Pillow plates: Introduction

Pillow Plate Heat Exchangers (PPHE) are a special heat exchanger design in which two sheets of metal are welded together to form a closed pressure chamber. A laser-welded or resistance-welded spot pattern is defined within the pressure space in order to achieve the pressure resistance required by the customer and also to generate maximum fluid mechanical and thermodynamic efficiency. The spot pattern is the characteristic element of pillow plate heat exchangers and has been defined to perfection by the engineers at BUCO for many decades.

Figure 1: Sample welding spot pattern

After the pillow plate heat exchanger is completely welded, the volume of the pressure chamber is defined by pneumatic cold deformation of the inner space under high pressure. This parameter complements the variables mentioned above and is determined by engineers at BUCO. The so-called press-on height, maximum clear distance between both inner sides of the plate, defines the flow cross-sections in the pre-welded spot pattern and forms the characteristic visual design of a pillow plate heat exchanger. In addition, strain hardening during the pneumatic deformation process creates the stability of the pillow plate.

 

Figure 2: Flow channel inside the pillow plate

According to the customer's requirements, we define the welding spot pattern and the internal channel height as a result of different variables, such as:

  • Sheet metal material (e.g. DC01, DC04, 304, 304L, 316L, 316Ti, 318LN, 904L, 254 SMO, etc.)
  • Sheet wall thicknesses (e.g. 0.6 mm, 0.8 mm, 1.0 mm, 1.5 mm, 2.0 mm, 3.0 mm, etc.)
  • Maximum allowable pressure
  • Maximum allowable temperature
  • Fluid inside the pillow plate
  • Fluid outside the pillow plate
  • Flow rate through the pillow plate
  • Maximum allowable pressure drop.

Our decades of expertise in interacting with the above variables can be applied to all conceivable fluids in the heat exchange plate. These include:

  • Single-phase fluids such as water, thermal oils, or highly viscous fluids such as flycol-water mixtures
  • Single-phase superheated gases
  • Fluids that evaporate in the pillow plate (e.g. natural refrigerants, freons)
  • Fluids that condense in the pillow plate (e.g. steam)

Maximum allowable pressures PS >70bar or maximum allowable temperatures TS > 400°C can be achieved thanks to BUCO's outstanding know-how.

The versatility of pillow plate heat exchangers leads to an almost unlimited range of applications, especially where complex pillow plate geometries are required due to complicated base bodies and mounting options.

Allowable pressure and burst pressure test

The allowable pressure inside the pillow plates is verified by burst pressure tests. These burst pressure tests are usually carried out on samples of the pillow plate with the same strength-determining properties as the pressure vessels to be manufactured (material, sheet thickness, weld spot pattern, etc.). To determine the burst pressure PB, the pressure in the test specimen is gradually increased in the presence of an expert from the responsible testing organization until it bursts. This procedure thus belongs to the destructive tests. The weld spot pattern of the pillow plate which is required achieve the required burst pressure is determined by the engineers at BUCO. Burst pressures > 450 bar are readily achievable. The calculation of the maximum allowable pressure PS from the achieved burst pressure PB is done e.g. according to AD 2000 Merkblatt S5 or the ASME code. In very simplified terms, the burst pressure must be more than five times the maximum allowable pressure.

 

Flow and heat transfer

To ensure that the pillow plate heat exchanger works as efficiently as possible for all fluid groups, optimum flow arrangement within the pressure chamber is a top priority. To achieve this, among other things, seam welds are placed in the welding spot pattern to guide the fluid through the pillow plate. An optimum combination of welding spot pattern and channel height support the best possible flow distribution by specifically influencing the flow cross-sections. This minimizes dead spaces and poorly flowed areas in the pillow plate. This has been confirmed for decades by verifications on test rigs and customer installations, making BUCO's customized pillow plate geometries the most efficient on the market.

Due to the flow within the three-dimensional pillow profile with regularly repeating changes in cross-section and direction, turbulent flows can be generated even at low Reynolds numbers, resulting in comparatively high internal heat transfer coefficients at an early stage. As a result, under the same conditions outside the heat exchange plate, the overall heat transfer coefficient increases and the required heat exchange area decreases. This advantage is particularly evident in comparison with tube flow in tube bundles, which are significantly more inefficient for geometric reasons. As a result, BUCO's pillow plate heat exchangers are also more resource-efficient and sustainable, as the amount of steel required is significantly lower than for solutions with tubes. In addition, solutions with tubes, coils or tube bundles are extremely inflexible in design, making them inferior to pillow plates in many applications. The above advantages can be applied to all the fluid groups mentioned above, which BUCO has mastered to perfection for many decades.

Figure 3: Simulation of the flow in the pillow plate by CFD (M. Piper et al., International Journal of Thermal Sciences, 120 (2017), 459-468)
Figure 4: Exemplary flow guidance with weld-offs