Monday, 14 January 2019

Heatless Desiccant Dryers Overview

Heatless Desiccant Dryer

The Working Principle - Flow Direction

Wet air from the compressor flows UP the tower. This means that gravity will encourage the water, already collected from the air, down the tower and away from the dry desiccant. More water exists at the bottom of the tower than at the top.

A small amount of the dried air, is feed through the purge line DOWNWARDS through the “wet” tower. Water is concentrated at the bottom of the tower due to the direction of the wet air.

The downward direction of the purge air means that the bulk of the water in the tower does not have to pass through the bulk of the desiccant. The moisture only has to travel a short distance before it’s ejected.

Performance Recalculation - A Question

Why is it that you cannot up rate the dryer for inlet temperature below 35°C ?

The Answer:

  • You cannot push more than the nominal flow through the dryer because the pressure drop increases dramatically.
  • Additionally, the speed of the air increases, meaning that the air is in contact with the desiccant for less time, which results in less effective drying and an increased PDP
  • The additional air speed also disrupts the desiccant bead and causes “channels” to be formed - resulting in less effective drying. The extra friction also reduces the desiccant lifetime.
  • Up-rating the dryer for a lower inlet temperature means that after 3 minutes the desiccant bed is saturated as normal. However, the purge air is also cooler than normal, meaning that it has less capacity to hold water. The purge flow (nozzle) is sized based on a purge air temp of 35°C, meaning that if the temperature of the air is only 30°C, it will not have the capacity to dry the desiccant bed. 

Heatless Desiccant Dryer :

Choice Of Desiccant 

Why Use Different Desiccant ?

Different desiccants have different levels of water attraction
The greater the attraction, the better pressure dew point is possible, but the more purge is needed to regenerate the desiccant.
Most desiccant cannot provide a -70°C PDP
Desiccant that can provide a -70°C PDP is more expensive
We don’t want the customer to pay for something that’s not needed

What Desiccant Do We Use in Heatless Desiccant Dryers

For PDP’s of -20 to -40°C PDP Activated Alumina
For PDP’s of -70°C PDP Molecular Sieve
We balance performance with capital investment and ongoing running cost through purge air
For smallest dryers Molecular Sieve is used as due to small amount of desiccant the cost of it is not critical.

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