Transfer lag

Heat transfer lags can be significant and the nature of the problem can be quite different in various processes. If there is a sensor lag, it is mostly due to heat transfer between the sensor and the fluid medium. (Thermocouples, depending on how we make them, can have very fast response times.) The overall response is sluggish and PI control will make it more so. It is unlikely we can live with any offsets. PID control is the appropriate choice. 

Note that, for the dynamic response, both t -dL and t rai are transfer lag time, also called pure lag time. For convenience, the total lag time, )a , is defined as... 

The onset times for the traces with time as the x-axis were artificially lined up. The onset of the melting endotherm for slower heating rates would normally be much later, since it would take a longer time for the furnace to reach the melting temperature. For the reference temperature as the ordinate, the higher temperature onset for faster heating rates is caused by the heat transfer lag from the sample interior to the thermocouple junction. During the limited amount of time needed for... 

Recent computational work has suggested the existence of a mechanism for aminolysis that bypasses the tetrahedral intermediates. Transition structures corresponding to both stepwise addition-elimination through a tetrahedral intermediate and direct substitution were found for the reaction of methylamine with methyl acetate and phenyl acetate. There is considerable development of charge separation in the direct displacement mechanism because proton transfer lags rupture of the C—O bond. 

Mercury column damping occurs at 30 cycles/minute, while heat-transfer lag should be influenctial at 46 cycles/minute according to our best estimates. 

A first-order process with a transfer function given by eq. (10.3) is also known as first-order lag, linear lag, or exponential transfer lag. 

The response of the overdamped multicapacity system to step input change is S-shaped (i.e., initially changes slowly and then it picks up speed). This is in contrast to a first-order response which has the largest rate of change at the beginning. This sluggishness or delay is also known as transfer lag and is characteristic of multicapacity systems. 

Exponential transfer lag system (see First-order lag system)... 

The control valve on the steam has linear installed characteristics and passes 500 Ib/min when wide open. An electronic temperature transmitter (range 50-250° F) is used. A temperature measurement lag of 10 seconds and a heat transfer lag of 30 seconds can be assumed. A proportional-only temperature controller is used. 

Sampling, sample conditioning, and transfer of a sample from the column to the analyzer are common weak spots in analsrzer control systems, and frequently the cause of poor control. Plugging, two-phase sampling, reaction, and excessive transfer lags are common trouhle-spots. A partial phase change must be avoided upstream of the analyzer, because fractionation will result, and the analyzer will not see the entire sample. 

A typical overall lag is 10 to 20 minutes, and it is not unusual to find a 30-minute lag or higher. It has been recommended (258, 309, 332,422) to restrict the sample transfer lag to less than 1 minute, and the sample interval to less than 2 minutes. In many situations, these targets are not readily achievable. Because of limitations associated with the electrical classification of the analyzer and the hazard rating of the area, and in order to ease maintenance, analyzers frequently need to be mounted in a protected shed, a fair distance from the column. The time it takes the samples to travel the distance can easily exceed 1 minute, even with high-velocity samples. 

With analyzer controllers, it is essential to minimize the sample transfer lags. Vapor samples are preferred, since they can travel faster. A liquid sample is often vaporized upon withdrawal if sample lines are long. Heat tracing and insulation are usually required to keep the sample vaporized. To maintain high velocities, and at the same time avoid miniscule sample transfer lines that tend to break, the sample drawn is often much larger than the analyzer requires, with its unused portion returned to the process. This is considered an absolute must for unvaporized liquid samples (362). 

We have also foimd that the sensible heat effect of the liquid mass in the column base or separator is small. Heat-transfer lags are typify only several seconds vapor flow fixMn the separator follows steam flow aln instantaneously. 

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