Environmental Chambers and Space Simulation Systems

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Walk-in Humidity Systems

Bemco Integral and Modular Walk-in and Drive-in Environmental rooms are available with a number of different humidification and dehumidification systems.

The maximum humidity range offered over the temperature range of 5 C to 85 C is 5% RH to 95% RH with the ability to reach 5% RH at the low end requiring a very tightly sealed system with desiccant dehumidification combined with, a drained vapor generator, a drained mass transfer system, or a steam generator for humidification. A more usual specification is 20% RH to 95% RH limited by a 5 C dew point and an 85 C maximum dry bulb temperature. This specification is achievable with mechanically cooled dehumidification and a standard vapor generator.

On Bemco systems, the humidity conditioning system selected is optimized to the required process conditions.

Bemco Modular Chamber Humidity System with Steam Generator and Desiccant Drier

The common definition of Relative Humidity (RH) is the ratio of the partial pressure of water vapor in a gaseous mixture of air and water to the saturated vapor pressure of water at the same temperature. Bemco chambers include a direct reading Percent RH sensor that utilizes a dielectric polymer film, sensitive to vapor pressure, to monitor and control humidity. These sensors are electronically compensated for temperature variation.

Bemco recommends the use of softened water rather than deionized or distilled water with all humidity systems. Prior to selecting a water demineralization system, we suggest that you contact a local water conditioning company for a survey of your laboratory’s available water supply.

The selected filtration system should remove all ionized contaminants, except free carbon dioxide and silica, and produce water with a resistance of over 20,000 ohms/cm3 but less than 50,000 ohms/cm3. These systems are sometimes described as a "Two Bed, Weak Base" system. The capacity required and the expected life between regenerations varies with the hardness of your water. Once you have determined your local water conditions, an optimum solution can be chosen in discussion with your Bemco Applications, Engineer.

Some of the standard Bemco systems available for humidification are:

Humidification System
Vapor Generator
Lowest cost, best with small systems
Hysteresis, continues to humidify
Steam Generator
Precise control, high capacity to humidify
Higher first cost, adds superheat
Spray System
Very high capacity, best with large loads
Over spray on walls, medium first cost
Mass Transfer
Lowest energy input, very precise control
Hysteresis, continues to humidify

Some of the Bemco standard systems available for dehumidification are:

Dehumidification System
Low Dp
Refrigerated DH Coil
+3 C
Lowest cost, best with small systems
Hysteresis, continues to dehumidify
Pressurized Dry Air Injection
-73 C
Precise control, very low dewpoint
Utility cost, adds thermal load
Rotary Desiccant Drier
-60 C
High capacity, best with large flows
Utility cost, adds large thermal load
Loop Cooling to Dewpoint
+3 C
Controlled dewpoint process
Highest first cost, limited low humidity

Vapor Generator:

Bemco vapor generators are stainless steel insulated tanks connected to the chamber conditioning system by a carefully sized insulated tube that allows water vapor to migrate into the conditioned process air. They include a sight glass, a low water cutoff, and automatic drain freeze protection. They can also be shutoff and drained when very low humidity control is desired.

Humidity is increased by the proportional cycling of the vapor generator heater. Vapor generators are very reliable. They can be useful on smaller systems but they must be drained if continuous operation at very low humidity is desired.

Steam Generator :

Bemco steam generators are electric fired low pressure units, operating at pressures less than 15 psig. Bemco furnishes well designed commercial units optimized to provide "dry steam." These units are UL listed, bear a National Board Boiler Stamp, and are rated ASME CSD-1.

10 psig steam has an entry temperature of approximately 115 C. Because steam vapor enters the conditioned process at a temperature higher than the usual process conditions, this extra energy must be removed by the cooling system. Because the process of cooling also condenses water at controlled higher humidities, this process is often counterproductive when high quantities of vapor are needed to maintain process conditions. On the other hand, because the flow of steam can be fully shutoff and isolated from process conditions, steam is sometimes an excellent choice when medium to very low humidity conditions are to be continuously or precisely maintained.

Spray System:

Bemco water spray systems utilize air atomizing nozzles and pressurized water to spray a precisely controlled flow of preheated water into the process flow.

Because the sprayed atomized water must evaporate into the process air flow, a certain amount of cooling takes place while humidifying. This cooling must be compensated for by process reheat. Atomized water spray is a good choice on higher humidity systems with large live loads since large quantities of water can be easily added to the process air stream. By taking advantage of the cooling effect of evaporation, this water spray can also be used for limited cooling by evaporation.

Mass Transfer:

Bemco Mass Transfer Humidity System

Bemco mass transfer systems work by heating or cooling a water surface directly exposed to the process air. Since very small differential temperatures between the water surface and the air passing over it result in a humidity change, this type of system can achieve very precise humidity control.

Since water freezes at 0 C, at sea level pressure, this technique is limited to approximately a 5 C Dewpoint.

The photograph shows the rear view of a Bemco FL-M Modular Walk-in Chamber with the conditioning compartment access doors open. This system has a mass transfer temperature and humidity conditioning system and a HEPA filter system intended for high humidity preconditioning of medical products prior to insertion in an ethylene oxide sterilization system. The preconditioning reduces the sterilization processing time and improves product throughput.

Refrigerated DH Coil:

Bemco Refrigerated Dehumidification (DH) Coils remove water from the process by condensation on a refrigerated temperature controlled surface operating at just above freezing. This surface is protected by a baffle to prevent convective heat transfer from raising its temperature above approximately 3 C. Coil temperature is usually maintained by an automatic evaporator pressure regulator and proportionally controlled by the cycling of a solenoid valve.

This system works well on small systems requiring limited water vapor removal at above 5 C dewpoint.

Pressurized Dry Air Injection:

Bemco pressurized dry air injection systems utilize dual bed, heatless desiccant driers with an effluent dewpoint of -40 C to -73 C to remove water vapor from compressed air. This air is directly injected into the workspace to reduce chamber humidity. These systems include prefilters and afterfilters and are proportionally modulated to maintain process conditions. As the dry air enters the chamber workspace, air containing water vapor at a higher dewpoint is vented.

Extremely low humidities can be achieved using this technique. Systems of this type are ideally suited to small to medium sized facilities.

Rotary Desiccant Drier:

Desiccant Drier System Inside Bemco Conditioning System

Bemco rotary desiccant drier systems utilize commercial rotary desiccant driers, either in the primary conditioning system or in a secondary loop, to remove process moisture. These systems are often furnished with a precooling system to reduce inlet moisture content and and aftercooling refrigeration system to remove desiccant regeneration heat. They sometimes incorporate a fold back flow of already dried air through the regeneration circuit to achieve even lower dewpoints.

Rotary desiccant driers work by the gradual rotation of either a drum or wheel, where a large portion of the rotated medium is exposed to the process and a smaller portion of the drum or wheel is exposed to a heated, secondary flow, of regeneration air drawn from the outside. The heated regeneration air drives the adsorbed water from the desiccant. The process of heating the wheel or drum for regeneration adds residual heat to the active air loop. This heat must be removed to maintain process conditions.

Systems of this type are recommended when continuous outside flows of outside air are required of when large amounts of water are generated as part of the process. Examples include larger man rated low humidity systems that need make-up air for breathing and automobile test facilities where air is needed for engine operation and potential flammable vapor (gasoline) dilution.

Loop Cooling to Dewpoint:

Bemco Loop Cooling Systems utilize mechanical single stage, air or water cooled refrigeration systems and carefully sized heat exchangers to cool the entire loop airflow to a controlled dewpoint. By chilling the airflow to a controlled temperature and then reheating the air to the desired process temperature any humidity down to a dewpoint of approximately 3 C can be achieved.

Systems of this type are often recommended when steady state, one setpoint temperature operation is contemplated and the reheat from the dewpoint selected is close to the operating temperature.

Available Literature Includes:

FL Bulletin Download Bemco FL Series Walkin Integral and Modular Rooms. Temperature, humidity and altitude, integrally constructed and prefabricated walk-in environmental test equipment, drive-in room and environmental test facilities from 6 feet by 6 feet by 7 feet to large building sized systems. Document is 8 pages, 674 KB download.

All of these systems are also available as independently, either for addition to existing facilities or as part of a larger facility. The systems described above are our Standard Models. We make custom equipment to your specifications.  If you have any questions, need assistance in preparing a specification, or have a technical problem you want help with, we are ready.  Our quotations and advice are free.

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