Design for Lower hp in Air Systems

By R. Scot Foss, Plant Air Technology

Proper system balance, including storage and pressure differential, keeps hp needs down.

As long as compressed air systems have existed, there has been a perception that all problems and solutions lie in adequate supply horsepower.  A lesser school of thought sees the system as having grown over a period of time and that an inadequate piping system is the reason why the system’s pressure cannot be maintained.  Neither way of thinking is usually correct.  The intent of this article is to explain how the air system really works.

Basic Operation:

Turning on a compressor in the system without a demand will elevate the pressure in the system to the unloaded pressure set on the compressor.  With demand added, stored air in the system will be drawn off until the pressure drops to a preset load pressure in the throttling band of the machine, after which the unit reloads.  This assumes that the unit on-line is in the “load/no load” operating mode.

If the compressor is capable of modulating, it will throttle inversely proportionately to the signal pressure in an effort to maintain a preset system pressure.  This assumes that the compressor is larger than demand.  This method will work, but it requires that the compressor always have excess capacity and be part loaded.  Because motor efficiency decreases considerably under part load conditions, this can be a very expensive way to operate.  Neve3rtheless, it is in general use, and I believe this is one of the reasons why most systems have compressors that are larger than they need to be.  The amount of time that it takes from “add supply” to “response” to a “stable condition” is a function of the size of the unit on-line, the permissive time on the compressor controls, and the amount of storage capacity in the system.

Examples:

In the following illustration demonstrating this, Figure 1, a number of items have been omitted regarding time, pressure and volume relationships to simplify the example.  We will assume that the system is balanced and that 100% of the demand including leaks to controlled in terms of pressure and temperatures.

Unfortunately, many systems are supply controlled and therefore unbalanced.  The demand volume in the supply controlled system would fluctuate with the supply pressure and keep the compressor loaded despite the real requirement.

In Figure 1, if we assume that the demand is loaded with system’s storage at 110 psig, it will be necessary to deplete storage until the pressure in storage upstream of “P3” reaches the load pressure of 100 psig.  Table 1 lists the events in terms of demand, storage condition, system pressure and time.

Event item 4 (see Table) required the 1,000 cfm compressor at 110 psig to run for 37.4 seconds to replace the equivalent of 500 cubic feet (cf) at 90 psig or 411.3 cf at 110 psig plus the 124.45 cf at 110 psig that was depleted from storage.  The system’s cycle time is 14.9 seconds unloaded (9.9 + 5.0) plus 37.4 sec loaded for a total of 52.3 sec.

Let’s examine the same event in a system where there is 250 cf of capacity for storage (Figure 2).  The demand will be the same, but we will use a 500 cfm compressor rated at full flow at 125 psig.  The throttling band for the compressor will be set for 125 psig unloaded and 110 psig load.  The formula for useful storage is the capacity times the control pressure differential (psid) divided by the barometric pressure.  In this case we have 250 cf of capacity times 35 psid divided by 14.7 psia or 595 cf of useful storage (250 x 2.38 = 595).

By using significantly increased storage, 100 % demand control and a wider throttling band for the compressor, we were able to satisfy the same load with half the horsepower and never approach the “P5” control pressure.  What was very different was the way that we chose to use time and storage and their effect on energy.  It should also be pointed out that the longer cycle time on the smaller compressor will significantly reduce the maintenance potential, as opposed to the shorter cycle time for the larger compressor for the same demand.

You do not have to size storage for the initiation of production, but you will have to put on more than the base load to start in order to recover from the initial depletion of storage.  Once the system is recovered, you only need to size storage and compressors (and their throttling band) for the largest single event that could hit the system over any base load at any given time.  Remember that an air system is a time machine measured in parts of a minute.  You cannot forget the element of time if you wish the system to work properly.

R. Scot Foss is president of Plant Air Technology, Charlotte, N.C., a company specializing in system auditing and designing. This series of articles is based on his book, “Compressed Air System Solution Series”.  A portion of the proceeds from sales of the book is donated to children’s charities.  The book can be ordered through Southern Corporation.