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HEATING VENTILATING AIR CONDITIONING GUIDE 1940
With flat plate panels it is common practice to use a frame of plaster, wood, metal or composition to allow for expansion. These plates may be heated with either hot water or steam and connected to an ordinary radiator system.
5. By electric heated metal plates or panels. These plates or panels are either placed in insulated recesses of walls or ceilings or fastened to the construction, as found desirable. They should not have a surface temperature much above 200 F; some have a much higher surface temperature but a lower temperature gives a more comfortable condition and is more efficient.
6. By electrically heated tapestry mounted on screens and on the wall. For this purpose the screen is woven with an electric continuous conductor. Such screens are useful to plug in at any position for emergency local heating without taking care of a large room or office.
Note. If all of a heating panel is installed at one end of a large room there may be a marked difference between the BET on the two sides of the body. It is usually desir able, therefore, that the heat be distributed at different parts of the walls and ceilings so that no uncomfortable effects will be felt from unequal heating.
PUN Air ducts in floor space
Fig. 3. Diagram of Air Ducts for Floor Heating
CALCULATION PRINCIPLES
The calculations for radiant heating are entirely different from those for convective heating. The purpose of the latter is to determine, and compensate for the rate of heat loss from the room, when maintained in the desired condition; but radiant heating involves the regulation of the rate of heat loss from the human body.
The first step in the calculations for radiant heating of a given room is to ascertain the desired MRT; next, to decide at what temperature the heating surface shall operate; then, to compute the size and disposition of the heating surfaces required to produce this MRT; and last, to provide converted heat for the required number of, air; changes.
Mean Radiant Temperature ' If the entire interior surface of a room were at the sanie temperature,
this would be the MRT. Such a condition seldom exists, however, since in different parts of a room, with some surfaces exposed to the outer air, the actual surface temperature varies greatly with the construction and exposure of different sides of the enclosure. It is therefore necessary to calculate the thermal mean of these interior surface temperatures.
This is not the arithmetic average of the various actual surface tem peratures, but the radiant temperature which corresponds to the average
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CHAPTER 42. RADIANT HEATING
of the several rates of heat emission (Btu per square foot) from the several surfaces. The emission at any given surface temperature, for any stated "emissivity factor, and also the MRT corresponding to any average emission, can be obtained directly from Table 1. For example; if the emissivity of the surface is 0.9, lsq ft of surface at 50 F will emit 104.9 Btu per square foot per hour to surroundings at absolute zero.
Table 1. Total Black Body Radiation to Surroundings at Absolute Zero3
' Bodt- " Radiation in Bto per square foot per boor
OB - emitted to surroundings with a tempera
Mbak ture of absolute zero by bodies at various
Radiant temperatures and with emissivity. factor $
Tbmpbb-
`'ATVBB Deg Fahr
e 1.00
0.95
e 0.90
e 0M
1 Boor OB
Mban [Radiant
Tbmpbb-
atubb
Deg Fahr
Radiation in Btu per square foot per hour emitted to surroundings with a temperature of-absolute zero by bodies at various temperatures and with emissivity factor .
1.00
0.95 * 0.90
0.80 '
30 99.3 94.3
35 103.5 98.3 40 107.6 102.4 45 112.1 106.5
..-.46 .,112.9 107.3 . 47 `113.9 108.2 '`48' 114.8 109.1
:49: .115.6 109.9 50 116.5. 110.6
51 117.5 111.6 52 118.4 112.5 53 119.4 113:4
54 120.2 114.2 55 121.1 115.1
: -56 122.1. 11610 57. 123.1 117.0
. 58 124.0 117.8 59 124.9 118.6
: 60 125.8 119.5 61 126.6 120.3.
- 62 127.7 121:4 63 128.6' 122.2
1 .64 129.6 123.1 65 130.5 124.0
66- 131.6 125.0 67 132.5 125.9 68 133.5 126.8 69 134.5 127.8 v.70 135.5 .128.8.
89.4 79.4 71
136.5
129.6
93.2 82.8 72
137.4
130.5
96.8 86:1 73
138.4
131.5
100.9. 89.7 74 139.6 132.6
101.6. 90.4 75
141.0
133.9
102.5' 91.1 80 146.6 139.4
103.4 91.9 85
152.3
144.6
104.1. 92.4. 90 : 157.9 149.9
104.9 93.2 100 169.6 161.1
105.8 94.0 no 181.6 172.5
106.5 94.7 120
194.8
185.0
107.4 95.5 130
210.1
199.6
108.2 96.2 140 223.2 . . 212.1
109.0 96.9 150 237.1' ' 225.2
.109'. 9: :97.7 160 . 251.1' :238.8
110.9. 98.5 170 270.5.. .257.0
111.6 99.2 180 288.0 273.8
112.4 99.9 190 306.5 291:0
133:4; 100.7 200 "325.2 309.0
114.0 101.4 210 .348.0 330.6
.114.9 102.2. 220 , 371.5 . 353.0
115.8 102.9' 250 437.8 ' 415.9
116.7 103.7 '300 .575.0 1 546.1
117.5 104.4 350 740.0 - 703.0
118.4 -105.4 400 942.1 : 895.0
119.3 106.0 450 1176.0. . 1117.0
120.1 106.8 500 1464.0 1390.0
321.1. :107.6.1 550 1791.0 1701.0
.121.9 . 108.4 666 2405.0 2284.0
122.9 109.3
123.6 109.9' 124.5. 110.6
125.6 111.7
126.9 112.8
132.0 117.4 1371 121.9 142.1 126.4
152.6 135.7 163.5 ' 145.4 175.4 155.9 189.1 .168.1
201.0 178.5 . 213.5 189.7 226.0 201.0 243.5 . 216.4 259.1 230.4 275.8 245.1 292.8 260.3 313.1 278.4 334.4 297.1 394.0 350.2 517.5 460.0
666.0 592.0 847.5 753.5 1059.0 94i.O 1318.0 1171.0 1613.0 1434.0 2165.0 1925.0 .
. These factors arc calculated from the formula
where
Q
T
Q
/ 0.1723 X T*\
L loo.ooo.ooo )
total black body radiation, Btu per square foot per hour.:
emissivity. .
_;
absolute temperature, degrees Fahrenheit.
Such a determination of the amount of radiant heating surface needed in a room (to maintain a desired MRT), requires knowledge of the type of heating, and the surface temperatures of the unheated surfaces, which latter can only be estimated-^--but with a considerable degree of accuracy after some experience. -------------------- - .... --...... ..... --......-..... .. -... -
721.