BOILER / WATER HEATER INSIDE COATING WITH WET ENAMELY
Hans-Jürgen Thiele
E.I.C Group GmbH, member of DEV

Introduction
This paper will show and explain the wet enamel application technique with flooding and lowpressure spraying systems, which is currently used. The main interesting point is the general concept of nowadays systems and the process technique, used for bigger manufactures series.
It could be asked if the quite popular wet coating process is still an economical solution in comparison with the powder coating process?
Usually, manufacturers of electrical hot water heaters use both, coating processes wet and powder for small production rates. It was often necessary to adapt the boiler construction and shape to the powder coating process.
Can the wet coating process be still considered as an actual application process compared to the powder coating systems installed in the recent years?
Surely yes, because of:

1) The generally increasing production rates for water heater with internal pipes
2) Change of production from stainless steel to normal steel
3) Modifications in boiler construction
4) Increase of boilers above 100 lit. Volume
5) New wet application techniques with increased automation concepts
6) and finally the necessity of producing a higher quality among competitors

This led to multiple investments, since end of the nineties in new wet coating lines. These investments were planned according to actual economical and ecological aspects. In the last 5 years boiler coating lines with wet enamel have been installed in Europe, North America and North Africa in accordance with the following criterions:

A) The economical aspect
A.1) The demand for a highest flexibility in a technical solution, which is requested for electrically heated boiler as well as for those with pipes inside (heat exchanger), was forced by the following aspects:
- The rate of using boilers with internal pipes increased to approx. 70 %
- The boiler size now is between 80 and 500 litres, with an average of being between 200 - 300 lit.
- The minimum production capacity raced above 800 boilers/shift 7.5 h, some installations do have an average of more than 1.500 boilers/shift 7.5 h.

The automation of coating process according to the wide range of boiler shapes, as well as the diameter and the length are generally required and installed.
Also the variation of pipes shapes with the various outlet pipe connections does have an extreme effect on the technical plant solution.

A.2) The degree of automation to the entire production process is based on:
I. Formation possibilities direct from coil
II. Welding immediately upon formation
III. The following pre-treatment 2 x degreasing and 2 x rinsing with spraying nozzles and the water drier
IV. Enamel coating process by spraying or flow coating with drier
V. At present a manual cleaning of pipes outlet connection
VI. The furnace oven design
VII. A floor or over-head transport to and from individual plant process stations are automated
VIII. Enamel slip circulation is now in a closed circuit

B) The ecological aspect
The ready to use enamel delivered by various enamel manufacturers in big bags - avoids the use of mills and considerably improves the environmental aspect of the wet application.
Similar to powder, wet enamel is used in closed circuit.

C) Selection criterion for the application and process determination
The entire global concept of the actual coating lines designed for high production is interesting.
Most of the tasks are running fully automatically, except for the worker cleaning the pipe outlet connection, the operator setting specific parameters for coating of the inside of the boiler and possibly the loading/unloading of the boilers.

The remaining process steps are fully automatic:
1) Boilers are mostly transported automatically from the welding to the application plant with the help of an over-head conveyor. The loading can be either at a zero point position up or down. The opening for entering with lances or spraying gun are always at down position.
Main advantages are that the boilers are loaded from the beginning directly to the furnace conveyor. This means that the boiler is loaded at the welding and unloaded after firing.
2) In contrary to former pre-treatment techniques with a dipping process, currently all actual investments for pre-treatments are designed for 2 x degreasing at 60 -70 °C and ca. 90 -120 sec., 2 x rinsing with cold water and ca. 50 sec. Also the following pickling with an attached rinsing is effected by a spraying process and eventually 1 x passivation at 40 – 50 °C and 110 – 120 sec. by spraying with nozzles.
Also the blasting technique is still in use if the internal pipes do allow for blasting.
3) After the wet pre-treatment, the boilers must drop off and are dried by a drier at 120 – 140 °C and 8 – 10 min.
4) The wet application by automatically driven flow coating are done at a boiler position of 30° and 110 °. The coating happens during the boiler rotation.
Slip transfer into the boiler requires pumps with high fluid volume transfer in shortest time.
The production time for one boiler like transport, swivelling, flooding, draining and distribution is finally depending on the process time required, determined by
:: The boiler’s size
:: Diameter of heating pipes, position, construction and distances of pipes
:: The slip adjustment
:: Required number of coatings with thickness and quality
The spreading out of the enamel onto the boiler’s inside surface is effected – depending on the system’s capacity – after the flooding stations in one or two stations. In order to spread out the slip, the boiler rotates. In order to do so, the boilers are moved – in accordance with the internal heat exchangers` position – to the diagonal positions as programmed.
5) The outside coating can be applied directly after the inside coating.
6) Air nozzles with heated air (120-140 °C) will dry the inner area of the boiler during approx. 8- 10 min.
7) Before firing, the applied inner surface is controlled manually.
8) Even the outside flange and pipe connections are cleaned manually in most cases.
9) Depending on the capacity and customer requirements, the transfer to the furnace conveyor is affected automatically or manually.
10) The Burning process
11) Automatic or manual boiler unloading from the furnace conveyor.

Main features for the slip transfer of the actual automatic wet flow coating systems of boiler are:

1) Constant filtration and circulation in closed circuit of the slip
2) The slip is automatically proportioned according to the boiler size and its configuration in order to coat the coils properly. The working tanks are equipped with flow level control sensors, which will call for fresh material, transferred to the tank with diaphgram pumps.
3) The slip amount for the inside coating can be automatically transferred volume controlled into the boiler depending on the boiler size and the position of the heat exchange pipes.
4) In addition, pipe surfaces which are very difficult to reach, are applied with coating material
5) The slip can be set up in accordance with the individual requirements
6) A constant slip consistence offers a regular thickness and surface quality.
All above-mentioned criterions have been taken into consideration for our decisions during the
planning of our boiler coating lines installed in recent years.

The boiler inside coating with low-pressure spraying technique
In general, E.I.C. developed enamel systems, which are exclusively used in the manufacturing industry for products to be enamelled.
Our focus is based and concentrated on:

:: The improvement of the surface quality.
:: Perfectly automated productions run – from the loading to a conveyor up to the unloading for assembly.
:: Perfect enamel materials transfer efficiency.
:: A reduction of wareness of parts, which are in contact with enamel – e.g. nozzles, needles, pumps and valves.

Independent of the above mentioned flow coating technique, the inside of boilers can be coated with automatic spray guns with low pressure. In this case, the boiler bottom is separated from the boiler body. Both parts are welded together after the coating. This process is successfully used inNorth America.
Also for this application process the boilers are transported with an overhead conveyor via spray re-treatment and the following dryer.

In contrast to the flooding technique in which the boiler is retreated, coated and fired at the furnace hanger the boiler is now taken from the re-treatment line onto the coating line and then onto the firing line.
Independent of these transfer requirements there is the advantage of a high manufacturing output.
More than 1.500 boilers / shift / 7.5 hours can be enamelled.
The low-pressure spraying technique – talking about a material pressure of approx. 0.2 - 0.3 bar(3.0 to 5.0 psi) and an atomisation pressure of approx. 1.0 – 2.0 bar (15 – 30 psi) at the spray gun) offers the advantage of a high material transfer efficiency within a short time and with a low over spray. The inner surface’s thickness of the boiler is between 150 – 220 micro m.
This application technique is based on the enamel slip circulation via the spray gun head and back into the working tank which allows the pre-mentioned low material pressure of approx. 0.2 – 0.3 bar inside the spray gun head. This circulation process (the slip is always in motion and enamel can not settle) avoids locking inside the hoses and achieves constant manufacturer parameters.
Additionally, the cleaning system, developed by E.I.C., returns the remaining slip, still resting in the material hoses, back to the working tank after production stop or colour change.
The flow rate at the spray guns can be individually adjusted for each spray gun in accordance with the customer requirement.

To allow the inside spraying, the boiler is separated into 2 parts:
a) The tank shell with welded top
b) The bottom with and without flue pipe
Depending on the different parts` shape and in order to achieve an ideal automatic application process the pre-mentioned parts will be coated in different systems.

Re: a) System technique for tank shells with top
Tank shells are loaded with an opening up side down on an overhead conveyor. Each tank will be detected before entering the spray booth. The automatic spraying guns do not spray if hangers are not loaded. The tank shells are fixed automatically to the spray position inside the spray booth.
When the parts are in the spraying position, the hangers will start spinning and the guns are moved up into the tank shells. To spray the upper area inside tanks, the guns are automatically moved from horizontal to vertical spraying position. Altogether 4 spraying stations equipped each with 2 guns are proposed. The speed of stroke machine and spin frequency of hangers are adjustable.
After tanks are sprayed, the tank fixation assembly opens and the conveyor moves one step forward. 4 tank shells are sprayed at the same time.

Re: b) The bottom with and without flue pipe
Flue bottoms are carried on a floor conveyor. Parts are detected before entering the spray booth.
Positionable and movable spray guns will coat the bottom outside surface. In this installation, 3 parts are enamelled on a stop and go basis. Following the coating, the welding area is automatically cleaned from enamel with air nozzles. Servomotors are automatically positioning the air nozzles depending on the bottom ring`s diameter.
The E.I.C. automatic gun operates always with a continuous enamel circulation through the gun head. This system avoids blockages, which are normally frequent by spraying enamel out of pressure feed tanks or without circulation system. The enamel circulation is working like a hydraulic principle. The enamel output depends on the pressure difference between enamel in and the output of the gun. Opening with the needle the fluid tip passage at the gun will activate the material flow. Increasing or reducing the material pressure/speed of the material inside the hoses can regulate the material output.

Slip supply system
The slip supply system from the storage tank placed inside the mill room up to the spraying guns is automatic. All storage tanks are equipped with fluid level controls. By reaching the minimum level information is given to the main control panel to refill a tank or to transfer material to another tank.
Each tank is equipped with an electrically driven agitator to avoid material settlement of the slip.
Mixing times of the slip material are programmable in order to keep the slip ready to use. Each tank has two outlets: one deeper than the bottom level to empty and clean the tank and the other one for the enamel supply to the working tank.

Enamel supply system from the working tank to the spraying guns
The enamel supply from the working tank to the spray system is also running by diaphragm pumps with pulsation chamber in order to avoid pulsation inside the hoses. The pumps will work continuously and without interruption. They forward the enamel to the guns through the gun head and back to the tank. Each pump on/off mode is controlled from the control panel. The material amount ca be adjusted by the material pressure – which is not really precise – or - more precisely – by the stroke frequency of the pump.

Material supply cleaning system
Cleaning means a time-controlled pulsing on 2 valves.
Our cleaning system water/air has been designed for reusing effectively the slip inside pumps and fluid hoses. To avoid any risks of production stops we recommend the use of this cleaning process between 2 working shifts as well as daily production, or during production stops: longer than 20 minutes. Duration of the cleaning process is no longer than 5 min.

CUSTOMER BENEFITS
With E.I.C. Material Saving Intelligent Spraying System
1) Photocell/Computer Controlled Gun Triggering System
a) The gun will spray only if there are parts loaded to the hanger
Advantage: Lower production cost
b) The guns will be triggered individually according to the height and length of the parts
Advantage: Optimal and quick adaptation to the part dimension
c) Due to lower over-spray and controlled triggering, there will be a smaller quantity of reclaimed enamel in the cabin
Advantage: Enamel material saving
Advantage: Less cabin cleaning
d) Due to less reclaimed enamel mixing ratio of fresh enamel to reclaimed enamel will be higher
Advantage: Better quality of sprayed enamel, better surface quality
Advantage: Reduction of waste disposal costs

2) Patented Low-Pressure Spraying Atomizer
a) Very low atomization pressure needed
Advantage: Less air atomising, less air energy
Advantage: Spray pattern and coated surface are more even and thickness smoother, less orange peel effect
b) During atomization, very low bounce-back from the part
Advantage: The exhaust air speed inside the spray booth can be reduced by nearly 50 % which saves energy accordingly.
c) Very low material pressure and fluid speed at the nozzle
Advantage: The lifetime of the nozzle is much longer (minimum 6 months, usual nozzles will wear-out after 6 weeks).
Advantage: Production loss and maintenance times are less.

3) The Dual Material Supply and Circulating System is the most important part of this intelligent spraying technique
a) Automatically controlled fluid slip transfer from the mill room to the individual working tanks
Advantage: No staff needed for slip transfer via forklift.
b) Working tanks with integrated fluid level control
Advantage: Smaller working tanks which can be cleaned quicker and with less water consumption.
Advantage: Slip saving due to less surface contact
c) Slip circulation
Advantage: No production interruption or losses by settlements in the hoses and spray guns.

Summary
When choosing and determining new system technique, the most important criterions are the following:
:: A quick adaptation of production systems to constructive changes of working parts without having to make high and cost intensive new investments;
:: High grade of automation;
:: Flexibility in enamel adjustment;
and – last but not least –
:: A reduction of the ecological damage
These criterions were and are parts and deciding factors for boiler systems, installed in the last years, and will also be for new systems.

 


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