WO2019144242A1

WO2019144242A1 - Micro capillary-assisted low-pressure evaporator

Info

Publication number: WO2019144242A1
Application number: PCT/CA2019/050102
Authority: WO
Inventors: Poovanna THIMMAIAH; Majid Bahrami
Original assignee: Simon Fraser University
Priority date: 2018-01-29
Filing date: 2019-01-29
Publication date: 2019-08-01

Abstract

A micro capillary-assisted low-pressure evaporator comprises a plurality of microchannels disposed within a capillary tube, and has a plurality of substantially vertically oriented fins having a variegated exterior surface. The plurality of fins form grooves between adjacent fins for inducing capillary action of a refrigerant and the formation of a thin film of refrigerant disposed on the exterior surface of the fins. The microchannels reduce a flow resistance of a working fluid flowing through the evaporator while the plurality of fins increases the efficiency of evaporation of the refrigerant disposed thereon.

Description

MICRO CAPILLARY-ASSISTED LOW-PRESURE EVAPORATOR

FIELD

The present disclosure relates generally to evaporators, and in particular, to environmentally friendly, high efficiency, low pressure micro capillary evaporators.

BACKGROUND

Conventional vapor-compression refrigeration (VCR) technology has been the dominant technology of air conditioning systems for close to a century, but its environmental impact and high energy consumption are contrary to sustainable development. The synthetic refrigerants used in VCR systems contribute strongly to climate change and the energy consumption of heating, ventilation, and air conditioning (HVAC) equipment (including VCR systems) is considerable. For example, HVAC equipment contributed to over 38% of the total primary energy consumption in the U.S. buildings sector in 2013. In the transportation sector, VCR-based air conditioning (A/C) and refrigeration systems cause a 20% increase in fuel consumption. Also, in an internal combustion engine (ICE) of a light-duty vehicle, 65-70% of the total fuel energy released is dissipated as waste heat. Furthermore, as the world moves to meet its greenhouse gas (GHG) emissions targets, applications for novel refrigerants must be developed to replace the existing refrigerants with high global warming potential (GWP). Therefore, using water as an alternative refrigerant provides substantial environmental benefits over HFCs including no toxicity and significantly lower GWP.

Low grade heat (LGH) or waste heat [sources with temperatures less than 100C], available from many sources such as solar thermal, geo-thermal, oil and gas, ICE, power plants or any other industrial facilities, can be utilized to run waste-heat driven adsorption chillers as a viable alternative to VCR systems.

A portion of the waste heat of an ICE is sufficient to run an adsorption chiller and generate the cooling power required for the vehicle A/C applications. In addition to environmental benefits and energy savings, an adsorption chiller has the benefits of simple construction, no moving parts (except valves), no vibration, quiet operation, and low operating cost. As a result, waste heat-driven adsorption cooling is a promising technology.

However, there are some technical limitations, including the need to maintain a high vacuum, and the low specific cooling power (SCP) and low coefficient of performance (COP) of these systems. These limitations lead to a large size and heavy system compared to a conventional VCR system.

Most current LP evaporators use commercially available industrial tubes, and the remainder use modified commercial tubes including those with thermal spray coatings on them. These tubes are not optimized for LP applications and they are subject to significant technical limitations such as high internal thermal resistance due to large internal diameter and, high external resistance due to poor capillary performance due difficulties in manufacturing the desired spacing between fins. The current LP technology is limited to commercially available tubes. Also, industrial evaporator tubes are only desirable for high chilled water mass flow rate applications, which is

disadvantageous in applications such as low pressure evaporators.

Designing a highly efficient and compact evaporator is key in reducing the mass and size of adsorption chillers. Water has been traditionally used as the refrigerant in adsorption chillers because of its high enthalpy of evaporation. However, since evaporators typically operate between 3 and 20 degrees Celsius, the operating pressure (caused by water) varies between 0.76 and 2.34 kPa. This low operating pressure dictates or requires a special evaporator design due to accumulation of liquid water in the evaporator, which negatively impacts the evaporation rate, and renders the currently known evaporators increasingly inefficient as they operate. The water column in evaporators creates a hydrostatic pressure that increases the saturation temperature or boiling point of natural refrigerants such as water. For an evaporator operating at 1.0 kPa (6.9 deg C), the hydrostatic pressure of the water used as refrigerant in the system increases the saturation temperature to approximately 13 degrees Celsius at a depth of 5 cm of water column, where the pressure (operating + hydrostatic) is approximately 1.49 kPa. This temperature variation inside currently known evaporators reduces the generation of cooling power in the adsorption chiller. All conventional evaporators available in the market, therefore, fail to perform efficiently as low pressure evaporators in adsorption chillers.

In practical applications of known evaporators, chilled water in CALPEs typically exchanges heat within an Air Handling Unit (AHU), and while large flow rates remove the limiting factor of internal heat transfer resistance, pressure drops between chilled water (working fluid) inlet and (working fluid) chilled water outlet increase significantly with higher velocities through the evaporator, piping, and AHU, thereby requiring powerful pumps to maintain an appropriate operating pressure.

One method for overcoming this limitation is utilizing falling film evaporators which utilize pumps to spray a thin film of water on the external surface of the evaporator tubes. One of the benefits of falling film evaporators is a high overall heat transfer coefficient (U). However, there are drawbacks due to system complexity and added the power consumed by internal pumps. In addition, non-uniform thin films typically cause dry-out on sections of the evaporator tubes, due to the impossibility of proving an absolutely uniform film thickness using spray nozzles and other available methods, which has detrimental effects on the performance.

The main challenge with CALPEs is the internal heat transfer resistance of the liquid chilled water flowing inside the evaporator tubes.

There is a need, therefore, for a novel low pressure (LP) evaporator that optimizes heat transfer by exploiting the evaporation of water in a thin film.

SUMMARY

Embodiments of the present invention generally relate to micro capillary- assisted low-pressure evaporators. More specifically, embodiments of the present invention generally relate to micro capillary-assisted low-pressure evaporators having a plurality of elongated, partially hollow evaporator tubes adapted to induce evaporation of a thin film of refrigerant disposed on an exterior surface thereof.

In a broad aspect, an elongated, partially hollow evaporator tube, for evaporating a refrigerant about the evaporator tube by fluidly communicating a working fluid therethrough, comprises at least one microchannel disposed longitudinally through the elongated, partially hollow evaporator tube, and a plurality of substantially vertically oriented fins disposed on an exterior surface of the evaporator tube for increasing a surface area of the evaporator tube thereby increasing an efficiency of evaporation of the refrigerant from the plurality of substantially vertically oriented fins.

In another broad aspect, an evaporator tube, for evaporating a refrigerant about the evaporator tube by a working fluid flowing therethrough, comprises a plurality of microchannels disposed longitudinally through the evaporator tubes, the evaporator tubes being comprised of a material selected from the group of: aluminum, copper, steel, ceramic-plastics, synthetic graphite, and polymeric composites.

In another broad aspect, a micro capillary assisted low-pressure evaporator comprises a housing and at least one capillary tube disposed within the housing. The housing, adapted to store a pool of refrigerant therein and define a vacuum chamber, further comprising an inlet port and an outlet port. In embodiments, at least one capillary tube is disposed within the housing and in the pool of refrigerant, and is in fluid communication with the inlet port and the outlet port of the housing. In embodiments, the at least one capillary tube is adapted to receive a working fluid through at least one inlet port, fluidly travel through the capillary tube, and exit through at least one outlet port. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a sectional perspective view of an embodiment of the present invention illustrating a portion of an evaporator tube having microchannels therethrough and two adjacent substantially vertically oriented fins, the portion of the evaporator tube being partially submerged in a pool of refrigerant.

Figure 1 a is a plan cross-sectional view of Fig. 1 , illustrating the profile of the refrigerant at a particular height.

Figure 1 b is a plan cross-sectional view of Fig. 1 , illustrating the profile of the refrigerant at a particular height lower than the height shown in Fig 1 a. Figure 2 is side view of an embodiment of the present invention, illustrating an evaporator tube disposed within a low pressure evaporator housing, the housing having a vapour outlet.

Figure 3 is a plan view of the embodiment in accordance with Fig. 2, illustrating the elongated, partially hollow evaporator tube being fluidly connected to an inlet port and an outlet port of the evaporator housing.

Figure 4 is a perspective view of an embodiment of the present invention, illustrating a plurality of evaporator tubes with inlet and outlet headers, inlet and outlet ports, and intermediate headers in a serpentine configuration.

Figure 5 is a perspective view of the embodiment in accordance with

Fig. 4.

Figure 5a is a detailed view of the embodiment of the invention in accordance with Figs. 4 and 5, illustrating the inlet header, the inlet port and showing a plurality of microchannels, fins and grooves.

Figure 5b is a partial cut-away sectional view of the embodiment of the invention in accordance with Figs. 4 and 5, illustrating a plurality of microchannels disposed in the evaporator tube.

Figure 6 is a perspective view of an embodiment of the present invention, illustrating an extended header comprising an inlet header, an inlet port, an outlet header, an outlet port and open headers with the evaporator tubes in a parallel configuration. In such an embodiment, the plurality of microchannels are disposed in the horizontal or straight portions of the evaporator tubes, and the headers.

Figure 7 is a perspective view of the embodiment in accordance with

Fig. 6.

Figure 7a is a detailed view of the embodiment of the invention in accordance with Figs 6 and 7, illustrating the inlet port and showing a plurality of microchannels.

Figure 7b is a partial cut-away sectional view of the embodiment of the invention in accordance with Figs. 6 and 7, illustrating a plurality of microchannels disposed in the elongated, partially hollow evaporator tube. Figure 8 is a perspective view photograph of an embodiment of the present invention, showing an elongated, partially hollow evaporator tube, an inlet header, an outlet header, and open headers.

Figure 8a is a close up perspective view photograph of the embodiment in accordance with Fig. 8, showing a plurality of substantially vertically oriented fins and grooves between adjacent fins.

Figure 8b is a close up perspective view photograph of the embodiment of the present invention in accordance with Fig. 8, showing the inlet port.

Figure 9 is a photograph of an embodiment of the present invention, showing the variegated exterior surface of the fins.

Figure 10 is a schematic drawing of an embodiment of the present invention, illustrating an evaporator system.

Figure 11 is a system in use schematic drawing of an embodiment of the present invention.

DETAILED DESCRIPTION

This invention relates to a micro capillary-assisted low-pressure

evaporator (pCALPE), which draws a refrigerant (such as water) from pool of refrigerant into grooves, thereby covering most of the exterior or external surface of the evaporator with a thin film of refrigerant. Capillary forces draw the water along the grooves without the use of external power, thereby passively and uniformly distributing a working fluid, typically water along the tubes. This invention overcomes the challenge of internal heat transfer resistance and provides significantly more efficient evaporation rates with high effectiveness and high compactness compared to known CALPEs.

One aspect of this invention was inspired by the natural phenomenon that occurs in the water circulation and evaporation process in trees. Trees are natural hydraulic pumps. They absorb water through roots, pump the water using a capillary effect, and evaporate liquid through the leaves. Inspired by the capillary assisted evaporation in trees, this invention achieves efficient, in a biomimetic fashion, passive evaporation in the micro capillary-assisted low-pressure evaporator (pCALPE). The present invention includes embodiments of a highly efficient and compact evaporator which plays a significant role in reducing the mass and size of adsorption chillers, due to its efficiency of 400-450% over traditional CALPEs. The present invention, the pCALPE, has a system SCP and COP which is significantly better than traditional CALPEs, thereby reducing operational costs and environmental impacts.

The present invention provides a system that can be operated at lower mass flow rates (1 -3 L/min) than known CALPEs, with a sizable temperature difference between the inlet and outlet ports. The present invention discloses a pCALPE with greater effectiveness compared to the prior art.

The present invention provides benefits over the prior art, including: at least 50% less chilled water usage; significant overall heat transfer (U factor), which is increased nearly threefold compared to commercially available LP systems; a smaller and lighter weight; durability, in that the (in one embodiment) aluminum pCALPE design is not susceptible to formicary corrosion like some copper evaporator tubes; ease of manufacture using additive manufacturing (AM) technology; less waste or zero waste during manufacturing and little lead time to manufacture.

The benefits of the present invention over the known CALPE designs is supported by research results showing the 10 times improvement in cooling power density, including additional information on the size of the fins, size of the gaps, ranges for optimal performance, and optimal thickness of the thin film of refrigerant.

To show the improvement, the present pCALPE invention was compared with a CALPE built with commercially-available tubes. It is important to note that the footprint (area & volume occupied) of CALPE is four times larger than that the present pCALPE invention providing a similar cooling capacity.

8000 7,706 W/m³

7000

6000

Cooling density (j)

Compactness (b)

5000

4,084 m²/m³

4000

3000

2000

1000 789 W/m³

77 m²/m³

0

CALPE m CALPE

The above chart is a comparison of the cooing density [W/m³] and compactness [m²/m³] of pCALPE and the CALPE built with Turbo Chil™-40 FPI (Wolverine™ Tube Inc.) commercially available tubes, which are a typical present industry norm.

The present invention pCALPE provides 7,706 W/m³ of cooling or power density, which is ten times higher compared to a typical CALPE.

An evaporator is considered compact in the industry when it has a heat transfer area to volume ratio [m²/m³] greater than 400 m²/m³. The compactness (b) for pCALPE is 4,084 m²/ m³, which is 5,223% higher than a CALPE in comparison (see above). The higher b is a good indication of performance, the higher compactness leads to higher the effectiveness for a given pumping power. For instance, in one embodiment, the present invention could be dimensioned with channels having a diameter of 1 -2 mm and would provide a cooling power of 1 kW and an effectiveness of 75%-85%, compared to known designs which would be 14.45 mm channel diameter in order to provide the same output of 1 kW yet also resulting low effectiveness between 25%-40%.

Another advantage of the present invention is the scalability for various application, including mobile applications such as onboard vehicles, as a functioning embodiment of the present invention could be as small as a wristwatch, but also scaled up to the size of a car, or larger, in order to service large industrial applications.

In a broad aspect of the invention, a micro capillary-assisted low-pressure evaporator (pCALPE) comprises a vacuum chamber or housing to hold capillary tubes, or core, placed in a pool of refrigerant, for example water. To produce a refrigerant vapor, capillary tubes with high density fins disposed on the outside of the tube or tubes including rows of mini/micro channels on the inside of the tube. An optimally designed heat exchanger (evaporator) with optimized number, arrangements, cross-sectional shape, and diameter of mini-/microchannels, manifold(s) (or header) to distribute the chilled water into the microchannels, and an inlet port and an outlet port are also provided.

In one embodiment, the CALPE employs evaporator or capillary tubes 20 in the range of 12.70 mm to 16.93 mm, which have, optimally, a 14.45 mm inner diameter (ID). These are commercially available tubes with a high fin density (40 fins per inch) which provides a corresponding fin spacing of 0.635 mm. A high fin density increases the area of the thin film region due to capillary action and thereby increases the external heat transfer coefficient. However, the large diameter ID of the

commercially available tubes 20 (not shown) results in a poor internal heat transfer coefficient. To overcome the bottleneck, restriction or challenge of the internal heat transfer restriction, in an alternate, preferred embodiment, smaller tubes with ID’s of 6.35 mm to 9.52 mm are used, with an optimal ID of approximately 7.9 mm ID, which is ideal because sufficient of number fins are easily placed on the outside of this tube.

Commercially available finned tubes with a 7.9 mm ID have a maximum fin density of 26 fins per inch, which results in a weakened capillary effect, compared to pCALPE embodiments disclosed below, causing the outer heat transfer coefficient to suffer. Finally, the finned 7.9 mm ID tubes 20 are coated with a porous copper coating, in one embodiment, that effectively reduces fin spacing and increases the area for thin film evaporation to occur, leading to improved higher cooling power by reducing internal and external resistances and increasing internal and external heat transfer coefficients. However, the internal heat transfer was still the main bottleneck with respect to heat transfer and hinders performance. pCALPE embodiments of the present invention overcome the CALPE challenge of heat transfer as explained below.

To evaluate the internal convective heat transfer coefficient, h , for port diameters above 1 mm Gnielinski correlation is used. The flow of chilled water 60 inside the tube 20 is characterized by the Reynolds number, Re_/:

_D- _ P j, tube where, V is the chilled water 60 velocity. For Reynolds numbers higher than 2,300 (2,300 < Re, < 5x10⁶ ) Gnielinski correlation [58] can be used to calculate the Nusselt number:

With f = 0.078 Re

Finally, the internal heat transfer coefficient, hi, is calculated as follows: h _ Nu, k_waier

' ‘ D-'i.tube

Employing the figures relevant to the present invention, it was determined that the optimal total heat transfer coefficient of 3250 W/m².K, which is 2.4 times higher than known designs was achieved by reducing both the ID of the channels and the width of the grooves. The following figures depicts the overall heat transfer coefficient of pCALPE and the CALPE built with known designs as function of chilled water mass flow rate

1.0, [Re=1 100] 1.5, [Re=1650] 2.0, [Re=2200]

Chilled water mass flow rate, m _in (kg/min)

The following chart depicts the thermal resistances of plain and commercial tubes employed in known CALPE systems.

Ext. convection Conductive Int. convection Overall thermal resistance resistance resistance resistance

In comparison, the following chart depicts the relatively negligible internal resistance in the present pCALPE invention.

0.006

o

c

TO

« w 0.003

External thermal Conductive Internal thermal Overall thermal resistance thermal resistance resistance

resistance

Now referring to Fig. 1 , a section of the evaporation or microchannel tube 20 in a pCALPE is shown in perspective, including two fins 26 and refrigerant 50, which may be water or other fluids. In this embodiment of the present invention, plurality of microchannels 24 and transfers the thermal energy from the chilled water 60 to a refrigerant (working fluid) 50 disposed on an exterior surface of the capillary tube 20, causing the refrigerant 50 to evaporate. The microchannels optimally have an internal diameter of 100-200 micrometers, for the most efficient flow characteristics. A micro capillary tube, then, is optimally a capillary tube according to the present invention, with an internal diameter of 100-200 micrometers. Below 100 micrometers, high pressure drop occurs, which increases the pumping power required to push the chilled water, and over 200 micrometers in diameter results in poor heat transfer (details provided herein).

The capillary tube 20 is also and interchangeable called an evaporator tube 20, or a microchannel tube 20, or evaporator core 20 and may be an elongated hollow cylinder, or, as in the embodiments depicted herein, an elongated, partially hollow (due to the internal microchannels 24) rectangular prism or hollow cuboid or partially hollow rectangular parallelepiped or other shape. The evaporator tubes are optimally 3-4 mm in thickness or diameter, including the fins 28, to provide structural integrity.

The fins 26 are thin, substantially vertically disposed rectangular shapes abutting the exterior wall of the evaporator tubes 20 in the present embodiment, but may be other shapes and may, for instance, extend radially around a circular or hollow cylindrical tube 20. The fins and evaporator tube 20 consist, optimally of aluminum, or any other lightweight material with favorable thermal characteristics, such as copper, steel, ceramic-plastics, natural/synthetic graphite, or others. The fins 26 are optimally in groups of 1 -2 inches in length, in order to achieve sufficient capillary rise. The fins are optimally 1.5-2 mm high, because it is easier to manufacture and avoid dry-out regions in such a configuration. Each fin 26 is optimally 0.5-1 mm thick. The spacing between fins 28 is optimally 0.1 -0.5 mm, which creates a groove 28 between fin 26 spaces, which is optimal because it ensures that if sufficient refrigerant is provided, capillary action will occur and cover the entire length of the fin with a thin film of refrigerant (calculations and geometry provided herein).

As shown in Fig. 1 , the capillary tube 20 is placed within a pool of refrigerant (such as water) 50. The capillary tube 20 further comprises a plurality of fins 26 disposed along the exterior of the capillary tube 20, and the space between each two adjacent fins 26 is called a groove 28 or microgroove or slot. The width of the grooves 28 is sufficient to cause a portion of the refrigerant 50 to be drawn up by surface tension against the gravitational forces to travel within the groove 28 through capillary action, thereby creating a thin film of refrigerant 50 within the groove 28. The transfer of thermal energy from the working fluid 60 to the refrigerant 50 is sufficient to cause the thin film of refrigerant 50 to evaporate to create a refrigerant vapor which is temporarily received and stored within a vacuum chamber (not shown). The stored refrigerant vapor may then be fluidly communicated through a vapor outlet to be condensed in a downstream condenser (not shown). The evaporative thin film portion is merely the top approximately 1 -5 microns of the fin 26.

In a preferred embodiment, the working fluid 60 is chilled water, but need not be limited to chilled water and can comprise any other fluid. Chilled water is optimal due to its high specific heat capacity, availability and environmental friendliness.

The refrigerant 50, through capillary action at low pressure, runs along and up within the grooves 28 from the refrigerant 50 pool to create a thin film on the exterior surface of the evaporator tube 20. The refrigerant 50 may be any suitable liquid, but is optimally water, or optionally, methanol, ammonia, ethanol, n-butane, and LiBr, or a combination of the above designed for various applications, e.g. air conditioning, heat pumping, thermal energy storage, and adsorption water desalination. The refrigerant 50 can also be selected from a wide range of other fluids, but preferably, the refrigerant 50 is readily available, ozone friendly, environmentally friendly, and has a high enthalpy and easy to evaporate.

The capillary action is achieved by creating narrow channels or grooves 28, having an optimal gap for capillary effect, using a high density (a high number of fins by unit length) of fins 28. The fins 28 optimally have a grainy, variegated, rough, uneven, textured or irregular surface, which provides additional heat transfer surface area and further enhances the performance of the evaporator 10. The variegation of the fins 26 and entire exterior of the tubes 20 is achieved by applying a thermal spray, such as wire-frame or plasma spray or laser sintering process. Any form of thermal spray, such as plasma, wire arc, flame, high velocity oxy-fuel, or HVAF coating may be employed to add a compatible coating to the substrate material of the fins and exterior of the tubes. Etching or other techniques may also be employed. The variegation is shown in a close up detail drawing in Fig 9.

The optimal number of ports is 8-12, with a diameter of 1 -2 mm. These transition chambers or ports are adapted to receive a flow of fluid from a source external to the evaporator 10, or to hold an exiting fluid, in an open chamber or port.

An advantage of evaporation through capillary action is to draw the refrigerant 50 from the pool to cover the outside exterior surface of the capillary tube 20 and to produce uniform distribution of refrigerant 50 along the tubes in a thin film.

Drawing water through capillary action avoids the negative hydrostatic pressure impacts present in LP evaporators. The capillary action is completely passive and does not require additional power in order to distribute the film on the outside surface of the evaporator. Another advantage of utilizing mini/micro channels 24 is to achieve internally high Reynolds number flow and to overcome the major bottleneck of internal heat transfer resistance.

The optimal groove 28 dimensions are determined by creating the optimal thin film thickness at the upper portion of the fins 26. The grooves 28 are, optimally, 0.5 mm wide, and 1 mm deep and 25.4 mm in height, calculated between the average coating depth on the fins 28. Below the variation of d along z-direction is shown.

00025

0002

E,

io

8 00015

w

c

0001

ip

c

£ 00005

0

O OOE+OO 500E-06 1 00E-O5 1 50E-05 200E-05

z [m]

In a rectangular open microchannel, the channel length is several orders of magnitude larger than the thin film region. The film thickness remains at the initial value to mark the non-evaporating region I before it increases sharply to meet the bulk region III. When curvature of the thin film region meets bulk curvature, the length of the thin film region is determined. The thin film region also corresponds to the region with a high evaporative heat flux. In the extended meniscus, the thin film region is seen to be the major contributor to the overall heat transfer. In a rectangular open

microchannel, the channel length is several orders of magnitude larger than the thin film region.

The film thickness remains at the initial value ( S₀ ) to mark the non- evaporating region I before it increases sharply to meet the bulk region III. The meeting of the curvature of the thin film region and the bulk curvature determines the optimal length of the thin film region, which is 5-10 microns. The thin film region also

corresponds to the region with a high evaporative heat flux. In the extended meniscus, the thin film region is seen to be the major contributor to the overall heat transfer. The evaporation of the thin film refrigerant only occurs in the top 1 -2% of the height, or at the apex, of the grooves 28 between the fins 26.

The evaporation rate increases from zero (at the non-evaporating region) to a maximum in the thin film region and then decreases to zero (at the bulk region). The evaporative heat flux rises in correspondence with the decreasing film thickness. The heat transfer is almost negligible up to a channel depth of 1 mm and it increases sharply close the apex of the channel. This is due to a huge resistance to the heat transfer coming from the thick film below the bulk curvature. The contribution from the bottom wall is negligible due to the large resistance and therefore it is neglected.

The heat transfer per unit length in the thin film region

100

90

c

<u 60

c

3 50

w

Q. 40

o 30

tn

c

00

OOOE+OO 500E-04 100E-03 150E-03 200E-03

y[m]

The heat transfer per unit length in the bulk region <¾uik decreases along channel. This is because ¾_u,_k is also increasing along the ^-direction. The higher ¾_u,_k results in a smaller contact angle, which results in the thinner film along the channel.

000 079 157 236 314

f [radians]

The collective effect of the heat transfers from all the regions on the heat transfer coefficient and total heat transfer rate of the CALPE.

The utilization of microchannels 24 in the present invention provides a more compact CALPE and higher internal heat transfer coefficients through high surface area density. Flowever, the main challenges of integrating microchannels in CALPE are: i) the manufacturing complications, and ii) high pressure drop which leads to higher pumping power required to flow the chilled water through the microchannels 24. Therefore, the channel (port) diameter and the headers were carefully designed as follows.

Fig. 1 (a) is a plan view cross section of two fins 26 and a portion of the tube 20 wall, showing the decreased cross sectional area of refrigerant 50 in the upper portions of the grooves 28 due to the increased water column overcoming the capillary action induced by surface tension.

Fig. 1 (b) is a plan view cross section of two fins 26 and a portion of the tube 20 wall, showing the greater cross sectional area of refrigerant 50 in the lower portions of the grooves 28 relative to the higher areas.

The open grooves 28 with a rectangular cross-sectional area draws water 50 in a vertical direction when the lower portion of the evaporator 10 is placed in the liquid. For a capillary channel of width W and depth of the groove L, the capillary height (H) is given by:

_H _ a[(2L + W) cose - W]

P,[LW]g

where s (sΐn) is the surface tension between the liquid-vapor interface, Q is the contact angle and pi is the liquid density.

The capillary tubes 20 can be arranged in a serpentine fashion with multiple passes as shown in Fig. 3, or in a parallel fashion with multiple passes in Fig. 6. In a serpentine configuration, the microchannels are disposed in the evaporator tube and in the headers, while in the parallel configuration, the microchannels are disposed in the straight or horizontal portions of the evaporator tube. The grainy surface of the exterior of the evaporator is shown in Fig. 9. An advantage of the parallel layout is less pressure drop due to leaks.

Fig. 4 show a perspective view of the serpentine configuration.

Fig. 5 shows a perspective view of the serpentine tube 20 formation and provides exploded detail drawings of the inlet header 30 and port 32 in Fig. 5a, and of the fins 26 and microchannels 24 in Fig. 5b.

Fig. 6 is a perspective view of the parallel tube 20 configuration with Fig. 7a showing details of the inlet header 30 and port 32 and Fig. 7b of the fins 26 and microchannels 24.

Referring to Fig. 8, the perspective view of a serpentine flow evaporator 10, the single-phase chilled water or working fluid 60 follows through the inlet header 30 and is distributed into a row of microchannels 24 of a capillary tube 20 and flows through the intermediate headers 44 at each elbow in the flow path and finally exits through the outlet port 42. The pressure drop for the entire pCALPE evaporator 10 is the sum of pressure drops in the inlet header 30 (part of the inlet port for serpentine flow type pCALPE), port inlet 32 contraction, along the port, port exit expansion, exit header, and the outlet port 42.

Fig. 10 is a schematic view of an embodiment of an evaporator system. The system comprises a control system (TCS) 70 and a variable speed fluid pump 120 which provides a constant temperature thermal fluid (chilled water) 60 to the evaporator 10 at different, controllable and selectable mass flow rates by the controller 70. A vacuum pump 90 selectively, using the controller 70, lowers the system pressure from the ambient? and control valves 110 controllably, by the controller 70, regulate the pressure inside the evaporator 10. To protect the vacuum pump 90 from water vapor produced by the evaporator 10, at least one and optimally two cold traps (dry ice and isopropyl alcohol) 100 sit inline and adjacent to the vacuum pump 90. Acronyms for Fig. 10

CALPE: Capillary-assisted Low Pressure Evaporator

pCALPE: Micro Capillary-assisted Low Pressure Evaporator

F: Flow sensor

T: Temperature sensor

P: Pressure sensor

TCS: Temperature Control System

EV: Expansion Valve

In operation, the evaporator tubes 20 are partially submerged in a pool of refrigerant 50. The evaporator tubes 20 are in contact the refrigerant 50 and the pressure in the evaporator 10 housing is maintained at approximately1 -2 kPa with chilled water 60 inlet temperatures of approximatelyl O to 20°C, and a thermal fluid flow rate of approximately 2.0 kg/min. The present pCALPE invention shows a significantly higher heat transfer rate and heat transfer coefficients relative to CALPE built with industrial tubes. The working fluid 60 may be pumped or drawn, for instance in Fig. 11 , into the pCALPE, where it circulates through an inlet port, into the plurality of

microchannels through the evaporator tube, through a plurality of headers to maintain advantageous flow, and out an outlet port. Meanwhile, the refrigerant is driven up the grooves via capillary action, and evaporation occurs at the top portion of the grooves between the fins. The evaporator may be used in combination with other components as described herein.

The present invention may be employed in desalination plants to decrease carbon emissions and reduce power consumption compared to reverse osmosis (RO) where global desalination capacity was 65.2 million m³/day requiring 75.2 TWh/year of energy in 2012.

The present invention may also take advantage of the temperature gradient available in power plants, where large quantities of warm cooling water are discharged from the plant, reducing the energy input needed to create a temperature gradient. The present invention may also be employed in multi-stage flash distillation, where changing salt content and harmful algal blooms can stall reverse osmosis (RO) production. Hybrid desalination techniques such as adsorption desalination (AD) promise lower costs and up to 85% less C02 emissions compared to RO. The present invention may also be employed in hybrid Ocean Thermal Energy Conversion (OTEC) plants can add (LGH driven desalination) desalination functionality and reduce thermal pollution that harmfully affects coastal aquatic ecosystems.

The present invention may also be employed to significantly mitigate carbon footprint and global warming by increasing the performance viability of renewable and waste-heat driven air conditioning, and provide reliable access to clean water through desalination methods with low power consumption. The present invention may also be employed in the oil and gas industry where substantial amounts of fresh water are required for production processes.

The illustrative embodiments herein described are not intended to be exhaustive or to limit the scope of the invention to the precise forms disclosed. They are chosen and described to explain the principles of the invention and their application and practical use. Many alterations and modifications are possible in the practice of this invention without departing from the scope of the invention, which is defined by the claims appended hereto.

Claims

CLAIMS WHAT IS CLAIMED IS:

1. At least one elongated, partially hollow capillary tube, for evaporating a refrigerant about the capillary tube by fluidly communicating a working fluid therethrough, the capillary tube comprising:

at least one microchannel disposed longitudinally through the elongated, partially hollow capillary tube; and

a plurality of substantially vertically oriented fins disposed on an exterior surface of the capillary tube for increasing a surface area of the capillary tube thereby increasing an efficiency of evaporation of the refrigerant from the plurality of

substantially vertically oriented fins.

2. The at least one capillary tube of claim 1 , wherein the fins are spaced apart from one another defining a groove between adjacent fins, the groove being adapted to induce capillary action of the refrigerant and forming a thin film of the refrigerant disposed about an exterior surface of the plurality of substantially vertically oriented fins.

3. The at least one capillary tube of claim 2, wherein the grooves have a spacing in between the range of 0.1 mm to 0.5 mm.

4. The at least one capillary tube of claim 2 or 3, wherein the thin film of the refrigerant has a uniform thickness of 1-4 pm.

5. The at least one capillary tube of any one of claims 1 to 4, wherein the exterior surface of each of the plurality of substantially vertically oriented fins is variegated.

6. The at least one capillary tube of any one of claims 1 to 5, wherein the capillary tubes are disposed in a serpentine fashion with a header at each bend in the capillary tube design, such that the microchannels continue through the headers into the next capillary tube in a sequence.

7. The at least one capillary tube of claim 5 or 6, wherein each of the plurality of micro-channels have an inner diameter in between a range of 100 pm to 200 pm, thereby rendering the capillary tube a micro capillary tube.

8. The at least one capillary tube of any one of claims 1 to 7, wherein the capillary tubes are adapted to be used in a low pressure capillary housing having an internal operating pressure below ambient pressure.

9. The at least one capillary tube of claim 8, wherein the housing is disposed about the at least one capillary tube, and the operating pressure is in the range of 1 kPa to 2 kPa.

10. The at least one capillary tube of any one of claims 1 to 9, wherein the capillary tubes further comprise an elongated, partially hollow rectangular prism, cuboid, parallel pipe, or other shape, including an elongated, partially hollow rectilinear tubing.

11. At least one capillary tube, for evaporating a refrigerant about the capillary tube by a working fluid flowing therethrough, the capillary tubes comprising:

a plurality of microchannels disposed longitudinally through the capillary tubes, the capillary tubes being comprised of a material selected from the group of: aluminum, copper, steel, ceramic-plastics, synthetic graphite, and polymeric

composites.

12. The at least one capillary tube of claim 11 , wherein the at least one capillary tube further comprise a plurality of substantially vertically oriented fins disposed on an exterior surface of the capillary tubes for increasing a surface area of the capillary tube, thereby increasing the efficiency of evaporation of a thin film of the refrigerant from the plurality fins.

13. The at least one capillary tube of claim 12, wherein each substantially vertically oriented fin of the plurality of substantially vertically oriented fins are spaced apart from one another to create a groove between adjacent substantially vertically oriented fins thereby adapted to induce capillary action of the refrigerant and forming a thin film of the refrigerant disposed on an exterior surface of the plurality of substantially vertically oriented fins.

14. The capillary tube of claim 13, wherein the grooves have a spacing in between the range of 0.1 mm to 0.5 mm.

15. The at least one capillary tube of claim 12 or 13, wherein the thin film of the refrigerant has a uniform thickness of 2-3 pm.

16. A micro capillary assisted low-pressure evaporator comprising: a housing for storing a pool of refrigerant therein and defining a vacuum chamber, the housing having an inlet port, and an outlet port;

at least one micro capillary tube, in fluid communication with the inlet port and the outlet port, disposed within the housing and in the pool of refrigerant,

wherein the micro capillary tube is adapted to receive a working fluid through at least one inlet port, through the capillary tube, and exiting through at least one outlet port.

17. The evaporator of claim 16, wherein the at least one micro capillary tube further comprises a plurality of substantially vertically oriented fins for creating grooves between adjacent, substantially vertically oriented fins,

wherein the grooves are adapted to receive a vertical flow of the refrigerant from the pool of refrigerant driven by capillary action thereby creating a thin film of refrigerant disposed on an exterior surface of the plurality of substantially vertically oriented fins.

18. The evaporator of claim 17, wherein the grooves between adjacent substantially vertically oriented fins are in a range of 0.1 mm to 0.5 mm.

19. The evaporator of claim 16, 17 or 18, wherein as the working fluid flows through the at least one micro capillary tube, heat from the working fluid transfers from the working fluid through the micro capillary tube and heats the thin film of refrigerant disposed thereon, thereby causing the thin film of refrigerant to evaporate into the vacuum chamber as refrigerant vapor.

20. The evaporator of any one of claims 16 to 19, wherein the housing further comprises a vapor outlet in fluid communication with a condenser to condense vapor evaporated from the refrigerant, and

wherein the vacuum chamber temporarily receives and stores the refrigerant vapor for fluidly communicating the refrigerator vapor to the condenser.

21. The evaporator of any one of claims 16 to 21 , wherein the evaporator operates at an internal operating pressure below ambient pressure.

22. The evaporator of claim 21 , wherein the internal operating pressure is in between a range of 1 kPa to 2 kPa.

23. The evaporator of any one of claims 16 to 22, wherein the refrigerant is selected from the group of water, methanol, ammonia, ethanol, n-butane, lithium bromide, or a combination thereof.