High-viscosity products do not behave like water. They resist flow, form thick boundary layers, and stick to hot or cold surfaces. A standard shell and tube or plate heat exchanger often fails in this service. It plugs, fouls, or loses heat transfer capacity within hours.
A scraped surface heat exchanger (SSHE) solves that problem mechanically. A rotating blade continuously removes the product film from the heat transfer surface. Fresh product touches the wall. Heat moves. The process keeps running.
This guide explains how scraped surface heat exchangers work, where they fit, how to select one, and what to watch out for in high-viscosity service.
Why High-Viscosity Fluids Are Hard to Heat and Cool
Heat transfer depends on fluid movement near the wall. With low-viscosity fluids, turbulence renews the boundary layer. With high-viscosity fluids, flow is usually laminar. The layer near the wall acts like insulation.
Common problems:
Low heat transfer coefficient. The product film resists heat flow.
Hot spots and burn-on. Product overheats at the wall while the bulk stays cool.
Fouling and coking. Deposits build up quickly and reduce capacity.
High pressure drop. Thick products need high pump pressure to move through narrow gaps.
Uneven residence time. Some product spends too long at temperature and degrades.
Crystallization and solidification. Cooling can form crystals or wax that block passages.
Poor cleanability. Viscous residue clings to surfaces and resists CIP.
A scraped surface heat exchanger addresses the root cause: the boundary layer.
What Is a Scraped Surface Heat Exchanger?
A scraped surface heat exchanger is a cylindrical heat exchanger with a rotating shaft inside. Blades or scraper elements are mounted on the shaft. They sweep the inner surface of the cylinder as the shaft turns.
The product flows through the annulus between the shaft and the cylinder wall. The heating or cooling medium flows through the jacket outside the cylinder.
Key names you may see:
Scraped surface heat exchanger (SSHE)
Scraped surface cooler
Votator or mutator in older food and chemical literature
Thin-film scraped heat exchanger
Continuous crystallizer when used for cooling and crystallization
The working principle is simple: keep the heat transfer surface clean and renew the product film.
How It Works
Product enters one end of the cylinder. The rotor turns. Blades push product against the wall and scrape away the film that has already been heated or cooled.
This does three things:
Reduces thermal resistance. The wall is always in contact with fresh product.
Prevents burn-on and fouling. Deposits are removed before they harden.
Improves uniformity. Product is mixed and moved through the unit with controlled residence time.
Heat transfer coefficients in an SSHE can be several times higher than a conventional heat exchanger handling the same viscous product. That is why SSHEs are common in processes where ordinary heat exchangers fail.
Where Scraped Surface Heat Exchangers Fit
SSHEs are used when the product is viscous, sticky, heat-sensitive, crystallizing, or prone to fouling.
Typical applications:
Food and dairy
Sauces, ketchup, mayonnaise, jams, fruit preparations, chocolate, peanut butter, shortenings, margarine, ice cream mix, processed cheese, and dairy desserts.
Chemical and polymer
Resins, adhesives, sealants, greases, waxes, polymer melts, and viscous intermediates.
Pharmaceutical and personal care
Ointments, creams, gels, lotions, and sterile viscous products where sanitary design matters.
Petrochemical and industrial
Asphalt, bitumen, heavy fuel oil, wax, sludge, and high-viscosity byproducts.
Environmental
Thick slurries, sludges, and waste streams that would plug other heat exchangers.
If your product can be pumped but not easily cooled or heated in a plate or shell and tube unit, an SSHE is a strong candidate.
Main Design Choices
Not all scraped surface heat exchangers are the same. Selection depends on product rheology, duty, cleaning, and pressure.
Horizontal vs Vertical
Vertical SSHE
Good drainage, smaller floor footprint, and often preferred for sanitary service. Useful when CIP and complete drainability are required.
Horizontal SSHE
Easier access for maintenance and blade inspection in some designs. Common in industrial and chemical service where floor space allows.
Single vs Double Tube
A single tube SSHE has one product annulus. A double tube design has two concentric product passages or two heat transfer surfaces. Double tube units can provide more area in a smaller footprint, but they are more complex and harder to clean.
Rotor and Blade Design
Blade type affects shear, mixing, product damage, and wear.
Fixed blades: Simple and effective, but higher wear and shear.
Floating or spring-loaded blades: Maintain contact while reducing wear.
Plastic or composite blades: Reduce metal contamination risk in food and pharma.
Metal blades: Durable for abrasive or high-temperature service.
Rotor speed is a major variable. Higher speed improves heat transfer and reduces fouling, but it increases shear, power consumption, and blade wear. For shear-sensitive products, low speed with optimized blade geometry may be better.
Materials and Surface Finish
Product contact materials must handle the product, cleaning chemicals, and temperature.
Common choices:
316L stainless steel
Duplex and super duplex for corrosive service
Titanium for chlorides and seawater
Nickel alloys for aggressive chemicals
FDA-compliant elastomers for seals and gaskets
Sanitary units often require Ra ≤ 0.8 µm or better on product contact surfaces. Some pharmaceutical and food applications specify electropolishing.
Seals
Shaft seals are a critical point. A leaking seal can contaminate product or allow product to escape.
Options include:
Single mechanical seals
Double mechanical seals with barrier fluid
Flushed seals for abrasive or crystallizing products
Sanitary seals for CIP/SIP service
Choose seals based on pressure, temperature, product, and cleaning method. Do not treat seals as an afterthought.
How to Select a Scraped Surface Heat Exchanger
Sizing an SSHE is not just a matter of heat transfer area. Rheology and residence time matter as much as duty.
Collect this data:
Product viscosity curve vs temperature and shear rate
Flow rate and allowable pressure drop
Inlet and outlet temperature
Heating or cooling medium temperature
Heat load (kW or BTU/hr)
Residence time limits
Solids content and particle size
Fouling, crystallization, or burn-on tendency
Cleaning method: CIP, SIP, manual
Pressure and temperature ratings
Materials and sanitary standards
Available space and utility limits
For viscous products, a single viscosity number is not enough. Many products are shear-thinning. They behave differently at different rotor speeds and flow rates. A rheology curve is far more useful.
Pilot testing is often the fastest way to confirm performance. A small SSHE test can measure heat transfer, pressure drop, fouling rate, and product quality before you commit to a full-scale unit.
Common Pain Points and Fixes
Burn-on and fouling
Reduce the temperature difference between the product and the heating medium. Increase rotor speed slightly. Check blade condition. For cooling, avoid surface temperatures that cause crystallization or sticking.
Poor heat transfer
Check blade contact, rotor speed, product flow, and jacket flow. Air in the jacket or fouling on the water side can reduce performance. Verify that the product is not bypassing the scraped surface.
High pressure drop
Increase annular clearance, shorten the flow path, or reduce flow rate. Cold product is much harder to pump. Preheat if the process allows.
Blade wear
Abrasive products and high rotor speeds wear blades quickly. Use wear-resistant materials, floating blades, or adjustable clearance. Monitor product for metal particles.
Seal leakage
Check seal type, barrier fluid pressure, and shaft alignment. Crystallizing or abrasive products need flushed or double seals.
Vibration
Usually caused by imbalance, worn bearings, or product buildup. Inspect the rotor, bearings, and blades.
Uneven cooling or crystallization
Control jacket temperature and rotor speed. For crystallization, residence time and cooling rate determine crystal size. Adjust for the product, not just the equipment.
Cleaning difficulty
Choose a sanitary design with full drainage, smooth surfaces, and accessible seals. Validate CIP coverage. If the product is hard to clean, manual access may be needed.
Maintenance and Cleaning
An SSHE needs regular inspection. Blades wear. Seals age. Bearings need lubrication or replacement.
Routine checks:
Blade clearance and wear
Seal condition and leakage
Rotor alignment and bearing condition
Jacket fouling and water quality
Product outlet temperature and pressure drop
CIP/SIP effectiveness
For sanitary service, use ASME BPE, 3-A, or EHEDG guidance where applicable. Document cleaning validation. Do not assume a smooth surface alone solves cleaning.
FAQ
What viscosity can a scraped surface heat exchanger handle?
It can handle products from a few thousand cP to pastes and gels that are difficult to pump. The limit depends on rheology, flow, pressure drop, and blade design. Pilot testing is recommended.
Can an SSHE handle particulates?
Yes, but blade clearance and rotor speed must be selected to avoid crushing or damaging particles. Large or fragile particles may need a different design.
Is a scraped surface heat exchanger good for crystallization?
Yes. It is widely used for cooling and crystallizing fats, waxes, chocolate, ice cream, and chemical products. Scraping keeps crystals from building on the wall.
How do I size an SSHE?
Use viscosity curves, flow rate, temperatures, heat load, residence time, and fouling data. Do not size by area alone. A pilot test gives the most reliable result.
Why not use a plate heat exchanger instead?
Plate heat exchangers are excellent for clean, low-viscosity fluids. They struggle with high viscosity, fouling, and crystallization. An SSHE handles those conditions better.
Can an SSHE be CIP and SIP cleaned?
Many sanitary models can. It depends on drainage, surface finish, seals, and validation. Confirm cleaning requirements before purchase.
What causes fouling in an SSHE?
High surface temperature, low scraper speed, product burn-on, crystallization, and poor flow. Correct operation and maintenance reduce fouling.
Bottom Line
A scraped surface heat exchanger is the practical answer for high-viscosity heating, cooling, and crystallization. It keeps the heat transfer surface clean, maintains product quality, and handles products that would foul or plug ordinary heat exchangers.
But selection must be based on real rheology, flow, temperature, residence time, and cleaning requirements. There is no universal size or speed. The right design comes from the product, not a catalog.
If you are selecting a scraped surface heat exchanger for a viscous product, send your viscosity curve, flow rate, inlet and outlet temperatures, heat load, solids content, and CIP/SIP requirements. We will help you choose the right rotor, blade, seal, and tube design for your process.