raise the boiling point. The boiling pt curve is actually relatively flat below ~50wt% glycol (I'm looking at it right now) and doesn't really get steep until you're upwards of 80wt%+, and you don't want to run there (a mixture that high in EG) because other heat transfer properties will suffer.
In the range of mixtures you want to be running raising the system pressure has much much more affect on raising the boiling point than adding more ethylene glycol.
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