Winter is coming…

The cold weather bring a particular set of problems to compressed air systems – and these problems start even before temperatures reach negative values (I’m talking degrees Celsius here).

Now that temperatures are dropping, it’s time to do some basic checks on your system to prevent problems.

Cold weather can affect the air compressor itself, compressed air equipment (like filters and dryers), compressed air lines running outside and also the equipment at the point of use.

The main problem in cold weather is freezing of condensate water that is in the compressed air.

And this happens even above freezing temperatures!

Freezing above freezing temperatures

Why does this happen?

Air compressors generate a lot of heat. This heat is removed in the compressor in the oil cooler and after cooler of the compressor (for oil-injected rotary screw compressors), or in the intercooler and after cooler (for oil-free screw compressors).

Removing this heat, and keeping the compressor running cool, can be a challenge all by itself – especially in warm summer months.

But where is all this heat coming from? And why are we talking about heat when this articles is about freezing problems?

The heat comes from the compression of the air – compressing air generates heat. The air heats up.

When we use compressed air, the opposite happens: the air cools down as it expands.

It’s basic thermodynamics, and there’s nothing you can do about it.

The problem in cold weather is that at any point where compressed air is blown of, the temperature will drop – and if there’s any water in the compressed air, it will freeze.

This can happen even if you don’t have liquid condensate water in your compressed air, because the part where compressed air is blown off will get very cold and water vapor in your compressed air will condense into liquid water and later freeze to ice.

This happens even before ambient temperatures reach negative values. I call this ‘local freezing’ as the freezing happens very locally, only right at the part that is blowing off air.

Generally speaking below 10 degrees Celsius ambient temperature, this can happen. And the colder it gets, the more this will become a problem.

Common local freezing problems

Here are some common locations where local freezing can lead to problems.

Air Compressor Control system

The control system of an air compressor has several blow-off points. Remember that the compressor control system works on compressed air that is not filtered or dried in any way – for this reason it contains lots of moisture. At each of the blow-off points of the control system, local freezing problems can happen.

For example, air control air is blown off when the compressor switches from load to unload. At the same time, internal pressure is blown off to lower unload energy consumption. When the compressor stops, the internal pressure is completely blown off.

This air is blown off through pilot valves on the inlet valve or at the bottom of the screw element (operated by outlet pressure). Or, it is blown off through a vent-hole in the load/unload solenoid valve.

All of these blow-off holes can freeze shut, causing problems. The most susceptible to this is the load/unload solenoid.

Solenoids that blow off air can become so cold they complete freeze up themselves, making them stuck in ‘open’ or ‘closed’ position.

Desiccant Air Dryers

With desiccant compressed air dryers, local freezing problems happen again at any point where air is blown off.

Compressed air filters and condensate traps

Compressed air filters and condensate traps all have drains to purge condensate water from the system.

Again, at these points local freezing can occur – blocking the drain. In this case, the flow of compressed air is not blocked, but the condensate drains don’t operate anymore, leaving all the condensate water in your compressed air (giving you lots of problems if not noticed in time).

Point of use

Where compressed air is used, air is expanded. Any machine that uses compressed air will get cold.

When ambient temperature are cold, this can cause local freezing on machine or equipment that use compressed air – especially at any point where compressed air is blown off.

How to prevent or fix local freezing problems in compressed air systems

Here is some advice to prevent and fix problems caused by local freezing of compressed air.

Dry air

To prevent these problems, the main thing is to keep the compressed air as dry as possible.

The lower the (pressure) dew point of the air is, the less problems you will have with local freezing.

The local freezing will still occur, but with little water in the compressed air, the chance of it freezing shut is smaller.

Bigger vent holes

Local freezing is more of a problem with small vent holes, as they are easier to freeze shut compared to bigger vent holes.

If you have particular vent holes that freeze, try adding a compressed air damper/muffler to it. The compressed air will expand over a larger area which makes the cooling effect spread out over a larger area – often eliminating the problem. And even if local freezing still occurs, the blow-off area of the damper is often so large it won’t freeze shut completely.

In general: increase the size of the blow-off hole – this will prevent them from freezing shut completely very quickly.

Condensate drains on air receivers are sometimes really small – replacing them with bigger versions can prevent these problems.

Compressed air heater

Sometimes it might be necessary to install a compressed air heater at the point of use.

The compressed air heater heats up the air – at the point of use – to prevent problems with condensation and local freezing.

This is only an option if the flow of air is relatively small and of course this only works for equipment at the point of use.

Fix freezing problems in compressor control systems

To prevent blow-off problems in the compressor control system – the easiest option is to raise the ambient temperature in the compressor room.

Even if the compressor takes cold air from outside (through ducting), this is not a problem – as the solenoids and other vent-holes are now in warmer ambient temperatures, preventing freeze-ups.

In my experience, freeze-ups of solenoid valves and other vent holes happen more often with machine that frequently switch between load and unload, or start and stop – simply because air is blown off more often and there’s little time in between for the part to warm up again (even if its to only a few degrees in the plus).

It sometimes helps to adjust the pressure setpoints so the compressor loads and unloads less often –

Re-use compressor heat

If possible, recirculate the hot cooling air into the compressor room to raise the temperature – a great way to re-use the otherwise unused heat of the air compressor.

There are systems available that automatically open and close dampers in the cooling-air ductwork to automatically adjust the compressor room temperature.

If this is not easy to do – another option is to add heat tracing to every blow-off point. The heat tracing will warm up the part. When compressed air is blown off, the part will cool down, but since it was warm to start with, it won’t reach freezing temperatures.

Other cold weather problems affecting your compressed air system

Until now I have only focused on what I call ‘local freezing’ problems. But there are some more problems associated with cold ambient temperatures.

Here are some more things to consider when outside temperatures are dropping.

Compressor running temperature

Keep an eye on the running temperature of your air compressor. Just like overheating, running at low temperatures is also bad for your air compressor.

The main problem here is excessive condensate formation in the air compressor, with lots of problems as a result – I will talk more about this in my next article.

Compressor startup

Compressor oil is thicker when it’s cold. If a screw compressor starts up when it’s cold, it can have a hard time pushing around this cold oil.

The thick cold oil will cause higher than normal startup currents. Some compressors might trip on this high current – other compressors have a low ambient temperature sensor and simply refuse to startup when it’s too cold.

Refrigerated air dryers

With refrigerated compressed air dryers, the dryer can become too cold and freeze up internally.

This can cause a severe pressure drop, or even completely stop the flow of compressed air.

External compressed air lines

If you have compressed air lines running outside (between the compressor room and the factory for example), water vapor can condense and freeze up.

The only real solution is to dry the compressed air.

A refrigerated compressed air dryer is not enough in this case – as it will only lower the pressure dew point to 4 degrees Celsius typically. That means that if the compressed air temperature drops below 4 degrees Celsius, condensation will still happen.

To protect compressed air lines running outside, you need a desiccant compressed air dryer. Desiccant dryers are capable of reaching pressure dewpoints of -40 degrees Celsius or even lower.

With a pressure dew point of -40 degrees Celsius, condensation will only happen if the temperature drops below -40 degrees Celsius, which of course is very unlikely to happen.

Check your compressor

Now that temperatures are dropping, pay an extra visit to your compressor room and check if everything is still running smoothly.

Here are some things that you can check:

  • Note the ambient temperature in the compressor room
  • Check that the compressor still loads and unloads correctly
  • Check all condensate drains for correct operation
  • Check the running temperature of the air compressor
  • Check the dewpoint indicator of refrigerated air dryers

Even if you have no problems with freeze-ups – it’s bad for a screw compressor to run at low temperatures (element output temperature) for a long time.

More about that in my next article!

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