Heat Recovery Devices

When the cooling tower was operating as a heat recovery device, its capacity was considerably higher because of the high temperatures and humidities. In case (a) we had... 

Waste heat Heat from a process that is surplus to requirement this heat may be supplied to a heat recovery device for use in another part of the plant or process. 

Preheater vibration. Air preheaters or any type of waste-heat recovery device designed for horizontal flow across vertical tubes, may be subject to vibration produced by the velocity of gas across the tube banks. The velocity produces a vortex-shedding wave pattern that could correspond to the natural harmonic frequency of the tube bank. If the natural harmonic frequency is reached, excessive vibration of the tubes will occur. Redesign of the internal baffle system by inserting dummy baffles can stop the vibration. 

Heat Recovery Devices The heated air used for seed cotton drying is normally exhausted to the atmosphere after one pass through a tower drier. Since the exhaust air is still relatively... 

The exhaust gases are generally discharged into dust and fume knockdown equipment to avoid contamination of the atmosphere. Gas-cleaning equipment includes cyclones, settling chambers, scrubbing towers, and electrical precipitators. Heat-recovery devices are utilized both within and outside the kiln. These result in an increase in kiln capacity or a decrease in fuel consumption. Waste-heat boilers, grates, coil systems, and chains are used for this purpose. 

A special indirect heat-recovery device is the themosiphon system. The steam produced in the oil cooling section is sent in a closed loop to the oil heating section. The steam will condense there, and the water is returned to the cooling section. 

There are six components that may be important in industrial combustion processes (see Figure 1.16). One component is the burner that combusts the fuel with an oxidizer to release heat. Another component is the load itself that can greatly affect how the heat is transferred from the flame. In most cases, the flame and the load are located inside of a combustor, which may be a furnace, heater, dryer, or kiln that is the third component in the system. In some cases, there may be some type of heat recovery device to increase the thermal efficiency of the overall combustion system, which is the fourth component of the system. The fifth component is the flow control system used to meter the fuel and the oxidant to the burners. The sixth and last component is the air pollution control system used to minimize the pollutants emitted from the exhaust stack into the atmosphere. The first four system components are considered next. 

Heat recovery devices are often used to improve the efficiency of combustion systems. Some of these devices are incorporated into the burners, but more commonly they are another component in the combustion system, separate from the burners. These heat recovery devices incorporate some type of heat exchanger, depending on the application. The two most common types have been recuperators and regenerators that are briefly discussed next. Reed (1987) predicts an increasing importance for heat recovery devices in industrial combustion systems for increasing heat transfer and thermal efficiencies [43]. 

Reed, R. J. "Future Consequences of Compact, Highly Effective Heat Recovery Devices." In Heat Transfer in Furnaces, edited by C. Presser and D. G. Lilley, 23-28, Vol. 

J.C. Ashworth, Energy performance of drying and application of heat recovery devices, in The Scientific Approach to Solids Drying Problems, J.C. Ashworth (Ed.), Drying Research Ltd., Wolverhampton, U.K. (1982). 

Phase 3. The furnace gases may then be directed through some heat recovery device (covered later in this chapter), and maybe through some induced draft device, then finally to the stack. 

Particulates are a pain in many heat recovery devices, but especially in check-erworks and other packed tower type recovery equipment. Dust deposits cause difficulties in furnace operation by choking flow passages, necessitating higher pressure drops to maintain flows of air and poc. The necessary higher pressures can cause leaks of air, poc, and heat through walls and by dampers. 

When the last two sentences are related to heat transfer within heat recovery devices (instead of within furnaces), the low volume and velocity do present concerns with oxy-fuel firing. Heat recovery equipment with larger flow passage cross sections can benefit more from the triatomic gas radiation with oxy-fuel firing. A good example of this is the double-pipe stack or radiation type recuperator. However, they must have parallel flow at the recuperator s waste gas entrance to prevent overheating there. 

At a particular gas velocity, excessive vibration of the lubes can result. Operators have been forced to bypass hot flue gas around the heat recovery device to control vibration. Redesign of the internal baffling by insertion of dummy baffles can stop the vibration by changing the natural frequency of the lube bank. 

Air preheaters, or any type of waste-heat recovery devices that are designed for horizontal flow of fuel gas across vertical tubes, are subject to vibration produced by the velocity of the gas across the tube banks. 

Inserting air filtration, silencing, evaporative coolers, chillers, and exhaust heat recovery devices in the inlet and exhaust systems causes pressure drops in the system. The effects of these pressure drops are unique to each design. Shown in Fig. 6.62 are the effects on the MS7001EA. 

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