How to Clean a Scraped Surface Heat Exchanger

Abstract: The scraped surface heat exchanger is the core equipment in the industrial production of peanut butter; its operating condition directly affects product quality and production efficiency. Due to the characteristics of peanut butter, such as high viscosity, high oil content, and easy coking, long-term operation of the equipment can lead to problems like decreased heat transfer efficiency, scraper wear, and seal failure. This article systematically elaborates on the key points of daily maintenance and care for scraped-surface heat exchangers, covering daily inspection, cleaning and maintenance, key component upkeep, and shutdown protection. Combined with the actual needs of food processing enterprises, it provides practical operating guidelines and maintenance cycle recommendations to help companies extend equipment life, ensure product quality, and reduce operating costs.

Keywords: Scraped surface heat exchanger; Peanut butter; Equipment maintenance; Cleaning and care; Seals

I. Introduction

In the peanut butter production process, the scraped-surface heat exchanger performs the critical tasks of cooling and crystallization. The ground peanut butter slurry has a relatively high temperature (typically 60-80°C) and needs rapid cooling to partially crystallize the fat, forming a stable semi-solid structure. However, the high viscosity characteristics of peanut butter (viscosity can exceed 50,000 cP at room temperature) and its high oil content pose many challenges during long-term equipment operation: fouling on the heat transfer surface, scraper wear, seal aging, and other issues gradually emerge, leading to decreased heat exchange efficiency, unstable product quality, and even equipment failure.

As the saying goes, “A small leak will sink a great ship.” In the field of food processing, especially in ensuring that heat treatment equipment operates optimally, proactive maintenance is particularly important. A tiny crack or leak could disrupt the sterile environment, cause product loss, and jeopardize the brand’s reputation. Equipment failure causes production interruptions, resulting in unnecessary downtime and economic losses. Therefore, establishing a scientific, standardized daily maintenance system is of paramount importance to ensuring the continuity, stability, and safety of peanut butter production.

This article will provide a comprehensive set of maintenance and care guidelines for scraped-surface heat exchangers from a practical operational perspective for peanut butter manufacturers.

II. Daily Inspection: The Foundation of Preventive Maintenance

2.1 Monitoring Operating Parameters

Daily inspection is a fundamental part of maintaining the scraped-surface heat exchanger, requiring regular, comprehensive observation of the equipment’s operating status. It is recommended to record and analyze operating parameters at least once per shift (8 hours). Focus on monitoring the following parameters:

• Inlet-Outlet Pressure Difference: A sudden increase in pressure difference usually indicates internal channel blockage or severe fouling; a decrease in pressure difference may suggest leakage or bypass.
• Temperature Changes: Record the inlet and outlet temperatures of the product and the heat-transfer medium, and calculate the heat-exchange efficiency. A decrease in the temperature difference indicates a decline in heat transfer performance.
• Motor Current: Abnormal fluctuations in the main shaft motor current may reflect scraper jamming, bearing damage, or changes in product viscosity.
• Vibration and Noise: Abnormal vibration or noise is often a precursor to mechanical failure.

2.2 Visual Inspection

Perform a visual inspection of the equipment daily:

• Check all connection points for leaks, especially at the shaft seal and flange connections.
• Observe whether instruments such as pressure gauges and thermometers are functioning correctly.
• Check if the lubricating oil level is normal and if there are any leaks.
• Listen to the equipment during operation to detect any abnormal friction or impact sounds.

2.3 Establish Inspection Records

Create an equipment operation log that records detailed data from each inspection and any issues found. This not only helps track trends in equipment performance changes but also provides a basis for fault diagnosis and maintenance planning.

III. Periodic Maintenance of Key Components

3.1 Scraper System Maintenance

The scraper is the core executing component of the scraped-surface heat exchanger, directly contacting the heat-transfer surface and bearing the dual forces of product pressure and centrifugal force. It is a wear part.

Scraper Inspection Interval: It is recommended to check the scraper wear condition every 500 operating hours or monthly.

Inspection Points:

• Wear Level: Measure the scraper’s thickness. If wear exceeds 1/3 of the original thickness, replacement should be considered.
• Edge Integrity: Check the scraper edge for chips, cracks, or deformation.
• Elasticity Status: For scrapers using spring pre-tension, check if the springs are fatigued or failed.

Replacement Precautions: When replacing scrapers, choose original manufacturer parts or products of equivalent quality. During installation, ensure the scraper can float freely on the holder—neither too tight nor too loose.

3.2 Seal System Maintenance

The seal system is critical for preventing product leakage and external contamination. For food-grade applications, scraped-surface heat exchangers typically use steam- or aseptic water-type shaft seals.

Seal Inspection Interval: It is recommended to check the seal condition every 3 months.

Inspection Points:

• Shaft Seal Leakage: Observe if there are traces of product seepage at the shaft seal.
• Seal Face Wear: Regularly check if the sealing surface is smooth and flat, without scratches or grooves.
• Aging Condition: Rubber seals tend to harden, become brittle, or crack at high temperatures over prolonged periods.

Replacement Criteria:

• Gaskets should generally be preventively replaced after 1-2 years of use.
• Immediately replace if hardening, cracking, or poor compression rebound is observed.
• When replacing, ensure the new seal fits the groove precisely and tighten to the specified torque.

3.3 Bearing and Drive System Maintenance

Lubrication Care:

• Regularly add or replace lubricating grease according to the equipment manual’s instructions.
• Be sure to use food-grade lubricants to avoid contamination risks.
• Over-lubrication can, conversely, cause bearing overheating.

Inspection Points:

• Bearing Temperature: Bearing temperature should be stable during normal operation; abnormal increases require attention.
• Operating Sound: Use a listening rod or electronic stethoscope to check for unusual noises from the bearings.

3.4 Fastener Inspection

During long-term operation, connecting bolts may loosen due to temperature changes and vibration. It is recommended to check monthly:

• Check if the tightening bolts are loose; if so, tighten them evenly.
• When tightening, ensure it is parallel to the pressure plates on both sides to avoid uneven loading that could lead to seal failure.
• Note: It is not recommended to perform clamping operations while the equipment is operating at high temperature.

IV. Cleaning and Maintenance: The Core of Ensuring Heat Exchange Efficiency

During heat exchange, fat components and proteins in peanut butter tend to form a fouling layer on the heat-transfer surface, severely degrading heat-transfer efficiency. Regular cleaning is the most effective means of restoring equipment performance.

4.1 Determining Cleaning Frequency

Cleaning frequency depends on production intensity, raw material quality, and equipment operating conditions. General recommendations:

• Light production: Clean every 2-3 months.
• Continuous high-intensity production: Clean monthly.
• Based on pressure difference: When the inlet-outlet pressure difference rises by more than 30% compared to the initial value, cleaning should be carried out promptly.

4.2 Cleaning Method Selection

Depending on the degree of fouling and product characteristics, different cleaning methods can be chosen:

1. CIP (Clean-in-Place)

For scraped surface heat exchangers, CIP is the most common cleaning method:

1. Alkaline wash: Use a 1-2% sodium hydroxide solution at 70-80°C for 30-60 minutes to effectively remove fats and proteins.
2. Acid wash: For mineral fouling, use 0.5-1% nitric acid or phosphoric acid solution.
3. Water rinse: After each chemical cleaning, thoroughly rinse with clean water to prevent chemical residues from affecting product quality.
4. Dismantling and Manual Cleaning

When fouling is severe or CIP is ineffective, the equipment needs to be disassembled for manual cleaning:

2. Preparation before disassembly: Measure and record the compressed length of the plate pack, and mark it.
3. Cleaning tools: Use soft fiber brushes, bristle brushes, or high-pressure water jets (0.1-0.2 MPa).
4. Prohibited actions: Never use wire brushes or steel wool, especially on stainless steel plates, to avoid accelerating corrosion.
5. Stubborn stains: For difficult deposits, cleaning powder can be used.
6. Chemical Cleaning (without dismantling)

For harder deposits (oxides or carbides), chemical cleaning can be adopted:

3. Choose the appropriate cleaning agent based on the plate material.
4. For stainless steel, a formic acid solution can be used, with buffering agents and surfactants added.
5. The pickling temperature should be controlled around 60°C.
6. First, soak statically for 2 hours, then circulate dynamically for 3-4 hours.
7. After pickling, passivation must be performed to prevent corrosion of the metal surface.

4.3 Cleaning Precautions

• Water used for cleaning must be softened or deionized water to avoid introducing new minerals.
• After cleaning, carefully inspect the plates to confirm no damage before reassembly.
• During reassembly, ensure the plates are installed in the correct sequence and orientation.
• Tighten the clamping bolts evenly, symmetrically, and crosswise to the specified dimension.

V. Protective Measures During Shutdown

5.1 Short-term Shutdown (1-7 days)

• Drain the product and media from the equipment.
• Rinse the interior of the equipment with clean water to prevent residual product from drying and hardening.
• Keep the equipment ventilated to avoid a humid environment.

5.2 Long-term Shutdown (over 7 days)

When not in use for an extended period, take the following protective measures:

• Drain media: Thoroughly drain all media from inside the heat exchanger.
• Anti-corrosion treatment: Fill with dry nitrogen or add anti-freeze/anti-corrosion fluid to prevent internal corrosion.
• Loosen bolts: Appropriately loosen the clamping bolts (by about 5-10 mm) to relieve long-term pressure on the gaskets and extend their service life.
• Environmental requirements: Store in a dry, well-ventilated place, away from direct sunlight and humid environments.
• Regular checks: Inspect the equipment status monthly to prevent accidental damage.

5.3 Preparation Before Restarting

• Check the condition of all seals; replace if necessary.
• Retighten the bolts to the required dimension per specifications.
• Conduct a hydrostatic test at 1.25 times the operating pressure, hold for 30 minutes.
• For food-grade applications, perform cleaning and disinfection.

VI. Fault Diagnosis and Common Problem Handling

6.1 Decreased Heat Exchange Efficiency

Possible Causes:

• Fouling on the heat transfer surface → Action: Clean promptly.
• Severe scraper wear → Action: Replace scrapers.
• Abnormal product flow or temperature → Action: Adjust process parameters.

6.2 Leakage Issues

Possible Causes:

• Aged or damaged gasket → Action: Replace the seal.
• Loose clamping bolts → Action: Tighten bolts evenly.
• Plate corrosion or cracks → Action: Inspect and replace damaged plates.

Note: If medium is found leaking from the signal hole, first analyze the cause. If it is due to loose bolts, appropriate tightening may help; if the gasket is aged, replacement is necessary.

6.3 Abnormal Vibration or Noise

Possible Causes:

• Bearing damage → Action: Replace bearings.
• Scraper jamming or detachment → Action: Check the scraper system.
• Unstable product flow → Action: Check the feed system.

6.4 Motor Overload

Possible Causes:

• Excessively high product viscosity → Action: Check raw material quality or preheating temperature.
• Increased resistance due to excessive scraper wear → Action: Replace scrapers.
• Poor bearing lubrication → Action: Replenish lubricating grease.

VII. Maintenance Plan and Record Management

7.1 Recommended Maintenance Schedule

Table 1 Recommended Maintenance Schedule for Scraped Surface Heat Exchanger

Component/Item	Maintenance Activity	Frequency
Scraper System	Check wear, edge integrity	Every 500h / Monthly
Seal System	Check for leaks, aging, wear; replace if necessary	Every 3 months (check); Replace every 1-2 years
Bearings / Transmission	Remove fouling from the heat transfer surface	Monthly (check); Lubricate per manual
Fasteners (Bolts)	Check tightness, re-tighten if needed	Monthly
Cleaning (CIP/Manual)	Drain, loosen bolts, anti-corrosion, and environment control	Every 1-3 months or based on ΔP
Short-term Shutdown	Drain, rinse, ventilate	As applicable
Long-term Shutdown	Drain, loosen bolts, anti-corrosion, environment control	Before and during prolonged inactivity

7.2 Contents of Maintenance Records

Establish a detailed equipment maintenance file, recording the following information:

• Maintenance date and content.
• Model and quantity of replaced parts.
• Abnormalities found and corrective actions taken.
• Recommendations for next maintenance.
• Operator’s signature.

Complete maintenance records help track equipment performance trends and provide a basis for preventive maintenance.

VIII. Conclusion

The scraped-surface heat exchanger, as the core equipment in peanut butter production, has its maintenance directly related to product quality, production efficiency, and the enterprise’s economic benefits. By establishing a scientific daily inspection mechanism, standardized cleaning procedures, timely maintenance of key components, and comprehensive shutdown protection measures, equipment service life can be effectively extended, high heat exchange performance maintained, and failure rates and operating costs reduced.

For food processing companies, equipment maintenance should not be viewed as reactive “repair work” but as a proactive means of quality assurance. As Tetra Pak’s maintenance philosophy states: “Through proactive maintenance, the energy consumption of heat exchangers can be minimized, thereby reducing the carbon footprint.” Investing in equipment maintenance is investing in product quality and brand reputation.

It is hoped that the maintenance guidelines provided in this article will assist peanut butter manufacturers in establishing a sound equipment management system, ensuring smooth, efficient production processes, and ultimately achieving a dual enhancement of product quality and production efficiency.