PAINT BOOTH CONSULTANT

2819 Pickering Rd
Greensboro, NC 27407
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Drying Waterborne Paint

Drying Waterborne Paint

By Andras Uhlyarik

History of Waterborne Paint Drying Systems

Even though the first attempt to introduce waterborne finishes into the collision repair market failed in the ‘90s but we learned quite a bit. Paints had to be reformulated. It also became clear that waterborne finishes need a different environment to dry properly. People realized that as the water exits the paint, the moisture laden air just lingers over the painted surface. This saturated, humid layer of air severely slows the drying process. The solution was to build auxiliary air moving systems that blew fresh, lower relative humidity air onto the painted surface. As the process became better understood, turbulence creation became the goal instead of just trying to move more air over the surface.

I like to read patents related to a process. Patents represent original ideas or new solutions to a problem.

In this field, the first patent was awarded to Christopher Jeffs in March, 1995. The system used compressed air delivered through nozzles to increase airflow over the painted surface. The nozzles were mounted on a frame below the ceiling. The nozzles accelerated the airflow over the painted surface. However, this airflow was smooth, not turbulent. This system was designed for down draft spray booths only.

The next patent was awarded to Rick Farnan in October 1995 for the Advanced Cure paint booth. The Advanced Cure paint booth was the first version of the current Global Finishing Solutions waterborne paint drying system. It is an adjustable, nozzle based system that takes warm, spray booth air to dry the coating.

Junair UK improved on the Advaced Cure design and was awarded a US patent in February 2001 (QADS). They had the same corner mounted, adjustable nozzle design. Junair claimed that the system creates turbulence by crashing the airflow from the nozzles into the downdraft airflow of the booth. Junair added doors and took air out of the plenum of the spray booths. Mounting the fans externally is a big step toward great performance although QADS do not take advantage of this. The blower does not have to be designed to fit the corner enclosure. It can be designed to produce the correct air volume at the correct pressure. Towers can be made smaller in order to make retrofits easier.

Garmat was awarded a patent in May, 2006 for their AcceleCure system. They mounted ceiling fans under the intake filter of the spray booth. The fans increase airflow in a downdraft spray booth.

The California Pulse system is the latest innovation in the evolution of waterborne paint drying systems. We have a nozzle based system that takes spray booth air and delivers it back onto the painted surface. We have several patents pending to cover various aspects of our system. Our three main features are:

Semi stationary nozzles
Creation of directional turbulence
Controlling the evaporation rate of water

Semi Stationary Nozzles

Our nozzles are semi stationary. They are not intended to be adjusted between jobs since nozzle adjustment wastes a lot of time. The California Pulse system is designed to cover the center of spray booth with turbulent airflow.

Creation of Directional Turbulence

We built turbulence into the air flow that is coming from the nozzles. We do not blow air in a continuous stream. We pulse the airflow. Starting and stopping the airflow creates tremendous amount of turbulence.

Controlling the Evaporation of Water

We have the only system on the market that can finally guarantee drying times in an energy efficient manner. We built a control panel that regulates the evaporation rate of the water. California Pulse is the only company that provides a 100% solution to drying waterborne paints.

Problems and Solutions - How Our System Was Designed

There are theoretical and practical problems that need to be addressed when designing a waterborne paint drying system.

The two theoretical problems are: the development of the Boundary Layer and constantly changing evaporation rate of water. The evaporation rate of the water changes continuously as the level of relative humidity changes.

Boundary Layer

The spray booth has non turbulent or laminar air flow. Laminar air flow follows the contour of the object it encounters. This sounds good but laminar airflow also creates a boundary layer.

How does the boundary layer develop?

A layer of paint is actually rough on the surface at the molecular level. As laminar air flows around an object the air molecules get “stuck” on these surface imperfections. A boundary layer of slow moving air develops between the painted surface and air moving at full speed.

How does the boundary layer block the drying process?

As water evaporates from the coating, it moves into the boundary layer. The boundary layer saturates with moisture and prevents the rest of the water from transitioning out of the paint. Think about it as a layer of insulation between the painted surface and the low relative humidity spray booth air. As the boundary layer saturates with water, the paint stops drying. Wet air does not dry wet paint.

How can we eliminate the boundary layer?

The simple solution is to create turbulent air flow that breaks up the boundary layer.

Turbulence eliminates the boundary layer and prevents the boundary layer from forming again. Starting and stopping the airflow is a very effective way to deliver low relative humidity air onto the surface of the paint.

Evaporation Rate of Water

The second problem is that high relative humidity conditions slow down the drying process. Relative humidity varies greatly not just within geographic areas but also within seasons and throughout the day. It is a continuously changing variable.

We designed a control panel that varies spray booth heat in order to stabilize the evaporation rate of water. Consistency is the only thing that is acceptable in a production environment. The California Pulse system provides unparalleled consistency.

Practical Problems

Will the system work without a car in the booth?

Most systems project a weak air stream. They were designed with the assumption that there will be a car in the booth. The air stream looses a lot of momentum when it has to travel through the spray booth air. Once the air stream reaches the car body, it has much less frictional losses. The air stream will follow the curved surface of the car (Coanda effect) with relative ease.

Most systems were designed with the assumption that the air stream from the nozzles will reach the car body within 5’ from the nozzle. However, many times there will be only parts in the booth. If the booth has parts toward the middle, most systems will not be able to reach it with airflow and turbulence.

This is one reason why California Pulse uses a heavy duty blower with a 2HP motor. A 2 HP motor with a larger diameter blower wheel will be able to produce more pressure than two 1 HP motors on smaller diameter blower wheels. More pressure projects the air stream further.

Air speed on the surface of the vehicle is not an issue until it reaches about 1,000 ft/min. The Pulse system reaches 550-600 ft/min peak speeds on the surface of the vehicle.

How much room does it take in the corners?

The second reason for using a large, external blower is to be able to build a nozzle plenum with a small footprint. For best performance, you have to mount the nozzle plenums in the corners. Many booths have man doors and other obstacles that make corner mounting difficult. We designed a nozzle plenum we call the Slim Tower. Slim Towers only need 4-3/8” corner space to be mounted. With the large external blower it produces outstanding results.

Advantages of Nozzle Based Systems

Multiple, horizontal air streams from the corners provide by far the most even distribution of air or turbulence over the vehicle. Cars have very complex shapes including horizontal and vertical surfaces, indentations, etc. Creating turbulent air movement over every surface is a fairly difficult task. Directing air at the car from only one direction does not do as good of a job as surrounding the car with the sources of turbulence. Our system creates turbulent airflow in the difficult to reach areas like under styling lines, door handle indentations, under side view mirrors, etc.

California Pulse System Components

Nozzle plenum

Delivers the pressurized air onto the vehicle. Semi stationary nozzles do not require adjustment after initial setup. Nozzle adjustment is a waste of time. System should be designed in a way to cover the center of the spray booth with turbulent airflow regardless of the size of the object getting coated.

Doors

Doors are necessary in a horizontal airflow spray booth in order to protect the nozzles next to the exhaust plenum from overspray.

Control Panel

The control panel provides the timing functions and an interface to the spray booth heater. Auxiliary air moving systems should be operated in the bake mode to save energy. Auxiliary air moving systems eliminate the boundary layer that insulates the vehicle from the hot spray booth air in bake mode. Heat transfer between the hot spray booth air and the car body becomes much more efficient without this insulation. The car body, or parts, heat up faster without the insulation and the bake cycle can be shortened.

Blower

The blower is the heart of the system. A well engineered blower is absolutely crucial for good performance. Forward curved, “squirrel cage” type blowers are best suited for this application. They produce the right volume at the right pressure for this specific application. These blower wheels do this very efficiently. Bathroom fans will not get the job done!

Pulsing Mechanism

We use two pneumatically operated dampers to direct the air from the blower to two towers at a time. We switch between the two pairs of towers every 5 seconds.

California Pulse System Operation

The California Pulse system takes filtered and heated spray booth cabin air. The air goes through a paint arresting filter and is ducted to the blower. From the blower the air flows to two towers at a time. There are two pneumatically operated blast gates built into the system. Each blast gate controls the air flow to two towers. One blast gate is open and the other is closed. They alternate between open and closed every 5 seconds. Starting and stopping the air flow is an excellent way to create turbulence.

There are many different ways to duct the air from the top of the booth to the tower. You can go through the gable, intake plenum, side walls, front wall or the rear wall of the spray booth. Each spray booth needs to be evaluated for best installation method.

Summary

The first part of the solution is not air movement but it is turbulence. The easiest way to generate turbulence is through moving air.

Controlling the evaporation rate of water is the second part of the perfect solutions to drying waterborne paint. You have to have some control over the evaporation rate of the water if you want consistent results.

Nozzle based systems provide the most even coverage of a vehicle.

The Future:

VOC, Greenhouse Gas,and Energy Efficiency Regulations

I believe that the low VOC emission legislations are just the beginning of tighter regulations of the refinishing industry. I think the next set of regulations will limit greenhouse gas emissions, CO2, NOx, and methane, and set energy efficiency standards. One of the easiest way to lower emissions is to make spray booths more energy efficient. We designed a spray booth control system that uses 45% less energy than an 80/20 recirculating system. The good news is that energy efficient spray booth heating systems can be manufactured at a reasonable cost.

Waterborne paint drying systems are an integral part of our energy efficient spray booth control system. One aspect of energy saving through using waterborne paint drying systems has already been demonstrated. Operating waterborne paint drying systems in the bake mode increases the rate of panel temperature rise by about 30%. The bake cycle can be shortened by about 5-10 minutes.

There is another aspect of waterborne paint drying systems that has not been used yet in our industry. This aspect will yield further savings. California Pulse has a patent pending on this process. For example in NY State this equates to a savings of $2,700 per year over a conventional spray booth with an 90/20 recirculation system!

We believe that they waterborne paint drying systems will become a standard part of every heater spray booth sold in the United States during the next few years.

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