1、2.1CHAPTER 2AMMONIA REFRIGERATION SYSTEMSSystem Selection. 2.1Equipment 2.2Controls 2.6Piping. 2.7Reciprocating Compressors . 2.10Rotary Vane, Low-Stage Compressors. 2.12Screw Compressors 2.12Condenser and Receiver Piping. 2.14Evaporative Condensers 2.15Evaporator Piping 2.17Multistage Systems. 2.20
2、Liquid Recirculation Systems. 2.21Safety Considerations. 2.25USTOM-ENGINEERED ammonia (R-717) refrigeration sys-Ctems often have design conditions that span a wide range ofevaporating and condensing temperatures. Examples are (1) a foodfreezing plant operating from +50 to 50F; (2) a candy storagereq
3、uiring 60F db with precise humidity control; (3) a beef chillroom at 28 to 30F with high humidity; (4) a distribution warehouserequiring multiple temperatures for storing ice cream, frozen food,meat, and produce and for docks; and (5) a chemical process requir-ing multiple temperatures ranging from
4、+60 to 60F. Ammonia isthe refrigerant of choice for many industrial refrigeration systems.The figures in this chapter are for illustrative purposes only, andmay not show all the required elements (e.g., valves). For safetyand minimum design criteria for ammonia systems, refer toASHRAE Standard 15, I
5、IAR Bulletin 109, IIAR Standard 2, andapplicable state and local codes.See Chapter 24 for information on refrigeration load calculations.Ammonia Refrigerant for HVAC SystemsThere is renewed interest in using ammonia for HVAC systemshas received renewed interest, in part because of the scheduled phas
6、e-out and increasing costs of chlorofluorocarbon (CFC) and hydrochlo-rofluorocarbon (HCFC) refrigerants. Ammonia secondary systemsthat circulate chilled water or another secondary refrigerant are a vi-able alternative to halocarbon systems, although ammonia is inappro-priate for direct refrigeration
7、 systems (ammonia in the air unit coils)for HVAC applications. Ammonia packaged chilling units are avail-able for HVAC applications. As with the installation of any air-con-ditioning unit, all applicable codes, standards, and insurancerequirements must be followed.SYSTEM SELECTIONIn selecting an eng
8、ineered ammonia refrigeration system, sev-eral design decisions must be considered, including whether to use(1) single-stage compression, (2) economized compression, (3)multistage compression, (4) direct-expansion feed, (5) floodedfeed, (6) liquid recirculation feed, and (7) secondary coolants.Singl
9、e-Stage SystemsThe basic single-stage system consists of evaporator(s), a com-pressor, a condenser, a refrigerant receiver (if used), and a refriger-ant control device (expansion valve, float, etc.). Chapter 2 of the2009 ASHRAE HandbookFundamentals discusses the compres-sion refrigeration cycle.Econ
10、omized SystemsEconomized systems are frequently used with rotary screw com-pressors. Figure 1 shows an arrangement of the basic components.Subcooling the liquid refrigerant before it reaches the evaporatorreduces its enthalpy, resulting in a higher net refrigerating effect.Economizing is beneficial
11、because the vapor generated during sub-cooling is injected into the compressor partway through its com-pression cycle and must be compressed only from the economizerport pressure (which is higher than suction pressure) to the dis-charge pressure. This produces additional refrigerating capacitywith l
12、ess increase in unit energy input. Economizing is most bene-ficial at high pressure ratios. Under most conditions, economizingcan provide operating efficiencies that approach that of two-stagesystems, but with much less complexity and simpler maintenance.Economized systems for variable loads should
13、be selected care-fully. At approximately 75% capacity, most screw compressorsrevert to single-stage performance as the slide valve moves such thatthe economizer port is open to the compressor suction area.A flash economizer, which is somewhat more efficient, mayoften be used instead of the shell-and
14、-coil economizer (Figure 1).However, ammonia liquid delivery pressure is reduced to econo-mizer pressure. Additionally, the liquid is saturated at the lowerpressure and subject to flashing with any pressure drop unlessanother means of subcooling is incorporated.Multistage SystemsMultistage systems c
15、ompress gas from the evaporator to thecondenser in several stages. They are used to produce temperaturesof 15F and below. This is not economical with single-stage com-pression.Single-stage reciprocating compression systems are generallylimited to between 5 and 10 psig suction pressure. With lubrican
16、t-injected economized rotary screw compressors, where the dischargetemperatures are lower because of the lubricant cooling, the low-suction temperature limit is about 40 F, but efficiency is very low.Two-stage systems are used down to about 70 or 80F evaporatortemperatures. Below this temperature, t
17、hree-stage systems shouldbe considered.The preparation of this chapter is assigned to TC 10.3, Refrigerant Piping.Fig. 1 Shell-and-Coil Economizer ArrangementFig. 1 Shell-and-Coil Economizer Arrangement2.2 2010 ASHRAE HandbookRefrigerationTwo-stage systems consist of one or more compressors that ope
18、r-ate at low suction pressure and discharge at intermediate pressureand have one or more compressors that operate at intermediate pres-sure and discharge to the condenser (Figure 2).Where either single- or two-stage compression systems can beused, two-stage systems require less power and have lower
19、operat-ing costs, but they can have a higher initial equipment cost.EQUIPMENTCompressorsCompressors available for single- and multistage applications in-clude the following:ReciprocatingSingle-stage (low-stage or high-stage) Internally compoundedRotary vaneRotary screw (low-stage or high-stage, with
20、 or without economizing)The reciprocating compressor is the most common compressorused in small, 100 hp or less, single-stage or multistage systems. Thescrew compressor is the predominant compressor above 100 hp, inboth single- and multistage systems. Various combinations of com-pressors may be used
21、 in multistage systems. Rotary vane and screwcompressors are frequently used for the low-pressure stage, wherelarge volumes of gas must be moved. The high-pressure stage may bea reciprocating or screw compressor.When selecting a compressor, consider the following:System size and capacity requirement
22、s.Location, such as indoor or outdoor installation at ground level oron the roof.Equipment noise.Part- or full-load operation.Winter and summer operation.Pulldown time required to reduce the temperature to desired con-ditions for either initial or normal operation. The temperaturemust be pulled down
23、 frequently for some applications for a pro-cess load, whereas a large cold-storage warehouse may requirepulldown only once in its lifetime.Lubricant Cooling. When a reciprocating compressor requireslubricant cooling, an external heat exchanger using a refrigerant orsecondary cooling is usually adde
24、d. Screw compressor lubricantcooling is covered in detail in the section on Screw Compressors.Compressor Drives. The correct electric motor size(s) for amultistage system is determined by pulldown load. When the finallow-stage operating level is 100F, the pulldown load can be threetimes the operatin
25、g load. Positive-displacement reciprocating com-pressor motors are usually selected for about 150% of operatingpower requirements for 100% load. The compressors unloadingmechanism can be used to prevent motor overload. Electric motorsshould not be overloaded, even when a service factor is indicated.
26、For screw compressor applications, motors should be sized by add-ing 10% to the operating power. Screw compressors have built-inunloading mechanisms to prevent motor overload. The motorshould not be oversized, because an oversized motor has a lowerpower factor and lower efficiency at design and redu
27、ced loads.Steam turbines or gasoline, natural gas, propane, or diesel inter-nal combustion engines are used when electricity is unavailable, orif the selected energy source is cheaper. Sometimes they are used incombination with electricity to reduce peak demands. The poweroutput of a given engine si
28、ze can vary as much as 15% depending onthe fuel selected.Steam turbine drives for refrigerant compressors are usually lim-ited to very large installations where steam is already available atmoderate to high pressure. In all cases, torsional analysis is requiredto determine what coupling must be used
29、 to dampen out any pulsa-tions transmitted from the compressor. For optimum efficiency, aturbine should operate at a high speed that must be geared down forreciprocating and possibly screw compressors. Neither the gearreducer nor the turbine can tolerate a pulsating backlash from thedriven end, so t
30、orsional analysis and special couplings are essential.Advantages of turbines include variable speed for capacity con-trol and low operating and maintenance costs. Disadvantagesinclude higher initial costs and possible high noise levels. The tur-bine must be started manually to bring the turbine hous
31、ing up totemperature slowly and to prevent excess condensate from enteringthe turbine.The standard power rating of an engine is the absolute maximum,not the recommended power available for continuous use. Also,torque characteristics of internal combustion engines and electricmotors differ greatly. T
32、he proper engine selection is at 75% of itsmaximum power rating. For longer life, the full-load speed shouldbe at least 10% below maximum engine speed.Internal combustion engines, in some cases, can reduce operatingcost below that for electric motors. Disadvantages include (1) higherinitial cost of
33、the engine, (2) additional safety and starting controls,(3) higher noise levels, (4) larger space requirements, (5) air pollu-tion, (6) requirement for heat dissipation, (7) higher maintenancecosts, and (8) higher levels of vibration than with electric motors. Atorsional analysis must be made to det
34、ermine the proper coupling ifengine drives are chosen.CondensersCondensers should be selected on the basis of total heat rejectionat maximum load. Often, the heat rejected at the start of pulldown isseveral times the amount rejected at normal, low-temperature oper-ating conditions. Some means, such
35、as compressor unloading, canbe used to limit the maximum amount of heat rejected during pull-down. If the condenser is not sized for pulldown conditions, andcompressor capacity cannot be limited during this period, condens-ing pressure might increase enough to shut down the system.EvaporatorsSeveral
36、 types of evaporators are used in ammonia refrigerationsystems. Fan-coil, direct-expansion evaporators can be used, but theyare not generally recommended unless the suction temperature is0F or higher. This is due in part to the relative inefficiency of thedirect-expansion coil, but more importantly,
37、 the low mass flow rateof ammonia is difficult to feed uniformly as a liquid to the coil.Instead, ammonia fan-coil units designed for recirculation (overfeed)systems are preferred. Typically, in this type of system, high-pressureammonia from the system high stage flashes into a large vessel at theev
38、aporator pressure, from which it is pumped to the evaporators at anoverfeed rate of 2.5 to 1 to 4 to 1. This type of system is standard andvery efficient. See Chapter 4 for more details.Fig. 2 Two-Stage System with High- andLow-Temperature LoadsFig. 2 Two-Stage System with High- and Low-Temperature
39、LoadsAmmonia Refrigeration Systems 2.3Flooded shell-and-tube evaporators are often used in ammoniasystems in which indirect or secondary cooling fluids such as water,brine, or glycol must be cooled.Some problems that can become more acute at low temperaturesinclude changes in lubricant transport pro
40、perties, loss of capacitycaused by static head from the depth of the pool of liquid refrigerantin the evaporator, deterioration of refrigerant boiling heat transfercoefficients caused by lubricant logging, and higher specific vol-umes for the vapor.The effect of pressure losses in the evaporator and
41、 suction pipingis more acute in low-temperature systems because of the largechange in saturation temperatures and specific volume in relation topressure changes at these conditions. Systems that operate near orbelow zero gage pressure are particularly affected by pressure loss.The depth of the pool
42、of boiling refrigerant in a flooded evapo-rator exerts a liquid pressure on the lower part of the heat transfersurface. Therefore, the saturation temperature at this surface ishigher than that in the suction line, which is not affected by the liq-uid pressure. This temperature gradient must be consi
43、dered whendesigning the evaporator.Spray shell-and-tube evaporators, though not commonly used,offer certain advantages. In this design, the evaporators liquid depthpenalty can be eliminated because the pool of liquid is below theheat transfer surface. A refrigerant pump sprays liquid over the sur-fa
44、ce. Pump energy is an additional heat load to the system, and morerefrigerant must be used to provide the net positive suction head(NPSH) required by the pump. The pump is also an additional itemthat must be maintained. This evaporator design also reduces therefrigerant charge requirement compared t
45、o a flooded design (seeChapter 4).VesselsHigh-Pressure Receivers. Industrial systems generally incorpo-rate a central high-pressure refrigerant receiver, which serves as theprimary refrigerant storage location in the system. It handles refrig-erant volume variations between the condenser and the sys
46、tems lowside during operation and pumpdowns for repairs or defrost. Ideally,the receiver should be large enough to hold the entire system charge,but this is not generally economical. The system should be analyzedto determine the optimum receiver size. Receivers are commonlyequalized to the condenser
47、 inlet and operate at the same pressure asthe condenser. In some systems, the receiver is operated at a pres-sure between the condensing pressure and the highest suction pres-sure to allow for variations in condensing pressure without affectingthe systems feed pressure. This type is commonly referre
48、d to as acontrolled-pressure receiver (CPR). Liquid from the condenser ismetered through a high-side control as it is condensed. CPR pres-sure is maintained with a back-pressure regulator vented to an inter-mediate pressure point. Winter or low-load operating conditionsmay require a downstream press
49、ure regulator to maintain a mini-mum pressure.If additional receiver capacity is needed for normal operation,use extreme caution in the design. Designers usually remove the in-adequate receiver and replace it with a larger one rather than installan additional receiver in parallel. This procedure is best becauseeven slight differences in piping pressure or temperature can causethe refrigerant to migrate to one receiver and not to the other.Smaller auxiliary receivers can be incorporated to serve assources of high-pressure liquid for compressor injection or thermosi-phon
