Compressed air/gas Dryer Outdoor Operation

A deliquescent compressed air or natural gas dryer can be installed outdoors and operated in low ambient conditions.  Indeed, this is among a deliquescent dryer’s most important characteristics.

One challenge of operating a water saturated pressurized gas system outdoors is, of course, the threat of forming ice within pipes, valves, regulators, and points-of-use.  It’s why a dryer is needed .  But should you be concerned about freezing within the dryer itself?  A deliquescent dryer after all is a liquid collection point.

The answer is both yes and no.

First, I recommend insulating a deliquescent dryer’s drain line when operating outdoors in low ambient conditions, as the drain line is not warmed by the flow of gas through the dryer.

The liquid produced with a deliquescent dryer is salty.  And it’s true that a salty water (brine) freezes at reduced temperatures compared to pure water.   So why should the dryer’s drain be a concern in low ambient temperatures?  One reason is that not all salt liquids behave the same.  The freezing point of brine depends on the ratio of salts to water and the types of salts in the brine.  Low ambient temperatures can increase the volume of water condensed in upstream coolers.  Depending on the presence (or absence) of liquid separators and coalescing filters, a high volume of incoming liquid water to the deliquescent dryer will dilute the brine drain solution, raising the freezing point.

In many cases the salt and water solution will have enough salt content to reduce its freezing point to very low temperatures, but at the same mixture the brine has a crystallization temperature  at or near  32° F.  Keeping the brine in the drain pipe above the freezing point of water helps keep the liquid flowing well.

Does the freezing point of the brine drop as system operating pressure increases?

It is true that the freezing point of most brine solutions  will be reduced with increased pressure, but in most cases this relationship is negligible unless the pressures are very high.  While increased pressure can reduce the freezing point it is also true that the crystallization point can increase with increased pressure.  So don’t assume that high pressure will spare you from the need to insulate an outdoor drain line.

Either way it is good insurance to insulate the drain and drain piping if the dryer is to be operated in ambient conditions near or below 32°F.

What about insulating or heat tracing of the dryer vessel itself?

The dryer itself -- that is the internal sump area at the tank's base -- is not usually susceptible to freezing, especially in applications where relatively warm compressed air or gas is flowing through the dryer vessel.  There are very few cases where heat tracing is required because most dryers operate with constant flowing compressed gas or air at above freezing temperatures keeping the brine level in the dryer above freezing.

Installations that only operate during single shifts should be drained of all brine during non-working hours and drain lines should be kept as short as possible to minimize low temperature exposure.