Chances and Limitations of a Hybrid Refrigerant System for Vehicle Air Conditioning

According to the motor vehicle directive of the European Community, as of January 2017 new cars will only be allowed to use air conditioning systems with refrigerants of GWP below 150. A moratorium to significantly extend this deadline is not to be expected considering the recently negotiated F-Gas-Regulation. One option will be flammable refrigerants of the ASHRAE classification A3 or a future classification A2L. However they are subject to safety reservations when used in systems with direct evaporation. R744 requires for a wide use and a complete realization with development, industrialization and validation probably more time than the remaining available time horizon. Therefore an option shall be discussed, which is based on a hybrid solution with R744 and a flammable refrigerant and which may allow a sufficiently rapid implementation. The utilization of R744 refrigerant mixtures shall not be considered. Instead a small and compact refrigerant loop will be operated with A3 or A2L refrigerants. Compactness, leak tightness and significant reduced charge amounts already contribute to the enhanced safety. R744 will be used as a secondary refrigerant. For the distribution in branched systems, new approaches have to be discussed. Additional safety aspects arising from the use of R744 as secondary refrigerant and at the same time in the function as fire extinguishing medium. Efficiency, safety, time-to-market and scenarios for the possible full transition to R744 as a refrigerant will be discussed

Theoretical and Experimental Investigations on the Recirculation of the Solution (Mix-Flow) in an Absorption Resorption Refrigeration System

Ammonia water absorption resorption refrigeration systems do not need a rectification column like conventional absorption refrigeration systems do. In this case such systems need a recirculation of the solution. This so called “mix flow” is necessary to prevent the dilution of the solution in the “cold cycle” of the system which is caused by the relative high concentration of water in the hot vapor. The influence of this recirculation of the solution on the performance of the system could be shown in previous studies and is now modelled and investigated more in detail. As a result of the theoretical investigation, a preferred internal piping layout, especially for the mix flow pipe, is proposed. In addition, the theoretical foundations are verified with extended experimental investigations at an existing test cycle

Noise Effects In Capillary Tubes Caused By Refrigerant Flow

The preferred cooling process for household refrigeration appliances is a vapor compression refrigeration process with a capillary tube as expansion device. The vapor compression refrigeration system requires a phase change of the refrigerant inside the condenser and evaporator. Through a direct connection of condenser outlet and capillary tube inlet, which is commonly practiced for household refrigeration cycles, the condenser outlet defines directly the refrigerant state at the capillary tube inlet. Due to unsteady operation conditions the refrigerant state can change from subcooled liquid to saturated liquid with partially a vapor phase at the capillary tube inlet. The refrigerant flow inside the capillary tube is either adiabatic or non-adiabatic (by utilizing internal heat exchange). In both cases the refrigerants state changes during the expansion with an increase of vapor quality towards the capillary tube outlet. A variable vapor quality at the capillary tube inlet causes different flow patterns, especially at the capillary tube outlet. These flow patterns change periodically depending on the refrigerant state at the capillary tube inlet. Associated with the periodical changing flow patterns the occurrence of noise effects with the same periodicity and remarkable variations of the sound pressure level can be observed at the capillary tube outlet. Â This paper presents the experimental investigations on the simultaneous occurrence of refrigerant flow patterns and corresponding noise effects at the outlet of a capillary tube installed in a refrigeration test cycle. The discussion of the experimental results leads to an explanation of causal relation between distinguishable flow patterns and corresponding noise effects

Pneumatically Driven Environmental Control System In Aircrafts Based On A Vapor-compression Cycle

The environmental control system is an essential subsystem of an aircraft, which has to comply some vital tasks. These include the setting of the desired temperature, humidity and cabin pressure, as well as ensuring the fresh air or oxygen supply and air quality. The requirements must be achieved in very different ambient conditions. This refers to the environmental conditions in the flight and ground cases, and also to the different climate zones. In addition, the existing interfaces to the aircraft must be taken into account. State of the art in currently used commercial aircrafts is an environmental control system, which is based on the air cycle and is driven by bleed air from the engine. This technology allows only a significantly lower coefficient of performance for the given boundary conditions compared to a vapor-compression refrigeration system. Therefore the potential application of the vapor cycle in the aircraft air conditioning system is investigated in a research project with the cooperation partner Airbus. The new system has to be developed as a replacement of the existing air cycle system. In a detailed simulation model the thermodynamic state variables are calculated for every element and different operating cases. The calculation results are then used to investigate and size the main components, e.g. heat exchangers and turbomachinery. After the optimization of the system parameters and the individual components the new system is compared with the existing system. The benchmark shows a considerably decrease of bleed air mass flow for the reference cases on ground and in flight, both at hot ambient conditions

Design of a cascade refrigeration system for applications below -50°C using CO2-sublimation

Trifluoromethane (R-23) is currently the most widely used non-flammable refrigerant for applications down to about -80°C (193K). However, R-23 has an enormous global warming potential (GWP100=14800) and recent price increases lead to a loss of attractiveness in industry. As an alternative, some studies in recent years indicated the possibility of extending the application temperature of the natural refrigerant carbon dioxide (CO2) beneath its triple conditions in a refrigeration cycle. Thus there would be a great potential for CO2 to replace R-23. In present paper, the design of a cascade refrigeration machine using CO2-sublimation is introduced. R-452A is used as refrigerant for the upper stage. The lower stage can be switched between the CO2-sublimation cycle, featuring a two-stage compression with intercooling as well as an internal heat exchanger, and a conventional R-23 cycle. Both the CO2-sublimation as well as the R-23 lower stage cycles can be compared directly under the same ambient conditions. Stationary process simulations were made in order to compare the CO2 and R-23 cycles under various boundary conditions. It turned out that the overall system efficiency of the CO2-sublimation cycle could exceed the conventional R-23 cycle in many operation points. The design of this cascade refrigeration machine is currently used for building-up a demonstrator to prove the concept experimentally

An Investigation Into The Dynamics Of Self-Acting Compressor Valves

In principle, a reciprocating compressor stage consists of a cylinder, the working volume which varies periodically due to a piston moving inside a liner. Latter is sealed by two sets of valves. On the one hand the suction valve for admitting the gas to be compressed and on the other the discharge valve for allowing the high pressure gas to be delivered to the downstream process. The typical type of valves used within compressors for refrigeration plants are reed valves. The key feature of these mentioned valves - simultaneously affecting both their sealing performance as well as the reliability - is that they are not actuated. They are held close by elastic forces and they open and close automatically, in accordance to the balance of gas pressure forces as well as the previously mentioned elastic ones. The former is primarily characterized by the static pressure difference across the closed valve and the aerodynamic drag brought about by the gas flow impinging onto the sealing reed in a partially or fully open valve. Evidently, a high degree of coupling between the gas flow and the sealing element motion, today referred to as fluid-structure interaction (FSI), can be expected. The reed and the absence of a reliable guide that would provide for the plan-parallel motion give rise to non-parallel impacts with other parts of the valve assembly, leading to dynamic stress effects. This not only complicates the prediction of the reed valve dynamics, it also causes premature sealing element fracture, decreases the compressor efficiency and ultimately leads to unscheduled maintenance shutdowns. Consequently pursuing the optimization of the reed valve movement is a necessity. The measurements of reed valve dynamics as well as their movement in compressors can be performed by commercial proximity sensors, strain gauges or in house developed sensors. Although timing and lift information is available through these methods, other modes of fluttering such as torsional movement are more difficult to be observed. This work tries to address these aspects by providing real operation data obtained in a test rig. An in-depth comparative analysis of three measurement methodologies, represented by an optical, resistive as well as an eddy current sensor system will be carried out regarding their possible adoption in a state-of-the-art refrigeration compressor. Furthermore, the acquired data will be evaluated with respect to its potential application for collecting mentioned torsional movements. In the end a comparison against a numerical approach shall be conducted

Surface Tension Of Low-viscous Lubricants In High Pressure Carbon Dioxide Atmospheres

The optimization of refrigeration systems can be achieved by increasing the isentropic efficiency of the compressor. One possible way is the reduction of the friction and leakage losses which occur at the piston rings. The viscosity and the surface tension of the lubricant must be known for this purpose. From the literature only a small amount of measured values for the surface tension of commonly used lubricants are known. Additionally natural refrigerants like R-744 are more common according to the latest (European) environmental regulation. The thermophysical properties of lubricant-refrigerant-mixtures can currently only be calculated by generalized calculation methods. For specific mixtures, these equations need to be confirmed by measurements. In this paper the measurement results of the surface tension of different low-viscous lubricants are shown. A test bench was designed to measure the surface tension of different liquids especially under high pressure atmospheres. The investigation was performed by using the pendant drop method and analyzing the results with new algorithms for the solution of the Young-Laplace equation. Effects of the geometry of the used capillaries are discussed which leads to possible optimizations of this measurement method. Dilution effects of the surface tension of the lubricants in carbon dioxide atmospheres are shown and discussed. The effect of higher pressures and different temperatures on the surface tension is shown as well as a comparison of the measurements with generalized methods

Energy Saving Potential of a Temperature Test Chamber by implementing a Heat-Pump

Energy savings become more and more important – also for the rather small industry sector of environmental simulation. So far the technical focus was mainly to realize challenging test cycles which are often gives as temperature profiles. The energy efficiency of the test devices are not specified by any standard. Nevertheless, especially test cycles with intermittent cooling and heating over a wide temperature range need a substantial electrical energy input. As state-of-the-art technology for cooling a vapor compression cycle and for heating an electrical heater is used. In this work a standard temperature test chamber with a specified temperature range from -75 °C to 180 °C using a cascade cooling system and an electric heater is investigated. As a first step the baseline chamber was investigated experimentally and the energy consumption quantified. In the next step the refrigeration cycle was modified with a heat pump capability as well as further cycle modification that indicated energy saving potential in preliminary studies. A comparison to the baseline chamber is carried and reveals substantial energy saving potential

Experimental Investigation of a Heat Pump Tumble Dryer with a Zeotropic Refrigerant Blend

The restrictions on the use of hydrofluorocarbons with a high global warming potential (GWP) are increasing on a global scale. They can be cost-driven or politically enforced. In Europe, the EU regulation 517/2014 restricts the refrigerants currently used in household appliances, such as R-134a, so new systems and working fluids must be investigated as replacements. The trend in domestic appliances with heat pumps, such as tumble dryers, is towards using propane as a refrigerant. This paper presents an experimental comparison of different refrigerants in a heat pump tumble dryer test rig. The original working fluid is R-134a, and the tests performed investigate a zeotropic refrigerant blend. The retrofit of the system for each new refrigerant is minimized. Modifications made to the system focus on adapting the throttling device to create a stable operating point. The cycle performance is evaluated using various parameters such as refrigerant mass flow, compressor input power, energy consumption and the duration of a standard drying cycle

Experimental Analysis Of Natural Refrigerant Blends For Household Application

Hydrofluorocarbon (HFCs), due to their high global warming impact, are not considered a desirable solution for future household application. In accordance to the Kigali Amendment to the Montreal Protocol, numerous countries committed the significant face- down of the production and consumption of HFCs over the next two to three decades. Natural refrigerants as well as their blends are considered viable alternatives due to their low global warming potentials. Additionally, the use of non-azeotropic refrigerant blends can positively influence the cycle performance of refrigerators and heat pump tumble dryers. Moreover, the knowledge of the behaviour of the oil-refrigerant-mixture is a necessity to enable a more precise component design and dimensioning. While most pure refrigerants are experimentally well investigated, the properties of specific refrigerant blends and oil-refrigerant-mixtures need to be confirmed by measurements. This paper describes the experimental investigation of a mixture of a propane (R290) / iso-butane (R600a) refrigerant blend and a mineral oil (MO) for application within household appliances. Several test benches were built to investigate the properties of interest, such as vapour pressure, viscosity as well as densities of the mixtures. Furthermore, the experimentally investigated properties are used to validate previous component design calculations