When process fluids are sticky, fouling, clogging and heat-sensitive, many types of heat exchangers reach their limits. Viscous, sticky, chunky, crystallizing and particulate-laden products in food processing, cosmetics and chemical process industries often require rapid heating or cooling. The very nature of the product prevents optimal heat transfer. A scraped-surface heat exchanger's construction makes it suitable for cooling and heating a wide range of products with difficult consistencies.
The vertical design of the scraped-surface heat exchanger has features that optimize performance and maintenance. Scraped-surface heat exchangers are used in processing operations such as:
- Heating.
- Pasteurizating.
- Slush freezing.
- Sterilizing.
- Cooling.
- Crystallizing.
How It Works
The heat exchanger is designed to provide even temperature distribution and product uniformity. In a vertically installed scraped-surface heat exchanger, product enters the heat exchanger through the port at the bottom of the cylinder. Steam, glycol or water enters the heat exchanger from the top end while refrigerants such as ammonia and Freon enter from the lower end. As it flows upward through the cylinder, the product is agitated continuously and removed from the inside wall of the cylinder by scraping blades. This scraping action removes fouling material from the heat transfer surface and produces mixing and turbulent flow that can be controlled via rotational speed to optimize product quality and heat transfer efficiency. The rotor can be driven by either a top-mounted electric motor (belt and sheave or direct-coupled) or hydraulic motor drive. Either drive can be adjusted for varied rotor speeds, which is important when a number of different products are being processed.At the same time, the heating or cooling media flows in a narrow annular channel between the heat exchange cylinder and the insulated jacket. When liquid media or steam is used, a spiral coil is installed in the annulus. This provides higher heat transfer efficiency.
Preserving the freshness and quality of delicate ingredients is vital in the processing of food products, and all equipment must comply with strict hygienic standards. Scraped-surface heat exchangers have been integrated into many food processing lines.
Benefiting Poultry Processing
Foster Farms, a poultry-processing plant in northern California, decided to abandon the traditional method of cooling mechanically deboned meat with carbon dioxide (CO2). The company's goal was to eliminate uneven cooling. For its new system, Foster Farms chose two scraped-surface heat exchangers that use ammonia as the cooling medium. The result was an end product with higher and more consistent quality."We used to use the traditional method of cooling the mechanically deboned meat with CO2," said Jim Theis, director of operations at Foster Farms. "That method had a lot of disadvantages, including uneven cooling. In practice, this meant that to ensure a consistently low temperature throughout the batch, we had to cool some of the products so drastically that they froze. This led to inconsistent product quality and occasionally to production shutdowns."
Foster Farms processes turkey products. Food temperature increases during the deboning process, so to comply with strict hygienic requirements and prevent the growth of bacteria, the product must be cooled rapidly and efficiently from 55 to 36.5oF (13 to 2.5oC).
The combined capacity of the two scraped-surface heat exchangers is approximately 8,000 lb of product per hour. Theis noted, "Our two scraped-surface heat exchangers have radically reduced processing costs. The investment paid for itself in less than two years. And, at the same time, we have much better control, which means that now we can cool all the deboned meat to exactly the right temperature. This results in a consistently high quality end product."
Rice Pudding Processing
When Procordia Food, Orebro, Sweden, was looking for equipment to expand its rice pudding processing line, it needed a heat exchanger designed to handle viscous products. The process involves first heating the rice pudding without burning and sticking, then cooling it before packaging.During processing, the heat-sensitive rice pudding is heated gently in three heat exchangers, which subject the product to heat as the blades scrape the internal surfaces. Then, the product is cooled in three additional scraped-surface heat exchangers. The units made it possible for Procordia Food to switch from batch operation to continuous operation.
According to Christer Norefors, who is responsible for fruit and berry products at Procordia Food, "The scraped-surface heat exchangers make it possible to operate five shifts continuously and still be able to increase production when needed."
"Maintenance is minimal," said Thomas Wretlund, who works with processing and maintenance at the plant. "We inspect the heat exchangers once a week and sharpen the knives when needed. This is a great advantage when running a continuous operation because we only have time for a couple of hours of planned shutdown per week."
Cooling Sticky Variegates
The marbled sauces in ice cream, known as variegates, present quite a problem for a heat exchanger. Lowering the temperature of the variegates to a specified temperature before they are injected into ice cream is essential. The sauces' viscosity, in combination with the required low temperature, makes this a particularly difficult task. A sauce that is too hot will melt the ice cream; a sauce that is too cold will not swirl. In 1995, one ice cream producer installed two scraped-surface heat exchangers using ammonia as a cooling media after a competitive type failed to perform. Three years later, two more were installed. According to the company, the four scraped-surface heat exchangers allow production with double variegates on two production lines. The plant now produces 20,000 gal per day of these difficult ice cream flavors.Processing Skin Lotion
When Japanese cosmetics manufacturer Shiseido wanted to cool milky skin lotion with 50% brine in its Osaka plant, eight scraped-surfaced heat exchangers were purchased. "The scraped-surface exchangers are reliable and easy to maintain. We have been using them successfully, not only in full production but also in various development projects. As a matter of fact, installation was completed faster than scheduled and the units worked flawlessly right from the start," explained Masaya Hosaka, plant technical director at Shisedo. Subsequent to installing these units, Shiseido has ordered eight additional units for cooling face cream with 45% glycol.The vertical design of the scraped-surface heat exchanger allows it to fit into a 3.6 ft2 footprint, thereby saving valuable floor space. One worker is all that is needed to raise and lower the rotor, simplifying assembly and maintenance and minimizing downtime.
As the food processing sector expands, processing sensitive products with delicate ingredients is expected to rapidly grow. One solution to handling sticky, fouling and clogging products is selecting a scraped-surface heat exchanger dimensioned to perform in this demanding environment. PCE
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