Document a69OLEzxE1n1mrqX650Rp4NR

American Society of Heating and Ventilating Engineers Guide, 1934 under conditions of high vacuum is not only reduced in proportion to the difference in the steam temperature between that for 2 lb gage and for 25 in. of vacuum but it is reduced still further by a reduction in the amount of steam which can pass through the orifice when the steam is expanded due to the vacuum. This renders possible the control of heat emission from the radiators to a point not indicated entirely by the difference in steam temperatures. The high-vacuum pumps on this system are equipped with receivers having float control so that the pump can be placed on a receiver-returnpump basis at night if desired so no high vacuum will be carried. One radical difference between this system and the ordinary vacuum system is that no lifts can be made in the return line. The returns must grade downward constantly and uninterruptedly from the radiator return outlet to the inlet on the high-vacuum pump receiver. No attempt should be made to heat service water on this system unless the steam line for water heating is taken off the boiler header back of the heating system control valve, and then only when 2 lb or more will be carried on the boiler at all times. ORIFICE SYSTEM Orifice systems of steam heating may have piping arrangements identical with vacuum systems but some of these systems omit both the radiator thermostatic traps and the vacuum pump in cases where the returns are wasted to a sewer or delivered to some type of receiver in which no back pressure exists. The principle on which they operate is embodied in the well-known fact that an orifice will deliver varying velocities when the ratio of the absolute pressures on the two sides of the orifice exceeds 58 per cent. If the absolute pressure on the outlet side is less than 58 per cent of the absolute pressure on the inlet side no further increase in velocity will be obtained. v As a result, if an orifice is so designed in size as to exactly fill a radiator with steam at 2 lb gage on one side and )4 lb gage on the other, the abso lute pressure relation is 14.7 + 0.25 14.7 + 2.0 90 per cent Should the steam pressure be dropped to 34 lb gage, the pressure on each side of the orifice would be balanced and no steam flow would take place. From this it will be seen that if an orifice of a given diameter will fill a given radiator with steam when there is a given pressure on the main, it is simply a question of dropping this main pressure so as to fill any desired portion of the radiator down t.o the point where the main pressure equals the back pressure in the radiator, at which time no steam will be supplied at all. If orifices throughout a job are designed on a similar basis, all radiators will heat proportionately to the steam pressure within the limits for which the orifices are designed. Some systems use orifices not only in radiator inlets but also at different points on the main; thus balancing the system to a greater extent. For example, the system may be designed for a particularly long run involving an initial pressure of 3 lb gage on the main and 2 lb at the end of the main, 432 Chapter 31--Steam Heating Systems. but each branch from the main may have an orifice for reducing the pressure at it to 2 lb gage. This is particularly useful for branches near the boiler where the drop in the main has not yet been produced. Orifice systems using a vacuum pump operate successfully with the ordinary low vacuum type of pump producing 8 to 10 in. of vacuum. They are controlled by various means to regulate the steam pressure.One method is by a thermostat located on the roof to govern the steam pressure by a combination of outside and inside temperatures; another, useful on systems without traps and vacuum pumps, controls the steam pressure manually from temperature indication stations in the building, or automatically by a thermostatically-controlled pressure reduction valve or draft regulator on the boiler; with oil or gas firing, the on-and-off control or a boiler pressure control may be used. ZONE CONTROL Certain portions of a building may require more heat at times than others but if the whole building is on one general control, such as would occur with a single piping system with an on-and-off control or with the sub-atmospheric or the orifice systems, it would be necessary to supply sufficient heat to accommodate the coldest portion of the building even though some sections would be overheated. By zoning, each section of a building may be controlled separately. The sides of the building with different exposures should be considered first, because of the varying effects of the wind and sun. With the pre vailing winter winds from the northwest, a simple zoning would place the north and west sides of the building on one system.and the south and east sides on another. If the building is large enough to justify 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. In case of high buildings, the lower 8 or 10 stories may be well protected from wind by surrounding buildings, the next 10 stories may have moderate exposure, and above this there may be an unobstructed exposure to gales. On still days the heat demands vertically will vary little, but on windy days there will be a marked difference in the heat requirements for the different horizontal sections. In addition, the 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 will give an air movement which will require zoning to correct. Where such conditions are encountered, the building should be divided horizontally as well as vertically. An arrangement of this character would give 12 zones, namely: north, east, south, and west lower zones; similar middle zones; and similar top zones. Each zone should constitute an individual and separate system of piping with its own supply steam valve (controlled by thermostats in its respec tive zone) and with its own return or vacuum pump, if one is used. 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 results are to be secured. Zoning has advantages even where individual thermostatic radiator 433