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SMART ENERGY Compressed Air: Fix the Leaks



ompressed air leakage is an important oppor- tunity to reduce energy use and improve profit. Substantial resources are often wasted on elec-

tricity used to power air compressors to “fill the leaks” throughout a facility. But, compressed air leakage rates are very difficult and expensive to measure directly in a foundry environment. Fortunately, indirect measurement approaches can be used to determine the aggregate compressed air leakage rate at a facility to within about 20% of the true value. Using actual baseline compressed air system measure-

ments from a foundry, we determined several important parameters of the compressed air system used for their operations.

Static Air Leaks Static leaks are the compressed air losses that exist

regardless of whether production equipment connected to the compressed air system is in operation. Table 1 summarizes leak rates associated with vari-

ous size circular leaks. Although leak rates from more common, irregularly shaped leaks are more difficult to quantify, this table shows the relative importance of focusing repair efforts on larger leaks. Fixing a single, large leak may consume the same amount of labor and supplies as fixing a single, small leak but the larger leak may allow 100 times more air to escape compared to a smaller leak in the same compressed air system. It is im- portant to prioritize leak repair efforts to focus first on the leaks that are perceived to allow the greatest amount of air to escape. Static leaks exist in virtually every foundry and manu-

facturing operation and are often overlooked or ignored for repair. In contrast, hydraulic fluid leaks are repaired with higher priority due to the problems with fluid on the floor. The cost impacts of these different types of fluid and air leaks may not be used to determine the repair priority. Many of these air leaks are “low-hanging fruit” and can be repaired in a very cost-effective manner.

Dynamic Air Leaks Dynamic leaks are the compressed air losses in

equipment that are only apparent when equipment con- nected to the compressed air system is in operation. It is difficult to separate the anticipated operational use of compressed air in a machine from the excess use of com- pressed air caused by leaks in the actuators and valves in the machine. These leaks are only exposed to compressed air when the machine is operating. Depending on the age and maintenance record of

the equipment, dynamic air leaks can be very significant and may be of the same order of magnitude as the static leaks. In simplest terms, dynamic air leaks result in a piece of equipment using significantly more compressed air during operations than it did when it was new.

Foundry Example

Te following example only pertains to static air leaks. It is possible to calculate an aggregate compressed air leakage rate from measuring the time and pres- sure during the compressed air system discharging and charging processes. Measurements were made during a non-production

day. In addition to enabling various calculations, these measurements can serve as a baseline set of conditions for future reference to show progress in removing leaks. When a future system discharge test is performed, the time that it takes the system to change pressure from 100 psig to 90 psig could be a benchmark to rate the overall progress of efforts to remove leaks from the compressed air system. The longer it takes for the pressure to fall from 100 psig to 90 psig in future measurements com- pared to the current value, the more progress made in eliminating costly leaks. This leak-down time from 100 psig to 90 psig can be used as a monthly or quarterly metric of the building’s compressed air system. From these measurements, we calculated an approxi- mate air leakage rate from the compressed air system including header pipes, compressed air drops, attached equipment, and the air compressor hardware. We esti-

Table 1. Leakage Rates (cfm) for Different Supply Pressures and Approximately Eqivalent Orifice Size* Pressure (psig)

70 80 90

100 125

44 | MODERN CASTING July 2018

1/64 0.29 0.32 0.36 0.40 0.48

1/32 1.16 1.26 1.46 1.55 1.94

Orifice Diameter (inches) 1/16 4.66 5.24 5.72 6.31 7.66


18.62 20.76 23.10 25.22 30.65


74.4 83.1 92.0

100.9 122.2


167.8 187.2 206.6 227.0 275.5

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