Physics Part 31

Sometimes radiators are placed in a box or room in the bas.e.m.e.nt. Air from out of doors is then driven by a fan over and about the hot radiators. The air thus heated is conducted by pipes to the several rooms. This arrangement is called _indirect heating_. (See Fig. 144.) The latter method, it may be observed, provides both heat and ventilation, and hence is often used in schools, churches, court houses, and stores. Since heated air, especially in cold weather, has a low _relative humidity_ some means of moistening the air of living rooms should be provided. Air when too dry is injurious to the health and also to furniture and wood work. The excessive drying of wood and glue in a piece of furniture often causes it to fall apart.

[Ill.u.s.tration: FIG. 145.--An automatic air valve.]

[Ill.u.s.tration: FIG. 146.--An automatic vacuum valve.]

=163. Vacuum Steam Heating.=--In steam heating, air valves (Fig. 145) are placed on the radiators to allow the air they contain to escape when the steam is turned on. When all the air is driven out the valve closes.

Automatic vacuum valves (Fig. 146) are sometimes used. When the fire is low and there is no steam pressure in the radiators the pressure of the air closes the valve, making a partial vacuum inside. The boiling point of water falls as the pressure upon it is reduced. As water will not boil under ordinary atmospheric pressure until its temperature is 100C.

(212F.), it follows that by the use of vacuum systems, often called vapor systems, of steam heating, water will be giving off hot vapor even after the fire has been banked for hours. This results in a considerable saving of fuel.

[Ill.u.s.tration: FIG. 147.--Plenum hot-blast system with temperature regulation.]

=164. The Plenum System of Heating.=--In the plenum system of heating (see Fig. 147) fresh air is drawn through a window from outdoors and goes first through tempering coils where the temperature is raised to about 70. The fan then forces some of the air through heating coils, where it is reheated and raised to a much higher temperature, depending upon the weather conditions. Both the hot and tempered air are kept under pressure by the fan in the plenum room and are forced from this room through galvanized iron ducts to the various rooms to be heated.

The foul air is forced out of the room through vent ducts which lead to the attic where it escapes through ventilators in the roof.

[Ill.u.s.tration: FIG. 148.--A thermostat. (Johnson System.)]

A thermostat is placed in the tempered-air part of the plenum room to maintain the proper temperature of the tempered air. This thermostat operates the by-pa.s.s damper under the tempering coils, and sometimes the valves on the coils. The mixing dampers at the base of the galvanized-iron ducts are controlled by their respective room thermostats. Attic-vent, fresh-air, and return-air dampers are under pneumatic switch control. A humidifier can be provided readily for this system. This system of heating is designed particularly for school houses where adequate ventilation is a necessity.

=165. The Thermostat.=--One of the many examples of the expansion of metals is shown in one form of the thermostat (Fig. 148) in which two pieces of different metals and of unequal rates of expansion, as bra.s.s and iron, are securely fastened together.

The thermostatic strip _T_ moving inward and outward, as affected by the room temperature, varies the amount of air which can escape through the small port _C_. When the port _C_ is completely closed (Fig. 148_a_) the full air pressure collects on the diaphragm _B_ which forces down the main valve, letting the compressed air from the main pa.s.s through the chamber _D_ into chamber _E_ as the valve is forced off its seat. The air from chamber _E_ then pa.s.ses into the branch to operate the damper.

When port _C_ is fully open (Fig. 148_b_) the air pressure on diaphragm _B_ is relieved, the back pressure in _E_ lifts up the diaphragm and the air from the branch escapes out through the hollow stem of the main valve, operating the damper in the opposite direction from that when _C_ is closed.

Important Topics

1. Transmission of heat in fluids.

2. Convection. Drafts of a chimney. Land and sea breezes.

3. Heating and ventilation of buildings.

(a) By hot air.

(b) Hot-water heating.

(c) Steam heating.

(d) Direct and indirect heating.

(e) Vacuum steam heating.

(f) The plenum system.

(g) The thermostat.

Exercises

1. Is a room heated mainly by conduction, convection, or radiation, from (a) a stove, (b) a hot-air furnace, (c) a steam radiator?

2. Name three natural convection currents.

3. Explain the _draft_ of a chimney. _What_ is it? _Why_ does it occur?

4. Make a _cross-section_ sketch of your living room and indicate the convection currents by which the room is heated. _Explain_ the heating of the room.

5. Make a sketch showing how the water in the hot-water tank in the kitchen or laundry is heated. Explain your sketch, indicating convection currents.

6. Is it economical to keep stoves and radiators highly polished?

Explain.

7. If you open the door between a warm and a cool room what will be the direction of the air currents at the top and at the bottom of the door?

Explain.

8. If a hot-water heating system contains 100 cu. ft. of water how much heat will be required to raise its temperature 150F.?

9. Why does a tall chimney give a better draft than a short one?

10. Explain how your school room is heated and ventilated.

11. Should a steam or hot-water radiator be placed near the floor or near the ceiling of a room? Why?

12. In a hot-water heating system an open tank connected with the pipes is placed in the attic or above the highest radiator. Explain its use.

(6) THE MOISTURE IN THE ATMOSPHERE, HYGROMETRY

=166. Water Vapor in the Air.=--The amount of water vapor present in the air has a marked effect upon the weather and the climate of a locality.

The study of the moisture conditions of the atmosphere, or hygrometry, is therefore a matter of general interest and importance. The water vapor in the atmosphere is entirely due to evaporation from bodies of water, or snow, or ice. In the discussion of evaporation, it is described as due to the gradual escape of molecules into the air from the surface of a liquid. This description fits exactly the conditions found by all careful observers. Since the air molecules are continually striking the surface of the liquid, many of them penetrate it and become absorbed. In the same manner many vapor molecules reenter the liquid, and if enough vapor molecules are present in the air so that as many vapor molecules reenter the liquid each second as leave it, the s.p.a.ce above the liquid is said to be _saturated_ as previously described. (See Art. 18.)

=167. Conditions for Saturation.=--If a liquid is evaporating into a vacuum, the molecules on leaving find no opposition until they reach the limits of the vessel containing the vacuum. Evaporation under these conditions goes on with great rapidity and the s.p.a.ce becomes saturated almost instantly. If, however, air be present at ordinary pressure, many of the ordinary water vapor molecules on leaving are struck and returned to the water by the air molecules directly above. Those escaping gradually work their way upward through the air. This explains why it is that our atmosphere is not often saturated even near large bodies of water, the r.e.t.a.r.ding effect of the air upon the evaporation preventing more than the layers of air near the water surface becoming saturated.

Just as the amount of salt that can be held in solution in a liquid is lessened by cooling the solution (Art. 26), so the amount of water vapor that can be held in the air is lessened by lowering its temperature. If air not moist enough to be saturated with water vapor is cooled, it will, as the cooling continues, finally reach a temperature at which it will be saturated or will contain all the water vapor it can hold at this temperature. If the air be still further cooled some of the water vapor will condense and may form fog, dew, rain, snow, etc., the form it takes depending upon where and how the cooling takes place.

=168. The Formation of Dew.=--If the cooling of the atmosphere is at the surface of some cold object which lowers the temperature of the air below its saturation point, some of its moisture condenses and collects upon the cold surface as _dew_. This may be noticed upon the surface of a pitcher of ice-water in summer. At night, the temperature of gra.s.s and other objects near or on the ground may fall much faster than that of the atmosphere owing to the radiation of heat from these objects. If the temperature falls below the saturation point, dew will be formed. This natural radiation is hindered when it is cloudy, therefore little dew forms on cloudy nights. Clear nights help radiation, therefore we have the most dew on nights when the sky is clear. If the temperature is below freezing, _frost_ forms instead of dew.

=169. Formation of Fog.=--If the cooling at night is great enough to cool the body of air near the earth below the saturation temperature, then not only may dew be formed, but some moisture is condensed in the air itself, usually upon fine dust particles suspended in it. This const.i.tutes a _fog_. If the cooling of the body of air takes place above the earth's surface as when a warm moist current of air enters a colder region, _e.g._, moves over the top of a cold mountain, or into the upper air, then as this air is cooled below its saturation point, condensation upon fine suspended dust particles takes place, and a _cloud_ is formed.

If much moisture is present in the cloud, the drops of water grow in size until they begin to fall and _rain_ results; or if it is cold enough, instead of rain, snowflakes will be formed and fall. Sometimes whirling winds in severe thunderstorms carry the raindrops into colder and then warmer regions, alternately freezing and moistening the drops or bits of ice. It is in this way that _hail_ is said to be formed.

=170. The Dew Point.=--The temperature to which air must be cooled to saturate it or the temperature at which condensation begins is called the _dew point_. This is often determined in the laboratory by partly filling a polished metal vessel with water and cooling the water by adding ice until a thin film of moisture is formed upon the outer surface. The temperature of the surface when the moisture first forms is the dew point.

=171. The Humidity of the Atmosphere.=--After the dew point has been obtained, one may compute the _relative humidity_ or _degree of saturation of the atmosphere_, from the table given below. This is defined as the _ratio of the amount of water vapor present in the air to the amount that would be present if the air were saturated at the same temperature_.

For example, if the dew point is 5C. and the temperature of the air is 22C., we find the densities of the water vapor at the two temperatures, and find their ratio: 6.8/19.3 = 35 per cent. nearly.

Determinations of humidity may give indication of rain or frost and are regularly made at weather bureau stations. They are also made in buildings such as greenhouses, hospitals, and schoolhouses to see if the air is moist enough. For the most healthful conditions the relative humidity should be from 40 per cent. to 50 per cent.

WEIGHT OF WATER (_w_) IN GRAMS CONTAINED IN 1 CUBIC METER OF SATURATED AIR AT VARIOUS TEMPERATURES (_t_)C.

--------+------ _t_C. | _w_ --------+------ -10 | 2.1 - 9 | 2.4 - 8 | 2.7 - 7 | 3.0 - 6 | 3.2 - 5 | 3.5 - 4 | 3.8 - 3 | 4.1 - 2 | 4.4 - 1 | 4.6 0 | 4.9 1 | 5.2 2 | 5.6 3 | 6.0 4 | 6.4 5 | 6.8 6 | 7.3 7 | 7.7 8 | 8.1 9 | 8.8 10 | 9.4 11 | 10.0 12 | 10.6 13 | 11.3 14 | 12.0 15 | 12.8 16 | 13.6 17 | 14.5 18 | 15.1 19 | 16.2 20 | 17.2 21 | 18.2 22 | 19.3 23 | 20.4 24 | 21.5 25 | 22.9 26 | 24.2 27 | 25.6 28 | 27.0 29 | 28.6 30 | 30.1 --------+------

=172. Wet and Dry Bulb Hygrometer.=--A device for indicating the relative humidity of the air is called an _hygrometer_. There are various forms. The _wet_ and _dry bulb hygrometer_ is shown in Fig. 149.

This device consists of two thermometers, one with its bulb dry and exposed to the air, the other bulb being kept continually moist by a wick dipping into a vessel of water. An application of the principle of cooling by evaporation is made in this instrument. Unless the air is saturated so that evaporation is prevented, the wet-bulb thermometer shows a lower temperature, the difference depending upon the amount of moisture in the air, or upon the relative humidity. Most determinations of relative humidity are made with this kind of instrument. It is necessary in order to make an accurate determination, to fan or set the air in motion about the thermometers for some time before reading them.

The relative humidity is then found by using tables giving the relative humidity that corresponds to any reading of the thermometers.