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Unsuitable substitutions: Learning the hard way about absorption
Mercy Medical Center is a 476-bed hospital serving
several counties in northeastern Ohio.
This is a story about product substitutions, and the problems caused by two humidifiers that
saturated a filter bank and left water on an AHU floor. Read on to learn how you can avoid a similar costly — and frustrating — scenario.
THE STORY BEGINS
Managers at Mercy Medical Center, a regional
medical center in Ohio, were planning to refurbish an
air handling unit. The design engineering firm, LRM
Engineers in Akron, specified replacing many of the
AHU components, including the humidifier.
There was 5 psi steam available on site, and so for
humidification the engineer chose a steam dispersion
panel to disperse boiler steam directly into the
airstream. This type of humidifier is called “steam
injection” and, depending on the load and absorption
requirements, includes a dispersion tube or set of tubes.
Several manufacturers make this type of product, as it
can be a fairly straightforward and economical way to
provide humidity.
The distance from a steam injection dispersion tube(s) to
the point of absorption varies greatly among products
and manufacturers, with short absorption (non-wetting)
distances the most difficult to achieve. If there is plenty
of room in a duct or AHU for absorption to occur, a
humidifier that requires a long non-wetting distance may
be adequate. But in a duct or AHU with elbows, fans
or filters downstream of the humidifier, it is absolutely
essential that absorption occurs before steam hits those
objects or it will condense and cause dripping, which
can cause microbial growth resulting in foul smelling,
unhealthy air. In addition, if humidification steam is not
absorbed, desired conditions will not be met.
At Mercy, the new humidifier was to be installed about four feet
upstream of a filter bank within an AHU, and so absorption had to
occur completely before the filters to ensure nonwetting. The engineer
understood this and specified DriSteem’s Ultra-sorb® humidifier panel,
a multiple-tube assembly with separate steam supply and condensate
headers. He sized this panel to span the width and height of the
available airflow in the AHU to maximize air/steam mixing. He also
wrote a performance specification that stated the humidifier must
provide absorption to preclude water accumulation on any surfaces
within 15 inches downstream of the humidifier.
AND THEN WHAT HAPPENED?
STEAM ABSORPTION COMPARISON
The drawings below show how increasing the
number of steam discharge points shortens the
non-wetting distance.
Single-tube humidifier
Of the three examples shown here, the singletube
humidifier will have the longest nonwetting
distance. In addition to the number of
steam discharge points, non-wetting distance is
determined by entering and leaving RH, load,
airflow and duct temperature.
Multiple-tube humidifier
With the same conditions, absorption will
occur in a shorter distance with a multipletube
humidifier because it has more steam
discharge points.
Multiple-tube dispersion panel
(Ultra-sorb)
With the same conditions, this humidifier will
provide the shortest non-wetting distance. The
humidifier not only has multiple tubes, it has
two rows of discharge points on each tube,
and also has an additional header for
managing condensate.
First, a non-DriSteem single-tube humidifier, bought as part of a
package, was installed and started up. It proceeded to completely
saturate the filter bank. On-site staff quickly removed the filters and
noted that steam was absorbing several feet downstream of the filter
bank location.
A second humidifier — this time, a multiple-tube assembly provided by
the same manufacturer to replace the first humidifier — was installed.
When started up, it absorbed a bit better than the first humidifier, but
it still fully saturated the filters, causing dripping. Again, staff promptly
removed the filters and noted that the non-wetting distance was two
feet past the filter bank – much more than the required 15 inches
from discharge.
Finally, the originally specified DriSteem Ultra-sorb panel was installed,
started up, and provided full absorption within 12 inches, actually
improving on the engineer’s specification of 15 inches.
FACTORS THAT AFFECT ABSORPTION
In the construction world, product substitutions are made all the
time, and often the substituted product works as well as the specified
product. To make a successful humidifier substitution, however, requires
an understanding of some often-misunderstood absorption basics.
Absorption is affected primarily by three things:
1. Duct or AHU temperature. Cool air absorbs less than warm air
and will require a longer non-wetting distance.
2. The difference between entering and leaving RH. The more
humidity that needs to be dispersed into the airstream, the longer
the non-wetting distance.
3. Mixing of air and steam. Uneven airflow, non-uniform mixing
of steam with air, and the number of steam discharge points in
the airstream will affect non-wetting distance. How steam and air
mix to affect absorption is often misunderstood, and so it is not
surprising that it was a misunderstanding of this principle that
caused the selection of an inadequate substitution.
WHY WAS THE ULTRA-SORB JUST RIGHT?
The first humidifier installed was a single tube (not made by DriSteem).
There is no single-tube humidifier on the market that could have met
the absorption requirements of this job.
The second humidifier installed (another not made by DriSteem) was
a multiple-tube humidifier installed to cover about one-third of the
available airflow. Even if this humidifier had covered the full airflow, it
would not have been able to provide absorption before the filter bank
because each tube had only one row of steam discharge points.
The third installed humidifier easily met the specification of absorption
within 15 inches for many reasons. First, the Ultra-sorb has two rows
of steam orifices on each tube, allowing superior mixing. Second, the
panel was sized to span the height and width of the available airflow,
optimizing mixing. And third, the Ultra-sorb has a second header
designed for managing condensate, so discharged steam is always
dry, with no entrained condensate.
WHY DID THIS HAPPEN?
Why are product substitutions made? Usually because another product
is available at a lower up-front cost, or, as in this case, was available
as part of a package. We’re confident that if all parties involved had
a better understanding of the often misunderstood complexities of
absorption, none of this would have happened.
LESSONS LEARNED
1. Find a representative you can trust. Most
engineers don’t design humidification
systems nearly as often as they design
heating systems, and so they are not as
familiar with humidification system design.
And given the complexities of providing
proper humidification, it is important to
work with a manufacturer’s representative
who really understands humidification.
This is especially important with critical
applications, where either short non-wetting
distances or accurate control are required.
2. Write a performance specification. Because
the design engineer wrote a performance
specification, the contractor was required to
meet that specification, regardless of how
many times he tried a substitution. Anticipate
substitutions and cover yourself with a
performance specification.
3. Consider the reputation of the manufacturer.
The manufacturer of the first two humidifiers
did not stand behind their product, at great
cost in time and money to the contractor and
engineer.
4. Insist on published non-wetting distances.
Only the last installed humidifier had a nonwetting
distance guarantee with absorption
data published in a printed catalog. One
way the engineer might have improved his
specification would have been to add a note
to the “acceptable manufacturers” section
stating, “The manufacturer must provide
published data guaranteeing the required
non-wetting distance.” Don’t settle for a
note on the product order guaranteeing
absorption. Insist on data printed in a
catalog or published on a web site.
5. Improve mixing action to reduce non-wetting
distance. Add tubes, or add tubes that have
double rows of steam orifices, to increase
the amount of steam mixing with available
airflow. To ensure absorption upstream
of a filter bank, install a fan or heating
coil downstream of steam discharge to
improve air and steam mixing action. And
size dispersion assemblies to span the full
height and width of the available airflow to
maximize air/steam mixing.
6. Manage condensate. Some condensation
is inevitable in steam dispersion, but
condensate can be controlled and directed
away from where it will cause problems. A
double-header multiple-tube dispersion panel
uses gravity to remove condensate. Steam
enters the top header, escapes through the
steam orifices, and condensate drains out the
bottom header. Many large-capacity steam
injection systems will have strategicallyplaced
steam traps to ensure that only the
hottest, driest steam is discharged into the
airstream.
THE MORAL OF THE STORY
When making a product substitution, consider not just the price or
convenience of the substitution, but the long-term value as well. As the
saying goes, “The bitterness of poor quality remains long after
the sweetness of low price.”
AHU at Mercy Hospital showing the Ultra-sorb dispersion panel installed on the left and the filter bank on the right, four feet downstream.
THE SUPERIOR ULTRA-SORB TAKES STEAM
ABSORPTION TO A NEW LEVEL
ULTRA-SORB STEAM DISPERSION PANEL
ULTRA-SORB PANEL WITH HIGH EFFICIENCY TUBES
Highest capacity, shortest non-wetting distance. With the
High-efficiency Tube option for new installations or as a
retrofit for existing installations.
The Ultra-sorb steam dispersion panel established industry
standards for meeting short absorption requirements.
Ultra-sorb performs to guaranteed non-wetting distances
and can be installed within inches of downstream devices
without condensation worries. It can be factory assembled
for easy installation or shipped unassembled.
- Works with any steam pressure. From just ounces of
steam pressure to boiler pressures. Steam capacity up
to 4000 lbs/hr (1815 kg/h).
- No unnecessary heat gain. Because the tubes operate
drip-free without steam jackets, no extra heat is added
to the airstream when the humidifier is idle.
- Two rows of tubelets on each tube, perpendicular
to airflow. Just another way to ensure complete
absorption.
- Guaranteed and published absorption data. Years of
testing and thousands of successful installations back
up our absolute guarantee. If we say it will work, it will
work!
- Requires less than 1 psi of pressure to disperse steam
- Models available from 12” x 12” up to 144” x 144”
(305 x 305 mm to 3658 x 3658) for horizontal or
vertical airflow.
- Available with High-efficiency dispersion tubes