Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco. Chapter : Process For additional information on this subject, contact File Reference: CHE21001 R.A. Al-Husseini on 874-2792 Engineering Encyclopedia Saudi Aramco DeskTop Standards Refrigeration Systems
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Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.
Chapter : Process For additional information on this subject, contactFile Reference: CHE21001 R.A. Al-Husseini on 874-2792
TYPES OF REFRIGERATION SYSTEMS USED BY SAUDI ARAMCO .......................................1
SELECTING APPROPRIATE REFRIGERATION SYSTEMS .........................................................2
Vapor Compression-Expansion Refrigeration Systems .....................................................................2Operation .............................................................................................................................2Size and Achievable Temperatures ......................................................................................2Refrigerants..........................................................................................................................3
Figure 5. Vertical Shell, Small Capacity, Lithium Bromide Cycle WaterChiller for Solar Cooling ............................................................................................................8
Figure 6. Steam Jet Refrigeration System ......................................................................................................9
Figure 7. (Page 1 of 3). ASHRAE Standard Designation of Refrigerants(ANSI/ASHRAE Standard 34-78) ..............................................................................................12
Figure 7. (Page 2 of 3). ASHRAE Standard Designation of Refrigerants(ANSI/ASHRAE Standard 34-78) ..............................................................................................13
Figure 7. (Page 3 of 3). ASHRAE Standard Designation of Refrigerants(ANSI/ASHRAE Standard 34-78) ..............................................................................................14
Figure 8. Frequently Used Refrigerants .........................................................................................................14
Figure 9. Latent Heat of Vaporization Versus Boiling Point..........................................................................18
Figure 10. Effects of Temperature on Capacity [Constant CompressorDisplacement = 2 L/s (4 cfm)] ....................................................................................................20
Figure 11. Effects of Temperature on Theoretical kW (HP) per kW (Ton)of Refrigeration...........................................................................................................................21
Figure 12. Purge Unit and Piping for Noncondensable Gas...........................................................................27
Figure 13. Summary of Refrigerant Applications ..........................................................................................29
Figure 14. Refrigeration Systems Schematic..................................................................................................30
Figure 15. Refrigeration System and Equipment Limits ................................................................................31
Figure 16. Temperature Limits for Refrigerants .............................................................................................32
Figure 17. Physical Properties of Refrigerants ...............................................................................................33
Figure 18. Specific Gravity of Aqueous Solutions of Lithium Bromide ........................................................34
Figure 19. Specific Heat of Aqueous Solutions of Lithium Bromide.............................................................35
Figure 20. Viscosities of Aqueous Solutions of Lithium Bromide.................................................................36
Figure 21. Velocity of Sound in Refrigerant Vapors, m/s (ft/s) .....................................................................37
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Figure 22. Comparative Refrigerant Performance Per kW (Ton)[Based on 258°K Evaporation (5°F) and 303°K Condensation (86°F)]......................................38
Figure 23. (Page 1 of 4). Comparative Refrigerant Performance Per kW(Ton) at Various Evaporating and Condensing Temperatures ....................................................39
Figure 23. (Page 2 of 4). Comparative Refrigerant Performance Per kW(Ton) at Various Evaporating and Condensing Temperatures ....................................................40
Figure 23. (Page 3 of 4). Comparative Refrigerant Performance Per kW(Ton) at Various Evaporating and Condensing Temperatures ....................................................41
Figure 23. (Page 4 of 4). Comparative Refrigerant Performance Per kW(Ton) at Various Evaporating and Condensing Temperatures ....................................................42
Figure 24. Relative Safety of Refrigerants .....................................................................................................43
Figure 25. Underwriters Laboratories Classification of Comparative Hazard toLife of Gases and Vapors............................................................................................................44
Figure 26. ASHRAE SI for HVAC and R Conversions .................................................................................47
Types of Refrigeration Systems Used by Saudi ARAMCOFigure 1 lists some of the refrigeration systems currently used by Saudi Aramco. All of these systems use vaporcompression-expansion refrigeration systems. All of these systems, except one, use hydrocarbons (ethane,propane, butane, etc.) for their refrigerants. Most of the systems use only propane, but a few of them arecascade systems that use propane/ethane.
In addition to the refrigeration systems listed in Figure 1, Saudi Aramco uses many air-conditioning systems forbuildings. These air-conditioning systems may use absorption, steam jet, or vapor compression-expansion(with freon) refrigeration systems.
SelectING appropriate refrigeration SYSTEMSThis section introduces and briefly describes the following types of refrigeration systems that are covered inChE 210.
The information contained in the following sections is used to narrow the choice of refrigeration systems. Thefinal selection of the most appropriate system will be made by comparing the capital and operating costs of thesystems and determining schedule availability.
Vapor Compression-Expansion Refrigeration Systems
OperationFigure 2 shows a simplified schematic of a vapor compression-expansion system. The compressor compressesthe vaporized refrigerant and then the condenser condenses the vapor to a liquid. The liquid from theaccumulator then expands across an expansion valve which causes it to flash and its temperature to dropsubstantially. The liquid portion of this flash is the refrigerant which then goes to a heat exchanger (the chiller).
In the chiller, the liquid refrigerant absorbs heat from the process stream, which lowers the process stream’stemperature. The heat vaporizes the liquid refrigerant. The system then returns this vapor and the vapor fromthe flash to the compressor to repeat the cycle.
Size and Achievable TemperaturesVapor compression-expansion systems can be built in very large sizes. These systems can also reach very lowtemperatures. A cascaded system consists of two or more vapor compression-expansion systems that areconnected in a series with each system having a different refrigerant. These systems can achieve temperatureslow enough to liquefy natural gas.
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Figure 2. Vapor Compression-Expansion Refrigeration System
RefrigerantsVapor compression-expansion systems typically use hydrocarbons, freons, ammonia, and sulfur dioxide forrefrigerants.
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Absorption Refrigeration SystemsAbsorption refrigeration systems are similar to vapor compression-expansion systems, except that they absorbthe refrigerant with a carrier liquid. Carrier liquids can use a pump instead of a compressor to increase thepressure of the system. This section covers ammonia-water and lithium bromide-water absorption systems.
OperationAmmonia Absorption Systems – Figure 3 shows a schematic of a simplified absorption refrigeration systemthat uses water to absorb the ammonia refrigerant.
Figure 3. Ammonia Absorption Refrigeration Systems
The system in Figure 3 uses ammonia for its refrigerant and water for its liquid carrier. In the absorber, water(the carrier liquid) absorbs ammonia vapor (the refrigerant). Next, the ammonia-water solution enters the pumpwhere the pressure of the solution is increased to a high level. After the solution leaves the pump, the generatorcolumn vaporizes the refrigerant (ammonia) from the water (the carrier liquid). The system returns the water tothe absorber and sends the ammonia vapor to the condenser.
At this point in the cycle, absorption systems are very similar to vapor compression-expansion systems. Theammonia vapor is condensed to form the liquid refrigerant stream, which is stored in the accumulator drum.The expansion valve flashes the liquid refrigerant, which reduces its temperature. In the chiller, the refrigerantabsorbs heat from the process stream and the absorbed heat boils the refrigerant. As in a vapor compression-expansion system, the boiled refrigerant and the flash vapor are sent to the absorber column to begin the cycleagain.
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Lithium Bromide-Water Single-Stage Absorption Systems – Figure 4 shows a lithium bromide-waterabsorption system.
The system shown in Figure 4 uses a lithium bromide-water solution as its absorbent (carrier liquid) and wateras its refrigerant. In this system, hot concentrated lithium bromide-water solution (in equilibrium at condenserpressure) leaves the generator and enters the heat exchanger. In the heat exchanger, the incoming solutioncools the concentrated lithium bromide-water solution. The cooled solution is then throttled through therestrictor to the absorber.
In the absorber, the cold concentrated lithium bromide-water solution absorbs low-pressure refrigerant (8 and 9)that is in equilibrium at evaporator pressure. This diluted solution is then pumped to the heat exchanger. In theexchanger, the diluted solution absorbs heat from the hot solution that leaves the generator. The heatedsolution then flows to the generator (5).
The generator then heats the hot, diluted solution and strips the refrigerant from the carrier liquid. The distilledrefrigerant (diluted solution) flows to the condenser (6) and the hot, concentrated solution flows to the heatexchanger (1).
The condenser condenses the hot, high-pressure refrigerant into hot, high-pressure liquid (7). The restrictorexpands the hot, high-pressure liquid to low-pressure, low-temperature liquid and vapor. In the evaporator, thelow- temperature, low-pressure refrigerant absorbs heat from the space being refrigerated. The low-pressurerefrigerant then flows to the absorber (3) where it is absorbed by the cold, concentrated lithium bromidesolution (the carrier liquid).
ApplicationsAbsorption refrigeration systems were once popular for a variety of cooling tasks in industry and for foodstorage. Recently, the absorption refrigeration cycle has found renewed use in medium- and large-size air-conditioning systems. Currently, these systems generally use lithium bromide for their absorbent and potablewater for their refrigerant.
Absorption refrigeration systems are popular where one or more of the following conditions exists:
• Electric rates are high.• Fuel costs are low.• Low-pressure heating boilers are not used during summers (the cooling season).• Steam or gas utility companies promote summer loads.• Waste steam is available.
Absorption refrigeration systems can be installed in almost any building with adequately strong and levelfloors. In absorption refrigeration systems, the absence of heavy moving parts minimizes noise levels andpractically eliminates vibration.
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Small lithium bromide-water units of 10.5 kW to 105 kW (3 tons to 30 tons) capacity are designed forresidential or small commercial use. Indirect- and direct-fired liquid chiller, chiller-heater, and air-conditioningequipment (both single-stage and dual-effect configurations) have been produced. Currently, lithium bromide-refrigeration systems have one or more of the following unique features:
• Flat-plate solar collector heat sources with good efficiency, but with lower capacity, can be used tofire the refrigeration cycle.
• Heat can be derived from the cooling cycle by stopping the flow of the cooling water. In somedesigns, when the cooling water stops, a solution trap between the high and low sides opens andallows refrigerant vapor to flow to the evaporator coil. In the evaporator coil, the water that flowsinside the evaporator tubes is condensed and heated.
• Solution can be circulated either thermally by vapor-lift action in a pump tube, or by a mechanicalpump.
Lithium bromide absorption systems are available as package units. Figure 5 shows an example of a packagedlithium bromide-absorption system.
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Source: 1988 Equipment
Figure 5. Vertical Shell, Small Capacity, Lithium Bromide Cycle Water Chiller for SolarCooling
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AdvantagesAbsorption refrigeration systems have the following advantages:
• They do not require compressors.
• Uninsulated, small-diameter, low-pressure lines can circulate the liquid refrigerant and absorbent.
• Lithium bromide systems are not flammable.
• Low-level waste heat can frequently be used as the heat source for the generator.
• Many remotely-located refrigeration systems can be connected to one, central generator that islocated near a source of waste heat. Small diameter liquid refrigerant and adsorbent lines connectthe remotely-located refrigeration systems.
Steam Jet Refrigeration SystemsSteam jet refrigeration systems use a steam jet ejector to produce vacuum pressures. At these lower pressuresthe water flashes and its temperature is reduced. Steam jet refrigeration systems then use this chilled water fortheir refrigerant. Figure 6 shows a schematic of a simplified steam jet refrigeration system.
Figure 6. Steam Jet Refrigeration System
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OperationIn Figure 6, chilled water is pumped from the evaporator to the chiller. In the chiller, the chilled water absorbsheat from the process stream that is being refrigerated and then flows to the evaporator. The evaporator flashesthe water and lowers its temperature. The flashed vapor leaves the evaporator through the steam jet ejector.Steam flowing through the steam jet ejector maintains the low-pressure (vacuum) in the evaporator.
The steam from the steam jet ejector and the flashed water from the evaporator flow to the condenser and arecondensed. The system diverts enough water to the evaporator to replace the flash vapor. The excess water canbe used for boiler feed water or other uses.
AdvantagesSteam jet refrigeration systems have the following advantages:
• The refrigerant (water) is not flammable.
• The refrigerant is not toxic.
• The refrigerant is a low-pressure liquid that can be transported in small lines.
• Waste steam can be used in the steam jet ejector.
DisadvantagesSteam jet refrigeration systems have the following disadvantages:
• The freezing point of water limits the temperatures these systems can achieve.
• The low operating pressures of the evaporator and the condenser require large equipment.
ApplicationsSteam jet refrigeration units are used for some cooling loads of 175 kW (50 tons) or more. These systems areused in many applications in which steam is available. Typical applications include comfort air conditioning,industrial process cooling, and other similar types of service. Recently, some large office buildings have steamjet refrigeration systems installed on the roof. These units provide the cooling needed for the buildings’ airconditioning systems.
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SELECTING the most appropriate refrigerantsOnce a designer has selected a refrigeration system, he must choose a refrigerant. The following points must beconsidered when selecting the most appropriate refrigerants.
• The minimum temperature the refrigerant must achieve (refrigerant level).
• The suitability of the refrigerant.
• Environmental regulations and concerns.
The minimum temperature that the refrigerant must achieve (refrigerant level) narrows the list of possibleappropriate refrigerants and sets the system pressure. The suitability of the refrigerant derives from theavailability, cost, flammability, and toxicity of the refrigerant. Freon, for example, is becoming difficult toobtain because of environmental regulations and concerns which restrict its manufacture and use.
Hydrocarbons should be used in refinery applications because the material is readily available, can be ventedinto the refinery system during turnaround, and have a low cost. Flammability is not a large factor in thenormal refinery environment. Water is a good choice as long as freezing is not a constraint. Ammoniaabsorption could be considered if waste heat is available for solution regeneration.
Refrigerants often affect the materials they contact in refrigeration systems. Therefore, the consideration of arefrigerant’s thermal stability and its compatibility with other materials is important. Elastomers and theelectrical insulation of refrigeration systems must also be chosen carefully.
It is important to choose a refrigerant that will not operate under vacuum at the lowest pressure point in thesystem. Operation at sub-atmospheric conditions with a flammable refrigerant could cause an explosivemixture to form if there is any leakage. In the case of non-flammable material (freon), the in-leakage of air willcause problems when the refrigerant is condensed.
Commonly Used Refrigerants
Typically, a refrigerant is referred to by the refrigerant number ("R" Number) assigned to it by ASHRAE. Thetable in Figure 7 lists this refrigerant number, the chemical name, and the chemical formula of each refrigerant.
a Methane, ethane, and propane appear in the Halocarbon section in their proper numerical order, but these compounds are not halocarbons.b Ethylene and propylene appear in the Hydrocarbon section to indicate that these compounds are hydrocarbons, but they are properly
identified in the section Unsaturated Organic Compounds.Source:1989 Fundamentals
Figure 7 (Page 1 of 3). ASHRAE Standard Designation of Refrigerants(ANSI/ASHRAE Standard 34-78)
a Methane, ethane, and propane appear in the Halocarbon section in their proper numerical order, but these compounds are not halocarbons.b Ethylene and propylene appear in the Hydrocarbon section to indicate that these compounds are hydrocarbons, but they are properly identified in the section
a Methane, ethane, and propane appear in the Halocarbon section in their proper numerical order, but these compounds are not halocarbons.b Ethylene and propylene appear in the Hydrocarbon section to indicate that these compounds are hydrocarbons, but they are properly identified in the section
Figure 7 (Page 3 of 3). ASHRAE Standard Designation of Refrigerants(ANSI/ASHRAE Standard 34-78)
When not presented in a table, the prefix "R" is normally used to indicate that the number is an ASHRAEnumber. The following are the refrigerants most frequently used:
Freons Hydrocarbons Other AbsorptionR-12 R-170 R-717 Li BrR-13 R-290 R-718 Ammonia (R-717)R-13 B1 R-1150 R-728R-22 R-1270 R-744R-502 R-764R-503
Figure 8. Frequently Used Refrigerants
Figure 16 in Work Aid 2 shows the approximate ranges of temperatures in which the more common refrigerantsare used. Industrial systems with temperatures down to about -50°C (-45°F) frequently use R-717 or R-22 insingle- or two-staged systems. Two-stage systems with temperatures in the -73°C to -101°C (-100°F to -150°F)range frequently use R-503 in the low range and R-22, R-12, or R-502 in the high range.
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The ozone depletion potential of freons will probably cause their use to decline.
At any given temperature, R-503 has a higher suction pressure and density than R-13. Therefore, a givencompressor with a suction temperature of - 84°C(-120°F) and a condensing temperature of -29°C (-20°F) that uses R-503 instead of R-13 has 57% morecapacity. R-503 also maintains a positive suction pressure at temperatures lower than R-13. At about -87°C (-125°F), R-503 has a suction pressure of 0 kPa (gauge). In comparison, R-13 has a suction pressure of 0 kPa at -79°C (-110°F).
R-503, however, has a higher operating pressure at ambient temperature. At 16°C (60°F), R-503 has anoperating pressure of 3.9 MPa (560 psig) but R-13 has an operating pressure of only 2.77 MPa (402 psig). Theuse of R-503 or R-13 requires special designs and special handling. Systems that use these refrigerants requirean expansion tank to hold the refrigerant during periods of shutdown or power failure.
Physical PropertiesThe critical properties of a refrigerant represent the conditions under which the distinction between liquid andgas is lost. At temperatures above the critical point, a separate liquid phase is not possible. For refrigerationcycles that use condensation, a refrigerant must be chosen that will change between liquid and gas phases belowthe critical point of the refrigerant. Figure 17 in Work Aid 2 lists physical properties of refrigerants that areused in vapor compression systems.
Figure 17 lists the refrigerants in order of increasing boiling points. The boiling point of a refrigerant directlyindicates the temperature level at which a refrigerant can be used. The freezing point of a refrigerant must belower than any temperature in its intended application.
Figures 18, 19, and 20 show density, specific heat, and viscosity of lithium bromide solutions.
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FlammabilityTo prevent the drawing of air into the refrigerant system, the use of extremely flammable refrigerants (propane,for example) should be limited to applications where pressures will remain above atmospheric pressure.
The Safety Code for Mechanical Refrigeration ANSI/ASHRAE 15-1598 and the IIAR Bulletin 109 MinimumSafety Criteria for a Safe Ammonia Refrigeration System (1988) should be referred to for safety and minimumdesign criteria of common refrigerants.
Compromises Between Conflicting Desirable Thermodynamic PropertiesOperating-Pressure – Desirable refrigeration systems have the highest evaporator pressures and the lowestcondenser pressures. Systems with high evaporator pressures have high vapor densities. Therefore, for a givencompressor, the higher the evaporator pressure, the higher the system capacity. However, increasing theevaporator pressure also decreases the efficiency of the system, especially as the condenser pressure approachesthe critical temperature.
Latent Heat of Vaporization – On a molar basis, fluids with similar boiling points have almost the same latentheat of vaporization. Since compressors operate on volumes of gas, refrigerants with similar boiling pointsproduce similar capacities in the same compressor. On a mass basis, the latent heat of fluids varies widely. In atheoretical vapor compression cycle, fluids with low vapor heat capacities produce the maximum efficiencies.Fluids with simple molecular structures and low molecular weights are associated with low vapor heatcapacities.
Transport Properties – The thermal conductivity and the viscosity of a refrigerant affect the performance ofheat exchangers and piping. Refrigerants with high thermal conductivity and low viscosity are desirable.
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Velocity of Sound - Figure 21 in Work Aid 2 lists the velocities of sound in several refrigerants. The velocityof sound through a gas limits the practical velocity of a gas through piping and openings. The velocity ofsound increases as the temperature of the gas increases, and decreases as the pressure of the gas increases.
Equation A (shown below) can be used to calculate the velocity of sound in gases.
va = Gckdρdρ
T
For a perfect gas: va = GckRT/M (EQN
A)
where:
Va = Velocity of sound, m/s (ft/s)
Gc = Constant, 32.17 m/s2 (32.17 ft/s2)
k = cp/cv ratio of specific heats
R = Gas Constant, 8.314 gr-m/mole °K (1,546 lb-ft/mole °R)
T = Temperature, °K (°R)
M = Molecular Weight, gr/mole (lb/mole)dρdρ
T= Rate of change of pressured/rate of change of density, atconstant temperature
Latent HeatAccording to an empirical rule of chemistry (Trouton’s Rule), the latent heat of vaporization of a material at itsboiling point (on a molar basis) divided by the boiling temperature (in absolute units) is a constant for mostmaterials.
Figure 9 tabulates this constant for several refrigerants. Note that while this rule applies fairly well to theserefrigerants, the “constant” varies from 96.64 η/K to 73.36 η/K (23.086 η/°R to 17.52 η/°R). This rule canbe used to compare different refrigerants and to understand the operation of refrigeration systems.
a Not at normal atmospheric pressure.b Normal boiling temperatures.1 Thermodynamic Properties of Refrigerants (Stewart et al. 1986)2 Handbook of Chemistry and Physics (CRC 1987)3 ASHRAE (1977)4 Chemical Engineer's Handbook (1973)
Source: 1989 Fundamentals (SI)
Figure 9. Latent Heat of Vaporization Versus Boiling Point
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PerformanceFigure 22 in Work Aid 2 tabulates the theoretical calculated performance of several refrigerants. Thetheoretical performance of these refrigerants is based on a US standard cycle of 258 K (5°F) evaporation and303 K (86°F) condensation. Figure 23 in Work Aid 2 tabulates calculated data for other operating conditions.
Both Figure 22 and Figure 23 can be used to compare the properties of different refrigerants. Actual operatingconditions, however, usually differ from those that are tabulated. Most of the tabulated data was generated withthe assumption that the suction vapor is saturated and that the compression is adiabatic (constant entropy).
In Section F of Figure 22, the temperature of the suction gas is 291 K (65°F). A comparison of Section F withSection E illustrates the effect of suction gas superheating on the properties of refrigerants.
Figure 10 tabulates the effect of different evaporating and condensing temperatures on the theoretical capacityof refrigeration systems using R-12 and R-22. Figure 11 tabulates the effect of different evaporating andcondensing temperatures on the horsepower of compression of refrigeration systems when R-12 and R-22 isused. Figure 10 and Figure 11 below assume constant compressor displacement.
Figure 10. Effects of Temperature on Capacity [Constant CompressorDisplacement = 2 L/s (4 cfm)]
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EVAP. TEMP. CONDENSING TEMPERATURE
°C 15°C 25°C 35°C 45°C
R-12 THEORETICAL kW/kW OF REFRIGERATION
-30 0.22 0.28 0.36 0.45
-20 0.15 0.21 0.27 0.34
-10 0.11 0.16 0.22 0.28
5 0.04 0.08 0.13 0.18
R-22 THEORETICAL kW/kW OF REFRIGERATION
-30 0.22 0.28 0.36 0.44
-20 0.16 0.22 0.28 0.36
-10 0.11 0.16 0.21 0.28
5 0.04 0.08 0.13 0.18
EVAP. TEMP. CONDENSING TEMPERATURE
°F 60°F 80°F 100°F 120°F
R-12 THEORETICAL hp/ton OF REFRIGERATION
-20 1.03 1.38 1.80 2.32
0 0.71 1.01 1.37 1.80
20 0.44 0.70 1.00 1.36
40 0.20 0.43 0.68 0.99
R-22 THEORETICAL hp/ton OF REFRIGERATION
-20 1.02 1.35 1.75 2.22
0 0.71 1.00 1.34 1.75
20 0.44 0.69 0.99 1.34
40 0.20 0.43 0.68 0.98
Source: 1989 Fundamentals (SI)
Figure 11. Effects of Temperature on Theoretical kW (HP) per kW (Ton)of Refrigeration
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Figure 24 in Work Aid 2 summarizes the toxicity and flammability of many refrigerants. ANSI/ASHRAEStandard 15-78 classifies refrigerants according to the hazard involved in their use. Figure 24 tabulatesrefrigerants by their ANSI/ASHRAE Standard 15-78 safety code group and by their Underwriters’ Laboratories(UL) classification of comparative hazard to life. Group 1 refrigerants are the least hazardous and Group 3refrigerants are the most hazardous. Figure 25 in Work Aid 2 tabulates the UL group classifications.
Effect on Construction MetalsUnder normal conditions, halogenated refrigerants can be used with most metals such as steel, cast iron, brass,copper, tin, lead, and aluminum. Under more severe conditions, various metals affect the properties ofrefrigerants such as hydrolysis and thermal decomposition. Metals affect refrigerant properties in varyingdegrees. For instance, the metals listed below promote thermal decomposition of halogenated refrigerants. Themetals are listed in the approximate order of their promotion of thermal decomposition:
These metals can have a similar effect on the hydrolysis of halogenated refrigerants. In systems when a trace ofwater may be present, magnesium, zinc, and aluminum alloys containing more than 2% magnesium are notrecommended for use with halogenated refrigerants. When water is present in sulfur dioxide systems, sulfurousacid is formed and attacks iron and steel rapidly and other metals more slowly.
Warning: Never use methyl chloride with aluminum in any form.This combination produces a highly flammable gas andis a great explosion hazard.
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Ammonia (R-717)Ammonia is generally used in large, open-type compressors for industrial and commercial applications. Theusage of ammonia may increase, as it continues to become a substitute for freons. Ammonia has the followingadvantages as a refrigerant:
• High refrigerating capacity per unit of compressor displacement• Low-pressure losses in connecting piping• Low reactivity with refrigeration oils
In contrast with freon, ammonia is toxic and flammable. Ammonia is a strong irritant. Humans can detect it atconcentrations below 5 ppm. Within a narrow range of concentrations (16% to 25% by volume), ammonia airmixtures are flammable. These mixtures can explode if heated above 650°C (1200°F), but the maximumexplosive pressure is relatively low 350 kPa (50 psi). Therefore, ammonia is not considered a serious fire orexplosion hazard.
The thermal stability of ammonia often confuses people. At atmospheric pressure, at 300°C (570°F), and in thepresence of active catalysts such as nickel or iron, ammonia starts to dissociate into nitrogen and hydrogen.However, because such high temperatures are unlikely to occur in open type compressors, thermal stability isnot a problem in ammonia systems.
When moisture is present, ammonia attacks copper. Therefore, except for some specialty bronzes, copper-bearing metals must be excluded from ammonia systems.
Ammonia is a powerful solvent that strips any remaining dirt and scale from piping. To protect against thedebris that results from stripping. Most compressors are equipped with suction strainers, disposable strainerliners, or both.
Liquid ammonia that returns with the suction gas causes serious cylinder and ring wear. Ammonia slugs breakvalves or cause more serious damage. Low steady flow rates of ammonia liquid reduce cylinder lubrication.This loss of lubrication causes extremely rapid cylinder wall and ring abrasion.
A carefully and properly installed system, in which no foreign matter or liquid enters the compressor, willoperate for many years. When new piping is installed, it should be wire brushed and blown out withcompressed air.
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Ammonia Piping RequirementsRecommended Material – Ammonia piping should conform to ANSI/ASME Code for Pressure Piping. Thiscodes specifies the following:
• Liquid lines 40 mm (1.5 in) and smaller shall not be less than schedule 80 carbon steel pipe.
• Liquid lines 50 mm (2 in) through 150 mm (6 in) shall not be less than schedule 40 carbon steelpipe.
• Vapor lines 150 mm (6 in.) and smaller shall not be less than schedule 40 carbon steel pipe.
Fittings – Coupling, elbows, and tees for threaded pipe should be built for a minimum design pressure of 14MPa (2000 psi) and constructed of forged steel.
Tongue and groove or ANSI flanges should be used in ammonia piping.
Pipe Joints – Joints installed between lengths of pipe or between pipe and fittings can be threaded if the size ofpipe is 32 mm (1.25 in) or smaller. Joints in piping that is sized 40 mm (1.5 in), or larger, should be welded.
To prevent contamination of the ammonia refrigeration system, excessive amounts of sealant should not beapplied to any joint, nor should any sealant be applied to female threads.
When flanges are used, 1.6 mm (0.063 in) fiber gasket should also be used. The pipe and bolt holes should beproperly aligned before tightening flange bolts to valves, controls, or flange unions. The use of flanges tostraighten pipe may put stress on adjacent valves, compressors, and controls. This stress can cause theoperating mechanism to break.
Steel (14 MPa = 2000 psi) ground joint unions should be used for gauge and pressure control lines withscrewed valves and for joints up to 20 mm (0.75 in).
Pipe Location – Where possible, piping for ammonia systems should be located at least 2.3 m (7.5 ft) abovethe floor. Piping for ammonia systems, especially if they are to be insulated, should be located carefully inrelation to other piping and structural members. Pipe hangers should be placed no more than 2.5 m to 3 m (8 ftto 10 ft) apart.
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Oil in Ammonia Systems – Oil is miscible in liquid ammonia only in very small proportions. As temperaturesdecrease, oil separates from the liquid ammonia. The evaporation of ammonia increases the concentration of oiland causes the oil to form a separate layer below the ammonia liquid.
The periodic or continuous removal of oil prevents it from covering the evaporator’s heat transfer surface andreducing performance. Because the density of oil is greater than that of ammonia, oil will settle in low areas ofammonia systems. Unless temperatures are too low to allow the oil to flow, draining oil from these systems isnot difficult. In low-temperature situations, maintaining oil receivers at higher temperatures may be beneficial.
Oil Separators should be located in the discharge line of a compressor. A high-pressure float valve drains oilback into the compressor crankcase or oil receiver. To cool the discharge gas (ammonia vapor), oil separatorsshould be placed as far from the compressor as possible. The cooling of the ammonia vapor increases theeffectiveness of the separator.
Compressor Piping –The objective of suction mains is to return clean, dry gas to the compressor.
A good suction line system has a total friction drop 1 K (2°F) to 3 K (5°F) pressure drop equivalent. Practicalfriction losses should not exceed 0.01 K per meter equivalent length (0.5°F equivalent per 100 ft equivalentlength). The main suction line should slope toward a suction trap (1% vertical drop per horizontal lengthminimum).
A well designed discharge main has a total friction loss of 20 kPa to 35 kPa (3 psi to 5 psi). To hold downdischarge pressure and discharge temperature, a slightly oversized discharge line is generally used. To protectthe compressor, high- and low-pressure cutouts and gauges are installed on the compressor side of stop valves.
Discharge Check Valves located on the downstream side of each oil separator will prevent high-pressure gasfrom flowing into an inactive compressor and condensing. To prevent reverse rotation, screw compressors mustbe equipped with a check valve.
If the temperature of the engine room is below the condensing temperature of the refrigeration system, gas mayleak past the check valve and condense in the discharge line and oil separator. Liquid ammonia then drains intothe compressor from the float under the oil separator.
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When the temperature of the engine room is below the condensing temperature of the refrigeration system,ASHRAE recommends the use of a positive method of equalizing the fluid level of the discharge and suctionlines. ASHRAE recommends that equalization always be used when compressors are located outside or inunheated spaces. Equalization can be done automatically or manually.
Accessories – Any ammonia vessel that can be valved off must have an automatic relief valve. The sizing ofthe valve should conform to Safety Code for Mechanical Refrigeration ANSI/ASHRAE 15-1989.
The use of dual relief valves allows the testing of each valve. The use of three-way stop valves allows one to beremoved for repair or testing.
A noncondensable gas separator (purge unit) is useful in most plants, especially when an ammonia system’ssuction pressure is equal to atmospheric pressure.
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Source: 1990 Refrigeration
Figure 12. Purge Unit and Piping for Noncondensable Gas
In Figure 12, the high-pressure liquid expands through the coil in the purge unit. This expansion cools theliquid and the coil. The line from the coil then goes to a high-temperature suction main. Ammonia vapor andnoncondensable gas are drawn up into the purge drum. The ammonia then condenses on the cold surface of thecoil. The air and other noncondensable gases fill the purge drum. When the drum is full, a float valve opensand releases the noncondensable gases to the open water bottle.
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The receiver should be equipped with an armored gauge glass to allow the measurement of the ammonia in thereceiver. The valves on each side of the gauge glass should be equipped with excess flow check valves toprevent the loss of refrigerant if the gauge glass breaks.
Enough liquid ammonia needs to be charged into the system to seal the outlet pipe of the receiver and toprevent hot gas from blowing from the high side to the low side. The lowest liquid level a receiver shouldoperate is the equivalent of two diameters of the outlet pipe for both the top and bottom outlet receivers.
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REFRIG-ERANT
FLAM-MABLE TOXIC
USESLOW-
LEVELHEAT
MINIMUMACHIEVABLE
TEMPER-ATURE TYPICAL APPLICATIONS
R-717(ammonia)
Yes Yes No 241 K (-26°F) Used extensively in large, opencompressors for industrial andcommercial applications.
R-11
R-13B1
R-500
No Low No 297K +76°F216K -70°F241K -26°F
Used widely for specializedapplications.
R-12 No Low No 240 K (-20°F) Most widely used refrigerants.
R-22 No Low No 233 K (-40°F) Most widely used refrigerants.
Water(ammonia
absorption)
Yes Yes Yes 275 K (35°F) Used where absorption systemsare practical, except whereammonia’s flammability is notacceptable (occupied buildings,for example).
Water(Li Br
absorption)
No No Yes 275 K (35°F) Used where absorption systemsare practical. No toxicity orflammability limitations.
Water(steam jet)
No No Yes 275 K (35°F) Large number of applications inwhich steam is available.Applications include comfort airconditioning, industrial processcooling, and other similar typesof service.
Ethane Yes No 186 K (-126°F) Used for refinery operations.
Propane Yes No 232 K (-42°F) Used for refinery operations.
Butane Yes No 274 K (33°F) Used for refinery operations.
Ethylene Yes No 171 K (-153°F) Used for refinery operations.
Propylene Yes No 227 K (-51°F) Used for refinery operations.
Figure 13. Summary of Refrigerant Applications
Work Aid 2 describes the procedures and provides the resources required to select appropriate refrigerants.
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Work Aid 1: Resources For Selecting appropriate refrigeration SYSTEMSDirections: Use Figure 14 and Figure 15 to help select an appropriate refrigeration system.
Figure 14. Refrigeration Systems Schematic
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ABSORPTION
CONSIDERATIONVAPOR
COMPRESSION AmmoniaLithiumBromide STEAM JET
Minimum refrigeranttemperature
No limit 2°C (35°F) 2°C (35°F) 2°C(35°F)
Refrigerant toxic? Yes/No Yes No No
Flammable? Yes/No Yes No No
Reciprocating compressor Up to 1 400 kW (400tons*)
None None
Centrifugal compressor >1 400 kW(400 tons*)
None None
*tons of refrigeration
Figure 15. Refrigeration System and Equipment Limits
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Work Aid 2: PROCEDURES AND RESOURCES FOR Selecting the mostappropriate refrigerants
To select the most appropriate refrigerant, analyze the refrigeration system (already selected) and the followingoperating considerations and refrigerant properties.
• Availability of low-level (waste) heat• Availability of steam• Availability (both generally and for the specific application)• Capital cost• Compatibility with system building materials• Environmental regulations (both existing and anticipated)• Flammability• Minimum achievable temperature• Operating cost• Thermal stability• Toxicity
Determine the relevance of each consideration and property for the application. For each relevant property,refer to the following appropriate Figures for more information. Based on the information in the relevantFigure and the information covered in the Information Sheet, select the most appropriate refrigerant.
Notes References aData are from Thermodynamic Properties of Refrigerants (Stewart et al. 1987) 1Kirk and Othmer (1956) bThe temperature of measurement (Celcius) is shown in parentheses. The 2Matheson Gas Data Book (1966) data are from Handbook of Chemistry and Physics (CRC 1987) unless 3Bulletin D-27 (duPont 1967) otherwise noted. cFor the sodium D line. 4Bulletin B-32A (duPont) dSublimes. 5Bulletin T-502 (duPont 1980) eAt 527 kPa. 6Handbook of Chemistry (1967) fRefrigerants 23 and 13 (40.1/59.9% by mass). 7Bulletin G-1 (duPont) gRefrigerants 32 and 115 (48.2/51.82% by mass). 8CRC Handbook of Chemistry and Physics (CRC 1987) hRefrigerants 22 and 115 (48.8/51.2% by mass). iData from Electrochemicals Department, E.I. duPont de Nemours & Co. jRefrigerants 12 and 152a (73.8/26.2% by weight). kDielectric constant data.
Source: 1989 Fundamentals (SI)
Figure 17. Physical Properties of Refrigerants
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% LiBr By Mass
Source: 1989 Fundamentals (SI)
Figure 18. Specific Gravity of Aqueous Solutions of LithiumBromide
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Source: 1989 Fundamentals (SI)
Figure 19. Specific Heat of Aqueous Solutions of LithiumBromide
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Source: 1989 Fundamentals (SI)
Figure 20. Viscosities of Aqueous Solutions of LithiumBromide
1130 Dichloroethylene 2 4a 5.6 to 11.4a Underwriters Laboratories Report MH-3134 c Underwriters Laboratories Report MH-3072b Underwriters Laboratories Report MH-2360 d Underwriters Laboratories Report MH-6138
Source: 1989 Fundamentals
Figure 24. Relative Safety of Refrigerants
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GROUP DEFINITION EXAMPLES
1 Gases or vapors which in concentrations of about 1/2to 1 percent for durations of exposure of about 5minutes are lethal or produce serious injury.
Sulfur Dioxide
2 Gases or vapors which in concentrations of about 1/2to 1 percent for durations of exposure of about 1/2hour are lethal or produce serious injury.
Ammonia
Methyl Bromide
3 Gases or vapors which in concentrations of about 2 to2 1/2 percent for durations of exposure of about 1 hourare lethal or produce serious injury.
Carbon Tetrachloride
Chloroform
Methyl Formate
4 Gases or vapors which in concentrations of about 2 to2 1/2 percent for durations of exposure of about 2hours are lethal or produce serious injury.
Dichloroethylene
Methyl Chloride
Ethyl Bromide
Between
4 and 5
Appear to classify as somewhat less toxic than Group4.
Methylene Chloride
Ethyl Chloride
Much less toxic than Group 4 but somewhat moretoxic than Group 5.
Refrigerant 113
5a Gases or vapors much less toxic than Group 4 butmore toxic than Group 6.
Refrigerant 11
Refrigerant 22
Carbon Dioxide
5b Gases or vapors which available data indicate wouldclassify as either Group 5a or group 6.
Ethane
Propane
Butane
6 Gases or vapors which in concentrations up to at leastabout 20 percent by volume for durations of exposureof about 2 hours do not appear to produce injury.
Refrigerant 12
Refrigerant 114
Refrigerant 13B1
Source: 1989 Fundamentals
Figure 25. Underwriters Laboratories Classification of Comparative Hazard toLife of Gases and Vapors
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GLOSSARY
ANSI American National Standards Institute
ASHRAE American Society of Heating, Refrigerating, and Air Conditions Engineers
ASME American Society of Mechanical Engineers
cascade Two refrigerating systems connected in series to produce a lower temperaturerefrigerant.
CFC A fully halogenated compound.
chiller Heat exchanger that provides the refrigeration to the process stream beingrefrigerated.
coefficient of performance(COP)
In a refrigeration cycle, the ratio of the heat energy extracted by the heat engineat the low temperature to the work supplied to operate the cycle.
critical temperature The temperature of the liquid-vapor critical point. The temperature abovewhich a substance has no liquid-vapor transition.
generator The stripper tower in an absorption refrigeration system. It separates therefrigerant from the carrier fluid.
halocarbon A compound of carbon and a halogen, sometimes with a hydrogen.
receiver Vessel, container, or tank, used to receive and collect liquid material from aprocess unit.
steam-jet ejector A fluid acceleration vacuum pump or compressor using the high velocity of asteam jet for entrainment.
suction line A pipe or tube feeding into the inlet side of a compressor.
ton of refrigeration 12 000 Btu/hr = 2.5 kW
UL Underwriters’ Laboratories
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ADDENDUM
TABLE OF CONTENTS
Addendum A: ASHRAE SI for HVAC and R Conversions 47
Addendum B: Conversion Factors 48
Addendum C: Symbols Used in ChE 210.01 49
Addendum D: Equation Used in ChE 201.01 50
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ADDENDUM A: ASHRAE SI FOR HVAC AND R CONVERSIONS
MULTIPLY BY TO OBTAIN
bar 100 kPa
Btu, IT 1.055 kJ
Btu/ft3 37.3 Kj/m3 = J/L
Btu • ft/hr • ft2 • °F 1.731 W/(m • K)
Btu • in/(hr • ft2 • °F)
(thermal conductivity, k)
0.144 W/(m • K)W/(m • °C)
Btu/hr 0.293 W
Btu/ft2 11.4 kJ/m2
Btu/(h • ft2) 3.15 W/m2
Btu/(hr • ft2 • °F)(overall heat transfer coefficient, U)
(thermal conductance, C)
5.68 W/(m2 • K)W/(m2 • °C)
Btu/lb 2.33 kJ/kg
Btu/(lb • °F)(specific heat, c)
4.19 kJ/(kg • K)kJ/(kg • °C)
centipoise, viscosity, m(absolute, dynamic)
1.00 mPa • s
centistokes, kinematic viscosity, u 1.00 mm2/s
dyne/cm2 0.100 Pa
in of mercury (60°F) 3.38 kPa
in of water (60°F) 249 Pa
kW • hr 3.60 MJ
mm of mercury (60°F) 0.133 kPa
mm of water 9.80 Pa
ppm (by mass) 1.00 mg/kg
ton of refrigeration (12,000 Btu/hr) 3.52 kWRounded to three or four significant figures.
Figure 26. ASHRAE SI for HVAC and R Conversions
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ADDENDUM B: CONVERSION FACTORS
Pressurepsi In. Hg atm mm Hg bar kg/cm2 dyne/cm2 pascal1 2.0360 0.068046 51.715 0.068948 0.07030696 68,948 6894.8
Volume in3 ft3 gal litre metre3
1 5.787 x 10-4 4.329 x 10-3 0.0163871 1.63871 x 10-5
Energy Btu ft • lb calorie joule watt • sec1 777.65 251.9957 1054.35 1054.35
Density lb/ft3 lb/gal g/cm3 kg/m3(g/L)1 0.133680 0.016018 16.018463