The invention relates to a cooling device, in particular for use in motor vehicles.
Modern motor vehicles are often equipped with airconditioners containing a cooling device. The cooling device is used for cooling air, which is supplied to the passenger compartment, or is recirculated therein. In connection with this, the demand for cooling output is subject to great fluctuations. If the vehicle has been heated by the rays of the sun over an extended period of time and is subsequently put into operation, a large cooling output is demanded from the cooling device. With low ambient temperatures, however, the cooling device need only operate at low output. Therefore the cooling device requires a large control range, wherein it is required that it operate stably with any output requirement within the control range.
As a rule, cooling devices have a compressor driven by the vehicle engine. Thus, the number of revolutions of the compressor is inevitably the result of the number of revolutions of the engine. The cooling device must operate stably, independently of the respectively actual number of revolutions of the compressor.
Lately, alternative coolants have been tested, for example CO2 (carbon dioxide), hydrocarbons or others. The employment of such coolants often requires an uncommonly high operating pressure in the cooling devices, for example more than 20 bar. Depending on the layout, operating pressures between 70 and 150 bar can occur. The respective compressors are designed for the required operating pressure and are also equipped to build up the required pressure difference. However, they react very sensitively to impermissible operating conditions. Such a compressor is designed to aspirate coolant in the form of a vapor and to also pass it on in the form of a vapor to the condenser. If the aspirated vapor contains drops of fluid, these can damage the compressor. For example, the inlet valve can be damaged. This must be prevented in all operational states of the compressor, i.e. at a high, as well as at a low number of revolutions, and at high cooling output, as well as low cooling output.
The object on which the invention is based, to produce an operationally dependable cooling device, is derived from this.
This object is attained by a cooling device in accordance with the characteristics of claim 1. Moreover, by means of the method in accordance with claim 14 a method is disclosed, which can be used to generate cold air in an operationally dependable manner.
The cooling device in accordance with the invention has a coolant circuit containing a section leading to the compressor, in which the coolant is essentially present in the form of a vapor. The collector provided there removes fluid which has possibly been carried along by the coolant vapor. This can be droplets of fluid coolant as well as oil droplets. Coolant droplets can be contained in the line coming from the condenser if incompletely vaporized coolant is carried along with the vapor coming from the condenser, or if coolant recondenses in the line leading away from the condenser. Moreover, oil droplets can be contained therein. Besides the coolant, the coolant circuit can contain a defined amount of oil circulating with the coolant for lubricating the compressor.
The condenser provides wet coolant vapor, which can carry droplets of fluid along. These are precipitated by the collector to a large.extent, i.e. in a quite predominant amount, and are collected in a reservoir. The coolant is therefore conducted from the collector to the compressor in the form of a vapor, which is relieved to a large extent of the fluid phase. As a rule it is possible to provide a heat exchanger (a so-called interior heat exchanger) between the collector and the compressor, which slightly warms the coolant conducted to the compressor. In this way the coolant vapor arriving at the compressor is dry in every case and contains no fluid particles.
An inlet conduit, which is regulated by a control device, is provided in the cooling device in accordance with the invention between the reservoir of the collector and the section leading to the compressor. Fluid coolant is added in a controlled and tolerable manner to the vaporous coolant via the inlet conduit, but only so much that the added fluid coolant will become completely vaporized in the remaining section from the inlet location to the compressor, or at least does not form large drops. The coolant is preferably introduced into the section upstream of the above mentioned interior heat exchanger, so that it has a chance to become vaporized in the interior heat exchanger. A control device is provided for controlling this process, which monitors the state of the coolant (in particular the phase state) via a sensor device.
The cooling device in accordance with the invention conducts the condensate, which was precipitated in the collector and collected in the reservoir and as a rule contains oil, back into the section leading to the compressor. In this way it is assured that the connected compressor receives the required amount of oil as continuously and steadily as possible, and that on the other hand no fluid particles can cause its destruction. The reservoir in the collector acts as a buffer, which receives excess coolant and oil if the arriving coolant vapor contains too much of the fluid phase. In this case the reservoir in the collector is slowly filled with fluid coolant and with oil, while only the required and admissible amounts are returned into the section via the inlet conduit. It is assured in this way that the compressor does not run dry. However, if the coolant vapor coming from the condenser does not contain a fluid phase and the vapor is already dry, for example in the case where the condenser must cool very warm air, no fluid phase is precipitated in the collector. But it is nevertheless possible to introduce fluid coolant and oil from the reservoir into the section, for one, in order to empty the reservoir again, and also to supply oil to the compressor. The cooling device in accordance with the invention can make do with a smaller amount of stored oil than is customary, if this is desired. The return flow of fluid from the collector into the section not only prevents the entry of fluid into the compressor, but furthermore provides the prerequisites for stabilizing the operating point of the cooling device. This in particular in view of the vapor quality (pressure p, temperature T) of the vapor aspirated by the compressor.
It is therefore possible to operate the cooling device in accordance with the invention over a very large operating range without the appearance of excess wear or damage, in particular to the compressor.
An apparatus for slightly heating the coolant coming from the condenser is preferably provided between the collector and the compressor. This apparatus can be, for example, an interior heat exchanger, in which the coolant aspirated by the compressor exchanges heat with the coolant released by the condenser, which is at least at ambient temperature, before it is conducted to the expansion valve. In this way the expansion valve receives pre-cooled coolant on the one hand, and on the other the compressor as a whole operates at a higher temperature level, which in turn increases the efficiency of the cooling device as a whole.
Moreover, the employment of an interior heat exchanger is also advantageous in that appreciable amounts of coolant and oil can be provided to the coolant vapor coming from the collector upstream of the heat exchanger, which undergo an intimate mixing with the vaporous coolant in the heat exchanger, as well as warming, so that the mixture as a whole arrives in vapor form at the inlet of the compressor. However, in principle it would also be possible to introduce the fluid coolant into the section downstream of the heat exchanger, provided the section leading to the compressor is sufficient for the re-vaporization of the fluid coolant.
The control device is connected with a sensor device which is used to detect whether the entire coolant supplied to the compressor is present in the gaseous phase (i.e. as a vapor). In the simplest case the sensor device contains a pressure sensor and a temperature sensor, both of which are arranged at the inlet of the compressor or the outlet of the interior heat exchanger (or between them). If a material which in the fluid phase conducts electricity, but does so to a lesser degree or not at all in vapor form, is used as the coolant, a conductivity sensor can be used as the sensor device. The control device contains a table or a computing module. The phase interface line in the pressure-temperature diagram which describes the pressure and temperature values in which the fluid phase borders the gaseous phase, can be stored as a formula or a table. The computing module can compare the actually measured temperature and pressure values with the temperature and pressure values of the phase interface line and determine in this way whether a sufficient safety margin from the fluid phase has been maintained.
Various control strategies can be based on this. For example, it is possible to introduce the fluid (coolant with oil) kept in the reservoir as continuously as possible in order to keep the fill level in the reservoir as low as possible. In such a case the control device regulates the introduction of fluid coolant into the section possibly in such a way that the required safety margin of the phase state at the compressor inlet from the phase interface line is just maintained. If, however, it is desired to always introduce fluid from the reservoir into the section if possible, so that the compressor is operated as little as possible or never with only a small amount of oil or without any amount of oil in the aspirated coolant, the control device regulates the control member in such a way that a minimal flow of fluid coolant is always introduced into the section, wherein this minimal flow is reduced to even lower values only in those cases where the state of the coolant vapor does not permit this, i.e. if otherwise too close an approach to the phase interface line would occur. If necessary, this control strategy can be supplemented in such a way that the introduction of fluid coolant into the section is increased in those cases where it is permissible and the fill level in the reservoir is too high. Permissibility can be determined on the basis of the measured pressure and temperature values of the coolant vapor at the compressor inlet. A further control strategy can lie in always keeping the distance of the operating point from the phase interface line constant, or in keeping the operating point itself always constant. The latter also includes keeping the pressure constant, which is made possible, if required, by the additional control of the expansion valve.
Monitoring the fill level is possible if a fill level sensor, which monitors the fill level in the reservoir of the collector, is a part of the sensor device in addition to the temperature sensor and the pressure sensor. Because of this it becomes possible in addition to make the volume of the reservoir so small that only a fraction of the coolant contained in the coolant cycle as a whole can be received in fluid form by the reservoir. By means of this it is possible to create a collector which is small and space-saving. Moreover, with appropriately small dimensions it can be designed with only little difficulties for very large burst pressures, which is of particular importance in connection with alternative coolants, such as CO2.