More and more industrial compressed air users are embracing a systems approach with respect to energy efficiency. Gone are the days of simply comparing the electrical efficiency of compressors and buying the one with the lowest input energy. Smart operators are looking beyond individual components. They are beginning to understand that true efficiency is more than the sum of the system’s parts – it’s how everything works together as a whole.
We’ve seen this with system master controls ushering in the industrial internet of things (IIoT), carefully orchestrating interplay between system components to ensure energy efficiency and make predictive maintenance possible. And it’s not just “new” technology that a systems approach makes possible – it's also ideas that may have been around for awhile but hadn’t been thoroughly considered or acted on.Industrial compressed air users are also getting more aggressive about managing leaks, reducing operating pressure, and finding alternatives to using compressed air for some applications. The results are stronger, leaner, and more-efficient compressed air systems.
There is another opportunity for major energy savings in compressed air; however, it depends on thinking about compressors in a different way. So let’s shift a paradigm and save some money while we’re at it.
These are not the compressors you are looking for
For users and system designers to expand their systems approach and take the next big leap in cost reduction requires embracing a new paradigm: Industrial compressors are just electric heaters that happen to make compressed air. A system with two 250 hp compressors, then, can be thought of asa 375kW heater. Taking a 375kW air heater (air-cooled compressors) and venting it to the outside all year long makes no economic sense. Taking a 375kW water heater (water-cooled units) and running the hot water to a cooling tower to vent the heat to atmospherebefore reheating the cooled water, only to cool it again, is even worse.
A little bit of physics goes a long way
To make this paradigm shift, it is important to understand a bit of physics. All of the input power to a compressor is converted to heat – all of it. That input power does not add any energy to the air. This often prompts the question, “If all of the input power were converted to heat, how is any energy left in the compressed air to do work?” The simple answer is that the energy was already there before the air was compressed. The fact is, at the same temperature, 1 pound of air at atmospheric pressure contains exactly the same amount of energy as 1 pound of air at 100 PSIG.
Air at a temperature above absolute zero contains energy. If air is at 60°F, it is really 521° above absolute zero. We can use gas laws to find that 1 pound of air requires about 123.7 BTUs to heat it from absolute zero to 60°F. That is the intrinsic energy of 1 pound of air at 60°F.
When air expands, it gives up heat. When air expands from 100 psig to atmospheric pressure (14.7 PSIA), its temperature will drop by about 231°F. Here is the formula for calculating the temperature change:
T1 is the absolute temperature after expansion
T is the absolute temperature of the air before expansion
P1 is the absolute ambient pressure in pounds per square inch
P is the absolute pressure of the compressed air in pounds per square inch
n is the ratio of specific heat at a constant pressure and specific heat at a constant volume (1.4 for air)