Document 44zrQjvBE39BgpZpqLGKq4y9x

Heating Ventilating Air Conditioning Guide 1939 Chapter 15. Steam Heating Systems a similar basis for a given system and proportioned to the heating capacitv of the radiators they serve, all radiators will heat proportionately to the steam pressure. The range of pressure variation is limited by the per. missible noise level of the steam flowing under the pressure difference required for maximum heat output. The control of the steam supply js obtained by a valve placed in the steam main, which maintains a deter mined pressure. These valves are frequently manually set from a remote location, guided by temperature indicating stations in the building; 0r & thermostatically controlled from a thermostat on the roof, which auto matically measures the differential of outside and inside temperatures; or by a boiler pressure control. Since the range through which the pressures !4 L i fmay be varied is limited, usually from 0 to 4.0 lb gage, the control should be capable of maintaining close regulation to maintain the desired space j f temperatures, particularly in mild weather. 31 Some systems use orifices not only in radiator inlets but also at different points in the steam supply piping for the purpose of balancing the system to a greater extent. In this manner the difference between the initial and terminal pressure in the steam main may be compensated to a great extent. For example, if the initial pressure was 3 lb gage and the pressure at the end of the main was 2 lb,.an orifice could be used in each branch for the purpose of obtaining a more uniform pressure throughout the system. Such a provision may be particularly, useful in this system for branches close to the boiler where the drop in the main has not yet been produced. Orifice systems are proprietary. ZONE CONTROL Often certain portions of a building may require more heat than others even if occupied for the same daily periods and the same maintained temperatures exist. Sometimes an entire building is on one general control, which results in overheating some sections when sufficient heat is supplied to accommodate the coldest portion. By separating a building into zones, each with its own piping system, each zone may be controlled separately. Systems are zoned to care for ex posure, hours of occupancy, stack effect and the requirements of occu pancy activity. In large buildings it is important to consider zoning for exposure because of the varying effects of the wind and sun. With the prevailing winter winds from the northwest, for example, a simple zoning would si i1 Vj; i ? `r i' 1 !'f ; | place the north and west sides of the building on one zone and the south 41 and east sides on another. If the size of the building justifies the expendi ture, a better arrangement would be to place all north walls on one zone, all west walls on a second, all east walls on a third, and all south walls on a fourth. Certain interior areas, such as basements, light well walls and other locations where sun and wind do not affect the conditions, should be placed in still another zone if the most economical operation is to be secured. In high buildings it is often important- to consider zoning for stack or chimney effect, caused by the difference in density between the warm air on the inside of a building and the colder air on the outside. Where the lowest eight or ten stories are protected from winds by surrounding buildings, it may accentuate the need for zoning to correct'for the chimney i* 298 t i k ' -I:'. On still days the heat demands vertically will vary little, but on Hv days there will be a marked difference in the heat requirements for "hdifferent horizontal sections at different elevations. An arrangement "ie rovi(je for difference in heat requirement for exposure and chimney P, would give 12 zones; namely, north, east, south, and west lower e*eC~s. similar middle zones; and similar top zones. Every type of steam h ting system may be zoned. The extent to which any system may be ;s governed by the limitations of the given installation and the Zrticular type of system. Zone control is used principally with subatmospheric, orifice and vacuum heating systems. Each zone should constitute an individual and separate system with its own control valve (controlled by thermostats in its respective zone), steam supply and return piping and preferably its own return pump or vacuum pump. It is possible for a single vacuum pump to serve several zones with a controller for each section connected in parallel so that the zone in which the lowest differential or vacuum is produced may start the pump. Zoning has advantages even where individual thermostatic radiator control is installed, whether this he of pneumatic, electric, or the self contained radiator valve type. The control secured by zoning in supply ing heat in parallel with its outside temperature and wind fluctuations removes a large part of the load from such individual thermostatic controls; they operate less frequently and the radiators follow a more even temperature instead of fluctuating from extreme hot to extreme cold. CONDENSATION RETURN PUMPS Condensation return pumps are used for gravity systems when the local conditions do not permit the condensation to return to the boiler under the existing static head. The return of the condensate permits the water to repeatedly go through the cycle of vaporization, with subsequent condensation and return to the boiler. During such repeated cycles any incrustants or other substances in. solution are precipitated and the water de-activated to a considerable extent so that corrosion of a serious nature is seldom ever encountered where the condensate is repeatedly used. Serious corrosion is more frequently found in systems where the conden sation is not repeatedly used but is wasted and fresh make-up water is continually being introduced. The most generally accepted condensation pump unit for low pressure heating systems consists of a motorrdriven centrifugal pump with receiver and automatic float control.. Other types in use include rotary, screw and reciprocating pumps with steam turbine or motor drive, and directacting steam reciprocating pumps. The receiver capacities of these automatic units should be sized so as not to cause too great a fluctuation of the boiler water line if fed directly to the boiler and at the same time not so small as to cause too frequent operation of the unit. The usual unit provides storage capacity between stops in the receiver of approximately 1.5 times the amount of condensate returned per minute and the pump generally has a delivery rate of 3 to 4 times the normal flow. This relation of receiver and pump size to heating system condensing capacity takes account of the peak condensation rate. 299