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 TABLE 4-1. Heat Storage Comparison
TABLE 4-2. Density and Volume
of HTW and Steam
Variations of HTW
With Temperature
Saturated
Heat
Density
Spec. Volume
Total Heat Content
Saturated
Content
Tempera-
Temperature
(Btu/ft')
(lb/ft')
Ratio
ft'/lb
Absolute
ture
Pressure
HTW/
(F)
(psia)
HTW
Steam
Steam
62.30
0.01606
70
0.01700
58.82
250
29.82
12,852
84.22
152.6
250
0.01709
58.51
260
35.43
260
13,378
99.23
134.8
58.24
270
0.01717
41.86
270
13,910
116.35
119.5
57.94
0.01726
280
49.20
14,430
135.77
280
106.3
0.01735
57.64
290
57.56
14,946
157.73
290
95.17
57.31
300
0.01745
67.01
15,449
182.44
300
84.68
0.01755
56.98
310
77.68
15,950
310
210.18
75.88
56.66
320
0.01765
89.66
16,446
320
241.19
68.19
330
56.31
0.01776
103.06
16,930
330
275.76
61.39
0.01787
340
55.96
118.01
17,411
340
314.18
55.42
350
55.59
0.01799
134.63
17,878
350
356.76
50.10
153.04
0.01811
360
360
18,342
392.29
55.22
55.70
0.01823
370
54.85
370
173.37
18,803
455.73
41.26
195.77
19,251
0 .0183b
380
513.10
380
54.47
37.52
0.01850
390
54.05
220.37
19,685
390
34.21
575.73
400
0.01864
400
53.65
247.31
20,116
31.21
644.55
410
0.01878
410
53.25
276.75
20,545
719.82
28.54
420
420
0.01894
308.83
20,949
52.80
802.07
26.09
430
0.01910
52.36
430
343.72
21,350
891.76
23.94
440
0.01926
51.92
440
381.59
21,750
989.48
21.98
450
0.01940
51.50
450
422.6
22,170
1,095.79
20.23
difference between the supply and the return water consistent with economical
considerations.
The amount of heat extracted from a given amount of water
e f f e c t s the temperature differential between the supply and the return water.
The greater the temperature difference, the more heat obtained per unit of
water. Higher heat extraction permits cutting down the flow rates, which
reduces the size requirements of the distribution pipes and pumps, and the
pump horsepower. The usual temperature differential range for an HTW system
is 100F to 150F. A maximum differential of 200F is practical with forced
c i r c u l a t i o n b o i l e r s . F o r natural circulation boilers, a maximum differential
o f 250F is considered practical.
4 EXPANSION TANKS. All HTW systems require expansion tanks to allow for
variations in the system water volume caused by temperature changes. Volume
expansion is not based on cold water conditions because that extreme variation
only occurs when starting up cold. Rather, the tank is sized to handle
changes in water volume resulting from normal load changes. I n general, the
f o l l o w i n g factors are considered in sizing a tank.
Sludge and suction space at the bottom of the tank. From 6 to 9
inches of elevation are reserved for this purpose.
4-3
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