Two-stage Air Compressor

Cooper reported on development work by the former dbb Company aimed at developing a methanol-fiielled bus [581]. Two NECAR 3 fuel processors switched in parallel were coupled to six Ballard Mark 7 stacks resulting in 100 kW of net continuous power. A turn down ratio of 8 1 could be achieved. A two stage air compressor with heat and pressure energy recovery was used as the air supply. 

Figure 10.42 A two-stage air compressor, engine power 13.0 HP. Free air flow at maximum pressure is 25.1 CFM (ftVmin), maximum pressure 175 psi, and tank capacity is 30 gallons.

Figure 2-1. Two-water cooled intercoolers on a three-stage air compressor. Ck)urtesy of Elliott Company)...

Estimate the power required to compress 1000 m3/h air from ambient conditions to 700 kN/m2 gauge, using a two-stage reciprocating compressor with an intercooler. 

Two-stage air intake filter/silencers are usually of the panel type and are suited for centrifugal and axial compressors. 

Single-acting air-cooled and water-cooled air compressors are available in sizes up to about 75 kW (100 hp). Such units are available in one, two, three, or four stages for pressure as high as 24 MPa (3500 IbFin"). These machines are seldom used for gas compression because of the difficulty of preventing gas leakage and contamination of the lubricating oil. 

Figure 3-2. Trunk-piston type two-stage compressor with fins for air cooling.

A more rare form of driver is the steam cylinder. Most arrangements combine the steam driver and compressor on the same frame with the steam cylinder opposite the compressor cylinder. Each cylinder s connecting rod is connected to a common throw on the crankshaft. A flywheel is used to provide inertia. For air service, the units are built as single- and two-stage units, with other combinations available for process service. 

For the a/s example quoted earlier, with this form of two stage cooling (with a = 2.79, Ah = 1.22, i//h = 0.1, i/ l = 0.05), the thermal efficiency is reduced from 0.4442 (uncooled) to 0.4257, i.e. by 0.0185, still not a significant reduction. If the second step of cooling uses compressor delivery air rather than air taken at the appropriate pressure along the compressor, then the analysis proceeds as before, except that the expansion work for the processes 7, 11 in Fig. 4.7a is replaced by that corresponding to 7, 11 in Fig. 4.7b. It may be shown [5] that the efficiency may then be written as... 

Compressed air is needed for general use and for the pneumatic controllers that usually seiA e for chemical process plant control. Air is often distributed at a pressure of 100 psig. Rotary and reciprocating single-stage or two-stage compressors are used. Instrument air must be dry and clean (free from oil). 

Generally, rotary and reciprocating single-stage or two-stage compressors generate compressed air for tlie operation of tlie pnemnatic controllers lluit may be used in chemical process plants. Compressor malfunctioning or air lliat is not clean (oil-free) and dry may cause loss of the system. 

Figure 12-34. Typical air compressor performance, single- and two-stage. (Adapted and used by permission Dresser-Rand Company)...

In conventional compressed air systems, vapor and liquid removal is limited. Most two-stage compressors will include an intercooler between stages. On air-cooled units for 100 to 200psig service, the air between stages is not cooled sufficiently to cause substantial liquid drop out and provision is not usually made for its removal. Water-cooled intercoolers used on larger compressors will usually cool sufficiently to condense considerable moisture at cooler pressure. Drainage facilities must always... 

Air at 290 K is compressed from 101.3 kN/m2 to 2065 kN/m2 in a two-stage compressor operating with a mechanical efficiency of 85 per cent. The relation between pressure and volume during the compression stroke and expansion of the clearance gas is PV1-25 = constant. The compression ratio in each of the two cylinders is the same, and the interstage cooler may be assumed 100 per cent efficient. If the clearances in the two cylinders are 4 per cent and 5 per cent respectively, calculate ... 

Ambient air (stream 200) is compressed in a two-stage compressor with intercooling to conditions of approximately 193°C (380°F) and 8.33 atmospheres (122.4 psia). The majority of the compressed air (stream 203) is utilized in the fuel cell cathode however, a small amount of... 

Atmospheric air at the rate of 6.1 million SCFD is compressed to 160 psig in a two-stage compressor JJ-1 that is provided with an intercooler and a knockout drum. Then it proceeds to a packed tower T-l where it is scrubbed with recirculating caustic soda solution. Overhead from T-l is cooled to 14°F in a refrigerated exchanger. After removal of the condensate, this stream proceeds to a dryer system that consists principally of two vessels F-l and F-2 packed with solid desiccant. 

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