Chilled Water is used in 2-pipe systems with separate dehumidifiers or in 4 pipe systems with reheat from heat recovery chillers and/or boilers.
When air cooled chillers are used, the water is circulated outside and precautions must be taken to avoid water ever freezing in the outdoor piping and within the chillers (which will destroy the evaporator of the chiller).
One precaution that is taken is putting antifreeze in the chilled water, in the form of "glycol". Ethylene glycol and propylene glycol are both commonly used to prevent freezing in chilled water systems. Propylene glycol is less toxic and generally considered safe around food, making it a better choice in grow facilities where a consumable product is being cultivated and processed.
Propylene glycol is usually used in concentrations of 20 to 50% depending on the outdoor temperature.
However, propylene glycol changes the properties of chilled water in more ways than just lowering the freezing point.
One other way it effects water is through viscosity. Glycol is more viscous (i.e. thicker) than water. This increases the pressure drop through piping and coils but more importantly also reduces heat transfer at the coil. This is because the increased viscosity increases the size of the boundary layer and reduces turbulence.
With separate dehumidifiers in 2-pipe systems, this is less of an issue but when the chilled water coils are fully relied on for dehumidification, it has a larger effect. We might think that glycol impacts capacity on the order of 15-25%, but it can reduce dehumidification performance by a much larger percentage, since reduction in cooling reduces latent removal disproportionately.Â
To reduce this effect, coils should be selected specifically for dehumidification with glycol. This usually means deeper coils, higher velocity water flow, and higher pressure drop. Lower chilled water temperatures can also help and is feasible due to the antifreeze properties of glycol, but chilled water with no glycol in the low 40s will usually dehumidify significantly better than 30% glycol in the high 30s.
In larger systems, designing for freeze protection without using glycol in the indoor chilled water look can significantly improve performance.
Since glycol impacts viscosity more at low temperatures than high temperatures, glycol has negligible impact on hot water heating.