Consulting-Specifying Engineer - April 2008 - (Page 28) Figure 1: The cooling tower as depicted on the contract documents cooling tower piping (left) contrasted with the installed cooling tower piping (right) and also showed that the installation might not match the design intent. Note the symmetrical arrangement proposed by the design versus the non-symmetrical arrangement implied by the tee in the installed piping. Source: Based on design plans prepared by Taylor Engineering. Photo: David Sellers • Ensures uniform flow distribution to each tower cell. If flow is distributed uniformly to each cell, then the cells will perform predictably. • Minimizes the potential for problems with level control. Maintaining tower basin levels can be deceptively difficult—discover why later in this article. The critical detail is the configuration of the tee in the installed piping that splits the flow between the cooling towers. The loss through the tee and the branches it serves will be related to the flow through them. In the limit, if there is no flow through a branch, then there is no loss. If the tee is applied so the water comes in the branch and exits through the run in both directions—or comes into the run from both directions and exits via the branch—then the loss through either branch will be the same and can be more than six times the loss of one of the other configurations (see Figure 2). In this particular situation, the higher loss is actually an advantage because it is equally difficult for the water to split in either direction—the flow will equalize between the two branches if the same difference in pressure is applied across them. It is important to recognize that in this situation, the constraints of physics will dictate that the pressure difference across both branches of the tee will the same. On one end, both branches are exposed to atmospheric pressure while at the other end, they are connected to the same pipe by virtue of the tee. What will happen if the tee is applied where it is easier for water to flow in one direction and the pressure difference to drive flow is the same in either direction? The flow in the constrained direction will drop off, as will the associated loss due to flow. At the same time, the flow in the less constrained direction will increase until everything balances with the pressure drop, due to flow in the less constrained 100 A 50 40 30 B C B C B C B C A A A Number of equivalent elbows 20 15 10 5 4 3 2 1.5 1.0 0.5 0.4 0.3 0.2 0.15 0.10 10 20 A A C 30 40 50 60 70 80 B C B 90 100 Percentage of water flowed through circled branch Figure 2: ASHRAE tee pressure drops in various configurations. Note that there is a factor of 6.7 difference between the highest loss and lowest loss configuration with a 50/50 flow split. Source: ASHRAE Handbook of Fundamentals, 2001, Figure 4, page 35.7 28 Consulting-Specifying Engineer • APRIL 2008
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