Page 298 - Applied Process Design For Chemical And Petrochemical Plants Volume III

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260 Applied Process Design for Chemical and Petrochemical Plants

with the prorata share of such cooling facilities unless the c. Fin bond efficiency.
specific plant account of costs dictates otherwise. d. Header type and pressure.
The overall economics of an air-cooled application e. Type of piping connections.
depends upon the following: 2. Air Moving Equipment
a. Power source (electricity, gas, etc.).
1. Quantity and quality of available water. b. Power transmission to fan (direct, gear, belt, etc.).
2. Ambient air and water temperature. c. Number of fans.
3. Fluid inlet as well as exit temperatures. d. Fan material and design.
4. Operating pressure. 3. Structure
5. First costs. a. Slab or pier foundation.
6. Maintenance and operating costs. b. Forced or induced draft.
7. Physical location and space requirements. c. Structural stability.
d. Ladders, walkways, handrails.
Mukherjee 265 presents an interesting examination of fac- e. Type of construction.
tors that can influence operating problems with air-cooled f. Belts, reducing gears, shaft and fan guards.
heat exchangers. 4. Controls
a. Temperature control instruments.
Advantages—Air-Cooled Heat Exchangers b. Power.
c. Louvers, rolling doors.
1. Generally simple construction, even at relatively high
d. Mixing valves.
pressure and/or high temperatures. Amount of special
metals often is reduced.
Factors to consider in evaluating the selection between
2. No water problems, as associated with corrosion, algae, 24
induced and forced draft include the following:
treating, scale, spray, etc.
3. Excellent for removing high level temperatures, partic-
1. Induced Draft
ularly greater than 200°F. a. Recirculation of air is less (exit air velocity 2–3 times
1
4. Maintenance generally claimed to be / 3 or less than
forced draft).
water coolers. Clean fins by compressed air and
b. Air distribution over exchanger is better.
brushes, sometimes while operating.
c. Sections are closer to ground and easier to main-
5. Lower operating costs under many conditions, depend-
tain, provided driver mounted below cooler.
ing upon the type of water system used for comparison.
d. Maximum weather protection for finned tubes
6. Ground space often cooling towers; can also serve
(rain, hail, freezing).
dual purpose by mounting air-cooled units above other
e. Few walkways needed, mounting easier overhead.
equipment or on pipe ways or roofs of buildings. Vibra-
f. Connecting piping usually less.
tion is no problem.
2. Forced Draft
a. Mechanical equipment more easily accessible.
Disadvantages
b. Isolated supports for mechanical equipment.
1. Rather high limitation on outlet fluid temperature. c. Simpler structure.
2. Generally most suitable only for liquids or condensing d. Easier to adapt to other than motor drives.
vapors in tubes, with limited application for gas cooling e. Fan horsepower less for same performance (due to
due to low inside coefficient. difference in air density).
3. First capital costs may range from only 25–125% above f. Exchangers are easier to remove for repairs.
water-cooled equipment for same heat load. Each situ-
ation must be examined on a comparative basis. Bid Evaluation
4. Fire and toxic vapor and liquid hazard, if leaks occur to
atmosphere. Manufacturer’s specification sheets, Figure 10-185, are
5. Not too suitable for vacuum services due to pressure important for proper bid evaluation, and purchaser’s speci-
drop limitations but are used in application. fications may be offered on a form as in Figure 10-186.
Optimum design is not often achieved in all respects;
24
Chase lists these factors affecting the overall costs: however, the fundamentals and application cost factors of
87
Nakayama are of real value in selecting goals and design
1. Exchanger Sections features.
a. Tube material and thickness. In addition to the items listed on the specification sheets
b. Fin material size, shape. and in other paragraphs of this section, it is important for