ASHRAE REFRIGERATION SI CH 29-2010 INDUSTRIAL FOOD-FREEZING SYSTEMS《工业食品冷冻系统》.pdf

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 18to 35C. Chap

3、ter 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

4、of 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.

5、Values 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, fi

7、rst 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 Chap

8、ters 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 (conv

9、ection). 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 extr

10、acted 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 60C by spraying liquidnitrogen or liquid carbon dioxide into the freezing chamber.Cryomechanical fre

11、ezing (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 withex

13、tra 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 produ

14、ction 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 HandbookRefrigeration (SI)high-quality, co

15、st-effective 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

16、 inexceptional 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 product

17、s may 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

18、 over 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 b

19、ypass 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. How

20、ever, 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 18C or below the temperature of palletized, cased prod-ucts that have previously been frozen through the latent heat offusion

21、 zone by other means. The flexibility of a blast cell makes itsuitable for small quantities of varied products; however, laborrequirement is relatively high and product movement is slow.Push-Through Trolley FreezersThe push-through trolley freezer (Figure 3), in which the racksare fitted with wheels

22、, incorporates 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

23、widelyused 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, whi

24、ch satisfied 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 th

25、e product 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 be

26、ltinitially 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 18C or below. Transfer between belts helps to redistributethe product on the belt and prevents product adhesion to the belt.To ensure uniform cold air co

27、ntact and effective freezing, prod-ucts should be distributed uniformly over the entire belt. Two-stage freezers are generally operated at 9 to 4C refrigeranttemperatures in the precool section and 32 to 40C in the freez-ing section. Capacities range from 0.9 to 45 Mg of product perhour, with freezi

28、ng times from 3 to 50 min.When products to be frozen are hot (e.g., French fries from thefryer at 80 to 95C), another cooling section is added ahead of thenormal precool section. This section supplies either refrigerated airat approximately 10C or filtered ambient air to cool the productand congeal

29、the fat. Refrigerated air is preferred because filteredambient air has greater temperature variations and may contami-nate the product.Multipass Straight Belt FreezersFor larger products with longer freezing times (up to 60 min)and higher capacity requirements (0.5 to 5.4 Mg/h), a singlestraight bel

30、t freezer would require a very large floor space.Required floor space can be reduced by stacking belts above eachother to form either (1) a single-feed/single-discharge multipasssystem (usually three passes) or (2) multiple single-pass systems(multiple infeeds and discharges) stacked one on top of t

31、he other.The multipass (triple-pass) arrangement (Figure 5) providesanother benefit in that the product, after being surface frozen onBatchCold storage roomsStationary blast cellsPush-through trolleysContinuous/Process-LineStraight belts (two-stage, multipass)Fluidized bedsFluidized beltsSpiral belt

32、sCarton (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 29.3the first (top) belt, may be stacked more deeply on the lower

33、 belts.Thus, the total belt area required is reduced, as is the overall sizeof the freezer. However, this system has a potential for productdamage and product jams at the belt transfers.Fluidized Bed FreezersThis freezer uses air both as the medium of heat transfer and fortransport; the product flow

34、s through 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 tec

35、hnique 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 40C or lower, giving air temperatures of 30C orlowe

36、r. Fluidized bed freezers are normally manufactured as pack-aged, factory-assembled units with capacity ranges of 0.9 to 9 Mg/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

37、 belt 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, th

38、e product can be loadeddeeper for greater efficiency on the second belt. Two-stage fluidizedbelt freezers operate at 34 to 37C refrigerant temperature and incapacity ranges from 0.9 to 45 Mg/h. A good order-of-magnitudeestimate of total refrigeration load for individually quick-frozen(IQF) freezers

39、is 155 kW of refrigeration per megagram of productper hour. Small freezers require about 10 to 15% more capacity perton of product 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

40、during freezing. 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 de

41、signs use 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 arang

42、e of belt 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 sp

43、iral freezers (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-veloci

44、ty cold 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 u

45、p or down 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 FreezerF

46、ig. 7 Horizontal Airflow Spiral Freezer29.4 2010 ASHRAE HandbookRefrigeration (SI)Another 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

47、transfer and 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,

48、from 0.5 to 10 Mg/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 relativ

49、ely high velocity. 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 25 mm thick).Carton Free