Document 0gKd8z4ZZZkdZqKxwpdmXbLxd

For indirect or vento heating install service pressure and Heating Ventilating Air Conditioning Guide 1939 788 Chapter 42. District Heating 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 amount of heating surface so that the building can 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 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. 2. 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. Fig. 6 shows a typical steam service installation for high pressure steam, complete for steam flow metering, water heating, preheating, automatic heating control, and for using steam for other purposes. 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. The number of square feet of heating surface in the economizer should be approximately equal to one per cent of the equivalent square feet of heating surface in the building. 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. This may be done in several ways, as by the use of temperature controls of various, types or by orifice systems. Another method which is very simple is the use of an ordi nary vacuum return line system in which the pressure in the radiators is varied between a high vacuum and a few pounds pressure, thus producing some control over the heat output of the heating system by varying the temperature of the steam in the radiators. Several proprietary systems are on the market which accomplish this automatically, either with outdoor or indoor controls or a combination of both. One form of control which appears to be well suited for controlling district steam service to a building is the weather compensating control. It regulates the steam supply automatically ac cording to the outdoor temperature, and gives frequent short intervals of intermittent steam supply, and at the same time insures delivery of steam to all the radiators. This type of control can be equipped with time clocks and thermostats to provide a warmingup period in the morning. Another form of regulation, known as the time-limit control, is sometimes employed for regulating the steam supply from the central station main to the building. Such a control provides an intermittent supply of steam to the radiators either throughout the 24 hours of the day or during the daytime hours only. The setting of a switch may provide no service, continuous service, or periodic service. For the latter, by means of several intermittent settings, steam will be supplied during each period in increments 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 inter mittent supply called for by the day switch setting, or may be set to interrupt the opera- 789 /