Document NZJp389jBy2BB0RwqeKO4vmw

522 CHAPTER 48 1962 Guide And Data Book Table 7 .... Air Travel with Ejector Nozzles Approx. Noxdo Oatiet VetaoJjr, Fpm 400 500 600 700 800 900 1,000 1,100 1,200 1,300 1,400 1,500 1,600 Static Pmwra, In. of Water 0.01 0.016 0.023 0.031 0.04 0.051 0.063 0.076 0.09 0.106 0.123 0.141 0.16 Approx. Effective Air Trove# from NotA, ft 12 15 17 20 23 26 28 31 34 37 39 42 45 metal and must be insulated with about one inch cork to pro* vent sweating if the combination system is used. Nozzles made of insulating board in the shape of a truncated pyramid with square inlet and outlet areas have also been used successfully. Table 7 gives nozzle pressures, velocities, and air travel for ejector nozzles in storage rooms of 10 to 13 ft ceiling height. The air travel or throw from the air nozzles should not exceed 35 to 40 ft for best results. The air velocity in the duct branches or headers for the ejector nozzles should not exceed 1200 fpm for sheet metal ducts or 1000 fpm for wood or cane fiber ducts. If a combina tion system is used, the usual design is to provide two ejector nozzles and one humidifying spray nozzle per 20 ft bay. The humidifying nozzle is placed above and between the two ejec tor nozzles, which are spaced about 18 in. apart. Compressed air and water lines are required for each spray nozzle. Return ducts are sized in the Mmp manner as supply ducts, except that velocities are generally somewhat lower. For sheet metal ducts, the return velocity at the apparatus should not exceed 1500 fpm; for wood or cane fiber ducts this velocity should not exceed 1300 fpm. Return ducts are often omitted with the combination system, and one central return used with the apparatus located centrally with respect to the storage room. Outdoor air connection and all damper areas should be based on a velocity of about 1000 fpm. Intake louvers and screen should be based on 750 fpm velocity. Relief dampers with a total area about 25 percent greater than the outdoor air dampers should be provided. These are needed for exhaust when large quantities of outdoor air are introduced. They should be uniformly distributed and located as far away from the supply outlets as possible. Aluminum vaned dampers, which open on a slight increase in air pressure in the storage room, are used for this purpose. The air thus discarded may often be used to advantage to help cool adjacent spaces in the warehouse. If some shrinkage is not permitted to occur in stor age, it will happen rapidly in transit anyway. A five percent loss of weight in 60 days of storage is a reasonable allowance to add to the latent load. In addition, the respiration an(i out door air will contribute to the latent ln*d. System Design The engineering problem of designing a suitable storage for preserving the quality of lemons and grapefruit for a con siderable holding period requires an understanding of the biological as well as the engineering aspects.- All citrus fruits suffer from simiiar_diseases and require similar treatment in storage except as to temperatures maintained. Lemons and grapefruit should be held at 58 to 60 F, whereas oranges are immediately precooled and stored at 34 to 36 F. An exception is grapefruit from regions where stem-end rot is prevalent which can only be stored for short periods and at temperature from 32 to 34 F--about the same as fororanges. Uniformly high relative humidities, not less than 80 percent in any case, are re quired for all citrus fruit. Usually relative humidities are main tained between 84 and 88 percent. A uniformlylow carbon dioxide concentration must be main tained in the storage room by means of adequate ventilation. This concentration should be not higher than 0.2 percent and preferably about 0.1 percent. The latter condition requires from 70 to 75 cm of outdoor air per carload of fruit. Two types of air distribution systems have been outlined herein. One is the conventional system using a supply &ir quantity of 650 cfm per car (1 cfm per storage box). The other is the combination system using direct humidification in the storage rooms and much less supply air (250 to 350 cfm per car). The latter system is considerably cheaper in first cost but is not recommended by some large growers for best results. With either type of system, the refrigeration requirements are about 0.6 to 0.7 tons refrigeration per car, considerably less than for oranges. Air conditioning principles are involved in the proper handl ing of lemons and grapefruit in storage. Careful consideration must be given to air volumes, air temperatures, humidity, ventilation, and air distribution. Systems must also be de signed with suitable automatic controls in order to be flexible and responsive to varying load demands. Therefore all the basic factors of air conditioning design are involved. Load Calculations Refrigeration load consists of heat gain through room sur faces, internal load, and outdoor air load. Heat gain through the exposed walls, floor, and ceiling is sensible load and is cal culated in the usual manner from the product of the transmis sivity area, and the temperature difference. The transmissivity for one inch of wood or fiber is usually taken as 0.33 Btu per (hr) (sq ft) (Fdeg per in.). Internal load consists of fruit cooling and respiration, fruit box cooling, fan and pump motor heat, and heat from lights and people (usually insignificant and often omitted). Part of this internal load is latent heat and about 10 percent of the cooling-down load may be considered as such. In these calcu lations the fruit and the boxes should be considered separately due to the difference in specific heats. Lemons have a specific heat of 0.94, grapefruit 0.87, and wooden boxes about 0.40. Outdoor air load will consist of the heat and moisture gain from infiltration of air into the conditioned space, and from outdoor air taken in at the air washer. If the outdoor air drawn in through the washer is equivalent to, or greater than, a one hour air change in the conditioned space, no additional out door air need be figured for infiltration. Sensible and latent heat loads should be calculated sepa rately because the supply air quantity is a function of the former only.'A certain amount of shrinkage and moisture loss occurs normally in the storage room. If humidities are carried so high as to prevent all shrinkage, mnlffc will appear on the fruit and boxes. Example illustrates typical load dinhii.inn for a lemon storage basement. Example S continues the calculations as ap plied to a conventional system using high air volumes. Ex ample 4 gives calculations for a combination system using low air quantities and auxiliary humidification. Example f: A lemon storage basement is 152 ft long X 114 ft wide X 13 ft ceiling height. The long wall of the building faces Gtrus Fruit Storage rth. The lower 9 ft of the walls is 8 in. of concrete below and the floor is 6 in. of concrete on the ground. Above MAiind'the walls and ceiling are of frame construction insulated of fiber board. The packing floor is located above tte storage basement. fiondiSons to be maintained in the storage space are 58 F dry- Wh temperature and 88 percent rh. The floor area of the rtaraee ia 17,328 sq ft, andthe volume is 225,264 eu ft. The RtAmee c&nacitv is 70 cars, entering at the rate of OUTDOOB 80 F avg max 45% 56.5 F 68 13.81 Conditions Dry-bulb temp, F Wet-bulb temp, F Relative humidity Dew-point temp, F Grains per lb Btu per lb Cu ft per lb Indoob 58 F 88% 54.5 F 63 13.23 523 Respiration load of 68 care max in storage (68) (32,500) (192) (24) (2000) 8,850 Moisture loss from lemons (assume 5% of weight in 60 days) (68) (32,500) (0.05) (1060) (60) (24) 81,200 Total Room Latent Heat Load - 95,660 Outdoor air load for 5040 cfm; outdoor dew point, 56.5 F, 68 gr per lb; indoor dew point 54.5F, 63 gr per lb; moisture gain, 5 grains per lb. Lomd (5040) (60) (5) (1060) (13.23)(7000) 17,340 Total Latent Heat Load TM 113,000 Sensible Heat Load (1) Heat TBANSMrrr&D Through Subtaces Btuh Walls below ground (4,788)(0.10)(70 - 58) - Walla above ground (2,128)(0.18)(80 - 58) - Sun effect on S. wall (608)(0.18)(12Q -- 80) - Floor - (17,328 sq ft)(0.10)(65 - 58) ruling - (17,328 sq ft)(0.18)(80 - 58) - 5,745 8,420 4,380 Total Tbansmitted Heat - 99,375 ruling 2 cars of lemons per day, 650 boxes (50 lb each) (90% of load assumed to be sensible) (2) Fruit load (2) (650) (50) (0.94) (80 - 58) (0.90) 50,500 (3) Box load - (2)(650)(6)(0.40)(80 - 58) (4) Respiration load of 68 cars maximum in storage (Refer to Table 5). (68 cars) (32,500 lb) (2,754 Btu) < (24 hi-)(2000 Jb) 127,000 Total Room Sensible Load "279,735 (5) Outdoor air load, 70 cars, 72 cfm per car for 0.1% CO, (70)(72)(80 - 58)(0,24)(60) 13.23 (6) Fan motor heat (20 hp) (20 X 2545) - 442,335 Total Sensible Heat Load = 442,335/12,000 " 36.6 Tons refrigeration Latent Heat Load Cooling 2 care of lemons per day (fruit only) (10% of load assumed to be latent) (2)(650)(50)(0.94)(80 - 58)(0.10) ; 24 Example S: For the 70 car storage basement of Example g calculate the requirements for a conventional system supplying 1 cfm air per box or 650 cfm per carload. Solution: Air Quantities For the 70 car storage basement of Example , provide 650 cfm of supply air per car * (70) (650) " 45,500 cfm or 45,500 /13.23 - 3,440 lb per min. To maintain oot more than 0.1% CO, concentration, provide not less thon 72 cfm outdoor air per car " (70) (72) TM 5040 cfm or 5040/13.23 = 380 lb per mip. Storage volume Time for 1 air change " O_ utd-o-o--r--a:--ir c--fm 225,264 co ft 44.7 min 5040 cfm Since this outdoor air quantity is greater than one air change ir hour in the conditioned space, no additional outdoor air need considered as infiltration in the beat load calculations. Return (or recirculated) air " supply air -- outdoor air = 2440 - 380 - 3060 lb per min - 45,500 - 5040 = 40,460 cfm Apparatus Selection Supply fane required for tffiOO efm. Assuming an arrangement requiring two systems with the resistance of the apparatus and duct system equal to 1 ^ in. water the fan motors needed will be 10 hp each for 22,750 cfm fans. Air washers required for 22,750 cfm at 500 fpm face velocity - 22,750/500 " 45.5 sq ft. Make washer 8 ft wide X 6 ft high above the pan. Assum ing an 18 in. deep pan, the air washers will be 7.5 ft high X 8 ft wide X 8 ft long. With two spray banks, a water circulating pump of about 250 gpm capacity at 65 ft head will be required. A 5 hp pump motor is necessary for each washer. Air Temperatures Total room sensible load (from Example g) * 279,735 Btu Add for 20 bp fan motor (from Example g) = (20) (2545) = 50,900 Btu Greet sensible heal load = 330,635 Btu Gross sensible heat load, Btu Lb supply air per min (60 min)(sp ht)(temp range' Temp range 330,635 (60) (0.24) (temp range) 330,635 (60) (0.24) (3440) 3440 lb 6.68 F deg, say 7 F deg 58 F room temp -- 7 deg range -- 51 F indicated supply air temp