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.
Learn more about the aftercoolers eCompressedair.com offers by
viewing our product selection of aftercoolers.
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.
Functions of compressed air aftercoolers
- 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.
Recommended location of
aftercoolers
The aftercooler should be located as
close as possible to the discharge of the compressor.
Types of
aftercoolers
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 Aftercooler

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.
Water-Cooled Pipe Line
Aftercooler

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.
Aftercooling sizing
considerations
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.
Maintenance
requirements
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.
Rules of Thumb
- 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.
Learn more about the aftercoolers eCompressedair.com offers by
viewing our product selection of aftercoolers.