Desiccant Dryers

ATEX Desiccant dryer

Desiccant Dryers and dehumidifiers – A guide to selecting and using desiccant dryers

Dehumidifier or Desiccant Dryer?

VRF300 desiccant dryer

VRF300 desiccant dryer

In order to start to appreciate the capacities of desiccant dryers, it helps to first understand the limitations of refrigerant based dehumidifiers. We know that the performance of conventional dehumidifiers depends largely on the temperature of the ambient air and its relative humidity. The higher the ambient humidity and temperature, the greater the extraction rate for a particular dehumidifier. From this it follows that the dehumidifier is working at its’ most efficient when air is 100% RH and temperature is as high as it reasonably can be, say 35°C. Therefore it is also true that dehumidifiers become less effective and less efficient as humidity and temperature decrease. At lower temperatures and humidity they become both ineffective and inefficient. Desiccant dryers, using a completely different mechanism for reducing humidity operate at much higher efficiencies at lower temperature that compressor based dehumidifiers.

large VRF desiccant dryer

Large VRF desiccant dryer

The role of Desiccant Dryers

Within various areas of industry and commerce there are numerous applications where dry air is critical to maintain the integrity of processes and to retain particular characteristic of products and raw materials. These types of applications are often referred to as controlled humidity environments, or CHE for short.

In these applications either a conventional refrigerant based dehumidifier would be either too blunt an instrument to maintain an exact humidity level or more often they simply cannot efficiently get the humidity down to a low enough level. In these applications and those where ambient temperatures are low, desiccant dryers are used rather than dehumidifiers as they offer exacting control of humidity and ability to achieve the sometimes very low humidity levels demanded by process applications, and can operate at lower temperatures.

 Where processes have to take place in an environmentally exact condition or where research establishments rely on precise conditions of humidity and temperature in order to guarantee the accuracy of their instruments, air handling based on desiccant dryer technology is vital to maintaining the exact conditions required in the close control unit (CCU). Direct expansion (DX) based systems by comparison are just not controllable enough to serve this purpose, partly because they don’t have the range of drying capability required to treat all ambient conditions, and partly because their performance tends to be ‘spikey‘, corresponding to the hydrostat switching and re-entrainment of moisture from the wet coils back to the process air flow.

Desiccant dryers are found in a vast variety of applications and the following examples are just a few;

Material processing  

Prevention of Mould

Powder silos
Pneumatic conveying
Mechanised handling
Production processes (e.g. pressing, moulding, coating)

VRF1000 desiccant dryer

VRF1000 desiccant dryer

 

Healthcare
Cargo protection
Flood & water damage

Brewing
Archives

Seed and grain
Cotton and Fabric
Foodstuffs

Product Drying  

Prevention of Condensation and Corrosion

Cured meat drying
Smoked fish
drying
Production of cultures based additives incl. yeast

Confectionery coating processes
Pharmaceutical materials handling and product finishing
Food drying pre packing e.g. herbs
 

Trotec TTR700 Desiccant dryer

Trotec TTR700 Desiccant dryer

 

Military storage depots
Generator storage
Pumping stations 
Power distribution
Bulk metal
storage
Metal component storage
Museum artefacts
Lay up of Marine vessels, aircraft and vehicles
Storage silos and tanks whilst out of use
Meat processing
Ice rinks
Plastics manufacturing  

Cold stores
Silos
Water treatment and pumping stations

Desiccant Dryers – How they work

The basic desiccant dryer mechanism

The basic desiccant dryer mechanism

Humid process air enters the desiccant dryer and passes through a slowly rotating fluted wheel. The surface of the wheel is coated with an absorptive material, usually silica gel, so that as the air passes through the flutes on the wheel there is effective contact between the moist air and absorptive gel. The gel absorbs moisture from the air therefore reducing its humidity.  The rotating absorption wheel has during the majority of its cycle process air moving through its flutes.  The rotating wheel has a smaller section of its cycle dedicated to desorption, where a reverse flow of hot air is blown through the flutes. This forces the water held by the silica gel to evaporate thus drying that section of the wheel in preparation for its cycling through the process air drying section.

This desiccant drying process results in a drying process that is less temperature dependant than drying by refrigerant based mechanisms, but rather than condensate, the collected moisture from the process air is discharged as water vapour on the heated regeneration air flow. Although this describes the basic desiccant dryer mechanism, there are a number of variations to the theme which are used in different dryers depending on the requirements of the application. Desiccant dehumidifiers are ideally suited to use in cooler ambient air, or when low humidity levels are required.

The process is continuous and closely controlled with the wheel turning approximately 3-4 times per hour and the regeneration section being about 25% of the overall wheel face area

Variants to the basic Desiccant Dryers

 

Desiccant dryer with DX precooler

Desiccant dryer with DX precooler

VRF Special desiccant dryer

VRF Special desiccant dryer

The regeneration heater is usually an electric unit but can also be a coil based running of hot water, steam or compressed refrigerant. In the case of using compressed refrigerant the setup is usually part of a compete refrigerant cycle including precooler coil and heater coil. This hybrid system is in effect a two stage unit with the higher humidity inlet being dried by the precooler coil and the more challenging drying lower down the RH scale being done by the desiccant wheel.

In some applications there can be safety issues with using electric elements, for example component temperature restriction in ATEX applications. Depending on the Temperature class, elements can either be low temperature electrical or water fed coil.

Desiccant dryer with preheater, precooler, after cooler, and filter stages

Desiccant dryer with preheater, precooler, after cooler, and filter stages

 

Desiccant Dryers – Puravent Capabilities and range

We work very closely with the leading manufacturers of desiccant dryers, which means that as well as being able to offer standard units from stock can also offer a wide range of capabilities and features in units built to order, incorporating design specifications that are needed in your particular application. This includes but is not limited to;

  • High airflow versions
  • High pressure versions
  • Various filter efficiencies up to HEPA and ULPA
  • Coils including preheating, precooling, post heating and post cooling
  • Control options, including remote switching, monitoring, humidistat, and BMS interfaces
  • ATEX rated units including both dust and gas categories.
Trotec TTR5000 desiccant dryer

Trotec TTR5000 desiccant dryer

 

If you need help with a drying problem, however complex, give is a call, 0845 688 0112

 

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