It's a Control Issue
By definition, efficiency is the ratio of the energy, or work, output to the energy input. It also is defined as a comparison of the production output with that of cost, in terms of energy, time or money. Increasing efficiency means one of three scenarios needs to be accomplished:
- Increase output while maintaining cost levels.
- Maintain output while reducing cost levels.
- Increase output while reducing cost levels.
It is better said than done in most cases, but there are ways to increase output with little or no additional cost.
Temperature control is one of those ways. It is accomplished by using a mechanical or electrical device that will alter the temperature output through an input. The efficiency of a system that uses some form of temperature control is directly related to how close the actual temperature is to the desired temperature. In other words, if the system can maintain a constant temperature, it is going to be more efficient -- no energy will be wasted in producing the output.
Cooling Tower SystemsFigure 1 shows a basic cooling tower system with a separate pump tank that is split into a hot side and a cold side. The hot-side (or recirculation) pump sends water up to the cooling tower and back to the cold side of the tank.
Without temperature control, the recirculation pump and the tower fan could be on all the time -- wasting electricity and water -- while the tank temperature swings with changes in the outside conditions. By adding a simple electromechanical thermostat to the recirculation pump and tower fan, they can be set up to operate only when the water temperature in the cold side is above the desired temperature. For example, suppose the desired tank temperature is 85oF (29oC). The system can be set up with a thermostat on the recirculation pump set at 85oF and the thermostat on the tower fan set at 95oF (35oC). Only on the hottest, most humid days would the pump and fan run constantly.
Of course, the thermostat settings depend on how quickly the temperature in the cold side of the tank changes, which is dependent on the heat load and the amount of water in the system. They also depend on the duty cycle of the motor so that the motor is not cycled on and off within the time it takes for them to slow to a stop after being turned on.
Going a step further, a variable frequency drive can be used on the tower fan so that the fan only runs as fast as needed to maintain the desired temperature. By doing this, the amount of energy used can be cut to a minimum, therefore reducing the expense of running the fan.
Even in a system in which the cooling tower basin is used as the pump tank, a thermostat can be used to control the fan to reduce the energy cost of running the tower, maintaining a more consistent temperature.
Chilled Water SystemsMost chilled water systems have a temperature control device that will control the compressors turning on and off (hermetic reciprocating and scroll), unloading cylinders (semihermetic reciprocating) or unloading screws. In each one of these cases, as the amount of compression changes, the amount of energy used changes, therefore reducing costs and increasing efficiency. A more complex control system can improve the ability of the system to reach its optimum performance. Many manufacturers have moved from electromechanical devices to full electronic control systems that precisely control system components -- for example, compressors, unloading devices, regulating valves on water-cooled condensers, fan regulation on air-cooled condensers -- to precisely control the temperature of the water being produced.
Process ImprovementAnother way to improve system efficiency is to measure the temperature of the material that needs to be controlled. If this is feasible, measuring the material vs. controlling the liquid temperature will produce quicker results. For instance, the temperature of a mixture of chemicals in a jacketed kettle can be measured to control the amount of cooling fluid that passes through the jacket (figure 2).
Instead of merely controlling the temperature of the fluid flowing through the jacket, the temperature of the chemicals are measured. This helps to compensate for the thermal lag across the kettle wall. This type of application can be used for both exothermic and endothermic chemical reactions.
Adding or improving the use of temperature control can improve the efficiency of most applications that transfer heat from one media to another. Not only does the efficiency of the system increase, but also the process becomes more stable. The payback period to justify the additional expense will be based on energy savings or production increases. It may even pay for itself through reduced maintenance expenses.
Sidebar: Keep It Clean for Fluid Quality, EfficiencyNo matter how much technology is used to control a system, the efficiency of a liquid-circulating heat transfer system is most affected by how well the heat is transferred from one media to another. Keeping the heat transfer surfaces as clean as possible allows the system to perform most efficiently.
The two primary ways to reduce the amount of scale that will attach itself to the surfaces are mechanical and chemical. The mechanical means of keeping the fluid clean is through filtration. Good filters will stop solid particles 5 to 20 micron (1 micron is 0.00004"), but typically these are side-stream devices and may not remove all of the scale. Chemicals can be used in water to help change the physical structure of calcite so that it will not attach itself to other calcite or heat transfer surfaces. These suspended solids then can be removed through mechanical means.
One note regarding cooling tower systems: Biological growth must be controlled to eliminate the possibility of spreading waterborne diseases such as legionnella. Traditionally, chemicals has been used to control the growth. Contact your local water treatment company for more details.