DE102019135437B4 - Process for indirectly determining pressure in refrigeration circuits - Google Patents

Abstract

Method for determining the pressure of the refrigerant in a refrigeration circuit, the refrigeration circuit having at least one component in which heat transfer takes place between the refrigerant and a heat transfer fluid, comprising at least the following method steps: a) measuring the temperatures of the refrigerant and of the heat transfer fluid at the inlet one and at the outlet of a component of the refrigeration circuit in which heat transfer takes place between the refrigerant and the heat transfer fluid (TR,in, TR,out; TW,in,TW,out);b) measurement of an intermediate temperature of the refrigerant TR,mand an intermediate temperature of Heat transfer fluid TW,min of the component of the refrigeration circuit in such a way that these intermediate temperatures record the same heat transfer process that has taken place up to there, once in relation to the refrigerant and once in relation to the heat transfer fluid; c) Determination of the relationship between pressure pR and enthalpy hR of the refrigerant at the measured temperatures of refrigerant els (TR,in, TR,out, TR,m) at least in the pressure range provided for the operation of the refrigeration circuit by means of a data processing device using a material database;d) determination of the pressure pR present at the time of the measurements in method steps a) and b). * by determining the dependency of the pressure pR of the refrigerant on a relationship between the enthalpies of the refrigerant resulting from the manipulation of a system of equations of at least two different balance equations of the heat transfer process in the component of the refrigeration circuit in which heat transfer takes place between the refrigerant and the heat transfer fluid in the process steps a) and b) determined temperatures of the refrigerant, the relationship of the enthalpies present at the time of the measurements in method steps a) and b) from the corresponding relationship of the temperatures of the heat transfer fluid determined in method steps a) and b). is determined; where, if more than one pressure value pR* is assigned to the relationship between the temperatures of the heat transfer fluid, which corresponds to the relationship between the enthalpies of the refrigerant, to determine the pressure of the refrigerant present at the time of the measurements in method steps a) and b), a plausibility check is carried out on the more than a pressure value is carried out.

Description

The invention relates to a method for determining the pressure of the refrigerant in a refrigeration circuit, the refrigeration circuit having at least one component in which heat transfer takes place between the refrigerant and a heat transfer fluid. Furthermore, the invention relates to the preferred use of such a method.

The invention describes a method that determines the pressure from multiple temperatures in a refrigeration cycle. Even though the method according to the invention is described below for use in a refrigeration circuit, the method according to the invention is not only suitable for refrigeration circuits, but equally for various types of clockwise and counterclockwise thermodynamic cycles or heat transfer processes.

For example, in commercially available heat pumps, many of the temperatures used for the process according to the invention are already being used for other purposes, e.g. B. for monitoring the compression end temperature recorded. Particularly advantageously, the high pressure can also be determined indirectly in transcritical refrigeration circuits using a few, in many cases only two, additional cost-effective temperature sensors. This means that energy balancing, which will be required by law in the future, can be implemented more cost-effectively using the refrigeration cycle process variables. In this way, the use of external energy recording systems or pressure sensors that are expensive and difficult to install can be omitted.

Due to the ban or the reduction in the permitted quantity of F-gases that can be sold, the trend in new refrigeration systems or heat pumps is towards refrigerants with a low GWP (global warming potential). CO ₂ (R744) is of particular importance as it is neither flammable nor toxic, does not lead to the depletion of ozone in the atmosphere and does not have to be disposed of separately or recycled. Since the critical point of CO ₂ is at the comparatively low temperature T = 304 K (31 °C) (and a pressure p = 7.4 MPa = 74 bar), heat pumps in particular with CO ₂ as refrigerant are generally operated transcritically. In a transcritical process, the condensation of the refrigerant at constant pressure and constant temperature that occurs in the subcritical process is replaced by isobaric cooling of the refrigerant on the high-pressure side at a strongly sliding temperature in a high-pressure heat exchanger, also known as a gas cooler.

For the purposes of this application, the term “transcritical” includes processes in which the thermodynamic change of state on the high-pressure side of the refrigerant takes place in the supercritical area of the phase diagram, i.e. the wet-steam area is not passed through; on the low-pressure side, however, the pressure can be lower than the critical pressure of the refrigerant and the wet vapor region can be passed through.

In order to determine the annual performance factor of a heat pump heating system, i.e. the ratio between the energy supplied and the heat actually generated, either expensive external measurement technology (e.g. a heat meter) must be used, or the annual performance factor is calculated using cycle processes based on thermodynamic state variables (e.g. B. p and T) calculated at inner working points. In the case of purely subcritical systems, due to the i. A. Relatively low pressure levels simple pressure measurement technology can be used. Since condensation of the refrigerant takes place in subcritical heat pumps and the pressure in the wet steam area can be clearly assigned to the associated condensation temperature, the pressure can also be determined in a particularly simple manner simply by measuring the (condensation) temperature.

EP 2 196 740 B1 discloses a method for determining the COP (Coefficient Of Performance) of a refrigerating machine, in particular a heat pump, which includes a closed circuit having a refrigerant, in which an evaporator, a compressor, a condenser and an expansion valve are arranged. In the method, at least three temperatures of the refrigerant are determined with the aid of temperature sensors arranged in the circuit. Alternatively, at least two temperatures and at least one pressure of the refrigerant are determined with the aid of sensors arranged in the circuit. The enthalpies of the circuit and from this the heating capacity and the consumed electrical power of the refrigerating machine are calculated from the determined refrigerant temperatures or pressures in order to determine the coefficient of performance of the refrigerating machine from the quotient of the calculated heating capacity and the calculated consumed electrical power. The invention also relates to a refrigeration machine for carrying out such a method.

This in EP 2 196 740 B1 The procedure described relates to subcritical refrigeration circuits. A phase change therefore takes place in the condenser, and by measuring the temperature at the outlet of the condenser, a reference can be made to the high pressure or directly to the required enthalpy of the refrigerant at the outlet of the condenser. This one-to-one assignment does not apply to supercritical processes, so that a determination z. B. the enthalpy for a COP determination a simple temperature measurement analogous to EP 2 196 740 B1 not possible.

It is therefore state of the art to use pressure sensors for pressure measurement on the high-pressure side in transcritical process control, with different types of sensors being able to be used. All sensor principles determine the pressure indirectly by exposing a measuring element to the pressure, which then changes its specific properties according to the pressure. This specific property, such as B. a small resulting electrical voltage in the piezo effect is detected by electronics and passed on in a suitable form to a control or regulation. In pressure measurement technology, for example, there are the variants of inductive, capacitive, piezoresistive pressure measurement and pressure measurement using thin-film or thick-film strain gauges.

Pressure sensors, especially for high pressures, are significantly more expensive and more prone to failure than temperature sensors. Furthermore, the cooling circuit must be opened when installing a pressure sensor.
WO 2006/112 924 A2 describes a method for optimizing the COP of a transcritical compression refrigeration system using CO ₂ as the refrigerant. The COP is maximized by setting an optimal high pressure. The optimal high pressure is determined as a function of the gas cooler outlet temperature, i.e. at least at outside temperatures below 37 °C, there is exactly one high pressure value for each gas cooler outlet temperature at which the COP of the system is optimal, and this high pressure value is determined by measuring the gas cooler -Outlet temperature determined. However, the dependency of the optimal high pressure value on the gas cooler outlet temperature must be determined experimentally for each system or at least calculated based on the performance data of its components, in particular the compressor and the gas cooler. The disadvantage of this method is that it does not actually determine the pressure, because the optimum high pressure value can be deduced from the measured gas cooler outlet temperature alone, but not the high pressure that actually exists in the system.

Similarly, US 2014/0 116 075 A1 discloses a method for controlling transcritical compression refrigeration systems, which is based on the use of an approximation formula for determining a reference high pressure from the measured value for the outlet temperature of the refrigerant from the gas cooler. Here, too, the high pressure actually present in the system is not determined.

In contrast, US 2005/0 066 675 A1 actually discloses a method for determining the high pressure of the refrigerant in a transcritical compression refrigeration system, namely by determining that temperature of the refrigerant in the gas cooler at which the refrigerant has its maximum specific heat. A pressure value is assigned to this temperature using the assumption of maximum specific heat. The refrigerant has the maximum specific heat where its temperature changes least over the length of the gas cooler. The measuring method therefore requires that the temperature of the refrigerant is measured at several points along the gas cooler in order to determine between which two measuring points the temperature change is lowest. The mean value of the temperatures at these two measuring points is then assumed to be the temperature of the maximum specific heat. In order to be able to carry out the method with sufficient accuracy, a number of equidistant temperature measuring points must therefore be provided along the gas cooler, which disadvantageously involves considerable measuring effort.

For transcritical refrigeration circuits, there is no known way to determine the high pressure using a few inexpensive temperature sensors.

The object of the invention is therefore to overcome the disadvantages of the prior art and to provide an advantageous method for determining the pressure in refrigeration circuits using temperature sensors.

The object is achieved by a method having the features of claim 1. Developments of the invention are specified in the dependent claims. A preferred use of the method is specified in claim 8.

1. a) Messung der Temperaturen des Kältemittels sowie des Wärmeträgerfluids jeweils am Eintritt und am Austritt aus einer Komponente des Kältekreises, in der ein Wärmeübergang zwischen Kältemittel und Wärmeträgerfluid stattfindet (T_R,ein, T_R,aus, T_W,ein, T_W,aus);
2. b) Messung einer Zwischentemperatur des Kältemittels T_R,m und des Wärmeträgerfluids T_W,m in der Komponente des Kältekreises so, dass diese Zwischentemperaturen den gleichen bis dorthin vollzogenen Wärmeübertragungsprozess, einmal bezogen auf das Kältemittel und einmal bezogen auf das Wärmeträgerfluid, erfassen;
3. c) Ermittlung des Zusammenhangs zwischen Druck p_R und Enthalpie h_R des Kältemittels, h_R(p_R, T = konstant), zu den gemessenen Temperaturen des Kältemittels, also T = T_R,ein; T = T_R,aus, T = T_R,m, zumindest im für den Betrieb des Kältekreises vorgesehenen Druckbereich mittels einer Datenverarbeitungseinrichtung unter Nutzung einer Stoffdatenbank;
4. d) Ermittlung des zum Zeitpunkt der Messungen in den Verfahrensschritten a) und b) vorliegenden Drucks p_R* durch Ermittlung der Abhängigkeit des Drucks p_R des Kältemittels von einem aus der Manipulation eines Gleichungssystems mindestens zweier voneinander verschiedener Bilanzgleichungen des Wärmeübertragungsprozesses in der Komponente des Kältekreises, in der ein Wärmeübergang zwischen Kältemittel und Wärmeträgerfluid stattfindet, resultierenden Zusammenhangs der Enthalpien des Kältemittels bei den in den Verfahrensschritten a) und b) ermittelten Temperaturen des Kältemittels, wobei der zum Zeitpunkt der Messungen in den Verfahrensschritten a) und b) vorliegende Zusammenhang der Enthalpien aus dem entsprechenden Zusammenhang der in den Verfahrensschritten a) und b) ermittelten Temperaturen des Wärmeträgerfluids bestimmt wird;

wobei bei Zuordnung von mehr als einem Druckwert p_R* zu dem dem Zusammenhang zwischen den Temperaturen des Wärmeträgerfluids entsprechenden Zusammenhang zwischen den Enthalpien des Kältemittels zur Bestimmung des zum Zeitpunkt der Messungen in den Verfahrensschritten a) und b) vorliegenden Drucks des Kältemittels eine Plausibilitätsprüfung der mehr als einen Druckwerte durchgeführt wird.According to the invention, the object is achieved by a method for determining the pressure p _R * of the refrigerant in a refrigeration circuit, which comprises at least the following steps:

1. a) Measurement of the temperatures of the refrigerant and the heat transfer fluid at the inlet and outlet of a component of the refrigeration circuit in which heat transfer takes place between the refrigerant and the heat transfer fluid (T _R,in , T _R,out , T _W,in , T _{W, off} );
2. b) Measurement of an intermediate temperature of the refrigerant T _R,m and of the heat transfer fluid T _W,m in the component of the refrigeration circuit so that these intermediate temperatures have the same heat transfer process that has taken place up to there, once based on the refrigerant and once based on the heat transfer fluid, detect;
3. c) determination of the relationship between pressure p _R and enthalpy h _R of the refrigerant, h _R (p _R , T = constant), to the measured temperatures of the refrigerant, ie T = T _R,in ; T=T _R,out , T=T _R,m , at least in the pressure range provided for the operation of the refrigeration circuit by means of a data processing device using a material database;
4. d) Determination of the pressure p _R * present at the time of the measurements in method steps a) and b) by determining the dependency of the pressure p _R of the refrigerant on one of at least two different balance equations of the heat transfer process in the component of the refrigeration cycle resulting from the manipulation of a system of equations , in which heat transfer takes place between the refrigerant and the heat transfer fluid, resulting relationship between the enthalpies of the refrigerant at the temperatures of the refrigerant determined in method steps a) and b), the relationship between the enthalpies present at the time of the measurements in method steps a) and b). is determined from the corresponding relationship between the temperatures of the heat transfer fluid determined in method steps a) and b);

where, if more than one pressure value p _R * is assigned to the relationship between the enthalpies of the refrigerant, which corresponds to the relationship between the temperatures of the heat transfer fluid, a plausibility check of the more is carried out as a pressure value.

• di) Ermittlung der Abhängigkeit des Drucks p_R vom Verhältnis der mittels zweier voneinander verschiedener Bilanzgleichungen des Wärmeübertragungsprozesses ermittelten Enthalpiedifferenzen des Kältemittels bei den in Verfahrensschritten a) und b) ermittelten Temperaturen des Kältemittels unter Nutzung des in Verfahrensschritt c) ermittelten Zusammenhangs zwischen Druck p_R und Enthalpie h_R des Kältemittels zumindest für den für den Betrieb des Kältekreises vorgesehenen Druckbereich;
• dii) Ermittlung des zum Zeitpunkt der Messungen in den Verfahrensschritten a) und b) vorliegenden Verhältnisses der Enthalpiedifferenzen aus dem über die genannten Bilanzgleichungen ermittelten entsprechenden Verhältnis von Differenzen der Temperaturen des Wärmeträgerfluids und Zuordnung mindestens eines Druckwerts p_R* zu diesem Verhältnis der Enthalpiedifferenzen unter Nutzung des in Verfahrensschritt di) ermittelten Zusammenhangs zwischen dem Druck p_R und dem Verhältnis der Enthalpiedifferenzen;

wobei bei Zuordnung von mehr als einem Druckwert zu dem dem Verhältnis der Differenzen der Temperaturen des Wärmeträgerfluids entsprechenden Verhältnis der Enthalpiedifferenzen des Kältemittels zur Bestimmung des zum Zeitpunkt der Messungen in den Verfahrensschritten a) und b) vorliegenden Drucks des Kältemittels eine Plausibilitätsprüfung der mehr als einen Druckwerte durchgeführt wird.In one embodiment of the method according to the invention, method step d) comprises the following sub-steps:

• di) Determination of the dependence of the pressure p _R on the ratio of the enthalpy differences of the refrigerant determined by means of two different balance equations of the heat transfer process at the temperatures of the refrigerant determined in method steps a) and b) using the relationship between pressure p _R and determined in method step c). Enthalpy h _R of the refrigerant at least for the pressure range intended for the operation of the refrigeration circuit;
• dii) Determination of the ratio of the enthalpy differences present at the time of the measurements in method steps a) and b) from the corresponding ratio of differences in the temperatures of the heat transfer fluid determined using the balance equations mentioned and assignment of at least one pressure value p _R * to this ratio of the enthalpy differences using use the relationship determined in method step di) between the pressure p _R and the ratio of the enthalpy differences;

Where more than one pressure value is assigned to the ratio of the enthalpy differences of the refrigerant, which corresponds to the ratio of the differences in the temperatures of the heat transfer fluid, a plausibility check is carried out on the more than one pressure value to determine the pressure of the refrigerant present at the time of the measurements in method steps a) and b). is carried out.

• di') Ermittlung der Abhängigkeit des Drucks p_R vom Verhältnis der Differenz zwischen den Enthalpien bei T_R,ein und T_R,m und der Differenz zwischen den Enthalpien bei T_R,aus und T_R,m, unter Nutzung des in Verfahrensschritt c) ermittelten Zusammenhangs zwischen Druck p_R und Enthalpie h_R des Kältemittels zumindest für den für den Betrieb des Kältekreises vorgesehenen Druckbereich;
• dii') Ermittlung des zum Zeitpunkt der Messungen in den Verfahrensschritten a) und b) vorliegenden Verhältnisses der Enthalpiedifferenzen des Kältemittels aus dem entsprechenden Verhältnis der Differenz zwischen T_W,ein und T_W,m und der Differenz zwischen T_W,aus und 7_W,m, und Zuordnung mindestens eines Druckwerts p_R* zu diesem Verhältnis der Enthalpiedifferenzen, unter Nutzung der in Verfahrensschritt di') ermittelten Abhängigkeit des Drucks p_R von dem Verhältnis der Enthalpiedifferenzen;

wobei bei Zuordnung von mehr als einem Druckwert zu dem dem Verhältnis der Differenzen der Temperaturen des Wärmeträgerfluids entsprechenden Verhältnis der Enthalpiedifferenzen des Kältemittels zur Bestimmung des zum Zeitpunkt der Messungen in den Verfahrensschritten a) und b) vorliegenden Drucks des Kältemittels eine Plausibilitätsprüfung der mehr als einen Druckwerte durchgeführt wird.The sub-steps di) and dii) described can be carried out, for example, as follows:

• di′) determining the dependency of the pressure p _R on the ratio of the difference between the enthalpies at T _R,in and T _R,m and the difference between the enthalpies at T _R,out and T _R,m , using the method in step c ) determined relationship between pressure p _R and enthalpy h _R of the refrigerant at least for the pressure range provided for the operation of the refrigeration circuit;
• dii') Determination of the ratio of the enthalpy differences of the refrigerant at the time of the measurements in method steps a) and b) from the corresponding ratio of the difference between TW,in and TW _,m and the difference between _{TW ,out} and 7 _{W ,m} , and assignment of at least one pressure value p _R * to this ratio of the enthalpy differences, using the dependence of the pressure p _R on the ratio of the enthalpy differences determined in method step di′);

Where more than one pressure value is assigned to the ratio of the enthalpy differences of the refrigerant, which corresponds to the ratio of the differences in the temperatures of the heat transfer fluid, a plausibility check is carried out on the more than one pressure value to determine the pressure of the refrigerant present at the time of the measurements in method steps a) and b). is carried out.

A temperature measurement at the entrance to and at the exit from a component of the refrigeration circuit, in which heat transfer takes place between the refrigerant and the heat transfer fluid, is to be understood as measuring the temperature at a position in the refrigeration circuit that is between the last component that influences the temperature of the refrigerant and the component of the refrigeration circuit, in where heat transfer takes place between refrigerant and heat transfer fluid. A refrigerant temperature influencing component is z. B. the compressor. The temperature can be measured in a manner known to those skilled in the art using commercially available temperature sensors, e.g. B. resistance thermometers or thermocouples are carried out.

Process steps a) and b) can be carried out simultaneously.

The component of the refrigeration circuit in which heat transfer takes place between the refrigerant and the heat transfer fluid is largely thermally insulated from the environment, so that the heat given off by the refrigerant is largely completely absorbed by the heat transfer fluid. The component of the refrigeration circuit in which heat transfer takes place between the refrigerant and the heat transfer fluid is also referred to below as the heat exchanger. In the context of this invention, the term “heat exchanger” also includes a series connection of two or more heat exchanger elements, in each of which heat transfer takes place between refrigerant and heat transfer fluid.

The measuring point for the intermediate temperature of the refrigerant T _R,m and the measuring point for the intermediate temperature of the heat transfer fluid T _W,m must be selected in such a way that these intermediate temperatures record the same heat transfer process that has taken place up to that point, once in relation to the refrigerant and once in relation to the heat transfer fluid . This means that the heat flow released from the refrigerant entering the heat exchanger to the measuring point of the intermediate temperature of the refrigerant to the heat transfer fluid must be equal to the heat flow absorbed from the measuring point of the intermediate temperature of the heat transfer fluid to the exit of the heat transfer fluid from the heat exchanger. Similarly, the heat flow transferred from the measuring point of the intermediate temperature of the refrigerant to the point at which the refrigerant exits the heat exchanger to the heat transfer fluid must be equal to the heat flow absorbed from the entry of the heat transfer fluid into the heat exchanger to the measuring point of the intermediate temperature of the heat transfer fluid. The measurement of the intermediate temperature allows the heat transfer to be balanced in two separate sub-processes, each with its own heat flow. Does the heat exchanger z. B. two series-connected heat exchanger elements, the intermediate temperature can be advantageously measured between the two heat exchanger elements.

und $${\dot{Q}}_{\text{R}\mathrm{,2}}={\dot{Q}}_{\text{W}\mathrm{,1}}$$

The following conditions must therefore be met: $${\dot{Q}}_{\text{R}\mathrm{,1}}={\dot{Q}}_{\text{W}\mathrm{,2}}$$

and $${\dot{Q}}_{\text{R}\mathrm{,2}}={\dot{Q}}_{\text{W}\mathrm{,1}}$$

und $${\dot{m}}_{\text{R}}\left(h_{\text{R},\text{m}}-h_{\text{R},\text{aus}}\right)={\dot{m}}_{\text{W}}{c}_{\text{p},\text{W}}\left(T_{\text{W},\text{m}}-T_{\text{W},\text{ein}}\right)$$

The following balance equations of the heat transfer process apply: $${\dot{m}}_{\text{R}}\left(H_{\text{R},\text{one}}-H_{\text{R},\text{m}}\right)={\dot{m}}_{\text{W}}{c}_{\text{p},\text{W}}\left(T_{\text{W},\text{out}}-T_{\text{W},\text{m}}\right)$$

and $${\dot{m}}_{\text{R}}\left(H_{\text{R},\text{m}}-H_{\text{R},\text{out}}\right)={\dot{m}}_{\text{W}}{c}_{\text{p},\text{W}}\left(T_{\text{W},\text{m}}-T_{\text{W},\text{one}}\right)$$

where ṁ _R is the refrigerant mass flow; h _{R,in is} the enthalpy of the refrigerant entering the heat exchanger; h _R,m is the enthalpy of the refrigerant at the measuring point of the intermediate temperature of the refrigerant; h _R,aus is the enthalpy of the refrigerant when it leaves the heat exchanger; ṁ _w is the heat transfer fluid mass flow; c _p,w is the specific heat of the heat transfer fluid at constant pressure; T _W,out is the temperature of the heat transfer fluid when it leaves the heat exchanger; T _W,m is the intermediate temperature of the heat transfer fluid and T _{w,in is} the temperature of the heat transfer fluid entering the heat exchanger.

wobei gilt Δh_R,1 = h_R,ein - h_R,m; Δh_R,2 = h_R,m - h_R,aus; ΔT_W,2 = T_W,aus - T_W,m und ΔT_W,1 = T_W,m - T_W,ein.By dividing equation (1') by equation (2'), where dividend and divisor can also be chosen in reverse, it follows: $$\frac{\Delta H_{\text{R}\mathrm{,1}}}{\Delta H_{\text{R}\mathrm{,2}}}=\frac{\Delta T_{\text{W}\mathrm{,2}}}{\Delta T_{\text{W}\mathrm{,1}}}$$

where Δh _R,1 = h _R,in - h _R,m; Δh _R,2 = h _R,m - h _R,off ; ΔT _W,2 = T _W,out - T _W,m and ΔT _W,1 = T _W,m - T _W,in .

The ratio of the enthalpy differences of the refrigerant related to the two separate sub-processes of heat transfer therefore corresponds to the ratio of the temperature differences of the heat transfer fluid determined from the corresponding measured temperature values.

It goes without saying that the use of other balance equations, e.g. B. ṁ _R (h _R,in - h _R,out ) = ṁ _W c _p,w (T _W,out - T _W,in ), instead of equation (1') or (2'), or another A sign convention for calculating the heat flows according to equations (1') and (2') is possible within the meaning of the invention, provided that it is applied consistently, e.g. Δh _R,1 = h _R,m - h _R,in and ΔT _W,2 = T _W,m - T _W,off ..

Method step c) includes determining the relationship between pressure p _R and enthalpy h _R of the refrigerant at least in the pressure range that is provided for the operation of the refrigeration circuit, at the measured temperatures of the refrigerant T _R , _in , T _R,m , T _{R ,off} , i.e. h _R (p _R ) at T = T _R, on ; h _R (p _R ) at T = T _R,m ; h _R (p _R ) at T = T _R,off ; with the help of a substance database. Suitable substance databases are known to those skilled in the art, e.g. B. the substance database of the NIST (National Institute of Standards and Technology) at webbook.nist.gov.

The relationship between pressure p _R and enthalpy h _R of the refrigerant determined in method step c) at the measured temperatures of the refrigerant is used in method step d) to determine the relationship between pressure p _R and the ratio of the enthalpy differences Δ _hR,1 /Δ _{hR ,2} to calculate. This means that at least in the pressure range intended for the operation of the refrigeration circuit, the relationship between pressure and Δh _R,1 /Δh _R,2 from the values stored in a substance database for the refrigerant, where Δh _R,1 = h _R (T _{R ,} on ) - h _R (T _R,m ) at a pressure value p _R and Δh _R,2 = h _R (T _R,m ) - h _R (T _R,off ) at the same pressure value p _R .

The ratio of the enthalpy differences at the time of the temperature measurements is calculated using equation (3) from the measured temperatures of the heat transfer fluid, i.e. Δ _hR,1 /Δ _hR,2 = ΔT _W,2 /ΔT _W,1 From the method step d ) determined relationship between the pressure p _R and the ratio of the enthalpy differences Δ _hR,1 /Δ _hR,2 , the pressure p _R * of the refrigerant to be determined at the time of the temperature measurements can now be determined via p _R * = p _R (Δh _{R, 1} /Δh _R,2 = ΔT _W,2 /ΔT _W,1 ).

It can happen that the ratio of the enthalpy differences Δh _R,1 /Δ/h _R,2 = ΔT _W,2 /ΔT _W,1 can be assigned more than one pressure value. In this case, a _plausibility check, e.g. B. on the basis of cycle process considerations by balancing energy or enthalpies. An advantageous possibility for this is the comparison of the enthalpy differences over another component present in the refrigeration circuit with heat transfer, into one side of which the refrigerant flows with the pressure p _R *, for example an internal heat exchanger. In refrigeration circuits without splitting the refrigerant mass flow, the enthalpy change of the refrigerant on the high-pressure side of the internal heat exchanger must correspond to the enthalpy change of the refrigerant on the low-pressure side of the internal heat exchanger. In order to determine the enthalpy change, in addition to the pressure p _R * to be tested, the temperatures of the refrigerant at the inlet and outlet of the internal heat exchanger and the pressure in the refrigeration circuit on the other side of the heat exchanger that does not have p _R * must be known.

The person skilled in the art is also readily familiar with suitable options for plausibility considerations for refrigeration circuits with variants such as an intermediate injection or multi-stage compression of the refrigerant.

The method according to the invention advantageously allows the pressure, particularly advantageously the high pressure, to be determined in a refrigeration circuit using commercially available temperature sensors without having to use expensive and susceptible pressure measurement technology.

The method according to the invention can advantageously be embedded in an algorithm, the required relationships between temperature, pressure and enthalpy z. B. can be queried from a multi-dimensional map, so that the inventive method can be performed without direct access to a substance database.

In one embodiment of the method according to the invention, the refrigeration cycle is a refrigeration cycle with transcritical process management.

In a further embodiment of the method according to the invention, the heat transfer between the refrigerant and the heat transfer fluid takes place without a phase change of at least the refrigerant. Since in this case there is no one-to-one relationship between pressure and temperature and the pressure cannot be determined from a measured temperature value with the aid of this relationship, the use of the method according to the invention has particular advantages.

In a further embodiment of the method according to the invention, the component of the refrigeration circuit in which heat transfer takes place between the refrigerant and the heat transfer fluid is a gas cooler.

The method according to the invention can also be used in refrigeration circuits in which the component in which heat transfer takes place between the refrigerant and the heat transfer fluid and the pressure on the refrigerant side is to be determined, in particular the gas cooler or the condenser, is not accessible for temperature measurements. B. by inserting a parallel section in the refrigerant flow. At this parallel street cke it can be e.g. B. be a two-part bypass heat exchanger, advantageously a z. T. anti-segregation devices provided as standard.

The method according to the invention can also be applied to heat transfer processes in which the component in which heat transfer takes place between the refrigerant and the heat transfer fluid and the pressure on the refrigerant side is to be determined is a heat exchanger made up of two or more heat exchanger elements connected in series, the Heat exchanger elements work with different mass flow. Here, only the corresponding ratio of the mass flows has to be inserted into equation (3) and taken into account in the following process steps.

In a further embodiment of the method according to the invention, the refrigerant is CO ₂ (R744).

The method according to the invention is particularly suitable for monitoring, specifically for energy balancing, of refrigeration systems or heat pumps. Therefore, a preferred use of such a method relates to use for a refrigeration system or a heat pump system. The method according to the invention can preferably be used for monitoring transcritical heat pump systems without a high-pressure sensor.

The method according to the invention is also suitable for optimizing the operating point of refrigeration system systems or heat pump systems, in particular for determining the optimal high pressure in the systems.

The invention is not limited to the illustrated and described embodiments, but also includes all embodiments that have the same effect within the meaning of the invention. Furthermore, the invention is not limited to the combinations of features specifically described, but can also be defined by any other combination of specific features of all individual features disclosed overall, provided that the individual features are not mutually exclusive, or a specific combination of individual features is not explicitly excluded.

The invention is explained below using exemplary embodiments with reference to figures, without being restricted to these.

• 1 das Grundfließschema einer transkritisch arbeitenden Wärmepumpe mit innerem Wärmeübertrager sowie das zugehörige Druck-Enthalpie-Diagramm,
• 2a die prinzipielle Anordnung der Temperatursensoren an einem Gaskühler,
• 2b die prinzipielle Anordnung der Temperatursensoren an einem Gaskühler, der zwei in Reihe geschaltete Gaskühlerelemente 1' und 1" umfasst,
• 3 das Druck-Enthalpie-Diagramm für CO₂ mit den Isothermen der Eintrittstemperatur in den Gaskühler, der Zwischentemperatur im Gaskühler und der Austrittstemperatur aus dem Gaskühler für CO₂ in einem beispielhaften Kältekreis,
• 4 die Abhängigkeit des Drucks vom Verhältnis der aus 3 entnommenen Enthalpiedifferenzen,
• 5 die Anordnung von Temperatursensoren am inneren Wärmeübertrager,
• 6 einen Ausschnitt des Druck-Enthalpie-Diagramms der 3 zur Erläuterung der Plausibilitätsprüfung.

The

• 1 the basic flow diagram of a transcritical heat pump with an internal heat exchanger and the associated pressure-enthalpy diagram,
• 2a the basic arrangement of the temperature sensors on a gas cooler,
• 2 B the basic arrangement of the temperature sensors on a gas cooler, which comprises two gas cooler elements 1' and 1" connected in series,
• 3 the pressure-enthalpy diagram for CO ₂ with the isotherms of the inlet temperature in the gas cooler, the intermediate temperature in the gas cooler and the outlet temperature from the gas cooler for CO ₂ in an exemplary refrigeration cycle,
• 4 the dependence of the pressure on the ratio of the out 3 extracted enthalpy differences,
• 5 the arrangement of temperature sensors on the internal heat exchanger,
• 6 a section of the pressure-enthalpy diagram 3 to explain the plausibility check.

1 shows the basic flow diagram of a heat pump with an internal heat exchanger and the associated idealized pressure-enthalpy diagram (logp,h diagram) for transcritical process control. In comparison to the standard heat pump cycle, the heat-emitting component in the basic flow diagram is not referred to as a condenser, but as a gas cooler 1, since the associated change of state from β to γ does not run through the wet-steam region 6. As a result, there is no condensation, but pure gas cooling. The coolant is then further cooled from state γ to state δ by the internal heat exchanger 2 with the high pressure p _HD remaining the same. The subsequent expansion from point δ to ε to the low pressure p _ND by means of the expansion valve 3 and the subsequent evaporation up to the state point ζ in the evaporator 4 are equivalent to the standard cold steam cycle of a heat pump. In comparison, however, the refrigerant is then heated by the internal heat exchanger 2 from the ζ state to the α state. The internal heat exchanger 2 is used, among other things, to use the evaporator 4 more effectively and to achieve higher compression end temperatures in the compressor 5. The refrigerant then runs through the cycle process again.

• - der Eintrittstemperatur des Wärmeträgerfluids in den Gaskühler 1, T_W,ein;
• - der Zwischentemperatur des Wärmeträgerfluids im Gaskühler 1, T_w,m;
• - der Austrittstemperatur des Wärmeträgerfluids aus dem Gaskühler 1, T_W,aus;
• - der Eintrittstemperatur des Kältemittels in den Gaskühler 1, T_R,ein;
• - der Zwischentemperatur des Kältemittels im Gaskühler 1, T_R,m,
• - der Austrittstemperatur des Kältemittels aus dem Gaskühler 1, T_R,aus .

the 2a and 2 B show the arrangement of temperature sensors on the component of the refrigeration circuit in which heat transfer takes place between the refrigerant and the heat transfer fluid and the pressure on the refrigerant side 7 is determined by measuring temperatures target. In the exemplary embodiments described here, this component is the gas cooler 1, with 2 B the gas cooler 1 comprises two gas cooler elements 1' and 1" connected in series. The search is for the high pressure present in the gas cooler 1 on the refrigerant side 7. The refrigerant flows into the gas cooler 1 on the refrigerant side 7 at _{a high temperature T R,} in and at a low temperature T _R,out from the gas cooler 1 and in the gas cooler 1 gives off its heat to the heat transfer fluid which, on the heat transfer fluid side 8, enters the gas cooler 1 with _{a low temperature T W,} in and out of the gas cooler with a high temperature T _W,out 1. The direction of flow of the fluids is indicated by arrows on both sides of the gas cooler 1. Temperature sensors for measuring the following temperatures are arranged on the gas cooler 1:

• - The inlet temperature of the heat transfer fluid in the gas cooler 1, T _W, ein ;
• - the intermediate temperature of the heat transfer fluid in the gas cooler 1, T _w,m ;
• - The outlet temperature of the heat transfer fluid from the gas cooler 1, T _{W, from} ;
• - the inlet temperature of the refrigerant in the gas cooler 1, T _R,in ;
• - the intermediate temperature of the refrigerant in gas cooler 1, T _R,m ,
• - The outlet temperature of the refrigerant from the gas cooler 1, T _R,aus .

Advantageously can in 2 B the intermediate temperatures T _R,m on the refrigerant side 7 and T _W,m on the heat transfer fluid side 8 between the two gas cooler elements 1' and 1'' are measured.

$$T_{R,m}=\mathrm{317,15}\text{K}\left(44°\text{C}\right);$$

$$T_{R,aus}=\mathrm{313,15}\text{K}\left(76°\text{C}\right).$$

Die in 3 aufgetragenen Stoffdaten werden dazu genutzt, im zum Betrieb von CO₂-Wärmepumpen relevanten Druckbereich zwischen 1 und 15 MPa die Abhängigkeit des Drucks vom Verhältnis der Enthalpiedifferenz Δh_R,1 bei Temperaturänderung von T_R,ein = 349,15 K auf T_R,m = 317,15 K zur Enthalpiedifferenz Δh_R,2 bei Temperaturänderung von T_R,m = 317,15 K auf T_R,aus = 313,15 K zu berechnen. Die Werte für Δh_R,1 und Δh_R,2 bei einem beispielhaften Druck p = 9,48 MPa sind in 3 als Doppelpfeile eingezeichnet. 3 shows the pressure-enthalpy diagram for CO ₂ with isotherms of various exemplary temperatures (dashed lines) and the wet steam region 6. The material data used to generate the pressure-enthalpy diagram using a data processing unit were taken from the material database “NIST Standard Reference Database 23, Version 9.1 “ taken. In 3 are also the isotherms of the inlet temperature into the gas cooler, T _R,in , the intermediate temperature in the gas cooler, T _R,m , and the outlet temperature from the gas cooler, T _R,out , as solid lines, which were measured for CO ₂ in a refrigeration circuit as an example , drawn. The temperature readings are: $$T_{R,ein}=349.15\text{K}\left(76°\text{C}\right);$$

$$T_{R,m}=317.15\text{K}\left(44°\text{C}\right);$$

$$T_{R,a\mathrm{and}s}=313.15\text{K}\left(76°\text{C}\right).$$

In the 3 The material data applied are used to determine the dependence of the pressure on the ratio of the enthalpy difference Δh _R,1 with a temperature change from T _R,in = 349.15 K to T _R,m in the pressure range between 1 and 15 MPa relevant to the operation of CO ₂ heat pumps = 317.15 K to the enthalpy difference Δh _R,2 with a temperature change from T _R,m = 317.15 K to T _R,aus = 313.15 K. The values for Δh _R,1 and Δh _R,2 at an exemplary pressure p = 9.48 MPa are in 3 marked as double arrows.

$$T_{\text{W},\text{m}}=\mathrm{315,45}\text{K}\left(\mathrm{42,3}°\text{C}\right);$$

$$T_{\text{W},\text{aus}}=\mathrm{325,25}\text{K}\left(\mathrm{52,1}°\text{C}\right),$$

kann über Gleichung (3) das zum Zeitpunkt der Temperaturmessungen vorliegende Verhältnis der Enthalpiedifferenzen Δ_hR,1/Δ_hR,2 bestimmt werden, das im Ausführungsbeispiel 2,88 beträgt. 4 shows those from the in 3 plotted data determined as a function of the pressure p _R on the ratio of the enthalpy differences Δh _R,1 /Δh _R,2 . From the measured values of the temperatures of the heat transfer fluid: $$T_{\text{W},\text{one}}=312.05\text{K}\left(38.9°\text{C}\right);$$

$$T_{\text{W},\text{m}}=315.45\text{K}\left(42.3°\text{C}\right);$$

$$T_{\text{W},\text{out}}=325.25\text{K}\left(52.1°\text{C}\right),$$

Equation (3) can be used to determine the ratio of the enthalpy differences Δ _hR,1 /Δ _hR,2 present at the time of the temperature measurements, which is 2.88 in the exemplary embodiment.

As 4 it can be seen that the ratio Δ _hR,1 /Δ _hR,2 = 2.88 corresponds to two possible pressure values, namely p _R '= 9.48 MPa and p _R ''= 8.56 MPa. The pressure actually present on the CO ₂ side at the time of the temperature measurements must therefore have a means 5 and 6 explained plausibility check are determined.

The plausibility check is carried out using the example of a heat pump with an internal heat exchanger by comparing the enthalpy changes on the two sides of the internal heat exchanger. The explanation of the plausibility check is based on the associated, in 1 cycle process shown. In order to be able to determine the enthalpy changes, the values for the inlet temperatures into and the outlet temperatures from the internal heat exchanger for both sides of the internal heat exchanger as well as the value of the low pressure must be available, whereby the low pressure value p _LP is recorded as standard in most refrigeration circuit systems, e.g. B. to regulate the superheat on the evaporator.

• - die der Eintrittstemperatur des Kältemittels in das Expansionsventil entsprechende Temperatur T_Ex,ein;
• - die der Austrittstemperatur des Kältemittels aus dem Verdampfer entsprechende Temperatur T_o,aus;
• - die der Eintrittstemperatur des Kältemittels in den Verdichter entsprechende Temperatur T_V,ein.

5 shows the arrangement of temperature sensors on the internal heat exchanger 2. The direction of flow of the refrigerant on both sides of the internal heat exchanger 2, on the high-pressure side 9 and the low-pressure side 10, is indicated by arrows. In addition to the outlet temperature of the refrigerant from the gas cooler T _R,out the following temperatures are to be measured:

• - the temperature T _Ex, in corresponding to the temperature at which the refrigerant enters the expansion valve;
• - the temperature T _{o,out corresponding to the outlet temperature of the refrigerant from the evaporator;}
• - the temperature T _V, in corresponding to the inlet temperature of the refrigerant into the compressor.

$$T_{\text{o},\text{aus}}=\mathrm{294,25}\text{K}\left(\mathrm{21,1}°\text{C}\right)$$

$$T_{\text{V},\text{ein}}=\mathrm{299,35}\text{K}\left(\mathrm{26,2}°\text{C}\right)$$

$$p_{\text{ND}}=\mathrm{5,5}\text{MPa}$$

6 shows a section of the pressure-enthalpy diagram 3 . In addition to the isotherms for T _R,in = 349.15 K (76°C); T _R,m = 317.15K (44°C); T _R,off = 313.15 K (40°C), the state changes of the cyclic process are entered using the following measured values, both for a high pressure value p _R ' = 9.48 MPa and for a high pressure value p _R '' = 8 .56 MPa: $$T_{\text{Ex},\text{one}}=310.95\text{K}\left(37.8°\text{C}\right)$$

$$T_{\text{O},\text{out}}=294.25\text{K}\left(\mathrm{21:1}°\text{C}\right)$$

$$T_{\text{V},\text{one}}=299.35\text{K}\left(26.2°\text{C}\right)$$

$$p_{\text{ND}}=5.5\text{MPa}$$

For the high pressure present at the time of the temperature measurements, the enthalpy changes on the high-pressure side 9 and on the low-pressure side 10 of the internal heat exchanger 2 must be the same. As 6 can be seen, the enthalpy change Δh _HD '' between the entry of the refrigerant into the internal heat exchanger with T _R,off and the exit from this with T _o,off for the lower high pressure value p _R ''= 8.56 MPa is clear greater than the enthalpy change Δh _ND on the low-pressure side 10 of the internal heat exchanger 2. The enthalpy change Δh _HD' for the higher high-pressure value p _R ' = 9.48 MPa between the entry of the refrigerant into the internal heat exchanger with T _R,out and the exit out this with T _o,aus for the higher high-pressure value p _R '=9.48 MPa, however, is equal to the enthalpy change Δh _ND on the low-pressure side 10 of the internal heat exchanger 2. As a result of the plausibility check, p _R * = 9.48 MPa than the The high pressure value that was actually present at the time of the temperature measurements can be identified.

Reference List

1

gas cooler

1', 1"

gas cooler elements

2

Internal heat exchanger

3

expansion valve

4

Evaporator

5

compressor

6

wet steam area

7

refrigerant side

8th

heat transfer fluid side

9

high pressure side

10

low pressure side

α, β, γ, δ, ε, ζ

Conditions in the refrigeration circuit

TW, a

Inlet temperature of the heat transfer fluid in the gas cooler

TW, w

Intermediate temperature of the heat transfer fluid in the gas cooler

TW, off

Outlet temperature of the heat transfer fluid from the gas cooler

TR,a

Inlet temperature of the refrigerant in the gas cooler

TR, w

Intermediate temperature of the refrigerant in the gas cooler

TR, out

Outlet temperature of the refrigerant from the gas cooler

7'Ex,a

Inlet temperature of the refrigerant in the expansion valve

Hey, off

Outlet temperature of the refrigerant from the evaporator

TV, a

Inlet temperature of the refrigerant in the compressor

pHD

high pressure of the refrigerant

pND

low pressure of the refrigerant

PR

pressure of the refrigerant

pR'

First value for the high pressure of the refrigerant

pR''

Second value for the high pressure of the refrigerant

PR*

Refrigerant pressure to be determined

Mr

enthalpy of the refrigerant

ΔhND

Enthalpy on the low-pressure side of the internal heat exchanger

ΔhHD'

Enthalpy on the high-pressure side of the internal heat exchanger at p _R '

ΔhHD''

Enthalpy on the high-pressure side of the internal heat exchanger at p _R ''

ΔhR,1

Enthalpy difference with temperature change from T _R, in to T _R,m at p _R

ΔhR,2

Enthalpy difference with temperature change from T _R,m to T _R,off at p _R

Claims (10)

Method for determining the pressure of the refrigerant in a refrigeration circuit, the refrigeration circuit having at least one component in which heat transfer takes place between the refrigerant and a heat transfer fluid, comprising at least the following method steps: a) measuring the temperatures of the refrigerant and of the heat transfer fluid at the inlet one and at the exit from a component of the refrigeration circuit in which heat transfer takes place between the refrigerant and the heat transfer fluid (T _R,in , T _R,out ; T _{W,in ,} T _W,out ); b) Measurement of an intermediate temperature of the refrigerant T _R,m and an intermediate temperature of the heat transfer fluid T _W,m in the component of the refrigeration circuit in such a way that these intermediate temperatures capture the same heat transfer process that has taken place up to there, once in relation to the refrigerant and once in relation to the heat transfer fluid ; c) Determination of the relationship between pressure p _R and enthalpy h _R of the refrigerant at the measured temperatures of the refrigerant (T _R,in , T _R,out , T _R,m ) at least in the pressure range provided for the operation of the refrigeration circuit using a data processing device Use of a substance database; d) Determination of the pressure p _R * present at the time of the measurements in method steps a) and b) by determining the dependency of the pressure p _R of the refrigerant on one of at least two different balance equations of the heat transfer process in the component of the refrigeration cycle resulting from the manipulation of a system of equations , in which heat transfer takes place between the refrigerant and the heat transfer fluid, resulting relationship between the enthalpies of the refrigerant at the temperatures of the refrigerant determined in method steps a) and b), the relationship between the enthalpies present at the time of the measurements in method steps a) and b). is determined from the corresponding relationship between the temperatures of the heat transfer fluid determined in method steps a) and b); where, if more than one pressure value p _R * is assigned to the relationship between the enthalpies of the refrigerant, which corresponds to the relationship between the temperatures of the heat transfer fluid, a plausibility check of the more is carried out as a pressure value. procedure after claim 1 , characterized in that method step d) comprises at least the following sub-steps: di) determination of the dependence of the pressure p _R on the ratio of the enthalpy differences of the refrigerant determined by means of two different balance equations of the heat transfer process at the temperatures of the refrigerant determined in method steps a) and b). Use of the relationship between pressure p _R and enthalpy h _R of the refrigerant determined in method step c) at least for the pressure range provided for the operation of the refrigeration circuit; dii) Determination of the ratio of the enthalpy differences of the refrigerant at the time of the measurements in method steps a) and b) from the corresponding ratio of differences in the temperatures of the heat transfer fluid determined using the balance equations mentioned and assignment of at least one pressure value p _R * to the ratio of the enthalpy differences using the relationship between the pressure p _R and the ratio of the enthalpy differences determined in method step di); where when more than one pressure value p _R * is assigned to the ratio of the differences in temperatures of the heat transfer fluid corresponding ratio of the enthalpy differences of the refrigerant to determine the pressure of the refrigerant present at the time of the measurements in method steps a) and b) a plausibility check of more than one pressure value is carried out. procedure after claim 2 , characterized in that the sub-steps di) and dii) comprise the following procedure: di') determining the dependence of the pressure p _R on the ratio of the difference between the enthalpies at T _R,in and T _R,m and the difference between the enthalpies at T _R,off and T _R,m , using the relationship between pressure p _R and enthalpy h _R of the refrigerant determined in method step c), at least for the pressure range provided for the operation of the refrigeration circuit; _dii ') determination of the ratio of the enthalpy differences present at the time of the measurements in method steps a) and b) from the corresponding ratio of the difference between TW,in and TW _,m and the difference between _TW,out and _TW,m and assignment of at least one pressure value p _R * to the ratio of the enthalpy differences, using the dependence of the pressure p _R on the ratio of the enthalpy differences determined in method step di′); where, if more than one pressure value p _R * is assigned to the ratio of the enthalpy differences of the refrigerant, which corresponds to the ratio of the differences in the temperatures of the heat transfer fluid, a plausibility check of the more is carried out as a pressure value. Procedure according to one of Claims 1 until 3 , characterized in that the refrigeration cycle is a refrigeration cycle with transcritical process management or with supercritical process management. Procedure according to one of Claims 1 until 4 , characterized in that the heat transfer between the refrigerant and the heat transfer fluid takes place without phase change at least the refrigerant. procedure after claim 5 , characterized in that the component of the refrigeration circuit, in which a heat transfer between the refrigerant and the heat transfer fluid takes place, is a gas cooler (1). Method according to one of the preceding claims, characterized in that the refrigerant is CO ₂ (R744). Use of a method according to one of the preceding claims for a refrigerating machine system or a heat pump system. use after claim 8 for high-pressure sensor-free monitoring of transcritical heat pump systems. use after claim 8 for optimizing the operating point of refrigeration systems or heat pump systems, in particular for determining the optimum high pressure.

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