AN EVAPORATOR ASSEMBLY

Abstract

An evaporator assembly comprises at least one plate evaporator, each plate evaporator comprising first and second sheets that are joined together and that define an internal conduit between the sheets. The internal conduit is aligned parallel to the sheets and is configured to carry refrigerant through the plate evaporator, each plate evaporator further comprising a first electrically insulative layer applied on the first sheet, an electrically resistive layer applied on the first electrically insulative layer, and a second electrically insulative layer applied on the electrically resistive layer. The electrically resistive layer is an elongated track that follows a meandering path traversing along the first sheet.

Description

FIELD OF THE INVENTION

The present invention relates to an evaporator assembly, typically for use in refrigeration systems.

BACKGROUND OF THE INVENTION

Evaporators for refrigeration systems are typically one of two different types, coil evaporators and plate evaporators. A coil evaporator generally comprises a fluid pipe for refrigerant and a large number of fins extending transversely from the fluid pipe, to provide a large external surface area for cooling.

A plate evaporator generally comprises a plate formed by two sheets that define a conduit between the sheets, parallel to the plate. Compared to an equivalent coil evaporator, a plate evaporator has a smaller external surface area for cooling. The smaller external surface area is compensated for by a lower thermal resistance between the external surfaces and the flow of refrigerant, allowing the external surfaces of the plate evaporator to drop to a lower temperature than the external surfaces of the fins of the coil evaporator.

Plate evaporators are often claimed to be 30% more efficient than equivalent coil evaporators, however due to the colder external surfaces plate evaporators suffer more from build-up of ice on the external surfaces of the evaporator than coil evaporators do, and so this precludes use of plate evaporators except for in small size refrigerators. The build-up of ice on evaporator surfaces reduces the efficiency of the evaporator, and so is to be avoided.

It is known to provide heaters that intermittently blow hot air through the fins of coil evaporators to clear ice build-up, however this does not work sufficiently well for plate evaporators where ice build-up may be greater, and where too much heat would need to be pumped into the refrigerated space to defrost the plate evaporators. The extremes of heat and cold experienced by evaporators that use blown hot air to defrost them also reduces the reliability of the evaporators and results in more frequent breakdowns and increased maintenance requirements.

One of the problems with coil evaporators and the often complex system of fins and heaters that blow hot air, is that the blown air circulates any stray particles with the result that the fins can become congested to the point that air circulation is blocked, greatly reducing efficiency and the ability to cool the refrigerated space.

Another heating technique for coil evaporators is described in US 2020/0248952 A1, and comprises an electrically resistive coating that is applied substantially all over the coil evaporator, through which current can be passed to produce heating. However, the electrically resistive coating along with its accompanying electrically insulative coatings increases the thermal resistance between the external surfaces of the evaporator and the refrigerant, reducing the effectiveness of the coil evaporator, and rendering the method ineffective for use in plate evaporators which require a low thermal resistance between their external surfaces and the refrigerant to realise their efficiency advantages.

Cleaning a coil evaporator is extremely messy and difficult. During any cleaning process fins can become distorted and bent and in effect lose their efficiency. This does not apply to plate evaporators which are very simple set up and very easy to clean. The fact that plate evaporators are significantly stronger is one advantage, and the straight gaps between the plates also makes cleaning out any debris a quick and simple task.

It is therefore an object of the invention to provide an improved plate evaporator that retains efficiency advantages over coil evaporators but which does not suffer from excessive build-up of ice.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided an evaporator assembly comprising at least one plate evaporator. Each plate evaporator comprises first and second sheets that are joined together and that define an internal conduit between the sheets, the internal conduit aligned parallel to the sheets and configured to carry refrigerant through the plate evaporator. Each plate evaporator further comprises a first electrically insulative layer applied on the first sheet, an electrically resistive layer applied on the first electrically insulative layer, and a second electrically insulative layer applied on the electrically resistive layer, wherein the electrically resistive layer is an elongated track that follows a meandering path traversing along the first sheet.

The electrically resistive layer acts as a heater when an electrical current is passed along it, reducing any build-up of ice on the external surfaces of the plate evaporator.

Since the electrically resistive layer is an elongated track that follows a meandering path traversing along the first sheet, the electrically resistive layer does not significantly affect the thermal resistance between the external surfaces of the evaporator and the refrigerant, since most of the surface area of the first sheet is not covered by the electrically resistive layer. The electrically resistive layer may be a resistive oxide layer formed on the first electrically insulative layer, allowing efficient transfer of heat from the electrically resistive layer to the first sheet.

Preferably, the width and length of the elongated track is such that the electrically resistive layer is present over less than 10% of the whole surface area of the first sheet, more preferably less than 5% of the whole surface area of the first sheet.

The second sheet is preferably devoid of any electrically resistive layer, maintaining a low thermal resistance between the external surface and the refrigerant.

The second electrically insulative layer may extend over and encapsulate both the first and the second sheets, to both electrically insulate the electrically resistive layer from the surrounding environment and to protect the first and second sheets from the surrounding environment. Preferably the second electrically insulative layer is applied directly to the second sheet without any first insulative layer or electrically resistive layer between the second electrically insulative layer and the second sheet, to maintain a low thermal resistance between the external surface and the refrigerant. The second electrically insulative layer may form the external (exterior) surface of the plate evaporator.

The first electrically insulative layer may also be an elongated track that follows the meandering path, between the first sheet and the electrically insulative layer. Then the first electrically insulative layer only covers a small area of the first sheet, and so does not significantly affect the thermal resistance between the external surfaces of the evaporator and the refrigerant. The elongated track of the first electrically insulative layer is preferably wider than the elongated track of the electrically resistive layer and protrudes outwardly from beneath the electrically resistive layer, beyond the lengthwise edges of the electrically resistive layer, to ensure that the electrically resistive layer will not come into contact with the first sheet and cause an electrical short-circuit.

The meandering path and elongated track may traverse over at least 50% of the whole area of the first sheet, more preferably at over at least 75% of the whole area of the first sheet, and still more preferably over substantially the whole area of the first sheet. Then, heat conduction away from the elongated track and through the first sheet will heat the full area of the first sheet to defrost any ice that has begun to form over the first sheet. Heat conduction from the first sheet to the second sheet will also defrost any ice that has begun to form over the second sheet.

The first and second sheets may be metal sheets, preferably aluminium sheets, which aids the heat conduction from the electrically resistive layer on the first sheet to the second sheet, as well as the heat conduction from the external surfaces of the evaporator to the refrigerant.

The internal conduit is configured to carry refrigerant through the plate evaporator, and preferably follows at least a portion of the meandering path. Then, at least some of the electrically resistive layer will be in close proximity to the internal conduit and flow of refrigerant, at the location where ice build-up is most likely to occur.

The first electrically insulative layer may adhere to the first sheet, for example if the electrically insulative layer is a paint, which may be sprayed or brushed onto the first sheet during manufacturing. Painting the first electrically insulative layer onto the first sheet ensures good thermal contact between the first electrically insulative layer and the first sheet, and ensures that the first electrically insulative layer conforms to the surface of the first sheet. The first electrically insulative layer is preferably a polymer, for example a polymer based paint.

The electrically resistive layer may adhere to the first electrically insulative layer, for example if the electrically resistive layer is a paint, which may be sprayed or brushed onto the first electrically insulative layer during manufacturing. Painting the electrically resistive layer onto the first electrically insulative layer ensures good thermal contact between the electrically resistive layer and the first electrically insulative layer, and does not require the use of heat, which could otherwise damage the first electrically insulative layer.

The paint of the electrically resistive layer may comprise a nickel-chromium powder, to render the paint electrically resistive and therefore capable of generating heat when a current is passed through it.

The second electrically insulative layer may perform the dual function of electrically insulating the electrically resistive layer from the surrounding environment and providing a sanitary external surface of the plate evaporator. The second electrically insulative layer may for example be an enamel coating.

The first sheet may therefore define a substrate, upon which the first electrically insulative layer, electrically resistive layer and second electrically insulative layer are successively built up. There is no need for any other supporting substrates, and accordingly the first electrically insulative layer, electrically resistive layer and second electrically insulative layer may be only supported by the first sheet.

The evaporator assembly may comprise a heater control unit that is electrically connected to opposing ends of the elongated track of the electrically resistive layer, and configured to drive an electric current along the elongated track to heat the plate evaporator in a defrost mode. The heater control unit may gently heat the plate evaporator by passing a low current through the elongated track of the electrically resistive layer, to reduce ice build-up on the plate evaporator.

The heater control unit may be configured to heat each plate evaporator for a fixed period of time, sufficient to melt any ice build-up that may have occurred since the last time the plate evaporator was heated. In some embodiments each plate evaporator may be fitted with a temperature sensor, which is connected to the heater control unit. The heater control unit may be configured to heat the plate evaporator until the temperature sensor reports a temperature reaching a threshold temperature, for example 5 degrees centigrade.

The plate evaporator may be heated to a maximum temperature of 5 degrees centigrade by the heater control unit. This avoids adding excessive heat into the refrigeration space, which would otherwise need to be subsequently removed by the refrigerant, and which could otherwise undesirably raise the temperature of other contents in the refrigeration space. Accordingly, the heater control unit is preferably configured to supply an electrical power of less than 100 W to the elongated track of the electrically resistive layer.

The heater control unit may be configured to receive a signal indicating when the flow of refrigerant through the plate evaporator has ceased, and to only enter the defrost mode of the plate evaporator when the signal indicates the flow of refrigerant through the plate evaporator has ceased. Then, the heating of the electrically resistive layer will not occur when the evaporator is actively refrigerating the refrigerated space, and can occur during the intermittent off-cycles of the refrigeration process in which the temperature inside of the refrigerated space is below a temperature setpoint such that the flow of refrigerant is temporarily turned off.

The evaporator assembly may comprise a plurality of the plate evaporators arranged parallel to one another, to form a larger evaporator assembly suitable for commercial refrigeration purposes. The evaporator assembly may comprise more than three of the plate evaporators, for example five or more, or ten or more of the plate evaporators. This would provide a large refrigeration capacity equivalent to coil evaporators having much greater numbers of fins, the fins being much more susceptible to clogging with foreign objects due to their closer proximity than the plate evaporators, and much more susceptible to damage from impacts due to their thinner thicknesses than the plate evaporators. The plate evaporators are preferably all parallel to one another and their major surfaces are substantially planar, providing straight gaps between plates that face one another, which are easy to clean between.

The plate evaporators may be arranged with the first sheet and the second sheet of immediately adjacent plate evaporators facing towards one another. Then, the heat from the electrically resistive layer on the first sheet of one plate evaporator radiates to the second sheet of an immediately adjacent plate evaporator which may not have any electrically resistive layer on the second sheet.

The evaporator assembly may comprise an inlet manifold for connecting to a compressor, an outlet manifold for connecting to a condenser, and pipes connecting the internal conduits of the plate evaporators to the inlet and outlet manifolds, for flow of refrigerant.

The heater control unit may be configured to drive an electric current through the electrically resistive layers of the plate evaporators in sequence such that the plate evaporators are not all in the defrost mode at a same time as one another. Then the heater control unit can be a lower power unit since it only needs to power a limited number of the electrically resistive layers at any one time, preferably only one of the electrically resistive layers at a time. The heater control unit may be configured to enter the defrost mode of at least one of the plate evaporators every time that the signal is received indicating the flow of refrigerant has ceased.

There is further provided a refrigeration system comprising the evaporator assembly, and the refrigeration system may comprise a temperature sensing element configured to sense temperature within a refrigeration space to be refrigerated by the at least one plate evaporators. A controller may be configured to control the flow of refrigerant through the at least one plate evaporator based on the temperature sensing element, wherein the temperature sensing element comprises a food simulant material and a temperature probe embedded within the food simulant material, wherein the food simulant material is preferably a solid wax in which are distributed a plurality of gas-filled polymeric particles, for example polystyrene balls.

The temperature of the food simulant material does not fluctuate as much as the air within the refrigeration space, for example when a door of the refrigeration space is opened the air temperature rises rapidly but the temperature of food inside the refrigeration space does not rise as quickly, and may hardly rise at all if the door is quickly closed. Therefore the placement of the temperature probe within the food simulant material reduces the on & off cycles of the refrigeration to govern the actual temperature of food inside the refrigerated space, rather than the more rapidly fluctuating air temperature, saving power. These concepts are described in more detail within the Applicant's European Patent No. EP 2352976 B1.

The use of the plate evaporator with the electrically resistive layer disclosed herein, in combination with a temperature sensing element comprising a food simulant material and a temperature probe embedded within the food simulant material, is particularly advantageous because the extended off-cycle that the temperature sensing element provides gives more time for a low-power heater control unit to heat the plate evaporator, before the next on-cycle occurs.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the invention will now be described by way of non-limiting example only and with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic diagram of a plate evaporator assembly comprising a plate evaporator in accordance with an embodiment of the invention;

FIG. 2 shows a schematic cross-sectional diagram of the plate evaporator of FIG. 1;

FIG. 3 shows a schematic diagram of a plate evaporator assembly comprising a plurality of the plate evaporators of FIG. 1; and

FIG. 4 shows a schematic diagram of a refrigeration system comprising the plate evaporator assembly of FIG. 3.

The figures are not to scale, and same or similar reference signs denote same or similar features.

A first embodiment of the invention will now be described with reference to FIGS. 1 and 2, and FIG. 1 shows a plate evaporator assembly comprising a plate evaporator 10. The plate evaporator 10 comprises a first sheet 11 which may be aluminium, and second sheet 12 which may also be aluminium. The first and second sheets may be substantially planar and fixed on top of one another in a stacked arrangement.

The first and second sheets 11 and 12 have shapes that define an internal conduit 16 between them when the first and second sheets are joined together. The internal conduit 16 may meander over the area between the first and second sheets, parallel to the first and second sheets, traversing over the majority of the area between the first and second sheets. The internal conduit 16 may be connected to an inlet 15 at one end of the internal conduit 16 for receiving refrigerant from a compressor, and connected to an outlet 17 at an opposite end of the internal conduit 16 for out letting the refrigerant to a condenser when the evaporator is in use.

The first sheet 11 may have a first electrically insulative layer 20 of a material that is painted onto the outer surface of the first sheet and that follows a meandering path over the area of the first sheet. The first electrically insulative layer 20 may be a polymer based paint, which may have been applied to the outer surface of the first sheet by a spray process. The first electrically insulative layer 20 is preferably in the form of an elongated track running parallel to and conforming to the surface of the first sheet, as shown.

The first electrically insulative layer 20 may have an electrically resistive layer 21 painted upon it, the electrically resistive layer 21 being an elongated track that follows the same meandering path as the first electrically insulative layer 20. The electrically resistive layer 21 may be a layer of paint comprising a nickel-chromium powder that allows flow of electric current through the paint, however other types of electrically resistive paint could alternatively be used. The width of the elongated track of the electrically resistive layer 21 may be less than the width of the elongated track of the first electrically insulative layer 20, such that the first electrically insulative layer 20 extends beyond the electrically resistive layer 21 at both side edges of the electrically resistive layer 21, as shown. The elongated track of the electrically resistive layer 21 may traverse over (across) approximately 75% of the whole area of the first sheet 11, as shown.

The internal conduit 16 may follow a least a portion of the meandering path taken by the electrically resistive layer 21, for example FIG. 1 identifies a portion P1 where the internal conduit 16 follows the meandering path of the electrically resistive layer 21 vertically, and another portion P2 where the internal conduit 16 follows the meandering path of the electrically resistive layer 21 horizontally.

The elongated track of the electrically resistive layer 21 has a positive electric terminal pad 22 at one end of the elongated track and a negative electric terminal pad 24 at an opposite end of the elongated track. The positive and negative electric terminal pads 22 and 24 are connected to positive and negative wires 23 and 25, respectively, for connection to an electrical power source. When the electrical power source drives electric current through the elongated track of the electrically resistive layer 21, ohmic heating raises the temperature of the electrically resistive layer 21, and the heat is conducted through the first electrically insulative layer 20 to the first sheet 11 and subsequently to the second sheet 12. The first electrically insulative layer 20 prevents shorting of the electrical current to the first sheet 11.

The plate evaporator also has a second electrically insulative layer 28 which may be applied to and over the electrically resistive layer 21, the first electrically insulative layer 20 and the first and second sheets 11 and 12. This is best seen in FIG. 2 which shows a cross-sectional view of the plate evaporator 10 taken along line XS1 marked on FIG. 1. As shown in FIG. 2, the first and second sheets 11 and 12 may each have complimentary ridges 16a and 16b which coincide with one another to define the internal conduit 16 between the first and second sheets. Alternatively, the ridges could only be formed on one of the two sheets if desired.

The rightmost part of the internal conduit shown in FIG. 2 is part of the plate evaporator that may have the first electrically insulative layer 20 painted on the outside surface of the first sheet 11, and the electrically resistive layer 21 painted on the first electrically insulative layer 20, as shown. The second electrically insulative layer 28 may be formed as an enamel coating over the first and second sheets 11 and 12, the first electrically insulative layer 20, and the electrically resistive layer 21, to provide a sanitary exterior surface of the plate evaporator that protects the interior of the plate evaporator from the external environment. The second electrically insulative layer 28 may encapsulate the first and second sheets 11 and 12, surrounding them on all sides.

The second plate 12 may be devoid of any electrically resistive layer, and the second electrically insulative layer 28 may directly contact the second plate 12 over the whole outer surface of the second plate 12.

It would alternatively be possible to apply the first electrically insulative layer 20 over the whole of the surface of the first sheet rather than in the shape of an elongated track. For example, the first electrically insulative layer could be applied as a coating over the whole of the plate evaporator, and the second electrically insulative layer could be formed as an elongated track over the meandering path of the electrically resistive layer, with the exterior surface of the plate evaporator then being formed by the first electrically insulative layer, except for the areas where the elongated track of the second electrically insulative layer was present.

In use, a compressor pumps refrigerant into the inlet 15 and through the internal conduit 16, absorbing heat from the first and second sheets 11 and 12 as the refrigerant evaporates within the internal conduit. This rapidly lowers the temperature of the first and second sheets, and also the exterior of the plate evaporator. There is minimal thermal resistance between the exterior of the plate evaporator and the refrigerant, since the first insulative layer 20 and the resistive layer 21 are only present over a small area of the overall exterior surface area of the plate evaporator, and heat only needs to conduct through the second electrically insulative layer 28 and the sheets 11 and 12 to reach the refrigerant over the majority of the exterior surface area of the plate evaporator.

After a period of operation ice may begin to form on the exterior surface of the plate evaporator, and the electric current may be passed through the electrically resistive layer 21 in a defrost mode that defrosts the ice and prevents it from building up significantly over time. The electric current provides gentle heating of the electrically resistive layer 21, the heat conducting though the plate evaporator and melting any ice present on the exterior surfaces of the plate evaporator. The temperature of the exterior surfaces of the plate evaporator may for example be raised to up to 5 degrees centigrade, sufficient to defrost any ice without significantly raising the temperature inside the space that is being refrigerated.

The schematic diagram of FIG. 3 shows another plate evaporator assembly 6 in which five of the plate evaporators 10 of FIG. 1 may be assembled together to form a multi-plate evaporator assembly. Each plate evaporator may have four threaded holes 36 around the periphery of the plate evaporator, which receive four threaded rods 32, 33, 34 and 35 to secure the plate evaporators 10 together. In an alternate embodiment the holes 36 may not be threaded and nuts may be screwed onto the threaded rods 32, 33, 34 and 35 to secure the plate evaporators 10 in the desired positions.

The inlets 15 of the plate evaporators 10 may be connected to a common inlet manifold 30, and the outlets 17 of the plate evaporators 10 may be connected to a common outlet manifold 31. The inlet manifold 30 may be for connecting to a compressor and the outlet manifold 31 may be for connecting to a condenser.

The positive wires 23 of the evaporator plates 10 may be bundled together into a cable 23a, and the negative wires 25 of the evaporator plates 10 may be bundled together into a cable 25a. The cables 23a and 25a may be connected to a heater control unit, to control the heating of each of the plate evaporators 10. Clearly there are a myriad of different ways in which the positive and negative wires could be bundled together into cables for connecting to the heater control unit.

In this embodiment there are five of the evaporator plates 10, however alternative embodiments may have alternate numbers of evaporator plates.

The schematic diagram of FIG. 4 shows a refrigeration system comprising the plate evaporator assembly 6 of FIG. 3. The plate evaporator assembly 6 is shown being used to refrigerate a refrigerated space 40. The refrigerated space 40 may for example be a commercial chiller, freezer or cold room, from the very small to the very large.

The bundles of cables 23a and 25a of the plate evaporator assembly 6 may be connected to a heater control unit 50, which is configured to control when the plate evaporators are heated. The heater control unit may enter a defrost mode of one or more of the plate evaporators, in which the heater control unit applies a voltage across the wires 23 and 25 connected to the plate evaporator, to drive an electric current through the electrically resistive layer of the plate evaporator. The heater control unit may for example apply a voltage of 24V and current of 3 A to each pair to wires 23 and 25 corresponding to one of the plate evaporators. The heater control unit may apply the voltage and current for long enough to raise the temperature of the plate evaporator to approximately 5 degrees centigrade, which may take a minute or two. Preferably, the heater control unit only heats one plate evaporator at a time, and so the heater control unit does not have to supply any more current that that which is drawn by a single plate evaporator.

The inlet manifold 30 may be connected to an outlet of a compressor 55, and the outlet manifold 31 may be connected to an inlet of a condenser 56. An outlet of the condenser may be connected to an inlet of the compressor, forming a closed path around which refrigerant can be pumped by the compressor, to refrigerate the space 40.

The refrigeration system may also comprise a temperature sensing element 45 that is configured to sense the temperature within the refrigeration space 40. The temperature sensing element comprises a food simulant material 46 and a temperature probe 47 embedded within the food simulant material. The food simulant material may be a solid wax in which are distributed a plurality of gas-filled polymeric particles, for example polystyrene balls. The temperature sensing element 45 may also comprise a cable 48 for outputting temperature measurements, the temperature measurements being similar to the temperatures of any foods that are stored within the refrigerated space.

The refrigeration system may also comprise a controller 57, which is connected to the cable 48 of the temperature sensing element 45. The controller 57 may also be connected to the compressor 55, and to the heater control unit 50. The controller sends a signals to the compressor 55 instructing the compressor to turn on and off, and also sends signals 58 to the heater control unit 30, informing the heater control unit 30 of whether the compressor is turned on or off.

The controller 57 may control the flow of refrigerant through the at least one plate evaporator by turning the compressor on and off, based on the temperature reported by the temperature sensing element 45, to regulate the temperature of any food that may be stored in the refrigerated space 40.

In use, when the temperature reported by the temperature sensing element 45 rises too high, the controller 57 instructs the compressor 55 to start pumping, and informs the heater control unit 50 that flow of refrigerant has begun, causing the heater control unit 50 to exit any defrost modes that were active. Once the temperature reported by the temperature sensing element 45 has lowered sufficiently, the controller 57 instructs the compressor 55 to stop pumping, and informs the heater control unit 50 that flow of refrigerant has ceased. The heater control unit 50 then enters a defrost mode of one of the plate evaporators, passing current through the electrically resistive layer of that plate evaporator, and may continue for a minute or so, before ceasing and entering the defrost mode of the next plate evaporator. The heater control unit 50 steps through the defrost modes of the plate evaporators in sequence, until all the of the plate evaporators have been defrosted, or until the controller 57 signals that the compressor is going to start pumping again. It would also be possible for the heater control unit to repeatedly cycle though all of the plate evaporators whilst the flow of refrigerant has ceased, spending a short time (for example 5 or 10 seconds) in the defrost mode of each plate evaporator before moving on to the next plate evaporator. Then, all the plate evaporators are defrosted at a similar time and rate to one another.

Many other variations of the described embodiments falling within the scope of the invention will be apparent to those skilled in the art.

Claims

1. An evaporator assembly comprising at least one plate evaporator, each plate evaporator comprising first and second sheets that are joined together and that define an internal conduit between the sheets, the internal conduit aligned parallel to the sheets and configured to carry refrigerant through the plate evaporator, each plate evaporator further comprising a first electrically insulative layer applied on the first sheet, an electrically resistive layer applied on the first electrically insulative layer, and a second electrically insulative layer applied on the electrically resistive layer, wherein the electrically resistive layer is an elongated track that follows a meandering path traversing along the first sheet.

2. The evaporator assembly of claim 1, wherein the first electrically insulative layer is also an elongated track that follows the meandering path, between the first sheet and the electrically insulative layer.

3. The evaporator assembly of claim 1, wherein the second electrically insulative layer extends over and encapsulates both the first and the second sheets.

4. The evaporator assembly of claim 1, wherein the internal conduit follows at least a portion of the meandering path.

5. The evaporator assembly of claim 1, wherein the first electrically insulative layer adheres to the first sheet, and wherein the electrically resistive layer adheres to the first electrically insulative layer.

6. The evaporator assembly of claim 5, wherein the first electrically insulative layer is a paint layer that is painted on the first sheet.

7. The evaporator assembly of claim 5, wherein the electrically resistive layer is a paint layer that is painted on the first electrically insulative layer.

8. The evaporator assembly of claim 7, wherein the paint of the electrically resistive layer comprises a nickel-chromium powder.

9. The evaporator assembly of claim 1, wherein the second electrically insulative layer forms an exterior surface of the plate evaporator.

10. The evaporator assembly of claim 1, wherein the second electrically insulative layer is applied directly to the second sheet without any first insulative layer or electrically resistive layer between the second electrically insulative layer and the second sheet.

11. The evaporator assembly of claim 1, wherein the first and second sheets are metal sheets.

12. The evaporator assembly of claim 1, comprising a heater control unit that is electrically connected to opposing ends of the elongated track of the electrically resistive layer, and configured to drive an electric current along the elongated track of the electrically resistive layer to heat the plate evaporator in a defrost mode.

13. The evaporator assembly of claim 12, wherein the heater control unit is configured to receive a signal indicating when the flow of refrigerant through the plate evaporator has ceased, and to only enter the defrost mode of the plate evaporator when the signal indicates the flow of refrigerant through the plate evaporator has ceased.

14. The evaporator assembly of claim 1, comprising a plurality of the plate evaporators arranged parallel to one another, and further comprising an inlet manifold for connecting to a compressor, an outlet manifold for connecting to a condenser, and pipes connecting the internal conduits of the plate evaporators to the inlet and outlet manifolds.

15. The evaporator assembly of claim 14, comprising a heater control unit that is electrically connected to opposing ends of the elongated track of the electrically resistive layer of each plate evaporator, and configured to drive an electric current along the elongated track of the electrically resistive layer to heat the plate evaporator in a defrost mode, wherein the heater control unit is configured to drive an electric current through the elongated tracks of the plate evaporators in sequence such that the plate evaporators are not all in the defrost mode at a same time as one another.

16. The evaporator assembly of claim 15, wherein the heater control unit is configured to receive a signal indicating when the flow of refrigerant through the plate evaporators has ceased, and to only enter the defrost mode of each plate evaporator when the signal indicates the flow of refrigerant through the plate evaporator has ceased, wherein the heater control unit is configured to enter the defrost mode of at least one of the plate evaporators every time that the signal is received indicating the flow of refrigerant has ceased.

17. The evaporator assembly of claim 15, wherein the heater control unit is configured to enter the defrost mode of only one plate evaporator at a time.

18. The evaporator assembly of claim 16, wherein the plate evaporators are arranged with the first sheet and the second sheet of immediately adjacent plate evaporators facing towards one another.

19. An evaporator system comprising at least one plate evaporator, each plate evaporator comprising first and second sheets that are joined together and that define an internal conduit between the sheets, the internal conduit aligned parallel to the sheets and configured to carry refrigerant through the plate evaporator, each plate evaporator further comprising a first electrically insulative layer applied on the first sheet, an electrically resistive layer applied on the first electrically insulative layer, and a second electrically insulative layer applied on the electrically resistive layer, wherein the electrically resistive layer is an elongated track that follows a meandering path traversing along the first sheet.

20. The system of claim 19 including a temperature sensing element configured to sense temperature within a refrigeration space to be refrigerated by the at least one plate evaporator, and a controller configured to control the flow of refrigerant through the at least one plate evaporator based on the temperature sensing element, wherein the temperature sensing element comprises a food simulant material and a temperature probe embedded within the food simulant material, wherein the food simulant material is preferably a solid wax in which are distributed a plurality of gas-filled polymeric particles.