Document Yrger9ZE9G6qJx21bZNj391NK

702 CHAPTER 66 1962 Guide And Data 3^ below 10 F. Distilled water can be frosen with temperature as low as 6 F. Brine velocity has a marked influence on freezing t-lmp. m] should not exceed about 35 fpm. Brine movement is caused by the hydraulic gradient, best designed for about lyi to 1> in. in the tank length. This permits submergence of water in the ice cans below the brine level. It is important to main tain brine movement, insuring a temperature as nearly uni form as possible. A variation of not more than F deg is desirable, though 1 F deg is permissible. Uniform rate of ice harvesting on exact t?m< arhadple is likewise necessary for maimnm yield. The rate of ice freezing drops rapidly as the ice becomes thicker. An 11X 22 in. can holding 320 lb of water in 12 F brine will make 280 lb of ice in 24 hr and consume an additional 14 hr to freeze the 40 lb balance. When brine agitation is moderate, 15 to 25 fpm, the total freezing time for US standard cans, 11 in. thick, may be ex pressed by Equations 1 and 2 from: 7a* 0 32 - t 3im 0 250 (1) (2) where 0 -- total freezing time for the block, hours. a " thickness of ice block, inch** t -- temperature of brine, Fahrenheit, n -- number of cans per ton of ioe produced in 24 hours. w -- weight of ice block, pounds. Equation 3 is obtained by combining Equations 1 and 2 and solving for the brine temperature. t -32 683-3o* ton (3) Thus, with a given plant, the dally output is a function of the brine temperature only. The number of cans per ton is the usual unit for rating tank capacity. It indicates the number of c*ng that are work ing to produce one ton of salable ice per 24 hr day, when ice is harvested uniformly. The number of cans per ton estab lishes the necessary brine temperature for a given daily output. Table 1 .... Normal Number of 300-lb Cam Required r*. Ton of Ice per Day (Based on 30 fpre brine velocity) Awogt trio* Temp, f 22 20 18 16 15 14 13 12 11 10 9 8 1M4X22K fa. Con fWor 22.5 19 16 14.5 13.6 12.5 12 11.5 11 10.6 10 9.5 Tine, Hr 81 68.4 57.6 52.2 48.6 45.0 43.2 41.4 39.6 37.8 36.0 34.2 J IX22 to. CeT~~~ Nembor 21 18.0 15.5 13.5 12.5 11.5 11 10.5 10 9.5 9.0 8.5 64 8 41.4 37.8 34.2 30.6 Table 1 shows observed ice can performance when v&rbas brine temperatures are maintained in UntTM designed for 300lb cans, with all can water below the brine level. Fig. l qs chart prepared by Rasori for calculation of freezing tank paformance when the can width is not less than twice the ice block thickness. In tanleu with a high brine velocity, the constant 7 in Equation 1 reduces to 6.3 for 11 X 22 in. cam. Warm water in 11H X 22H size ice cans will cool in tlx brine tank at a beat transfer rate of 50 Btu per-(sq ft) (hi) (F deg) up to the time when ice first forms on the can gWW Ice first appears when the water reaches 40 F with 14 F brine and at 38 F with 18 F brine. In 14 F brine, water will drop from 70 to 40 F in 15 min, but chilling thereafter down to 32 F will require an additional 40 to 50 min. The extent of both brine and water turbulence will influence the cooling rates to a marked extent. Can Ice Freezing Time The freezing time (Equation 1) applies principally to standard American freezing with dimensions of 11 X 22 in., or approximations to them. For block thicknesses above 12 in. of below 10 in., Equation 1 should not be used because: (1) it assumes that all resistance to heat flow is dependent WOSKT OF KC SUX*-l MINE TEMPOATUtt - r Fig. 1 .... Freezing Tank Performance Ice Manufacturing For redongutar earn wUb dda afb In ratio of 1:2 to X Fig. 2 .... Freezing Time for Can Ice* upon ice thickness, whereas the resistance actually consists of (a) ice resistance (thickness), and (5) surface resistance (from brine to ice), which is independent of ioe thickness; aod (2) it does not fadr into consideration the flow of heat tcross the narrow sides of the can, which becomes increasingly important as the shape of the approaches a square. Equation 4 is more exact for rectangular cans having a ratio of rides a/b =* where a = short side and 6 -- long side of can. Fig. 2 shows a graphical solution of Equation 4. 156r6 -r 1.2626* (4) ofccre f, = total freezing time, houm. t coefficient surface of resistance, brine to ioe, (F deg) (hr) (sq ft) per Btu. b * Long ride of can, inches. The value of r varies with brine velocity. For very high wiae velocities r=0.02; for ordinary ice tanks with relatively moving brine r--0.04. To the freezing time established, it is necessary to add a Ic*s>nable time for the core pulling and refilling operation, v*tying in time from to 1 hr. BRINE COOLING The cooling of brine in ice tanka is accomplished by various of evaporators. Modern t*nk use either open end shell- and-tube horizontal coolers, trunk coils, or verti-flow or race way coils. The older coil types appear to find acceptance in the new small tanka using low pressure refrigerants. An enclosed cooler external to the tank may be used only when the brine is pumped from one end of the brine tank and discharged into an agitator at the opposite end. The thermal efficiency of this system is not equal to that of the usual design. Typical freezing coil Hfrrigna, until about 1925, had or 2 in. black steel pipe coils arranged-in single vertical stands running longitudinally set up between each row of along the tank sides and center partition. Pipe coils were nlgn arranged in various vertical and horizontal circuits, with in. pipe more commonly used. Vertical stands 8 and 10 pipes high, containing 180 to 200 ft per ton of daily ice capacity were usual. More efficient tanks of the -10 pipe class used 240 to 310 ft of in. pipe per ton of daily ice produc tion. Open shell-end-tube coolers and trunk evaporators provide a large circulating brine volume in a single-pass operation. The cooling range at best does not exceed to 1 F deg estab lishing the variability of brine temperature in the tank. Tanks of over 30 tons are usually divided into multiple sec tions so as to insure email temperature ranges. Brine velocities vary between 3 and 5 fps through the shell oooler tubes, in order to keep the cooler diameter reasonable and agitation hp low. In shell-end-tube coolers, beat transfers of 60 to 80 Btu per (sq ft) (hr) (F deg) may be procured, with brine tem- i-: t ! r{ Lr ! fi