A cooling water system comprises a cooling tower, heat exchangers, pumps and all necessary piping to move water through the system. Control of a cooling water system is based on balancing the desire to run the cooling water system at the highest concentration cycles possible without incurring detrimental scaling, corrosion, fouling or microbiological control situations.
A concentration cycle is defined for a specific species as: ##EQU1##
For example, when the specific species is the calcium ion (hereinafter "Ca.sup.+2 " or "Ca.sup.+3 " depending on what context it is used in),: if a cooling water system is running at 500 ppm Ca.sup.+2 with 150 ppm Ca.sup.+2 in the makeup water, the cooling water system is running at 3.3 concentration cycles. In operating a cooling water system it is desirable to achieve the maximum number of concentration cycles to avoid unnecessary loss of water in blowdown as well as unnecessary overfeeding of treatment chemicals including but not limited to, treatment polymers. The maximum concentration cycles for a cooling water system are limited by the undesirable events, such as scaling and corrosion, which occur when the amount of specific species in the cooling water tower reaches a certain level, such that the species contributes to these problems.
There are several currently known ways used to control the concentration cycles in cooling water systems. Controlling the concentration cycles is typically done by controlling the flow rate of "fresh" water (from one or more sources) known as make-up water into the system and by controlling the main flow rate out of the system, referred to as blowdown. In order to control makeup water flow, a pump or valve controls the flow of make-up water into the cooling tower and a level controller is typically used in the cooling tower reservoir or "sump". The level controller is linked to the make-up water pump or valve and when the water in the sump decreases to a point lower than the setpoint for the level controller the make-up water pump or valve is activated.
Conductivity is the typical method of blowdown control. For purposes of this patent application, conductivity is defined as the measurement of electrical conductivity of water with electrical conductivity being present in the water due to ionic species being present in the water. Conductivity can be used to control bleed of blowdown because conductivity can readily be used to estimate the overall amount of ionic species present in the water and a simple controller can be set to open a valve or pump and start blowdown when the conductivity of the reservoir water reaches above a certain setpoint. There are limits to how useful conductivity is for control of a cooling water system as conductivity is nothing more than an indirect measure of the amount of ionic species present. Therefore, conductivity cannot provide information about scaling tendency or actual scaling and use of conductivity can cause "catastrophic failure", where scaling causes the cooling water system to overcycle and scale further.
Alternatively, a timer can control bleed of blowdown without actually measuring any of the specific species in the water. In addition to or in place of the above control schemes, water flow meters on the make-up and blowdown can be used, in conjunction with a microprocessor controller to do an overall cooling water mass balance.
A problem with these known control schemes, is that when the blowdown is controlled by conductivity and the make-up is controlled by a level controller, if the composition of the usual make-up water is variable, or if there are alternate sources of make-up water that are significantly different from the usual make-up water source, level controllers and conductivity cannot account for all events that are occurring in the system. In these cases, the cooling water system is typically controlled by the operator being conservative with the conductivity setpoint which thus causes extra undesirable expense due to non-optimal use of treatment chemicals and water.
Many cooling water systems use treatment products to control undesirable events such as scaling, corrosion, fouling and microbiological growth. These treatment products comprise polymers and other materials and are known to people of ordinary skill in the art of cooling water systems. A cooling water control system can be set up to feed treatment product based on either a bleed/feed mechanism where the action of blowdown triggers a chemical feed pump or valve that feeds treatment product; or, in the alternative, the cooling water control system feeds treatment product based on timers using a "feeding schedule" or flow meters on the make-up water line trigger the pumping of treatment product based on a certain amount of make-up water being pumped. A limitation of these control methods is that none of these systems measure the treatment product concentration directly online, so if there is a mechanical problem, for example, if a pump fails, a drum empties, or high, low or unknown blowdown occurs, system volume changes or makeup water quality changes; the correct treatment product concentration is not maintained. Because this problem is common, typically cooling water systems are either overfed to ensure the level of treatment product in the system does not drop too low as a result of high variability in product dosage or the treatment product is unknowingly underfed. Both overfeeding and underfeeding of treatment product are undesirable due to cost and performance drawbacks.
One aspect of known control schemes is an inert fluorescent chemical being added to the cooling water system in a known proportion to the active component of the treatment product feed and the use of a fluorometer to monitor the fluorescent signal of the inert fluorescent chemical. This is commercially available as TRASAR.RTM.. TRASAR.RTM. is a registered trademark of Nalco Chemical Company One Nalco Center, Naperville, Ill. 60563 (630) 305-1000). The fluorescent signal of the inert fluorescent chemical is then used to determine whether the desired amount of treatment product is present in the cooling tower (and to control blowdown).
Many current cooling towers use inert fluorescent tracers to control the treatment product level in the system and also use a conductivity controller to measure the conductivity in the water.
Cooling towers that use both inert tracer(s) and conductivity typically use the following types of information in order to control the tower. For example, a cooling tower with typical makeup water having: 150 ppm Ca.sup.+2, 75 ppm Mg.sup.+2, and 100 ppm "M alkalinity"; with a conductivity of 600 .mu.S/cm (note that conductivity is expressed in units of microsiemans per centimeter), is set to run at 500 ppm Ca.sup.+2. In order to operate within acceptable levels, the cycles of concentration for this cooling water system are 3.3 (calculated by dividing 500 by 150). Running the system at 500 ppm Ca.sup.+2 corresponds to a conductivity setpoint of 3.3 times 600 or 1980 .mu.S/cm. When the conductivity exceeds this setpoint the system is configured to automatically blowdown a portion of "concentrated" cooling water ("concentrated" referring to system water with an unacceptably high level of ions) which is replaced with "fresh" makeup water (where "fresh" is defined as having a lower level of scaling ions than the "concentrated" cooling water). This decreases the conductivity and hardness (Ca.sup.+2 and Mg.sup.+2) ions via dilution. Dilution also reduces the amount of inert tracer chemical in the system. Decreasing the amount of inert tracer in the system decreases the fluorescent signal from the inert tracer. When the fluorescent signal from tracer decreases, the tracer control system is set up to feed a fresh mixture of treatment product and inert tracer chemical to makeup for the decrease in fluorescence that was lost in the blowdown.
A known method of control of product feed to a cooling water system involves the use of another aspect of tracer technology. This involves using a treatment product containing a polymer that has been "tagged" with a fluorescent moiety. These tagged treatment polymers are not inert, rather, they are supposed to be consumed as they function to treat whatever performance-related condition it is that they are designed to treat. Thus, by measuring the fluorescent signal of the tagged treatment polymer it is possible to determine the amount of consumption of the tagged treatment polymer. By knowing the amount of consumption of the tagged treatment polymer it is possible to use that information to control the feeding of new treatment product containing tagged treatment polymer.
A recent reference in this area is entitled "High Cycle Cooling Tower Operation: Hurdles and Solutions", Hoots et al, pp. 388-397, was presented at the 60.sup.th Annual Meeting of the International Water Conference held on Oct. 18-20, 1999.
New methods and techniques for control of cooling water systems are always desirable.