EP3601894A1

EP3601894A1 - Method to counteract build-up of frost on a heat reclaimer arranged in an air treatment unit

Info

Publication number: EP3601894A1
Application number: EP18777332.0A
Authority: EP
Inventors: Johan BJÄRKLEV; Daniel Berg
Original assignee: FlaektGroup Sweden AB
Current assignee: FlaektGroup Sweden AB
Priority date: 2017-03-31
Filing date: 2018-03-01
Publication date: 2020-02-05
Also published as: SE1750396A1; SE540735C2; EP3601894A4; RU2019128646A3; WO2018182479A1; RU2760419C2; RU2019128646A

Abstract

Method to counteract build-up of frost on a heat reclaimer (1) arranged in an air treatment unit (2), wherein the method being characterized in that in every point of operation to counteract build-up of frost on the heat reclaimer (1) in an energy efficient manner as follows. By measuring the temperature and moisture of the exhaust air (5) and calculating its dew point, and then establish a validation temperature and a temperature of frost build-up, which depends, inter alia, on the type of heat reclaimer (1), the validation temperature is checked against at least one of two criteria K₁, K₂. If at least one of the criteria K₁ or K₂ a step is taken to influence the point of operation of the heat reclaimer (1) by reducing its efficiency η and/or raising the temperature of the exterior air (7) before the heat reclaimer (1), just as much that any of the criteria K₁, K₂ no longer is fulfilled, for achieving an optimal recovery and at the same time counteract build-up of frost.

Description

Method to counteract build-up of frost on a heat reclaimer arranged in an air treatment unit Field of the invention

The present invention relates to a method of use in an air treatment unit which is provided with a heat reclaimer of some kind for the recovery of energy from an exhaust air flow and transfer this energy to an intake air flow. The aim is, in every point of operation, to prevent frost build-up on the surface of the heat reclaimer, but still use the heat reclaimer to the maximum, preferably by a minimum number of sensors, thereby minimizing the energy loss that occurs if defrosting of the heat reclaimer must be done.

Description of the prior art

A well-known problem in the technology of air treatment is that frost easily is build-up on the surfaces of the heat reclaimer under certain conditions such as high moisture content in the exhaust air in combination with cold exterior air temperature which exterior air reaches the heat reclaimer essentially unheated in many cases. Since build-up of frost on surfaces in the heat reclaimer affects the recycling rate negative, it is desirable to either totally prevent frost build-up, or to permit a certain occurrence of frost or build-up of ice and subsequently defrost by various methods and devices. Known methods usually comprises that a number of measurements being carried out to indicate frost formation or risk of frost build-up. For example, one method is to measure the temperature and moisture content of the exhaust air and as well as the heat reclaimer's input exterior temperature and to allow it to be an indicator to determine whether the heat recovery is to be reduced, thereby defrosting or avoiding frost build-up. Another known way is among others to measure pressure drop over the heat reclaimer in order to record when frost or ice begins to be build-up on the heat reclaimer wherein then the pressure drop gradually increases over the same. The latter is very common and the most common and relatively simple way is to defrost the heat reclaimer, when frost or ice is build-up, and the defrosting takes place either by reduced heat recovery or by that a preheating battery heats up the exterior air, that reaches the heat reclaimer, so that there is no ability for the condensed water to freeze. This is not energy-efficient, and energy efficiency requirements are constantly increasing. The requirements for energy efficiency have also recently led to an increasing number of air treatment units having so- called VAV regulation. VAV means "Variable Air Volume" and means demand-controlled ventilation, that is, if a room or a space is not used, the ventilation flow is pulled down to a minimum, and then returned to normal operation or forced operation when the room is used. Overall, this affects the flow and pressure drop across the components of the air treatment unit. To measure the drop of pressure and to use it as an indicator of frost formation is then difficult or not even possible. In many cases solutions with preheating battery must be used, which usually means that an electric heating battery with a relatively high power-consumption and thus high energy consumption is installed before the reclaimer in the exterior air flow for defrosting. As mentioned above, a high energy consumption is undesirable. Another problem with known solutions is to measure the temperature in a representative place, for example on, in or after the heat exchanger, to know if it in the coldest part of the heat exchanger is a risk of ice formation or build-up of frost. The frost is actually build-up on the surfaces of the heat reclaimer, but for practical reasons the air temperature is usually measured in or after the heat exchanger. For so-called plate heat exchangers, such as cross-flow or counter flow coupled, the most critical temperature is in the industry referred to as the "cold corner", which then represents the position where the lowest temperature is or is expected and where a temperature thus should be measured. For rotating heat exchanger there is another problem - the temperature varies in principle over the entire cross section of the rotor. The point at which it is the most representative to measure is a problem, since, firstly, it is undesirable to have many different sensors for registering the most critical position in each operating case, and secondly, an "optimal" position may vary with the operating case that prevails at the moment. With known technology, measurements must be taken either at many points, or at a point which may be considered representative, but which therefore in reality is a compromise, since the position may vary depending on the rpm of the rotor, the rotor size, the airflow, temperature conditions etc. Today's methods and solutions are "generalised" and are used with relatively large safety margins where the solution often is to defrost instead to preclude frost build-up, and if the latter is used, this is also with good margins, because it is difficult to measuring correct and in the correct position. There is therefore a need for a more precise method that works in the case of varying pressure drop in the plant and with as much recovery as possible to meet future energy savings targets.

Summary of the invention

According to the present method the above problems in an air treatment unit are solved with the preamble of claim 1 , wherein the solution of the above problems is by the method continuous or intermittent at first measure the temperature and moisture content of the exhaust air in one position, preferably just before the heat exchanger, and then calculate the current dew point temperature Td_P of the exhaust air based on measured values. After this, a so-called validation temperature T_v is determined, either by calculation or by measurement, for the current point of operation and based on the measured and calculated values. For different types of heat reclaimers, the validation temperature will be determined in different ways, which is why at this stage it is talked about "determining" the validation temperature. Most preferred is to calculate the validation temperature, since, for most different types of heat reclaimers, the number of sensors can be minimized and that one becomes independent of in which position the measurement must take place, since measurement is

superfluous. The idea is thus to calculate when the risk of frost build-up comes up and not start too early or too late with actions, but rather to "accompany" with current operational data to optimally recover as much energy as possible and for as long as possible without frost building up on the heat exchanger and this without too large safety margins. For many cases, the validation temperature may be the same as a "critical temperature", that is, a temperature in which condensation is formed or the freezing point is passed and is in other cases selected to have a certain margin to freezing point and/or condensation point for the operating case. For some applications, measurement instead of calculation for the determination of the validation temperature may be relevant, for example in the case of so-called battery heat exchangers where measurement of the temperature of the inlet water to the exhaust air battery may be done instead of calculation because this is a relatively simple, cost- effective, safe and representative measurement to do at liquid coupled heat reclaimers. Other heat reclaimers such as a rotor has, as mentioned above, varying temperatures over virtually the entire surface why it is difficult to place a sensor in a representative place. In addition to the validation temperature, also a so-called frost-build temperature Tf, is determined, which is a value that depends on one or more properties or characteristics of the application in question and in the current operating case, such as the type of heat reclaimer, the temperature efficiency of the reclaimer and, where appropriate, its moisture efficiency; moisture content and temperature of the exhaust air, the temperature of the exterior air, and possibly the speeds of air in intake air and exhaust air. This value Tf is thus a function of one or more of these characteristics and can be produced either empirically or analytically. The temperature of frost build-up can thus be a constant or consist of an equation descriptive of a straight line or curve, or other model which is based on empirically determined data for all possible points of operation or is based on algorithms. For a plate heat exchanger as an example, the value can be a single constant, while a battery heat exchanger can be described by an interval (line).

In order to insert action, in an appropriate manner, to prevent frost build-up, the established validation temperature Tv is checked against at least two criteria Κι, K2, as:

K₂: Tv < Td + Ai°C, wherein Δ1≥ 0.

If at least one of the two criteria are met an action is taken in order to influence the point of operation of the heat reclaimer to prevent frost build-up on the heat reclaimer, until at least one criteria no longer are fulfilled or no longer is fulfilled with a certain margin or after a predefined period of time. The first criteria relate to if a check if the validation temperature is below the so-called frost build-up temperature and the second relates to if the validation temperature is below the point of condensation (dew point). To the latter a "safety factor" Δ1 can be selected that is set to be zero or larger. In other words, there is a changeable margin to be entered in the control unit before the action is to be taken to prevent frost formation/frost build-up or the action to be stopped. If the safety factor is selected to be 0, minimal energy loss is obtained, but in practice a certain margin is likely to be chosen, preferably a small margin, to take a measure to prevent frost formation and frost build-up. Thus, if at least one of the criteria is met, a measure is activated, whereby the point of operation of the heat reclaimer is affected. This is done either by reducing the efficiency of the heat reclaimer and/or by raising the temperature of the incoming exterior air flow to the heat reclaimer. The degree of efficiency referred to is in the preferred case the temperature efficiency of the heat reclaimer, but to change the temperature efficiency also changes the efficiency of moisture content, which is why the term efficiency is used to cover both types. By the method it is also possible to choose how precise the control for prevention of frost build-up should be and in this way the control follows the current operating conditions very precisely with optimal utilization of the heat recovery, unlike known solutions which have a rough and generalized control of the defrosting and/or the frost protection measures. According to a preferred embodiment, the heat reclaimer is a liquid coupled heat reclaimer, which comprise at least one exhaust air battery in the first air flow and at least one intake air battery in the second air flow. Between these batteries, then a controllable liquid flow is circulating, with respect to the flow rate, in a liquid circuit (piping system), which liquid flow transfer energy between the air flows for recycling. When the air treatment unit is arranged with such a heat exchanger, the method according to the preferred embodiment comprises the additional steps of measuring the liquid temperature of the controllable incoming liquid to the exhaust air battery, and to determine the validation temperature to be equal to the measured temperature of the incoming liquid to the exhaust air battery. Furthermore, if at least one of the criteria Ki or K2 are met, the measure occurs - to influence the point of operation of the heat exchanger - by reducing its efficiency. The latter is done by that a control signal, which in various ways controls the flow of liquid, is changed, either by controlling a circulation pump for increased or decreased flow through the battery, or that one in the circulatory circuit included valve is adjusted, whereby the flow through the battery changes. A combination of circulation pump and control valve can also be used for controlling the flow. By controlling the temperature of the input liquid according to any of the measures and the efficiency is changed, to prevent that frost is build-up on the exhaust air battery. In this case, it is in practice good enough to measure the temperature of the input liquid instead of calculating the same. This is because it is sufficient to use one sensor, which is arranged to measure either directly in the liquid flow or on the outside of the tube. The measurement is reliable and the measurement position, before the liquid flows into the exhaust air battery, is definitely the coldest position. However, it does not exclude that a calculated value can be used instead of the measured to determine the validation temperature. The proposed method at the liquid coupled reclaimer provides a more precise control with maximum utilization of the heat recovery without the build-up of frost, compared to existing methods.

According to another preferred embodiment, the heat reclaimer is a liquid coupled heat reclaimer, which comprises at least one exhaust air battery in the first air flow and at least one intake air battery in the second air flow. Between these batteries then is circulating, with respect to the flow rate, a controllable liquid flow is circulating in a piping system, which liquid flow transfer energy between the air streams, as described above. In addition, the air treatment unit also comprises a pre-heater, which is arranged in the other air flow, in the flow direction, before the exhaust air battery. The method also comprises the steps that, just as in the previously presented case, measure the temperature of the input liquid to the exhaust air battery, and further to determine the validation temperature to be equal to the measured temperature of the input liquid to the exhaust air battery. Furthermore, if at least one of the criteria Ki or K2 are met, the measure occurs - to influence the point of operation of the heat exchanger - by increasing the temperature of the exhaust air before the intake air battery. This is done by changing the control signal to the pre-heater to thereby increase the power output from the same, and thereby preventing frost build-up in the exhaust air battery. By combined control of the heat recovery and the pre-heater, along with constantly checking the validation temperature against the criteria Ki and the K2, optimum operation with low energy consumption is obtained despite that prevention of frost build-up occurs and thus there is no need of energy-consuming defrosting sequence of a reclaimer covered with ice, as in prior art. According to another preferred embodiment the heat reclaimer is a plate heat exchanger, either in the form of a cross-flow heat exchanger or a counter flow heat exchanger, which is arranged to exchange energy between the first and the second airflow. The method thus comprises the step to measure the temperature in the so called "cold corner", Tec, at the plate heat exchanger. The cold corner is a known term in connection with plate heat exchangers and corresponds to the coldest position where the air meets the extract air. Furthermore, the method comprises the step to set the validation temperature to being equal to the measured temperature in the cold comer, Tec, which is considered to be a safe and cost-effective way to measure the temperature, i.e. with a correct placed temperature sensor in the "cold corner". This is because the position certainly is the coldest position unlike the uncertainty that prevails by measuring, for example, by a rotating heat exchanger. The point of operation of the heat exchanger is influenced in this case by reducing the efficiency of the plate heat exchanger by changing a control signal to the control devices of the throttle, which in conventional manner are arranged at the exterior air side of the plate heat exchanger and at a so-called "by-pass-section". By that the throughput of air through the recycling part of the heat reclaimer by by-pass at least a subset of the exterior air, wherein the efficiency decreases to prevent frost build-up in the heat reclaimer. By the method the validation temperature is controlled against above criteria with the aim of constantly preventing frost build-up but with simultaneous maximum possible heat recovery, that is, to be as close to the limit of frost as desired for the current plant, which saves energy and eliminates the need for recurrent defrosting sequences, such as in prior art at plate heat exchangers.

According to an alternative preferred embodiment the heat recycler is also in this embodiment a plate heat exchanger, which is arranged to exchange energy between the first and second airflow. The method thus comprises the step to measure the temperature in the so called "cold corner", Tec, at the plate heat exchanger, just as mentioned above. The method also comprises the step of setting the validation temperature to be equal to the measured temperature in the cold corner which is considered to be a safe and cost-effective way to measure the temperature, i.e. with a correct placed temperature sensor in the "cold corner". The point of operation of the heat exchanger is influenced in this case by that the air treatment unit also comprising a pre-heater, which is arranged in the first air flow in the flow direction before the plate heat exchanger. The method then comprises the step of influencing the point of operation of the heat reclaimer by increasing the exterior air temperature before the plate heat exchanger by changing the control signal to the pre-heater to increase the power output from the same, to prevent frost build-up in the heat exchanger. By the combined control of the heat recovery and the pre-heater and by constantly checking the validation temperature against the criteria Ki and the K2, optimum operation is obtained with low use of energy supplied despite the prevention of frost build-up. In this way, there is no need of energy-consuming defrosting sequence of a reclaimer covered with ice, as in prior art.

According to another preferred embodiment the heat reclaimer also in this embodiment is a plate heat exchanger, which is arranged to exchange energy between the first and the second air stream. The method then comprises the steps to measure exhaust air flow, intake air flow, the temperature and moisture of the exhaust air and the temperature and moisture content of the exhaust air before the plate heat exchanger. Using these, the dew point is calculated as well as the efficiency of the plate heat exchanger in a known manner for the current point of operation. Subsequently, according to this embodiment, the validation temperature is determined by a theoretical calculation of the temperature of the coldest point based on measured and calculated measures, instead of using at least a slightly more uncertain measurement value. This results in a temperature to use for controlling, and the validation temperature is checked against the set frost-build temperature and/or the dew point, as described earlier. Finally, actions are made to influence the point of operation of the heat reclaimer and this is also achieved in this embodiment by reducing the efficiency of the plate heat exchanger by a changed control signal to the throttle controlling devices as described above. By that the efficiency of the prevention of frost build-up in the heat exchanger is reduced.

According to another embodiment with a heat reclaimer of the type plate heat exchanger, the validation temperature is calculated theoretically instead of measuring it, as described above. In this case, however, the measure is instead to use a pre-heater, which in this embodiment is arranged in the first air flow in the flow direction before the plate heat exchanger. The method then comprises, just as previously described, the step to influence the point of operation of the heat exchanger by raising the exterior air temperature before the plate heat exchanger by changing the control signal to the pre- heater to increase the power output from the same, to prevent frost build-up in the heat reclaimer.

According to another preferred embodiment, the heat reclaimer is a rotating heat reclaimer, and the method comprises then the steps, in a known manner, to measure exhaust air flow, intake air flow, the temperature and moisture of the exhaust air and the temperature and moisture content of the exhaust air before the rotating heat reclaimer. Using this, the dew point as well as the efficiency of the heat reclaimer are calculated in a known manner, for the current point of operation. Subsequently, according to this embodiment, the validation temperature is determined by calculation based on measured and calculated values. As mentioned in the description of prior art, the measurement must, according to known technology be done either in many points, or in one point which may be considered to be representative to be able to measure the coldest point. In reality, this becomes a compromise, because the position can vary depending on the rpm of the rotor, rotor size, air flow, temperature conditions etc., why in many cases reliable safety margins are chosen to begin defrosting in time or large power is used to shorten the defrost time. To avoid using a large number of sensors or to need to take a chance with the placement of one or a few sensors, the idea, as mentioned, is to theoretically calculate the temperature in the coldest point for the current case and to use this instead of an uncertain measurement value. By this a safe temperature to control from the outside is obtained, wherein the validation temperature then being checked against the set frost-building temperature and/or the dew point, as previously described. Finally, an action is set to influence the point of operation of the heat exchanger and this is achieved in this embodiment by reducing its efficiency by modifying the control signal that controls the speed of the rotating heat reclaimer, for optimal recovery and at the same time preventing frost formation on the rotating heat reclaimer. By this method a large number of sensors is avoided or a highly uncertain placement and measurement of a representative validation temperature, which is advantageous compared to known methods.

A possible alternative to the just described embodiment is to measure the exhaust air temperature in a representative position instead of calculating the same and to allow this to constitute the validation temperature. Otherwise, the influence of the point of operation of the heat reclaimer is in the same way as above by changing the rotor speed.

Another alternative embodiment is to use a pre-heater also for the alternative when the heat reclaimer is a rotating heat reclaimer. The heater is arranged in the second air flow (the intake air) in the direction of flow before the rotating heat reclaimer and the method then comprises the steps, in a known manner, to measure exhaust air flow, intake air flow, the temperature and moisture of the exhaust air and the temperature and moisture content of the exhaust air before the rotating heat reclaimer. By these, the dew point and the efficiency of the heat reclaimer are calculated, in a known manner. Subsequently the validation temperature is determined by calculation based of measured and calculated values, as described above, and finally an action is made to influence the point of operation of the heat exchanger. This is achieved by changing the control signal to the pre-heater to increase the power output from the same and by the controlling the recovery is optimized while preventing frost build-up on the rotating heat exchanger.

According to an alternative embodiment bye use of pre-heaters in connection with the rotor (as most recently) it is instead of calculating the validation temperature to measure the extract air temperature and to determine this as being the validation temperature. If the check of the criteria shows that an action must be done change, as is described before, the control signal to the pre-heater to increase the power output from the heater. Control and coordination of recovery and pre-heater works as previously described for optimal recovery without the need to remove frost already built up on the rotor.

By the invention, a number of advantages over known solutions are obtained:

Prevent ice formation/frost build-up on the reclaimer using a refined method which does not consume as much energy as previously known solutions do.

The need for defrosting is eliminated as frost build-up is not allowed.

By in different manners determine the validation temperature and estimate it according to different criteria and with the pre-defined possible margin, the sensitivity of the operation of defrosting can be better adjusted than in older methods, and that consideration can be taken to optimal heat recovery.

The method works well in installations with demand-controlled ventilation (VAV), unlike older solutions that use the pressure drop over the heat exchanger as an indicator for freezing.

The embodiments where the validation temperature is calculated gives a secure value and excludes the danger of measuring the wrong place at the heat exchanger, which, unlike older solutions, results in relevant and secure data, and minimizes the number of sensors. Especially in the case of rotating heat reclaimer, it is particularly advantageous because it is difficult to measure the relevant data with one or a few sensors.

- The method works for several different types of heat reclaimers.

Minimizes the need for multiple sensors in different positions to provide secure data.

Brief description of the drawings

The following schematic drawings show:

- Fig. 1 shows a symbolic figure of an air treatment unit arranged with a liquid coupled heat

reclaimer.

Fig. 2 shows a symbolic figure of an air treatment unit arranged with a plate heat exchanger. This can be either a counter flow heat exchanger or a so-called cross-flow heat exchanger.

Fig. 3 shows a symbolic figure of an air treatment unit arranged with a rotating heat exchanger.

The constructive design of the present invention is shown in the following detailed description of three examples of embodiments ef the invention, referring to the accompanying figures showing preferred, but not limiting examples of embodiments of the invention. Detailed description of the drawings

Fig. 1 Shows symbolically an air treatment unit 2 with a heat reclaimer 1 of the type liquid coupled heat reclaimer 1. The heat reclaimer 1 is arranged to transfer energy between a first flow 3 and a second flow of air 4, wherein the first exhaust air flow 3 comprises, in the flow direction, first exhaust air 5, which when it passes the heat reclaimer 1 is referred to as extract air 6. The second air flow 4 comprises in the flow direction first exhaust air 7, which when it has passed the heat reclaimer 1 is referred to as intake air 8. The air treatment unit 2 also comprises an exhaust air fan 9 which drives the first air flow 3 and an intake air fan 10 which drives the second air flow 4. For the control and monitoring of the air treatment unit 2 and its components, and thus the implementation of the invented method, a control equipment 1 1 is arranged. The heat reclaimer 1 In turn, comprises at least one exhaust air battery 12 in the first airflow 3 and at least one intake air battery 13 in the second air flow 4. In order to transfer energy between batteries 12, 13, i.e. recover heat (or cool), a liquid flow is circulated which is controllable regarding the flow rate in a liquid circuit 14 between the batteries 12, 13. The liquid is circulating using a circulation pump 15 which preferably can be controlled regarding the rotational speed to vary the flow through batteries 12, 13. The liquid circuit is in the figure also arranged with a control valve 16, which, alternatively or together with the speed control of the pump 15, can be controlled to regulate the flow rate through the exhaust air battery 12. For the

measurement of air temperatures and moisture in different positions are symbolically a number of sensors arranged (without designations) for measuring the temperature Ts and moisture Xs of the exhaust air 5, the temperature T7 of the exterior air 7 and the temperature Te of the intake air 8. Thus, according to this method, the temperature T5 and moisture X5 of the exhaust air 5 is measured wherein the current temperature Tsd_P of dew point of exhaust air 5 is calculated. After this, the so- called validation temperature T_v is determined which at this type of heat reclaimer 1 is determined to be equal to a measured liquid temperature T_Wi of the input fluid to the exhaust air battery 12, which symbolically is shown as a temperature sensor on the pipeline.

The liquid temperature T_Wi is a good and reliable temperature, which safely measures the coldest temperature with sufficient accuracy, why this preferably is used to obtain a simple plant and control. According to the method it's also determined a so-called frost-build temperature TF which depends on at least one, but preferably several characteristics and/or condition si ... ss for the current operating case and the heat reclaimer in question 1. The frost build-up temperature Tf is based either on empirical data produced by testing the respective heat reclaimer in a large number of operating cases, whereby the current operating point gives a critical value, that is the frost build-up temperature Tf, alternatively, it is determined analytically by calculation. The data that is the basis for the function (which gives the frost build-up temperature Tf) are generally as follows:

s1 = type of heat reclaimer (1 ),

s2 = efficiency of temperature ητ,

s3 = efficiency of moisture η_χ,

s4 = moisture content xs of the exhaust air (5),

s5 = the temperature Ts of the exhaust air (5),

s6 = the temperature T7 of the exterior air (7)

s7 = the air speed V4 of the intake air (8)

s8 = the air speed V4 of the exhaust air (5).

Depending on the type of heat reclaimer 1 , a varying number of the above is used by determining of frost build-up temperature Tf. For liquid coupled heat reclaimer 1 preferably s1 , s2, s4, s5 and s6 are used. Other conditions that may have influence are s7 and s8, which can be added as parameters of importance in the calculations. When the validation temperature T_v and frost build-up temperature TF been determined, the validation temperature T_v is compared against at least one of the two criteria Ki , K2 where Ki : T_v≤ Tf, i.e. the validation temperature is checked if it is equal to or lower than the frost build-up temperature. In addition, the second criterion K2 may also be checked according to K2: T_v≤ T5d_P + Δ1⁰ C, i.e. if the validation temperature is equal to or lower than the calculated dew point. This criterion also comprises an optional constant Δ1 to add to the calculation that gives a certain margin to how close to the dew point the condition should be, wherein Δ1≥ 0. If at least one of the criteria Ki or K2 are fulfilled is according to one alternative the temperature efficiency ητ of the heat reclaimer reduced, by changing the flow of liquid through the exhaust air battery 12, by speed control of the circulation pump 15 or by controlling the valve 16, so that the input liquid temperature T_Wi to the exhaust air battery 12 increases. By this the point of operation of the heat reclaimer 1 is changed, that at least any of the criteria Ki, K2 no longer is fulfilled. Another alternative is to increase the

temperature T7 of the exterior air 7 by using a pre-heater 17, which in this case is arranged in the second air flow 4 (intake air), before the heat reclaimer 1 , wherein the point of operation of the heat reclaimer 1 is changed just as much that at least one of the criteria Ki , K2 no longer is met, for optimal recovery and at the same time counteracting frost build-up on the heat reclaimer 1. Both of these alternatives are made by a control signal from the control equipment 11 to the circulation pump 15, and/or the control valve 16, alternatively the pre-heater 17.

Fig. 2 shows symbolically the air treatment unit 2 with a heat reclaimer 1 of the type plate heat exchanger, which can be in the form of a cross-flow heat exchanger or a counter-flow heat exchanger. The plate heat exchanger 1 is arranged with a number of controllable throttle devices 18, according to known technology, which can be controlled to completely or partially close or open the heat reclaimer 1 for throughput of exterior air 7 in combination with to open and close a so called by-pass-throttle arranged to, if necessary, bypass a certain part of the exterior air 7. This is the possibility of regulating the heat reclaimer 1 via the control equipment 1. The air treatment unit 2 is constructed in the same way as the above described with a first and a second air flow 3, 4, fans 9, 10, exhaust air 5, extract air 6, exterior air 7 and intake air 8. The difference is that which is unique to the type of heat reclaimer 1. For the measurement of air temperatures and moisture in different positions is symbolically in the same way as in figure 1 a number of sensors are arranged (without designations) for measuring the temperature Ts and moisture Xs of the exhaust air 5 and, the temperature T7 of exterior air 7 and the temperature Ts of intake air 8. In the same way as above, the temperature Tsd_P of the current dew point of the exhaust air 5 based on measurement data of the exhaust air 5. After this also here the validation temperature T_v is determined which by this type of heat reclaimer 1 is set to be equal to a measured temperature Tcc in the so called "cold corner". This position is well known and easy to determine as the coldest position at plate heat exchangers and is the position where the cold exterior air 7 meets the extract air 6. The temperature in the cold corner T∞is a good and reliable temperature, which can be measured with sufficient accuracy, which is why this preferably is used to obtain a simple plant and control. However, it can also be chosen to use a calculated temperature, which is described more extensively in connection with the rotating heat exchanger, wherein a number of other measurements also has to be done (see further description on calculation of the validation temperature by rotating heat reclaimer, figure 3). The temperature of frost Build-up is determined in the same way as described above by a function that depends on at least one, but preferably a number of characteristics and/or conditions si ... ss for the current case of operation and for the current heat reclaimer 1. For plate heat reclaimers 1 preferably s1 , s2, s4, s5 and s6 is used. Other conditions that may affect are s7 and s8, which can be added as parameters of importance in the calculations.

Further, the method also in this case continues in the same way, that when the validation temperature Tv and the temperature Tf of frost build-up is determined check the validation temperature T_v against the two criteria Κι , K2 and take measures if at least one of these is met. As described before decreases by an alternative heat reclaimer 1 the temperature efficiency ητ, but in this case by the control signal from the control equipment 11 , which makes sure to change the throttle devices 8 for reduced heat recovery. By this the point of operating of the heat reclaimer 1 is changed, so that at least one of the criteria Κι , K2 no longer is fulfilled. The second alternative is to increase the temperature T7 of the exterior air 7 by a pre-heater 17, which is arranged in the second air flow 4 (intake air), before the heat reclaimer 1 , wherein the point of operating of the heat reclaimer 1 is changed just as much that at least one of the criteria Κι, K2 no longer is met, for optimal recovering and at the same time counteracting frost build-up on the heat reclaimer 1. The power output of the pre-heater 17 is changed by that a control signal is output from the control equipment 1 to the heater 17.

Fig. 3 shows symbolically the air treatment unit 2 with a heat reclaimer 1 of the type rotary heat reclaimer 1. The air treatment unit 2 is constructed in the same way as previously described with a first and a second air current 3, 4, fans 9, 10, exhaust air 5, extract air 6, exterior air 7 and intake air 8. What differs also here is what is unique for the type of heat reclaimer 1 , in this case the rotating heat exchanger 1. For measurement of air temperatures and moisture in different positions is symbolically in the same way as in figure 1 and figure 2 a number of sensors arranged (without designations) for measuring the temperature Ts and moisture Xs of the exhaust air 5, the temperature T7 of the exterior air 7 and the temperature Te of the intake air 8. In the same way as above, the temperature Tsdp of the current dew point of the exhaust air 5 based on measurement data of the exhaust air 5. In addition to these measurement data, measurement data is also collected to the control equipment 1 1 regarding the current intake air flow V4, current exhaust air flow V3, and the temperature T7 of exterior air 7 and moisture content X7 in a position before the rotating heat reclaimer 1. Furthermore, the 1 efficiency η of the rotating heat reclaimer 1 is calculated for the current point of operation. After that the validation temperature T_v is determined which at this type of heat reclaimer 1 is determined to be equal to an estimated temperature entirely based on a number of the just listed measured values. By calculating instead of measuring, one avoids the compromise as a measured value from a - hopefully representative - sensor yields, which is still not reliable because the optimal position varies depending on the rpm of the rotor, rotor size, air flow, temperature conditions, etc. which is particularly problematic, for example, in VAV systems. The calculated validation temperature T_v is thus calculated and is the coldest temperature and it is this that is then checked against the determined temperature Tf of frost build-up and/or the dew point Tsdp, as previously described. For rotating heat exchanger 1 , preferably s1 -s6 are used and other conditions that may influence the use are s7 and s8, which can be added as parameters of significance in the calculations to determine the temperature TF of frost buildup. If then at least one or both criteria Κι, K2 for action are fulfilled, the action is done to influence the point of operation of the rotating heat reclaimer 1. The alternatives which are available for this are as before - to reduce efficiency η of the heat reclaimer 1 by changing the control signal that controls the speed of the rotating heat exchanger 1 , for optimal recovery and at the same time preventing frost formation on the rotating heat reclaimer 1. The second alternative is to make use of a pre-heater 17 in the exterior air 7 for heating this in a position in the second air flow 4 (intake air) before the rotating heat reclaimer 1. The control signal will then make sure to increase the power output from the heater 17. An alternative to calculating the validation temperature T_v may be to instead measure the temperature in the extract air Τε, a bit away from the rotor and let this constitute T_v. The temperature sensor for this alternative is shown as a dashed sensor in the extract air in Figure 3. COMPONENT LIST

1 = heat reclaimer

= air treatment unit

= first air flow

= second air flow

= exhaust air

= extract air

= exterior air

8= intake air

9= exhaust air fan

10= intake air fan

11 = control equipment

12= exhaust air battery

13= intake air battery

14= liquid circuit

15= circulating pump

16= control valve

17= pre-heater

18= throttle devices

T = temperature

x= moisture content

F = function

K = criterion

η= efficiency

V = flow

v = speed

s = specific property

Claims

C L A I M S

1. Method of preventing frost build-up on a heat reclaimer (1 ) arranged in an air treatment unit (2), said heat reclaimer (1 ) is arranged to transfer energy between a first air flow (3) and a second air flow (4), and the first air flow (3) comprises in the flow direction at first exhaust air (5) which passes the heat reclaimer (1 ) and hereafter referred to as extract air (6), and the second air flow (4) comprises in the flow direction at first exterior air (7) which passes the heat reclaimer (1 ) and hereafter referred to as intake air (8), said air treatment unit (2) also comprises an exhaust fan (9) which operates the first airflow (3) and an intake air fan (10) which drives the second air flow (4), and for the control and monitoring of the air treatment unit (2) and its components a control equipment (1 1 ) is arranged, characterized in to counteract build-up of frost in each point of operation on the heat reclaimer (1 ) by that the method comprises the following steps: a) measure:

a1. temperature Ts of the exhaust air (5),

a2. moisture content xs of the exhaust air (5),

b) calculate:

b1. current dew point temperature Tsd_P of the exhaust air (5) based on measured values, c) determine:

c1. a validation temperature T_v for the current operating point,

c2. a frost-build-up temperature Tf, whereby Tf = F(s1 , s2,... s8), i.e. Tf is a

function F dependent on at least one of the following:

s1 = type of heat reclaimer (1 ),

s2 = temperature efficiency ητ,

s3 = moisture efficiency η_χ,

s4 = moisture content xs of the exhaust air (5),

s5 = temperature Ts of the exhaust air (5),

s6 = temperature T7 of the exterior air (7)

s7 = air speed 4, of the intake air (8)

s8 = air speed V3 of the exhaust air (5),

d) check Tv against at least one of two criteria Κι , K2 where:

K2: Tv < T5d + Ai°C, wherein Δ1≥ 0, and

e) if at least one of the criteria Ki or K2 are fulfilled lower the efficiency η of the heat reclaimer (1 ) and/or raise the temperature T7 of the exterior air (7) before the heat reclaimer (1 ), wherein the validation temperature T_v is raised just as much as at least one of the criteria Ki , K2 no longer are fulfilled, for optimal recycling and at the same time to counteract frost build-up on the heat reclaimer (1 ).

2. Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a liquid coupled heat reclaimer which comprises at least one exhaust air battery (12) in the first airflow (3) and at least one intake air battery (13) in the second airflow (4), and between these batteries (12, 13) a liquid flow is circulating, controllable with respect to the flow quantity, in a liquid circuit (14) arranged between the batteries (12, 13), which liquid flow transfer energy between the air flows (3, 4), and the method also comprises the step of:

a) measure:

a3. the input liquid temperature Tvw of the controllable liquid flow Ts to the exhaust air battery (12), and in that at the step c1 ) determine the validation temperature T_v to being equal to the measured input liquid temperature Twi to the exhaust air battery (12),

and in that at step e) influence the point of operation of the heat reclaimer (1 ) by to reduce its efficiency η, by changing the fluid flow by controlling the rotational speed of a pump (15) or by controlling a bypass valve (16), so that the input fluid temperature Twi to the exhaust air battery (12) increases.

3. Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a liquid coupled heat reclaimer which comprises at least one exhaust air battery (12) in the first airflow (3) and at least one intake air battery (13) in the second airflow (4), and between these batteries (12, 13) a liquid flow is circulating, controllable with respect to the flow quantity, that transfer energy between the air flows (3, 4), and air treatment unit (2) comprises a pre-heater (17), which is arranged in the second air flow (4) in the flow direction before the intake air battery (13) and the method also comprises the step of: a) measure:

a3. input liquid temperature Twi to the exhaust air battery (12), and in that at the step c1 ) determine the validation temperature T_v to being equal to the measured input liquid temperature Twi to the exhaust air battery (12),

and in that at step e) influence the point of operation of the heat reclaimer (1 ) by to raise the exterior air (7) temperature T7 before the intake air battery (13) by increasing the power output from the pre- heater (17).

4. Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a plate heat exchanger, and the method also comprises the step to:

a) measure:

a4. the temperature in the so-called "cold corner" Τ∞ at the plate heat exchanger (1 ), which is the coldest position in the plate heat exchanger (1 ) where the exterior air (7) meets the extract air (6), and in that at the step c1 ) determine the validation temperature Tv to being equal with the measured temperature in the cold corner Τ∞, and in that at the step e) influence the point of operation of the heat reclaimer (1 ) by to reduce its efficiency η by Changing a control signal to throttle devices (18) which are arranged at the exterior air side of the plate heat exchanger (1 ), to reduce the throughput of air (7) through the heat changing part of the plate heat exchanger (1 ) and instead direct at least a subset of the exterior air (7) through a by- pass part of the plate heat exchanger (1 ).

5. Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a plate heat exchanger, and the air treatment unit (2) comprises a pre-heater (17), which is arranged in the first airflow (3) in the flow direction before the plate heat exchanger (1), and the method also comprises the step to:

a) measure:

a4. the temperature in the so-called "cold corner" Τ∞ at the plate heat exchanger (1 ), which is the coldest position in the plate heat exchanger (1 ) where the exterior air (7) meets the extract air (6), and in that at the step c1 ) determine the validation temperature T_v to being equal with the measured temperature in the cold corner Tec,

and in that at the step e) influence the point of operation of the heat reclaimer (1 ) by to raise the exterior air (7) temperature T7 before the intake air battery (13) by increasing the power output from the pre-heater (17).

6. Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a plate heat exchanger, and the method also comprises the steps to:

a) measure:

a5. the intake air flow V4,

a6. the exhaust air flow V3,

a7. the temperature of the exterior air (7) T7 before the heat reclaimer (1 ),

a8. the moisture content of the exterior air (7) X7 before the heat reclaimer (1 ), b) calculate:

b2. the efficiency η of the heat reclaimer (1 ) for the current point of operation, and in that at step c1 ) determine the validation temperature Tv by calculation based on measured and calculated values,

and in that at step e) influence the point of operation of the heat reclaimer (1 ) by decreasing its efficiency η by changing a control signal to the throttle devices (18) which are arranged at the exterior air side of the plate heat exchanger (1 ), in order to reduce the throughput of air (7) by the heat exchanging part of the plate heat exchanger (1 ) and instead direct at least a subset of air (7) through a by-pass part of the plate heat exchanger (1 ).

7. Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a plate heat exchanger, and the air treatment unit (2) comprises a pre-heater (17), which is arranged in the first airflow (3) in the flow direction before the plate heat exchanger (1), and the method also comprises the steps to:

a) measure:

a5. the intake air flow \ ,

a6. the exhaust air flow V3,

a7. the temperature of the exterior air (7) T7 before the heat reclaimer (1 ),

b2. the efficiency η f the heat reclaimer (1 ) for the current point of operation, and in that at step c1 ) determine the validation temperature T_v by calculation based on measured and calculated values,

and in that at step e) influence the point of operation of the heat reclaimer (1 ) by to raise the temperature T7 of the exterior air (7) before the plate heat exchanger (1 ) by increasing the power output from the pre-heater (17).

8. Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a rotating heat reclaimer, and the method also comprises the steps to:

a) measure:

a5. the intake air flow V4,

a6. the exhaust air flow V3,

a7. the temperature T of the exterior air (7) before the heat reclaimer (1 ),

a8. the moisture X7 content of the exterior air (7) before the heat reclaimer (1 ), b) calculate:

b2. the efficiency η f the heat reclaimer (1 ) of the current point of operation, and in that at step c1 ) determine the validation temperature T_v by calculation based on measured and calculated values,

and in that at step e) influence the point of operation of the heat reclaimer (1 ) by decreasing its efficiency η by changing the rotation speed of the rotating heat reclaimer (1 ).

9. Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a rotating heat reclaimer, and the method also comprises the steps to:

a) measure:

a9. the temperature Τβ of the extract air (6), and in that at the step c1 ) determine the validation temperature T_v to being equal with the temperature Te of the extract air (6),

and in that at the step e) influence the point of operation of the heat reclaimer (1 ) by decreasing its efficiency η by changing the rotation speed of the rotating heat reclaimer (1 ).

10. Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a rotating heat exchanger, and the air treatment unit (2) comprises a pre-heater (17), which is arranged in the second airflow (3) in the flow direction before the rotating heat reclaimer (1 ), and the method also comprises the step to:

a) measure:

a5. the intake air flow V ,

a6. the exhaust air flow V3,

a7. the temperature T7 of the exterior air (7) before the heat reclaimer (1 ),

b2. the efficiency η f the heat reclaimer (1 ) of the current point of operation, and in that at the step c1 ) determine the validation temperature Tv by calculating based on measured, and calculated values,

and in that at step e) influence the point of operation of the heat reclaimer (1 ) by increasing the temperature T7 of the exterior air (7) before the rotating heat reclaimer (1 ) by increasing the power output from the pre-heater (17).

11 Method according to claim 1 , characterized in that the heat reclaimer (1 ) is a rotating heat exchanger, and the air treatment unit (2) comprises a pre-heater (17), which is arranged in the second airflow (3) in the flow direction before the rotating heat exchanger (1), and the method also comprises the step to:

a) measure:

a9. the temperature Τβ of the extract air (6), and in that at the step c1 ) determine the validation temperature T_v to being equal with the temperature Τε of the extract air (6),

and in that at the step e) influence the point of operation of the heat reclaimer (1) by increasing the temperature T7 of the exterior air (7) before the rotating heat reclaimer (1 ) by increasing the power output from the pre-heater (17).