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POWER EQUIPMENT INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO. AIR COMPRESSORS & AIR HANDLING SYSTEMS. Learning Objectives. Understanding of compressed air systems and air compressor types . Discuss compressed air uses, volume, pressure, and temperature relationship.
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POWER EQUIPMENT INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO AIR COMPRESSORS & AIR HANDLING SYSTEMS
Learning Objectives • Understanding of compressed air systems and air compressor types. • Discuss compressed air uses, volume, pressure, and temperature relationship. • Discuss safety issues and hazards associated wit compressed air. • Discuss heat removal and moisture removal requirements for compressed air systems. • Examine and determine classifications of air compressors. • Define pressure ratings and capacity determination on reciprocating compressors. • Examine the compressor unloader system and determine why it is needed and how it functions.
Air Compressor Systems • Compressed air has several applications on ships and in power plants. • Operation of pneumatic tools • Pneumatic control • Automatic control of machinery • Diesel engine start and control • Compressed air is stored energy or potential energy • We do work (add energy) on air during the compression process and store it for future use. • Air compression takes place in one of two ways:
Adiabatic process • It is a thermodynamic process in which there is no transfer of heat to or from the system during the process. • When compressed adiabatically, all the work done on the air goes to increase its internal energy. • The air temperature still increases, but there is little time for heat transfer to occur.
Adiabatic process • Adiabatic compression is very difficult to achieve • One of the closest mechanical devices capable of approaching this is the gas turbine. • The gas turbine compression phase involves a constant flow process at a rapid speed and thus approaches adiabatic compression.
Isothermallyprocess • Temperature remains constant throughout the compression process. • To achieve an isothermal compression you must have an external cooling medium. • As the compressor gets bigger, more cooling becomes necessary and it gets to the points where it is no longer practical to strive for a isothermal compression.
Isothermallyprocess • Compression inherently produces a large amount of heat . • Look at the pressure, volume, and temperature relationship given by Boyle’s, Charles’ and the ideal gas laws. P1V1T2= P2V2T1 • Isothermal compression T = C P1V1 = P2V2 • Isobaric process P = C V1T2 = V2T1 • Isometric process V = C P1T2 = P2T1 • Adiabatic compression No heat transform
Boyles law calculation • Question: 4 ft3 of Nitrogen @ 100 Pisg. If we allow the Nitrogen to expand to 6 ft3 , what is the new gage pressure reading? • Answer: • Convert gauge to absolute pressure • 100 psig + 14.7 = 114.7 psia • Use the equation P1V1 = P2V2 • 4 ft3 × 114.7 psia = 6 ft3 × P2 • P2 = 4 ft3 × 114.7 psia / 6 ft3 • P2 = 76.46 psia = 61.76 psig • 61.76 psig is the new gage pressure reading
Practice process • In practice, a combination of both is used for most of the applications we would use. • Air is compressed nearly adiabatic within any one stage, and isothermally when considered from start to finish. • To receive benefits of both, most compressors have more then one stage, and have cooling arrangements after both. • Pre-coolers are used before entering the compressors, inter-coolers are used between stages and after-coolers are found after all stages of compression.
Compressor Classifications • Compressors are classified according to: • How much pressure is produced • Low Pressure : 0 - 150 psi • Medium Pressure: 151 - 1000psi • High pressure- Over 1000 psi • The type of compressing element. • Centrifugal • Rotary • Reciprocating
Centrifugal or kinetic compressors • Used for capacities greater than 10,000 CFM and 100 psi to 150 psi (low to medium pressure) • It is a dynamic compressor which depends on transfer of energy from a rotating impeller to the air. • It produce high-pressure discharge by converting angular momentum imparted by the rotating impeller. • In order to do this efficiently, centrifugal compressors rotate at higher speeds than the other types of compressors. • They are also designed for higher capacity because flow through the compressor is continuous. • Capacity or compressor output is often regulated by controlling air flow into the inlet eye of the impeller or fan
Rotary- Positive displacement • Capacities up to 10,000 CFM and pressures 0-150 psi (low to medium pressure • Air enters a sealed chamber where it is trapped between two contra-rotating rotors. • As the rotors intermesh, they reduce the volume of trapped air and deliver it compressed to the proper pressure level. • RECIPROCATING - POSITIVE DISPLACEMENT • Capacities below 1000 CFM and pressures above 100 psi (medium to high psi)
Compressor Classifications • Number of stages • Single stage • Several stages • Stages are in series, with one stage discharging to the next stage • For medium and high pressures • Sources of power • Electric motor driven • Most common for power plant and shipboard applications • Steam turbine • Gas turbine • Drive connections • Reciprocating applications • V-belt drive very common for small low and medium pressure. • Rotary compressors • Direct drive • Driven through solid couplings • Driven through flexible couplings
Reciprocating Compressorsmajor components • Flywheels • Crankshaft • Connecting rods • Pistons • Trunk type • Driven directly by connecting rods, connectingthe crankshaft and the piston through the piston wrist pin. • Differential or piggy back type. • They are trunk type pistons with two or more diameters fitted in specially designed cylinders. Each diameter is designed to deliver different pressures, and would be used in multiple stage systems. • In multiple stage systems, the pistons of each stage will be smaller as the pressure increases; the low pressure piston will the larger than the high pressure system.
Reciprocating Compressorsmajor components • Valves • Compressor valves are made of special spring steel • Opening and closing of the valves is caused by the difference in pressure between the air in the cylinder and atmospheric air. • The pressure of the discharge valve causes it to open on the upward stroke of the piston. • The suction valve opens as the piston strokes downward, creating a vacuum in the cylinder. • Two common types • Feather type - Made of spring steel, usually used in low pressure systems • Disc type – usually used for high pressure applications. • Cylinders • Air cooled • Water cooled
Cooling systems • Two cooling mediums • Air • Water • Most compressors will have some sort of cooling. • Between each stage – called interstage coolers • After the last stage – called afterstage coolers • A typical water cooled two stage compressor might have the cooling water entering the afterstage cooler, then proceeding to the high pressure cylinder, to the interstage cooler, and finally to the low pressure cylinder. • This is counter flow cooling. • Low pressure systems typically use air as cooling means
Cooling systems • Interstage cooling functions to: • Remove moisture from the compressed air by dropping the air below the dew point corresponding to the pressure of the air at that time • Reduces the temperature in each cylinder • Lower cylinder temperatures helps with cylinder lubrication • Increases the volumetric efficiency of each of the stages. • Additional cooling is used to facilitate air drying for air being used for control functions • These air driers typically use refrigeration systems to drop the compressed air below the dew point of any moisture left in the air.
Reciprocating Compressorstwo stage and intercooler • Water cooled heat exchangers are usually shell and tube type heat exchangers. • 1 first stage • 2 intercooler • 3 second stage • 4 afterstage cooler
Compressor P-V diagram • Red line --- an isentropic compression process (n=k) • Blue line --- a polytropic compression process (1<n<k) • Green line --- an isothermal compression process (n=1) • Yellow area represents the air compressor work required during compression process of an isothermal process • Because the area from each line to the left is the required air compressor work, we can see that an isothermal process requires loweer amount of energy than polytropic process and isentropic process respectively.
Lubricating Systems • Compressor lubrication systems for low, medium, and high pressure systems are usually forced systems, with an oil pump attached to the shaft. • A complete system will have a filter, and an oil cooler.
Compressed Air Storage • For compressed air to be useful potential energy, we must store it until it is needed. • Air is stored in an accumulator or receiver. • Accumulators function to: • Act as a storage device when demand for air exceeds compressor capacity. • It helps eliminate pulsations in air supply systems. • Allows the compressor to shut down or rest during periods of light load. • Acts as a place to remove moisture from the system and also cools the air. • Moisture comes in with air.
Control Systems • Control systems include: • Control system modes of operation • Start – stop operation • Compressor is on or off based on receiver pressure. • pressure gets to the set point, the compressor shuts off • pressure gets to the cut in set point, the compressor restart • The pressure is sensed via a pressure switch mounted on the receiver. • Constant speed control • The compressor run constantly, but loads or unloads based on the pressure in the receiver. • Safety shutdown • Low oil pressure • High air outlet temperature • Low cooling water pressure
Unloading systems • Unloading systems remove all but the friction loads on the compressor • The compressor motor is still turning, but is not pumping air • This is accomplished by holding the cylinder suction valves open • Unloaders automatic remove load during • Start ups and shut downs • When the air set pressure is met during constant speed operation. • There are several types of unloaders • Using oil pressure to hold open the suction valves • Or using air pressure
Moisture Removal • Most air systems will have an additional means to remove moisture between the compressors and the receiver. • The State of Maine has two refrigeration/filter units known as refigerfilters. • They use a refrigeration process to lower the temperature of the compressed air. • When the temperature of the air is lowered, the moisture in the air reaches the dew point, and condenses. • They also have filters to clean the air.
H P A C • High pressure air compressor
SHIP SERVICE LP AIR SYSTEM • Question: If atmospheric pressure = 14.7 psia and we pump up the flask to 100 psig, How much air can be forced into an air flask? • Note: Use the equation P1V1 = P2V2 • Answer: 5407in3 air must be forced into the air flask.
