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.
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.
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
Some of the standard Bemco systems available for humidification are:
Lowest cost, best with small
Hysteresis, continues to humidify
Precise control, high capacity
Higher first cost, adds superheat
Very high capacity, best
with large loads
Over spray on walls, medium
Lowest energy input, very
Hysteresis, continues to
Some of the Bemco standard systems available for dehumidification are:
Refrigerated DH Coil
Lowest cost, best with
Pressurized Dry Air Injection
Precise control, very
Utility cost, adds thermal
Rotary Desiccant Drier
High capacity, best
with large flows
Utility cost, adds large
Loop Cooling to Dewpoint
Highest first cost,
limited low humidity
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.
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.
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:
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:
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.