Cooling waters

Cooling water which removes heat from surface coolers and condensers is the type of water most frequently required by industry. To avoid the deterioration of heat transfer by cooling water and to ensure the adequate service life of equipment, it should not cause 

The composition of sediments leading to a decrease in the heat transfer depends on the temperature and composition of the water. Up to 40 C only calcium carbonate is separated, and at higher temperatures calcium sulphate also. The requirements for cooling water depend on the type of the circulation system, of which there are only two types  

In coohng water the content of calcium, sulphates and hydrogen carbonates is hmited. Sulphates should be avoided because of the problems with concrete structures. Calcium sulphate can react with a component of cement — calcium aluminate — and form with it etteringite (CagAl2[(0H)4(S04)]3.2H20) whose crystals are more voluminous than those of the original aluminate. Changes in the volume can cause destruction of concrete. 

The content of hydrogen carbonates is quite low because of the intensive ventilation of coohng water in an open circulation system and thus, the majority of free CO2 is removed, so that the foUowing calcium carbonate equilibrium shifts to the left  

The electricity supply of a plant must be designed with sufScient redundancy, so that safety-relevant systems (e.g., pumps for cooling water) are in different independent electrical circuits. 

---

Separation of low-molecular-weight materials. Low-molecular-weight materials are distilled at high pressure to increase their condensing temperature and to allow, if possible, the use of cooling water or air cooling in the column condenser. Very low... 

If the vapor stream consists of a mixture of unconverted feed material, products, and byproducts, then some separation of the vapor may be needed. The vapor from the phase split is difficult to condense if the feed has been cooled to cooling water temperature. If separation of the vapor is needed, one of the following methods can be used ... 

Where the cold composite curve extends beyond the start of the hot composite curve in Fig. 6.5a, heat recovery is not possible, and the cold composite curve must be supplied with an external hot utility such as steam. This represents the target for hot utility (Q niin)- For this problem, with ATn,in = 10°C, Qnmin 7.5 MW. Where the hot composite curve extends beyond the start of the cold composite curve in Fig. 6.5a, heat recovery is again not possible, and the hot composite curve must be supplied with an external cold utility such as cooling water. This represents the target for cold utility (Qcmin)- For this problem, with AT in = 10°C, Qcmm = 10-0 MW. 

Analogous effects are caused by the inappropriate use of utilities. Utilities are appropriate if they are necessary to satisfy the enthalpy imbalance in that part of the process. Above the pinch in Fig. 6.7a, steam is needed to satisfy the enthalpy imbalance. Figure 6.86 illustrates what happens if inappropriate use of utilities is made and some cooling water is used to cool hot streams above the pinch, say, XP. To satisfy the enthalpy imbalance above the pinch, an import of (Q mjj,+XP) is needed from steam. Overall, (Qcmin+AP) of cooling water is used. ... 

An alternative inappropriate use of utilities involves heating of some of the cold streams below the pinch by steam. Below the pinch, cooling water is needed to satisfy the enthalpy imbalance. Figure... 

Not all problems have a pinch to divide the process into two parts. Consider the composite curves in Fig. 6.10a. At this setting, both steam and cooling water are required. As the composite curves are moved closer together, both the steam and cooling water requirements decrease until the setting shown in Fig. 6.106 results. At this setting, the composite curves are in alignment at the hot end,... 

In design, the same rules must be obeyed around a utility pinch as around a process pinch. Heat should not be transferred across it by process-to-process transfer, and there should be no inappropriate use of utilities. In Fig. 6.13a this means that the only utility to be used above the utility pinch is steam generation and only cooling water below. In Fig. 6.136 this means that the only utility to be used above... 

A refrigeration system is a heat pump in which heat is absorbed below ambient temperature. Thus the appropriate placement principle for heat pumps applies in exactly the same way as for refrigeration cycles. The appropriate placement for refrigeration cycles is that they also should be across the pinch. As with heat pumps, refrigeration cycles also can be appropriately placed across utility pinches. It is common for refrigeration cycles to be placed across a utility pinch caused by maximizing cooling water duty. 

Most refrigeration systems are essentially the same as the heat pump cycle shown in Fig. 6.37. Heat is absorbed at low temperature, servicing the process, and rejected at higher temperature either directly to ambient (cooling water or air cooling) or to heat recovery in the process. Heat transfer takes place essentially over latent heat profiles. Such cycles can be much more complex if more than one refrigeration level is involved. 

Total power for heat rejection to cooling water = 0.38 -I- 0.44 = 0.82 MW... 

Process cooling by level 2 by this arrangement across the pinch is 0.54 — 0.14 = 0.40 MW. The balance of the cooling demand on level 2, 0.8 — 0.4 = 0.4 MW, together with the load from level 1, must be either rejected to the process at a higher temperature above the pinch or to cooling water. 

Figure 6.40 A two-level refrigeration system for Example 6.6 with heat rejection to cooling water.

Solution Figure 7.2 shows the stream grid with the pinch in place dividing the process into two parts. Above the pinch there are five streams, including the steam. Below the pinch there are four streams, including the cooling water. Applying Eq. (7.3),... 

Solution First, we must construct the balanced composite curves using the complete set of data from Table 7.1. Figure 7.5 shows the balanced composite curves. Note that the steam has been incorporated within the construction of the hot composite curve to maintain the monotonic nature of composite curves. The same is true of the cooling water in the cold composite curve. Figure 7.5 also shows the curves divided into enthalpy intervals where there is either a... 

In early designs, the reaction heat typically was removed by cooling water. Crude dichloroethane was withdrawn from the reactor as a liquid, acid-washed to remove ferric chloride, then neutralized with dilute caustic, and purified by distillation. The material used for separation of the ferric chloride can be recycled up to a point, but a purge must be done. This creates waste streams contaminated with chlorinated hydrocarbons which must be treated prior to disposal. 

Cooling water systems are dosed with corrosion inhibitors, polymers to prevent solid deposition, and biocides to prevent the growth of microorganisms. 

Cooling tower blowdown can be reduced by improving the energy efficiency of processes, thus reducing the thermal load on cooling towers. Alternatively, cooling water systems can be switched to air coolers, which eliminates the problem altogether. 

Reducing wastewater associated with cooling water systems. 

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

There are two esdsting steam mains. These are high-pressure steam at 41 bar superheated to 270°C and medium-pressure steam at 10 bar saturated at 180°C. Boiler feedwater is available at 80°C and cooling water at 25°C to be returned at 30°C. 

Following the pinch rules, there should be no heat transfer across either the process pinch or the utility pinch by process-to-process heat exchange. Also, there must be no use of inappropriate utilities. This means that above the utility pinch in Fig. 16.17a, high-pressure steam should be used and no low-pressure steam or cooling water. Between the utility pinch and the process pinch, low-pressure steam should be used and no high-pressure steam or cooling water. Below the process pinch in Fig. 16.17, only cooling water should be used. The appropriate utility streams have been included with the process streams in Fig. 16.17a. 

Example 16.4 The stream data for a process are given in Table 16.5. Steam is available condensing between 180 and 179°C and cooling water between 20 and SO C. All film transfer coefficients are 200Wm C For lO C, the... 

Given a network structure, it is possible to identify loops and paths for it, as discussed in Chap. 7. Within the context of optimization, it is only necessary to consider those paths which connect two different utilities. This could be a path from steam to cooling water or a path from high-pressure steam used as a hot utility to low-pressure steam also used as a hot utility. These paths between two different utilities will be designated utility paths. Loops and utility paths both provide degrees of freedom in the optimization. ... 

Utilities (fuel, steam, electricity, cooling water, process water, compressed air, inert gases, etc.)... 

---