1、29.1CHAPTER 29INDUSTRIAL FOOD-FREEZING SYSTEMSFreezing Methods 29.1Blast Freezers 29.1Contact Freezers . 29.4Cryogenic Freezers . 29.5Cryomechanical Freezers . 29.5Other Freezer Selection Criteria 29.5Refrigeration Systems 29.7REEZING is a method of food preservation that slows theFphysical changes
2、and chemical and microbiological activitythat cause deterioration in foods. Reducing temperature slowsmolecular and microbial activity in food, thus extending usefulstorage life. Although every product has an individual idealstorage temperature, most frozen food products are stored at 0 to30F. Chapt
3、er 21 lists frozen storage temperatures for specificproducts.Freezing reduces the temperature of a product from ambient tostorage level and changes most of the water in the product to ice.Figure 1 shows the three phases of freezing: (1) cooling, whichremoves sensible heat, reducing the temperature o
4、f the product tothe freezing point; (2) removal of the products latent heat of fusion,changing the water to ice crystals; and (3) continued cooling belowthe freezing point, which removes more sensible heat, reducing thetemperature of the product to the desired or optimum frozen storagetemperature. V
5、alues for specific heats, freezing points, and latentheats of fusion for various products are given in Chapter 19.The longest part of the freezing process is removing the latentheat of fusion as water turns to ice. Many food products are sensitiveto freezing rate, which affects yield (dehydration),
6、quality, nutri-tional value, and sensory properties. The freezing method and sys-tem selected can thus have substantial economic impact.When selecting freezing methods and systems for specific prod-ucts, consider special handling requirements, capacity, freezingtimes, quality, yield, appearance, fir
7、st cost, operating costs, automa-tion, space availability, and upstream/downstream processes.This chapter covers general freezing methods and systems. Addi-tional information on freezing specific products is covered in Chap-ters 23, 30 to 33, and 38 to 42. Related information can be obtainedin Chapt
8、ers 19 and 20, which cover thermal properties of foods aswell as their cooling and freezing times. Information on refrigerationsystem practices is given in Chapters 1 to 4.FREEZING METHODSFreezing systems can be grouped by their basic method ofextracting heat from food products:Blast freezing (conve
9、ction). Cold air is circulated over the prod-uct at high velocity. The air removes heat from the product andreleases it to an air/refrigerant heat exchanger before beingrecirculated.Contact freezing (conduction). Food, packaged or unpackaged, isplaced on or between cold metal surfaces. Heat is extra
10、cted bydirect conduction through the surfaces, which are directly cooledby a circulating refrigerated medium.Cryogenic freezing (convection and/or conduction). Food isexposed to an environment below 76F by spraying liquid nitro-gen or liquid carbon dioxide into the freezing chamber.Cryomechanical fr
11、eezing (convection and/or conduction). Foodis first exposed to cryogenic freezing and then finish-frozenthrough mechanical refrigeration.Special freezing methods, such as liquid immersion (e.g., brinesfor packaged products), are covered under the specific productchapters.BLAST FREEZERSBlast freezers
12、 use air as the heat transfer medium and depend oncontact between the product and the air. Sophistication in airflowcontrol and conveying techniques varies from crude blast-freezingchambers to carefully controlled impingement freezers.The earliest blast freezers consisted of cold storage rooms withe
13、xtra fans and a surplus of refrigeration. Improved airflow controland mechanization of conveying techniques have made heat transfermore efficient and product flow less labor-intensive.Although batch freezing is still widely used, more sophisti-cated freezers integrate freezing into a continuous prod
14、uction line.This process-line freezing has become essential for large-volume,The preparation of this chapter is assigned to TC 10.9, Refrigeration Appli-cation for Foods and Beverages.Fig. 1 Typical Freezing CurveFig. 1 Typical Freezing Curve29.2 2010 ASHRAE HandbookRefrigerationhigh-quality, cost-e
15、ffective operations. A wide range of blastfreezer systems are available, includingCold Storage RoomsAlthough a cold storage room is not considered a freezing sys-tem, it is sometimes used for this purpose. Because a storage roomis not designed to be a freezer, it should only be used for freezing ine
16、xceptional cases. Freezing is generally so slow that the quality ofmost products suffers. The quality of the already frozen productsstored in the room is jeopardized because the excess refrigerationload may raise the temperature of the frozen products considerably.Also, flavors from warm products ma
17、y be transferred.Stationary Blast Cell Freezing TunnelsThe stationary blast cell (Figure 2) is the simplest freezer that canbe expected to produce satisfactory results for most products. It is aninsulated enclosure equipped with refrigeration coils and axial orcentrifugal fans that circulate air ove
18、r the products in a controlledway. Products are usually placed on trays, which are then placedinto racks so that an air space is left between adjacent layers of trays.The racks are moved in and out of the tunnel manually using a palletmover. It is important that the racks be placed so that air bypas
19、s isminimized. The stationary blast cell is a universal freezer, becausealmost all products can be frozen in a blast cell. Vegetables andother products (e.g., bakery items, meat patties, fish fillets, pre-pared foods) may be frozen either in cartons or unpacked andspread in a layer on trays. However
20、, product losses from spillage,damage, and dehydration can be greater, and product quality can bereduced. In some instances, this type of freezer is also used toreduce to 0F or below the temperature of palletized, cased productsthat have previously been frozen through the latent heat of fusionzone b
21、y other means. The flexibility of a blast cell makes it suitablefor small quantities of varied products; however, labor requirementis relatively high and product movement is slow.Push-Through Trolley FreezersThe push-through trolley freezer (Figure 3), in which the racksare fitted with wheels, incor
22、porates a moderate degree of mechani-zation. Racks are usually moved on rails by a pushing mechanism,which can be hydraulically or electrically powered. This type offreezer is similar to the stationary blast cell, except that labor costsand product handling time are decreased. This system is widelyu
23、sed to crust-freeze (quick-chill) wrapped packages of raw poultryand for irregularly shaped products. Another version uses a chaindrive to move the trolleys through the freezer.Straight Belt FreezersThe first mechanized blast freezers consisted of a wire meshbelt conveyor in a blast room, which sati
24、sfied the need for continu-ous product flow. A disadvantage to these early systems was thepoorly controlled airflow and resulting inefficient heat transfer.Current versions use controlled vertical airflow, which forces coldair up through the product layer, thereby creating good contactwith the produ
25、ct particles. Straight belt freezers are generally usedwith fruits, vegetables, French fried potatoes, cooked meat top-pings (e.g., diced chicken), and cooked shrimp.The principal design is the two-stage belt freezer (Figure 4),which consists of two mesh conveyor belts in series. The first beltiniti
26、ally precools or crust-freezes an outer layer or crust to condi-tion the product before transferring it to the second belt for freez-ing to 0F or below. Transfer between belts helps to redistributethe product on the belt and prevents product adhesion to the belt.To ensure uniform cold air contact an
27、d effective freezing, prod-ucts should be distributed uniformly over the entire belt. Two-stage freezers are generally operated at 15 to 25F refrigeranttemperatures in the precool section and 25 to 40F in the freez-ing section. Capacities range from 1 to 50 tons of product per hour,with freezing tim
28、es from 3 to 50 min.When products to be frozen are hot (e.g., French fries from thefryer at 180 to 200F), another cooling section is added ahead ofthe normal precool section. This section supplies either refriger-ated air at approximately 50F or filtered ambient air to cool theproduct and congeal th
29、e fat. Refrigerated air is preferred becausefiltered ambient air has greater temperature variations and maycontaminate the product.Multipass Straight Belt FreezersFor larger products with longer freezing times (up to 60 min)and higher capacity requirements (0.5 to 6 ton/h), a single straightbelt fre
30、ezer would require a very large floor space. Required floorspace can be reduced by stacking belts above each other to formeither (1) a single-feed/single-discharge multipass system (usuallythree passes) or (2) multiple single-pass systems (multiple infeedsand discharges) stacked one on top of the ot
31、her. The multipass(triple-pass) arrangement (Figure 5) provides another benefit inthat the product, after being surface frozen on the first (top) belt,BatchCold storage roomsStationary blast cellsPush-through trolleysContinuous/Process-LineStraight belts (two-stage, multipass)Fluidized bedsFluidized
32、 beltsSpiral beltsCarton (carrier) Fig. 2 Stationary Blast CellFig. 2 Stationary Blast CellFig. 3 Push-Through Trolley FreezerFig. 3 Push-Through Trolley FreezerFig. 4 Two-Stage Belt FreezerFig. 4 Two-Stage Belt FreezerIndustrial Food-Freezing Systems 29.3may be stacked more deeply on the lower belt
33、s. Thus, the totalbelt area required is reduced, as is the overall size of the freezer.However, this system has a potential for product damage and prod-uct jams at the belt transfers.Fluidized Bed FreezersThis freezer uses air both as the medium of heat transfer and fortransport; the product flows t
34、hrough the freezer on a cushion ofupward-flowing cold air (Figure 6). This design is well suited forsmall, uniform-sized particulate products such as peas, diced vege-tables, and small fruit.The high degree of fluidization improves the heat transfer rate andallows good use of floor space. The techni
35、que is limited to well-dewatered products of uniform size that can be readily fluidized andtransported through the freezing zone. Because the principle dependson rapid crust-freezing of the product, the operating refrigerant tem-perature must be 40F or lower, giving air temperatures of 20F orlower.
36、Fluidized bed freezers are normally manufactured as pack-aged, factory-assembled units with capacity ranges of 1 to 10 ton/h.Particulate products generally have a freezing time of 3 to 15 min.Fluidized Belt FreezersA hybrid of the two-stage belt freezer and the fluidized bedfreezer, the fluidized be
37、lt freezer has a fluidizing section in the firstbelt stage. An increased air resistance is designed under the first beltto provide fluidizing conditions for wet incoming product, but thebelt is there to help transport heavier, less uniform products that donot fluidize fully. Once crust-frozen, the p
38、roduct can be loadeddeeper for greater efficiency on the second belt. Two-stage fluidizedbelt freezers operate at 30 to 35F refrigerant temperature and incapacity ranges from 1 to 50 ton/h. A good order-of-magnitudeestimate of total refrigeration load for individually quick-frozen(IQF) freezers is 4
39、0 tons of refrigeration per ton of product per hour.Small freezers require about 10 to 15% more capacity per ton ofproduct per hour.Spiral Belt FreezersThis freezer is generally used for products with long freezingtimes (generally 10 min to 3 h), and for products that require gentlehandling during f
40、reezing. An endless conveyor belt that can be bentlaterally is wrapped cylindrically, one tier below the last; this con-figuration requires minimal floor space for a relatively long belt.The original spiral belt principle uses a spiraling rail system tocarry the belt, although more recent designs us
41、e a proprietary self-stacking belt requiring less overhead clearance. The number oftiers in the spiral can be varied to accommodate different capaci-ties. In addition, two or more spiral towers can be used in series forproducts with long freezing times. Spiral freezers are available in arange of bel
42、t widths and are manufactured as packaged, modular,and field-erected models to accommodate various upstream pro-cesses and capacity requirements.Airflow varies from open, unbaffled spiral conveyors to flowthrough extensive baffling and high-pressure fans. Horizontal air-flow is applied to spiral fre
43、ezers (Figure 7) by axial fans mountedalong one side. The fans blow air horizontally across the spiral con-veyor with minimal baffling limited to two portions of the spiral cir-cumference. The rotation of the cage and belt produces a rotisserieeffect, with product moving past the high-velocity cold
44、air near thedischarge, aiding in uniform freezing. Several proprietary designs are available to control airflow. Onedesign (Figure 8) has a mezzanine floor that separates the freezerinto two pressure zones. Baffles around the outside and inside of thebelt form an air duct so that air flows up or dow
45、n around the productas the conveyor moves the product. The controlled airflow reducesfreezing time for some products.Fig. 5 Multipass, Straight Belt FreezerFig. 5 Multipass, Straight Belt FreezerFig. 6 Fluidized Bed FreezerFig. 6 Fluidized Bed FreezerFig. 7 Horizontal Airflow Spiral FreezerFig. 7 Ho
46、rizontal Airflow Spiral Freezer29.4 2010 ASHRAE HandbookRefrigerationAnother design (Figure 9) splits the airflow so that the coldest aircontacts the product both as it enters and as it leaves the freezer. Thecoldest air introduced on the incoming, warm product may increasesurface heat transfer and
47、freeze the surface more rapidly, which mayreduce product dehydration.Typical products frozen in spiral belt freezers include raw andcooked meat patties, fish fillets, chicken portions, pizza, and a varietyof packaged products. Spiral freezers are available in a wide range ofcapacities, from 0.5 to 1
48、0 ton/h. They dominate todays frozen foodindustry and account for the majority of unpackaged nonparticulatefrozen food production, as well as many packaged products.Impingement FreezersIn this design (Figure 10), cold air flows perpendicular to the prod-ucts largest surfaces at a relatively high vel
49、ocity. Air nozzles withcorresponding return ducts are mounted above and below the con-veyors. The airflow constantly interrupts the boundary layer that sur-rounds the product, enhancing the surface heat transfer rate. Thetechnique may therefore reduce freezing time of products with largesurface-to-mass ratios (thin hamburger patties, for example). Im-pingement freezers are designed with single-pass or multipassstraight belts. Freezing times are 1 to 10 min. Cost-effective applica-tion is limited to thin food products (less than 1 in. thick).Carton FreezersThe carton (o
