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This primer discusses the thermal design of ACHEs and the optimization of the thermal design, and offers guidance on selecting and designing ACHEs for various applications.
Based on bare-tube heat-transfer area, air-cooled heat exchangers (ACHEs) cost two to three times more than water-cooled heat exchangers for the same heat duty (hardware costs only). There are two main reasons for this. First, the thermal conductivity of air is considerably lower than that of water, which results in a much lower heattransfer coefficient. Second, since design ambient temperatures are always higher than design water temperatures, the mean temperature difference (MTD) is always lower for an ACHE, especially at relatively low process-fluid outlet temperatures.
As a result of these two factors, the heat-transfer area of an ACHE is considerably larger than that of a water-cooled heat exchanger for the same duty. In addition, the larger area requires an elaborate structural support system, which increases the cost further.
However, as all engineers know, the capital (or fixed) cost of equipment is only part of the story. What is important is the total cost - the sum of the fixed cost and the operating cost.
The operating costs for water cooling are much higher than those for air cooling. These include the costs of the initial raw water itself, makeup water, and treatment chemicals, the apportioned cost of the plant cooling tower, and the pumping cost. For ACHEs, the operating cost is just the cost of the power required to make the air flow across the tube bundles. As water becomes scarcer, the operating costs of water-cooled heat exchangers increase, thereby tilting the economics further in favor of air cooling.
This article outlines the advantages and disadvantages of ACHEs, explains and illustrates what ACHEs are, elaborates the various construction features available to accommodate different application requirements, discusses the thermal design of ACHEs and the optimization of the thermal design, and examines several special applications.
Pros and cons of ACHEs
Air-cooled heat exchangers offer several important advantages over water-cooled exchangers.
Some of these are a direct result of water not being used as the cooling medium. The high costs of using water, including the costs for the raw water, makeup water, and treatment chemicals, are eliminated. The location of the...