Pillow Plate Heat Transfer

Anyone holding a pillow plate for the first time understands the name immediately: two metal sheets, welded together with refrigerant or heating medium flowing between them, bulge outward into a pillow-like, three-dimensional structure. What looks like a simple design principle is actually the result of decades of development work on a heat transfer element that today replaces tube bundle and coil systems in food processing, the pharmaceutical industry, chemical processing, and industrial refrigeration, anywhere efficiency, cleanability, flexibility, and durability matter.

This article explains how pillow plates are built, how they're engineered, which configurations suit which applications, and where they deliver concrete advantages over conventional heat exchanger technologies in industrial practice. HTT AG's product page provides a technical overview. This article covers the background: the physics, the design principles, and a solid basis for decision-making for planners and plant operators.

A pillow plate consists of two thin metal sheets welded together in a defined pattern. After welding, the space between them is pressurized, and the sheets bulge outward between the weld points. This creates a network of channels through which the heat transfer medium flows, along with a three-dimensionally structured outer surface that's active on the product or process side.

The design difference from other heat exchangers comes down to three characteristics:

Geometric flexibility. A pillow plate is not a standard product with fixed dimensions. Length, width, wall thickness, weld pattern, and channel routing are all engineered for the specific application. HTT AG uses CNC-controlled laser welding systems that translate any geometry from the design drawing directly into production. That makes it possible to build pillow plates as rectangles, segments, cylinders, conical shapes, or custom geometries for non-standard vessels.

Use as a structural element. Pillow plates can be used not only as suspended heat exchangers but also as structural elements and as the vessel wall itself. A single-embossed pillow plate can serve as a structural element and as a tank jacket that functions simultaneously as heat exchanger, structural element, and vessel wall in one. That eliminates the need for a separate heat exchanger and significantly reduces the space required.

Behavior with difficult media. The open, channel-like structure on the process side has no tight gaps where deposits can build up. Media with particulate content, highly viscous products, and media containing solids can be cooled or heated without clogging or elevated pressure drop becoming a limitation. That sets pillow plates apart from plate heat exchangers with their narrow, profiled channels.

The fundamental distinction among pillow plates is between the single-embossed and double-embossed versions. Both work on the same principle but are engineered for different applications.

Single-Embossed Pillow Plate

In the single-embossed version, one plate is flat and the other is embossed. The flat side rests against the vessel wall and forms a structural element or a tank jacket. The heat transfer medium flows through the pillow-shaped channels between the flat wall and the embossed plate.

Typical applications:

• Structural element: Single-embossed plates, whether small or large, are fitted as structural elements into heating or cooling constructions. They efficiently heat or cool a specific surface area.
• Dimple jacket / dimple plate: Single-embossed pillow plates mounted as an outer jacket around cylindrical or rectangular tanks. Pressure drop on the jacket side is low thanks to the parallel, flat channels, which means large surface areas can be circulated with low-power pumps.
• Tank integration: The pillow plate is built directly into the vessel wall. Tank and heat exchanger form a single unit, eliminating the capital and installation cost of a separate heat exchanger.
• Wall cooling in cold rooms and storage facilities: Large-area single-embossed plates are installed as wall panels in cold storage warehouses and cold rooms. They cool the space directly through the wall surface, without air coolers and the air movement that comes with them.

Double-Embossed Pillow Plate

In the double-embossed version, both plates are embossed, symmetrically to one another. This produces a significantly larger channel cross-section, higher turbulence in the heat transfer medium, and therefore higher heat transfer coefficients.

Typical applications:

• Structural element: Double-embossed plates, whether small or large, are fitted as structural elements into heating or cooling constructions. They efficiently heat or cool a specific surface area.
• Standalone heat exchangers: Double-embossed plates are installed as suspended heat exchangers inside tanks or vessels, either stacked at defined spacing or as individual panels.
• Cooling at high heat flux densities: Anywhere a large amount of heat needs to be removed from a small surface area, the double-embossed version delivers higher k-values thanks to its greater turbulence.
• Gas–liquid applications: The larger channel cross-section is necessary when partial evaporation occurs on the heat transfer medium side (for example, with NH₃ used directly as the refrigerant).

A heat exchanger's heat transfer coefficient depends critically on the flow velocity and turbulence of the medium at the transfer surface. In smooth pipes or flat channels, a laminar boundary layer forms near the wall that impedes heat transfer.

The embossed, three-dimensional surface of a pillow plate continuously disrupts this boundary layer. The medium is repeatedly redirected, and eddies form even at low flow velocities and with highly viscous media. That's why pillow plates achieve turbulent heat transfer even at low Reynolds numbers, in flow regimes where conventional heat exchangers remain laminar.

In concrete terms: the overall heat transfer coefficient (k-value) of a double-embossed pillow plate in liquid-to-liquid operation is typically higher than that of comparable coil systems of similar size. At an equivalent k-value, the required transfer surface is smaller, making for a more compact unit.

HTT AG calculates k-values, pressure drops, and transfer surfaces based on the actual process parameters: mass flow rates, inlet and outlet temperatures on both sides, and the viscosities, thermal conductivities, and densities of the media involved. The result is a design that fits the process, not a standard design stacked with safety margin on top of safety margin.

The standard material for pillow plates in the food and pharmaceutical industries is stainless steel 1.4301 (AISI 304) or 1.4404 (AISI 316L). The latter offers improved corrosion resistance against chlorides thanks to its molybdenum content, relevant when seawater is used as a cooling medium or when chloride-containing cleaning agents are involved.

For more aggressive media or higher purity requirements (such as pharmaceutical production), special alloys are used. Carbon steel is used where food-grade requirements don't apply and where higher strength or lower material cost is the deciding factor.

As for pressure rating: in HTT AG's design, pillow plates withstand pressures of up to 50 bar on the heat transfer medium side, and higher in special designs. On the process side (the outside of the jacket), the pressure load is typically much lower. That makes pillow plates suitable for applications that would otherwise require a pressure-rated custom build in a conventional heat exchanger.

The fully welded stainless steel construction contains no gaskets on the product-contact side. That's a significant hygiene benefit: gaskets are a potential source of contamination and need to be replaced regularly. With pillow plates, that maintenance burden disappears entirely.

The food industry is today the largest field of application for pillow plates. The reasons lie in the combination of hygienic design, cleanability, geometric flexibility, and the ability to reliably hold temperatures near 0°C.

Milk Cooling and Dairy Processing

In milk tanks, double-embossed pillow plates are installed directly around the vessel as a dimple jacket. Glycol or a direct refrigerant flows through the jacket and keeps the milk at cooling temperature. The low pressure drop in the jacket allows for large-area jackets with small circulation pumps. With direct expansion, ammonia can be used as the refrigerant, since the fully welded construction rules out leaks on the product side.

Dough Cooling in Baked Goods Production

Kneading dough generates friction heat that raises the dough temperature and can trigger uncontrolled yeast fermentation. Pillow plate heat exchangers cool process water to temperatures near 0°C, which is then supplied to the mixer as a cooling medium. Single-embossed pillow plates shaped to the vessel can also be built directly into the mixing bowl itself.

Fish and Seafood Processing

Fish processing plants cool product water, bulk material, and vessels to temperatures between 0 and 2°C. Pillow plates, used as structural elements or tank jackets in process tunnels or vessels, hold these temperatures precisely while staying easy to clean. The open, smooth stainless steel surface meets the hygienic design requirements that apply to food-contact surfaces in the fish industry.

Beverage Industry and Brewing

Fermentation and storage tanks in breweries are cooled and temperature-controlled through pillow plate jackets. Precise temperature control is critical to the quality of the fermentation process. Since breweries often operate several hundred tanks, the low pressure drop of the pillow plate jacket has a direct effect on the pumping costs of the overall operation.

Chemical and Pharmaceutical Industry

In the chemical industry, pillow plates remove reaction heat from exothermic processes, condense distillates, or bring products to a defined temperature after synthesis. They continue to function reliably even with contaminated media, vapors, or particulate-laden streams, without risk of clogging. In pharmaceutical production, cleaning requirements are especially strict: the fully welded, gasket-free construction in high-grade stainless steel is designed for CIP (Cleaning in Place) and SIP (Sterilization in Place).

One application that's still underestimated in planning practice: using pillow plates to recover heat from contaminated streams. Wastewater, process waste heat, and condensate streams contain recoverable energy that can't be efficiently captured with conventional heat exchangers because of the risk of clogging.

Pillow plate heat exchangers with unobstructed cross-sections on the outside can handle these streams, transferring the heat to heating media and saving primary energy in the process. In the pulp and paper industry, biomass processing, and large industrial laundries, HTT AG systems are already in use that recover heat from wastewater above 30°C and demonstrably reduce heating costs as a result.

In many production processes, the heat load fluctuates significantly over the course of the operating day. Batch operations in the chemical industry, varying fill volumes in food production, or seasonal peaks in the beverage industry all create situations where a heat exchanger has to deliver significantly more than its nominal design point for a few hours at a time.

Pillow plate systems can be engineered to buffer these fluctuations. Double-embossed pillow plates, used as suspended elements or in tanks, can use the vessel's own temperature mass as thermal storage: the product mass in the production process or tank is held at a target temperature, and the pillow plates deliver more capacity during demand peaks than the design accounts for under continuous operation.

This characteristic makes pillow plates especially attractive in combination with ice storage tanks or chillers that are likewise designed for buffered operation. The ice storage tank supplies water at 0.5°C to 1°C on demand, the pillow plates transfer this cooling precisely to the process medium, and the chiller runs at steady, efficient partial load instead of fluctuating full load. That lowers energy consumption, protects the refrigeration system, and improves the controllability of the entire system.

When designing this kind of combined operation, HTT AG takes into account both the instantaneous output of the pillow plates and the storage capacity of the vessel. Share your load profiles and production data with us, and we'll deliver a design that tunes both components to work together.

A pillow plate isn't chosen from a catalog, it's calculated. The input data needed for a reliable design are:

Process side:

• Medium (liquid, gas, vapor, suspension)
• Volumetric or mass flow rate
• Inlet temperature and required outlet temperature
• Viscosity, thermal conductivity, density, specific heat capacity
• Allowable pressure drop

Heat transfer medium side:

• Medium (glycol, water, NH₃, CO₂, steam)
• Available flow rate
• Inlet temperature
• Allowable operating pressure

Structural constraints:

• Vessel geometry (diameter, height, shape)
• Available mounting area
• Installation conditions (tank jacket, insert exchanger, freestanding)
• Material requirements (stainless steel grade, wall thickness)

Based on these parameters, HTT AG calculates the k-value, the required transfer surface, and the pressure drop on both sides, and defines the pillow plate's channel geometry. The result is a drawing and a data sheet documenting all design parameters, which then serve as the basis for the order.

We'll tell you whether a pillow plate is the right choice for your process, and if so, with which geometry and which material. Tell us about your process parameters.

The choice between a pillow plate and conventional heat exchanger technologies comes down to three core questions:

1. How difficult is the medium?

Plate heat exchangers and tube bundles clog with particulate-laden or highly viscous media. Pillow plates don't. If the medium on the process side contains solids, has high viscosity, or tends to foul, the pillow plate is the more reliable choice.

2. What cleaning requirements apply?

In the food and pharmaceutical industries, every heat transfer surface has to be CIP-capable. Tube bundles have narrow tubes that are difficult to clean. Pillow plates have smooth, accessible outer surfaces and a gasket-free construction on the process side.

3. Does the heat exchanger need to integrate into an existing vessel design?

Pillow plates can be implemented as a tank jacket, a built-in ceiling panel, a coil replacement, or a standalone unit. Tube bundles are standalone units that require floor space and pipe connections. Anywhere the tank and heat exchanger need to form a single unit, the pillow plate is the only flexible alternative.