Dehumidification in manufacturing
Controlling humidity levels in
manufacturing and packaging operations can have a major impact on the bottom line. Whether
it is candy coating, meat processing, battery storage or glass making, maintaining the
optimum level of humidity reduces production costs through greater efficiency and fewer
defects.
Dehumidification solves four common
problems typically encountered in manufacturing:
Moisture regain or clogging and
sticking - Dehumidification prevents moisture regain from occurring in powder processing
and product handling activities such as granulated sugar storage silos, packaging rooms,
ammonium nitrate fertilizer storage buildings and candy wrapping.
Condensation or sweating -
Condensation, which can lead to mold, fungus growth and contamination from overhead
dripping, occurs when cold surfaces such as pipes, silos and ceilings in manufacturing
plants are surrounded by moist air. Dehumidification systems prevent condensation by
keeping the air surrounding the cold surface at a constant dew point set just below the
temperature of the coldest surface.
Corrosion - Removing moisture from the
air prevents rust from developing on metal surfaces and organic material from rotting.
Drying heat-sensitive products - Many
types of products must be dried to low moisture levels but cannot stand excessive heat,
including pharmaceutical diagnostics, thermo-set resins, industrial enzymes and most
proteins. Using a dehumidifier to speed up
drying time without damaging the product is most beneficial when the product’s
temperature limit is around 120 F and the limit is 95
F or below.
These are the operating principles
used in consumer-grade dehumidifiers which one might use to dehumidify a damp basement. Air passes across a cold coil, which cools and
dries the air. Then the saturated air passes
through a second coil, where the heat from the compressor and refrigerant is added back
into the air stream, lowering the relative humidity before air is supplied to the space.
Conventional air conditioning systems
also accomplish dehumidification the same way, but such equipment is usually configured
for optimal heat removal, and dehumidification is a by-product of the primary function of
cooling the air. For industrial purposes,
cooling-based dehumidification is accomplished by custom-engineered air handling units
optimized for removing moisture rather than removing heat.
These units cool small amounts of air very deeply, as opposed to cooling
large amounts of air only slightly. The
deeper cooling condenses more moisture from the air.
Dessicant-based dehumidification
When
the required dew point is low, or when very low relative humidities are needed, desiccant-based
dehumidification is usually the most cost-effective alternative. This equipment uses differences in vapor pressure
to remove moisture from air by chemical attraction. The
surface of dry desiccant has a very low vapor pressure compared to the much higher vapor
pressure of humid air. Water vapor moves out
of the humid air onto the desiccant surface to eliminate the vapor pressure difference. Eventually, the desiccant surface collects enough
water vapor to equal the vapor pressure of the humid air.
Then the desiccant must be dried (reactivated) by applying heat, before it
is re-cycled to remove more moisture from the air stream.
There are many ways of presenting a
desiccant to an air stream. In most modern, atmospheric pressure industrial dehumidifiers,
the desiccant is held in a lightweight matrix in the shape of a wheel, which rotates
between two separate air streams.
The desiccant is contained in the
walls of thin air channels that extend through the depth of the wheel. The diameter of the channels varies, but is
usually about two millimeters. The diameter
of the wheel depends on how much air must pass through it. Larger airflow requires a
larger-diameter wheel. The process air passes
through the desiccant wheel giving off its water vapor to the desiccant contained in the
walls of the air passages. The dry air leaves
the wheel and is carried to the point of use by fans or blowers.
While that is happening, part of the
wheel is rotating through a second, smaller air stream -- the reactivation air that has
been heated. The hot reactivation air heats
the wheel, driving water vapor out of the desiccant. As each section of the wheel rotates
out of the reactivation air, its desiccant is dry, and can once again remove moisture from
the process air.
As air is dried, the temperature of
the process air rises in proportion to the amount of water removed. Drier air means warmer air. This is the reverse of the more familiar process
of evaporative cooling. When water is
evaporated into air, the heat needed for evaporation comes
from that same air, so its sensible
temperature falls. Conversely, when air is
dehumidified, the heat needed to evaporate the water originally is liberated, raising the
temperature of the air stream.
Because a desiccant dehumidifier
removes water from the air as a vapor rather than as a condensed liquid, there is no risk
of freezing condensate. So this type of
equipment is most often used for applications where dew points below 50 F are required.
An extreme example is lithium foil
processing. Lithium metal is used for
batteries. In its pure form, the metal
surface is energetic enough to break water molecules apart forming gaseous hydrogen and
oxygen, and liberating heat. With enough
water vapor, that heat can ignite the hydrogen. Consequently,
lithium foil processing takes place in rooms kept at dew points between -30 and -40 F. Plutonium and calcium metal processing require
similar environments. Desiccant-type
dehumidification systems maintain rooms at that level of dryness even with a moisture load
of 30 to 60 people working in the room.
Desiccant vs. cooling-based dehumidification
Engineers new to dehumidification technology frequently question which method of cooling
or desiccant is the best choice. In most
manufacturing/processing applications, the simple answer is that both technologies are
used together so they cooperate rather than compete.
Cooling-based dehumidification handles the moisture load occurring at high
dew points, and desiccant-based dehumidification removes the moisture load at lower dew
points. The specific mix of the two
technologies will depend on the characteristics of the application in question.
Factors include the following:
Evaluating the dew-point level
When the required moisture
control level is comparatively high (above a 50 F dew point), cooling-based
dehumidification is very economical in terms of both operating cost and initial equipment
cost. Low-cost, high-volume, standard
equipment is available for this control level and above.
Below that control level, the cooling approach begins to be less economical,
primarily because of the precautions needed to avoid freezing the condensed water on the
cooling coil.
Although water does not freeze until
temperature falls below 32 F, a dehumidification system may well have to deliver air below
that dew point in order to maintain a room below 50 F dew point. (This is analogous to home heating, where air must
be supplied at 120 F in order to maintain a cold house at 70 F.) So, a cooling-based dehumidifier providing air at
low dew points can freeze unless special precautions are taken in the design of the unit. Such features result in higher-cost, custom
equipment, and equipment that has a higher operating cost per kilogram of water removed,
so desiccants become more economical than cooling-based systems at low dew points.
Gauging relative humidity
sensitivity
When a process needs low moisture level in absolute terms, but can tolerate a high
relative humidity, a cooling-based dehumidification can be very cost-effective without the
need for desiccants. An example is fruit and
most vegetable storage. The ideal temperature
might be 40 F, so of course the dew point must be below that level. But if relative humidity is below 90%, the fruit
can dry-out in storage and lose value. Since
the product needs both low temperature and high humidity, cooling-based systems are ideal
for the application. In contrast, other
processes might demand a low relative humidity in addition to a low dew point.
Narrow or wide temperature
tolerance
If an application has a narrow temperature tolerance, then cooling and heating will be
essential in addition to dehumidification. If
the application can tolerate wide temperature variations, such as occur in un-heated
storage, then dehumidification equipment alone may be sufficient.
Designing The Ideal Dehumidification System
Industrial dehumidification systems
are custom-engineered for each particular project. Consequently,
manufacturers have developed a near-infinite variety of possible components to serve the
near-infinite variety of possible applications. These
components make it easy to optimize a system design, but the variety also presents the
project engineer with many decisions at a early stages of a project usually before the
cost/benefit implications of those decisions are completely clear.
Define the purpose of the project
The designer must clearly
understand and document the purpose of the project. This
understanding sorts all other design decisions in order of their true importance. For example, if the purpose of the project is to
prevent the growth of mold on starch in a storage silo, there is no need to maintain a
strict tolerance of 1% relative humidity (RH). The
only real concern is that the humidity does not exceed 60% and that condensation does not
occur. The system can be simple and
inexpensive.
Conversely, if the purpose of the
project is to prevent the corrosion of lithium, there is no point to try to save money by
using a control which has a tolerance of 5%
RH. Above 2% RH, lithium corrodes, giving off
hydrogen, which eventually explodes. A sensor
with a tolerance greater than the critical control level itself could not start the system
in time to prevent that explosion. Understanding the purpose of the project in these
terms helps the system designer avoid both unnecessary expense and false economy.
Establish control levels and
tolerances
After the purpose or purposes of the project are clearly defined, the designer must decide
what humidity and temperature control levels and tolerances will achieve those purposes. These decisions may require research, but in many
cases, the relationship between a process and moisture is understood clearly enough to
allow the project to proceed. For example, if
a process bogs down during summer, but not during spring, fall or winter, one can assume
that the humidity tolerance is quite wide, and that only summer extremes of humidity must
be removed by the dehumidification system. In
other cases, the supplier of a problem material may be able to recommend optimal
environmental conditions for processing the product.
The control set point must be
established to allow calculation of the peak heat and moisture loads, and without loads,
there is no way to estimate equipment sizes and costs.
Loads are relative to the temperature and moisture levels maintained. All other variables being equal, a system to hold
humidity at 72 F, 35% RH will be much smaller than one which must hold 72 F, 25% RH. The lower the humidity level, the more costly the
system will be. Higher moisture loads also
increase system cost. Therefore, calculating
these loads is the next critical step in designing a system.
Calculate moisture loads
In most cases, the application engineer employed by the dehumidification supplier will
assist the project engineer in calculating moisture loads.
In order from largest to smallest, typical loads come from ventilation air,
air infiltration, miscellaneous openings, people, products/packaging and vapor permeation. Lower loads mean less expensive equipment.
Consequently, the most cost-effective adjustment to building operation is to reduce the
exhaust air to the minimum, reducing the cost of dehumidifying the air brought into the
space to replace the exhaust. After that,
sealing up cracks in the building greatly reduces the cost of dehumidification for a very
modest investment in caulking material.
Fresh/ventilation air is essential in
most controlled spaces. In most cases, codes require a certain amount of air per person or
per square foot of occupied space. Often,
less attention is paid to making sure all exhaust air is made up by the ventilation
system. This is especially a problem in large
spaces, where the exhausts may not be obvious. Also,
engineers used to designing commercial buildings may not be fully aware of the effect of
insufficient make-up air on humidity-controlled spaces.
The next load source is miscellaneous
openings. Each time a door is opened, moist
air is pulled into the room. When possible,
spend time observing the number of times a door is opened during the busiest production
period.
Air locks greatly reduce moist air
infiltration (as long as one door is not propped open by the occupants). As the humidity control level goes lower, air lock
doors become very advantageous economically. The
assumption behind an air lock is that equilibrium is reached half way between the inside
and outside conditions and all the air enters the room each time the lock opens.
Often, product must enter or leave a
humidity-controlled room on a conveyor. This
type of conveyor opening should not be overlooked as a possible infiltration source. To reduce infiltration of moist air through large
openings such as ducts, engineers often supply a slight overpressure of make up air so
that dry air leaks out of cracks rather than moist air leaking in.
When people exhale or perspire
moisture is given off, creating another load source.
The rate depends on the level of exertion -- more metabolism equals more
moisture. When calculating loads in a room,
be sure to allow for "visitors" flowing in and out of the room. Experienced engineers often double their
"people" estimates to allow for changes in room use and "visitors".
The load from products and packaging
varies greatly by application. In large
storage applications, moisture released from product can represent the single largest load
component. The load is the difference between the products initial wet weight, and its
weight when at equilibrium with the lower humidity.
Vapor permeation through building
components is typically the smallest portion of the load, accounting for less than the 2%
of the total (as long as the walls, floor and ceiling are solid surfaces without air
leaks). The permeation load becomes more
worthy of attention when the building is extremely large, so that moisture permeates
across a large surface area, or if the control condition is very low. Below 5% RH, every leak, no matter how small
becomes critical.
The peak design weather conditions are
a very important element in the load calculations. The
owner must decide how conservatively the system should be sized. If extreme weather data is used, the system will
control humidity throughout all 8,760 hours in a typical year. Such a system will also be very costly. If some out-of-spec hours can be risked, the
system may cost 20 to 30% less, but if all moisture loads peak at the same moment during
extreme weather, the humidity may rise above set point.
Air conditioning engineers quantify
these choices in the ASHRAE Handbook of Fundamentals according to the percent of annual
hours that weather conditions will be above certain values.
For example, the 0.4% values are likely to be exceeded for only 35 hours per
year (8,760 x 0.004). A less conservative
design point would be the 1% or 2.5% values, which may be exceeded for 70 and 219 hours
respectively.
The decision as to which data to use
is made by the end user, who is in the best position to assess the economic consequences
of being slightly above specification for short periods.
Lithium processing, for example, usually demands a more conservative design
than starch silos, because the consequences of high humidity with lithium are hazards not
just expenses.
Evaluating dehumidification
technology
The project engineer investigating the use of dehumidification systems will likely be
working closely with equipment suppliers to determine costs and benefits of
dehumidification versus alternate means of solving problems. Dehumidification suppliers can be most helpful and
respond quickly when key aspects of the potential project are well-defined. These include: clearly communicating the nature of
the problem and its consequences; defining the purpose of the project in a simple,
declarative sentence describing measurable results; and researching available utilities
and physical characteristics of the site.
A Host of Applications
Dehumidification
systems are widely used throughout the manufacturing and processing industries, but
abundant opportunities remain for further use of the technology. A project engineer would do well to consider its
use whenever weather variations affect production rate or product quality, when corrosion
or condensation cause problems, or whenever product must be dried at low temperatures.
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