhibitor forms a surface barrier that protects the metal from attack.
The inhibited glycol solution are generally stable and relatively non-corrossive to most standard
materials. The exception is galvinised steel
because the zinc coating will react with the inhibitor
part found in most formulated inhibited glycol sold commercially.
Performance effects of qlycol solutions:
a. Higher concentrations of glycol will retard heat transfer coefficient in the BPHE. For
example, a 30% glycol concentration may cause a drop of capacity by about 20%. Refer to
the attached graph depicting this drop of performance (Appendix 16).
b. As the concentration increases, the viscosity of the solution also increases. Therefore, the
friction losses of the piping system will also increase. A correction factor must be used to
evaluate this increase. See Appendix 16.
c. Similarly, the viscosity increase will also affect the water pump performance. The flow rate and
efficiency will reduce with higher concentrations. Thus, the head pressure developed will also
drop. See also Appendix 16.
Due consideration of these effects must be given when designing and selecting the chiller unit
for such a system. Failure to do so may result in insufficient capacity and water flow rate.
Guidelines:
1. Before applying the glycol solutions, thoroughly clean and flush the system. Reaction with
sludge, rust, deposits and oil may retard the inhibitor function. This is especially so after using
cleaning agents in the BPHE. Complete removal of the cleaning agent is necessary before
charging in the glycol.
2. Calculate the total water volume in the system and determine the amount of glycol needed.
The solution can be mixed outside the system in drums or barrels and then pumped into the
system.
Use clean and soft water (low in chloride and sulfate ions) for the mixing. Distilled water,
deionized water or condensate water may be used for this purpose.
3. The system must not be connected to an automatic make-up water pipeline. This may
cause dilution of the concentration and increases the water freezing point.
The inhibitor portion in the solution may also deplete over time due to reaction with
contaminants in the system.
Therefore, it is vital that a maintenance programme is in place to determine the glycol
concentration in the system. This can be measured by using refractometer (which measures
the refractive index of the solution), gas chromatography or density bottle (though not so
accurate).
4. Do not mix different inhibited glycol formulations together.
Section 9 Page 114
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