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PARTS CLEANING USING FLUIDIZED BED
HEAT-CLEANING SYSTEM
Shahrani Hj. Anuar, Wan Ahmad Najmi Wan Mohamed, Azmi Masripan
Department of Fluids and Thermodynamics
Faculty of Mechanical Engineering
Universiti Teknologi MARA (UiTM)
40450 Shah Alam
Abstract
Cleanliness of a machinery component is an important consideration to
avoid machinery breakdown and application failures. Currently, industries
are turning away from using chemical compounds and solvents for
cleaning purposes due to its adverse effect on health and the environment.
Efficiency and cleaning speed are two parameters that are demanded by
the industry. Thermal cleaning using fluidized bed is an attractive
alternative, offering higher cleaning efficiency and flexibility in process
control. This paper gives an overview of the parts cleaning technology and
the progress in the design of a lab-scale fluidized bed heat-cleaning system
for commercial purposes.
Keywords: Cleaning, machinery parts, heat, fluidized bed
1. Introduction
Machineries consist of components
subjected to various modes of operation.
Industries depend heavily on their process
machineries, where profits and losses are
mainly subjected to the efficiency of
industrial machineries to work as they are
supposed to. However, machinery life and
efficiency depends on many inter-related
factors.
One of the factors which is the subject
of interest of this paper is the cleanliness of
the machinery components. Rotating
components requires lubricants such as
grease and oil for smooth operation, but due
to design failure, process conditions and
other factors, the greases and oils is
transferred to other parts of the machinery,
therefore contaminating them with unwanted
organic substances. A working environment
with dust, dirt, tar, resins, carbonized matter,
and chemical substances also contributes to
contamination.
Failure to properly address the
problem of unwanted contaminates,
especially on critical machinery parts, is a
major cause for machinery breakdown and
reduced efficiency. Conventional methods
of cleaning metal, or the removal of these
contaminants, normally apply chemical
based solvents. Due to problems
encountered using chemical cleaning agents,
especially the effect on human health and
the environment, other methods of cleaning
are developed throughout the years based on
specific industrial requirements. Cleaning
with heat, or thermal cleaning, is one of the
alternative method that is suitable for a wide
range of applications.
1.1 Research Objectives
The objective of the research project is to
develop a rig and process capable of
cleaning metal parts from machineries. It is
understood that an efficient cleaning process
has a high commercial value, and localized
expertise in the field can be developed. This
paper addresses the basic issues in cleaning
technology, and discusses the considerations
critically required for the design of a
fluidized bed heat-cleaning system. The
scope of the parts to be cleaned are
automotive parts exposed to oil, greases, and
carbon deposits.
2. Parts Cleaning Technology
Cleaning methods used in the industrial
sector is based on the items for cleaning and
process requirements such as the food
industries, industrial plants, and heavy
machineries. Industrial requirements can be
categorized into two categories;
industrial/metal cleaning and critical
cleaning. The first category is defined for
cleaning parts that are generally large, and if
small, have relatively low unit value. These
parts are often described as having a
typically high per-piece cleaning load.
Examples of components falling in this
category are fabricated metal products,
hardware (tools etc), commercial equipment,
aviation and automotive (structural
components), and machined parts.
The second category is defined for
parts with higher workloads than the
industrial group, and unsatisfactory results
could cause problems in performance and
product quality. The component includes
hydraulics, automotive, marine and aircraft
components (nonstructural), magnetic
devices, and heavy duty machinery
components.
It is not economical to remove a small
amount of organic contaminant by cleaning
it with large amounts of chemicals, merely
transferring the contaminant from the
substrate to the cleaning solvent. Repeated
rinses to achieve good cleaning greatly
increase the volume of chemical waste to be
dealt with. Most importantly, chemical
solvents and wastes are harmful to human
health as well as the environment [1].
Ultrasonic assisted multi-stage solvent
cleaning is applied in the aviation industry.
British Airways uses this method to increase
the efficiency of removing all traces of oil,
grease and carbon deposits from the
hydraulics component of their aircraft liners.
2.1 Industrial Requirements
A study among 6000 panelists from the
industrial sector by Cleaning Technology
Group at Witter Publishing Corporation was
made in 1998 to understand the cleaning
requirements of the industrial sector. The
importance of cleaning was stressed, but the
standard of cleanliness is varied. Efficiency
and speed of cleaning equipment are
demanded by those who practice industrial-
type cleaning [2]. The biggest challenge for
the development of an advanced, more
efficient cleaning systems are:
(i) removal of oil, grease, dirt
etc.
(ii) cost, efficiency and speed
(iii) environmental issues
Cleaning processes must also be safe
to the technical personnel and environment.
Surface damage of the cleaned parts must
also be avoided.
3. Cleaning with Heat
Thermal cleaning is an alternative to
chemical solvents. Heat is applied to remove
the contaminants by a combination of
vaporization, thermal decomposition, and
oxidation. As the part temperature rose, the
organic contaminants decompose to
combustible smoke and vapors. Once the
part is cooled, small amount of ashes or
residues remains which can be normally
removed with a quick wipe. No hazardous
chemical compounds are involved in the
process.
Environmental concerns are pushing
industries to turn to thermal cleaning.
Removing the organic contaminants with
heat offers the advantage of eliminating
chemical cleaning completely. However, the
high temperatures inherent in thermal
cleaning rules out this technology for a
variety of applications, such as for electronic
circuit boards or plastic parts. It is suitable
only for cleaning metal parts, ceramics and
glassware [3].
The electric motor rewinding industry
was among the first to adopt heat cleaning as
mean of removing varnish and insulating
coatings from electric motor rewindings to
be rebuilt. Removal of oils and greases from
automotive engines, blocks and related
metal parts using heat is also gaining
popularity, replacing caustic tanks and
chemical based solvents. Paint and coatings
industry also favors heat cleaning to remove
paint residues from the automated paint lines
such as hangers and hooks. Plastics and
polymers used in the plastics industry can
also be removed by heat, especially on
extruder and molding machines, dies, and
piping. Because many plastics are not
readily soluble in chemicals or solvents,
thermal cleaning assumes even greater
prominence in this industry. In the
laboratories, heavy residues of tars, waxes,
gums, polymers, resins, carbonized matter,
and organic contaminants are common, and
laboratory-glassware is another major
application for heat cleaning.
Diverse types of thermal cleaning
systems are available, where some are gas-
fired, others electric, as well as a
combination of both. Among them are
molten salt baths, high temperature vacuum
ovens, “burn-off” furnaces, specialty
cleaning ovens, and fluidized bed systems.
Furnace design can be adapted to the
specific applications, for example long, thin
furnaces in lengths of up to 10 meters are
available for cleaning extruder tubes in the
plastic industry. Most furnaces have
chamber sizes ranging from as small as 250
by 250 mm to 8 meters long cleaning
furnaces large enough to hold heat
exchangers weighing as much as 12 tons.
The operating temperature of heat cleaning
is usually in the range of 300oC to 550oC,
depending on the type and concentration of
organic substance to be removed.
Thermal cleaning is primarily limited
to the removal of organic contaminants or
residues. Because inorganic compounds not
affected by the heat of cleaning are not
removed, secondary cleaning operations are
often required. Ash residues are normally
fairly easily removed by mechanical means,
such as tapping the hangers and water
washing.
For automotive parts such as engine
block and heads, thermal cleaning removes
oils, greases, and some of the carbonized
residues. To achieve the high level of
cleaning necessary, glass-beading or shot
blasting is often the method of choice for
secondary cleaning. When post-cleaning of
thermally cleaned parts is necessary, the
methods used normally do not involve
chemicals or solvents. Therefore, the
primary advantage of thermal cleaning is
generally not compromised by the need for
secondary cleaning.
Temperature control and residence
time are important parameters in thermal
cleaning equipment design. Simple systems
apply timers to hold the unit at processing
temperature for the required cleaning period.
Sophisticated cleaning equipment
automatically control the cleaning times
required for different loads. These advanced
control methods usually rely on sensing
when smoke production from the cleaning
process has ended, and adjusting the time
cycle accordingly.
Designing industrial cleaning
equipment presents certain technological
challenges. Contaminated metal parts often
contain substantial amounts of heat-
degradable residues. The large amount of
organic contaminants involved in a large
heat-cleaning unit creates the potential for
ignitions or fires from the fumes created
during cleaning. Pollution control is also an
important issue.
Fluidized bed systems with alumina as
the fluidizing medium eliminates the
concern of ignition. Alumina is inert and
avoids chemical reactivity with the fumes.
Fluidization is the principle by which inert
particles of alumina oxide, loosely packed in
a closed chamber, are set into motion by a
rising stream of air. With the proper air
flow, the bed of alumina particles becomes
fluidized and exhibits many of the properties
of a true liquid, including excellent heat
transfer. Because the alumina is inert,
fluidized beds have on melting point, and no
vapors or odors are emitted from the media
at the cleaning temperature used. High
efficiency in cleaning is also achieved due to
a very fast rate of heat transfer.
Afterburners and post-combustion
chambers located directly above the bed is
commonly used to burn the smoke and
combustible organic gases rising from the
cleaning process. An alternative is to install
pollution control equipments such as
scrubbers and thermal oxidizers, but these
are costly and may increase the economic
aspect greatly. Developing cleaning methods
that minimizes hazardous emissions is the
requirement of today’s industry.
4. The Design of Fluidized Bed Heat-
Cleaners
The fluidized bed design is based on a
cleaning process requiring intimate gas-
solids contact, in order to promote chemical
reactions and physical processes. The
fluidizing particles is required to be inert,
therefore design is based on alumina oxide
particles that will not react chemically with
the organic substances of the cleaned parts.
The solid particles are only required as a
heat transfer medium from the fluidizing hot
air to the parts suspended in the reactor.
Design of a complete system requires
many considerations. The fluidized bed
reactor size, in terms of bed-depth (H) and
diameter (D) effects the fluidizing behavior
and process efficiency. For compact reactors
where D=H, high pumping power is
demanded to supply gases at velocities
higher than the minimum fluidization
velocity, due to elutriation of the alumina
particles. Lower pumping power can be
achieved for shallow depths where bed-
depth is smaller than the bed diameter
(H<D), but a larger floor area is required for
this type of reactor [4]. Mean residence time
of the fluidizing particles is influenced by
the size of the reactor, therefore affecting the
speed aspect of the cleaning process. Wire
baskets and steel hangers are used to
suspend the parts inside the fluidized
reactor.
Suitable particle sizes are also
required. Excellent fluidizing particle sizes
suitable for cleaning is estimated in the
range of 40-500μm and density range of
1400-4500 kg m-3. Beds of these particles
exhibit stable expansion with free bubbling
when the minimum fluidizing velocity is
reached. The sphericity (φ) of alumina, a
non-dimensional quantity of the particle
shape depending on the volume and surface
area of the particles, is in the range of 0.3 –
0.8 [5]. Large particles are unsuitable for
cleaning due to the fear of surface damage,
while smaller size particles have the
advantage of being able to clean difficult-to-
reach areas. The minimum fluidizing
velocity and temperature required for an
efficient heat-cleaning process are related to
the shape, and size of the reactor, as well as
contaminant type and concentration on the
components.
The design of distributors is important
for heat transfer efficiency. Common
designs are the porous plate design and
stand-pipe design. Hot air is supplied
through the distributor, supplied by a fan or
compressor. Energy input to the system is
achieved with the use of thermal heaters
connected to the air supply line.
Other considerations in design include
[6]:
(1) feed mechanism for the supply of
solid particles into the reactor
(2) separation of entrained particles from
the exhaust gases using a cyclone
(3) installation of hangers and wire
baskets at the suitable height within
the reactor
(4) fan or compressor power required for
pumping the gases
(5) integrated instrumentation and
controls (thermocouples, pressure
gages, flow meters etc)
(6) analysis on the concentration of
hazardous gases from emission of the
cleaning process
Overall, the design of a fluidized bed
heat-cleaner system must be based on
performance, reliability, safety, emission of
pollutants, and meets the economic criteria
with minimum technical risks.
5. Current Progress
The project is in the phase of finalizing the
details of the system design. Figure 1
illustrates the conceptual design of the
reactor, which consists of 8 main
components:
(1) Reactor Bed
(2) Wire Baskets ( or Hangers) for parts
suspension
(3) Feeder for solid particles feeding
(4) Cyclone Separator with Gas Exhaust
(5) Gas Distributor
(6) Heater for energy input
(7) Air supply line
(8) Slots for temperature probes, pressure
gages, and gas analyzer.
The scope of experimental tests to be
carried out is based on the performance of
cleaning automotive parts from oil, grease,
and carbonized residues. These parts include
engine blocks, gear boxes, and power shafts.
Parameters of concern are suitable particle
size, optimum fluidizing velocity and
temperature, gas residence time, pressure
distribution and most importantly, cleaning
performance.
6. Conclusions
Thermal cleaning is a suitable method for
removing unwanted organic contaminants
from machinery parts by vaporization,
thermal decomposition, and oxidation. A
fluidized bed heat-cleaner system offers the
capability to efficiently clean parts but
requires many critical considerations in the
system design process. Economic aspect as
well as pollution control are the two main
challenges in designing an efficient fluidized
bed heat-cleaning system suitable for
industrial applications.
References
[1]. Seelig S., The Chemical Aspects of
Cleaning, CleanTech Magazine,
Vol. 11/1995, pp. 33-40, 1995.
[2]. Kenneth M., Cleaning with Heat:
Old Technology with a Bright New
Future, CleanTech Magazine, Vol.
7/2001, pp. 10-15, 2001.
[3]. Daniels, R., ed., New Cleaning
Strategies: Environmental Issues
and Technology Development,
Miller Freeman Books, San
Francisco, 1994.
[4]. Davidson, J.F., Clift, R. and
Harrison, D., (ed.) Fluidization 2nd
Edition, Academic, London, 1985.
[5]. Howard, JR., Fluidized Bed
Technology: Principles and
Applications, Adam Hilger IOP
Publishing, New York, 1989.
[6]. ASM International, Guide to
Mechanical Cleaning Systems, ASM
International, Ohio, 1996.
Figure 1. Conceptual Design of the lab-scale Fluidized Bed Heat-Cleaning System
Exhaust gas
Recycled
particles
Cyclone
Flow meter
Residues
discharge
Gas analyzer
Distributor
Fan or
compressor
Heating
element
Hot gas
Wire basket
Fluidized Bed
reactor
Solid feeder