Document 2qmXpa2x2YVMYyo9YX6BV7zeN

Fit The Air This concluding article shows how to solve design problems. Part I gave reasons lor studying building requirements and making a complete physical survey before designing a system V J D CONSTANCE, Protuihncl fnflnm the simple exrcotBHT of installing leu cooling surface and operating It at lower temperature sometimes materially reduces the first cost of equipment. Be cause the combined condensing unit and evaporator fit the load at a lower suction pressure, the horsepower re quirements per ton of refrigeration in crease, Fig. 7. But .this increased power use is offset by the attendant improve ment in comfort conditions at the lower Indoor humidities. Change in Humidity. Conversely, a alight increase in design-indoor humid ity may use greater coil surfaces and, therefore, a more expensive evaporator assembly, but lower power-consuming refrigeration equipment Of course, leu comfortable conditions of humidity pre vail. Since condeasing-unil capacity varies in steps according to motor sue, a change in design-indoor humidity may easily determine economies in first cost of the condensing unit Variations in design-indoor dry-bulb temperature af fect equipment selection similarly but to a different degree. Economic selection of equipment thus requires a balance between operating expense and fixed charges. Although total cost may be reduced by slight changes in design-indoor conditions, these are beyond the control of the application engineer in industrial and process applications, and often in com fort installations, because conditions are' either defined by the process or set by contract. Ho must confino his efforts, therefore, to variations in air flow and characteristics of heat-transfer surface in arming at the most economical ar rangements. Conditions That Naed Reheat. The straight line joining points 1 and 2 in Fig. 8 may fail to intersect the satura tion line (Part (, Jan 1948, p 80) be cause of high latent-heat load require ments. This indicates that the ratio of latent- to sensible-heat loads (curve slope) is so great as to require on addi tional air-treating device. Remember that surface-type equipment has definite limitations with respect to dehumidification. Once enterlng-oir conditions, point 1, are set, the maximum ratio of latent to sensible capacity attainable with that particular coil Is fixed by the steepest line that can be drawn through initial point 1, and yet terminate on the saturation curve. This is a tangent to the 100% saturation curve. Thus, with the entering-air point fixed, changes in surface-cooler design and modifications to air flow, such as by passing, Fig. 9, will not further In crease the amount of dehumidificstion with respect to sensible cooling. This holds true for extended surface coolers of all types. Reheat or Sorbent. Only alternative is to supplement the surface cooler by some device that altera the temperature or humidity of the air. One method Is to reheat the air. Fig. 10, after it has been cooled and dehumidified. Another way is to dehumidify the air chemically. Fig. 11, before passing it to the surface cooler. Reheat is commonly used when the ratio of latent to sensible heat U greater than can be handled simul taneously in a single process. Air en tering the Conditioner may be cooled by a surface maintained at some temper ature below the initial dewpoint cor responding to 2 on Fig. 8, to tome point where the correct amount of moisture lias been removed. In removing the latent heat end sweating out moisture from the air stream, more than the re quired amount of sentiblo heat Is re moved. This results in s tow dry-bulb temperature. Watch Dry.Otilb Temperature. If air is introduced to the conditioned space without further treatment, dry-bulb temperature will be below the design value. With normal ceiling height, architectural treatment and supplygrille setting, dry-bulb temperature of entering air should not he more than M (72) IS F below design value. To avoid tos ^ ^ low a temperature or too great a diffo- > sion, (he air stream leaving the cooling 1 coil must be warmed by a reheat caQ l in tho conditioner or in the supply docL If point 3, Fig. 8, Is the fixed coadi- j tlon of air supply to the conditioned j ; space, length of line 2 to 3 represents . ! reheat rcqulrementa. Ideal setup is U ( condition the air down to the inuoduo f , lion point, but this Is seldom done la l practice. Problems on Selection. The follow- ! ing example illustrates the principle jj involved in selecting equipment for i cooling and dehumltUfying. For com fort cooling, the design conditions could apply to areas in and around New York Gty and Tampa, Florida. DESIGN CONDITIONS j Outdoor*, 95 F dry bulb, and 78 F wt bulb ledeora, SO F dry bulb, ud 50% reUtlro bualditf Outdoor olr (apply, 1340 dm Tout Knilbto boot gain, 87,000 Bid per hr Total latent-beat gale, <9300 Otaper hr Hollo. Utrut-io-ocaiiblo-but gates 0.56 Total air circulation, 4000 efm * | The absolute humidities at design ; outdoor (o) and Indoor (I) conditions , are determined from the psychrometrfc chart (see Jan 1948 Powea, p 83, for exploitation of symbols): uu " 117 grains per lb \ti 77 grains per lb | j POWER * February I9 I The conditions o( air entering the cooling surface are determined from: *. I, -h- #<!.-<-> 56% ratio between latent- and sensibleheat capacities when extracting 87,000 + 48,300 = 135,300 Biu per hr. This is equivalent to removing heat Accordingly, our figures show that: DC o.w* 85 " 1.06X 4000 04JP. to. < - W| -Mjpi (, t* -- *) equal to 135,300/12,000 = 11.28 tons refrigeration from about 4000 efts of oat W y-300-- 0.06 X 4000 72.1 gr per Ih. and thus, h-80+|^(95 - 80)-MP vi " 77 + $535^77 - 77) - 90.4 gr per tb Air-Flow Rotes. On the psychrometifcchon this corresponds to a wet-bulb temperature of 70.8 F. Note that we used volume-flow rates Instead of air initially at 85 F dry-bulb and 70.8 F wet-bulb temperature. Required temperature and humidity ol the air leaving the cooling surface, point 2 in Fig. 8, may be determined from*. II- It-ls aod 12. Wi -- Surface Temporatare. Locating points 1 and 2 on a psychrometric chart, ikelo- ton form Fig. 8, and extending the straight line joining them until it inter* sects the saturation curve, we find the required aurface temperature, is 54 F. Another common term for this value Is "apparatus dewpoint" Mean temperature difference between weight-flow rates called for In the equa tion!. Resulting error is so slight that this method may bo used without ef fecting the final result within measur able limits. 1 Rating,fables for cooling surface may . be used if. available. They give latent- sad sensible beat capacities for various dry- and wet-bulb ( temperatures, re-1 frigerant temperatures and airflow rates. Together with condensing-unit ratings f auction' temperature or pressure, *peed, eondensing-water requirements utd capacity, a graph similar to Fig. 6 Pan 1 may be developed. For this type of application use on ***poramr temperature of'45 F. All "tat is necessary Is to plot' two points *nd draw a straight line through them, J^**'"* both condensing unit and coil "* same manner, it Is merely nec- SURFACE COOLER con be supplemented by reheating the air after It hoj been cooled end dehumidifiod when the ratio of latent to sensible hoot Is high eu*tJ to `select a surface having a fOW8R February 1940 173) !