7. Dehumidification 8. Air-change rates 9. Positive-pressure areas 10. Negative-pressure areas 11. Balanced-pressure areas 12. Contaminated exhausts 13. Chemical exhausts and fume hoods 14. Energy conservation devices 15. Economizer system 16. Enthalpy control system 17. Infiltration 18. Exfiltration 19. Controls 13.3.2 Design Criteria Accuracy Some engineers apply much effort to determination of design conditions with great accuracy. This is usually not necessary, because of the great number of variables involved in the design process. Strict design criteria will increase the cost of the necessary machinery for such optimum conditions and may be unnecessary. It is generally recognized that it is impossible to provide a specific indoor condition that will satisfy every occupant at all times. Hence, HVAC engineers tend to be practical in their designs and accept the fact that the occupants will adapt to minor variations from ideal conditions. Engineers also know that human comfort depends on the type and quantity of clothing worn by the occupants, the types of activities performed, environmental conditions, duration of occupancy, ventilation air, and closeness of and number of people within the conditioned space and recognize that these conditions are usually unpredictable. 13.3.3 Outline of Design Procedure Design of an HVAC system is not a simple task. The procedure varies considerably from one application or project to another, and important considerations for one project may have little impact on another. But for all projects, to some extent, the following major steps have to be taken: 1. Determine all applicable design conditions, such as inside and outside temperature and humidity conditions for winter and summer conditions, including prevailing winds and speeds. 2. Determine all particular and peculiar interior space conditions that will be

Rate of return expected from the type of business Interest rate for borrowed money Rate for government bonds or notes Rate for highly rated corporate bonds The client is concerned with interest rates because all costs represent money that must be borrowed or that could otherwise be invested at a current interest rate. The client also has to be concerned with time, measured from the date at which an investment is made, because interest cost increases with time. Therefore, in economic comparisons of systems, interest rates and time must be taken into account. (Effects of monetary inflation can be taken into account in much the same way as interest.) An economic comparison usually requires evaluation of initial capital investments, salvage values after several years, annual disbursements and annual revenues. Because each element in such a comparison may have associated with it an expected useful life different from that of the other elements, the different types of costs and revenues must be made commensurable by reduction to a common basis. This is commonly done by either: 1. Converting all costs and revenues to equivalent uniform annual costs and income 2. Converting all costs and revenues to present worth of all costs and revenues at

For any large project, significant savings can be made through use of quality control to reduce the overdesign otherwise required by a building code (law). When FIGURE 9.3 Curves show variation of 28-day compressive strength of non-air-entrained concrete with type of aggregate and watercementitious materials ratio, except that strengths exceeding 7000 psi were determined at 56 days. All mixes contained a water-reducing agent and 100 lb / yd3 of fly ash. Calculation of watercementitious materials ratio included two-thirds of the fly-ash weight in the cement content. the owners project specifications include a minimum content of cementitious materials, however, much of the economic incentive for the use of quality control is lost. See Fig. 9.3 for typical water-cementitious materials ratios. Note that separate procedures are required for selecting proportions when lightweight aggregates are used, because their water-absorption properties differ from those of normal-weight aggregates. (Building Code Requirements for Structural Concrete, ACI 318 Standard Specifications for Structural Concrete, ACI 301; Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete, ACI 211.1; Standard Practice for Selecting Proportions for Structural Lightweight Concrete, ACI 211.2; Recommended Practice for Evaluation of Strength Test Results of Concrete, ACI 214, American Concrete Institute, P.O. Box 9094, Farmington Hills, MI 48333, Design and Control of Concrete Mixtures, EB001TC, Portland Cement Association, 5420 Old Orchard Road, Skokie, IL 60077.) Questions often arise between concrete suppliers and buyers regarding yield, or volume of concrete supplied. A major reason for this is that often the actual yield may be less than the yield calculated from the volumes of ingredients. For example, if the mix temperature varies, less air may be entrained; or if the sand becomes drier and no corrections in batch weights are made, the yield will be under that