Compressed air aftercoolers · Air- or water-cooled, placement after the compressor, separator and drain, temperature monitoring, condensate load
An aftercooler is a heat exchanger fitted after an air compressor that cools the hot discharge air, condensing water vapour so it can be separated and drained before the air reaches the receiver, dryer and pipework. Ask three things first: whether it will be air- or water-cooled, the flow, pressure and outlet temperature it must achieve, and how the condensate will be separated and drained.
Air-cooled and water-cooled aftercoolers and intercoolers for industrial air compressors, with their moisture separators and drains. Refrigerated and desiccant dryers (see air dryer), receivers (see air receiver tank) and general process heat exchangers are outside this page.
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01
HSE's Compressed air safety guide (HSG39) says intercoolers and aftercoolers lower the temperature of compressed air and are water or atmospheric-air cooled; with very small compressors the receiver performs the aftercooler's role of lowering the dew point, otherwise “a separate aftercooler will normally be necessary”. It recommends, particularly for medium and large compressors, fitting the aftercooler on the discharge side immediately after the compressor, which HSE says “are frequently provided with separators to remove oil and moisture from the air”, and that “Many aftercoolers are pressure vessels and the associated hazards are similar to those of air receivers”. [1]
For air-cooled coolers HSE asks for good circulation of external air over the heat-exchange surface, space around them for cleaning, unobstructed inlets and outlets, and where possible an air inlet on the north side of the building. Where a fluid is the cooling medium, it asks for a coolant drain valve, corrosion protection of the cooling circuit and a way to measure inlet and outlet temperatures of both air and coolant; it lists the aftercooler outlet among the air-temperature points to monitor and the inlet and outlet of each cooling circuit among water-temperature points. On multi-stage compressors, a safety valve should be fitted, where appropriate, in the inter or after cooler circuit. [1]
Give the supplier the compressor free air delivery and discharge pressure, discharge temperature, required outlet temperature or approach, cooling medium (ambient air temperature or water temperature and flow), allowable air and water pressure drop, materials, separator and drain requirements, and site conditions (freezing, dust).
02
HSE calls for a condensate separator with an automatic drain valve at a suitable drainage point at the cooler's compressed air outlet, protected against freezing in cold conditions. [1]
A US Department of Energy in-plant training session shows why the aftercooler must work: a 25 hp compressor producing about 100 scfm at 100 psig delivers about 240,000 scf in a 40-hour week. If the air leaves the aftercooler saturated at 80 °F (1.42 grains per scf) and then cools to 70 °F (1.03 grains), the 0.39 grain difference condenses 93,600 grains, about 13.4 lb or 13 pints, in the piping each week, and the air is still saturated with a 70 °F dew point. [2]
Editorial calculation: the same arithmetic per hour is 100 scfm × 60 × 0.39 ≈ 2,340 grains, about 0.33 lb of water an hour that a separator and drain downstream of the aftercooler must handle for each 10 °F of further cooling. [2]
03
Sullair, a compressor maker, states in a technical article (J. Moore) that 75% of the water vapour drawn in by a compressor should be removed by its aftercooler, moisture separator and drain. It says a properly sized air-cooled aftercooler cools the air to within 20 °F of ambient, so a compressor room held at 85 °F gives an aftercooler discharge of about 105 °F, and that air-cooled units are “most susceptible to high ambient conditions”. A water-cooled shell-and-tube aftercooler cools the air to within 10–25 °F above the inlet water temperature, for example 85 °F discharge with 60 °F water. Sullair adds that fouled fins reduce effective surface, that a properly maintained cooler should last 10 to 15 years, and that water should be sampled before installing water-cooled units because chlorine can degrade stainless steel coolers quickly. [4]
These figures sit beside HSE's placement and condensate-drain guidance and the US DOE training material above: the approach temperature decides how much water condenses before the dryer. [1] [2] [3]
Editorial calculation: 20 °F is about 11 K and 10–25 °F about 5.6–13.9 K; 85 °F ambient and 105 °F discharge are about 29 °C and 41 °C.
| Sullair figure | Value |
|---|---|
| Water vapour removed at aftercooler, separator and drain | 75% of intake |
| Air-cooled approach to ambient | Within 20 °F (e.g. 85 °F room → 105 °F air) |
| Water-cooled approach to inlet water | 10–25 °F (e.g. 60 °F water → 85 °F air) |
| Recommended compressor-room ambient | 85 °F |
| Service life, maintained cooler | 10–15 years |
Ask the supplier for the approach temperature at your flow and ambient or water temperature, the separator efficiency and drain type, and whether fins or tubes can be cleaned on site.
04
The supplier linked to this page lists a compressed air cooler, in carbon or stainless steel, with other heat exchangers; its card links to the page reviewed. [3]
Ask any supplier for rated air flow at stated inlet temperature and pressure, outlet temperature or approach at your ambient or water temperature, air and water pressure drop, design pressure and code, materials, fan power for air-cooled units, separator efficiency and drain type, and dimensions and weight.
05
This is a functional reference built from the sources cited; it is not one maker's specification.
| Part | Role | Fix in the order |
|---|---|---|
| Heat-exchange core | Cool the compressed air. [1] | Air- or water-cooled; material. |
| Fan or coolant circuit | Remove heat. [1] | Fan power; water flow. |
| Moisture separator | Separate condensate. [1] | Type; efficiency. |
| Automatic drain | Discharge condensate. [1] | Type; freeze protection. |
| Temperature points | Monitor air and coolant. [1] | Gauges or sensors. |
| Coolant drain | Empty the water side. [1] | Valve. |
06
An aftercooler quotation needs values for your compressor. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Cooling Capacity (kW) | Heat removed. | Capacity at your conditions. |
| Air Flow Rate (m³/min) | Compressor free air delivery, e.g. 100 scfm example. [2] | Rated flow. |
| Inlet Air Temperature (°C) | Compressor discharge. | Maximum inlet. |
| Outlet Air Temperature (°C) | Sets downstream condensation. [2] | Outlet or approach. |
| Operating Pressure (MPa) | e.g. 100 psig example. [2] | Design pressure. |
| Cooling Water Flow Rate (m³/h) | Water-cooled units. [1] | Flow and temperature. |
| Cooling Water Pressure Drop (MPa) | Water side. | Value. |
| Air Pressure Drop (MPa) | Air side. | Value at rated flow. |
| Heat Transfer Coefficient (W/(m²·K)) | Core design. | Value. |
| Material | Core and shell. | Materials. |
| Weight (kg) | Mass. | Mass. |
| Dimensions (L×W×H) (mm) | Footprint and clearances. [1] | Size and service space. |
| Approach temperature | Examples: within 20 °F of ambient (air-cooled); 10–25 °F above inlet water (water-cooled) (Sullair). [4] | Approach at your conditions. |
07
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Compressor: [free air delivery; pressure; discharge temperature] Cooling: [air or water; ambient or water temperature; water flow] Target: [outlet temperature or approach] Condensate: [separator; automatic drain] Site: [space; freezing; dust]
This company lists compressed air coolers.
Lists a compressed air cooler with finned-tube, plate and air-to-air heat exchangers.
Product scope checked 2026-10-11
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HSE recommends, particularly for medium and large compressors, fitting it on the discharge side immediately after the compressor. [1]
Yes: HSE calls for a condensate separator with an automatic drain at the cooler's compressed air outlet, protected against freezing. [1]
A US DOE training example shows a further 10 °F drop after the aftercooler condensing about 13 pints of water per 40-hour week from a 100 scfm compressor. [2]
Sullair says a properly sized air-cooled aftercooler cools air to within 20 °F of ambient, and a water-cooled one to 10–25 °F above the inlet water temperature. [4]
Sources checked 2026-10-12. The UK HSE's HSG39 is regulator guidance; the US DOE in-plant training is government training material; Jiema is the supplier linked to this page.
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