ENERGY SAVING SCREW COMPRESSOR TECHNOLOGY
Foreword
Any activity we carry out during a day is closely linked to energy . Even just turning on a light means to consume the electricity needed to power it. That is what happened to dozens of other small everyday actions now performed almost automatically, without even realizing all the production processes that are upstream and the energy consumption arising from them. All this causes a huge impact on the entire terrestrial ecosystem, whose full extent is not immediately obvious. Only the information provided daily by the media is bringing to our attention the possible catastrophic consequences of continued increase in consumption. The greater consumption results in an increase of polluting emissions into the atmosphere, in particular of the so called greenhouse gases. The challenge for the future is this: create wealth reducing energy consumption and pollution level. In other words be able to achieve the goal of sustainable development. The main sources of energy currently used are fossil fuels, coal, oil and gas. Unfortunately, in all cases, these are non-renewable sources and the processing of which generates pollution. So the problems associated with their use, is concerning the supply costs and environmental impact. In particular the cost aspect is the one who has generated some sort of “new awarness” of the value of energy. The big run of the prices of crude oil in 2008, made many governments decide for a restructuring of their energy politics. Even more important was the change of mentality of operators using energy, having production processes heavily depending on it. They have got a preview of what could happen if the cost of energy is running away, and want to be prepared for the worst. For the motto is: energy saving!
Energy saving potential in the AC&R systems
In a AC&R circuit we can find many parts that need energy to perform their function, ensuring the effectiveness of the refrigerating cycle. The main components are the compressor, in addition to which we find the fans on the heat exchangers, guaranteeing the heat exchange with the air, or pumps, doing it with other heat transporting media. It is a paradox, but the compressor is the last component that started his efficiency optimizing process, even though it is the most energy consuming.
Apart from the introduction of new refrigerants with lower GWP, nowadays the reduction of the CO2-Emissions (and therefore of the energy consumption) is the mandatory issue of refrigeration systems. As the systems are, during the year, mostly working in part load condition, the research and development is since ever strongly committed to seek for the highest possible efficiency exactly in this condition. The new compressor generations are therefore designed for higher part load efficiency, and not for the highest output at 100% condition. Although this aspect is not neglected, it stays now only on the second position of the design priority list.
Up to today the main goal was the simple regulation of the system capacity, without considering the real efficiency of the system at those conditions. Nowadays, due to the mentality change inducted by high energy costs, the main target is to maintain the highest possible degree of efficiency at given capacity needs. The calculation methods have changed. Earlier the compressors were calculated for condensing temperatures of +30-+40°C. Nowadays the technicians try to optimize further their systems running them down to condensing temperatures of +25°C, maybe even +20°C. With such a small pressure differential the design of a compressor, especially of the compact screw compressor type, is of extreme importance. A lower condensing temperature means a higher capacity output, so that the capacity regulation, at the end, has to be proportionally bigger than before. At the very end of the thing, what really matters, what really influences the consumption of a compressor, is the amount of work we have to put in to overcome the pressure differential between suction and discharge pressure. The optimisation of the efficiency is only a function of how much I optimize my electrical and mechanical losses. The first are influenced by the type of motor and the electric network, the others are influenced by the internal fluid-dynamical conditions and mechanization / assembly of the compressor parts.
The state of the art
As we have seen before, the rationalization of the compressors’ energy consumption is the matter with the highest saving potential in a refrigeration system. Until few time ago the only way to capacity regulate a screw compressor was normally through a slide valve system. And here we are immediately hitting on the first cause of reduced efficiency. As we know, every screw compressor is designed with a built in volumetric ratio, that is normally optimized on nominal conditions (considered to be the extreme ones) and at full load. As we can see in table 1, the full load operation condition, according to ASHRAE ESEER calculation method, influences the efficiency only with a factor of 0,03. This means that the calculation assumes that only for 3%
of the operation time the compressor is working at his full load condition. If on top of that we consider that only a small part of this 3% will be performed at design temperatures, than we get a felling for how much time in a year this machine will work at it’s best efficiency. It’s really only a very small fraction of time.
As a matter of fact, the “built in volumetric ratio” is a parameter that tends to be only theoretical. Whenever the screw compressor starts to regulate this value is anyway lost,
due to the movement of the slide valve itself. As you can see in figure 1, the so called Vi (intrinsic volumetric ratio) is a function of the geometrical volume at the discharge port. The smaller the volume, the higher the Vi, and therefore the pressure difference between suction and discharge. Whenever the slide valve starts regulating the cooling capacity, this geometry is lost, and therefore the residual efficiency, too. This is one of the causes for the efficiency drop at partial loads in screw compressors with slide valve control. At this losses you have also to add the ones due to electrical inefficiency and mechanical resistances, to have, as a result, the total loss of efficiency. This argument may seem of secondary importance, but the reality shows us how much influence this parameter has. Laboratory tests are giving a clear picture of how much energy gets lost due to an inappropriate Vi value.
The above graphic is considering a standard compact screw compressor, witch’s Vi is fixed at the production stage. The evaporation temperature is always +2°C, so we could imagine this is related to an air cooled water chiller, as it is used in the most AC applications. In this conditions the normal condensing temperature, considered in the mid- southern European Continent is +50°C. This graph, result of laboratory and extensive field tests, is showing following:
The standard calculation conditions are giving a working point @ +2/+50°C. Therefore the optimized Vi result in the value of 3,2. As we can recognize in the graph, @+50°C condensing temperature the Vi=3,2 the graph curve is very near to it’s maximum.
If this compressor, optimized for the theoretical working point, works at a different condensing temperature, let’s say at +40°C, the efficiency difference, compared to the optimal Vi (2,6), is about 5% in efficiency. If you consider a condensing temperature of +30°C, the difference compared to the optimal Vi (2,2) is about 12% in efficiency.
The same happens, in case of a heat pump, when you have to condense @ +65°C. The efficiency difference is about 5%.
The conclusion is, that in every days operation, the unit, calculated for the nominal working point, is operating at approximately 8-10% less than it’s full efficiency potential. An integrated Vi control is something that could save just 10% of energy, with a minimum of investment. As we know, the easiest earned Euro is the saved one.
The second big issue in increasing the efficiency of compressors is the termination of the electrical inefficiencies. This, as know, can be done with the use of a frequency control, that is varying the rotation speed of the screws, in order to recover at least the energy waste, due to mechanical and electrical losses. A regulation throughout a classical slide valve control allows a good control of the cooling capacity (output), but the power consumption (input) still stays disproportionally over the output. If you cut down to 40% (so -60%) of the cooling capacity, the absorbed capacity in a standard compressor is not decreasing with the same value, but it drops only by 44%, so that the efficiency drops by overall more than 25%.
This phenomena can be easily solved by the use of a VFD (variable frequency drive). This kind of system can operate as a separate system to the compressor, although there are compressors nowadays, that have an integrated VFD. Variable frequency drive controllers are solid state electonic power conversion devices. The usual design first converts AC input power to DC intermediate power using a rectifier or converter bridge. The rectifier is usually a three-phase, full-wave-diode bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit. The inverter circuit is probably the most important section of the VFD, changing DC energy into three channels of AC energy that can be used by an AC motor. These units provide improved power factor, less harmonic distortion, and low sensitivity to the incoming phase sequencing than older phase controlled converter VFD’s.
The advantage of using such a frequency controlled compressor is mainly the proportional cut of cooling and absorbed capacity (what I am not getting as output I am not consuming in input), and in case of integrated VFD compressors, also a higher motor efficiency, with cosφ of approximately 0,99. As you can see in the above graphic, @50% of the capacity the advantage, in terms of efficiency, is about 20%.
The state of the art is therefore a compressor that is integrating a frequency control and a Vi control to. These compressors have been already designed and are in series production, integrating both of the systems, in order to have a fully integrated product, purchasable as a single item, including screw compressor, oil separator, frequency drive and Vi control in one.
Advantages of the integrated Vi controlled VFD-Screw Compressor
The first, and most interesting advantage, is the power saving. As you can read in table no. 2, the efficiency difference of a standard compressor, compared to a last generation compressor, is about 26%. This result is achieved considering the same working method all over the year. Integrating a sliding condensing and evaporation temperature control system, can easily raise this saving up to 50% of the electrical consumption considered as normal until the day before yesterday. But these figures are only considering the operational lifetime of the compressor. A lot ore of consumption happens not during operation, but during the start-up of the compressor.
When a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Thus starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. After the start of the VFD, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while the VFD is drawing less than 50% of its rated current from the mains in the low speed range. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed. Note, however, that cooling of the motor is usually not good in the low speed range. Thus running at low speeds even with rated torque for long periods is not possible due to overheating of the motor. Even here, in the last generation compressors, the cooling of the VD is performed via expansion of the refrigerant itself, so totally disconnected from the outdoor conditions, in a constant manner, all over the temperature range. This integrated soft-starter function is also one of the main advantages of this new generation compressor.
The same efficiency rate can be kept all over the speed range, or capacity regulation range, nearly in a constant way. As you can read in the graphic nearby, the oscillation at a given condensing temperature is limited between 7% and 10%, so almost constant. This is the advantage also of another type of compressor with fascinating technologies, but the application range of these VFD Screw Compressors is far wider than that one, allowing in practices a use the same as a standard compressor (see graphic below).
The compactness of these compressors is saving to operators a lot of wiring, control and space inside of the switchboard. These compressor needs only a drive signal, the rest (protection and control) is performed by the integrated electronics itself.
All in all there is a solution, which nowadays is setting the benchmark of screw compressor technology, allowing to control all aspects (thermodynamically and electrically) of the most power- intensive component of a refrigeration system.
Alexander Möller
RefComp SpA
