Off-design conditions

Most ethylene plants operate continuously with the expander functioning at or near design point. However, by using inlet guide vanes, the expander can still provide optimum performance at off-design conditions. Also, the expansion process generates power, which is used by the compressor. The ethylene enters the expanders at approximately 26 bar (377 psia) and exits at approximately 6 bar (87 psia). The expanders generate over 2,000 hp for gas compression. 

CASE 14 IMPROVING FCC EXPANDER RELIABILITY UNDER OFF-DESIGN CONDITIONS ... 

Rotating machinery usually performs efficiently if it works under design point conditions. However, off-design conditions require a predictive model of the machine s performance. In a FCC power train system, mass flow deviation is quite common for adjusting production capacity to meet the requirements of petrochemical product markets. 

The calculation of the performance of an axial-flow compressor at both design and off-design conditions requires the knowledge of the various types of losses encountered in an axial-flow compressor. 

Combustion control is currently a hot area of research in the U.S. and abroad. In addition to basic research at a number of universities, joint industry-university S T efforts are also underway to implement the control strategies developed by researchers in industry applications. Though some of the demonstrations have been made using gaseous fuels, the techniques can be extended to liquid fuels as well, and efforts are underway to accomplish this. It is hoped that future engines will perform equally well in off-design conditions, with improved reliability and easier maintenance, and reduced operational costs. 

Wishart, D.P. 1995. The structure of a heated supersonic jet operating at design and off-design conditions. Ph.D. Thesis. Tallahassee, FL Department of Mechanical Engineering, Florida State University. 

Mechanical stresses exist in the MEA as a combination of built-in stresses, assembly stresses, and operation stresses. The cell/stack fabrication and assembly process result in the built-in and the assembly stresses. During operation, stresses can be induced by the change of water content in the membrane. There are possibly other sources of stresses due to various off design conditions. 

It is also important to check the column performance at off-design conditions to determine the operable ranges. The above calculations could be repeated for different liquid and vapor rates to check for excessive or inadequate pressure drops. 

It will be seen that the recalculated value of nozzle efficiency deviates increasingly from the true value at the lower pressure ratios, with the deviation being greater when the original efficiency value is lower. However, the errors introduced will normally be small even in off-design conditions because of the combinations of pressure ratio and nozzle efficiency that are likely to be encountered in practice. 

Estimating the efficiency of a nozzle depends ultimately on the results of tests on similar systems, and the manufacturer will supply a figure at the design point, either explicitly or from the thermodynamic data that accompany the turbine flowsheet. However, the control engineer will need to consider off-design conditions, including particularly the sonic limitations that may occur if the pressure drop falls below a critical value. 

Hgura14.8 Nozzle efficiency for off-design conditions for a convergent-divergent nozzle, (outlet to throat area ration 1.5, diatomic gas). 

As a rough rule of thumb, equation (15.27) is approximately valid for (po - Pi)/po < 0.3 or pi/po > 0.7. But the wider significance is that equation (15.27) is independent of the precise values of m and m and hence we may calculate the mid-stage pressure ratio for off-design conditions using either the design values of m and nis in equation (15.21), or indeed setting... 

Off-design conditions in an impuise stage blade efficiency and stage outlet velocity in the absence of blade and nozzle inlet loss... 

Off-design conditions in an impulse blade typical corrections for kinetic energy losses... 

Blade efficiency at off-design conditions for a 50% reaction stage... 

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