Using ISO Standards and Heat Recovery to Reduce Operating Costs
Refrigerated air dryers are using brazed plate heat exchangers to help increase system efficiency and lower system operating costs.
Industrial compressed air is so widely used that it is often regarded as the fourth utility, after electricity, natural gas and water. Employed in thousands of applications, compressed air is vital to productivity around the globe for uses such as powering rotary and reciprocating equipment. In addition, it used to atomize, spray, cool, sand blast and agitate. From an application standpoint, compressed air can be divided into process air, control air and general plant air.
Water condensate is a common problem in compressed air systems. It can cause corrosion and other damage to the pneumatic equipment. For this reason, compressed air must always be dried if it is to be used in any kind of machinery. A common solution to this challenge is a refrigerated air dryer, which prevents the condensation of water by reducing the relative humidity of the air.
ISO 8573.1: 2010 is a group of international standards relating to the quality of compressed air, and these standards consist of 10 separate standards classes. Each class specifies the quality requirements of the compressed air in relation to solid particulate, water and oil.
Table 1 is the primary document used from ISO 8573.1: 2010. The standard allows the user to specify the air quality or purity required at crucial points in a compressed air system with a simple number class. It should be noted that the purity level for each contaminant is shown in separate tables within the standard.
For ease of use in this article, however, the three main contaminants are combined into one table.
Understanding and Relating the ISO Standards
It is important to understand and apply these standards. Keep in mind that while the solid particulate portion typically is the most straightforward, the rest often are complicated. Also, remember that your choice of equipment can limit the ISO class your system can achieve. For instance, an oil-flooded rotary screw compressor will never deliver an ISO Class 1 oil rating; instead, it will require an oil-free compressor to stay within the limits.
Some unique brazed plate heat exchanger solutions, including brazed plate heat exchangers for air dryers, offer versions with and without an integrated separator.
Similarly, it is impossible to achieve an ISO Class 2 water rating with a refrigerated air dryer. A compressed air system rated for an ISO Class 2 water rating typically will require a desiccant air dryer. The initial cost required for an oil-free compressor is substantial compared to a standard, contact-cooled compressor, and the operating cost for a desiccant air dryer is significantly more than that for a refrigerated
Also, it is critical to understand how ambient conditions may affect equipment choice. For instance, if the compressed air lines are outdoors, and the facility is in an area that experiences sustained ambient temperatures below freezing (for example, near the city of Chicago) you may need an ISO Class 2 water rating to avoid freezing in the lines. At the same time, you might not necessarily require an ISO Class 2 solid particulate or oil rating. Understanding these details will help the company save money in the initial capital purchase, operating budget and reliability. All of these can be measured using some standard rules of thumb for compressed air.
This brazed plate heat exchanger combines the energy recovery unit and the refrigerated cooler, sandwiching an integrated separator in a modular design. This design offers stable performance, convenient drainage and simple installation.
The Cost of Making Compressed Air
A recent survey by the U.S. Department of Energy showed that for a typical industrial facility, approximately 10 percent of the electricity consumed goes to generating compressed air. For some facilities, compressed air generation may account for 30 percent or more of the electricity consumed.
Compressed air is an on-site generated utility — and it is one of the most expensive sources of energy in a plant. Very often, the cost of generation is not known; however, some companies use a value of $0.15 to $0.30 per 1,000 ft3 of air. In addition, the overall efficiency of a typical compressed air system can be as low as 10 to 15 percent. For example, to operate a 1 hp air motor at 100 psig, approximately 7 to 8 hp of electrical power is supplied to the air compressor. To calculate the cost of compressed air in your facility, use this formula:
where bhp is the compressor shaft horsepower, which frequently is higher than the motor nameplate horsepower (check equipment specification); percent time is the percentage of time running at this operating level; percent full-load bhp is the compressor shaft horsepower (bhp) as percentage of full-load bhp at this operating level; and motor efficiency is the motor efficiency at the specified operating level (typically 85 to 96 percent).
Refrigerated air dryers use brazed plate heat exchangers that help increase the system efficiency to condense moisture while lowering system operating costs. An integrated heat recovery process inside the brazed plate heat exchanger is coupled with the refrigerant cooler in the dryer.
Let us assume our previous example in Chicago has a 100 hp (450 scfm) air-cooled compressor that is running at 100 percent load for 75 percent of the time, and unloaded 50 percent for 25 percent of the time. Also assume the facility typically works two shifts (from 6 a.m. to 11 p.m.) five days a week, for 50 weeks each year. Finally, also assume the aggregate electric rate is approximately $0.07 per kilowatt-hour. The cost for compressed air would be as follows:
If the same facility in Chicago has connection points outside, which require compressed air, they would need an ISO Class 2 compressed air system, and a desiccant air dryer would be required.
The Cost of Drying Compressed Air
A heatless desiccant air dryer uses approximately 18 percent of the rated flow for purged air. Based on compressor data, a 100 hp compressor will produce approximately 450 scfm at 100 psig. Using our formula, this would cost about $7,000/year just for the compressed air to run the desiccant dryer for the ISO Class 2 rating.
As an alternative, the facility can instead use a refrigerated air dryer to obtain an ISO Class 4 water rating for the system and use a small desiccant point-of-use dryer for the outdoor lines. The ISO Class 4 system would use a 450 scfm refrigerated dryer with a small 20 scfm desiccant dryer using less than 4 scfm of purged air. Doing this would save the company more than $5,300 per year and still allow for a reliable system with a similar capital cost. In addition, the facility could use the 80+ scfm for production instead of purging a large desiccant dryer.
Brazed Plate Heat Exchangers Improve Air Dryer Applications
Why is this possible? Many refrigerated air dryers now use brazed plate heat exchangers (BPHEs). This heat exchanger technology has increased the system efficiency to condense moisture while lowering system operating costs.
An integrated heat recovery process inside the brazed plated heat exchanger is coupled with the refrigerant cooler in the dryer. This allows the incoming warm, moist air to be precooled before the refrigerant system, thereby reducing the power needed to condense the moisture and heat the air returning to the compressed air system.
Some unique brazed plate heat exchanger solutions, including those for air dryers, offer versions with and without an integrated separator. These brazed plate heat exchangers are also some of the most compact air dryer heat exchangers on the market. They combine the energy recovery unit and the refrigerated cooler, sandwiching an integrated separator in a modular design. This solution offers stable high performance, convenient drainage and simple installation.