Recovery and reheat process simplifies compressed air drying

Key concepts Warm, dry air is critical to many manufacturing processes.

07/01/2000


COMPRESSED AIR

Key concepts&/HEADLINE>

Warm, dry air is critical to many manufacturing processes.

A packaged reheat drying system cools compressed air in an aftercooler, removes moisture using a separator, and then reheats the air using a regenerative heat exchanger.

A reheat drying system saves energy and requires little maintenance.

Warm, dry air is critical to many manufacturing processes. The traditional method of drying com-pressed air typically includes an aftercooler and refrigerated dryer (Fig. 1). This arrangement, although effective, can be expensive to operate and maintain.

As a result, many plants are now considering a reheat drying system (Fig. 2) to perform this function. The approach is popular because of its energy savings, low-capital equipment cost, and maintenance-free operation.

Operation of the system

A packaged reheat drying system cools compressed air in an aftercooler, removes the moisture using a separator, and then reheats the air using a regenerative heat exchanger. It operates in the manufacturing process without using an external energy source. Essentially, a reheat system supplies free heat to a process while significantly reducing plant operating expenses.

Cooling compressed air is essential to condense moisture present in an airstream. However, this process robs the energy or volume from the compressor system. Although the amount varies, it is not unusual to lose 30% of the total energy available from the compressed air system.

A reheat drying system adds this energy or volume back into the compressed air system using the heat of compression from the air compressor. A particular advantage of such a system is that it requires no external energy to reheat the air, which results in significant savings in plant operating expenses.

In most cases, the system is efficient enough to replace a refrigerated dryer, depending upon pressure dewpoint desired and coolant available. Typical dewpoint temperatures achieved range from 35%%MDASSML%%90 F. The system can reheat process air to temperatures as high as 300 F, depending on the air discharge temperature out of the compressor. However, 90%%MDASSML%%120 F is more commonly achieved for most applications.

In Fig. 1, a reheat system replaces the traditional equipment in the shaded area. The reheat system (see Fig. 2) consists of an air-to-air regenerative tubular exchanger, aftercooler(s), cyclone separator, automatic moisture trap, interconnecting piping, and controls, all mounted on a support rack.


Fig. 1. The traditional method of drying com-pressed air typically includes an aftercooler and refrigerated dryer.


Fig. 2. Many plants are now considering a reheat drying system for drying compressed air.

Key components

A key component in the reheat system is the air-to-air regenerative tubular heat exchanger. Regeneration is a method of exchanging heat between the same fluid at different intervals in the process. For example, compressed air must be cooled to condense the moisture present, then be reheated.

Instead of handling the entire cooling load in an aftercooler, a reheat system uses a regenerative heat exchanger upstream of the aftercooler to reduce the cooling load on the aftercooler. The regenerative heat exchanger's cooling source is the same compressed air that has already been cooled by the reheat system aftercooler. A cyclone separator efficiently removes up to 99% of the condensed liquid from the airstream before it returns to the regenerative heat exchanger.

The system is arranged in this manner for two reasons. First, it conserves the amount of coolant needed in the aftercooler because the regenerative heat exchanger is handling part of the cooling load. Second, compressed air is reheated using its own energy from the heat of compression generated by the air compressor. No external energy source is used to reduce the dewpoint of the process air. The "free" heat of compression increases the air temperature above the dewpoint at the aftercooler.

Configuration and performance

Reheat drying system configurations can vary. One arrangement (Fig. 3A) uses a single aftercooler. The aftercooler cooling medium can be cooling tower water, chilled water, river water, lake water, city water, or ammonia refrigerant.

When chilled water is needed to dry the air, a secondary aftercooler (Fig. 3B) can be added to minimize water consumption. The primary aftercooler uses tower, river, or lake water to handle part of the cooling load, while the secondary chilled water aftercooler performs the final cooling. This configuration reduces chilled water consumption and operating expenses.


Fig. 3. Reheat drying system configurations can vary. One arrangement (A) uses a single aftercooler. When chilled water is needed to dry the air, a secondary aftercooler can be added to minimize water consumption (B).

In the food industry, ammonia refrigerant instead of chilled water is commonly used as a cooling source. A significant amount of moisture can be condensed using chilled water or ammonia refrigerant. When 33%%MDASSML%%45-F coolant is readily available, a reheat system can take the place of a refrigerated dryer. This arrangement saves on system capital equipment costs and annual operating expenses because a refrigerated dryer operates on electricity. When a desiccant dryer is used, a reheat system can dramatically improve its performance.

Figure 4 illustrates system performance when air is compressed, cooled, and reheated. In this example, 7100 scfm of atmospheric air at 14.7 psia and 70 F enters a two-stage compressor and discharges at 125 psig. The resulting volume represents 1000 cfm of air at 125 psig and 250 F. It takes 1200 hp to compress the air.


Fig. 4. This drawing illustrates system performance when air is compressed, cooled, and reheated.

When the air is cooled to 70 F in the aftercooler, its volume shrinks to 748 cfm, a 25% reduction. This loss equals 302 hp of work that could have been performed if the air temperature was at 250 F. By reheating the air to 200 F, using the waste heat of compression from the compressor, a gain in air volume of 182 cfm is achieved. The reheat action increases air volume by 24% and reclaims 291 hp of work. In addition, reheating the air eliminates the chance for additional condensation in the air distribution system.

Moisture is a major problem in any compressed air process because the compressor uses ambient air that contains manmade pollutants. Carbon dioxide, sulfur dioxide, chlorine, and similar contaminants combine with the moisture to form weak acids that are concentrated and corrosive in a compressed state. Figure 5 shows the moisture content of compressed air at various pressure dewpoints. Moisture is significantly reduced by cooling the air as much as possible. How cold the air gets depends on the coolant source available.


Fig. 5. Moisture content of compressed air at various pressure dewpoints.

If compressed air is cooled to 47 F using chilled water instead of 85 F using tower water, the moisture content in the compressed air is 11 gal. of water/day as opposed to 43 gal./day. This action reduces the total amount of moisture present in the stream by almost 75%.

The aftercooler(s) in a reheat drying system should be sized to ensure that moisture in the compressed air is minimized. The result is a low air dewpoint that can range from 35%%MDASSML%%90 F, depending on the cooling source available.

—Edited by Jeanine Katzel, Senior Editor, 630-320-7142, jkatzel@cahners.com

&HEADLINE>More info&/HEADLINE>

Technical questions about this article may be directed to the author by phone at 716-877-2608 or by e-mail at info@rpadams.com . The company web site is located at www.rpadams.com .

For additional articles on this and related topics, see the Compressors and prime movers channel at www. plantengineering.com.

&HEADLINE>Advantages of a reheat system&/HEADLINE>

  • Provides dry, high-temperature compressed air for a production or manufacturing process

      • Incurs no external power costs, which saves annual plant operating expenses

          • Increases compressed air volume from a given air compressor by keeping pressure constant and reheating the air

              • Achieves low dewpoints of the compressed air ranging from 35%%MDASSML%%90 F, depending upon the coolant available

                  • Eliminates condensate and external line sweating in the air distribution system

                      • Offers a virtually maintenance-free system with no moving parts or control mechanisms


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