In this article, we’ll explore the evolution of pillow plates, from their early manual production to modern fully automated laser welding techniques. We’ll also discuss their applications in various heat transfer systems, the materials used, and the unique calculations involved in optimizing their performance.
By Ir. Henk-Jan Keppels, Keppels Laser Welding BV
Pillow plates are two metal plates welded together with a pattern and closed borders around the pattern then inflated to achieve an internal space. These plates were manually produced in the past and now produced on fully automatic laser welding machines. This inflated plate can be used in all kinds of heat transfer applications, from cooling or heating tanks, making cold water or cooling sugar or fertilizers with bulk solid heat exchangers. The tanks are made of single embossed pillow plates, a thicker base and a thinner HE top plate. All immersion applications are using double embossed HE pillow plates, both plates are of the same thickness. Heat transfer calculations with pillow plates are specific based on the application and made in spreadsheets. Nowadays the pillow plates are mostly made of stainless steel, but Keppels Laser Welding just introduced a copper laser welded pillow plate.
History of pillow plate
Pillow plate is an inflated plate from two plates welded together and by applying pressure between the two plates the inner space is made. With a repetitive pattern the inflated plate will look like a pillow, therefore is the commonly used popular name “pillow plate or pillowplate” or the old fashion name dimple jacket. The space between the plates can be used for all kinds of heat transfer applications. In the past, for example to cool a tank, the tank was surrounded by a pre-dimpled plate and manually welded to the wall of the tank with TIG or MIG. When the resistance spot welding was invented, this production method was applied to produce flat plates with resistance welded spot patterns, flow and border lines. These plates were welded on multi gun spot welders and produced in flat shape. The resistance welded lines are added manually. Following the welding process, the spot-welded plate is molded into the tank, which is then inflated to create a heat exchanger on the tank wall.
The resistance spot welding needs two electrodes to guide the electricity to the plates and the welding meld pool will be in the middle of the metal plates. For two plates of the same thickness, the weld spot is on the right middle position, using a thick base plate and a thin top plate the weld spot will not be at the plate intersection. Using different diameter of electrodes can move the weld pool toward the thinner plate, but not perfect on the intersection. During the spot welding, both electrodes are pressing hard in the material and make an unwanted undercut in the plate surface, which has to be grinded away if not allowed in the application.
To eliminate the disadvantages of spot welding, a pillow plate laser welding equipment was created using a CO2 laser for the first time in 1995. A keyhole penetrates the plates and focuses the laser beam on the upper plate. By moving this keyhole around and closing it after the beam with the liquid weld metal, it is possible to weld the plates together with shapes like circles, arcs or lines.
This method leaves the base plate without welding spots by welding from one side and it is even possible to process the base plate with plastic protection on the backside. Because the XY moving laser beam is flexible, the heat exchanger can be produced exactly according to the needs of the design engineer with respect to the heat exchange application. Starting from 2010 the CO2 laser (6% efficiency) was replaced by the YAG laser (4% efficiency). A few years later, the IPG fiber laser hit the market. With its exceptional beam quality and extremely high 35% energy efficiency in converting electricity to laser light, it was the better kind of laser to employ. Furthermore, the laser sources’ 100.000 hours of lifetime have made them flawless in pillow plate production equipment.
Pillow plate types
The space between two metal plates is formed by plastic deformation of the metal into a pressure vessel, that space can be used for the flow of liquid or air/gasses. This gives a very strong plate with cooling or heating capacity. The medium to be cooled or heated can be located in or outside the pillow plate. The metal heat exchanger plate can be constructed and formed or shaped to what is needed for the heat exchanger construction, process or application. The pillow plate can also be the construction. The pressure of inflation of the pillow plate is defined by the welding pattern of the laser spots and the thickness of the plate to inflate. The range of inflation pressure in general is between 8 bar and 450 bar, resulting in allowed ASME working pressures in the pillow plate of 3 – 300 bar. For example, a 1.0 mm top plate with a small pattern can burst the top plate at 150 bar, making a 80 bar carbon dioxide refrigerant application possible with 1.0 mm stainless steel.
Depending on the plate thickness, there are 2 types of pillow plates: single and double embossed pillow plates.
Single embossed plates are used for application where a thicker plate holds a product and transfers heat or cooling to the object through the thicker plate. Applications include rolls, fl at plates, walls, and bottoms of tanks, as well as various types of thermal equipment.
Tanks:
- Tanks general
- Wine and beer tanks
- Milk and cheese tanks, bulk milk coolers (BMC)
- Fermenting tanks
- Chocolate tanks
Others:
- Flat cooling plates under transport belts (chocolate lines)
- Trough for olive oil or ironing textile
- Rectangular basins
- Cooled containers
- Climate chambers
- Hoppers
- Food cookers
- Clamp on plates (add a heat exchanger to an existing tank wall)
- Pipes
- Vibration feeders
- Freeze dry plates, ammonia at -40°C
Double embossed plates are two inflated plates with the same thickness and holding one medium in the plate and the other medium around the plate. It is therefore possible to use the outside or inside of the plate for the medium/product to be cooled or heated.
Example: Steam or hot water inside – product outside. The goal is to heat the product on the outside of the plate.
Example: Water inside – river water outside. The goal is to cool the process water inside the plate. The majority of the plates used in these applications are immersed in liquid and are referred to as immersion plates. However, it is also possible to recover heat from gases or exhaust air by condensation of the latent heat or to cool or heat solid granulates, bulk solids, with the plates, such as sugar or fertilizers (NPK, CAN). The continuous production of ice water at 0.5°C using a falling film water chiller and the creation of an ice buffer using an ice bank are two major industries. These are applied in foodindustry where constant or instant cooling is needed.
Ice water makers: Falling film water chillers.
Cold storage: Ice banks, Flake ice machines.
Solid cooling: Bulk solid heat exchangers (BSHE).
Chemical: Crystallizers, Black liquor.
Heat recovery: Plate banks in solids, liquid and gasses.
Cooling / evaporation plates are used with:
- Ice water 0.5°C
- Glycol
- Freon, R12, R22, R143a, R404a, etc.
- Ammonia R717
- CO2
Pillow plate heat transfer calculations
The heat transfer K-value from pillow plates is based on experience values. The K-value can differ from 5 W/m2 with natural fl ow of gasses up to 1400 W/m2 for a forced steam application.
Over the last 20 years, there have been calculations developed for dimensioning the heat transfer area for tanks, bulk solid heat exchangers, falling fi lms and ice banks. These calculations are available at Keppels Laser Welding. The calculations are from simple heat transfer equation with the K-value, calculations with Reynold values, calculations with ice growth rates and differential equations for heat transfer in solid applications. The simple calculation is used for cooling or heating tanks, whereas the more complex calculations are used for the double embossed plates, submerged in liquids or solids.
What we also see is that companies with pillow plate products develop their own calculations for their specific pillow plate applications, for example in the condensing, chemical and pulp & paper industry. Over the course of our twenty-four-year pillow plate business venture, we have seen that, in order to ensure the application’s functionality, an additional reserve heat transfer area is included after the heat transfer calculation. A classic failure in dimensioning is a different K-value in heating or cooling of oils, leading to an under dimensioned heat exchanger.
The future of pillow plates
In 2024, 99% of the pillow plates are made of stainless steel, grade 304 to SMO254, duplex and exotic stainless steel version. The rest is made of titanium or carbon steel. The stainless steel grades are welded perfectly with the fiber laser and the materials that can handle thermal changes in the applications.
With the relatively bad heat transfer from stainless steel, we are looking for other materials to produce pillow plates. Until 2024, we have never seen a pillow plate in copper or aluminum because they are difficult to weld and process afterwards to a pillow plate. This year, Keppels Laser Welding has achieved laser welded pillow plates from copper and aluminum. We advise to do a welding and inflation test on the type of material for your application with possible following top plate thicknesses:
- Copper: from 0.1 to 1.5 mm thickness;
- Aluminum: from 1 – 3 mm thickness
About this Technical Story
This Technical Story was first published in Heat Exchanger World Magazine in October 2024. To read more Technical Stories and many other articles, subscribe to our print magazine.
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