Compressed air refrigeration dryer

Abstract

Compressed air refrigeration dryer for dehumidifying compressed air, comprising a refrigerant circuit with carbon dioxide as the refrigerant, comprising at least one compressor (1); at least one gas cooler (3);The refrigerant receiver (11) comprises at least one evaporator (2) configured to dehumidify compressed air passed over its cold surface on the secondary side, a refrigerant receiver (11), and at least one expansion valve (6) or an ejector, characterized in that the refrigerant receiver (11) is connected in parallel to the expansion valve (6) or ejector and has at least one inlet and one outlet, wherein a first control or solenoid valve (12) is provided upstream of the inlet of the refrigerant receiver (11) and a second control or solenoid valve (13) is provided downstream of the outlet of the refrigerant receiver (11), by means of which both the inflow of refrigerant into the refrigerant receiver (11) and the outflow of refrigerant from the refrigerant receiver (11) can be controlled as a function of the refrigerant pressure on the high-pressure side in high-pressure transcritical operation and / or the temperature of the refrigerant on the high-pressure side and / or the cooling water temperature.

Description

[0001] The invention relates to a compressed air refrigeration dryer that operates with a natural refrigerant and achieves high COP (Coefficient of Performance) values. The refrigeration dryer is particularly suitable for drying compressed air, e.g., in industrial plants.

[0002] In many industrial sectors, such as the automotive industry, compressed air-operated tools and equipment are used. Compressed air is generated by compressing air in compressors. Due to the pressure increase, the air's capacity to hold water vapor decreases. To prevent water from condensing during compressed air use, caused by temperature drops in the compressed air distribution network or during expansion in a tool or the operating device, the compressed air must be dried. Without drying, the resulting condensation can cause corrosion in the compressed air system. Furthermore, ice formation during pressure drops and / or at low ambient temperatures can lead to the failure of tools or components of the compressed air system.

[0003] To avoid this, the compressed air is dried to a predetermined pressure dew point, e.g., 3 °C. The following discussion will focus exclusively on refrigerated dryers that operate on the principle of "condensing water from the air on cold surfaces" (so-called condensation dryers).

[0004] In known refrigerated dryers (e.g., DE 10 2010 033 187 A1, WO 2011 / 128317 A1 and DE 10 2004 056 483 A1), the cold surface is an evaporator or condenser surface, with the evaporator / condenser integrated into a refrigeration unit (refrigerated dryer). To improve the process, an air-to-air heat exchanger is installed upstream, in which the warm, humid air coming from the air compressor is pre-cooled and pre-dehumidified. The heat is then transferred to the dried, cold air. Refrigerants typically used are HFCs, such as R134a, or HCFCs, such as R22.

[0005] HFCs and HCFCs are not natural refrigerants, meaning they are not environmentally neutral; HCFCs are also ozone-depleting. Refrigeration machines that use the less harmful carbon dioxide as a refrigerant are described in US 2006 / 0218943 A1, DE 10 2009 020 062 A1 and US 5 099 655 A.

[0006] The invention is based on the objective of providing a refrigerated dryer operated with a natural refrigerant, with which high COP values ​​can be achieved and with which partial or complete heat recovery at a high temperature level is possible if required.

[0007] The object of the invention is achieved by the characterizing features of claim 1. Further advantageous embodiments of the invention are set forth in claims 2 to 10.

[0008] The starting point is a refrigerated dryer for dehumidifying air using a vapor-combustion refrigeration machine (condenser dryer). The refrigerated dryer has (at least) one compressor, (at least) one gas cooler or condenser, (at least) one evaporator designed to dehumidify a medium, e.g., air, passed over one side, (at least) one expansion valve, and, in the case of heat recovery, (at least) one internal heat exchanger located between the suction side of the compressor and the inlet of the expansion valve (i.e., serving for heat transfer between the refrigerant on the suction side of the compressor and the refrigerant before the inlet of the expansion valve).

[0009] Depending on the performance requirements, the components of a refrigerated dryer can also be composed of several individual components cascaded in parallel and / or in series. For the sake of simplicity, however, the following will always refer to individual components; that is, the "at least" will be omitted.

[0010] According to the invention, the refrigerated dryer operates with carbon dioxide (CO2) as a refrigerant. Refrigerated dryers using carbon dioxide as a refrigerant are not previously known. The refrigerated dryer according to the invention has three further significant differences compared to conventional dryers using HFC and HCFC refrigerants.

[0011] Firstly, depending on the cooling water temperature (at the gas cooler or condenser), the refrigerated dryer according to the invention is operated under- or supercritically on the high-pressure side, whereas conventional refrigerated dryers are operated exclusively undercritically on the high-pressure side.

[0012] Secondly, the operating pressures in the refrigerated dryer according to the invention are 2 to 5 times higher than in conventional refrigerated dryers, which do not exceed 10 bar on the low-pressure side and 40 bar on the high-pressure side.

[0013] Thirdly, the refrigerated dryer according to the invention comprises a refrigerant receiver connected in parallel to the expansion valve and having at least one inlet and one outlet, wherein the inflow of refrigerant into the refrigerant receiver and the outflow of refrigerant from the refrigerant receiver can be automatically controlled. The conventional refrigerant receiver, through which refrigerant constantly flows in conventional refrigerated dryers, is omitted.

[0014] Unlike conventional refrigerant receivers with continuous flow, the refrigerant in the refrigerant receiver of the refrigerated dryer according to the invention is always kept below the critical point, which creates a density difference between liquid and gaseous carbon dioxide and thus ensures a high storage capacity of the refrigerant receiver, which increases with decreasing temperature and thus increasing distance from the critical point.

[0015] The advantage of this circuit is particularly beneficial in high-pressure transcritical process control, as the optimal refrigerant charge is automatically set regardless of the operating point, especially when operating points change depending on the heat recovery and / or recooling situation.

[0016] Another advantage of this circuit is that when selecting the components that determine the internal volume (heat exchanger, oil separator, pipes...) there are more degrees of freedom regarding the internal volume of these components.

[0017] This offers the further advantage that the internal volume of the refrigerant circuit can be designed in such a way that the permissible standstill pressure of the system is not exceeded under practically relevant temperature conditions, e.g. in machine rooms with temperatures up to 60°C, and thus no pressurized safety discharge lines to the outside are required, or safety discharge lines can even be omitted entirely.

[0018] Thus, when using overflow valves for pressure equalization between the high and low pressure sides, safety relief lines and, if necessary, safety valves can be completely dispensed with, since the refrigeration circuit is intrinsically safe.

[0019] To ensure that the refrigerant temperature in the receiver remains sufficiently low (below the critical point) during operation of the refrigerated dryer, the receiver can be equipped with a refrigerant cooling system (active cooling). Additionally, the receiver can include a refrigerant heating system. Heating the refrigerant increases the pressure in the receiver, thereby accelerating the refrigerant return to the refrigerant circuit, while cooling the refrigerant (pressure reduction) enables rapid refrigerant withdrawal from the circuit.

[0020] Depending on the application, the temperature control of the refrigerant receiver can be achieved in various ways, e.g. by means of a heat sink / heat source, by heating and cooling using the refrigerant of the refrigerated dryer, or by using a heating mat / external cooling unit.

[0021] The refrigerant receiver can be implemented in various ways, e.g., as a receiver with only a heating mat and thermal insulation, as a receiver with air cooling, as a receiver surrounded by a coil (or with at least one integrated coil) that serves for both heating and cooling, as a receiver with a heating mat and a coil for cooling (refrigerant, external chiller, or using waste heat as a heat source), or as a receiver designed as a shell-and-tube heat exchanger, where the refrigerant is contained in the jacket and the temperature control medium is contained within the tubes. The use of two separate tube bundles, each used for heating and cooling respectively, is also conceivable.

[0022] To control the refrigerant receiver of the refrigerated dryer according to the invention, electrically actuated valves, e.g., motor-driven control valves or solenoid valves, are installed upstream and downstream of the refrigerant receiver's outlet. These valves (with the aid of a suitable control system) regulate both the refrigerant flow into and out of the receiver during transcritical high-pressure operation, depending on the refrigerant pressure on the high-pressure side. During subcritical high-pressure operation, the refrigerant subcooling at the outlet of the gas cooler, which in this case functions as a condenser, is used as the control variable.

[0023] In high-pressure transcritical operation, the control valve upstream of the refrigerant receiver opens as the high pressure rises. Since the high pressure is higher than the pressure in the refrigerant receiver, refrigerant flows from the circuit into the receiver. This process can be further assisted by cooling the refrigerant receiver (the refrigerant within it).

[0024] As the high pressure drops, the control valve behind the refrigerant receiver opens, allowing refrigerant to flow back into the circuit because the suction pressure is lower than the pressure in the receiver. This process can be assisted by heating the refrigerant receiver (the refrigerant within it).

[0025] In high-pressure subcritical operation, if the refrigerant is not sufficiently subcooled at the condenser outlet, refrigerant is pumped from the collector into the refrigerant circuit as described above; if the subcooling is too high, it is pumped from the circuit into the collector.

[0026] Furthermore, the refrigerated dryer is to be equipped with a control system that allows it to automatically switch between subcritical and supercritical operation on the high-pressure side, depending on the cooling conditions. A reciprocating, screw, scroll, or turbo compressor can be used in the refrigerated dryer.

[0027] To improve the process in the case of heat recovery, the refrigerated dryer according to the invention can optionally include an air-to-refrigerant heat exchanger in addition to the internal heat exchanger. The air-side inlet of this heat exchanger is connected to the outlet of the portion of the air-to-air heat exchanger through which the air to be dried flows, and its air-side outlet is connected to the air-side inlet of the evaporator. On the refrigerant side, the air-to-refrigerant heat exchanger is connected between the outlet of the internal heat exchanger and the suction side of the compressor. The heat exchanger raises the compressor inlet temperature and thus also the compressor outlet temperature.

[0028] Alternatively, an external heat source (e.g., existing warm process wastewater) can be used to preheat the air instead of an air-to-air heat exchanger. Using warm water would have the advantage of allowing the use of a smaller and more cost-effective heat exchanger (than an air-to-air heat exchanger).

[0029] To further improve the process, an ejector can be used instead of the expansion valve for work-generating pressure relief.

[0030] To increase ease of servicing, all those sides of the heat exchangers of the refrigerant dryer that are exposed to air or water during operation (of the refrigerant dryer), such as the evaporator, the condenser and, if applicable, the air-refrigerant heat exchanger (but not: the internal heat exchanger), can be designed in such a way that they can be cleaned easily.

[0031] In an advantageous embodiment of the refrigerated dryer according to the invention, heat is recovered by cooling the transcritical pressurized gas to the lowest possible values. For this purpose, the refrigerated dryer is equipped with at least one heat exchanger on the high-pressure side, the heat from which can be transferred to a heat consumer (which does not serve the purpose of heat dissipation to the environment). In the simplest case, the heat consumer is connected directly or indirectly to the gas cooler via an intermediate circuit or safety heat exchanger. However, several heat exchangers / gas coolers connected in series and / or parallel can also be used, which can recover heat at different temperature levels. If only partial heat recovery takes place, a cooling tower or other suitable recooling device may also be required.

[0032] A continuously flowing, conventional (no flooded evaporator) refrigerant receiver in combination with a state-of-the-art high-pressure control valve for CO 2- Refrigeration systems can only be used effectively if the operating conditions of the application, and therefore the required CO2 charge, change only within narrow limits. This is because, due to the comparatively small difference between the density of liquid and vaporous carbon dioxide near the critical point, the continuously flowing, conventional refrigerant receiver, while having a very low, is sufficient storage capacity under these conditions. This applies particularly to designs with shell-and-tube gas coolers and screw compressor systems.

[0033] If the high-temperature heat demand cannot be met by the described dryer design, the air-to-air heat exchanger can be omitted. The dryer is then designed for the entire cooling and dehumidification capacity of the compressed air. Reheating of the dried compressed air takes place in an additional heat exchanger located downstream of the gas coolers on the high-pressure side. In this case, subcritical operation on the high-pressure side is impossible and only occurs briefly during dryer start-up.

[0034] The invention is explained in more detail below with reference to five exemplary embodiments; shown here are: Fig. 1: Refrigerated dryer (state of the art), Fig. 2: Refrigerated dryer (state of the art, with refrigerant collector flowing through it), Fig. 3: Refrigerated dryer with flooded evaporator (not according to the invention), Fig. 4: Refrigerated dryers using carbon dioxide as a refrigerant, Fig. 5: Refrigerated dryers using carbon dioxide as a refrigerant and with heat recovery, Fig. 6: Refrigerated dryers using carbon dioxide as a refrigerant for maximum heat recovery at high temperature levels

[0035] At the in Fig. In the refrigerated dryer shown in Figure 1, which uses HFC or HCFC refrigerants, the compressor 1 draws refrigerant from the evaporator 2 and compresses it to a high-pressure level; this also increases the temperature of the refrigerant. In the downstream condenser 3, the refrigerant is liquefied in a subcritical state at nearly constant pressure and temperature, transferring heat to the secondary medium (e.g., cooling water) of the circuit. Via the receiver 5 and the expansion valve 6, the refrigerant enters the subsequent evaporator 2, where it evaporates and superheats, absorbing heat in the process. An internal heat exchanger 7 is incorporated to improve the process. Before the air to be dehumidified is directed into the evaporator 2, some of its moisture is removed by means of an air-to-air heat exchanger 8. To remove the condensate, both the evaporator 2 and the air-to-air heat exchanger 8 are equipped with a condensate separator 9.

[0036] At the in Fig. The refrigerated dryer described in section 2, which uses carbon dioxide as a refrigerant and has relatively unchanging operating conditions, must be compared to the dryer according to... Fig. 1. In high-pressure supercritical operation, the high pressure must also be regulated. This is done by control valve 4.

[0037] In the circuit with flooded evaporator ( Fig. 3) The refrigerant flowing from the internal heat exchanger 7 is expanded directly to evaporation pressure in the high-pressure control valve 4. The control variable used is the high pressure during supercritical operation and the refrigerant subcooling at the gas cooler / condenser outlet during subcritical operation. The expanded refrigerant is fed to the refrigerant receiver 5.1, which is part of a flooded evaporator. The compressor 1 draws in saturated refrigerant vapor via the internal heat exchanger 7 and supplies this high-pressure compressed volume flow to the gas cooler / condenser 3. This completes the refrigerant cycle, which is simple from a control perspective.

[0038] The refrigerant receiver 5.1 is dimensioned to store (compensate for) the operating-point-related fluctuations in the refrigerant mass in the evaporator 2, the gas cooler / condenser 3, and the internal heat exchanger 7. The storage effect (greater storage capacity) is better than in receiver 5 because the refrigerant (carbon dioxide) assumes its evaporation pressure and temperature. Under these conditions, the density difference between liquid and gaseous carbon dioxide is greater than in receiver 5, which is located at the higher intermediate pressure (and higher temperature) level. The embodiment of the refrigerated dryer with flooded evaporator according to Fig. 3 is not in accordance with the invention and serves only for general understanding.

[0039] The dryer suitable for variable operating conditions according to Fig. 4. The continuously flowing refrigerant receiver 5 is eliminated. Instead, a refrigerant receiver 11, temperature-controlled by a heating / cooling unit 10, is connected in parallel to the expansion valve 6. A first control valve 12 is installed before the inlet of the refrigerant receiver 11, and a second control valve 13 is installed after the outlet. These control valves regulate the refrigerant charge in the circuit to achieve the optimal high pressure in the dryer. Ejectors can be used to improve the process.

[0040] Fig. Figure 5 shows a refrigerated dryer, which is basically analogous to the refrigerated dryer according to Fig. 4 is built (a structure according to would also be possible) Fig. 3), however, with the difference that, firstly, it is equipped with an air-to-refrigerant heat exchanger 14 for further process improvement, which is connected on the air side between the air-to-air heat exchanger 8 and the evaporator 2 (alternatively, the order of the air-side series connection can be reversed, or water can be used as a heat source), and that, secondly, the gas cooler 3 has been replaced by a refrigerant-side series connection consisting of a heat exchanger for high-temperature utilization 15, a heat exchanger for low-temperature utilization 16, and a heat exchanger 17, which can be connected to a cooling tower, for example, and which serves to remove the heat that is no longer usable (too low a temperature level).

[0041] The in Fig. The refrigerated dryer shown in section 6 differs from the previous refrigerated dryers ( Fig. 1, Fig. 2, Fig. 3, Fig. 4 to Fig.5) by eliminating the air-to-air heat exchanger 8. This makes it possible to raise all the energy extracted from the air to be dried to a high, usable temperature level. The reheating of the dried carbon dioxide takes place here in the air-to-O₂ heat exchanger 18. List of reference symbols used 1 compressor 2 evaporators 3 gas coolers 4 High-pressure control valve 5 refrigerant collectors 5.1 Refrigerant receiver, designed as part of a flooded evaporator 6 Expansion valve 7 Internal heat exchanger 8 air-to-air heat exchangers 9 condensate separators 10 Heating / cooling unit 11 Temperature-controlled refrigerant collector 12 First control valve 13 Second control valve 14 air-to-refrigerant heat exchangers 15 heat exchangers for high-temperature use 16 heat exchangers for low-temperature use 17 Cooling tower 18 air-CO2 heat exchangers for air reheating

Claims

[1] Compressed air refrigeration dryer for dehumidifying compressed air, comprising a refrigerant circuit with carbon dioxide as the refrigerant, comprising at least one compressor (1); at least one gas cooler (3); at least one evaporator (2) configured to dehumidify compressed air passed on its cold surface on the secondary side, a refrigerant receiver (11) and at least one expansion valve (6) or an ejector, characterized by, that - the refrigerant receiver (11) is connected in parallel to the expansion valve (6) or ejector and has at least one inlet and one outlet, wherein a first control or solenoid valve (12) is installed before the inlet of the refrigerant receiver (11) and a second control or solenoid valve (13) is installed after the outlet of the refrigerant receiver (11), by means of which both the inflow of refrigerant into the refrigerant receiver (11) and the outflow of refrigerant from the refrigerant receiver (11) can be controlled depending on the refrigerant pressure on the high-pressure side in high-pressure transcritical operation and / or on the temperature of the refrigerant on the high-pressure side and / or the cooling water temperature. [2] Compressed air refrigeration dryer according to claim 1, characterized by , that the refrigerant collector (11) is equipped with a device (10) which serves to cool and / or heat the refrigerant in the refrigerant collector (11). [3] Compressed air refrigeration dryer according to one of claims 1 or 2, characterized by , that it has an internal heat exchanger (7) which serves to transfer heat between the refrigerant on the suction side of the at least one compressor (1) and the refrigerant before the inlet of the expansion valve (6). [4] Compressed air refrigeration dryer according to one of claims 1 to 3, characterized by , that it has a compressed air CO2 heat exchanger (18) for heating the dried compressed air, which is connected downstream of the gas cooler (3) or several gas coolers (15, 16) on the refrigerant side and through which the cold, already dried compressed air is passed on the compressed air side, whereby the entire power for cooling and dehumidifying the compressed air is supplied by the evaporator (2). [5] Compressed air refrigeration dryer according to any one of claims 1 to 4, characterized by , that at least one compressor (1) is a reciprocating, screw, scroll and / or turbo compressor. [6] Compressed air refrigeration dryer according to any one of claims 1 to 5, characterized by , that it has a compressed air-to-compressed air heat exchanger (8) which is connected upstream of the compressed air-side inlet of the evaporator (2) and which serves, on the one hand, to reduce the moisture content of the compressed air to be dehumidified by means of a heat exchange with the cool and dried compressed air from the refrigerated drying process before entering the evaporator (2) and, on the other hand, to increase the temperature of the dried compressed air. [7] Compressed air refrigeration dryer according to any one of claims 1 to 6, characterized by, that it has a compressed air-refrigerant heat exchanger (14) whose compressed air-side inlet is connected either directly to a supply line of the compressed air to be dried or to the outlet of the part of the compressed air-compressed air heat exchanger (8) through which the compressed air to be dried flows, and whose compressed air-side outlet is connected to the compressed air-side inlet of the evaporator (2), wherein the compressed air-refrigerant heat exchanger (14) is connected on the refrigerant side between the outlet of the inner heat exchanger (7) and the suction side of the at least one compressor (1). [8] Compressed air refrigeration dryer according to any one of claims 1 to 6, characterized by, that it has a water-refrigerant heat exchanger which is connected on the refrigerant side between the outlet of the internal heat exchanger (7) and the suction side of the at least one compressor (1) and which is permeable on the water side with hot water supplied from outside, which has been heated by means of a heat source. [9] Compressed air refrigeration dryer according to any one of claims 1 to 8, characterized by , that it includes at least one heat exchanger (15, 16) on the high-pressure side, with which process heat can be fully or partially extracted during transcritical process operation on the high-pressure side. [10] Compressed air refrigeration dryer according to any one of claims 1 to 9, characterized bythat it is equipped with internal bypass valves, wherein the storage capacity of the refrigerant receiver (11) and the amount of carbon dioxide in the refrigerant circuit are selected such that the stagnation pressure of the system, up to the maximum temperatures permitted for the installation room of the refrigerant dryer, cannot rise above the respective design pressure of the high and low pressure sides, while fully utilizing the internal volume of the refrigerant dryer, thereby enabling the refrigerant dryer to be operated intrinsically safely without pressurized safety discharge lines or safety valves.

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