One of the most effective ways to save energy when it comes to compressed air is to reduce system flow. This requires dealing with the end uses and abuses of compressed air in an effort to eliminate or reduce the flow of compressed air that results from wasteful practices.
One often-cited opportunity for improvement is leak reduction – a valuable endeavor, but more can be done. Additional significant savings can be achieved by finding and fixing “inappropriate uses,” which we can define as some sort of process that could be more cost-effectively supplied by another energy source. This is not a simple process: There are hundreds if not thousands of individual uses of compressed air in most plants. With that in mind, though, let’s explore some of the more common potentially inappropriate uses of compressed air and look at examples of how plants have addressed these energy-wasters.
If enough investigation is done, you will become skilled at finding ways to amend end uses to reduce compressed air costs. The following case studies are examples of actual projects done over the past few years; they illustrate what can be done with some careful thought and action to reduce inappropriate compressed air use.
Conversion of compressed air agitation
A compressed air audit was performed at a parts machining plant that made transfer case parts for large farm machinery. Once the parts are machined, they’re painted, but to ensure the paint adheres properly to the metal, the part must be carefully cleaned and treated. The part is dipped into a bath to remove any contamination. To ensure proper mixing of the chemicals in the bath, the plant chose to agitate the liquid with 10 cfm of 100 psi compressed air as shown in Figure 1. The energy consumption of this flow of compressed air was 2.8 kW, equivalent to about $2,450 in annual energy consumption.
It was discovered that the actual pressure requirement at the submerged nozzles was about 2 psi; the flow of compressed air was being throttled off with a ball valve. Plant personnel investigated installing a regenerative-style blower to use in place of the compressed air agitation as per Figure 2. Trial with a test apparatus found that the submerged piping actually produced more agitation air than was previously provided, causing a better mixing action. The blower’s required power was measured at 0.42 kW – about 15% of the equivalent compressed air power consumption.
This particular plant had about four other containers of chemicals with agitation in place. Conversion of the compressed-air-powered agitation with blower style achieve a simple payback on the project of slightly more than two years, which was reduced to one year by a utility incentive.
More-efficient cabinet cooling
A furniture-making facility had a number of complex CNC machines that were controlled by PLC. The PLCs were installed within sealed metal enclosures near the machine in an often-warm environment, with the enclosure’s internal temperature reaching high levels thanks to a power transformer installed within the same cabinet. The high temperature often caused thermal failure of the control, and plant personnel recognized that they needed some sort of cooling to prevent this problem. Compressed-air-powered coolers were chosen because of their simplified design and ease of installation. To save costs, these coolers had no temperature controls installed, and therefore they ran 24/7, even when the plant was down during evenings and weekends.
A compressed air audit found that this style of cooling was quite expensive. Each cooler consumed a continuous 20 cfm of compressed air while producing 1,500 btu/hr of cooling – equivalent to about 4 kW of air compressor power. There were about 12 similar coolers, operating in the plant, contributing about $42,000 per year in operating costs.
The plant sourced some refrigerated panel coolers to use in place of the compressed-air-powered units. The units were installed with temperature controllers to ensure that the cooling circuits turned off when their use wasn’t required. Each of these coolers produced 3,000 btu per hour of cooling, with the average power consumption measured at about 4% of that of the compressed-air-powered coolers. Estimated payback for this conversion was 1.3 years based on $40,000 per year savings.
Through the years a number of similar cooling applications have come up. It is very common to see cabinets with a small cooling fan installed but no secondary ventilation hole to allow the air to circulate. In some of these cases simply providing better fan-powered ventilation with no refrigeration cooling provides the necessary solution.