ASHRAE Journal - September 2009 - 60

ASHRAE 19 50 59–2009 YE A R S JOURNAL As the water level falls to the nipple, steam fig. 3 pressure and the height of the column of water in the boiler are exactly balanced in the loop, preventing further flow. Hartford Loops are always installed where condensate is returned to the boiler by gravity. Many heating systems employing variations of the basic design ideas shown in Figure 3 still operate today. For example, a steam main in the basement could supply radiators on upper floors through vertical upfeed risers, or a steam main in the attic could supply lower radiators through a downfeed riser. A whole body of piping installation practices was developed by contractors and steamfitters, who recognized the requirements for steam flow, condensate drainage, thermal expansion and support. Large multistory buildings were successfully heated by systems incorporating these ideas and practices. However, as systems got larger, we eventually ran out of vertical distance to get the necessary A-dimension, and gravity condensate return no longer worked. It was time to add a pump to return condensate to the boiler. Low pressure steam system terminology. Condensate Pumps Simple in-line pumps installed in the return piping could not work because of uneven condensate flow; for example, heavy flow on start-up and no flow during the off-cycle. Those pumps are designed for closed, flooded systems where they always have water to pump. Steam systems require a condensate transfer unit composed of a receiver or storage tank, and a pump that is controlled by a float switch mounted in the receiver. It sounds simple enough, but the addition of a pump receiver changes system operation in several important ways. By its nature, the condensate pump is a cycling pump. It comes on as the level in the receiver rises and turns off when the level drops, so it increases the time lag in condensate return. In gravity return systems, a drop of condensate falling into the vertical A-dimension column immediately forces another drop of water into the boiler with no time lag. A drop of condensate falling into the pump receiver will not be returned to the boiler until the condensate level rises high enough to turn on the pump. The receiver adds vol60 ASHRAE Journal ume to the system that must be filled before condensate can return to the boiler, and that takes time. The receivers in these low pressure condensate pumps are not designed to operate with internal pressure. It is dangerous to operate them unless they are vented to the atmosphere to prevent pressure build-up. The receiver vent acts to prevent system air binding, but it also provides an opening in the system where steam can be lost. Therefore, another component—the steam trap—has to be added to the system. The trap is a kind of automatic valve installed between the end of the main and the vented condensate receiver. It closes to prevent the passage of steam and opens to allow condensate and air to flow to the receiver. The float and thermostatic trap (or F&T trap, as shown in Figure 4) is widely used in all kinds of steam systems. It has a float that rises and falls with condensate level in the trap body. The float operates a valve, opening it as the float rises to allow condensate to flow, but closing it as the level falls to keep steam from flowing past the trap. A small amount of condensate always covers the trap orifice, providing an effective seal to keep steam from escaping. Of course, that water seal also prevents venting which, as we have seen, is important to avoid air binding. For this reason, a separate thermostatic vent element is included in the trap body. This is a small bellows assembly, filled with a volatile fluid, and attached to a separate vent 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
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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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