Cooling duty

After maximizing heat recovery in the heat exchanger network, those heating duties and cooling duties not serviced by heat recovery must be provided by external utilities. The outer-most layer of the onion model is now being addressed, but still dealing with targets. 

The gaseous reactor product is cooled first by boiler feedwater before entering a cooling water condenser. The cooling duty provided by the boiler... 

The dominant heating and cooling duties associated with a distillation column are the reboiler and condenser duties. In general, however, there will be other duties associated with heating and cooling of feed and product streams. These sensible heat duties usually will be small in comparison with the latent heat changes in reboilers and condensers. 

Turning now to the cold-end design, Fig. 16.6a shows the pinch design with the streams ticked off. If there are any cold streams below the pinch for which the duties eu e not satisfied by the pinch matches, additional process-to-process heat recovery must be used, since hot utility must not be used. Figure 16.66 shows an additional match to satisfy the residual heating of the cold streams below the pinch. Again, the duty on the unit is maximized. Finally, below the pinch the residual cooling duty on the hot streams must be satisfied. Since there are no cold streams left below the pinch, cold utility must be used (Fig. 16.6c). 

Cooling water costs tend to be low relative to the value of both fuel and electricity. The cost of cooling duty provided by cooling water is on the order of 1 percent that of the cost of power. For example, if power costs 0.07 kW h, then cooling water will typically cost 0.07 X 0.01/3600 = 0.19 x 10 kJ or 0.19 GJ L... 

Determine the theoretically required KaV/L value for a cooling duty from 105 F inlet water, 85 F outlet water, 78 F amhient wet-hiilh temperature, and an UG ratio of 0.97. 

Cooling Water System. A list of cooling duties will be available at this point so the cost estimate for this system can be factored or estimated based on a similar operating system. For a more definitive estimate based on initial or detailed layout, it is probably best to use a contractor or consultant skilled in these designs. If a cooling tower is involved, the groundwork will already have been set. This basis can be passed along on specification sheets provided in the Appendix to a vendor for quotes. 

The solution involves the use of two water substreams for scrubbing at 293 and 298 K. The first coolant is employed to undertake the cooling duty. Several conceptual flowsheets can be developed to implement this solution. Figures 9.18a and b illustrate such configurations. The developed mass exchanger is composed two sections. The top section employs Si,3 to reduce the ammonia composition in the gas from 0.13% to 0.10% and operates at an average water temperature of 293 K. In Fig. 9.18a, temperature is reduced by indirect contact cooling of the... 

The power fed to the fan shaft (or the total electrical power fed to the motor if within the duct) appears as sensible heat in the system. During a heating cycle this is useful. As a mle of thumb, fan shaft power is 17 x flow m x mm swg watts. This commonly lies between 2 per cent of the cooling duty for small systems to 10 per cent for large systems. 

The use of dry coolers cannot take advantage of the lower cooling temperatures available by the evaporation of the cooling water, and is limited by the ambient dry bulb temperature, rather than the wet bulb Higher power is therefore required, and a given size compressor will perform less cooling duty. 

Example 10.1 A suction pipe for an R.502 system, evaporating at -40°C and having a cooling duty of 50 kW, is to be run in copper tube. What size should it be Reference 32, Chapter 3, Table 3,... 

Example 13.2 In the rating table shown in Table 13.1 for a water-cooled condensing unit, what is the cooling duty at -20°C evaporation, with water onto the condenser at 25°C ... 

All assumptions regarding the load and estimated cooling duty should be recorded as the design parameters of the system, and agreed with the user. 

While all these factors affect the good running of positive-displacement compressors, the effect is far worse with centrifugal machines, which can approach stall condition and so give a much reduced cooling duty... 

The cooling duty can be provided by either making the draught tube an internal heat exchanger or with a heat exchanger in an external circulation loop. The mass transfer coefficient for external loop airlift Fermenter is estimated as 8... 

If the reactor conversion is changed so as to optimize its value, then not only is the reactor affected in size and performance but also the separation system, since it now has a different separation task. The size of the recycle will also change. If the recycle requires a compressor, then the capital and operating costs of the recycle compressor will change. In addition, the heating and cooling duties associated with the reactor and the separation and recycle system change. 

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