From Wikipedia:  “An Air Cycle Machine (ACM) is the refrigeration unit of the environmental control system used in pressurized turbine-powered aircraft. Normally an aircraft has two to three of these machines arranged in a system called a "pack". The cooling process uses air instead of freon in a gas cycle. No condensation or evaporation is involved, and the cooled air output from the process is used directly for cabin ventilation.”

The usual compression cooling and expansion seen in any refrigeration cycle is accomplished in the ACM by a centrifugal compressor, two heat exchangers and an expansion turbine.

Engine bleed air, which can be in excess of 150°C, is directed into a primary heat exchanger before going through the compressor. Once the hot air is cooled, it is then compressed which heats the air back up and it is then sent to the secondary heat exchanger. The pre-cooling through the first heat exchanger increases the efficiency of the ACM because of the heating from compression. The air is again cooled in the secondary heat exchanger and travels through the turbine which expands the air and cools it even further. It is possible for the ACM to to prod

Here is a .pdf from Boeing on their ACS machine.

Source: Internet

What is air cycle?

Air is by nature the safest and cheapest refrigerant. Environmental concerns about ozone depletion, global warming and increasingly stringent legislation have renewed interest in alternative refrigeration technologies.

Air cycle systems have specific advantages that apply to all potential applications:

•The working fluid (air) is free, environmentally benign, totally safe and non-toxic.

•Air cycle equipment is extremely reliable, thereby reducing maintenance costs and system down-time.

•The performance of an air cycle unit does not deteriorate as much as that of a vapour-compression unit when operating away from its design point.

•When operating in a refrigeration cycle, an air cycle unit can also produce heat at a useful temperature.

The use of air as a refrigerant is based on the principle that when a gas expands isentropically from a given temperature, its final temperature at the new pressure is much lower. The resulting cold gas, in this case air, can then be used as a refrigerant, either directly in an open system, or indirectly by means of a heat exchanger in a closed system. The efficiency of such systems is limited to a great extent by the efficiencies of compression and expansion, as well as those of the heat exchangers employed. Originally, slow speed reciprocating compressors and expanders were used. The poor efficiency and reliability of such machinery were major factors in the replacement of such systems with vapour compression equipment. However, the development of rotary compressors and expanders greatly improved the isentropic efficiency and reliability of the air cycle. Advances in turbine technology, together with the development of air bearings and ceramic component offer further efficiency. Combining this with newly available compact heat exchangers with greatly improved heat transfer characteristics makes competition with many existing vapour compression, and certainly liquid nitrogen systems, quite feasible.

History and potential applications

Air cycle is not a new technology. At the turn of the century air cycle or 'cold air machines' were available from companies such as J & E Hall (see picture). These were used on board ships and by food producers and retailers to provide cooling for their food stores.

However, the development of vapour compression cycles, based initially on ethyl ether, ammonia or sulphur dioxide, but superseded by chlorofluorocarbons (CFCs). These led to a much more favourable thermodynamic efficiency which led to the gradual replacement of the majority of air cycle systems, except in the field of aircraft air conditioning.

With a fuller understanding of the potential of CFCs for environmental damage, and of the limitations of vapour compression systems, attention has once again focused on alternative refrigeration cycles. The use of air is one of these, offering a benign substitute for CFC refrigerants as well as reduced energy consumption and capital costs for targeted applications.

Further benefits may be gained from its special performance characteristics. For example, in the field of meat chilling evaporative weight loss from the meat is worth between 20 and 50 times the cost of the energy consumed. In many countries powerful refrigeration is employed with low air temperatures and high air velocities to both cool the meat rapidly and increase yield. Although the increased yield reduces the cost of the chilling operation, it is only achieved at the expense of increased energy consumption, capital and running costs and frequently a deterioration in product quality. Alternatively, systems have been developed which use high humidity air, but their commercial application is limited by the constraints imposed by the performance of the refrigeration plant currently available. By contrast, using air cycle, supersaturated air at chill temperatures can be produced, extending the range of conditions and enabling greater overall cost savings than when using rapid chilling, without damaging product quality.

Environmental control in buildings

Until recently the use of air cycle has been largely restricted to aircraft cabin air conditioning systems. A recent trial has demonstrated the advantages that air cycle technologies can offer to passenger train air conditioning systems. An important conclusion of this trial was that air cycle train air conditioning systems will have lower overall life cycle ownership costs than comparable vapour compression systems. The successful demonstration of these units in Germany’s ICE2.2 high speed trains by Normalair-Garrett Ltd. led to the company receiving the Engineering Council’s Environmental Award for Engineers in 1996.

Studies carried out by the Buildings Research Establishment (BRE) and frperc have demonstrated that air cycle systems in buildings would have a number of advantages. These include -

•Limination of the need to use environmentally damaging CFC, HCFC or other alternative refrigerants in building air conditioning systems

•Use of high grade heat recovery from air cycle cooling systems resulting in lower energy consumption

•Improved reliability and reduced maintenance compared with conventional systems

•Maintenance of near full load efficiency at part load conditions

•No susceptibility to refrigerant leakage

Further work is now taking place to develop a practical system to provide heating, cooling and hot water services in buildings which is funded by the UK Department of Trade and Industry, DTI Sector Challenge. As part of the dissemination activities a full scale air cycle system will be built as a demonstration unit. This is a crucial part of the project and the most effective way of promoting this new technology. The work to construct the unit is being carried out by frperc with substantial 'in-kind' help from Allied Signal Normalair-Garrett who will donate a working prototype train unit (see picture) to be used as the basis for the demonstration system. The work is being overseen by a steering committee made up of construction small to medium sized enterprises (SMEs).

Previous work has demonstrated that for building applications, air cycle refrigeration is best applied by utilising both the cooling and heating features available. Although the cold air provided by the air cycle system could be directly applied for building cooling, it is less easy to utilise the hot air rejected by the intercooler. Although central all-air systems were common in the earlier days of air conditioning, the style of buildings has changed, especially where glazing areas are concerned, making it necessary to provide both heating and cooling for much of the year, to suit the demands of different zones within the building. Dual-duct (hot and cold) high velocity air conditioning systems were popular during the 1960s, but their use has largely been superseded by systems requiring even smaller distribution pipes. One type of installation which is now being widely used is the variable refrigerant flow rate (vrf) system. In this system refrigerant fluid is circulated around a building to cassettes which can provide either heating or cooling. Yet another system circulates water to heat pump/refrigerator units which use the water either as a sink, when refrigerating, or as a source, when heat pumping. Perhaps the most appropriate option to use with air cycle systems is that in which chilled and hot water are circulated to fan coil units within the building, thereby providing cooling or heating, as required. Another option is to use air cycle systems in buildings with high hot water requirements, including hotels, leisure centres and hospitals.

A detailed study conducted by frperc and the BRE showed that an air cycle system could meet a range of building's heating and cooling demands. The equipment selected was a 100 kW water chiller incorporating heat recovery. The system was designed to provide chilled water at 6°C and hot water at 81°C across all load and ambient conditions.

The results indicated that the system could be easily controlled to provide 81 kW of cooling coincident with 210 kW of heating. At the summer design case of 100 kW of cooling , a requirement of 10 kW water heating was assumed. The design was technically viable, incorporating a centrifugal compressor powered by an electric motor, and a centrifugal compressor on the same shaft as the expansion turbine (Bootstrap unit). However, for a 100 kW cooling capacity the turbo-machinery required is larger than that used in aerospace environmental cooling, but not as large as that used in the chemical process sector. An additional part of the project is to disseminate information via a free buildings air cycle special interest group which will provide members with information about the progress of the project and invitations to attend workshops throughout the project (see clubs for more infromation).

Food freezing system

Currently frperc are working on the design, construction and installation of an air cycle fluidised bed freezer for food freezing. The air cycle plant will operate with air as the refrigerant delivering it to the freezer bed at -75°C.

Fluidised beds have a number of useful characteristics. Heat and mass transfer rates to and within the bed are high and there is a good uniformity of treatment of the particles to yield high quality individually quick frozen products. Freezing food faster can increase turnover on an existing footprint, reduce the freezing cost and produce a higher quality of frozen food. Freezing food with an air cycle refrigeration plant has two advantages;

•The air can replace toxic, inflammable or environmentally unfriendly refrigerants and replace it with a safe and replaceable refrigerant

•It is capable of producing freezing temperatures far colder than vapour compression plant for less energy consumption, size and cost. Freezing temperatures as low as those produced by cryogenic refrigeration are possible but without the high running costs and energy consumption inherent in such systems.

The project will be carried out by three partners, the University of Ancona, a group of SMEs close to Ancona and frperc. The contract is supported by an Italian Government grant which helps support developments by SMEs. The project has three phases -

Phase 1 - prototype design.

This stage will define a specification for the freezing process. frperc will be responsible for providing design data on the process freezing times versus freezing temperature and air speed. These will be produced by a combination of computer simulation and practical measurement. A mathematical model will be developed to model heat transfer in different shaped food (sphere, cylinder, slab) at the low process temperatures achieved using air cycle.

In addition frperc will be involved in assessing different system configurations and their ability to meet the process specification. Assessments will be based on practicality, energy efficiency and availability of equipment.

From the above information the freezer specification will be developed. The air cycle refrigeration plant will then be designed and drawings and specifications produced for the equipment and fabricated components.

Phase 2 - prototype construction.

During this phase the components specified for the freezing plant will purchased. FRPERCs role will be to find suppliers of the air cycle equipment or companies that can manufacture one off components should this be necessary and to assist in the negotiations and inspection of equipment once delivered.

The prototype system will be assembled at a testing facility at one of the company sites near Ancona under the supervision of FRPERC and the University of Ancona. Once built tests will be carried out to measure the performance of the system and further developments carried out if necessary.

Phase 3 - plant installation and performance monitoring.

The air cycle plant and freezing tunnel will be moved from the test facility and reassembled at the company's production site. The plant will be fully commissioned and performance checked under full operational conditions. The performance of the plant will be monitored for 16 weeks to assess temperatures, pressures and flow rates of the air, food temperature and throughput, energy consumption and reliability. The results will then be evaluated and compared to those predicted.

Results to date. A small fluidised bed has been built and tests carried out in order to determine values for the heat and mass transfer coefficients between the air and different food products (peas, carrots, prawns, squid, fish slabs). The mass and volume flow required for fluidisation, which depends on the product, the product bed height, and the air temperature has also been measured. The graph above shows air velocities with different bed heights that were achieved with frozen and fresh product.

Prawns and fish were very difficult to fluidise as they required greater velocities. Peas were the easiest product to fluidise. Fresh product has been found to require greater air flows than frozen product to fluidise the product. When product was fresh the measured heat transfer coefficients were higher because of mass transfer from the surface of the product. Once the product was frozen, the air flow required to fluidise the product was lower and the heat transfer coefficients measured were lower due to freezing of water in the products which restricted mass transfer.

Based upon these results, food freezing times will be determined in order to finally design the fluidised bed freezer. The unit will be built in partnership with the University of Ancona and ANCOOPESCA Spa, Italy. frperc are currently designing the air cycle system to deliver the required mass flow and air temperature and are sourcing turbo-expander equipment to meet the required duties.

CFC free heat pumps

frperc are a partner in a European Commission JOULE programme to develop heat pump systems for heating and cooling of buildings. The project partners are the Netherlands Organisation for Applied Scientific Research (TNO), the University of Ljubljana, Atlas Copco and Kugl Tekniska Hogskolan (KTH).

The objective of the project is to develop heat pump systems, to be used in existing as well as new buildings, using air as the environmentally benign working fluid to improve the primary energy ratio of heating and cooling systems. The project is divided into four main phases - generation and development of background knowledge and general design tools, development of innovative air cycle components and the demonstration and evaluation of a closed and an open air cycle heat pump system.

The development of background knowledge and general design tools will be carried out by TNO and frperc who will use models to predict optimal system designs in relation to specific building applications.

At the same time as background knowledge is being generated TNO, Atlas Copco and the University of Ljubljana will be working to improve innovative air cycle components. In addition KTH will carry out an investigation into a novel air cycle concept which does not use turbine technology.

To improve the efficiency of air cycle systems the (isentropic) efficiency of the rotating equipment (expanders and compressors) is crucial. High efficiency equipment is available in other application fields such as pressurised air systems and energy recovery systems but conditions of operation differ largely from air cycle applications. Atlas Copco will therefore develop high efficiency and cost effective rotating equipment based on existing equipment design for other application fields, specifically targeted at air cycle applications.

The efficiency of air cycle systems is also affected by the efficiency of the heat exchangers. Although some development in this area has been carried out in previous projects, further improvements of equipment currently available are possible concerning internal friction losses and local air flow resistances. The University of Ljubljana will carry out work to develop novel and innovative heat exchangers.

The project will ultimately build and demonstrate an open and a closed air cycle heat pump system. frperc will be responsible for building the closed system and TNO the open system. The work that frperc will carry out will use as a basis a Normalair-Garrett air conditioning pack developed for high speed trains. The main components will be the turbo-machinery, high speed, high efficiency electric motor and air-air, and air-water heat exchangers plus an air-air recuperator. The test rig will be fully instrumented to enable measurement of energy consumption, temperatures, pressures, flow rates, humidity and rotational speeds. Controlled loads for both heating and cooling, using either air or water, will be supplied by fans and pumps. A range of high and low side heat exchangers will be used to assess the flow configuration, fluid path and pressure drop effects on the cycle performance. The practical data will then be used to verify the models developed earlier.

The rig developed by frperc will be controlled to operate across a range of building heating and cooling loads from 100% heating, 0% cooling to 100% cooling, 0% heating. As few buildings have constant loads different methods of system control must be investigated by adjustment of the mass flow rate, pressure ratios, machinery speed and circuit pressure. At the same time TNO will work on developing an optimal open air cycle heat pump. Using the data generated from the modelling parts of the work TNO will design and build a prototype system. The rotating equipment developed by Atlas Copco will be used as part of the system.

Both test rigs will be used to obtain data on energy usage and related carbon dioxide and refrigerant emissions for air cycle heat pump systems which can then be compared to conventional plant used in building applications. The project aims to produce several feasible concepts for air cycle heat pump systems for heating and cooling buildings which will be proven by experimental results in prototype rigs. The involvement of industrial partners in the project should help ensure that results are exploited and that commercialisation of the systems developed may begin.

Supermarket retail display cabinets

Air cycle refrigeration has been installed in two purpose-built supermarket display cabinets, a chilled multi-deck and a frozen well, and developed to satisfy standard performance specifications. The AFM LINK programme utilises a system in which the expansion of compressed air through a turbine delivered power to a shaft which was absorbed by a fan moving air external to the refrigerant air stream; such a machine is commonly called a ‘turbofan’. The temperature of the air leaving the turbine depends on the amount of power generated and the inlet temperature; generated power depends on the pressure ratio across the turbine, which is established by the supply from a remote air compressor.

Cold air can be used to provide refrigeration in one of two ways. It can be passed through a heat exchanger to cool the space air, without mixing with it - a ‘closed’ system closely resembling vapour compression systems; alternatively it can be delivered directly into the cold space, thereby eliminating the need for a heat exchanger. The latter ‘open’ system is particularly suitable for application to supermarket display cabinets, and has been used throughout the present programme (see picture above). The cooling duty provided by an air cycle system depends on sensible heat transfer. It is therefore affected by air flow rate, the turbine delivery temperature, and the exit temperature from the heat exchanger or cold space. Air cannot be bettered as a 'green' fluid having zero environmental impact. However, the Joule (or Brayton) air refrigeration cycle, on which the present work is based, has a theoretical coefficient of performance (COP) below that of the theoretical Rankine vapour compression cycle. It was, therefore, recognised that an energy consumption penalty might be encountered unless the use of air cycle refrigeration could be combined with a reduction of cooling loads.

Chilled multi-deck cabinet.

Typically, the cooling load is 2 kW for a 2.44 m chilled multi-deck cabinet. This is from conductive/convective heat transfer through the structure, fan power required to achieve circulation through the evaporator, radiation from the surroundings and any lighting, heat input to achieve periodic defrosting and infiltration of ambient air (typically more than 65% of the total load). When applying an open air cycle system to a refrigerated multi-deck cabinet, experimental trials have shown that a significant part of the infiltration load can be eliminated. This is possible because the net outflow of air from the cabinet will carry some, if not all, of the infiltration air with it before this ambient air can enter the circulation system.

Using ejector technology to introduce the air to the cabinet, jets of high velocity air are created which entrain a quantity of air either from inside or outside the cabinet cool space (see diagram right), and additionally avoid very cold air impinging on the product, obviating the need for fans.

As the energy is removed during expansion in the turbine prior to the air being released from the ejector nozzles, there is no requirement for a heat exchanger to remove the heat load. This allows greater flexibility with regard to space and the application of cooling within the cabinet.

Initial development work simulated typical directions and mass flows of air within conventional cabinets. The results indicated that lower air temperatures could be achieved by keeping air velocities over the shelves as low as possible. In conventional cabinets it is common to utilise one or more high velocity air curtains to isolate the cooler internal air. However, the air curtain itself acts as an ejector which entrains ambient air and carries it, by recirculation, to the evaporator. The application of a positive input of air suggested that an air curtain would not be necessary when using open air cycle. This formed the basis of the purpose built cabinets supplied by one of the industrial partners in the programme.

In the multi-deck cabinet, a minimum expansion temperature of -20°C was selected as providing the best compromise between refrigerant air flow and acceptable mixed air temperature after the ejectors. To prevent the formation of ice or snow in the ducting between the turbine and the ejector nozzles, the primary compressed air supply was equipped with a cycling absorption dehumidifier capable of ensuring a free air dew point of less than -50°C.

The only frost encountered was located around the outside of the nozzles, from the infiltration of a small amount of ambient air. The very dry air leaving the nozzles ensured that their orifices remained open by evaporating any nearby ice, therefore no extra energy was required for defrosting.

Frozen well cabinet.

The thermal load characteristics of a frozen well are significantly different from those of a multi-deck. Convective/conductive and radiative heat transfer take precedence, with infiltration of ambient air being of much less significance. One especially valuable characteristic of air cycle refrigeration is that very low delivery temperatures can be achieved if recuperation is used. After being cooled to near-ambient temperatures, the compressed air from the source is further cooled by air leaving the cold space, the temperature drop across the turbine remaining essentially unaltered so that the temperature of the air leaving the turbine will be depressed by whatever temperature reduction occurs in the recuperator. The use of a turbofan is particularly beneficial in this type of system, because the air from the cabinet can be drawn through the recuperator by the fan.

The best experimental performance was achieved by introducing the air into the cabinet along the front of the well, and extracting the recuperation air from the back. Although this meant that the temperature of the air entering the well was substantially lower than the maximum allowable product temperature, the rapid mixing of entry air with air within the well rapidly dissipated these very low temperatures.

Although the air leaving the recuperator is cold, when it is discharged back into the surroundings it is above ambient temperature, offsetting any cooling effect due to the presence of the frozen well.

Noise levels.

The noise level encountered during the initial stages of the work was approximately 95 dBA, requiring ear protection. Noise levels were reduced by ensuring minimum pipe lengths and bends. In both cabinets the ducting for the turbine and fan were independent. From this it was established that there were two sources of noise. The turbine generated noise at a very high frequency (peak 1-2 kHz), but the fan generated a coarser noise at lower frequencies (peak 200-400 Hz). A purpose-designed silencer box for the fan outlet was designed and constructed and, after development, noise values were close to 70 dBA, which is the design target for commercial cabinets.

Results.

The prototype versions of each cabinet operated within the temperature specification. Mean air temperatures in the multi-deck cabinet varied between -0.5 to 4.5°C. This equated to a cooling load of 0.57 kW/m. Mean air temperatures in the frozen well were in the range -28 to -45°C. This equated to a cooling load of 0.32 kW/m. These cooling loads are based on maximum performance and not on averaged values taking account of control. The effectiveness of the recuperation was 60%. This performance was achieved with a turbine efficiency of 56%. The turbofans used to provide the cold air were ex-aircraft units, and were therefore not representative of the most recent technology. The current level of technology in small commercial turbo machinery allows efficiencies of up to 85%. Improvements in turbine efficiency and cabinet design will facilitate reduced supply pressures and therefore reduce operating costs.