WO2025083483A1

WO2025083483A1 - Evaporative cooler using hollow fibers to produce chilled water and having performance factors

Info

Publication number: WO2025083483A1
Application number: PCT/IB2024/058993
Authority: WO
Inventors: Sarah A. TUPY; Anne N. De Rovere; Michael D. ZENNER; Sankar MUTHUKRISHNAN; Ravi KOLAKALURI; Neeraj Nitin SINAI BORKER
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2023-10-17
Filing date: 2024-09-16
Publication date: 2025-04-24
Anticipated expiration: 2026-04-17

Abstract

A unit for use in evaporative cooling includes a first capped frame and a second open frame opposite the first frame. Posts are located between and coupled to the first and second frames. A porous hollow fiber membrane extends around the posts between and coupled to the first and second frames to form an interior volume. The first and second frames are configured for flow of water between them via the membrane. The membrane is configured to transport the water between the first and second frames and to provide for air flow from the interior volume through the membrane for evaporative cooling. The cooling unit can produce chilled water. The cooling unit performance factors can be controlled by water flow rate, air flow rate, and water temperature and pressure. The unit can control air humidification.

Description

EVAPORATIVE COOLERS USING HOLLOW FIBERS TO PRODUCE

CHILLED WATER AND HAVING PERFORMANCE FACTORS

BACKGROUND

Evaporation is a cost and energy efficient way of cooling and is used for regulating temperatures in data centers, food processing plants, or office buildings. Currently, cellulosic pads are used to perform evaporative cooling on a large scale such as in a data center. Hot dry air is cooled by evaporating water flowing over the cellulosic pads yielding cool, humid air on the output. Large amounts of water are required for this type of cooling, and the media must be maintained either in a dry state or wet state to prevent degradation due to fouling or crystalline salt deposition. The humidity level of the air discharged into the data center can be controlled using louvers or dampers which direct the input air through only a portion of the media or completely around the media in a bypass duct. Accordingly, a need exists for an improved evaporative cooling system.

Data centers are exploring options for cooling by use of evaporative cooling. In evaporative cooling, there are two fluids, air and water. As water is evaporated, the air gathers more moisture. Evaporative cooling can be done directly or indirectly. Evaporative cooling can be used to cool the air. This air is directly used to cool a data hall. This is one definition of direct evaporative cooling within the HVAC industry. For indirect evaporative for cooling building air, one definition is that there are two air sources physically separated, and the overall system typically contains an evaporative cooler and a heat exchanger. The heat exchanger could be air-to-air or air-to-liquid. A cooled fluid (air or water) from the evaporative cooler can be used in the heat exchanger to cool the building air.

SUMMARY

A unit for use in evaporative cooling includes a first capped frame and a second open frame opposite the first frame. A plurality of mechanical supports are located between and coupled to the first and second frames. A porous hollow fiber membrane extends around the supports between and coupled to the first and second frames to form an interior volume. The first and second frames are configured for flow of water between them via the membrane. The membrane is configured to transport the water between the first and second frames and to provide for air flow through the membrane for evaporative cooling.

In one embodiment, the unit produces chilled water through the evaporative cooling process. In another embodiment, the unit performance factors can be controlled by water flow rate, air flow rate, and water temperature and pressure. In another embodiment, the unit can control air humidification. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a front sectional view of a rounded square shape evaporative cooling unit. FIG. IB is a side sectional view of the rounded square shape evaporative cooling unit. FIG. 2 is a diagram of the water path through an evaporative cooling unit.

FIG. 3 is a side view of an air handling unit.

FIG. 4 is a diagram of a water recirculation system for a flat panel evaporative cooling unit.

DETAILED DESCRIPTION

Embodiments include an evaporative cooler using a membrane having hollow fibers with porous walls, which provides enhanced evaporative cooling and reduced pressure drop. This construction includes an array of knitted fibers rolled into an annular circular cylinder, rounded square, or other shapes and potted at both ends to allow flow of liquid water through the fibers. One end of this annular cylinder is open for the passage of air and the other end is capped, which forces the air to flow through the fiber array to cool the incoming air. This construction could provide for ease of manufacturability compared to a folded design. This construction also provides for improvement of the panel performance by systematically increasing the length of the panel. Additionally, adding folds in the fiber array around the cylinder can also improve the performance due to increase in the surface area. This construction with hollow fibers with non-porous walls could also work as a heat exchanger. Using porous walled fibers can also work as a heat exchanger when the air is very humid.

Examples of evaporative cooling units are disclosed in PCT Application Publication No. WO 2023/037287, which is incorporated herein by reference as if fully set forth.

Rounded Square Shape Cooler

FIGS. 1A and IB are front and side sectional views of an evaporative cooling unit 10 panel construction which includes a knitted fiber array using a rounded square shape, as an example. A perspective view of unit 10 is illustrated in FIG. 2. As shown in FIGS. 1A and IB, this panel construction also works for any other cross-sectional shape as well. Unit 10 includes a front open frame 12, mechanical supports such as posts 14, a porous hollow fiber membrane 16, and a capped rear frame 20. Frame 12 is open in that frame 12 has an opening to allow for the passage or flow of air into unit 10. Frame 20 is capped in that frame 20 at least partially, and preferably completely, blocks the passage or flow of air in unit 10. As an optional alternative, unit 10 can include another membrane wrapped around another set of mechanical supports inside of membrane 16 and spaced apart from it. Unit 10 can be portable unit or non-portable.

A liquid such as water flows (22) between front frame 12 and rear frame 20. An air stream or air flow (24) from front frame 12 is forced by rear frame 20 through the fibers of membrane 16 to cool the air. Alternatively, air can flow in the other direction from outside unit 10 to the interior volume. Unit 10 preferably has no core, such that the interior volume is open between the frames, for more effective air flow through the interior volume. The air can be induced into a radial flow through the fibers of membrane 16. Frame 12 can be mounted in a horizontal direction in an air duct, and have mechanical structures for attachment to the air duct, with a fan to pull air from outside through membrane 16.

A plurality of units, each configured as or similar to unit 10, can be coupled together in series via the interior volume as a way to increase the membrane surface area without significantly changing the module design.

Posts 14 extend between and are coupled to frames 12 and 20, either directly or through other mechanical structures. Posts 14 can have optional perforations such as perforation 15. Only a single perforation 15 is shown for illustrative purposes; the posts have multiple perforations while still maintaining the mechanical stability of the posts. The perforations can provide for air flow through the posts. Posts 14 can be connected to one another to provide more support. For example, posts 14 can include an optional cross brace 18 located between frames 12 and 20, such as at a midpoint between the frames or other location. Cross brace 18, or other mechanical connection between posts 14, can divert the air flow through the interior volume of unit 10. One of the standoff posts can optionally be used as a pipe to facilitate the servicing and installation of the unit.

Although the module is shown with 4 posts 14, it can alternatively include more or fewer posts configured to provide different cross-sectional shapes. For example, the module can have only 2 posts to provide for a substantially flat panel of membrane 16, or a plurality of posts arranged along the same plane and configured to provide for a substantially flat panel of membrane 16.

Posts 14 can have a circular cross-sectional shape, as shown, or other shapes such as the following alternatives and options. The posts can be a round comer rectangular bar, for example 0.75 inch X 0.25 inch where each comer is radiused with a 0. 125 inch radius and set at a 45° angle to the circumference for a square. The posts can be a folded post, where a 1.5 inch X 0.125 inch piece of material is folded such that the cross section becomes 0.75 inch X 0.25 inch. A post can be a comer post that is a 0.5 inch X 0.5 inch X 0. 125 inch angle iron “L” shaped piece. One or more of the posts can be a hollow pipe to facilitate all of the water connections on one end (frame), for example.

Posts 14 are preferably constmcted of ABS plastic. Alternatively, the posts can be formed from stainless steel, aluminum, or fiberglass. Frames 12 and 20 are preferably constmcted of ABS plastic. Alternatively, the frames can be formed from PVC, styrene, polycarbonate, or metal(s). Materials of unit 10 can optionally have a Flame Retardant (FR) rating.

Membrane 16 (e.g., a knitted fiber mat) extends around the four posts 14 (e.g., wrapped around) to form an interior volume and can be mechanically held in place between posts 14 and the frames, as illustrated in FIG. 1A, or between an inner and outer frame assembly. Membrane 16 preferably forms a continuous loop around posts 14, as shown in FIG. 1A, to create the interior volume; alternatively, membrane 16 can form a discontinuous loop around the posts. The hollow fibers in membrane 16 are potted at the two ends of the frame. For example, the fibers of membrane 16 can be held in an epoxy in the frame with open ends of the hollow fibers to receive water or other liquid. As another example, the ends of the fibers in membrane 16 can be held by an adhesive, the adhesive can then be cut to open the ends of the fibers, and an end plate can be fixed over the open ends of the fibers. Alternatively, unit 10 can have a frame construction where the framework supports the open end of the hollow fibers, which are then attached to an air handler unit in a system that has water channels for use in circulating the water through the hollow fiber membrane.

Membrane 16 can include multiple layers, for example 27-33 layers wrapped around posts 14. Alternatively, a length of membrane 16 (Lf) can be increased to reduce the number of layers. The membrane is hydrophobic (at least on the inside) for water. Air flows from the front of the panel and through the fibers where evaporation cools the air. The air flow velocity through the fibers is reduced due to enhanced surface area. The following are exemplary parameters for the hollow fiber membrane: a pore size of 0.01-0.2 microns and preferred of 0.03-0.04 microns; a porosity of 25%-80%; a wall thickness (single layer) of 15-75 microns and preferred of 25-50 microns; and a knitting density of 15-65 fibers per inch, or 20-60 fibers per inch, or 35-53 fibers per inch. An example of a hollow fiber membrane is disclosed in U.S. Patent No. 9,541,302. Examples of hollow fiber membranes are also included in the following products: the LIQUI-CEL MM Series Membrane Contactor from 3M Company (product ID B5005009013) and the LIQUI-CEL SP Series Membrane Contactor Cartridge from 3M Company (product ID B5005009016).

FIG. 2 is a diagram of a water recirculation system for evaporative cooling unit 10. A water tank 30 provides water on an intake line 32 to a pump 34, which circulates the water through a water filter 36 to an inlet 38 in frame 12. An outlet 40 on frame 20 provides the water to a water return line 42 back to water tank 30. Alternatively, the water can flow in the other direction with frame 20 receiving the water. Optionally, one frame can include both the inlet and the outlet. The water can have a particular type of quality. The water recirculation system can optionally include an anode/cathode feature to control mineral buildup within the water loop.

For the construction shown in FIGS. 1A and IB, the velocity O of the incoming air is greatly reduced by the enhancement of the area, and the local air velocity going across the fibers is approximately given by

where A is the panel frontal area of the construction, P is the approximate perimeter of the fiber mat and Lf is the length of the exposed fiber. The frontal area for the panel described herein is IF , PF being the length of the side as shown in FIG. IB. The local velocity Lfi can be systematically reduced by increasing L . The effect of other design variables, such as open area post size, can be obtained from numerical simulations or experiments. The velocity reduction factor /L is defined as ratio of the mean local velocity passing through the fiber stack and the air velocity incoming on the frontal face of the panel and is mathematically given by:

The value of 1 is the characteristic feature of the design and is fixed for a given construction. The local velocity of air passing through the fibers is approximately given by

= ME The effectiveness of the panel (hollow fiber membrane) should increase and the pressure drop decrease with decreasing value of . The air cooling effectiveness e is given by:

where is the inlet air temperature, 7^. is the outlet air temperature and

is the wet bulb temperature at the inlet air temperature and relative humidity. The value of quantifies the fraction of maximum available evaporative cooling from the cooling device. The flow of air through the panels can also be in the reverse direction to the one shown in FIG. IB.

EXAMPLES

These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims.

Table 1. Material (rounded square and flat panel)

Table 2. Air Handling Unit (AHU) and Equipment - Set-Up 1

Table 3. Air Handling Unit (AHU) and Equipment - Set-Up 2

EQUIPMENT

Construction of rounded square evaporative cooling module

FIGS. 1A and IB show the evaporative cooling unit. This was constructed by assembling 4 posts (15.5 inch long, % inch diameter - ABS from International Plastics) into bottom and top end frames (7.5 inch inner opening - ABS). An 18.5 inch-wide array of knitted HFPM was wound around the support posts and held in place with a bead of adhesive at the top and bottom edge. The evaporative cooling unit was built with 33 wraps of knitted HFPM. Next, an outer frame (ABS) was attached around both top and bottom frames and potted in place with adhesive. After the adhesive was set, an end cap was attached to each end of the inner/outer frame assembly to form a ! inch water channel that communicated with the HFPM. Tapped ports (1/4 inch NPT) were made on the top and bottom of the end caps to facilitate water flow to and from the evaporative cooling unit. A 1/8 inch aluminum cap was attached to the downstream side frame of the evaporative cooling unit closing off this end of the unit.

Construction of flat evaporative cooling panel

A flat panel module of about 4 in x 17 in x 0.5 in was constructed with 30 layers of knitted HFPM to fit a small scale AHU. Open fiber area was 16 inch by 3.5 inch. The frame was 19.5 inches by 4.5 inches.

Air handling unit set-up 1

In another embodiment, an air handling unit was assembled to test performance of a rounded square evaporative cooler, as shown in FIG. 3. The AHU included a first enclosure 50 mechanically coupled to a second enclosure 52 with an evaporative cooler 54 between the enclosures. A fan 56 is coupled to an end of enclosure 50, and a heat source 58 is coupled to an end of enclosure 52. Evaporative cooler 54 was constructed as described in rounded square evaporative cooling module section above. Table 2 lists the parts description.

Ductwork started at 5 inches diameter circle and expanded to ~ 18 inches by 18 inches square for the enclosures. The second piece was 18 inches by 18 inches for about 7 inches and then narrowed down to a 5 inch diameter circle. A plate with cut-out hole was mounted in the inside center square section of the duct. The evaporative cooling unit (module 54) was mounted on this plate. A blower (fan 56) was attached to the outlet side of the air duct. A hot air gun (heat source 58) was located at the inlet side of the air duct to provide hot air. Air and humidity sensors were located upstream and downstream of the evaporative cooler. The hot air gun was operated on the high setting. Air temperature and humidity sensors were installed at the inlet and outlet of the air duct. Air velocity was measured using a hot wire anemometer from TSI, and air velocities were converted into volumetric air flows based on duct cross section area.

FIG. 2 is a diagram of the water path for once through an evaporative cooling unit, except that in this embodiment the water was not recirculated and no fdter was used. A 10-gallon plastic water tank with stand (30) was used to hold the inlet water. Under the tank, the gravity feed was plumbed with plastic tubing (32) to deliver water to the pump (34). The pump sent water through the inlet flow meter and into the water inlet of the evaporative cooler (38). The water moved through the HFPM to the outlet side of the evaporative cooler (40). Plastic tubing connected the outlet of the evaporative cooler to a 5 -gallon plastic container (outlet water tank). The temperature of the inlet and outlet water tanks were measured using temperature sensors with two k-type thermocouples in each tank. The average value of the two readings were used.

A temperature control unit (TCU) and shell and tube heat-exchanger were used to pre-heat the water before experiments. The water in the inlet tank would recirculate through the heat exchanger until at desired experimental temperature. The lines were then disconnected from the heat exchanger and connected to the inlet and out of the module. The line at the exit of the module was then connected to the outlet tank.

Air handling unit set-up 2

A small air handling testing unit was assembled to test performance of a flat panel evaporative cooler made of HFPM (see FIG. 4). The air duct is shown with the flat panel evaporative cooler module mounted in the center. An air blower was attached to the inlet of the air duct. Heat cartridges with temperature controller were located at the inlet side of the air duct to provide hot air. Air and humidity sensors were located upstream and downstream of the evaporative cooler. Table 3 summarizes the parts used in this set-up.

In FIG. 4, the water recirculation loop is shown with a glass vessel (70), a copper coil connected to a TCU (62), and a pump (72) connected to an evaporative cooling module (66). The air duct (68) has an air inlet, a heater section (64) for providing heated dry air to the module (66), and an outlet section (60) for providing cooled moist air. A coil of copper tubing connected to the TCU was submerged in the water of the glass vessel. The glass vessel was insulated. The TCU was used to heat the water in the vessel to the desired temperature. Two k-type thermocouples connected to sensors were placed in the glass vessel to measure the water temperature. Plastic tubing from the glass vessel was connected to the water pump. The pump sent water through the inlet flowmeter, through a tee containing a K-type thermocouple (inlet water temperature), and then to the inlet of the evaporative cooler unit. The water moved through the HFPM to the outlet side of the evaporative cooler. Plastic tubing connected the outlet of the evaporative cooler unit to a tee containing a K- type thermocouple (outlet water temperature), and the tubing was connected to the water vessel.

This formed a closed system to recirculate the water while also maintaining the water in the vessel at a desired temperature.

Example 1:

Air handling unit set-up 1 was used for this example of a single water pass through a rounded square module. The experiment ran for 10 minutes. Average values from the last 4 minutes are reported, first 6 minutes are considered as transition to reach steady state. The pre-heated DI water was pumped through the evaporative cooler at a rate of 0.3 gal/min. The blower was turned on to setting 3. The air velocity was measured and the calculated volumetric flowrate was 362 ft³/min. The hot air gun was operated on the high setting. The water temperature of the inlet and outlet tanks, change of water temperature, and inlet air and outlet air temperature and relative humidity values are shown in Table 4.

Table 4: Results - Measured Air and Water Properties

This shows that using evaporation of warm water through a rounded square hollow fiber membrane module, the inlet water can be cooled to a lower temperature. Additionally, due to evaporation, the outlet air is cooled as well as humidified. The surface area of HFPM in the module can be optimized to meet specific capacity requirements of a chiller. For example, to increase the length of the interior volume, two rounded square modules could be coupled in series if the end cap of the first module is modified or removed. Example 2:

The Air Handler Unit Set-up 2 was used in this example of water recirculation through a flat panel module and was run for about 1 hour. Representative average values of air and water conditions are reported in Table 5. The TCU setpoint was set to 128.6 deg F and the temperature of the DI water in the water tank measured 102.6 deg F. The heated water was pumped through the evaporative cooler at a rate of 300 mL/min (0.08 gal/min). The water temperature at the inlet and outlet of the module are shown in Table 5. The blower was turned to setting 1 and the variac set to 38. The air velocity was measured, and the calculated volumetric flowrate was 31 ft³/min. The heater was setpoint was 98 deg F. The inlet and outlet air temperature, relative humidity and water temperatures were measured and are shown in Table 5.

Table 5: Results - Measured Air and Water Properties

This shows that using evaporation of warm water through a flat hollow fiber membrane module, the inlet water can be cooled to a lower outlet water temperature.

Example 3:

Sets of operating conditions have been developed that allow for control over air humidification via porous hollow fiber evaporative cooling.

Modules and panels with various shapes, dimensions and characteristics (square module, flat panel, various hollow fiber membrane surface area, etc.) can be used to produce and release humid air. Operating conditions can be optimized based on multiple factors such as inlet air characteristics, hollow fiber membrane surface area, and target conditioned air characteristics. Other factors such as air and water pressure drop can also be taken into consideration.

Air handling unit set-up 2 was used for this example. The TCU for water heating was not used, and water was recirculated. Air flow was set to 70 CFM. Inlet air temperature was controlled to -90-95 deg F. Water flow was set to 0.025 gpm for all samples.

Air relative humidity was measured and captured over several hours. Datapoint were collected every 1 to 5 min. Table 6 recites the experimental conditions and measurements. Table 6: Experimental Conditions and Measurements

Claims

The invention claimed is:

1. A unit for use in evaporative cooling, comprising: a first capped frame; a second open frame opposite the first frame; a plurality of mechanical supports between and coupled to the first frame and the second frame; and a porous hollow fiber membrane extending around the mechanical supports between the first frame and the second frame to form an interior volume, and coupled to the first frame and the second frame, wherein the first and second frames are configured for flow of water between the first and second frames via the membrane, and the membrane is configured to transport the water between the first and second frames and to provide for air flow through the membrane for evaporative cooling, wherein the unit is configured to produce chilled water during the evaporative cooling, where the chilled water output from the unit has a lower temperature than water input to the unit.

2. A unit for use in evaporative cooling, comprising: a first capped frame; a second open frame opposite the first frame; a plurality of mechanical supports between and coupled to the first frame and the second frame; and a porous hollow fiber membrane extending around the mechanical supports between the first frame and the second frame to form an interior volume, and coupled to the first frame and the second frame, wherein the first and second frames are configured for flow of water between the first and second frames via the membrane, and the membrane is configured to transport the water between the first and second frames and to provide for air flow through the membrane for evaporative cooling, wherein the unit performance can be controlled by at least one of a flow rate (air or water), a temperature, or a pressure of the water during the evaporative cooling.

3. A unit for use in evaporative cooling, comprising: a first capped frame; a second open frame opposite the first frame; a plurality of mechanical supports between and coupled to the first frame and the second frame; and a porous hollow fiber membrane extending around the mechanical supports between the first frame and the second frame to form an interior volume, and coupled to the first frame and the second frame, wherein the first and second frames are configured for flow of water between the first and second frames via the membrane, and the membrane is configured to transport the water between the first and second frames and to provide for air flow through the membrane for evaporative cooling, wherein the unit is configured to control at least some air humidification during the evaporative cooling.

4. The unit of any of claims 1-3, wherein the mechanical supports are configured to provide for a substantially flat panel of the membrane.

5. The unit of claim of claims 1-3, further comprising a plurality of the units coupled together in series via the interior volume.

6. The unit of any of claims 1-3, wherein the plurality of mechanical supports comprise posts.

7. The unit of claim 6, wherein one of the posts comprises a pipe for transporting the water.

8. The unit of any of claims 1-3, wherein the membrane has a plurality of layers.

9. The unit of any of claims 1-3, further comprising a pump coupled to the first frame and the second frame for circulating the water through the membrane.

10. The unit of any of claims 1-3, wherein the unit is configured to circulate air from the interior volume through the membrane.

11. The unit of any of claims 1-3, wherein the second frame includes an inlet, and the first frame includes an outlet.

12. The unit of any of claims 1-3, wherein the first or second frame includes an inlet, an outlet, a first channel for the inlet, a second channel for the outlet, and flow separation elements between the first and second channels.

13. The unit of claim 12, wherein the first or second frame includes a continuous channel.

14. The unit of any of claims 1-3, wherein the membrane has a pore size of 0.01-0.2 microns.

15. The unit of any of claims 1-3, wherein the membrane has a porosity of 25%-80%.

16. The unit of any of claims 1-3, wherein the membrane has a wall thickness of 15-75 microns.

17. The unit of any of claims 1-3, wherein the membrane has a knitting density of 35-53 fibers per inch.

18. The unit of any of claims 1-3, wherein the membrane has a knitting density of 15-65 fibers per inch.