Heated vs heaterless dessicant air dryer 1

[rickfer]

I'm looking to buy a new dessicant type air dryer for a very cold area, and I wonder, when is an area too cold?
When do I need a heater for the regeneration of the air dryer and how much can I stand without the heater?

 

[crjones]

It is my impression that heatless dryer uses more compressed air than does one with a heater. As far as temperature freezing seems to be the challenge.

 

[quark]

Ambient temperatures are not a criteria for selection of air dryer (This is because when you compress a gas, it heats up and then you cool it). The regeneration air flow rate, as suggested by crjones, is higher for heatless air dryer.

 

[Montemayor]

Quark is exactly correct. The ambient air temperature is not the criteria you should use for selecting an adsorption air dryer. The main criteria you should be looking at is your specified dew point on the product air. If you are proposing to install a TSA (Temperature Swing Adsorption) system, that is one thing. If you are looking at a so-called "heatless" adsorption dryer, that is another thing. The heatless models will consume a lot of product dry air. That is not my opinion; that is an empirical fact. You never get something for nothing (or, "there are no free rides or free lunches"). Therefore, be ready to pay a lot of money in regenerating the adsorbent with product dry air if you select a heatless model. Personally, I would never recommend a heatless dryer in today's world. I have proven their exorbitant operating cost in the field many times.

The ideal situation is where you can scavenge waste heat - such as 35 to 50 psig waste steam. This "waste" heat coupled with a TSA type of adsorber will give the best, low operating costs you can get.

My advice is to look to apply a TSA type of dryer, but look for existing waste heat sources - if possible. However, all this has to do with the dew point you are trying to achieve and maintain. You haven't told us that, plus a lot more of the basic data.

 

[zdas04]

To expand slightly on that point, you really need to know what your true goal is. If you can live with 10-20F dew point reduction the a deliquescent dryer (possibly in conjunction with a chiller) is often the most cost effective approach. If your process requires less than zero C then you have to use something that can be regenerated. I've done this with a gas-to-gas heat exchanger wrapped around the exhaust stack on a compressor driver, and I've seen it done with electric heaters and hot rocks.

Bottom line is that on any dryer you have to be able to reject the water you've taken out of the stream. With a deliquescent dryer you drain it as brine. With a "heatless" unit you slowly evaporate it into a dry stream and reject it to atmosphere (these units are still TSA, and are never the best economics). With a regenerative unit you apply enough BTU's to the bed to cook the water back into vapor. With a TEG unit you reject it in the reboiler. In every case the water does not simply vanish and all the wishful thinking in the world won't make it so.

Ambient temperature is not always a non-factor in this analysis. On the North Slope of Alaska any air lines leaving a heated building have to have a dew point lower than -50 F. In the northern U.S. a dewpoint around 0F is usually adequate for lines leaving heated spaces. On the Gulf Coast dewpoints below 50F are often a waste of money.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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