Cooling: Water-cooled vs. Air-cooled Compressors

Dimensioning Compressor Installations Compressed Air Wiki Compressors

Just like with computers, there are several ways to cool a compressor. The first choice you have to make is between an air-cooled or a water- cooled compressor. If you pick the latter, know that there are three different water-cooling methods. Let us take a look at these different cooling options and which one you should consider in your compressed air system.

The different methods of water cooling a compressor installation

cooling a compressor installation

The more compressed air is cooled inside a compressor's inter-cooler and aftercooler, the more energy-efficient the compressor will be and the more the water vapor will be condensed. A water-cooled compressor installation puts little demand on the compressor room ventilation system, as the cooling water contains, in the form of heat, approximately 90% of the energy taken up by the electric motor. Compressor water cooling systems can be based on one of three main principles:

  • Open systems without circulating water (connected to an external water supply) 

  • Open systems with circulating water (cooling tower) 

  • Closed systems with circulating water (including an external heat exchanger/ radiator)

Open system without circulating water

In an open system without circulating water, water is supplied by an external source: municipal water mains, lake, stream, or well and after passing through the compressor, this water is discharged as wastewater. The system should be controlled by a thermostat, to maintain the desired air temperature as well as to govern water consumption.

Generally, an open system is easy and inexpensive to install, but expensive to run, especially if the cooling water is taken from the municipal water mains. Water from a lake or stream is normally free of charge, but must be filtered and purified to limit the risk of clogging the cooling system. Furthermore, water that is rich in lime can result in boiler scale forming inside the coolers, and causing gradually impaired cooling. The same applies to salt water, which may however be used if the system is designed properly and dimensioned accordingly.

Open system with circulating water

In an open system with circulating water, cooling water from the compressor is re-cooled in an open cooling tower. Water is cooled in the cooling tower by allowing it to sprinkle down into a chamber as surrounding air is blown through. As a result, part of the water evaporates and the remaining water is cooled to 2˚C below the ambient temperature (this may vary depending on the temperature and relative humidity).

Open systems with circulating water are primarily used when the availability of an external water supply is limited. The disadvantage to this system is that the water gradually becomes contaminated by the surrounding air. The system must be continuously diluted using external water due to evaporation. Dissolvable salts are deposited on the hot metal surfaces, reducing the thermal heat transfer capacity of the cooling tower. The water must be regularly analyzed and treated with chemicals to avoid algae growth in the water. During winter, when the compressor is not operating, the cooling tower must either be drained or the water must be heated to prevent freezing.

Closed system with circulating water

In a closed cooling system, the same water continuously circulates between the compressor and some form of external heat exchanger. This heat exchanger is in turn cooled either by means of an external water circuit or by the surrounding air. When the water is cooled using another water circuit, a flat plate heat exchanger is used. When the water is cooled using the surrounding air, a cooling matrix consisting of pipes and cooling fins is used. The surrounding air is forced to circulate through the pipes and fins by means of one or more fans. This method is suitable if the availability of cooling water is limited.

The cooling capacity of open or closed circuits is about the same, i.e. the compressor water is cooled to 5°C above the coolant temperature. If the cooling water is cooled by the surrounding air, the addition of an anti-freeze (e.g. glycol) is required. The closed cooling water system is filled with pure, softened water.

When glycol is added, the compressor system's water flow must be recalculated, as the type and concentration of glycol affects the water's thermal capacity and viscosity. It is also important that the entire system be thoroughly cleaned before being filled for the first time. A correctly implemented closed water system requires very little supervision and has low maintenance costs. For installations in which the available cooling water is potentially corrosive, the cooler should be designed in a corrosion-resistant material such as Incoloy.

Air-cooled compressors

Most modern compressor packages are also available in an air-cooled version, whereby the forced ventilation inside the air compressor package contains close to 100% of the energy consumed by the electric motor.

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