Heatexchanger Systems And Principles Of Operation

Heat is transferred to or from a food in batch and continuous systems. Batch systems involve unsteady-state heat transfer whereby the food being heated or cooled begins at a given temperature and increases or decreases until the desired temperature is reached. The heat-transfer medium can vary in temperature (eg, a hot surface or liquid that changes temperature as it gives or receives heat) or can be at steady state (eg, condensing steam).

Batch steady-state systems involve a series of batch systems that give the overall effect of steady state processing. For example, liquid-filled tanks in series can be stepwise heated by batch heating but can be connected so that the end result of the overall heating is a steady-state emission of constant-temperature liquid continuously flowing at the same rate as the cold liquid entering the first tank.

A true steady-state system is found in flowing liquids or viscous solids whereby mass flow rate, temperature, pressure, and physical properties of the food and the heat transfer medium are constant at any given cross section.

Heating and Cooling Liquids

Batch Heating. Typical batch heating of a liquid food takes place in a steam-jacketed kettle. While the food is being heated, the system is under unsteady-state conditions. After the food reaches the desired processing temperature and is heated at a constant temperature, steady-state conditions are reached. Assuming that the final temperature of the food is to be maintained somewhat below that of the condensing steam, steady-state conditions prevail as the steam flow rate is adjusted to maintain the processing conditions. As is the practice for all commercial heat-exchanger equipment, the outside of the kettle is lagged to minimize heat loss to the surroundings and isolate the environment of the unit operation.

Large kettles (tanks) being used for heating a liquid often have coils in the tank to transfer heat. This greatly improves the heat-transfer efficiency by increasing the heat-transfer surface above that of the outside wall receiving heat from the jacketed heating (or cooling) source. An additional improvement of the heat transfer in tanks can be realized by installing mechanical stirring equipment, which increases heat transfer above that of natural convection.

Tubular Heat Exchangers. The simplest continuous heat exchange occurs when two fluids of different temperature are flowing through concentric pipes or tubes. The flow in steady-state heat exchangers can be either cocurrent or cocurrent (parallel) flow. During countercurrent flow, one stream (liquid or vapor) is introduced at the opposite end of the unit. By controlling the flow rates, it is possible to heat the cold liquid above the outlet temperature of the entering hot stream. Conversely, when the two liquids are introduced at the same point, the stream being heated can never leave at a temperature above that of the stream being cooled. This cocurrent system is normally less efficient than a countercurrent system because the temperature difference driving force can become quite small as the temperatures of the two streams meet. However, there are circumstances whereby cocurrent flow can be used to ensure that a heat-sensitive material does not rise above a certain temperature during processing. In the case of using steam to heat a flowing liquid, the food is heated while the condensing steam is maintained at the saturation temperature of the steam. As in the case of a steam kettle cooker, the most common and efficient heating medium is condensing steam. Shell-and-tube heat exchangers are essentially improved tubular heat exchangers where a few to many tubes replace the single concentric inner tube.

Whereas the heat transfer coefficient is constant under these steady-state conditions, the temperature driving force (AT) is calculated as the log mean temperature difference

and the steady-state heat transfer is calculated as

where Ts = temperature of condensing steam, Tf = temperature of liquid food being heated, TJ = overall heat transfer coefficient, and ATU = log mean temperature driving force.

Plate Heat Exchangers. Plate heat exchangers solve one of the principal processing problems encountered with tubular and shell-and-tube heat exchangers, that of sanitation. It is virtually impossible to thoroughly clean and sanitize closed system exchangers when a food liquid or slurry is passed through the larger diameter or the shell side where velocity is low. The basic units of plate exchangers are stainless steel (for sanitation and corrosion resistance) plates that are pressed, machined, or formed to accomplish several special design features. The contour of a stack of the plates is such that, when a formed gasket is placed between each plate, a heat exchanger allowing two liquid streams to flow between plates is formed. In practical operation, the plates are suspended from horizontal rails or pipes that allow them to be brought together and tightly compressed during operation or separated for cleaning and maintenance.

These types of heat exchanger are used extensively in the dairy industry. The exchanger is highly efficient due to the high turbulence and minimum volume flowing between the channels. Capacity can be increased to any flow rate desired (eg, 10,000 kg/h) by increasing the number of plates in the frame. When operated at the proper flow rate, the velocity of liquid through the small channels decreases the tenacious baked-on deposits that are caused when colloidal suspended components (eg, proteins in milk) contact hot surfaces. Also, the plates can be separated for thorough cleaning and sanitizing during the maintenance periods.

The disadvantages of plate heat exchangers is the high initial cost compared to tubular type exchangers and the high cleaning and maintenance cost of taking the exchanger apart for each cleaning period. Also, the maximum velocity is limited by the small cross-sectional area and the pressure limitations of the gasketed plates. The minimum velocity is determined by the varying cross section, which allows dead spots of low velocity and subsequent bake-on of the suspended or dissolved solids.

Direct Steam Heating. Direct contact between steam and a food is the most efficient means of heating by directly transferring the latent heat of vaporization to the food. However, the product must be able to sustain the dilution effect caused by the added water resulting from the condensed steam that remains in the heated product. Furthermore, special consideration must be given to producing steam that is safe for human consumption.

When steam is added to a product, the process is known as steam injection. When the product is sprayed into a chamber of steam, thus adding the product to the steam, the process is called steam infusion.

Falling Film Evaporators. When a film of liquid is allowed to flow down a heated wall, the transfer of heat is extremely rapid. This type of unit is used for rapid heating of a fluid that is being evaporated. However, due to the limitation of flow rate as compared to other types of heat exchanger, this method is not often used solely for heating a product.

Heating and Cooling Solids

Batch Heating. 'Batch heating of solids is carried out in the type of heat-exchange facilities normally associated with cooking foods. Broiling, roasting, or baking operations are accomplished by a combination of radiant heat, convection, and conduction, the predominant mechanism of heat transfer depending on the specific commercial equipment. Ovens are heated by elements emitting radiation at a wavelength of 5 x 10"6 m at a temperature of 250-400°C. A more effective radiant heating occurs in chambers heated by an electric bulb infrared source. In this case the air is not heated by the source so that little heating of the food is due to convection or conduction heating.

Microwave heating is a specialized form of dielectric radiant heating that has many advantages over other dielectric methods because there is no requirement for critical spacing between the food and the capacitor plates. Heating is accomplished by the friction of excited molecules rubbing against each other as the strong alternating energy reverses the polarization of the molecules in the food many millions of times per second. The advantages of microwave heating include extremely rapid heating of the food, uniform distribution of the heat throughout the entire food mass, high efficiency, and good control of the energy being added to the food.

Many products are cooked and heated by convection heating when immersed in hot vegetable oil (deep frying) or poached in hot or boiling water. Batches of products are often heated by placing a container of the product on a hot surface (eg, hot plate or element of a stove) or in a steam environment. Steam retort canning of foods is a good example of batch heating by steam. Hermetically sealed cans are placed in baskets and placed in a steam chamber that can be closed and pressurized, normally at about 10 psi (117°C or 242°F). Condensing steam transfers energy to the outside of the can by conduction and convection, whereby the heat is conducted through the can by conduction and then to the food. Solid packs with no free liquid are heated in the can by conduction while a pack with free water transfers heat by both conduction and convection. After being held at the required temperature and time to accomplish sterilization, the cans are removed and air cooled. Sensitive products that tend to scorch or decrease in nutritional value during long air cooling periods are often pressure cooled with water prior to being removed from the retort.

Continuous Heating. More efficient production control, increased product throughput, and improved processed food quality can be accomplished when processes are upgraded from batch to continuous. Modern processing of solid foods involves large continuous production lines utilizing continuous baking ovens (eg, bakery products); deep-frying tanks (eg, french-fried potatoes), microwave ovens, radiant heat chambers, and steam chambers.

Batch Cooling or Freezing. Removing heat from foods, with the exception of cooling in tubular type exchangers, requires considerably different types of facilities than heating. This is due to the nature of the recycling refrigerant or the cryogenic liquids used to remove heat from a food. An additional factor is the psychrometric properties of air that are often recycled near the humidity saturation point in refrigeration facilities in which natural or forced convection is involved in the process.

The basic unit involved with refrigeration cooling of foods is a heat exchanger in which a refrigerant is introduced through an expansion valve into the coils that are located in the cooling or freezing chamber. Thus the cooling or freezing heat exchanger has a cold gaseous refrigerant on one side of the coil wall and the food or heat-transfer medium on the other. Air or liquid brines are the normal mediums used for transferring heat by convection from the food to the refrigeration coils. When the food product is in direct contact with the refrigeration coils, heat is transferred directly by conduction.

Blast Cooling and Freezing Facilities. Forced-air convection is used in blast refrigeration to transfer heat from refrigerated coils to the product. A blast freezer or cooler is actually a double heat exchanger. One exchange takes place between the product and direct contact with air flowing past, and the second is the cooling of the recirculating air as it passes over the freezing coils. During the first phase of this cycle, cold air is circulated over the product and through the freezing chamber, which results in an increase in humidity due to the humidity driving force H between the product environment and the colder refrigerant coil. Thus the air removes water from the freezing chamber and deposits it as ice on the refrigeration coils during the second phase when the air is cooled below the saturation point. If the product being frozen is not completely protected from the air, desiccation will occur in the product. Also, if the doors to the chamber are not completely sealed when closed, moist warm air will enter and further complicate the moisture transfer problem. Of course, as the moisture, in the form of ice, builds up on the coil it acts as a heat-transfer barrier and greatly reduces the efficiency of the system. Hence, the advantage of blast refrigeration is the relatively simple facility required while the main disadvantage is desiccation of the food and buildup of ice deposits that must be removed from the refrigeration coils. The ice greatly reduces the efficiency of heat transfer and increases the cost of operation.

Contact Plate Freezers. Many solid food products are frozen by conduction on freezer shelves, called plates, that contain circulating refrigerant. Thus plate freezing involves heat exchange between a solid food and a vapor refrigerant. Efficient plate freezing is limited to foods and food packages that have flat surfaces (eg, rectangular packages of vegetable) because irregular geometries (eg, turkeys) cannot contact the flat freezer plate. The efficiency of freezing suitable packages is further increased by plate freezer systems in which the plates can be adjusted after loading to contact both the top and the bottom of the package.

Immersion Freezing in Brine. Saturated brine solutions have freezing points well below the freezing point of water and can be used efficiently to freeze products, particularly irregularly shaped items such as turkeys and fish. The product is immersed in a cold brine solution that is maintained at the low temperature by freezing coils. As in the case of blast freezing, there is a two-phase heat transfer. The refrigerant takes heat from the brine and the brine removes heat from the food by conduction and convection. This heat exchange is between a solid food and a liquid.

Cryogenic Freezing. Immersion of a solid food in a liquid refrigerant is similar to freezing in a brine except that there is a much higher temperature driving force between the liquid (eg, liquid ammonia or freon) and the product. Thus the freezing is rapid. Batch freezing by this method is not ordinarily carried out commercially because the cost is prohibitive. Due to the extremely fast freezing, a cry-ogen immersion frozen product must be carefully tempered before further handling and processing or the internal stresses produced will cause cracking of the frozen item.

Continuous Cooling or Freezing. As in the case of heating a food, continuous freezing is much more efficient than batch freezing. Many modern freezing operations involve continuous lines whereby conveyors carry a food through a freezing apparatus. This includes blast freezing and cryogenic freezing. One improvement over immersion cryogenic freezing is the continuous freezing in tunnels in which a liquid cryogenic is flowed over the product. In this operation the liquid expands to a vapor through nozzles directed toward the moving product line. This freezing of a solid by direct contact with a vapor greatly reduces the amount of refrigerant used during processing. However, the cost of the liquid cryogens used (ammonia, freon, carbon dioxide, and nitrogen) are such that only continuous processing lines operating long hours can be justified economically.

There are many different types of heat exchanger and auxiliary equipment available to the food processor. The length of the processing season, the type of food or food product, the value of the raw materials, the cost of utilities at the processing location, environmental factors, and common sense are all factors that must enter into the plans for food-processing operations. Judicial selection of the equipment and facilities for a given food and a given process are necessary to insure that the highest quality product is produced efficiently and economically.

George M. Pigott University of Washington Seattle, Washington


In view of its complexity and variability and the need to carry out experimental work on a long-term basis under actual operating conditions, fouling remains a somewhat neglected issue among the technical aspects of heat transfer. Still, the importance of carefully predicting fouling resistance in both tubular and plate heat-exchanger calculations cannot be overstressed. This is well illustrated in Tables 1 and 2.

Note that for a typical water-water duty in a plate heat exchanger, it would be necessary to double the size of the unit if a fouling factor of 0.0005 were used on each side of the plate (ie, a total fouling of 0.001).

Although fouling is of great importance, there are relatively little accurate data available and the rather conservative figures quoted in Kern (Process Heat Transfer) are used all too frequently. It also may be said that many of the high fouling resistances quoted have been obtained from poorly operated plants. If a clean exchanger, for example, is started and run at the designed inlet water temperature, it will exceed its duty. To overcome this, plant personnel tends to turn down the cooling-water flow rate and thereby reduce turbulence in the exchanger. This encourages fouling and even though the water flow rate eventually is turned up to design, the damage will have been done. It is probable that if the design flow rate had been maintained from the onset, the ultimate fouling resistance would have been lower. A similar effect can happen if the cooling-water inlet temperature falls below the design figure and the flow rate is again turned down.

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