1 CO2 (R744) REFRIGERATION GUIDELINES C.J.Hewetson 7 january 05 Revised 23 April 05 1) General Many people perceive CO2 refrigeration systems as new technology, this perception is incorrect. CO2 along with SO2 was a widely used refrigerant in the late 19th century until the advent of safer Freon refrigerants, patented & introduced in the early 20th century. What's new is the fact that few people are familiar with the benefits, safety aspects, usage & application design requirements of CO2 refrigeration plant. Additionally, there are few refrigeration products & components commercially available suitable for system design pressures of 35-50barg, although if CO2 usage increases, no doubt products + sources will increase. What is CO2? it is Carbon Dioxide a chemical compound formed by 1 Atom of Carbon & 2 atoms of Oxygen. It is environmentally friendly with an ODP = 0, GWP =1. Under normal atmospheric temperature & pressure CO2 exists as colorless, odorless (depending on concentration), non toxic non flammable gas. Its specific gravity or relative density is 1.524 at 0degC, so it's heavier than air & will therefore tend to sink to the floor in a confined space or non ventilated area. If compressed & cooled to a low enough temperature the gas liquefies. Liquid CO2 is produced by compressing & cooling CO2 gas, liquid CO2 cannot exist as liquid at atmospheric pressure. It must be pressurized above 60.4lbf/in2 (4.1barg) (-56.6degC) to remain as liquid. At this pressure, which is the Triple point, CO2 can exist as a solid, liquid or gas. Below this pressure it will flash to a gas and solid. Above 1056lbf/in2 (72.8barg) 31.1degC which is the Critical point CO2 cannot exist as a liquid. Generally CO2 is delivered & maintained at or below 0degC 300lbf/in2 (20.68barg) Solid CO2 or dry ice is produced by expanding liquid CO2 to atmospheric pressure which forms very low temperature snow. The snow is then compressed to form blocks or pellets, the temperature of dry ice is 78.5degC at atmospheric pressure. Solid CO2 or dry ice sublimates directly from a solid to a gas. In view of the temperature of dry ice great care must be exercised in handling it. The need for this caution will occur if the relief valves relieve liquid CO2 or liquid leaks arise. CO2 liquid has a high heat of vaporization of 276.8Kj/Kg @ -16.7degC (20.8barg), at this temperature its density is 1010Kg/m3. 2) Safety issues The potential safety issues associated with high pressure CO2 refrigeration systems have not been studied in depth. Two main issues can however be identified: High pressure gas/low temperature liquid handling, the potential for tubing, vessels & hose ruptures to be quite violent. The potential consequences of an evaporator or condenser or piping rupture & rapid release of the entire CO2 charge into the plant room or freeze rooms. A rough estimate is that 10 kg CO2 charge released into a small freeze room or plant room would increase the CO2 concentration to 20%, a level that will result in rapid unconsciousness followed by death. Note that Tablia Biologica indicates that 9% is the lethal concentration of carbon dioxide for healthy adults. For elderly individuals & those with impaired health, the lethal concentration is probably around 5%. More work is needed to determine the likely duration of high CO2 concentrations compared to the range of human tolerance for these concentrations. Many people inaccurately believe that as the safety data sheets for CO2 describe the product as non toxic, non flammable it is safe. This is not correct; a CO2 leak can be fatal. As CO2 levels increase above 2% in air, you may feel drowsy, at 3% the gas has a narcotic effect, blood pressure rises & hearing may become impaired. When levels reach 5% difficulties in breathing will be experienced & as CO2 levels rise above 5% unconsciousness will occur. This is particularly of concern to plant operators & service 2 personnel who may be changing oil, overhauling compressors at low level, whereby unconsciousness arises without any forewarning apart from a feeling of drowsiness. Additionally CO2 storage systems may be under extremely high pressure due to the steep temperature – pressure curve for CO2, particularly during refrigeration plant shutdown or emergency shutdown & if the system is not designed correctly with piping & vessels correctly insulated & or standby chilling system to keep the CO2 below the system MAWP, the relief valves will lift resulting in loss of charge. Plant designers & plant owners therefore need to ensure that CO2 monitors are installed in the plant room plus any other cold rooms or confined spaces where CO2 could leak. Additionally regular leak checks need to be undertaken using electronic CO2 snuffer detectors or Infra Red CO2 detectors. Annual inspections of all CO2 plant must be undertaken by a competent engineer. Owners & designers have a duty of care to ensure the plant is correctly & safely designed to meet health & safety aspects & ensure they are not compromised. In this respect a "Risk Assessment" or "Hazop" study must be undertaken prior to commissioning any CO2 plant. Service engineers must be fully familiar with the special safety precautions of charging & evacuating CO2 system, for maintaining the system dry & checking charge levels without the aid of liquid line sight glasses. Commercially available CO2 should be 98% pure with no moisture content. During plant overhaul the system must be evacuated properly to at least 5micron or 5mmHg to ensure all moisture is removed. If the system experiences excessive levels of moisture, whilst tolerable in R717 or R22 pant, will lead to Carbonic Acid production & Carbonic Acid corrosion on the internal parts of the system. When designing CO2 plant it is vital that no trapped pockets are produced in the piping or instrumentation, failing which rupture of components could arise during plant shutdown if the trapped pocket has no means of relieving the pressure. 3) Plant shut down Industrial refrigeration CO2 systems will generally operate at sub critical temperatures & pressures or low pressure. The term "Low pressure" for our purposes is used to describe a sub critical system pressure below 35barg so it is not really low. Generally CO2 is only used in sub critical Cascade 2 stage systems in the low stage part of the system. The low stage CO2 compressed gas is condensed in the cascade condenser using a high stage R717 or R404A or R22 direct refrigeration system or using a glycol chiller circulating glycol through the condenser. However in hot climates like Saudi Arabia where summer design ambient or plant room temperatures >50degC are often encountered, extra storage & safety precautions need to be considered for emergency or planned shutdown of the plant, or a malfunction arises in the high stage system & controls. Loss of condenser coolant, power failures, compressor failure, controls failure, condenser tube leaks all have the potential to shut the plant down at any time & for extended periods. With the high stage inoperable, pressures in the low stage will increase rapidly beyond the relief set point and the CO2 charge will be lost. To counter this problem, which is not such a problem in European climates with an average ambient temperature of 15-18degC for most of the year, the low stage system must be fitted with a stand alone Copeland or Bitzer small hermetic chiller air cooled condensing system & the system must be fully insulated with a minimum of 50mm Urethane insulation. If the CO2 low stage system is designed for MAWP of 30barg, DWP 25.4barg, the CO2 must be maintained at a temperature no higher than -10degC. For a system designed for 30barg, the only way to guarantee the CO2 system pressure will remain within the MAWP during system shut down is to include a) Insulation of the suction, discharge, liquid feed piping plus the cascade condenser and or liquid receiver if used. The purpose of insulation is to reduce heat gain into the system & to prevent frost build up on the outer surface. If the storage vessel is insulated with 3" Urethane insulation fully vapor proofed & Aluminum jacketed, this should maintain the system temperature below the critical temperature or the MAWP for at least 4 days, which provides a reasonable duration to fix any problem. This is based on a heat transmission of approximately 100btu/sqft/24hr for a 55degC delta t. If it is anticipated that longer periods of shutdown will arise, system cooling can be achieved by venting gas from the receiver to 3 atmosphere to lower the pressure, alternatively the vessel should not only be insulated but also a standby chiller should be installed as described in b). b) Install a stand alone DX R404A or R134A air cooled condensing unit to cool the CO2 in the condenser or storage receiver via a cooling coil located in the storage vessel or cascade condenser. Based on the above insulation a small Blitzer or Copeland R404A or R134A condensing unit can be used. For Saudi Arabia the cooling system should be designed for -15degC ET + 65degC CT. For most CO2 systems a 1-3kw condensing unit should be sufficient to take care of the heat transmission into the system assuming 3" Urethane insulation. The cooling coil should be located in the storage receiver, below the liquid level & the system should include all necessary shut off, solenoid valves, filter driers, sight glass, TEV with automatic CO2 pressure cycling controls in order the unit will stop & start automatically whenever the system pressure rises above the set point. During evaporator defrost; the coils will need to be pumped down. It is unlikely that all of the system charge can be safely held in the condenser, which is another good reason for providing an HP receiver. If no receiver is included in the system, then it may not be possible to inspect the cascade condenser or carry out tube leak repairs/replacement without removing the CO2 to a separate refrigerated road tanker. CO2 refrigeration plant designed for Saudi requires stringent design & safety features to maintain the system temperature & pressure below the CO2 critical temperature of 31.1degC or 72barg. In any event it would be impractical to design the system for 120barg which is used for super critical automotive A/C. 4) Storage HP receivers Storage tanks or vessels for CO2 refrigeration differ little from commercial CO2 tanks for the Brewing & drinks industry, the same design & safety elements apply. The liquid storage vessel consist of a welded steel pressure vessel designed, constructed, and tested in accordance with the requirements of ASME V111 Div1 Boiler and Pressure Vessel Code applicable to storage unit construction, for Unfired Pressure Vessels or for EC sourced vessels PED. The storage vessel is constructed using high strength, fine grain carbon steel ASTM SA-612 or 516 grade 70N based on the design operating temperature & pressure. The storage vessel MAWP will depend upon the application, but generally would have to be 35barg at the minimum designed metal temperature (MDMT). Liquid CO2 storage vessels should be designed, constructed and tested for specific operating pressures and fill rates with full system charge storage @ 80% receiver volume & maximum expected standby temperature. CO2 vessels must never be allowed to become completely full at the pressure relief device setting. The MAWP of a CO2 storage vessel should be 10-20% above the design working pressure DWP. The relief valves must be set at the MAWP, the relief valves must be located such that only gas is discharged straight to atmosphere. No discharge relief piping should be included, to ensure that the relief valve discharge does not become blocked due to freezing or dry ice blocking the relief line. Instrumentation supplied with all CO2 storage vessels/systems should include the following: 1. 0 to 600 psi (4137 kPa) pressure gauge with a 6" (15.24 cm) dial face. This gauge gives the operating pressure of the storage unit at all times. 2. A gauge is used to indicate capacity in the storage unit. The differential liquid level gauge has a 6" (15.24 cm) diameter dial face, calibrated to read in kilograms and pounds of liquid carbon dioxide. 3. High and low audible pressure alarm. This alarm monitors the pressure of the storage unit. If the pressure fluctuates outside of normal operating conditions, an alarm will sound. 4) CO2 temperature & pressure sensors to automatically cycle the refrigeration standby unit. 5) CO2 plant room & freeze room monitoring system 4 CO2 is compatible with most metallic piping materials. There are piping hazards associated with liquid carbon dioxide storage units such as elevated pressure. Corrosion is typically a problem only on the exterior of piping that is subjected to moisture condensation from temperature cycling. However internal corrosion may arise depending on moisture content of the CO2 due to Carbonic Acid attack. All vessels should include at least a 2mm corrosion allowance or in accordance to the code. All piping and fittings furnished as a part of a CO2 storage unit should be seamless schedule 80 carbon steel. Fittings can be weld or screw type 2,000 Lbs (907 Kg) forged steel. All nozzles and valves that penetrate the outer jacket should be stainless steel. Depending on CO2 storage capacity/vessel size & code requirements, a man way should be included as standard equipment. The man way serves two purposes: 1. Internal inspection. 2. Cleaning inside of the storage vessel. There has been discussion within certain regulatory agencies in the United States that may require periodic hydrostatic testing and internal inspections of all storage vessels. If these regulations become mandatory, then the man way will play an important role in compliance with these regulations. A refrigeration coil should be installed as standard equipment on the Urethane insulated storage vessels as previously indicated. However as an alternative CO2 can be vented in gas form the storage vessel, a self refrigeration effect occurs as the pressure in the storage vessel is reduced. This self refrigeration effect is more pronounced in vapor venting. For every pound of vapour vented, approximately 120 BTU/lb is removed. 5) Compressors/components Most reciprocating & screw compressors will not be suitable for CO2 operation due to design pressure limitations of 25barg. Generally cast steel housings are required to withstand the pressure. Sabroe provide reciprocating compressors HPO/HPC range rated at 40barg & using Nodular Iron casing can use screw compressors rated at 35barg. Frick can provide optional cast steel housings as standard. Bitzer also have a good range of CO2 compressors. In terms of screw compressors the factory should check any selection in terms of radial/axial bearings L10 life, in view of the potentially high compression ratio of CO2 compressors operating at -45 & -5degC. They should also provide a recommendation for the correct lubricating oil. Danfoss have a total range of relief, stop, check, regulating, solenoid valves, filter driers rated at 50barg, there are also many other manufacturers of cast steel valves suitable for 50barg. Solenoid coils may need to be dc voltage 20watt to cope with the additional forces arising from high differential pressures. In terms of service shut off valves high pressure cast steel long neck ball valves should be used to allow for adequate insulation. These are readily available from many manufacturers. CO2 filter driers, valves, gauges, HP/LP controls are also readily available from conventional CO2 storage tank suppliers or CO2 suppliers. All suppliers must be advised the compressors & components are for CO2 usage in combination with POG synthetic oil to ensure the compatibility with materials of construction. CO2 piping can be carbon steel schedule 80 ASTMA SA 333 grade 6 which is suitable for operation from 0degC to -46degC. Forged steel flanges should be SA 350LF2, Wrought fittings should be SA 420WPL6. 316 stainless steel tube & fittings should also be considered for CO2 systems as this reduces the risk of corrosion externally due to condensation or internally due to Carbonic Acid attack. Charging hoses should be rated @ 120barG minimum & gauges + differential gauges used to check charge levels must be 6" dial type rated at 50barg minimum. The pressure relief valves on the high stage R717 system must be rated in accordance to API 526 & RP520 with the relief rate based on burst tube conditions which may arise if a tube in the cascade condenser leaks or bursts. The relief rate calculation should be based on CO2 not equivalent air flow rate. Evaporators can be standard Cu/Al air coolers with suitably modified circuitry for CO2 operation if considered necessary by the manufacturer. Most manufacturers provide an option of 316 stainless steel 5 tube coolers if the system designer perceives corrosion may be an issue during operation or service. In all cases the MAWP should be checked with the manufacturer as in general this information is not provided in their catalogues or computer printouts. If shut off valves are provided at the cooler inlet & outlet a suitably CO2 rated relief valve must be installed. Defrost can be similar to normal electric or hot gas system designs, however it is vital that the compressor can operate on a continuous pump down cycle to ensure evaporators are properly evacuated prior to energizing the defrost system so the evaporator pressure is kept below the relief valve set point during defrost. Generally electric defrost is preferable to Hot gas systems. With hot gas it may not be possible to provide enough gas with a high enough enthalpy to quickly defrost due to the cooler pressure limitations or relief valve settings e.g. if the system design condensing temperature is -5degC the discharge temperature will be approximately 67degC @ 30.8bara & the enthalpy difference around 244kJ/kg. On a normal R717 hot gas system the discharge gas is at a temperature of around 132degC @16.4bara with an enthalpy difference of around 1,056kJ/kg. The system saturated operating pressures must not drop below 5.5barg e.g. evaporating temperatures below –55degC are not possible as the CO2 will solidify & the plant will stop. 6) Supercritical/subcritical/Transcritical CO2 systems A supercritical fluid is defined as a substance that is at conditions of temperature and pressure that are above its vapor-liquid critical point. At supercritical conditions, a fluid does not meet the definition of a liquid because it can't be made to boil by decreasing the presssure at constant temperature. Also it is not a vapor because cooling at constant pressure won't cause it to condense. Water is a supercritical fluid above 374degC 22 MPa, CO2 is a supercritical fluid above 31degC & 72.9atm 5.5bar Supercritical fluids in general possess unique transport properties compared to liquids or gases. Supercritical fluids can have liquid-like densities, gas-like diffusivities, and compressibilities that deviate greatly from ideal gas behavior. Fig.1 Phase diagram for R 744. The three well-known phases: solid, liquid and vapor are shown in Fig. 1. The boundaries between them represent the well known phase change processes like evaporation and condensation for the boundary between liquid and vapor phases (vapor pressure curve). Two important state points are marked on the figure: the triple point and the critical point. The triple point represents the condition where all three phases can co-exist in equilibrium. At temperatures below the triple point temperature, liquid cannot exist. In other words, the triple point temperature sets the lower temperature limit for any heat transfer process based on evaporation or condensation. At the other end of the vapor pressure curve the critical point marks the upper limit for the heat transfer processes. At temperatures above the critical temperature, all heat transfer processes will be singlephase processes. The term “critical” is not used in the sense of "dangerous" or "serious", rather it is used to represent the condition where distinguishing between liquid and vapor becomes difficult. The termination of the vapor pressure curve at the critical point means that at higher temperatures and pressures no clear distinction can be drawn between what is called liquid and what is called vapor. Thus, there is a region extending indefinitely upward from, and indefinitely to the right of, the critical point that is called 6 the fluid region. This region is bounded by dashed lines that do not represent phase transitions, but instead conform to arbitrary definitions of what is considered a liquid and what is considered vapor. Table 1. Critical properties of selected refrigerants. Refrigerant Critical pressure Bar a R22 49.9 R134A 40.6 R404A 37.3 R410A 49.0 R600a 36.4 R717 113.3 R744 (CO2) 73.8 Critical temperature degC 96.1 101.1 72.0 71.4 134.7 132.3 31.0 All substances have a triple point and a critical point, but for most of the substances used as refrigerants, the triple point and critical point are found for conditions that lie outside the region where they are normally used. Table 1 compares the critical pressure and temperature of a number of common refrigerants. The critical temperature means that heat rejection processes by condensation can be established at temperatures up to the critical temperature, which is a temperature higher than necessary for rejecting heat to the ambient for almost all refrigeration applications. Most commonly used refrigerants have a critical temperature above 95 °C, but some of the refrigerants such as those indicated R 404A, R 410A & R 744,have critical temperatures below that which means normal heat rejection processes are not possible. For R 744, the critical temperature is only 31.0°C. This means that R 744 heat rejection process by condensation can only be established at temperatures up to 31°C, which is a temperature much lower than necessary for rejecting heat to the ambient applicable to most countries except some in Northern Europe. Considering the temperature difference needed in a heat exchanger, a practical upper limit for a heat rejection process based on condensation of R 744 is reached at ambient temperatures around 20°C. For many refrigeration applications, the ambient temperature will exceed this value, certainly here in Saudi Arabia. However, this doesn’t mean that carbon dioxide cannot be used as a refrigerant in applications rejecting heat to the ambient at temperatures above 20°C. Carbon dioxide can indeed be used as a refrigerant for these applications, but the heat rejection process from these applications cannot be based on condensation. There are three refrigeration cycle processes: Sub critical, where the pressure in all parts of the refrigeration system is kept below the critical pressure. Super critical, where the pressure during the heat rejection (condensation) process is above the critical pressure. Transcritical, where some parts of the refrigeration cycle process take place at pressures above the critical point & others parts below the critical pressure, the refrigeration cycle process is referred to as a transcritical cycle process. In the transcritical cycle process, the heat rejection takes place at pressures and temperatures above the critical point. The terminology used for the processes and the components are almost the same for the three cycle processes except for the heat rejection parts. In the transcritical cycle process, the heat rejection is called gas cooling and subsequently the heat exchanger used is called a gas cooler. 7 7) R744 cycle benefits In general terms CO2 will not be the 'Holy Grail' environmental refrigerant replacement. There are inherent dangers in its application & usage, few components are readily available, users & service engineers require specialist training in handling & servicing techniques. Its COP is no better than most other currently available environmentally friendly refrigerants such as R134a or R404A. Its COP is only possibly superior at low temperature e.g. -40degC. The best way to indicate the benefits of R744 applied to low temperature booster applications is to compare an R744 HPO 28 high pressure 40bar low stage compressor to the nearest equivalent SAB R717 25bar screw compressor, operating at the similar conditions as per Table 2. It can readily be seen that the major advantage of R744 at these conditions only, is not just its zero ODP & GWP of 1, but higher COP/EER or Kw/Kw which provides lower shaft power, smaller Kw rated drive motors-starters, cables + much lower compressor displacement e.g. for an equivalent duty using R717 you would require a compressor with 8 x larger displacement due to the difference in the suction gas specific volume. In terms of safety aspects between the two systems it is again fair to presume that R744 systems are overall probably safer than R717 systems, on the basis both require equal special safety precautions. R717 is flammable as well as toxic with a classification of B2, as against A1 for R744, which requires even greater safety precautions. In some countries system design may have to conform to Zone2 or Division 2 hazardous area requirements. In other countries R717 systems can be dealt with by "special protection" in a manned plant room e.g. IP55 TEFC motors, R717 leak detectors & monitoring system linked to an extract fan with exhaust stack. So for both R744 as well as R717, leak detectors & monitors plus ventilation extract fans & exhaust stacks would be a requirement. Description R744 value R717 value HPO28 SAB202L Compressor 289.4Kw 277.6Kw Compressor capacity 77.2Kw 98.7Kw Compressor shaft power 0.267 0.356 Kw/Kw 0.933 1.24 Kw/TR 8.16 BarA 0.53 BarA Suction pressure -45degC -45degC Suction temperature 26.83 BarA 2.97BarA Discharge pressure 46.6degC 77.15degC Discharge temperature (actual) -10degC -10degC Condensing temperature 1.121Kg/s 0.2232Kg/s Suction mass flow rate 0.0469M3/kG 2.0591m3/Kg Suction specific volume 267.64kJ/Kg 1,244.17kJ/Kg Enthalpy difference 226.3m3/h 1919m3/h Compressor swept volume 983.2Kg/m3 652 Kg/m3 Liquid density 3.29 5.6 Pressure ratio A1 B2 Gas safety group Table 2. Comparison of performance for R744 & R717 based on HPO 28 + SAB202L compressor & same operating conditions.