Document gDY1B413gyL7Qyk0gRgjwRNQ

534 CHAPTER 29 1946 Guide . 1. Provision should be made for conveniently shutting off the steam supply at night and at other times when heat is not needed. It has been thoroughly demonstrated that a considerable amount of heat can. be saved by shutting off steam at night. Although there is, in some cases, an increased consumption of heat when steam is again turned on in the morning, there is a large net saving which may be explained by the fact that the lower inside temperature maintained - during the night obviously results in lower heat loss from the building, and less heat need therefore be supplied. . ^ Steam can be entirely shut off at night in most buildings even in .very cold weather without endangering plumbing. It is necessary, however, to have an ample ampunt'of ' heating surface so that the building caii be quickly warmed in the morning. Where the hours of occupancy differ in various parts of the building, it is good practice to install separate supply pipes to the. different parts. For example, in an office building with stores or restaurants on the first floor which are open in the evening, a separate main supplying the first floor will permit the steam to be shut off from the remainder of the District Heating 535 ment of district heating as any.-.other one thing. Meters are classified into two groups: Quantity Meters and Rale of Flow.Meters. Quantity Meters The one type of quantity meter used is the condensation meter, which may be of the tilting bucket or revolving drum type. The condensation meter is a popular type for use on small and medium sized installations, where all the condensate can be brought to a common point for metering purposes. Its simplicity of design,, ease in testing, accuracy at all loads, low.cost, and.adaptability to low pressure distribu tion has made it standard equipment with many heating companies. Condensation meters should not be operated under pressure; they are made for either gravity or vacuum installations. Where bucket traps are used, a vented receiver is essential ahead of the meter. Where con tinuous flow traps are used, a vented receiver is not necessary, but is Fig. 5. Method of Installing a Water Heater and Economizer in a . Gravity Heating System building in the late.afternoon. The division of the building into zones each with a separately controlled heat supply is sometimes desirable, as it permits the heat to be adjusted according to variations in sunshine and wind. S. Residual heat in the condensate should be salvaged. This heat.may be.salvaged by means of a cooling coil, or as is more frequently done, by a water' heating economizer (see Fig. 5) which preheats.the hot water supply to the building. The condensation from the heating system, after leaving the trap, passes through the economizer. The supply to the hot' water heater passes through the economizer, ab sorbing heat from the condensate. If the hot water system in the building is of the recirculating type, the recirculating connection should be tied in between the economizer and the water heater proper, not at the economizer inlet, because the recirculated hot water is itself at a high temperature. Because of.the lack of coincidence between the heating system load and the hot water demand, a greater amount of heat can be extracted from the condensate if storage capa city is provided for the preheated water. Frequently a type of economizer is used in which the coils are submerged in a storage tank. S. Heat supply should be graduated according to variations in the outside temperature. .. . The maximum in economical operation and satisfactory heating can only be obtained by the use of some automatic temperature control system. . . The perfection of fluid meters has contributed as' much to the advance- Fig. 6! Gravity Installation for Condensation Meter Using Vented Receivers desirable. Fig. 6 illustrates a gravity condensation meter installation using a vented receiver. / Rate of Flow or Flow Meters Flow meters used for district heating work are of three types: Area. Meters, Head Meters and Velocity Meters. Area meters are those, in the operation of which, a variation in the cross-section of stream under constant head is used as an indication of the rate of flow. A tapered plug is suspended in an orifice and moves axially with the flow, which is vertically upward. The weight of the plug provides a definite pressure differential, and the plug floats at such a height as will provide enough orifice area to pass the flow at the pressure difference. The position of the plug'is transmitted by means of a lever and pencil and records the flow on a graduated strip chart. Head meters are those in which the stream of fluid creates a difference of pressure, or differential head. This head is created by an orifice, Venturi tube, flow nozzle', of Pitot tube and will depend upon the velocity and density of the fluid. , .The secondary element must contain a differential pressure gage, which will translate the pressure difference into rate of flow or total flow. This mechanism may be either mechanical or electrical. The electric, flow meter has the advantage of being able to locate the instruments at some distance from the primary element.,