Steam-Heated Exchangers

Steam-Heated Exchangers Steam, the most common heating medium, transfers its latent heat in condensing, causing heat flow to be proportional to steam flow. Thus a measurement of steam flow is essentially a measure of heat transfer. Consider raising a liquid from temperature Ti to T2 by condensing steam  

However, the steady-state process gain described by this derivative varies inversely with liquid flow adding a given increment of heat flow to a smaller flow of liquid produces a greater temperature rise. 

The variability of the process parameters with flow causes variability in load response, as shown in Fig. 8-50. The PID controller was tuned for optimum (minimum-IAE) load response at 50 percent flow. Each curve represents the response of exit temperature to a 10 percent step in liquid flow, culminating at the stated flow. The 60 percent curve is overdamped and the 40 percent curve is underdamped. The differences in gain are reflected in the amplitude of the deviation, and the differences in dynamics are reflected in the period of oscillation. 

If steam flow is linear with controller output, as it is in Fig. 8-50, undamped oscillations will be produced when the flow decreases by 

8-50 The response of a heat exchanger varies with flow in both gain and dynamics here the PID temperature controller was tuned for optimum response at 50 percent flow. 

8-51 Heat-transfer rate in sensible-heat exchange varies nonlinearly with flow of the manipulated fluid, requiring equal-percentage valve characterization. 

---

Tube failure in a water-cooled or steam-heated exchanger used in hydrocarbon service can result in the contamination of the effluent cooling water or the condensate by the process stream, especially if the latter is at a higher pressure. Such effluents must be disposed of in such a maimer that the hydrocarbon contaminations can be safely contained. The following are some safe design practices ... 

Reheat involves steam-to-steam heat exchange using steam at boiler discharge conditions. In the reheat cycle, after partially expanding through the turbine, steam returns to the reheater section of the boiler, where more heat is added. After leaving the reheater, the steam completes its expansion in the turbine. The number of reheats that are practical from a cycle efficiency and cost colisidcratioli is two. 

Mathur,J., Performance of Steam Heat-Exchangers, Chem. Eng, Sept. 3, (1973) p. 101. 

By putting the corrodent on the tubeside and the hot vapour on the shellside provided the vapour is compatible with the shell material this is the normal configuration for steam-heated exchangers. 

The use of electric powered steam generators, steam-to-steam heat exchangers, or reboilers to provide steam to steamtables, humidifiers, autoclaves, and for direct injection into the process. Where this type of approach is employed, it generally is necessary to provide a high quality water source to avoid rapid internal deposition of crystalline scales and foulants. 

The ammoxidacion reaction is carried out at about 800°F and 30 psi. Because it is highly exothermic, heat is removed continuously from the reactor by hear exchangers. The residence time of the reactants is about three seconds. The reaction gases are cooled as they pass by the water-to-steam heat exchanger in the reactor. The effluent is treated for removal of ammonia by scrubbing it with water acidified with sulfuric acid, forming ammonium sulfate, a marketable commodity that can be recovered by crystallization. But that s another story. 

Crude oil is pumped from storage through a steam heated exchanger and into an electric desalter. Dilute caustic is injected into the line just before the desalting drum. The aqueous phase collects at the bottom of this vessel and is drained away to the sewer. The oil leaves the desalter at 190°F, and goes through heat exchanger E-2 and into a furnace coil. From the furnace, which it leaves at 600°F, the oil proceeds to a distillation tower. 

The steam-heated exchanger is also nonlinear (Figure 2.106). The steady-state gain of such an exchanger is the derivative of its outlet temperature with respect to steam flow, having the dimension of °F(lb/h) ... 

The feedback control of a steam-heated exchanger and its characteristic equation. 

Convection methods involve contact of the wet textile with hot air and are the most common method used in textiles since they combine high process speeds with control of fabric dimensions during drying. Examples include tenter frames (Fig. 2.13). Air is heated to the desired temperature by gas- or oil-fired burners or steam heat exchangers and passed over the fabric by high velocity blowers. Fabric tensions are adjusted in both the width and length directions, allowing for complete control of final fabric dimensions. 

As the temperature of the miscella exiting the first stage evaporator is low, it is a good heat sink for heat recovery. In various facilities, heat from hot finished oil, heat from steam ejector exhausts, or recovered flash steam is used to preheat the miscella to approximately 75°C in temperature. The preheated, concentrated miscella is then typically heated to 110°C in a steam-heated exchanger prior to entering the second rising film evaporator. 

---