How to Choose a Scraped Surface Heat Exchanger

Abstract: The Scraped Surface Heat Exchanger (SSHE) is an indispensable core piece of equipment in the industrial production of peanut butter. Peanut butter, as a typical high‑viscosity, oil‑containing semi‑solid food, is prone to fouling on heat-transfer surfaces during cooling and crystallization, leading to a drop in heat-exchange efficiency and deterioration of product quality. The scraped-surface heat exchanger successfully addresses this industry problem through its unique mechanical scraping mechanism. This article systematically describes the basic structure, the functions of its core components, the working principle, and the key role of the scraped surface heat exchanger in peanut butter production. Combined with technical solutions from mainstream equipment manufacturers, it deeply analyzes how this equipment ensures the stability and consistency of peanut butter, providing a comprehensive technical reference for food engineering technicians.

Keywords: Scraped surface heat exchanger; Peanut butter; Heat exchange equipment; Crystallization; High‑viscosity fluid

I. Introduction

Peanut butter is a food loved by consumers worldwide. Its smooth texture and stable spreadability result from precise processing. During peanut butter production, the ground peanut butter slurry has a relatively high temperature (typically 60–80 °C) and must be rapidly cooled to partially crystallize its fat, forming a stable semi‑solid structure and preventing oil separation. However, peanut butter has extremely high viscosity — at room temperature, the viscosity of peanut butter is more than five times that of ketchup, and it is almost semi‑solid under low‑temperature conditions. This characteristic makes traditional heat exchange equipment (such as shell‑and‑tube or plate heat exchangers) face serious challenges in peanut butter cooling: high‑viscosity fluids flow slowly, the heat transfer coefficient is extremely low, and they are very prone to forming a scorched film or crystalline layer on the heat transfer surface, causing a sharp drop in heat exchange efficiency.

The scraped surface heat exchanger is a specialized device designed to solve this problem. By continuously scraping the heat-transfer surface with rotating blades, it both enhances heat transfer and prevents material fouling, making it the “heart equipment” for processing high‑viscosity foods such as peanut butter, margarine, and chocolate. This article will provide a detailed analysis of the structure and operating principle of this key piece of equipment.

II. Basic structure of the scraped surface heat exchanger

The structural design of a scraped surface heat exchanger centers on a core goal: achieving efficient, uniform heat transfer as the high‑viscosity material flows continuously. Its typical structure consists of several precisely designed subsystems.

2.1 Heat transfer cylinder

The heat transfer cylinder is the core component of the scraped surface heat exchanger, usually adopting a double‑tube structure:

• Inner tube (heat-transfer tube): the material flows through it. The tube material must combine excellent thermal conductivity and corrosion resistance. High‑end equipment uses AISI 316 stainless steel or higher-grade stainless steel to ensure food contact safety. Some designs use multi‑layer corrugated cooling tubes, which can increase heat transfer and prevent the internal accumulation of compressor oil.
• Outer tube (jacket): an annular layer is formed between the outer and inner tubes, through which the heat exchange medium flows. Depending on the process requirements, the jacket can be fed with cooling medium (e.g., ammonia, freon, CO₂, or cold water) or heating medium (e.g., steam, hot water).

The diameter and length of the inner tube are designed to match the processing capacity. Common heat transfer area specifications include 0.4, 0.5, 0.6, 0.7, 0.8, 1.0, 1.2, and 2.8 m², among others. The annular gap is typically 10 mm or 20 mm.

2.2 Rotor system

The rotor system is the “actuator” of the scraped-surface heat exchanger, installed at the center of the inner tube and driven by a motor. The design of the rotor system directly affects heat transfer efficiency and scraping effect:

• Shaft: the core component transmitting torque, made of high‑strength alloy steel. For high‑viscosity materials such as peanut butter, some heavy‑duty equipment (e.g., HRS RHD series) uses larger, strengthened scraper bars and is equipped with heavy‑duty bearings and lip seals to withstand increased torque.
• Scrapers (blades), mounted on the rotor, are the parts that directly contact the heat-transfer surface. Depending on the application, scraper materials can be metal or engineering plastic. In peanut butter cooling applications, scrapers need good wear resistance and appropriate elasticity.
• Scraper fixing mechanism: common “Bulldog” rotor systems use centrifugal force to press the scrapers against the cooling tube, ensuring close contact with the heat transfer surface regardless of changes in material viscosity.

2.3 Drive and transmission system

The drive system provides power for rotor rotation, including motor, reducer, and transmission:

• Motor: for high‑viscosity materials like peanut butter, the motor must have sufficient power. For example, the HRS standard R series is equipped with a 4 kW motor, while the heavy‑duty RHD series developed for peanut butter increases the motor power to 7.5 kW. SPX FLOW equipment uses low‑energy motors and cast‑iron gearboxes to generate maximum power while keeping noise levels to a minimum.
• Speed regulation mechanism: rotor speed can be adjusted according to material viscosity and process requirements. Common speed ranges are 0–350 r/min, and some models support wide‑range speed adjustment from 25 r/min to over 1000 r/min. Each cooling tube can be equipped with an independent motor, allowing separate control of different rotor shaft speeds.

2.4 Sealing and connection system

Because peanut butter production requires strict hygienic conditions, the sealing system is critical:

• Shaft seal: single or double mechanical seals can be used to prevent material leakage and external contamination.
• Connection ports: the material inlet often adopts a tangential design, enabling the material to form a swirling flow in the annular channel, thereby enhancing heat transfer.
• CIP cleaning interface: the equipment supports cleaning‑in‑place (CIP). The sealing system allows efficient cleaning cycles without disassembling the rotor, minimizing downtime.

2.5 Shell and insulation

The equipment shell is a fully sealed, fully insulated, corrosion‑resistant stainless steel structure, ensuring years of trouble‑free operation while reducing energy loss.

Table 1: Typical models and technical parameters of scraped surface heat exchangers

Series / Model	Heat transfer area (m²)	Motor power	Annular gap	Speed range	Features
HRS R series	0.4 / 0.5 / 0.6 / 0.8 / 1.0 / 1.2 / 2.8	4 kW	10 mm / 20 mm	0–350 r/min	Standard
HRS RHD series (heavy duty)	similar	7.5 kW	10 mm / 20 mm	0–350 r/min	Reinforced bars, heavy bearings, for peanut butter
SPX FLOW Votator II	various	low‑energy motor	–	25–1000+ r/min (adjustable)	Independent speed per tube, modular

III. Working principle of the scraped surface heat exchanger

The working principle of a scraped surface heat exchanger can be summarized as the synergistic action of three core processes: forced heat transfer, mechanical scraping, and continuous renewal.

3.1 Material flow and heat exchange process

The peanut butter production process typically includes the following stages:

1. Feeding: hot ground peanut butter (approx. 60–80 °C) is continuously pumped by a feed pump into the material inlet of the scraped surface heat exchanger. The inlet is tangentially designed so that the material enters the annular channel in a swirling manner.
2. Annular space flow: the material flows in the annular gap between the inner wall of the heat transfer tube and the outer surface of the rotor. This annular gap is usually only 10–20 mm, ensuring the material forms a thin layer conducive to rapid heat transfer.
3. Counter‑current heat-exchange medium: the heating or cooling medium flows counter‑currently in the jacket, forming the maximum temperature gradient with the material to maximize heat-conduction efficiency. For cooling peanut butter, common cooling media include ammonia, freon, CO₂, or chilled water.
4. Crystallization process: When the temperature of the peanut butter drops below the melting point of its fat, the oils begin to crystallize. This crystallization process determines the final texture and stability of the product.

3.2 Scraping mechanism

The scraping mechanism is the core feature that distinguishes the scraped surface heat exchanger from other heat exchange equipment:

1. Centrifugal force action: as the rotor rotates, the scrapers mounted on it are pushed radially outward by centrifugal force, pressing tightly against the inner wall of the heat transfer tube. This design ensures that, regardless of changes in material viscosity, the scrapers maintain close contact with the heat-transfer surface.
2. Continuous scraping: as the rotor turns, the scrapers sweep over the heat-transfer surface, removing the layer of material that has just cooled and may begin to crystallize or adhere. This process achieves two important functions: preventing fouling on the heat-transfer surface and maintaining high heat-exchange efficiency; and mixing the cooled material with the bulk material, thereby promoting temperature uniformity.
3. Boundary layer renewal: the scraping action continuously disrupts the thermal boundary layer on the heat transfer surface, exposing fresh material to the heat transfer surface and greatly increasing the heat transfer coefficient.

3.3 Temperature control and regulation

Modern scraped surface heat exchangers are equipped with precise temperature control systems:

• Independent temperature control circuits: e.g., the Kombinator 250S has automatic temperature-adjustment devices for each cooling tube, allowing accurate regulation of cooling temperature.
• Variable speed adjustment: rotor speed can be adjusted based on product viscosity and process requirements to achieve optimal heat transfer.
• Multi‑stage cooling: By connecting multiple heat-exchange units in series, multi‑stage temperature control can be achieved, meeting the process requirements for the fractional crystallization of peanut butter fat.

3.4 Three‑stage process

In peanut butter production, the scraped surface heat exchanger accomplishes a three‑stage process:

1. Cooling stage: gently lowers the temperature of the peanut butter while maintaining optimal quality. The cooling rate must be precisely controlled; too fast may cause insufficient crystallization, and too slow may affect production efficiency.
2. Control stage: precisely manages temperature and viscosity to achieve ideal consistency. By adjusting the cooling rate and scraping intensity, the morphology and size of fat crystals are controlled.
3. Thickening stage: as the temperature drops, the viscosity of peanut butter gradually increases, forming a stable semi‑solid structure. Continuous scraping ensures uniform thickening throughout the material system, preventing local over‑thickening or clumping.

IV. Key technical characteristics of scraped surface heat exchangers

4.1 Adaptability to high‑viscosity materials

The high viscosity of peanut butter places special demands on heat-exchange equipment. Studies show that the viscosity of peanut butter at room temperature is more than five times that of ketchup, and it is almost semi‑solid at low temperatures. Adding stabilizers (such as partially hydrogenated vegetable oil) can prevent oil separation, but it forms a crystalline structure that further increases viscosity.

Scraped surface heat exchangers address this challenge through the following designs:

• Powerful drive system: e.g., the HRS RHD series increases motor power from 4 kW to 7.5 kW.
• Reinforced scraper bars and bearings: heavy‑duty bearings and lip seals to withstand increased torque.
• Additional support structures: added supports at the scraper ends and the motor end to handle additional weight and forces.

4.2 Hygienic design

Food‑grade hygienic design is an important feature of scraped surface heat exchangers:

• Material selection: all product‑contact parts are made of AISI 316 stainless steel or higher-grade stainless steel, meeting food-industry hygiene standards.
• Surface finish: contact surfaces are smooth and free of dead corners, preventing material residue and microbial growth.
• CIP capability: supports cleaning‑in‑place, allowing efficient cleaning cycles without disassembling the equipment.
• Sealing performance: fully sealed stainless steel shell prevents external contamination.

4.3 Heat transfer enhancement technologies

Advanced heat transfer designs improve equipment efficiency:

• Multi‑layer corrugated cooling tubes: specially designed to increase heat transfer and prevent internal accumulation of compressor oil.
• CO₂ refrigeration technology: when using CO₂ as the refrigerant, the external heat transfer coefficient is 2–3 times higher than that of ammonia, and the total heat flux increases by up to 40%.
• Heat recovery systems: with CO₂ refrigerant, a heat recovery system can be installed to provide up to 90 °C usable water, recovering up to 70% of energy.

4.4 Modularity and scalability

Modern scraped surface heat exchangers adopt a modular design for easy capacity expansion:

• Multi‑tube configurations: e.g., the R3 multi‑tube option significantly saves floor space and operating costs compared to a single-tube system.
• Parallel expansion: the practice of an Argentine peanut butter producer shows that adding a single Votator II heat exchanger to a modular line can increase capacity from 1000 kg/h to 2000 kg/h.
• Replaceable cooling tubes: easy to inspect and replace, facilitating maintenance and upgrades.

V. Application of scraped surface heat exchangers in peanut butter production

5.1 Typical application process

In industrial peanut butter production, scraped surface heat exchangers are typically integrated into a continuous production line:

Peanut kernels → Roasting → Skin removal → Coarse grinding → Fine grinding → (Ingredient mixing) → Scraped surface heat exchanger cooling → Homogenizing/kneading → Filling

Multiple A‑unit series devices can be connected in series to achieve seamless integration of preheating, sterilization, and cooling, followed by aseptic packaging.

5.2 Application benefits

According to SPX FLOW application data from the Argentine peanut butter industry, after adopting the Votator II scraped-surface heat exchanger, most producers’ output increased by an average of more than 1,000 kg/h. Specific benefits include:

• Capacity increase: modular design supports scaling from 1000 kg/h to 2000 kg/h.
• Quality assurance: precise temperature control and gentle scraping treatment maintain product flavor, color, and texture.
• Enhanced stability: By controlling the cooling rate and crystallization process, the product does not separate or oil off during storage.
• Continuous production: supports 24/7 continuous operation, meeting large‑scale industrial production needs.

5.3 Application precautions

When using scraped surface heat exchangers in peanut butter production, attention should be paid to:

• Material pre‑treatment: ensure uniform grinding fineness to avoid large particles damaging the scrapers.
• Temperature control: precisely control the cooling rate to prevent rapid crystallization, causing rough texture.
• Cleaning and maintenance: Perform regular CIP cleaning to prevent residual material from spoiling.
• Scraper wear monitoring: regularly check scraper wear and replace them on time to ensure effective scraping.

VI. Technology development trends

6.1 Application of environmentally friendly refrigerants

As environmental regulations become stricter, natural refrigerants such as CO₂ are increasingly used in scraped surface heat exchangers. The Nexus series was among the first to adopt the environmentally friendly CO₂ refrigerant, reducing energy consumption by 30% at -10 °C compared to traditional ammonia systems and offering lower investment costs.

6.2 Intelligent control

Modern scraped surface heat exchangers integrate advanced control systems:

• Independent temperature control for each cooling tube, enabling precise temperature curve management.
• Variable frequency speed regulation automatically adjusts rotor speed according to material viscosity.
• Data acquisition and remote monitoring, supporting Industry 4.0 applications.

6.3 Heavy‑duty design trends

For extremely high‑viscosity materials such as peanut butter, equipment manufacturers continually strengthen their mechanical design. The HRS RHD series represents this trend: larger motors, reinforced scraper bars, heavy‑duty bearings, and additional supports ensure reliability under extreme conditions.

VII. Conclusion

The scraped-surface heat exchanger is an indispensable piece of equipment in the industrial production of peanut butter. Its unique structural design and mechanical scraping mechanism effectively address the fouling problem in heat exchange with high‑viscosity materials. Through the precise coordination of the heat transfer cylinder, rotor system, drive unit, and sealing system, the scraped surface heat exchanger precisely controls the peanut butter cooling and crystallization process, ensuring stability of the final product and uniform consistency.

From SPX FLOW’s Perfector and Kombinator series to HRS’s RHD heavy‑duty series, equipment manufacturers continuously innovate, optimizing material selection, heat-transfer enhancement, energy savings, and environmental protection. Modular design and scalability enable scraped-surface heat exchangers to adapt to production needs across different scales, providing solid technical support for the large‑scale, standardized development of the peanut butter industry.

For food engineering technicians, a deep understanding of the structure and operating principle of scraped-surface heat exchangers helps make more informed decisions in equipment selection, process optimization, and fault diagnosis, ultimately improving both product quality and production efficiency.