Document YrgMgNzb1jVJZd46mX9a4DdMn

American Society of Heating and Ventilating Engineer Guide, 1935 time hours only. The setting of a switch may provide no service, con tinuous service, or periodic service. For the latter, by means of several intermittent settings, steam will be supplied during each period in in crements of a certain number of minutes for each successive setting of the switch, steam being shut off during the balance of the period. These settings afford from 15 to 80 per cent of the maximum heating effect required on days of zero temperature. A night switch with a variety of settings may be adjusted so as to maintain throughout the night the intermittent supply called for by the day switch setting, or may be set to interrupt the operation of the day switch and entirely cut off the supply of steam to-the radiation at night during "certain hours which are selected by the operating engineer. FLUID METERS No one thing has contributed more to the advancement of district heating than the perfection of fluid meters, which may be classified as follows: 1. Positive Meters: The fluid passes in successive isolated quantities--either weights or volumes. These quantities are separated from the stream and isolated by alternately filling and emptying containers of known capacity. 2. Differential Meters: The fluid does not pass in isolated separately-coqnted quan tities but in a continuous stream which may flow through the line without actuating the primary device of the meter. In the differential meter, the quantity of flow is not determined by simple counting, as with the positive meter, but is determined from the action of the steam on the primary element. Additional subdivisions of these two general classifications can be made as follows: Fluid Meters Positive - quantity Weighing Vo!,,metric { { Quantity - Current - Turbine ,, Differential Rate of flow Area _ (Geometric) Venturi I Flow nozzle Orifice Pitot tube ( Orifice and plug \ Cylinder and piston Head area (Weir) f V-notch \ Special notch In selecting a meter for a particular installation, the number of different makes and types of meters suitable for the job is usually limited by one or more of the following considerations: 1. Its use in a new or an old installation. 2. Method to be used in charging for the service. 3. Location of the meter. 4. Large or small quantity to be measured. 5. Temporary or permanent installation. 648 Chapter 37--District Heating Fig. 7. Orifice Meter Steam Supply Connection 6. Cleanliness of the fluid to be measured. 7. Temperature of the fluid to be measured. 8. Accuracy expected. 9. Nature of flow: turbulent, pulsating, or steady. 10. Cost. (o) Purchase price. (6) Installation cost. (c) Calibration cost. (d) Maintenance cost. 11. Servicing facilities of the manufacturer. 12. Pressure at which fluid is to be metered. 13. Type of record desired as to indicating, recording or totalizing. 14. Stocking of repair parts. 15. Use of open jets where steam is to be metered. 16. Metering to be done by one meter or by a combination of meters. 17. Use as a check meter. 18. Its facilities for determining or recording information other than flow. Condensation Meters The majority of the meters used by district heating companies in the sale of steam to their customers are of the condensation or flow types. The condensation meter is a popular type for use on small and medium sized installations, where all of the condensate can be brought to a com mon point for metering purposes. Its simplicity of design, ease in testing, accuracy at all loads, low cost, and adaptability to low pressure distri bution has made it standard equipment with many heating companies. Two types of condensation meters are in general use: the tilting bucket meter and the revolving drum or rotor meter of which there are several makes on the market. Condensation meters should not be operated under 649