Document DGg7v1nqvnXovOwaB131rrdod

518 CHAPTER 48 1962 Guide And Data Book have the disadvantage of requiring frequent strengthening (concentration) of the brine. Brine spray air washers for this type of work must have spe cial eliminators (with more bends than for water sprays) in order to prevent excessive brine carryover. The air velocity through the washer should be from 500 to 750 fpm per sq ft of eliminator face area. The range of brine spray quantities varies from 2 to 6 gpm per sq ft ofcross-sectional area per sprayhank. Some designs call for one spray bank, but most designs use two spray banks to secure the highest possible degree of satu ration. The brine temperature range may vary from 2 to 10 F. For ordinary calculations, the refrigeration ton capacity of a brine system may be obtained by multiplying the gpm circu lated by the brine temperature range and then dividing by 28 i.e., 28 gal deg (1 ton refrigeration). The air quantity usu ally runs from 1000 to 1500 cfm per ton refrigeration. Calcium chloride or sodium chloride (salt) is used for brine, with sodium chloride most common in citrus work because of its cheapness. Both brines have a lower vapor pressure than water, and therefore have a drying effect on the air passing through the washer. Calcium brine has the lower vapor pres sure and is the more drying. There may also be a precipitation of CaCOj from the carbon dioxide in the air. Salt brine will give fairly high saturation due to the high air-brine ratio and the low sodium chloride concentration required. It is not pos sible to obtain complete saturation when dehumidifying, espe cially with a brine spray, but it can be assumed to be about 95 percent rh for the air leaving a two-bank washer. Cor rosion must be carefully guarded against with a washer system and, consequently, a pH value (hydrogen ion concen tration) between 7.5 and 8.5 (a pH of 7.0 indicates a neutral brine) should be maintained for best results. The refrigerant most commonly used in orange precooling work is ammonia. However, *mmnnU fumes are very injurious to citrus fruits, and precautions must be taken to eliminate leaks. It is most desirable to locate the ammonia compressors and all ammonia-containing vessels in a separate room or building, tightly closed off from the storage space. Refrigerant 12 is a safer refrigerant to use for unitary systems with sepa rate conditioners in each room, as this refrigerant is not in jurious to the fruit. An indirect system using c&lrinm chloride brine for the cooling units located in the precooler rooms makes the best unitary system when Ammonia is the refriger ant With a central air washer, the brine may be cooled in a separate shell-and-tube brine cooler, or by direct expansion coils placed between the sprays in the air washer. Normally the refrigeration machine will be thermostatically controlled from the brine temperature. AIR DISTRIBUTION The quantity of supply air should be based on 3000 cfm for each car undergoing precooling, plus 1000 cfm for each addi tional car in storage only. The temperature rise in the supply air should be hurII, about 6 to 8 F deg at the beginning to 2 or 3 F deg at the end of precooling. This temperature rise may be determined mathematically by dividing the sensible beat loss in Btu per hour by the product of pounds of supply air per hour and the specific heat (tee Example 1). The amount of outdoor air is that required to keep the car bon dioxide generated by respiration to the desired minimum concentration for best keeping qualities of the fruit. The differ ence between the supply air quantity and the outdoor airquantity ib the return or recirculated air quantity. Wooden, wood fiber, or masonry ducts are used in most cases instead of sheet metal which will sweat and rust in the temperature and humidity conditions of an orange precoob ing room. Smooth wooden supply ducts are ordinarily de- signed for a maximum velocity of 1500 fpm at the fan with velocities tapering off in the branches to 800 or 1000 fpm. For masonry ducts, lower velocities should be used, say 1200 fptn maximum. Damperssbould be easily adjustable with positions marked for precooling and storage air quantities. Supply and return dampers should be connected so as to operate as a unit for each room. Damper areas should be based on not more than 1000 fpm air velocity. Dampera are often made of wood. Metal dampers and metal transformation pieces or guide vanes, where required, should be protected with a heavy coating of bituznastic paint. In precooling rooms, full height supply and return ducts are often formed by false walls at opposite ends of the room. Open ings are provided near the floor, and another set near the ceil ing. Dampers or adjustable slides over the openings in the supply chamber admit air to the room, and similar dampers are provided in the return chamber at the opposite end of tile room. The space over the corridor is often utilised for the return duct. The boxes of oranges are stacked so that the bulge or crown leaves an air space in the direction of air flow from one end of the room to the other. Where practicable the ducts are run inside the refrigerated space to save insulation. Ducts which are not inside refrigerated rooms should be in sulated with the equivalent of four inches of corkboard. In the final analysis refrigeration for oranges is an air condi tioning problem. In designing a plant, careful attention must be given to the following important factors: (I) air volumes, (2) air temperatures, (3) humidity, (4) ventilation, (5) air dis tribution, (6) flexibility, and (7) controls. Consideration of these factors, in the manner outlined in the text, should result in a properly designed refrigeration plant. LOAD CALCULATIONS The heat gain through the exposed walls, floor and railing of the precooling of storage room is figured in the same manner that any refrigerating or air conditioning load is calculated from the exposed area, transmission coefficient and the tem perature difference. Insulation should be four inches of corkboard or other material having an equivalent insulating value. The heat gain from leakage through the insulation, infiltra tion, or outdoor air, fan motors, pump motors, etc. is calcu lated in the usual manner. To these heat gains must be added the cooling down load for the fruit precooled each day and the respiration load for all the fruit held in storage. The fruit and the boxes should be considered separately because the former has a specific heat of 0.9 and the latter only 0.4. About 90 per cent of the cooling down load will be mgihlA heat and the re maining 10 percent latent heat. Example 1 gives typical calcu lations for an orange precooling plant for five cars per day pre cooling capacity and 50 cars total storage capacity. Example 1: Illustrate the calculations for a precooling plant for oranges having space and capacity to handle precooling of 5 cars of oranges per day. The maximum storage capacity is 50 carloads. The precooling time is 72 hr. Solution. With three days allowed for precooling, maximum refrigeration load will oecur with 15 cars undergoing precooling, five cars being loaded in, five cars being loaded out, ana 25 cars in storage. Therefore, provide 10 rooms offive carloads capacity each for a maximum storage of 50 rarlrmHa Allow 240 sq ft floor area per carload. Boxes are stacked 3 hip) in precoolers. (5 cars) (240 sq ft) 1200sq/t floor area per room-1 The floor space required for 5 cars wul be 5 X 240 -- 1200 sqft. Make rooms 20 X 60 X 10H ft high, plus 5 ft for air duds at ends, or 20 X 65 ft gross floor area. Insulation should be equiv alent to 4 in. of corkboard. With 10 rooms, arranged five on each side of a 10 ft corridor, total refrigerated floor area will be ap* proximately 100 X 140 ft - 14,000 sq ft. The cubic contents will be 14,000 X lOHft - 147,000 cu ft. The computations are baaed on the following conditions: Gfrus Fruit Storage 519 Qtjtdoob (Summer) 80(24-hr avg) 63 52 37 57.6 28.22 13.78 Conditions Dry-bulb temp, F Wet-bulb temp, F Dew-point temp, F Relative humidity, % Grains per lb Btu per lb . Cu ftperib Indoor 34.5 32 85 28.5 12.81 12.57 Sensible Heat Load Btuh (1) Heat transmitted through the surfaces. The room surface areas are: walls -- 480 X 10H ,, 5040 sq ft: floor: 140 X 100 * 14,000 sq ft; snd ceding: 140 X 100 = 14,000 sq ft. Walls (5040) (0.075) (80 - 36) = (14,000)(0.075) (80 - 36) 'Ceding ; C?4,000)(0.075)(80 - 38) - Total Transmitted Heat 16,000 46,200 46,200 109,000 (2) Precooling fruit from 80 to 36 F in 5 cara of 462 fav* with 74 lb of fruit per box. Cooling period ta 24 hr. Specific heat is 0.09. Assume 90% of load to be (5)(462)(74)(80 - 36)(0.09)(0.90) 24 253,850 (3) Cooling boxes at 7 lb each and 0.40 sp bt. (S)(462)(7)(80 - 36K0.40) 24 11,860 (4) Heat from respiration of 15 cara precooling at 58 F average temperature (see Table 5). (15) (462) (74)(4700) (24) (2000) 50,200 (5) Holding 25 cars at 36 F. (25) (462) (74) (1150) (24)(2000) 20,475 (6) Total Internal Room Load = 445,385 (7) Outdoor air: 10 cfm per car for 40 cars. (40) (10) (60) (0.24)(80 - 36) 13.78 (8) Fan motor (25 hp not in air stream). 25 X 2545 - 63,625 B) Subtotal -- [10) Adding 5% for duct and apparatus Josses -- 527,410 26,665 Total Sensible Heat Load -- 554,075 Latent Heat Load (1) Precooling 5 cars per day, --wnming that 10% of load is latent heat. (5)(462)(74)(80 - 36)(0.90)(0.10) Btub . (2) Respiration for 15 cars per day. See Table 5 and interpolate for 58 F average temperature. (15) (462) (74) (323) _ (24) (2000) Table 5 . Respiration Data for Citrus Fruits lAtl factor* ore psr 24 fcr per 2000 lb of frail) Oranges Grapefruit Temp, F 32 40 60 80 32 40 60 SO 32 40 60 80 Suribto (mat produced, Btu 900 1400 5000 8000 580 810 2970 6200 460 1070 2770 4180 Latent heat produced, Bto 63 97 348 557 40 207 432 32 74 193 291 (3) Holding 25 cars at 36 F. (25){462X74)(80) (24) (2000) 1430 (4) Outdoor air at 10 cfm per car. Load to be based on 40 cars. Moisture removal per pound of. air from 57.6 to 26.5 gr. (40) (10) (60) (57.6 - 26.5X1060) (13-78) (7000) Total Latent Heat Load * 8220 41,300 Cooling Load Summary Total sensible beat " Total latent heat = Washer pump motor heat (20 hp) (20)(2545) = 554,075 41,300 50,900 Total Refrigeration Load 1- 646,275 53.9 Tons Internal Sensible Heat Load fob One Precooling Room at Five Carloads Capacity and Three Days to Prbcool Transmission through surfaces = (Yu)(109,000) - 10,900 Precooling - (M)(253,850) - 84,620 Cooling boxes = (})(11,860) = 3,850 Respiration -- ()^)(50,200) - 16,730 Total -116,200 Internal Sensible Heat Load for One Storage Room of Five Carloads Caracity : Transmission through surfaces -- (1/i#)(109,000) - 10,900 Respiration -- (K)(28,400) => 5,280 Total - 16,180 Sensible Heat Gain from Fan and Duct System S3 63,625 (Fan) 28,665 (Ducts) -- 90,290 Aib Quantities and Temperatures Base supply air quantity on 3000 cfm for each carload under going precooling plus 1000 cfm for each car in storage. Air quantity is to be based on 15 ears for precooling, 25 carafor storage and 40 cars total for outdoor air. For precooling -- (15 cars)(3000 cfm) For storage = (25 care)(l000 cfm) " 45,000 cfm -- 25,000 cfm Total supply air quantity Outdoor air => (40 cars)(10 cfm) 70,000 cfm 400 cfm Return air -- Supply air -- outdoor air 69,600 cfm