Low heat load

When the integration of sequences of simple columns was considered, it was observed that sequences with higher heat loads occurred simultaneously with more extreme levels. Heat integration always benefits from low heat loads and less extreme levels, as we shall see later in Chap. 12. Now consider the effect of thermal coupling arrangements on loads and levels. Figure 5.18 compares a... 

Pipe Coil Pipe coil for submersion in coil-box of water or sprayed with water Is simplest type of exchanger. Condensing, or relatively low heat loads on sensible transfer. Transfer coefficient is low, requires relatively large space if heat load is high. 0.5-0.7... 

Open Tube Sections (Water cooled) Tubes require no shell, only end headers, usually long, water sprays over surface, sheds scales on outside tubes by expansion and contraction. Can also be used in water box. Condensing, relatively low heat loads on sensible transfer. Transfer coefficient is low, takes up less space than pipe coil. 0.8-1.1... 

Figure 9.2 Diagram showing the proper flow allocations during low heat load operation, with water flow diverted from the fill section.

Although this can use a chilled drum, it is not essential, as the process involves a relatively low heat loading. The process again uses a vacuum chamber provided with a helium—oxygen mixture and a silicon-based monomer such as tetramethyldisiloxane or hexamethyldisiloxane. An applied power creates a plasma which activates the oxidation of the silicon gas, creating reactive chemical molecules which form the SiO coating on the film surface. 

Change In tube vapor volume with change in heat input. Stepping up heat input generates more vapor and expands the volume of fluids in the tubes. Fluid swell following an abrupt increase in heat input may displace liquid back into the column base. If the liquid to the reboiler comes from a trapout pan, this can flood the bottom tray. This problem is most pronounced at low heat loads (67, 68). 

Steam-heated air is recirculated by a fan over and parallel to the surface of the trays. Electrical heat is also used, especially for low heating loads. About 10 to 20% of the air passing over the trays is fresh air, the remainder being recirculated air. 

The simplest type of heat exchanger is a pipe coil, in which tubes are bent to form and then submerged in water or sprayed with water. This process is very effective in low-volume, low-heat-load operations. A double-pipe heat exchanger, which has a pipe-within-a-pipe design, provides better temperature control. 

For a fixed liquid level in the column base, the volumetric percent vapor in the tubes increases as steam (or other heating medium) flow rate is increased. As shown by Figure 4.2, this effect is very pronounced at low heat loads but tapers off" at higher loads. The end result is similar to boiler swell (Chapter 10, reference 6) an abrupt increase in steam flow causes liquid to be displaced back into the column base, with a resulting temporary increase in column base level. 

For a constant liquid level in the column base, as heat load increases, percent vapor in the tubes increases. The effect is most marked at low heat loads. At a given operating point ... 

The tenn k 2 is normally both negative and small. But as pointed out in Chapter 4, k i is often large at low heat loads the control system designer must be carefiil about the term ... 

At low adsorbate loadings, the differential heat of adsorption decreases with increasing adsorbate loadings. This is direct evidence that the adsorbent surface is energetically heterogeneous, ie, some adsorption sites interact more strongly with the adsorbate molecules. These sites are filled first so that adsorption of additional molecules involves progressively lower heats of adsorption. 

Scale control can be achieved through operation of the cooling system at subsaturated conditions or through the use of chemical additives. The most direct method of inhibiting formation of scale deposits is operation at subsaturation conditions, where scale-forming salts are soluble. For some salts, it is sufficient to operate at low cycles of concentration and/or control pH. However, in most cases, high blowdown rates and low pH are required so that solubihties are not exceeded at the heat transfer surface. In addition, it is necessary to maintain precise control of pH and concentration cycles. Minor variations in water chemistry or heat load can result in scaling (Fig. 12). 

Cogeneration systems will not match the varying power and heat demands at all times for most applications. Thus, an industrial cogeneration systems output frequently must be supplemented by the separate on-site generation of heat or the purchase of utility-supplied elec tric power. If the on-site electric power demand is relatively low, an alternative option is to match the cogeneration system to the heat load and contract for the sale of excess electricity to the local utihty grid. 

It is important that the formaldehyde addition rate be balanced with the alkali content of the system and the engineering control capability. At high alkali contents, the exotherm will be more vigorous and create more load on the heat exchangers. At low alkali contents, the reaction rate may be quite slow. While this temporarily reduces the difficulty in instantaneous heat load, it may permit potentially hazardous levels of unreacted formaldehyde to accumulate. Such accumulations could become dangerous as batch temperature rises. In both cases. 

As with the electrical load profile, it is also necessary to analyze the heat load over the daily and annual cycles. Ideally, the heat load will match the available heat from the electrical generator (however, this is rarely the case). There will be periods when supplementary output will be necessary which can be achieved by, say, supplementary firing the waste heat gases of a gas turbine, or heat output reduction is necessary by the introduction of bypass stacks. For a steam turbine installation bypass pressure-reducing valves will be necessary to supplement steam output, while a dump condenser may be needed at low-process steam demands. The nature of the electrical and heat load will obviously have significant influence in the development of the scheme and scope of equipment. 

As discussed earlier (Section 15.3), steam turbines providing low-pressure steam are more established for CHP. However, for lower heat loads the diesel and gas turbine with waste heat recovery offer an attractive alternative. 

Virtually no mnning cost Self-regulation (flow rate increases with heat load) Low maintenance cost Provides daylight when open (roof vent) Psychological appeal of clear sky (roof vent) Easy installation... 

This is the exchanger where heat flows from the room return or mixed air to cold refrigerant or to chilled water. It is an arrangement of finned tubes normally of aluminum fins on copper tubes, but copper fins can be specified for corrosive atmosphere. Performance characteristics are controlled by fin and tube spacing. If the room rh is high, dehumidification may be brought into use by operating the coil or one of a number of parallel coils at a low temperature. If the room s sensible heat load is low reheat must be allowed to operate at the same time. 

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