ASHRAE Journal - September 2009 - 62

ASHRAE 19 a heater filled with steam at 215°F (102°C) and surrounded by air at 70°F (21°C) is measured in the capacity rating test. By definition, each 240 Btu/h (70 W) given off under test conditions defines a capacity of one square foot of equivalent direct radiation (EDR). The EDR definition allows direct comparison of the capacity of different types of heating units. It also provides a way to describe a system’s size. Because each different type of equipment was rated by the same test and described by using the same EDR definition, the sum of the system’s EDR could be used for determining the size of the boiler required and the condensate flow rate to be expected. 50 59–2009 YE A R S JOURNAL tial temperature, the trap’s thermal element cannot sense a difference between steam and condensate until the condensate has subcooled, or has fallen below the steam temperature. Then, the boiling in the bellows stops and the trap opens. Condensate subcooling is an important feature of all thermostatic traps. It means that the condensate must have some place to collect and cool off upstream of the trap (a cooling leg), so that it will be cool enough to open the trap and flow out to the return piping. Notice that the float and thermostatic trap used at the end-of-main does not require condensate subcooling because it is a mechanical trap. The main trap orifice is operated by a float and linkage that responds to condensate level, regardless of condensate temperature. Both traps are required in this system, even if condensate is returned by gravity and there is no vented pump receiver. If either trap allows steam leakage into the return piping, condensate drainage could be hampered. The improvement in condensate drainage that was achieved by the development of two-pipe systems did not entirely solve the problems of noise and water hammer. Although we would like to have all the steam condense and give up its latent heat in the system radiators, it does not happen that way in a real system. As steam flows through a hot pipe, some small amount of heat will be lost from the pipe wall to the cooler surrounding air. This loss of heat is accompanied by the formation of a small amount of condensate. This running load of condensate must be September 2009 Two-Pipe Systems The next major advance in steam system design came when someone recognized the continuing problems that occurred whenever steam and condensate were handled together in the same pipe. Even parallel flow is not totally satisfactory because, for example, piping that was properly pitched at the time of installation may sag due to broken pipe hangers or lose its pitch as the structure settles. When that happens, condensate will collect at low points, and there will be noise, water hammer or flow blockage. Two-pipe systems go a long way toward solving these problems by adding a second pipe to carry condensate away from the radiator. Of course, the price for this improved condensate drainage is an increase in cost for the extra pipe. A two-pipe radiator has two connections: a steam supply located at the 62 ASHRAE Journal top, because it is now one-way flow; and a separate condensate return connection, with another steam trap at the bottom. Separate air vents are not usually installed in two-pipe systems. Therefore, the radiator trap now handles all of the venting jobs that were accomplished by the vent in the onepipe system. The trap also provides a way for condensate to drain without interfering with the steam flow. Zoning is more straightforward and more effective in two-pipe systems because the flow of steam can be throttled by a partly closed valve at the radiator inlet without causing condensate build-up. For example, the nonelectric zone valve shown in Figure 2b can automatically maintain a selected room temperature by regulating the flow of steam into the radiator. More Steam Traps Radiator traps are usually a different design, with smaller capacity than the end-of-main trap because they handle only the radiator’s condensate load. Most radiator traps are simply larger versions of the thermostatic vent in the F&T trap. In the presence of air, liquid in the thermostatic element will not boil and the trap will be open because the air or air/steam mixture is cool. This vents air from steam to enter the radiator. As steam fills the radiator and trap body, the volatile liquid in the trap bellows boils and the trap closes. In fact, the trap will continue to stay closed until the condensate has subcooled by losing some sensible heat through the trap body. Because steam and condensate at a given pressure have the same inia s h r a e. o rg
ASHRAE Journal - September 2009

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