The refrigerant NH3 has been used for more than a century in industrial and larger refrigeration plants. It has no ozone depletion potential and no direct global warming potential. The efficiency is at least as good as that of R22, in some areas even more favourable; the contribution to the indirect global warming effect is therefore small. In addition, its price is exceptionally low. Is it therefore an ideal refrigerant and an optimum substitute for R22 or an alternative for HFCs?
NH3 has indeed very positive features, which can be exploited quite well in large refrigeration systems and heat pumps.
Unfortunately there are also negative aspects, which restrict the wider use in the commercial area or require costly and sometimes new technical developments.
A disadvantage with NH3 is the high isentropic exponent (NH3 = 1.31 / R22 = 1.19 / R134a = 1.1), which results in a discharge temperature even significantly higher than that of R22. Single stage compression is therefore already subject to certain restrictions below an evaporating temperature of around -10°C.
The question of suitable lubricants is also not satisfactorily solved for smaller plants in some kinds of applications. The most commonly used mineral oils and poly-alpha-olefins are not soluble with the refrigerant. They must be separated with complex technology and seriously limit the use of "direct expansion evaporators" due to the deterioration in the heat transfer.
Special demands are made on the thermal stability of the lubricants due to the high discharge gas temperatures. This is especially valid when automatic operation is considered where the oil is supposed to remain in the circuit for years without losing any of its stability.
NH3 has an extraordinarily high enthalpy difference and thus a very small circulating mass flow (approx. 13 to 15% compared to R22). This feature, which is favourable for large systems, makes the control of the refrigerant injection more difficult with small capacities.
Further to be considered is the corrosive action on copper containing materials; pipe lines must therefore be made of steel. This also hinders the development of motor windings resistant to NH3 as basis for semi-hermetic constructions. Another difficulty arises from the electrical conductivity of the refrigerant in case of higher moisture content.
Additional characteristics include toxicity and flammability, which require special safety measures for the construction and operation of such systems.
Based on the present "state of technology", industrial NH3 systems demand a completely different plant technology, compared to usual commercial systems.
Due to the insolubility with the lubricating oil and the specific characteristics of the refrigerant, high efficiency oil separators and flooded evaporators with gravity or pump circulation are usually employed. Because of the danger to the public and to the product to be cooled, the evaporator often cannot be installed directly at the cold space and the heat must be transported by a secondary refrigerant circuit.
Due to the thermal behaviour, two stage compressors or screw compressors with generously sized oil coolers must be used even at medium pressure ratios.
Refrigerant lines, heat exchangers and fittings must be made of steel; welded joints in pipelines of larger dimensions are subject to inspection by a certified inspector. In some cases, aluminium can also be used as a material.
Depending upon the size of the plant and the refrigerant charge, corresponding safety measures and special machine rooms are required.
The refrigeration compressor is usually of "open" design, the drive motor is a separate component.
These measures significantly increase the expenditure for NH3 plants, especially for medium and smaller capacities.
Efforts are therefore being made world-wide to develop simpler systems which can also be used in the commercial area.
A part of the research programs is dealing with partly soluble lubricants, with the aim of improving oil circulation in the system. Simplified methods for automatic return of non-soluble oils are also being examined as an alternative.
BITZER is strongly involved in these projects and is operating a larger number of compressors. The experiences up to now have revealed that systems with partly soluble oils are difficult to manage. The moisture content in the system has an important influence on the chemical stability of the circuit and the wear of the compressor. Besides, high refrigerant solution in the oil (wet operation, insufficient oil temperature) leads to strong wear on the bearings and other moving parts. This is due to the enormous volume change when NH3 evaporates in the lubricated areas. These research developments are being continued, with focus also on alternative solutions for non-soluble lubricants.
Various equipment manufacturers have developed special evaporators, allowing significantly reduced refrigerant charge. There is a strong trend towards "low charge" systems, i.a. with regard to safety requirements, which are also largely determined by the refrigerant charge.
In addition to this, there are developments for the "sealing" of NH3 plants: compact liquid chillers (charge below 50 kg), installed in a closed container and partly with an integrated water reservoir to absorb NH3 in case of a leak.
This type of compact unit can be installed in areas which were previously reserved for plants with refrigerants of safety group A1 due to safety requirements. An assessment of NH3 compact systems – instead of systems using HFC refrigerants and conventional technology – is only possible on an individual basis, taking into account the particular application. From a merely technical viewpoint and presupposing an acceptable price level, a wider range of products will supposedly become available in the foreseeable future.
The product range from BITZER today includes an extensive selection of optimized NH3 compressors for various types of lubricants:
The refrigerant NH3 is not suitable for the conversion of existing (H)CFC or HFC plants; they must be constructed completely new with all components.
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The previously described experiences with the use of NH3 in commercial refrigeration plants with direct evaporation caused further experiments on the basis of NH3 by adding an oil soluble refrigerant component. Main goals were improved oil transport and heat transmission with conventional lubricants, along with a reduced discharge gas temperature for the extended application range with single stage compressors.
The result of this research project is a refrigerant blend of NH3 (60%) and dimethyl ether "DME" (40%). It was developed by the Institute of Air Handling and Refrigeration (ILK) in Dresden, Germany, and has been applied in a series of real systems. As a largely inorganic refrigerant it received the designation R723 due to it its average molecular weight of 23 kg/kmol in accordance to the standard refrigerant nomenclature.
DME was selected as an additional component for its good solubility and high individual stability. Its boiling point is -26°C, the adiabatic exponent is relatively low, it is not toxic and available in high purity. In the above-mentioned concentration NH3 and DME form an azeotropic blend characterised by a slightly higher pressure level than pure NH3. The boiling point lies at -36.5°C (NH3 -33.4°C), 26 bar (abs.) of condensing pressure corresponds to 58.2°C (NH3 59.7°C).
The discharge gas temperature in air conditioning and medium temperature ranges decreases by about 10 to 25 K (Comparison of discharge gas temperatures) and allows for an extended application range to higher pressure ratios. Thermodynamic calculations conclude a single-digit percent rise in refrigerating capacity compared to NH3. The coefficient of performance is similar and is even more favourable at high pressure ratios, confirmed by experiments. Due to the lower temperature level during compression, an improved volumetric and isentropic efficiency can be expected, at least with reciprocating compressors in case of an increasing pressure ratio.
Due to the higher molecular weight of DME, mass flow and vapour density increase by nearly 50% compared to NH3, although this is of little importance to commercial plants, especially in short circuits. In conventional industrial refrigeration plants, however, this is a substantial criterion with regard to pressure drops and refrigerant circulation. These considerations again show that the preferred application area of R723 is in commercial applications and especially in liquid chillers.
Material compatibility is comparable to that of NH3. Although non-ferrous metals (e.g. CuNi alloys, bronze, hard solders) are potentially suitable, provided minimum water content in the system (< 1000 ppm), a system design similar to typical ammonia practice is recommended.
Mineral oils or (preferred) polyalpha olefin are suitable lubricants. As mentioned before, the proportion of DME leads to improved oil solubility and partial miscibility. Furthermore, the relatively low liquid density and an increased DME concentration in the oil enhances oil circulation. PAG oils would be fully or partly miscible with R723 for typical applications, but are not recommended because of the chemical stability and high solubility in the compressor crankcase (strong vapour development in the bearings).
Tests have shown that the heat transfer coefficient at evaporation and high heat flux is significantly higher in systems with R723 and mineral oil compared to NH3 with mineral oil.
Further characteristics are toxicity and flammability. The DME content lowers the ignition point in air from 15 to 6%. However, the azeotrope is ranked in safety group B2, but may receive a different classification in case of a revised assessment.
Experiences with the NH3 compact systems described above can be used in plant technology. However, the component layout has to be adjusted considering the higher mass flow. By appropriate selection of the evaporator and the expansion valve, a very stable superheat control must be ensured. Due to the improved oil solubility, “wet operation” can have considerable negative consequences compared to NH3 systems with non-soluble oil.
With regard to safety regulations, the same criteria apply to installation and operation as for NH3 plants.
Suitable compressors are special NH3 versions which possibly have to be adapted to the mass flow and the continuous oil circulation. An oil separator is usually not necessary with reciprocating compressors.
BITZER NH3 reciprocating compressors are suitable for R723 in principle. An individual selection of specifically adapted compressors is possible on demand.
R290 (propane) can also be used as a substitute refrigerant. Being an organic compound (hydrocarbon), it has no ozone depletion potential and a negligible direct global warming effect. To take into consideration however, is a certain contribution to summer smog.
Pressure levels and refrigerating capacity are similar to R22, and its temperature behaviour is as favourable as with R134a.
There are no particular problems with material. In contrast to NH3, copper materials are also suitable, so that semi-hermetic and hermetic compressors are possible. Common mineral oils of HCFC systems can be used here as a lubricant over a wide application range. Polyol esters (POE) and polyalpha-olefins (PAO) offer even more favorable properties.
Refrigeration plants with R290 have been in operation world-wide for many years, mainly in the industrial area – it is a "proven" refrigerant.
Meanwhile R290 is also used in smaller compact systems with low refrigerant charges like residential air-conditioning units and heat pumps. Furthermore, a rising trend can be observed in its use with commercial refrigeration systems and chillers.
Propane is offered also as a mixture with Isobutane (R600a) or Ethan (R170), in order to provide a similar performance to halocarbon refrigerants. Pure Isobutane is mostly intended as a substitute for R12 in small systems (preferably domestic refrigerators and freezers).
The disadvantage of hydrocarbons is their high flammability, therefore they are classified as refrigerants of "Safety Group A3". Based on the refrigerant charge quantities commonly used in commercial systems, the system design and risk analysis must be in accordance with explosion protection regulations.
Semi-hermetic compressors in so-called "hermetically sealed" systems are in this case subject to regulations for hazardous zone 2 (only seldom and short term risk). Safety demands include special devices to protect against excess pressures and special arrangements for the electrical system. In addition, measures are required to ensure hazard free ventilation to effectively prevent a flammable gas mixture in case of refrigerant leakage.
Design requirements are defined by standards (e.g. EN378) and may vary in different countries. For systems applied in the EU, an assessment according to EC Directive 94/9/EC (ATEX) may become necessary as well. With open compressors, this will possibly lead to a classification in zone 1 ‒ which demands, however, electrical equipment in special flame-proof design.
Apart from the measures mentioned above, propane systems require practically no special features in the medium and low temperature ranges compared to a usual (H)CFC and HFC system. When sizing components, however, the relatively low mass flow should be considered (approximately 55 to 60% compared to R22). An advantage here is that the refrigerant charge can be greatly reduced. From the thermodynamic point of view, an internal heat exchanger between the suction and liquid line is recommended as this will improve the refrigerating capacity and COP.
Owing to the particularly high solubility of R290 (and R1270) in common lubricants, BITZER R290/R1270 compressors are charged with special oil of a high viscosity index and particularly good tribological properties.
Again, an internal heat exchanger is of advantage as it leads to higher oil temperatures, lower solubility and therefore improved viscosity.
Due to the very favourable temperature behaviour (Comparison of discharge gas temperatures), single stage compressors can be used down to approximately -40°C evaporation temperature. R290 could thus also be considered as an alternative for some of the HFC blends.
A range of ECOLINE compressors and CS. compact screws is available for R290. Due to the individual requirements a specifically equipped compressor version is offered. Inquiries and orders need a clear reference to R290. The handling of the order includes an individual agreement between the contract partners. Open reciprocating compressors are also available for R290, together with a comprehensive program of flame-proof accessories which may be required
Due to the special safety measures when using R290, a conversion of existing systems only seems possible in exceptional cases. They are limited to systems, which can be modified to meet the corresponding safety regulations with an acceptable effort.
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For some time there has been increasing interest in using propylene (propene) as a substitute for R22 or HFC. Due to its higher volumetric refrigerating capacity and lower boiling temperature (compared to R290), applications in medium and low temperature systems are of particular interest, e.g. liquid chillers. On the other hand, higher pressure levels (> 20%) and discharge gas temperatures have to be taken into consideration, restricting the possible application range.
Material compatibility is comparable to propane, as is the choice of lubricants.
Propylene is also easily inflammable and belongs to the safety group A3. The same safety regulations are therefore to be observed as with propane (R290 as alternative refrigerant).
Due to the chemical double bond, propylene reacts quite easily, risking polymerization at high pressure and temperature levels. However, tests by hydrocarbon manufacturers and stability tests in real applications show practically no reactivity in refrigeration systems. Doubts have occasionally been voiced in literature regarding possible carcinogenic effects of propylene. These assumptions have been disproved by appropriate studies.
With regard to system technology, experience gained from the use of propane can widely be applied to propylene. However, component dimensions have to be altered due to higher volumetric refrigerating capacity (Comparison of performance data). The compressor displacement is correspondingly lower, as are the suction and high pressure volume flows. Because of higher vapour density, however, the mass flow is almost the same as for R290. As liquid density is nearly identical, the same applies to the liquid volume in circulation.
As with R290, an internal heat exchanger between suction and liquid lines is of advantage. However, due to the higher discharge gas temperature of R1270, restrictions are partly necessary at high pressure ratios.
A range of ECOLINE compressors and CS. compact screws is available for R1270. Due to the individual requirements a specifically equipped compressor version is offered. Inquiries and orders need a clear reference to R1270.
The handling of the order includes an individual agreement between the contract partners. Open reciprocating compressors are also available for R1270, together with a comprehensive program of flame-proof accessories which may be required.
(see also https://www.bitzer.de)
CO2 has a long tradition in the refrigeration technology reaching far into the 19th century. It has no ozone depleting potential, a negligible direct global warming potential (GWP = 1), is chemically inactive, non-flammable and not toxic in the classical sense. Therefore, CO2 is not subjected to the stringent containment demands of e.g. HFCs (F-Gas Regulation), flammable or toxic refrigerants. However, compared to HFCs the lower critical value in air has to be considered. For closed rooms, this may require special safety and detection systems.
CO2 is also low in cost and doesn't require recovery and disposal. In addition, it has a very high volumetric refrigerating capacity: depending on operating conditions, approx. 5 to 8 times as high as R22 and NH3.
Above all, the safety relevant characteristics were an essential reason for the initial widespread use. The main focus for applications were e.g. marine refrigeration systems. With the introduction of the "(H)CFC Safety Refrigerants", CO2 became less popular and had nearly disappeared by the 1950s.
The main reasons for that are its relatively unfavourable thermodynamic characteristics for usual applications in refrigeration and air conditioning.
The discharge pressure with CO2 is extremely high, and the critical temperature at 31°C (74 bar) very low. Depending on the heat sink temperature at the high pressure side, transcritical operations with pressures far beyond 100 bar are required. Under these conditions, energy efficiency is often lower than in the classic vapour compression process (with condensation), therefore the indirect global warming effect is higher.
Nonetheless, there is a range of applications in which CO2 can be used very economically and with favourable eco-efficiency. These include subcritical cascade plants, but also transcritical systems, in which the temperature glide on the high pressure side can be used advantageously, or the system conditions permit subcritical operation for long periods. It should further be noted that the heat transfer coefficients of CO2 are considerably higher than of other refrigerants – with the potential of very low temperature differences in evaporators, condensers, and gas coolers. Moreover, the necessary pipe dimensions are very small, and the influence of the pressure drop is comparably low. In addition, when used as a secondary fluid, the energy demand for circulation pumps is extremely low.
In the following section, a few examples of subcritical systems and resulting design criteria are described. An additional section provides details on transcritical applications.
From energy and pressure level points of view, very beneficial applications can be seen for industrial and larger commercial refrigeration plants. For this, CO2 can be used as a secondary fluid in a cascade system and if required, in combination with a further booster stage for lower evaporating temperatures (Cascade system with CO₂ for industrial applications).
The operating conditions are always subcritical which guarantees good efficiency levels. In the most favourable application range (approx. -10 to -50°C), pressures are still on a level where already available components, e.g. for R410A, can be matched with acceptable effort.
For the high temperature side of such a cascade system, a compact cooling unit can be used, whose evaporator serves on the secondary side as the condenser for CO2. Chlorine-free refrigerants are suitable, e.g. NH3, HCs or HFCs, HFO and HFO/HFC blends.
With NH3, the cascade heat exchanger should be designed in a way that the dreaded build-up of ammonium carbonate in the case of leakage is prevented. This technology has been applied in breweries for a long time.
A secondary circuit for larger plants with CO2 could be constructed utilising, to a wide extent, the same principles for a low pressure pump circulating system, as is often used with NH3 plants. The essential difference is the condensing of CO2 in the cascade cooler, while the receiver tank (accumulator) only serves as a supply vessel.
The extremely high volumetric refrigerating capacity of CO2 (latent heat through the changing of phases) leads to very low mass flow rates, allows for small cross sectional pipe and minimal energy needs for the circulating pumps.
There are different solutions for the combination with a further compression stage, e.g. for low temperatures.
The following figure (Cascade system for industrial applications) shows a variation with an additional receiver, which one or more booster compressors will bring down to the necessary evaporation pressure. Likewise, the discharge gas is fed into the cascade cooler, condenses and is carried over to the receiver. The feeding of the low pressure receiver (LT) is achieved by a level control device.
Instead of conventional pump circulation the booster stage can also be built as a so-called LPR system. The circulation pump is thus not necessary, but the number of evaporators is then limited with view to an even distribution of the injected CO2.
In the case of a system breakdown where a high rise in pressure could occur, safety valves can vent the CO2 to the atmosphere with the necessary precautions. As an alternative, additional cooling units for CO2 condensation are also used where longer shut-off periods can be bridged without a critical pressure increase.
For systems in commercial applications, a direct expansion version is possible as well.
Supermarket plants with their usually widely branched pipe work and shock freezer offer an especially good potential in this regard: The medium temperature system is carried out in a conventional design or with a secondary circuit, for low temperature application combined with a CO2 cascade system (for subcritical operation). A system example is shown in the figure below (Konventionelle Kälteanlage kombiniert mit CO₂-Tiefkühlkaskade).
For a general application, however, not all requirements can be met at the moment. It is worth considering that system technology changes in many respects and specially adjusted components are necessary to meet the demands.
The compressors, for example, must be properly designed because of the high vapour density and pressure levels (particularly on the suction side). There are also specific requirements with regard to materials. Furthermore only highly dehydrated CO2 may be used.
High demands are made on lubricants as well. Conventional oils are mostly not miscible and therefore require costly measures to return the oil from the system. On the other hand, the viscosity is strongly reduced if miscible and highly soluble POE are used. Further information: Lubricants for compressors.
For subcritical CO2 applications BITZER offers two series of special compressors.
(see also https://www.bitzer.de)
Transcritical processes are characterized in that the heat rejection on the high pressure side proceeds isobar but not isotherm. Contrary to the condensation process during subcritical operation, gas cooling (desuperheating) occurs, with corresponding temperature glide. Therefore, the heat exchanger is described as gas cooler. As long as operation remains above the critical pressure (74 bar), only high-density vapour will be transported. Condensation only takes place after expansion to a lower pressure level – e.g. by interstage expansion in an intermediate pressure receiver. Depending on the temperature curve of the heat sink, a system designed for transcritical operation can also be operated subcritically ‒ with higher efficiency. In this case, the gas cooler becomes the condenser.
Another feature of transcritical operation is the necessary control of the high pressure to a defined level. This "optimum pressure" is determined as a function of gas cooler outlet temperature by means of balancing between the highest possible enthalpy difference and at once minimum compression work. It must be adapted to the relevant operating conditions using an intelligent modulating controller (Example of a transcritical CO₂ Booster system).
As described before, under purely thermodynamic aspects, the transcritical operating mode appears to be unfavourable in terms of energy efficiency. In fact, this is true for systems with a fairly high temperature level of the heat sink on the high pressure side. However, additional measures can improve efficiency, such as the use of parallel compression (economiser system) and/or ejectors or expanders for recovering the throttling losses during expansion of the refrigerant.
Apart from that, there are application areas in which a transcritical process is advantageous in energy demand. These include heat pumps for heating of sanitary water or drying processes. With the usually very high temperature gradients between the discharge temperature at the gas cooler intake and the heat sink intake temperature, a very low gas temperature outlet is achievable. This is facilitated by the temperature glide curve and the relatively high mean temperature difference between CO2 vapour and secondary fluid. The low gas outlet temperature leads to a particularly high enthalpy difference, and therefore to a high system COP.
Low-capacity sanitary water heat pumps are already manufactured and used in large quantities. Plants for medium to higher capacities (e.g. hotels, swimming pools, drying systems) must be planned and realised individually. Their number is therefore still limited, but with a good upward trend. Apart from these specific applications, there is also a range of developments for the classical areas of refrigeration and air-conditioning, e.g. supermarket refrigeration. Installations with parallel compounded compressors are in operation to a larger scale. They are predominantly booster systems where medium and low temperature circuits are connected (without heat exchanger). The operating experience and the calculated energy costs show promising results. However, the investment costs are still higher than for conventional plants with HFCs and direct expansion.
On the one hand, the favourable energy costs are due to the high degree of optimized components and the system control, as well as the previously described advantages regarding heat transfer and pressure drop. On the other hand, these installations are preferably used in climate zones permitting very high running times in subcritical operation due to the annual ambient temperature profile.
For increasing the efficiency of CO2 supermarket systems and for using them in warmer climate zones, the technologies described above using parallel compression and/or ejectors are increasingly used.
Therefore, but also because of very demanding technology and requirements for qualification of planners and service personnel, CO2 technology cannot be regarded as a general replacement for plants using HFC refrigerants.
Detailed information on this topic would go beyond the scope of this publication. In any case, the system and control techniques differ substantially from conventional plants. Already when considering pressure levels as well as volume and mass flow ratios specially developed components, controls, and safety devices as well as suitably dimensioned pipework must be provided.
The compressor technology is particularly demanding. The special requirements result in a completely independent approach, e.g. considering design, materials (bursting resistance), displacement, crank gear, working valves, lubrication system, as well as compressor and motor cooling. The high thermal load severely limits the application for single-stage compression. Low temperature cooling requires 2-stage operation, whereby separate high and low pressure compressors are particularly advantageous with parallel compounded systems.
The criteria mentioned above in connection with subcritical systems apply to an even higher degree to lubricants. Further information: Lubricants for compressors.
Further development is necessary in various areas, and transcritical CO2 technology cannot in general be regarded as state-of-the-art yet.
For transcritical CO2 applications, BITZER offers a wide range of special compressors. Their use is aimed at specific applications, therefore individual examination and assessment are required.
(see also https://www.bitzer.de)
Within the scope of the long-discussed measures for reducing direct refrigerant emissions, and the ban on the use of R134a in MAC systems within the EU, the development of CO2 systems has been pursued intensively for several years.
At first glance, efficiency and therefore indirect emissions of CO2 systems under typical ambient conditions appear to be unfavourable. But it must be considered that previous R134a systems are less efficient than stationary plants of the same capacity, because of specific installation conditions and high pressure losses in pipework and heat exchangers. With CO2, pressure losses have significantly less influence. Moreover, system efficiency is further improved by the high heat transfer coefficients in the heat exchangers.
This is why optimized CO2 air conditioning systems are able to achieve efficiencies comparable to those of R134a. Regarding the usual leakage rates of such systems, a more favourable balance is obtained in terms of TEWI.
From today's viewpoint, it is not yet possible to make a prediction as to whether CO2 can in the long run prevail in this application.
It certainly also depends on the experience with "low GWP" refrigerants that have meanwhile been introduced by the automotive industry (R1234yf). Among others, operating safety, costs, and global logistics will play an important role.