Design point

Fig. 1. Effect of energy use on total cost where total cost is the sum of capital and energy costs for the lifetime of the plant, discounted to present value. Point D corresponds to the design point if the designer uses an energy price that is low by a factor of four in projected energy price. Effects on costs of (a) pressure drop in piping, (b) pressure drop in exchangers, (c) heat loss through insulation, (d) reflux use, and (e) energy recovery through waste-heat boiler...

Fig. 9. Impact of excess design capacity on pump energy use, where A corresponds to actual operating flow, and D to the design point if the designer calls...

A sample is taken at several designated points in the stack, which represent equal areas. The sampling probe is placed at each samphng... 

Engine type Size range, MWe/iinit Efficiency at design point Part-load efficiency Multi fuel capability Maximum temperature of recoverable heat, op (OC). Recoverable heat, Btii/ kWhf Typical power-to-heat ratio... 

At design head, on the other hand, capacity does not change markedly with speed, so that once the design point has been passed the pump-turbine acts as a restriction in the hue. Since most or these units operate on a relatively fixed pressure differential, they then tend to act Eke an orifice to limit flow, and little or no benefit can be realized from any overcapacity in terms of fluid flow available to the unit in the actual iustaUatiou. 

If properly established, the management of change mutual-review agreement and designated points of contact should preclude most... 

The eomplex FCC system involves not only turbomaehinery, but also related proeess eomponents. All of these must be properly designed and sized to operate within system parameters from startup to steady state design point, and through shutdown. System response to emergeney eonditions is also mandatory. Computer simulation is, therefore, an integral part of the design proeess. A eomputer program eapable of this simulation is deseribed below. 

Sensitivity to process gas inlet temperature. Figure 7-13b shows TTE variation with changes in process gas inlet temperature. The sensitivity of variable speed machines to temperature variation is less than constant speed machines. The pattern and symmetry around the design point, however, are the same for constant speed machines. For example, note that a 3% decrease in the inlet temperature causes TTE to drop to 98% and that the same percentage drop in TTE occurs when gas inlet temperature rises by 3%. 

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. 

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. 

As long as the volume flow is kept near design point, both the deflection angle and pressure drop can be corrected. Temperature differential increase is limited by metallurgy, so it is neglected in analytical calculations. This evaluation is based on inlet pressure changes. The new volume at a different pressure is calculated by the ideal gas equation ... 

NACA has given design points for eaeh easeade tested. Eaeh design point is ehosen on the basis of the smoothest pressure distribution observed on the blade surfaees if the pressure distribution is smooth at one partieular ineidenee at low speed, it is probable that the seetion will operate effieiently... 

The off-design characteristics of a turbine are as important to define as the design-point characteristics. Figure 9-29 shows the effect of the speed-to-... 

Loaded speed-torque curve (compressor design point)... 

At the design point the volume flow is such that the relative velocity is parallel to the blade at the inlet of the impeller. The impeller losses at the design point are... 

Outside the design point the direction of the relative velocity is not parallel to the blade, and shock losses are generated. 

In practice, the fan characteristic curve—i.e., the total pressure difference dependency on the volume flow—is determined experimentally. The measured results are then for the impeller and casing together. Since the losses are greater outside the design point, the fan efficiency is high at the design point. 

Figure 9.42 shows the typical characteristic curve of a centrifugal fan, where the blades are backward curved. The figure also shows the theoretical characteristic curve when the slip factor is 1 and when it is smaller than 1. Characteristic curves for a real fan are closer to the isentropic one at the design point. At this point the efficiency is maximum. 

The second kind of modeling is focused on the needs of the planner and HVAC engineer, who has to comply with certain criteria for heat delivery or removal for comfort and energy efficiency and from this has to select a certain type of component available on the market. Once the component is selected, only the performance of this component under variable load is of interest. This kind of modeling normally requires much less input, because actually only the change in performance from a given design point to a point for the actual load has to be determined. 

Degraded failure A failure that is gradual or partial it does not cease all function but compromises that function. It may lower output below a designated point, raise output above a designated point or result in erratic output. A degraded mode might allow only one mode of operation. If left unattended, the degraded mode may result in a catastrophic failure. 

A driver selected to just handle the power requirements of the design point (other than maximum) is usually a poor approach to economy. Of course, there are applications wiiere the control system takes care of the possibilities of power overload. 

Some manufacturers furnish 70°F air equivalent curves to allow the purchaser to convert performance to actual plant conditions. It is almost impossible to operate on one design point. 

The pressure drop of these trays is usually quite low. They can be operated at an effective bubbling condition wnth acceptable efficiencies and low pressure drops. For more efficient operation the clear liquid height on the tray appears to be. similar to the sieve tray, i.e., 1.5-2-in. minimum. This is peculiar to each system, and some operate at 1 in. with as good an efficiency as when a 2-in. is used. When data is not available, 2 in. is recommended as a median design point. 

Figure 9-28. Packing comparison at design point for IntaJox ceramic saddles, metal pall rings, and metal IntaJox . Reproduced by permission of American Institute of Chemical Engineers, Strigle, R. F., Jr. and Rukovena, F., Chemical Engineering Progress, Mar. (1979) p. 86 all rights reserved.

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