What does the future hold for compressor manufacturers?

What does the future hold for compressor manufacturers? A B Pearson Star Refrigeration Ltd, UK

ABSTRACT This paper explores some future opportunities in refrigeration for compressor designers and manufacturers. Some of the most interesting concepts are in expanders and in particular how to integrate them into systems, how to target suitable applications and how to convince non-technicians that this is necessary and worthwhile. In some cases the compressor may hold the key to a problem experienced in another part of the system; at other times the solution to a compressor design challenge may be found by making changes elsewhere. 1

INTRODUCTION

Industrial refrigeration covers a wide variety of applications and sizes of system. The United Nations report on Refrigeration Technical Options (1) takes the range of sizes for industrial systems as 10 kW to 10 MW of refrigerating effect, at cooling temperatures ranging from -50 °C to +20 °C, with the added criterion that failure of the cooling system would jeoparise the operation of the facility that it serves. For example an office air-conditioning system may not be essential to continued operation but the cooling plant serving a data centre is “mission critical”. Commercial refrigeration covers some of the same ground, usually in a temperature band from -30 °C to +5 °C and a capacity range from 5 kW to 500 kW in point-ofsale facilities, for example supermarkets, mini-markets and local shops. The compressors that currently serve these market sectors are usually of the piston, screw or scroll type with suction swept volumes from about 1 m3 hr-1 up to 10,000 m3 hr-1. They are most often direct drive, with the small to medium sized ones of a “semi-hermetic” construction. The drive is usually a 2-pole, 4-pole or 6-pole motor (2950 rpm, 1450 rpm or 975 rpm on a 50 Hz supply) but there is a growing trend in variable speed drive, either using a frequency inverter on the direct drive motor or with a permanent magnet motor. The working fluid is typically a halogenated hydrocarbon, with an increase in the use of hydrocarbons in small systems, carbon dioxide in small to medium-sized systems and ammonia in medium to large-sized systems. Much of the development effort in refrigeration compressors over the last twenty years has been centred on the transition from chlorinated hydrocarbons. This imperative has to an extent diverted attention away from more radical forms of development, although there has been a flurry of recent innovation in compressor type which bodes well for the future. For example Orosz et al (2) describe a novel rotary compressor based on an oscillating spool, Teh and Ooi (3) show a form of rotary vane compressor where the cylinder casing rotates eccentrically with the

___________________________________________ © The author(s) and/or their employer(s), 2013

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rotor thereby creating a variable volume, and Wissink (4) uses a torsional effect on a free piston compressor to improve efficiency. Other researchers have applied improvements to more long-established technologies, for example the oil-flooded scroll compressor in Bell et al (5), the application of permanent magnet motors to swing compressors in Sekiguchi et al (6) and the implementation of a water-cooled hermetic motor to an ammonia reciprocating compressor in Boone (7) go well beyond the incremental refinement of existing designs. Future development needs will be driven by more than the transition from chlorinated hydrocarbons. Three interlinked drivers were identified by the UK Government report “The Future of Food and Farming” under the Foresight (8) programme. They are population growth/migration, energy demand and food security. By 2050 the world population will have increased to about 9.3 Billion, of which about 75% will live in cities according to the Institution of Mechanical Engineers (9) – this means that the population of cities by then will match the total population at the present time; more than double the present population of cities. This rapid increase will place unprecedented demands on energy supply and the food chain. Resultant threats include uncontrollable climate change, energy price volatility and food deficiencies; meaning that there may be sufficient food but insufficient nutritional value, or in the wrong place or at the wrong price. Converting this overview into a set of priorities for the refrigeration world is not easy. Pearson (10) outlines some of the implications. Transition from high global warming potential (high GWP) working fluids must be achieved without causing increased energy or water consumption. Investment decisions must be based on life cycle climate performance not lowest capital cost, and equipment must be easy to maintain in peak operating condition without skilled intervention. 2

TRENDS IN REFRIGERATION

Vapour compression accounts for the great majority of refrigeration systems in the world, using variants of the Perkins cycle of compression, condensation, expansion and evaporation. The compressor’s role is to raise the pressure of dry gas (usually superheated gas) from a pressure at which the working fluid evaporates from liquid when heat flows to it from the surroundings. The gas is compressed to a higher pressure which is sufficient to allow heat rejection to ambient (or to some process which derives benefit from the heat input) either by desuperheating and condensing the gas back to liquid or, in a few cases, by the removal of sensible heat from a gas in the supercritical state, for example in transcritical CO2 systems. The working fluids are generally stable chemicals or blends of chemicals which have been mixed in order to give more appropriate properties for a given operating condition. Most compressors are oil lubricated and the oil is used for a number of ancillary functions in addition to lubricating the moving parts in bearings, cylinders, vanes, volutes or rotors. Oil is used to seal the compression path and improve efficiency, it is used as hydraulic fluid to drive actuated components such as capacity control gear, it feeds the drive shaft seal keeping it lubricated and cool and it is used as a heat transfer fluid to cool the compression process. The oil might be miscible with the working fluid, which poses some challenges in the compressor and lubrication circuit, but keeps life simple in the evaporator, or it may be immiscible which gives more stable oil condition to the compressor but makes return from the low side of the system more difficult. In a few tough cases, for example high temperature heat pumps using ammonia with polyalfaolefin or hydrocracked mineral oil, the oil may be miscible in the bearings and lubricant circuit, but immiscible in the evaporator – the worst of both worlds.

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Recent developments in compressors have included various efficiency improvements. In piston compressors this has been achieved through material selection, valve design and segregation of high and low temperatures as described by Bon (11). In screw compressors optimisation of screw profile and orientation of the economiser and oil injection ports have been the most common developments. In both types of compressor the improvements are incremental as the basic designs are by now very well understood. Other developments have centred on raising the maximum speed, lowering the minimum speed and ensuring that the machine operates across the full range without excessive resonance. Screw compressors running up to 6,000 rpm and piston compressors up to 3,000 rpm have been recently introduced in larger sizes than previously seen at those speeds and the availability of cost-effective inverters in larger sizes have enabled efficient part-load operation to be achieved. A medium-sized centrifugal compressor designed for HFC-134a was introduced about ten years ago by Conry (12) and has recently been redesigned for the unsaturated HFC-1234ze(E) as described by Pearson (13). Similar compressors are now on offer from several manufacturers, with capacities at water chilling conditions ranging from 200 kW of cooling up to 2,000 kW. These machines use either magnetic or gas bearings to achieve oil-free operation and usually have the speed control electronics integrated into the compressor assembly. This offers the possibility of far greater monitoring and diagnostic capability than is available with more traditional compressor types. One manufacturer of small piston compressors has followed suit and included a diagnostic module with their compressor, but the full capability of this package is not yet in widespread use. The term “Not in kind refrigeration” is used to capture a wide range of technologies that offer an alternative to vapour compression. These include magneto-caloric refrigeration, thermoelectric (Peltier effect) and thermoacoustic as well as various forms of absorption and adsorption. Apart from the traditional absorption systems used in heat-powered chillers and camping fridges the only one of these technologies to be used in mainstream commercial products is the Peltier effect which is used for drinks coolers and small portable fridges. Absorption (where the refrigerant vapour is absorbed into a liquid and pumped to high pressure) has been used for over a century in industrial systems. Adsorption (where the vapour is adsorbed into a solid material) has gained popularity more recently and is not as widely commercialised. It seems to be less suitable for application to large cooling loads, so it is perhaps less of a threat to compressor manufacture, at least in the industrial and commercial sectors. The thermophysical effects are unlikely to be scaled up to industrial size due to low inherent efficiency and high capital cost. Other gas cycles such as the Stirling cycle and the Brayton cycle offer opportunities for manufacturers, but are unlikely to be used for industrial systems. In the Stirling cycle the heating and cooling occurs on opposite ends of the “engine”, so a secondary fluid would be required to transfer the cooling capability to the heat source (in the freezer or cold room). The cost of incorporating suitable heat exchangers on an industrial scale into the Stirling engine would be extremely high; it would be more apropriate to think in terms of heat exchangers with Stirling engines built into them. The Brayton cycle (also often known as air cycle refrigeration) requires a compressor and expander combination, frequently done in the style of a turbocharger. These devices are potentially cheap to make, but they require relatively expensive and bulky air to air heat exchangers to make the efficiency anywhere close to acceptable and they are most suited to applications where both the heat source and heat sink are served by a large temperature change which can be arranged in counterflow to the airstream. This is also difficult and expensive and does not fit the normal requirement for refrigeration, where the product temperature is to be kept at as steady a temperature as possible.

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It therefore seems probable that the not-in-kind technologies do not present any threat to the manufacturers of compressors for industrial refrigeration. Indeed one recent development, sometimes called the hybrid cycle offers an intriguing crossover between the two camps. A two component solution, usually comprising water and ammonia, is used in an absorption system with generator and desorber, but the evaporator outlet gas is compressed in a typical refrigeration compressor, while the weak solution is pumped to high pressure and then mixed with the compressor outlet. This system is attractive for heatpumps as it allows high temperatures to be reached in the heat sink circuit, but without the high pressures that would be required in a Perkins cycle ammonia heat pump. For example to heat water to 90 °C requires an ammonia discharge pressure of about 5 MPa, whereas the hybrid cycle would operate at about half that pressure, well within the range of a standard machine. A disadvantage of the hybrid compression-absorption cycle is that, like the air cycle, there is a large temperature change in the heat source and sink as the cycle progresses, so to take full advantage requires an unusual heat source profile and an expensive pure counterflow evaporator. 3

ADDITIONAL CHALLENGES

Addressing the drivers identified in the Foresight report will require more than just incremental change to existing technology. It is useful to take a step back from current technology and consider what we would ideally like a compressor to be (or not to be)! Systems must use less energy and less water, but must be highly reliable without requiring specialist intervention. To achieve this we require compressors to achieve more complex tasks, but to be simpler than current technology. They should have higher efficiency but be lower cost. They should be integrated into the system operation rather than being a stand-alone component. Oil free operation has been achieved in the medium sized centrifugal compressors described earlier, but they are presently restricted to a few working fluids and only for operation in chill conditions (evaporating temperature above -10 °C). Piston, screw and scroll compressors have been available for many years in other applications, but not for refrigeration. However the problems introduced on the low pressure side of a refrigeration plant by the accumulation of oil, not least the environmental impact of inefficient operation and the hazard associated with oil draining from a live system, mean that providing oil-free refrigeration machines would be a major benefit. Wet operation is also a desireable goal, since it would deliver benefits not only to the compressor but to the rest of the system too. Traditionally operating with liquid in the suction has been problematic; the liquid may cause damage to valves or cylinder heads and can also wash lubricant away from bearings. It also may reduce the discharge temperature to the point where more liquid forms during the compression process. However liquid in the gas stream would provide at least two of the functions currently provided by oil; sealing and cooling, so may help towards the goal of oil free operation. Liquid refrigerant can also in some cases provide bearing lubrication itself. The challenge would be to get the right amount of liquid in the right place at the right time. It could also increase the suction gas mass flow by ensuring maximal density at the compressor inlet. The benefit to the system of a compressor that is tolerant of some liquid is that evaporator flow control can be made much simpler, evaporating temperatures can be raised (helping to improve efficiency) and suction separation vessels could be made a bit smaller (and cheaper) if the standard of separation were to be relaxed. Hermetic operation for ammonia compressors would also be a significant factor in facilitating the use of ammonia in a wider range of applications. At present a few

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machines have been demonstrated, including piston compressors in Germany, screw compressors in Germany and a scroll compressor in Japan, but none of these have been made available to the wider market and have tended to suffer from low efficiency. The recent announcement by Boone (7) of a water cooled piston compressor brings new hope of a significant breakthrough in this field, and the greater use of permanent magnet motors may also offer new ways to reach this goal. Wider use of ammonia is a relatively simple route to higher efficiency systems, particularly if efficient heat exchangers (for example microchannel aluminium panels) are also used. 4

ADDITIONAL OPPORTUNITIES

The expansion process in the Perkins cycle was once described by Lorentzen as “the internal haemhorrhage of the refrigeration process” (14). Expanders have been discussed over many years, but there is a strong disincentive to their adoption, as they are as complicated as a compressor (the most sophisticated mechanical component in the cycle) but replace a component that may be as rudimentary as a small orifice plate and is never more complicated than a ballcock. There needs to be a strong imperative to install the complexity, cost, risk and maintenance overhead inherent in an expander. The means to employ the recovered work is also not well defined. It could be converted to electric power in an alternator, but then there is the question of how to connect to the mains supply; the safety devices to ensure synchronisation and to protect the expander from a sudden loss of mains connection are expensive but necessary. Alternatively the expander could be directly connected to the compressor shaft, either within a single “compander” or on the opposite end of a double shaft motor so that the expander work output reduces the motor power input. The latter arrangement is more readily achieved in the short term but rather goes against the previously described goal of achieving hermetic operation. The compounded machine (either two devices on a single shaft within a common housing or else with both compression and expansion occurring on a common device such as the “expressor” described, for example, by Hansen et al (15) and Brasz (16). In these compound machines the compression and expansion processes take place in the same location, which does not suit the traditional arrangement of many large refrigeration systems where the compressors are typically in a machinery room and the expansion takes place at the surge drum, which might be at the other side of the machinery room, or outdoors beside the condenser, or out on the plant, close by the evaporators. This presents a further problem; large plants typically have multiple compressors and a requirement to function at low part load for much of the time. A fixed capacity expander on the back of one of the compressors will not match the part load capacity of the whole plant, and may not run at all if its partner compressor is switched off. One solution to the part load problem would be to pipe high pressure liquid (perhaps highly subcooled to avoid flash gas in the liquid line) out to the multiple evaporators on the plant and have an expressor at each evaporator, sized to match the load requirement of that unit alone. The energy recovered from the pressure reduction of the subcooled liquid would not be enough to raise the evaporator outlet gas to condensing pressure, but it would get part way there, thus reducing the power requirement of the main compressors in the machinery room. The expressor would only run when the evaporator control called for cooling, so part load would be managed by switching individual units on and off. It ought to be possible to deliver sufficient energy from subcooled high pressure liquid to raise the outlet pressure of cold store evaporators to match the suction pressure of the high stage compressors serving chill rooms, thus removing booster compressors from a two-

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stage system. It would also be possible in this way to avoid the wasteful practice of running cold store coolers at very low evaporating pressure when freezers are in operation. It is likely that expanders will either be two-phase devices, in which case there are a number of possible platforms (including all of the compressor variants mentioned previously) or else they will draw in subcooled liquid which produces little or no flash gas as it expands. These devices could not be positive displacement machines as there is very little volume change to drive them. The type of expander required to power an organic Rankine cycle power generation system is also different, as it operates in the superheated gas area of the pressure-enthalpy map. Expanders can therefore be categorised according to whether they have liquid, vapour or both at the inlet and whether they have liquid, vapour or both at the outlet. The most attractive opportunities in both refrigeration and power generation lie in the areas that require devices handling both liquid and vapour. If wet expansion is necessary then perhaps wet compression should be revisited at the same time, although the challenge of incompressible liquid in the machine is greater. A further opportunity for compressor manufacturers lies in the chance to pump heat from a low temperature to a higher one. Of course all vapour compression refrigeration systems do this, but the output at the high temperature end is usually called “waste heat” and is literally thrown away. In future, as the energy demand outlined in the Foresight report begins to bite, venting heat may become as unacceptable as venting refrigerant. There is a higher demand for heating than for refrigeration in any developed society so the wastefulness inherent in our failure to connect the two processes is unsustainable. The main barrier to this connected approach is logistical; the heating and cooling demands are not coincident (in time or space). As we build new cities over the next thirty years in order to double the urban population they will be less haphazard and more organised than our current urban sprawls which have evolved over centuries to their current size. Industrial parks can contain heating and cooling utility loops with smart meters to enable individual companies operating within the park to draw off or feed in heating and cooling as their processes require it. Provided there is a mix of industry on the estate the loads will be reasonably balanced, so that the data centre can reject its heat into a heating circuit which serves an office block or hospital, or an industrial laundry running a heat pump on its water supply can sell cooling to a drinks bottler. For a variety of reasons the most suitable fluids for use in these types of system are ammonia and carbon dioxide, but they both require compressors capable of running higher pressures than we have been used to in the refrigeration world in the past. 75 bar is the design benchmark for ammonia and sub-critical carbon dioxide, and if the carbon dioxide system discharges heat from supercritical gas a design pressure of 100 bar to 140 bar is likely to be needed. 5

NEW TECHNOLOGIES

5.1 Sensors The development of raw computing power delivered by the continued expression of Moore’s Law has already resulted in huge leaps in sensing capability in recent years. In future it will be feasible to provide sufficient pressure, temperature and acceleration probes on a dynamic machine to monitor its condition cost effectively in real time. This will enable more sophisticated compressor control to be implemented, perhaps including the injection of oil at specific points of the compression cycle in measured doses that are the bare minimum to achieve the required effect, rather than flooding the compression chamber as we do at present. They might also permit the use of measured doses of liquid refrigerant in just the right spots for cooling, sealing and lubricating, eliminating the need for oil

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completely. Greater use of powerful magnets, perhaps even in ambient superconductors, will allow far greater use of actuators within machines, enabling more of the parameters affecting the compression process to be carefully controlled, in the manner that combustion is now more carefully controlled in a car engine. It will be possible to vary capacity, pressure ratio and discharge temperature by modulating the size and shape of the compression chamber, whether it is a piston, screw or scroll machine – or one of the new generation currently called “novel”. There is now a capacitance based sensor on the market that can measure the relative proportions of vapour and liquid in a refrigerant pipe. If the compressor were liquid-tolerant then this sensor would eliminate the need for superheat control of thermostatic expansion valves, thereby raising the suction condition in a typical application by about 5K and improving the system CoP by about 15%. The expansion valve would be set to ensure a 2% - 5% overfeed from the evaporator rather than 6K superheat as is current practice. This would also reduce evaporator size by about 15%, the heat transfer surface typically required to provide the superheat. 5.2 Diagnostics The proliferation of sensors will enable an entirely new approach to compressor maintenance, based upon real time measurement of the condition of all the moving parts in the machine. Significantly improved software will be required to maximise the benefit, and as Ron Conry said of the Turbocor compressor, it will be better to think of it as a computer that pumps gas rather than as a compressor with a lot of controls built in. The compressor will be able to monitor its performance and advise when preventive remedial action is required not only to avoid expensive breakdowns but more importantly to keep it running at peak efficiency. In effect the compressor becomes its own calorimeter, enabling system efficiency to be accurately calculated in real time based on a small number of refrigerant state measurements. Real time sensing could also be used to greatly reduce vibration levels by fine-tuning compressor ports to match the cylinder and discharge manifold pressures more accurately. 5.3 Materials Plastics are already being widely used in some elements of compressors, including valve gear, piston rings and seals. They will become increasingly used in structural elements such as housings, cylinder linings and ports, including “memory materials” which change shape at different temperatures and could be used to ensure much tighter fits (and hence better efficiency without oil) over a wider temperature range. They could also make compressors far lighter, revolutionising the way in which they are built into systems. 5.4 Production techniques When plastic materials are more widely adopted for the wetted parts of a compressor they will enable machines to be produced in entirely new ways. For example, intricate interlinked pieces can already be made in 3-D printers that are only as sophisticated as a dot-matrix printer was in the 1980s. They already print in a variety of plastics (in full colour), bronze and stainless steel. As these machines become more refined the quality of their output will improve to the point that pieces can be taken straight from the printer to the assembly line. 5.5 New concepts 3-D printing would also open up the possibility of making novel types of compressor that are not cost effective to machine at present, such as the “cylindricalcylindrical” single screw compressor (as opposed to the more common “cylindricalplanar” arrangement) described by Heidrich (17). The linear-torsional and spool

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compressors mentioned earlier would also benefit from these advanced production techniques, as would more traditional scroll compressors. 6

JUSTIFICATION FOR THE CHANGE

The main imperative for making this change will be energy efficiency, driven in part by increased energy cost and cost volatility. Financial assessment will become more realistic, requiring an assessment of full life energy cost that factors in the balance between cost of maintenance and cost of performance degradation. Life cycle costing will not assume as-new efficiency for the duration. The winning technology will therefore be the one that does maintain its “out-the-box” performance and does so without significant manual intervention, but also is seen to do so and therefore can validate the energy cost savings claimed for it. The level of instrumentation, engine management software and diagnostics found under the bonnet of an average family car would almost be enough to enable this to be done today, so it is not such a big leap to foresee it in refrigeration compressors in ten or twenty years time. Those that do not provide this level of provable performance will not sell, no matter how cheap they are. 7

CONCLUSIONS

There are exciting times ahead for compressor manufacturers. A demand for higher operating pressures and temperatures to service the needs of a renewed heat pump market, coupled with a requirement for low-maintenance systems and energy efficiency will drive the development of oil-free, liquid tolerant, hermetically driven compressors and expanders (sometimes combined into a single unit). The continued advances expected in sensor technology will enable a raft of energy saving control and monitoring techniques that revolutionise the way that we interact with these machines, enabling them to report on system efficiency and advise precise intervention intervals. All this needs to be done at a price that is significantly lower than the present day and in a manner that enables the compressor manufacturers to make a healthy profit from their expertise. REFERENCE LIST (1) (2) (3) (4) (5) (6)

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UNEP, “Assessment Report of the Refrigeration, Air-Conditioning and Heat Pumps Technical Options Committee”, Nairobi, 2010 Orosz, J., Kemp, G., Bradshaw, C. and Groll, E., Performance and Operating characteristics of a Novel Rotating Spool Compressor, International Compressor Engineering Conference, Purdue, 2012 Teh, Y.L., Ooi, K.T., Analysis of internal leakage across radial clearance in the improved revolving vane (RV-i) compressor, International Compressor Engineering Conference, Purdue, 2008 Wissink, E., Dedicated Compressor Technology for a next generation domestic heat pump – free piston with oil free torsion drive, GL2012 IIR Conference, Delft, 2012 Bell, I., Groll, E., Braun, J., Horton, T., Experimental Testing of Oil-Flooded Hermetic Scroll Compressor, International Compressor Engineering Conference, Purdue, 2012 Sekiguchi, T., Development of Lightweight and High Efficiency Swing Type Compressor using New Interior Permanent Magnet Synchronous Motor, International Compressor Engineering Conference, Purdue, 2012

(7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)

Boone, J., Ammonia Chillers in different industrial plants in Switzerland, International Institute of Refrigeration conference “Ammonia Refrigeration Technology”, Ohrid, 2013 Foresight, The Future of Food and Farming (2011) Final Project Report. The Government Office for Science, London, 2011. Institution of Mechanical Engineers “Population: One planet, too many people?” London, 2011 Pearson, A., The role of refrigeration in the future of food and farming, 42nd Congress on HVAC&R, KGH, Belgrade, 2011 Bon, G., New high efficiency piston compressors for ammonia, GL2012 IIR Conference, Delft, 2012 Conry, R., A brief overview of the Turbocor compressor – the Road to Discovery, Proc Inst Ref, London, 2009 Pearson, A., R-1234ze for variable speed centrifugal chillers, Proc Inst Ref, London, 2013 Lorentzen, G., Throttling, the internal haemhorrhage of the refrigeration process, Proc Inst Ref, London, 1983 Hansen, T., Smith, I., Stosic, N., Combined Industiral Cooling and Heating with Transcritical CO2 Heat Pumps Utilising the Work of Expansion, GL2004 IIR Conference, Glasgow, 2004 Brasz, J.J., Single Rotor Expressor as Two-Phase Flow Throttle Valve Replacement, US Patent, n.006185956, 2001 Heidrich, F., Water Flooded Single Screw (SSP) Compressor Technology, International Compressor Engineering Conference, Purdue, 1996

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