Bitzer refrigerant report: Part 4

Bitzer refrigerant report: Part 4

Published with permission of Bitzer

Stratospheric ozone depletion as well as atmospheric greenhouse effect due to refrigerant emissions have led to drastic changes in refrigeration and air conditioning technologies since the beginning of the 1990s. For reference to earlier information, please see Part 1, 2 and 3 published in the previous issues of Cold Link Africa.

NH₃ (Ammonia) as alternative refrigerant

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 plants. 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 NH₃₃ 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 polyalpha-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 (approximately 13 to 15% compared to R22).

Bitzer Refrigerant Report A 501 20 29 fig1Figure 1: Comparison of discharge gas temperatures. →

This feature, which is favourable for large plants, makes the control of the refrigerant injection more difficult with small capacities. A further criteria to be considered is the corrosive action on copper containing materials; pipelines must therefore be made of steel. The development of motor windings resistant to NH₃ is hindered, too. Another difficulty arises from the electrical conductivity of the refrigerant with higher moisture content.

Additional characteristics include toxicity and flammability, which require special safety measures for the construction and operation of such plants. Resulting design and construction criteria 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 un-favourable 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; larger size pipe-lines must be examined by a certified inspector.

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 programmes is dealing with part 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 has a large number of operating 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.

Bitzer Refrigerant Report A 501 20 29 Fig2← Figure 2: NH₃/R22 – comparison of pressure levels

Various equipment manufacturers have developed special evaporators, allowing significantly reduced refrigerant charge. There is a strong trend towards so-called ‘low charge’ systems, and especially 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 50kg), 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 optimised NH₃ compressors for various types of lubricants:

Single stage open reciprocating compressors (displacement 19 to 152 m³/h with 1450 rpm) for air conditioning, medium temperature and booster applications ;

Open screw compressors (displacement 84 to 1015 m³/h – with parallel operation to 4060 m³/h – with 2 900 rpm) for air conditioning, medium and low temperature cooling.

Options for low temperature cooling:

  • Single stage operation
  • Economiser operation
  • Booster operation

R723 (NH₃/DME) as an alternative to NH₃

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.

Bitzer Refrigerant Report A 501 20 33 Fig3Figure 3: R744(CO₂) – pressure/enthalpy diagram Figure 3: R744(CO₂) – pressure/enthalpy diagram →

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 with the standard refrigerant nomenclature. Conversion of existing plants. 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.

Carbon dioxide R744 (CO₂) as an alternative refrigerant and secondary fluid

CO₂ has had a long tradition in 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 HFCs (F-Gas Regulation) and 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 there is no necessity for 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 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.

Bitzer Refrigerant Report A 501 20 33 Fig4← Figure 4: R744(CO₂)/R22/R404A – comparison of pressure levelsFigure 4: R744(CO₂)/R22/R404A – comparison of pressure levels

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. Subcritical CO₂ 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 (Figure 5).

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 or items in development (such as for R410A), can be matched with acceptable effort.

Resulting design criteria

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, for example 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.

Bitzer Refrigerant Report fig5Figure 5: Cascade system with CO₂ for industrial applications →

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, for example for low temperatures.
Figure 5 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 (MT). 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.

Bitzer Refrigerant Report fig6← Figure 6: Conventional refrigeration system combined with CO₂ low temperature cascade

As an alternative, additional cooling units for CO₂ condensation are also used where longer shutoff 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 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 Figure 6. 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 must 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, if miscible and highly soluble POE are used, the viscosity is strongly reduced.

Transcritical CO₂ applications

Transcritical processes are characterised 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 – for example 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 sub-critically - 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 (see system example, Figure 7).

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 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 (for example hotels, swimming pools, drying systems) must be planned and realised individually. Their number is therefore still limited, but with an upward trend. Apart from these specific applications, there is also a range of developments for the classical areas of refrigeration and air-conditioning, for example 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 optimised 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.

Resulting design criteria

Detailed information on this topic would go beyond the scope of this publication. In any case, the system and control techniques are substantially different 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. For example, this involves design, materials (bursting resistance), displacement, crank gear, working valves, lubrication system, as well as compressor and motor cooling. Hereby, 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 for lubricants. Further development is necessary in various areas, and transcritical CO₂ technology cannot in general be regarded as state-of-the-art.

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. Suplementary BITZER information concerning compressor selection for transcritical CO₂ systems.

"As an alternative, additional cooling units for CO2 condensation are also used where longer shutoff periods can be bridged without a critical pressure increase."

CO₂ in mobile air-conditioning systems 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 since several years. At first glance, efficiency and therefore indirect emissions from CO₂ systems under typical ambient conditions appear to be unfavourable. But it must be considered that present 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.

Bitzer Refrigerant Report A 501 20 36Figure 7: Example of a transcritical CO₂ Booster system

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 optimised 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 been introduced by the automotive industry. Further aspects such as operating safety, costs, and global logistics will play an important role. 

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