EP3093583B1 - Method and device for defrosting an evaporator of a refrigeration installation and use of the defrosting device as calorimeter - Google Patents

Description

The invention relates to a method and a device for defrosting an evaporator of a refrigeration system that serves as an air cooler, wherein the evaporator can be used, for example, to cool refrigerators, cold rooms or deep-freeze cells. The invention further relates to the use of the defrosting device in refrigerated cabinets as a calorimeter.

Air moisture condenses on the evaporator of a refrigeration system, which serves as an air cooler, so that after certain operating times below the freezing point, the evaporator ices up and has to be defrosted so that the cooling performance can be maintained. For this purpose, it is known to interrupt the cooling process and to defrost the evaporator, for example by means of electric heating elements, which are arranged between the pipes of the evaporator carrying refrigerant. It is also known to provide the evaporator with hot gas by reversing the refrigeration circuit, or to defrost the evaporator using a separate heat transfer circuit, the pipes of which are arranged between the pipes of the evaporator that carry refrigerant.

GB 998,719 A describes a refrigeration circuit with means for defrosting ice, wherein refrigerant flows through a pipe in the evaporator that is guided in turns through a stack of plate-shaped fins and an inner pipe is provided within the refrigerant pipe for the heat transfer medium intended for defrosting, which has radial ribs in the space between the inner and outer tubes is in heat-conducting contact with the outer tube.

The invention is based on the object of improving the defrosting process on an evaporator of a refrigeration circuit with a simple structure.

According to the invention, the refrigerant-carrying pipes of the evaporator are at least partially or partially surrounded by a pipe jacket, through which a heat transfer medium is guided along the surface of the refrigerant-carrying pipes during the defrosting process.

Because the evaporator pipe carrying refrigerant is in close thermal contact with the heat transfer medium over its longitudinal dimension during the defrosting process, a uniform temperature distribution in the evaporator is achieved during the defrosting process and excessive local temperatures are avoided during the defrosting, as is the case, for example a defrost heater that uses electric heating elements.

In a method for defrosting an evaporator of a refrigeration circuit serving as an air cooler, in particular for refrigeration cabinets and cold rooms, frost or ice formation on the evaporator is defrosted by a heat transfer medium which is passed through a defrost circuit and heated for defrosting while the cooling process of the refrigeration circuit of the evaporator is switched off , wherein the heat transfer medium of the defrosting circuit is guided at least in sections along the outer circumference of the refrigerant lines in the evaporator during defrosting operation.

The heat transfer medium can be guided in sections over the longitudinal extent of the refrigerant lines and/or also in sections around the circumference of the refrigerant lines.

In order to guide the heat transfer medium of the defrost circuit at least in sections along the coolant lines in the evaporator, a cavity is formed on the outer circumference of the coolant lines, through which the heat transfer medium is guided.

Advantageously, the flow of the heat transfer medium in the defrosting circuit is interrupted during the cooling operation and the flow of the refrigerant in the refrigerant line is interrupted during the defrosting operation.

The fan is conveniently switched off during defrosting so that the heat of the heat transfer medium is only conducted to the iced-over fins of the evaporator via thermal contact of the pipe walls.

In a device for defrosting an evaporator of a refrigeration circuit serving as an air cooler in refrigeration cabinets or cold rooms, a fan is provided for blowing air to be cooled through the evaporator, the pipes of the refrigerant and the heat transfer medium being guided through the evaporator and a pipe casing being at least partially or is formed in sections around an inner tube, which has a cavity on or around the inner pipe, so that there is good thermal contact between the pipe and the surrounding cavity.

Preferably, fins are provided on the outer circumference of the outer tube in heat-conducting contact with the outer tube, through which air is blown by the fan.

The inner tube of the evaporator is expediently guided in turns through a stack of plate-shaped fins and the outer tube is only provided in the area of straight sections of the inner tube, so that outer tube sections lying essentially parallel to one another result, at the ends of which the arches of the inner tube laid in turns protrude .

However, the inner tube, which is guided in turns through the evaporator, can also be surrounded by the outer tube over its entire length, including bends.

At least one heat-conducting spacer element is advantageously provided in the cavity surrounding the inner tube in order to improve the heat-conducting contact between the inner and outer tubes.

Such a spacer element can expediently be designed as a corrugated sheet metal which rests in the cavity both on the inner circumference of the outer tube and on the outer circumference of the inner tube.

The described structure of a defrosting device of an air cooler can advantageously be used as a calorimeter in a refrigerated cabinet, in which the air to be cooled is guided by the fan through the hollow walls of the refrigerated cabinet in such a way that the cool air sweeps over the open top of the refrigerated cabinet.

If the fan is switched off in such a refrigeration cabinet and the flow of refrigerant through the refrigeration circuit is maintained as in refrigeration mode by the operation of the compressor, while at the same time the heat transfer medium flows through the defrost circuit and is heated, this operation can be adjusted so that an equilibrium is achieved arises between the amount of heat supplied to the defrost circuit on the one hand and the constant temperature of the refrigerant in the refrigeration circuit on the other. As a result, when this equilibrium is reached, the performance of the evaporator can be determined in a simple manner based on the amount of heat supplied.

Fig. 1

schematisch einen Kältekreis mit einem ein Wärmeträgermedium führenden Abtaukreis,

Fig. 2

schematisch ein Kühlmöbel mit einem Verdampfer als Luftkühler,

Fig. 3

den Abtaukreis nach Fig. 1 mit einer schematisch wiedergegebenen erfindungsgemäßen Abtauheizung,

Fig. 4

eine schematische Detailansicht der erfindungsgemäßen Abtauheizung,

Fig. 5

eine Schnittansicht zu Fig. 4,

Fig. 6

eine Draufsicht auf eine Rohrleitungsführung in einem Verdampfer mit der erfindungsgemäßen Abtauheizung,

Fig. 7

eine Stirnansicht der Rohrleitungsanordnung von unten in Fig. 6,

Fig. 8

im Querschnitt eine Ausführungsform mit Wärmeübertragungselement zwischen Innenrohr und Außenrohr,

Fig. 9, 9a

eine andere Anordnung zwischen Innen- und Außenrohr,

Fig. 10

eine schematische Schnittansicht durch eine Rohrleitung bei einer weiteren Ausführungsform des Verdampfers, und

Fig. 11

ein Messprotokoll zur Erläuterung des Betriebs bei einer kalorimetischen Messung.

For example, embodiments of the invention are explained in more detail below with reference to the drawing. Show it

Fig. 1

schematically a refrigeration circuit with a defrosting circuit carrying a heat transfer medium,

Fig. 2

schematically a refrigeration cabinet with an evaporator as an air cooler,

Fig. 3

the defrost circuit Fig. 1 with a defrost heater according to the invention shown schematically,

Fig. 4

a schematic detailed view of the defrost heater according to the invention,

Fig. 5

a sectional view Fig. 4 ,

Fig. 6

a top view of a pipeline in an evaporator with the defrost heater according to the invention,

Fig. 7

a front view of the pipeline arrangement from below Fig. 6 ,

Fig. 8

in cross section an embodiment with a heat transfer element between the inner tube and outer tube,

Fig. 9, 9a

a different arrangement between the inner and outer tube,

Fig. 10

a schematic sectional view through a pipeline in a further embodiment of the evaporator, and

Fig. 11

a measurement protocol to explain the operation of a calorimetric measurement.

Fig. 1 shows a refrigeration circuit K with a compressor 1, from which a hot gas line 2 leads to a condenser, for example a plate heat exchanger 3. 4 denotes a condensate line and 5 denotes a refrigerant collector, from which a pipeline 6 leads to an expansion valve 7. The refrigerant expanded in the expansion valve flows in line 6 through an evaporator 8, on which a fan 80 is provided, which blows air through a package of fins 8.1.

In the exemplary embodiment shown, a heat exchanger 9 is provided in the hot gas line 2, through which heat is transferred from the high-pressure refrigerant to a circuit V carrying a heat transfer medium such as brine in order to cool the hot gas coming from the compressor before it enters the condenser 3.

With A is in Fig. 1 a defrosting circuit with a circulation pump 20 and a heating device 21 for the heat transfer medium, for example brine, flowing through the defrosting circuit A. From the circulation pump 20, the heat transfer medium flows via a flow line 22 through the fin area 8.1 of the evaporator 8 parallel to the refrigerant line 6 and via a return line 23 back to the circulation pump 20. 24 denotes a safety valve and 25 denotes an expansion vessel in defrost circuit A.

With this known arrangement Fig. 1 The compressor 1 of the refrigeration circuit K is switched off when a certain degree of frost or ice formation is detected by suitable sensors on the evaporator 8, whereupon, after the refrigeration circuit K has been switched off, the circulation pump 20 with heating device 21 in the defrosting circuit A is switched on, so that the heating device 21 heated heat transfer medium flows through the fin area 8.1 of the evaporator 8 in order to heat it and defrost the ice. If sensors have detected a sufficient defrosting process, the defrosting circuit A is shut down by switching off the circulation pump 20 and the heating device 21, whereupon the refrigeration circuit K starts operating again in order to cool the air flowing through the evaporator via the evaporator 8, which has been freed from frost and ice .

Fig. 2 shows a schematic cross-section of a refrigerated cabinet as used in department stores to display frozen goods. 30 denotes the outer insulation of a housing 31 with a U-shaped cross section, the walls of which are hollow to form a cooling air circuit, with air outlet and inlet slots 32 being provided in the upper region of the side walls. The evaporator 8 is arranged between the inner floor surface and the floor insulation, through which air is blown by means of the fan 80. The cooling air circuit is indicated by arrows. A shut-off valve in the refrigerant line 6 is designated by 33.

The evaporator 8 can also be arranged at another location in the closed area of the cooling air circuit within the refrigerated cabinet, in which the cooling air flow between the air slots 32 sweeps over the open upper area of the refrigerated cabinet.

Fig. 3 shows a defrosting circuit A designed according to the invention, in which the heat transfer medium of the defrosting circuit A flows through a pipe casing 40 which surrounds the refrigerant line 6 in the fin area 8.1 of the evaporator 8. The remaining defrost circuit A corresponds to that in Fig. 1 .

Fig. 4 shows schematically in a detailed view the in Fig. 3 The structure of the defrost heater shown here.

The pipe casing 40 through which a heat transfer medium such as brine flows is provided with the slats 8.1 on the outer circumference. The inner pipe 6 leading to the refrigerant causes the brine to enter the cavity 41 of the pipe casing during cooling operation 40 cooled, the cooling temperature being transferred to the pipe casing 40 around which air flows and the plate pack 8.1 through which air flows.

With this tube-in-tube arrangement Fig. 4 the pipe casing 40 forms an outer pipe and the refrigerant line 6 forms an inner pipe.

The refrigerant line 6 and the pipe casing 40 are preferably made of the same material in order to avoid tensions between the pipe casing and the refrigerant pipe at the connection points, which could arise due to different linear expansions due to the temperature changes that occur. Copper is preferably used for the refrigerant line 6 as well as for the pipe casing 40.

The slats 8.1 can be designed in the shape of a plate and they are provided with punched outs into which the pipe casing 40 is inserted. A collar indicated at 8.11 can be provided on the punched outs of the slats 8.1, through which the contact area between the slat 8.1 and the pipe casing 40 is increased. Fig. 5 shows a front view of the structure in Fig. 4 .

Fig. 6 shows schematically a view of an evaporator 8 with plate-shaped spaced fins 8.1, through which the refrigerant line 6 runs in turns, which is essentially only surrounded by the pipe casing 40 on the straight sections extending through the fin pack 8.1.

At the entry of the refrigerant into the evaporator 8, a shut-off valve 10 is shown in the refrigerant line 6.

The heat transfer medium of the defrosting circuit A is supplied through the line 22, which opens into a distribution pipe 42, from which the pipe jacket 40 extends along the refrigerant line 6. In the area of the winding bends 6a of the refrigerant line 6, the pipe casing 40 runs, for example, in arches 40a between the straight sections of the refrigerant line 6, without the refrigerant line 6 being encased. In the same way, the refrigerant line 6 runs outside the pipe casing 40 in the area of the curved guide 6a.

At the outlet of the heat transfer medium from the pipe casing 40, a collecting pipe 43 is provided, from which the line 23 ( Fig. 3 ) leads to the heating device and the pump in defrost circuit A.

At the outlet of the refrigerant from the evaporator 8 can be in Fig. 6 a collecting pipe 6.1 can be provided, into which the individual refrigerant lines 6 of several layers of refrigerant lines open. The refrigerant line 6 runs from the collecting pipe 6.1 Fig. 6 to compressor 1.

Fig. 7 shows schematically a front view of the arrangement in Fig. 6 from below, with, for example, three layers of the refrigerant line 6 lying one above the other in turns. For the sake of clarity, the pipe casing 40 is shown by a dashed line, while the arches 40a of the pipe casing between winding strands of the refrigerant line 6 are shown by solid lines.

In the same way, the bends 6a of the refrigerant line 6 are shown by solid lines and the further course of the refrigerant line by dashed lines.

It is also possible to guide the pipe casing 40 along the bends 6a of the refrigerant line 6, so that the entire refrigerant pipe 6 running through the evaporator is surrounded by the pipe casing 40. Here, in the area of the bends, the refrigerant line 6 can rest on the inner circumference of the pipe casing 40, like this 9 and 9a shows

Fig. 9a shows a top view, with a lamella 8.1 being arranged between the arch 40a and the straight section of the pipe casing 40. The pipe casing 40 can be bent together with the internal refrigerant line 6.

In the embodiment according to Fig. 6 and 7 The bends 6a can be soldered onto the straight sections of the refrigerant line 6, which are surrounded by the pipe casing, so that when the evaporator is manufactured, the pipe casing 40 is pushed onto a straight section of the refrigerant pipe 6 and on the end faces for sealing on the circumference of the refrigerant pipe 6 is soldered, whereupon the bends 6a are soldered onto the individual jacketed sections of the refrigerant line 6.

To defrost the evaporator 8, the refrigeration circuit K is interrupted and the defrost circuit A is put into operation, whereupon the heat transfer medium such as brine heated by the heating device 21 initiates the defrosting process, with the fins 8.1 of the evaporator 8 being pushed through the cavity 41 via the pipe casing 40 Pipe casing 40 flowing brine is heated.

Here, the refrigerant in the refrigerant line 6 is heated in the evaporator 8, which is brought back to a lower temperature after the cooling operation of the refrigeration circuit K has resumed. During cooling operation, the heat transfer medium or brine is in the pipe casing 40 is cooled, the fins 8.1 being brought to the low temperature of the air cooler or evaporator via the cooled heat transfer medium.

The volume of the heat transfer medium in the cavity 41 of the pipe casing 40 in the evaporator 8 is preferably kept low in order to promote the cooling effect on the fins 8.1 during cooling operation.

Preferably, a refrigerant with a low specific volume is used to operate the refrigerant circuit K.

In the defrosting circuit A, a shut-off valve can be provided before the heat transfer medium enters the evaporator 8 and at the exit from the evaporator, so that a flow of the heat transfer medium in the defrosting circuit A can be prevented during cooling operation.

The heat transfer between refrigerant line 6 and pipe casing 40 during cooling operation can be improved by heat-conducting spacer elements in the cavity 41 between refrigerant pipe 6 and pipe casing 40.

As an example is in Fig. 8 a corrugated sheet 44 in the cavity 41 is shown in a cross-sectional view, which rests on the inner circumference of the pipe casing 40 and on the outer circumference of the refrigerant line 6. This enables an accelerated reduction in temperature on the slats 8.1 during cooling operation.

Fig. 9 shows an embodiment in which the refrigerant line 6 rests on the inner circumference of the pipe casing 40. In this embodiment, too, the heat transfer from the refrigerant line 6 to the pipe casing 40 is improved during cooling operation. In this arrangement, the refrigerant line 6 is only partially surrounded by the cavity 41 on the circumference.

Fig. 10 shows schematically another embodiment of the pipe arrangement in the evaporator 8, in which, in contrast to the previously described embodiment, the refrigerant flows through the cavity 61 of a refrigerant pipe 60, within which a pipe 22 of the defrosting circuit A carrying the heat transfer medium runs. In this tube-in-tube arrangement, the fins 8.1 arranged on the outer tube 60 are cooled more effectively in cooling mode, while in defrosting mode the heat from the fins arranged in the inside Pipe 22 flowing heat transfer medium must be transferred via the refrigerant in the cavity 61 to the fins 8.1.

Also with the pipe arrangement Fig. 10 can follow the embodiments 8 and 9 be provided.

When arranged according to Fig. 10 the refrigerant line 60 forms the outer pipe and the pipe 22 carrying the heat transfer medium forms the inner pipe.

The described design of an evaporator with a refrigerant line 6 that is at least partially sheathed or a brine line 22 that is sheathed by the refrigerant line 60 ( Fig. 10 ) can be used as a calorimeter. Here, the refrigeration circuit is operated in cooling mode, for example with CO2 as a refrigerant, with the fan 80 switched off, with the evaporator having previously cooled the air flowing through to, for example, minus 20 ° C.

During further operation of the refrigeration circuit K with the fan 80 switched off, counterheating is simultaneously carried out by the brine heated in the defrost circuit A, with the heated brine passing through the pipe casing 40 ( Fig. 4 ) or through the inner tube 22 in Fig. 10 flows. Here, the heating output can be varied by the heating device 21 until the refrigerant temperature in the evaporator becomes essentially constant. In this way, a balance is established between the heat supplied on the one hand and the refrigerant temperature in the evaporator on the other. The heating power supplied can be measured using a wattmeter. The refrigerant temperature within the evaporator can be determined using a measuring device.

As soon as the equilibrium is set by appropriately varying the heating output, the cooling output of the evaporator can be determined based on the heating output supplied, because due to the balance, the heating output supplied corresponds to the cooling output of the evaporator.

In practice, the manufacturer's information on evaporator performance is not uniform for refrigeration systems in refrigerated cabinets, so that the operator of a refrigeration system in refrigerated cabinets cannot compare the information on evaporator performance. He is also not able to easily check the evaporator performance.

In order to enable a simple check of the evaporator performance, the defrosting device described is used as a calorimeter according to the invention.

Here, one in Fig. 2 shown refrigeration cabinet, in which the evaporator 8 is arranged in a largely closed cavity, the fan 80 is switched off, so that in particular when the evaporator 8 is arranged in the bottom area of the refrigeration cabinet There is no air flow through the evaporator, at most a slight convective air flow, which does not influence a calorimetric measurement. The compressor 1 continues to deliver refrigerant through the refrigerant line 6. At the same time, the defrosting circuit A is switched on, so that the pump 20 delivers heat transfer medium (eg glycol) heated by the heating device 21 through the evaporator 8.

With this simultaneous operation of the refrigeration circuit and defrost circuit with the fan 80 switched off, a temperature balance can be set between the amount of heat supplied by the defrost circuit using heated glycol as a heat transfer medium and the refrigerant temperature in the evaporator, as shown in Fig. 11 is reproduced using a measurement protocol. Here, the heating output of the heating device 21 is kept constant while the evaporator 1 continues to run and maintains the cooling operation of the refrigeration circuit. The room temperature in the largely closed cavity in which the evaporator is arranged is kept constant over a certain period of time. This is facilitated by the fact that the largely closed cavity is, on the one hand, provided with thermal insulation 30 ( Fig. 2 ) and on the other hand is provided with only relatively small openings in the form of slots 32. Furthermore, the temperature of the refrigerant in the evaporator is kept constant.

1. a) die Leistungsaufnahme des Verdichters knapp unter 800 Watt,
2. b) die durch die Heizung eingebrachte Wärmemenge in der Größenordnung von 1.900 bis 2.000 Watt,
3. c) die Temperatur von - 20°C in dem den Verdampfer umgebenden Hohlraum,
4. d) die Temperatur von - 30°C des im Verdampfer verdampfenden Kältemittels,
5. e) die Temperatur von etwa - 13°C des Wärmeträgermediums am Eintritt in den Verdampfer, und
6. f) die Temperatur von etwa - 23°C des Wärmeträgermediums am Austritt des Verdampfers.
7. g) Gibt den Verdampfungsdruck wieder.

In detail there is Fig. 11 a calorimetric measurement in a defrosting device according to the invention, in which CO ₂ flows as a refrigerant in the inner tube and glycol as a heat transfer medium in the outer tube of the evaporator. This shows

1. a) the power consumption of the compressor is just under 800 watts,
2. b) the amount of heat introduced by the heater in the order of 1,900 to 2,000 watts,
3. c) the temperature of -20°C in the cavity surrounding the evaporator,
4. d) the temperature of - 30°C of the refrigerant evaporating in the evaporator,
5. e) the temperature of approximately -13°C of the heat transfer medium at the inlet to the evaporator, and
6. f) the temperature of approximately -23°C of the heat transfer medium at the outlet of the evaporator.
7. g) Represents the evaporation pressure.

With the operating state set in this way, the easily measurable heating output can be used to determine which corresponds to the evaporator output, with the refrigerant in the evaporator absorbing the amount of heat supplied.

When using the defrosting device as a calorimeter, no heat is absorbed from the outside because the cavity in which the evaporator 8 is arranged is thermally insulated from the outside and the air blowing and air intake openings 32 largely close off the cavity from the environment when the fan is switched off. In this state, the amount of heat supplied by the defrosting circuit corresponds to the amount of heat for cooling by the refrigerant (preferably CO ₂ ), the temperature of which is kept constant during operation as a calorimeter.

Claims (9)

1. Method for defrosting an evaporator (8), serving as an air cooler, of a cooling circuit (K) with refrigerant lines (6), in particular in items of refrigeration equipment and cold storage rooms, wherein a build-up of frost or ice at the evaporator (8) is defrosted by a heat transfer medium which is guided through a defrosting circuit (A) and heated for defrosting, while the cooling process of the cooling circuit (K) of the evaporator (8) is switched off,
   characterised in that
   in defrosting operation, the heat transfer medium of the defrosting circuit (A) is guided through a cylinder guiding the refrigerant and surrounding, at least in sections, the piping of the evaporator (8) along the surface of the outer circumference of the refrigerant lines (6) over its longitudinal dimension in the evaporator (8).
2. Method according to claim 1, wherein a hollow space (41) is formed, at least in sections, on the refrigerant line (6) in the evaporator (8), through which space the heat transfer medium of the defrosting circuit (A) is guided.
3. Method according to claim 1 or 2, wherein, during the cooling operation, the flow of the heat transfer medium in the defrosting circuit (A) is interrupted and, during the defrosting operation, the heat transfer medium circulates in the defrosting circuit (A), while the flow of the refrigerant in the refrigerant line (6, 60) in the evaporator (8) is interrupted.
4. Device for defrosting an evaporator (8), serving as an air cooler, of a cooling circuit in items of refrigeration equipment or cold storage rooms, comprising

   an evaporator (8),

   a fan (80) for blowing air to be cooled through the evaporator (8), a cooling circuit (K) connected to the evaporator (8), with refrigerant lines (6) and a defrosting circuit (A) guiding a heat transfer medium, the piping of which circuit leads through the evaporating region of the evaporator (8),

   wherein in the evaporator (8) a tube jacket (40, 60) is designed, at least partially or in sections, about an inner tube (6, 22) of a refrigerant line of the cooling circuit (K), forming a hollow space (41, 61) about the inner tube (6, 22) connected to the piping of the defrosting circuit (A) for guiding through the heat transfer medium, and wherein in the hollow space (41, 61) at least one heat-conducting distancing element (44) is provided between inner tube (6, 22) and outer tube (40, 60).
5. Device according to claim 4, wherein fins (8.1) of the evaporator (8) abut against the outer circumference of the outer tube (40, 60) in heat-conducting contact.
6. Device according to claim 4 or 5, wherein the inner tube (6, 22) is guided, in coils, through a stack of plate-shaped fins (8.1) and the outer tube (40, 60) surrounds the inner tube (6, 22) running through the stack of fins in coils only in the region of straight sections.
7. Device according to claim 4 or 5, wherein the inner tube (6, 22) guided in coils through the evaporator is surrounded, over its whole length, including curves (6a) by the outer tube (40, 60).
8. Device according to claim 4, wherein a corrugated sheet (44) is provided in the hollow space (41, 61), which element abuts against the inner circumference of the outer tube (40, 60) and against the outer circumference of the inner tube (6, 22).
9. Use of the device according to claim 4 as calorimeter, wherein the evaporator (8) is arranged in a hollow element which is largely closed and insulated against heat dissipation outwards, such as for example in an item of refrigeration equipment, and wherein, during operation of the cooling circuit (K), the fan (80) is switched off at the evaporator (8) and heated heat transfer medium is guided through the defrosting circuit (A) until an equilibrium between the added quantity of heat and a quantity of heat absorbed by refrigerant in the evaporator is set, while the temperature of the refrigerant is held substantially constant, whereby the refrigerating performance of the evaporator is ascertained using the added heating capacity for the defrosting circuit (A) of the device.

Images