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Basic Sizing

There are three fundamental elements necessary for designing and selecting an evaporative hot-gas cooling and conditioning system.

  • 1. A sound understanding of the dynamics of droplet evaporation under varying conditions.
  • 2. Spray nozzles capable of producing extremely fine water or liquid droplets over a wide flow modulation range and with the ability of creating finer droplets with turndown.
  • 3. A control system and overall systems view, which takes full advantage of the design data and modulation capability of the spray nozzles while recognizing and designing for the environment into which it is to be applied.

Evaporative cooling involves the use of fine water sprays to cool a hot gas stream. The cooling section is located between the heat source (furnace or process) and the APC device (dust collector equipment) and, in its simplest form, consists of a straight section of ductwork, or a chamber (usually cylindrical) with spray nozzles inserted through the walls. At times, the inlet gas temperatures exceed the temperature limits of ductwork or chamber steel. When this occurs, refractory-lined ductwork or chambers are used. When chambers are used, they are usually cylindrical and mounted vertically with the gas inlet transition at the top or bottom depending on the overall system design. In all cases, spray nozzles are positioned for maximum gas/water contact.

When using a cooling chamber, one must provide adequate residence time for droplet evaporation. The diameter of the cooling chamber is sized to limit gas velocity from 700 to 1200 feet per minute based on the average gas volume rate at the inlet and outlet.

Water usage calculations are made by performing an energy balance on the system. Using readily available enthalpy tables or specific heat data, the required flow rates of water are calculated for the expected hot gas flow rates.

A close estimate of the water usage requirements to cool hot gases can be made using results shown in Figure 6.4. The calculated data plotted in Figure 6.4 were obtained for the cooling of hot, dry air by water evaporation assuming constant specific heats for air and water vapor. Given the normal degree of fluctuations and uncertainty in the measured hot gas flow rates in industrial practice, the graph shown in Figure 6.4 yields quick information, good enough for most preliminary equipment sizing and design purposes.

More accurate predictions of water usage must be made using enthalpies for each gaseous constituent present in the hot-process gas stream.

One of the major shortcomings of traditional evaporative gas cooling and conditioning systems of old has been the lack of good quantitative data,

FIGURE 6.4

Water injection rates (Hart Environmental, Inc.).

which would allow accurate determinations of residence time. Accurate calculations for residence time determine the size requirements of either the cross-sectional area of ductwork or a cooling chamber and the length necessary to accomplish the total evaporation.

Many suppliers of spray towers base their designs either on data that were available for spray drying from Marshall but were applicable to only a small range of temperatures, or on other parameters that were based on other limited field experience or a good understanding of nozzle geometry, or both.

In 1972, an exhaustive computer analysis of evaporation rates was performed. The study analyzed evaporation rates of various droplet distributions with inlet temperatures ranging from 650°C to 1370°C under various conditions of inlet humidity and velocity The most significant findings of the study are as follows.

  • 1. The largest droplet in each spray distribution required the longest time to evaporate. As simple and intuitive as that sounds, the importance was not previously recognized.
  • 2. An excess of fine droplets in the presence of a few large ones increased evaporation time (te) by lowering the temperature (driving force) surrounding the larger droplets.
  • 3. A determination of residence time cannot be made by a consideration of the largest droplet or the mean droplet diameter alone but must consider the entire droplet distribution. Effective droplet diameter (Dct() for a given distribution is defined as the equivalent droplet of a perfectly homogeneous spray that would evaporate at the same time. The actual value of Deff must be determined from the gas cooling supplier or from experienced gas cooling spray nozzle experience. The formula for determining effective droplet size is shown as follows.

where:

te = evaporation time

Dcff = effective droplet diameter

T, = initial temperature

Ts = saturation temperature

Cj, C2, C3 = constants

Tg = average temperature

The moisture content of the gases to be cooled cannot be neglected in the determination of evaporation time when the outlet temperature approaches Ts as in those cases f(Ts) approaches 0 and tc approaches infinity.

All-Important Atomization

Effective and reliable evaporative gas cooling and conditioning must begin with carefully selected and applied atomization. All atomizing nozzles are not created equal especially when using them for evaporative gas cooling and conditioning.

An atomizing nozzle used in this application (gas cooling and conditioning) should have the following characteristics.

  • 1. Efficiently produce water droplets with small maximum droplet diameters and relatively uniform size distributions (minimum Dctf) at maximum flow rates.
  • 2. It should have a wide-flow modulation characteristic while producing finer droplets with turndown. This is important because evaporation time increases as inlet temperature decreases.
  • 3. It should be designed to minimize maintenance; that is, the nozzle materials of construction and design must be suited to operate and live in aggressive hot gas environments, utilize relatively large liquid ports to minimize internal pluggage, and be relatively self-cleaning to avoid external buildups of gas-laden dust, which would interfere with its atomizing characteristics. Figure 6.5 shows a photo of a heavy-duty gas cooling nozzle.

There are two types of atomizing nozzles that can satisfy the requirements for hot gas cooling and conditioning applications. These nozzles are first and foremost robust in construction, and second, capable of producing the kind

FIGURE 6.5

Heavy-duty atomizing nozzle designed for evaporative gas cooling (Hart Environmental, Inc.).

FIGURE 6.6

External mix nozzle (left); internal mix nozzle (center, right) (Hart Environmental, Inc.).

of droplet size distributions necessary for effective atomization. These nozzle designs are referred to as dual-fluid atomizers. This is where a liquid, usually water, and a compressible gas, usually compressed air, are pumped into the nozzle in combination to supply the liquid and energy for the required atomization. The two nozzles, both dual-fluid types, which will be discussed here, differ in geometry. One is referred to as an external mix device while the other nozzle is an internal mix device. See Figure 6.6 for a photograph of the two types of gas cooling atomizing nozzles.

External mixing is where the liquid, usually water, is introduced externally into the compressed air. Mixing the liquid externally with the accelerated air stream shatters the liquid into exceptionally fine droplets. In the internal mix device, the liquid and compressed air are mixed internally in a multiport fashion before exiting the nozzle outlet orifice.

The external nozzle generally uses more compressed air consumption but can produce turndown capabilities of as much as 20 to 1. The internal mix type nozzle will be more of an energy saver, but the turndown is lower at 10 to 1. Each nozzle design can be produced in many sizes, and both have their strengths and weaknesses. Depending on the process and system requirements, one nozzle type may have some advantages over the other. However, during the selection process, a systems analysis must be completed to decide which atomizing technology is best for a given application. Because both the external and internal mix nozzles do not rely on hydraulic energy to atomize, the liquid ports are relatively large, and wear does not affect performance within broad limits.

A significant advantage of the nozzles presented here is that controlling the ratio of energy to flow with turndown can control the size of the liquid droplets. This is an important aspect of the nozzle selection because it allows the cooling duct or chamber to be sized as a function of maximum temperature conditions without risk of low-end problems. Although other nozzles produce a similar degree of atomization, that is, extremely high-pressure hydraulic nozzles, these nozzles pose mechanical and operational problems, which preclude their general use. They use extremely small liquid ports that plug and wear, are limited in their maximum flow capability, do not offer adequate turndown ratios, and where droplets increase in size as the nozzles are turned down. Furthermore, these nozzles produce higher momentum directional sprays, which impinge on duct or chamber walls creating corrosion and dust buildup problems.

Although the data and spray nozzles provide the major technical components to this gas cooling and conditioning technology, they cannot stand alone. Each component of the overall system must be designed to survive the plant environment, to function through the full range of operating conditions, and to minimize maintenance. Some parameters that should be considered in this technology are as follows.

  • 1. Gas inlet design: Gas flow through the inlet section of a cooling chamber or into a duct section where the spray nozzles are located must be straight to avoid washing of the walls. The use of internal distribution devices should be avoided.
  • 2. Gas velocity: Gas flow direction through the duct or chamber is maintained using relatively high velocity, minimizing the potential of wall buildups.
  • 3. Controls: Processes modulate on a continuous curve, not as a step function. Controls must modulate on the same curve and must be as rapid as the process to ensure that exactly the correct quantity of water is injected at any given time. Depending on several variables such as inlet and outlet temperatures, fluctuations in process conditions, and flexibility and adjustability requirements, the scope of the control can vary. Control schemes will vary from single-loop feedback control to feed-forward/cascading-type control. Some applications may be able to use pressures of both fluids to control flows, while other difficult gas streams require more powerful controls of actual flow measurements and specific algorithms for more precise control. The best control philosophy for a specific application must be determined during the review of the specifications and process conditions. Figure 6.7 shows a typical flow control scheme.
  • 4. Redundancy: In certain critical areas, there is a need to provide redundant equipment to minimize downtime, which affects production. Utilities and measuring devices are generally the most vulnerable components and require the greatest attention.

FIGURE 6.7

A typical process and instrumentation diagram for flow control (Hart Environmental, Inc.).

5. System layout: A good system review and layout will serve to minimize installation costs and maintenance by packaging related materials close to each other, minimizing ductwork and structural steel, and facilitating access.

The system considerations listed previously are universal and do not apply solely to evaporative gas cooling and conditioning systems. Evaporative gas cooling and conditioning systems, whether duct cooling or cooling chambers, have a unique reputation for misapplication of the principle. They typically do not get the attention they deserve compared with the selection process of APC devices. Evaporative gas cooling and conditioning systems are an important and integral part of the gas handling system. They should be coordinated and thoroughly thought out as a key element to the overall performance of any APC system scheme.

What does an evaporative gas cooling and conditioning system look like? Figure 6.8 shows an evaporative cooling tower in use ahead of a baghouse

at an aluminum plant. The evaporative cooler is located at the center of the frame and the baghouse is to the left of center.

Case History Example

One of the earliest applications of improved dual-fluid atomizing nozzles to evaporative hot-gas cooling occurred at a mining company that processes copper ore into a refined product.

The roasting operation resulted in an 1100°F gas, which had as its major contaminating constituent sulfur dioxide and combustible dust particles, which would ignite if the cooling sprays were turned off. Because the dry electrostatic precipitator that was installed at that time could not tolerate temperatures above 800°F, an evaporative gas cooling tower was used to reduce the gas temperature to the desired level and to provide additional moisture for subsequent conversion of sulfur dioxide to sulfuric acid.

The original installation used 10 high-pressure, single-fluid, 350-psig spray nozzles in a vertical cooling chamber 31 feet high by 11 feet in diameter. The liquid droplets produced covered a wide size range, and their velocities were excessive due to the high atomizing liquid pressure. Approximately 15,000 cubic ft/min of hot gas with a velocity of 2.6 ft/s did not provide enough residence time for complete droplet evaporation due to the high velocities of the larger droplets produced by the high-pressure spray nozzles. The large droplets remained in the cooling chamber due to incomplete evaporation or impaction and run-off down the chamber internal walls together with entrained dust buildup, fouling sludge deposits in the collector's hoppers. Frequent shutdowns of the system were required to permit cleaning out the sludge buildup.

The changeover to correct and/or improve the evaporative cooling operation was to three dual-fluid external mix nozzles. The dual-fluid atomizers were adequate to replace 10 high-pressure nozzles that were originally used. The retrofitted dual-fluid nozzles operated at 60-psig compressed air and 58-psig water pressure. The resulting spray of approximately 3.5 gallons per minute (GPM) per nozzle produced droplets that minimized the unevaporated water fallout and sludge buildup. The unscheduled shutdowns due to problems associated with the original high-pressure spray nozzles were eliminated.

Cost Considerations

Experience over the years has indicated that the installed cost of a complete evaporative gas cooling system generally runs about 10% of the complete APC or gas handling system price. Because of this low cost, the evaporative gas cooling system has received much less attention than the APC devices. However, the design, selection, and performance of the evaporative gas cooling system can have a major impact on the success of the overall gas handling or gas cleaning system. This performance can complement the emission as well as the maintenance of the plant's operation.

A complete evaporative gas cooling system will consist of a cooling chamber or duct, the spray nozzles and supporting lance assemblies, air and liquid valve rack trains, compressed air, pumping station, and the necessary piping and wiring to connect the components.

The major consideration and operating cost for a dual-fluid nozzle system will be in the compressed air usage. Compressed air is the second fluid necessary to produce the most efficient atomization and droplet sizes for effective evaporative gas cooling. Depending on the type of nozzle (internal or external mix) applied, the compressed air requirement can be from 4 to 10 standard cubic feet per minute (scfm) per GPM of cooling liquid required. For estimating purposes, it will take about one horsepower of compressed air to produce about 5 scfm.

Evaporative coolers can be quite large because adequate time must be provided to allow the atomized sprays to dry to completion. Figure 6.9 shows a

large evaporative cooler on a dry process cement kiln in use ahead of a dry electrostatic precipitator.

 
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