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					all about air | Thomas W. Kreher
and bigger is better. There are some applications like a bag house that uses blasts of compressed air to shake the precipitate from the bags. No air is used for possibly 10 minutes or more. During the, one bag at a time, discharge sequence the air usage is high. A large receiver can be recharged during the idle period with a small compressor dryer package. ACCESSORIES for the primary receiver should include a pressure gauge, drain and relief valve. In addition a pressure switch would provide means of monitoring the compressed air supply. If a pipe broke or the compressor stopped the receiver pressure would drop below set point and trip an alarm. OTHER USES FOR THE PRIMARY RECEIVER include the following: 1. BACK UP: Like the battery back up for safe computer shut down if power is lost the receiver can provide enough air for an orderly and safe shut down. Use a check valve to hold pressure in the receiver. 2. COMPRESSOR CHECK: When the receiver is charged shut down the compressor. Close the outlet valve to the plant. Observe the receiver gauge to check for leakage back through the compressor. 3. CHECK LEAKS: When the plant is idle: (1) Note the receiver pressure. (2) Close the inlet valve to the receiver to prevent air being added. (3) Time the pressure decay until the pressure drop is about 10%. With this information you can determine the loss from leaks in the plant without expensive equipment. (P1-P2) / 14.7 x Cubic Feet of Receiver = SCF. Then (SCF x 60) / Time (seconds) = SCFM (Std Cu Ft per min.) Lost. Be prepared for a number of Standard Cubic Feet per Minute of air being lost that is hard to believe. 4. CHECK FLOW : The same technique used in number 3 above might also determine the average flow during production unless the decay time is too brief. Shut off the supply to the receiver. Time the pressure decay until a safe margin above the minimum operating pressure and open the supply valve immediately. SECONDARY AIR RECEIVERS are usually smaller than primary receivers. Most often they are dedicated to a single task. Receivers may store compressed air like an electrical capacitor stores electrical energy. The stored compressed air dumps back into the source if the inlet pressure falls. It is important to note that a receiver does not increase compressor capacity. If the demand for air exceeds the compressor capacity for more than brief period the pressure for the entire system will fall. The size of the receiver compared to the instantaneous or temporary demand for air and a recovery period to recharge will determine how well the receiver helps smooth out fluctuations and regulate pressure.

Air Receivers

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ressurized Air Receivers contain significant potential energy. The regulations of OSHA, ASME, City, State, Federal or other empowered agency supercede this data if contradictory. Receivers, tanks, reservoirs are used to store a volume of compressed air. The sizes of these receivers are often rated in gallons. To readily convert from gallons to cubic feet, divide the number of gallons by 7.48 (7.48 gallons = 1 cubic foot) Wow! We could pour 7 ½ gallons of liquid into a 12” cubed tank. Also you may multiply gallons by 13.4% (.1337) to get cubic feet. 300 gallons = approximately 40 SCF. The most common air receiver in a compressed air system is a “primary” receiver located near the compressor in the main air supply line. On smaller systems the compressor may be “tank mounted”. In this case the tank is a primary receiver. The pressure in the primary receiver is approximately equal to the compressor discharge pressure for both types of primary receivers, “Wet” and “Dry”. A WET PRIMARY RECEIVER is located following the compressor and possibly after an air cooler but before filtering contaminants and drying. The wet primary receiver stores air, aids air cooling and water dropout and limited contaminant fallout. Air in the wet receiver is usually above ambient temperature from the heat of the discharged compressor air. A DRY PRIMARY RECEIVER is also usually located near the compressor but after air preparation devices which might include filters, water separators and a ‘after cooler’ or dryer. Compressed air in a dry primary receiver is at or near compressor discharge pressures but cleaner, dryer and possibly cooler than the raw compressor discharge air. TO SIZE A RECEIVER use a common rule of thumb for minimum receiver capacity of 1 gallon for each ACFM of compressor capacity. Actual Cubic Feet Per Minute (ACFM) is the free air ingested by the compressor which would be the same as (SCFM) at “standard” conditions. The term ACFM is commonly used in the compressor business. Those who use the compressed air are more apt to call the same airflow SCFM when both are referring imprecisely to the ambient inlet air to the compressor. Another rule of thumb for the compressor output is a common discharge rate of approximately 4 SCFM per horsepower. Then combined rule of thumb approximations imply a minimum receiver volume of 4 gallons per compressor horsepower. If our compressor has 100 horsepower and the standard volume size falls between 300 and 500 gallons the 500-gallon or next larger size might be a better choice than a smaller unit. The rule of thumb is for minimum size

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Thomas W. Kreher | all

about air

SYSTEM SUPPORT: Most new air systems are sized for the current demand with limited room for growth. The air required increases almost continually over time with each new application or process. It is unusual not to out grow the compressed air system within a few years. Receivers are often easier to install and less expensive than re-plumbing or other alternatives. A SECONDARY SYSTEM RECEIVER about the size of the primary receiver plumbed in at the far side of the plant air loop may back fill when the flow is high to reduce pressure drop during peak demands. Part of the airflow comes from two directions. Pressure loss in transmission lines from both volume and the distance of air travel is improved. SUGGESTION: Consider a Valve to block the Secondary Receiver when the compressor is shut down. This could provide a source of compressed air for maintenance, blowguns etc. or just hold until start up. Both Primary and Secondary receivers will bleed down over night through air leaks in the plant unless isolated. CLOSE COUPLED air tanks or receivers are invaluable

when a huge blast is required for speed or consistency. SAFE SHUT DOWN air is available in a receiver with an inlet check to allow critical functions to complete one last cycle on critical or dangerous systems. SPECIAL PURPOSE receivers are useful for instrument air, air that may require sterilization, exceptional filtration, extreme regulation, lubricants or other added ingredients or to store vacuum. TIME DELAYS using a receiver or volume regulated to a specific pressure will bleed down through a fixed orifice with gratifying regularity. Changing the regulated pressure changes the bleed down time and regulator provides adjustment. DO THE MATH 1. FLOW FROM A RECEIVER TO CHECK LEAKS OR THE FLOW THROUGH COMPONENTS. (P1-P2) x Cu Ft (Receiver) x 4.08 / T (seconds) = SCFM (Std Cu Ft per Min.) 2. THE REMAINING PRESSURE AFTER A DECREASE IN AIR WITHIN THE RECEIVER. P1 – 14.7 / Cu Ft (Receiver) x SCF (decrease) = P2

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