Heat rejection

By contrast. Fig. 13.46 shows an endothermic reactor integrated below the pinch. The reactor imports Qreact from part of the process that needs to reject heat. Thus integration of the reactor serves to reduce the cold utility consumption by Qreact- There is an overall reduction in hot utility because, without integration, the process and reactor would require (Qumin + Qreact) from the utility. 

Simplified Cycle. A simplified fossil steam cycle appears in Figure 19. The water accumulates in the bottom of the condenser, called the hotweU. It goes through a feed pump to pressurize it. The pressurized water passes through one or more feedwater heaters, which raise the temperature. The water then enters the boiler where heat from the fuel converts it to steam. The steam expands through the engine, usually a turbine, which extracts work. In the middle of the turbine some of the steam is extracted to supply heat to the feedwater heater. The remainder expands through the turbine and is condensed. The rejected heat is carried away by the condenser coolant, which is usually water, but sometimes air. The condensed steam then returns to the... 

Hea.t Pumps. The use of heat pumps adds a compressor to boost the temperature level of rejected heat. It can be very effective in small plants having few opportunities for heat interchange. However, in large faciHties a closer look usually shows an alternative for use of waste heat. The fuel/steam focus of energy use has led to appHcation of heat pumps in appHcations where a broader examination might suggest a simpler system of heat recovery. 

Thus, a cogeneration system is designed from one of two perspectives it may Be sized to meet the process heat and other steam needs of a plant or community of industrial and institutional users, so that the electric power is treated as a by-produc t which must be either used on site or sold or it may be sized to meet electric power demand, and the rejected heat used to supply needs at or near the site. The latter approach is the likely one if a utility owns the system the former if a chemical plant is the owner. 

Fig. 3 shows an idealised solar collector (generator) containing adsorbent which is connected to a condenser that rejects heat to the environment and an insulated box containing a liquid receiver and a flooded evaporator. Fig. 4 shows the p-T-x (pressure - temperature - concentration or Clapeyron diagram) for the adsorbent-adsorbate pair with typical temperatures. 

Conventional refrigeration is based on the evaporation of a refrigerant. The evaporator produces cooling, the compressor generates high-pressure energy, and the condenser rejects heat to the environment. An absorption machine differs from a vapor compressor only in the way the refrigerant is compressed from the evaporator to the condenser. 

A comparison of wet and dry bulb readings allows the relative humidity to be determined from a psychrometric chart. The wet bulb temperature is always lower than the dry bulb value except when the air is already saturated with water - 100% relative humidity. This is when the wet and dry bulb temperatures are the same. Tlie air will no longer accept water and the lack of evaporation does not allow the wetted bulb to reject heat into the air by evaporation. This situation would be... 

Heat pump A reversed heat engine or refrigerator that takes in heat from a body at low temperature and by the expenditure of mechanical work rejects heat to a body at a higher temperature. 

Fig. 2.3 shows such a fully reversible steady flow through the control volume CV. The heat transferred [GrevIx. supplies a reversible heat engine, delivering external work [( c)rev]x and rejecting heat [(2o)rev1x to the environment. 

As is shown in Fig. 4.3a, the lower pressure cooling is fed by air i/(l at state 7, at a corresponding pressure p-j and a temperature T, and this mixes with air (1 + 4>w) froni the HP exhaust at temperature T,) to produce a temperature Tg as indicated in the diagram. The full turbine gas flow (1 + i/() then expands through a pressure ratio Al to a temperature Tio, and subsequently rejects heat, finishing at Tj = Ta. 

Industry needs both electricity and heat. It is possible for an industry to produce its electricity from gas-fired turbogenerators and use the rejected heat for industrial purposes. The rejected heat can be at relatively high temperature, making it more useful if some sacrifice is made in the efficiency of the... 

This type of exchanger is used to reject heat from a fluid inside the tubes (and associated headers) directly to ambient air. To be effective, the air must flow in forced convection to develop acceptable transfer coefficients. Figures 10-174, 10-175, and 10-176 illustrate the two types, designated by the type of air movement, induced draft or forced draft. 

Polytropic system. Figures 12-37D and 12-37E, gets nearer to the conditions of a practical system, with polytropic process having n = 1.2. The gray area Wi compresses air from 1 to 2 while the air rejects heat to atmosphere through compressor cylinder walls. equals area 1-2-y-z in 4e. In the aftercooler-receiver, the air rejects heat Q i equal to gray area 2-3-w-y. 

The efficiency of all reversible cycles absorbing heat from a single-constant higher temperature and rejecting heat at a single-constant lower temperature must be the same. 

The purpose of a heat engine is to remove heat Q, from a thermal reservoir at a higher absolute temperature T, extract useful work W and reject heat to a second thermal reservoir at a lower absolute temperature T. The device used to obtain the useful work is the heat engine. 

Apart from such requirements for head pressure control, winter precautions are needed to prevent freezing of the water while the plant is not rejecting heat to it. These commonly take the form of an electric immersion heater in the water tank, together with lagging and possible trace heating of exposed pipes. In some systems, the evaporative condenser itself may be within the building, with air ducts to the outside. In severe climates, external tanks need to be lagged to conserve the heat provided by the immersion heater. 

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