Units converting

Some systems can give quantitative results from known pieces of data complete with proper units. For example, these systems can take all the starting information and then determine a set of equations from the available list that can yield the desired result. The program could subsequently convert units or algebraically solve the equations if necessary. 

Fig. 3. Process flow diagram for a continuous falling film SO sulfonation plant, equipped with a sulfur-burning SO converter unit. See text.

The conversion factors for other compound units not Hsted can easily be generated from numbers given in the alphabetical Hst by the substitution of the converted units eg, to find the conversion factor from Ib-ft/s to kg-m/s ... 

A converter unit is used for the control of d.c. machines and also to provide a d.c. source to an inverter unit controlling an a.c. machine. In d.c. drives the d.c. voltage after tlie converter unit should be variable, whereas for in a.c. drive it is kept fixed. The voltage is varied by the invener unit. A convener unit is the basic power conversion scheme to convert an a.c. supply to a d.c. supply. Conventionally they are also known us rectifier units and c in be arranged in four different modes to suit dilTerent applications of a motor as follows ... 

Both V and/can be varied with the help of pulse width modulation (PWM) in the inverter circuit. The converter unit normally is an uncontrolled pow-cr diode rectifier. 

Depending upon the mode of operation of a motor, the type of converter unit can be decided. For. simplicity, the operation (conduction) of a motor can be represented by four quadrants as illustrated in Table 6.4. 

Figure 6.24(b) Thyristor cubicles for high-power static converter units (Courtesy Siemens)... 

Table 6.4 Operation of a motor in different modes and the corresponding conducting quadrants of a controlled converter unit...

Figure 6.32 Regenerative energy feedback arrangement for a converter unit...

When there is no dedicated transformer and these circuits are connected on the system bus directly a large inductor will be essential at the incoming of the static circuits, sufficient to absorb the trapped charge within the transformer and the interconnecting cables up to the converter unit. The size of the inductor can be calculated depending on the size (kVA) of the distribution transformer, its fault level and the characteristics of its current limiting protective device. An inductor sufficient to absorb //, L of the transformer and the cables may be provided at the incoming of the sialic circuits. 

Determine expansion or contraction losses, if any, including tank or vessel entrance or exit losses from Figures 2-12A, 2-15, or 2-16. Convert units to psi, head loss in feet times 0.4331 = psi (for water), or adjust for Sp Gr of other liquids. 

In this book I will follow these recommendations as far as is reasonable and SI units will be used throughout. I cannot accept the argument of some of my colleagues who are twenty or more years younger than I that they are accustomed to (for example) kcal, and that they think it sufficient just to tell students in courses once that the SI unit is kJ, that 1 kJ = 0.239 kcal, and to suggest that students convert units if they want ... 

SI (le Systeme International d UniUs) units are used in many countries to express clinical laboratory and serum drug concentration data. Instead of employing units of mass (such as micrograms), the SI system uses moles (mol) to represent the amount of a substance. A molar solution contains 1 mol (the molecular weight of the substance in grams) of the solute in 1 L of solution. The following formula is used to convert units of mass to moles (mcg/mL to pmol/L or, by substitution of terms, mg/mL to mmol/L or ng/mL to nmol/L). 

Program DGC04 solves the time-dependent problem. Subroutine EQUATIONS evaluates the coefficients of the unknown delys in the manner just outlined, and then subroutine GAUSS solves for the values of dely. Subroutine STEPPER steps forward in time by incrementing x and y. Subroutine SPECS sets the values of the parameters of the problem, converting units where necessary, and PRINTER writes the results to a file for plotting. 

When working with measurements, you often have to convert units before performing other calculations. There are two methods of converting measurements. One is using proportions and the other is using a scientific method called dimensional analysis. 

To use the proportion method to convert units, set up a proportion as described in Chapter 5. Keep the units consistent on both sides of the proportion. For example, if you want to convert 50.8 centimeters to inches, set up a proportion, such as meter = centimeter and substitute in tie given values on one side, tie conversion factor on tie other yyq = -yyy. Cross-multiply to get 2.54 xs=lx 50.8. Now, divide 50.8 by 2.54 to get n = 20 inches. 

You may be wondering Why bother with dimensional analysis at all There is a very good reason to know this method. It is the method scientists prefer because it allows you to do several conversions at once. Study the next example. Both methods of converting units will be used. 

---