Document 3eDyqm06gJ767bQjq0b5k1jex

520 CHAPTER 48 1962 Guide And Data Book 70,000 cfm suppiy air 53.9 toos refrigeration 1300 cfm per too Air Temperature Range (Btu per hr sensible load) (ep bt) (lb per hr supply air) (Btu per hr)(cu ft per lb) (sp ht)(cfm)(60 min) Overall Air Temperature Range (560,000 Btu)(12.57 cu ft per lb) (0.24) (70,000 cfm) 60 min) Average Air Temperature Range in Rooms 7.0 Fdeg (451,310 Btu) (12.57 cu ft per lb) ,, ,, _ . (0.24)(70,000 cfm)(60 min) 6 Average Air Temperature Range in Precoolers (116,200 Btu)(12.57 cu ft per lb) * 6.7 Fdeg (0.24) 5 care)(3000 cfm) (60 min) Air Temperature Range in Precooler at End of Precooling (16,180 Btu)(12.57 cu ft per lb) (0.24)(5 cara)(3000 dm)(60 min) > 0.94 F deg Air Temperature Range in Storage Room (16,180 Btu)(12.57 cu ft per lb) 2.8 F d (0-24)(5 cara)(1000 cfm)(60 min) Air Temperature Rise in Fan and Ducts ' (90,290 Btu)(12.57 cu ft per lb) (0.24)(70,000 cfm) (60 min) 1.1 Fdeg Design the brine spray washer for 32 F leaving air tempera ture. With 1.1 F deg rise in fan and ducts the sir entering rooms will be 32 -f 1.1 = 33.1 F deg. Assume that air leaves washer at 31 F dew point and 95 percent relative humidity at which condition (from a psychrometrie chart) the air will have a mois ture content of 25-3 gr per pound. Average Moisture Gain in Rooms (Btu per hr latent load) (7000 gr per lb) (1060 Btu per lb)(lb per min supply air) (Btu per hr) (7000 gr)(cu ft per lb) (1060 Btu)(dm)(60 min) (41,300 Btu)(7000 gr)(12.57 cu ft per lb) (1060 Btu) (70,000 cfm) (60 min) = 0.8 gr per lb supply air The return air will then have a moisture content of 25.3 + 0. = 26.1 gr per lb and will have a dew-point temperature of 31.71 The return air dry-bulb temperature, with a-7 F deg range i temperature, will be 32 + 7 39 F. The average temperature of air leaving precoolers 33. + 6.7 ** 39.8 F dry-bulb and 31.7 F dew point. The relativ humidity will then be 73 percent. The average air temperature leaving precoolers at end of pn cooling -- 33.1 + 0.94 = 34.04 F dry-bulb, and 31.7 dew poini Tie relative humidity will be 90 percent. The air leaving the storage rooms *= 33.1 + 2.8 = 35.9 1 dry-bulb and 31.7 dew point. The relative humidity will be 8 percent. Revbiqehation System Amrtinte a 2 F deg brine temperature range. _ (53.9)(28galdeg) .. Then------------ ------------- --- 755 gpm brine With a 2 F deg temperature difference between leaving air and leaving brine in washer the having brine temperature will be 32 -- 2 - 30 F. The temperature of the leaving brine will be 30 -- 2 * 28 F. The average brine temperature is (30 + 28) + 2 29 F. Select a refrigerant temperature 8 to 10 F deg below average brine, say 29 -- 9 20 F. By using Ammonia, the evaporator pressure corresponding ^ 29 F is 33.5 psig. Select a washer for 500 fpm air velocity, with conrequent 7(1000 + 000 " 140 sq ft washer cross sectional area. The approximate washer dimensions will be 14 ft wide X 11.5 ft high (allowing 1M ft for the water pan) X 8 ft long. The brine spray per square foot mil be 755 -5- 140 -- 5.4 If there are two spray banks the brine per square foot per spray bank is 2.7. Select a brine pump with capacity of 755 gpm at 65 ft brine head at 25 paig spray pressure with a 20 hp motor. LEMON AND GRAPEFRUIT STORAGE Air Conditioning Equipment For lemon and grapefruit storages, an air conditioning sy&. tem to perform the following functions is required: 1. Heat or cool to maintain a constant temperature 2. Humidify to maintain a high relative humidity 3. Ventilate with outdoor air to maintain low CO* concen tration 4. Distribute the air uniformly 5. Wash or filter the air The type ofair conditioning system commonly used consists of a fan, an air washer, and ductwork for air distribution. The conventional system uses large quantities of air, about 1 cfm per storage box or 650 cfm per carload. This results in a low temperature rise in the supply air which makes it possible to maintain the high humidities required. A variation of this method is termed in this article the combination system. It em ploys much smaller air quantities (about l cfm per sq ft of floor area or 300 cfm per carload) but requires the addition of auxiliary humidifying nozxles in the room to maintain high humidity. The conventional system using large air volumes seems to give better results as high air circulation tends to give uniform temperatures and humidities throughout the storage room. The air circulation must reach all parts of the room to avoid dead spots and pockets of carbon dioxide. Normally fans of the high-speed, backward-curved-blade, centrifugal type are used as quietness of operation is not im portant. This type of fan is nonoverloading. Depending upon the arrangement and application, fans may be either single width single-inlet or double-width double-inlet. Total system air resistance usually is between 1 and 2 in. water gage depend ing upon the type of equipment and length of duct system. The air washer consists of a spray chamber about eight feet long having two banks of spray nozzles opposing each other and with moisture eliminators on the outlet end. The air veloc ity across the face area of the washer should be between 400 and 600 fpm. At 25 to 30 psig spray pressure, a high degree of air cleaning is accomplished and the air will leave the washer very close to saturation at the water temperature. The outdoor air connec tion should be sized for the full washer capacity so that evapo rative cooling with outdoor air can be utilized during cool nights or when the outdoor wet-bulb temperature is low. Since a saturating type washer is used, saturated or dew point air is circulated in the duct system. However, the supply air must be below the room temperature to absorb the heat load. This heat pickup in the storage space lowers the relative humidityTo maintain 84 to 88 percent rh, the supply air temperature rise must be small, thus necessitating large air volumes, unless. auxiliary humidification is used. The spray water is cooled by refrigeration. Ammonia, Re frigerants 11 or 12 are commonly used. If ammonia is used, the equipment should be sealed in a room or building separate from the storage rooms as ammonia fumes are injurious to the Citrus Fruit Storage 521 fruit. With ammonia and Refrigerant 11, indirect refrigeration gystemsare used. Water is cooled in a shell and tube cooler (or other type of evaporator) and is then circulated through the rashersprays. An indirect system may be used with Refrigerjnt 12, but generally direct expansion coils are placed directly in the air washer between the two banka of sprays. Since Refrigerant 12 leaks are not injurious to the fruit, a direct ex pansion design can be used at a considerable saving, in most compared with an indirect Refrigerant 11 or ammonia job. For a carbon dioxide concentration of 0.1 percent, about 72 efm of outdoor air per carload must be provided for lemons stored at 58 F or grapefruit stored at 60 F. The difference be tween the total supply air quantity 0.e., 650 cfm per carload for conventional or 300 cfm for combination system) and the minimum outdoor air quantity will be the recirculated or return air quantity. For temperature control both winter and summer, preheat- ' mg and reheating coils should be provided as well as refrigera tion) Heating coils may besteam, hot water, orelectricdepend ing upon sources of heat available and energy costs. The re frigeration should be controlled by a dew point thermostat in the air washer discharge. On an indirect system it would con trol a three-way miring valve on the suction of the air washer spray pump. If a room thermostat is used, it would control the air washer bypass dampers and also the reheat coiL A thermostat in contact, with the mixed recirculated and oatdoor air would control the preheat coil, and either an out door dry-bulb economizer thermostat or an outdoor wet-bulb thermostat would control the outdoor and return air mixing dampers. Other control combinations may be used, but con stant dew-point control to maintain the required relative humidity is the most important control function with a con ventional system. With a combination system, the humidity may be easily and positively controlled by means of a room hiimidistat acting upon a solenoid valve in the air line feeding the atomizing water spray nozzles. Capacities of humidifying nozzles are given in Table 6.- Air Distribution Air distribution ducts may be made of galvanized sheet metal or aluminum but, unless insulated, they will sweat be cause of the high humidities in the storage rooms. Therefore, it has proved economical to build the ducts of one inch thick in sulating board with a protective coating on both sides. The high humidity apparently does not cause deterioration of the insulating board if properly coated, and the ducts dry out dur ing the off season when the storage is empty. The duct systems are sized in the usual manner for a con stant pressure drop per 100 ft of length with the total pressure drop based on the longest run. The starting supply duct veloc ity at the fan should be not more than 1700 fpm for sheet metal ducts or 1500 fpm for wood or cane, fiber ducts. Highvelocity type distributing outlets or nozzles, which induce a high secondary air movement (about one and one half to two times the primary air), are often used. These ejector nozzles are generally conical in shape with an angle of convergence of 12 to 14 degrees (Fig. 11). The nozzle length should be about three times the outlet diameter, giving a coefficient of dis charge of 0.94. The conical nozzles are usually made of sheet *a* Dorvatmoa mmro rex onus wow HAVE AMSLC A a TO H DEB Fig. 11 .... Design Factors for Typical Air.Supply Nozzles Binks Parks Cramer Armstrong Table 6 ... .Capacities of Humidifying Nozzles Type Water Syphon Water Pressure Water Syphon Steam (*/u in. orifice) Gi in. orifice) (lj^ in. orifice) Sire 164 174 N4 N6 N8 N10 N12 N15 K-2 K-2 K-4 PrwAsur*, Ut per Sq la Air Sloan 30 40 60 40 40 60 30 30 30 30 30 80 2 5 10 15 2 5 2 5 10 15 Water 0 0 0 30 30 30 0 0 0 0 0 0 Moisture lb per Hr 4.5 4.9 5.7 17.0 23.0 37.0 4.0 6.0 8.0 10.0 12.0 15.0 9.0 15.0 25.0 35.0 18.0 33.0 240.0 510.0 960.0 1410.0 five Air Required, Cfm . 1.00 1.25 1.75 1.00 1.25 1.75 1.0 1.3 1.5 1.7 2.0 2.6