Document 5LZrapB8e0op1NygX486V8boR

646 CHAPTER 29 1953 Guide Diversity Factor. The ratio of the sum of the individual demands of a number of buildings to the actual composite demand of the group. Types of Rates The various types of rates to be found in use in district heating systems -are: ' ' 1. Straight-Line Meter Rate. The price charged per unit is constant, and the con sumer pays, in direct proportion to his consumption without, considering the dif- ference in costs of supplying the individual customers^ ' * `'5 2. Block Meter Rate. The pounds of steam consumed by a customer are divided into blocks of thousands of pounds each, and lower, rates are charged .for each suc cessive block consumed. This type of charge predominates in steam heating rate schedules, having the advantage of proportioning the bill according to the consump tion and the cost of service. It has the disadvantage of not discriminating between customers having,a high loadfactor (relatively low demand), and those having a low load factor (relatively high demand). The utility company must maintain sufficient capacity to serve the high demand customers, and the cost of the increased plant investment is divided equally among the users, so the high demand customers are ' benefited at the expense of the others. 3. Demand Rates. These refer to any method of charge based on a measured maximum load during a specified period of time. The fiat demand rate is usually expressed in dollars per thousand pounds of demand per month or per annum. It is based on the size of a customer's installation, and is seldom used except where a meter is hot practicable. The Wright demand rate is similar in calculation to the block rate, except that it is expressed in terms of hours' use of the maximum demand.' It is seldom used, but forms the basis for other forms of rates. The Hopkinson demand rate is divided into two elements:. () A charge based upon the demand, either estimated or measured. () A charge based upon the amount of steam consumed. This rate may be modified by dividing the quantities of steam demanded and con sumed into blocks charged for at different rates. The Doherty rate is divided into three elements: (o) A charge based upon demand. (6) A charge based upon steam consumed. (c) A customer charge. In the Hopkinson rate, the last two elements are combined into one element. Demand rates are comparatively new and are not yet widely, used. While they are equitable and competitive, they are-difficult for the average layman to understand. They are of benefit to utility companies and to consumers because the investment and operating costs can be divided, to suit the particular circumstances, into demand, customer, and consumption groups through the use of some modification of the Hopkinson rate. De mand rates are an advantage to the customer in that the use of such a rate reduces the rate per thousand pounds to the long-hour user. Fuel Price Surcharge. It is usually desirable to establish a rate upon a specified basic cost of fuel to the utility company. Where there are wide variations in the price of fuel, it is also desirable to add a definite charge per thousand pounds of steam sold for each increment of increase in the price of fuel. This surcharge automatically compensates for the variations without necessitating frequent changing of the whole rate structure. Some utility companies include a labor surcharge as well as a coal sur charge. CHAPTER 30 CENTRAL SYSTEMS FOR AIR CONDITIONING Features of Systems, Zoning; Humidity Control, Cooling Load, Heating Load, Air Quantity and Effectual Temperature Difference, Low and High Pressure Induction Convectors, Evaporative Cooling, Precooling, Sensible Cooling with Unwetted Coils, Run-Around System, Selection of Type of System, Location of Apparatus, Design Procedure THE term, central, applied to an air conditioning system implies that the equipment such as fans, coils, filters and their encasement are designed for assembly in the field rather than in a factory as a unit. As a central system usually serves several different rooms, individual con trols are required for each room. FEATURES OF CENTRAL SYSTEMS One advantage of a central air supply system is that one apparatus serving many rooms may involve^ a lower investment cost than that for a number of self-contained plants, each serving a single room. A central system may occupy basement or attic space that is relatively unimportant, whereas individual factory-assembled apparatus placed in each room may occupy otherwise valuable, space. Another advantage of a central system is accessibility for servicing, since it is possible to provide doors in the encasement for cleaning and inspecting all of the component parts in a manner usually superior" to that practicable with compact factoryassembled equipment. Central air conditioning, systems usually are connected by ducts with the various rooms served, and preferably have exhaust fans that may effect complete removal and disposal of any desired proportion of the air. The exhaust fan may return air-to the supply system for recirculation, as a measure of economy'of fuel or refrigeration. Central air conditioning systems are served by heating and refrigerat ing equipment which may. be located at some distance from the air supply apparatus, and which may serve one or more central air supply systems. Year-Round Air Supply System Fig. 1 is a plan of a year-round air supply system. Outside air may enter from the left at A, desirably from an intake On the side of the build ing least exposed to solar heat, and not close to the ground or to a sunheated or dust-gathering roof. The damper B for proportioning the volume of outside air, is interlocked with the return air damper C in such manner that as the outside air volume increases the.return air volume decreases. The return air duct D, shown diagrammatically, comes from the exhaust fan. All the air, it will be observed, must pass through the filters E, and there is ample room on both the inlet and outlet sides of the filters for servicing them. The filters may be of mechanically cleaned type, of replaceable cell type, or may be electronic, as described, in Chapter 34. The cleaned air passes to the equipment that changes its temperature and humidity. Except in very warm, climates, a heating or tempering coil F is required to warm the air to a temperature above freezing. Usu- 647