Staged dry out control for evaporative media systems

A staged dry out process and control system for an evaporative media cooling system having a plurality of media stages that are selectively activated and deactivated by a control system is disclosed. The staged dry out process ensures that wet media stages are appropriately dried with minimal disruption to the staging strategy implemented by the control system. In one aspect, the staged dry out process monitors deactivated media stages to determine if the media stages reach a dry state before being activated. In another aspect, the staged dry out process locks out a media stage that has been in a wet state beyond a predetermined maximum time limit until the media stage attains a dry state. With this strategy the cooling system can operate without being required to completely shut down for a drying process.

BACKGROUND

Evaporative media systems, for example direct evaporative coolers, are frequently used in commercial and industrial HVAC systems, including applications for data centers and power plant turbine inlet cooling. Evaporative media systems consume less energy than conventional cooling equipment and are increasingly being used to supplement and occasionally replace conventional cooling equipment. In operation, evaporative media systems use the enthalpy of vaporization of water as a means to cool and humidify air. Typically, this is accomplished by flowing air directly through a media wetted with water. As air passes through the wetted media, water evaporates by taking energy from the air to vaporize the water. Accordingly, the air temperature exiting the wetted media is reduced and the humidity is increased while the energy or enthalpy of the exiting air remains the same as the entering air. This type of a process is often referred to as adiabatic cooling.

Evaporative media systems typically use a water pump to transfer water in a tank below the media to the top of the media. The water flows down through the media where a small portion of the water evaporates and a relatively larger portion drains out the media bottom into the tank below. The water continues to be recirculated using the water pump, or re-circulation pump, with make-up water added to replace the evaporated water. Tank water is periodically drained and replaced with additional make-up water to control tank water concentration and minimize scale fouling, biological fouling and corrosion.

The air flowing through an evaporative media system allows for the introduction of algae. Additionally, the continuously wet environment may allow that algae to propagate. If measures are not taken, algae can fowl the media and contaminate the air being conditioned by the system. To prevent the continuous growth of algae, it is common practice to allow the media to completely dry out at least once in any given 24 hour period in a dry out cycle. However, by requiring the media in an evaporative media system to completely dry out, the system is necessarily inactive and no longer flowing water. Thus, the desired cooling and humidification effects are temporarily lost during the dry out cycle. The length of a dry out cycle, and subsequent delay in desired output, is dependent on incoming air conditions. The dry out cycle is often a scheduled event that occurs regardless of past system output. Improvements are desired.

SUMMARY

A staged dry out process and control system for an evaporative media cooling system having a plurality of media stages that are selectively activated and deactivated by a control system is disclosed. One step of the process includes monitoring the condition of each media stage to determine whether the media stage is in a wet state or a dry state while another step includes starting a wet condition timer for each stage that is in a wet state. In one step, each media stage is monitored as being activated or deactivated by the control system. For any deactivated media stage, the process can include assigning a demand based drying status to the media stage as long as the media stage is in a wet state and assigning a dry status to the media stage if or when the media stage reaches a dry state. For any activated media stage that has not been in a wet state for greater than a maximum predefined time period, the process may include assigning a wet status to the media stage. For any activated media stage that has been in a wet state for greater than a maximum predefined time period, the process may include locking out the media stage from activation by the control system until the media stage has reached a dry state and then assigning a dry status to the media stage. In one embodiment, the maximum predefined time period is 24 hours. In one embodiment, the control system is a demand based control algorithm that activates and deactivates the media stages as necessary to satisfy a cooling demand. In one embodiment, only one media stage is allowed to be locked out at any given time to ensure that the evaporative media cooling system can stage or activate all remaining stages that might be needed to satisfy the cooling demand.

DETAILED DESCRIPTION

General Evaporative Media System Description

Referring toFIG. 1, an air handling system1comprising an evaporative media system10is shown.FIG. 2shows a three stage version of the evaporative media system10in additional detail. As shown, the air handling unit may be additionally provided with a supply fan5, a damper section6, a filter7, a heating coil8, and a cooling coil9. It should be understood that various other components and alternative configurations may be applied to air handling system1without departing from the concepts disclosed herein. In operation, the supply fan5draws air through the evaporative media system10to result in adiabatically cooled air when the evaporative media system10is activated.

In one aspect, the evaporative media system10shown atFIG. 2includes an evaporator tank14having a sidewall15and a bottom side17that together define an interior volume11for holding a fluid12, such as water. As shown, the tank14defines a single compartment with a single interior volume11for holding a fluid12. The sidewall15may have various cross-sectional shapes as dictated by the requirements of the evaporator and air handling unit, for example square, rectangular, and circular cross-sectional shapes. The bottom side17may also be provided with various shapes to accommodate the perimeter defined by the sidewall15.

The storage tank14may be provided with a drain opening16located in one of the bottom side17and the sidewall15. In the particular embodiment shown, the drain opening16is provided at the bottom side17of the tank14. In one aspect, a drain valve30is provided to selectively drain water from the tank14while a fill valve40is provided to selectively add water to the tank14. The drain and fill valves30,40may be provided as automatic control valves operated by a controller, such as electronic controller500discussed below.

As presented, evaporative media system10also includes a plurality of media stages4A,4B,4C through which air is drawn via the operation of fan5.FIG. 2shows a three stage system having stages4A,4B,4C of a generally equal size and capacity. It should be appreciated that the evaporative media system10may include fewer or more media stages of same or different sizes without departing from the concepts disclosed herein. Furthermore, each media stage may include multiple subsections of media. As shown, each media section4A,4B,4C is separated from the other by a gap, or alternatively a frame or barrier to prevent moisture from communicating from one section to the other. This configuration allows for an individual media section to be dry out without being subjected to wicking moisture from an adjacent section.

Each individual media stage4A,4B,4C is shown as being provided with an individual corresponding distribution pump3A,3B,3C. A spray distribution apparatus2A,2B,2C is in fluid communication with each pump3A,3B,3C such that each pump3A,3B,3C can deliver fluid12, such as water, from the storage tank14to a spray distribution apparatus2A,2B,2C to wet the associated media stage4A,4B,4C. One suitable pump for pumps3A,3B, and3C is a Little Giant F-Series F10-1200 (manufactured by Franklin Electric of Oklahoma City, Okla.). This type of pump has a wet rotor design without a shaft seal to separate the motor from the pump wherein water circulates around the armature.

In operation, when a pump3A,3B,3C is activated (e.g. turned on or modulated to a speed greater than zero), the associated media stage4A,4B,4C is wetted with fluid12. When a media stage4A,4B,4C is being actively wetted with water, for example when the associated pump3A,3B,3C is in operation, that media stage4A,4B,4C can be referred to as being activated. Likewise, when a media stage4A,4B,4C is not being actively wetted with water, for example when the associated pump3A,3B,3C is shut off and not in operation, that media stage4A,4B,4C can be referred to as being deactivated.

Control System

Referring toFIG. 2, the evaporative media system may also include an electronic controller500. The electronic controller500is schematically shown as including a processor500A and a non-transient storage medium or memory500B, such as RAM, flash drive or a hard drive. Memory500B is for storing executable code, the operating parameters, and the input from the operator user interface502while processor500A is for executing the code. The electronic controller is also shown as including a transmitting/receiving port500C, such as an Ethernet port for two-way communication with a WAN/LAN related to an automation system. A user interface502may be provided to activate and deactivate the system, allow a user to manipulate certain settings or inputs to the controller500, and to view information about the system operation.

The electronic controller500typically includes at least some form of memory500B. Examples of memory500B include computer readable media. Computer readable media includes any available media that can be accessed by the processor500A. By way of example, computer readable media include computer readable storage media and computer readable communication media.

Electronic controller500is also shown as having a number of inputs/outputs that may be used for implementing desired operational modes of the evaporative media system10and/or the air handling system1. For example, electronic controller500provides outputs for commanding individual evaporator stage pumps3A,3B,3C such that they can be staged as needed to meet the output demands of the system10(e.g. a leaving air temperature or relative humidity set point). Controller500may also provide outputs for controlling the tank fill valve40, an output for controlling the tank drain valve30, and an output for controlling a circulation/drain pump20. Status inputs can be provided for each of the aforementioned control components as well. Additionally, inputs for entering and leaving air temperature and humidity, outdoor air temperature and humidity, tank water level, tank water temperature (which can serve as a proxy for entering and leaving air wet bulb temperatures), and an airflow switch (or a fan status input signal) may be provided as well. The controller500can also include the necessary inputs and outputs for desirable operation of the remaining components of the air handling system1, for example, inputs and outputs to operate the fan5, damper section6, and the coils8,9.

In one aspect, the controller500may be programmed to operate with a demand based control algorithm that activates and deactivates the media stages as necessary to satisfy a cooling demand. By use of the term “demand based” it is meant to include any algorithm which selectively activates and deactivates stages or groups of stages to meet a current output demand setting or load of the system1. For example, a demand based algorithm could be an algorithm that activates and deactivates the stages to satisfy a temperature set point or relative humidity set point for the air leaving the system1via fan5. It is noted that a single pump serving individual valves associated with each stage may be provided, wherein valves replace pumps3A,3B,3C. The valves may be either modulating valves or two-position type control valves, depending upon application.

Staged Dry Out Process Description

Referring toFIG. 4, an example of a staged dry out process1000in accordance with the disclosure is presented. Staged dry out process1000operates to eliminate or greatly minimize system shut downs due to dry out requirements, minimizes interruptions in desired system output, and better ensures acceptable system output during required dry out periods.

It is noted that although the figures diagrammatically show steps in a particular order, the described procedures are not necessarily intended to be limited to being performed in the shown order. Rather at least some of the shown steps may be performed in an overlapping manner, in a different order and/or simultaneously. It is also noted, that the described process steps can be performed with respect to individual media stages or with media stages placed in groups (e.g. pairs or larger groups), which may or may not include the same number of media stages. Furthermore, the process shown inFIG. 4is exemplary in nature and other steps or combinations of steps may be incorporated or altered without departing from the central concepts disclosed herein.

In one aspect, the algorithm is started or initialized, for example when the system is enabled, and includes an update block1002wherein the status updates are received. In a step1004, an evaluation is determined as to whether the stage should be enabled. Where the stage should not be enabled, for example, when the stage is locked out in a forced drying mode or is otherwise unavailable or not needed, the algorithm proceeds to step1008where the stage is moved to a disabled state or held in a disabled state. Where the stage should be enabled, the system enables the stage and sets the status of the stage, StageStatus equal to “wet” at a step1006. Additionally, once the StageStatus changes from “dry” to “wet” a wet timer, WetTime, is initiated at step1010to track the duration for which the media stage has been in a non-dry state according to the selected parameters utilized for the determination at step1010.

In a step1010, it is determined whether the condition of the media stage is in a dry state. The determination as to whether the media stage is in a dry condition or state can be accomplished with a variety of approaches, for example, by comparing the entering and leaving air temperatures through the media stage. Dry out times can be determined in at least two ways. For example, when the change in temperature between the incoming air and outgoing air through a particular stage are the same (+/−3 degrees), the media stage can be considered to be dry. Alternatively, the media stage can be considered to be dry after a specified amount of time with no addition of water (2 hours), which would equate to the status of the media stage not being in a “wet” condition for at least the specified period of time.

If the media stage is assessed to be in a dry condition at step1010, a step1012is implemented in which the status of the media stage, StageStatus, is set to “dry” and the wet timer, WetTime, is set to zero. Subsequently, the process returns to step1002for that media stage. If the media stage is assessed to be in a wet condition at step1010, the wet condition timer is updated at step1014.

At a step1016, the current wet timer value, WetTime, is compared against a maximum wet time value, MaxWetTime. In one example, the maximum wet time value is set to 24 hours. If the timer has not reached the maximum wet time value, the process simply returns to steps1002and1004for an evaluation as to whether the media stage should be enabled. As long as the stage is and should be enabled and the wet condition timer, WetTime, has not reached the maximum wet time value, MaxWetTime, the process will loop through steps1002-1006,1010,1014, and1016.

Once the wet condition timer, WetTime, for a media stage has reached or exceeded the maximum wet time value with the stage enabled, the process moves to step1018. At step1018, it is evaluated whether any other stage has been locked out of operation. By use of the term “locked out” it is meant to include any condition in which the associated control valve and/or pump for the media stage being evaluated is prevented by the control system from operating in the normal sequencing of the system such that it is not possible for the media stage to be wetted with water and is allowed to dry out. If another media stage is locked out of operation, then the process returns to steps1002and1004for continued monitoring of the media stage activation status and continued running of the wet condition timer. As long as the stage is activated, the wet condition timer is at or beyond the maximum time period, and another stage is locked out, the process will loop through steps1002-1006,1010, and1014-1018.

If no other media stage is locked out, or once no other media stage is locked out, the process is allowed to move to step1020wherein the status for the media stage is set to “forced dryout” and the media stage is locked out from operation. The “forced dryout” status is an indication that the media stage is being forced to dry out to a dry condition because the stage has been active with a “wet” status for at least the maximum time value. With reference to the evaluation at step1018regarding whether another stage is in a “forced dryout” condition, the determination at step1018may alternatively evaluate the status of the other media stages to determine if any other media stage is in a “locked out” condition. In either case, the process could easily be modified to allow more than one media stage to be in a “forced drying” condition. For example, step1018could determine whether a maximum number of other media stages are in the “forced dryout” mode. Alternatively, step1018could be eliminated such that every media stage could be placed in the “forced dryout” mode.

Once the media stage has been placed in the forced dryout mode at step1020, the media stage remains disabled until the media stage has been in the forced dryout mode for a predetermined period of time or until the stage has attained a dry condition in the same manner as at step1010. In one example, once the locked out stage has reached a dry state or a predetermined time period has expired, the status of the media stage is set to “dry” and the wet condition timer for that stage is stopped and reset to zero, similar to the actions at step1012. Once the forced dryout mode has concluded, the media stage is unlocked and the process can return back to step1002for that stage.

In practice, the algorithm1000allows for disabled stages that are not needed to meet the system demand to be monitored as they transition from the wet state to the dry state. These disabled stages can be characterized as being in a “demand based drying” mode, wherein such status is an indication that the media stage is not needed at that time to maintain the desired output of the evaporative media system, and is thus inactive or offline for that reason. As the control loop for each stage continually passes through step1010, the stage(s) in the demand based drying mode are evaluated for whether the media stage(s) has attained a dry condition, thereby avoiding the need to lock out the stage(s) in the forced dryout mode.

The above described process1000, ensures that the evaporative media system will never be forced into a zero output condition to fulfill a dry out requirement, unless desired. Rather, only one stage (or a selected number of stages) is permitted to go through a forced dry out cycle at any given time. If one media stage reaches the maximum wet time period while another is in a forced dry out cycle, it will continue operation until a time when it will be the only stage in a forced dry out cycle. While required dry out cycles are being fulfilled, the system will continue to operate with the remaining stages by turning on the minimum number of stages needed to meet or exceed the demand. If demand cannot be met, all remaining unlocked stages will remain turned on. It is also possible that the cyclical demand cycle (e.g. system load) is such that none of the media stages must be placed into a “forced drying” status. During such times, system output will not be interrupted as long as this appropriate cyclical demand cycle continues.