Figure 3. Use a booster for high equipment such as this filter press instead of keeping the entire system at high pressure.
Pressure regulators operate full open
Often the process regulator is wide open at the header pressure because the “droop” wasn’t allowed for. The regulator may be correctly sized but the gulp of air required is more than the line can supply. If, during the delay, there isn’t enough volume to hold the demand pressure, then the pressure falls. Production is negatively affected and the operator opens the regulator up to full line pressure. This is called “artificial demand.”
A fix for this problem is a receiver. This will hold the pressure until the regulator opens. One example is an air-driven agitator used for a settling pond. Three minutes of agitation at 150 cfm are required every 30 minutes to promote digestion. The pressure drop, measured for the cycle, is from 90 psig to 40 psig. The total air, based on the inlet pressure, required for agitation is 450 ft3 (3 minutes × 150 cfm). Therefore, the volume of the receiver should be at least:
VR = (450) (14.7)/ (40) = 165 ft3
Inappropriate uses for compressed air
This can be the most difficult problem to solve. The previous example could represent an inappropriate use of compressed air if electrical power is available. Some other applications could be:
- cooling for electric cabinet;
- vacuum (eductors);
- blowing, or blasting;
- pumps (air-diaphragm);
- air hoists;
- air vibrators; and
- sparging (agitating with air);
Table 1 summarizes typical savings possible by eliminating inappropriate uses of compressed air. Some applications, such as using an air eductor, or ejectors, to prime a suction-lift pump require compressed air only periodically. Still, it is worth noting that to produce 10 in. of Hg vacuum, at a flow of 10 cfm educted air (14.7 psia, 70°F), requires 8 cfm of compressed air (100 psig, 100°F). Often eductors are hard to replace with electric-driven vacuum pumps. Eductors can pump a corrosive fluid and pose lower risk in an intrinsically safe environment.
Table 1: Equivalent Uniform Annual Cost
For other applications, such as cooling electric cabinets, it’s easier to justify less-expensive alternatives. Cabinet cooling is often required to prolong the life of electronic equipment. Compressed air seems like a tempting choice. However, it’s apparent from Table 1 that it doesn’t make economic sense. There are several cost-efficient methods of cooling that are available, including vortex cooling, heat pipe, thermoelectric refrigeration, ambient air, and standard refrigeration.
In vortex cooling, compressed air is chilled by expansion and bled into a cabinet. Hot air in the cabinet is educted to the outside by the flow of the chilled compressed air. To economize on air and prevent condensation of moisture, vortex units should be equipped with an automatic temperature shutoff. Although the cost saving isn’t as significant as other alternatives, such as heat pipe and refrigeration, vortex cooling works up to 200°F.
The heat pipe is another substitute for compressed air. Cores of a heat pipe are made of a highly conductive material for cooling. As in a refrigerator, heat is transferred by changing the phase of a liquid. Heat is absorbed by the core, inside the cabinet, and passed to the fluid inside the pipe. The fluid is vaporized and flows outside of the cabinet where it is condensed. In the air-cooled version, an exterior fan draws heat away from the core, which cools the internal cabinet without exposing the interior to ambient air. The fluid is condensed and returned to the core inside the cabinet for another cycle. The air-cooled version is limited by the ambient temperature of the outside air. The heat pipe offers the best economic choice, where physical limits allow (Table 1).
Thermoelectric refrigeration offers the greatest flexibility and has the second lowest annual cost. This type of refrigeration operates using the Peltier effect: when two dissimilar metals or semiconductors are connected together, current drives heat from one terminal to another. Refrigeration units consist of an array of p-type and n-type semiconductor elements connected (Figure 4).
Figure 4. Thermoelectric refrigeration works by forcing electrons to overcome resistance.
Heat is carried by electrons, in the n-type, and the holes created by electrons, in the p-type. The heat pumping capacity of a module is proportional to the current and is dependent on the element geometry, the number of couples, and the material properties. Current technology is limited; 1,500 BTU appears to be the largest available — now.
Other options for cabinet cooling have their own niche. Fan cooling and standard refrigeration can certainly compete for large cabinets. Fans are limited by ambient air temperature but are inexpensive. One unique place for fans is in coolers or other areas that are already cooled. Refrigeration may be the only choice for large cabinets in hot zones.
Blowing cash away
Another common waste of compressed air, apparent to anyone who has visited a packaging area, is the use of air to blow products or dust. A ¼-in. line will flow 32 cfm at 80 psig. An open-ended blow tube represents an annual loss of more than $3,800 (Table 1). Regardless of the application, several guidelines should be applied to compressed air being used for open blow-off: 1) use anything else whenever possible; 2) use low pressure air from a blower — the lower the pressure the lower the cost; 3) regulate the flow to the lowest effective pressure — know the pressure! 4) use a venturi nozzle or air inducer to reduce compressed air used; and 4) shut-off blow-off air (automatically) when not needed for production.