Calculating the Water Costs of Water-Cooled Air Compressors
Compressed air systems are sometimes called the “4th Utility” due to their presence in almost all industrial processes and facilities. The objective of this article is to focus on the opportunity to reduce the water consumption of compressed air systems. Water consumption has leveled off in the U.S. as reductions in the power, irrigation, and industrial segments have offset increases in the public-supply segment driven by population growth. Energy managers should understand how much cooling water is required for the inventory of air compressors in their factories along with the related costs. An evaluation can then be made, of the different types of cooling systems, to ascertain water and cost reduction strategies.
Compressed air systems are present in almost all industrial processes and facilities. They have been correctly identified as an area of opportunity to reduce electrical (kW) energy costs through measures like reducing compressed air leaks and identifying artificial demand and inappropriate uses. Water-cooled air compressors can also be significant consumers of water and reducing these costs can represent a second area of opportunity.
A very “typical” industrial plant running two 125 horsepower, water-cooled, single-stage, rotary screw, air compressors can consume 11.4 million gallons of water per year. A larger installation, with a 350 horsepower rotary screw under similar circumstances, can consume 17 million gallons per year¹.
Many older facilities continue to use two-stage, water-cooled, reciprocating air compressors. Pulp and paper mills and steel mills are perfect examples. Facilities, like these, can require 550 million gallons per year of cooling water for the air compressors.
Both air compressors and compressed air dryers can be water-cooled. We highly recommend that energy managers, at multifactory corporations, take an inventory of the water-consumption of all the installed air compressors and of how the water-cooling systems function. An evaluation should be made, in each facility, of the feasibility and benefits of switching to an air-cooled air compressor or switching to a different watercooling system.
¹Figures taken from a data sheet of a single stage, lubricant cooled, rotary screw air compressors at 100 psig pressure, 8600 working hours, and 70 °F water temperature.
How Much Cooling Water is Required by Air Compressors?
The standard rating, for air compressor cooling water requirements, is how many gallons of water per 1,000 btu/hr is rejected into the cooling water flow. Air compressors generate a high rejection load due to their very basic inefficiency — i.e. it takes 7 to 8 input horsepower to supply 1 hp of work in compressed air. This creates a heat-of-compression generated during the process reflecting this inefficiency. Energy input not converted to work shows up as heat. This heat has to be removed for the equipment to run and for the plant to be able to use the air. Particularly today, where dry compressed air is often critical, it must be reliably and effectively after-cooled and dried to a specified pressure dew point using compressed air dryers.
Calculating the required gallons-perminute (gpm) is dependent upon several critical variables:
- Intake cooling water temperature to the air compressor or dryer.
- The allowable compressor discharge temperature — i.e. reciprocating, oil-free rotary screw and centrifugals easily handle 350 to 400 ˚F discharge. Lubricant-cooled units are limited by the cooling lubricant fluid but are usually a maximum of about 200 ˚F.
- Other critical data is needed such as OEM rated air flow (acfm at full load pressure), compressor shaft power (BHP), and motor input HP/KW inclusive of motor/drive losses.
- GPM is typically provided, relative to cooling water requirements, by the manufacturers of air compressors.
Cooling Water Requirements for Single-Stage Rotary Screw Air Compresors
* This data is general in nature and should not be used to select equipment. It is necessary to look at the specific engineering data for all equipment being used.
**System at 100 psig
Cooling Water Requirements for Two-Stage Reciprocating Air Compresors
*This data is general in nature and should not be used to select equipment. It is necessary to look at the specific engineering data for all equipment being used.
Three Primary Sources of Industrial Cooling Water
Public Supply Water
Discussed in the U.S. Geological Survey as the category that continues to grow in water use. Over the last 40 years these costs have escalated rapidly reflecting the scarcity of water and the cost of water treatment. It is becoming the exception to the rule today to see a compressed air system’s cooling-water supply coming from the municipal utility. True costs are not always evident. Additional costs, like sewer chargers, can’t be ignored. Often, city water will still require water treatment for effective performance in industrial cooling. These costs must also be considered.
Well-water has varying site-specific characteristics but it is generally not “Free”. After the well is drilled, in most parts of the world the good news is that the water is usually cool. The bad news is that it usually requires significant intake filtration and water treatment for industrial use. Electric energy is required to pump it out of the ground and through the equipment.
Today, the cost of disposing of the heated cooling water has escalated as various agencies may limit the dumping of the heated water into streams, rivers and lakes due to potential thermal pollution. This is what has driven the thermoelectric power plants to alternative cooling systems. In many areas well-water supply is diminishing as the water tables are lowering and as the well gets older, the total flow in gallons-per-minute (gpm) falls off.
River Water/Lake Water
River and lake water have the same limitations as well-water with regards to intake filtration and water treatment. In many, if not most, areas today it is no longer “free” and there often is a charge for the discharge-heated water to the local body of water. There are also EPA Clean Water Act regulations to be met and monitored with any water being discharged to this type of water supply.
|“Rule of Thumb” Formula
to Calculate Water-Cooling
Costs of Air Compressors
Calculating the Basic Water Costs for Compressed Air Systems
Regardless of what “rule of thumb” number is used for water cost (not including energy use) it will probably not be right for each particular location. This cost is very site specific and should be the first factor identified when embarking on a water cost, energycontrol program. All of these escalating costs, along with the man hours required to measure and manage the process, have created a great incentive for industrial plants to supply or replace all their own plant water utility.
The net result of these cost factors for cooling water have resulted with most design engineers using a “default cost” of $3.00 (USD) per 1,000 gallons of cooling water — when the actual site situation is unknown. The accompanying water treatment cost is about specific situations and can be much higher depending on site conditions and maintenance diligence — $1.20 per 1,000 gallons based on 40 grains of hardness, alkalinity 10, and biocide treatment included.
Switching to Air-Cooled Air Compressors
In light of escalating water costs and regulations, plants have sought to decrease their cooling water requirements. Compressed air systems have become prime targets for continuous duty air-cooled air compressors. Some air-cooled rotary screw compressors have seen design improvements and can accept higher ambient temperatures than in the past.
The benefits of switching are readily apparent in terms of reduced water consumption and costs. Using the prior example, an air-cooled, two-stage, rotary screw, lubricant-cooled, 100 horsepower air compressor will have 85% lower water and electrical energy costs — compared to a similar water-cooled unit. The air-cooled unit will simply deploy a 5 kW fan creating an energy cost of $2,850.00 per year. Assuming 8600 working hours per year to take into account down-time, the water-cooled unit, with cooling water at 70 °F, will use 5,676,000 gallons per year at a cost of $17,028.00 per year. Add the small compressor enclosure vent fan and the total annual cost is $17,597.00.
Using a similar example, an air-cooled, 200 horsepower oil-free, air compressor will see 89% lower water and electrical energy costs — compared to a similar water-cooled unit. The air-cooled unit will simply deploy a 10 kW fan creating an energy cost of $5,160.00 per year. Assuming 8600 working hours per year to take into account down-time, the water-cooled unit, with cooling water at 70 °F, will use 7,236,600 gallons of water per year at a cost of $44,892.00 per year. Add a small compressor enclosure vent fan and the total annual cost is $45,279.00.
Both of these examples used conservative water temperatures of 70 °F, did not add the potential cost of $1.20 per thousand gallons for water treatment, and did not add pumping circulation costs.
When evaluating a switch to an air-cooled air compressor, however, particularly on units larger than 100 horsepower, it is important to seriously evaluate all the application variables like room ventilation, ambient temperatures, after-cooler and dryer performance capabilities, maintenance personnel and processes.
All this considered, looking at the differences, it is obvious why plants are looking for aircooled units whenever possible and/or looking at how to minimize water cooling costs when air is not viable.
For more information, please contact Hitachi America
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