Refrigeration drying is the most important treatment method for compressed air, yet in the past it was a relatively energy-intensive process, with only limited prospects for the integration of energy-saving options. Now, however, exciting new thermal mass technologies enable significant savings when it comes to providing effective refrigeration drying of compressed air for compressor systems delivering up to 34 m³/min and also allow significantly more compact unit design.
Under normal conditions, ambient air contains water vapour. The quantity of water vapour the air is capable of retaining depends on the ambient temperature. When the air temperature rises, the air’s capacity to absorb water vapour also increases. The degree to which the air is saturated with water is described as “relative humidity”. If this normal ambient air is then taken in by a compressor and compressed, the proportion of water vapour per volumetric unit of compressed air also increases. If the temperature of the compressed air then drops, as generally occurs in the compressor’s aftercooler, the compressed air becomes oversaturated, causing the excess water to condense – this water is then termed “condensate”.
More water than expected
The amount of condensate that comes out of compressed air is often underestimated. Let’s take the example of a compressor in Germany. It has a flow rate of, say, 10 m³/min and a working pressure of 9 bar, takes in ambient air at a temperature of 20 °C with 60 percent humidity, the amount of condensate resulting over a 24-hour period will total around 140 litres. If the temperature increases to a high of around 35 °C with 30 percent humidity, the volume of condensate increases to 160 l per day. For similar prevailing temperatures in Asia however, where humidity is often much higher at around 80 percent, the resulting amount of condensate can easily triple this figure common for Germany.
The compressor itself only compresses the ambient air. If the compressed air is not treated properly downstream from the compressor, the condensate it contains will pass unhindered into the compressed air system. Subsequent cooling of the compressed air down to the ambient temperature of the compressed air system causes additional condensate to accumulate on an ongoing basis. This can result in corrosion of the pipework, or even damage production machinery, not to mention potential adverse effects on the quality of the products being produced. For all of these important reasons, it is therefore crucial to remove the moisture directly within the compressed air station itself in order to prevent condensate formation in the downstream compressed air system and reduce the relative humidity of the compressed air down to around 30 percent.
A range of drying methods is available to treat compressed air, each being less or more suitable, based on the requirements of the specific production process in question. Of the many drying methods available, refrigeration drying is the most common, as it delivers sufficient performance and compressed air quality for most uses.
Peak values are decisive
Although temperature conditions vary over the course of the year, refrigeration dryers should always be designed for continuous performance under the most extreme conditions in order to reliably provide the required compressed air quality, even on the hottest days of the year. This means that in terms of their overall drying output, they should be designed to be capable of reliable operation even if peak temperature conditions were to prevail 365 days a year. Of course, since this is never the case, more energy must necessarily be invested in compressed air drying than is actually required to meet actual demand – unless the dryers are equipped with technology capable of adjusting their output to actual daily demand.
If the total energy requirements of a compressed air system are examined, the process of drying the compressed air accounts for only around 3 percent of the total, assuming the station is running at 100 percent of capacity. If this is not the case and the compressed air station is only running at partial load, the compressed air dryer will also be running at less than capacity. This ratio has an especially disadvantageous result when production facilities are operating with just one or two shifts, with the dryers left to dry only the compressed air for smaller consumer points or leaks during periods when production activities are not required.
Letting dryers run continuously
To ensure reliable compressed air quality, it is recommended that compressed air dryers run continuously. Otherwise, the dryer has to be pre-cooled up to one hour in advance of compressor system start-up, depending on the size. Ultimately this means that the more that the compressors are running at anything less than full capacity, the greater the energy waste resulting from a compressed air dryer left running continuously 24 hours per day, configured to deal with maximum temperatures. Under such conditions, the energy requirements for the compressed air dryer can spike dramatically – and account for up to 20 percent of the total energy required for compressed air production.
Thankfully refrigeration dryers have benefited from technical innovations over the years. Up until the early 1990s, efficient refrigeration dryers featured an air/air heat exchanger, in which first the cold outgoing compressed air cooled the incoming compressed air, thereby creating a kind of energy recovery system. At the same time, the consequent re-warming process served to reduce the relative humidity of the outgoing compressed air to below 30 percent and prevented condensate accumulation on the exterior of the pipework carrying the cooler compressed air. After passing through a pre-cooling phase in the air/air heat exchanger, an air/coolant heat exchanger cooled the compressed air down to 3 °C in most cases.
The coolant circuit was often equipped with a “hot gas bypass control”, which diverted coolant back into the circuit when less cooling was required as a result of low air consumption. This system resulted in energy loss to varying degrees since the coolant compressor was always running at basically the same output (to circulate the coolant circuit). The only way to reduce the load somewhat on the refrigeration dryer when the compressor was running at partial capacity, was to switch off the coolant fan; this only yielded minor results, however, since this condenser fan consumed relatively little energy in comparison to the system as a whole.
For large systems, i.e. those with a flow rate in excess of 50 m³/min, a method to adjust refrigeration dryer performance to match actual demand during periods of lower compressed air consumption had been known for some time. This was accomplished using coolant compressors with multiple cylinders, which were switched off individually as appropriate during partial-load operation. This still remains an effective option today for obtaining efficient performance from these large-scale refrigeration dryers during partial-load operation.
Energy optimisation measures
The mid-1990s saw the development of three different methods for improving the performance of refrigeration dryers operating with air flow rates of less than 50 m³/min in partial load:
- “Digital scroll” coolant compressors
- Variable-speed coolant compressors
- Thermal mass dryers
The digital scroll method involved modifying the clearance losses within the scroll compressor used for the coolant, which in turn regulated the flow rate of coolant to adjust it to the quantity required to cool the compressed air. The advantage of this method was that it allowed for a relatively large control range; yet the technical difficulty of implementing it made it less attractive. Furthermore, in addition to a scroll compressor to cover the base load, this method also employed a controlled scroll compressor which was switched off completely during periods of very low demand.
In terms of compressors with variable-speed control, one disadvantage was that it was only possible to regulate the coolant compressor within a relatively small range; it was then necessary to transition again to hot gas bypass control. This system was therefore capable of saving only a small amount of energy during partial-load operation.
Thermal mass delivers greater savings
Buffer dryers were the preferred technology for compressed air flow rates of less than 20 m³/min. Some systems relied on a tank, similar to the way a compressed air receiver works within a compressed air system, to buffer load fluctuations while keeping the pressure relatively constant and reducing compressor switching to a minimum. This means the larger the tank, the smaller the pressure fluctuations and therefore, less switching is required. Other systems relied on a thermal mass, which absorbs heat energy, instead of incorporating a cool air buffer tank.
These dryers generally use mineral materials to store the cooling energy. In order to keep the switching frequency of the coolant compressor within economical bounds and to ensure a consistent pressure dew point, the amount of mass required rises in direct proportion to the system capacity. Furthermore, heat distribution within the thermal mass requires precise regulation.
A limiting factor: excessive weight
Weight considerations impose certain size restrictions on these types of compressed air dryers, although the thermal mass system itself is essentially ideal. It involves no mechanical loads or switching of any type of system, since it is only necessary to switch the refrigeration dryer on and off as needed based on compressed air production, and its operation is extremely reliable. Moreover, when the thermal mass is saturated, the system maintains safety reserves in order to accommodate short-term overload periods. Yet thermal mass dryers suitable for relatively modest compressed air capacities of 17 to 20 m³/min are extremely heavy, weighing some 850 kilos. Until recently, larger systems had to be equipped with digital scroll systems and were precluded from benefiting from the advantages of thermal mass dryers.
A solution thanks to new technology
Thankfully, this hurdle was overcome by a new technology that entered the market just recently, in 2013: a refrigeration dryer equipped with a totally different type of thermal mass – a phase changing material (PCM). Phase changing materials can store and release vast quantities of energy if they are harnessed at the precise point at which they undergo a phase change, such as between liquid and solid states. These materials work according to the same principle by which ice cubes keep a drink cool for an extended period in the summer. The temperature of the drink remains constant as long as the ice cubes remain melting in the glass. They are capable of absorbing a significant amount of heat before melting completely; consider that the same amount of energy is required to change solid ice with a temperature of 0 °C to a liquid as is needed to heat water from 0 °C to 80 °C.
These thermal masses are also known as latent heat thermal masses owing to their capacity to store thermal energy virtually invisibly for long periods with only minor losses and their ability to accommodate any desired repetition cycle. Some familiar applications of this technology include the heat pillows used by some athletes in winter, as well as cool packs that allow refrigerators to continue to provide cooling during periods of power failure, and paraffin-filled storage elements in the tanks of solar-thermal systems.
Paraffin for consistency
Latent heat thermal masses usually employ special salts or types of paraffin as the storage medium since these materials can absorb huge amounts of thermal energy (such as heat of fusion). When the thermal energy is discharged, the thermal mass solidifies. During this process, the thermal mass returns the large amount of heat it previously absorbed back into the environment. The temperature remains constant during the transition from one state of matter to another since all the heat entering the system is invested in the change of state. Just think of the drink kept cool by ice cubes – at normal pressure, a mixture of water and ice maintains a constant temperature of 0 °C. These innovative refrigeration dryers exploit the analogous principle of liquefying and solidifying for thermal management purposes.
At a basic level, these dryers function as follows: when compressed air requires cooling, from a starting temperature of 5 °C, for example, the coolant compressor is switched on. The refrigeration dryer first cools the paraffin to a temperature of around 3 °C while the compressed air cools simultaneously. During this extended period, the temperature remains constant because the paraffin is undergoing a phase change from fluid to solid. Once this process is complete, the material is cooled somewhat more, to around 2 °C. The coolant compressor then switches off the supply current. The compressed air then flows into the heat exchanger, which is surrounded by the solidified paraffin, where the air gradually warms the paraffin, which in turn keeps the compressed air cool as it changes from the solid to fluid state. This process continues until a set maximum temperature threshold is reached, at which point the coolant compressor switches on the supply current and the whole cycle begins afresh.
Greater storage density
Since refrigeration dryers cannot use water as a medium due to its high expansion factor (a sealed water bottle will explode when frozen), the new refrigeration dryers for compressed air employ a paraffin-based system. This material offers the dual advantages of having a low expansion coefficient as well as 98% better thermal density than the materials previously used as thermal masses. This may impose a range of other design requirements on the dryer, but, far more importantly, offers distinct advantages.
The higher storage density of the PCM meant that the heat exchanger in the refrigeration dryer could be completely redesigned. While earlier refrigeration dryers used copper spiral heat exchangers, the first thermal mass dryers relied on plate heat exchangers. The new refrigeration dryers, on the other hand, work with an aluminium heat exchanger that combines these two different heat exchanger systems – an air-air heat exchanger between the cold, outgoing compressed air and the warm, incoming compressed air, as well as another compressed air-PCM heat exchanger. In addition to energy efficiency advantages, this new dual heat exchanger design has also resulted in significantly less space requirement.
While it was important to ensure precise distribution of the PCM within the heat exchanger, the Kaeser engineers additionally succeeded in equipping the system with an integrated water separator. This plays a key role in avoiding pressure losses by reducing the amount of pipework the system requires – a feature which also makes it easier to insulate the entire system. Naturally this keeps any heat or cooling losses to an absolute minimum.
Reduced pressure loss, reduced energy requirement
Moreover, the recently developed thermal mass offers further advantages in terms of energy efficiency. The compact design has allowed pressure losses to be reduced to a mere 0.15 bar, compared to values of 0.20 bar and more characteristic of conventional models. The input energy requirements of PCM thermal mass dryers are also exceptionally low: the system requires less than 87 watts per m³/min to dry compressed air.
Furthermore, the new thermal mass technology means that the entire dryer can be significantly lighter and more compact – the new units require up to 46 percent less installation space and weigh around 60 percent less than conventional thermal mass dryers on the market. These size advantages are made possible by new, smaller components as well as intelligent component layout.
When the heat exchanger system was redesigned, the entire cooling system received a design upgrade along with the air heat exchanger. Now a highly efficient scroll compressor has replaced the previously used reciprocating compressor. The capillary tube often used in refrigeration dryers has been replaced by an expansion valve, a development mirrored in many other refrigeration dryers today. In earlier machines, a clearly graduated capillary injected a specific quantity of coolant; however, this method resulted in poor filling outside of the design point. To balance this disadvantage, capillaries require greater quantities of coolant and higher output from the coolant compressors. Expansion valves completely avoid this disadvantage because they can regulate the quantities dynamically, depending on the load. As a result, significantly less coolant is required and the coolant compressors can run at a much lower output.
The refrigeration dryer’s condenser has been replaced by a micro-channel condenser which features outstanding mechanical stability and excellent effectiveness in accomplishing heat transfer with the cool air. Refrigeration dryer performance is enhanced still further by this innovation. The total volume of coolant required by the systems has also been drastically reduced thanks to the more precise dosing system. A further impressive engineering achievement is the total power consumption of these new refrigeration dryers, which require 50% less power than comparable conventional equipment available on the market.
Internal controller ensures optimal performance
Like other components of any state-of-the-art compressed air station, PCM thermal mass dryers can be controlled to great effect and integrated into a compressed air system. These refrigeration dryers are equipped with a microprocessor controller which regulates and controls thermal mass operation while also delivering a wide range of efficient analysis and monitoring functions. Unit operation is straightforward and intuitive thanks to a generously dimensioned colour display and language-neutral menu navigation; the controller also features a memory bank and error code display functionality while also allowing the user to connect a separate dew point measurement device. Even without it, however, during all load phases the refrigeration dryer maintains a stable pressure dew point and keeps coolant compressor switching to a minimum.
The system also includes a service message feature, P&I diagram and supports communication with master control systems via alarm contacts. This makes it easy to integrate the unit into an overall system involving monitoring, controlling and coordination of the complete compressed air production process. Connecting a master control system allows data from the refrigeration dryer to be analysed, monitored and documented for the long-term via PC. This makes it possible, for example, to perform detailed, long-term evaluation of key data to document energy management in accordance with ISO 500001.
The system is equipped with an automatic condensate drain as standard, so that the resulting condensate can be separated out and subsequently treated in an environmentally-friendly manner. The valves are highly accessible for straightforward maintenance, and the condenser can be cleaned with ease. The new refrigeration dryer models are designed to allow for setup adjacent a wall, further reducing the space required for the whole compressed air station.
Gateway to further development
The recent development of this new thermal mass system serves as a gateway that will enable even larger dryers to be equipped with this innovative thermal mass technology. This will primarily benefit systems in the capacity range beyond the technical limits of thermal mass dryers, for which weight and size considerations have previously made this technology an impractical option. In the coming years therefore, even more models with greater capacities can be expected to enter the market. Although this technology can also be applied to the lower capacity range, the fact that thermal mass dryers are already an excellent solution in this area means that the greater capacity range is expected to be the primary focus of development efforts.