Don't Err With Air-Fin Exchangers

Deciding on the maximum air temperature is one of the important steps in defining the basis for air-fin heat exchanger design. Too low a maximum air temperature may result in an exchanger that fails to perform. Too high a maximum air temperature can lead to excessive costs — and to an exchanger so large it becomes a problem when air temperature is lower than normal.

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Air temperature selection affects both process and mechanical design of a fin-fan. A vendor will design to whatever temperature limit you give. It's your job as the purchaser to set the correct air temperature.

Setting maximum design air temperatures involves finding answers to three important questions:
1. How critical is meeting your process temperature?
2. What are the climate data for your location?
3. What special or local factors should you include?

The more critical meeting your process temperature is, the more stringent your air temperature selection guidelines.

The most conservative approach is to pick an air temperature that's never exceeded. However, this can be surprisingly high. For instance, at one snow-belt site, temperatures occasionally reach 70°F (24°C) in December (Figure 1). This strategy is needed for extreme cases — if exceeding the temperature will cause a plant shutdown or pose safety or environmental consequences.

The second approach, which applies to plants that have significant step changes in operation tied to temperature, is to set the maximum to an air temperature that's exceeded only a specific number of days per year. This is a stringent requirement that's usually imposed to forestall major plant problems. As a result the percentage target generally is small, e.g., 1% or 2% of the days.

Historical Data

Figure 1. December temperatures for location were obtained inexpensively from NOAA website.

Usually, I use a temperature that's exceeded only a certain number of hours per year, typically 1% to 2% of the hours. This approach is less stringent because it's based on the temperature being topped at any time during a day, not for the full day — 1% of the days where the high temperature goes beyond a certain value is a lot less than 1% of the hours where the temperature passes the same value. The strategy particularly suits plants where high air temperatures pose an economic cost but don't create safety, environmental or major operational problems. Detailed data are needed for this.

Another approach is to use the average high temperature for the coldest month(s) of the year. This is a relatively low temperature for most locations. For example, the "mean max" temperatures for that snow-belt plant are 24–25°F (~13–14°C) colder than the maximum monthly temperatures (Figure 1). This strategy suits processes with minor cost or operational consequences from air temperatures higher than the air-fin's design basis.

There are sources of general data for local climate conditions. For the U.S., the National Oceanic and Atmospheric Administration (NOAA) has a superb website that gives climate data (www.weather.gov/climate/ or http://lwf.ncdc.noaa.gov/oa/climate/stationlocator.html). Find the weather station closest to your location that has the same climate conditions. The station you choose should be physically nearby, at similar elevation, and in the same general type of location — e.g., on the water or in the same valley (if a mountain location). All of these factors can significantly affect local temperature patterns. Data available will vary between different stations.

You can download the best data for a trivial fee. Current pricing for the data used to generate this starts at $3 and goes up gradually. Figure 1 shows an extract from a NOAA report.

Another excellent source is the ASHRAE Fundamentals Handbook, which lists worldwide summaries of temperatures that are exceeded 35 hours (0.4%), 88 hours (1%) or 175 hours (2%) during an average year.

Once you've selected a base climate temperature, you must adjust it to account for any special factors — such as air recirculation and potential air maldistribution because of wind and the effect of nearby equipment.

Will layout considerations force placing the new air-fin adjacent to an existing air-fin? If so, this may require adding some temperature allowance due to the neighboring unit. Watch out in particular for induced-draft and forced-draft air-fin exchangers close to each other and adjacent exchangers at different elevations.

Large tanks, vessels, buildings and fired heaters near the air-fin can starve the exchanger of air or force hot air toward it.

The typical design method for handling such issues is to adjust the design air temperature. Often a 5–10°F (3–5°C) allowance will deal with most of these special factors. If you really think a larger allowance is needed, you should reexamine the entire location selection logic for the exchanger.

Selecting maximum air design temperatures for air-fins can dramatically change process economics: too high and the equipment becomes very large and expensive, too low and process performance suffers. Select a temperature strategy consistent with your requirements. Find good climate data on local conditions. Then allow for special circumstances for your plant and exact equipment location.

Andrew Sloley is a Chemical Processing Contributing Editor. You can e-mail him at [email protected].