Pressure Drop In Fittings

In addition to the pressure drop that occurs in straight pipe, fittings such as elbows and tees also cause pressure drop. The physical layout of the piping system determines where elbows and branch tees are located. In addition there are noticeable drops in pressure through valves, even when they are wide open. Judgment must be used on where to install valves and what type of valve to choose. Even valves that are rarely shut off may be invaluable in isolating a certain component or even another valve in rare instances. On the other hand, extra valves placed in vapor lines may represent a persistent demand for extra compressor power whenever the system operates.

The magnitude of the pressure drop in fittings or open valves is proportional to the density of the fluid flowing and to the square of the velocity. The pressure drop coefficients, PDC, in SI and I-P are applied as indicated in Eq. 9.8.

When used in the expressions of the form in Eq. 9.8, the same values of PDC apply to both SI and I-P units. Values8 of PDC are presented in Table 9.3.

An important observation that can be made from Table 9.3 is the relative pressure drops of globe and angle valves. An angle valve causes anywhere from 1/2 to 1/6 the pressure drop of a globe valve (depending upon the valve and pipe size) and should be considered if the physical arrangement permits.

Many designers find it convenient to express the pressure drop of fittings and open valves as equivalent length of straight pipe. This equivalent length, Leq, can then simply be added to the actual length of straight pipe to determine the total pressure drop. The form of Eq. 9.8 and the data in Table 9.3 provide the tools to compute Leq, since the PDC term for fittings is comparable to the fL/D group in Eqs. 9.1 and 9.2. A typical value of the friction factor f for refrigeration applications is approximately 0.02. Thus,

Applying Eq. 9.9 to the data in Table 9.3 yields the equivalent lengths of Table 9.4.

The equivalent lengths increase slightly as the pipe diameter increases, which is due to change in ratio of pressure drop of straight pipe and fittings. Several important observations can be made from Table 9.4. One is that the pressure drop in the fittings can be a significant contributor to the total pressure drop. Another is the relative pressure drops of globe and angle valves. A globe valve causes anywhere from 2 to 6 times the pressure drop of an angle valve (depending upon the valve and pipe size). When the physical arrangement permits, an angle valve should be chosen over a globe valve.

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