Compressed Air Demand Storage (Dry) Part II: Application and Use in an Efficient Compressed Air System
In continuation of our series examining the application of compressed air demand storage, this article will focus on another type of supply side storage concept: demand (dry) storage. Before discussing the definition, application and use of demand storage, I would like to reiterate that contrary to current thought, consideration should be given to control (wet) and demand (dry) storage being separate storage solutions based on their primary purpose.
When applying control storage in a compressed air system, the main objective should be improving the functionality and efficiency of the air compressor controls. Other issues such as moisture removal or supplementing demand storage should be considered ancillary benefits, not a rationalization for its application. Moisture removal, while desirable, is more effectively achieved by a properly engineered separation and condensate drain system. Utilizing control storage as a supplement to demand storage may seem logical however, it must be noted that control storage due to its location in the system is unfiltered and wet. This raises the possibility that intermittent flow levels, in response to demand events in the system, could exceed the capacity of air preparation equipment resulting in unfiltered and wet air being delivered to the distribution system.
Definition of compressed air demand storage The definition of demand storage for the purpose of this article is a pre-conditioned volume of compressed air located on the supply side of the system, which is generated by the intentional creation of a pressure differential at a level above system pressure.
Should you consider demand storage? It is not a question of will a compressed air system function without demand storage, it is a question of how well. If reliability, productivity and efficiency are not a priority then a compressed air system can operate without demand storage. Many examples of systems operating without demand storage in industrial plants today are easily identifiable because they usually exhibit one or more of the following conditions.
- Operating at a higher system pressure than necessary
- Increased system air demand
- More on-line compressor horsepower than required
- Excessive fluctuation in system pressure levels
- High maintenance costs
The negative impact caused by the lack of adequate demand storage on the compressed air system can be attributed to the original intent of compressor controls and system dynamics. Compressor manufacturers design compressor controls to first and foremost protect the compressor, then to satisfy the discharge pressure setting and in some cases provide for limited part load efficiency and networking capability. Without demand storage, the controls of an air compressor are required to support all demand events in the time frame which they occur within the compressed air system. This was not the original intent of compressor controls. It does not matter what type of compressor or compressor control is involved, the expectation of reacting effectively and efficiently to the changes that may occur in a compressed air system is impractical. With the proper amount of demand storage applied to the air system the benefits should include:
- Lower and consistent system pressure
- Reduction in system air demand levels
- Reduction in on-line horsepower
- Improvement in overall system efficiency
- Consistent quality of compressed air
- Reduction in maintenance costs.
What role does Demand Regulation (flow controller) play in the creation of demand storage? One of the critical factors in the creation of compressed air demand storage is the intentional creation of a significant pressure differential. If we rely solely on naturally occurring pressure drop, very little if any, demand storage will be generated. For example, a 1000 gallon receiver with a 2 psid will produce only 18 cf of demand storage. Point of use regulation could be utilized as an alternative to demand regulation in the creation of demand storage if each point of use was properly regulated at its lowest effective pressure level. Statistics and experience have shown however, that less than 50% of point of use applications have regulators installed and many of those are adjusted incorrectly.
Without the installation of demand regulation (flow controller) in combination with a storage receiver, demand storage cannot be created effectively or efficiently. The application of a flow controller allows the system designer to not only establish a pressure differential but also a separation between the supply and demand sides of the compressed air system. No longer is it the responsibility of the compressor controls to react to each demand event in the system, its concern is simply to maintain demand storage. Supporting demand events now becomes the responsibility of the flow controller in conjunction with demand storage. This allows the system designer to more effectively control the rate of change and reaction time within the compressed air system by controlling the expansion of air across the flow controller. More importantly, he is able to achieve this at a greater speed and capacity level than on-line horsepower alone.
Determining the amount of demand storage required varies greatly due to system dynamics and constraints, as well as ancillary requirements of the system i.e. supporting a failure of an online compressor. System considerations might also dictate the utilization of other storage concepts i.e. general, point of use or off-line storage in conjunction with demand storage. It is extremely important that sound engineering principles be involved in the design and application of demand storage. Without a thorough audit of the complete system the results might not meet expectations. It serves no useful purpose to either over or under create demand storage. If performed properly, you should be able to identify from the audit results the optimum amount of demand storage required to produce the results you expect.
The information needed to determine the amount of demand storage required would include some or all of the following:
Minimum system pressure required A properly performed audit should include the demand as well as the supply side of the compressed air system with each compressed air user individually evaluated for the minimum required system pressure. It is very common that the perceived pressure requirement is higher than necessary due to conductance issues which occur most often within 10 ft. of the compressed air user. It is critical that these issues are addressed in order to determine the true minimum pressure requirement.
Pressure limitations of the air compressors Confirm the design pressure range of each compressor in the system. If it is a load/no load control, what are the cut in and cut out pressures? If you are utilizing a type of compressor network control system, identify the lowest pressure level expected within the parameters of the control system. *
Size and duration of demand side events What is the magnitude, cycle time and duration of the demand event? The longer the event duration, the shorter the recovery time and the greater the magnitude of the event, the more problematic it becomes to utilize only demand storage.
Time required for trim machines to start and load If there is a multi-compressor installation where one or more compressors will be considered for trim, it is critical to know how long it will take a compressor to reload or come online. This is especially critical for larger horsepower compressors such as centrifugal, where the time needed to go from an off position to a loaded position could exceed one minute. Other considerations would be a control permissive that would delay start-up or reloading sequences.
Supply side pressure drop The capacity to create demand storage is determined by the available useful differential between the storage vessel, flow controller and the volume of the storage receiver itself. Once the pressure capability at the discharge of the compressor is identified, two factors must be considered:
Virtually all compressed air systems have purification equipment (i.e. filters, valves, and air dryers) installed between the compressor discharge and the storage vessel. Purification equipment is a source of pressure drop that could range from 5 to 15 psi. It is critical that this pressure drop be minimized as much as possible for it will reduce the differential that is required in the creation of demand storage.
A 3-5 psig differential should be maintained between supply pressure and the system pressure setting on the demand regulator. If the upstream pressure and the system pressure at the demand regulator equalize, the compressed air system goes dynamic with the supply side tracking the demand side and system pressure control is lost.
The goal of this article is to provide basic information on the definition, purpose and advantages of demand storage. The limitations of available space dictate the amount of information which can be provided and my hope is that the information contained in this article gives you a base of information to start the process. That being said, I cannot stress enough the importance of working with qualified individuals and companies who can provide you with the data you will need to make informed decisions. If you would like additional information, please feel free to contact me.
Ron Nordby, Vice President Sales and Marketing John Henry Foster 651.681.5724 firstname.lastname@example.org
Footnotes: * VSD compressors have an advantage of being able to increase the system pressure, but be aware that raising this pressure level above the design point will reduce the capacity of the compressor.