Patent Application
20110120693
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:
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.
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.
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.
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.
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:
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.
The present invention will be described with greater specificity and clarity with reference to the following drawings, in which:
a is a psychrometric chart showing the projected performance of the two stage evaporative cooler system of
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
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.
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.
The process flow of the two stage cooler is shown in
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:
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 (
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.
The process flow of the Building Energy Recovery Cooler is shown in
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.
The process flow of the Indirect Evaporative Cooler is shown in
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 60807414 | Jul 2006 | US |
