AZFlow coolers and Reduced media scaling and water savings
Patent-Pending AZFlow controls are unlike recirculating coolers where water is stored in a sump and the water level in this sump is maintained by a float valve with this sump water pumped to a spray bar where it is distributed over the media to evaporate or return to the sump. AZFlow controls are designed to meter fresh water from the potable water system directly onto the evaporative cooler's media by opening and closing a control valve. The water flows from this control valve through a special valve that assures a constant volumetric flow of water per unit of time is delivered to the media.
With this relationship established, the control system is able to meter controlled quantities of water onto the media by controlling the length of time the control valve is open and the time between openings. The media itself acts as a storage reservoir for water as it absorbs enough water as it is wetted to almost double its dry weight. This media feature is what gives the AZFlow coolers the ability to continue to operate, albeit for a short time period, with the control valve in the off position and water flow stopped. Not only can the cooler continue to operate with the water flow stopped but an air to wet media heat transfer surface is provided which has better heat transfer characteristics resulting in improved cooling capabilities.
The control system on / off times are setup to add water at a rate that accounts for the local water quality, the instantaneous evaporation rate, the media water content, and water needed to have the excess water leave the media at the desired cycles of concentration. The instantaneous water evaporation rate is sensed and computed in AZFlow coolers using installed sensors to monitor inlet temperature and humidity and stored design air flow rate information. The desired cycles of concentration is set into the control system by the system administrator.
The local evaporation rate changes with the wet bulb, dry bulb, evaporative cooler efficiency, and air flow rate. These change dramatically over the course of a day, from day to day, from week to week, and from month to month such that the local evaporation rate can change by a factor of 5 or more over the full season. AZFlow coolers are the only coolers that sense and compute the instantaneous evaporation rate and have the resolution in the control system to achieve a consistent cycles of concentration over the course of this constantly changing cooling season. A solid base of experience has been established with installed AZFlow coolers where they have performed well for multiple seasons using untreated local hard water and running at 10 or more cycles of concentration.
In coolers with a recirculating water system two competing processes are at work that drives the equilibrium concentration of dissolved solids found throughout the system. The first of these is the feed and evaporation process which is at work to increase the concentration of dissolved solids in the system. This increase is occurring since the water that is entering contains dissolved solids and the water that evaporates does not such that if this were the only process the concentration of dissolved solids in the system would continue to build up with time until some mechanism such as scale formation started to take dissolved solids out of the system. The rate of this buildup is dependent on a) the initial concentration of dissolved solids in the system, b) the volume of the system, c) the rate of feed and evaporation, and d) the concentration of dissolved solids in the feedwater. The rate of increase in system dissolved solids concentration is slower in systems with larger volumes and faster where the rate of evaporation increases and /or the feedwater gets harder (contains more dissolved solids). The competing process that is at work to decrease the level of dissolved solids in the system is the bleed process. This is a process where water at the system concentration of dissolved solids is bleed from the system and is replaced by feed water containing a much lower concentration of dissolved solids thereby diluting the concentration of dissolved solids. The rate of decrease in system dissolved solids is dependent on a) the initial concentration of the dissolved solids, b) the volume of the system, c) the rate of feed and bleed, and d) the concentration of dissolved solids in the feedwater. At all rates of feed and bleed the equilibrium dissolved solids concentration will be above that of the feedwater or greater than 1 cycle of concentration. This shows why the ability to set the bleed rate to predict the equilibrium cycles of concentration and therefore minimize scaling on recirculation coolers is dependent on a) knowing the volume of the system, b) knowing the feedwater water quality, and c) knowing the evaporation rate. Those familiar with the performance of recirculating coolers will recognize from the rapid rate and degree of scaling that takes place on these coolers that these factors have not been addressed effectively and that bleed rate control has been ineffective in avoiding media scaling.
Prior to placing an AZFlow cooler in operation a water sample is taken and analyzed to determine the local water quality in terms of the practical scaling index. This experimentally based scaling index is used as the basis for setting the upper limit for cycles of concentration for cooler operations. Once the cycle of concentration limit is determined this value is entered into the control system where it is combined with other sensed and input variables to compute the amount of water to be metered onto the media so that the required volume of water will exit at this level of concentration. For example to achieve 10 cycles of concentration, if the instantaneous evaporation rate as sensed by the temperature and humidity sensors and computed by the control system processor is 9 gallons per hour, 10 gallons of water per hour must be applied to the top of the pad in order that 9 gallons will evaporate and the remaining 1 gallon of water will exit the bottom of the pad. Since this is a very simple system the chemical concentration and volumetric relationships are the same and the minerals in the water discharged is 10 times the concentration they entered the cooler.
Unlike the AZFlow coolers, the guidance provided in operations and care manuals for establishing the rate of feed and bleed in other commercial coolers does not account for feedwater water chemistry, season timing, system volume, and startup and shutdown requirements. Suppliers of commercial recirculating coolers not only do not vary the bleed rate with temperature and humidity; their instructions have you set the bleed rate based on the horsepower of the unit which clearly does not account for these factors, system volume, water quality, or airflow.
Media scaling in AZFlow coolers is minimized by using the control system to control the cycles of concentration as discussed above and by incorporating rinse cycles to assure cooler operations are started and ended with the pads cleaned to one cycle of concentration and to periodically rinse the pads during operation. These rinse cycles are particularly effective on the AZFlow coolers since clean one cycle of concentration water is metered onto the media and the concentration of minerals increases as the water moved down the pad from the top to the bottom reaching the desired concentration just before exiting at the bottom. With this configuration of mineral concentration it only takes a small amount of rinse water to clean the pad. By making sure the pads are wet on startup and by cleaning the pads to one cycle of concentration before shutdown the pads are clean and saturated on startup and cleaned before they are allowed to dry.
Another factor contributing to media scaling is that media in evaporative coolers is a very effective particulate filter for particulate larger than 1 micron and capture of this particulate on the media provides a site for scale formation to start. This is a positive attribute of evaporative coolers in that they improve the quality of the air delivered to the conditioned space and since most airborne city and industrial dust is larger than this 1 micron much of this health challenging dust is removed by the evaporative cooler. While AZFlow coolers flush dust and particulate matter removed from the air down the drain, recirculating coolers capture this material in the sump where it is recirculated and redistributed back to the media where it can speed scale formation.
While somewhat counter-intuitive, the AZFlow cooler is able to minimize scaling while significantly reducing the quantity of bleed water used. The reasons AZFlow coolers are able to minimize scale while reducing bleed water quantities are: a) instantaneous evaporation rate is constantly updated and used as the basis for controlling the water application, b) the metered once through system yields a direct relationship between volume in and out and concentration in and out, c) cooler cycles of concentration limits are based on local water conditions, and d) clean water rinses are used to avoid any buildup. An AZFlow cooler will use 3 to 40 times less bleed water in a season than any other commercially available cooler.
The following table was constructed to compare the volume of bleed water that would be discharged from coolers set up and operating per the manufactures guidance. Since all cooler manufactures supply different sized coolers and the guidance changes as the circumstances and cooler selection change, we normalized the performance of all coolers to 20,000 cfm and a static pressure of 0.75 "W.G. The recirculating cooler was set up on 20% of the maximum expected evaporation rate while the Aspen Pad and the MasterCool were set up based on recommended bleed rate per horsepower. This table shows the projected water savings for one cooler and for ten units. The AZFlow number includes both bleed water and rinse water.
To enlarge table click here.
The water use for cooling with mechanical chillers is equal to or more than that used to cool with evaporative cooling in cases other than that where the mechanical chiller is air cooled and in this case it is not that much different. Several government reports show that more than half of the commercial buildings use water cooled mechanical chillers rather than air cooled mechanical chillers. Water use in the case of water cooled mechanical chillers is greater than used with evaporative coolers alone since the local heat load rejected is greater and the water used in the generation of the electricity to power the pumps, fans, and chiller adds significantly to this water use. To get a feel for this water use consider that electric power plant efficiencies for non combined cycle plants range from 30% to 40% such that 60% to 70% of the power plant's energy is rejected to the environment through wet cooling towers. These wet cooling towers operate between 2 and 3 cycles of concentration. The energy for the chiller alone at 0.61 kW per ton in the case of the 20,000 cfm cooler would require 182,000 gallons of water be evaporated in the power plant cooling tower.
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