What is in this article?:
- Improving Compressed Air System Efficiency: Part 2
- Demand control
How demand-side management, general storage considerations, and point-of-use logic can help you to properly design a compressed air system.
The conventional approach toward the development of a new compressed air system is filled with guesswork and usually involves little effort. The users simply guess what capacity they need and then add a little more.
To do the job properly, it is critical to develop an agenda with priorities that you wish to apply to the design and can live with comfortably. However, keep in mind that what is important to production people may not be important to maintenance people. These interactive issues should be considered by all parties involved:
• operating cost of the system, (Remember, it will cost more to operate the system in the first year than what it cost to buy and install it.)
• accuracy and repeatability of the system, which will produce the desired end results,
• maintainability of the system for day-to-day operation,
• minimizing the risk of interruption to the system, and
• capital cost of the project.
Point of use
The point of use is the logical place to begin developing a profile of the standards that will be used in the design. As demand volume is a function of supply pressure, it is important to control the pressure at the point where the air is used.
The differential established between the highest pressure at which air is removed from the system and the lowest supply pressure is called general storage, or stored energy in the overhead piping system and receiver. Properly maintained, general storage prevents one air user from interfering with another.
An important criterion for the design of the point-of-use standards is the differential of the installed components. All components have a resistance to flow — this resistance is called pressure drop, or ∆P. Pressure drop can be inconsequential or devastating to a system, often depending upon the original equipment suppliers. Pressure drop cannot be ignored, because a corresponding amount of supply energy will have to compensate for it.
Unlike electricity, where installation components can be matched to the amperage and voltage of the user, little thought is usually given to the proper selection of compressed air point-of-use components. Hose, fittings, disconnects, filters, and regulators are all rather standard fare for the user. Lubricators, check valves, dedicated storage, and metering valves may also be required.
Selection criteria should be based on the differential pressure generated at the highest flow and lowest supply pressure. Selection should be made one component at a time backwards from the article pressure, which is the pressure required at the inlet to the pneumatic device. A maximum allowable differential should be established for application between the lowest initial pressure, P4, and the highest article pressure in the system, P5.
Often, these components such as miniaturized or interlock filters, regulators, and lubricators are selected based on ease of installation. Hose, tube, and fittings are too often chosen for ergonomic, spatial, or appearance issues. If the consequences of differential are not considered, the system may have to operate at a much higher pressure to compensate. The financial consequences of uninformed installation decisions could result in a six-figure utility operating penalty.
One note about the regulator — the differential appears on the upstream side. If there is a 10 psig differential at rated flow and pressure across the regulator, and the desired setting of the regulator is 80 psig, the regulator will need to be supplied 90 psig. If the supply drops from 90 psig to 89 psig, the regulator will drop from 80 psig to 79 psig. This is called tracking supply.
Of the components at the point of use, the regulator is the most sensitive to differential. This problem is compounded with miniature regulators. There are many systems that must operate at 30 psig higher pressure because of a miniature regulator on an application.
Between the highest initial pressure and the lowest initial pressure there is a differential referred to as general storage, Figure 2. The purpose of this storage is to provide transparency between users in the system and to support demand events until control storage from the supply can reach a new event (a usage of air) in the system.
The new event will deplete volume, causing a drop in pressure. It is assumed that there is a source of stored energy at the supply at a higher pressure, which can spill over to the lower general storage. The integrity of the users is dependent on keeping the overhead system's pressure from dropping into the highest initial pressure requirement. Useful storage is a function of the capacity to store air plus the controlled differential pressure — in this case, the capacity of the piping plus any supplementary vessels plus the maximum change in pressure between the demand controller and the lowest overhead or initial pressure. The approach towards solving this requirement requires the following information:
• The largest event which could occur in the system when demand exceeds supply. This should be measured as both volume in scf per cycle and rate of flow in scfm. Example: 300 std ft3 for a cycle time of 30 sec. Rate of flow of 600 scfm.
• the distance from the event and the supply in linear feet of piping. Example: 1250 linear ft from demand event to supply.
• the fastest delivery speed needed to support a particular event. Assuming the overhead piping system will have a nominal 1 psig differential, the speed of the air will be about 250 ft/sec. Example: Any event in the system needs to be supported with a ramp in ½ sec. This implies that the initial volume requirement can be met within 125 ft of the unit location and can be supported continuously until the event is completed. Any effort to reduce or slow down the ramp rate into the event would also reduce the needs for general storage and the event's effect on other users.
• the linear feet of header and subheader piping in the overhead system. Example: 2000 ft of loop header and 4000 ft of subheader.
A critical issue at this time is the flexibility needed in the overhead design. If equipment must be moved around the system in the future, the entire system must be designed to accommodate this possibility. Otherwise, sectors and storage can be designed specifically for the events in each sector of the system.