Document VG4B0Z191VqwrN2wVV2Gya7gK

694 CHAPTER 42 1965 Guide And Data Book Table 3 .... Net Refrigeration Effect Conversion Factors* 10 15 16 18 : * 19 23 27 .. -30 -25 3.94 3.58 3.03 2.81 3.80 3.45 3.17 2.92 2.71 2.62 2.29 '2.17 2.54 2.34 2.22 2.10 2.06 2.00 1.97 1.90 1.89 1.82 1.81 1.75 1.73 1.67 1.00 1.60 1.57. 1.52 1.47 1.47. 1.42 1.41 1.37 1.31 1.27 -20 -15 -to. -5 0 5 10 3.69 3.35 3.08 2.84 2.63 3.60 3.27 3.00 2.77 2.57 3.52 3.20 2.93 2.71 2.51 3)45 3.13 2.87 2.65 2.48 3.39 .3.08 2.82 2.60 2.41 3.32 3.01 2.76 2.55 2.37 3.27 2.95 2.71 2.51 2.33 2.46 2.31 2.17 2.05 2.40 2.35 2.25 2.18 2.11 2.06 2.00. 1.95 2.SO 2.12 2.01 1.90 2.24 2.07 1.96 1.85 2.20 2.03 1.92 1.82 2.16 1.99 1.89 1.79 1.94 1.91 1.86 1.81 1.78 1.73 1.70 1.85 1.80 1.76 1.72 1.68 1.65 1.62 1.78 1.74 1.69 1.65 1.61 1.58 1.55 1.71 1.66 1.62 1.58 T.54 1.61 1.49 1.63 1.56 1.48 1.43 1.59 1.52 1.44 1.40 1.55 1.48 1.40 1.36 1.51 1.44 1.38 1.33 1.47. 1.41 1.34 1.29 1.45 1.38 1.32. 1.27 1.42 1.36 1.29 1.25 1.38 1.35 1.33 1.30 1.28 .1.25 1.23 1.20 1.32 1.27 1.22 1.18 1:28 1.24 1.19 1.15 1.25 1.20 1.16 1.12 1.23 1.20 1.18 1.16 1.14. 1.12 1.10 1.0S 15 3.21 2.91 2.67 2.47 2.29 2.12 1.96 1.85 1.75 1.67 1.59 1.53 1.46 1.40 1.33 1.27 1.22 1.18 1.14 1.10 1.06 * To obtain- oat rrfricwatian affwt ctikcr titan buio rating conditions, tfvida tt- rating bj iy'"t 20 25 3.15 2.87 2.63 2.43 2.26 3.12 2.83 2.59 2.40 2.23 2.10 1.94 1.83 1.74 2.08 1.92 1.82 1.72 1.65 1.58 1.52 1.45 1.64 1.56 1.49 1.43 1.39 1.32 1.26 1.21 1.37 1.31 L.24 1.20 1.17 1.13 1.09 1.05 1.16 1.12 1.08 1.04 (ran trklT 30 2.79 2.2 2.04 1.69 1.53 1.46 1.40 1.34 1.28 1.22 1.18 1.10 1.08 1.02 35 2.17 1.67 1.39 1.17 1.01 40 2.14 1.66 1.38 1.16 1.00 45 2.12 1.64 1.36 1.14 0.99 Ratings of both the compressor and the condenser must be plotted on the same basis. For example, if the-condenser is rated for 15 F subcooling, the compressor rating must be based bn 15 F subcooled liquid at the .evaporator or.appropii- ate corrections as suggested by themanufacturer must be applied to the ratings. Compressor saturated discharge tem perature must be corrected for pressure drop in-the hot gas line -to obtain the' saturated condehsing temperature for plotting of the air-cooled condenser'rating. The pressure drop is usually assumed to be the equivalent of 2 F deg drop in saturated temperature. The condensing capacity.-and con densing temperature are read at the intersection of the com pressor curve. (Qc) and the condenser curve (GHR). The net refrigeration effect (NRE) and power - (F) demand is then read by transposing the result to the (NRE) and P carves for the particular compressor^ as shown in Fig. 16. The same compressor, at 90 F ambient temperature, will operate at approximately 127 F condensing temperature and'250,000 Btu/hr net refrigeration effect with a power consumption of 28,000 watts, using the smaller condenser; or at 118 F con densing temperature and 270,000 Btu/hr (NRE) with a power consumption of 26,200 watts, using the next larger condenser. The ultimate condenser selection must satisfy the. anticipated cooling requirements at design ambient conditions at. the lowest possible annual operating cost of the total compressorcondenser system. CONTROL OF AIR-COOLED UNIT CONDENSERS In the amplest form of control, the fan on the unit con denser starts and stops with the compressor. This method was first used for air-cooled condensing units where the condenser fan was driven by the compressor motor. It is satisfactory where the air temperature at the condenser remains Condensers 695 Fig. Ifl .... Multiple Condenser Fans Controlled in Sequence reganlless of the time of day or season. Thisis seldom the Mg with remote condensers. During winter operation, the condenser capacity is greatly increased, due to low entering air-temperatures, so that the pressure falls to an abnormally low level. While this is from the point of view of compressor efficiency, the following problems arise: (1) insufficient pressure-difference Mrnfig the expansion valve due to low receiver pressure-de-- Kvere insufficient refrigerant to the evaporator, (2) -cold liquid with little or no subcooling evaporates in the liquid line, if that line runs through a warm space or must overcome a vertical lift before reaching the expansion 'valve-; vapor at the expansion valve inlet can also result.in insufficient feeding of the evaporator, (3) hot gasdefrost systems cannot function without adequate head pressure at the onset of defrost, and (4) whena low-pressure switch controls compressor operation, the pressure switch may not be actuated.when the pressure corresponding to the air temperature at the oondenser is .at or below the pressure settings of the low pressure'switch. - To prevent excessively4ow head pressure, the condenserdan may be stopped by a reverse acting high-ipressure switchwhen- ever the head pressure falls below .'a predetermined hmit^ -This control method is adequate where the condenser unpro tected bom winds and where -pronounced fluctuation Of head pressure, with -corresponding evaporator pressure -changes,, can be tolerated (Fig. 17). ` Head pressure and evaporator pressure .fluctuations caused by fan cycling can be minimized by using multiple fans con- Bypass Method* trolled in sequence. Fan control may be either in response to ambient temperatures or to condenser-pressure. Headpressure control with-multiple fans will rapid cycling of the fan motor and rapid fluctuation -in. head pFessure:-(see Fig. 17). Fig. 18 illustrates this type of fan control. -When multiple fans are cycled in response to ambient-tem perature, one fan -must be in operation at all times to prevent excessive.condensing, pressure.. Thus, a tworfan unit.may cycle only .one fan and a three-fan unit may cycle -two fans. The capacity, of two^fan .condensers will be: reduced to.-apr .proximately 55 percent capacity by cycling one fan,1while -the capacity of-a -three-fan condenser will be .-reduced-to ap proximately 40 percent by cycling two fans. Further, reducr .tion in capacity to approximately 15 percent is possible.-by modulating the air flow through the uncontrolled fan section with speed control or with dampers on the air intake or disc charge side (see Fig. 19). When used with propeller fan units, the closing of the damper may have to be limited or an over- m Proprietary naAcd tU 5.' VaHtllMo. 2,9B6j99l Fig.-i.21...... Modulated Bypass GontecficjtlSng .a :.ThreeWoy-Valve