Evaporators and the Refrigeration System OBJECTIVES Aer studying this unt you should be able define high, medium, and low-temperature refrigeration, + determine the boling tempecatute in an evaporator. + identify diferent types of evaporators + descrivea paraliel-low, plate-anc-fn evaporator. + describe multple- and single-crcult evaporators CEE eee SAFETY CHECKLIST [14 ear goggles and gloves when attaching arremaving gauges to transfer eigerant oto check pressures, a veaesrarm clothing when working ina wallcincooleror freezer 21.1 REFRIGERATION Refrigeration is the process of removing heat from a place where itis nor wanted and transferring thar heat roa place where it makes litle or no difference. Commercial refrigera- tion is similar to the refrigeration thar occurs in your house- hold refrigerator. The food that you keep in the refrigerator is stored ata temperature lower than the room temperature. Typically, the fresh-food compartment temperature is about 35°K Heat trom the room (typically noves through the walls of the refrigerator to the cooler temperature in the refrigerator. Heat travels normally and naturally from a ‘warm t0 a cool medium. If the heat that is transferred into the refrigerator re- mains in the refrigeratos, it will warm the food products and spoilage will occur. This heat may be removed from the refrigerator by mechanical means using the refrigerator’ refrigeration equipment, which requires energy, or work Figure 21.1 shows how heat is removed with the com- pression eyele, Because itis 35°F in the box and 75°F in the room, the mechanical energy in the compression cycle MoToR DRIVEN ‘COMPRESSOR \ at ere oom a 756) tal Conta fexrraow INSIDE THE Box ier to na sorin the rel that forces the ots ate nosmaly driven by elect actually pumps the heat to & warmer environment from the box to the room, The heat is transferred into a cold refrigerant coil and pumped by the system compressor to the condenser, where it Js released into the room. This is much like using. sponge to move water from one place to another. When a dry sponge is allowed to absorh water in a puddle and you take the wet sponge to a container and squeeze t, you exert energy, much like a compressor in the refrigeration system, Figure 21.2. Another example of refrigeration is a central air- conditioning system in a residence. It absorbs heat from thev2 Figure 21.2 A sponge absorbs wate. The water can then be carted in the sponge to another place. When the sponge i squeeze the water Isrejected to another place. The squeezing ofthe sponge may be con- Sdeted the energy that it akesto pump the watec oom supty AIR (55°F) OUTSIDE (95°6 COIL TEMPERATURE Gor) <== ROOM RETURN AIR (75°F) Figure21.3 An aiccondiloning example of etigeaton, home by passing indoor air at about 75°F over a coil that 1s cooled to about 40°E. Heat will transter from the room air to the coil cooling the air. This cooled air may be mixed with the room air, lowering its temperature, Figure 21.3. ‘This process is called air-conditioning, but itis also refrig- eration at a higher temperacure level than that of the house hold refrigerator, so itis frequently called high-temperacure refrigeration. ‘Commercial refrigeration is used in commercial busi- ness locations. The food store, fast-ood restaurant, drug store, flower shop, and food processing plant are only a few of the applications. Some of the commercial systems are plug-in appliances, such as a small, reach-in ice storage bin at the local convenience store. The system is entirely located within the one unit. Some systems consist of individual boxes with single remore condensing units, and some are complex systems with several compressors in a rack serving several reach-in display cases, as in a supermarket. Most commercial refrigeration is installed and serviced by a spe cial group of technicians who work only with commercial refrigeration and the food-service business. ‘aa nthe tigen Str 21.2 TEMPERATURE RANGES, OF REFRIGERATION ‘The temperature ranges for commercial retrigeration may refer to the temperature of the refrigerated box or the boil- ing temperature of the refrigerant in the coil. The following, temperatures illustrate some of the guidelines used in the industry when discussing box temperacures. HIGH-TEMPERATURE APPLICATIONS. High-temperature refrigeration applications will normally involve box tem- peratures of 47°F to 60°F. Storing such products as flowers and candy may require these temperatures. MEDIUM-TEMPERATURE APPLICATIONS. The household relrigerator fresh-food compartment is a good example of ‘medium-temperature refrigeration, which typically ranges from 28°F co 40°F. Many different products are stored at the medium-temperacure range, For most products, the ‘medium-temperature refrigeration range is above freezings few products are stored below 32°F. Items such as eggs, lettuce, and tomatoes lose their appeal if they freeze in a refrigerator LOW-TEMPERATURE APPLICATIONS. Low-temperature re- {rigeration produces temperaturce below the freezing point of water, 32°F One of the higher low-temperature applica- tions is the making of ice. Low-temperature food storage applications generally start at O°F and go as low as ~20°F. Ar this temperature ice eream would be frozen hard. Frozen meats, vegetables, and dairy products are ony a few of the foods preserved by freezing, Some foods may be kept for long periods of time and are appetizing when thawed for cooking, provided they are frozen correctly and kept frozen. 21.3 THE EVAPORATOR ‘The evaporator in a refrigeration system is responsible for absorbing heat into the system from whatever medium is to be cooled. This heat-absorbing process is accomplished bby maintaining the evaporator coil ata lower temperature than the meditim to be cooled. For example, if a walk-in cooler is to be maintained at 35°F to preserve food prod- ucts, the col in the cooler must be maintained at a lower temperature than the 35°F air that will be passing over it Irigure 21.4 shows the refrigerant in the evaporator boiling at 20°F, which is 15°F lower than the entering ar. The evap- ‘orator operating at these low temperatures removes latent and sensible heat from the cooler. Operating at 20°F, as in the preceding example, the evaporator will collect moisture from the air in the cooler, latent heat: The removal of sen- sible heat reduces the food temperature am.8 sects RETURN AIR (25°F) HAS PASSED ‘OVER FOOD. PRODUCTS. ‘TEMPERATURE DIFFERENCE (15°F) ‘COIL TEMPERATURE (20°F) FAN AND MOTOR THE COMPRESSOR IS RUNNING — THIS LOWERS COIL TEMPERATURE. Figure 21.4 The relationship ofthe ails bling temperature tothe ai passing over the col while it operating inthe design range, 21.4 BOILING AND CONDENSING ‘Two important factors in understanding refrigeration are the (1) boiling temperature and (2) condensing tempera ture. The boiling temperatuee and its relationship to the system involve the evaporator. The condensing tempera- ture involves the condenser and will be discussed in the next unit. These temperatures can be followed by using the temperatirelpressure chat in conjunction with a set of re- frigeration pressure gauges, Figure 21.5 and Figure 21.6. 21.5 THE EVAPORATOR AND BOILING TEMPERATURE ‘The boiling temperature of the liquid refrigerant deter- ‘mines the coil operating temperature In an air-conditioning, system a 40°F evaporator coil with 75°F air passing over ir produces conditions used for airconditioning or high- temperature refrigeration. Boiling is normally associated with high temperatures and water. Unit 3, “Refrigeration and. Kefrigerants,” discussed. the fact that water boils at 2I2PF at atmospheric pressure. It also discussed the fact that water boils at other temperatures, depending on the pressure. When the pressure is reduced, warer will boil at 40°E. This is still boiling—changing a liquid to a vapor. In a refeigeration system, the refrigerant may boil at 20°F by absorbing heat from the 35°F food. ‘The service technician must be able to determine what ‘operating pressures and temperatures are correct for the various systems being serviced under different load con- ditions. Much of this knowledge comes from experience. ‘When taking readings from thermometers and gauges, the Commer Retgertn readings must be evaluated. There can be as many differ ent readings as there are changing conditions. Guidelines ‘can help the technician know the pressure and temperature anges at which the equipment should operate. There are relationships between the entering ai temperature and the evaporator for each system, These relationships are similar from installation to installation. 21.6 REMOVING MOISTURE Dehumidifying the air means to remove the moisture, and this is frequently desirable in refrigeration systems, Moisture removal is similar from one refrigeration system to another. Knowing what the coibto-air relationship is can help the technician know what conditions to look for. The load on. the coil would rise or fall accordingly as the return-air tem- perature rises or falls. Warmer return aie in the box will also have more moisture content, which imposes further load on the ail, the cooler is warm die ro food added ta it the col would have more heat to remove because it has more load on it Iewould be much like boiling water in an open pan on. the stove, The warer boils atone rate with the burner on me- dium and at an inereased rate with the burner on high. The boiling pressure stays the same in the boiling water in a pan because the pan is open to the atmosphere. When this same boiling process occurs in an enclosed coil, the pressures will rise when the boiling occurs at a faster rate. This causes the operating pressure of the whole system to rise, Figure 21.7. When the evaporator removes heat from air and lowers the temperature of the aig, sensible heat is removed. When moisture is removed from the air latent heat is removed “The moisture is piped to a drain, Figure 21.8, Latent heat is called hidden heat because it does not register on a ther- rmometer, but it is heat, like sensible heat, and it must be removed, which takes energy. ‘The refrigeration evaporator is a component that ab- sorbs heat from the conditioned space into the refrigeration system. The evaporator ean be thought of as the sponge of the system. Iris responsible fora heat exchange between the conditioned space or product and the refrigerant inside the system. Some evaporators absorb heat more efficiently than | others. Figure 21.9 illustrates the heat exchange between air and refrigerant. 21.7 HEAT EXCHANGE CHARACTERISTICS OF THE EVAPORATOR ‘The following are conditions that govern the rate of heat exchange: 1, The evaporator material theough which the heat has to be exchanged. Evaporators may be manufactured from copper, steel, brass, stainless steel, of aluminum,a9, Unt 21- ana andthe eigen Sse idsoden pue 774 erase vEare YOU PUE VCS sunssaua 2919sects Figure 21.6 Many pressure gauges have temperatue/pressure rel ‘Nonships printed onthe gauge. AIR RETURNING. TOCOIL 50°F DUE TO WARM FOOD BEING ADDED TO BOX © sv4 psi ' eicea i CORRESPONDS Soastcon TewPeRATURE & Figure 21.7 The colHo-air temperature relationship under increased oo Corrosion is one factor that determines what material is used. For instance, when acidic materials need to be cooled, copper or aluminum evils would be eaten away. Stainless steel may be used instead, bur stainless steel does not conduct heat as well as copper. Some evapo- rators are even coated with a plastic-hke substance to protect the metal underneath fom rust or oxidation. “This application is often seen in restaurants with smaller commercial, medium-temperature coolers that store salad preparation materials, which are acidic because they often have a vinegar base for added flavor and for ‘extended shelf life Commer Retgertn MOISTURE DRIPPING. a ‘LOR DRAIN 3 Figure 21.8 The cooing collcondenses masture tam the a FINS RETURN AIR (95°F) WITH HEAT FROM FOOD PRODUCTS REFRIGERANT BOILING. ‘AT 20°F ABSORBS HEAT THROUGH THE WALL OF ‘THE COPPER TUBING. TUBING THE FINS GIVE THE COPPER TUBING MORE SURFACE AREA FOR GREATER HEAT EXCHANGE, Figure21.9 The hestcichange lstionchip betwicen stand rekigeant 2. The medium to which the heat is exchanged. Giving heat up from air to refrigerant is an example. The best heat exchange oceurs between two liquids, such as wa- ter to liquid refrigerant. This is because liquids are more dense than vapors and usually have a higher specific heat. However, this isnot always practical because heat frequently has to be exchanged between air and vapor refrigerant. The vapor‘to-vapor exchange is slower than the liguid-to-liquid exchange, Figure 21.10. 3. The film factor. This isa relationship between the me- dium gig up heat and the heat exchange surface. ‘The film factor relates to the velocity of the medium passing over the exchange surface. When the velocity is too slows; the film berween the medium and the surface becomes an insulator and slows the heat exchange, ‘The velocity keeps the film to a minimum, Figure 21.11. ‘The correct velocity is chosen by the manufacturer.v2 WATER (65°F) — VAPOR REFRIGERANT 100% VAPOR APPROXIMATELY 25% VAPOR 79% UQUIO CHILLED WATER 7 LIQUID REFRIGERANT WATER (45°F) METERING’ DEVICE Figure 21.10 The heat exchange relationship between a liquid in a Theaverahange axlersingeant fsa te wa Figure 21.11. One of the deterring factors in anormal heat exchange. “Tre fm factors the fy of ai oF guid next to the tube inthe heat ‘aa nthe tigen Str 4. The temperature difference between the two mediums in which the heat exchange is taking place. The greater the temperature difference between the evaporator coil and the medium giving up the heat, ehe faster the heat exchange will occur, 21.8 TYPES OF EVAPORATORS Numerous types of evaporators are available, and each hhas its purpose. The first evaporators for cooling air were of the natural-convection type. They were actually bare pe evaporators with refrigerant circulating through them, Figure 21.12. This evaporator was used in carly walk-in cool= crs and was mounted high in the ceiling. Ie relied on the air being cooled, falling to the floor, and setting up a natural ir current. The evaporator had to be quite lage forthe particu- lar application because the velocity ofthe air passing over the coil was 30 slow. Natural convection evaporators are stil oc casionally used today. The use ofa blower co force or induce air over the coil improved the efficiency of the heat exchange. ‘This meant that smaller evaporators could be used to do the same job, Design trends in the industry have always been to- ‘ward smaller, more efficient equipment, Figure 21.13. ‘The expansion of the evaporator surface to a surface larger than the pipe itself produces a more efficient heat exchange. The stumped etuporstor was one of the Brae de signs to create a large pipe surface. It consisted of two pieces ‘of metal stamped with the impression of a pipe passage through it, Figure 21. ‘A pipe with fins attached, called a fnned-tube evapo- zator,is today used more than any other type of heat ex changer between air and refrigerant. This hear exchanger is exchanger efficient because the fins are in good contact with the pi nay ficient by the fi good h the pipe ACONVECTION CURRENT INSULATED BARE PIPES WITH IS SET UP WHEN THE COLDER, CEILING REFRIGERANT HEAVIER AIN DISPLACE THE CIRCULATING WARMER AIR NEAR THE FLOOR. CONDENSATE CONDENSATE =—_ —— wee )F Figure 21.12 A bare-pipe evaporator \s AIR2 Figure 21.13 forced cia evaporator cunmyl carrying the refrigerant. Figure 21.15 shows an example of a finned-tube evaporator, Figure 21.15(B) shows a finned- tube evaporator used in a low-temperature, commercial, well-type freezer. The leading fins come in contact with the aur first and must have wider spacing between the fins. Its these leading fins that are more prone to frost accumula: tion. The wider fin spacing can accumulate more frost be- fore becoming plugged and impeding the airflow. The fins toward the back of the evaporator then encounter drier air ‘with less frost and so are more closely spaced. Multiple circuits improve evaporator performance and efficiency by reducing pressure drop inside the evaporator. Even though the pipes inside the evaporator might be pol ished smooth, they still offer resistance to the flow of both liguid and vapor refrigerants. The shorter the evaporator is, the less resistance there is to this flow. The “U" bends at the ends of the evaporator also offer a great deal of re- sistance to the flow of refrigerant. As evaporators hecome longer, they have more and more pressure drop associated Commer Regen ®) Figure 21.15 (AIA commerell reng evaporator for a medi Cate W) rinned tube vapor with refrigerant flowing through them. The manufacturer will design the evaporator so thatthe tubing bundles are in parallel ro one another, Figure 21.16 The evaporator for cooling liquids of making ice oper ates under the same principles a¢ one for cooling air but is designed differenely. It may be strapped on the side of a cylinder with liquid inside, submerged inside the liquid container, or be a double-pipe system with the refrigeranc inside one pipe and the liquid to be cooled circulated inside fn enter pipe, Fig 21.17, Every central splir-cooling system manufaerured in the United States today must have a Seasonal Energy Efficiency Ratio (SEER) of atleast 13. This energy require- ‘ment was mandated by federal law as of January 23, 2006 Also, with the phascoue of R-22 just around the comner, ‘manufacturers of HVACIR equipmcat have been looking for energy-efficient methods to apply to their equipment to meet these new energy requirements, The timeline for R22 is as follows: © 2010—R-22 use is banned at the original equipment manufacturer (OEM) level with a 75% reduction of HEC production. + 2015—90% reduction of HCFC production, © 2020—Total ban of R-22 productionUne2t asain tigen Stem 6 Z suchow EXPANSION LINE TO 2 VALVE COMPRESSOR “ : r | ‘ EVAPORATOR PIPING F AROUND CYLINDER 2 Oy) PLATE-TYPE EVAPORATORS Liguip LeveL uaUID REFRIGERANT TO EXPANSION VALVE | Oy ‘GUCTION LINE TO « ‘COMPRESSOR © e) © Figure 21.16 (4) A duce pressure cr evaporator. Thes in paral to reduce BeS- Figure 21.17 Liquid heat exchangers. (A) A drum-type evaporator sure drop Couey Spoon Compu ard) Cues Fer Ste rier (8) A plate type evaporator na tank {C) A ppe-in pine evaporstor Sy Prtos by hee (Ccouner/ nora Met ates re ©sects The equipment covered by this federal mandate includes *# unitary equipment from 1.5 to 5 tons, + splitipackaged air conditioners and heat pumps. Equipment not covered includes + commercial equipment greater than 6 tons, * spaceeonstrained units smaller than 3 t0ns (room air conditioners), SEER is caleulated on the basis of the total amount of cooling {in Bru} the system will provide over the entire sea- son, divided by the total number of wart-hours it will con- swine, Higher SEERS reflow « snore efficient cooling system TThe federal mandate impacts 95% of che unitary market in the United States, which is about $ million units manufac tured at the time of this writing. Because of the new federal ‘mandate of 13 SEER, most airconditioning and heat pump manufacturers are looking for more efficient evaporator and condenser designs, more efficient compressors and fan ‘motors, and more sophisticated control systems in order to reer the new energy efficiency requirement ‘One such evaporator design incorporates an aluminum parallel-flow, flat-plate-and-fin configuration with small parallel channels inside the flat plate. The plates are flat- tened, streamlined tubes each one of which is split into smalls, parallel ports, Viguse 24.18. % Refigerant willphase ‘change or evaporate from a liquid toa vapor inside the channels in the plate, while strategically shaped fins (extended surfaces) will enhance heat transfer fromthe air into the evoporator, The plates and fins are bonded or soldered to increase heat transfer ‘and to eliminate any contact resistance (air gaps) that will reduce heat transfer Headers at the inlet and outlet of the heat texchanger are also bonded to the plates through soldering, Heat is transferred from the air to the evaporating re- Irigerane in three steps, as follows: 1. Air side—hetween the fins and the air to be cooled 2. Heat conduetion—berween the fins and the tubes 3. Refrigerant side—hetween the tubes and the evaporat- ing refrigerant {The air side of the heat exchange can be enhanced through fin geometry. Louvres, lances, and ippled edges all increase heat transfer Figure 21.19, The conduction between the fins andthe tubes is enhanced through the application of a metalic bond (soldering) that eliminates any airgaps.%9 The refrigerant side of the heat transfer deals with how much surface area of the incide n the bes will eame intr enmact with the phace changing refrigerant. This intemal surface area is often re- ferced to as a wetted perimeter. As the internal surface area of the tubes increases, the heat transfer increases. Internal surface area can be increased by: * increasing the number of parallel channels inside the flat plates, Commer Regen ©) Figure 21.18 An aluminum paralle-Tow, fat plate-and fin heat ex AMEE cure Meine Manacting Ca are + increasing the number of fae plates (decreasing the spac ing between chem) Te capacity (tonnage) of the heat exchanger ean vary with its height and length. The plaes can be oriented vert eally for an evaporator application oF horizontally for con denser applications. The Vertical orientation of the flar platesv2 © Figure 21.19 Fin geometry fer increasing heat wansie. sy ene Natt Co, aie allows condensate removal to occur naturally, alleviating any water drainage issues from the evaporator Figure 21.20. This technology is being used with condensers as well as evapora- tors, Figire 21.21. Applications in the HVACR fed inclnde residential airconditioning, rooftop airconditioning, chillers, geothermal heat pumps, electronic cooling, packaged termi- nal air conditioners (PTAC), ice machines, beverage dispens- cr, refrigerated display cases, and foodservice refrigeration, Some of the benefits of this parallelflow; plate-and-fin heat ‘exchanger technology are as follows: ‘+ Reduces static pressure through the coil—which means fewer fan watts and less horsepower ‘+ Reduces coil depth for the evaporator and condenser, which leads to easier cleaning and less airside static pressure ‘+ Reduced internal volume reduces refrigerant charge ‘+ Reduced face area of condenser and evaporator ‘+ Smaller foorprin forthe condensing unit ‘aa nthe tigen Str MOUNTING WITH VERTICAL HEADERS. HEADER i MA GAEnen TUBE SIDE += FIN SIDE ‘TuBe sive view LRAT “FIN ORIENTATION IS NOT + CONDENSATE ‘CRITICAL ‘WILL MOVE “AIRFLOWMUST ENTERTUBE ——_-WITHAIRFLOW SIDE OF EVAPORATOR AND DRAIN Down Fit SIDE “ MOUNTING WITH HORIZONTAL HEADERS FIN ORIENTATION MUST BE FLOW DIRECTION CONCAVE SHEY FACE). SNOT CRITICAL =. =, l FINORIENTATION OAS ‘CONDENSATE WILL ORAIN THROUGH FIN LOUVERS © Figure 21.20 Condensate removal for vertical and horzontal header ‘configuration, Coun Medre trating Co. Rate Mt ‘+ 4.30% reduction in coil weight and size ‘+ A reduction in packing costs, sizes, and weights ‘+ The allaluminum coil, header, and fins enhance corro- *+ Lower system costs ‘+ Higher system efficiencies than a round, copper-tube heat exchanger with aluminum fins, Figure 21.16(C) ‘+ Lower operating costs + Quieter operations Field repair of leaking heat exchangers, Figure 21.22, can bbe accomplished by recovering the refrigerant, cleaning the leaking area with a solution, ‘brushing the area with a wire brush, using a utility knife blade to remove any fins that may be in the local area,6 Sexton Commer Reign Oy) ® Figure 21.21 (A) Condenser (8) Evaporator Couns Figure 21.22 Feld repair of aleaking heat exchanv2 + palling wacunm with a vacuses pomp, + applying a two-part epoxy that will be sucked into the fla plate where the leak exists, ‘+ applying heat with an clecri blow dice wntl the epoxy is cured, 1» evacuating ro 2 500-micron vacuum, ‘charging with the appropriate refrigerant. Field cleaning heat exchangers can be accomplished using. the same methods as for a standard round copper-tube heat exchanger with aluminum fins: ‘+ Elevate the temperature of the mixed cleaner t0 120° ‘© Use a power washer with a broad spray pattern. ‘+ Use nonacidic cleaners (pH < 10.5). = When che heat exelianger is el clean water. ringe ule coil with 21.9 EVAPORATOR EVALUATION Knowing the design considerations helps in evaporator evaluation. When the service technician arrives atthe job, iemay be necessary 3 evaluate whether a parscular evapo ator is performing properly. This can be considered one of the starting points in organized teoubleshooting. The evaporator absorbs heat, the compressor pumps it, and the condenser rejects it. The following example pertains to a fumicmperalace walk-in bos, Lloweves the procede ‘would be about the same for any typical application, SUCTION LINE TO COMPRESSOR, 170 psig 100% PURE LiguiD) 35% VAPOR 155% LIQUID PERE oe rev ersrune arruicanon, yorwas oreninion tad psig ‘aa nthe tigen Str Evaporator Specifications Copper pipe coil Aluminum fins attached to the copper-pipe coil Forced draft with a prop-type fan One continual refrigerant circuit Re134a Evaporator to maintain space temperature at 35°F Evaporator clean and in good working condition First we describe how the evaporator functions when itis working correctly Entering the evaporator isa partial liquid-partal vapor mixture at 20°F and 18.4 psig; it is approximately 65% liquid and 35% vapor. Approximately 35% of the liquid cenvering the expansion devive at che evaporavor is changed to a vapor and cools the remaining 65% of the liquid to the evaporator's boiling temperature (20°F). This is accom plished by the pressure drop across the expansion device. ‘When the warm liquid passes through the small opening in the expansion device into the low pressure (184 psig) of the evaporator side, some of the liquid flashes to a gas, Figure 21.23. ‘As the parcial liguidepartial vapor mixture moves through the evaporator, more of the liquid changes to a va- pot. This is called boiling and is a result of heat absorbed into the col from whatever medium the evaporator is cool ing, Finally, near the end of the evaporator the liquid is all boiled away wo a vapos. At this point the teftigerant is known as saturated vapor. This means that the refrigerant 184 psi EVAPORATOR COIL. i Flqure 21.23 When the | 1Fliquid passes through the eansion vale rice, some ofthe liquid Rashes toa vanor and cools the remainina ligld tothe evaparator temperature a 20° aaSexton Commer Reign vapor is saturated with heat. Ifany more heat is added to it, it will rise in temperature. If any heat is taken away from it, it will stare changing back to aliquid. This vapor is satu- rated with heat, but i is still atthe evaporating temperature corresponding to the boiling point, 20°F. 9 This isa most important point in the function of an evoporator becouse all of the quid must be boiled away as close to the end ofthe col as possible. This s necessary to (1) keep the coil efficiency up and (2) ensure that liquid refrigerant doesnot leave the evaporator {and move into the compressor: For efficient operation, the evaporator must be as full of liquid as possible without al- lowing liquid to reach the outlet of the coil, because the bese heat exchange is between the liquid refrigerant and the air passing over the col ‘The pressurefemblpy chart in Figure 21.24 shows sraphically what happens inside the evaporator of the pre- ceding walk in cooler example. The refrigerant enters the evaporator at point A (after leaving the expansion valve) ‘The liquid pressure is 18.4 psig and contains 48.7 Beu/lb of heat at this point. Approximately 33% of the liquid flashed to a vapor when passing through the expansion valve. As the liquid proceeds through the evaporator, it is changing to a vapor. All of it has changed to a vapor at point B, but the vapor temperature is still 20°F and capable of absorbing, heat; in the form of superheat. The vapor temperature starts ‘orice while iis ll n the evaporator until the temperature is 30°F (containing 10°F of superheat). The vapor leaves the evaporator at point C with a heat content of 108.1 Btuflb, ‘The usable refrigeration in the evaporator is from points A to CG, where the refrigerant absorbed 59.4. Bra/lb (108.1 Bru/lb ~ 48.7 Bro/Ib) of the circulating refrigerant. You only need roknow how many Brufh capacity is needed %6 determine the amount of refrigerant that needs to be circu- lated. For example, ifthe evaporator needs to have a capacity of 33,000 Buu, it must have 389.2 Ib of refsiges ‘culate through it per hour (35,000 Brwh + 59.4 Bru/lb 5389.2 lbh). This sounds like a lot of refrigerant, bue it is only 9.82 Ihvmin (589.2 Ibvh + 60 minfh = 9.82 lbvmin). “The siz of the compressor and the system operating condi- tions determine how much refrigerant can be pumped. g Pressure (psia) 5888 3 EFFECT Figure 21.24 The refgeration effect inthe evaporator. outer | Dont NET REFRIGERATION 108.1 =487 504 Blus NET REFRIGERATION EFFECT ° soav2 21,10 LATENT HEAT IN THE EVAPORATOR ‘The latent heat absorbed during the change of state is much ‘more concentrated than the sensible heat that would be added to the vapor leaving the coil. Refer to the example in Unie 1, Section 1.3, that showed how it takes 1 Bru to change the temperature of 1 Ib of 68°F water to 69°F water. Section 1.8 also showed that itrakes 9/0 btu to change 1 1D (of 212°F water to 212°F steam, The change of state is where the greatest amount of heat is absorbed into the system. The preceding example showed that 59.4 Bru of heat were absorbed for every 1 Ib of refrigerant that was circulated {59.4 Bru/lb). This happened at a boiling temperature of 20°F, without a change in pressure. 21.11 THE FLOODED EVAPORATOR “To get the maximum effcieney from the evaporator heat exchange, some evaporators are operated full of liquid, oF flooded, and are equipped with a device to keep the liquid refrigerant from passing to the compressor. These flooded evaporators are specially made and normally use a float me- tering device to keep the liquid evel as igh as possible inthe evaporator This text will not go into detail about ths system Ihecause itis not a device often encountered, The manufactur ers literature should be consulted for any special application. RETURN AIR es 2F 35°F ——> 37 so} 39°F ——> 39°F METERING DEVICES 100% LIQUIDS 184 psig Liou. REFRIGERANT UNE t f (sucTion une) Figure 21.25 The evaporator operating under normal load. |-—VAPOR REFRIGERANT LINE. ‘aa nthe tigen Str When an evaporator is flooded, it would operate much like water boiling in a pot with a compressor taking the vapor off the top of the liquid. There would always be a lig uid level. Ifthe evaporator is not flooded, that is, when the refrigerane starts out as a partial liquid and boils away toa vapor in the heat exchange pipes; it ix known as a dry-type, ‘or direct-expansion, evaporator, 21.12 DRY-TYPE EVAPORATORS, PERFORMANCE ‘To check the performance of a dry-type evaporator, the ser- vice technician would first make sure that the refrigerant col is operating with enough liquid inside the eoil. To de- termine this, the technician must calculate the evaporator superheat. This is generally done by comparing the boiling temperature of the refrigerant inside the coil with the line temperature leaving the coil. The difference in temperatures is usually 8°F to 12°F For example, in the coil pictured in Figure 21.25, the superheat in the coil was arrived at by converting the coil pressure (suction pressure) to tempera- ture, In this example, the pressure is 18.4 psig, which eor- responds to 20°F. The suction pressure reading is important to the technician because the boiling temperature must be known to arrive at the superheat reading for the coil. In the following example, the evaporator supetheat reading is 10°F (30°F = 20°F), [HIS HEFHIGEHAN CONIAINS LAST POINT ‘OF LIQUID REFRIGERATED — _AR — Spy ones CONDENEATE DRAIN ‘APPROXIMATELY 35% VAPOR, 165% LIQUID \ 8sects 21,13 EVAPORATOR SUPERHEAT ‘The difference in temperature between the boiling relriger- ant temperature and the evaporator outlet tempcrature is known as evaporator superheat. Superheat is the sensible heat added to the vapor refrigerant ater the change of state hhas occurred. Supetheat is the best method of checking to see when a refrigerant coil has a proper level of refrigerant. When a metering device is not feeding enough refrigerant ta the coil the col is said to be a starved coil, and the super- hreat is greater, Figure 21.26. Itcan be seen from the exam- ple that all of the refrigeration takes place at che beginning of the col. The suction pressure is very low; below freezing, but only a portion of the coil is being used effectively. This coil would freeze solid and no air would pass through ‘The freeze line would creep upward until the whole coil was a block of ice, and the refrigeration would do no good. ‘The refrigerated box temperature would rise because ice is 21.14 HOT PULLDOWN (EXCESSIVELY LOADED EVAPORATOR) ‘When the refrigerated space has been allowed ro warm up considerably, the system must go through a hot pulldown. On a hot pulldown the evaporator and metering device are not expected to act exactly as they would in a typical design condition, For instance, if a walk-in cooler supposed ~. AB AUF AIR 40°F FAN AND MOTOR Commer Retgertn to maintain 35°F were allowed to warm up to 60°F and had some food or beverages inside, it would take an extended time to pull the air and product temperacure down. The coil ‘may be boiling the refrigerant 3 fast thatthe supetheat may: not come down to §°F to 12°F until the box has cooled dlown closer to the design temperature. ‘A superheat reading on a hot pulldown should be in- terpreted with caution, Figure 21.27. The reading will be ETHAN AIR (65°F) Figure 21.27 1lot pulldown witha coll Thisis@ medium temperature evaporator that should be operating at 18a psig 38a, 20°F The e- turn airs 55 instead of 35° This causes the pressure in the colo a, The warm box balls the refrigerant a faster Fate. The thermostatic exe pansion vaive isnat abl to ee the evaporator quickly enough ta keep the supatheat at 10% The evaporator has 15 of superhoat THIS 30°F GAS CONTAINS, -F SUPENHEAT, f= OF LIQUID gy ase FEFHIGEHATED AIH 104 \, NAPPRONATELY G5 VAPOR, 65% LOUID = | t \ \wererine ovce 4 100% LIQUID i HIGH PRESSURE m LOW TEMPERATURE coo, tow § (WARM LIQUID LOW-PRESSURE LIQUIDNAPOR PRESSUREVAPOR Figure 21.26 A starved evaporator col showing 38°F (40°F ~ 27) of evanoratarsuperheat,v2 AIR (95°F) FAN Figure 21.28 The evanorator is Fading because the thermostatic fearsion device s nat conteing religeant flow propery correct only when the coil is at or near design conditions However, many modern thermostatic expansion valves (TXVs) have wide temperature control ranges. Some ean control evaporator superheat from +20*F to ~20°F, and they are advertised to do this effectively even when un- der heavy or light heat loadings of the evaporaror. TXVs should control superheat under most normal conditions However, when a system is under a hot pulldown, the technician should let the system get past this heavy load period and reach a somewhat stabilized condition before trying to calculate an evaporator superheat reading. Hot pulldowns are not considered normal conditions, and the technician must be patient when calculating evaporator supetheat. Ir eakes time for a TXY to fill aut the evapora tor with refrigerant even when itis wide open during a hot pulldown. When a dey.cype col is fed too much sofigerant, not all the refrigerant changes to a vapor. This coils thought of asa flooded coil—flooded with liquid refrigerants, Figure 21.28 Do not confuse this with a coil flooded by design. This is rmptom that can cause real trouble because unless the Tiquid fn the snction line hails tr a vaper before it reaches the compressor, compressor damage may occur. Remember, the evaporator is supposed to boil al ofthe liquid to a vapor. ‘Therefore, a thermostatic expansion devies that is not oper- ating correctly can cause compressor failure, 21,15 PRESSURE DROP IN EVAPORATORS “Mulkicircuie evaporators are used when the coil would be- come t00 long for a single circuit, Figure 21.29, The same evaluating procedures hold true for a multicircut evapora- tor as fora singlecireuit evaporator. ‘aa nthe tigen Str cameras JCONDENSEA VALVE REWOTE BULB Figure 21.29 A mulcicuit evaporator counry “pink Company A dry-type evaporator has to be as full as possible with refrigerant to be efficient. Each circuit should be feeding che same amount of refrigerant. If this needs to be checked, the service technician can check the common pressure tap for the boiling pressure, which can be con- verted ra temperature. Then the temperature will have to be checked at the outlet of each circuit to see whether circuit is overfeeding or starving, Figure 21.30 and Figure 21.31. Some reasons for uneven feeding of a mulkicireuit evap- orator are the following: Blocked distribution system Direy coil Uneven air distribution Coil circuits of different lengths In larger commercial and industrial-type evaporators, an associated pressure drop usualy is caused from friction as the refrigerant eravels the length of the evaporator and down a long suction line to the compressor. This causes the pressure at the compressor to be a bit lower than the pres- sure at the evaporator outlet. With larger evaporators and longer suction lines, itis importanc to measure the refriger- ant’s pressure atthe evaporator outlet—not at the compres- sor service valves—when measuring evaporator superheat, Figure 21.29 and Figure 21.30, It is best to measure the refrigerant pressure at the same location that che evapora- tor outlet temperature is taken when measuring evaporator supecheat. This will give the service technician a more ac- curate evaporator superheat reading and, therefore, better evaporator efficiencies. Schrader taps are often provided at the outlet of larger evaporators for this reason. Line taps also can be used to gain access to evaporator outler pres- sure. This method also will protect the compressor from flooding or slugging problems caused by inaccurate evapo- rator superheat readings.2 Secon Comme ei ecHnapen VALVE AND GAGE THERMOSTATIC EXPANSION VALVE REFRIGERANT SENSING ELEMEN DISTRIBUTOR FIN. EXTERNAL EQUALIZER TUBE —~ SLOPE DOWN — CoMPRESSOR |-—REFRIGERANT VAPOR {oF SUBEANEAT UST PONT OF TOUD NEROTEOIL Liquip f tne] A [ {_EQUAL-LENGTH DISTRIBUTOR TUBES FOR EXACT COIL FEEDING $ THERMOSTATIC EXPANSION VALVE j Figure 21.30 The 2pcaarcec amultdicut oapactr onthe sie when ted comet reels soxtleapnctr plpedin pal TO COMPRESSOR SUPERHEAT IN THIS CIRCUIT IS 20°F AND THE OTHER CIRCUITS: ARE 10°F THIS IS A STARVED CIRCUIT. CRIMP IN LINE CAUSING THIS CIRCUIT TO SLIGHTLY STARVE FOR REFRIGERANT LIQUID LINE THERMOSTATIC t EXPANSION VALVE Figure 21.31 The appearance ofa mulicicuit evaporator onthe inside wien itis nt fetne21 fa 2 -16 LIQUID COOLING EVAPORATORS (CHILLERS) A ditferent type of evaporator is required for liquid cooling. Ir functions much like the one for cooling air and is nor- ‘mally a dry-type expansion evaporator in smaller systems, Figure 21,32(A). Evaporators for larger-tonnage chiller are usually the flooded type. They have sarurated liquidivapor refrigerant in the shell and the water to be chilled flows in the tube bundles, Figure 21.32(B). They use a low side float to meter the refrigerant into the shell of the evaporator to maintain the proper refrigerant level, Liquid cooling evaporators have more than one refrigerant circuit to prevent pressure drop. These evaporators sometimes have to be checked to see whether they are absorbing heat as THERMOMETER — Vv WATER our, er THERMOSTATIC EXPANSION VALVE — LQUID-LINE REFRIGERANT IN Figure 21.3218) Adi OPERATING THERMOSTAT \, / ‘or cooing liquids sand the ageraton Sse 2 they should. Using refrigeration gauges and some accurate method for checking the temperature of the suction line are very important. These evaporators have anormal superheat range similar to airtype evaporators (8°F to 12°F). When the superheat is within this range and all circuits in a mulicircuit evaporator are performing alike, the evaporator is doing its job ‘on the refrigerant side, However, this does not mean that it will cool properis. The liquid side of the evaporator must be clean so that the guid will come in proper contact withthe evaporator, “The following problems are typical on the liquid side of the evaporators Mineral deposits may build up on the liquid side and cause ‘a poor heat exchange. They would act like an insulator 2. Poor circulation of the liguid to be cooled where a cir culating pump is involved, THERMOMETER, PRESSURE TAPS ;-FREEZE ae * STAT WATER, IN REFRIGERANT OUT = SUCTION LINE TO COMPRESSOR HARDWARE AND CONTROL BOARD COMPARTMENT CHILLED WATER RETURN 100-tn F134) flooded type evaporator Use chil water for cooling al CONTROLLER WATER, INSULATED EVAPORATOR (OF CHILLER BARREL CHILLING WATERsects 51.3 psig OH 55° 15°F SUPERHEAT. | ‘SUCTION LINE LIQUID LINE Commer Retgertn CHILLED WATER CHILLED WATER OUT (75°F) INTO THE (esr) EVAPORATOR CONTAINS HEAT FROM BUILDING. Figure 21.33 Ahot pulldown on aliquid evaporator aving up its heat to refrigerant This evaporator notmaly has 55°F water in and 45°F weer out Tre hot plldowin th 75 ner inated of $5 ater bo the efigeront ata fate rae The expansion valve ry not beable to feed the evapo ter quichlyenaugh to maintan 10" ‘When the supetheat is correct and the coil is feeding cor rectly in a multicireuit system, the echnician should con- sider the temperature ofthe hquid. the superheat may not be within the prescribed limits ifthe liquid to be cooled is not close to the design temperature. On a hot pulldown of a liquid product, the heat exchange can be such that the coil appears t0 be starved for refrigerant because itis so loaded up that itis boiling the refrigerant faster than normal. The technician must be patient because a pulldown cannot be rushed, Figure 21.33. Airto-refrigerant evaporators do not have quite the pronounced difference in pulldown that lig- uid heat exchange evaporators do because of the excellent heat exchange properties ofthe liquid to the refrigerant. 21,17 EVAPORATORS FOR LOW-TEMPERATURE APPLICATIONS Low-temperature evaporators used for cooling space or product to below freezing are designed differently because they require the coil to operate below freezing. In an airflow application, the water that accumulates on the coil will freeze and will have to be removed, The design of the fn spacing must be carefully chosen, because a very small amount of ice accumulated on the fins will restriet the airflow. Low-temperature coils have fin spacings that are wider than medium-temperature coils, Figure 21.34. Other than the airflow blockage due to ice buildup, these low-temperature evaporators perform much the same as :medium-temperature evaporators. They are normally dry- type evaporators and have one or more fans to circulate the air across the coil. The defrosting of the coil has to be done: by raising the coil temperature above freezing to melt the ice. Superheat. No conclusions should be made untl the system aparoaches design cantons, @ Figure 21.4 Fin scacng, (8) Law-tempertue evaccat (8) Mediu: temperature evaporate Peasy Bereanv2 “Then the condensate water has to be drained aff and kept from freezing. Defrost is sometimes accomplished with heat from outside the system. Electric heat can be added to the evaporator to melt the ie but this heat adds to che load of the system and needs to be pumped out after defrost. 21.18 DEFROST OF ACCUMULATED MOISTURE Defrost can be accomplished with heat from inside the sys- tem using the hot gas from the discharge lin of the compres sor by routing a hot gas lin from the compressor discharge line to the outlet of the expansion valve and insalling a solenoid valve to control the flow. When defrost is needed, hot gas is released inside the evaporator, which will quickly smelt any ice, Figure 21.35 ‘When the hor gas enters the evaporator, tis likely chat liquid refrigerane will be pushed out of the suction line to- ward the compeessoe. In fact, when hor gas enters an evapo- rator and starts to cool. as it melts ice or frst, it wll soon lose all of its supesheat and turn to liquid or condense. This ig- uid will goto the aecumolator and fall to ts hortom. Dense saturated vapors will be drawn into the compressor’ suction stroke. The compressor will see an increased load from these dense vapors and may draw a highce amp than during the normal running cycle. Ifthe system does not have an accu ‘mulator, this condensed liquid may flood the compressors crankcase and foaming of the oil in the crankcase may occu, Figure 21.36. This can lower the oil level in the crankease SOLENOID VALVE ~ TO CONTROL ‘SUCTION-LINE HOT GAS, ACCUMULATOR RECEIVER, 7 DRIER SIGHT GLASS Figure 21.35 Using hot gas to defrost an evaporator ‘aa nthe tigen Str and cause scoring of bearing surfaces in the compresion. This condition is often referred to as bearing washout Flooding the compressor’ crankcase with liquid refi «rant ean also cause the foaming refrigerant and oil mixtare to pressurize the crankcase, which causes the mixture of lig vid and vapor refrigerant and ol foam to be forced through any crevice available, including the compressor’s piston rings. The mixture is often pumped into the high side of the system by the compressor. The compressor’ discharge tem perature will decrease from the wet compression of the rich mixture of refrigerant and oil foam, As soon as this mixture is compressed, it will vaporize and absorb heat away from the cylinder walls. This is what causes a lower-than-normal discharge temperature on the compressor's discharge line. Some manufaciurers of systems incorporating hoc yas de frost place a thermistor on the discharge line of the compres- sof f0 sense this cooler than normal discharge temperacure ) e Figure 21.36 (A) A compressors clear cl sight glass. (6 A sight lass from loading crankcase foams with Iquidreigeant. EVAPORATOR REFRIGERATED ‘SPACE + CONDENSATE DAAIN LINE eSsects while in the defrost mode. The thermistor relays a mescage t0 a control circuit ro deenergize the hot gas solenoid; clos- ing the hor gas solenoid temporarily prevents any more hot 1 from entering the evaporator and dus turning to liquid [As the compressor’ discharge temperature rises again, the thermistor rensct the dae in temperature and relays a mes: sage to the control circuit to continue with hot gas defrost by energizing the hot gas solenoid again. This sequence of events may continue until defrost is com plete. The sequence simply protects the compressor from bear- ing washout and wet compression while in defrost I may also protect the compressor during the refrigeration eycle by shut ting down the compressor if the discharge temperature gets 100 low indicating wer compression. To prevent this liquid from eter the Compressor, ofcn a sui line wecunulstor will be added to the suction piping, Figure 21.35. ‘The hot gos defost system is economical because power does not have to be purchased for defost using external heat, such as electric heaters that will heat the evaporator. The heat is aready in thesystem Electric defrost is accomplished using electric heat- ing elements located at the evaporator. The compressor is stopped and the heaters are energized on a call for defrost and allowed to operate until the frost is melted from the coil Figure 21.37. These heaters are often embeded in the actual evaporator fins and cannot be removed if they burn ‘out. Peequently, in uhe event that the heaters do bun out, hhot gas defrost can be added to the system and the electric heat defrost procedures discontinued. ‘When either system is used for defrost, the evaporator fan is often turned off during defrost; if iti not, wwo things will happens 1. ‘The heat from defrost will be transferred directly to the ‘conditioned space. 2. The cold, conditioned air will slaw down the defrost process, Commer Retgertn : 3 Figure 21.37 A heater used for electric defiost of low-temperature evaporators However, some manufacturers design their open frozen- food cases so that the evaporator fans are left on during defrost. Ths allows the supply-air and return-air ducts t0 be defrosted along, with the coil. The warm defrost airs dis ‘charged from the supply duct and rises out ofthe case, hav- ing litte effect on the product temperature. Fans are always shut off on closed, glass-door cases because of the problem of fogging on glass or mirrored surfaces. Evaporators in some ice-making processes have similar defrost methods. They must have some method of applying, heat to the evaporator to free the ice. Sometimes the heat is electric or hot gas. When the evaporator is being used t0 make ice, the makeup water for the ive maker is sometimes used for defrost. Evaporator defrosting is covered in more detail in Section 22, (Defrost Cycle) oF Unit 25, “Special Refrigeration System Components.” In summary, when checking an evaporator remember that its job isto absorb heat into the refrigeration system. SUMMARY ‘+ Heat travels normally from a warm substance to a cool ‘+ For heat to travel from a cool substance to a warm sub- stance, work must be performed. The motor that drives the compressor in the refrigetation eyele does this work. ‘+ The evaporator is the component that absorbs the heat into che refrigeration system. ‘+ The evaporator must be cooler than the medium to be cooled to have a heat exchange. ++ The refrigerant boils to a vapor in the evaporator and absorbs heat because it is boiling at a low pressure and low temperature. + The boiling temperature of the refrigerant in the evapo- savor determines the evaporator (low side) pressure ‘+ Medium-temperature systems can use offcycle defrost. ‘The product is above freezing, and the heat from it can be used to initiate the defrost. ‘+ Low-temperature refrigeration must have heat added to the evaporator to mele the ice. ‘+ For the same types of installations, evaporators have the same characteristies regardless of location, ‘+ Most refrigeration coils ace copper with aluminum fins, ‘© The starting point in organized troubleshooting is to decermine whether the evaporator is operating elv2 Checking the superheat is the hese mthod the service technician has for evaluating evaporator performance. ‘+ Some evaporators are called dry-type because they use a sninimam of refrigecant. ‘* Dry-type evaporators ae also called direct expansion evaporators. ‘+ Some evaporators are flooded and use a float to meter the refrigerant. Supetheat checks on these evaporators should be interpreted with caution, ‘aa nthe tigen Str Some evaporators have a single cireuit, and some have multiple circuits. Mulicircuit evaporators keep excessive pressure drop from occurring in the evaporator ‘There is a relationship between the boiling temperature of the refrigerant in the evaporator and the temperature of the medium being cooled. ‘The coil normally operates at temperatures from 10°F to 20°F eolder than the temperature of the air pasting over it REVIEW QUESTIONS 1. What is the function of the evaporator in the refrigera- sion system? 2. Refrigerant in the evaporator A. changes from vapor to liquid B, changes from liquid to vapor. C.stays in the vapor state, D. stays in che liquid state. 3. The sensible heat that is added to a saturated vapor af- ter all of the liguid has boiled away is referred to as 4, What determines the pressure on the low-pressure side of the system? 5. A refrigerant systems evaporator typically runs about ‘degrees of superheat 6. What docs a high evaporator superheat indicate? 7. A low evaporator supetheat indicates ‘A. undercharge, B, system restriction C. overcharge. D. dirt buildup. 8. Why is a molticircuit evaporator used? 10. " 13, 4, 9. Flooded evaporators use a device. An evaporator that is wo« flooded is thought of xs what type of evaporator? ‘When an evaporator experiences a heat-load increase, the suction pressure A. remains constant. B. decreases. C. varies up and down, D. increases, . What is commonly used to defrost the ice from a low- temperature evaporator? A medium-temperature refrigeration box operates ‘within whae remperature range? A. 28°F t0 40°F B. 40°F to 60°F C.0°F to 20°F D. O°F to 50°F List seven advantages that an aluminum paralle-flow, platen evaporator has over a standard round, copper tube platefin evaporator. type of expansion a