Basics of Compressed Dry Air System Design Part 2

Basics of Compressed Air System Design and Troubleshooting Part Two

September 7, 2017 9:17 pm

Last week, we gave you a taste of what goes into compressed air system design and troubleshooting, touching on everything from system components to fixing common issues.

It’s a topic that can be difficult to digest, so we split it up. Let’s continue with part two in our series.

Benefits of Master Controllers

A typical CDA system uses multiple compressors to supply the flow of compressed air. But most plants experience wide fluctuations in demand, so the systems can experience compressor cycling and air pressure drops. Master controllers can prevent these problems as well as deliver other benefits for CDA systems, making them a smart investment for compressed air operators. Master controllers save energy because they run compressors less and on lower pressure, which reduces air lost to leaks and lowers artificial demand. Master controllers reduce maintenance costs because there is less cycling and switching, which extends valve life; and causes fewer motor starts, leading to longer motor life. Master controllers also improve production equipment performance and result in less downtime due to pressure alarms.

The master controller can also be used to implement “system splitting.” Depending on demand, space and redundancy requirements, system splitting is an excellent way to increase system efficiency and equipment utilization. System splitting is accomplished by replacing one large compressor with three smaller ones that can be staged to meet demand. Just remember that there should be a redundant machine that is large enough to meet maximum system demand if any one machine fails. For even more efficiency, adding a compressor with a variable speed drive (VSD) along with system splitting is an excellent way to meet exact facility demand, while reducing air loss through leaks and fittings, and achieve very high (>95%) equipment utilization. The VSD accomplishes this by slowing down and speeding up to match all the compressors output capacity to the existing demand capacities without control gaps.

Condensate Management

Condensate management is crucial to the reliable, efficient operation of CDA systems. Condensate accumulates in receiver tanks, filters, dryers, and piping. If not removed, it’s carried downstream with compressed air and contaminates production equipment and products at the points of use. Condensate can:

  • Saturate filter elements and render them useless
  • Increase maintenance costs
  • Reduce system efficiency

Drain traps are a critical but often overlooked component in compressed air systems. They are an effective way to remove the condensate that accumulates in the receivers, filters and dryers. There are two types of drain traps: manual traps with hand operated valves and automatic, which are either timed or demand operated. The manual systems are inexpensive while the timed traps are moderately priced at about $150. The demand systems are a larger investment at close to $400.

Heat Recovery

One way to think of compressors is as extremely efficient space heaters that produce compressed air as a by-product. This is because 100 percent of the electrical energy input into a compressor is turned into heat. As a result, air compressors produce approximately 2550 Btu per hp. The good news is that up to 96 percent of that heat is recoverable, and the heat recovery can be achieved by air or water. Recovering all this heat is a more efficient practice that simply exhausting it into the air, as is commonly done.

One simple way to recover heat is to duct the compressor so it recirculates the hot air inside the facility during the cold months to provide free heating. Automated dampers tied into building control system are the preferred method to direct the air, but many places use manual dampers to achieve the same result. Other uses for the hot air include space heating, product drying and makeup air for the heating, ventilation and air conditioning (HVAC) system. Removing the heat benefits the machines as well because compressors operate most efficiently and have a longer lifespan at moderate temperatures, which regulation requires be between 38°F and 104°F. Note that a small 25 hp compressor generates enough thermal energy to heat a typical family home. Water cooled compressors are an option for 25 hp and above and can heat water to a max temp of 160°F.

FIGURE 1
Here is how much an operator of a twin 250-hp compressed air system could save by taking some basic steps. (Based on U.S. Department of Energy Estimates, with a blended electrical rate of $0.10/kWh and annual electrical cost of $328,500)

Strategy Annual Savings
Repair Leaks $98,550
Reduce Pressure $32.850
Eliminate Artificial Demand* $32,850
Install Split System with Master Controls $41,062.50
TOTAL SAVINGS $266,906.25**

*System now at 1,250 CFM
**Annual electrical cost: $123,187.50 / Heat recovery offset gas cost: $61,593.75

Compressed air is often taken for granted, like a fourth utility. But neglecting a CDA system can be a costly mistake. Generating more air pressure than is necessary for the particular application wastes money and can cause system failure. Investing in master controls opens up new opportunities to use the CDA system to its best advantage at the lowest cost. Likewise, not utilizing all the heat generated by the system is another missed opportunity to reduce operating costs. Understanding these basics of compressed air systems enables users to get the most from their systems while saving money.