Compressed air concerns I have serious problems with some of the content of the article, "Recovery and reheat process simplifies compressed air drying" (PE, July 2000, p 82, File 4020).

By Staff December 1, 2000

Compressed air concerns

I have serious problems with some of the content of the article, “Recovery and reheat process simplifies compressed air drying” (PE, July 2000, p 82, File 4020). It might give a false impression to many users of compressed air.

The article talks of the changes in volume at different temperatures. The fact remains that the mass flow of air coming from the air compressor has not changed, only the volume at given conditions.

It is stated that air temperatures as high as 300 F can be obtained. While a small number of process applications may benefit from this figure, there are materials in most compressed air systems that would not be amenable to such a temperature, including O-rings and polycarbonate bowls. One certainly would not want an air supply at this temperature to be supplied to hand tools or through typical hoses.

Even 90-120 F coming from the reheater would not be maintained for long in a compressed air distribution system, with receivers and piping exposed to normal ambient conditions. Much of any benefit of the reheat would be lost by radiant cooling before it reached the points of use.

Most refrigerant-type dryers use the inlet air to reheat the dried air leaving the dryer, but this is limited by the temperature of the inlet air. This approach also precools the inlet air, reducing the dryer load.

A life cycle cost analysis should be done to determine if a reheat system could be justified.-David McCulloch Author’s reply: The article “Recovery and reheat process simplifies compressed air drying” is about an efficient way to dry and reheat compressed air, as compared to a refrigerated dryer, and how the reheat system performs these functions.

The observation that mass flow remains constant and the volume changes with temperature is correct and is best illustrated in the article by Fig. 3 (below). This figure shows the changes in volume during the cooling and reheating conditions and indicates that the airflow rate is a constant 7100 scfm during this process. The point is the constant mass flow of air has a greater volume and will do more work when it is warmer. Work, resulting from the use of compressed air, is dependent upon the volume and pressure at the point of use. Hot air has a greater volume than cold air. Therefore, hot air will do more work at the same cost than cold air. According to Charles Law, if the pressure is held constant, the volume will vary directly as the absolute temperature. So if the pressure and power requirements are constant and the air is reheated, then its volume will increase.

Compressed air is used for a wide variety of applications with the most common use being for pneumatic controls and power tools. It is also used in many process-related applications. Examples are companies that use large volumes of compressed air to pneumatically convey product, pharmaceutical plants that use the air in the fermentation process to make drugs, the powder coatings industry is a large volume user, and glassmakers require large volumes of hot, dry air to manufacture glass. Reheat systems are in successful operation for compressed air systems in all of these applications.

Reheat systems can provide a dew point as low as 35 F, depending on the coolant available. Therefore, any increase in temperature, even to temperature near ambient conditions (about 70 F in a typical plant), will increase the compressed air volume and provide additional work at the same cost in power when compared to air at 35 F.

Each application has its own unique needs and will require different air outlet temperatures. The glass and pharmaceutical industries prefer hotter air, as compared to powder coatings applications or air for pneumatically conveying product. A reheat system is custom designed to meet those needs and provide the desired air outlet temperature. The typical application is for compressed air systems that require large volumes of dry, warm air and have flow rates greater than 500 scfm.

While not every compressed air system will require a reheat system, there are numerous applications that need large volumes of warm, dry air. A total cost analysis of a reheat system, taking into account the capital investment and operating expense, is a good way to demonstrate the merits of these systems.-Carl Kozacki, R.P. Adams Co., Buffalo, NY