Alternative overhead system for pressure column
The suggested approaches to avoiding these difficulties are as follows:
1. Select the number of degrees of subcooiing so that the sensible heat load will be at least 5 percent of the total heat load. This will have a secondary advantage of reducing the probability of cavitation in control valves and pumps.
2. If water-header pressure fluctuations are a problem, use a cascade temperature water-flow control system.
3. If summer—winter heat-load variations are sufficiendy severe, use dual, split-range water valves. The smaller valve should open first and will provide adequate winter cooling. The ratio of cooling water rate for the maximum summer heat load to that for the minimum winter load is often two to three times as great as process turndown. (See also discussion in Chapter 11, Section 6.)
4. For the horizontal condenser, the temperature detector preferably should be located in the liquid line just beneath the condenser for maximum speed of response. For the vertical condenser, the temperature detector should be located in a trough at the lower end of a drip collector just below the tube bundle and above the reflux drum. (See Figure 3.8.)
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