US20180172333A1

US20180172333A1 - Insulating separators for self-defrosting evaporator coil optimized for frost-free and frost-loaded conditions

Info

Publication number: US20180172333A1
Application number: US15/843,184
Authority: US
Inventors: Victor F. Petrenko; Sameh M.I. Saad; Kazi Kased L. Ahmed
Original assignee: Betterfrost Technologies Inc
Current assignee: Betterfrost Technologies Inc
Priority date: 2016-12-21
Filing date: 2017-12-15
Publication date: 2018-06-21

Abstract

Described herein is an evaporator and evaporator system. The system comprises a refrigerant tube formed from an electrically conductive material, the refrigerant tube being shaped to comprise a plurality of parallel tube portions; an upstream refrigerant conduit for supplying a refrigerant to the refrigerant tube; a downstream refrigerant conduit for receiving the refrigerant from the refrigerant tube; and a plurality of separators for securing the plurality of parallel tube portions in a plurality of relative positions to provide a plurality of airflow gaps separating adjacent parallel tube portions, and to impede electrical contact between different tube portions in the plurality of parallel tube portions.

Description

RELATED APPLICATIONS

This application claims priority from U.S. Patent Application No. 62/437,321, filed Dec. 21, 2016 entitled “INSULATING SEPARATORS FOR SELF-DEFROSTING EVAPORATOR COIL OPTIMIZED FOR FROST-FREE AND FROST LOADED CONDITIONS”, the disclosure of which is incorporated, in its entirety, by reference.

FIELD

The described embodiments relate to refrigeration systems. In particular, the described embodiments relate to systems and methods for providing separation of tube portions of a self-defrosting evaporator coil.

BACKGROUND

During the operation of a refrigeration system such as a refrigerator or an air conditioner unit, cooling may be accomplished by cycling a refrigerant liquid through a heat exchanger system in which the refrigerant liquid is allowed to evaporate as it passes through an evaporator coil located in the environment being cooled. During, the process of evaporation, heat energy surrounding the evaporator coil may be absorbed by the refrigerant liquid thereby reducing the temperature of the air in the surrounding environment. The cooled air may be circulated to provide cooling. The evaporated refrigerant can then be cycled to a compressor located away from the environment being cooled to be compressed and cooled back to a liquid (which disperses the energy absorbed by the liquid as heat) so that the refrigerant liquid can be recycled back into the evaporator coil for further cooling.

SUMMARY OF VARIOUS EMBODIMENTS

In a broad aspect, at least one embodiment described herein provides an evaporator. The embodiment comprises a refrigerant tube formed from an electrically conductive material, the refrigerant tube being shaped to comprise a plurality of parallel tube portions; an upstream refrigerant conduit for supplying a refrigerant to the refrigerant tube; a downstream refrigerant conduit for receiving the refrigerant from the refrigerant tube; at least one current delivery connector for delivering an electrical current from an electrical current source and at least one current return connector for returning the electrical current to the electrical current source, wherein the at least one current delivery connector and the at least one current return connector are coupled to the refrigerant tube to provide at least one electrical flow path between the at least one current delivery connector and the at least one current return connector; and a plurality of separators for securing the plurality of parallel tube portions in a plurality of relative positions to provide a plurality of airflow gaps separating adjacent parallel tube portions, and to impede electrical contact between different tube portions in the plurality of parallel tube portions.
In some embodiments, the refrigerant tube is a helically arranged refrigerant tube formed from an electrically conductive material, the refrigerant tube being curled around an axial airflow path axis to define a plurality of loops, each loop in the plurality of loops spanning a 360° rotation about a central axis, such that each point along a length of the refrigerant tube lies in a corresponding loop, and is axially positioned parallel to the central axis and radially positioned away from the central axis.
In some embodiments, each separator in the plurality of separators is electrically insulating having a resistance greater than 100 Ω·m, to electrically insulate different segments of the refrigerant tube in contact with the separator, and wherein the separator is electrically and mechanically stable even at temperatures below −30° C.
In some embodiments, each separator in the plurality of separators has a corresponding radial dimension and an axial dimension; intersects with at least two loops in the plurality of loops at a plurality of corresponding tube segments; and comprises a plurality of tube engagements for engaging the refrigerant tube at, for each loop in the at least two loops, a corresponding tube segment; wherein each corresponding tube segment is separated by at least one loop from every other corresponding tube segment in the plurality of tube segments.
In some embodiments, each separator in the plurality of separators comprises a thickness dimension substantially orthogonal to and much smaller than the radial dimension and the axial dimension, such that that separator has a substantially planar configuration.
In some embodiments, for each separator in the plurality of separators, the thickness dimension is between 1 mm and 20 mm.
In some embodiments, for each separator in the plurality of separators, the thickness dimension is between 3 mm and 10 mm.
In some embodiments, the plurality of airflow gaps comprises a plurality of radial air flow gaps and axial air flow gaps, the plurality of separators block less than 10% of an airflow cross-sectional area, and the airflow cross-sectional area is one of a radial air flow cross-sectional area through the plurality of radial air flow gaps, and an axial airflow cross-sectional area of an axial airflow path parallel to the central axis.
In some embodiments, the plurality of airflow gaps comprises a plurality of radial air flow gaps and axial air flow gaps, the plurality of separators block less than 2% of an airflow cross-sectional area, and the airflow cross-sectional area is one of a radial air flow cross-sectional area through the plurality of radial air flow gaps, and an axial airflow cross-sectional area of an axial airflow path parallel to the central axis.
In some embodiments, for each separator in the plurality of separators, the plurality of tube engagements define a gap distance for separating each loop in the at least two loops engaged by the plurality of tube engagements from a closest other loop in the at least two loops.
In some embodiments, the gap distance varies along at least one of the axial dimension and the radial dimension.
In some embodiments, each tube engagement in the plurality of tube engagements comprises a coupler for detachably coupling the corresponding tube segment in the plurality of tube segments, such that the separator maintains a target separation distance between the plurality of tube segments.
In another broad aspect, at least one embodiment described herein provides an evaporator system comprising the evaporator as defined above; and an airflow subsystem for providing an airflow relative to the plurality of parallel tube portions, wherein the plurality of parallel tube portions comprises an upwind layer relative to the airflow for first receiving the airflow, and at least one downwind layer relative to the airflow for subsequently receiving the airflow; wherein the plurality of tube engagements of at least one separator in the plurality of separators includes an upwind row of tube engagements, and a downwind row of tube engagements configured such that the upwind row of tube engagements define an upwind layer gap between adjacent corresponding segments in the upwind layer, and the downwind row of tube engagements define a downwind layer gap between adjacent corresponding segments in the downwind layer, the upwind layer gap being larger than the downwind layer gap.
In some embodiments, the airflow flows at least along one of the radial dimension and the axial dimension.
In some embodiments, the plurality of separators block less than 10% of an airflow cross-sectional area through the plurality of radial air flow gaps and/or axial air flow gaps; and the airflow cross-sectional area is one of a radial air flow cross-sectional area through a plurality of radial air flow, and an axial airflow cross-sectional area of an axial airflow path parallel to the central axis.
In some embodiments, the plurality of separators block less than 2% of an airflow cross-sectional area through the plurality of radial air flow gaps and/or axial air flow gaps; and the airflow cross-sectional area is one of a radial air flow cross-sectional area through a plurality of radial air flow, and an axial airflow cross-sectional area of an axial airflow path parallel to the central axis.
In some embodiments, each separator in the plurality of separators is electrically insulating having a resistance greater than 100 Ω·m, to electrically insulate different segments of the refrigerant tube in contact with the separator, and wherein the separator is electrically and mechanically stable even at temperatures below −30° C.
In yet another broad aspect, at least one embodiment described herein provides an evaporator system comprising the evaporator as defined above; and an airflow subsystem for providing an airflow relative to the plurality of parallel tube portions, wherein the plurality of parallel tube portions comprises an upwind layer relative to the airflow for first receiving the airflow, and at least one downwind layer relative to the airflow for subsequently receiving the airflow; wherein the plurality of tube engagements of each separator in the plurality of separators includes an upwind row of tube engagements, and a downwind row of tube engagements configured such that the upwind row of tube engagements define an upwind layer gap between adjacent corresponding segments in the upwind layer, and the downwind row of tube engagements define a downwind layer gap between adjacent corresponding segments in the downwind layer, the upwind layer gap being larger than the downwind layer gap.
In some embodiments, the plurality of separators comprises at least three sets of separators, separated from one another by at least 80°.
In another broad aspect, at least one embodiment described herein provides a method of configuring an evaporator coil, the method comprising: providing a refrigerant tube formed from an electrically conductive material, the refrigerant tube being shaped to comprise a plurality of parallel tube portions, an upstream refrigerant conduit for supplying a refrigerant to the refrigerant tube, and a downstream refrigerant conduit for receiving the refrigerant from the refrigerant tube; providing at least one current delivery connector to the refrigerant tube for delivering an electrical current from an electrical current source and at least one current return connector for returning the electrical current to the electrical current source, the refrigerant tube providing at least one electrical flow path between the at least one current delivery connector and the at least one current return connector to generate heat to defrost the refrigerant tube during a defrost cycle; configuring an airflow subsystem for providing an airflow relative to the plurality of parallel tube portions; determining an air flow gap size between a parallel tube portion and a next closest parallel tube portion based at least on: a) the defrost cycle being less than two minutes; b) maintaining a heat exchange rate of the plurality of parallel tube portions and air pressure drop between the parallel tube portion and the next closest parallel tube portion that varies linearly between a frost layer thickness of 0 mm to 25% of the gap size; and configuring and providing a plurality of separators to secure the plurality of parallel tube portions in a plurality of relative positions to maintain the determined airflow gap size and to impede electrical contact between different tube portions in the plurality of parallel tube portions.
Other features and advantages of the present application will become apparent from the following detailed description taken together with the accompanying drawings. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the application, are given by way of illustration only, since various changes and modifications within the spirit and scope of the application will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described in detail with reference to the drawings, in which:

FIG. 1 is a diagram of an evaporator system in accordance with at least one example embodiment;

FIG. 2A is a graph showing energy transfer as a function of gap size in accordance with at least one example embodiment;

FIG. 2B is a graph showing energy transfer as a function of frost layer thickness in accordance with at least one example embodiment;

FIG. 3A is a graph showing the change in air pressure drop as a function of gap size in accordance with at least one example embodiment;

FIG. 3B is a graph showing the change in air pressure drop as a function of frost layer thickness in accordance with at least one example embodiment;

FIG. 4 is a graph showing defrost time as a function of gap size in accordance with at least one example embodiment;

FIG. 4A is a diagram showing a cross-sectional view of a helical evaporator with anti-radial and axial air flow in accordance with at least one example embodiment;

FIG. 5 is a diagram showing a cross-sectional view of a helical evaporator with axial air flow in accordance with at least one example embodiment;

FIG. 6 is a diagram showing a cross-sectional view of a helical evaporator with radial/anti-radial air flow in accordance with at least one example embodiment;

FIG. 7 is a diagram showing a cross-sectional view of a helical evaporator with varying gap spacing along the radial direction with mixed axial/anti-radial air flow in accordance with at least one example embodiment;

FIG. 8 is a diagram showing a cross-sectional view of a helical evaporator with varying gap spacing along the axial direction with axial air flow in accordance with at least one example embodiment;

FIG. 9 is a diagram showing a cross-sectional view of a helical evaporator with varying gap spacing along the radial direction with radial/anti-radial air flow in accordance with at least one example embodiment;

FIG. 10 is a diagram of a helical coil with a plurality of separators in accordance with at least one example embodiment; and

FIGS. 11 and 12 are diagrams showing various separator designs in accordance with at least one example embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Various embodiments in accordance with the teachings herein will be described below to provide an example of at least one embodiment of the claimed subject matter. No embodiment described herein limits any claimed subject matter. The claimed subject matter is not limited to devices or methods having all of the features of any one of the devices or methods described below or to features common to multiple or all of the devices and or methods described herein. It is possible that there may be a device or method described herein that is not an embodiment of any claimed subject matter. Any subject matter that is described herein that is not claimed in this document may be the subject matter of another protective instrument, for example, a continuing patent application, and the applicants, inventors or owners do not intend to abandon, disclaim or dedicate to the public any such subject matter by its disclosure in this document.
It will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. Also, the description is not to be considered as limiting the scope of the embodiments described herein.
It should also be noted that, as used herein, the wording “and/or” is intended to represent an inclusive-or. That is, “X and/or Y” is intended to mean X or Y or both, for example. As a further example, “X, Y, and/or Z” is intended to mean X or Y or Z or any combination thereof.
It should be noted that terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree may also be construed as including a deviation of the modified term if this deviation would not negate the meaning of the term it modifies.
Furthermore, the recitation of numerical ranges by endpoints herein includes all numbers and fractions subsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.90, 4, and 5). It is also to be understood that all numbers and fractions thereof are presumed to be modified by the term “about” which means a variation of up to a certain amount of the number to which reference is being made if the end result is not significantly changed, such as 10%, for example.
The efficient operation of a refrigeration system generally relies at least on the efficiency of the evaporator coil, which carries refrigerant liquid to capture excess heat in the environment being cooled in a heat exchange process. The evaporator coil is generally a finned tube heat exchanger so as to increase the surface area of the coil for improved heat transfer and to fit within a confined space (e.g. window mounted air conditioners have limited space for the evaporator coil). However, there is also a desire to maintain a separation or gap between tube portions of the coil to allow air flow through the coil, which is needed for convective heat transfer. As will be described in further detail below, the cooling efficiency of the evaporator coil may depend on the gap size of the coil. As such, there is a desire to maintain or control the gap size to improve the cooling performance of the refrigeration system.
FIG. 1 is a diagram of an embodiment of a self-defrosting evaporator system 100 using resistive heating similar to the system described in co-pending U.S. patent application 62/404,536, and Issued U.S. Pat. No. 8,424,324 which are hereby incorporated by reference. The evaporator comprises a refrigerant tube 101 to carry refrigerant liquid. In the present embodiment, the refrigerant tube 101 can be made from a conductive material and wound to form a number of parallel portions. In the present case, the refrigerant tube 101 may wound in a helical fashion. The axis around which the tubing can be referred to as the axial direction 120. The direction oriented transversely to the axial direction may be referred to as the radial direction 122. Relative to this frame of reference, the coil may be curled around an axial axis to define a plurality of loops. These loops may span a 360° rotation about a central axis in the axial direction 120 such that each point along the refrigerant tube lies in a corresponding loop and is axially positioned parallel to the central axis and radially positioned away from the central axis. It may be noted from FIG. 1 that the helical coils comprise coil layers in which portions of the tubing may be separated in the axial and radial directions. An example of the axial and radial coil layers is shown in FIG. 4A. Each tube portion may be separated by a gap in the axial and radial direction to permit air flow through the coil. It may also be noted that the refrigerant tube may be shaped in any other desirable manner to increase the surface area for effective refrigeration depending on various design factors such as space constraints. For example, in some embodiments, the refrigerant tube 101 may be wound in an elliptic, spiral, or oval shapes.
A fluid conduit may be used to direct refrigerant fluid between the refrigerant tube to/from a compressor component of the refrigeration system. It should be noted that the conduit used for providing the flow of refrigerant liquid may have a single or multiple directing outlets, depending on the desired flow control. In circumstances in which an electrical signal is applied to the refrigerant tube, electrical separation between the refrigerant tube and the rest of the evaporator system may be desirable. In the present embodiment, the conduit may be a dielectric union. Specifically, the first end of the refrigerant tube may be coupled to an inlet refrigerant tube 115 carrying refrigerant liquid from the compressor (not shown) via an inlet conduit or first dielectric union 114 which may be regarded as an upstream refrigerant conduit. The second end of the refrigerant tube may be coupled to an outlet refrigerant tube 117 carrying evaporated refrigerant to the compressor via an outlet conduit or a second dielectric union 116, which may be regarded as a downstream refrigerant conduit.
Electrical wires may be used to provide electrical connections for resistive or electromagnetic heating of the refrigerant coil. In the present embodiment a first wire segment 102 can be used to connect power provided to an alternating current (AC) or direct current (DC) supply connector 104 to the refrigerant tube 101 via a first electrical clamp 103 at the second end of the refrigerant tube. The first wire segment may be connected to a third wire segment 109 to a drip pan 108 via a fourth electrical clamp 110, in which the drip pan 108 may be connected, via a third electrical clamp 107, to the first end of the refrigerant tube through a second wire segment 106 and a second electrical clamp 105. In the present embodiment, the current may be delivered to the coil by the first electrical clamp 103, flow through the coil and be returned to the alternating current supply through the second electrical camp. Alternatively, the current may be delivered to the coil by the second electrical clamp 105, flow through the coil and be returned to the alternating current supply through the first electrical clamp 103. In some embodiments, more than two electrical clamps may be used to divide the refrigerant tube into parallel electrical portions. For example, two clamps may be used to configure the entire refrigerant tube as one electrical portion as shown in FIG. 1; whereas three clamps may be used (one at each end and one at the middle of the refrigerant tube) to divide the tube into two parallel electrical portions. For a two-portion coil, a 3-connector configuration can be used in which a current delivery clamp may be placed in the middle portion and two current return clamps can be coupled to the two ends of the coil. Alternatively, a current return clamp may be placed in the middle and two current delivery clamps may be located at the two ends of the coil. In different embodiments, to separate the coil into multiple electrical portions, adjacent current clamps should not be of the same type (i.e. adjacent clamps should not be both current delivery or both current return clamps). In these embodiments, current delivery clamps and current return clamps can be arranged in an alternating configuration. The 3-connector examples described above have this alternating configuration. Two parallel current flows, each flowing along one half of the length of the tube may be achieved using these 3-connector configurations to create two parallel electrical portions of the refrigerant tube. Electrically, the electrical length of the coil can thus be reduced by 2. For much longer tubes, n clamps may be used to divide the tube into n−1 parallel electrical portions as needed with the condition that adjacent clamps cannot be of the same type (i.e. adjacent clamps cannot be both current delivery or both current return). While electrical clamps are described for the present embodiment, any appropriate electrical connectors may be used to provide the alternating current to the refrigerant coil. For example, in some embodiments, plug-and-socket electrical connectors with complementary mating portions may be used in place of electrical clamps 103 and 105. In other embodiments, the electrical connectors may be terminal blocks or posts. In yet other embodiments, blade-type connectors may be used.
Along the first wire segment, an interlock switch 113 may be installed to allow manual disconnection of the electrical circuit (e.g. by unplugging the interlock) to allow access to the refrigerant coil upon opening the evaporator cover. A fuse link 112 comprising a temperature sensitive thermal fuse may also be installed such that the electrical circuit may be disconnected upon the resistive or electromagnetic heating exceeding a threshold temperature. A switch 111 may be installed between the first wire segment 102 and third wire segment 109 to control (i.e. initiate and terminate) the defrost cycle.
With respect to configuring or winding an evaporator coil such as refrigerant tube 101 of FIG. 1 for operation, one design consideration is the spacing or the gap between different radial layers and between the different axial layers. One reason for considering the gap is because it is generally known that the cooling efficiency of a wound evaporator coil such as the one shown in FIG. 1 may be sensitive to the size of the gap between tube portions. In particular, the gap size may directly affect the heat exchange between the air and the coil (i.e. the cooling capacity of the refrigerant tube) and the air pressure drop across the evaporator coil, which will be described in more detail subsequently.
Referring now to FIG. 2A, shown therein is a graph showing heat energy transfer Q in Watts (W) of a wound evaporator coil as a function of gap size in millimeters (mm) with no frost and with different thicknesses of frost buildup (0.5 mm, 1.0 mm, and 1.5 mm) for a given air flow and a selected fan. For example, in some embodiments, the air flow may flow axially. In other embodiments, the airflow may be radially oriented. In yet other embodiments, the airflow may flow axially and radially. The graph in FIG. 2 suggests that for the given air flow configuration and the selected fan, the cooling capacity may increase as the gap size increases from a lower value (e.g. roughly twice the frost layer thickness), and reach a maximum before gradually decreasing with increasing gap size. Given this relationship, there is an optimum gap size that may preferably be maintained in the evaporator coil to maximize heat transfer.
FIG. 2B is a graph showing heat energy transfer Q in Watts (W) as a function of frost layer thickness in millimeters (mm) for the same evaporator described in association with FIG. 2A with different gap sizes (2 mm, 4 mm, 6 mm, and 8 mm). FIG. 2A suggests that for a given gap size, the evaporator coil design may allow a relatively constant and linearly varying heat transfer performance for a range of frost thicknesses from 0 mm to about 25% of the gap size. In other words, for a given gap size, the frost buildup may not significantly affect the heat transfer performance up to a certain frost thickness that is roughly 25% of the gap size. It may be noted that this characteristic is different from conventional finned tube evaporator coils in which even small quantities of frost buildup may reduce the heat transfer performance. The ability to maintain a relatively constant energy transfer over a range of frost thicknesses may be useful in reducing the frequency of defrosts cycles. It may be noted that the optimal gap size can be selected based on maximizing heat transfer as well as an expected range of frost buildup in the system. For example, a gap of 6 mm and no frost may have Q=152.3 W. A frost layer thickness of 0.5 mm may increase Q to 157.0 W (+3.1%); a frost layer thickness of 1 mm may increase Q to 159.6 W (+4.8%); a frost layer thickness of 1.5 mm may increase Q to 152.9 W (+0.4%); and a frost layer thickness of 2 mm, however, may decrease Q to 132.3 W (−13.1%). Therefore, in this case a frost buildup of up to 1.5 mm may maintain the designed frost-free heat transfer performance of the evaporator coil.
FIG. 3A is a graph showing the change in air pressure drop in Pascals (Pa) as a function of gap size in mm for the same evaporator described in association with FIG. 2A, with no frost and with different thicknesses of frost buildup (0.5 mm, 1.0 mm, and 1.5 mm). The graph shows the difference in air pressure measured at one tube portion and a neighboring tube portion. FIG. 3B is a graph showing the change in air pressure drop in Pascals (Pa) as a function of frost layer thickness in millimeters (mm) for the same evaporator with different gap sizes (2 mm, 4 mm, 6 mm, and 8 mm). The waveforms suggest that the gap size may also affect the pressure drop across the evaporator coil. Specifically, the figure shows that, for a given air flow configuration and selected fan, the air pressure drop may decrease with increasing gap size.
FIG. 4 is a graph showing required defrost time (s) as a function of gap size in millimeters (mm) for the same evaporator described in association with FIG. 2A and with a frost layer thickness that is 25% of the gap size (as discussed above) with different power densities (W/m) per total tube length of the evaporator coil. The power density per tube length can be calculated as R*I²where R is the electrical resistance of the coil per unit length (Ω/m) and I is the maximum allowable current (A) defined by a circuit breaker to which the refrigerator is connected. The graph suggests that for a given power density, as the gap size (as well as the allowable frost thickness) increases, the time required for defrost also increases. In practice, it is generally preferable to keep the defrost time below 5 minutes, and more preferably under 2 minutes, to avoid any unnecessary heat loss to the air surrounding the evaporator coil. This requirement may impose a limit on the maximum gap that can be used as shown in the figure. Such limit will also depend on the power density per unit length used for defrost as shown. Therefore, may be important that the selected gap size should not only satisfy the heat transfer and pressure drop requirements under both frost and frost-free conditions but also should satisfy the condition that the defrost time is under 5 minutes, or preferably under 2 minutes, under frost conditions.
It may also be appreciated that in the context of self-defrosting evaporator coils based on resistive or electromagnetic heating, maintaining a gap between coil portions in the axial and radial layers also prevents electrical shorting due to different portions of the coil touching. Electrically shorting coil portions, especially between radial layers, may cause a large decrease in the total resistance of the evaporator coil. Not only does the decrease in resistance lower the heating efficiency of the coil, there may also be a sudden increase in the current drawn from the power source.
Furthermore, evaporator coils generally operate at refrigerant temperatures below the frost point of water. Thus, frost may develop on the surface of the evaporator coil during operation as moisture in the circulating air comes in contact with the coil surface. As frost builds up on the coil surface, the gap between coil portions decrease. As a result, the performance of the evaporator may deteriorate after significant frost buildup (e.g. frost thickness greater than 25% of the gap size), as shown in FIG. 2B (i.e. reduced heat transfer), and the flow of air through the evaporator coil may become restricted, as shown in FIG. 3B (i.e. greater air pressure difference and the variation is no longer varies linearly with frost layer thickness). In extreme cases of frost build up, air flow may even be halted. As such, careful consideration of gap size as an evaporator coil design parameter under both frost and frost-free conditions may be warranted.
To address the disadvantages of frost buildup, the use of energy-efficient direct defrosting such as the system described in FIG. 1 to facilitate removal of frost build up. By using a self-defrosting evaporator coils such as the multi-layered helical evaporator coil shown in FIG. 1, the gap size can be selected to improve heat transfer (i.e. reducing or enlarging the gap so that it reaches a peak energy transfer as shown in FIG. 2).
Referring now to FIGS. 4A-6, shown therein are diagrams of various air flow patterns flowing through an evaporator coil such as the one described in the system of FIG. 1. The air flow may be provided by an airflow subsystem to circulate air through the evaporator coil and through the parallel tube portions. FIG. 4A shows a cross-sectional view of a helical evaporator coil such as the one shown in FIG. 1 with constant radial gaps 402 and constant axial gaps 404. In embodiments such as that of FIG. 4A, the airflow subsystem may be configured to provide a mixed flow comprising radial air flow 406 and axial air flow 408 using appropriate brackets (not shown) to direct the air flow.
In some embodiments, the airflow subsystem may be configured to provide airflow flowing along one direction as described in FIGS. 5 and 6. FIG. 5 shows a cross-sectional view of a helical evaporator such as the one shown in FIG. 1 with constant radial gaps 502 and constant axial gaps 504. In the embodiment of FIG. 5, the airflow subsystem may be configured to provide purely axial air flow 508 using appropriate brackets (not shown) to direct the air flow. Specifically, the direction of airflow indicated by the arrows in FIG. 5 shows that the airflow may enter the coil at one axial end, flow through the coil along the axial direction, and exit the coil at another axial end. It may be noted that the airflow may flow past several layers or rows of tube portions along the axial direction. For example, the embodiment of FIG. 5 shows five rows of tube portions in the axial direction. The first row 510 may receive the airflow first, then followed by the second row 512 and so on. As such, the first row 510 of tube portions may be considered an upwind row. The second row 512, which receives airflow subsequently, may be regarded as a downwind row. It may be observed that any pair of rows in the axial direction may be defined as an upwind row and downwind row relative to each other.
FIG. 6 shows a cross-sectional view of a helical evaporator such as the one shown in FIG. 1 with constant radial gaps 602 and constant axial gaps 604. In this embodiment, the airflow subsystem may be configured to provide a purely radial air flow 606 entering the coil at one radial end or side and exiting at another radial end or side using appropriate brackets (not shown) to direct the air flow. The embodiment of FIG. 6 shows four rows of tube portions in the radial direction for each half of the coil being considered. The same considerations regarding upwind and downwind rows may be applicable to the embodiment of FIG. 6 for any pair of rows (regardless of which half the row is from) in the radial direction. For example for the left half of the cross section, the first row 610 may be regarded as an upwind row relative to fourth row 616 which may be regarded as a downwind row (in one sense, the first row 610 is the upwind row, as it is upwind to all of the other rows in this configuration). Similar determinations of upwind and downwind rows may be considered in a mixed airflow environment such as the one shown in FIG. 4A for each of the axial and radial airflow (e.g. up/downwind axial rows and up/downwind radial rows).
In some embodiments, it may be preferable to maintain a certain gap size to achieve the desired cooling capacity (i.e. heat transfer rate) and a reasonable air pressure drop. In the present disclosure, “separators” 1002 as shown in FIG. 10 may be attached to the evaporator coil to secure and maintain the relative positions of tube portions of the coil to define the desired gap size that separates one loop from another closest loop. The plurality of gaps maintained by the separators may be used to permit air flow. For a self-defrosting evaporator coil based on resistive or electromagnetic heating, the separators may be fabricated using electrically insulating or weakly conductive material with an electrical resistivity of at least 100 am to substantially avoid electrical contact between loops via the separators so that no more than 0.1% of the current is allowed to flow through the separators. In another embodiment, the separators 1002 can be made from a thermally conductive dielectric material, where they may improve the heat transfer performance by acting as extended surfaces or evaporator fins.
In addition to allowing the evaporator coils to be separated at a desired distance, the use of separators 1002 may be relevant in at least two other respects. First, the separators 1002 may provide structural support for the evaporator coil to help the evaporator coil maintain a constant gap size, at the desired value, to help each pair of tube portions to obtain 1) a satisfactory air flow for efficient cooling (i.e. heat exchange) and 2) a desired pressure drop. Second, the separators 1002 may provide protection again electrical shorts by holding adjacent coil axial/radial coil layers apart to impede them from coming into contact with each other.
Under circumstances in which frost accumulates on a helical evaporator coil, the formation may tend to accumulate more on the upwind layers (e.g. outer radial layers in the case of mixed radial/axial air flow) since those layers correspond to the first surface to come in contact with moisture in the air. Also, frost may generally tend to accumulate in areas where there is low air flow velocity.
To take these factors into account, in some embodiments, the overall convective heat exchange can be improved by winding a multi-layer coil with spatially varying gap sizes between different radial or/and axial layers (depending on the direction of air flow). The gap sizes or pitch along both axial and radial directions can be made variable as shown in FIGS. 7-9.
Designing a coil with spatially variable pitch may be advantageous in at least two more aspects: In a first aspect, in the case of mixed radial/axial air flow, the axial coil pitch of the coil layers can be varied so that the average air velocity and/or the convective heat transfer coefficient may the same for all coil layers. In another aspect, the axial coil pitch of the coil layers can be designed so that the air temperature drop may be the same for all coil layers (typically, in a bundles of coils, the heat exchange rate may decrease downwind due to decreasing temperature difference between air and tubes). By varying the gap width, the heat exchange rate can be made to be the same for all coil layers, thus reducing the total coil length required to meet the cooling power requirement. In general, considering both axial and radial tube pitches as design parameters may allow for the design of an evaporator coil with higher thermal performance, higher frost accumulating capacity, and lower air and refrigerant pressure drops.
Referring now to FIGS. 7-9 shown therein are diagrams of helical coil configurations with spatially varying pitch in accordance with at least one embodiment. In some embodiments, the pitch may vary along the axial direction. In other embodiments, the pitch may vary along the radial direction. In yet other embodiments, the pitch may vary in both the axial and radial directions.
FIG. 7 shows a helical coil configuration that relies on separators to produce spatially varying gap spacing or pitch along the radial direction in the presence of axially and anti-radially flowing air. Specifically, in the helical coil of FIG. 7, it may be noted that larger gaps are present near the outside or upwind radial layers of the helical coil and smaller gaps near the inside or downwind radial layers of the helical coil. This configuration may allow the outside or upwind radial layers to collect more frost, since the moisture from the cooling air may condense and freeze on the coil as a deposit of frost on these layers first. In the present embodiment, more frost can be accumulated, given the larger pitch near the outside or upwind layers, before the coil performance starts to degrade so that fewer defrost cycles may be needed. The variability of gaps will depend on the direction of the air flow.
FIG. 8 shows a sample cross-section of a helical evaporator coil with spatially variable pitch along the axial direction with a pure axial air flow. FIG. 9 shows a sample cross-section of a helical evaporator coil with spatially variable pitch along the radial direction with a pure radial air flow.
As described previously, FIG. 10 shows a helical coil with a plurality of separators engaged with the coil portions in accordance to at least one embodiment. Also shown in FIG. 10, each separator may have a corresponding radial and axial dimension and a substantially orthogonal thickness dimension. Each separator can engage or intersect with at least two loops at their corresponding tube segments in which the tube and separator come in contact. As shown in FIG. 10 and in some embodiments, for each separator, each corresponding tube segment engaged by that separator may be separated by at least one loop from every other corresponding tube segment. Shown in FIG. 10 are the sets or groups separators 1002 being distributed around the coil to provide uniform support across the coil. The separators may be designed in separate pieces as shown in FIGS. 11 and 12 that can be installed onto the helical coil. The separators may be made from an electrically insulating material to prevent short-circuiting as discussed previously. The separator material may also be selected to withstand low temperatures such as −30° C. or below without significant change in mechanical and electrical properties.
As shown in FIG. 11(A) the separator piece 1102 may be shaped to fit within the spacing of the coil. Each separator piece 1102 may have a number of tube engagements 1106 to engage the evaporator coil. In the present embodiment, each tube engagement may be a circular groove. Each separator piece 1002 may engage the coupler 1104 associated with the engagements to detachably couple to corresponding tube segments of the coil. In the present embodiment, the separators may be coupled to the evaporator coil by clipping onto a group of tubes as shown in FIG. 10 to define a row of tube portions. The spacing between the engagements may be chosen to correspond to the desired gap spacing between adjacent to portions in the row of tube portions. Each separator piece can thus be used to maintain the gap between coils of the same row at a desired value. Depending on the direction of airflow applied to the coil, each row may be regarded as an upwind or downwind row as described previously. Furthermore, the air gaps between adjacent tube segments in each of the rows of tubing may correspondingly be regarded as an upwind layer gap or downwind layer gap. In other embodiments, the couplers may be other shapes such as squares or rectangles or triangles. In some cases, when the gap sizes vary spatially, the upwind layer gap may be made larger than the downwind layer gap as shown in FIGS. 7-9.
In the present embodiment, four sets of separators 1002 are used. However, in some embodiments fewer separators 1002 may be used. In yet other embodiments, more separators 1002 may be used. Generally, however, it may be preferable to use at least three sets of separators 1002 to support the evaporator coil and the separators 1002 may be spread out so that the angle between each set of separators 1002 is at least 80° to provide rigidity of the coil from all sides. However, with more separators, the flow of air along the radial and axial directions through the radial and axial air gaps may be impeded. Impedance of airflow through the evaporator coil may lower the heat transfer efficiency unless the separators are good heat conductors. Generally, the number and positioning of the separators should be selected partly based on avoiding unnecessary or excessive impedance of airflow: the positioning of separators 1002 should not block more than about 10% and preferably should not block more than about 2% of an airflow cross-sectional area determined by the air flow through tube gaps in the axial direction (i.e. flow of air through an axial cross-section area), radial direction (i.e. flow of air through a radial cross-section area), or axial and radial directions, so that the thermal performance and the heat exchange rate may be maintained by the position of the separators. Additionally, the angle of each separator face may be oriented so that they are not horizontal relative to the ground so that water from melted frost or ice would flow down without accumulating on the separators.
FIGS. 11 and 12 show different examples of separator designs according to at least some embodiments. The shape or design of the separators may be dictated by the gap style (e.g. constant or variable sizing) and the direction of air flow (e.g. mixed radial/axial, pure axial or pure radial). Additionally, the thickness of the separator can be made substantially smaller than its radial or axial dimension. In some embodiments, the thickness of the separators may range from 1 mm to 20 mm. In other embodiments, the thickness of the separators may range from 3 mm to 10 mm. FIG. 11(A) shows one design in which the separators define substantially constant gaps (i.e. axial gaps are all the same and radial gaps are all the same, but the axial and radial gaps may not necessarily be the same). These separators may be used in mixed radial/axial air flow as shown in FIG. 4A, pure axial air flow as shown in FIG. 5, or pure radial air flow as shown in FIG. 6. FIG. 11(B) shows another design of separators that can be used to obtain variable gaps sizes. These separators may be installed to obtain gap variations directions that can be used with mixed radial/axial air flow as shown in FIG. 7 or pure radial air flow as shown in FIG. 9. FIG. 11(C) shows yet another possible design that can be used to obtain variable gaps for pure axial air flow as shown in FIG. 8.
For helically wound evaporator coils, minor modifications in the separators may be necessary to accommodate for minor changes or variations in tube locations as a result of the turning angle of the helical coil. FIG. 12 shows example embodiments of separators which may be made to account for these minor changes. For example, for a helically wound evaporator coil with four separator sets of separators as shown in FIG. 10, separator sets located on one axis can be fabricated to fit tube portions that are in an in-line position (e.g. the top two sets of separators shown in FIG. 12). Separator sets located on an orthogonal axis can be fabricated to fit tube portions that are in a staggered position (e.g. the bottom two sets in FIG. 12). For example the coil of FIG. 10 may be configured in a way such that both in-line and staggered separators may be needed. Specifically, separator sets 1006 and 1008 may be inline while separator sets 1002 and 1004 are staggered.
While the gap size and separators discussed in the preceding sections were described with respect to helical coil evaporators, the same concepts and design consideration can be applied to other styles of coil evaporators including but not limited to elliptic, spiral or oval coil evaporators. Helical and spiral coils may be more suitable to fit in square areas, while elliptic and oval coils may be more suitable to fit in rectangular areas. During manufacture, helical and elliptic coils may be made from a single tube (or two tubes in the case of a double wound coil) wound axially and radially. Spiral and oval coils may be made from parallel tubes wound radially only. Elliptic, spiral or oval coils may have elements that are in common with helical coils such that design considerations applicable to helical coils may similarly be applicable to these other coil styles.

Air Pressure Drop Calculations

Air pressure calculations for air flow around the tube portions of the evaporator coil are presented herein. For the case of mixed radial/axial air flow, the air pressure drop can be calculated by:
$\begin{matrix} Δ P_{a} = \sum_{i = 1}^{N_{rd}} \frac{1}{2} ρ_{a} {f (\frac{V_{a}}{A_{i, op}})}^{2} & (1) \end{matrix}$
where N_rdis the number of radial layers of the coil, ρ_ais the air density, V_ais the air volume flow rate, A_i,opis the total area of the i^thradial layer open for air flow in the coil, and f is the friction coefficient. The friction coefficient can be calculated as follows:
$\begin{matrix} f = f ({Re}_{a}) = f (\frac{ρ_{a} d_{out} V_{a}}{μ_{a} A_{i, op}}) & (2) \end{matrix}$
where Re_ais the Reynolds number for air flow, μ_ais the dynamic viscosity of air, and d_outis the outside diameter of the tube used to make the coil. The relationship between f and Re_amay depend on the tube configuration, for example staggered or in-line. Since the coil can be made from a single tube that is wound to make up the axial and radial layers, the friction coefficient f can be calculated as an average of both configurations. The air pressure drop, ΔP_a, can be calculated for a given air volume flow rate, V_a, using equation 1. For a given fan with a known fan performance curve ΔP_fan=f(V_a), an optimization procedure can be applied to determine the operating conditions, V_aand ΔP_a=ΔP_fan, of the coil when used with the specified fan.
For the case of pure radial air flow, equations 1 and 2 can still be used for one half of the coil as a first step and then solved again for the other half of the coil as a second step using the results of the first step as a boundary condition.
For the case of pure axial air flow, the air pressure drop can be calculated by:
$\begin{matrix} Δ P_{a, ax} = \frac{1}{2} ρ_{a} {{fN}_{ax} (\frac{V_{a}}{A_{ax, op}})}^{2} & (3) \end{matrix}$
where N_axis the number of axial layers of the coil, A_ax,opis the total area of an axial layer open for air flow in the coil, and f is the friction coefficient which can be calculated as follows:
$\begin{matrix} f = f ({Re}_{a, ax}) = f (\frac{ρ_{a} d_{out} V_{a}}{μ_{a} A_{ax, op}}) & (4) \end{matrix}$
The relationship between f and Re_a,axcan depend on the tube configuration, for example staggered or in-line. For the pure axial air flow case, the friction coefficient can be calculated based on the in-line configuration as shown in FIG. 5.

Air Heat Transfer Calculations

For the case of mixed radial/axial air flow, the total heat transfer Q_abetween air and the evaporator coil can be calculated by:
Q _a=ρ_a c _a V _a ΔT _a (5)
where c_ais the air specific heat capacity and ΔT_ais the air temperature drop from inlet to outlet and can be calculated by:
$\begin{matrix} Δ T_{a} = TD (1 - \prod_{i = 1}^{N_{rd}} \exp (\frac{- A_{i} h_{i}}{ρ_{a} c_{a} V_{a}})) & (6) \end{matrix}$
where TD is the temperature difference between the evaporation temperature and the air inlet temperature, A_iis the surface area of tubes in the i^thradial layer of the coil, and h_iis the local heat transfer coefficient of the i^thradial layer of the coil. The local heat transfer coefficient can be calculated as follows:
$\begin{matrix} h_{i} = \frac{k_{a} {Nu}_{a}}{d_{out}} f_{cor} & (7) \end{matrix}$
where k_ais the thermal conductivity of air, f_coris a correction factor used to compensate for the small thermal resistance between the tube and boiling refrigerant that can usually be calculated from refrigerant properties, refrigerant flow rate, and tube inner diameter, and Nu_ais the Nusselt number and can be calculated as follows:
$\begin{matrix} {Nu}_{a} = {\begin{matrix} f_{c} [\frac{1}{2} \times 0.35 {Re}_{a}^{0.6} + \frac{1}{2} \times 0.27 {Re}_{a}^{0.63}] {\Pr_{a}^{0.36} (\frac{\Pr_{a}}{\Pr_{a, s}})}^{1 / 4} & {Re}_{a} \geq 1000 \\ 1.1882 f_{c} [0.3 + \frac{0.62 {Re}_{a}^{1 / 2} \Pr_{a}^{1 / 3}}{{(1 + {(\frac{0.4}{\Pr_{a}})}^{2 / 3})}^{1 / 4}} {(1 + {(\frac{{Re}_{a}}{282000})}^{5 / 8})}^{4 / 5}] & {Re}_{a} < 1000 \end{matrix} & (8) \end{matrix}$
where Pr_ais the Prandtl number of air, Pr_a,sis the Prandtl number of air calculated at the tube surface temperature, and f_cis a correction factor for Nu_awhen the number of tube layers N_rd<20. The correction factor f_cmay also vary depending on the number of tube layers, N_rd. Additionally, the relationship between fc and N_rdmay depend on the tube configuration, for example staggered or in-line. Since the coil can be made from a single tube that is wound to form the axial and radial layers, f_ccan be calculated as an average of both configurations. A constant factor of 1.1882 can be added to remove a discontinuity in Nu_awhen the calculation of Nu_aswitches from a first case (Re_a≥1000) to a second case (Re_a<1000). An average convective air heat transfer coefficient can be calculated as:
$\begin{matrix} h_{a} = \sum_{i = 1}^{N_{rd}} \frac{π D_{i} N_{ax}}{L} h_{i} & (9) \end{matrix}$
where D_iis the average diameter of the i^thradial layer of the coil, N_axis the number of axial layers of the coil, and L is the total tube length of the coil.
For the case of pure radial air flow, equations 5 to 8 can be used for one half of the coil as a first step and then solved again for the other half of the coil as a second step using the results of the first step as a boundary condition.
For pure axial air flow, the total air heat transfer can be calculated by:
Q _a,ax=ρ_a c _a V _a ΔT _a,ax (10)
where ΔT_a,axis the air temperature drop from inlet to outlet and can be calculated by:
$\begin{matrix} Δ T_{a, ax} = TD (1 - {[\exp (\frac{- A_{ax} h_{a, ax}}{ρ_{a} c_{a} V_{a}})]}^{N_{ax}}) & (11) \end{matrix}$
where A_axis the surface area of tubes in an axial layer of the coil and h_a,axis an average convective air heat transfer coefficient that can be calculated as:
$\begin{matrix} h_{a, ax} = \frac{k_{a} {Nu}_{a, ax}}{d_{out}} f_{cor} & (12) \end{matrix}$
where Nu_a,axcan be calculated as follows:
$\begin{matrix} {Nu}_{a, ax} = {\begin{matrix} f_{c} [0.27 {Re}_{a, ax}^{0.63} {\Pr_{a}^{0.36} (\frac{\Pr_{a}}{\Pr_{a, s}})}^{1 / 4}] & {Re}_{a, ax} \geq 1000 \\ 1.1572 f_{c} [\begin{matrix} 0.3 + \\ \frac{0.62 {Re}_{a, ax}^{1 / 2} \Pr_{a}^{1 / 3}}{{(1 + {(\frac{0.4}{\Pr_{a}})}^{2 / 3})}^{1 / 4}} {(1 + {(\frac{{Re}_{a, ax}}{282000})}^{5 / 8})}^{4 / 5} \end{matrix}] & {Re}_{a, ax} < 1000 \end{matrix} & (13) \end{matrix}$
where f_cis a correction factor for Nu_a,axwhen the number of tube layers N_ax<20. The correction factor f_cmay also vary depending on N_ax. Additionally, the relationship between fc and N_axmay depend on the tube configuration, for example staggered or in-line. For the pure axial air flow case, f_ccan be calculated based on the in-line configuration. A constant factor of 1.1572 can be added to remove a discontinuity in Nu_a,axwhen the calculation of N_ua,axswitches from a first case (Re_a,ax≥1000) to a second case (Re_a,ax<1000).
The present invention has been described here by way of example only. Various modification and variations may be made to these exemplary embodiments without departing from the spirit and scope of the invention.

Claims

1. An evaporator comprising

a refrigerant tube formed from an electrically conductive material, the refrigerant tube being shaped to comprise a plurality of parallel tube portions;

an upstream refrigerant conduit for supplying a refrigerant to the refrigerant tube;

a downstream refrigerant conduit for receiving the refrigerant from the refrigerant tube;

at least one current delivery connector for delivering an electrical current from an electrical current source and at least one current return connector for returning the electrical current to the electrical current source, wherein

the at least one current delivery connector and the at least one current return connector are coupled to the refrigerant tube to provide at least one electrical flow path between the at least one current delivery connector and the at least one current return connector;

and

a plurality of separators for securing the plurality of parallel tube portions in a plurality of relative positions to provide a plurality of airflow gaps separating adjacent parallel tube portions, and to impede electrical contact between different tube portions in the plurality of parallel tube portions.

2. The evaporator as defined in claim 1, wherein the refrigerant tube is a helically arranged refrigerant tube formed from an electrically conductive material, the refrigerant tube being curled around an axial airflow path axis to define a plurality of loops, each loop in the plurality of loops spanning a 360° rotation about a central axis, such that each point along a length of the refrigerant tube lies in a corresponding loop, and is axially positioned parallel to the central axis and radially positioned away from the central axis.

3. The evaporator as defined in claim 2, wherein each separator in the plurality of separators is electrically insulating having a resistance greater than 100 Ω·m, to electrically insulate different segments of the refrigerant tube in contact with the separator, and wherein the separator is electrically and mechanically stable even at temperatures below −30° C.

4. The evaporator as defined in claim 3, wherein each separator in the plurality of separators,

has a corresponding radial dimension and an axial dimension;

intersects with at least two loops in the plurality of loops at a plurality of corresponding tube segments; and

comprises a plurality of tube engagements for engaging the refrigerant tube at, for each loop in the at least two loops, a corresponding tube segment;

wherein each corresponding tube segment is separated by at least one loop from every other corresponding tube segment in the plurality of tube segments.

5. The evaporator as defined in claim 4, wherein each separator in the plurality of separators comprises a thickness dimension substantially orthogonal to and much smaller than the radial dimension and the axial dimension, such that that separator has a substantially planar configuration.

6. The evaporator as defined in claim 5, wherein for each separator in the plurality of separators, the thickness dimension is between 1 mm and 20 mm.

7. The evaporator as defined in claim 5, wherein for each separator, the thickness dimension is between 3 mm and 10 mm.

8. The evaporator as defined in claim 5, wherein the plurality of airflow gaps comprises a plurality of radial air flow gaps, the plurality of separators block less than 10% of an airflow cross-sectional area, and the airflow cross-sectional area is one of a radial air flow cross-sectional area through the plurality of radial air flow gaps, and an axial airflow cross-sectional area of an axial airflow path parallel to the central axis.

9. The evaporator as defined in claim 5, wherein the plurality of airflow gaps comprises a plurality of radial air flow gaps, the plurality of separators block less than 2% of an airflow cross-sectional area, and the airflow cross-sectional area is one of a radial air flow cross-sectional area through the plurality of radial air flow gaps, and an axial airflow cross-sectional area of an axial airflow path parallel to the central axis.

10. The evaporator as defined in claim 4, wherein for each separator in the plurality of separators, the plurality of tube engagements define a gap distance for separating each loop in the at least two loops engaged by the plurality of tube engagements from a closest other loop in the at least two loops.

11. The evaporator as defined in claim 10, wherein the gap distance varies along at least one of the axial dimension and the radial dimension.

12. The evaporator as defined in claim 4, wherein each tube engagement in the plurality of tube engagements comprises a coupler for detachably coupling the corresponding tube segment in the plurality of tube segments, such that the separator maintains a target separation distance between the plurality of tube segments.

13. A system comprising:

the evaporator as defined in claim 4; and

an air flow subsystem for providing an airflow relative to the plurality of parallel tube portions, wherein the plurality of parallel tube portions comprises an upwind layer relative to the airflow for first receiving the airflow, and at least one downwind layer relative to the airflow for subsequently receiving the airflow;

wherein the plurality of tube engagements of at least one separator in the plurality of separators includes an upwind row of tube engagements, and a downwind row of tube engagements configured such that the upwind row of tube engagements define an upwind layer gap between adjacent corresponding segments in the upwind layer, and the downwind row of tube engagements define a downwind layer gap between adjacent corresponding segments in the downwind layer, the upwind layer gap being larger than the downwind layer gap.

14. The system of claim 13, wherein the airflow flows at least along one of the radial dimension and the axial dimension.

15. The system of claim 13, wherein

the plurality of separators block less than 10% of an airflow cross-sectional area through the plurality of radial airflow gaps; and

the airflow cross-sectional area is one of a radial air flow cross-sectional area through a plurality of radial air flow, and an axial airflow cross-sectional area of an axial airflow path parallel to the central axis.

16. The system of claim 13, wherein

the plurality of separators block less than 2% of an airflow cross-sectional area through the plurality of radial airflow gaps; and

17. The system of claim 13, wherein each separator in the plurality of separators is electrically insulating having a resistance greater than 100 Ω·m, to electrically insulate different segments of the refrigerant tube in contact with the separator, and wherein the separator is electrically and mechanically stable even at temperatures below −30° C.

18. A system comprising:

the evaporator as defined in claim 4; and

an airflow subsystem for providing an airflow relative to the plurality of parallel tube portions, wherein the plurality of parallel tube portions comprises an upwind layer relative to the airflow for first receiving the airflow, and at least one downwind layer relative to the airflow for subsequently receiving the airflow;

wherein the plurality of tube engagements of each separator in the plurality of separators includes an upwind row of tube engagements, and a downwind row of tube engagements configured such that the upwind row of tube engagements define an upwind layer gap between adjacent corresponding segments in the upwind layer, and the downwind row of tube engagements define a downwind layer gap between adjacent corresponding segments in the downwind layer, the upwind layer gap being larger than the downwind layer gap.

19. The system of claim 18, wherein the plurality of separators comprises at least three sets of separators, separated from one another by at least 80°.

20. A method of configuring an evaporator coil, the method comprising:

providing a refrigerant tube formed from an electrically conductive material, the refrigerant tube being shaped to comprise a plurality of parallel tube portions, an upstream refrigerant conduit for supplying a refrigerant to the refrigerant tube, and a downstream refrigerant conduit for receiving the refrigerant from the refrigerant tube;

providing at least one current delivery connector to the refrigerant tube for delivering an electrical current from an electrical current source and at least one current return connector for returning the electrical current to the electrical current source, the refrigerant tube providing at least one electrical flow path between the at least one current delivery connector and the at least one current return connector to generate heat to defrost the refrigerant tube during a defrost cycle;

configuring an airflow subsystem for providing an airflow relative to the plurality of parallel tube portions;

determining an air flow gap size between a parallel tube portion and a next closest parallel tube portion based at least on:

a) the defrost cycle being less than two minutes;

b) maintaining a heat exchange rate of the plurality of parallel tube portions and air pressure drop between the parallel tube portion and the next closest parallel tube portion that varies linearly between a frost layer thickness of 0 mm to 25% of the gap size; and

configuring and providing a plurality of separators to secure the plurality of parallel tube portions in a plurality of relative positions to maintain the determined airflow gap size and to impede electrical contact between different tube portions in the plurality of parallel tube portions.