Author Topic: drying air BEFORE it goes into the compressor?  (Read 818 times)

Offline howsitwork?

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drying air BEFORE it goes into the compressor?
« on: March 15, 2020, 01:40:46 PM »
Guys

how practical is it to dry air before the compressor ? Reason is Iíve just bought a 6 month old compressor ( the guy ,I am sure NEVER drained in in the time he had it, so there was a fair amount of rusty water in the tank. I have drained it, allowed to stand open but mus5 admit Iíd like to sluice inside with isopropyl or something to ensure itís dry.

Is there any practical way t9 dry the air before the compressor pressurises it? I know I should drain after every session. but sometimes forget and an nervous about a 50l tank at 150psi sitting there. OK itís only 1 year old now but  avoidance of a problem always seems a good idea to me.

i do have water trap on output etc so no issues with tools in use.

all suggestions welcome provided theyíre legal.

regards Ian

Offline awemawson

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Re: drying air BEFORE it goes into the compressor?
« Reply #1 on: March 15, 2020, 01:47:20 PM »
Fit a timed electric drainer. They cost peanuts on eBay and work really well
Andrew Mawson
East Sussex

Offline RussellT

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Re: drying air BEFORE it goes into the compressor?
« Reply #2 on: March 15, 2020, 02:22:12 PM »
I used to worry about corrosion in the air tank as well.  Then someone told me (it might have been on here) that corrosion failures aren't normally catastrophic - there'll be a leaking pinhole well before the tank explodes.  I tried to find a definitive source for that and although I couldn't find a definitive source I couldn't find anything that contradicted it either.

I think getting the air dry enough to eliminate damp in the tank would be difficult and probably expensive.  It might be better to save your money and inspect the tank regularly and use the saved money for the next air tank.

Russell
Common sense is unfortunately not as common as its name suggests.

Offline John Rudd

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Re: drying air BEFORE it goes into the compressor?
« Reply #3 on: March 15, 2020, 03:55:35 PM »
When I worked at the gas terminal, we had 3 big Bellis and Morcom compressors for site air.
The sets were served by 3 sets of air driers that were electrically regenerated.
No pre drying of inlet air.....Oh and we had air dewpoint meters checking how dry the air was prior to going to the receivers..-29 degress I think we aimed for..,
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Offline beeshed

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Re: drying air BEFORE it goes into the compressor?
« Reply #4 on: March 15, 2020, 06:39:49 PM »
If you can dry it as it comes out why can't you dry it before it goes in? Is it because the compression has removed a lot - that is now in the tank so there is less to remove at the output?. If you pass the compressed air through a dryer into a balloon (as in weather balloon) then compressed it again with a second dry tank compressor would that work?
Not completely daft if you have it all inside and recover the heat from both compression and the motors.

Offline Will_D

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Re: drying air BEFORE it goes into the compressor?
« Reply #5 on: March 15, 2020, 07:57:34 PM »
My new, silent, dental compressor lives next to my central heating oil boiler. Nice warm place, so very low humidity in the air intake so very very little condensate in the air tank.

If worried about moisture why not just build a big air box intake for the compressor with a load of silica gel plates to dry the air on the way in?

A bit like a fractionating column!


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Offline mc

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Re: drying air BEFORE it goes into the compressor?
« Reply #6 on: March 15, 2020, 08:03:12 PM »
Performance and efficiency.

Any kind of restriction on the intake of a compressor, reduces it's performance. If you create any kind of below atmospheric pressure on the inlet side, that is a pressure drop that has to be recovered. Even taking a 0.1bar drop, you'll typically lose 10% of capacity.

Then there is the issue, of air flow. On the intake side, you have far more airflow to have to dry. You would need a lot of surface area to effectively achieve sufficient drying.

Offline RussellT

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Re: drying air BEFORE it goes into the compressor?
« Reply #7 on: March 16, 2020, 05:50:42 AM »
Nice warm place, so very low humidity in the air

Surely that only reduces the relative humidity rather than the absolute amount of water in the air, and wouldn't it be more efficient to feed the compressor on cold air?

Russell
Common sense is unfortunately not as common as its name suggests.

Offline velocette

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Re: drying air BEFORE it goes into the compressor?
« Reply #8 on: March 24, 2020, 02:33:20 PM »
Lots of industrial compressors had finned pipes to lower the air temprature before it entered the holding reservoir. Something using this principal to lower the temprature as low as possible.
 Then through an after cooler with the temprature of 3 or  4 deg Celsius I.E. a coil of copper pipe from the compressor  through a  refrigerator.
 With a catch tank with a drain valve where it exits the fridge.
Working on large industrial compressed air with this principal for a number of years using this setup to feed a factory with all machines fitted with water traps and drains a clean and dry air supply. To dry the air first before feeding the compressor Dry Air Is Heavier Than Wet Air.

Offline awemawson

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Re: drying air BEFORE it goes into the compressor?
« Reply #9 on: March 24, 2020, 03:00:32 PM »
Dry Air Is Heavier Than Wet Air.

That sounds counter intuitive  :scratch:

Surely a given volume of dry air, that then is saturated by water MUST weigh more by the weight of the water that it has taken up.  :scratch: :scratch: :scratch:
Andrew Mawson
East Sussex

Offline velocette

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Re: drying air BEFORE it goes into the compressor?
« Reply #10 on: March 24, 2020, 08:14:36 PM »
"Surely a given volume of dry air, that then is saturated by water MUST weigh more by the weight of the water that it has taken up"
The best way I can explain without masses of mathematics. How do clouds form? The lighter wet air rises as it reaches an altitude were the temprature is lower the water vapour that was in the air stream become visible as tiny water droplets that reflect light and become visible. Then inthe turbulent airstream  these tiny droplets bump into each other become heavier and fall as rain. Another explaination is in wet-humid weather which I know you get lots in Britain I used to live there. The thick smoke from chimneys exits rises until cool then falls to the ground and settles at low levels as smog.

Offline awemawson

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Re: drying air BEFORE it goes into the compressor?
« Reply #11 on: March 25, 2020, 04:09:11 AM »
I can only think that either the volume or pressure of your air is not remaining constant.

A fixed volume of air at S T P has a certain number of molecules that weigh a fixed amount. If then water molecules are introduced such that they occupy the spaces between the air molecules so that the volume stays the same, the weight of the system has increased by the weight of the water molecules. Hence the density of the water plus air is greater than the original air.

Probably what is happening in the systems that you have experienced is that the volume is NOT staying constant as the water molecules effectively are pushing the air molecules apart disproportionally to their mass, and the density of the system thus decreases.

Andrew Mawson
East Sussex

Offline John Rudd

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Re: drying air BEFORE it goes into the compressor?
« Reply #12 on: March 25, 2020, 04:16:49 AM »
Just think about the jet engine....how the performance changes with air density and the amount of fuel it consumes...
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Offline velocette

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Re: drying air BEFORE it goes into the compressor?
« Reply #13 on: March 25, 2020, 09:26:43 PM »
My ability to understand how things work far exceeds my ability to explain why due to the way my brain processes information.
Called in  this help from Google
"Dry air consists mostly of nitrogen and oxygen molecules, which weigh more than water molecules. This means that when a given volume of air is made more moist by adding water molecules, heavier molecules are replaced with lighter molecules".

BroomWade Web Site gives this
Compressed air leaving the compressor after-cooler and moisture separator is normally warmer than the ambient air and fully saturated with moisture. As the air cools, the moisture will condense in the compressed air lines. Excessive moisture can result in pipe corrosion and contamination. For this reason it makes sense to fit a BroomWade air dryer. Some end use applications require very dry air, such as compressed air distribution systems where pipes are exposed to winter conditions. Drying the air to dew points below ambient conditions is necessary to prevent ice build up.

My Thoughts
By super cooling the after cooler it was not absolutely necessary to fit air driers.


Offline RodW

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Re: drying air BEFORE it goes into the compressor?
« Reply #14 on: March 29, 2020, 05:54:14 AM »
When I researched this, Commercial systems typically had a compressor, a precooler, a storage tank in that order followed by a refrigerated drier.
So the key to this is to get the air inlet temperature down low enough so the temperature drop that the drier can achieve hits the dew point to maximise water removal.

So my solution to this was to add a transmission cooler and some flexible hydraulic hoses between the compressor and storage tank so that the fan in the compressor pulley draws air though the precooler. I placed an auto drain at the lowest point. This is extremely effective in cooling the air entering the tank and removes some water. An electric autodrain  drains the tank at regular intervals and is not expensive at all. The inlet to the pre-cooler is too hot to touch and the hose into the tank is always cool to the touch. I've been meaning to alter the plumbing so the air travels up from the auto drain so there is more chance of water flowing back to the drain.

So when the air hits my refrigerated drier, it is reasonably dry and the dewpoint is always hit. The drier came with self draining pre and post filters and no further filtering is required I now have perfectly dry air all the time.
RodW
Brisbane, Australia

Offline mattinker

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Re: drying air BEFORE it goes into the compressor?
« Reply #15 on: March 29, 2020, 06:00:49 AM »
Not very exotic, but as a start, I plan to move my compressor outside, but, I will pick up the air from inside the workshop where it's less damp!

Regards, Matthew