Wayne T. Hartshorn
Hart Environmental, Inc., Lehigh ton, Pennsylvania
Evaporative gas coolers use the controlled application of a liquid (usually water) to a hot gas stream to reduce that gas stream's temperature through the evaporation of that liquid. The liquid is often applied in the form of an air-atomized mist or fog.
Typical Applications and Uses
Evaporative coolers are designed to reduce a hot gas stream's temperature to a level suitable for further treatment. They are also used to "condition" the particulate before capture in another device.
When a gas stream requires treatment by a device that is sensitive to gas temperatures as well as gas humidity (such as a fabric filter collector), an evaporative gas cooler is often used to reduce the gas stream temperature to a tolerable level above the saturation temperature. Through the careful application of the liquid, the outlet temperature can be reduced, yet the bulk stream quality can be maintained safely above the water saturation temperature and/or acid dewpoint.
The evaporative gas cooler is sometimes also used ahead of devices such as electrostatic precipitators or spray dryers to temper or condition the gas stream before particulate separation or gas absorption onto a sorbent. For boiler applications, the addition of moisture often favorably reduces the resistivity of the fly ash.
Evaporative coolers are often used as the first stage of a gas cleaning system on hot gas applications such as thermal oxidizers, incinerators, furnaces, cal- ciners, and kilns. Figure 6.1 shows an evaporative cooler (to the right) ahead of a pulse-type baghouse equipped with dry lime injection on a medical
Evaporative cooler on pulse type baghouse (Bundy Environmental Technology, Inc.).
waste incinerator. The evaporative cooler reduces the flue gas temperature to less than 500°F to protect the filter medium in the collector and to reduce the treated gas volume.
Primary Mechanisms Used
Evaporative coolers use the heat of vaporization of a liquid to extract heat from the gas stream and thereby reduce the mixture temperature.
The evaporation rate is dictated by the temperature and humidity differential between the desired outlet gas conditions and the given inlet gas quality. The droplet size produced by the evaporative cooling nozzles or spray system dictates the evaporation time and therefore the physical size of the evaporative cooler.
Over the years, much progress has been made in the further development and improvements of air pollution control (APC) devices, such as electrostatic precipitators (wet and dry), fabric filters (baghouses), scrubbers (wet and dry), as well as other types of collection equipment. However, far less attention has been given to the cooling and conditioning of hot-process gases before being treated in APC devices. Every APC device installed on a high-temperature application is affected in some way by the cooling technique used. Because of this effect, the area of cooling and conditioning becomes significant and indeed important when designing an overall gas handling or pollution control system.
Evaporative cooling can be applied to hot-process gases in many industries and applications. Some of those industries are ferrous and nonferrous metals, rock products, industrial and utility power, and incineration. When evaporative cooling systems are properly engineered, they can provide the most cost-effective method of dealing with increased heat loads from these sources.
Types of Gas Cooling
The three most used techniques for cooling hot-process gases are dilution cooling, convection/radiant cooling, and evaporative cooling. Figure 6.2 shows the effect of evaporative and dilution cooling on resulting gas volume when cooling to 400°F. When selecting an APC device to be installed downstream of the gas cooling system, it is important to note the lower gas volume that results using evaporative cooling versus dilution cooling.
Dilution cooling is the use of ambient air to dilute the total heat content of a hot gas stream so that its resulting temperature is lower, that is, fewer British thermal units (BTUs) per pound of gas.
Convection/radiant cooling implies the use of heat exchanger surface to exchange BTUs from the hot gas stream to a suitable receiver fluid, which is normally air, or water in the case of waste heat boilers. The receiving fluid may be forced across the heat exchanger surfaces by means of fans or pumps, or natural convection currents can be used as in the case of hairpin-type radiant coolers.
Evaporative gas cooling is the use of the heat of vaporization of water to absorb BTUs from the hot gas stream and thus reduces its temperature. Evaporative cooling systems can be either wet or dry, depending on the design and the process requirements.
When we discuss evaporative gas cooling, it is commonly understood that the concept is used to cool hot gases. However, evaporative cooling technology does more than lower gas temperatures.
The term gas conditioning can refer to many processes, but the result is to affect the nature of the gas in some way beneficial to the APC device. The purpose may be to change the gas or dust electrical resistivity, dust surface
Effect of evaporative and dilution cooling (Hart Environmental, Inc.).
conditions, corrosion characteristics, odor, or many other functions. Gas conditioning is accomplished by the addition of water, acid, ammonia, or some other type of chemical. Figure 6.3 shows the effect of moisture added on fly- ash resistivity. Reducing the resistivity of fly ash can improve the performance of electrostatic precipitators.
The basic reason for cooling hot gases is to allow the gases to be collected by conventional APC devices, which have temperature limitations. There are some other reasons, however, which are somewhat less apparent and should be considered in the design of any APC system: to improve collection efficiency of the APC device, to reduce the size of the APC device and associated
Effect of moisture on fly-ash resistivity (Hart Environmental, Inc.).
equipment, to reduce maintenance and thus downtime in the collection and related equipment, to increase production, and to improve reliability and service life of the APC device and components.
Both terms evaporative cooling and evaporative conditioning imply the injection of water into a hot gas stream. The purpose may be to reduce the gas volume by reducing gas temperature, to alter gas or dust properties by changing humidity, or to reduce temperature to allow less-expensive filter materials and/or materials of construction. Whatever the reason, the problems remain the same. Reviewing technologies around the world revealed two general groupings of problems with some types of technologies: problems of original design generally related to the sizing of the equipment, atomizing nozzle type and placement, and ability or inability to turn down; and mechanical and maintenance problems associated with the type of spray nozzles selected and the gas velocities in the systems.
Due to the history and problems associated with evaporative gas cooling and conditioning, efforts were put forth to improve the design and reliability of water spray systems on all industrial applications. Those efforts included a better understanding of why the systems were being used. In some cases, only cooling of the hot gases was required and it was not desirable to affect the properties of the suspended dust particles or gases. In other cases, field experience has shown that the real object of water sprays was to affect the electrical resistivity of the dust particles or gases, and cooling was simply a secondary function. In many industrial applications, the temperature and electrical resistivity level is critical when using a hot/dry electrostatic precipitator as the APC device.
When considering an evaporative gas cooling and conditioning system, one must bear in mind process requirements. Cooling equipment and components can be selected on the following basis: collection or APC device requirements, process outlet temperature, temperature cycles from the process, and the nature of the gas stream.
The first step is to select or determine the type of collection equipment or APC device that will be used for control of emissions. The properties of the emissions, the particulate loading, and the nature of the emissions will affect the type of APC device used. After the collection device has been selected or determined, the operating temperature must be determined. In the case of a dry electrostatic precipitator, electrical resistivity will be a factor. In the case of a baghouse (fabric filter), the type of filter material will be a critical factor; and the maximum temperature of the inlet gases will be a function of materials used and capabilities in the case of a scrubber or wet electrostatic precipitator.
After the collection equipment and the inlet operating temperature are known, the designer must consider the process outlet temperature to determine the amount of cooling required.
Another factor, which is important in the selection, design, and control of the evaporative gas cooling and conditioning system, is the gas temperature profile. Very constant profiles are easier to handle, but rapidly cycling temperatures are more difficult to handle and control. Knowing the process temperature profile will allow the designer to select the right control for the evaporative cooling and conditioning system. The control system or method of controlling rapidly changing temperatures can provide a very constant outlet temperature. It is extremely important to maintain a very constant outlet temperature from a cooling system to protect and maximize the efficiency of the APC device. A meticulously designed evaporative gas cooling and conditioning system can accomplish this requirement.
A rather serious consideration regarding the cooling system design is the effect of the cooling process on the chemical composition of the gas stream. There may be vaporous constituents, which condense at certain temperatures, through which cooling must be affected; and if a plastic phase is involved with that condensation process, extreme fouling or plugging of ductwork or other equipment may result unless cooling is effected rapidly. An intelligently designed and applied evaporative gas cooling and conditioning system can accomplish this rapid cooling or quenching.