APPLICATION OF HYDROCARBON REFRIGERANTS IN LOW TEMPERATURE CASCADE SYSTEMS

Transcription

1 APPLICATION OF HYDROCARBON REFRIGERANTS IN LOW TEMPERATURE CASCADE SYSTEMS B. M. ADAMSON Refrigeration Engineering Pty Ltd PO Box 1197 Unanderra NSW 2526 Australia Fax ; ben_adamson@refeng.com.au ABSTRACT The paper reviews choice of hydrocarbon refrigerants for cascade refrigeration systems, and criteria to be considered in selecting various system components. It also discusses the author s experience of problems in application of hydrocarbon refrigerants at temperatures down to -100 C, and solutions to many of these problems. Two case studies are included, one for a small direct expansion system using scroll compressors, and one for a large system with flooded evaporators using oil-flooded screw compressors. 1. INTRODUCTION Refrigeration Engineering is active in the process refrigeration market, mainly for oil, gas and petrochemical industries, and particularly in refrigerated vapour recovery systems. The vapours for recovery are generally hydrocarbons the most common application is gasoline (petrol) vapour recovery at loading terminals, but these systems are also used to recover other hydrocarbons, such as benzene, propylene oxide, naphtha, acrylonitrile, methylene chloride etc. In order to meet the required emission limits, it is necessary to cool vapour streams typically to -75 C or lower, to condense out the vapours to be recovered. As most applications are in areas already classified as hazardous due to flammability of the vapours handled by the vapour recovery system, and in industries which are comfortable with handling flammable materials, there is generally no objection to hydrocarbon refrigerants on the grounds of flammability, nor additional cost involved. Refrigeration Engineering uses hydrocarbon refrigerants as standard in these vapour recovery units, rather than hydrofluorocarbons, for reasons of lower cost of refrigerants and lower environmental effects in case of leakage. Power cost is generally not a major factor, although there can be small advantages for hydrocarbons in some cases. These refrigerants have been applied in cascade refrigeration systems over a wide range of sizes, using screw, reciprocating and scroll compressors. 2. REFRIGERANT PROPERTIES Properties of refrigerants relevant to cascade systems, comparing hydrocarbons to other refrigerants, are summarised in table CHOICE OF REFRIGERANT For the high stage of the cascade, typically working between about -30 C evaporating and +45 C condensing, commonly available hydrocarbons are propane (R-290) and propylene (R-1270). Refrigerant grade high purity propane is readily available in some countries, but in others only commercial grade is available. Commercial propane varies quite significantly in composition it

2 may be as high as 99% or as low as 90% propane. The balance is mainly ethane and butane, and depending on relative amounts of these, the properties as a refrigerant may be significantly affected, with large temperature glide in direct expansion systems and partial fractionation in flooded systems. Propylene is widely used in making polypropylene, and polymer grade which is better than 99% propylene is readily available in most areas where refineries and petrochemical plants are located. For this reason we generally prefer to use propylene rather than propane, unless there are other particular reasons favouring propane. The higher pressure of propylene, allowing effectively a 5K lower evaporating temperature than propane, is also useful in cascade systems, although can be a disadvantage in air-cooled systems operating areas with high summer temperatures. Choice of refrigerant for the low stage of the cascade system follows similar logic to the high stage. Ethylene is widely used in making polyethylene and high-purity polymer grade is readily available, although obtaining the relatively small quantities for refrigerant is occasionally difficult. (Ethylene is also commercially used in fruit ripening, especially bananas and tomatoes.) Ethane has fewer direct uses, and is less readily available, so we have standardised on ethylene as the low stage refrigerant when using hydrocarbons. The higher pressure of ethylene also allows effectively a 15K lower evaporating temperature than ethane, which is critical in the temperature ranges where we commonly work. The main disadvantage of ethylene is its high specific heat ratio (see table 1) which leads to relatively high discharge temperature. This can be controlled easily in oil-flooded screw compressors, but can be a problem at high compression ratios in scroll and reciprocating compressors if suction superheat is high. Table 1. Refrigerant properties relevant to cascade systems (Coolpack) High temperature refrigerants Boiling point* at 1 atm, C Sat. temp* at 2000 kpag, C Specific heat ratio, k = C p /C v Suction vol. flow, m 3 /hr for 10 kw load at -30/45 C Propane Propylene R-134a R-404a R-407c R Low temperature refrigerants Boiling point* at 1 atm, C Sat. temp* at 2000 kpag, C Specific heat ratio, k = C p /C v Suction vol. flow m 3 /hr for 10 kw load at -80/-25 C Ethane Ethylene R R-508a * Dewpoint used for zeotropic refrigerants ** Isentropic efficiency 0.7 for all cases 4. COMPONENT SELECTION Power**, kw for 10 kw load at -30/45 C Power**, kw for 10 kw load at -80/- 25 C Selection of components such as compressors, expansion valves, condensers and other components for hydrocarbon service is complicated by the fact that there are generally no published ratings for

3 these, particularly in the smaller sizes of equipment. In addition, many manufacturers do not want to be involved in applications for their products with hydrocarbon refrigerants or in hazardous areas. Application assistance, as well as any warranty, is usually not available from manufacturers and techniques have been developed within our company for calculation of capacity ratings on hydrocarbon refrigerants. Capacity rating techniques are regarded as proprietary information, and are not discussed here. The discussion below is confined to mechanical issues. 4.1 Compressors The required compressor capacities for the common sizes of vapour recovery systems can mainly be handled with either fully hermetic scroll compressors or semi-hermetic reciprocating compressors, with up to two in parallel. The main mechanical issue is ensuring discharge temperature is kept within acceptable limits, especially for the low stage ethylene compressors. Compatibility between compressor materials (particularly motor winding insulation for hermetic compressors) and refrigerants was checked as far as possible before trials were started, but full information was not always available. These compressors are designed to operate with the windings exposed to lubricating oils which are a mixture of many hydrocarbons, and have also been operating in domestic refrigeration in Europe and other areas for many years. On this basis, compatibility of windings with hydrocarbon refrigerants was not expected to be a problem, and this has been proven in operation. There are no problems between hydrocarbon refrigerants and the metals commonly used in refrigeration compressors, including steel, aluminium, zinc, copper, tin, lead and their alloys. 4.2 Lubricating oils Selection of lubricating oils for hydrocarbon applications operating down to about -50 C is not difficult. The solubility of hydrocarbons in oils is similar to solubility of common HFC and HCFC refrigerants in their respective lubricants, so similar oil viscosity grades are generally used. For the low side of the cascade, operating at -70 C to -100 C, there are again few problems with the oil as long as some care is taken. Oil return in DX systems has proven trouble-free in systems down to -85 C (the limit of our DX experience) provided normal good practice is followed in piping design for oil return. Although the oil may be returning at temperatures well below its rated pour point, the remaining dissolved refrigerant in oil returning from DX evaporators reduces viscosity and pour point and allows normal oil return. Oil return from flooded evaporators can also be managed using normal techniques, but after the oil-refrigerant mixture has been distilled to remove excess liquid refrigerant, care must be taken to avoid exposing this oil to low temperatures in the suction line or inlet area of the compressor, as oil freezing will occur. Oil flooded screw compressors operating in the low temperature stage of cascade systems can experience several problems due to the low temperatures at the suction end, unless special precautions are taken. Oil management in the low temperature sections of cascade systems with oilflooded screw compressors and flooded evaporators is critical to successful operation of these systems. Oil-flooded screw compressors are mainly used in larger systems, where flooded evaporators are also more common. In a screw compressor, oil flows into the suction gas inlet area from various sources - escaping from the suction end bearings, lubricating oil from the slide valve, hydraulic oil from the slide valve cylinder, as well as oil returning from the evaporators. This oil is carried back through the compressor by the suction vapour, which may be at -70 C to -100 C. The lubricating oil is initially warm, which reduces solubility of hydrocarbon, and after pressure is reduced to suction pressure,

4 solubility is further reduced, with the result that the oil may contain quite low concentrations of dissolved refrigerant, and so pour point may be close to that of the pure oil. When this oil contacts the cold surfaces of the compressor in the suction area, the oil may freeze and significant accumulations of the solid oil can occur. For screw compressors in the low stage of cascade systems, another problem can occur if the capacity control slide valve remains in one position for a long period typically, this can occur either at maximum or minimum capacity. On many common makes of screw compressor, the hydraulic cylinder used to drive the slide valve is below the suction inlet, and attached to the suction casting. Heat transfer from the cylinder to the cold suction area can result in excessively viscous oil in the cylinder, leading to slow or no response when the capacity control system tries to move the slide valve. Heating of the capacity control cylinder may be required. 4.3 Expansion and solenoid valves As for compressors, there are no metal compatibility issues with standard components such as expansion and solenoid valves when used with hydrocarbon refrigerants. In the author s experience, obtaining mechanical thermostatic expansion (TX) valves with suitable charges for hydrocarbon service ranges from difficult to impossible, especially for ethylene some manufacturers will not supply them at all, and the few who do generally quote long lead times and high prices, such that they are impractical. Future availability of spare parts (such as TX valve power elements with special charges) is also a significant consideration. In direct expansion hydrocarbon systems, the author s company has used only electronic expansion valves, and the problem with these can be obtaining them with suitable certification for use in hazardous areas. Because the large majority of our work is in hazardous areas, many years ago we developed solenoid coils with hazardous area certification to fit Danfoss solenoid valves. The same coils can also be used on Danfoss electronic expansion valves, and for this reason, we have used only these valves on our direct expansion systems. In our particular systems, the electronic expansion valves also offer us other advantages in controllability down to extremely low load conditions, which is an essential feature of vapour recovery systems where load can vary anywhere between 100% and zero. Sizing of expansion valves usually must be done from basic principles, as manufacturers generally will not provide ratings on hydrocarbons, particularly for the low temperature side of the cascade. 4.4 Seals and O-rings Elastomers which are compatible with mineral oils are generally also compatible with ethane, propane and butane, which are similar molecules to natural or synthetic hydrocarbon oils, but smaller. Some literature suggests that excessive swelling may occur when some elastomers are exposed to propylene, although the literature is not unanimous. In the case of O-rings, limited swelling is not a major problem as long as the O-ring remains confined, but if the item is disassembled then re-assembly may be difficult or impossible with a swollen O-ring. A less well-known effect may occur when an elastomer absorbs significant amounts of hydrocarbon under pressure, and is then rapidly depressurised. The hydrocarbon absorbed within the elastomer may not be able to desorb sufficiently rapidly by diffusion through the elastomer, and gas bubble formation within the elastomer can cause an explosive deterioration (Parker), leading to a shredded or blistered appearance. The effect is uncommon, but if it occurs, it can cause leakage from an O-ring without the O-ring being disassembled or otherwise disturbed. Higher pressure systems are more prone to this problem, so it should be considered for the low temperature stage of the cascade, where shutdown pressures may be substantially higher than is usually seen in normal refrigeration systems. The relatively small molecule size of ethane and ethylene (molecular weight

5 30 and 28) also increases diffusion of the gas into the O-ring material (compared to R-23 for example, molecular weight 70) and therefore increases the tendency of this type of failure. Another problem can occur on screw compressors with O-ring seals between casing sections. The O-ring sealing the joint between the suction section and the rotor section of the compressor casing may see temperatures close to suction temperature in some parts of the joint. O-ring elastomers generally lose their elasticity at very low temperatures, and leakage can result from hardening of the O-ring at low temperatures. (The Challenger space shuttle disaster was attributed to this problem.) Selection of O-ring material which will retain elasticity at very low temperatures and will resist explosive decompression failure as well as meeting all the other compatibility criteria is not always possible, and redesign to ensure that O-rings never see these temperatures may be required in some cases. 4.5 Heat exchangers There are no particular problems with heat exchangers for hydrocarbon refrigerant service. Commercial software, such as that from HTFS and HTRI, is available for thermal design of shell and tube chillers with hydrocarbon refrigerants. Commercial software is not generally available for hydrocarbons in direct expansion evaporators, and these must be sized using basic design principles, where ratings with hydrocarbon refrigerants are not available from manufacturers. 5. CASE STUDIES 5.1 Gasoline vapour recovery A system was commissioned in October 2005 at an oil refinery for recovery of mixed gasoline and benzene vapour from air. In order to meet the required emission standard, the vapour was cooled in three stages to -75 C, all at atmospheric pressure. Propylene was used in the 0 C and -30 C stages, and ethylene was chosen for the -80 C stage, for reasons outlined in section 3 above. Basic data for the system is shown in table 2, and the system is shown in Fig. 1. Table 2. Gasoline vapour recovery unit refrigeration data First stage Second stage Third stage Process vapour outlet temperature, C Refrigerant evaporating temperature, C Refrigerant Propylene Propylene Ethylene Process cooling load, kw 50 11* 6 Compressor type Hermetic scroll Hermetic scroll Hermetic scroll Evaporator type * excludes condensing load from third stage 2 x 50% Direct expansion, plate fin & tube 2 x 50% Direct expansion, plate fin & tube 2 x 50% Direct expansion, plate fin & tube The compressors were all hermetic scroll type. The only modification from standard was addition of a certified Ex d terminal box for hazardous area operation. High discharge temperatures were anticipated under some conditions on the low stage, and for this reason the third stage compressors were provided with liquid injection (this was a manufacturer s standard option.) Oil used in all compressors was an ISO VG32 grade polyol ester (POE) oil, with antiwear additives, specifically formulated for scroll compressors.

6 The load on the system can fluctuate rapidly and the system was designed to run anywhere between 100% and zero load, while remaining on line. Hot gas bypass maintains suction pressure at low or zero load conditions for all stages. Some intermittent flooding from the TX valves was anticipated after load changes, so suction separators have been used on all stages. The system operates over wide load range and can spend long periods at or close to zero capacity, but there have been no problems with oil return under any conditions. Figure 1. Gasoline vapour recovery unit (not complete, before shipment). 5.2 Ethylene liquefaction This system was commissioned in January 2001, for liquefaction of ethylene produced in a nearby petrochemical plant. The ethylene is stored as liquid at atmospheric pressure, at its saturation temperature of -104 C. The liquefaction process takes place under a slight pressure, normally at about -82 C, and the resulting liquid is then flashed into the storage tank with flash gas returned to the process stream via a compressor. A closed-loop ethylene two-stage refrigeration system is used as the low stage of a cascade refrigeration system, with propane used in the higher temperature stages, again in a two stage system. The process ethylene is cooled in four stages, two by the propane system and two by the ethylene system, and remains separated from the refrigerant ethylene at all times. (An open ethylene system, using the process ethylene also as refrigerant, was not used in order to prevent contamination of the process ethylene by oil used in the ethylene compressor.) High quality propane (>97%)was readily available for the high stage refrigerant. Basic data for the system is shown in Table 3, and the two-stage ethylene compressor is shown in Fig.2.

7 Table 3. Ethylene liquefaction system refrigeration data 1st stage 2nd stage 3rd stage 4th stage Refrigerant Propane Propane Ethylene Ethylene Refrigerant evaporating temperature, C Compressor type Single-stage screw, 1 x 100% Single stage screw, 1 x 100% Two stage screw 1 x 100% Evaporator type All evaporators flooded shell & tube, TEMA K shell type Design process cooling load is 685 kw at -88 C, with additional smaller loads at the intermediate temperatures. Ethylene compressor motor is 900 kw and propane compressor motor 1600 kw. The normal operating temperature for the low stage is -88 C, but the system must operate down to -100 C under some conditions. Use of an oil-flooded screw compressor in the low stage was not considered initially by the end user, due to concerns over oil management down to -100 C, despite the large cost saving compared to oil-free compressors which are normally used at these temperatures. Only after all the problems outlined above had been addressed in detail was the oilflooded screw concept accepted. This system is thought to be the lowest temperature application of an oil-flooded screw compressor anywhere in the world. Oil used was a VG150 polyglycol in the propane system, selected for its low hydrocarbon solubility and good resistance to dilution, and a VG68 synthetic hydrocarbon in the ethylene system, selected for its stability and low pour point. Figure 2. Two-stage oil-flooded screw ethylene compressor

8 The system has been operating continuously since commissioning, and demonstrates that hydrocarbon refrigerants can be successfully applied in large cascade systems with flooded evaporators. REFERENCES Coolpack software V.1.46, Department of Mechanical Engineering, Technical University of Denmark. Parker Hydraulics, Technical bulletin 5704B1-USA Explosive decompression resistant fluorocarbon polymers