Scraped surface heat exchangers

Scraped surface heat exchangers are used for processes likely to result in the substantial deposition of suspended solids on the heat transfer surface. In has been used for many years, and still are used, in aseptic processing systems to heat and cool products.

In principle, a scraped surface heat exchanger is a monotube equipped with a rotating internal scraper. The scraper keeps the heating surface free from any deposits and also promotes turbulence.

Scraped surface heat exchangers can be employed in the continuous, closed processing virtually any pumpable fluid involving cooking, slush freezing, cooling, crystallizing, mixing, plasticizing, gelling, polymerizing heating, aseptic processing, etc.

Use of a scraped surface heat exchanger prevents the accumulation of significant buildup of solid deposits. Unlike tubes that usually operate with water as medium, scrape surface heat exchangers tend to use steam for greater heat transfer efficiency.

Due to the mechanical construction and the moving parts, scraped surface heat exchangers are more expensive than any other heat exchanger use in food processing. They can be justified only for heating/cooling fluid foods that cannot be handled in normal tube or plate heat exchangers.
Scraped surface heat exchangers

Shell and tube heat exchanger

Heat exchangers are designed for a rated or nominal condition. The most commonly used heat exchanger is the shell and tube type. It is the ‘workhorse’ of industry. They are rugged, reliable, simple in construction and available in such exotic alloys as Hastelloy, titanium and zirconium as well as all the common metals of construction.

It has many applications on the power generation, petroleum refinery, chemical industries, and process industries. They are used as steam generator, oil cooler, condenser, feedwater heater, and in single phase processes.

Shell-and-tube heat exchangers are built of round tubes mounted in large cylindrical shells with tube axis parallel to that of the shell.

The major components of a shell and tube exchanger are tubes, baffles, shell, front head, rear head and nozzles. Expansion joint is an important component in the case of fixed tubesheet exchanger for certain design conditions.

In the simplest form of a horizontal shell-and-tube type: one fluid stream flows through the tubes while the other flows on the shell side across or along the tubes.
Shell and tube heat exchanger

Tubular heat exchangers

The simplest heat exchanger is the double pipe type which consists of two centric pipes with the cold stream flowing usually in the inner tube and the heating medium in the annulus. It operates in either co-current or counter-current flow.

The concentric tube heat exchanger is the most common type of the tubular heat exchanger. The hot liquid, or steam, flows through the outer jacket and the cold liquid flows through the inner tube.

The shell and tube heat exchangers are less expensive than other types of food heat exchangers especially when high loads are transferred. They can be operating at higher temperature and pressures (e.g. steam at 6 bar and 160 ° C) than plate heat exchangers. Shell and tube exchangers are tubular consisting of bundles of parallel tubes inside a larger cylindrical jacket (shell).

In a type of tubular exchanger known as a Joule effect heater, the tube wall is electrically heated. The length of most commercial tubular system has been standardized at six meters; therefore, tubular heat exchangers are long and thin in terms of their geometry.

Most tabular heat exchanger now use corrugations on the shell and tubes to enhance heat transfer with the heating and cooling media, typically water. Tubular heat exchangers are suitable for heating or cooling highly viscous products and where relatively high pressure must be applied.

They are therefore utilized for the bulk in-flow sterilization of products containing solid particles or for the heat treatment of cooling of tomato paste prior to aseptic packaging.

For fruit juices with fibers of up to 15 mm length and for relatively water-like foods, a multitube tubular heat exchanger is preferably used. Also fluid of moderate to high viscosity with only small particulates will flow through a multitube heat exchanger without problem.
Tubular heat exchangers

Steam infusion heat exchanger

Direct heating involves direct contact between the food and the heating medium by steam injection or steam infusion.

Steam injection introduced steam into the product of an injection chamber as product is pumped through the chamber, while steam infusion introduces product through a steam-filled infusion chamber.

A steam infusion heat exchanger provides a direct contact between steam and the product. A product in liquid state is pumped to the top of the heat exchanger and then allowed to flow in thin sheets in the heating chamber.

It is heated to 142 – 146 ° C in 0.3 seconds and is held for 3 seconds in a holding tube before flash cooling in a vacuum chamber to 65 -70 ° C.

The flash cooler has dual purposes. First, it reduces the sterilization temperature in a very short time and second, it removes water from the product.

The viscosity of the liquid determines the size of the spreader. Products containing particulates, such as diced vegetables, meat chunks and rice can be handled by specially designed spreaders.

High rates of heat transfer are archived when steam contacts tiny droplets of the food. The temperature of the product rises rapidly due to steam condensation.

Steam infusion has advantages over injection methods because the liquid does not contact hotter surfaces and burning-on is therefore reduced.
Steam infusion heat exchanger