Motors in the 50-hp range should not be expected to start more than 3 or 4 times/hr. If the compressors are running under automatic start/stop control, the compressor should be running about one-third of the time and each motor should start not more than 6-8 times/hr. If the compressors are loaded up for long periods of time and unloaded only for short periods, the system demand is probably too high for the connected compressors. Compressors should also be timed during a period of the day known to have the highest compressed air demand.Įven without noting system pressures and computing cfm flows, the relative load-to-unload ratio can indicate if the compressor equipment is adequate or not. Ideally, a timing session of a couple of hours helps determine an overall average load. It is very important to note that the calculated value of C in the equation is in cfm of free air referenced to ambient pressure and temperature, not cfm at system pressure.Īs more cycles are timed, a more complete picture can be obtained. Air in the receiver is also assumed to behave as an ideal gas and that the compressibility effects of a real gas are negligible.Īir compressed under very high pressures or at very low temperatures deviates significantly from the ideal gas relationship, but high-pressure systems up to 3000 psig at ambient temperatures can be relied upon to behave as an ideal gas. ![]() In other words, it is assumed that the heat of compression is small and transferred quickly to the environment. Pressure, mass, and temperature all change with time as air is drawn from or added to the receiver.īut the equation assumes that the temperature in the receiver is nearly constant and at standard atmospheric temperature. Some key assumptions are made when using the equation for air demand, C, as shown in the accompanying sidebar, “Estimating compressed airflow rate.” Compressed air in the receiver undergoes a polytropic thermodynamic change as air is added to and drawn from the receiver. Although less accurate than the previous two methods, timing a few of the compressor load/unload cycles with a stopwatch and noting the receiver volume and load/unload pressures (sometimes called cut-in and cut-out pressures) can yield useful information. The third method is a quick estimate of a compressor’s capacity and the demand of the connected system. When a quick estimate is needed, the time and expense of hiring a field specialist are usually not warranted. The second method provides a reasonably accurate measurement of the compressed air load of a system without having to invest in fixed equipment, but it still means paying a specialist and having data available only during the time the survey is conducted. However, plants usually don’t have expensive flow elements and datalogging software, particularly if the compressed air system is a relatively minor utility. Such flow elements include annubars, orifice plates, and turbine meters. Facilities engineers at some manufacturing plants have installed fixed flow elements in central compressed air systems with electronic datalogging features that can download information on demand. The first method provides the most accurate long-term load profile for a system, especially if historical data is collected and stored over an extended period of time. (See accompanying sidebar, “Determining load profiles.”) ![]() There are several methods for analyzing compressed air system loads. And it can also help to troubleshoot a system experiencing problems. It can provide evidence of how much reserve capacity a system has for possible expansion. It can give an engineer a better idea of how actual compressed air usage relates to the theoretical load estimates used to size equipment. However, determining the actual load profile of an installed and functioning compressed air system, even if it is a rough estimate, is of considerable value. As a vital support utility, the proper design and operation of air compressors, receivers, and dryer-filter assemblies are critical.Īir compressor equipment is sized for an application using standard load estimating techniques found in any engineering handbook. Compressed air systems are widely used for a variety of applications, including pneumatic controls and power, breathing, laboratory, and process air. pressurized air receiver to drop from 125 to 115 psig, what is the volumetric flow rate of air out of the tank? Does this question sound like something from the professional engineer’s Principles and Practices qualification exam? Or does the ability to come up with the answer have practical value in the field?Įngineering design and field evaluations often involve the estimation of loads on existing equipment to verify if the installed machinery is below, at, or above capacity. If it takes 20 sec for the pressure in a 240-gal.
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