ASHRAE Journal - September 2009 - 68

ASHRAE 19 This device senses water level in the boiler and enables the burner to fire only if a minimum level exists. If the water level falls below this minimum, the burner circuit is interrupted, and the burner cannot be lighted until the level is restored. In some areas of the country, local codes require that the low water cut-off must be manually reset before the burner can be re-lighted. The two most popular types of low water fuel cut-offs are float-operated switches and conductance probes. The float-operated control is mounted so that boiler level can cause a float to rise and fall. With a satisfactory level in the boiler, the float closes a switch, enabling the burner to fire under the control of the combustion control system. If the boiler level drops, the switch opens to disable the burner. The conductance probe low water cut-off completes an electrical circuit to enable the burner as long as the probe is covered with boiler water. If the boiler level drops and uncovers the probe, the electrical signal to the burner is interrupted and the burner shuts down. While it may not be required, a feeder is often found on low pressure steam boilers. Its job is to maintain at least a minimum level by adding water from the city water supply. Feeders and low water cut-offs are sometimes built into a single combination control. Other common feeder designs use a separate solenoid valve that gets an electric signal from the float-operated low water cut-off, as shown in Figure 6. The efficiency of a steam boiler can be improved by reducing its water content and size. Smaller boilers can generate steam much faster than a larger 68 ASHRAE Journal 50 59–2009 YE A R S JOURNAL until the boiler shuts down due to normal action of its control system. When the thermostat once again calls for heat, we find that the boiler has flooded and cannot generate steam. Because the feeder is responsible for adding water, we conclude that it is faulty and replace it. Unfortunately, the new feeder does not make any difference because the boiler floods again after the next shutdown. To solve this system problem, we must realize that before we replaced the original pump receiver, the water volume in the boiler just happened to be sufficient to fill the system with steam and condensate, and get the pump to start before the boiler level dropped to the point where the feeder added new water. With the newer, much larger pump receiver, it takes more water from the boiler to fill the system and get the pump to start. As steam leaves the boiler, the level continues to drop, opening the feeder and adding new water. Eventually, the pump will start and the feeder will stop adding water, but now the total amount of system fluid is greater. The next time the boiler stops generating steam, all of the condensate from that large receiver will be pumped back to the boiler, flooding it and causing the system to fail. In other words, the amount of boiler water between the lighting-off level and the feeder operating level was sufficient for the original system, but is no longer sufficient for the system with the larger pump receiver. This problem is actually pretty common, but it usually occurs as systems are enlarged, or because of September 2009 boiler, which has to bring a larger volume of water to the boiling point before the first pound of steam can be generated. Smaller volume also means less heat loss to the surrounding air. For these boilers, electronically timed feeders designed to work with the conductance probe low water cutoff should be used. A large flow of water from a solenoid-operated feeder is ideal for a larger volume boiler, but it could flood the smaller boiler. The timing cycle of a modern electronic feeder prevents boiler flooding by waiting for condensate to return from the system before adding just enough water to allow the burner to operate. Feeders are connected between a cold water supply and the boiler condensate returns. Because water supply pressure is usually much greater than 15 psig (103 kPa), feeders can force water into a low pressure steam boiler when the feeder opens. Causes of Boiler Flooding In any engineering system, at least two conditions must be met for satisfactory operation: each component must do the job it was designed to do; and all the components must work together. The simple steam systems described, so far, illustrate the point, because changes that may seem unimportant can cause the system to fail. For example, a two-pipe system equipped with a condensate transfer unit, a boiler and feeder has been operating satisfactorily for years. Suppose that the original condensate unit is replaced by a new one, which is identical in all respects to the original except that it has a much larger receiver. As the heating season starts, the system operates with no problems a s h r a e. o rg

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 - 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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