Method For Operating A Liquid-To-Air Heat Exchanger Device

Abstract

A method for operating a liquid-to-air heat exchanger device comprises: determining a dew point temperature of ambient air; determining whether the dew point temperature is higher than a temperature of the liquid, and as long as this is the case operating the heat exchanger device in a pulsed operation mode according to: allowing the liquid to flow through a first heat exchanger stage during a predetermined period of time; preventing that the liquid flows through the first heat exchanger stage, monitoring the temperature of the air at the exit of the first heat exchanger stage, said temperature indicating a first increase, remaining awhile constant and then showing a second increase; detecting the second increase and terminating the preventing that liquid flows through the first heat exchanger stage once the second increase was detected.

Description

TECHNICAL FIELD

The invention concerns a method for operating a liquid-to-air heat exchanger device.

BACKGROUND OF THE INVENTION

The method is suitable for operating a liquid-to-air heat exchanger device which comprises a passive heat exchanger stage in which the air is guided through a first flow channel extending in the vertical direction and the liquid is guided through a second flow channel, wherein the two flow channels in this stage are separated by a thermally passive separating wall. The term “thermally passive” means that the exchange of heat occurs without performing any work. The flow channels contain a plurality of plate fins which are in good thermal connection with the thermally passive separating wall. The distances between the plate fins in the flow channel for the air are relatively small in relation to the size of their surface so that the heat exchange is efficient.

If the air has a relatively high atmospheric humidity it may occur especially on hot summer days that the dew point temperature of the air is higher than the temperature of the liquid. This leads to the consequence that humidity contained in the air will deposit as condensate on the plate fins. Since the overall size of the heat exchanger device is usually subject to narrow limits, it is difficult to form the plate fins in such a way that the produced water will drop and drain off completely, especially in the case of vertical guidance of the air flow. This leads to the consequence that the water will increasingly block the intermediate spaces between the plate fins and will effectively prevent further effective cooling of the air as a result of the thereof arising air resistance.

From GB 2461365, a central heating system with at least one radiator is known, which can also be used for cooling. In cooling operation, heat is withdrawn from the liquid circulating through the radiator by means of a heat exchanger. The extracted heat is supplied to a heat storage unit by means of a second heat exchanger. The two heat exchangers are part of a compressor-operated heat pump. In order to prevent that humidity can condensate on the radiator, the dew point of the air is determined by measurement of temperature and humidity in the ambient environment of the radiator and when the determined dew point temperature moves towards the temperature of the radiator the cooling output is reduced.

From EP 508766, a method for controlling an air-conditioning system is known, in which the temperature is determined in cooling operation in which condensation of water occurs on a test element and in which it is ensured in this case that the temperature of the cooling liquid is higher than the condensation temperature. This occurs for example by stopping the cooling operation.

The solutions known from this state of the art have the joint objective of preventing the accumulation of condensate and achieve this by reducing the cooling performance or interrupting cooling operation.

BRIEF DESCRIPTION OF THE INVENTION

The invention is based on the object of remedying the aforementioned problem.

The object as mentioned above is achieved in accordance with the invention by the features of claim 1. Advantageous embodiments are provided in the dependent claims.

The invention relates to the operation of a liquid-to-air heat exchanger device, which comprises a first flow channel for the air and a second floor channel for the liquid. The heat exchanger device contains a first passive heat exchanger stage in which the first flow channel and the second flow channel are separated by a thermally passive separating wall, and optionally a second heat exchanger stage in which the air is actively cooled or heated, i.e. by pumping of heat from one side to the other. The thermally passive separating wall consists of a material that conducts heat very well. A matching condensate drainage system is advantageously built into the second heat exchanger stage. The first and second flow channel can each also be a plurality of flow channels extending in parallel. The flow channel or channels for the air contain(s) plate fins.

The invention proposes a method in order to achieve the aforementioned object. The method comprises two parts, namely a first part in which it is determined whether the dew point temperature of the air is higher than the temperature of the liquid. This occurs by the following steps:

Determination of the dew point temperature of the ambient air, i.e. the dew point temperature of the air before it enters the first heat exchanger stage;

• • comparing the determined dew point temperature of the air with the temperature of the liquid which is measured or transmitted by a superordinate control device.

The dew point temperature of the air can be determined by the following for example:

• • Measurement of the temperature of the air and the humidity of the air before the entrance of the air into the first heat exchanger stage, and subsequently
  • determining the dew point temperature of the air from the measured temperature and the measured humidity of the air.

The determination of the dew point temperature of the air from the measured temperature T and the measured humidity of the air can occur for example by means of a Mollier diagram. The dew point temperature, designated as T_p1, can alternatively be determined by calculation by means of the equation

$T_{\mathrm{pl}}=\frac{241.2*\ln \left(\frac{\mathrm{phi}}{100}\right)+\frac{4222.03716*T}{241.2+T}}{17.5043-\ln \left(\frac{\mathrm{phi}}{100}\right)-\frac{17.5043*T}{241.2+T}}$

wherein the unit of measurement of the temperatures T and T_p1 is degrees Celsius and the air humidity phi is entered as a relative air humidity stated in percent.

It is also possible to measure two other quantities of the h-x diagram of the air (h designates enthalpy, x designates absolute humidity), e.g. two from the dry bulb temperature, wet bulb temperature, specific enthalpy and density of the air, and therefrom the dew point temperature of the air.

If and as long as the dew point temperature of the air is higher than the temperature of the liquid, the second part of the method is performed, which consists of operating the heat exchanger device in an operating mode designated as pulsed operation. The pulsed operation comprises the following steps which are continuously repeated in the same sequence:

• • allowing the liquid to flow through the first heat exchanger stage during a predetermined period of time;
  • preventing that the liquid flows through the first heat exchanger stage, and measuring and monitoring the air temperature after exiting from the first heat exchanger stage, wherein the air temperature measured after exiting from the first heat exchanger stage indicates a first rise in the temperature, and then remains for a certain period of time at a level which is constant in good approximation and which corresponds to the wet bulb temperature of the incoming air, and then indicates a second rise in the temperature;
  • detecting the second temperature increase and terminating the preventing that liquid flows through the first heat exchanger stage once the second temperature increase was detected, and
  • repeating these steps as long as the dew point temperature of the air is higher than the temperature of the liquid.

The condition whether the dew point temperature of the air is higher than the temperature of the liquid is checked periodically or aperiodically in pulsed operation in that the first part of the method is carried out.

In the pulsed operation, a phase of the removal of condensate by evaporation periodically follows an accumulation phase, while cooling of the air continues in an uninterrupted fashion. Although the pulsed operation allows a temporary accumulation of water between the plate fins, it still prevents blockage of the plate fins by condensate which would lead to blocking of the air flow, reduces the time of switching off the water flow to a minimum and thus increases the efficiency of the heat exchanger device.

In order to ensure that the method in accordance with the invention can be performed, the heat exchanger device is equipped with the temperature and humidity sensors necessary for this purpose.

If the heat exchanger device comprises a second active stage in which heat is pumped between the liquid and the air by supply of energy, the step of preventing that the liquid flows through the first heat exchanger stage further ensures according to a first variant that the liquid will also not flow through the second heat exchanger stage and that the second heat exchanger stage is deactivated, or the step of preventing that the liquid flows through the first heat exchanger stage ensures according to a second variant that the liquid is guided past the first heat exchanger stage (bypass), so that it can still flow through the second heat exchanger stage.

The invention is subsequently explained in more detail by means of exemplary embodiments and the drawing. The drawings are not drawn to scale.

DESCRIPTION OF THE DRAWING FIGURES

FIGS. 1, 2 schematically show a side view or top view of the parts of a liquid-to-air heat exchanger device necessary for understanding the invention, which heat exchanger device is set up for operation according to the method in accordance with the invention, and

FIG. 3 shows three diagrams for illustrating the method in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 schematically show a side and top view of the parts of a liquid-to-air heat exchanger device 1 necessary for the understanding of the invention, which heat exchanger device comprises a first passive heat exchanger stage 2 and optionally a downstream active heat exchanger stage 3. The first heat exchanger stage 2 comprises at least one flow channel 4 (preferably several thereof) for the air and at least one flow channel 5 (preferably several thereof) for the liquid. The flow channels 4 for the air and the flow channels 5 for the liquid are arranged in alternating succession and are separated by thermally passive separating walls which conduct heat well. The flow channels 4 for the air contain a plurality of plate fins 6 which are in good thermal connection with the thermally passive separating walls. The distances between the plate fins 6 are small, so that heat exchange between the air and the liquid is efficient. The flow channels 4 for the air extend in the vertical direction in this example.

The optional second active heat exchanger stage 3 can be formed in different ways. It can contain a cooling circuit with a compressor for example, in which a cooling liquid circulates, wherein the air exchanges heat with the cooling circuit.

In the example shown in FIGS. 1 and 2, the second heat exchanger stage 3 is formed in such a way that heat can be exchanged between the liquid and the air by supplying electrical energy, i.e. by means of at least one Peltier element 10. The second heat exchanger stage 3 contains at least one flow channel 7 for the air, at least one flow channel 8 for the liquid, and the at least one interposed Peltier element 10 which pumps heat from the liquid to the air when the air is to be heated and which pumps heat from the air to the liquid when the air is to be cooled. The liquid is not subjected to any change in the aggregate state in this example. In the illustrated example, the air flows between plate fins 9 which are arranged in parallel and which are in good thermal contact with the at least one Peltier element 10.

The heat exchanger device 1 further comprises a valve 11 and optionally a bypass line 12 whose purpose will be described below in closer detail.

Persons skilled in the art often use the term “thermoelectric element” or the term “Peltier heat pump”as a synonym for the term “Peltier element”. The thermoelectric elements are especially based on the Peltier effect, but they can also be based on another thermoelectric effect such as the principle known as thermo tunnelling.

The heat exchanger device 1 comprises an inlet 13 and an outlet 14 which can be connected to an external liquid circuit. The liquid circulating in the liquid circuit is heated or cooled by an external central device to a predetermined temperature. The liquid that is used is usually water or a liquid on water basis, but it is possible to use any other suitable liquid. The flow channels 4 for the air extend in the vertical direction. The flow channels for the liquid are designed as a line system which connects the inlet 13 and the outlet 14 to each other. The heat exchanger device 1 further contains a fan and the necessary baffle plates and guide elements for the forced guidance of the air through the first heat exchanger stage 2 and the second heat exchanger stage 3 (if present), and a drain 15 for the condensate accumulating in the second heat exchanger stage 3. The direction of flow of the liquid is shown by arrows 16 and the direction of flow of the air is indicated by arrows 17.

The heat exchanger device 1 further comprises the sensors which are necessary for operation in accordance with the invention, which are at least one temperature sensor 18 for the measurement of the temperature and a humidity sensor 19 for measuring the humidity of the air, which are arranged before the first heat exchanger stage 2, a temperature sensor 24 measuring the temperature of the air which is arranged after the first heat exchanger stage 2, and a control device 21. The temperature of the liquid is either measured by means of a temperature sensor 22 which is arranged at the inlet for example or is transferred from an external central device to the control device 21. The control device 21 evaluates the data transmitted by the sensors and controls both the flow rate of the liquid through the first heat exchanger stage 2 and also the at least one Peltier element 10.

FIG. 3 shows three diagrams arranged one above the other, which illustrate the following features of the method in accordance with the invention in function of time t on the basis of an example.

The middle diagram shows the flow rate of the liquid through the first heat exchanger stage 2. The flow rate of the liquid through the first heat exchanger stage 2 is permitted for a predetermined period of time T₁ and is then interrupted, wherein the interruption in the flow of the liquid through the first heat exchanger stage 2 occurs either by closing the valve 11 or, if there is a bypass line 12, by changing over the valve 11, so that the liquid flows through the bypass line 12 and is therefore guided past the first heat exchanger stage 2.

The bottom diagram shows the current flowing through the at least one Peltier element 10 in the case that the interruption of the flow of the liquid through the first heat exchanger stage also produces the interruption of the flow of the liquid through the second heat exchanger stage 3. The current flowing through the at least one Peltier element 10 is deactivated either simultaneously or with a time delay whenever the flow of the liquid through the first heat exchanger stage 2 is interrupted, so that the at least one Peltier element 10 will not overheat. In the other case that the flow of the liquid through the second heat exchanger stage 3 is not interrupted, the at least one Peltier element 10 is not deactivated.

The upper diagram shows the progression of the temperature of the air after exiting the first heat exchanger stage 2, i.e. the progression of the temperature measured by the temperature sensor 20. The illustration clearly shows a first temperature increase 23 (in the example from 18° C. to approximately 22° C.), an approximately constant level 24 and a second temperature increase 25 (in the example from approximately 22° C. to approximately 27° C.).

The progression of the temperature as shown in the diagram consists of the following repeated phases A-D:

Phase A: The flow of the liquid through the first heat exchanger stage 2 is not interrupted. The air is cooled, in the example to approximately 18° C. Water gradually condensates between the plate fins 6, which increasingly increases the flow resistance of the air.

Phasen B to D: The flow of the liquid through the first heat exchanger stage 2 is interrupted.

Phase B: The temperature of the air increases to the approximately constant level 24.

Phase C: The temperature of the air remains at the level 24, since the water accumulated between the plate fins 6 evaporates and cools the air adiabatically in this process.

Phase D: The temperature of the air increases further once the water has evaporated between the plate fins 6.

FIG. 3 shows the pulsed operation very clearly. Since the duration of the individual cycles (a cycle comprises a sequence of the phases A-D) typically lies in the range of a few minutes or a few 10 minutes and the dew point temperature of the air usually only changes slowly, the dew point temperature only needs to be measured occasionally during the pulsed operation, e.g. once per half hour or hour, or also in other intervals.

Claims

1. A method for operating a liquid-to-air heat exchanger device, in which air flows through at least one first flow channel at least in a first passive heat exchanger stage (2), which flow channel comprises plate fins, and a liquid flows through at least one second flow channel which is separated from the at least one first flow channel by a thermally passive separating wall, comprising the following steps:

determining a dew point temperature of ambient air;

determining whether the dew point temperature of the ambient air is higher than a temperature of the liquid, and if this is the case operating the heat exchanger device in an operating mode designated as pulsed operation according to the following steps:

allowing the liquid to flow through the first heat exchanger stage during a predetermined period of time;

preventing that the liquid flows through the first heat exchanger stage, and measuring and monitoring the temperature of the air after exiting of the air from the first heat exchanger stage, wherein the temperature of the air measured after exiting the first heat exchanger stage indicates a first temperature increase, remains subsequently at an approximately constant level for a specific period of time, and then shows a second temperature increase;

detecting the second temperature increase and terminating the preventing that liquid flows through the first heat exchanger stage once the second temperature increase was detected, and

repeating these steps as long as the dew point temperature of the ambient air is higher than the temperature of the liquid.

2. The method of claim 1, wherein the dew point temperature of the ambient air is determined by

measuring the temperature of the air and the humidity of the air before entrance of the air into the first heat exchanger stage, and

determining the dew point temperature of the air from the measured temperature and the measured humidity of the air.

3. The method of claim 1, in which heat is pumped between the liquid and the air by supply of energy in a second active heat exchanger stage, wherein the step of preventing that the liquid flows through the first heat exchanger stage also ensures that the liquid does not flow through the second heat exchanger stage, and that the second heat exchanger stage is deactivated.

4. The method of claim 1, in which heat is exchanged between the liquid and the air by supply of energy in a second active heat exchanger stage, wherein the step of preventing that the liquid flows through the first heat exchanger stage ensures that the liquid bypasses the first heat exchanger stage so that it still flows through the second heat exchanger stage.

5. The method of claim 2, in which heat is pumped between the liquid and the air by supply of energy in a second active heat exchanger stage, wherein the step of preventing that the liquid flows through the first heat exchanger stage also ensures that the liquid does not flow through the second heat exchanger stage, and that the second heat exchanger stage is deactivated.

6. The method of claim 2, in which heat is exchanged between the liquid and the air by supply of energy in a second active heat exchanger stage, wherein the step of preventing that the liquid flows through the first heat exchanger stage ensures that the liquid bypasses the first heat exchanger stage so that it still flows through the second heat exchanger stage.