Scraped-Surface Heat Exchanger

Overview

A scraped-surface heat exchanger (SSHE) heats or cools viscous, sticky, or particulate foods that would burn, freeze, or foul onto a plain heat-transfer wall. Products such as margarine, shortening, ice cream, cream cheese, peanut butter, tomato paste, custards, and meat slurries are pumped through a vertical cylinder while a rotating shaft drags spring-loaded blades along the inside wall. The blades continuously wipe the layer of product off the surface, which keeps the boundary film thin, sustains a high heat-transfer rate, and prevents the fouling that would otherwise stop a conventional tube exchanger within minutes. The original commercial design is the Votator, and the working principle is shared across margarine cooling units, freezers, and aseptic cookers.

The machine described here is a single vertical cylinder unit. Larger plants stack two or three cylinders in series so the product passes through several heating or cooling stages. Capacity for one cylinder runs from roughly 300 to 4,000 kg/h depending on diameter, the temperature swing required, and product viscosity.

How it works

Product enters at the bottom head through the Product Feed Pump, a positive-displacement pump that meters viscous material at controlled pressure regardless of how stiff the product becomes during processing. The product fills the thin annular gap between the rotating Mutator Shaft and the inner cylinder wall. The heating or cooling medium flows through the jacket surrounding that wall. As product climbs the cylinder it exchanges heat through the chromed wall, and the scraper blades wipe the film off the wall on every revolution so fresh product is always in contact with the surface. Treated product leaves through the top head.

Mutator speed sets both heat transfer and product shear. Higher speed renews the film faster and raises the heat-transfer coefficient, but it also works the product mechanically, which matters for fat crystallization in margarine and shortening where the blades nucleate and break crystals to set the final texture. The Variable-Frequency Drive lets the operator tune speed to the recipe, and the motor current monitor flags rising torque as the blades wear or as product over-thickens.

Cylinder and jacket

The heat-transfer cylinder is the core of the machine. The Inner Product Cylinder is a hard-chromed nickel tube, typically 102 to 203 mm inner diameter and 0.9 to 1.8 m long. Nickel resists corrosion from acidic and salty foods, and the hard chrome plating gives a smooth, wear-resistant surface that the blades ride against without scoring. Around it the Outer Jacket Shell forms an annular space for the medium, fed through the inlet nozzle and drained through the outlet nozzle. For cooling duty the jacket runs flooded ammonia or R507 evaporating at low temperature; for heating it carries steam, hot water, or glycol. A Pressure Sensor on the jacket inlet and outlet tracks the media side, and the insulation jacket with stainless cladding keeps the surface cool and stable.

Mutator shaft and scraper blades

The Mutator Shaft, also called the dasher, is a hollow stainless shaft centered in the cylinder by journals running in Ball Bearing supports at each head. The shaft carries 2 to 6 axial blade rows, and each row holds 6 to 8 Scraper Blade elements. Each blade pivots on a Scraper Blade Pin and is pushed against the wall by a Coil Spring or by centrifugal force, so wear is taken up automatically and the wipe stays effective as the edge dulls. Blades are the main wear part and are replaced on a schedule, usually every few weeks to a few months depending on product abrasiveness. Plastic blades are common for soft products and steel for hard fats. The hollow shaft can be heated or cooled in some designs to add transfer area, and balance discs limit whirl at high speed.

Drive and seals

The Drive Unit uses a TEFC induction motor of 7.5 to 37 kW feeding a helical Gearbox Housing with a Helical Gear Pair, then a Drive Belt set on V-belt pulleys to the shaft. The belt drive absorbs shock when product stiffens and is simple to retension. Where the shaft passes through each head it is sealed by the Shaft Sealing Assembly, a double mechanical seal with silicon carbide or carbon faces, an O-Ring Set for the secondary seals, and an Oil Seal backing the assembly. A sterile water or condensate flush at the seal port lubricates the faces and stops product from drying into the gap, which is what keeps the unit clean enough for dairy and aseptic service.

Materials and sanitation

All product-contact surfaces are stainless steel or chromed nickel with sanitary finishes, no crevices, and full drainability so the cylinder cleans in place. Seals, O-rings, and blades use food-grade elastomers and polymers. The machine is built to 3-A Sanitary Standards and EHEDG hygienic design rules, while the pressure-bearing jacket and refrigerant circuit meet PED 2014/68/EU. CIP runs caustic and acid cycles through the product path with the mutator turning slowly so the blades scrub the wall during cleaning.

Variants and use

Cooling units chill and crystallize margarine, shortening, and ice cream; the ice cream variant is the continuous freezer that whips in air while freezing. Heating and cooking units pasteurize or sterilize sauces, processed cheese, and ready meals with particulates that plate exchangers cannot pass. Aseptic SSHE lines feed directly into sterile filling for shelf-stable products. The Control Panel runs the cycle on a PLC with temperature and back-pressure sensors so each batch hits the target temperature and texture.