Document g20a7z91xmMGbvMnR8QvoqLgL

American Society of Heating and Ventilating Engineers Guide refrigeration increases due to the higher compression ratio, but the/ other considerations which make such a system desirable. In the^ place, where a toxic refrigerant is undesirable or cannot be used H '31 fire or other risks especially in densely populated areas, the brine cacL cooled in an isolated room or building and then be circulated throuehtt air conditioning equipment in perfect safety because it is used t<!> e<v)|l water or air, without any possibility of direct contact between the air ' refrigerant. | When an indirect system of cooling is used, it will be found that the)/ transfer rate of the water cooler is considerably higher, as a general than that of a direct expansion cooler for the same requirements. direct expansion interchangers, it is almost impossible to keep the * system-flooded with~liquid7 whereas with brine interchangers'the cool medium completely fills the space of the interchanger and perfect conti is insured. TM Ice may be used for chilling water or air for conditioning work. {< application is limited because of the cost of ice, although the efficiency cooling is higher than any other water cooling system. The word "waj cooling" is used advisedly in that the direct cooling of air by ice is, whij not impossible, rather impractical. It might be said that ice coolersai economical for systems requiring a maximum of 20 tons per 24 h<r where the load fluctuates considerably, and it is possible to introduce only as it is required to cool water. The most general method of cooli water with ice is to spray the water over the surface of the ice, insuring') much contact as possible and approximating the same performance as td Baudelot type of cooler. Because of the large fluctuations in load in tint air conditioning system, the higher cost of refrigerating effect when icej used is offset by the fact that there are no motor and condenser ij efficiencies under partial load. Also, because the cost of the mechanic! refrigeration equipment for the small system is so much higher per unite effect, the fixed charges are small enough to overbalance the extracts of the ice. Condensers ./ Condensers are usually either the double pipe type or the shell and tut type. Shell and tube, condensers are almost identical with coot Double pipe condensers are arranged so that water passes through inner of two concentric pipes, and refrigeration passes through annular space in the outer pipe. Where possible, there should be coun flow of the refrigerant and the condensing water to maintain maximp temperature differences. The amount and temperature of the condensing water determine d condensing temperature and pressure, and indirectly the power reqi for compression. It is, therefore, necessary to strike a balance so that quantity of water insures economical compressor operation. As part of the condenser, or attached to it, there must be storage s| for liquid refrigerant. The installation of all equipment should be accessible for inspection, repair, and cleaning. Both the coolers condensers should have space for pulling tubes. Because there is a decided tendency to conserve the water in city and most large cities are restricting the use of water, in order to use' 178 vstems and refrigeration equipment it is often necessary to conditioning s^towers. The cooling towers, unfortunately, produce the install cooling . water at the time when the load on the system is tfannest con t refrigeration equipment must be designed to meet greatest, so jrnum ioaci at normal conditions, but also the maximum not only the condensing water temperatures. If properly designed, load ataDn difference in the efficiency of operation throughout the this, makes ^ t^ose t;mes when the condensing water temperature is year excep ^ occurs only for 5 per cent of the entire cooling period it ^^disregarded as a factor in establishing yearly operating costs. ca" ooiing tower has a certain advantage over the use of water from mains in that the temperature of the condensing water varies with the outdoor temperature and, as pointed out, the refrigera- irednad also varies with this temperature. Certain economies are pos- when a cooling tower is used which cannot be achieved by the use of 9b Hehsine water from city mains, even where the city water temperature ?!vtrpmelv low. Normally, the lowest city water temperature met during fhfsummer months is from 65 to 70 F. This temperature range takes lace for the entire cooling period, regardless of what the outdoor tempera tures are. With the cooling tower, the temperature of the condensing water may rise to 80 or 85 F under maximum conditions, but under less than maximum conditions the temperature of the water off the cooling tower drops considerably, and it has been established that 50 per cent of the time the outdoor wet-bulb temperature varies from 60 to 70 F and the cooling tower water, therefore, for the same periods, varies from 65 to 75 F. When the outdoor wet-bulb temperature drops below 60 F, which occurs approximately 30 per cent of the time, the condensing water temperature is still lower. The cost of water used for condensing is negligible, as the only water required is that used to make up the loss by evaporation in the cooling tower itself. See also Chapter 11. PROBLEMS IX PRACTICE Ulna locality where the electric power rate is based on a demand charge, it U desired to install the smallest possible compressor motor which will provide summer cooling for a 300-seat restaurant which operates 6 hours per day from 11 a.m. to 2 p.m., and from 5 p.m. to 8 p.m. The refrigeration load at the peak is 28 tons. If the load factor for both the noon and evening meals is 70 per cent, discuss the type of equipment which would take the greatest advantage~of the reduced power rate at low kilowatt demand. A storage system using a chilled water storage tank would permit the installation of a "refrigeration system having the smallest motor. For a 28-ton system operating 6 hours per day at a 70 per cent load factor, on the maxi mum day the total heat removed would be, 28 tons X 6 hr X 0.7 = 117.5 ton-hours per day. Hacompressor were to operate 24 hours at a constant rate, its average capacity would be 117.5 ton-hours *""24 jj0urs------- 4-9 tons, or approximately 5 tons. If operated 12 hours per day, the compressor capacity would have to be increased to 10 tons. ^ storage tank would store the refrigeration and allow off-peak operation, so a *ler ajmpressor motor could be used. However, the suction temperature at which the tempicssor would be operated would be lowered approximately 5*to 10 F. This would horsepower per ton of refrigeration, when dichlorodiflouromethane is used, approximately 10 per cent for a 5 F reduction and 24 per cent for a 10 F reduction in the 179