Calculating the Water Costs of Water-Cooled Air Compressors

Compressed air systems are often described as “the “4th Utility” due to their usage in virtually every industrial process and facility. The goal of this post is to concentrate on the potential to cut the amount of water used by compressed air units. The consumption of water has slowed across the U.S. as reductions in the irrigation, power, and industrial sectors have been offset by increases in the public-supply sector driven by the growth in population. Energy managers must be aware of the amount of cooling water needed to maintain the inventory of air compressors at their facilities, as well as the associated cost. An assessment can then be done, of various cooling systems to determine the need for water and strategies to reduce costs.

Compressed air systems are found throughout every industrial facility and process. They are recognized as a potential area to lower electricity (kW) energy expenses through measures such as the reduction of leaks in compressed air and identifying fake demands and unsuitable use. Air compressors with water cooling can be significant water users and reducing their costs could be a further possibility.

A “typical” industrial plant running two 125 horsepower single-stage, water-cooled rotary screw, air compressors will consume 11.4 million Gallons of water each year. A bigger installation, which has the rotary screw with a 350 horse power in similar conditions, could consume 17 million gallons of water per year1.

Some older facilities continue to utilize two-stage, water-cooled air compressors that reciprocate. Paper and pulp mills as well as steel mills are great examples. These types of facilities could require 550 million gallons every entire year to cool water to Air compressors.

Air compressors and air dryers are able to be water-cooled. We strongly recommend that energy managers at multi-factory enterprises make a list of the consumption of water by all air compressors that are installed and of the way that water cooling systems work. A thorough evaluation must be conducted for each facility of the benefits and feasibility of changing to an air-cooled compressor or changing to a different water cooling system.

1Figures derived from a data sheet for one stage, lubricant-cooled air compressor with rotary screw at 100 psig pressure, 8600 work hours, and 70°F temperature for water.

How Much Cooling Water is Required by Air Compressors?

The most common rating for cooling water requirements for air compressors is the amount of Gallons of water per 1000 BTU/hr are rejected by cool water. Air compressors create a significant rejection load because of their basic inefficiency that is i.e. it takes between 7 and 8 input horsepower to provide 1 horsepower of compressed air. This generates heat by the process that reflects the inefficiency. The energy input that is not converted into work appears as heat. The heat must be removed in order for the machine to function and to utilize the air. Particularly in today’s world, where the use of dry compressed air is essential, it has to be effectively and efficiently after-cooled and dried to a specific pressure dewpoint using compressors drying compressed air.

The calculation of the required gallons-per-minute (gpm) is contingent on several crucial factors:

  • Take in cooling water temperature and then transfer it to the air dryer or compressor.
  • The maximum discharge temperature for compressors is -the temperature at which the compressor discharges – i.e. rotating, oil-free rotary screws and centrifugals are able to handle up to 400 °F discharge. Lubricant-cooled units are restricted by the cooling fluid, but typically have capable of handling 200 @F.
  • Other vital information is required for example: OEM recommended airflow (acfm in full load) as well as the power of the compressor shaft (BHP) and the motor input HP/KW, including the motor and drive losses.
  • GPM is normally supplied, in relation to cooling water requirements by the makers of air compressors.

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