Refrigeration Manual - HVAC and Refrigeration Information Links

More documents

Recommendations

Info

Section 6EVAPORATORSThe evaporator is that part of the low pressure side ofthe refrigeration system in which the liquid refrigerantboils or evaporates, absorbing heat as it changes into avapor. It accomplishes the actual purpose of the system,refrigeration.TYPES OF EVAPORATORSIn other types of systems, secondary refrigerants suchas chilled water or brine may be used for the actualspace or product refrigeration while the evaporator is thewater or brine chiller. A complete packaged water chiller,designed to furnish chilled water for air conditioning orother cooling applications is shown in Figure 22.Evaporators are made in many different shapes andstyles to fill specific needs. The most common styleis the blower coil or forced convection evaporator inwhich the refrigerant evaporates inside of finned tubes,extracting heat from air blown through the coil by a fan.However, specific applications may use bare coils withno fins, gravity coils with natural convection air flow,flat plate surface, or other specialized types of heattransfer surface.Direct expansion evaporators are those in which therefrigerant is fed directly into the cooling coil through ametering device such as an expansion valve or capillarytube, absorbing the heat directly through the walls ofthe evaporator from the medium to be cooled. Figure21 shows a direct expansion coil of one manufacturerprior to assembly in a blower unit.BLOWER COIL CONSTRUCTIONA typical blower coil is made up of a direct expansioncoil, mounted in a metal housing complete with a fan forforced air circulation. The coil is normally constructedof copper tubing supported in metal tube sheets, withaluminum fins on the tubing to increase heat transferefficiency.If the evaporator is quite small, there may be only onecontinuous circuit in the coil, but as the size increases,the increasing pressure drop through the longer circuitmakes it necessary to divide the evaporator into severalindividual circuits emptying into a common header. Thevarious circuits are usually fed through a distributor whichequalizes the feed in each circuit in order to maintainhigh evaporator efficiency.The spacing of fins on the refrigerant tubing will varydepending on the application. Low temperature coils mayhave as few as two fins per inch, while air conditioningcoils may have up to twelve per inch or more. In general© 1967 Emerson Climate Technologies, Inc.All rights reserved.6-1
if the evaporator temperature is to be below 32° F. sothat frost will accumulate, fin spacings of 4 per inch orless are commonly used, although closer fin spacingsare sometimes used if efficient defrost systems areavailable. In air conditioning applications, icing of thecoil is seldom a problem, and the limit on fin spacingmay be dictated by the coil’s resistance to air flow.Since the heat transfer efficiency of the coil increaseswith an increase in the mass flow of air passing throughit, high velocities are desirable. However at face velocitiesgreater than 500 to 600 FPM, water collecting on the coilfrom condensation will be blown off into the air stream,and except for specialized applications, these velocitiesare seldom exceeded.PRESSURE DROP AND OTHER FACTORS INEVAPORATOR DESIGNAs mentioned previously, pressure drop occurring in theevaporator results in a loss of system capacity due tothe lower pressure at the outlet of the evaporator coil.With a reduction in suction pressure, the specific volumeof the gas returning to the compressor increases, andthe weight of the refrigerant pumped by the compressordecreases.However there are other factors which must also beconsidered in evaporator design. If the evaporator tubingis too large, refrigerant gas velocities may become solow that oil will accumulate in the tubing and will not bereturned to the compressor. The only means of assuringsatisfactory oil circulation is by maintaining adequate gasvelocities. The heat transfer ability of the tubing may alsobe greatly decreased if velocities are not sufficient toscrub the interior tubing wall, and keep it clear of an oilfilm. The goals of low pressure drop and high velocitiesare directly opposed, so the final evaporator design mustbe a compromise.Pressure drops through the evaporator of approximately1 to 2 psi are acceptable on most medium and hightemperature applications, and 1/2 to 1 psi are commonin low temperature evaporators.EVAPORATOR CAPACITYThe factors affecting evaporator capacity are quite similarto those affecting condenser capacity.1. Surface area or size of the evaporator .2. Temperature difference between the evaporatingrefrigerant and the medium being cooled.3. Velocity of gas in the evaporator tubes. In the normalcommercial range, the higher the velocity the greaterthe heat transfer rate.4. The velocity and rate of flow over the evaporatorsurface of the medium being cooled.5. Material used in evaporator construction.6. The bond between the fins and tubing is quiteimportant. Without a tight bond, heat transfer will begreatly decreased.7. Accumulation of frost on evaporator fins. Operationat temperatures below freezing with blower coils willcause the formation of ice and frost on the tubesand fins. This can both reduce the air flow over theevaporator and reduce the heat transfer rate.8. Type of medium to be cooled. Heat flows almost fivetimes more effectively from a liquid to the evaporatorthan from air .9. Dewpoint of the entering air. If the evaporatortemperature is below the dewpoint of the enteringair, latent as well as sensible cooling will occur.TEMPERATURE DIFFERENCE ANDDEHUMIDIFICATIONSince for a given installation, the physical characteristicsare fixed, the primary variable as in the case of thecondenser, is the temperature difference between theevaporating refrigerant and the medium being cooled,commonly called the TD. For a blower coil, the colderthe refrigerant with respect to the temperature of the airentering the evaporator, the greater will be the capacityof the coil.Temperature differences of 5° F. to 20° F. are commonlyused. Usually for best economy, the TD should be keptas low as possible, since operation of the compressorwill be more efficient at higher suction pressures.The amount of moisture condensed out of the air isin direct relation to the temperature of the coil, and acoil operating with too great a differential between theevaporating temperature and the entering air temperaturewill tend to produce a low humidity condition in therefrigerated space. In the storage of leafy vegetables,meats, fruits, and other similar perishable items, lowhumidity will result in excessive dehydration and damageto the product. For perishable commodities requiring avery high relative humidity (approximately 90%) a TDfrom 8° F. to 12° F. is recommended, and for relativehumidities slightly lower (approximately 80%) a TD from12° F. to 16° F. is normally adequate.6-2© 1967 Emerson Climate Technologies, Inc.All rights reserved.
Page 3 and 4: Part IIREFRIGERATION SYSTEM COMPONE
Page 5 and 6: Section 4COMPRESSORSThe compressor
Page 7 and 8: CROSS-SECTIONAL VIEW OF COPELAMETIC
Page 9 and 10: When the piston starts down on the
Page 11 and 12: COMPRESSORS WITH UNLOADERSIn order
Page 13 and 14: flow by means of louvers is also an
Page 15 and 16: Figure No.20 shows parallel circuit
Page 17: and the total pressure existing in
Page 21 and 22: In modern refrigeration practice, a
Page 23 and 24: CAPILLARY TUBE SELECTION R-22HIGH T
Page 25 and 26: CAPILLARY TUBE SELECTION R-12MEDIUM
Page 27 and 28: CAPILLARY TUBE SELECTION R-502LOW T
Page 29 and 30: multiple evaporators are used. On l
Page 31 and 32: which may be caused by pulsations i
Page 33 and 34: WATER REGULATING VALVESOn water coo
Page 35 and 36: CONDENSER FAN CYCLING CONTROLIn ord
Page 37 and 38: 1. The continuous VA (volt-amperes)
Page 39 and 40: A. Continuous VA requirement2 Conta
Page 41 and 42: The desired pulley size or the resu
Page 43 and 44: un winding is wound with relatively
Page 45 and 46: TWO PHASE MOTORSTwo phase power is
Page 47 and 48: 1 = 1 + 1MFD total MFD1 MFD2For exa
Page 49 and 50: voltage torque, and star-delta star
Page 51 and 52: 10-6© 1967 Emerson Climate Technol
Page 53 and 54: Regular Copeland® brand motor prot
Page 55 and 56: contact prematurely due to shock or
Page 57 and 58: The effect of an open phase in the
Page 59 and 60: vulnerable to liquid damage such as
Page 61 and 62: REFRIGERATION GAUGESPressure gauges
Page 63 and 64: © 1967 Emerson Climate Technologie

Refrigeration Manual - HVAC and Refrigeration Information Links

Create successful ePaper yourself

Delete template?

Save as template?