5.2: Compressors and the Need for Aftercoolers, Driers, and Receivers
- Page ID
- 116630
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Compressor Air
After passing through the compressor intake filter, air moves into the piston chambers. In two-stage compressors, air undergoes an initial compression in the first stage, resulting in a temperature rise. It then passes through an aftercooler where the temperature decreases before moving to the second stage for further compression and an additional temperature increase.
At this point, the compressed air has increased potential energy but contains excessive heat and water vapor. Air compressor outlet temperatures can range between 400°F to 500°F (204°C to 260°C). The air may also carry some dirt not removed by the intake filter and oil vapor picked up while passing through the compressor. Both contaminants are undesirable, and the high heat could cause issues in the distribution system, such as expansion and contraction of pipes, leading to leaks and reduced efficiency. For this reason, instead of discharging air directly into a receiver for storage, it is first passed through an aftercooler.
Aftercoolers
As the name suggests, an aftercooler cools the compressed air after compression is complete. This cooling process is typically achieved by passing cooling water or air over the aftercooler chamber. In addition to cooling the air, the aftercooler is where some dirt and oil vapor fall out of suspension, and a significant amount of entrained water vapor condenses. The aftercooler must be equipped with a moisture separator, preferably one with an automatic drain.
How an Aftercooler Condenses Water Vapor
When gas cools, its specific volume decreases, which typically leads to a decrease in gas pressure. Additionally, as air cools, its ability to carry water vapor diminishes. This phenomenon can be compared to how cooler air during a summer day often leads to rainfall; similarly, in the aftercooler, as compressed air cools, water vapor condenses and "rains" out.
By the time the air exits the aftercooler and enters the air receiver, it is cooler, cleaner, and contains significantly less water. Since compressed air is typically very humid, aftercoolers are essential for removing much of the water vapor. A well-functioning water-type aftercooler can cool the air to within 15°F of the cooling water temperature and condense up to 90% of the water vapor initially present in the air.
The removal of water is as crucial as the removal of heat. For example, 1,000 cubic feet (28 m³) of compressed air can release up to 1.4 quarts (1.3 liters) of water. A modestly sized system of 100 cfm (472 dm³/s) can produce more than 50 gallons of condensed water in just 24 hours. For systems requiring even drier air, further processes can be employed.
Air Drying Methods
Several methods are available to further dry compressed air:
- Over Compression
In this method, the air is compressed until the partial pressure of the water vapor exceeds the saturation pressure. The air is then expanded, causing it to become drier. While this is a simple method, it consumes a lot of power and is primarily used for small systems, making it less common in industrial applications. - Refrigeration
As mentioned earlier, reducing air temperature decreases its ability to hold water vapor. In a refrigerant-type cooler, hot incoming air exchanges heat with cold outgoing air in a heat exchanger, cooling the air to around 32.4°F (0.6°C) to prevent frost formation. This method provides dry air at relatively low initial and operating costs. - Absorption Process
In the absorption process, water vapor is absorbed either by solid chemicals or by deliquescent agents that liquefy as they absorb water. Common solid chemicals include dehydrated chalk and magnesium perchlorate. Deliquescent agents like lithium chloride and calcium chloride react with water vapor, liquefying in the process. This method is cost-effective but can lead to corrosion issues due to the highly corrosive nature of the drying agents. Maintenance typically involves periodic replacement of the deliquescent agents. - Adsorption (Desiccant Drying)
Adsorption involves trapping water vapor in the pores of desiccant chemicals, such as silica gel or activated alumina. This process is not fully understood but is effective in industrial applications. Although adsorption dryers have moderate to high initial and operating costs, they often have lower maintenance costs than absorption systems due to the absence of moving parts. In heat-regenerative dryers, however, high regenerative temperatures may increase maintenance costs.
Receiver Tank
After the air has left the aftercooler and/or dryer, it is directed to a receiver or storage tank. The receiver tank is used to store a supply of compressed air and assure a steady supply without excessive line pulsations or frequent loading and unloading of the compressor.
A receiver tank is a pressure vessel for air storage, provided with a drain and flow ports and a flow baffle to assure optimum contaminant fallout and to provide ready access to devices that will drain off contaminants. Codes are quite strict as to the strength and testing of such devices. ASME (American Society of Mechanical Engineers), state and local codes require a safety valve to be installed on most units. The valve must be capable of discharging the full rated flow of the compressor. Safety valves are normally set 10 psi (0.7 bar) above the normal receiver pressure, but always below the rated working pressure of the receiver. Always be sure the receiver has a drain, preferably an automatic one, at its lowest point. If the receiver is mounted horizontally, it is a good practice to tilt it slightly to ensure good drainage.
Sizing a Receiver Tank
Receiver tanks are sized according to the compressor output, the size of the system, and air demand cycles. It is better to oversize the receiver than to undersize it. This adds very little to installation costs and by averaging varying flow demands, provides increased capacity to meet demand surges. It also lowers air velocity in the tank, which improves the fallout and removal of condensed droplets of solvents and oils that may be in the air stream.