Water

Water is a chemical compound of two gases, oxygen and hydrogen, in the proportion of two parts by weight of hydrogen to 16 parts by weight of oxygen, having mixed with it about 5 percent of air by volume at 14.7 lb absolute pressure. It may exist as ice, water, or steam due to changes in temperature (water freezes at 32°F and boils at 212°F when the barometer reads 29.921 in).

One cubic foot of water weighs 62.41 lb at 32°F and 59.82 lb at 212°F. One U. S. gallon of water (231 in3) weighs 8.33111 lb (ordi­narily expressed as 8V3 lb) at a temperature of 62°F. At any other temperature, of course, the weight will be different (Table 2-2).

Table 2-2 Weight of Water per Cubic Foot at Different Temperatures

Temp.,

°F

Lb per ft3

Temp.,

°F

Lb per ft3

Temp.,

°F

Lb per ft3

Temp.,

°F

Lb per ft3

32

62.41

55

62.38

78

62.23

101

61.98

33

62.41

56

62.38

79

62.22

102

61.96

34

62.42

57

62.38

80

62.21

103

61.95

35

62.42

58

62.37

81

62.20

104

61.94

36

62.42

59

62.37

82

62.19

105

61.93

37

62.42

60

32.36

83

62.18

106

61.91

38

62.42

61

62.35

84

62.17

107

61.90

39

62.42

62

62.35

85

62.16

108

61.89

40

62.42

63

62.34

86

62.15

109

61.87

41

62.42

64

62.34

87

62.14

110

61.86

42

62.42

65

62.33

88

62.13

111

61.84

43

62.42

66

62.32

89

62.12

112

61.83

44

62.42

67

62.32

90

62.11

113

61.81

45

62.42

68

62.31

91

62.10

114

61.80

46

62.41

69

62.30

92

62.08

115

61.78

47

62.41

70

62.30

93

62.07

116

61.77

48

62.41

71

62.29

94

62.06

117

61.75

49

62.41

72

62.28

95

62.05

118

61.74

50

62.40

73

62.27

96

62.04

119

61.72

51

62.40

74

62.26

97

62.02

120

61.71

52

62.40

75

62.25

98

62.01

121

61.69

53

62.39

76

62.25

99

62.00

122

61.68

54

62.39

77

62.24

100

61.99

123

61.66

Table 2-2 (continued)

Temp.,

°F

Lb per ft3

Temp.,

°F

Lb per ft3

Temp.,

°F

Lb per ft3

Temp.,

°F

Lb per ft3

124

61.64

160

60.99

196

60.21

380

54.47

125

61.63

161

60.97

197

60.19

390

54.05

126

61.61

162

60.95

198

60.16

400

53.62

127

61.60

163

60.93

199

60.14

410

53.19

128

61.58

164

60.91

200

60.11

420

52.74

129

61.56

165

60.89

201

60.09

430

52.33

130

61.55

166

60.87

202

60.07

440

51.87

131

61.53

167

60.85

203

60.04

450

51.28

132

61.51

168

60.83

204

60.02

460

51.02

133

61.50

169

60.81

205

59.99

470

50.51

134

61.48

170

60.79

206

59.97

480

50.00

135

61.46

171

60.77

207

59.95

490

49.50

136

61.44

172

60.75

208

59.92

500

48.78

137

61.43

173

60.73

209

59.90

510

48.31

138

61.41

174

60.71

210

59.87

520

47.62

139

61.39

175

60.68

211

59.85

530

46.95

140

61.37

176

60.66

212

59.82

540

46.30

141

61.36

177

60.64

214

59.81

550

45.66

142

61.34

178

60.62

216

59.77

560

44.84

143

61.32

179

60.60

218

59.70

570

44.05

144

61.30

180

60.57

220

59.67

580

43.29

145

61.28

181

60.55

230

59.42

590

42.37

146

61.26

182

60.53

240

59.17

600

41.49

147

61.25

183

60.51

250

58.89

610

40.49

148

61.23

184

60.49

260

58.62

620

39.37

149

61.21

185

60.46

270

58.34

630

38.31

150

61.19

186

60.44

280

58.04

640

37.17

151

61.17

187

60.42

290

57.74

650

35.97

152

61.15

188

60.40

300

57.41

660

34.48

153

61.13

189

60.37

310

57.08

670

32.89

154

61.11

190

60.35

320

56.75

680

31.06

155

61.09

191

60.33

330

56.40

690

28.82

156

61.07

192

60.30

340

56.02

700

25.38

157

61.05

193

60.28

350

55.65

158

61.03

194

60.26

360

55.25

159

61.01

195

60.23

370

54.85

Water changes in weight with changes of temperature. That is, the higher the temperature of the water, the less it weighs. It is this property of water that causes circulation in boilers and in hot-water heating systems. The change in weight is due to expansion and a reduction in water volume. As the temperature rises, the water expands, resulting in a unit volume of water containing less water at higher temperature than lower temperature.

Fill a vessel with cold water and heat it to the boiling point. Note that boiling causes it to overflow due to expansion. Now let the water cool. You will note that when the water is cold, the vessel will not be as full because the water will have contracted.

The point of maximum density of water is 39.1°F. The most remarkable characteristic of water is its expansion below and above its point of maximum density. Imagine 1 lb of water at 39.1°F placed in a cylinder having a cross-sectional area of 1 in2 (Figure 2-13). The water having a volume of 27.68 in3 will fill the cylinder to a height of 27.68 in. If the water is cooled, it will expand, and at, say, 32°F (the freezing point) will rise in the tube to a height of 27.7 in before freezing. If the water is heated, it will also expand and rise in the tube; and at the boiling point (for atmospheric pressure 212°F) it will occupy the tube to a height of 28.88 in.

The elementary hot-water heating system in Figure 2-14 illus­trates the principle of thermal circulation. The weight of the hot

Water

Figure 2-13 The point of maximum density.

HIGH TEMPERATURE

Water

And expanded water in the upflow column C, being less than that of the cold and contracted water in the downflow column C’, upsets the equilibrium of the system and results in a continuous circula­tion of water as indicated by the arrows. In other words, the heavy, low-temperature water sinks to the lowest point in the boiler (or system) and displaces the light, high-temperature water, thus caus­ing continuous circulation as long as there is a temperature differ­ence in different parts of the boiler (or system). This is referred to as thermal circulation.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers