Hygienic Air Handling Systems

Desiccant Dehumidifiers - CPA-DW

Desiccant Dehumidifiers - CPA-DW

Desiccant Dehumidifiers - CPA-DW

The CPA-DW integrates Evapco's Critical Process Air (CPA) Systems with a desiccant dehumidifier to meet the demanding requirements of temperature, pressurization and air purity control with humidity and condensation control.  Designed specifically for the industrial refrigeration market, CPA-DW offers complete flexibility in design, capacity and features to fulfill the most demanding applications.  Every CPA-DW is constructed with a comprehensive choice of quality components integrated together to create a state of the art, custom air handling unit backed by the industry's mostexperienced critical process product design and system application engineers.

Principle of Operation

Desiccant Dehumidifiers - CPA-DW

The operation of the dehumidification system is accomplished by exposing the air to the desiccant in a sealed air stream. After the desiccant has absorbed moisture it is exposed to a second air stream at a higher temperature. This causes the moisture to be driven out of the desiccant preparing it for more moisture adsorption. This cycle is done on a continuous basis, providing a constant drying process. One aspect of this process can be confusing. As the air dries, its temperature rises in proportion to the amount of water removed. As water vapor is removed from the air, the dry bulb temperature rises because the process is exothermic. This process is the reverse of evaporative cooling. When water is evaporated into air, the heat needed for evaporation comes from the same air and the sensible temperature falls. Conversely, when air is dehumidified, the heat needed to evaporate the water originally is liberated and the temperature of the air stream rises. The greater the amount of water removed, the greater the temperature rise. The amount of heat gain in BTUH on the process side equals the heat loss in BTUH on the reactivation side.

Desiccant Dehumidifiers - CPA-DW

The application of desiccant systems throughout the industrial market is growing rapidly and the applications are almost unlimited. While the systems add initial cost to the air unit and can be expensive to operate, they can produce dramatic results which warrant their expense.
Evapco’s “DW” desiccant product line provides unlimited flexibility and design capabilities for almost any application.  The units are configured for the specific application providing the desired results with the most efficient operation.  And this includes an extensive list of options from double wall construction with stainless liners, coils for any refrigerant, including ammonia, any filtration system, and heating and regeneration to match the plants source of heating energy.  All with an open platform PLC control system which can be easily integrated into the plants HMI system.  All with Evapco’s world class service and support

The design features of the CPA-DW are as follows:

The desiccant cassette is designed with two air streams flowing in opposite directions to maximize energy efficiency. The two air streams are separated by seals, in contact with the desiccant media. The seals are critical to prevent moisture from the reactivation air stream contaminating the process air stream.  To enhance the system performance, the process air blower is upstream of the wheel and the reactivation blower is downstream of the wheel. The arrangement creates a positive air pressure on the process side of the seal and a negative pressure on the reactivation side of the wheel.

The desiccant wheel rotates using a chain drive system. The rotor speed runs at approximately 15 revolutions per hour. Under normal conditions, the drive system parts should last the lifetime of the system.

Desiccant wheel capacities range from as little as 200 CFM to 47,000 CFM.  However, in most systems only a portion of the air is dehumidified, so total unit air volume capacities can be much larger. 

Many applications will include a damper to bypass air around the desiccant wheel. This is used when the dehumidification rate requires only a portion of the total unit air volume be desiccant dehumidified. Bypass dampers can be used to bypass all the air during certain cycles in special applications.

The desiccant rotors used in Evapco products have an operating lifetime of 87,600 hours while maintaining a greater than 90% performance. The wheels have an 82% desiccant loading which are the industries highest. The operating temperature limit is 500 degrees with a mechanical integrity limit of 2000 degrees. The rotors can be cleaned by using 100 psi dry air blown through the wheel or washed with non alkaline water. Exposure from petroleum based or other organic oils can clog the wheel.

The reactivation air is typically brought in from the outside though a louver or an inlet hood. At this point the air is filtered with a 30% filter, the same pre-filter as used in the inlet of the air handling unit. Once the air passes through the system the reactivation blower discharges the air through a back draft damper back to the outside of the unit. On applications where very low dew points are required, the reactivation air is actually pulled from a portion of the process air after it has passed through the desiccant wheel. This is known as a purge cycle and is rare in food plant applications.

The reactivation exhaust blower is selected and sized by Evapco specifically for each application. These blowers are single width, single inlet direct drive blowers rated for high temperature applications.

Double wall, high temperature air ducts within the unit is fabricated by Evapco for the reactivation air. The ducts are constructed with the same material as the unit liner material. The reactivation air can be heated using any heating source capable of producing the required air temperature. The most common type of heating source used is direct fired natural gas.

About the CPA

Desiccant Dehumidifiers - CPA-DW

The environmental system controlling critical process room air of course includes diffuses and ducting.  However, the most important element is the CPA critical process air-handling unit itself.   It has the system’s air moving devices and provides the rooms heating, cooling, dehumidification or humidification, pressurization, and the air filtration requirements.  The CPA unit is designed with ultimate flexibility so it can be designed to match each room’s particular needs.

Many design decisions are necessary to select the proper size CPA unit (see “CPA – Formal Selection” section), however once the size unit is selected, the particular features of the unit must then be determined.  The following discussion will aid in selecting the proper CPA features.

Each unit can be supplied with an open platform PLC control system with the ability to communicate with the plant control system.

 The following are some of the many options, which are available for the control system:

 Main control panel, control transformer and terminal blocks.

  • Outside air and recirculated air controls which use outside air on an economizer.
  • Outside air and recirculated air controls providing a fixed amount of outside air.
  • Outside air damper controls for systems with100% makeup air.
  • Room thermostats or RTD’s
  • Return air thermostat or RTD’s
  • Outside air thermostat or RTD’s
  • Motorized steam valve (field mounted).  Trap by others.
  • Magnetic motor starters mounted in the control panel.
  • Disconnect switches mounted in the control panel.
  • Remote stainless steel control panels which can include (just a few of the possible options):
  • blower on/off switch
  • blower indicating light
  • cleanup switch
  • cleanup indicating light
  • dirty filter light
  • burner Indicator light
  • Steam coil freezestats.
  • Hot water or glycol 3-way control valves.
  • Chilled water or glycol 3-way control valves.
  • Room de-humidistat located in the remote control panel (or return air).
  • Natural gas heating controls – either direct or indirect fired.
  • Discharge air ammonia detector.
  • Smoke detectors.
  • CO2 detectors
  • Interior service lights.
  • Service receptacles.
  • Room pressurization control.
  • Heating coil face and bypass dampers and damper controls.
  • Two-speed motors and starters.
  • VFD motor control
  • Exhaust motor starters.
  • Cleanup cycle system and controls.
  • UVC coil/drain pan light systems.
  • Blower door interlock switches
  • Dirty filter lights/contacts.
  • RVS, ASME surge drums mounted on top of the unit complete with dual relief valves.
  • LC panel view screens 

Although food plants are not classified as “clean rooms” by the classical sense, they certainly are operated as rooms that have similar filtration levels.  What makes them different is possibly the level of access to them, the amount of traffic, the type of processes, and the clothing worn by the personnel. 

Clean room air cleanliness levels are defined by the number of particles of a certain size and larger per cubic foot of air when the room is tested with a particle counter. An ISO 5 (old class 100) clean room can have a maximum of 100 particles .5 micron and larger in a cubic foot of air. The number and location of sampling points within a room should be based on the specified cleanliness level and the number of critical locations within the room.

The table below will define Class of room and the type filters for a traditional clean room.

 ISO 3 (Class 1)
 ISO 4 (Class 10)
 ISO 5 (Class 100)
 ISO 6 (Class 1000)
30% TO 55%
 ISO 7 (Class 10,000)
10% TO 20%
 ISO 8 (Class 100, 000)
5% TO 10%


 Recently the methods for testing and rating filters have been changed to what is called the MERV (Minimum Efficiency Reporting Value).   The following table gives the current filter ratings:


Standard 52.5


Dust Spot



Typical Controlled














<0.30 micron

Virus (unattached)

Carbon Dust

Combustion Smoke













0.3 to 1.0 micron

All Bacteria

Most Tobacco Smoke














1.0-3.0 micron


Milled Flour






The CPA will always use a filter at the inlet to the unit.  This is typically used as a pre-filter however can be the only filter too, as in the CPA-XX-30 unit.  A pre-filter is used to help keep the interior of the unit clean and to extend the life of the final filter.  


There are several cooling options available:

There are several heating options:

The motors shall be Epact efficient TEFC duty, 1800 nominal RPM with an option for premium efficiency or wash down duty.  Motors are mounted on an adjustable motor base.  

The blowers are a centrifugal backward curved or airfoil plenum type blower, either belt or direct drive.

The blower have inlet screen as a standard component.

The on belt drive units the wheel is be supported by two (2) self-aligning outboard bearings, which shall be self-aligning, ball bearing, pillow block and shall be designed for at least 200,000 hours of average life. The blower shaft are ground and polished.

The blower wheel, inlet funnel and support are of a steel construction (aluminum optional), completely coated with an air-dry enamel coating.  Other coatings available for special applications.

The overload service factor used for the V-belt drive selection is at least 1.50. 

Fixed sheaves are standard and are a cast iron type (optional – adjustable motor sheaves for plus or minus 5% adjustability).  The V-belt drives are of a standard capacity and furnished with reinforced rubber belts.

Optional - the blower, motor and drive shall be spring isolated and be complete with a flexible connection.
Optional - the drive shall be complete with a belt guard.
Optional - the blower shall be complete with a discharge screen.
Optional – The bearings shall have plastic extended greaselines run to the outside of the unit casing.
Optional – The shaft shall be stainless steel.

The dampers are parallel blade (optional opposed blade), with nylon bearings and stainless steel spring loaded side closures. 

The construction will be a minimum of 16 gauge galvanized steel single blade design, with 304 stainless as an option, and a 12-gauge casing.   Airfoil blades have a hollow core, with are typically not sealed (and therefore becomes a cleaning issue), and therefore are not recommenced.

The dampers have a low leak rating of less than 6.5 CFM per square foot at 4” of differential static pressure.   The standard operator is a direct drive motor requiring no linkage.  Lower leakage rating dampers are also available.

Access doors are provided with an extruded aluminum full perimeter frame.  The frame extrusions shall be miter cut at the corners and assembled by mechanically staking the frame into concealed gussets made of extruded aluminum.

Each door is pressure injected with 2.5# of polyurethane foam per cubic foot of door.
The interior and exterior panels are constructed of 24 gauge, G90 hot dipped galvanized steel with the option of 304 stainless steel. 

The doors are mounted to the frame by a minimum of two (2) polycarbonate adjustable hinges.

The doors can have an optional 8” x 8” plastic window.

On units operating below 55°F, the doors and door frames will have a thermal brake.  This is standard on both the 2” and 4” cam lock construction.

Each door is complete with a minimum of two (2) 90-degree polycarbonate handles, operable from the interior and the exterior of the unit. 

Each section of the unit, no matter what the wall construction, shall be complete with drain pans, which shall be a minimum of 16 gauge, G235 galvanized steel.  Each drain pan shall be fully welded and individually tested to ensure they are leak free. 

On all units with a cooling coil section, the drain pan under the cooling coil shall be constructed of 304 stainless steel.

When the stainless liner option is selected, all the drain pans are stainless steel.
The drain connection for the drain pans are designed so the pans drain out the bottom of the drain pan for full drain pan drainage with the drain connection then extend out the side of the unit.  This construction prevents standing water in the drain pans.

An optional fully welded floor/pan assembly is also available.

The CPA unit has an option of two panel types and three insulation options.

The basic CPA exterior casing is constructed of 18 gauge, G235 galvanized steel, unpainted, with 2” polyurethane.  The inner liner of the unit consists of a watertight, sanitary liner constructed of 16 gauge, G235 galvanized steel.  The panel construction is a self-framing construction with the panels forming the structure for the unit casing.  Casing fasteners are internal to the panel, or outside the unit, with no panel fasteners on the inside of the unit.  All interior seams are sealed air and water tight with an FDA approved sealant so the interior is watertight.  Unless customer specified, the fiberglass insulation would be used down to a 55°F discharge air temperature.

An alternate of 2” to 4” foamed in place polyurethane panels are also available.  Again, unless a customer specifies differently, this insulation would be used down to 32°F discharge air temperature.

With either of the 2” panel designs there is the option of an 18 gauge 304 stainless steel inner liner.  With this option, the inner walls, all drain pans, closures, filter frames and dampers are stainless steel.  Unless specified differently, the blower, motor, and coil will be of a standard construction.   

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