US20110120693A1

US20110120693A1 - Integrated evaporative cooler and flat plate air to air heat exchanger

Info

Publication number: US20110120693A1
Application number: US11/777,971
Authority: US
Inventors: Larry L. Kammerzell; Donald L. Kammerzell
Original assignee: AZ EVAP LLC
Current assignee: AZ EVAP LLC
Priority date: 2006-07-14
Filing date: 2007-07-13
Publication date: 2011-05-26

Abstract

This invention is an integrated system comprised of a flat plate, air-to-air heat exchanger, ducting and controls, and an AZFlow™ direct evaporative cooler in configurations where the system is able to provide air that has been cooled well below the ambient wet-bulb to temperatures approaching the dew point. These include: A) a two stage cooler where one evaporative cooler performs the stage one cooling by direct evaporative cooling, of the ambient air. This air is directed to the secondary side of the air to air heat exchanger where it indirectly and sensibly cools primary air which is then cooled by a second evaporative cooler which is able to produce the desired cool air approaching the dew point; B) an energy recovery cooling system where building exhaust is collected and directed to the secondary side of the air to air heat exchanger where it indirectly and sensibly cools primary air which is then cooled by an evaporative cooler which is able to produce the desired cool air approaching the dew point; C) an indirect evaporative cooler where an evaporative cooler performs direct evaporative cooling of the ambient air. This air is directed to the secondary side of the air to air heat exchanger where it indirectly and sensibly cools primary air. This sensibly cooled primary air can be used as input to a broad number of applications including but not limited to: inlet air to cooling towers producing water chilled to temperatures approaching the dew point which itself has a broad range of uses, and building makeup air in areas other than the most humid where cooling the air below ambient conditions reduces the energy required to maintain the building environment.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to and claims priority of a provisional application entitled “INTEGRATED EVAPORATIVE COOLER AND FLAT PLATE AIR TO AIR HEAT EXCHANGER SYSTEMS”, filed Jul. 14, 2006, and assigned Ser. No. 60/807,414, disclosing an invention by the present inventor.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to systems and components specifically designed and configured to systematically condition the indoor air of buildings and facilities by evaporatively cooling air and directing this air into the building, the air gains sensible heat from lighting and other heat loads and is then recovered and directed through a flat plat heat exchanger where it is still cool enough to sensibly cool the inlet air prior to its being evaporative cooled. By sensibly cooling the ambient air the dry bulb and wet bulb are lowered such that the system is able to produce desirable cool temperatures even under conditions of relatively high relative humidity. Unique features of the invention include:

- A two stage cooling system that for stage one uses either the cooled exhaust from a building or the discharge of a direct evaporative cooler to sensibly cool ambient air in an air to air heat exchanger before this ambient air is direct evaporatively cooled by an independent evaporative cooler (the second stage). By sensibly cooling the ambient air the dry bulb and wet bulb temperatures of this air are lowered thereby lowering the temperature that the stage two evaporative cooler can produce.
- The design features and fabrication methods for a large surface area, thin plate, and combination cross and counter flow, air-to-air heat exchangers capable of efficiently transferring sensible heat from one air stream to another.
- An integrated system of ducts, fans, and dampers that establish flow paths, control capability, and motive force to direct and route building exhaust or stage 1 evaporatively cooled air to the inlet of the sensible heat exchanger to achieve a controlled temperature air stream out of the stage 2 evaporative cooler adjustable dry-bulb temperature from ambient air to a dry bulb temperature a few degrees above dew point.
- Integration of a control system with features to adjust damper position and fan speed to control air flow and the air flow path in such a way as to control the temperature of air delivered to the conditioned space.

2. Description of Related Art
Indirect, two stage, and energy recovery evaporative coolers principles of operation are discussed in the ASHRAE Handbook and various patents discuss indirect and direct evaporative coolers. The ASHRAE Handbook and Journals are the technical society's comprehensive authoritative source of technical information for Heating Ventilation and Air Conditioning technology such as indirect and direct evaporative coolers and the state of the art of these pieces of equipment. Patents provide additional information on new developments affecting the state of the art.

State of the Art Discussion—2003 ASHRAE Applications Handbook—Chapter 51

The 2003 ASHRAE Applications Handbook—Chapter 51 discusses operating principals for direct evaporative cooling which they define as an adiabatic cooling process. By definition an adiabatic process is one where no heat is added to, or extracted from the process. In this regard, the inlet and exit conditions of the evaporative cooling process have air at unchanged enthalpy since the amount of energy removed from evaporatively cooled air to lower its temperature is equal to energy added in the form of latent heat of vaporization for the water vapor present in the exit air stream. Several types of apparatus cool by evaporating water directly in the airstream including: a) wetted media (Aspen pads and cellulose); b) spray or other wetted air washers; c) sprayed-coil units; and d) humidifiers. Indirect evaporative cooling is defined as a process where evaporative cooling takes place in a secondary air stream with a heat exchanger physically separating this secondary air stream from the primary air stream while transferring heat from the primary air stream to the secondary air stream.
The handbook also discusses indirect evaporative cooling with heat recovery where outside supply air passes through an air-to-air heat exchanger and is cooled by evaporatively cooled air exhausted from the building or application. Applications discussed in the handbook cover a very broad range since evaporative cooling is a proven form of effective cooling for most of the areas where cooling is desired. These applications include: Manufacturing facilities; Wood and Paper products; Mines; Animals; Power-Generation Facilities; Kitchens; Athletic Facilities; Laundries; Office buildings; Schools; Laboratories; Produce; Greenhouses; and Gas Turbines. Gas Turbine performance is affected directly by the inlet air temperature because of the relationship of air temperature to density and mass flow rate. As the temperature of air increases the density and mass flow rate decreases such that the work that must be performed by the compressor must is increased. The output of the Gas Turbine (without turbine air inlet cooling) goes down about 0.4% per degree with the increase in output achieved by using direct evaporative cooling ranging from 5.8% in Albany, N.Y., to 14% in Yuma, Ariz. An average size combined cycle Gas Turbine plant is 50 Megawatts such that the output increase for Albany would be 2,900 Kilowatts installed capacity and 7,000 Kw in Yuma. With an average cost of $800 per installed kw producing this additional power by building additional gas turbine installations would cost $2.3 million for Albany and $5.6 million for Yuma.
Relevance of State of the Art Discussion to Invention:
A direct evaporative cooler forms a key part of this invention. An area of uniqueness not discussed in the state of the art discussions but included in the invention is incorporation of design features in between the stage one evaporative cooler outlet and stage two evaporative cooler inlet designs to limit bypass flow and parasitic heat gain. While the AZFlow cooler is not the only evaporative cooler able to support two stage cooler operations, the better performance of the AZFlow cooler results in better performance of the combination direct and indirect or two stage coolers.

State of the Art Discussion—2000 ASHRAE HVAC Systems and Equipment Handbook—Chapter 19

The ASHRAE HVAC Systems and Equipment Handbook section 19 describes and discusses evaporative air cooling equipment notably the physical principles and general engineered features of direct, indirect, and combination evaporative coolers as well as air washers. The indirect evaporative air cooler is defined in this section as a cooler where outdoor air or exhaust air from the conditioned space passes through one side of a heat exchanger. This air (the secondary airstream) is cooled by evaporation by one of several methods: 1) direct wetting of the heat exchanger surface, 2) passing through evaporative cooling media, 3) atomizing spray, 4) disk evaporator, etc. The surfaces of the heat exchanger are cooled by secondary airstream. On the other side of the heat exchanger surface, the primary airstream (conditioned air to be supplied to the space) is sensibly cooled by the heat exchanger surfaces.
Although the primary air is cooled by secondary air, no moisture is added to the primary air. Hence, the process is known as indirect evaporative cooling. The supply (primary) air may be recirculated room air, outside air, or a mixture of these. The enthalpy of the primary airstream decreases because no moisture is added to it as it is cooled. The usefulness of indirect evaporative cooling is identified as being related to the depression of the wet-bulb temperature of the secondary air below the dry-bulb temperature of the entering primary air. A package indirect evaporative air cooler includes a heat exchanger, a wetting apparatus, a secondary air fan assembly, a secondary air inlet louver, and an enclosure. The heat exchanger may be constructed with folded metal or plastic sheets, with or without a corrosion-resistant or moisture-retaining coating; or it may be constructed with tubes, so that one airstream flows inside the tubes and the other flows over the exterior tub surfaces.
The secondary air is evaporatively cooled with water evaporation taking place on the secondary side of the heat exchanger plates or tubes. With evaporation of water comes the need for a continuous bleed-off and fresh water makeup to keep the concentration of minerals and contaminants in the secondary side of the heat exchanger water from rising. In all evaporative cooling systems water quality should be controlled to avoid scale and other deposits. Water treatment may be necessary to control corrosion of heat exchanger surfaces and other metal parts.
The packaged indirect evaporative air cooler may be either self-contained, with its won primary air supply fan assembly, or part of a built-up or more complete packaged air-handling system. The cooler may include a single stage of indirect evaporative cooling, or it may include indirect evaporative cooling as the first stage, with additional direct evaporative cooling and/or refrigerated cooling stages.
Indirect evaporative cooling has been applied to a number of heat recovery systems including plate type heat exchangers; heat pipe heat exchangers; two phase thermosiphon loop heat exchangers. Indirect evaporative cooling/heat recovery can be used as a retrofit on existing systems, which results in lower operational cost and reduced peak demand relative to standard direct refrigeration systems. For new installations, the refrigeration equipment can be downsized, resulting in lower overall cost of the project as well
Relevance of State of the Art Discussion to Invention:
This document provides general information on the theory and basic principles of design for direct, indirect, and multi-stage evaporative coolers. However, no details concerning configuration and critical design features of the components are provided. Plate heat exchangers and their use in configuring a two stage cooler and the associated psychrometric parameters for a successful design are not discussed.

State of the Art Discussion—2000 ASHRAE HVAC Systems and Equipment Handbook—Chapter 44

ASHRAE Chapter 44—Air-to-Air Energy Recovery discusses heat exchangers and system technology for Air-to-Air energy recovery. These systems are categorized by their application and particularly whether the application is: 1) process to process, 2) process to comfort, and 3) comfort to comfort.
ASHRAE Chapter 44 discusses the theoretical airflow arrangements and effectiveness for plate type heat exchanges as being A) parallel-flow with a 50% effectiveness limit; B) counter-flow where effectiveness can approach 100%; C) cross-flow where the nominal effectiveness ranges from 50% to 70%; and D) Multipass cross-flow where the nominal effectiveness in two passes can achieve 60% to 85% effectiveness. The handbook notes that as a practical matter, design and manufacturing limitations associated with getting air into and out of the heat exchanger have resulted in configurations of transverse or cross-flow.
ASHRAE Chapter 44 provides generic performance data for industry flat plate heat exchangers showing pressure loss in inches of water as a function of face velocity. While the actual numbers are a function of several features and parameters, the general curves show 0.2″ loss at 350 fpm; 0.3″ loss at 450 fpm; 0.4″ at 500 fpm; and 0.6″ at 620 fpm. This data also shows the effectiveness of the heat exchanger approaching 68% effectiveness as it approaches a face velocity of zero and 58% as it approaches a face velocity of 700.
ASHRAE Chapter 44 discusses controls for heat recovery systems where it is stated that heat exchanger controls may function to control frost formation or to regulate the amount of energy transferred between primary and secondary airstreams at specified operating conditions. For example, ventilation systems designed to maintain specific indoor conditions at extreme outdoor design conditions may require energy recovery modulation to prevent overheating ventilation supply air during cool to moderate weather or to prevent over humidification. Modulation may be achieved in a plate to plate configuration by bypassing the heat exchanger with one airstream using face and bypass dampers such that it changes the supply-to-exhaust mass airflow ratio.
ASHRAE Chapter 44 discusses indirect evaporative cooling where exhaust air from a building is passed through water spray where it becomes saturated. As the water evaporates, it absorbs sensible energy from the air, lowering its temperature. This process follows a constant wet-bulb line on a psychrometric chart. Thus, the enthalpy of the airstream remains nearly constant, moisture content increases, and dry-bulb temperature decreases. The evaporative cooled exhaust air can then be used to cool the supply air through an air-to-air heat exchanger. The heat exchanger may be applied either for year-round energy recovery or exclusively for its evaporative cooling benefits.
Indirect evaporative cooling has been applied with flat-plate heat exchangers and other heat transfer methods for summer cooling. Energy recovery is enhanced by improved heat transfer coefficients due to wetted exhaust-side heat transfer surfaces.
ASHRAE Chapter 44 discusses indirect evaporative cooling equipment including Fixed-Plate Exchangers. This discussion states that alternate layers of plates, separated and sealed form the exhaust and supply airstream passages. Plate spacing ranges from 0.1 to 0.5 in., depending on the design and the application. Heat is transferred directly from the warm airstreams through the separating plates into the cool airstreams. Design and construction restrictions inevitably result in cross-flow heat transfer, but additional effective heat transfer surface arranged properly into counterflow patterns can increase heat transfer effectiveness. Plate exchangers are available in many configurations, materials, sizes, and flow patterns. Many are modular, and modules can be arranged to handle almost any airflow, effectiveness, and pressure drop requirement. Plates are formed with integral separators (e.g. ribs, dimples, ovals) or with external separators (e.g. supports, braces, corrugations) airstream separations are sealed by folding, multiple folding, gluing, cementing, welding, or any combination of these, depending on the application and manufacturer.
State of the Art Discussion—U.S. Pat. No. 4,023,949
U.S. Pat. No. 4,023,949 describes an evaporative refrigeration system. In this evaporative refrigeration system air is evaporatively cooled by water with two streams separated such that a cooled air stream is produced without the addition of water vapor. A heat exchanger is discussed which operates by movement of working air internally through tubular conduits counter-currently to water flowing downwardly on inner surfaces thereof while the air to be cooled passes externally across the conduits.
Relevance of State of the Art Discussion to Invention:
While both embodiment ‘949’ and the invention apply the principal that sensibly cooling ambient air reduces the air's wet-bulb and dry-bulb, embodiment ‘949’ is significantly different than that of this invention in that embodiment ‘949’ has evaporation taking place in the inside tubular conduits as a combination evaporative cooler and heat exchanger. The invention uses a modern evaporative cooler (preferably but not constrained to an AZFlow™ cooler) where this cooler has designed to operate effectively and efficiently in an environment of hard water and changing environmental conditions. The invention disclosed herein uses a hybrid cross and counter flow thin plate heat exchanger rather than the tubular heat exchanger of the reference patent. With these innovations the invention addresses issues of mineral buildup, saturation pressure and temperature change, water usage, heat transfer between fluids with significantly different thermal characteristics, complexity and cost of assembly, and component life.
State of the Art Discussion—U.S. Pat. No. 4,137,058
U.S. Pat. No. 4,137,058 describes an indirect evaporating film heat exchanger. In this indirect evaporative film heat exchanger air is evaporatively cooled by water with two streams separated such that a cooled air stream is produced without the addition of water vapor. An embodiment of the patent uses the dry air as input to a power turbine air compressor.
Relevance of State of the Art Discussion to Invention:
While both embodiment ‘058’ and the invention apply the principal that sensibly cooling ambient air reduces the air's wet-bulb and dry-bulb, embodiment ‘058’ is significantly different than that of this invention in that embodiment ‘058’ uses tubular conduits with water flowing in the inside of these conduits as a combination evaporative cooler and heat exchanger. The invention discussed herein uses a modem evaporative cooler (preferably but not constrained to an AZFlow™ cooler) where this cooler has designed to operate effectively and efficiently in an environment of hard water and changing environmental conditions. The invention disclosed herein uses a hybrid cross and counter flow thin plate heat exchanger rather than the tubular heat exchanger of the reference patent. With these innovations the invention addresses issues of mineral buildup, saturation pressure and temperature change, water usage, heat transfer between fluids with significantly different thermal characteristics, complexity and cost of assembly, and component life.
State of the Art Discussion—U.S. Pat. No. 4,156,351
U.S. Pat. No. 4,156,351 describes a depressed wet bulb water cooler that utilizes an indirect evaporative cooling film heat exchanger with general features similar to those described in the patent above. Namely, a heat exchanger that has a wet and a dry side is formed with the surfaces of elongated hollow tubular conduits. Like the discussion above, ambient air is passed through the dry side of the tubes where it gives up sensible heat to the wet side of the tubes. A portion of the air cooled on the dry side of the tubes is routed to the wet side of the tubes where it supports evaporative cooling at a reduced wet bulb temperature. Similar to the patents above, evaporation takes place in the wet channels of the heat exchanger and has an objective to provide air cooled by the heat exchanger.
Relevance of State of the Art Discussion to Invention:
While both embodiment ‘351’ and the invention apply the principal that sensibly cooling ambient air reduces the air's wet-bulb and dry-bulb, embodiment ‘351’ is significantly different than that of this invention in that embodiment ‘351’ uses tubular conduits with water flowing in the inside of these conduits as a combination evaporative cooler and heat exchanger. The invention discussed herein uses a modern evaporative cooler (preferably but not constrained to an AZFlow™ cooler) where this cooler has designed to operate effectively and efficiently in an environment of hard water and changing environmental conditions. The invention disclosed herein uses a hybrid cross and counter flow thin plate heat exchanger rather than the tubular heat exchanger of the reference patent. With these innovations the invention addresses issues of mineral buildup, saturation pressure and temperature change, water usage, heat transfer between fluids with significantly different thermal characteristics, complexity and cost of assembly, and component life.
State of the Art Discussion—U.S. Pat. No. 4,380,910
U.S. Pat. No. 4,380,910 describes a multi-stage indirect-direct evaporative cooling process and apparatus wherein air is cooled through at least three or more stages of direct and indirect cooling. The heat exchanger leading to the second stage has a wet and a dry side that is formed with the surfaces of elongated hollow tubular conduits. Like the discussion above, ambient air is passed through the dry side (outside) of the tubes where it gives up sensible heat to the wet side (inside) of the tubes. The recirculating water from the evaporative cooling cycle is pumped through tubes that are placed in the air stream such that these tubes serve as a stage of air cooling.
Relevance of State of the Art Discussion to Invention:
While both embodiment ‘910’ and the invention apply the principal that sensibly cooling ambient air reduces the air's wet-bulb and dry-bulb, embodiment ‘910’ is significantly different than that of this invention in that embodiment ‘910’ uses tubular conduits with water flowing in the inside of these and tubular conduits where air flows on the inside of the tubes. The invention discussed herein uses a modern evaporative cooler (preferably but not constrained to an AZFlow™ cooler) where this cooler has designed to operate effectively and efficiently in an environment of hard water and changing environmental conditions. The invention disclosed herein uses a hybrid cross and counter flow thin plate heat exchanger rather than the tubular heat exchanger of the reference patent. With these innovations the invention addresses issues of mineral buildup, saturation pressure and temperature change, water usage, heat transfer between fluids with significantly different thermal characteristics, complexity and cost of assembly, and component life.
State of the Art Discussion—U.S. Pat. No. 5,664,433
U.S. Pat. No. 5,664,433 describes an indirect and direct evaporative cooling system. In this evaporative refrigeration system air is evaporatively cooled by water with two streams separated such that a cooled air stream is produced without the addition of water vapor. A heat exchanger is discussed which operates by movement of working air upward through formed plates with water flowing downwardly on inner surfaces thereof while a second stream of air to be cooled passes externally across these formed plates.
Relevance of State of the Art Discussion to Invention:
While both embodiment ‘433’ and the invention apply the principal that sensibly cooling ambient air reduces the air's wet-bulb and dry-bulb, embodiment ‘433’ is significantly different than that of this invention in that embodiment ‘433’ uses tubular conduits with water flowing in the inside of these conduits as a combination evaporative cooler and heat exchanger. The invention in this patent uses a modem evaporative cooler (preferably but not constrained to an AZFlow™ cooler) where this cooler has designed to operate effectively and efficiently in an environment of hard water and changing environmental conditions. The invention disclosed herein uses a hybrid cross and counter flow thin plate heat exchanger rather than the tubular heat exchanger of the reference patent. With these innovations the invention addresses issues of mineral buildup, saturation pressure and temperature change, water usage, heat transfer between fluids with significantly different thermal characteristics, complexity and cost of assembly, and component life.
State of the Art Discussion—U.S. Pat. No. 6,854,278
U.S. Pat. No. 6,854,278 describes an indirect evaporative cooling process and indirect evaporative cooling apparatus employing a dry side channel and a wet side channel. The wet side channel removes sensible heat from the gas on the dry side channel with a portion of this dry side gas being sent on as conditioned gas. The cooling in the wet side takes place as water is evaporated. To lower the temperature of this evaporation process, the remaining portion of the dry side gas that has been cooled is sent to the wet side either through holes in the plate separating the wet and dry sides or through a return passage connecting the dry side to the wet side. As discussed in Chapter 19 of the ASHRAE Handbook, the sensible cooling that is taking place on the dry side of this indirect evaporative cooler lowers both the dry bulb and wet bulb temperature supporting a reduced evaporation temperature of the water in the wet channel resulting in lower dry side gas temperatures.
Relevance of State of the Art Discussion to Invention:
While both embodiment ‘278’ and the invention apply the principal that sensibly cooling ambient air reduces the air's wet-bulb and dry-bulb, embodiment ‘278’ is significantly different than that of this invention in that embodiment ‘278’ has evaporation taking place in the narrow wet channels of the heat exchanger and has the end objective of providing gas cooled by the heat exchanger. In the invention that is the subject of this patent, the air entering the cooling tower is cooled by liquid water that has been cooled by evaporation in the cooling tower and the end objective of the invention is to provide water for cooling at a temperature approaching the ambient dew point.

SUMMARY OF THE INVENTION

This invention is an integrated system comprised of a flat plate, air-to-air heat exchanger, ducting and controls, and an AZFlow™ direct evaporative cooler in configurations where the system is able to provide air that has been cooled well below the ambient wet-bulb to temperatures approaching the dew point. These include: A) a two stage cooler where one evaporative cooler performs the stage one cooling by direct evaporative cooling of the ambient air. This air is directed to the secondary side of the air to air heat exchanger where it indirectly and sensibly cools primary air which is then cooled by a second evaporative cooler which is able to produce the desired cool air approaching the dew point; B) an energy recovery cooling system where building exhaust is collected and directed to the secondary side of the air to air heat exchanger where it indirectly and sensibly cools primary air which is then cooled by an evaporative cooler which is able to produce the desired cool air approaching the dew point; C) an indirect evaporative cooler where an evaporative cooler performs direct evaporative cooling of the ambient air. This air is directed to the secondary side of the air to air heat exchanger where it indirectly and sensibly cools primary air. This sensibly cooled primary air can be used as input to a broad number of applications including but not limited to: inlet air to cooling towers producing water chilled to temperatures approaching, the dew point which itself has a broad range of uses, and building makeup air in areas other than the most humid where cooling the air below ambient conditions reduces the energy required to maintain the building environment.
The unique features of the invention include:

- 1. AZFlow™ evaporative coolers that are the subject of another patent application but are important to this system in that they are able to achieve and maintain performance efficiencies in the 85% to 90% range where other evaporative coolers experience a performance droop after the first month of operation to the 70% to 75% range. These coolers are able to consistently achieve approach temperatures within a couple of degrees of the inlet wet bulb temperature which is very important in this pursuit of cooler air temperatures.
- 2. An air-to-air heat exchanger that brings features to: a) minimize pressure drop with higher velocities to limit power losses and improve heat transfer; b) direct the air across the plates to achieve counter flow conditions for a significant flow fraction to achieve a close approach temperature with limited surface area, c) maintain a constant area flow channel, d) achieve a sufficiently rigid structure with light gauge aluminum plate to minimize vibration and support field assembly e) support assembly of large surface area units with reasonable costs, and f) support rapid assembly with limited labor hours.
- 3. Insulated and sealed ducting and wall panels to preserve the sensible cooling and associated temperature reductions as the air is directed from one area of the apparatus to another.
- 4. Integrated computer control system is deployed to a) monitor ambient conditions, conditioned space temperatures, and the performance parameters of the various system components; and b) compute and manipulate controls to achieve the desired performance from the system that will achieve the desired conditioned space conditions.

Objectives of the System:

A primary object of the present invention is to provide the ability to cool gas turbine plant inlet air temperature in order to recover power production capability lost as the ambient temperature increases over the design point temperature (59° F.).
Another objective of the present invention is to provide an efficient way (considering energy and water) to cool buildings in arid and semi arid areas of the world to comfortable temperatures without the necessity of operating mechanical air conditioning systems.
Still another object of the present invention is to provide a method of extending the environmental conditions where evaporative cooling can provide effective comfort cooling, in particular, those involving increased humidity levels.
A still further object of the present invention is to provide the ability to reduce the peak demands of electricity by reducing the heat load that must be removed by mechanical chillers.
Another object of the present invention is to provide an efficient means of converting a wet cooling tower to one that is able to achieve lower process discharge temperatures and thereby improve the efficiency of various heat cycles.
A yet further object of the present invention is to provide an efficient means of converting a wet cooling tower to one that is able to temperatures well inside the comfort range such that this coolant can be used as a source of radiant coolant in various buildings and homes.
An additional objective of the present invention is to provide the ability to cool facilities without adding moisture to the cooling air stream.
These and other objects of the present invention will become apparent to those skilled in the art as the description thereon proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with greater specificity and clarity with reference to the following drawings, in which:

FIG. 1 is a process flow chart illustrating the relationships of the components for the two stage cooler version of the present invention showing the relationships between the evaporative coolers, flat-plate air-to-air heat exchangers, and the connecting ducting. As characterized by the figure ambient air is first cooled by the stage 1 evaporative cooler with this evaporatively cooled air directed to the primary side of the Air-to-Air heat exchanger where it sensibly cools ambient air lowering both the wet bulb and dry bulb temperatures. This lower temperature air is directed to the second stage of evaporative cooling where it is cooled and delivered to the space to be conditioned.

FIG. 1 a is a psychrometric chart showing the projected performance of the two stage evaporative cooler system of FIG. 1. The starting point for this psychrometric chart is ambient air with these conditions being consistent with those experienced in Phoenix ahead of the monsoon season. The temperatures of 105° F. Dry-Bulb and 67° F. Wet-Bulb were selected as representative of a significant period of time ahead of the monsoon season where the humidity in the air increases significantly. The Stage 1 evaporative cooler cools this ambient air to 73° F. Dry-Bulb following the 67° F. Wet-Bulb line. This is representative of 90% evaporative cooler performance efficiency and is consistent with AZFlow cooler performance. Note that performance of this system is very dependent on the efficiency of the evaporative cooler and that the typical evaporative cooler performance (after a couple of weeks of operation) is approximately 70% to 75%. The evaporatively cooled air from the stage 1 cooler is directed to an air-to-air heat exchanger where it sensibly cools the ambient air being fed to the stage 2 evaporative cooler. The air-to-air thin plate heat exchanger has moist stage 1 cooler air on one side of these plates and sensibly cooled stage 2 inlet air on the other side of the plate. This sensible cooling lowers both the dry-bulb and wet bulb temperatures. In this example case the equipment was sized such that the dry-bulb is lowered to 75° F. and the wet-bulb is lowered to 56.5° F. The stage 1 and stage 2 air flow rates through the heat exchanger are the same in this sample case but can differ with the effect of raising and lowering the stage 2 inlet wet and dry bulb temperatures. In the example case the stage 2 inlet air is cooled by an AZFlow cooler to 58° F. Dry-Bulb. This conditioned air represents a cooling efficiency of 124% and is suitable for multiple applications as it is or it can be mixed with other air sources to achieve desired conditions.

FIG. 1 b is a psychrometric chart showing the projected performance of a two stage evaporative cooler system with inlet conditions experienced in Phoenix during the monsoon season. This chart represents performance derived from the same equipment and equipment arrangement as portrayed in FIG. 1 a above except that the ambient air temperature has been changed to be more consistent with ambient air conditions occurring during the Phoenix monsoon season. Namely, the moisture level in the ambient air is higher as seen in the 41° F. dew point in the first case and 60° F. dew point in the second case. In the first case the delivered air temperature is 56° F. while in the second case it is 67° F. While the delivered temperature for these two cases is clearly different the cooler temperatures achieved under the higher humidity monsoon conditions is clearly significant and supportive of a broad range of applications from facility comfort cooling to process and turbine inlet cooling.

FIG. 1 c is a psychrometric chart showing the projected performance of a two stage evaporative cooler system with inlet conditions experienced in the Houston area. These conditions are hot and significantly more humid with a dry-bulb of 97° F., a wet-bulb of 78° F., and a dew point of 70° F. Under these conditions the stage 1 evaporative cooler operating at 90% efficiency is able to cool the air to a 79° F. Dry-bulb. This 79° F. air is directed through the air-to-air heat exchanger where it sensibly cools the second stage air before it is further cooled in the second stage evaporative cooler. As seen in the psychrometric chart the air-to-air heat exchanger reduces the air temperature from a dry-bulb of 97° F. and a wet bulb temperature of 78° F. to a dry-bulb temperature of 81° F. and a wet-bulb of 73° F. The second stage cools the air to a dry bulb of 74° F. and wet-bulb of 73° F.

FIG. 2 is a process flow chart illustrating the relationships of the components and the process parameters for the building energy recovery version of the present invention showing the relationships between the evaporative cooler, flat-plate air-to-air heat exchangers, the baffles, connecting ducting, fans and controls. As seen in this figure, ambient air is drawn in across the air-to-air heat exchanger where it is sensibly cooled thereby lowering its dry-bulb and wet-bulb temperature with the level of cooling being dependent on the amount of heat removed from the building, the air flow rate, and the arrangement of the air-to-air heat exchanger. The sensibly cooled air from the air-to-air heat exchanger is directed to the evaporative cooler where it cools the air to a temperature close to the air's wet-bulb temperature and directs this cooled air into the building. Once in the building the air mixes with and adsorbs the heat energy available to warm the facility. Namely, the air picks up the energy that is input to the building from sources that vary dependent on their composition and include: a) the lighting load (typically about 5 BTU/hr per sq ft); b) the building sun load (typically about 3 BTU/hr per sq ft of roof area 1.5 BTU/hr per sq ft for an insulated manufacturing building); c) occupancy (typically about 250 BTU/hr per occupant), and d) heat load from equipment in the facility (typically 1.6 BTU/hr per sq ft for an office environment) Using these for an example in a 15,000 square foot facility with 20 foot tall walls one would get a total load in the building of 170,000 BTU/hr. This heat load would result in an approximate 8° F. temperature rise assuming an air supply of 24,000 cfm at 75° F. This temperature rise is sufficiently low that a large difference will exist between the exhaust temperature and that of ambient air. It is this large difference (25° F. during the monsoon season) that makes it attractive to take advantage of this source of relatively cool air to precondition the air supply to the evaporative cooler. For this to be effective requires a heat exchanger that is able to achieve a close approach temperature (Log Mean Temperature Difference) and an evaporative cooler that is able to maintain a high efficiency >85%. This is shown in the psychrometric charts 2 a and 2 b where 2 a shows the performance under early Phoenix summer conditions of high Dry-Bulb (105° F.) and low Wet-Bulb (68° F.) while 2 b shows performance under Phoenix monsoon conditions (100° F.) Dry-Bulb and (74° F.) Wet-Bulb. Under each of these sets of conditions the building temperature is well inside the comfort region.

FIG. 3 is a process flow chart illustrating the indirect evaporative cooler section of the present invention, in particular, showing the relationships between the evaporative cooler and the flat-plate air-to-air heat exchanger to set up a component combination able to evaporatively cool one side of a heat exchanger and sensibly cool the other such that an air stream is produced that is cooled and has the same moisture content as when it started. An illustrative example psychrometric chart showing this cooling performance under early summer (non-monsoon) Phoenix conditions is shown in FIG. 3 a with Phoenix monsoon conditions shown in FIG. 3 b. As can be seen in the two charts a significant dry-bulb and wet-bulb temperature reduction is achieved with this apparatus. Such a temperature reduction is beneficial to a wide variety of HVAC, power generation, petrochemical, and industrial process applications. For example, reducing the wet-bulb temperature of cooling tower inlet air such that the cooling tower discharge temperature approaches the dew point is a significant benefit to these various processes and cycles. Such a temperature reduction is also beneficial in HVAC systems particularly those of schools, commercial buildings, hospitals, and other facilities where high makeup air rates are beneficial and the humidity associated with direct evaporative cooling is not desirable. The indirect evaporative cooler can dramatically reduce the energy requirements and support achievement of the desired makeup air rates.

FIG. 4 is a cut away view of a section of cross-flow heat exchanger with the layer separator and locking strips. The unit is cross-flow in that the air flow streams run perpendicular to one another with a flat plate separating the flow streams.

FIG. 5 is a zoomed in view of the cooler separator strips and heat exchanger plate. The separator strips have an interference fit button with the male end shown in this figure that is threaded through the heat exchanger plate and inserted and snapped into the female side of the button to form a rigid assembly. The buttons have a hole through them that can be used with high strength low elasticity line to lock the assembly into place. This action creates a structure that able to withstand the pressure drop and flow rate without experiencing vibration and leakage normally associated with high air flow rates in plate heat exchangers.

FIG. 6 shows a sectional view of the cross-flow air-to-air heat exchanger inlet and outlet with the separator strips forming an effective entrance and exit configuration for the air flow streams. The typical velocity head loss coefficient at both entrance and exit for a well shaped entrance and exit as shown in the figure is about 3% while that of a square unshaped entrance is more like 90%. The head loss is exacerbated by the high velocity associated with the need to have a reasonably small heat exchanger air flow face area to have the flow stream contact the plate area.

FIG. 7 shows the energy recovery arrangement of a 18,000 cfm unit where building air is pulled out of the building in the duct by the fan on the bottom of the figure. This duct and fan push the building air up the duct between the two parallel heat exchangers to the control damper at the top of these cross-flow air-to-air heat exchangers. Outside air is drawn across and sensibly cooled in the air-to-air heat exchanger and then directed to the evaporative cooler pads where it is cooled and sent back into the building.

FIG. 8 shows the hybrid cross-flow/counter-flow heat exchanger that is able to achieve dramatically closer approach temperatures than the cross-flow heat exchanger. In the hybrid heat exchanger air inlet and exhaust are accomplished in a transition area that is formed to bring the air streams into the heat exchanger in order to have an arrangement where a core area of counter flow heat exchange can be achieved. The separator strips guide the air flow streams into, through, and out of the heat exchanger. These separator strips contain the same leading edge configuration of the cross-flow units in order to minimize entry and exit losses. The buttons on the separator strips perform a similar role in creating a stable structure for installation and operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention is an integrated system comprised of a flat plate, air-to-air heat exchanger, ducting and controls, and an AZFlow™ direct evaporative cooler in configurations where the system is able to provide air that has been cooled well below the ambient wet-bulb to temperatures approaching the dew point. Three configurations with two alternate air-to-air heat exchangers are discussed in this section. These are: a) Two stage evaporative cooling, b) Building energy recovery cooling, and c) Indirect evaporative cooling.
Anyone skilled in the art of cooling system design will both understand the significance and uniqueness of the invention and the broad range of applications and design options that can be deployed to implement the key features and derive the associated benefits. In this regard, the descriptive material presented below is not meant in any way to bound or otherwise limit the embodiment to an approach or set of parameters but is presented only as a way of communicating the critical attributes of the invention.

Two Stage Cooler

The process flow of the two stage cooler is shown in FIG. 1 which was described earlier. The psychrometric charts in FIGS. 1 a, 1 b, and 1 c show the performance of an 18,000 cubic feet per minute (cfm) AZFlow evaporative cooler as the stage 1 cooler, a hybrid air-to-air heat exchanger to use the stage 1 cooling to sensibly cool the stage 2 inlet air, and an AZFlow evaporative cooler as the stage 2 cooler with this discharge air being used in a selected application. One such application is as a module of gas turbine inlet air cooling, another is facility conditioned space cooling, and another is process cooling. The wet-bulb efficiency of this system as deployed in Phoenix in the early summer prior to the monsoon season is 124% with a dew-point efficiency of 74%. Dew Point Efficiency being defined as the ratio of the actual difference between the ambient air inlet dry bulb temperature and the second stage evaporative cooler air outlet temperature divided by the ambient air dry bulb temperature minus the dew point temperature.
The preferred evaporative cooler stages are modularized such that modules can be ganged together to achieve the desired capacity. The preferred module size is selected to be driven by media size and is therefore structured to accept eight sticks of 72″ cellulose media. This module face area results in a design flow rate of 24,000 cfm when the face velocity is held at 500 fpm and a flow rate of 36,000 cfm when the face velocity is increased to 750 fpm. The heat exchanger design and arrangement is driven by the heat transfer to produce the stage 2 product temperature and flow rate. The evaporative cooler efficiency tends to be constant even as the environment changes in humidity and temperature. This results in changes to the dry-bulb temperature out of the coolers. The result of this is that the LMTD changes such that the same heat exchanger surface area is able to transfer more or less heat consistent with the performance of the coolers in response to changes in ambient conditions.
Heat transfer in the heat exchanger follows the relationships:
BTU per hr=(mass flow rate)×(specific heat)×(differential temperature)
BTU per hr=(heat transfer coefficient)×(heat transfer area)×(log mean temperature difference)×(heat exchanger configuration correction factor)
Therefore, effective ways to control the delivery temperature of the system is by incorporating a flat plate air-to-air heat exchanger with particular area and configuration features and/or vary the primary side mass flow rate.
In this regard the heat exchanger area and configuration features are:

- Heat exchanger surface area given a particular heat transfer coefficient and log mean temperature difference
- Cross-flow air flow configuration and number of heat exchanger stages—a single cross-flow heat exchanger will transfer 75% to 85% of the available heat load with two heat exchangers in series would result in transferring >90% of this heat load
- Hybrid heat exchanger configuration with a significant counter flow core area result in transferring >90% of this heat load.
  Stage One Heat Exchanger flow rate adjustment:
- Sizing the stage one evaporative cooler larger than the stage 2 cooler
- Incorporating a Variable Frequency Drive (VFD) on the stage 1 fan to adjust flow through the stage 1 cooler
- Incorporating a heat exchanger bypass damper to spill stage one flow

The key features of the evaporative cooler design to meet the design and performance objectives of the two stage cooler are: a) water distribution features that avoid water entrainment issues, when using face velocities in the range of 500-750 fpm, by not flooding the media but rather metering water onto the media in a controlled manner; b) water metering and rinse features control the level of dissolved minerals on the pad such that the water saturation pressure and temperature are not raised; c) water metering and rinse features to keep the pad clean and the heat transfer coefficient constant in order to consistently perform at efficiencies greater than 85% when the cooler is operated with air velocities between 500 and 750 feet per minute, d) design and fabrication features to limit flow bypassing the media; and e) media wetting characteristics that support rapid and frequent transition from wet to dry operation and shutdown.
A critical component driving performance of the two stage evaporative cooler is the effectiveness of the air-to-air heat exchanger between the stage 1 and stage 2 air streams. This is because the transfer of heat in this heat exchanger determines the wet-bulb and dry-bulb temperatures of the input air to the second stage evaporative cooler. This transfer of heat or sensible cooling in the heat exchanger is performed to exploit a unique property of air which is that cooling air without adding moisture will reduce both the dry-bulb and the wet-bulb.
Lowering the wet-bulb temperature of the air is important since it is this air temperature that governs the outlet temperature and thermal performance of an evaporative cooler. In particular, the temperature of the air will cool as the air's sensible energy is transformed to latent energy with the evaporation of water. As this process takes place the temperature and relative humidity of the air will move along the wet-bulb temperature line in a psychrometric chart until it reaches the dew-point temperature and 100% relative humidity. Based on this characteristic performance of the evaporative cooler, the objective of conducting this sensible cooling is to lower the wet-bulb temperature so that cooler outlet temperatures can be achieved and applied to various processes as well as the cooling of facilities.
As can be seen in the psychrometric charts (FIGS. 1 a, 1 b, & 1 c) the rate of change in wet-bulb temperature per change in dry-bulb temperature reduces as the humidity increases such that at a 60° F. dew point temperature a 20° F. dry-bulb change results in a 5° F. change in wet-bulb temperature while at a 40° F. dew point temperature a 20° F. dry-bulb chance results in a 7° F. wet-bulb temperature change. Therefore, the value, of each degree of dry-bulb temperature drop achieved through sensible cooling, increases as the humidity increases.
The preferred heat exchanger design and configuration to transfer heat and thereby sensibly cool the inlet air to the second stage evaporative cooler is the hybrid thin flat plate air-to-air heat exchanger. This heat exchanger configuration is preferred since the core of this cooler is counter flow and the inlet and outlet form a series of cross-flow exchangers. The plate separators at the edge of the plates are shaped to yield a smooth rounded entrance and exit for both the stage one and stage two gas streams to minimize pressure drop across the heat exchanger and therefore fan power. The preferred material for the plate has properties that include: easily worked for fabrication, good heat transfer coefficient, and sturdy enough to maintain shape when configured as thin plates with air velocities in the 1000 fpm range. One readily available material that meets these criteria is 0.020 inch thick 5052 aluminum. The preferred gap between plates is ¼ inch and the surface area per plate (both sides) is 64 sq ft such that an 11,200 square foot heat exchanger surface fits in an 8 ft wide assembly. The air velocity through the resulting channels is approximately 1100 fpm.
For air flow rates in the range of 18,000 fpm-24,000 fpm the components are all made up of single modules such that the preferred arrangement is to duct these single modules together into a two stage cooler. For some applications such as turbine inlet cooling the second stage connects to a low pressure area like the compressor inlet such that no additional motive power is needed for the second stage. For the first stage a blower on the evaporative cooler is used to create and induced draft across the cooler discharging this air to the air-to-air cooler where it is then exhausted in a manner to limit the chance for this air to reenter the system. This creates a slight differential pressure, across the heat exchanger plates, that is dispositioned by the separation and locking strips. In cases where a greater air flow capacity is desired one has the option of building larger components by joining modules together and then ducting the larger modules together, which is the preferred arrangement, or connecting modules together to create larger modules and then connecting these larger modules together to form the system.
Additional stages involving additional heat exchanger and evaporative cooler can be added to the system to further sensibly cool the air and lower the wet-bulb temperature and dry-bulb temperature.

Building Energy Recovery Cooling

The process flow of the Building Energy Recovery Cooler is shown in FIG. 2 which was described earlier. The psychrometric charts in FIGS. 2 a, and 2 b show the performance of this system under low and moderate moisture conditions that represent real design conditions in the Phoenix, Ariz. area. In this system a hybrid air-to-air heat exchanger is fed the relatively cool building exhaust to sensibly cool the stage 2 inlet air with an 18,000 cubic feet per minute (cfm) AZFlow evaporative cooler serving as the stage 2 cooler which discharges air directly into the building as the source of building makeup and cooling. The cool air that has been added to the building via the evaporative cooler then absorbs and transports energy gained from the normal sources out of the building. Even though this air has absorbed this energy it is significantly cooler than the ambient air and is therefore used as a source of cooling in the hybrid heat exchanger. In a typical 15,000 square foot light office or store front in the southwest, the building heat load gain other than cooling makeup air would be on the order of 170,000 btu/hr (75,000 lighting, 10,000 occupancy, 25,000 equipment, and 60,000 environment).
By sensibly cooling the air before it is cooled in the evaporative cooler the dry-bulb temperature is reduced to 85° F. and the wet-bulb is reduced to 60° F. If this air temperature is too cool it can be mixed with ambient air or some of the building exhaust air can be directed to the atmosphere before it goes through the hybrid heat exchanger. The controls include a damper in the building exhaust that bypasses the heat exchanger and spills exhaust air when the building temperature is too cold
The preferred components used in this application are the same as those discussed above in the two stage cooler section. The difference is that only one stage of evaporative cooling is used and the building exhaust is used as the source of cooling for the hybrid heat exchanger. Additionally, an exhaust fan is used to withdraw air from the building and direct this air to the hybrid heat exchanger. To balance the pressure in the building at a slightly positive pressure in order to limit air in-leakage with associated energy loss and infiltration of dust, the fans are fitted with VFD controls and automatically adjusted based on building differential pressure instrumentation to maintain the volumetric flow rates that will deliver this condition.
This system is able to significantly extend the range of conditions where evaporative cooling is useful and can fully perform the cooling function in offices and commercial facilities without having to run mechanical air conditioning systems thereby saving significant energy resources and avoiding the generation of greenhouse gases.

Indirect Evaporative Cooler

The process flow of the Indirect Evaporative Cooler is shown in FIG. 3 which was described earlier. The psychrometric charts in FIGS. 3 a, and 3 b show the performance of this system under low and moderate moisture conditions that represent design conditions in the Phoenix, Ariz. area. In this system a stage 1 evaporative cooler discharges cooled air to a hybrid air-to-air heat exchanger creating a source of sensibly cooled air with no moisture added that has many uses. One of these uses is preconditioning makeup air for various buildings but particularly those having a requirement for large makeup fractions such as hospitals, casinos, and kitchens to name a few. This is also an effective source of air for improving the performance of wet cooling towers by discharging air that has been sensibly cooled to a lower dry-bulb and wet-bulb to the cooling tower inlet. This enables the cooling tower to produce a chilled water temperature that is closer to the dew point temperature.
The preferred components used in this application are the same as those discussed above in the two stage cooler section. The difference is that no second stage of evaporative cooling is used and the product is the stream of sensibly cooled air that can be generated with the existence of a cooled hybrid heat exchanger.

Claims

1. A multi-stage evaporative cooling system where multiple evaporative coolers are linked in series using an efficient flat plate air-to-air heat exchanger to thermally link the coolers to reduce the wet-bulb temperature at the evaporative cooler inlet and achieve cooler outlet dry-bulb temperatures for more effective cooling.

2. A facility evaporative cooling system that uses an efficient flat plate air-to-air heat exchanger to recapture and apply the cooling energy in the building exhaust to lower the wet-bulb temperature at the evaporative cooler inlet and achieve cooler outlet dry-bulb temperatures for more effective building cooling particularly in higher humidity conditions.

3. An indirect evaporative cooler stage that uses an efficient flat plate air-to-air heat exchanger with an evaporative cooler to create a system capable of sensibly cooling an air stream (cooling without adding moisture) to lower its dry-bulb and wet-bulb temperatures.

4. A hybrid thin plate air-to-air heat exchange apparatus formed by assembling flat plate aluminum sheets using separator strips with interlocking connectors where the strips form a combination cross and counter flow channel boundary between the plates.

5. The hybrid thin plate air-to-air heat exchange apparatus in claim 4 where the separator strips form a rounded edge air stream entry at the edges of the heat exchanger to limit the parasitic pressure loss and associated increase in fan power.

6. The air-to-air heat exchanger of claim 4 where the interlocking connectors are located at approximately 12″ increments on the periphery and on inside channel boundary positions, said interlocking connectors forming a structurally stable heat exchanger.

7. The interlocking connector of claim 6 where high tensile strength low elasticity spectra line is used to secure the heat exchanger plates together by running the line through a hole through the center of the connector such that the connector protects the line from chaffing and maintains the separation distance between the plates.

8. A control system for systems of claim 1 where a sensor and damper are used to control the volume of air flowing through the stage 1 side of the hybrid heat exchanger to control the wet-bulb and dry-bulb temperature exiting the second stage of the system.

9. A control system for systems of claim 2 where a sensor and damper are used to control the volume of air flowing through the stage 1 side of the hybrid heat exchanger to control the wet-bulb and dry-bulb temperature exiting the second stage of the system.

10. A control system for systems of claim 3 where a sensor and damper are used to control the volume of air flowing through the stage 1 side of the hybrid heat exchanger to control the wet-bulb and dry-bulb temperature exiting the second stage of the system.