Document EqXKyVbn3XNzd6dwqvGd27KrN

668 CHAPTER "40.qivf* ` ' 1,965 Guide'And Data Book (1) the steam valve throttles and supplies a reduced amount of steam which, in turn, reduces the degree of solution reconcentration, (2) the automatic solution valve reduces the quantity of solution reconcentrated which, in turn, reduces the amount of steam condensed, or (3) both effects are ob tained simultaneously. . In addition to controlling the amount of refrigeration pro duced, as outlined above, the temperature of the chillediwater must be controlled to meet the requirements of the load! The absorber, through' its effect on the evaporator pressure, de termines the evaporator temperature, and consequently con trols the leaving chilled water temperature. The.absorber, in turn, is controlled by the flow rate and concentration of the absorbent solution which it receives, and by the temperature of the oooling water. For installations where the design load is close to the ma chine capacity rating, the steam requirements will be 19 to 20 lb: of steam per ton-hour. Steam consumption at reduced loads'will depend upon-the type of capacity control used. Fig. 12 shows the' range in consumption for a typical unit. Auxiliaries Heat Exchanger. While the entire absorption machine is essentially a group of heat exchangers, one particular com ponent is known as the heat exchanger. This is located, .thermodynamically between the absorber,and the generator;, it preheats dilute solution flowing from the absorber, to the generator;.while cooling the concentrated solution flowing from the generator to the absorber. It may be of shell-and-tube or double pipe construction. . , ... Purge Unit. Because tire lithium bromide cycle operates at a high vacuum, a purge unit is included to maintain that -Vacuum by removing any nnn^ndpnaahitt gaa that might accumulate. The purge unit may be a vacuum pump,' an eductor or a palladium cell. - Expansion Valve. Mechanical expansion valves are not used in absorption units. The flow of refrigerant -liquid to the. evaporator is controlled by an orifice or other fixed restriction between the condenser and the evaporator. .Solution and Refrigerant Pumps. In large tonnage units, the dilute solution and. the refrigerant,are circulated through the unit by email pumps rather than by natural circulation. These pumps may be used to dilute the absorbent solution' in .the generator when cooling is no longer required, to elimi nate any possibility of crystallisation of the concentrated solution. , . . .. '. Refrigerant arid Solution Piping... In all refrigeration - systems, pressure losses in piping must be kept to a mini mum, consistent'with other design criteria. In an absorption machine, no consideration is needed for lubrication or lubricant recovery. Velocities-of solutions-and refrigerant vapors may be selected on the basis of heat transfer vs: pres, sure drop. Because of their inherently closed-cycle nature absorption machines are riipplied as complete air-conditions^ or water chillers (not as components). Complete refrigeration and solution interconnections are furnished as part of. the piping of the machine. *: Insulation. To prevent sweating, the chilled water headers tiie refrigerant pump, and refrigerant piping should be insu lated. Insulation may be provided for the absorber-evaporator shell when unusually high ambient conditions are encountered in the equipment room, and adequate ventilation is not avail able. "" - - Limit Controls Controls are provided to stop the machine whenever certain conditions are outside pre-eelected limits, as follows:' 1. Law temperature cut-out. This is a low limit thermostat which will stop the unit if the evaporator temperature falls tod.low.' When the refrigerant temperature warms to the cut-in point, the switch resets itself to start the machine again 2. Cooling water pressure switch. This, is a presure sensitive switch which will stop the unit if the cooling water supply failV The switch resets itself when the cooling water supply is restored. 3. Chilled water flow switch. This is a pressure sensitive'switch that will stop the unit if the chilled water flow is reduced below design limits. The switch resets itself when the proper chilled water flow is re-established. " 4. Liquid level switches. These are'float-type switches that will stop the unit if the refrigerant level in the evaporator is too low or too high. They are self-resetting. Protective Devices 1. Hick pressure cut-out switch. This is a pressure; sensitive switch that will stop the unit if the high side pressure exceeds a safe limit. It is only required on ammonia-water units. 2. Generator high-temperature -limit switch. Ibis'is a tem perature-sensitive switch that will stop, the unit if the generator operating temperature exceeds a safe limit. This is generally used only with direct-fired units. 3. Generator shell bursting disc.' On high temperature water apjjjteafums/a bursting disc may be furnished in the generator . EQUIPMENT SIZING AND SELECTION, ' The proper selection of equipment for any air-conditioning application is of utmost importance in obtaining satisfactory results from the system. The critical factors are: (1) the sire and nature of. the load, and (2) economics (the total owning and operating oqste). The method for calculating the cooling'load is shown in Chapter 27; It is important to emphasize that many of the heat gains' are variable and that they do not necessarily reach their, maximum values simultaneously.. Proper evalu ation of the timing of these peaks will avoid pyramiding the load. A discussion of the factors affecting the actual instan taneous cooling load will be found in Chapter 27. ' A method for preparing a comprehensive economic study appears in Chapter 86 of the 1964 Guinn And Data Book. - * Stooa cotaoapthn vorf**.froo a low of-17A ai 10% refed toa highoi 194 Am pot hr por too at, 100% '! h Utod'by pormiaion from fafatnw 3 - - Rg. 12 .... Steam.Consumption.for a . Typical Absorption Unit*1 Table 1 .... Normal Rating Condttions'-fbr Heat-Operated Units ' Leaving chilled water temperature 44 F Chilled water temperature differential 10 F Entering condenser water temperature 85 F , Steam pressure at control valve inlet 12 pdg (dry and .saturated) Scale factor for' evaporator, condenser, and absorber ` 0.0005 i Absorption Air-Goridiiidhirig and Refrigeration Equipment ;`V.i O 669 Proper preparation-of that part of;the study pertaining to operating-costs requires a.workmg knowiedge iof' the various oJasuroptiomrates. In the absence of specific `manufacturer's information, the following procedure will-furnish reasonable estimates of the required machine- are and the consumption rates for,energy and:wateh' . h- -- -The nominal capacity rating of an absorption-machine is its capacity under a given set of conditions.-These conditions'are g^^ally accepted-to' be as given:in-Table;l..'`' u?: v.- Therefore, to determine the required sire of unit it is neces sary to find the nominal capacity rating that will provide the required refrigerator capacity.jfi^r.;'; . Variations in capacity with varying conditions .of chilled water' temperatures,*condenser water entering .temperature, and eteam.pressure are given in.Fig.T3..` v. jy ,;? Capacity factor Pi is plotted as a fuhetion of th'e'entering pqnd,n^.f water temperature. Thus 'Fi``=^-1.0 for an entering oondenser water'temperature of 85F. Similarly,' Pb is plotted as a function of the tearing chilled water temperature and is 1.0 at 44 F. Fc is plotted'as a function of the chilled water temperature differential and is 1.0 at 10 F deg.Fp is plotted as a function,of inlet steam pressure andia'l.Oat 12 psig.- - V': `The overall capacity.factor,for,the,.unit is.the product of the four capainty factors found from Fig. 13. Therefore, for the conditions given in Table 1, the overall capacity factor Fo:=1.0,'and the.nominal capacity rating equals^the're quired refrigeration load; -' !* r!^ Selection of a unit for conditions other than those'giveh in Table l is illustrated by ExampUl. ' '! Example 1: A beat-operated air conditioner is to be installed in an office building having a refrigeration load of 200 tons. The chilli water temperaUire leaving the'unit is 42 F, with an 8 F deg differential between entering and-leaving temperatures. The water temperature entering the condenser is 87-F. Dry, saturated steam is available at 10 psig at the control valve inlet. Find: (a) the nominal capacity, rating of the unit, (b) the steam con sumption at full load,' (c) the'condenser water flow rate, (d) the chilled water flow rate, and (e). the heat rejected to the cooling tower. Solution: (a) The capacity factors are found from Fig. 13. For 87 F entering condenser water, F =* 0.96. For a 42 F chilled waiter temperature, F 0.935; For an 8 F deg temperature difference, F = 0.99. For 10 psig steam pressure; Pd " 0.98. The.overall capacity factor Fo is: ,- . -:- o* Fo,- WaHPsKPcXFd> - (0.96)(0.935)(0.99)(0.98) = 0.871- , i The nominal capacity rating required' is 200/0.871 = 230 tons. Any unit with a nominal capacity rating;of 230 tons or more could be.used for this application. , '1 (b) The steam consumption usually is estimated as 20 lb per (ton) (hr):'Therefore; the'steam'flow at fuirioad'(200 tone) 'is 200 X 20 = 4000 lb per hr. y Py ENTERING CONDENSER WATER TEMP - F' 40 42 44 4 B 46 LEAVING CHILLED WATER TEMP - F SO Rg. 13.... Capacity Factors for Equipment Selection