The Maintenance Manager at a major manufacturer of cast aluminum wheels recently called me. He reported problems with wet compressed air in both the casting and machining areas of the plant, and asked that I come in to look at the existing air dryers and make some recommendations.
It’s always good to get away from the office and directly work on an air drying application.
The plant’s compressed air system is big, with peak demand of 15,000 SCFM. There are three distinct compressor locations that supply the compressed air. The maintenance manager wanted to focus on improving the compressed air system in phases.
The first phase of the project deals with (3) 2000 SCFM oil flooded screw compressors with water cooled after-coolers that are fed with 60-65F closed loop chiller water. Downstream of the compressors are coalescing filters, refrigerated dryers, and another set of coalescing filters. All of the air treatment components were properly sized. I measured the inlet temperature to the dryers at 85F and the inlet pressure was 125 psig.
85F is pretty cool. A relatively low compressed air inlet temperature is always good to see. Cool compressed air contains less water vapor and means a reduced load on a dryer. A good cooler is a critical—and often overlooked—part of the air treatment process.
I also measured the ambient temperature in the compressor room at 85F, the casting area ambient was 111F, and the machining area ambient was 85F.
The Maintenance Manager said the refrigerated dryers have not been very reliable and repairs are extremely expensive. He said there are no backups to these dryers, so when one of them goes down they are pushing wet air downstream which results in product rejects. He also has issues with the pressure drop across each dryers being a bit more than 5 PSID. Pressure drop is a real cost in a compressed air system. Lost pressure amounts to lost energy. (Here is how to calculate pressure drop.)
Suggested Action Plan:
1. Check to make sure all drains are working properly on the after-cooler separators, coalescing filters, and refrigerated dryers.
2. Install a dry air receiver tank (downstream of dryers and filters) to eliminate slugs of water going downstream in the event of a catastrophic failure of an upstream drain, separator, or dryer.
3. Since the inlet temperature to the dryers is consistent year round (80-85F) the refrigerated dryers could be replaced with single tower deliquescent dryers. The single tower dryer will suppress the pressure dew point by 20F. This means a constant 60-65F outlet pressure dew point. That dew point is well below any of the ambient temperatures I measured inside the facility. So the airlines would remain liquid free at all times. Also I explained that a properly sized single tower dryer has less than 1 PSID – real energy savings.
1. Reductions in pressure drop of 4 psig which translates into energy savings of 2% per year. Since this is a 24/7/365 operation and their utility cost is $.0627/Kw the customer will save ~$12,000.00/year in electric costs on pressure drop savings alone. Of course, some of that savings will be diverted to replacement Dry-O-Lite desiccant. But net of that, the facility is still looking at almost $7000 of operating cost reduction, due to reduced pressure loss.
2. Elimination of repair bills and utility bills for refrigerated dryers.
3. Elimination of product rejects caused by wet air.
4. Downtime eliminated. Single tower deliquescent dryers have no moving parts.
We calculated that the return on investment for new equipment will be less than two years for the first phase of the project.
The next phases of the project will have us looking at their 6000 SCFM Centrifugal Compressor and their 3000 SCFM screw compressor applications. I look forward to getting back on site and working with these guys to improve the air system.