Water is the most common compound on Earth. Control of water temperature, flow rate, pressure, and purity affect virtually every manmade, biological, and physical system. The unique properties of water make it the most frequently used solvent, heat exchange fluid, and cleaning agent in industry. Control strategies are designed around the property of the water used in a given process.
The temperature of water in industrial applications is controlled by process controllers in heat exchangers. Water may be the cooling or heating agent. In a car’s radiator, the high heat capacity of water causes the water to absorb heat from the engine. It then transfers the heat to the ambient air by the movement of air across the radiator. Valve controllers either increase or decrease the flow of water based on the deviation from the desired temperature.
Flow rate of water is a concern in irrigation, processing plants, and cleaning applications. This parameter is controlled by monitoring pressure in closed systems. Because the diameter of the piping is known, pressure readouts can be converted to flow rates. Feedback loops within the electronics of the controller maintain a steady flow rate.
In open systems such as irrigation canals, spinning devices, like a propeller blade, capture the water velocity. By knowing the cross-sectional area of the water delivery system, and monitoring the flow over time, the total volume of water transferred can be obtained. The water control system includes manual and automatic gates, valves, and dams.
Pressure is the control parameter in the processing of many materials, as well as extraction of chemicals by water. The vapor form of water is often the acting agent in these control schemes. Pressure is the basis of this water control system and is adjusted by controlling the vapor flow rate and temperature.
Purity of the water, or concentration of a desired compound dissolved in the water, make up the control factors in another type of water control system. Online control systems monitor the dissolved substance in one of several different ways. The water may be analyzed for changes in electrical conductivity, transparency, acidity, or directly for concentration by compound-specific instruments, such as ion-specific electrodes. Offline control strategies are always used to back up and calibrate online controls, using periodic sampling and analysis.
People have been less successful at water control due to weather conditions. Hurricanes, tsunamis, floods, and other natural disasters wreak enormous havoc and result in the tragic loss of many lives each year. Dams, shore levies, and warning systems that attempt to create a water control system are often unable to prevent enormous destruction.
Biological systems employ multiple water control systems that regulate the flow of water in and out of cellular structures. Each water control system monitors the same properties as its industrial counterparts. Epidermal cells regulate temperature by the movement of water across the skin. The circulatory system uses pressure regulators to control blood pressure and the level of dissolved gases in the bloodstream. Liver and kidney cells control the level of nutrients and toxins in the blood.