ASHRAE Journal - September 2009 - 58

ASHRAE 19 50 59–2009 YE A R S JOURNAL tems is difficult, although not impossible. For example, the self-contained zone vent valve shown in Figure 2a can be installed with a straight shank vent in the radiator vent connection. It has a thermal element that compares actual room temperature to the desired temperature, and vents just enough air to admit just enough steam to maintain the desired temperature. f i g . 2 Self-contained zone vent valve (A); and non-electric zone valve (B). B. A. Piping Problems/Improvements Counterflow piping designs required large steam pipes and significant pitch to carry the two-way flow of steam and condensate. This limited the maximum size of the system, added to its cost and made it difficult to install. Noise is likely in counterflow piping as steam bubbles mix with condensate, condense and collapse. That is why most steam piping today is parallel flow; it is pitched away from the boiler, so steam and condensate flow in the same direction. Parallel flow designs allow the use of smaller pipe because there is less friction loss from steam and condensate interfering with each other. Less pressure drop allows longer pipe runs, and less pitch makes it easier to fit the piping in the structure. Of course, with parallel flow, the problem exists of getting condensate back to the boiler from the end of the system. The solution is to install return piping. Wet returns drop from the end of the main to a level below the boiler waterline, so they are always flooded and do not need to be pitched. Dry returns drop from the end of the steam main, but do not fall below the waterline, so they are not flooded and must be pitched in the direction of flow. They can carry steam, air and condensate. rises in the vertical column until the sum of steam pressure and pressure due to static head at one end of the system balances boiler pressure at the other end. • Water level at the end of the system continues to rise until enough static head is developed to overcome friction losses in the return piping. • Finally, some pipe volume must be allowed to accumulate the heavy load of condensate that forms each time the system piping warms up from the initial temperature to saturation temperature. As system size increased, the value of the A-dimension had to be increased to allow for greater steam pressure drop, more static head and more volume to ensure that the start-up condensate load would not flood the steam mains or radiators. Gravity Condensate Return Many early systems returned condensate to the boiler by using the static head of a vertical column of condensate. For any temperature, the higher the water column, the greater the pressure at its bottom. Steamfitters building a gravity return system provided a vertical pipe run connecting the farthest steam pipe to the returns. This column had to be long enough to contain the A-dimension, which is measured from the boiler level up to the bottom of the steam pipe. The A-dimension required is determined by three factors: • Steam pressure at the far end of the main will be less than boiler pressure due to friction losses. Condensate 58 ASHRAE Journal Hartford Loops and Variations In the early days, many boilers were ruined due to loss of water and consequent overheating. The Hartford Loop piping around the boiler was developed to keep the waterline in the boiler high enough to avoid damage. The Hartford Loop consists of an equalizing pipe connecting the steam header to the condensate inlet at the bottom of the boiler. The wet return is connected to that loop through a close nipple located 2 to 4 in. (5 to 10 cm) below the waterline. The loop allows steam pressure to build in the boiler without forcing water into the return line. It also protects the boiler against low water in the event of a return line leak because the boiler level cannot drop below the close nipple. a s h r a e. o rg September 2009

ASHRAE Journal - September 2009

Table of Contents for the Digital Edition of ASHRAE Journal - September 2009

ASHRAE Journal - September 2009
Contents
Commentary
Industry News
Letters
Meetings and Shows
Getting to Net Zero
Feature Articles
How High Can You Go? Building Height and Net Zero
Lab for Learning
Solar Hot-Water Heating System: Lessons Learned
50th Anniversary—Low Pressure Steam Heating Systems
Building Sciences
Products
Emerging Technologies
People
Classified Advertising
Advertisers Index
ASHRAE Journal - September 2009 - ASHRAE Journal - September 2009
ASHRAE Journal - September 2009 - Cover2
ASHRAE Journal - September 2009 - 1
ASHRAE Journal - September 2009 - 2
ASHRAE Journal - September 2009 - Contents
ASHRAE Journal - September 2009 - 4
ASHRAE Journal - September 2009 - Commentary
ASHRAE Journal - September 2009 - Industry News
ASHRAE Journal - September 2009 - 7
ASHRAE Journal - September 2009 - 8
ASHRAE Journal - September 2009 - 9
ASHRAE Journal - September 2009 - Letters
ASHRAE Journal - September 2009 - 11
ASHRAE Journal - September 2009 - 12
ASHRAE Journal - September 2009 - 13
ASHRAE Journal - September 2009 - 14
ASHRAE Journal - September 2009 - 15
ASHRAE Journal - September 2009 - Meetings and Shows
ASHRAE Journal - September 2009 - 17
ASHRAE Journal - September 2009 - Feature Articles
ASHRAE Journal - September 2009 - 19
ASHRAE Journal - September 2009 - 20
ASHRAE Journal - September 2009 - 21
ASHRAE Journal - September 2009 - 22
ASHRAE Journal - September 2009 - 23
ASHRAE Journal - September 2009 - 24
ASHRAE Journal - September 2009 - 25
ASHRAE Journal - September 2009 - How High Can You Go? Building Height and Net Zero
ASHRAE Journal - September 2009 - 27
ASHRAE Journal - September 2009 - 28
ASHRAE Journal - September 2009 - 29
ASHRAE Journal - September 2009 - 30
ASHRAE Journal - September 2009 - 31
ASHRAE Journal - September 2009 - 32
ASHRAE Journal - September 2009 - 32a
ASHRAE Journal - September 2009 - 32b
ASHRAE Journal - September 2009 - 33
ASHRAE Journal - September 2009 - 34
ASHRAE Journal - September 2009 - 35
ASHRAE Journal - September 2009 - 36
ASHRAE Journal - September 2009 - 37
ASHRAE Journal - September 2009 - Lab for Learning
ASHRAE Journal - September 2009 - 39
ASHRAE Journal - September 2009 - 40
ASHRAE Journal - September 2009 - 41
ASHRAE Journal - September 2009 - 42
ASHRAE Journal - September 2009 - 43
ASHRAE Journal - September 2009 - Solar Hot-Water Heating System: Lessons Learned
ASHRAE Journal - September 2009 - 45
ASHRAE Journal - September 2009 - 46
ASHRAE Journal - September 2009 - 47
ASHRAE Journal - September 2009 - 48
ASHRAE Journal - September 2009 - 49
ASHRAE Journal - September 2009 - 50
ASHRAE Journal - September 2009 - 51
ASHRAE Journal - September 2009 - 52
ASHRAE Journal - September 2009 - 53
ASHRAE Journal - September 2009 - 50th Anniversary—Low Pressure Steam Heating Systems
ASHRAE Journal - September 2009 - 55
ASHRAE Journal - September 2009 - 56
ASHRAE Journal - September 2009 - 57
ASHRAE Journal - September 2009 - 58
ASHRAE Journal - September 2009 - 59
ASHRAE Journal - September 2009 - 60
ASHRAE Journal - September 2009 - 61
ASHRAE Journal - September 2009 - 62
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ASHRAE Journal - September 2009 - 67
ASHRAE Journal - September 2009 - 68
ASHRAE Journal - September 2009 - 69
ASHRAE Journal - September 2009 - 70
ASHRAE Journal - September 2009 - 71
ASHRAE Journal - September 2009 - Building Sciences
ASHRAE Journal - September 2009 - 73
ASHRAE Journal - September 2009 - 74
ASHRAE Journal - September 2009 - 75
ASHRAE Journal - September 2009 - 76
ASHRAE Journal - September 2009 - 77
ASHRAE Journal - September 2009 - 78
ASHRAE Journal - September 2009 - 79
ASHRAE Journal - September 2009 - 80
ASHRAE Journal - September 2009 - 81
ASHRAE Journal - September 2009 - Products
ASHRAE Journal - September 2009 - 83
ASHRAE Journal - September 2009 - Emerging Technologies
ASHRAE Journal - September 2009 - 85
ASHRAE Journal - September 2009 - 86
ASHRAE Journal - September 2009 - 87
ASHRAE Journal - September 2009 - 88
ASHRAE Journal - September 2009 - 89
ASHRAE Journal - September 2009 - People
ASHRAE Journal - September 2009 - 91
ASHRAE Journal - September 2009 - 92
ASHRAE Journal - September 2009 - Classified Advertising
ASHRAE Journal - September 2009 - 94
ASHRAE Journal - September 2009 - 95
ASHRAE Journal - September 2009 - Advertisers Index
ASHRAE Journal - September 2009 - Cover3
ASHRAE Journal - September 2009 - Cover4
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