Compressors pressure-rating limits

Compressor Selection To select the most satisfactory compression equipment, engineers must consider a wide variety of types, each of which offers peculiar advantages for particular applications. Among the major factors to be considered are flow rate, head or pressure, temperature limitations, method of sealing, method of lubrication, power consumption, serviceability, and cost. 

Commercial sizes of the single-stage compressors have pressure ratings from 0.75 to 4 lb. per square inch and capacities from a lower limit of 500 cu. ft. per minute to a higher limit which ranges from 12,000 cu. ft. at 0.75 lb. pressure down to 3,000 cu. ft. at 4 lb. pressure. The multi-stage compressors are built of the following sizes ... 

In the case of compressors, the fundamental performance parameter is pressure ratio, which has a direct relationship to the head. A compressor of any type tends to be rated to achieve a certain pressure ratio for a given flowrate. A restriction in the suction system is more detrimental to performance than a restriction in the discharge. If a compressor is rated to compress a gas from 1 bar absolute to 10 bar absolute, it will be able to maintain a pressure ratio of 10 under varying suction pressure. The temperature ratio (often the limiting machine characteristic) corresponds to the pressure ratio. Thus a pressure loss of 0.2 bar in the suction line will result in a pressure of only (1.0 - 0.2) X 10 = 8 bar in the discharge for the same flow, or for the same temperature rise per stage. 

If casing limitations are fixed by user-supplied relief valves, this information should be conveyed to keep the vendor from rating the compressors on other data. Evaluations can be more of a problem if the same design basis isn t universal with all vendors. Startup and shutdown consideration influence various components, shaft end seals, seal system pressures, and even thrust bearings in some instances. The use of an alternate startup gas, or the desire to operate a gas compressor on air to aid in plant piping dryout should be covered. 

A suction pressure throttling valve can also be installed to protect the compressor from too high a suction pressure. This is typically a butterfly valve that is placed in the suction piping. As flow rate to the compressor increases, the valve will close slightly and maintain a constant suction pressure. This will automatically limit the flow rate to exactly that rate where the actual volume of gas equals that required by the cylinder at tlie chosen suction pressure setting. It will not allow the suction pressure to increase and the compressor cylinder to thus handle more flow rate. 

An actual split-shaft Brayton cycle receives air at 14.7 psia and 70° F. The upper pressure and temperature limit of the cycle are 60 psia and 1500°F, respectively. The turbine efficiency is 85% for both turbines. The compressor efficiency is 80%. Find the temperature and pressure of all states of the cycle. The mass flow rate of air is 1 Ibm/sec. Calculate the input compressor power, the output power turbine power, rate of heat... 

An engine operates on an actual reheat open Brayton cycle (Fig. 4.15a)). The air enters the compressor at 60°F and 14.7 psia, and exits at 120psia. The maximum cycle temperature (at the exit of the combustion chamber) allowed due to material limitation is 2000°F. The exit pressure of the high-pressure turbine is 50 psia. The air is reheated to 2000° F, and the mass flow rate of air is 1 Ibm/sec. The exit pressure of the low-pressure turbine is 14.7 psia. The compressor efficiency is 86% and the turbine efficiency is 89%. Determine the power required for the compressor, the power produced by the first turbine, the rate of heat added in the reheater, the power produced by the second turbine, the net power produced, back-work ratio, and the... 

Another operational limit in the CFB system involves gas suppliers. Three types of gas suppliers, i.e., a reciprocating compressor, a blower with throttle valve, and a compressor, are commonly used in the CFB system. For blower operation, as the gas flow rate decreases, the pressure head of the blower increases. For compressor operation, the pressure head of the compressor can be maintained constant with variable gas flow rates. The interactive behavior between a CFB system and a blower can be illustrated in Fig. 10.9, where dashed curves refer to the blower characteristics and solid curves refer to the riser pressure drop. At point A, the pressure drop across the riser matches the pressure head provided by a blower thus, a stable operation can be established. Since the pressure drop across the riser in fast fluidization increases with a decrease in the gas flow rate at a given solids circulation rate, a reduction in the gas flow rate causes the pressure drop to move upward on the curve in the figure to point B with an increase in the pressure drop of Spr. In the case shown in Fig. 10.9(a), with the same reduction in the gas flow rate, i.e., SQ, the increase in the pressure drop, Spr, from point A to point B is greater than that which can be provided by... 

In the PCU during rapid transients the rate at which inventory must be changed according to the load schedule may not be physically achievable. The helium fill and bleed system has limited capacity. In this case turbine bypass control is used to more quickly vary the power output of the shaft. In this scheme the power output of the turbine is changed by bypassing high pressure compressor outlet coolant to the exit of the turbine. The pressure drop across the turbine is reduced so power is reduced while at the same time the frictional losses through the rest of the PCU circuit increase. The result is a rapid reduction in shaft power. However, PCU efficiency is reduced under turbine bypass control and so control is typically transitioned back to inventory control over time. 

The catalyst support may either be inert or play a role in catalysis. Supports typically have a high internal surface area. Special shapes (e.g., trilobed particles) are often used to maximize the geometric surface area of the catalyst per reactor volume (and thereby increase the reaction rate per unit volume for diffusion-limited reactions) or to minimize pressure drop. Smaller particles may be used instead of shaped catalysts however, the pressure drop increases and compressor costs become an issue. For fixed beds, the catalyst size range is 1 to 5 mm (0.04 to 0.197 in). In reactors where pressure drop is not an issue, such as fluidized and transport reactors, particle diameters can average less than 0.1 mm (0.0039 in). Smaller particles improve fluidization however, they are entrained and have to be recovered. In slurry beds the diameters can be from about 1.0 mm (0.039 in) down to 10 Jim or less. 

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