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Aftercoolers
Why is aftercooling required in a compressed air system?
Functions of compressed air aftercoolers
Recommended location of aftercooler
Types of aftercoolers
Air Cooled Aftercooler
Water-Cooled Pipe Line Aftercooler
Aftercooling sizing considerations
Maintenance requirements
Rule of thumb
The compressed
air discharged from an air compressor is hot (fig.
AC1-1). Compressed air at these temperatures contains large quantities of water in
vapor form. As the
compressed air cools this water vapor condenses into a liquid form. As an example if an aftercooler is not used, a 200 scfm compressor operating at 100 psig introduces 45 gallons of water into the compressed air system each day.
Compressor Outlet Temperatures
| Compressor Outlet Temperatures |
| Type of Compressor |
Average Outlet Air Temperature (°F) |
Average Outlet Air Temperature (°C) |
| Oil Flooded Rotary |
200 |
93.3 |
| Oil Free Rotary |
350 |
176.6 |
| 2-Stage Reciprocating |
300 |
148.8 |
| Centrifugal |
225 |
107.2 |
| Figure AC1-1 |
Additionally by reducing the air temperature condensate forms. Most
air aftercoolers are sized to cool the air to within 5°F to 20°F (2.7°C to 11°C) of ambient air
temperature. As the compressed air cools up
to 75% of the water vapor present condenses to a liquid and can be
removed from the system.
A moisture
separator installed at the discharge of the aftercooler removes most of
the liquid moisture and solids from the
compressed air. Utilizing centrifugal force, moisture and solids
collect at the bottom of the moisture separator. An automatic drain should be used
to remove the moisture and solids.
- Cool air discharged
from air compressors via the heat exchanger
- Reduce risk of fire (Hot compressed air pipes can
be a source of ignition)
- Reduce compressed air moisture level
- Increase system capacity
- Protect downstream equipment from
excessive heat
Coolers are usually sized with a CTD (Cold Temperature Difference) of
5°F, 10°F, 15°F or 20°F (2.7°C, 5.5°C, 8.3°C, or 11°C). This means that the compressed air temperature
at the outlet of the aftercooler will be equal to
the cooling medium temperature plus the CTD when sized at the
specified inlet air temperature and flow.
The aftercooler should be located as close as possible to the discharge of the compressor.
There are two basic types of air aftercoolers:
The type of aftercooler you choose depends on your preference and requirements.
Compressor manufacturers may include aftercoolers within the compressor package. In
general these compressors are referred to as integral aftercoolers. A stand-alone or freestanding
aftercooler is
a separate unit installed downstream of the
compressor.
Air cooled aftercoolers (fig. AC1-2) use ambient air to cool the hot compressed air. The
compressed air enters the air cooled aftercooler. The compressed
air travels through either finned tubes or corrugated aluminum sheets of the
aftercooler while ambient air is forced over the cooler by a
motor-driven fan. The cooler, ambient air removes heat from the compressed
air.
Belt Guard Air Cooled Aftercooler
A belt guard air-cooled aftercooler mounts to the compressor's v-belt guard. The
compressor's belt pulley has fins designed to force ambient air over the
compressor and air cooled aftercooler. The air passing over
the aftercooler facilitates the heat transfer. The pulley also sends
air over the compressor helping maintain proper operating temperature.
A water-cooled pipe line aftercooler comes in different styles. The most common style for compressed
air service is a Shell and Tube Heat Exchanger/Aftercooler (fig. AC1-3). The pipe line aftercooler consists
of a shell with a bundle of tubes fitted inside. Typically the compressed air flows
through the tubes in one direction as water flows on the shell side in the opposite direction. Heat
from the compressed air is transferred to the water. Water vapor forms as the
compressed air cools. The moisture is removed by the moisture separator
and drain
valve. A modulating valve is recommended to maintain a
consistent temperature and reduce water
consumption. The tube bundles can be fixed or removable. Fixed tube bundles cost less but are more
difficult to maintain than bundles that can be removed for cleaning or service.
The disadvantages of a water-cooled aftercooler include high
water usage and difficult heat recovery. Advantages
to using a water-cooled aftercooler include better heat transfer and no required
electricity.
Most air aftercoolers are sized to cool the air to within 5 to 20°F (2.7°C to
11°C) of ambient
air or cooling water temperature. This is called the approach
temperature. Always size for the hottest day with 100% relative humidity.
Sizing is dependent on the temperature of the air coming from the
compressor.
Typically air exiting a compressor is between 180°F (82.2°C) to
350°F (176.6°C).
To select an aftercooler, determine the approach temperature
requirement (temperature above the cooling medium used) for your compressed air.
Specify 5, 10, 15, or 20°F (2.7°C, 5.5°C, 8.3°C, or 11°C) over the cooling medium
temperature. Consider the temperature requirements of downstream equipment like
dryers and your location's climate. Then, based on your compressor's CFM (cubic feet per minute) and compressed air
temperature,
choose an aftercooler.
Compressed Air System
Components Temperature Requirements.
| Component |
Normal Operating Temperature (°F) |
Normal Operating Temperature (°C) |
Average Maximum Operating
Temperature* (°F) |
Average Maximum Operating
Temperature* (°C) |
| Standard Refrigerated Air Dryer |
100 |
37.7 |
130 |
54.4 |
| High Temperature Refrigerated Air Dryer |
180 |
82.2 |
190 |
87.7 |
| Heated Desiccant Dryer |
100 |
37.7 |
120 |
48.8 |
| Heated Compression Desiccant Dryer |
350 |
176.6 |
375 |
190.5 |
| Pressure Swing Desiccant Dryer |
100 |
37.7 |
120 |
48.8 |
| Standard Particulate Filter |
100 |
37.7 |
150 |
65.5 |
| High Temperature Particulate Filter |
350 |
176.6 |
450 |
232.2 |
| Coalescer Filter |
100 |
37.7 |
150 |
65.5 |
| Vapor Filter (Activated Charcoal) |
100 |
37.7 |
150 |
65.5 |
| Oil / Water
Separators |
100 (condensate) |
37.7 (condensate) |
180 (condensate) |
82.2 (condensate) |
| Drain Valves |
100 |
37.7 |
180 |
82.2 |
| FRL’s (polycarbonate
bowl) |
100 |
37.7 |
125 |
51.6 |
| FRL’s (metal bowl) |
100 |
37.7 |
175 |
79.4 |
*These are industry average maximum operating temperatures provided for
reference. Before buying or installing any compressed air component,
check the actual maximum operating temperature for that
particular product.
Figure AC1-4
An oversized aftercooler is usually
cost-effective; it will produce cooler air with less moisture, reducing the
requirements for the air dryer. Additionally, a
larger aftercooler will have a lower pressure drop, lowering
the necessary discharge pressure
from the compressor. For the best
results, size the aftercooler for a 1 to 2 psi pressure drop.
Aftercooler (water-cooled): Routinely inspect, clean, check approach
temperature, pressure drop and monitor water quality.
Aftercooler (air-cooled): Inspect and clean on a regular basis.
Check drain valves
daily.
Proper maintenance will keep the aftercooler efficient. A dirty
aftercooler results in both warmer air temperatures and increased
pressure drop.
- Most aftercoolers are sized to cool the air to an approach
temperature of 5 to 20°F (2.7°C to 11°C) of ambient air
temperature.
- Size for the hottest day with 100% relative humidity.
- Compressed air aftercoolers are located directly downstream of the compressor.
- Proper maintenance will keep the aftercooler efficient. A
dirty aftercooler results in both warmer air temperatures and increased pressure drop.
- For every 20°F (11.1°C) rise in compressed air temperature, the
moisture content of the air doubles.
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