Method for operating a heat exchanger using temperature measurements to determine saturation level

A method for operating a heat exchanger, through which a heat transfer medium flows on a primary side, entering the heat exchanger with a first temperature and exiting the heat exchanger with a second temperature. The heat transfer medium emits on a secondary side a heat flow to a secondary medium flowing through the heat exchanger in the case of heating or, in the case of cooling, absorbs a heat flow from the secondary medium which enters the heat exchanger with a third temperature and exits the heat exchanger again with a fourth temperature. The heat exchanger is capable of transferring a maximum heat flow. At least three of the four temperatures are measured and the respective saturation level of the heat exchanger is determined from these measured temperatures and is used for controlling the operation of the heat exchanger.

TECHNICAL FIELD

The present invention refers to the field of air-conditioning technology. It relates to a method for operating a heat exchanger according to the preamble of claim1. It relates further to a HVAC installation for implementing said method.

PRIOR ART

Central installations, collectively referred to as HVAC installations, are normally used for heating, cooling, air-conditioning and venting of rooms in buildings. HVAC stands for Heating, Ventilation and Air Conditioning. In such HVAC installations, heat and/or cold are/is generated centrally and are/is fed via a suitable heat transfer medium, in most cases water, to the respective premises where the heat and/or cold are/is emitted into the room air via local heat exchangers, for example.

The heat flow which is emitted or absorbed via the local heat exchanger and which is required for achieving a predetermined room temperature is often controlled in such a manner that the mass flow on the primary side of the heat transfer medium is changed accordingly. A section of an exemplary HVAC installation is illustrated inFIG. 1. The HVAC installation10′ ofFIG. 1comprises a local heat exchanger15that is connected on the primary side to a superordinated flow line11via a flow branch line13and via return branch line14to a superordinated return line12. The flow line11and the return line12are connected to a central unit for heat and/or cold generation, which is not shown here. On the secondary side, an air flow16flows around the heat exchanger15, which air flow absorbs heat in the case of heating or emits heat in the case of cooling. For adjusting the mass flow of the heat transfer medium through the primary side of the heat exchanger15, a control valve17that is activated by a control21is arranged in the flow branch line13in the example ofFIG. 1.

The heat flow emitted in the heat exchanger15to the air flow16is determined by the mass flow on the primary side of the heat transfer medium, the inlet temperature TinWthereof at the inlet of the heat exchanger15and the outlet temperature ToutWthereof at the outlet of the heat exchanger15according to the simple relation {dot over (Q)}={dot over (m)}·cp·(TinW−ToutW), with the mass flow {dot over (m)} and the specific heat cpof the heat transfer medium. The mass flow is determined here via the corresponding volume flow {dot over (V)}, which is measured with a flowmeter18that is integrated in the return branch line14, for example. Measuring the two temperatures TinWand ToutWis carried out by means of two temperature sensors19and20, which advantageously are arranged at the inlet and the outlet, respectively, on the primary side of the heat exchanger15.

A comparable arrangement is known, for example, from the publication EP 0 035 085 A1, where said arrangement is used in connection with a consumption measurement. Moreover, in the room to be heated/air-conditioned, an additional temperature sensor is provided which controls the supply of the heat transfer medium on the primary side of the heat exchanger. If the room temperature sensor (RTS inFIG. 1) in this known arrangement signalizes increased heat requirement, the valve on the primary side of the heat exchanger is opened further (at constant flow temperature) in order to provide more heat.

The problem here is that the heat flow {dot over (Q)} transferred via the heat exchanger shows a progression as a function of the volume flow V on the primary side, which is illustrated inFIG. 2. The progression of the curve—as will be explained below—depends, on the one hand, on the construction of the heat exchanger (in particular on the heat transfer surface A, the heat transition coefficient k, a factor F and an exponent n) and, on the other, on the temperature, the mass flow and the heat capacity of the medium on the secondary side of the heat exchanger.

The curve, which first steeply rises in the case of small volume flows, flattens more and more as the volume flow increases and approaches asymptotically a limit value {dot over (Q)}max(saturation). The flattening of the curve means that for the same increases in heat, greater increases in volume flow and therefore increasing pump capacity has to be provided. In particular, the capacity to be provided for the pump increases with the third power of the volume flow, whereas the transferred heat increases only insignificantly. However, this makes little sense from an economic point of view.

It is therefore desirable within such a control configuration to limit the volume flow when a predetermined value in the ratio

Q.Q.max,
which is the saturation level of the heat exchanger, is reached. Such a value can be selected to be 0.8, for example, as marked inFIG. 2. By introducing such a limit value, the pump capacity to be provided by the system can be limited without having to accept major losses of transferred heat quantity, which results in advantages in design and operation of the installation. On the other hand, it is also conceivable to change the air flow on the secondary side of the heat exchanger.

As already mentioned above, the current heat flow in the heat exchanger and therefore the point on the curve shown inFIG. 2can be determined by measuring the volume flow and the temperatures on the primary side. For certain conditions on the secondary side of the heat exchanger, the curve and its asymptote can only be determined by the control21through measurements over an extended period of time. However, this requires a flowmeter which is relatively complex and can also be prone to faults if it contains movable parts.

For these reasons it would be advantageous to have a method by means of which the saturation level of the heat exchanger can be determined and monitored in a simplified manner.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to configure a method for operating a heat exchanger of the aforementioned kind in such a manner that the use of a flowmeter is not required.

Furthermore, it is an object of the invention to propose an HVAC installation for implementing the method.

These and other objects are achieved by the features of claims1and12.

The invention is based on a method for operating a heat exchanger through which a heat transfer medium flows on a primary side, which heat transfer medium enters the heat exchanger with a first temperature and exits the heat exchanger with a second temperature, and which emits on a secondary side a heat flow to a secondary medium flowing through the heat exchanger in the case of heating or, in the case of cooling, absorbs a heat flow from the secondary medium which enters the heat exchanger with a third temperature and exits the heat exchanger again with a fourth temperature, wherein the heat exchanger is capable of transferring a maximum heat flow.

The invention is characterized in that at least three of the four temperatures are measured and that the respective saturation level of the heat exchanger is determined from these measured temperatures and is used for controlling the operation of the heat exchanger.

One configuration of the method according to the invention is characterized in that the flow of the heat transfer medium on the primary side of the heat exchanger is controllable and that the flow of the heat transfer medium on the primary side of the heat exchanger is limited when the saturation level of the heat exchanger reaches a predetermined value.

Another configuration of the method according to the invention is characterized in that the flow of the secondary medium on the secondary side of the heat exchanger is controllable and that the saturation level of the heat exchanger is used for controlling the flow of the secondary medium.

It is principally possible, depending on application and demand, to use completely different media such as, e.g., water, ice, brine, ice slurry or similar media on both sides of the heat exchanger (primary side and secondary side).

In particular, however, the heat transfer medium can be water.

In particular, however, the secondary medium can be air.

Another configuration of the method according to the invention is characterized in that the heat exchanger is part of an HVAC installation.

According to another configuration of the invention, the first, second and third or fourth temperatures are measured, and a function of the kind

Q.Q.max=f⁡(T⁢⁢1,T⁢⁢2,T⁢⁢3)⁢⁢or⁢⁢Q.Q.max=f⁡(Ti⁢⁢nW,ToutW,Ti⁢⁢nL)
is used for determining the saturation level of the heat exchanger.

Within the scope of the invention, the heat exchanger can principally be operated in concurrent flow, cross-flow or counterflow or a combination of these types.

In particular, however, the heat exchanger is operated in counterflow and the function

Q.Q.max=1-12·T⁢⁢1-T⁢⁢2T⁢⁢1-T⁢⁢3⁢⁢or⁢⁢Q.Q.max=1-12·Ti⁢⁢nW-ToutWTi⁢⁢nW-ToutL
is used for determining the saturation level of the heat exchanger.

However, it is also conceivable that the heat exchanger is operated in counterflow and that the function

If the secondary medium is air, the moisture content of the air when entering the heat exchanger can additionally be measured in the case of cooling, wherein the saturation level of the heat exchanger determined from the temperatures is corrected accordingly so as to take account of a condensation taking place in the heat exchanger.

Another configuration of the method according to the invention is characterized in that the flow temperature of the heat exchanger is increased when the saturation level of the heat exchanger reaches a predetermined value.

The HVAC installation for implementing the method according to the invention comprises a heat exchanger which is connected on the primary side to a flow line and a return line of a central heating/cooling system that operates with a heat transfer medium and through which a secondary medium flows on the secondary side, and further comprises a control means for controlling the mass flow of the heat transfer medium on the primary side and/or for controlling the secondary flow, as well as a first temperature sensor for measuring the inlet temperature of the heat transfer medium entering the heat exchanger, a second temperature sensor for measuring the outlet temperature of the heat transfer medium exiting the heat exchanger, and a controller to which the first and second temperature sensors are connected on the inlet side, and which is connected on the outlet side to the control means.

The HVAC installation is characterized in that at least one third temperature sensor for measuring the inlet temperature and/or the outlet temperature of the secondary medium entering on the secondary side into the heat exchanger are/is provided, that the third temperature sensor is connected to an input of the controller and that the controller is designed such that it controls the control means in accordance with the temperature values measured by the at least three temperature sensors.

One configuration of the HVAC installation according to the invention is characterized in that a consumer is connected on the secondary side to the heat exchanger, and that the controller receives demand signals from the consumer via a demand signal line.

Another configuration of the HVAC installation according to the invention is characterized in that the heat transfer medium is water and the secondary medium is air.

Another configuration is characterized in that the control means is a control valve which is installed in a flow branch line or return branch line that leads to the primary side of the heat exchanger.

Another configuration is characterized in that the control means is a blower which is installed in an air duct that leads to the secondary side of the heat exchanger.

In particular, a humidity sensor for measuring the moisture content of the air flowing into the heat exchanger is provided, wherein the humidity sensor is connected to an input of the controller.

Another configuration of the HVAC installation according to the invention is characterized in that a flowmeter is provided which is installed in a flow branch line or return branch line that leads to the primary side of the heat exchanger, and that the flowmeter is connected to an input of the controller.

Yet another configuration of the HVAC installation according to the invention is characterized in that a plurality of heat exchangers are arranged in a plurality of consumer circuits, that the consumer circuits are supplied with energy by the central heating/cooling system or energy generator via a distributor, that the controller comprises a demand control, and that the controller is connected to the energy generator and the distributor via control lines.

WAYS OF CARRYING OUT THE INVENTION

The present invention is based on considerations which relate to a model-like heat exchanger, as illustrated inFIG. 5. The heat exchanger23ofFIG. 5transfers a heat flow {dot over (Q)} from a hydraulic side having a hydraulic channel24to an emission side25which, for example, is provided with ribs for increasing the emission surface and along which an inflow of a medium, in particular air, flows.

Water enters the hydraulic channel24from the left with a water inlet temperature TinWand exits the hydraulic channel24again on the right with a water outlet temperature ToutW. The water passes through the heat exchanger23with a mass flow714and a volume flow {dot over (V)}. The hydraulic channel24is provided with a surface Ainsidefor the transfer of the heat flow {dot over (Q)}. On the emission side25, the secondary medium (air) flows with an air inlet temperature TinLat the inlet side and an air outlet temperature ToutLat the outlet side and with a mass flow {dot over (m)}outsideand a volume flow {dot over (V)}outsidealong a surface Aoutside.

For the heat flow {dot over (Q)} flowing from the hydraulic channel24to the emission side25, the following equations (for a stationary state) are obtained:
{dot over (Q)}={dot over (m)}·cp·(TinW−ToutW)  (1)
with the heat capacity cpon the hydraulic side (water).
{dot over (Q)}={dot over (m)}outside·cp,outside·(TinL−ToutL)  (2)
with the heat capacity cp,outsideon the emission side (air).

Q.=k·AinsideKn-1·(Δ⁢⁢T)n⁢⁢(K=unit⁢⁢Kelvin)(3)
with a heat transition coefficient k according to the following known equation

k=11αinside+s·Ainsideλ·AMaterial+Ainsideαoutside·Aoutside,(4)a ΔT according to the following known equation (logarithmic mean)

(F=correction factor for taking account of the type of heat exchanger, i.e., concurrent, cross-flow, etc.) and a power n to be determined.

For the case n=1, these equations lead to the heat flow {dot over (Q)}:

Q.=Ti⁢⁢nW-Ti⁢⁢nL1k·A·F+12·V.·ρ·cp+12·V.outside·ρoutside·cp,outside(6)
and to the maximum value {dot over (Q)}maxasymptotically achieved for large volume flows {dot over (V)}:

For the simplified case with n=1, the following simple relation is obtained for the ratio {dot over (Q)}/{dot over (Q)}max, i.e., for the portion of the achieved saturation or the saturation level of the heat exchanger:

For a generalized case with a general n and a linearized equation (3), the following applies:

The two equations (8) and (9) can be replaced accordingly by a single equation of the form

Q.Q.max=1-B⁢Ti⁢⁢nW-ToutWTi⁢⁢nW-Ti⁢⁢nL(10)
with B depending on the type (but not the size) of the heat exchanger. For a pure counterflow heat exchanger, B=½ (see equation (8)); for a different heat exchanger, B can be determined with

It is essential for this result that under certain circumstances, the saturation level of the heat exchanger is a function of three temperatures, in the present case TinW, ToutW, TinL, which can be measured in a comparatively simple manner. Thus, if the control of an HVAC installation is to be limited such that the volume flow on the primary side of the heat exchanger is limited upon reaching a predetermined saturation level {dot over (Q)}/{dot over (Q)}max(of, e.g., 0.8) in the heat exchanger, this can be performed based on a simple measurement of three temperatures (at the inlet and outlet on the primary side and at the inlet on the secondary side) of the heat exchanger, provided that the functional dependency of the saturation level on the temperatures is known. If the saturation level is known, it is then also possible to determine the corresponding volume flow from a (known) curve according toFIG. 2. Thus, the relatively laborious use and installation of a flowmeter on the primary side of the heat exchanger is not required. Nevertheless, such a flowmeter can optionally be used for calibration.

FIG. 3shows an illustration of HVAC installation according to an exemplary embodiment of the invention, which is comparable to that ofFIG. 1. The HVAC installation10ofFIG. 3differs from the HVAC installation10′ ofFIG. 1in first instance in two substantial points: On the one hand, the use of a flowmeter8is not mandatory, but rather optional in order to be able to perform a calibration, if necessary. On the other hand, a third temperature sensor22is arranged at the heat exchanger's (15) inlet on the secondary side, said third temperature sensor being connected to a further input of the controller21. In contrast to the room temperature sensor27inFIG. 1, the third temperature sensor22does not measure a room temperature, but instead the air inlet temperature TinLof the air (air flow16) flowing into the heat exchanger15. It should be noted at this point that it is also possible, of course, to use a controllable pump or—if the heat transfer medium is gaseous—a blower (or an air flap) instead of the control valve17for influencing the volume flow on the primary side.

The controller21measures the three temperatures TinW, ToutWand TinLby means of the three temperature sensors19,20and22and determines therefrom the current saturation level

Q.Q.max
of the heat exchanger by means of a known functional dependency

Q.Q.max=f⁡(Ti⁢⁢nW,ToutW,Ti⁢⁢nL)
If this saturation level exceeds a predetermined limit value, which can be 0.8, for example, the volume flow {dot over (V)} on the primary side of the heat exchanger15is limited, even if the control requests a larger volume flow due to changing room temperatures.

In the simplest case, determining the saturation level is performed in accordance with the above-mentioned equation (8). The above-mentioned equation (9) can be more suitable in other cases. Other functional dependencies are also conceivable within the scope of the invention.

If the optional flowmeter18is additionally installed, the heat flow can be determined in a conventional way, and thus an assumed functional dependency

Q.Q.max=f⁡(Ti⁢⁢nW,ToutW,Ti⁢⁢nL)
can be checked or calibrated. It is in particular conceivable that such a flow meter18is used only during the startup procedure of an installation and is omitted during later operation.

In another configuration of the method according to the invention, it is detected with the described method that the heat exchanger has exceeded a predetermined saturation level or is in saturation, thus, can no longer transfer heat. In this case, the system is informed that the flow temperature needs to be increased. This can be carried out by increasing the temperature of the central flow in the flow line11. In circuits with constant volume flow, a special valve is located at each position where it is able to control the flow temperature of the consumer.

A special case occurs if an installation according toFIG. 3is intended to cool an air flow16that contains moisture which condensates during cooling in the heat exchanger15and can be discharged as condensed water from the heat exchanger15. This is in particular the case in tropical areas with high humidity where the installation can be used specifically for dehumidifying room air.

In this operating condition, a portion of the cold Δ{dot over (Q)} transferred to the air in the heat exchanger is used not for cooling the air, but instead for condensation of the moisture. The total cold flow is therefore larger and the limit value for associated volume flow on the primary side is therefore reached earlier than can be expected from the value of the cold flow for cooling the air ({dot over (Q)}1inFIG. 4) determined from the three temperatures. If this is to be taken into account, a correction can be made that also takes account of the moisture content of the air flowing through the heat exchanger15. For this purpose, a humidity sensor26which measures the moisture content of the air and transmits the measured values to the controller21can be arranged according toFIG. 3in the air flow16. From the measured temperature values and the measured moisture content, the controller21then determines the cold flow Δ{dot over (Q)} which is needed exclusively for the condensation and has to be added to the value ({dot over (Q)}1inFIG. 4) that is required for cooling the air so as to determine the correct associated volume flow according to the curve fromFIG. 4. A limit value for the volume flow in the case of condensation thus is reached earlier than without condensation.

Another possibility of operation in an HVAC installation30according toFIG. 6is to measure the inlet temperature TinLand the outlet temperature ToutLof the air in the air flow16on the secondary side of the heat exchanger15by means of the temperature sensors22and27and to use these measurements (analogously to the way described above) in connection with a temperature measurement on the primary side for deriving the heat exchanger's15saturation level, which depends on the volume flow on the secondary side, and therefore for deriving the volume flow on the secondary side (the heat exchanger15is viewed, as it were, in the opposite direction).

This variable can then be used to intervene in the volume flow on the secondary side of the heat exchanger15in a controlling or limiting manner. This can be carried out by means of a blower which is controlled by the controller21and is arranged in an air duct28that leads to the heat exchanger15(or away from the heat exchanger15). However, instead of the blower, a controllable air flap or—if the secondary medium is liquid, for example—a pump or a control valve can also be provided as a control means.

Such a control is particularly advantageous if—as it is often the case—a temperature sensor27is already installed at the outlet on the secondary side of the heat exchanger15in an HVAC installation.

However, it is principally also conceivable within the scope of the invention to measure only the temperatures TinW, ToutWand ToutLand to use them for controlling in the heat exchanger operation.

The present invention can be advantageously used in HVAC installations which comprise a so-called demand control and which become increasingly important with respect to increased energy efficiency.

FIG. 7shows in a schematic illustration the exemplary structure of an HVAC installation40with demand control. In the example, the HVAC installation40comprises five consumer circuits34a-ewhich are supplied with heat and/or cold energy by a central energy generator31via a distributor32and the corresponding supply lines47a, b. A heat exchanger35which transmits the fed energy to a consumer36is arranged in each of the individual consumer circuits34a-e.

Providing the energy by the energy generator31and distributing the energy by the distributor32is controlled by a demand control33via corresponding control lines41and42. Moreover, the demand control33can intervene in a controlling manner in the individual consumer circuits34a-eon the consumer side via corresponding control lines39in order to change the volume flow on the secondary side in the respective heat exchanger35, for example.

The demand control33receives demand signals from the consumer circuits34a-evia demand signal lines38in order to control the generation and distribution of energy in such a manner that the requested demand is covered in a way that is optimized according to predetermined criteria such as, e.g., energy efficiency.

For this optimization, information about the respective operating state of the heat exchangers35is needed, namely the inlet and outlet temperatures, the saturation level, the volume flows on the primary and secondary sides and—if air is used as the medium—the moisture content of the air.

According to the invention, this information can be derived from simple temperature and, optionally, humidity measurements without having to use complicated flowmeters. Accordingly, temperature values from the heat exchanger35are transmitted to the demand control33via temperature signal lines37(a signal line for the moisture measurement is not illustrated inFIG. 7).

The structure in the individual consumer circuit34nis illustrated inFIG. 8. The inlet and outlet temperatures T1, T3and T2, T4are measured on the primary and secondary sides by means of the temperature sensors43a-dand, optionally, the relative humidity is measured with a humidity sensor44. The secondary medium flows through the consumer36arranged on the secondary side of the heat exchanger35and is moved in a circuit by means of a feed device45such as, for example, a pump, a blower or the like. The volume flow of the secondary medium can be influenced either via the feed device45or via separate control means46, a valve, a flap or the like. A demand signal is output from the consumer36itself and is transmitted to the demand control33via the demand signal line38.

According to the invention, the saturation level of the heat exchanger35as well as the volume flows can be determined from the measured temperatures T1-T4. If the optimization requires intervention of the demand control33on the secondary side, this can be carried out by means of the control lines39a, bvia the feed device45and/or the control means46.

If the optimization requires intervention of the demand control in the distributor32, this can be carried out via the control line42. Intervention in the energy generator31is performed via the control line41. Such an intervention can include changing the flow temperature, for example. However, it is also conceivable to change the overall energy generation in stages if a plurality of similar modules in the energy generator (e.g. refrigerating machines) operate simultaneously and can be activated individually, as disclosed in the printed publication U.S. Pat. No. 7,377,450 B2, for example.

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