The amount of chilled water in gallons per minute to be circulated may be obtained from 24  tons of refrigeration where T is the temperature rise of water on passing through the cooling coil, usually 8 or 10
F. The smaller the temperature change, the more water to be circulated and the greater the pumping cost, but the better will be the heat transfer through the water chiller and cooling coil. Coil manufacturers catalogs usually give the procedure for picking the number of rows of coil necessary to do the required cooling with the water temperatures available. The water chiller should be sized to cool the required flow of water from the temperature leaving the cooling coil to that required by the coil at a given suction temperature. Assuming 45
F water supplied to the cooling coil and a temperature rise of 8
F, the water leaving the coil would be at 53
F. The chiller would then be picked to cool the required flow from 53 to 45
F at 37
F suction. The lower the suction FIGURE 13.44 Control circuit for a chilled-water system with evaporative

leads to a compression of the underlying porous soil or rock structure. Since subsidence is due to a secondary influence (extraction of oil or groundwater), its effect on the structure would be included in the second basic type of settlement. 6.5.3 Foundation Design Parameters Determining the settlement of the structure is one of the primary obligations of the geotechnical engineer. In general, three parameters are required: maximum total settlement (max), maximum differential settlement (), and rate of settlement. Another parameter that may be useful in the design of the foundation is the maximum angular distortion ( /L), defined as the differential settlement between two points divided by the distance between them. Figure 6.19 illustrates the maximum total settlement (max), maximum differential settlement (), and maximum angular distortion ( /L) of a foundation. Note in Fig. 6.19 that the maximum angular distortion ( /L) does not necessarily occur at the location of maximum differential settlement 6.5.4 Allowable Settlement The allowable settlement is defined as the acceptable amount of settlement of the structure and it usually includes a factor of safety. The allowable settlement depends on many factors, including the following (D. P. Coduto, Foundation Design, Principles and Practices, Prentice-Hall, Inc., Englewood Cliffs, N.J.): The Type of Construction. For example, wood-frame buildings with wood siding would be much more tolerant than unreinforced brick buildings. TABLE 6.13 Allowable Settlement Type of movement Limiting factor Maximum settlement

Effective Current and Voltage. The instantaneous value of an alternating current (or voltage) is continuously varying. This current has a heating effect on wire equal to the effective current I times the resistance R. Mathematically, the effective current is 0.707 times the maximum instantaneous current of the sine wave. The same relationship holds true for the effective voltage. Ohms law, E
IR, can be used in alternating circuits with E as the effective voltage, I, the effective current, A, and R, the resistance,
. Inductive Reactances and Susceptance. When alternating current flows through a coil, a magnetic field surrounds the coil. As the current decreases in instantaneous value from maximum to zero, the magnetic field increases in strength from zero to maximum. As the current increases in the opposite direction, from zero to maximum, the magnetic-field strength decreases to zero. When the current starts to decrease, a new magnetic field is produced that is continuously increasing in strength but has changed direction. The magnetic field, in changing, induces a voltage and current in the wire, but the phase, or timing, of the zero and maximum values of this induced voltage and current are actually 90 behind the original voltage and current wave in the wire. The induced voltage and current are proportional to a constant called the inductive reactance of the coil. This constant, unlike resistance, which depends on the material and cross-sectional area of the wire, depends on the number of turns in the coil and the material of the core on which the coil is wound. For example, a simple coil wound around an airspace has less inductive reactance than a coil wound around an iron core. Inductive voltage EL and inductive current IL, A, are