NO310321B1 - A method for defrosting a counter-current type heat exchanger and assembly comprising a heat exchanger in the form of such a counter-current heat exchanger - Google Patents

Description

This invention relates to a method for defrosting a heat exchanger of the counter-flow type and part of a ventilation unit where outside air is made to flow through the heat exchanger in a first direction, while exhaust air is made to flow through the counter-flow heat exchanger in the opposite direction, and where the counter-flow heat exchanger is periodically defrosted at low outdoor temperatures using the exhaust air.

The invention also relates to a ventilation unit comprising a heat recuperator in the form of a counter-flow heat exchanger, a supply pipe for outdoor air at a first side of the counter-flow heat exchanger and a supply air pipe with a supply air fan on the opposite side of the counter-flow heat exchanger, an extraction air pipe on said opposite side of the counter-flow heat exchanger and a outlet pipe for exhaust air on the aforementioned first side of the counter-flow heat exchanger.

After passing through the counter-flow heat exchanger, the outside air enters the room in the form of supply air. After emitting part of its heat energy in the heat exchanger, the exhaust air is released into the atmosphere above roof level.

Each of the two air flows thus passes through a number of parallel channels in the heat exchanger, some of which, for example every other one, are assigned exclusively for outside air passage, while the others are assigned exclusively for exhaust air passage.

The counterflow heat exchanger's outside air ducts are thereby completely separated from its exhaust air ducts, usually by means of mutually spaced duct delimiting slats or other thin plates whose walls serve as thermal transmission means. The advantage of counterflow type heat recuperators is their high thermal efficiency. These heat exchangers are likewise secured against odor transfer from the outgoing exhaust air to the incoming outside air. The individual ducts are laterally sealed against adjacent ducts, and outside air and exhaust air are each assigned an effective filter, both of which are placed upstream of the heat exchanger.

There are rotary heat exchangers, but these have a lower degree of efficiency than heat exchangers that work according to the counter-flow principle. In addition, with rotating heat exchangers there is a risk of odor and bacterial transfer between incoming and outgoing air flows.

A known air conditioning unit according to NO interpretation document no. 173,843 comprises two heat exchanger cells with flow channels for inlet air (outside air) or outlet air (exhaust air) as well as a shunt channel that adjoins the heat exchanger cells to be flowed through by inlet exhaust air which is led past the heat exchanger cells under certain operating conditions, i.e. for example during defrosting, where the defrosting function itself is temperature-controlled. This well-known air conditioning unit is both constructively and functionally rebuilt and based on the shunt channel which serves to enable the flow of inlet/outside air past the heat exchanger cells under certain operating conditions. With temperature control of this known air conditioning unit, there will be a continuous need for defrosting during the temperature range of -10 and -15°C, because the defrosting process is forcibly controlled by the outside temperature.

With the method and the ventilation unit according to the invention, the aim has been, among other things, to limit the defrosting function so that the heat exchanger is only defrosted when there is actually ice formation, and at the same time to make the defrosting itself more efficient when there is ice formation.

Said purpose is realized by using a method of the kind specified in the introductory part of patent claim 1, proceeding in accordance with the steps that appear in the characterizing part of this patent claim. A ventilation unit of the type specified in the introductory part of patent claim 3 is distinguished for the same purpose by the constructive features that appear in the characterizing part of this patent claim.

Ice formation is, in addition to low temperature, also dependent on humidity. With the ventilation unit according to the invention, it will thus be possible to vary quite a lot in time between each time there is a need for defrosting.

In the case of ventilation units according to the invention, financial advantages have also been intentionally achieved, as low-energy motors must be used as fan motors. Each fan motor must also be arranged so that it acts as a feeler/sensor that reacts to the load to which the motor is exposed at all times, as this reaction must be able to be used to register one of two special states of the heat exchanger: (1 ) That the heat exchanger has been exposed to frosting and should/must be defrosted or (2) that a filter (for example the outside air filter) has become so dirty that it exhibits a partially blocked/airflow retarding condition. According to experience, filter replacement takes place approximately once a year, and this aspect of the invention is consequently less critical than the registration that the heat exchanger has been exposed to freezing.

In the case of ventilation units/devices/systems for, for example, residential buildings, flats, office premises etc., the unit can be computer-controlled, as the unit can be designed to control the maintenance of a constant amount of air through the heat exchanger.

Heat exchangers of the counter-flow type are often exposed to freezing to the point of complete blockage, where the passage of air is prevented and the heat exchanger is put out of action.

Frost formation in heat exchangers of this type occurs when warm, moist exhaust air is cooled by cold outside air. Frost formation will normally occur at outside air temperatures of 5° C and lower.

Known defrosting technology has consisted of melting ice and frost on/in the heat exchanger, either when it has been ensured that cold outside air does not escape during the defrosting/thawing, or by then using a pre-heating battery, but none of these solutions genes are considered satisfactory, especially not from an energy-economic point of view.

It has thus previously been necessary to stop the supply air fan during the defrosting of the heat exchanger or to use a pre-heating battery that heats the outside air before it enters the heat exchanger during the defrosting period.

In the air conditioning unit according to the invention, the outside air supply pipe is assigned to two outside air chambers which are placed immediately upstream of the heat exchanger, each chamber being provided with a shutter and connected to each half of the heat exchanger block. Normally, both of these dampers are in the open state, so that outside air from both chambers is simultaneously supplied to each half of the heat exchanger block for flow through it at the same time. The continuous outside air channels of the heat exchanger block are thus kept tightly separated from each other by a dividing wall which extends in the direction of flow of the outside air through the block. The continuous exhaust air ducts are without such a division.

When a ventilation unit with heat recovery is to be used to defrost a cooled down heat exchanger block, first one damper assigned to an outside air chamber is activated to close the relevant chamber against supplied outside air, whereby the heat exchanger block is supplied with all incoming outside air via the other chamber whose damper is in the open position score.

As soon as defrosting of one half of the heat exchanger block is detected, the damper that has been in the open position until now is closed, while the closed damper is opened. Thereby, the incoming outside air is supplied to the heat exchanger block via the outside air chamber which was recently closed, but whose damper has now been brought into the open position. Full flow through the heat exchanger with warm exhaust air is maintained during the defrosting operation, which can take approx. 40 minutes for the entire heat exchanger.

Using built-in electronics in the supply air and extract air fan motor and control automation for the ventilation unit with heat recovery, the defrost function works as follows: The ventilation unit is designed to maintain a constant set air volume. When the heat exchanger rims and may be in the process of riming down, the pressure drop across the heat exchanger increases, and the speed of the fan motor increases to maintain said constant air volume. The speed increase/change is used to send a signal to said damper which is individually movable stored in the outside air intake, immediately upstream of the heat exchanger.

A larger increase in pressure drop that occurs over the course of a day indicates that there is frost in the heat exchanger block.

Furthermore, measurement of internal pressure drop is also regarded as an indication of and notification of filter replacement. In this case, you will get a steady increase in pressure drop over a longer period of time. During the time when the filters gradually become dirty, the pressure drop that builds up evenly will emit a signal, so that the fan motor speed is increased and a constant air volume is maintained, that is, the fan capacity increases when the filters become dirty. Between filter replacements, the right amount of air is thus ensured through the unit. The high efficiency of the heat recovery unit usually makes the use of a post-heating battery redundant. A non-limiting example of a preferred embodiment is described in more detail in what follows with reference to accompanying drawings, where the figures show the ventilation unit in the area of the heat recovery in a highly schematic representation, with numbered arrows showing the direction of the various air flows, and where: Fig. 1 represents a vertical sectional view where a heat exchanger of the counter-flow type is symbolized by a diamond-like circumference, and where a pipe cap is rendered as an inverted V to show that exhaust air which has passed through the heat exchanger and released a significant part of its heat energy there, is discharged into the atmosphere at a level above ceiling level; Fig. 2 shows largely the same as fig. 1, but in a side view from the outside, and where a special section III - III (for fig. 3, 3A and 3B) is marked; Fig. 3 shows a horizontal section along the line III - III in fig. 2, and where two outside air chambers located immediately upstream of the heat exchanger are each assigned a shutter that is in the open state, so that outside air flows through the heat exchanger from both chambers; Fig. 3A corresponds to fig. 3, but here the damper of one outside air chamber is brought into the closed position, so that the supplied outside air passes exclusively through one half of the heat exchanger via the other outside air chamber, after which it flows into the room as preheated supply air via a channel with a supply air suction fan; Fig. 3B corresponds to fig. 3A, but here there is another outside air chamber's damper which is in the closed position, so that supplied filtered outside air is exclusively brought to flow axially through the second half of the heat exchanger in relation to fig. 3A, the heat exchanger in its entire effective cross-section facing the hot exhaust air being open to its flow through during heat energy release; Fig. 4 shows an end view from the left side according to arrow IV in fig. 2; and Fig. 5 shows an end view from the right side according to arrow V in Fig. 2.

A heat recuperator of the counterflow type, which is symbolized by a diamond-like circumference, is denoted by the reference number 10 and is enclosed by a housing 12, short partitions 14 and 16 extending between the inner surfaces of the walls of the housing 12 and two of the opposite corners of the heat exchanger 10, as can be seen from fig. 1.

The housing 12 is joined by an outside air duct 18 which is provided with a filter 20.

The outside air, arrow 22, passes through continuous outside air channels in the heat exchanger in a heat-exchange relationship with the hot exhaust air which simultaneously flows through the heat exchanger in the opposite direction via the walls of the lamellae or similar thin plates of which the heat exchanger is built.

Preheated supply air is denoted by 24. The exhaust air flow is indicated by arrows 26.

From the housing 12 around the heat exchanger block 10, a channel 28 is connected for preheated supply air 24, in which the supply air fan 30 is mounted. The exhaust air 26 enters the house 12 from an exhaust duct 32 with filter 34, and leaves the house 12 via an exhaust duct 36 with an exhaust fan 38. As previously stated, the pipe cap 40 indicates that heat energy-reduced exhaust air is released into the atmosphere above the roof.

Fig. 3 shows a horizontal section along section line III - III in fig. 2. It appears that the outside air supply channel 18 is connected to two side-by-side outside air chambers 42 and 44, a dividing wall between the chambers 42, 44 being denoted by 46.

In the transition areas between the outside air duct 18 and each of

in the chambers 42, 44, a pivotable shut-off valve 48 and 50 respectively is arranged. In fig. 3, the dampers 48, 50 lie against each other and therefore do not appear as two separate dampers. The dampers 48,50 are driven by a damper motor 51.

Arrows 22 and 24, corresponding to the reference numbers in fig. 1, shows according to fig. 3 the passage of the outside air flow through the heat exchanger.

The heat exchanger 10 can be divided into separate, side-by-side compartments for parallel, continuous channels which are exclusively reserved for the flow of outside air, for example by a central dividing wall, while the intermediate parallel

channels which are exclusively intended for the flow of exhaust air are not divided in a corresponding way. exhaust air

thus flows through the entire cross-section of the heat exchanger in the continuous, parallel channels that are reserved for extract air flow.

When the temperature of the outside air is low, for example + 5° C and below, inflowing outside air in connection with warm, moist exhaust air will lead to the formation of frost in the heat exchanger, which must consequently be defrosted. In cold weather, there may be a need for defrosting, for example, once a day.

As mentioned, the ventilation unit is designed to maintain a constant set air volume. When the heat exchanger rims, the pressure drop across it increases and the speed of the fan motor increases to maintain a constant air volume. This speed increase is used so that said damper 50 or 4 8 is activated and swung over to close the passage of outside air into the outside air chamber 42, fig. 3A, or into the outside air chamber 44, fig. 3B.

Fig. 3A and 3B represent the two immediately following partial defrosting operations. From the one in fig. position shown in 3, the damper 50 is swung so that the chamber 44 is not supplied with outside air, as all supplied outside air is sucked through the half of the heat exchanger 10 in the corresponding location.

The fact that the outside air 22 in this case, fig. 3A, is supplied asymmetrically to the upstream end of the heat exchanger, is a sufficient measure to orient the outside air flow so that it remains in the left half of the heat exchanger 10 throughout the entire axial length of the heat exchanger; cf. arrows 22 and 24.

The same of course also applies to fig. 3B. It is thus not absolutely necessary in all embodiments of the invention to equip the heat exchanger 10 with an internal partition wall which is flush with the partition wall 46 and separates the heat exchanger's parallel, continuous channels which are reserved for supply air.

On the other hand, the exhaust air must under all circumstances flow through the full cross-section of the heat exchanger 10 connected to the exhaust air's separate, parallel, continuous channels which are hermetically separated from the corresponding channels of the outside air.

Fig. 3A and 3B show how a defrosted heat recovery unit of the counterflow type is defrosted in two operations. Exhaust air 26, see fig. 1, flows at all times through the heat exchanger 10 from right to left, and its heat energy ensures the defrosting of the heat exchanger half which, during defrosting, is not flowed through by the outside air at any time.

At all times during these defrosting operations, the part or parts of the heat exchanger that are not flowed through by the outside air are defrosted using the exhaust air. After the damper 50 has been closed during the first defrosting operation (fig. 3A), the programmed electronics built into the fan motors ensures that this damper 50 is brought into the open position and at the same time closes the damper 48, whereby the supply air flow (22 and 24) takes place as indicated in fig. . 3B, while the defrosting now takes place in the other half of the heat exchanger 10 which is not flowed through by the outside air.

De-icing initiation is preferably based on the above-mentioned increase in pressure drop, and a larger increase in pressure drop that occurs over the course of a day indicates that there is frost in the exchanger block, and that this is possibly in the process of becoming de-iced and functionally impaired. Defrosting can also be initiated after a signal from temperature sensors using a timer, etc.

Furthermore, such a measurement of internal pressure drop can be used to indicate functional deterioration of the filter and notify about the replacement of the filter.

Dirty/failing filters result in the pressure fan increase remaining constant for a longer time. At the time when the filters become dirty, the pressure drop that builds up evenly will emit signals to cause an increase in the fan motor speed and maintenance of a constant air volume.

Claims (5)

1. Method for defrosting a heat exchanger (10) of the counter-flow type and included in a ventilation unit where outside air (22) is made to flow through the counter-flow heat exchanger (10) in a first direction, while exhaust air (26) is made to flow through towards - the counter-flow heat exchanger (10) in the opposite direction, and where the counter-flow heat exchanger (10) is periodically defrosted at low outside temperatures using the exhaust air, characterized in that the counter-flow heat exchanger (10) is defrosted in areas, essentially corresponding to a longitudinal half of the counter-flow heat exchanger's cross-section in the direction of air flow at a time, with the outside air (22) without by-pass effect with respect to the heat exchanger being made to flow through a first counter-flow heat exchanger half part in a first defrosting operation, where a second longitudinal half part is blocked for outside air flow, and which is followed by a second corresponding defrosting operation without by-pass effect with consideration of the heat exchanger throughout said second, then blocked heat exchanger half, which is now open for outside air flow, while said first heat exchanger half is blocked for outside air flow, in that the exhaust air - in a manner known per se - flows through the counter-flow heat exchanger (10) over its entire cross-section during the entire defrost. 2. Method as stated in claim 1, characterized in that supplied outside air (22) is filtered before it is forced to flow through one of two side-by-side heat exchanger halves with parallel, continuous channels designed for exclusively outside air flow during defrosting, and that the supplied outdoor air (22) after defrosting the first heat exchanger half-part, is forced to flow through the half of the cross-sectional area of the counter-flow heat exchanger where only the exhaust air flowed during the first defrosting operation. 3. Ventilation unit comprising a heat recuperator (10) in the form of a counter-flow heat exchanger, a supply pipe (18) for outside air (22) on a first side of the counter-flow heat exchanger (10) and a supply air pipe (28) with supply air fan (30) on the opposite side of the counter-flow heat exchanger (10), an exhaust air pipe (32) on said opposite side of the counter-flow heat exchanger (10) and with a drain pipe (36) for exhaust air on said first side of the counter-flow heat exchanger, characterized in that the supply pipe (18) for outside air (22), upstream of the counter-flow heat exchanger (10), two closable/openable outside air chambers (42, 44) are assigned which are designed to either be kept open at the same time or - during defrosting - so that one outside air chamber (for example 44, Fig. 3A or 42, Fig. 3B) is kept closed, whereby all supplied outside air is oriented so that it flows forcibly through only a first area, corresponding to a longitudinal half of the cross-section of the counter-flow heat exchanger (10), after which a second area, answer end to a second heat exchanger half located next to the former and with the same flow direction as this, is brought to receive all the outside air flow, as the inlet of each outside air chamber (42, 44) facing the supplied outside air is assigned a closing flap in the form of a pivotable damper (48, 50) which, in the closed position, prevents access of outside air to the associated chamber. 4. Ventilation unit as stated in claim 2 or 3, characterized in that the supply pipe (18 respectively 32) for outside air (22) and extract air (26) respectively to the heat exchanger (10) is provided with a filter (20 respectively 34), and that the unit is equipped with means for measuring internal pressure drop which indicates a filter change notification if one is registered. steady increase in pressure drop over a longer period of time, as this pressure drop which builds up steadily due to gradual soiling of the respective filter will emit a signal so that the fan motor speed is increased to maintain a constant amount of air through the unit. 5. Ventilation unit as stated in claim 3, characterized in that the heat exchanger block (10) is divided into two or more adjacently placed parts with the same flow direction for outside air (22).