Tuesday, 29 January 2019

Types of Compressed Air Dryers

Types of Compressed Air Dryers

In the world of manufacturing, there are three utilities that are considered essential to most processes: water, electricity and natural gas. That said, there is one more utility considered nearly as important as these. Compressed air, is considered the fourth utility in manufacturing

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What Is a Compressed Air Dryer?

A compressed air dryer is a machine that sucks in and significantly reduces the moisture content of ambient air. In manufacturing, air dryers serve the vital role of rendering air more sanitary and conducive for workers and products alike. Depending on the moisture levels of a given environment, the use of a compressed air system can make all the difference between whether or not a production is even achievable. Compressed air dryers can be split into the following categories:
  • Refrigerated drying units:
    • Cycling
    • Non-cycling
  • Desiccant drying units:
  • Heatless
  • Heated — internal or external
The main difference between each dryer type is the agent placed inside the machine to dry the incoming air.

The Refrigerated Dryer

Out of all the compressed air dryers, the refrigerated dryer is the most popular and widespread type. True to its name, the refrigerated air dryer functions in a similar manner to home refrigerators in that a cooling process is employed that prevents moisture. The main difference is the objects or elements being cooled, because whereas a kitchen refrigerator cools and preserves perishable foods and beverages, a refrigerated air dryer maintains the quality of air in manufacturing plants.
In a refrigerated dryer, compressed air is cooled to around 35 degrees Fahrenheit. After the air has passed through the inlet valve, the moisture is extracted and drained, and the newly dried air is reheated by incoming air. At the end of each cycle, the outgoing compressed air has a dew point in the range of 35 to 40 degrees Fahrenheit. Overall, the process of the refrigerated dryer creates a more sanitary working environment that is also far more conducive for manufacturing.

In a non-cycling refrigerated dryer, the refrigerant perpetually circulates within the machine. This allows for quick responses to changes in the load, be it low or high densities of moisture within the incoming air. As such, factory environments with steamy engines can be de-moisturized in much the same way as low-moisture environments. The flow of the refrigerant is regulated with a bypass or unloader valve. In most units, the refrigerant is condensed by a heat exchanger after the compression process has completed a cycle.

CFC refrigerants like R12 and R22 were used in vintage refrigerant air dryers, though times have changed regarding the acceptability of such gases due to concerns over their impact on the environment. Therefore, in newer refrigerant dryers, a different range of chlorine-free refrigerants are used, such as the eco-friendlier R134A and R410A. The change was brought about by the Montreal Protocol, an international, anti-ozone-depletion treaty that was implemented in 1989.

Refrigerant dryers of the cycling variety use refrigerant to cool down a glycol or aluminum mass, which surrounds the air passage. The compressed air is then cooled by a thermostat-controlled heat sink. The advantages of a refrigerant air dryer break down as follows:
  • Advantages of refrigerant dryers in general:
    • Inexpensive to set up
    • Inexpensive to operate
    • Low maintenance costs
    • Resistant to airborne oil particles
  • Disadvantages of refrigerant dryers in general:
    • Minimal dew point capacity
  • Advantages of non-cycling units:
    • Consistent dew point
    • Continual operation
  • Disadvantages of non-cycling units:
    • Don’t conserve energy during low airflow
  • Advantagesof cycling units:
    • Conserve energy during low airflow
  • Disadvantagesof cycling units:
    • Variable dew point
    • Larger and heavier due to heat sink
    • Higher overhead cost

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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