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A heat exchanger is a piece of equipment that continually transfers heat from one medium to another in order to carry process energy. Food grade applications use indirect heat exchangers, those where both media are separated by a wall through which heat is transferred.
The natural laws of physics always allow the driving energy in a system to flow until equilibrium is reached. As long as there is a temperature difference, heat leaves the hottest fluid and will be transferred to the colder fluid.
A heat exchanger uses this principle in its endeavor to reach its desired result.
The theory of heat transfer from one media to another, or from one fluid to another, is determined by several basic rules.
- Heat will always be transferred from a hot product to a cold product.
- There must always be a temperature difference between the products.
- The heat lost by the hot product is equal to the amount of heat gained by the cold product, except for energy losses to the surrounding area
Reasons for SelectionProcessor or Kettle: The cheapest option and the option that requires the least floor space. This utilizes a jacketed tank for heating/cooling for a full range of products. Agitation can be from minimal to surface scraped. This option is used on a batch production basis.
Plate Heat Exchanger (liquid/liquid): The most efficient option and normally the cheapest option. Normally can only handle liquids with less than 7% of pulp and viscosities less than 20,000 cPs.
Tube in Tube Heat Exchanger (liquid/liquid; liquid/steam): An inexpensive option that can handle liquids with high solids (30 to 40%) with maximum size range of particulates at 0.125 to .5” in diameter if pressure conditions allow and high viscosities.
Triple tube (liquid/liquid; liquid/steam): Can utilize regeneration and can handle liquids with low solids (5 to 10%) in suspension.
Shell and Tube Heat Exchanger (liquid/liquid; liquid/steam): Restricted to solutions with low solids contents and viscosities under 10,000 cPs.
Scraped Surface Heat Exchanger (liquid/liquid; liquid/steam ): For high viscosity (up to 100,000 cPs) solids whose contents are greater than 75% of normal and particulates up to 1” maximum. For complex or sensitive products that require gentile processing.
Shell and Tube Heat Exchanger
Shell and tube heat exchangers consist of a bundle of parallel sanitary tubes with the ends expanded in tube sheets. The bundle is contained in a cylindrical shell. Connections are such that the tubes can contain either the product or the media, depending upon the application. The major limitation is that they cannot be used to regenerate, but they can transfer lots of heat due to the surface area.
Tube In Tube Heat Exchanger
The tube in tube heat exchanger, often called a double tube heat exchanger is an all welded heat exchanger. The process fluid passes through the inner tube, while the heating or cooling media goes through the outer tube. Because of the large size of the product tube, these heat exchangers have the ability to process very large particulates. They can handle high pulp products, create low product shear, have a low initial cost, can handle high pressure, and can CIP at the same flow rate as the process piping.
Triple Tube Heat Exchanger
Triple tube heat exchangers are designed with three concentrically mounted tubes. For heat transfer applications, the heating or cooling medium flows through the space between the inside and outside tubes while product travels in the opposite direction through the middle tube.
For sanitary regeneration applications, your heated product flows in one direction through the space in the inside and outside tubes while your cooled product travels in the opposite direction through the middle tube. Compared to traditional indirect regenerators the triple tube requires about one half of the surface area which in turn requires less heat exchangers, less floor space, and faster cooling of the product on the hot side of the system, decreasing heat damage due to shorter times in residence.
Plate heat exchangers have been using direct regeneration for many years for milk pasteurization. Regeneration is an efficient, safe and proven method for processing. A valuable option is using a triple tube system for direct regeneration. A triple tube heat exchanger provides a good amount of surface area as heat transfer takes place on all three tubes.
Advantages of triple tube heat exchangers is that they have the ability to process viscous and high pulp products, and that they are manufactured to sanitary standards on both the product and media side so they can be used as a direct regenerator.
Plate Heat Exchanger
Plate Heat Exchangers consist of a number of very thin corrugated stainless steel heat transfer plates clamped together in a frame. Every second channel is open to the same fluid. Between each pair of plates there is a rubber gasket, which prevents the fluids from mixing and from leaking to the surroundings. Heat is thus transferred from the warm fluid to the colder fluid via the thin stainless steel plate. The corrugations support the plates against differential pressure and create a turbulent flow in the channels. In turn, the turbulent flow provides high heat transfer efficiency, making the plate heat exchanger very compact compared with the traditional shell-and-tube heat exchanger. In most cases the plate type heat exchanger is the most efficient heat exchanger. Generally it offers the best solution to heating and cooling applications since it can better handle the widest pressure and temperature limits.
Advantages of a plate heat exchanger are that they utilize the thinnest material for the heat transfer surface that in turn gives optimum heat transfer, since the heat only has to penetrate thin material. Also, there is a high turbulence in the medium that in turn gives a higher convection, which results in efficient heat transfer between the media. The consequence of this higher heat transfer coefficient per unit area is not only a smaller surface area requirement but also a more efficient process. The high turbulence also gives a self-cleaning effect. Therefore, when compared to the traditional shell and tube heat exchanger, the fouling of the heat transfer surfaces is considerably reduced. This means that the plate heat exchanger can remain in service far longer between cleaning intervals. Since the plate heat exchanger consists of a framework of plates, more plates can easily be added to increase capacity, and the plates can easily be spread apart for cleaning.
Disadvantages of plate heat exchangers are their initial expense, they don't work well under high pressure rates and they are not well suited for processing pulpy products or product with particulates. The corrugated plate causes contact points that are required for rigidity, and "pinch" points are created which allow for retention of the pulp and particulates. This effectively creates an undesirable filter. This limits your ability to process more than one type of product on a single system, such as orange juice with pulp and a clear fruit drink that must contain no pulp. Trying to keep the plate heat exchanger clean before running a new product can prove very difficult, if not impossible.
Agitated Tank Heat Exchanger
A jacketed tank is a tank with an outer jacket designed to contain heating or cooling media. Product is heated or cooled while being mixed, blended or agitated. A dimple jacketed tank utilizes a simple heat transfer element. First, the heating element is created by pressing a dimpled profile into a flat sheet of stainless steel. This dimpled sheet, referred to as an embossing, is then spot welded to the non contact side of a stainless steel tank to create a flow passage for the heating or cooling media. The end result is a fully welded heat transfer element that is extremely thin (approximately ½" overall thickness). The coils can remain exposed, or they can be covered with an insulation material and then covered with a sheet of stainless steel. Jacketed tanks are not thermally efficient and they cannot be used in a continuous operation.
|Processor/Kettle||Plate Heat Exchanger||Tube in Tube||Triple Tube||Shell and Tube||Scraped Surface|
|Inspection for Fouling|
|Inspection for Leakage|
|Inspection for Corrosion|