Thumb guide

The choice of letters and numbers tends to be a national preference, see Table 6.1 as a rule-of-thumb guide. 

A good rule-of-thumb guide is to assume that between 15% and 25% extra capacity will be required. Hence the chosen rating will be 115% to 125% of the best-known estimate at the early design stage. This requirement also applies to power transformers and their main cables or overhead power lines, and to outgoing feeder cables to auxiliary switchboards and motor control centres. It does not usually apply to individual motor consumers, see Chapter 1. 

The presence of high frequency harmonics in the power supply lines leaving the switchgear can cause mutual coupling to electronic and telecommunication cables if they are routed in close proximity to the power cables. This can occur especially if the cable racks run parallel to each other over an appreciable distance. As a rule-of-thumb guide, derived from Table 13.1, the spacing d) between power and electronic cables should be at least,... 

A rule-of-thumb guide to the appropriate slow charging current is to divide the ampere-hour (AH) capacity of the battery (at a 10 hour rate) by about 7, e.g. a 100 AH battery would require a charging current of about 15 amps for 10 hours. 

For abnormal conditions, a security factor of 1.2 is advised, or perhaps 1.4 for extra wall heat for a cold startup. 1.4 x 10.6 kk = 15 kk Btu/hr. ( Rules of thumb may be very case specific or overly safe, but can be assuring ballpark guides thus coauthor Reed prefers to call them thumb guides. One such is 80 000 Btu/hr ft of hearth for large high-temperature car furnaces, which gives 80 0(X) x 20 x 10 = 16 kk Btu/hr for the job in this example). 

TABLE 7.3 Thumb guide generalizations relative to table 7.2. The first row is calculated as in example 7.2. The last row Is via equations (7.3), (7.4), and (7.5) using approximate velocity figures from the center three rows of table 7.2. 

A thumb guide for determining stack cross-sectional area (inside the lining) is to make it equal to about 60% of the sum of the areas of all exhaust ports or flues, provided that they were properly sized. This reduction to 60% is reasonable because the gases cool down on their way through the stack and because one large duct creates less frictional resistance than many small ducts of the same total cross-sectional area. 

A thumb guide for furnaces at or above 2000 F (1093 C) is that each foot (0.3 m) of furnace height will cause about 0.01 in. wc (0.25 mm wc) less pressure inside the hot furnace than In the surrounding room.)... 

As a rule-of-thumb guide for hand calculations, using a minimum of five slices and at least 8 to 10 time increments should give sufficient accuracy for most purposes. Only when very high accuracy is desired or several cases are to be solved is it desirable to solve the problem using a digital computer. In some cases the physical properties are not known with sufficient accuracy to justify a computer solution. 

Unfortunately, the complexity and extensive number of possible binding motifs precludes such a simplistic approach from providing anything other than a statistical rule-of-thumb guide. 

Selection of pump for a given appHcation is not a trivial task. Often more than one pump type can accomplish the required job. Thus a final choice on a pump type is often a result of personal experience and usage history. As a rule of thumb, the choice of a kinetic, such as centrifugal, or a positive displacement pump is made on the basis of the specific speed. Whereas specific speed is appHcable primarily for centrifugal but not positive displacement pumps, the US value can be used as a guide. Generally, for calculated values of specific speed, eg, nS > 10 [NS > 500), kinetic-type pumps are usually selected. For nS < 10 [NS < 500), positive displacement pumps are typically appHed. 

The most common rule of thumb is that a disc-shaped rotating part usually can be balanced in one correction plane only, whereas parts that have appreciable width require two-plane balancing. Precision tolerances, which become more meaningful for higher performance (even on relatively narrow face width), suggest two-plane balancing. However, the width should be the guide, not the diameter to width ratio. 

There are three rules of thumb to guide us in selecting the number of calibration samples we should include in a training set. They are all based on the number of components in the system with which we are working. Remember that components should be understood in the widest sense as "independent sources of significant variation in the data." For example, a... 

As a rough guide, the following rules of thumb suggested by Frank (1978) can be used to decide the design method to use for a partial condenser (cooler-condenser) ... 

This pocket guide contains selected rules of thumb and shortcut design methods meant to travel into the field as well as the office, even when the office is a hotel room. It contains updates on certain fast-moving technology and new material not found elsewhere. Miniaturization and easy retrieval of information are stressed. Those on the go can produce reasonable results quickly when using this book as a basic source. 

There are some rules of thumb which can guide the formulation of reaction mechanisms. 

Whether the system formed on mixing oil, water, and surfactant will be an oil-in-water or a water-in-oil emulsion is a central problem in emulsion technology. It was realized very early that the volume fractions of oil and water are not that important and that the type of emulsion is primarily determined by the nature of the surfactant. Simply speaking surfactants with Ns < 1 tend to form oil-in-water emulsions, while surfactants with Ns > 1 are more likely to form water-in-oil emulsions. Two more detailed guiding principles which are used for practical emulsion formulation are Bancroft s rule of thumb and the more quantitative concept of the HLB scale ... 

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