ASHRAE REFRIGERATION SI 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 1 to 4.5 mm. Continuousoperation (without a harvest cycle) requires less refrigeration capac-ity to produce a kilogram of ice than any other type of ice manufac-ture with simi

7、lar 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 tub

8、e 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 a

9、utomated 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 importan

10、t, 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 mecha

11、nical 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 assig

12、ned 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 HandbookRefrigeration (SI)water to enhance the release of ice from the refrigerated surface. Inrare cases, the presence of

13、salt in 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

14、produced on the outside of a tube, thefreezing cycle is normally 8 to 15 min, with the final ice thickness5 to over 13 mm, following the tubes curvature. The refrigeranttemperature inside the tube continually drops from an initial suctiontemperature of about 4C to the terminal suction temperature in

15、 therange of 12 to 26C. At the end of the freezing cycle, the circu-lating water is shut off, and hot discharge gas is introduced to har-vest the ice. To maintain proper harvest temperatures, typicaldischarge gas pressure is 1.1 MPa (i.e., an approximate saturatedtemperature of 28C for R-717 and 27C

16、 for R-22). This drives theliquid refrigerant in the tube up into an accumulator and melts theinside of the tube of ice, which slides down through a sizer andmechanical breaker, and finally down into storage. The defrost cycleis normally about 30 s. The unit returns to the freezing cycle byreturning

17、 the liquid refrigerant 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 9 Mg per 24 h and larger usually use R-717. Unit capacityincreases as terminal suction pressure decreases. A typ

18、ical unit with21C makeup water and R-717 as the refrigerant produces 17.5 Mgof ice per 24 h with a terminal suction pressure of 265 kPa andrequires 126 kW of refrigeration. This equates to 7.2 kW of refrig-eration per megagram of ice. The same unit produces 37.7 Mg of iceper 24 h with a terminal suc

19、tion pressure of 145 kPa and requires280 kW of refrigeration. This equates to 7.5 kW of refrigeration permegagram 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 20 to 5

20、0 mm, producinga cylinder that can be cut to desired lengths. The refrigeranttemperature outside the tube drops continually, with an initial tem-perature of 4C and a terminal suction temperature ranging from7 to 20C. At the end of the freezing cycle, the circulating wateris shut off and ice is harve

21、sted by introducing hot discharge gas intothe refrigerant in the freezing section. To maintain gas temperature,typical discharge gas pressure is 1.2 MPa (i.e., an approximate sat-urated temperature of 32C for R-717 and R-22). This releases theice from the tube; the ice descends to a motor-driven cut

22、ter plate thatcan be adjusted to cut ice cylinders to the length desired (up to40 mm). At the end of the defrost cycle, the discharge gas valve isclosed and water circulation resumes. Figure 3 shows the physicalarrangement for an ice maker that makes ice on the inside of thetubes.These units can use

23、 refrigerants R-717 and R-22; R-12 may befound in older units. Again, capacity increases as terminal suctionpressure decreases. A typical unit with 21C makeup water andR-717 as the refrigerant produces 39 Mg of ice per 24 h with a ter-minal suction pressure of 275 kPa and requires 262 kW of refriger

24、-ation. This equates to 580 kJ of refrigeration per kilogram of ice.The same unit produces 60 Mg of ice per 24 h with a terminal suc-tion pressure of 210 kPa and requires 475 kW of refrigeration. Thisequates to 684 kJ of refrigeration per kilogram of ice.Tubular ice makers are advantageous because t

25、hey 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 asmaller footprint. P

26、rovision 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 of either typ

27、e of tubular ice maker. If supply water temperatureis reduced from 21 to 4C, 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 commonly define

28、d 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 21 and 7C is contained in circuitinginside the plate. The length of the freezing cycle governs the thick-ness of

29、 ice produced. Ice thicknesses between 6 to 20 mm are quitecommon, with freeze cycles varying from 12 to 45 min. Figure 4shows 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 applied to the freez

30、ing 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 4 to10C, causing the ice surface touching the

31、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 ice on one or twosides o

32、f 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 of a plate only. Harv

33、estwater is chilled by passing over the plates. It is then collected in theFig. 3 Tubular Ice MakerFig. 3 Tubular Ice MakerIce Manufacture 43.3sump and recirculated to become precooled water for the next batchof ice.For plate ice makers, freezing time, harvest time, water, pump,and refrigeration are

34、 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 cycle involved, plate ice

35、 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 kilowatt of refrigeration is usually less than that of flakeice makers.

36、During the harvest cycle, suction pressure rises consid-erably, 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 compressor for th

37、e 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 sectionsor groups of

38、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 18C is usu-ally recommended to minimize both h

39、arvest 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 18C.Ice BuildersIce builders comprise various types of apparatus that produce iceon th

40、e refrigerated surfaces of coils or plates submerged in insulatedtanks of water. This equipment is commonly known as an ice bankwater chiller. Ice built on the freezing coils is not used as a manu-factured ice product but rather as a means of cooling water circulat-ing through the tank as the ice me

41、lts from the coils. The ice builderis most often used for thermal storage applications with high peakand intermittent cooling loads that require chilled water. See Chap-ter 50 of the 2008 ASHRAE HandbookHVAC Systems and Equip-ment for more information.Scale FormationPerformance of all ice makers is

42、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 may further

43、reduce 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 to freeze

44、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 concentration ofche

45、micals 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 precooled. Water th

46、at 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 in using i

47、ceto provide thermal storage of cooling capacity for air-conditioningFig. 4 Plate Ice MakerFig. 4 Plate Ice Maker43.4 2010 ASHRAE HandbookRefrigeration (SI)or process applications. The ice is produced and stored using loweroff-peak and weekend power rates. During the day, stored ice pro-vides refrig

48、eration for the chilled-water system. The design and fea-tures of thermal storage equipment are covered in Chapter 50 of the2008 ASHRAE HandbookHVAC Systems and Equipment.ICE STORAGEFragmentary ice makers can produce ice either continuously orin a constant number of harvest cycles per hour. Use of t

49、he 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, b