Abstrict A painting system includes a paint spray booth ventilated by process
air including a combination of outside make-up air and a portion
of the exiting process air which is returned to the booth, the returned
air being the main component of the combination. The humidity of
the air in the paint spray booth is controlled by routing a first
minor portion of the process air, exiting the booth, through a desiccant
dehumidifier, which removes moisture and volatile organic compounds
(VOCs) from this process air portion before it is sent back to the
paint spray booth. The dehumidifier is designed so that at any given
time the process air portion contacts only part of the desiccant
bed while air heated to approximately 250.degree. F. in a paint
cure oven is exhausted from the oven and directed through the remaining
part of the desiccant bed in order to dry, and thus regenerate,
the desiccant for continued use. The dehumidifier is designed so
that a continual exchange of dried desiccant for moisture laden
desiccant respectively takes place relative to the process and heated
air streams. The heated air also removes a large portion of the
volatile organic compounds removed from the first minor portion
of the process air. A second minor portion of the process air exiting
from the paint spray booth, equal to the volume of make-up air,
and the heated regenerative air exiting the dehumidifier are routed
through a control device, such as a thermal oxidizer, which destroys
the volatile organic compounds contained in this air before exhausting
it to the atmosphere.
Claims We claim:
1. A ventilating system for an enclosure defining a work space,
said enclosure containing a water scrubber and being provided ventilation
air intake and exhaust openings; an air supply fan coupled to said
enclosure for effecting a flow of ventilation air from said intake
opening to said exhaust opening, with said water scrubber being
located for contact by the flow of ventilation air; an air recirculation
fan having an inlet coupled to said exhaust opening and an outlet
coupled to said intake opening; a desiccant dehumidifier having
a recirculation air inlet coupled to said recirculation fan outlet;
flow control means for limiting the flow to said desiccant dehumidifier
inlet to a minor portion of the air flow exiting said recirculation
fan; said desiccant dehumidifier including a recirculation air outlet
coupled for returning dehumidified recirculation air to said ventilation
air intake opening.
2. The ventilating system defined in claim 1 wherein said desiccant
dehumidifier also includes a regeneration air inlet and a regeneration
air outlet; and a source of heated regeneration air being connected
to said regeneration air inlet.
3. A painting system, comprising: a paint spray booth; a spray
booth air supply fan means coupled for causing process air to flow
through and ventilate an interior of the booth; process air duct
means conveying said process air away from said spray booth interior
and dividing the process air into a major portion directed back
to said spray booth interior and into a minor portion; a desiccant
dehumidifier including a desiccant bed means; said process air duct
means coupling at least a first part of said minor portion of said
process air to said dehumidifier for flowing through and contacting
a first section of said desiccant bed means and then returning said
first part to said spray booth, whereby moisture and volatile organic
compounds are removed from said first part of said minor portion
of said process air.
4. The painting system defined in claim 3 wherein said painting
system includes a source of relatively hot regenerative air; regenerative
air duct means coupled between said source and said desiccant dehumidifier
for directing said regenerative air through a second section of
said desiccant bed means of the dehumidifier to thereby remove moisture
and volatile organic compounds from and, thus, regenerate said second
section of said desiccant bed means.
5. The painting system defined in claim 4 wherein said painting
system includes a paint cure oven for receiving painted articles
from the spray paint booth and drying the paint on the articles;
means for heating air in the oven to a paint cure temperature; said
heated air in said oven serving as said source of relatively hot
regenerative air; and an oven exhaust fan connected to said regenerative
air duct means for causing said regenerative air to flow through
said regenerative air duct means.
6. The painting system defined in claim 5 wherein said process
air duct means divides said minor portion of said process air into
said first part and into a second part; a volatile organic compound
control device means; and exhaust air duct means coupled for directing
said second part of said minor portion of said process air exiting
said paint spray booth and said regenerative air exiting from said
desiccant dehumidifier to said control device means, whereby volatile
organic compounds entrained in the air directed to said control
device means are destroyed; a source of fresh make-up air; and make-up
air means for directing a quantity of said make-up air, equal to
said second part of said minor portion of said process air, to said
spray paint booth.
7. In a painting system including a spray paint booth; a source
of fresh make-up air; an air supply fan coupled for effecting a
flow of process air including make-up air through said booth for
ventilating the booth; a major portion of said process air exiting
said booth being recirculated to said spray paint booth; and a dehumidifier
coupled for removing moisture from at least a first part of a minor
portion of said process air prior to said first part being returned
to said spray paint booth, the improvement comprising: said dehumidifier
being a desiccant dehumidifier acting also to remove a major portion
of volatile organic compounds entrained in said first part of said
minor portion of said process air exiting said spray paint booth.
8. The painting system defined in claim 7 and further including
a source of desiccant regeneration air connected to said dehumidifier
for removing moisture collected by desiccant of said dehumidifier.
9. The painting system defined in claim 8 and further including
a paint cure oven through which flows heated air serving as said
source of desiccant regeneration air; and means connecting said
heated air to said desiccant dehumidifier for regenerating the desiccant
thereof.
10. The painting system defined in claim 9 wherein a second part
of said minor portion of said process air exiting said booth is
combined with said regeneration air after it exits said dehumidifier
to form a stream of exhaust air; and a volatile organic compound
control device being coupled for receiving said stream of exhaust
air before the latter is exhausted to atmosphere.
11. The painting system defined in claim 9 wherein said spray paint
booth contains a water scrubber for removing heavy metals introduced
into the paint booth during spray painting.
Description BACKGROUND OF THE INVENTION
The present invention relates to painting systems including water-wash
paint spray booths ventilated by recirculating process air through
the booth.
It is conventional to ventilate a paint spray booth with a relatively
large volume of process air in order to maintain an acceptable working
environment within the spray booth. Two of the largest costs of
operating a spray painting facility are preconditioning fresh make-up
air to the required standards or characteristics, temperature and
humidity, e.g., for use in the spray booth and for decontaminating,
i.e., removing volatile organic compounds (VOCs) from that portion
of the process air discharged to the atmosphere after leaving the
spray booth. A known practice to reduce the costs associated with
preconditioning fresh make-up air entering, and removing VOCs from
air exhausted from, a spray booth is to reduce the volume of entry
and exit air required to be treated, while preserving total process
air flow through the spray booth work area or place, by directing
a large portion of the ventilating air back to the booth. In popular
use are water wash paint spray booths that include a sheet of water
for collecting over-sprayed paint and for converting isocyanates,
resulting from use of polyurethane paints, to harmless urea. In
these booths, the relative humidity may approach 100% causing serious
difficulties for paint application. While it is known to address
these difficulties by employing refrigeration-type dehumidification
to lower the humidity of the fresh air entering and/or process air
being recirculated to the spray booth, this manner of dehumidification
is very expensive, complex and difficult to maintain.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an improved
system for painting objects within a paint spray booth, and more
particularly there is provided an improved manner of maintaining
the quality (the humidity and temperature) of process air, used
to ventilate the paint spray booth, at an acceptable level, i.e.,
at a level which does not negatively impact the condition of the
paint sprayed onto an article being painted.
An object of the invention is to provide a spray painting system
wherein a major portion of the process air used in ventilating a
paint spray booth work place is recirculated to the work place and
to provide a relatively inexpensive way to efficiently dehumidify
the recirculated air so that, no matter what the humidity is of
fresh make-up or replacement air entering the work place, the humidity
of the totality of the air flowing through the work place is kept
at a level which does not negatively impact the quality of the coating
of the article being painted.
A more specific object of the invention is to provide a spray painting
system wherein about 90% of the total air flow used to ventilate
the paint spray booth is recirculated or returned to the booth and
wherein about 20% of this recirculated air is routed through a desiccant
dehumidifier prior to being returned to the booth, the dehumidified
air being recombined with the larger volume of unconditioned air
and with the fresh make-up air entering the booth so as to lower
the humidity of the total volume of process air passing through
the paint spray booth to an acceptable level.
Yet another specific object of the invention is to provide a painting
system, as set forth in the immediately preceding object, and further
including a paint cure oven with heated air from the oven being
routed to the desiccant dehumidifier so as to draw retained moisture
from, and thus regenerate or reactivate, the desiccant material
so that it is once again able to dehumidify a portion of the paint
booth process air leaving the spray booth before being returned
to the booth.
Still another object of the invention is to provide a painting
system, as set forth in one or more of the preceding objects, wherein
the desiccant dehumidifier also operates to remove a significant
amount of volatile organic compounds from that portion of the paint
spray booth process air routed to the dehumidifier after leaving
the booth.
A further object of the invention is to provide a painting system,
as set forth in one or more of the preceding objects, wherein the
paint spray booth is a water-wash booth which operates to remove
heavy metals from the over-sprayed paint.
These and other objects will be apparent from a reading of the
ensuing description together with the appended drawing.
BRIEF DESCRIPTION OF THE DRAWING
The sole FIGURE is a schematic showing of a painting system including
a water-wash spray paint booth, a paint cure oven, a desiccant dehumidifier,
a VOC abatement device, an air cooling device and various fans coupled
in ductwork arranged for effecting the desired air flow among these
components in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawing, there is shown a spray painting system
10 including a spray booth 12 paint cure oven 14 and desiccant
dehumidifier 16.
The spray booth 12 is, for example, of a water-wash type which
uses a water scrubber to wash away paint which is over-sprayed while
spray painting an article 18 to be painted, shown here suspended
from a conveyor 20 which would in most instances carry a plurality
of the articles 18 one after the other. The ventilation air flows
downwardly through the area where the article 18 is being spray
painted and aids in bringing the over-sprayed paint into contact
with a water scrubber 21 including a catch area 22 where the paint
solids, including hazardous heavy metals, separate from the water
and accumulate for removal from time to time.
The conveyed articles 18 are painted one after the other with the
conveyor 20 moving the painted articles 18 from the booth 12 and
into the paint cure oven 14 by way of openings (not shown) respectively
provided in the walls of the booth and oven. The oven 14 may be
of any well known construction including a heat source capable of
heating air in the oven to a paint cure temperature of about 250.degree.
F., for example.
While any desiccant dehumidifier system of a type having desiccant
bed arrangements which operate such that one portion of the bed
is being used for removing humidity from process air while a second
portion of the bed is being regenerated or reactivated by having
moisture removed therefrom, the desiccant dehumidifier 16 shown
here is of a type capable of continuously dehumidifying a given
process air stream so that the air stream has a substantially constant
relative humidity as it leaves the dehumidifier. Specifically, while
not shown in detail, the dehumidifier 16 includes a substantially
air tight, cubical housing 24 containing a driven carrousel (not
shown) rotatable about a fixed axis Y and including a plurality
of equi-spaced desiccant chambers 26 arranged annularly about the
axis Y with adjacent chambers 26 being interconnected by an air
impervious panel 27 extending from an inner corner of one chamber
to an outer corner of the adjacent chamber. The chambers 26 each
contain a supply of granular desiccant, e.g., silica gel, thus forming
a plurality of modular desiccant beds. A central cylindrical zone
is bounded by the chambers 26 and is divided into a major zone portion
30 and a minor zone portion 32 by an angular wall 34 having respective
legs joined so as to define a corner, which extends along the axis
Y, and having radially outer terminal edges joined to seal structures
36 disposed for engaging radially inner walls of the chambers 28
as the latter rotate past the seal structures. The legs of the angular
wall 34 form a right-angle, with the major and minor inner zone
portions 30 and 32 thus constituting three-fourths and one-fourth,
respectively, of the cylindrical zone. A pair of outer seal structures
38 are fixed to adjacent walls of the housing 24 so as to engage
radially outer surfaces of the chambers 28 as they rotate past the
seal structures, the seal structures 38 acting to divide an outer
zone, defined by and located between radially outer surfaces of
the chambers 28 and the housing 24 into outer major and minor zone
portions 40 and 42 respectively, constituting approximately three-fourths
and one-fourth of the total volume of the outer zone. The placement
of the seal structures 38 is such that the major inner and outer
zone portions 30 and 40 and the minor inner and outer zone portions
32 and 42 are respectively in fluid communication with each other
by way of respective paths through the granular desiccant contained
in the chambers 28. A process air inlet (not shown) is provided
in a wall of the housing 24 in direct fluid communication with the
inner major zone portion 30 while a process air outlet 46 is formed
in a wall of the housing 24 in communication with the outer major
zone portion 40. Similarly, a regeneration or reactivation air inlet
48 is formed in the housing 24 in direct fluid communication with
the outer minor zone portion 42 while a regeneration or reactivation
air outlet (not shown) is in direct communication with the inner
minor zone portion 32.
A known desiccant dehumidifier of the above-described type is the
MVB Series Dehumidifier marketed by Bry-Air, Inc. of Sunbury, Ohio.
However, it should be noted that other types of desiccant dehumidifiers,
for example those having separate fixed desiccant beds through which
process and regeneration air is alternately passed, may be used.
A spray booth air supply fan 52 has its inlet coupled, as by a
duct 54 to the process air outlet 46 of the dehumidifier 16 the
duct 54 containing a cooling element in the form of a water coil
56 for removing the heat of adsorption and the carry over heat resulting
from the regeneration of the desiccant, from the ventilation or
process air. The process air supply fan 52 has an outlet coupled,
as by a duct 58 to the booth 12 as at 60 while a spray booth
air recirculation fan 62 has an inlet coupled to the booth 12 as
at 64.
The relative humidity of the air at the outlet 64 which is just
downstream from the water scrubber 21 is always nearly 100%. The
temperature of this air depends on the temperature and relative
humidity of the air entering the scrubber 21 and to a lesser extent,
on the temperature of the water. Ordinarily, the water is reused
in the scrubbing system, with make-up water supplied to accommodate
evaporation. The water in the system generally reaches an equilibrium
temperature such that the most significant factor affecting the
air temperature is the evaporative cooling taking place in the scrubber
21.
The ventilation or process air drawn from the spray booth 12 by
the fan 62 first passes through a mist eliminator and filter assembly
66 and then enters the fan 62 where it is discharged and split into
a major part, about 70%, for example, which is returned to the inlet
60 of the booth, via a duct section 68 coupled to the duct 58 and
with a minor part, about 30%, for example, passing through a second
duct section 70. Approximately one-third of the air carried by the
duct section 70 or about 10% of the air exiting the recirculation
fan 62 is coupled, as by a duct section 74 to a system exhaust
duct 76 while approximately two-thirds of the air carried by the
duct section 70 or about 20% of the air exiting the recirculation
fan 62 is coupled, as by a duct section 77 to the process air
inlet (not shown) that is in direct communication with the inner
major zone portion 30 of the dehumidifier 16. It is to be noted
that the division of the process air flowing from the recirculation
fan 62 may be done by appropriately sizing the ductwork and/or by
using adjustable dampers (not shown), with the amount of ventilation
air sent through the dehumidifier determining the relative humidity
and temperature of the air in the spray paint booth 12. For example,
in the particular installation used to develop the present invention,
an operating condition in the 80% to 90% relative humidity range
at ambient indoor temperatures was achieved by routing about 25%
of the recirculating air through the dehumidifier 16. By removing
additional heat from the air exiting the dehumidifier 16 the spray
booth 12 can be operated at below ambient indoor temperature. By
passing a greater percentage of the recirculating air through the
dehumidifier 16 and cooling it, even lower booth relative humidity
can be realized.
The system exhaust duct 76 is connected between the outlet of an
oven exhaust fan 78 and an inlet of a paint system exhaust fan 80
the oven exhaust fan 78 having its inlet coupled to the regeneration
air outlet 50 of the dehumidifier 16. The system exhaust fan 80
has its outlet coupled to atmosphere by way of a suitable control
or abatement device 82 for destroying VOCs entrained in the exhaust
air, an example of such a device being a regenerative thermal oxidizer.
Since approximately 10% of the process air is exhausted to atmosphere,
this air has to be replaced and this is done by a fresh air make-up
fan 84 having its inlet coupled to atmosphere and having its outlet
coupled, as by a fresh air supply duct 86 to the process air supply
duct 58 at a location between the supply fan 52 and the spray paint
booth 12.
The desiccant in the dehumidifier 16 which is contacted by the
air flowing from the inner major zone 32 to the outlet 46 absorbs
moisture from the air resulting in the air becoming dried and heated
due to the exothermic action of absorption. Adequate removal of
this heat from the air exiting the dehumidifier was achieved in
the installation used for developing the instant invention by providing
the cooling coil 56 with 85.degree. F. cooling water. The reactivating
or regenerating air inlet 48 of the dehumidifier 16 is connected
to the paint cure oven 14 by a duct 88 so that air heated to about
250.degree. in the oven will be drawn through the dehumidifier 16
by the action of the fan 78 so as to pick up moisture from the desiccant
located in its flow path through the dehumidifier 16.
It is here noted that the desiccant not only removes moisture,
but also acts to remove a significant amount (up to 80% or more)
of the VOC's from the air stream delivered by the fan 62 to the
dehumidifier via the duct sections 70 and 77 as the air stream comes
into contact with the desiccant, these captured VOCs, in turn, being
removed from the desiccant by the regeneration or oven exhaust air
stream drawn through the dehumidifier 16 by the oven exhaust fan
78 for removing moisture from the desiccant.
With a reasonably constant inlet air temperature, the desiccant
dehumidifier 16 used in developing the present invention, achieved
a certain lower level of humidity regardless of the humidity of
the inlet air. As stated above, the relative humidity of the air
immediately downstream from the water scrubber 21 is nearly 100%.
Assuming the make-up air, because of its small volume, plays no
significant role in determining the relative humidity of the ventilation
air, the relative humidity in the spray booth 12 is determined by
the percentage of recirculating air which is passed through the
dehumidifier 16 and the final temperature of the mixed air supplied
to the booth. The temperature of the recirculating air not passing
through the dehumidifier is related only to the temperature and
humidity of the air in the booth, as these factors dictate what
evaporative cooling will occur in the water scrubber 21.
The operation of the paint system 10 is thought to be evident from
the forgoing description and for the sake of brevity is not repeated.
Suffice it to say, that by using the desiccant dehumidifier 16 to
dehumidify only a small first portion of the process air drawn from
the paint spray booth 12 by the recirculation fan 62 it is possible
to return a large second portion of the process air back to the
booth without any treatment at all with the humidity and temperature
of this returned air being maintained at respective values which
do not adversely affect the quality of the paint applied to an article
18. Further, only a minor second portion of the process air drawn
from the spray booth 12 is exhausted to atmosphere and therefore
only this small amount of air is required to be subjected to the
thermal oxidizer 74 for removing VOCs entrained therein. Further,
because only this small amount of air is removed from the painting
system, only an equal small amount of outside or fresh make-up air
is required to be added so that the process air adequately ventilates
the spray paint booth 12. Thus, costs associated with treating incoming
fresh air and air exhausted to the atmosphere are minimized. |