If you are just designing the equipment, the choice of ethylene glycol
(EG) or propylene glycol(PG) will be made for you, but if you are laying out
the entire system you may find yourself in the position of specifying the fluid
used in the heating and cooling coils. So what do you choose?
If the operating temperature of the system is below the freezing point
of water, you will probably find yourself having to choose between EG and PG.
Local regulations may require that you decide between the use of an
ethylene or a propylene glycol-based fluid. In the absence of regulations, how do you decide?
In most applications ethylene glycol-based fluids will be your first
choice because of their superior heat transfer efficiency. The lower viscosity
of ethylene glycol solutions is the key contributor to this efficiency.
This lower viscosity also leads to lower power consumption for pumps.
Further attributes making ethylene glycol a first choice is a lower
minimum operating temperature. Unfortunately, ethylene glycol has a very
high oral toxicity. Actually, the ethylene glycol itself isn’t toxic.
When the body metabolizes ethylene glycol, it produces glycolic and
oxalic acid, Which is the damaging toxins.
Which brings us to propylene glycol.
Propylene glycol is used in applications where incidental contact with
food or beverage is a possibility, or where a breach of the closed system could
contaminate a water supply. Propylene glycol is Generally Regarded as Safe by the US Food and Drug
Administration and is approved for immersion applications of wrapped food
product. On the downside, propylene glycol solutions are more
viscous than those of ethylene glycol, particularly at temperatures below 0°F.
This results in a reduced heat transfer efficiency at low-temperature
operation. In general propylene glycol is not used in systems operating
below -28°F due to this viscosity increase.
