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Did you buy the wrong type of compressed air dryer?

Do you have a desiccant compressed air dryer in your plant?
Yes?
Do you know the annual running cost of that dryer?
It's plugged into a power socked, so it probably uses a bit of electricity… and you need to replace the desiccant every so many years. Right?
True!
But for many types of desiccant air dryers, there's a hidden cost… and it is substantial!
In fact, the yearly cost will probably SHOCK you!
The culprit?
Purge air!
Purge air is air that is used to regenerate the desiccant. (the desiccant needs to be regenerated every few hours, to remove the water that it previously absorbed).
There are 2 main types of desiccant dryers:
Heated blower regenerated
This type uses an external blower and heater to regenerate the desiccant.

It requires only electricity (around 1 kW for every m3/min of compressed air).
Purge regenerated
This type doesn't use electricity to regenerate the desiccant...
Great, right! 
No! 
It uses precious compressed air!!!
This compressed air flow is between 10 and 20% of nameplate rating of the dryer.
So if we take a 90 kW air compressor (17 m3/min or 600 cfm) that runs for 8.000 hrs per year (almost fulltime, which is quite common). And we pay 10 cents per kWh… what do these two dryers cost us?
Running cost for heated blower generated dryer:
A dryer for 17 m3/min uses around 10 kW of electricity on average. At 8.000 hrs per years and 10 cents per kWh, 
… that's $ 8.000 per year in electricity
WOW!

That's the price you pay for dried air!
But let's take a look at the other type of dryer…
Running cost for purge air regenerated dryer:
Purge air regenerated dryers come in two flavors: heated purge and unheated purge.

Heated purge uses around 10% of compressed air (+ electricity for heating), while unheated purge uses around 20% of compressed air.
So… what does this cost us?
Our 90 kW air compressor, that runs for 8.000 hours at 10 cents per kWh costs us $72.000 to run per year!
(surprised?)
So what does our air dyer cost us? It uses up to 20% of our precious compressed air for the regeneration process!
What? Are you crazy?
20% of compressed air, that's $14.400 per year! 
Wow again! Almost double!
And that's the MINIMUM you'll pay through your desiccant compressed air dryer.
The purge air rating is *nameplate* rating – so if you have a bigger dryer than really needed (which by itself is a good thing actually), you will also use more purge air!
So if you have a 800 cfm dyer behind your 600 cfm air compressor… you will lose 160 cfm of compressed air. That is 1/4th of your precious compressed air!
And if you chose the cheapest option, without dewpoint control or anything like that … it will happily continue drying and purging, even if your compressor is stopped!

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