Heat recovery systems

Smith, R., and Jones, P. S., The Optimal Design of Integrated Evaporation Systems, Heat Recovery Systems and CHP, 10 341, 1990. 

The policy for waste heat recovery from the flue gas varies between incinerator operators. Incinerators located on the waste producer s site tend to be fitted with waste heat recovery systems, usually steam generation, which is fed into the site steam mains. Merchant incinerator operators, who incinerate other people s waste and... 

The steam generator is a balanced draft, controlled circulation, multichamber unit which incorporates NO control and final burnout of the fuel-rich MHD combustion gases. The MHD generator exhaust is cooled in a primary radiant chamber from about 2310 to 1860 K in two seconds, and secondary air for afterburning and final oxidation of the gas is introduced in the secondary chamber where seed also condenses. Subsequent to afterburning and after the gas has been cooled down sufftciendy to soHdify condensed seed in the gas, the gas passes through the remaining convective sections of the heat recovery system. 

Many units have waste heat recovery systems that generate low pressure steam from reaction heat. Such steam is often employed to drive adsorption refrigeration units to cool the reactor feed stream and to increase polymer conversion per pass, an energy-saving process that reduces the demand for electrical power. 

As a general rule, the optimum number of effects increases with an increase in steam cost or plant size. Larger plants favor more effects, partly because they make it easier to install heat-recovery systems that increase the steam economy attainable with a given number of effects. Such recoveiy systems usually do not increase the total surface needed but do require that the heating surface be distributed between a greater number of pieces of equipment. 

The code provides for the testing of gas turbines supplied with gaseous or liquid fuels (or solid fuels converted to liquid or gas prior to entrance to the gas turbine). Test of gas turbines with water or steam injection for emission control and/or power augmentation are included. The tests can be applied to gas turbines in combined-cycle power plants or with other heat recovery systems. 

Lubricating-oil governor and seal system cleaned Instrumentation and speed control checked Auxiliary heat recovery system Centrifugal Compressors... 

Goossens gives the following summary of heat recovery systems, subdivided by temperature range (in... 

Meunier, F., Second law analysis of a solid adsorption heat pump operating on reversible cascade cycles application to the zeolite-water pair. Heat Recovery Systems, 1985, 5, 133 141. 

It is prudent at this stage to briefly consider the problems that can be experienced in either refrigeration or heat recovery systems when water treatment is required to prevent freezing. The antifreeze treatment of pure water may be achieved by various means, typical ones being various brines, ethylene glycol, and propylene glycol. 

The influence of airflows from ventilating systems must also be considered. Processes using mediums of different physical qualities when mixed will have separation into different layers. Transmission of energy between molecules in flowing mediums takes place in the direction of the velocity. This strengthens the separation into parallel layers. The level of fluid in containers and tanks is due to stratification of horizontal temperature layers, while airflow after batteries, heat-recovery systems, and humidifiers or dehumidifiers will separate into parallel layers. 

The advantages of thermal incineration are that it is simple in concept, has a wide application, and results in almost complete destruction of pollutants with no liquid or solid residue. Thermal incineration provides an opportunity for heat recovery and has low maintenance requirements and low capital cost. Thermal incineration units for small or moderate exhaust streams are generally compact and light. Such units can be installed on a roof when the plant area is limited. = The main disadvantage is the auxiliary fuel cost, which is partly offset with an efficient heat-recovery system. The formation of nitric oxides during the combustion processes must be reduced by control of excess air temperature, fuel supply, and combustion air distribution at the burner inlet, The formation of thermal NO increases dramatically above 980 Table 13.10)... 

Considering the energy and maintenance costs during the life cycle, the cheapest investment is not always the best. It may, for instance, be profitable to buy a ventilation unit with a heat recovery system, which may increase the unit investment by 50%. The return on the investment in such a case may be in excess of 20%. 

LCC calculations frequently provide energy-efficient solutions. This gives reduced energy consumption and a reduction in environmental pollution. For example, the installation of a heat recovery system in a ventilation system may reduce the energy consumption and emissions by 50 to 80%. Figure 16.1 compares life cycle cost and life cycle assessment calculations. 

The investment costs should be estimated as accurately as possible. As can be seen in Fig. 16.2, a small difference in investment costs can influence the ranking of two heat recovery systems. 

Runaround coils The coils in a heat recovery system that collect hear from the hot w aste steam and supply it to the cold incoming stream via heat exchangers. 

Exhaust gases from the gas turbine are used to raise steam in the lower cycle without the burning of additional fuel (Fig. 7.3) the temperatures of the gas and water/steam flows are as indicated. A limitation on this application lies in the heat recovery system steam generator choice of the evaporation pres.sure (p ) is related to the temperature difference (Tft — T ) at the pinch point as shown in the figure, and a compromise has to be reached between that pressure and the stack temperature of the gases leaving the exchanger, (and the consequent heat loss ). ... 

Figure 11 -59. Representative expander refrigeration systems for temperature requirements and/or power recovery. (A) Expander coupled to a generator recovers pressure-loss energy for conversion to electric power. (B) System for power recovery from reactors in chemical processing plants. (C) Typical waste-heat recovery system using expansion turbines to generate electrical power. (Used by permission Bui. 2781005601. Atlas Copco Comptec, Inc.)...

Diesel engines with heat recovery systems... 

Having now established the space requirement for the boilers, we must consider the other items of plant to be housed in the same structure. These may include water-treatment plant, heat-recovery system, tanks and pump, instrument and controls. With oil- and gas-fired installations, there is little additional plant to consider within the boiler house. With solid fuel then the handling, distribution and perhaps storage could also form part of the building with provision for ash handling and removal. 

While vitally necessary, blowdown can be expensive in terms of lost heat. Therefore a point will be reached when it is economical to install a blowdown heat recovery system. Generally, the heat content in the blowdown water for a shell boiler will represent only about 25 per cent of the heat content in the same percentage of steam. Therefore, if a blowdown rate of 10 per cent is required this represents an approximate heat loss of 2.5 per cent from the boiler capacity. This differential reduces and eventually becomes insignificant on high-pressure watertube boilers. 

The aim of heat recovery is to avoid wastage of any quantity of heat or cold energy which has been generated within a system. Methods of recovery may be passive or active. Mechanical heat recovery systems will generally be found under the description of Heat Pumps. 

Blowdown Water and Flash Steam and Heat Recovery Systems... 

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