Problem Statement

PRODUCT: AFT Arrow

MODEL FILE: AroVerify6.ARO

REFERENCE: Crane Co., Flow of Fluids Through Valves, Fittings, and Pipe, Technical Paper No. 410, Crane Co., Joliet, IL, 1988, Page 4-13, example 4-20

GAS: Steam

ASSUMPTIONS:

Example does not specify the heat transfer conditions, but it appears from the calculation procedure that adiabatic flow is assumed. The AFT Arrow model assumes adiabatic.

RESULTS:

DISCUSSION:

Crane does not make a distinction between static and stagnation pressure, and it appears that static pressure is usually assumed. However, the problem statement is that the source steam comes from a header, where conditions are more likely stagnation. Therefore, stagnation pressure was assumed in the AFT Arrow model.

The Crane Sonic Velocity Formula yields a flow rate prediction that agrees well with the AFT Arrow prediction. The Crane Modified Darcy Formula appears to yield flow rates that are too high. If the inlet conditions in the AFT Arrow model are changed to static, the AFT Arrow flow rate prediction increases slightly to 11,498.4 (lbm/hr). AFT Arrow uses real gas properties for the steam as specified in the Fluid Panel.

The Crane sonic calculation assumes an isenthalpic process, which is why the exit static enthalpy is assumed to be constant at 1,196 Btu/lbm. With this assumption, the exit temperature is 317 deg. F.

However, an isenthalpic assumption turns out to be poor when the Energy Equation is applied. From the First Law,

If the process is adiabatic, the heat transfer is zero and therefore,

Thus, the static enthalpy will drop because of the velocity increase. When this is accounted for, the exit static enthalpy decreases to 1149. This yields an exit static temperature of 243.0 deg. F, which is 74 degrees cooler.

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