The subject matter disclosed herein relates to heat exchangers and their operation, and more particularly to heat exchangers that are subject to condensate formation on heat transfer surfaces.
Heat exchangers are widely used in various applications, including but not limited to heating and cooling systems including fan coil units, heating and cooling in various industrial and chemical processes, heat recovery systems, and the like, to name a few. Many heat exchangers for transferring heat from one fluid to another fluid utilize one or more tubes through which one fluid flows while a second fluid flows around the tubes. Heat from one of the fluids is transferred to the other fluid by conduction through the tube walls. Many configurations also utilize fins in thermally conductive contact with the outside of the tube(s) to provide increased surface area across which heat can be transferred between the fluids, improve heat transfer characteristics of the second fluid flowing through the heat exchanger, and enhance structural rigidity of the heat exchanger.
One of the primary functions of a heat exchanger is to transfer heat from one fluid to another in an efficient manner. Higher levels of heat transfer efficiency allow for reductions in heat exchanger size, which can provide for reduced material and manufacturing cost, as well as providing enhancements to efficiency and design of systems that utilize heat exchangers such as refrigeration systems. However, there are a number of impediments to improving heat exchanger system efficiency. One such impediment is the formation of condensate on heat transfer surfaces. When condensate forms, it can adversely impact the efficiency heat transfer between a flowing gas and the heat transfer surfaces on which the condensate has formed. In some applications such as refrigeration, the condensate can freeze, which can further adversely impact efficiency. In salty environments such as maritime environments, the presence of condensate can also provide liquid water to form an electrolyte that can lead to galvanic corrosion of heat exchanger components
According to some embodiments of this disclosure, a method of operating a heat exchanger comprises rejecting heat from a gas comprising water vapor on a heat rejection side fluid flow path to a heat absorption side of the heat exchanger. Liquid droplets of condensed water are formed at a first surface energy level on a hydrophobic surface of the heat exchanger on the heat rejection side fluid flow path that is in thermal communication with the heat absorption side of the heat exchanger. An electric field is applied to the hydrophobic surface to reduce a contact angle between the individual droplet surfaces and the hydrophobic surface, and to increase droplet surface energy to a second surface energy level. The electric field is removed to increase the contact angle between the individual droplet surfaces and the hydrophobic surface, and to reduce droplet surface energy to a third surface energy level. The third surface energy level is greater than the first surface energy level and greater than a surface energy level for a free droplet. A portion of the droplet surface energy is converted to kinetic energy to detach droplets from the hydrophobic surface. The detached droplets are removed from the heat rejection side fluid flow path.
In some embodiments of the above method, fluid flow on the heat rejection side fluid flow path is maintained at a steady state flow velocity that entrains detached droplets.
In some embodiments of the above method, fluid flow on the heat rejection side fluid flow path is pulsed in timed coordination with removal of the electric field to provide a pulse flow velocity that entrains detached droplets.
In any one or combination of the foregoing embodiments, further comprising capturing contaminants from the gas into the droplets.
In any one or combination of the foregoing embodiments, the method further comprises applying an electric field to impart an electrostatic charge to the contaminants.
In any one or combination of the foregoing embodiments, the electric field is applied in response to detection of condensed water on the hydrophobic surface.
In any one or combination of the foregoing embodiments, the electric field is applied in response to a pressure differential between a heat rejection side fluid flow path inlet and outlet.
In any one or combination of the foregoing embodiments, the electric field is applied in response to a differential between a temperature of the hydrophobic surface and an ambient dew point temperature higher than the hydrophobic surface temperature.
In any one or combination of the foregoing embodiments, the electric field is pulsed in a cycle pattern comprising alternating on and off periods wherein the duration of the off period is equal to or longer than the duration of the on period.
In some embodiments, a heat exchanger system comprises a heat exchanger comprising a heat rejection side fluid flow path and a hydrophobic surface in thermal communication with a heat absorption side of the heat exchanger and in fluid communication with the heat rejection side flow path. The system also includes a power source and a controller configured to apply an electrical field to the hydrophobic surface to reduce a contact angle between condensate droplet surfaces and the hydrophobic surface and increase droplet surface energy to a second level greater than a first surface energy level for condensate droplets on the hydrophobic surface in the absence of an electric field. The controller and power source are further configured to remove the electric field to increase the contact angle between the individual droplet surfaces and the hydrophobic surface, and reduce droplet surface energy to a third surface energy level greater than the first surface energy level and greater than a surface energy level for a free droplet, and convert a portion of the droplet surface energy to kinetic energy to detach droplets from the hydrophobic surface.
In some embodiments, the controller of the above heat exchanger system is further configured to maintain fluid flow on the heat rejection side at a steady state flow velocity that entrains detached droplets.
In some embodiments, the controller of the above heat exchanger system is further configured to pulse fluid flow on the heat rejection side fluid flow in timed coordination with removal of the electric field to provide a pulse flow velocity that entrains detached droplets.
In any one or combination of the foregoing embodiments, the heat exchanger system controller is further configured to apply an electric field to impart an electrostatic charge to contaminants in the heat rejection side fluid flow path.
In any one or combination of the foregoing embodiments, the heat exchanger system controller is further configured to apply the electric field in response to a pressure differential between a heat rejection side fluid flow path inlet and outlet.
In any one or combination of the foregoing embodiments, the heat exchanger system controller is further configured to apply the electric field in response to a pressure differential between a heat rejection side fluid flow path inlet and outlet.
In any one or combination of the foregoing embodiments, the heat exchanger system controller is further configured to apply the electric field in response to a differential between a temperature of the hydrophobic surface and an ambient dew point temperature higher than the hydrophobic surface temperature
In any one or combination of the foregoing embodiments, the heat exchanger system controller is further configured to apply the electric field in a pulsed cycle pattern comprising alternating on and off periods wherein the duration of the off period is equal to or longer than the duration of the on period.
In any one or combination of the foregoing embodiments, the hydrophobic surface is disposed on heat exchanger fins in thermal communication with the heat exchanger heat absorption side and in fluid communication with the heat rejection side fluid flow path.
In any one or combination of the foregoing embodiments, the heat exchanger fins individually comprise a portion comprising a hydrophilic surface.
In any one or combination of the foregoing embodiments, the hydrophobic surface comprises hydrophobic microstructural or nanostructural surface features.
In any one or combination of the foregoing embodiments, the hydrophobic surface comprises a hydrophobic coating disposed on a heat exchanger surface in thermal communication with the heat exchanger heat absorption side and in fluid communication with the heat rejection side fluid flow path.
In any one or combination of the foregoing embodiments, the heat exchanger hydrophobic surface comprises a heat exchanger structural feature formed from a hydrophobic polymer composition.
The subject matter which is regarded as the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
This disclosure can be applied to virtually any type of configuration of heat exchanger. An example embodiment of a round tube plate fin (RTPF) heat exchanger is schematically depicted shown in
Another type of exemplary heat exchanger that can be used according to the embodiments described herein is a micro-channel or mini-channel heat exchanger. The configuration of these types of heat exchangers is generally the same, with the primary difference being rather loosely applied based on the size of heat transfer tube ports. For the sake of convenience, this type of heat exchanger will be referred to herein as a micro-channel heat exchanger. As shown in
In some embodiments, the a heat exchanger can be used to cool a gas comprising water vapor flowing on a heat rejection side of a heat exchanger such as the heat exchangers depicted in
As stated above, condensed water droplets can be removed by selective application and removal of an electric field to change contact angles and surface energies of the droplets to cause them to detach from a hydrophic surface of the heat exchanger. An example water droplet 302 on a substrate 304 is schematically depicted in
γSG=γSW+γWG COS θ
where γSG is the interfacial tension between the substrate and the gas, γSW is the interfacial tension between the substrate and the water, γWG is the interfacial tension between the water and the gas, and θ is the contact angle between the water droplet and the substrate. Application of an electric field reduces the contact angle according to the Young-Lippmann equation:
COS θE=(γSC−γSW+CV2/2)γWG
as shown in
The first surface energy of a water droplet on a substrate surface before application of the electric field can be characterized by the formula
E1=γSW[2πR(θO)2(1−COS θO)−πR(θO)2COS θO sin2 θO]
where θO is the contact angle of the droplet in the absence of the electrical field and R is the radius of the droplet configured as a spherical cap on the surface, which can be determined according by conservation of volume according to the formula
Application of the electric field to the water droplet reduces the contact angle as described above, and increases the surface energy according to the formula
E2=γSW[2πR(θE)2(1−COS θE)−πR(θE)2COS θO sin2 θE]
When the electric field is removed, the capacitor formed by the droplet and the electrode discharges much faster than the shape of the droplet can change. Accordingly, the shape of the droplet is still largely as in
E3=γSW[2πR(θ)2(1−COS θ)−πR(θ)2COS θO sin2 θE]
However, the droplet configuration in
E0=γSW4πR2(θ)
where θ (in radians) approaches the value for π. In this condition, the excited energy level E3 provides sufficient energy to detach the droplet from the substrate as shown in
Electrode conductors can be integrated into the heat exchanger system in a variety of configurations, a few non-limiting examples of which are schematically depicted in
In still other embodiments, electrodes can be integrated into a surface layer on the heat exchanger surface (e.g., a fin surface) as depicted in
As can be appreciated from the above discussion, selection of a substrate having a target hydrophobicity is important for achieving detachment of water droplets from the substrate by applying and removing an electric field. Hydrophobicity can be achieved through various materials and material configurations for the substrate. In some embodiments, the substrate can be formed from a chemically hydrophic material or can comprise a surface layer formed from a chemically hydrophobic material. Chemically hydrophobic materials typically comprise nonpolar molecular structures that are incapable of forming hydrogen bonds with water. Introduction of such a non-hydrogen bonding surface to water causes disruption of the hydrogen bonding network between water molecules. The hydrogen bonds are reoriented tangentially to such surface to minimize disruption of the hydrogen bonded 3D network of water molecules and minimize the water-hydrophobe interfacial surface area. Examples of chemically hydrophobic materials include but are not limited to polyethylene, polypropylene, or polytetrafluoroethylene (PTFE). Hydrophobicity can also be provided through surface coating such as polyurethane or other hydrophobic coatings or by micro- or nano-sized features on the substrate surface. In some embodiments, the surface has hierarchical surface roughness with nanoscale or microscale structural or roughness features imparting a hydrophobic or superhydrophobic property to the surface. In some non-limiting examples, the microscale roughness may have Ra surface roughness values ranging from approximately 5 microns to approximately 100 microns and the nanoscale roughness may have an Ra value ranging from approximately 250 nanometers to approximately 750 nanometers. Surface roughness can be provided by chemical etching, spray coating, or sintering. In some embodiments, the heat rejection side fluid flow path heat exchanger surface can be formed from a chemically hydrophobic material or have a chemically hydrophobic surface coating, and have microscale or nanoscale surface features. In some embodiments, the surface can have microscale or nanoscale surface features and be formed from a hydrophilic material to provide hydrophilic sections such as sections 704 of
Droplets ejected from the hydrophobic surface as described above are removed from the heat rejection side fluid flow path. This can be accomplished by providing a flow velocity on the heat rejection side fluid flow path that entrains the detached droplets so that they can be carried out of the flow path along with the flowing gas. In some embodiments, the flow velocity is maintained at a steady state velocity that entrains the detached droplets. In some embodiments, the flow velocity is pulsed in timed coordination with the removal of the electric field to provide a temporary higher pulsed flow velocity to entrain the detached droplets. In some embodiments, contaminants can be captured in the water droplets and removed from the heat exchanger surface along with the detached water droplets. This can occur based on the surface tension interaction between the contaminants and the water droplets or, in some embodiments, the above-described electrodes, or separate electrodes disposed upstream along the gas flow path upstream of hydrophobic surface (either on the heat rejection side fluid flow path or upstream of the heat rejection side fluid flow path) can be used to apply an electric field to impart an electrostatic charge to the contaminants to facilitate their capture by the water droplets.
Various process control criteria can be utilized to trigger application and removal of the electric field to remove water droplets from the heat exchanger surface. In some embodiments, the electric field can be applied in response to detection of water on the hydrophobic surface (e.g., by a moisture sensor). In some embodiments, the electric field can be applied in response to a pressure differential (e.g., measured by pressure sensors) between a heat rejection side fluid flow path inlet and outlet, as the pressure drop differential can be indicative of accumulation of water on heat exchanger surfaces such as on closely-spaced fins. In some embodiments, the electric field can be applied in response to a differential between a temperature of the hydrophobic surface (e.g, measured by a temperature sensor either at the surface or measured for a working fluid on a heat absorption side fluid flow path) and an ambient dew point temperature (e.g., measured by a humidity sensor disposed at a heat rejection side fluid flow path inlet). In some embodiments, the electric field can be pulsed in a cycle pattern comprising alternating on and off periods. In some embodiments, the cycles are symmetrical with the duration of the off periods being equal to the duration of the on periods. In some embodiments, the duration of the off periods is greater than the duration of the on periods. Various waveforms can be used for cycling the electric field, including but not limited to square waves, saw waves, sinusoidal waves.
The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, “about” can include a range of ±8% or 5%, or 2% of a given value.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof.
While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.
This is a continuation of and claims priority to U.S. patent application Ser. No. 15/631,657 filed Jun. 23, 2017, which claims priority to U.S. Provisional Patent Application 62/354,571 filed Jun. 24, 2016, the disclosure of each of which is incorporated herein by reference in its entirety.
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20190137198 A1 | May 2019 | US |
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62354571 | Jun 2016 | US |
Number | Date | Country | |
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Parent | 15631657 | Jun 2017 | US |
Child | 16236977 | US |