ASHRAE REFRIGERATION IP CH 43-2010 ICE MANUFACTURE《冰加工》.pdf

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1、43.1CHAPTER 43ICE MANUFACTUREIce Makers. 43.1Thermal Storage 43.3Ice Storage 43.4Delivery Systems . 43.5Commercial Ice . 43.6Ice-Source Heat Pumps. 43.7OST commercial ice production is done with ice makers thatMproduce three basic types of fragmentary ice (flake, tubular,and plate), which vary accor

2、ding to the type and size required for aparticular application. Among the many applications for manufac-tured ice are Processing: fish, meat, poultry, dairy, bakery products, and hydro-coolingStorage and transportation: fish, meat, poultry, and dairy productsManufacturing: chemicals and pharmaceutic

3、alsOthers: retail consumer ice, concrete mixing and curing, and off-peak thermal storageICE MAKERSFlake IceFlake ice is produced by applying water to the inside or outsideof a refrigerated drum or to the outside of a refrigerated disk. Thedrum is either vertical or horizontal and may be stationary o

4、r rotat-ing. The disk is vertical and rotates about a horizontal axis. Typicalflake ice machines are shown in Figures 1 and 2.Ice removal devices fracture the thin layer of ice produced on thefreezing surface of the ice maker, breaking it free from the freezingsurface and allowing it to fall into an

5、 ice bin, which is generallylocated below the ice maker.Thickness of ice produced by flake ice machines can be varied byadjusting the speed of the rotating part of the machine, varyingevaporator temperature, or regulating water flow on the freezingsurface. Flake ice is produced continuously, unlike

6、tubular and plateice, which are produced in an intermittent cycle or harvest opera-tion. The resulting thickness ranges from 0.04 to 0.18 in. Continu-ous operation (without a harvest cycle) requires less refrigerationcapacity to produce a ton of ice than any other type of ice manufac-ture with simil

7、ar makeup water and evaporating temperatures. Theexact amount of refrigeration required varies by machine type anddesign.All water used by flake ice machines is converted into ice; there-fore, there is no waste or spillage. Flake ice makers usually operateat a lower evaporating temperature than tube

8、 or plate ice makers,and the ice is colder when it is removed from the ice-making sur-face. The surface of flake ice is not wetted by thawing duringremoval from the freezing surface, as is common with other types ofice. Because it is produced at a colder temperature, flake ice is mostadaptable to au

9、tomated storage, particularly when low-temperatureice is desired.Rapid freezing of water on the freezing surface entrains air in theflake ice, giving it an opaque appearance. For this reason, flake iceis not commonly used for applications where clear ice is important.Where rapid cooling is important

10、, such as in chemical processingand concrete cooling, flake ice is ideal because the flakes present themaximum amount of cooling surface for a given amount of ice.When used as ingredient ice in sausage making or other foodgrinding and mixing, flake ice provides rapid cooling while mini-mizing mechan

11、ical damage to other ingredients and wear on mix-ing/cutting blades.Some flake ice machines can produce salty ice from seawater.These are particularly useful in shipboard applications. Other flakeice machines require adding trace amounts of salt to the makeupThe preparation of this chapter is assign

12、ed to TC 10.2, Automatic Icemak-ing Plants and Skating Rinks.Fig. 1 Flake Ice MakerFig. 1 Flake Ice MakerFig. 2 Disk Flake Ice MakerFig. 2 Disk Flake Ice Maker43.2 2010 ASHRAE HandbookRefrigerationwater to enhance the release of ice from the refrigerated surface. Inrare cases, the presence of salt i

13、n the finished product may be objec-tionable.Tubular IceTubular ice is produced by freezing a falling film of water eitheron the outside of a tube with evaporating refrigerant on the inside, oron the inside of tubes surrounded by evaporating refrigerant on theoutside.Outside Tube. When ice is produc

14、ed on the outside of a tube, thefreezing cycle is normally 8 to 15 min, with the final ice thickness0.2 to over 0.5 in., following the tubes curvature. The refrigeranttemperature inside the tube continually drops from an initial suctiontemperature of about 25F to the terminal suction temperature in

15、therange of 10 to 15F. At the end of the freezing cycle, the circulatingwater is shut off, and hot discharge gas is introduced to harvest theice. To maintain proper harvest temperatures, typical discharge gaspressure is 160 psia (i.e., an approximate saturated temperature of83F for R-717 and 81F for

16、 R-22). This drives the liquid refrigerantin the tube up into an accumulator and melts the inside of the tubeof ice, which slides down through a sizer and mechanical breaker,and finally down into storage. The defrost cycle is normally about30 s. The unit returns to the freezing cycle by returning th

17、e liquidrefrigerant to the tube from the accumulator.This type of ice maker operates with R-717, R-404A, R-507, andR-22. R-12 may be found in some older units. Higher-capacity unitsof 10 tons per 24 h and larger usually use R-717. Unit capacityincreases as terminal suction pressure decreases. A typi

18、cal unit with70F makeup water and R-717 as the refrigerant produces 19.3 tonsof ice per 24 h with a terminal suction pressure of 38.5 psia andrequires 35.7 tons of refrigeration. This equates to 1.85 tons ofrefrigeration per ton of ice. The same unit produces 41.6 tons of iceper 24 h with a terminal

19、 suction pressure of 21 psia and requires80 tons of refrigeration. This equates to 1.92 tons of refrigerationper ton of ice.Inside Tube. When ice is produced inside a tube, it can be har-vested as a cylinder or as crushed ice. The freezing cycle rangesfrom 13 to 26 min. Tube diameter is usually 0.9

20、to 2 in., producinga cylinder that can be cut to desired lengths. The refrigerant temper-ature outside the tube drops continually, with an initial temperatureof 25F and a terminal suction temperature ranging from 20 to 5F.At the end of the freezing cycle, the circulating water is shut off andice is

21、harvested by introducing hot discharge gas into the refrigerantin the freezing section. To maintain gas temperature, typical dis-charge gas pressure is 180 psia (i.e., an approximate saturated tem-perature of 90F for R-717 and R-22). This releases the ice from thetube; the ice descends to a motor-dr

22、iven cutter plate that can beadjusted to cut ice cylinders to the length desired (up to 1.5 in.). Atthe end of the defrost cycle, the discharge gas valve is closed andwater circulation resumes. Figure 3 shows the physical arrangementfor an ice maker that makes ice on the inside of the tubes.These un

23、its can use refrigerants R-717 and R-22; R-12 may befound in older units. Again, capacity increases as terminal suctionpressure decreases. A typical unit with 70F makeup water andR-717 as the refrigerant produces 43 tons of ice per 24 h with a ter-minal suction pressure of 40 psia and requires 74.5

24、tons of refriger-ation. This equates to 1.73 tons of refrigeration per ton of ice. Thesame unit produces 66 tons of ice per 24 h with a terminal suctionpressure of 30 psia and requires 135 tons of refrigeration. Thisequates to 2.04 tons of refrigeration per ton of ice.Tubular ice makers are advantag

25、eous because they produce ice athigher suction pressures than other types of ice makers. They canmake relatively thick and clear ice, with curvatures that help preventbridging in storage. Tubular ice makers have a greater heightrequirement for installation than plate or flake ice makers, but asmalle

26、r footprint. Provision must be made in the refrigeration sys-tem high side to accommodate the volume of refrigerant requiredfor the proper amount of harvest discharge gas. Ice temperatures aregenerally higher than with flake ice makers.Supply Water. Supply water temperature greatly affects capac-ity

27、 of either type of tubular ice maker. If supply water temperatureis reduced from 70 to 40F, ice production of the unit increasesapproximately 18%. In larger systems, the economics of precoolingwater in a separate system with higher suction pressures should beconsidered.Plate IcePlate ice makers are

28、commonly defined as those that build ice ona flat vertical surface. Water is applied above freezing plates andflows by gravity over the freezing plates during the freeze cycle.Liquid refrigerant between 5 and 20F is contained in circuitinginside the plate. The length of the freezing cycle governs th

29、e thick-ness of ice produced. Ice thicknesses between 0.25 to 0.75 in. arequite common, with freeze cycles varying from 12 to 45 min. Figure4 shows a flow diagram of a plate ice maker using water for harvest.All plate ice makers use a sump and recirculating pump concept,whereby an excess of water is

30、 applied to the freezing surface. Waternot converted to ice on the plate is collected in the sump and recir-culated as precooled water for ice making.Ice is harvested by one of two methods. One method involvesapplying hot gas to the refrigerant circuit to warm the plates to 40 to50F, causing the ice

31、 surface touching the plate to reach its meltingpoint and thereby release the ice from the plate. The ice falls bygravity to the storage bin below or to a cutter bar or crusher that fur-ther reduces the ice to a more uniform size. Plate ice makers usingthe hot-gas method of harvesting can produce ic

32、e on one or twosides of the plate, depending on the design.In the second method of harvesting ice, warm water flows on theback side of the plate. This heats the refrigerant inside the plateabove the ice melting point, and the ice is released. Ice makers usingthis approach manufacture ice on one side

33、 of a plate only. Harvestwater is chilled by passing over the plates. It is then collected in thesump and recirculated to become precooled water for the next batchof ice.Fig. 3 Tubular Ice MakerFig. 3 Tubular Ice MakerIce Manufacture 43.3For plate ice makers, freezing time, harvest time, water, pump

34、,and refrigeration are controlled by adjustable electromechanical orelectronic devices. Using the wide variety of thicknesses and freez-ing times available, plate ice makers can produce clear ice. Thus,the plate ice maker is commonly used in applications requiringclear ice.Because of the harvest cyc

35、le involved, plate ice makers requiremore refrigeration per unit mass of ice produced than flake ice mak-ers. This disadvantage is offset by the ability of plate ice makers tooperate at higher evaporating temperatures; thus, connected motorpower per ton of refrigeration is usually less than that of

36、flake icemakers. During the harvest cycle, suction pressure rises consider-ably, depending on the design of the ice maker. When a commonrefrigeration system is used for multiple refrigerated requirements,a stable suction pressure can be maintained for all the refrigerationloads by using a dedicated

37、compressor for the ice machine, or byusing a dual-pressure suction regulator at each ice machine to min-imize the load placed on the suction main during harvest. This mayoccur in large processing plants, refrigerated warehouses, and soforth. Large plate ice makers can be arranged such that only sect

38、ionsor groups of plates are harvested at one time. Properly adjusting thetiming of harvesting each section can reduce the fluctuation in suc-tion pressure.Plate ice makers using the water harvest principle rely on thetemperature of the water for harvesting. A minimum of 65F is usu-ally recommended t

39、o minimize both harvest cycle time and harvestwater consumption. For installations in cold-water areas, or wherewintertime inlet water temperatures are low, it is advisable to pro-vide auxiliary means of warming the inlet water to 65F.Ice BuildersIce builders comprise various types of apparatus that

40、 produceice on the refrigerated surfaces of coils or plates submerged in insu-lated tanks of water. This equipment is commonly known as an icebank water chiller. Ice built on the freezing coils is not used as amanufactured ice product but rather as a means of cooling watercirculating through the tan

41、k as the ice melts from the coils. The icebuilder is most often used for thermal storage applications withhigh peak and intermittent cooling loads that require chilled water.See Chapter 50 of the 2008 ASHRAE HandbookHVAC Systemsand Equipment for more information.Scale FormationPerformance of all ice

42、 makers is affected by the characteristics ofthe inlet water used. Impurities and excessive hardness can causescale to be deposited on the freezing surface of the ice maker. Thedeposit reduces the heat transfer capability of the freezing surface,thereby reducing ice-making capacity. Deposited scale

43、may furtherreduce ice-making capacity by causing the ice to stick on the freez-ing surface during the harvest process. The rated capacity of all icemakers is based on the substantial release of all the ice from thefreezing surface during the removal period. Because the process offreezing water tends

44、 to freeze a greater proportion of pure water onthe ice makers freezing surface, impurities tend to remain in theexcess or recirculated water. A blowdown, or bleedoff, whereby aportion of the recirculated water is bled off and discharged, can beinstalled. The bleedoff system can control the concentr

45、ation ofchemicals and impurities in the recirculated water. The necessity ofa bleedoff system and the effectiveness of this concept for control-ling scale deposits depend on local water conditions. Some refrig-eration system loss occurs because recirculated water that is bled offto drain is precoole

46、d. Water that is bled off may be passed througha heat exchanger to precool incoming makeup water. Water condi-tions, treatment, and related problems in ice making are covered inChapter 48 of the 2007 ASHRAE HandbookHVAC Applications.THERMAL STORAGEInterest in energy conservation has renewed interest

47、 in using iceto provide thermal storage of cooling capacity for air-conditioning orprocess applications. The ice is produced and stored using lower off-peak and weekend power rates. During the day, stored ice providesFig. 4 Plate Ice MakerFig. 4 Plate Ice Maker43.4 2010 ASHRAE HandbookRefrigerationr

48、efrigeration for the chilled-water system. The design and features ofthermal storage equipment are covered in Chapter 50 of the 2008ASHRAE HandbookHVAC Systems and Equipment.ICE STORAGEFragmentary ice makers can produce ice either continuously orin a constant number of harvest cycles per hour. Use o

49、f the ice isgenerally not at a constant rate but on a batch basis. Batches varygreatly, based on user requirements. The ice must be stored andrecovered from storage on demand. Labor savings, economics,quantity of ice to be stored, amount of automation desired, and userdelivery requirements must all be considered in ice storage and stor-age bin design.Ice makers can produce ice 24 h a day. Making ice during off-shifts and weekends, as well as during work shifts, can lead to con-siderable savings in total ice-making and refrigeration systemrequirements. In addition, by