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CHAPTER 3

CARBON DIOXIDE REFRIGERATION SYSTEMS

Applications ............................................................................... 3.2

System Design ............................................................................ 3.3

System Safety.............................................................................. 3.5 Piping ......................................................................................... 3.6

Heat Exchangers and Vessels..................................................... 3.8

Compressors for CO2 Refrigeration Systems ............................. 3

Lubricants .................................................................................. 3

Evaporators................................................................................ 3 Defrost ...................................................................................... 3.1

Installation, Start-up, and Commissioning .............................. 3.1

ARBON dioxide (R-744) is one of the naturally occurringCcompounds collectively known as “natural refrigerants.” It isnonflammable and nontoxic, with no known carcinogenic, muta-genic, or other toxic effects, and no dangerous products of combus-tion. Using carbon dioxide in refrigerating systems can beconsidered a form of carbon capture, with a potential beneficialeffect on climate change. It has no adverse local environmentaleffects. Carbon dioxide exists in a gaseous state at normal tempera-tures and pressures within the Earth’s atmosphere. Currently, theglobal average concentration of CO2 is approximately 390 ppm byvolume.

Carbon dioxide has a long history as a refrigerant. Since the1860s, the properties of this natural refrigerant have been studied and tested in refrigeration systems. In the early days of mechanicalrefrigeration, few suitable chemical compounds were available asrefrigerants, and equipment available for refrigeration use was lim-ited. Widespread availability made CO2 an attractive refrigerant.

The use of CO2 refrigeration systems became established in the1890s and CO2 became the refrigerant of choice for freezing and transporting perishable food products around the world. Meat and other food products from Argentina, New Zealand and Australiawere shipped via refrigerated vessels to Europe for distribution and consumption. Despite having traveled a several-week voyage span-ning half the globe, the receiving consumer considered the condi-tion of the frozen meat to be comparable to the fresh product. By

1900, over 300 refrigerated ships were delivering meat productsfrom many distant shores. In the same year, Great Britain imported 360,000 tons of refrigerated beef and lamb from Argentina, NewZealand, and Australia. The following year, refrigerated bananaships arrived from Jamaica, and tropical fruit became a lucrativecargo for vessel owners. CO2 gained dominance as a refrigerant inmarine applications ranging from coolers and freezers for crew pro-visions to systems designed to preserve an entire cargo of frozen

products.Safety was the fundamental reason for CO2’s development and

growth. Marine CO2-refrigerated shipping rapidly gained popular-ity for its reliability in the distribution of a wide variety of fresh food

products to many countries around the world. The CO2 marinrefrigeration industry saw phenomenal growth, and by 1910 som1800 systems were in operation on ships transporting refrigeratefood products. By 1935, food producers shipped millions of tons food products including meats, dairy products, and fruits to GreBritain annually. North America also was served by CO2 marinrefrigeration in both exporting and receiving food products.

The popularity of CO2 refrigeration systems reduced once suable synthetic refrigerants became available. The development chlorodifluoromethane (R-22) in the 1940s started a move awafrom CO2, and by the early 1960s it had been almost entirelreplaced in all marine and land-based systems.

By 1950, the chlorofluorocarbons (CFCs) dominated the majoity of land-based refrigeration systems. This included a wide varieof domestic and commercial CFC uses. The development of the hemetic and semihermetic compressors accelerated the developmeof systems containing CFCs. For the next 35 years, a number CFC refrigerants gained popularity, replacing practically all othrefrigerants except ammonia, which maintained its dominant postion in industrial refrigeration systems.

In the 1970s, the atmospheric effects of CFC emissions wehighlighted. This lead to a concerted effort from governments, scentists, and industrialists to limit these effects. Initially, this took thform of quotas on production, but soon moved to a total phaseoufirst of CFCs and then of hydrochlorofluorocarbons (HCFCs).

The ozone depleting potential (ODP) rating of CFCs and HCFC prompted the development of hydrofluorocarbon (HFC) refrigeants. Subsequent environmental research shifted the focus froozone depletion to climate change, producing a second ratinknown as the global warming potential (GWP). Table 1 presenGWPs for several common refrigerants. Table 2 compares perfomance of current refrigerants used in refrigeration systems.

In recent years, CO2 has once again become a refrigerant of greinterest. However, high-pressure CO2 systems (e.g., 3.4 MPa atsaturation temperature of –1°C, or 6.7 MPa at 26.7°C) present somchallenges for containment and safety.

Advances in materials science since the 1950s enable the desigof cost-effective and efficient high-pressure carbon dioxide sytems. The attraction of using CO2 in modern systems is based on iThe preparation of this chapter is assigned to TC 10.3, Refrigerant Piping.

Table 1 Refrigerant Data

Refrigerant Number Refrigerant Group Chemical FormulaTemperature at101.3 kPa, °C Safety Group GWP at 100 Year

R-22 HCFC CHClF2 –40.8 A1 1700

R-134a HFC CF3CH2F –26.1 A1 1300

R-410A HFC blend HFC-32 (50%) –52.3 A1/A1 2000

HFC-125 (50%)

R-507A HFC blend HFC-125 (50%) –47.1 A1 3900

HFC-143a (50%)

R-717 Ammonia NH3 –33.3 B2 0

R-744 Carbon dioxide CO2 –78.4 A1 1

Source: ANSI/ASHRAE Standard 34. Note: –56.6°C and coincident pressure of 517.8 kPa (absolute) is triple point for CO2.

Related Commercial Resources

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3.2 2010 ASHRAE Handbook—Refrigeration (SI)

attractive thermophysical properties: low viscosity, high thermalconductivity, and high vapor density. These result in good heattransfer in evaporators, condensers, and gas coolers, allowingselection of smaller equipment compared to CFCs and HFCs. Car-

bon dioxide is unique as a refrigerant because it is being considered for applications spanning the HVAC&R market, ranging fromfreezers to heat pumps, and from domestic units up to large-scaleindustrial plants.

CO2 has been proposed for use as the primary refrigerant inmobile air conditioners, domestic appliances, supermarket displaycases, and vending machines. CO2 heat pump water heaters arealready commercially available in a many countries. In these appli-cations, transcritical operation (i.e., rejection of heat above the crit-ical point) is beneficial because it allows good temperature glidematching between the water and supercritical CO2, which benefitsthe coefficient of performance (COP). Large industrial systems useCO2 as the low-temperature-stage refrigerant in cascade systems,typically with ammonia or R-507A as high-temperature-stagerefrigerants. Medium-sized commercial systems also use CO2 as thelow-temperature-stage refrigerant in cascade system with HFCs or hydrocarbons as high-temperature-stage refrigerants.

A distinguishing characteristic of CO2 is its phase change prop-erties. CO2 is commercially marketed in solid form as well as in liq-

uid and gas cylinders. In solid form it is commonly called dry ice,and is used in a variety of ways including as a cooling agent and asa novelty or stage prop.

Solid CO2 sublimates to gas at –78.5°C at atmospheric pressure.The latent heat is 571 kJ/kg. Gaseous CO2 is sold as a propellantand is available in high-pressure cartridges in capacities from 4 g to2.3 m3.

Liquid CO2 is dispensed and stored in large pressurized vesselsthat are often fitted with an independent refrigeration system to con-trol storage vessel pressure. Manufacturing facilities use it in bothliquid and gas phase, depending on the process or application.

Bigger quantities of CO2 (e.g., to replenish large storage tanks)can be transported by pressurized railway containers and special-ized road transport tanker trucks.

CO2 is considered a very-low-cost refrigerant at just a fraction of

the price of other common refrigerants in use today. Comparingenvironmental concerns, safety issues, and cost differentials, CO2has a positive future in mechanical refrigeration systems, serving as

both a primary and secondary refrigerant.

In considering CO2 as primary or secondary refrigerant, thesematter-phase state conditions of solid, liquid, and vapor should bethoroughly understood. Of particular importance are the triple pointand critical point, which are illustrated in Figures 1 and 2.

The point of equilibrium where all three states coexist that isknown as the triple point. The second important pressure and tem-

perature point of recognition is the critical point where liquid and vapor change state. CO2 critical temperature is 31°C; this is consid-ered to be low compared to all commonly used refrigerants.

APPLICATIONS

Transcritical CO2 Refrigeration

In a transcritical refrigeration cycle, CO2 is the sole refrigerant.Typical operating pressures are much higher than traditional HFCand ammonia operating pressures. As the name suggests, the heatsource and heat sink temperatures straddle the critical temperature.

Development on modern transcritical systems started in the early1990s with a focus on mobile air-conditioning systems. However,early marine systems clearly were capable of transcritical operationin warm weather, according to their operating manuals. For exam-

ple, marine engineers sailing through the Suez Canal in the 1920sreported that they had to throttle the “liquid” outlet from the con-denser to achieve better efficiency if the sea water was too warm.They did not call this transcritical operation and could not explainwhy it was necessary, but their observation was correct.

The technology suggested for mobile air conditioning was alsoadopted in the late 1990s for heat pumps, particularly air-sourceheat pumps for domestic water heating. In Japan, researchers and manufacturers have designed a full line of water-heating-systemequipment, from small residential units to large industrial applica-tions, all incorporating transcritical CO2 heat pump technology. Awide variety of such units was produced, with many different com-

pressor types, including reciprocating, rotary piston, and scroll.

Current commercial production of pure transcritical systems is primarily in small-scale or retail applications such as soft drink vend-ing machines, mobile air conditioning, heat pumps, domestic appli-ances, and supermarket display freezers. Commercial and industrialsystems at this time tend to use CO2 as secondary refrigerant in a

Table 2 Comparative Refrigerant Performance per

Kilowatt of Refrigeration

Refrig-erant

Number

Evapora-tor

Pressure,MPa

Con-denser

Pressure,MPa

Net Refrig-eratingEffect,kJ/kg

RefrigerantCirculated,

kg/s

SpecificVolume of

Suction Gas,m3 /kg

R-22 0.3 1.19 162.2 1.7 × 10 –3 2.7 × 10 –3

R-134a 0.16 0.77 147.6 1.9 × 10 –3 4.2 × 10 –3

R-410A 0.48 1.87 167.6 1.7 × 10 –3

1.9 × 10 –3

R-507A 0.38 1.46 110.0 2.6 × 10 –3 1.8 × 10 –3

R-717 0.24 1.16 1100.9 0.26 × 10 –3 17.6 × 10 –3

R-744 2.25 7.18 133.0 1.1 × 10 –3 0.58 × 10 –3

Source: Adapted from Table 9 in Chapter 29 of the 2009 ASHRAE Handbook—Funda-

mentals. Conditions are –15°C and 30°C.

Fig. 1 CO2 Expansion-Phase Changes

Fig. 1 CO2 Expansion-Phase Changes(Adapted from Vestergaard and Robinson 2003)

Fig. 2 CO2 Phase Diagram

Fig. 2 CO2 Phase Diagram(Adapted from Vestergaard and Robinson 2003)


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Carbon Dioxide Refrigeration Systems 3.

two-phase cascade system in conjunction with more traditional pri-mary refrigerants such as ammonia or an HFC.

In a transcritical cycle, the compressor raises the operating pres-sure above the critical pressure and heat is rejected to atmosphere bycooling the discharge gas without condensation. When the cooled gas passes through an expansion device, it turns to a mixture of liq-uid and gas. If the compressor discharge pressure is raised, theenthalpy achieved at a given cold gas temperature is reduced, sothere is an optimum operating point balancing the additional energy

input required to deliver the higher discharge pressure against theadditional cooling effect achieved through reduced enthalpy. Sev-eral optimizing algorithms have been developed to maximize effi-ciency by measuring saturated suction pressure and gas cooler outlet temperature and regulating the refrigerant flow to maintain anoptimum discharge pressure. Achieving as low a temperature at thegas cooler outlet as possible is key to good efficiency, suggestingthat there is a need for evaporatively cooled gas coolers, althoughnone are currently on the market. Other devices, such as expanders,have been developed to achieve the same effect by reducing theenthalpy during the expansion process and using the recovered work in the compressor to augment the electrical input.

CO2 Cascade System

The cascade system consists of two independent refrigeration

systems that share a common cascade heat exchanger. The CO2 low-temperature refrigerant condenser serves as the high-temperaturerefrigerant evaporator; this thermally connects the two refrigerationcircuits. System size influences the design of the cascade heatexchanger: large industrial refrigeration system may use a shell-and-tube vessel, plate-and-frame heat exchanger, or plate-and-shelltype, whereas commercial systems are more likely to use brazed-

plate, coaxial, and tube-in-tube cascade heat exchangers. In chillingsystems, the liquid CO2 is pumped from the receiver vessel belowthe cascade heat exchanger to the heat load. In low-temperatureapplications, the high-pressure CO2 liquid is expanded to a lower

pressure and a compressor is used to bring the suction gas back upto the condensing pressure.

Using a cascade system allows a reduced high-temperaturerefrigerant charge. This can be important in industrial applications

to minimize the amount of ammonia on site, or in commercial sys-tems to reduce HFC refrigerant losses.

CO2 cascade systems are configured for pumped liquid recircu-lation, direct expansion, volatile secondary and combinations of these that incorporate multiple liquid supply systems.

Low-temperature cascade refrigeration application include cold storage facilities, plate freezers, ice machines, spiral and belt freez-ers, blast freezers, freeze drying, supermarkets, and many other food and industrial product freezing systems.

Some theoretical studies (e.g., Vermeeren et al. (2006)] have sug-gested that cascade systems are inherently less efficient than two-stage ammonia plants, but other system operators claim lower energy bills for their new CO2 systems compared to traditionalammonia plants. The theoretical studies are plausible because intro-ducing an additional stage of heat transfer is bound to lower the

high-stage compressor suction. However, additional factors such asthe size of parasitic loads (e.g., oil pumps, hot gas leakage) on thelow-stage compressors, the effect of suction line losses, and theadverse effect of oil in low-temperature ammonia plants all tend tooffset the theoretical advantage of two-stage ammonia system, and in the aggregate the difference in energy consumption one way or the other is likely to be small. Other factors, such as reduced ammo-nia charge, simplified regulatory requirements, or reduced operator staff, are likely to be at least as significant in the decision whether toadopt CO2 cascades for industrial systems.

In commercial installations, the greatest benefit of a CO2 cascadeis the reduction in HFC inventory, and consequent probable reduc-tion in HFC emission. Use of a cascade also enables the operator to

retain existing HFC compressor and condenser equipment wherefurbishing a facility by connecting it to a CO2 pump set anreplacing the evaporators and low-side piping. End users in Europand the United States suggest that CO2 cascade systems are simpleand easier to maintain, with fewer controls requiring adjustmenthan the HFC systems that they are replacing. This indicates thathey are inherently more reliable and probably cheaper to maintaithan conventional systems. If the efficiency is equivalent, then thcost of ownership will ultimately be cheaper. However, it is not cleaif these benefits derive from the higher level of engineering inpurequired to introduce the new technology, or whether they can bmaintained in the long term.

SYSTEM DESIGN

Transcritical CO2 SystemsRecent advances in system component design have made it pos

sible to operate in previously unattainable pressure ranges. Thdevelopment of hermetic and semihermetic multistage CO2 com

pressors provided the economical ability to design air-cooled transcritical systems that are efficient, reliable, and cost effectiveToday, transcritical systems are commercially available in sizefrom the smallest appliances to entire supermarket systems. Figure3 and 4 shows examples of simple transcritical systems. Heat rejection to atmosphere is by cooling the supercritical CO2 gas withou

phase change. For maximum efficiency, the gas cooler must be abto operate as a condenser in colder weather, and the control systemmust be able to switch from gas cooler operation (where outflowfrom the air-cooled heat exchanger is restricted) to condenser operation (where the restriction is removed, as in a conventional sys

tem). Compared to a typical direct HFC system, energy usage can breduced by 5% in colder climates such as northern Europe, but maincrease by 5% in warmer climates such as southern Europe or thUnited States. In a heat pump or a refrigeration system with hearecovery, this dual control is not necessary because the system opeates transcritically at all times.

CO2 /HFC Cascade Systems

Cascade refrigeration systems in commercial applications generally use HFCs, or occasionally HCs, as the primary refrigeranSupermarkets have adopted cascade technology for operational aneconomic reasons (the primary refrigerant charge can be reduced bas much as 75%). Liquid CO2 is pumped to low-temperature displa

Fig. 1 CO2 Expansion-Phase Changes

Fig. 3 Transcritical CO2 Refrigeration Cycle in Appliances

and Vending Machines


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cases and controlled via electronic expansion valve. The medium-

temperature display cases are supplied liquid from the same circuitor from a dedicated pump system (Figures 5 and 6). Cascade sys-tems in supermarkets have been designed to operate multitempera-ture display cases and provide heat recovery to generate hot water or space heating (Figure 7). In general, although a pump has beenintroduced, energy consumption is not significantly different from atraditional HFC system because the suction line losses are less and the evaporator heat transfer performance is better. This can result ina rise of up to 5 or 6 K in the evaporating temperature, offsetting the

pump’s power consumption and the temperature differential in thecascade heat exchanger.

Ammonia/CO2 Cascade Refrigeration System

Industrial refrigeration applications often contain large amountsof ammonia as an operating charge. Cascade systems provide an

opportunity to reduce the ammonia charge by approximately 90% percent compared to a conventional ammonia system of the samecapacity.

Another significant difference is the operating pressures of CO2compared to ammonia. The typical suction pressure at –28.9°C evap-orating temperature is 24.1 kPa (gage) for ammonia and 1582.4 kPa(gage) for CO2. In most industrial cascade systems, the ammoniacharge is limited to the compressor room and the condenser flat, reduc-ing the risk of leakage in production areas and cold storage rooms.

The cascade heat exchanger is the main component where thetwo independent refrigeration systems are connected in single ves-sel. CO2 vapors are condensed to liquid by evaporating ammonialiquid to vapor. This cascade heat exchanger vessel must be con-structed to withstand high pressures and temperature fluctuations tomeet the requirements of both refrigerants. Also, the two refriger-

ants are not compatible with each other, and cross-contaminationresults in blockage in the ammonia circuit and may put the systemout of commission for an extended period. The cascade heatexchanger design must prevent internal leakage that can lead to thetwo refrigerants reacting together. Figure 8 shows a simplified ammonia cascade system; note that no oil return is shown.

System Design Pressures

The system design pressure for a CO2 cascade system cannot bedetermined in the traditional way, because the design temperaturesare typically above the critical point. The system designer musttherefore select suitable pressures for each part of the system, and ensure that the system is adequately protected against excess

pressure in abnormal circumstances (e.g., off-cycle, downtime, power loss) .

For example, for a typical refrigerated warehouse or freezer cas-cade system, the following pressures are appropriate:

CO2 Side

• System design working pressure (saturated suction temperature):3.5 MPa (gage) (0.6°C)

• Relief valve settings: 3.4 MPa (gage)

• System emergency relief setting: 3.1 MPa (gage) (–3°C)

• CO2 discharge pressure setting: 2.2 MPa (gage) (–15°C)

Where the system uses hot-gas defrost, the part of the circuitexposed to the high-pressure gas should be rated for 5.2 MPa or higher.

Ammonia Side

• System design working pressure (saturated suction temperature):2.1 MPa (gage) (53°C)

• Relief valve settings: 2.1 MPa (gage)

• Ammonia suction pressure setting: 108 kPa (gage) (–18°C)

• Ammonia discharge pressure setting: 1.1 MPa (gage) (32°C)

• Temperature difference on the cascade condenser: (2.8 K)

On the CO2 side, the low-side temperature and coincident pres-sure must be considered. The triple point for CO2 is –56.6°C). Atlower pressure, liquid turns to a solid; thus, the low-side criteria of feasible applications are –56.6°C at a coincidental saturated suction

pressure of 414 kPa (gage). Therefore, the system must be dual-stamped for 3.5 MPa (gage) and –56.6°C at 462 kPa (gage). Toachieve suitable material properties, stainless steel pipe may beappropriate.

Fig. 2 CO2 Heat Pump for Ambient Heat to Hot Water

Fig. 4 CO2 Heat Pump for Ambient Heat to Hot Water

Fig. 3 R-717/CO2 Cascade System with CO2 Hot-GasDefrosting

Fig. 5 R-717/CO2 Cascade System with CO2 Hot-GasDefrosting

(Adapted from Vestergaard 2007)


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Valves

Valves in CO2 systems are generally similar to those in ammonia plants, but must be suitably rated for high pressure. Where equip-ment cannot operate at the required pressure differences, alternativetypes may be used (e.g., replacing solenoid valves with electricallydriven ball valves).

Expanding saturated CO2 vapor can solidify, depending on oper-ating pressure, so the relief valve should be located outside with nodownstream piping. If necessary, there should be a high-pressure

pipe from the vessel to the relief valve. This pipe should be sized toensure a suitably low pressure drop during full-flow operation.

The other very important consideration with the relief system isits discharge location. The relief header must be located so that, if there is a release, the discharge does not fall and collect in an areawhere it may cause an asphyxiation hazard (e.g., in a courtyard, or

near the inlet of a rooftop makeup air unit).CO2 relief valves are more likely to lift in abnormal circum-

stances than those used in ammonia or HFC systems, where thevalve will only lift in the event of a fire or a hydraulic lock. There-fore, care should be taken when specifying relief valves for CO2 toensure that the valve can reseat to prevent loss of the total refriger-ation charge. A pressure-regulating valve (e.g., an actuated ballvalve) may be installed in parallel with the safety relief valve toallow controlled venting of the vapor at a set pressure slightly lower than the relief valve setting.

For sizing relief valves, use the following equation:

C = f DL (1)

where

C = capacity required, kg/s of air

D = diameter of vessel, m L = length of vessel, m

f = refrigerant-specific constant (0.5 for ammonia, 1.0 for CO2)

Some special considerations are necessary for liquid feed valvassemblies to facilitate maintenance. Depending on the configuration, it may not be feasible to drain the liquid out of a valve assembl

before maintenance is needed. Liquid CO2

in the valve assemblcannot be vented directly to atmosphere because it will turn to drice immediately. Between any two valves that can trap liquid, a liquid drain valve should be installed on one side and a gas-pressurinvalve on the other. This facilitates pressurizing the valve train witgas, pushing the liquid out without it changing phase inside the pipe

CO2 Monitoring

CO2 is heavier than air, but the two gases mix well; it does notake much air movement to prevent CO2 from stratifying. The mo

practical place to measure CO2 concentrations is about 1.2 m abovthe floor (i.e., the breathing zone for most people). Where COmight leak into a stairwell, pit, or other confined space, an addtional detector should be located in the space to warn personnel ithe event of a high concentration.

Water in CO2 Systems

CO2, like HFCs, is very sensitive to any moisture within the system. Air must be evacuated before charging the refrigerant at initiastart-up, to remove atmospheric moisture. Maintenance staff mususe caution when adding oil that may contain moisture. Investigations of valve problems in some CO2 installations revealed thamany problems are caused by water freezing in the system; welldesigned and well-maintained CO2 systems charged with dry COand filter-driers run trouble free (Bellstedt et al. 2002).

Figure 9 shows the water solubility in the vapor phase of different refrigerants. The acceptable level of water in CO2 systems imuch lower than with other common refrigerants. Figure 10 showthe solubility of water in both liquid and vapor CO2 as function otemperature. (Note that solubility in the liquid phase is muc

higher.) Below these levels, water remains dissolved in the refrigerant and does not harm the system. If water is allowed to exceed thmaximum solubility limit in a CO2 system, problems may occuespecially if the temperature is below 0°C. In this case, the watefreezes, and ice crystals may block control valves, solenoid valvefilters, and other equipment.

If the water concentration in a CO2 system exceeds the saturatiolimit, it creates carbonic acid, which can cause equipment failureand possibly internal pipe corrosion. Filter-driers should be locateat all main liquid feed locations.

Because the entire CO2 system is at positive pressure during aoperating conditions, the most likely time for moisture penetratiois during charging. The appropriate specification for water contendepends on the size of the system and its intended operating tem

perature. Chilling systems are more tolerant of water than freezer

and industrial systems with large liquid receivers are likely to bmore tolerant than small direct-expansion (DX) circuits. It is imperative that the CO2 is specified with a suitable water content. Refrigerant grade, with a content less than 5 ppm, is suitable for smacommercial systems; larger plant may use cryogenic grade, with content less than 20 ppm. The content should be certified by the vendor and tested on site before installing in the system. On small systems, it may also be appropriate to charge through a filter-drier.

SYSTEM SAFETY

Safety is an important factor in the design of every refrigeratiosystem, and is one of the main reasons why carbon dioxide igaining acceptance as a refrigerant of the future. CO2 is a natura

Fig. 4 CO2 Cascade System with Two Temperature Levels

Fig. 6 CO2 Cascade System with Two Temperature Levels

(Adapted from Vestergaard 2007)


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refrigerant and considered environmentally safe. As a refrigerant, itis not without potential risks, but they are substantially smaller thanthose of other refrigerants. It is a slightly toxic, odorless, colorlessgas with a slightly pungent, acid taste. Carbon dioxide is a small butimportant constituent of air. CO2 will not burn or support combus-tion. An atmosphere containing of more than 10% CO 2 will extin-guish an open flame.

Mechanical failure in refrigeration equipment and piping cancourse a rapid increase in concentration levels of CO2. Wheninhaled at elevated concentrations, carbon dioxide may producemild narcotic effects, stimulation of the respiratory centre, and asphyxiation, depending on concentration present.

In the United States, the Occupational Safety and Health Admin-istration (OSHA) limits the permissible exposure limit (PEL) timeweighted average (TWA) concentration that must not be exceed dur-ing any 8 h per day, 40 h per week, to 5000 ppm. The OSHA short-term exposure limit (STEL), a 15 min TWA exposure that should not be exceeded, is 30 000 ppm. In other countries (e.g., the United Kingdom), the STEL is lower, at 15 000 ppm.

At atmospheric pressure, carbon dioxide is a solid, which sub-limes to vapor at –56.6°C. All parts of a charged CO2 refrigeratingsystem are above atmospheric pressure. Do not attempt to break pip-ing joints or to remove valves or components without first ensuringthat the relevant parts of the system have been relieved of pressure.

When reducing pressure or transferring liquid carbon dioxide,care is necessary to guard against blockages caused by solid carbondioxide, which forms at pressures below 517 kPa. If a blockageoccurs, it must be treated with caution. No attempt should be madeto accelerate the release of pressure by heating the blocked compo-nent.

In a room where people are present and the CO2 concentrationcould exceed the refrigerant concentration limit of 0.9 kg/10 m 3 inthe event of a leak, proper detection and ventilation are required.When detectors sense a dangerous level of CO2 in a room, the alarmsystem must be designed to make sure all people in the room areevacuated and no one is allowed to re-enter until concentration lev-els return to acceptable ranges. Protective clothing, including gloves

and eyewear, should be standard in locations that contain CO2equipment or controls, or where service work is done.

PIPING

Carbon Dioxide Piping Materials

When selecting piping material for CO2 refrigeration systems,the operating pressure and temperature requirements must be under-stood. Suitable piping materials may include copper, carbon steel,

stainless steel, and aluminum.Many transcritical systems standardize on brazed air-condition-

ing and refrigeration (ACR) copper piping for the low-pressure sideof the system, because of its availability. For pressures above 4.1MPa, the annealing effect of brazing can weaken copper pipe, so

pipework should be welded steel. Alternatively, cold-formed mechanical permanent joints can be used with copper pipe if the

pipe and fittings are suitably pressure rated. Small-diameter copper tubing meets the requirement pressure ratings. The allowable inter-nal pressure for copper tubing in service is based on a formula used in ASME Standard B31 and ASTM Standard 280:

(2)

where

p = allowable pressure

S = allowable stress [i.e., allowable design strength for continuouslong-term service, from ASME (2007)]

t m = wall thickness

D = outside diameter

Carbon Steel Piping for CO2

Low-temperature seamless carbon steel pipe (ASTM Standard A333) Grade 6 is suited for conditions within refrigeration systems.Alternatively a number of common stainless steel alloys provideadequate low temperature properties.

Fig. 5 Dual-Temperature Supermarket System: R-404 and CO2 with Cascade Condenser

Fig. 7 Dual-Temperature Supermarket System: R-404A and CO2 with Cascade Condenser

p2St m

D – 0.08t m------------------------------=


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Stainless steel, aluminum, and carbon steel piping require qual-ified welders for the piping installation.

Pipe Sizing

For the same pressure drop, CO2 has a corresponding tempera-ture penalty 5 to 10 times smaller than ammonia and R-134a have

(Figure 11). For a large system with an inherently large pressurdrop, the temperature penalty with CO2 is substantially less than thsame pressure drop using another refrigerant.

Because of CO2’s physical properties (particularly density), thvapor side of the system is much smaller than in a typical ammonisystem, but the liquid side is similar or even larger because CO2

Fig. 6 Dual-Temperature Ammonia Cascade System

Fig. 8 Dual-Temperature Ammonia (R-717) Cascade System

Fig. 7 Water Solubility in Various Refrigerants

Fig. 9 Water Solubility in Various Refrigerants(Adapted from Vestergaard 2007)

Fig. 8 Water Solubility in CO2

Fig. 10 Water Solubility in CO2(Adapted from Vestergaard 2007)


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lower latent heat requires more mass flow (see Table 3). The primarymethod of sizing CO2 pipe is to define the allowable temperature loss

that the system can handle, convert that to pressure loss, then size thesystem so that the total pressure drop is less than or equal to theallowable pressure drop.

HEAT EXCHANGERS AND VESSELS

CO2 operates at much higher pressures than most refrigerants for any given operating temperature. If a vessel contained liquid CO2and the pressure were lost through a refrigerant leak, the CO2 would continue to refrigerate while the pressure reduced to atmospheric.As the pressure dropped to 518 kPa, the liquid would change to asolid and vapor at –56.6°C. (Conversely, as the pressure rises, thesolid turns back to liquid.) The CO2 would continue to cool down to

–78°C at atmospheric conditions. For a vessel, the typical design isto be able to handle temperatures down to –56.6°C at 518 kPa. Nor-mal operation of pumps and valves is not affected by this phasechange in the long term, although the plant obviously cannot oper-ate when full of solid. The main hazard associated with this behav-ior is the effect of low temperature on the vessel materials.

Gravity Liquid SeparatorThis vessel is designed to separate the liquid out of two-phase

flow to protect the compressor from liquid entrainment. They can bein either a vertical or horizontal configuration. The vessel can bedesigned in accordance with Chapter 4, but using a factor of 0.03 for CO2 compared with 0.06 for ammonia.

Recirculator

This vessel is a gravity liquid separator, but it also contains amanaged level of liquid, which is pumped out to the evaporators ata specific flow rate. The circulating rate is the mass ratio of liquid

pumped to amount of vaporized liquid. A 4:1 circulating rate meansfour units of liquid are pumped out and one unit evaporates. The

three remaining units of liquid return to the vessel as two-phaseflow. The vessel then separates the two-phase flow, collecting theliquid and allowing the dry gas to exit to the compressors. The highgas density of CO2 means that liquid takes up a greater proportionof the wet suction volume than with ammonia, so there is a signifi-cant advantage in reducing the circulating rate. Typically 2:1 can beused for a cold store, whereas 4:1 would be preferred in this appli-cation for ammonia.

Design of a recirculator vessel must consider liquid flow rates.

When sizing pump flow rates, the pump manufacturer’s recommen-dations for liquid velocity should generally be followed:

• NH3 and most hydrocarbons (HCs): <1.0 m/s

• CO2, HCFCs, and HFCs: 0.75 m/s

Recirculator drop legs should be sized for a liquid velocity of lessthan 0.075 to 0.10 m/s to allow vapor bubbles to rise and to preventoil entrainment in the pump suction line.

CO2’s liquid density is typically higher than the oil’s density; typ-ical approximate values are 1200 kg/m3 for liquid CO2, 900 kg/m3

for oil, and 660 kg/m3 for liquid ammonia. Thus, unlike in ammoniasystems, oil that reaches the low side of a CO2 system tends to floaton the surface of the refrigerant. This makes oil recovery from therecirculator more difficult, but, conversely, it means that oil is morelikely to remain in the high-pressure receiver, if one is fitted, floating

atop the liquid there.

Cascade Heat Exchanger

The CO2 compressor discharge in a low-temperature system or the wet return in a pumped liquid chill system is piped to the cascadeheat exchanger, where the heat of rejection from the low stage isremoved by the high-stage system and condenses the CO2 dischargegas to high-pressure liquid. The high-stage system absorbs the heatof rejection from the low stage by evaporating the high-stage refrig-erant.

There are several configuration of the cascade heat exchanger.Industrial applications use conventional shell-and-tube, welded

plate-in-shell, and plate-and-frame heat exchangers. To reduce therisk of cross-contamination, some projects use shell-and-tube heatexchangers with double tube sheets, which are significantly moreexpensive than single-tube sheet heat exchangers. In commercialapplications system capacity influences design criteria; equipmentoptions include brazed-plate, tube-in-tube, coaxial, shell-and-tube,and plate and frame heat exchangers.

COMPRESSORS FOR CO2

REFRIGERATION SYSTEMS

Designing and manufacturing an efficient, reliable CO2 compressor represented a challenge that required extensive research tosatisfy the complex criteria dictated by operating pressures that far exceed those found in conventional refrigeration compressors.

Transcritical Compressors for CommercialRefrigeration

In transcritical CO2 systems, the design working pressure exceed 10 MPa (gage) in air-cooled applications. Construction techniquesand materials must withstand the pressure ranges that are essentialfor transcritical CO2 compression. With traditional reciprocatingcompressors, one challenge is to provide enough surface on thewrist pin and big-end bearings to carry the load created by the highdifferential pressure. Development of new compressor typesincluded two-stage rotary hermetic units, redesigned scroll and reciprocating compressors, and a hybrid piston configuration wherean eccentric lobe drives a roller piston rather than a connecting rod.These are often fitted with inverter-type dc motors designed tochange speeds from 1800 to 6000 rpm to satisfy part-load and effi-ciency requirements.

Table 3 Pipe Size Comparison Between NH3 and CO2

DescriptionCO2 at –40°C

NH3 at –40°C

Latent heat, kJ/kg 321.36 1386.83Density of liquid, m3/kg 4.34 2.69

Density of vapor, m3/kg 0.04 1.55

Mass flow rate for 70 kW refrigeration effect, kg/s 0.22 0.05

Liquid volumetric flow rate, m3/s 0.95 0.14

Vapor volumetric flow rate, m3/s 8.4 × 10 –3 78.6 × 10 –3

Liquid pipe sizes, mm (assumes 3:1 recirculationrate)

40NB 25NB

Vapor pipe sizes, mm 65NB 100NB

Fig. 9 Pressure drop for various refrigerants

Fig. 11 Pressure Drop for Various Refrigerants


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Compressor manufacturers generally use one of three conven-tional enclosure or housing styles (Figure 12): hermetic (used byappliance and heat pump manufacturers), modified semihermetic(used in compressors for supermarkets), or open-style belt-drivencompressors (used in transport and industrial refrigeration compres-

sors).As different segments of the refrigeration industry developed

CO2 equipment, each individual segment gravitated to designs thatevolved from their standard compressor arrangements. For exam-

ple, in the automotive industry, the typical R-134a vehicle air-con-ditioning compressor modified to operate with CO2 has a morerobust exterior enclosure, a more durable shaft seal arrangement,and stronger bearing configurations with reduced component clear-ances. However, the basic multiroller piston/swash plate, belt-driven compressor design remains fundamentally similar.

High-pressure screw compressors are also in development for commercial applications, in both single- and two-stage internallycompound versions.

Compressors for Industrial Applications

There are two primary types of compressors used for industrialapplications: rotary screw and reciprocating. These compressorshave been designed primarily for cascade systems with CO2 as thelow-temperature refrigerant. Modification requirements for the CO2cascade system compressors are less demanding because the tem-

perature and pressure thresholds are lower than those of transcriticalcompressors for commercial applications.

Depending on the operating parameters, the reciprocating compressor crankcase pressure may be considerably higher when usingCO2. Therefore, standard gray cast iron material may not meet thedesign specification criteria. Construction material strength may beincreased by selecting ductile cast iron for compressor casings in

both single- and two-stage versions. Internal moving componentsand bearing surfaces may also require new materials that tolerate theelevated pressures.

Typical screw compressors may also be modified to ductile castiron casings in lieu of gray iron for higher design working pressures.Shorter rotor lengths may be required to reduce deflection at thehigher operating pressures of CO2 applications, and the discharge

port may be enlarged to improve the compressor efficiency with thedense gas.

The same advantages and disadvantages apply to these two typesof compressors as with ammonia and most HFCs, with a few clari-fications. Because CO2 has a greater density than ammonia and HFCs commonly used in industrial applications, the displacementvolume needed in the CO2 compressor is comparatively less thanthat required for other refrigerants. For example, at –40°C saturated suction temperature, a CO2 refrigeration system’s displacement

requirement is approximately eight times less than ammonia for thsame refrigeration effect. Therefore, the compressors are approxmately eight times smaller for the CO2 system.

High-pressure screw compressors are also in development foindustrial applications, in both single- and two-stage internall

compound versions.

LUBRICANTS

There are several very suitable oils for use with CO2. Some oilare fully miscible with the refrigerant and some are nonmiscibleEach application requires a lubricant that meets specific temperature and miscibility characteristics. Lubricants include mineral oilalkyl benzene, polyalphaolefin (PAO), polyol ester (POE), an

polyalkyl glycol (PAG).

The development of a transcritical CO2 system requires specialtlubricants because of the high pressure and thus higher bearinloads. Antiwear properties and extreme pressures create a challengto provide a lubricant that achieves compressor longevity. Cascadsystems can use more traditional oils, and it may be possible t

reduce the risk of error by using the same lubricant in both sides othe cascade.

Currently, ASHRAE and other organizations are performinresearch with a variety of lubricants in different viscosity ranges tassess the oil structure and thermodynamic behavior in CO2 systems (Bobbo et al. 2006; Rohatgi 2010; Tsiji et al. 2004). POE anPAG oils are widely accepted in today’s CO2 systems; however, thdynamics of the refrigerant and oil mixture for different pressuretemperatures and buoyancy levels have yet to be established for aconditions. Chapter 12 covers details on CO2 lubricants.

In CO2, insoluble oils are less dense than the liquid refrigeranProviding a series of sampling points connected to an oil pot providea means of finding the level of stratification and removing the oil.

For fully soluble oils, a small side stream of liquid refrigerant i

passed through an oil rectifier, which can recover this oil from thlow temperature side and deliver it back to the compressor, as isome R-22 applications. The oil rectifier is principally a shell-andtube heat exchanger, which uses the high-pressure liquid to heat threfrigerant/oil sample. The tube side is connected to the bottom othe surge drum, so that low-pressure liquid is boiled off, and thremaining oil is directed to the suction line.

The oil rectifier liquid supply should be at least 1% of the plancapacity. The oil rectifier does not affect the plant efficiency becausthe liquid used subcools the remaining plant liquid. Typically, the orectifier is sized to maintain a concentration of 1% oil in the COcharge. Oil carryover from a reciprocating compressor with a standard oil separator is typically 10 to 20 ppm for CO2 operation.

Fig. 10 CO2 Transcritical Compressor Configuration Chart

Fig. 12 CO2 Transcritical Compressor Configuration Chart




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EVAPORATORS

Evaporator designs for CO2 cascade or transcritical systems aresimilar to those for other refrigerants. If the design pressure is lowenough, then standard air coolers/plate freezers for either ammoniaor HFCs can be used for CO2 and yield similar capacity at the sametemperatures. The heat transfer coefficients in CO2 evaporators aretypically double those found in R-134a systems, and about half of those in ammonia systems. However, the pressure/temperature

characteristic of CO2 offers the possibility to increase the mass fluxin the evaporator to achieve higher rates of heat transfer without suf-fering from excessive saturated temperature drop. Air units spe-cifically designed for CO2 with small stainless steel tube circuiting(16, 13, or 9.5 mm) and aluminum fins, increase heat transfer per-formance in industrial and commercial applications. Plate freezer design can be optimized with significantly smaller channels, and thus thinner plates, than are traditionally used for ammonia, en-abling up to 8% more product to be fitted into a given frame size.

Most CO2 evaporators control the liquid supply to coil distributor with liquid overfeed or electronic controlled direct expansionvalves, development in flow control technology is being studied inmany research facilities to provide optimal performance and super-heat conditions. Developments in microchannel evaporator technol-ogy for smaller capacity systems have also provided excellent heat

transfer capabilities.In low-temperature application where surface frosting accumu-lates and coil defrosting is required, hot-gas defrost air units requirethe design pressure to be in excess of 5.2 MPa (gage). If this is notfeasible, then electric defrost can be used. Provided the coil is

pumped down and vented during defrost, pressure will not riseabove the normal suction condition during an electric defrost.

For plate freezers, the low pressure drop (expressed as saturated suction temperature) is significantly less for CO2 than for any other refrigerant. This is because of (1) the pressure/temperature charac-teristic and (2) the lower overfeed ratio that can be used. Freezingtimes in plate freezers are dramatically reduced (up to one-third of the cycle time required with ammonia). Defrost in plate freezersmust be by hot gas.

Copper pipe and aluminum fin evaporators have been success-

fully used in commercial and supermarket applications for severalyears with CO2 in both cascade and transcritical installations. Com- pared to HFC evaporators, these new units are typically smaller,with reduced tube diameter and fewer, longer circuits to take fulladvantage of the pressure/temperature characteristic. Conversionfrom R-22 has been achieved in some installations by utilizing theoriginal electric defrost evaporators, rated for 2.6 MPa (gage). CO2has also been deployed in cooling coils for vacuum freeze dryersand in ice rinks floors. There are generally no problems with oilfouling, provided an oil with a sufficiently low pour point is used.

DEFROST

Perhaps the greatest diversity in the system design is in the typeof defrost used, because of the greater degree of technical innova-tion required to achieve a satisfactory result in coil defrosting. There

are significant differences in the installation costs of the differentsystems, and they also result in different operating costs. For sys-tems operating below 0°C where the evaporator is cooling air, effi-cient and effective defrost is an essential part of the system. Sometypes of freezers also require a defrost cycle to free the product at theend of the freezing process of service. Tunnel freezers may wellrequire a quick, clean defrost of one of the coolers while the othersare in operation.

Electric Defrost

The majority of small carbon dioxide systems, particularlythose installed in supermarket display cases in the early 1990s and later, used electric defrost. This technology was very familiar in the

commercial market, where it was probably the preferred method of defrosting R-502 and R-22 systems. With electric defrost, it isimperative that the evaporator outlet valve (suction shutoff valve) isopen during defrost so that the coil is vented to suction; otherwise,the high temperature produced by the electric heaters could causethe cooler to burst. It therefore also becomes important to pump outor drain the coil before starting defrost, because otherwise the ini-tial energy fed into the heaters only evaporates the liquid left in thecoil, and this gas imposes a false load on the compressor pack.

Exactly the same warnings apply to industrial systems, where elec-tric defrost is becoming more common.

If electric defrost is used in a cold store with any refrigerant, theneach evaporator should be fitted with two heater control thermo-stats. The first acts as the defrost termination, sensing when the coilrises to a set level and switching off the heater. The second is a safetystat, and should be wired directly into the control circuit for thecooler, to ensure that all power to the fans, peripheral heaters, trayheaters, and defrost heater elements is cut off in the event of exces-sive temperature. One advantage of electric defrost in a carbon diox-ide system is that, if the coil is vented, coil pressure will not riseabove the suction pressure during defrost. This is particularly appro-

priate for retrofit projects, where existing pipes and perhaps evapo-rators are reused on a new carbon dioxide system.

The electric system comprises rod heaters embedded in the coil

block in spaces between the tubes. The total electrical heatingcapacity is 0.5 times the coil duty plus an allowance for the drip trayheaters and fan peripheral heaters.

Hot-Gas Defrost

This is the most common form of defrost in industrial systems, particularly on ammonia plant. The common name is rather mislead-ing, and the method of achieving defrost is often misunderstood. Thegas does not need to be hot to melt frost, but it does need it to be ata sufficiently high pressure that its saturation temperature is wellabove 0°C. In ammonia plants, this is achieved by relieving pressurefrom the evaporator through a pressure regulator, which is factory-setat 0.5 MPa (gage), giving a condensing temperature of about 7°C.Despite this, it is common to find hot-gas defrost systems supplied

by a plant that runs at a condensing temperature of 35°C to deliver

the required flow rate. This equates to a head pressure of 1.3 MPa(gage), which means that there is a an 800 kPa pressure drop betweenthe high-pressure receiver and the evaporator. The real penalty paid with this error in operation is that the rest of the plant is running at theelevated pressure and consuming far more energy than necessary.With carbon dioxide compressors supplying the gas, there is no pos-sibility of the same mistake: the typical compressor used in thisapplication is likely to be rated for 5 MPa (gage) allowable pressure,and so runs at about 4.5 MPa (gage), which gives a condensing tem-

perature in the coil of about 10°C. Numerous applications of thistype have shown that this is perfectly adequate to achieve a quick and clean defrost (Nielsen and Lund 2003). In some arrangements, thedefrost compressor suction draws from the main carbon dioxidecompressor discharge, and acts as a heat pump. This has the benefitof reducing load on the high side of the cascade, and offers signifi-

cant energy savings. These can be increased if the defrost machine isconnected to the suction of the carbon dioxide loop, because it then

provides cooling in place of one of the main carbon dioxide compres-sors. A concern about this system is that it runs the compressor to itslimits, but only intermittently, so there are many starts and stops over a high differential. The maintenance requirement on these machinesis higher than normal because of this harsh operating regime.

Reverse-Cycle Defrost

Reverse-cycle defrost is a special form of hot-gas defrost in whichheat is applied by condensing gas in the evaporator, but it is delivered

by diverting all compressor discharge gas to the evaporator and supplying high-pressure liquid to the system condenser, thus producing


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Carbon Dioxide Refrigeration Systems 3.1

reverse flow in part of the circuit and operating the plant as a heat pump. Gas diversion is typically done with a single valve (e.g., afour-port ball valve). Reverse-cycle defrost is most appropriate intranscritical circuits, and is particularly suitable for use in low-

pressure receiver systems as described by Pearson (1996).

High Pressure Liquid Defrost

An alternative way of providing gas for defrosting is to pressur-ize liquid and then evaporate it, using waste heat from the high-pres-sure side of the cascade. This has the advantage that it does notrequire a high-pressure compressor, but uses a small liquid pumpinstead. The liquid evaporator stack is quite expensive, because itcomprises an evaporator, liquid separator, and superheater, butongoing development is helping to make this part of the systemmore economical. This type of system has been used very success-fully in cold and chill storage (Pearson and Cable 2003) and in a

plate freezer plant (Blackhurst 2002). It is particularly well suited tothe latter application because the defrost load is part of the productfreezing cycle and is large and frequent. The heat for evaporation is

provided by condensing ammonia on the other side of a plate-and-shell heat exchanger; in cold and chill applications, where defrostsare much less frequent, the heat is supplied by glycol from the oil-cooling circuit on the ammonia stage.

Water Defrost

Water defrost can be used, although this is usually limited to coilswithin spiral and belt freezers that require a cleandown cycle (e.g.,IQF freezers, freeze-drying plants).

INSTALLATION, START-UP, ANDCOMMISSIONING

It is imperative to take every feasible precaution to prevent mois-ture from entering the system. Because CO2 operates at positive

pressure about the triple point, the most likely times for contamina-tion are at start-up and during system charging.

When a system is complete and ready for pressure testing, a seriesof cleansing processes should be used to ensure a totally dry system.First, the system should be pulled into a deep vacuum (98 kPa) and held with a vacuum pump running for a minimum of 1 hour for each30 m3 of system to remove moisture. All low spots that are not insu-lated should be inspected for evidence of moisture (ice, condensa-tion) and the vacuum process continued until any moisture is gone.Hold the vacuum for 24 h. Break the vacuum with dry nitrogen to

bring the system up to design working pressure for 24 h. Soap-testevery joint and flange. Repair as needed and repeat. When confidentof the system integrity, pull the system back into a vacuum (98 kPa)and hold for 24 h to purge all nitrogen and other contaminates.

Break the vacuum with CO2 gas. On a large system, this can bevery cumbersome, but trying to charge a system with liquid cancause severe problems. First, as the liquid enters the vacuum, itimmediately solidifies and clogs the charging system. Secondly, theshock of such low temperatures can cause the metal of the system tocrack. Only charge a CO2 system with gas until the system is up toa minimum pressure of 1.4 MPa (gage). At this pressure, the corre-sponding temperature is about –30°C, which will not shock themetal of the system when liquid is introduced.

Daily maintenance and service of an ammonia/CO2 cascade sys-tem is very similar to a conventional ammonia system, but is typi-cally quicker and easier. When servicing equipment, remember thefollowing points:

• Do not trap liquid between two isolation valves. Trapped liquid CO2 expands very quickly when heated and can easily reach rup-ture pressure. CO2 gas can rise above design pressure whentrapped, so do not isolate gas where heat can be added to theequipment and superheat the gas.

• Pumpdown of a piece of equipment (e.g., an evaporator) followtypical procedure. The liquid isolation valve is closed, and thevaporator fans are run to evaporate all of the remaining liquidWhen all of the liquid is out, the fans are turned off, the suction iclosed, and the unit is isolated with gas on it at suction pressureIt is recommended to install service valves in the strainers of aliquid solenoids and at each piece of equipment to enable the technician to vent the remaining pressure to atmosphere in a controlled fashion. When service is complete, the unit must be pulle

back to a deep vacuum to remove all moisture. Break the vacuum by opening up the evaporator to suction and allow the unit to fiwith CO2 gas and pressurize the coil. Then open the liquid. If thliquid is opened before the unit is up to 1.4 MPa (gage), the liquiwill turn solid and clog the liquid supply line.

• Evacuation is particularly critical in CO2 systems because, unlikammonia, CO2 does not tolerate much water.

• It is not necessary to blow refrigerant out into a water containe(as with ammonia) or to pump refrigerant out with recovery unit(as with HFCs). After isolating a component, the CO2 containewithin can simply be released into the atmosphere. In additionwhen the component is opened for service, no extra time irequired waiting for the refrigerant smell to dissipate. The maicaution with releasing CO2 indoors is to ensure the room is weventilated and monitored by a CO2 detector to make sure the con

centration of CO2 does not get too high.• For systems that use soluble oils, an oil rectifier system distills th

oil out and sends it back to the compressors automatically.• With systems that use insoluble oils, sampling ports must b

added to the recirculator to drain off the oil, similar to an R-2system. Liquid CO2 is significantly more dense than lubricantso the oil tends to float on the surface of the liquid in the receive

• At initial start-up and during service, air and moisture may potentially contaminate a CO2 system. However, during normal operation, the CO2 side of the system always operates at a positiv

pressure in all areas of the plant, thereby preventing air and moiture from entering the system. Air purgers are not needed, but fiter-driers are.

Making sure the CO2 does not get contaminated is very important. Samples of the system CO

2

should be tested regularly to confirm the absence of water or other contaminants.

REFERENCES

ASHRAE. 2007. Designation and safety classification of refrigerantANSI/ASHRAE Standard 34-2007.

ASME. 2001. Refrigeration piping and heat transfer components. StandarB31.5. American Society of Mechanical Engineers, New York.

ASME. 2007. International boiler and pressure vessel code, section Power boilers. American Society of Mechanical Engineers, New York

ASTM. 2008. Standard specification for seamless copper tube for air condtioning and refrigeration field service. Standard B280-08. AmericaSociety for Testing and Materials, West Conshohocken, PA.

ASTM. 2005. Specification for seamless and welded steel pipe for low-tem perature service. Standard A333/A333M-05. American Society for Tesing and Materials, West Conshohocken, PA.

Bellstedt, M., F. Elefsen, and S.S. Jensen. 2002. Application of CO2

refrigerant in industrial cold storage refrigeration plant. AIRAH Journal: Ecolibrium 1(5):25-30.

Blackhurst, D.R. 2002. CO2 vs. NH3: A comparison of two systems. Proceedings of the Institute of Refrigeration, vol. 99.:29-39.

Bobbo, S., M. Scattolini, R. Camporese, and L. Fedele. 2006. Solubility oCO2 in some commercial POE oil. Proceedings of 7th IIR Conference

IIAR. 2010. The carbon dioxide industrial refrigeration handbook . International Institute of Ammonia Refrigeration, Alexandria, VA.

Nielsen, P.S. and T. Lund 2003. Introducing a new ammonia/CO2 cascadconcept for large fishing vessels. Proceedings of IIAR Ammonia Refrigeration Conference, Albuquerque, NM, pp. 359-396.

Pearson, A.B. and P.J. Cable. 2003. A distribution warehouse with carbodioxide as the refrigerant. 21st IIR International Congress of Refrigeration, Washington, D.C.


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Pearson, S.F. 2001. Ammonia low pressure receivers. Air Conditioning and Refrigeration Journal (January-March). Available at http://www.ishrae.in/journals/2001jan/article05.html.

Rohatgi, N.D. 2010. Stability of candidate lubricants for CO2 refrigeration.ASHRAE Research Project RP-1409, ongoing.

Tsiji, T., S. Tanaka, T. Hiaki, and R. Sato. 2004. Measurements of the bubble point pressure for CO2 and lubricants. Fluid Phase Equilibria219:87-92.

Vermeeren, R.J.F., A. Jurgens, and S.M. Van Der Sluis. 2006. Quick freezingwith carbon dioxide to achieve higher product quality. IIR Conference Proceedings.

Vestergaard, N.P. 2007. CO2 refrigerant for industrial refrigeration. Dan-foss Refrigeration and Air Conditioning Division. Available at http://www.danfoss.com.

Vestergaard, N,P. and M. Robinson. 2003. CO2 in refrigeration applications. Air Conditioning, Heating, and Refrigeration News (October).

BIBLIOGRAPHY

Bondinus, W.S. 1999. The rise and fall of carbon dioxide systems. ASHRAE Journal 41.(4):37-42.

Broderdorf, W. and D. Giza. 1993. CO2 subcooled refrigeration system. Pro-ceedings of IIAR Ammonia Refrigeration Conference.

Broesby-Olsen, F. 1998. International Symposium on HCFC AlternativeRefrigerants.

Christensen, O. 2006. System design for industrial ammonia/CO2 cascadeinstallations. Proceedings of IIAR Ammonia Refrigeration Conference.

Gillies, A.M. 2004. Design considerations when using carbon dioxide in

industrial refrigeration systems. Proceedings of IIR 6th Gustav Lorent- zen Conference, Glasgow.

Handschuh, R. 2008. Design criteria for CO2 evaporators. In Natural refrig-erants—Sustainable ozone- and climate-friendly alternatives to HCFCs, pp. 273-282. V. Hasse, L. Ederberg, and D. Colbourne, eds. GTZProklima, Eschborn, Germany. Available from http://www.gtz.de/en/themen/umwelt-infrastruktur/23898.htm.

IOR. 2009. Safety code of practice for carbon dioxide as a refrigerant . Insti-tute of Refrigeration, Carshalton, U.K.

Lorentzen, G. 1994. The use of natural refrigerants, a complete solution to theCFC/HCFC predicament. IIR Conference Proceedings: New Applicationsof Natural Working Fluids in Refrigeration and Air Conditioning .

Lorentzen, G. 1990. Trans-critical vapour compression cycle device. Patent WO/07683.

Miller, H. 1985. Halls of Dartford 1785-1985. Ebury Press, UK.

Pearson, A. 2000. The use of CO2/NH3 cascade systems for low temperaturefood refrigeration. IIAR 22nd Annual Meeting , Nashville, pp. 43-58.

Pearson, A. 2005. Evaporator performance in carbon dioxide systems. Pro-ceedings of IIAR Ammonia Refrigeration Conference.

Pearson, A. 2006. Defrost options for carbon dioxide systems. Proceedingsof IIAR Ammonia Refrigeration Conference.

Pearson, S.F. 2004. Rational design for suction pipes to liquid refrigerant pumps. Proceedings of IIR 6th Gustav Lorentzen Conference, Glasgow.

Pettersen, J. 1999. CO2 as a primary refrigerant. Presented at Institute of Refrigeration Centenary Conference, London.

Renz, H. 1999. Semi-hermetic reciprocating and screw compressors for carbon dioxide cascade systems. 20th International Congress of Refrigera-tion, IIR/IIF, Syndey. Available at http://www.equinoxe.hu/uploaded_ files/hutestechnika_letoltesek/av_9801_gb.pdf .

Saikawa, M. 2007. Development and progress of CO2 heat pump water heater “Eco-Cute” in Japan.

Vestergaard, N.P. 2004. Getting to grips with carbon dioxide. RAC (Refrig-eration and Air Conditioning).

Vestergaard, N.P. 2004. CO2 in subcritical refrigeration systems. Presented at IIAR Conference, Orlando.

Woolrich, W.R. 1967. The men who created cold: A history of refrigeration .Exposition Press, New York.

ACKNOWLEDGMENT

ASHRAE and International Institute of Ammonia Refrigeration(IIAR) joint members contributed both to this chapter and to IIAR’sCarbon Dioxide Industrial Refrigeration Handbook (IIAR 2010),material from which was used in this chapter’s development.

Related Commercial Resources


http://www.ishrae.in/journals/2001jan/article05.html


http://www.danfoss.com/


http://www.gtz.de/en/themen/umwelt-infrastruktur/23898.htm


http://www.equinoxe.hu/uploaded_files/hutestechnika_letoltesek/av_9801_gb.pdf


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