Refrigeration Manual - HVAC and Refrigeration Information Links

More documents

Recommendations

Info

indicates the dry bulb or ambient temperature, while if awick wetted with water is placed around the mercury bulband the thermometer is exposed to rapid air movement,the temperature indicated by the thermometer will be thewet bulb temperature. The difference between the drybulb and wet bulb readings is determined by the rate ofevaporation from the wet surface of the wick, and thisin turn is proportional to the moisture content or vaporpressure of the air. The wet bulb temperature is alwayslower than the dry bulb temperature, and for a given drybulb, the less the moisture content of the air, the lowerthe wet bulb temperature will be.Since the cooling is accomplished by evaporation of thewater, water consumption is only a fraction of that usedin conventional water cooled applications in which thewater once used is discharged to a drain. Evaporativecondensing is therefore widely used in hot, arid regionsof the world.Corrosion, scale formation, and the danger of freezingare problems that must be solved with both evaporativeand water cooled condensers. With both cooling towersand evaporative condensers, a bleed to a drain must beprovided to prevent the concentration of contaminantsin the cooling water.CONDENSER CAPACITYThe heat transfer capacity of a condenser dependsupon several factors:1. Surface area of the condenser .2. Temperature difference between the cooling mediumand the refrigerant gas.3. Velocity of the refrigerant gas in the condenser tubes.Within the normal commercial operating range, thegreater the velocity, the better the heat transfer factor,and the greater the capacity.4. Rate of flow of the cooling medium over or throughthe condenser. Heat transfer increases with velocityfor both air and water, and in the case of air, it alsoincreases with density.5. Material of which the condenser is made. Since heattransfer differs with different materials, more efficientmetals will increase the capacity.6. Cleanliness of the heat transfer surface. Dirt, scale,or corrosion can reduce the heat transfer rate.For a given condenser, the physical characteristicsare fixed, and the primary variable is the temperaturedifference between the refrigerant gas and thecondensing medium.CONDENSING TEMPERATUREThe condensing temperature is the temperature at whichthe refrigerant gas is condensing from a vapor to a liquid.This should not be confused with the temperature ofthe cooling medium, since the condensing temperaturemust always be higher in order for heat transfer to takeplace.In order to condense the refrigerant vapor flowing intothe condenser, heat must flow from the condenser at thesame rate at which heat is introduced by the refrigerantgas entering the condenser. As mentioned previously,the only way in which the capacity of the condensercan be increased under a given set of conditions is byan increase in the temperature difference through thecondenser walls.Since a reciprocating compressor is a positivedisplacement machine, the pressure in the condenserwill continue to increase until such time as thetemperature difference between the cooling medium andthe refrigerant condensing temperature is sufficientlygreat to transfer the necessary amount of heat. Witha large condenser, this temperature difference may bevery small. With a small condenser or in the event airor water flow to the condenser has been blocked, thenecessary temperature difference may be very large.This can result in dangerously high pressures, and it isessential that the condenser is operating properly anytime a refrigeration unit is in operation.The condensing temperature and therefore thecondensing pressure is determined by the capacityof the condenser, the temperature of the coolingmedium, and the heat content of the refrigerant gasbeing discharged from the compressor, which in turn isdetermined by the volume, density and temperature ofthe gas discharged.NON-CONDENSABLE GASESAir is primarily composed of nitrogen and oxygen, andboth elements remain in gaseous form at all temperaturesand pressures encountered in commercial refrigerationand air conditioning systems. Therefore, although thesegases can be liquefied under extremely high pressuresand extremely low temperatures, they may be consideredas non-condensable in a refrigeration system.Scientists have discovered that one of the basic laws ofnature is the fact that in a combination of gases, eachgas exerts its own pressure independently of others,© 1967 Emerson Climate Technologies, Inc.All rights reserved.5-5
and the total pressure existing in a system is the totalof all the gaseous pressures present. A second basiccharacteristic of a gas is that if the space in which it isenclosed remains constant, so that it cannot expand,its pressure will vary directly with the temperature.Therefore, if air is sealed in a system with refrigerant,the nitrogen and oxygen will each add their pressureto the system pressure, and this will increase as thetemperature rises.Since the air is non-condensable, it will usually trapin the top of the condenser and the receiver. Duringoperation the compressor discharge pressure will be acombination of the refrigerant condensing pressure plusthe pressure exerted by the nitrogen and oxygen. Theamount of pressure above normal condensing pressurethat may result will depend on the amount of trapped air,but it can easily reach 40 to 50 psig or more. Any time asystem is running with abnormally high head pressure,air in the system is a prime suspect .CONDENSING TEMPERATURE DIFFERENCEA condenser is normally selected for a system by sizing itto handle the compressor load at a desired temperaturedifference between the condensing temperature andthe expected temperature of the cooling medium.Most air cooled condensers are selected to operate ontemperature differences (commonly called TD) of 20°F. to 30° F. at design conditions, but higher and lowerTDs are sometimes used on specialized applications.Standard production air cooled condensing units areoften designed with one condenser for a wide rangeof applications. In order to cover as wide a range aspossible, the TD at high suction pressures may be from30° F. to 40° F., while at low evaporating temperaturesthe TD often is no more than 4° F. to 10° F. The designcondensing temperature on water cooled units is normallydetermined by the temperature of the water supply andthe water flow rate available, and may vary from 90° F.to 120° F.Since the condenser capacity must be greater than theevaporator capacity by the heat of compression and themotor efficiency loss, the condenser manufacturer mayrate condensers in terms of evaporator capacity, or mayrecommend a factor to allow for the heat of compressionin selecting the proper condenser size.5-6© 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: Figure No.20 shows parallel circuit
Page 19 and 20: if the evaporator temperature is to
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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?